Innovative Structural Materials: Reducing Weight of Transportation Equipment (Springer Series in Materials Science, 336) [1st ed. 2023] 9819935210, 9789819935215

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Springer Series in Materials Science 336

Teruo Kishi   Editor

Innovative Structural Materials Reducing Weight of Transportation Equipment

Springer Series in Materials Science Volume 336

Series Editors Robert Hull, Center for Materials, Devices, and Integrated Systems, Rensselaer Polytechnic Institute, Troy, NY, USA Chennupati Jagadish, Research School of Physics and Engineering, Australian National University, Canberra, ACT, Australia Yoshiyuki Kawazoe, Center for Computational Materials, Tohoku University, Sendai, Japan Jamie Kruzic, School of Mechanical and Manufacturing Engineering, UNSW Sydney, Sydney, NSW, Australia Richard Osgood Jr., Columbia University, Wenham, MA, USA Jürgen Parisi, Universität Oldenburg, Oldenburg, Germany Udo W. Pohl, Department of Materials Science and Engineering, Technical University of Berlin, Berlin, Germany Tae-Yeon Seong, Department of Materials Science and Engineering, Korea University, Seoul, Korea (Republic of) Shin-ichi Uchida, Electronics and Manufacturing, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan Zhiming M. Wang, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, China

The Springer Series in Materials Science covers the complete spectrum of materials research and technology, including fundamental principles, physical properties, materials theory and design. Recognizing the increasing importance of materials science in future device technologies, the book titles in this series reflect the state-ofthe-art in understanding and controlling the structure and properties of all important classes of materials.

Teruo Kishi Editor

Innovative Structural Materials Reducing Weight of Transportation Equipment

With contributions by Hiroshi Fukutomi, Tomoaki Hyodo, Sakurako Kotobuki

Editor Teruo Kishi Innovative Structural Materials Association Tokyo, Japan With Contrib. by Hiroshi Fukutomi Innovative Structural Materials Association Tokyo, Japan

Tomoaki Hyodo Innovative Structural Materials Association Tokyo, Japan

Sakurako Kotobuki Innovative Structural Materials Association Tokyo, Japan

ISSN 0933-033X ISSN 2196-2812 (electronic) Springer Series in Materials Science ISBN 978-981-99-3521-5 ISBN 978-981-99-3522-2 (eBook) https://doi.org/10.1007/978-981-99-3522-2 Translation from the Japanese language edition: “Kakushin Kozo Zairyo to Maruchi Materiaru: Yusoyokiki no Keiryouka no tameno Zairyo, Setsugo, Sekkei Gijustu” by Teruo Kishi et al., © Authors 2023. Published by Ohmsha, Ltd. All Rights Reserved. © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

On the Publication of this Book

Materials are classified as structural materials or functional materials. Historically, however, there has been strong demand for dramatic progress in the development of the structural materials that make up transportation equipment, general machinery, robots, and social infrastructure. In particular, exports play a crucial role in Japan, which has few natural resources, and the country’s three leading exports are first, automobiles, second, basic materials, and third, general machinery, all of which depend heavily on structural materials. Thus, achieving substantial weight reductions in transportation equipment by developing innovative structural materials, which will lead to a reduction in CO2 emissions in the future, is an urgent challenge for the industrial development of Japan. The “Future Pioneering Project: Technology Development of Innovative Structural Materials” (hereinafter referred to as “the Project” in this book) which was commissioned by Japan’s Ministry of Economy, Trade and Industry (METI) and began in October 2013, was launched as a project under the direct control of METI based on the establishment of the Innovative Structural Materials Association (abbreviated ISMA). From the second year, a “dream team” of industry, universities, and government organizations was assembled as a project of Japan’s New Energy and Industrial Technology Development Organization (NEDO) and continued its activities for 10 years, aiming at the development of innovative materials with drastically improved performance. As the key points here, while of course focusing on the mechanical properties required in structural materials, material development was carried out while also bearing in mind (i) LCA (Life Cycle Assessment) evaluation considering energy and the environment, (ii) Consideration of rare metals and rare earths (element strategy), and (iii) Construction of a recycling-based circulation society. With the participation of more than 30 automobile companies and material makers and 80 universities and research and development corporations, ISMA conducts research and development of ultra-high strength steels, ultra-high strength aluminum alloys and scandium-added aluminum alloys, heat-resistant magnesium alloys and

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titanium alloys as innovative metal materials, taking on the difficult challenge of overcoming the “banana curve” relationship, in which ductility (elongation) and toughness decrease with increasing strength. In addition to the development of carbon fiber materials with even higher strength, ISMA has also devoted considerable effort to the development of carbon fiber reinforced thermoplastics (CFRTP) using discontinuous fibers with the aim of improving the elastic modulus, strength, and formability. Since welding and joining are essential when using these materials, in the first 5 years of the Project, we focused mainly on welding and joining technologies for developed materials of the same type and worked to ensure bond strength by introducing innovative techniques in the resistance spot welding, arc welding, laser welding and friction welding methods. In the second half of the Project, we took up the challenge of multi-materials using various materials developed by ISMA, considering the optimum combination of materials for the optimum part. When joining dissimilar materials, mechanical fastening methods such as bolts and rivets have been widely used until now, but in addition to the welding and joining techniques developed in the first half of the Project, we also began the development of new adhesives. When using the developed materials, it is necessary to solve the problem of deterioration due to time-dependent fracture of joints of dissimilar materials. Concretely, this means deterioration due to corrosion, galvanic corrosion, hydrogen embrittlement, fatigue fracture, and similar failure modes. Therefore, ISMA proposed a galvanic corrosion evaluation method which takes into account the environments in which automobiles are actually used and evaluated durability in actual environments by applying computational science. We also clarified the behavior of deterioration and crack propagation due to simultaneous fatigue and corrosion fatigue acting on a joint, and are working toward the construction of a database. To inspect defects of welded joints and adhesive joints, we created prototypes of a new laser ultrasonic system and neutron beam analysis system, and have applied these devices in practical use. Simultaneously with material development, when aiming at auto body weight reduction, it is also necessary to develop CAE (computer-aided engineering) for obtaining the proper material arrangement and material geometry when using multimaterials. In the Project, ISMA newly developed a topology optimization technique based on the level set approach and conducted trials to obtain the optimal material arrangement for weight reduction. The results suggested that it is amply possible to achieve a weight reduction of 40–50%, including “outer lid” parts such as the roof and front fenders of an automobile. At the same time, we are developing CAE for auto collision safety (crashworthiness). On the other hand, we also developed a CAE technique for additive manufacturing (3D printing) materials which can be applied to casting of thick parts and verified this technique experimentally. To validate the results of these CAE techniques, we are conducting part prototyping using materials developed by ISMA. We are also developing recycling technologies with the aim of realizing a recycling-based society. As part of the Project, we carried out pioneering research and development on the feasibility of recycling aluminum alloys and CFRP. At the same time, we established a method for assessing environmental loads in terms of LCA (Life Cycle Assessment), which incorporates resource extraction, the

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manufacture of the product, distribution and use, and disposal and recycling in an integrated manner, and applied this assessment method to the developed materials. In promoting quick material development in the future, in addition to the theoretical, experimental, and simulation approaches, the establishment of a material integration technique for obtaining the desired material from an inverse problem analysis using a database will be indispensable. As one example, we attempted to apply this approach to the development of magnesium alloys. To achieve practical application and commercialization of the results of the Project, to accumulate research and development data and lead pioneering research in the field of structural materials, 5 research centers have been established to carry on and develop the spirit of the ISMA research association. At this time, however, I would like to add that the proposal of a TRL (Technology Readiness Level) evaluation method, which shows the degree of technical maturity of results as a quantitative index of the level of development outcomes, is an important item that should also be considered. Finally, I would like to note that the outline presented above was prepared bearing in mind that the first volume of this book shows guidelines for proceeding with the development of innovative structural materials, while the second volume integrates important data for developing structural materials and research outcomes contributing to database construction in light of the coming era of data science. Teruo Kishi

Contents

1 Background of This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Teruo Kishi and Tomoaki Hyodo

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2 Materials Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tomoaki Hyodo, Toshio Murakami, Kaori Kawano, Yuki Toji, Takao Horiya, Tadashi Minoda, Yasumasa Chino, Hiroaki Hatori, Ken-ichi Shida, Shu Yamashita, and Takashi Ishikawa

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3 Materials Integration—Data-Driven Approach to Materials Design Using Simulation and Database . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Manabu Enoki and Takao Horiya 4 Welding and Joining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Yoshinori Hirata, Hidetoshi Fujii, Chiaki Sato, and Hisashi Serizawa 5 Analysis and Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Sakae Fujita, Shusaku Takagi, and Yoshio Akimune 6 Structural Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 Shinji Nishiwaki, Shu Yamashita, Akihiko Chiba, and Takao Horiya 7 Prototyping of Multi-material Parts—Efforts to Realize Practical Application of Innovative Materials and Technologies . . . . . 319 Koji Chiba 8 Recycling and Lifecycle Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 Toshiyuki Seko, Shu Yamashita, Ken-ichi Shida, and Ichiro Daigo 9 Review and Future Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 Yoshio Akimune

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Contributors

Yoshio Akimune Innovative Structural Materials Association, YurakuchoTokyo, Japan Akihiko Chiba Tohoku University, Sendai, Miyagi, Japan Koji Chiba Innovative Structural Materials Association, YurakuchoTokyo, Japan Yasumasa Chino National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan Ichiro Daigo The University of Tokyo, Komaba, Tokyo, Japan Manabu Enoki The University of Tokyo, Hongo, Tokyo, Japan Hidetoshi Fujii Osaka University, Mihogaoka, Ibaraki, Osaka, Japan Sakae Fujita Innovative Structural Materials Association, YurakuchoTokyo, Japan Hiroaki Hatori National Institute of Advanced Industrial Science and Technology, Onogawa, Tsukuba, Ibaraki, Japan Yoshinori Hirata Osaka University, Suita, Osaka, Japan Takao Horiya Innovative Structural Materials Association, Chiyoda-ku, Tokyo, Japan Tomoaki Hyodo Innovative Structural Materials Association, Chiyoda-Ku, Tokyo, Japan Takashi Ishikawa Nagoya University, Nagoya, Aichi, Japan Kaori Kawano Nippon Steel Corporation, Shintomi, Futtsu, Chiba, Japan Teruo Kishi Innovative Structural Materials Association, Chiyoda-Ku, Tokyo, Japan Tadashi Minoda UACJ Corporation, ChitoseNagoya, Aichi, Japan Toshio Murakami Kobe Steel, Ltd, Takatsukadai, Kobe, Hyogo, Japan xi

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Shinji Nishiwaki Kyoto Univrsity, Nishikyo-ku, Kyoto, Japan Chiaki Sato Tokyo Institute of Technology, Yokohama, Kanagawa, Japan Toshiyuki Seko Innovative Structural Materials Association, Yurakucho, Tokyo, Japan Hisashi Serizawa Osaka University, Mihogaoka, Ibaraki, Osaka, Japan Ken-ichi Shida Innovative Structural Materials Association, Yurakucho, Tokyo, Japan Shusaku Takagi JFE Steel Corporation, Mizushima, Kurashiki, Japan Yuki Toji JFE Steel Corporation, Chiba, Japan Shu Yamashita Innovative Structural Materials Association, Chiyoda-ku, Tokyo, Japan

Chapter 1

Background of This Book Teruo Kishi and Tomoaki Hyodo

1.1 Background 1.1.1 Initiatives for Global Environmental Problems and Decarbonization Reduction of carbon dioxide (CO2 ) emissions and reduction of energy consumption are important international issues, and various initiatives to address these problems are underway in the industrial, transportation and consumer sectors. In 2019, Japan discharged approximately 3.2% of total global CO2 emissions and was the world’s fifth largest CO2 emitter by country, following China, the United States, India and Russia [1]. Of Japan’s total CO2 emissions of 1,044 million tons in FY 2020, by sector, emissions from the transportation sector (185 million tons) occupied 17.7% of this total, while all automobiles accounted for 87.6% of CO2 emissions in the transportation sector (15.5% of Japan’s total emissions). Of that amount, passenger vehicles discharged 48.4% of emissions in the transportation sector (8.5% of Japan’s total emissions) [2]. Thus, the development of technologies for reducing CO2 emissions from automobiles is a critical issue. All countries are grappling with reduction of CO2 emissions [3]. Japan has set a target of reducing CO2 emissions by 46% by FY 2030 in comparison with 2013, and will also continue the challenge of achieving a 50% reduction. Similar CO2 emission reduction targets have been announced by China, the EU, India, the Russia Federation, the United States and others, and it is particularly noteworthy that all T. Kishi · T. Hyodo (B) Innovative Structural Materials Association, 1-9-4, Yurakucho, Chiyoda-Ku, Tokyo 100-0006, Japan e-mail: [email protected] T. Kishi e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 T. Kishi (ed.), Innovative Structural Materials, Springer Series in Materials Science 336, https://doi.org/10.1007/978-981-99-3522-2_1

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countries have set targets of net zero CO2 emissions by 2050 to 2070, aiming at achieving this goal around the middle of this century.

1.1.2 Innovation and Materials Revolution There is a close relationship between materials and innovation. Historically, the development of materials, that is, the Stone Age, Earthenware Age, Bronze Age and Iron Age, was a source of innovation. In more recent years as well, materials innovation became a driving force for innovation, as shown in Fig. 1.1, as the development of iron and steel materials in the second half of the 1800s, polymer materials and aluminum alloys at the beginning of the 1900s, and ceramics, semiconductors and composite materials after the Second World War led to mass transportation, mass consumption, high economic growth and the IT revolution. What is important here is that the materials that gave rise to these great innovations did not disappear as new materials were created, but continued to be used and new materials were added to the mix. Around the beginning of the twenty-first century, nanomaterials attracted attention, centering on carbon nanomaterials. More recently, interest has focused on initiatives for realizing a sustainable society that considers the environment and energy, as exemplified by element strategy, the UN’s Sustainable Development Goals (SDGs), Life Cycle Assessment (LCA) and recycling, and high expectations are now placed on innovation through materials, in other words, the materials revolution. In the twentieth century, materials research was a trial-and-error process based on experimentation, but with the development of analytical instruments such as the transmission electron microscope (TEM) and the electron probe microanalyzer (EPMA), theoretical interpretation at the atomic and molecular levels became possible in the 1950s. As a result, rather than repeatedly conducting large-scale experiments such as automobile collision experiments, simulation-based performance prediction

Fig. 1.1 Relationship between materials and innovation

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(forward problem analysis) by simulation, following the introduction of theory, was actively utilized, and inverse problem analysis (materials integration) that seeks target materials has now become a focus of interest. Materials informatics links the structures with properties of materials, mainly by first-principles calculations [4], and is also considered to be a material search technique. In contrast, materials integration (MI) is defined as a comprehensive tool of materials technology which aims to support research and development of materials from the engineering point of view by introducing AI and other techniques, by integrating all science and technology, including theory, experiments, analysis, simulation, databases, etc., for the purpose of developing “materials must be used” [5]. The main target is to substantially reduce research and development time and costs when developing structural materials by making full use of computational material science, databases, informatics, etc. [6]. Figure 1.2 shows the relationship between MI and the component elements of research and development. Materials have long been considered to constitute the elements of processing, structure, properties and performance, but considered from the viewpoint of “materials must be used,” performance has a particularly large meaning. In making the best possible use of a material, the whole of the structure, properties, performance and processes of the material must be treated in an integral manner. While materials informatics encompasses the elements of structures and properties, in materials integration, stores information concerning the “hard” aspects of experiments (materials or atom) and “soft” information (digital or bit) in a database, and then reintegrates that information by a computational science approach. In future materials development, it is considered that promotion of the soft aspect will shorten development time and lead to major materials revolutions. It can truly be said that the materials science of the twenty-first century is an era of data science that integrates materials research and information science.

1.1.3 Significance of Research and Development of Structural Materials Unlike functional materials, which use the electrical, optical and magnetic properties, in structural materials particularly high importance is placed on mechanical properties. The points to note in research and development of structural materials are summarized in Fig. 1.3. Structural materials comprise mainly metals, ceramics, polymers and composites, and their mechanical properties are the highest priority. Considered from the viewpoint of “materials must be used,” performance has a large meaning. In the development of structural materials for use in transportation equipment, reduction of the weight of the material itself is frequently a target, but it is still necessary to develop lightweight structural materials that secure functions such as high strength, high ductility (elongation), flame retardance, corrosion resistance, and

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Fig. 1.2 Relationship between materials integration (MI) and 4 component elements of materials research

impact resistance. At the same time, the development of welding and joining technology, forming technology, and other application technologies that will not impair these functions is also demanded. Moreover, since the multi-materials approach of “using the optimum lightweight material at the optimum location” has been applied in the development of the automotive weight reduction technologies of recent years, it is also important to carry out research and development on welding and joining of dissimilar materials with greatly different material properties, performance evaluation techniques for joints, which are indispensable for assuring the safety of transportation equipment, CAE (topology optimization) and, as a major challenge for the future, evaluation techniques for recyclability and Life Cycle Assessment (LCA). From the industrial viewpoint, the automotive sector and the materials sector are especially important for Japan, as the auto industry and materials industry account for a large proportion of Japan’s exports. Figure 1.4 shows a summation by the authors of the data of foreign trade statistics (principal commodities) published by the Ministry of Finance, Japan [7]. Of the total values of exports of ¥83.1 trillion in 2021, transportation equipment (automobiles, parts, motorcycles, aircraft, ships, etc.) accounted for ¥16.2 trillion (19.5%) and industrial materials (sum of chemical products such as plastics and organic compounds plus products by raw material, such as steels, nonferrous metals, etc.) accounted for ¥20.5 trillion (24.6%). These two sectors occupied a large percentage of total exports, and were among Japan’s top-ranked exports. Moreover, since both general machinery (¥16.4 trillion, 19.7%) and electrical machinery (¥15.3 trillion, 18.4%) are also the result of having materials, the reader can understand the importance of the Project, which is responsible for materials for automobiles, but is also directly related to general machinery. By material, the export value of these materials is large in the order of iron and steel materials, plastics, nonferrous metals and organic compounds.

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Fig. 1.3 Development of innovative structural materials

Fig. 1.4 Export value by sector in Ministry of Finance trade statistics (principal commodities) [7]

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1.2 Effects of Auto Body Weight Reduction on Suppression of CO2 Emissions and Improvement of Fuel Consumption 1.2.1 Relationship of Body Weight and CO2 Emissions and Fuel Consumption Reduction of the body weight of automobiles is effective for improving fuel consumption, and in turn, reducing CO2 emissions. The relationship between auto weight and CO2 emissions is shown in Fig. 1.5 [8]. The issues related to reduction of CO2 emissions (improvement of fuel consumption) in automobiles include improvement of powertrain efficiency, beginning with the engine, reduction of body weight, reduction of air resistance, and others, but body weight reduction has a large CO2 emission reduction effect. Figure 1.6 shows the transition of regulations related to CO2 emissions in various countries [9]. The figure also shows fuel consumption (km/L) corresponding to CO2 emissions (g/km). In recent years, there have been active moves to strengthen CO2 regulations in many countries. Europe, which has the strictest regulations, requires a 55% reduction in fuel consumption from the level of 2021, to be achieved by 2030. These moves are not limited to the European countries, as China, India and other countries are also making similar moves.

1.2.2 Trends in Electrification of Automobiles In response to moves to achieve carbon neutrality in 2050, various countries have announced bans on the sale of gasoline vehicles and diesel vehicles taking effect in the 2030–2040s. Specifically in 2030, the United States will require that 50% of new cars sold be electric vehicles (EVs), fuel cell vehicles (FCVs) or plugin hybrid vehicles, and in 2035, Japan will require that all new cars be electrified vehicles, (which also include hybrids and plugin hybrids), Europe will in principle ban the sale of new gasoline-powered vehicles including hybrids (HV), and China will require that all new cars sold be electrified vehicles (HV: 50%, EV + PHV + FCV: 50%). In response to these regulations, auto makers have announced that they will increase their ratio of electrified vehicles to 30–100% [10]. However, the weight of these electrified vehicles tends to increase due to the battery and motor. Concretely, the weight of the battery increases when the maker increases battery capacity to extend a vehicle’s cruising range, and since the weight of the vehicle increases by 200–400 kg in comparison with conventional gasoline and diesel automobiles, the multi-materials approach is increasingly adopted to suppress this increase in the total weight of vehicles [11].

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Fig. 1.5 Relationship between auto body weight and CO2 emissions (Figure from the Ministry of Land, Infrastructure, Transport and Tourism website [8] with author’s additions)

Fig. 1.6 Transition of CO2 emission regulations in various countries/regions (Figure prepared by the author based on data from [9])

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Fig. 1.7 Example of multi-material application for vehicle weight reduction [12]

1.3 Initiatives Related to Multi-Material Design of Auto Bodies 1.3.1 Trends in Multi-Material Auto Bodies Application of multi-materials to automobile bodies, that is, use of the optimum high strength steels and lightweight materials in the optimum parts, has been promoted in the development of auto weight reduction technologies of recent years. Figure 1.7 shows a schematic diagram of the application of multi-materials for auto body weight reduction [12]. Multi-material design using ultra-high strength steel sheets, aluminum alloys and carbon fiber reinforced plastics (CFRP) has been actively applied in luxury automobiles and high-end sports cars, led by the European auto makers. For example, the BMW 7 Series, which was announced in 2015, was produced using hot stamped ultra-high strength steel materials, aluminum alloys and thermosetting CFRP. The BMW i3, which was put on the market in 2013, was the first mass-production to use CFRP in most of the auto body, and a large amount of CFRP was also used in the BMW i8, which was announced in the same year. In Japan as well, Toyota Motor Corporation adopted a multi-material body for the Lexus LC500/500 h. Moreover, because EVs and hybrids are heavier than gasoline vehicles, adoption of multi-material bodies is progressing in order to offset this weight increase.

1.3.2 Trends in Development of Automotive Materials Auto body weight reduction is an effective measure for both improvement of fuel consumption and reduction of CO2 emissions, and among steels for protection of passengers during a collision, moves to achieve higher strength in body frame parts by applying high strength steel sheets with tensile strength (TS) exceeding 980 MPa are accelerating [13, 14]. As a manufacturing method for obtaining part strength of TS 1.5 GPa or higher, widespread adoption of a hot press (hot stamp) process in which high strength is obtained by heating a high strength steel material to a high

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temperature, followed by quenching in the die began around 2013 [15], which was the year the “Innovative Structural Materials Research and Development Project” was launched, and cold-rolled steel sheets with 1.5 GPa class tensile strength have also attracted attention as auto body frame materials [16–18].

1.3.3 Trends in Automotive Welding and Joining Technologies For auto body weight reduction, it is necessary to develop welding and joining technologies corresponding to a multi-material structure in which the optimum materials are selected for the optimum parts. Specifically, appropriate welding and joining technologies must be established for pairs of ultra-high strength steel sheets, combinations of steel sheets and nonferrous metals, metals and resins/CFRP, etc. From the viewpoint of materials, these technologies can be broadly divided into joining technologies for ultra-high strength steel sheets and joining technologies for dissimilar materials. Techniques using various energy sources and mechanisms have been developed and applied practically in joining processes. Joining techniques are classified as fusion bonding, brazing, solid phase bonding and interfacial welding, adhesion processes and mechanical fastening.

1.4 National Project for Structural Materials for Drastic Weight Reduction of Transportation Equipment 1.4.1 Innovative Structural Materials Association (ISMA) and the New Innovative Structural Materials Research and Development Project The ISMA was established on October 25, 2013 with Dr. Teruo Kishi, Professor Emeritus of the University of Tokyo as President, for the purpose of promoting in an integrated manner the development of innovative welding and joining technologies for achieving drastic weight reductions in main transportation equipment, such as automobiles, aircraft, high speed railway trains, etc. and the development of technologies for achieving higher strength in the main structural materials for transportation equipment, including steel, aluminum, titanium and magnesium alloys, carbon fiber reinforced thermoplastics (CFRTP) etc. The ISMA is a mutual aid association (nonprofit public service organization) in which association members carry out joint research for themselves on technologies that are used in industrial activities, the members provide the researchers, research funds, equipment and other items used in conducting joint research, and the results are jointly and mutually utilized by

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the members. During the 10-year period of the Project, the ISMA had 52 member organizations, comprising 47 companies, 1 foundation, 2 national research institutes and 2 universities, and the project was carried out by 82 research subcontractors, consisting of 19 companies, 18 public research institutes and 45 universities. For details, please refer to the website of the Research and Development of Innovative Structural Materials Project [19] at the website of the New Energy and Industrial Development Organization (NEDO), the Basic Plan for Research and Development of Innovative Structural Materials [20] and related items in the [6, 21–25]. The Project being carried out by the ISMA is an extremely unique national project which has the following four distinctive features: (1) National project challenging the high-risk mid- and long-term research theme of reduction of the CO2 emissions of automobiles and other transportation equipment through weight reduction of structural materials, (2) Promotion of research and development related to multi-material design and technologies by a consortium of material manufacturers, including steels, nonferrous metals and resins makers, and automobile manufacturers and other users, thus extending beyond the walls of each industry, (3) Industry-academia-government “dream team” which conducts research and development in both the aspects of generic research and practical application technologies and (4) Fusion of competitive research in each company and each research institute and cooperative national research. The research and development items in the project can be broadly divided into (1) Development of multi-materials technologies, (2) Development of welding and joining technologies, (3) Development of innovative titanium materials, (4) Development of innovative aluminum materials, (5) Development of innovative magnesium materials, (6) Development of innovative steel sheets, (7) Development of thermoplastics CFRP (CFRTP), (8) Development of basic and generic technologies for innovative carbon fiber and (9) Strategic and generic research. In particular, (1) Development of multi-materials technologies was added to the top of the list of research and development items from FY 2018, when the second half of the project began. Figure 1.8 shows the research and development fields of the project. In the first half of the 10-year period of the project, effort was put into the development of the innovative materials of steel, aluminum, magnesium, titanium, CFRTP and carbon fiber as individual material development projects, together with innovative welding, joining and adhesion technologies. In the second half of the project, priority was placed on the development of multi-materials technologies, including multi-material structural designing, trial manufacture of parts and verification and analysis techniques using a small-scale neutron source, while the project also grappled with many other technology development issues that must be overcome in the future, including recycling and Life Cycle Assessment (LCA).

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Fig. 1.8 Research and development items in innovative structural materials research and development project

1.4.2 Relation of R&D Items in Innovative Structural Materials Research and Development Project Figure 1.9 shows the relation of the research and development items in the first 5 years and second 5 years of the Innovative Structural Materials Research and Development Project. In the 5 years of the first half of the project, “I. Development of Innovative Materials” and “III. Development of Joining Technologies for Innovative Materials” were carried out. The items in the second half of the project were “II. Analysis and Evaluatution of Innovative Materials” for practical application of the innovative materials and “IV. Joining of Dissimilar Materials,” which is essential for multi-material application of automotive materials. Referring to data on the body-in-white obtained by reverse engineering, “V. Multi-Material Design” and “VI. Trial Manufacture of Multi-Material Members” were carried out from the 3rd phase through the 4th phase. In addition, research and development on “VII. Recycling and LCA” was also started from the 3rd phase, and in “VIII. Innovative Manufacturing Processes, Application to Railway Cars, and MI (Materials Integration) of Magnesium Alloys,” which is continuing from the 1st period, research and development were carried out while selecting and prioritizing items the materials and technologies for development.

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Fig. 1.9 Relation of R&D items of innovative structural materials research and development project

1.5 Conclusion The Innovative Structural Materials Research and Development Project was started with two large goals: (1) To produce discontinuous research results that are not simply a linear extension of those to date and (2) To accomplish research by creating a dream team of industry, academia and government. Since the former is linked to competitive research in individual companies and research institutes, while the latter promotes cooperation by the nation as a whole, one challenge in this project is how to combine the mutually-contradictory requirements of “competition” and “cooperation.” At the same time, it can also be said that the project shows the importance of national projects which combine these two regions. In the 5-year period of the first half of the project, the emphasis was placed on competition, and innovative materials and innovative joining and adhesion technologies were developed. In the second half, the emphasis was shifted from competition to cooperation, and effort was devoted to optimized design technologies and joining technologies for dissimilar materials aiming at practical application of the multi-materials approach, corrosion and hydrogen embrittlement of ultra-high strength steel sheets, galvanic corrosion of dissimilar materials, materials integration as a fusion of materials science and computational science, and Life Cycle Assessment (LCA) and recycling. We are aiming at the creation of a research base, centering on neutral research institutes, that can maintain and effectively utilize the research data and results of this work even after the completion of the project.

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One particularly important direction for future materials development is recognizing that we are now in an era of data science, and realizing materials development of the desired materials in significantly shorter timeframes by constructing high-quality databases and introducing the inverse problem analysis approach of computational science, in addition to conventional experimental and analytical techniques.

References 1. Japan Center for Climate Change Actions (Website): Global CO2 emission (2019) https://www. jccca.org/download/66920?p_page=2#search 2. Ministry of Land, Infrastructure, Transport and Tourism (Website): CO2 emission in transportation sector https://www.mlit.go.jp/sogoseisaku/environment/sosei_environment_tk_000 007.html 3. Japan Center for Climate Change Actions (Website): https://www.jccca.org/download/13233? p_page=2#search 4. K. Rajan, Materialstoday 8, 38–45 (2005) 5. Japan Science and Technology Agency, Cross-ministrial Strategic Innovation Promotion Program (SIP), Structural Materials for Innovation (website and pamphlet): https://www.jst. go.jp/sip/k03/sm4i/project/project-d.html, https://www.jst.go.jp/sip/k03/sm4i/dl/pamph_d1_j. pdf 6. T. Kishi, Ferrum 20, 227–231 (2015) 7. Trade Statistics of Japan (principal commodity): http://www.customs.go.jp/toukei/suii/html/ time.htm 8. Ministry of Land, Infrastructure, Transport and Tourism (website): CO2 emissions by auto body weight: https://www.mlit.go.jp/common/000037099.pdf 9. International Council on Clean Transportation: Passenger Vehicle Fuel Economy: https://the icct.org/pv-fuel-economy/ 10. K. Chiba, Weld. Technol. 70(5), 2–7 (2022) 11. K. Hisajima, Lightweight Technology Innovation Forum Nagoya (Portmesse Nagoya, 2019) 12. Innovative Structural Materials Association: ISMA Report No.1 (2015) 13. T. Murakami, Lightweight Technology Innovation Forum (Tokyo Bigsight, 2017) 14. K. Seto, Lightweight Technology Innovation Forum (Tokyo Bigsight, 2018) 15. N. Nomura, M. Kubo, H. Fukuchi, M. Nakata, Nippon Steel Tech. Rep. 122, 21–27 (2019) 16. H. Kimura, T. Okubo, T. Shinmiya, Ferrum 27, 15–20 (2022) 17. JFE Steel Corporation: News Release (2021). https://www.jfe-steel.co.jp/release/2021/10/211 022.html 18. Futaba Industrial Co. Ltd.: News release (2023). https://www.futabasangyo.com/news/detail/ 001277.html 19. New Energy and Industrial Technology Development Organization (website): https://www. nedo.go.jp/activities/ZZJP_100077.html 20. Basic Plan for Research and Development of Innovative Structural Materials: https://www. nedo.go.jp/content/100749300.pdf 21. T. Hyodo, T. Kishi, Multi-Material Strategies for Automobiles (NTS, 2017), pp. 3–6 22. T. Hyodo, H. Yamashita, Y. Hirata, J. Soc. Automot. Eng. Jpn. 72, 4–9 (2018) 23. T. Kishi, Ferrum 25, 622–627 (2020) 24. T. Hyodo, 341st Technology of Plasticity Symposium (2021), pp. 1–8 25. T. Hyodo, T. Kishi, in 63rd Proceedings of Japan National Symposium on Strength, Fracture and Fatigue (2022), pp. 3–12

Chapter 2

Materials Development Tomoaki Hyodo, Toshio Murakami, Kaori Kawano, Yuki Toji, Takao Horiya, Tadashi Minoda, Yasumasa Chino, Hiroaki Hatori, Ken-ichi Shida, Shu Yamashita, and Takashi Ishikawa

Abstract It is well known that the trade-off relationship exists between strength and ductility in metallic materials. We developed technologies to obtain materials with both high strength and high ductility: innovative steel sheets with tensile strength of 1.5 GPa and elongation of 20% and high-strength innovative 5000 series and 6000 series aluminum alloys with Sc precipitates. For innovative magnesium alloys, we have completed the evaluation of a prototype hermetic fatigue test structure for a full-size (5 m long) high-speed rail car using a flame-retardant magnesium T. Hyodo (B) · T. Horiya · K. Shida · S. Yamashita Innovative Structural Materials Association, 1-9-4, Chiyoda-Ku, Yurakucho, Tokyo 100-0006, Japan e-mail: [email protected] T. Horiya e-mail: [email protected] K. Shida e-mail: [email protected] S. Yamashita e-mail: [email protected] T. Murakami Kobe Steel, Ltd, 1-5-5, Nishi-Ku, Takatsukadai, Kobe 651-2271, Hyogo, Japan e-mail: [email protected] K. Kawano Nippon Steel Corporation, 20-1, Shintomi, Futtsu 293-8511, Chiba, Japan e-mail: [email protected] Y. Toji JFE Steel Corporation, 1, Kawasaki-Cho, Chuo-Ku, Chiba 260-0835, Japan e-mail: [email protected] T. Minoda UACJ Corporation, 3-1-12, Minato-Ku, ChitoseNagoya 455-8670, Aichi, Japan e-mail: [email protected] Y. Chino National Institute of Advanced Industrial Science and Technology, 1-1-1, Umezono, Tsukuba 305-8560, Ibaraki, Japan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 T. Kishi (ed.), Innovative Structural Materials, Springer Series in Materials Science 336, https://doi.org/10.1007/978-981-99-3522-2_2

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alloy. For innovative titanium alloys, we developed innovative refining and manufacturing processes to reduce production costs. Thermoset carbon fiber reinforced plastic requires heating and curing in a high-temperature autoclave for several hours, resulting in an increase of the production costs and limited use in automotive applications. The LFT-D (Long Fiber Thermoplastics-Direct) process, in which thermoplastic resin and relatively long carbon fiber are mixed and pressed at high speed, was adopted to develop a high productive manufacturing process, and trial prototypes of chassis and floor panels were manufactured on trial. We succeeded in introducing innovative carbon fibers, by developing new precursor compounds to get flame resistant polymer threads for pre-carbonization, and also by developing new microwave carbonization process technology.

2.1 Steel Materials—World’s Highest Performance Automotive Ultra-High Strength Steel Sheets Using Medium- and High-Carbon Steels Tomoaki Hyodo, Toshio Murakami, Kaori Kawano and Yuki Toji

2.1.1 Introduction Steels are used in large quantities as structural materials for automobiles, machinery, buildings and other applications. It has long been known that iron has a high composition ratio in the Earth’s crust, little energy is required to reduce iron in comparison with other metallic elements, and iron possesses excellent mechanical properties and recyclability and does not cause environmental pollution, as it rusts and returns to the soil [1]. In recent years, it has also been reported that steels have an excellent LCA (Life Cycle Assessment) characteristics when assessed through the processes from the manufacture of basic materials to disposal and recycling [2]. Although susceptibility to rusting is a weakness of iron, this can also be considered an advantage from the viewpoint of recycling. The main reason the steels have been used in such a wide range of applications is because their mechanical properties can be controlled to meet the diverse requirements of the specific application. In terms of strength, in comparison with other H. Hatori National Institute of Advanced Industrial Science and Technology, 16-1, Onogawa, Tsukuba 305-8569, Ibaraki, Japan e-mail: [email protected] T. Ishikawa Nagoya University, Furo-Cho, Chikusa-Ku, Nagoya 464-8603, Aichi, Japan e-mail: [email protected]

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materials such as aluminum alloys, concrete and carbon fiber, steels are produced in various strength levels from easily-formed mild steels with tensile strength of around 300 MPa to ultra-high strength steels with tensile strength of approximately 4 GPa, depending on the selection of the manufacturing process [1]. Alloys of iron (Fe) and carbon (C) which do not contain elements other than C and those which unavoidably remain in the production process are called “carbon steel.” Among carbon steels, those with a carbon content of about 0.3% or less are called low-carbon steel, while those with carbon contents of approximately 0.3% to 0.7% are called medium-carbon steel and those with carbon contents of more than 0.7% are classified as high-carbon steel [3]. (In this book, the alloy composition is given in mass% unless otherwise noted.) As described in the following, the target of research in the project was steel materials with carbon contents of approximately 0.3–0.7%, which are called medium- and high-carbon steels. Since the high strength steels give a great potential for weight reduction in transportation equipment, represented by automobiles, the high strength steels with excellent formability have been used effectively to substitute conventional steels. In Japan, high tensile strength steel is frequently referred to as “high-ten,” using a shortened form of “high tensile strength steel” in the English term. Under the Japanese Industrial Standards, JIS G 3135:2018 establishes the provisions for “Cold-reduced high strength steel sheet and strip with improved formability for automobile uses,” which are used in automobiles, electrical machinery and other applications, and defines high tensile strength steel sheets as products with tensile strength of 340 MPa and higher [4]. The high-strength steel sheets for automobiles are generally defined as those with a tensile strength of 350– 790 MPa, although the definition differs depending on each country and company [5, 6]. Moreover, those with a tensile strength of 980 MPa or more are genrally called “ultra-high tensile strength steel sheets”. In this paper, we will also refer to steel materials with tensile strength of 980 MPa and higher as ultra-high tensile strength steel.

2.1.2 Representative Metallographic Structure of High Tensile Strength Steel Steel is an alloy consisting of iron (Fe) and carbon (C), to which various alloying elements such as silicon (Si), manganese (Mn), chromium (Cr), nickel (Ni), molybdenum (Mo), niobium (Nb), vanadium (V), titanium (Ti) and others are added for purposes such as improvement of strength, ductility (elongation) or corrosion resistance. The properties required in steel materials are obtained by utilizing these alloying elements together with various types of heat treatment based on the FeC alloy equilibrium phase diagram, as shown in Fig. 2.1 [7]. Fe has a bcc structure (ferrite, α phase) at low temperatures, but changes to the fcc structure (austenite, γ phase) by a temperature of 911 °C (the A3 point). The γ phase remains stable until 1392 °C, the A4 point, when it reverts to the bcc structure (now δ–phase) which

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remains stable up to the melting point of 1536 °C. The austenite/ferrite transition is one of the most important phase transformations in steel-making process [7]. The metal structure which is retained to room temperature by selecting the appropriate heat treatment for the austenite that exists at high temperature is called retained ( ) austenite (retained γ ), and the steel is called retained γ steel. A martensite α , structure is formed in quenched materials when steel is heated and then rapidly cooled from a high temperature. Martensite has extremely high hardness and is an indispensable metal structure for achieving high strength in steel materials, but it does not appear in the metastable phase diagram of the Fe–Fe3 C system. This is because it is a nonequilibrium metallographic structure induced by the martensitic transformation caused by increasing the cooling rate. A detailed description of the martensitic transformation may be found in [8–10]. The phenomenon of phase transformation shown in Fig. 2.1 is a distinctive feature of steel. Phase transformation can be divided into diffusional transformation and shear transformation (diffusionless transformation), depending on the transformation mode. Diffusional transformation is a type of transformation in which atoms move by diffusion and form a new phase. This type occurs more easily at high temperatures, as diffusion of the atoms becomes easier. The pearlite transformation shown in the equilibrium phase diagram is classified as this type. The martensitic transformation is a typical example of shear (diffusionless) transformation, and occurs even at low temperatures, as atomic diffusion is not necessary [6]. Figure 2.2 shows the metal structures when Fe–C alloys (steel materials) with various C contents are heated to the austenite region and then cooled to room temperature at different cooling rates [6], together with the names of the phases and structures

Fig. 2.1 Equilibrium diagram of Fe–C alloy (phase diagram of Fe–Fe3 C system) [7]

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Fig. 2.2 Relationship of metallographic structures of Fe–C alloys (steel materials) at room temperature and C content and cooling rate [6]

of the steel. Ferrite (α) is formed in pure Fe and compositions with extremely low C concentrations, while a structure comprising a type of α called pearlite and cementite (Fe3 C) is formed in Fe–0.77 mass% C steel (eutectoid steel), while a ferrite + pearlite structure is formed in the C concentration range from 0.02 to 0.77 mass% (hypoeutectoid steels). When a very high cooling rate is used, as in the case of water quenching, the structure changes to martensite, which is a completely different metallographic structure from ferrite or pearlite, and bainite can be formed by selecting an intermediate cooling rate. Steel (Fe–C alloy) has various transformation structures depending on the heat history applied in the manufacturing process. Because the strength of each transformation structure differs, as shown in Fig. 2.3, strength corresponding to many applications can be obtained by appropriate use of the transformation structure.

2.1.3 Functions of Carbon and Alloying Elements in High Strength Steel In practical applications, iron is almost never used in the form of pure Fe which does not contain other elements, but is widely used as steel (Fe–C alloy). In steel, small amounts of other atoms (solute atoms) are dissolved (as a solid solution) in a large

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Fig. 2.3 Strength ranges of various types of steel structures achieved in the laboratory stage [11]

amount of Fe atoms (solvent atoms, i.e., matrix). The solute atoms are introduced into a solid solution in two ways, that is, by substitutional solid solution, in which the Fe solvent atoms (matrix) are replaced by solute atoms by substitution, and interstitial solid solution, in which the solute atoms enter vacancies between the matrix atoms (i.e., interstitial positions). In steel, atoms which have a small atomic radius form interstitial solid solutions; these elements include H (atomic radius: 53 pm), B (87 pm), C (67 pm), N (56 pm) and O (48 pm). Other elements such as Si (111 pm), Mn (161 pm), Cu (145 pm), Ni (149 pm), Cr (166 pm), Mo (190 pm), Nb (198 pm), V (171 pm), Ti (176 pm), etc. form substitutional solid solutions. In this book, mass% is used to indicate the percentage of the alloy content, but in the case of C contents of 0.3–0.7 mass%, the content of interstitial C atoms is discussed in terms of mol%, which can be calculated by the following equation. X C (mol%) =

/ X C (mass%) Z C / / × 100 X Fe (mass%) Z Fe + X C (mass%) Z C

where, Z Fe and ZC are the atomic weights of Fe and C, respectively. When calculated using Z Fe = 55.85 and ZC = 12.0, the carbon content of 0.3 mass% is 1.38 mol% and the content of 0.7 mass% is 3.18%. This means the ratio of interstitial C atoms to Fe atoms is about 1: 70 in 0.3 mass% C steel and about 1: 30 in 0.7 mass% steel. Figure 2.4 shows the basic form of the binary system diagram (binary phase diagram) of Fe–M when another element (M) is added to Fe. (Here, A4 is the γ → δ transformation temperature, and A3 is the γ → α transformation temperature.) When the amount of Ni or Mn addition is increased, the A4 transformation temperature increases (in the binary system diagram of Fe–C in Fig. 2.1, A4 = 1 392 °C) and the A3 transformation temperature decreases (in Fig. 2.1, A3 = 911 °C). Therefore a) is called the “open γ field type.” In the case of interstitial type elements such as C, N, etc. and other elements such as Cu, Au, etc., when the concentration is large, a

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Fig. 2.4 Basic form of Fe–M binary phase diagram when another element (M) is added to Fe [6]

eutectoid reaction occurs and the γ region becomes narrower, but at low concentrations, the γ field expands, as in the case of (a); therefore, (b) is called the “expanded γ field type.” The elements that belong to types (a) and (b) are called “austenite forming elements” or “austenite formers.” On the other hand, with many elements, including Cr, Si, Al, Mo, Ti, V and P, the A4 transformation temperature decreases as the A3 point increases, resulting in a closed form, as shown in (c). This is called the “closed γ -field type.” Because the elements that belong to (c) destabilize austenite but stabilize ferrite, they are called “ferrite formers.”

2.1.4 High Strength Steel Sheets for Automotive Applications The properties required in mass-produced structural materials are “Strength corresponding to the application,” “Ease-of-use, including press-forming and other secondary processing” and “Ease-of-production in terms of productivity and cost.” In the automotive industry, auto body weight reduction is being promoted with the aim of improving fuel economy, but high strength is also required in automotive frame components to protect the passengers. To satisfy both “Auto body weight reduction” and “Strengthening of members,” auto makers are actively applying high strength and ultra-high strength steel sheets to automobile bodies. The transition of the development of high strength steel sheets for automotive applications since the 1950s is described in detail in [12], and the reader may refer to [13–15] for more recent research and development of high strength steel sheets for automotive use. Figure 2.5 shows the strengthening mechanisms used with steel products, the strengthening methods applied to automotive steel sheets and the corresponding tensile strength ranges [16]. In strengthening of steel, the movement of dislocations should be obstructed. When the strengthening mechanisms are classified by the

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type of obstruction and arranged by the size of the obstruction from the smallest to the largest, the strengthening mechanisms used with steel sheets are solid-solution elements (solid solution strengthening), precipitates (precipitation strengthening, particle dispersion strengthening), grain boundaries (grain boundary strengthening, crystal grain refinement strengthening) and hard phases (microstructural strengthening, dislocation strengthening). The general practice in ultra-high strength steel sheets is to use martensite, which is a high strength phase, as shown in Fig. 2.3. For details concerning the metal-strengthening mechanisms of solute strengthening, dislocation strengthening, particle dispersion strengthening and crystal grain refinement strengthening, see [17]. Martensite, which has a high content of C, is characterized by extremely high hardness and high strength. The strengthening mechanisms for Fe-based martensite include a supersaturated solid solution of C (solid solution strengthening), a high density of lattice defects such as dislocations (dislocation strengthening), a type of refined microstructure (crystal grain refinement strengthening), and in case of a high martensitic transformation start temperature (M S ), precipitation of fine carbides during cooling (precipitation strengthening). The hardness and strength of martensite are determined by the C content and the tempering temperature, as the strength of martensite increases as the C content increases or the tempering temperature decreases. Martensite in steel may take four morphologies, depending on the C content at the M S temperature: lath (internal strength: dislocation), butterfly (dislocation, twin),

Fig. 2.5 Strengthening mechanisms of steel products, strengthening methods applied to automotive steel sheets and tensile strength (TS) ranges [16]

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Fig. 2.6 Schematic diagram of lath martensite observed with optical microscope [19]

lens (dislocation, twin) or thin plate (twin) [6, 18]. However, the lath type is the basic structure in structural steels. Lath martensite is an extremely fine microstructure and tends to form with a large number of adjoining laths of almost the same orientation. Figure 2.6 shows a schematic diagram of the lath martensite structure observed with an optical microscope [19]. Lath martensite is made up of packets and blocks which consist of groups of laths with a specific arrangement. The individual laths are extremely minute and have a long, thin plate-like shape (lath shape) with a thickness of not more than 1 µm (normally, 0.2 µm), and a high density of dislocations exists in their interior. One austenite grain is divided into several packets composed of groups of parallel laths, and each packet is further divided into almost parallel blocks. Martensite does not grow beyond the grain boundary of the austenite parent phase which is its point of origin, and even if a martensite structure is achieved as a result of rapid cooling after heat treatment, the austenite grain boundaries that existed before the martensitic transformation are retained. This type of grain boundary is called a prior austenite grain boundary. In order to refine the crystal grain size of martensitic steel, it is necessary to refine the prior austenite grain. Researchers sought methods for imparting ductility (elongation) and toughness to high strength martensitic steel from an early date, and one such method utilizes TRIP (TRansformation-Induced Plasticity) of martensite. In 1967, Zackay et al. developed a high alloy steel with a representative composition of Fe-8.9%Cr-8.3%Ni-3.8%Mo2.0%Mn-1.9%Si-0.31%C which displayed both high strength and high ductility [20], and in 1970, Tamura published a commentary on TRIP [21]. Retained austenite and other types of austenite in a thermodynamically unstable state are called metastable austenite, and when shear stresses act on this metastable austenite as a result of tensile working, a martensitic transformation is induced, with these stresses as the driving force. The phenomenon of the martensitic transformation of metastable austenite under this type of working is referred to as the deformation-induced martensitic transformation. The reason why ductility and toughness are improved by TRIP is shown schematically in Fig. 2.7. When a tensile test of stable austenitic steel is performed, necking occurs after a certain degree of uniform deformation, and deformation becomes concentrated, resulting in fracture. However, when necking occurs in metastable austenite (i.e., retained austenite), stress increases in that portion, and the deformation-induced martensitic transformation occurs preferentially in the necking

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part. The necking part is strengthened by the high strength of the martensite, causing deformation to shift in other parts, and as a result, the growth of necking is suppressed and the material shows large uniform elongation. However, martensite must be formed with the proper amount of strain during tensile deformation of austenite in order to obtain large TRIP [6]. Because automotive steel sheets are formed at room temperature, a large quantity of metastable austenite (retained austenite) must exist at room temperature in order to utilize the room-temperature martensitic transformation. To increase the amount of retained austenite, it is necessary to reduce the martensitic transformation start temperature M S point to room temperature or lower; Zackay et al. achieved this by using a high alloy steel with a high content of added alloying elements [20]. However, since heavy addition of alloying elements also increases the production cost, this approach was judged to be unsuitable for automotive steel sheets, as cost reduction is always an issue. Thus, it was necessary to develop low alloy steels with reduced contents of alloying elements. In recent years, low alloy TRIP steels with addition of comparatively inexpensive Si have been developed. Retained austenite can be obtained by Si addition because Si suppresses cementite formation and promotes concentration of C to austenite accompanying an increase in bainitic ferrite [22]. Therefore, steels of this type are currently based on austempering treatment of Si-added steel (austempering is a heat

Fig. 2.7 Explanation of ductility (uniform elongation) improvement by deformation-induced martensite

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treatment process for obtaining the bainite structure by isothermal soaking at an appropriate temperature). These materials include steel in which bainite is obtained as the main structure by direct austempering treatment from the austenite single phase region in order to achieve high strength, and austempered low alloy TRIP steels with a structure consisting mainly of tempered martensite, which is heated (annealed) in the dual phase region of ferrite and austenite after obtaining the martensite phase by quenching [23]. As a different approach from austempering treatment, quenching and partitioning (Q&P) has also been proposed as a production process for obtaining a comparatively large amount of retained austenite in Si-added steels [24]. Q&P is a method in which Si-added steel is heated in the austenite single phase region or ferrite/austenite dual phase region, followed by quenching at a temperature between the martensitic transformation start temperature M S and finish temperature M f to form martensite and retained austenite, after which the material is immediately reheated to an appropriate temperature and then rapidly cooled. Since the C in the martensite diffuses to the surrounding retained austenite during tempering, C is concentrated and stabilized in the retained austenite.

2.1.5 Research and Development of Innovative Steel Sheets in the ISMA Project Figure 2.8 shows the final targets of the project carried out by the ISMA in terms of the relationship of the tensile strength and elongation of the main types of steels used in automotive steel sheets. When the project “Research and Development of Innovative Structural Materials” was launched in 2013, the steel sheets mostly used in automobiles were steels with a tensile strength of 590 MPa and elongation of 20%. Based on those values, the project set intermediate targets of 1,200 MPa (1.2 GPa) for tensile strength and 15% for elongation, and final targets of 1,500 MPa (1.5 GPa: 2.5 times the strength of conventional steels) and elongation of 20% (elongation equal to that of conventional steels) [25]. In achieving these targets, the project set a limit of not more than 10% for the total addition of rare metals, which are alloying elements that are added in large amounts to increase the strength of steel materials, and instead, effectively utilizing light elements, represented by C. It may also be noted that a similar project for the development of ultra-high strength, high ductility steel sheets was also carried out in the United States over a 4-year period from February 1, 2013 to January 31, 2017 under the name “Integrated Computational Materials Engineering Approach to Development of Lightweight Third-Generation Advanced High-Strength Steel Vehicle Assembly (ICME 3GAHSS)” [26]. Figure 2.9 shows an outline of research and development in three of the themes related to innovative steel sheets. These three R&D themes were successfully completed by using the techniques of (i) Achieve strength by medium- to highcarbon martensite and secure ductility (elongation) by controlling the retained

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Fig. 2.8 Main steel materials used in automotive steel sheets and final targets of the project

austenite structure and (ii) Achieve high ductility even in steel with a fully martensitic microstructure by optimizing grain boundary segregation elements. In (i), it is thought that high ductility with minimal necking can be achieved because plastic deformation is obtained by the deformation-induced martensitic transformation that occurs when retained austenite is subjected to tensile working, and in the case of (ii), high ductility is obtained by increasing grain boundary strength through ultrarefinement of the crystal grain size to a diameter of 1 µm or less, thereby shortening the diffusion distance and allowing the grain boundary strengthening elements to reach grain boundaries.

2.1.5.1

Development of 0.4% C Innovative Steel Sheet by Sophisticated Control of Retained γ

In Theme 22, material development was carried out by sophisticated microstructural control of retained austenite using 0.4% C steel [27, 28]. Figure 2.10 shows the tensile characteristics and stretch-flangeability of this steel. In contrast to the small elongation of conventional steels of 10% or less, with this steel, both Type A and Type B achieved the final target of tensile strength of 1.5 GPa and elongation of 20%. In (b), the hole expansion ratio is also considered, as this shows stretch-flangeability, which is an indicator of local formability. (b) shows that Type B is a well-balanced steel with excellent formability, which not only satisfies the final target for strength and elongation, but also has outstanding stretch-flangeability. The results of observation of the metallographic structure with a scanning electron microscope (SEM) are shown in Fig. 2.11. In Type A, it can be understood that the amount of retained γ was maximized, while in Type B, retained γ was controlled and the microstructure has also been refined. In the Type B steel, not only tensile strength and elongation but also excellent stretch-flangeability were achieved by

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Fig. 2.9 Outline of 3 R&D themes for innovative steel sheets

Fig. 2.10 Tensile characteristics and stretch-flangeability of 0.4% C innovative steel sheet

skillful control of the retained γ structure, which governs elongation, by fine control of the microstructure.

2.1.5.2

Development of 0.3% C Innovative Steel Sheet Utilizing Carbon

Figure 2.12 shows the results of a tensile test of the innovative steel sheet in Theme 24 using the developed 0.3% C steel. With this steel, microstructure control was

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Fig. 2.11 Images of metallographic structure of 0.4% C innovative steel sheets

performed to satisfy both excellent strength and ductility (elongation) with a mediumcarbon composition. A microstructure control technology for achieving high elongation in combination with tensile strength of approximately 1.2 GPa was developed from FY 2013 to FY 2015, leading to the successful development of a steel sheet that exceeded the final target for tensile strength x elongation, TS x El = 30 000 MPa%. That technical concept was then expanded to the 1.5 GPa class, and in FY 2017, a steel sheet with the final target properties of tensile strength of 1.5 GPa or more and elongation of 23% was successfully developed in the laboratory. When developing steel sheets utilizing carbon, an accurate understanding of the partitioning of C in the steel sheet is essential. Because highly accurate carbon analysis was not possible with the conventional carbon analyzers, a new C analyzer that enables precise analysis of carbon was developed. The configuration of the C analyzer is shown in Fig. 2.13. Although analysis of carbon at the 10 ppm level had been difficult with the conventional devices, highly accurate C analysis at that level Fig. 2.12 Results of tensile test of 0.3% C innovative steel sheet

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was achieved by utilizing three dedicated C detectors and incorporating a mechanism that suppresses deposition of organic material, i.e., contamination, on the specimen surface accompanying electron beam irradiation such as plasma irradiation, the heating stage, etc. [29]. Figure 2.14 is a quantitative carbon content map acquired using the newlydeveloped C analyzer. It can be understood that the retained γ with a high carbon concentration is dispersed uniformly, finely and in a large amount. As shown here, both high strength and high ductility were satisfied while utilizing inexpensive C atoms by optimizing the annealing conditions in the production process to achieve fine and homogenous dispersion of the high C concentration region. Fig. 2.13 Schematic diagram of newly-developed C analyzer

Fig. 2.14 Quantitative carbon concentration map acquired with the developed C analyzer

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Development of 0.5–0.7% C Innovative Steels by Effectively Utilizing Light Elements

Section 5.3 concerns research on generic technology for microstructure control of martensitic steels. Together with crystal grain refinement, every effort was made to avoid using rare elements and to use light elements as far as possible. Figure 2.15 shows the metallographic structure of the 0.5–0.7% C innovative steel material. In this steel, the microstructure was controlled to a fine martensitic structure with a grain diameter of 1 µm or less, and it was confirmed that the tensile properties of samples obtained with a laboratory testing machine satisfied tensile strength of 1.5 GPa and elongation of 20%. Since the key point of this microstructure control is refinement of austenite, which is the structure prior to the martensitic transformation, it is important to skillfully utilize carbon and light elements, which assist in the refinement of austenite and blocks.

2.1.6 Conclusion Steel materials are used not only in automobiles, but also in many other types of transportation equipment. Steel can be applied to a wide range of strength levels by addition of alloying elements and heat treatment methods corresponding to the application. Because resistance spot welding is widely used in joining automotive steel sheets, steel sheets with a carbon content of 0.2% or less had been applied as automotive steel sheets in the past. However, if it is possible to join medium- and high-carbon steel sheets with C contents of 0.3% or more, as described later in this book, we believe that this will enable practical application of the innovative mediumand high-carbon steel sheets developed in this research project, which simultaneously satisfy both ultra-high strength and high ductility. Fig. 2.15 Inverse pole figure crystal orientation map of 0.5–0.7% C innovative steel

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2.2 Aluminum Alloys—Development of High Strength Aluminum Alloys for Automobiles and Aircraft Takao Horiya, Tadashi Minoda

2.2.1 Directions of Alloy Development in the Project In this theme, in the first half of the project, the chemical composition of aluminum alloys was optimized based on the 7000 series alloys with the aim of developing high strength and high toughness innovative aluminum alloys targeting structural materials for aircraft, and process technologies that satisfy both strength and toughness were developed. In the second half of the project, the target was changed to the automotive field, considering the fact that the Project as a whole focused its aim on weight reduction of vehicles, centering on automobiles, and innovative 5000 series alloys and innovative 6000 series alloys were developed as innovative aluminum alloys for automotive parts. As technologies for practical application, a laser welding with filler wire technology was developed, and the optimum joining technologies for the innovative aluminum alloys were established. As an overview of this field, this introduction centers on the results of “Development of automotive parts using high strength aluminum alloys” in the second half of the project. Because the development of high strength and high toughness aluminum alloys in the first half is limited to a brief explanation, the reader is invited to refer to “Innovative Structural Materials and Multi-materials—Innovations in Materials, Joining and Design Technologies for Lightweight Transportation Equipment-” (in Japanese) published by Ohmsha Ltd. in June 2023 for details.

2.2.2 Classification and Applications of Aluminum Alloys Aluminum alloys are classified from the 1000 series to the 8000 series according to their major alloying elements, and can be further classified as non-heat treatable alloys (work-hardenable alloys) and heat-treatable alloys. Table 2.1 [30] shows the classification of aluminum alloys and examples of their applications. Alloys which are not classified from the 1000 series to 7000 series are classified as the 8000 series. Since non-heat-treatable aluminum alloys are non-age hardenable materials and their strength is not increased by quenching and aging treatments, the major strengthening mechanisms for this type are solution strengthening and work hardening, and these alloys have the characteristic of low to medium strength. The strength of heat-treatable alloys is increased by precipitation hardening by quenching and aging treatments, and they have the characteristic of medium to high strength. The

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strength ranges of each aluminum alloy are shown in Fig. 2.16 [30]. The 2000 series and 7000 series alloys are generally used in applications such as aircraft, in which weight reduction is the highest priority, as these alloys have high specific strength (strength-to-weight ratio), and corrosion resistance is assured by periodic maintenance. In automobiles, however, it is difficult to use the 2000 series and 7000 series alloys in applications which require a product life of 10 years or longer under nearly maintenance-free conditions due to the issue of corrosion resistance. In this case, the 5000 series and 6000 series alloys are used because they provide excellent corrosion resistance, even though they are medium strength materials.

2.2.3 Development of Automotive Materials As mentioned above, in the development of automotive alloys, it is necessary to satisfy both strength and corrosion resistance, and for this reason, mainly the 5000 series and 6000 series aluminum alloys are used. Figure 2.17 shows examples of the applications of aluminum alloys in automotive applications [31]. Since automotive structural materials must provide high strength while continuing to maintain corrosion resistance, the project targeted achievement of high strength in the existing 5000 series and 6000 series alloys in order to achieve further weight reductions in automotive members. The focus was then narrowed to scandium (Sc), which has become progressively less expensive in recent years, and development of Sc-added innovative aluminum alloys was carried out. Conventionally, transition elements such as manganese (Mn), chromium (Cr) and zirconium (Zr) have been added to aluminum alloys for microstructure control (suppression of recrystallization, control of the microstructural grain size, etc.). In homogenization treatment of billets, in addition to eliminating solidification segregation, these transition elements also precipitate as fine compounds with a size of several 10 nm to several 100 nm, thereby suppressing recrystallization during hot deformation and contributing to control of the grain size after recrystallization by heat treatment in the post-process. Alloys in which Sc is added as a transition element were also developed from around 1990, and practical application has been promoted [32]. Conventionally, the applications of Sc-added aluminum alloys were limited to bicycle frames, metal baseball bats and the like, as Sc was an extremely expensive element with a global production of only about 15 tons/year. However, in recent years, Ni and Co mining has been begun in Australia in anticipation of large increases in demand for use in automotive batteries. Since Sc is also produced accompanying the mining of those elements, production of Sc has increased, and a significant decrease in its price can be expected. When Sc is added to aluminum and heat treatment is performed at around 300– 400 °C, an extremely fine L12 -structured Al3 Sc compound with a size of several nm to several 10 nm is formed (an example of an Al3 Sc precipitate is shown in Fig. 2.18), and the precipitation strengthening mechanism appears. Furthermore, because the Al3 Sc compounds formed by homogenization and similar processes

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Table 2.1 Classification of aluminum alloys and examples of application [30] Alloy classification

Features

Applications (examples)

Representative alloys

1000 series Pure Al

Excellent workability, corrosion resistance, weldability and thermal conductivity. Low strength

Foils, reflective 1050 sheets, electrical 1070 wire, heat exchanger components

2000 series Al–Cu(–Mg) Duralumin and super Machine parts, duralumin have high aircraft strength, but a drawback in corrosion resistance

2017 2024

3000 series Al–Mn

Excellent Electric bulb bases, workability, beverage can bodies corrosion resistance and weldability. Slightly higher strength than pure Al

3003 3004

4000 series Al–Si

Excellent wear resistance. Small linear expansion coefficient. Low melting temperature

Brazing materials, pistons

4032

5000 series Al–Mg

Excellent corrosion resistance and weldability. Highest strength among non-heat treatable alloys

Ship-related applications, beverage can lids

5052 5083

6000 series Al–Mg–Si

Medium strength, good corrosion resistance and formability

Building materials, auto body parts

6061 6063

7000 series Al–Zn–Mg

Cu-added alloys are Aircraft, sports called extra-super goods, motorcycle duralumin and have rims high strength. Non-Cu-added alloys have excellent weldability

7075 7204

precipitate uniformly while maintaining coherence with the matrix over a wide temperature range of 200–500 °C and are comparatively stable even at high temperatures around 500 °C, they are known to have the effects of grain refinement, suppression of recrystallization, dispersion strengthening, promotion of precipitation, etc. [32].

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Fig. 2.16 Strength ranges of aluminum alloys (from Handbook) [30]

Fig. 2.17 Examples of applications of aluminum alloys in automotive applications [31]

Utilizing these advantages of Sc addition, the aim of the Project was to develop Sc-added automotive aluminum alloys and achieve weight reductions in automotive members in which they are used.

2.2.3.1

5000 Series Alloys

The 5000 series alloys are non-heat treatable aluminum alloys. As the content of the major alloying element magnesium (Mg) increases, strength is also increased by

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Fig. 2.18 Example of Al3 Sc precipitate

solution strengthening. Strength is not improved by quenching and aging treatment, but can be increased by work hardening, for example, by cold rolling or drawing, but if work hardening is to be used, it is limited to shapes that can be produced by cold working, and it is difficult to achieve strength improvement by work hardening in extruded shapes with complex cross-sectional geometries, such as those of automotive structural members. Although tensile strength can be improved by increasing the amount of Mg addition, high temperature deformation resistance increases, and the pressure necessary in extrusion of billets becomes excessive [33]. Therefore, when producing the hollow materials frequently used in automotive members by the extrusion process, it is necessary to increase strength while minimizing Mg addition as much as possible. As shown in Fig. 2.19, when the Mg content of an Al-1 mass% Mg alloy is increased to 3 mass%, tensile strength and proof stress increase by approximately 2 times, but the absolute value of proof stress is 73 MPa, which is only about 1/ 3 of tensile strength. In contrast to this, when Sc is added to Al-1 mass% Mg, both tensile strength and proof stress become higher than the Al-3 mass% alloy, and the improvement in proof stress is particularly remarkable. Moreover, a further improvement in strength can also be seen by increasing the Mg content to 2 mass%. In the project, the target value for the tensile strength of an innovative Al–Mg–Sc system alloy was set at 250 MPa or higher, using the strength of the conventional 6000 series alloy as a standard, but it was found that hollow extruded materials that satisfy this target value can be produced with a Sc-added alloy with Mg addition of 1.4 mass%. Figure 2.20 shows the cross-sectional microstructures of extruded materials of Al1 mass% Mg alloy without and with Sc addition observed with an optical microscope. Although a recrystallization structure can be seen in the sample without Sc addition, a fibrous structure elongated in the extrusion direction can be observed in the material with Sc addition. Since the interior of this fibrous structure contains sub-grains, it has the property of high strength in comparison with the recrystallization structure, due mainly to the Hall-Petch law [34]. Thus, in addition to the above-mentioned precipitation strengthening effect of Al3 Sc, it is thought that a strengthening mechanism involving this fibrous structure also acts to improve strength.

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Fig. 2.19 Effects of Mg content (mass%) and Sc addition on tensile properties of Al–Mg alloy in H112 condition (as-extruded) Fig. 2.20 Cross-sectional microstructure of Al-1 mass% Mg alloy extruded material observed with optical microscope, a without Sc, b with Sc addition

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Figure 2.21 shows the tensile properties of the innovative Al–Mg–Sc H112 material (as-extruded) before and after welding. In the case of laser welding with filler wire (F/L welding), there was no change in strength before and after welding, and joint efficiency of 100% was obtained, while joint efficiency of 92% was also achieved with MIG welding. Since the heat input is small and the welding time is short in laser welding with filler wire, there is almost no coarsening of the Al3 Sc grains and no change in strength. However, the MIG welding is characterized by a large heat input, which is thought to cause a decrease in strength due to coarsening of the Al3 Sc particles. With the conventional material, on the other hand, the base material strength of 6061-T6 material is higher than that of the Al-1.4 Mg–Sc alloy, but joint efficiency was 71% with laser welding with filler wire and 64% with MIG welding. Thus, strength after welding showed decreases in both cases, and was also substantially lower than that of the Al-1.4 Mg–Sc alloy. Table 2.2 shows a comparison of the properties of various aluminum alloys. Here, the welded joint efficiency of the general 5000 series alloy O material is 100%, there was almost no decrease in weldability or corrosion resistance due to Sc addition, and high strength was achieved. Based on these results, it was found that the innovative Al–Mg–Sc alloy has excellent performance as a hollow member for welded structures. When aluminum alloys are to be used in crash boxes and other impact absorbing members, a high deformation capacity is required because the absorbed energy of the member will decrease if cracking occurs during deformation. Figure 2.22 shows the condition of deformation of a hollow extruded specimen in an axial crushing test of the Al-2.0 Mg–Sc alloy H112 material (as-extruded). No cracking occurred during deformation, and the specimen displayed typical bellows deformation, demonstrating that this alloy also has excellent performance as an impact absorbing member. It is known that cold-worked materials in the 5000 series alloys show stress corrosion crack (SCC) sensitivity at Mg contents of 3.5% or more [35]. However, due to the

Fig. 2.21 Tensile properties of laser welded joints with filler wire and MIG welded joints of innovative 5000 series alloy and 6061-T6 material (filler metal: 5356 alloy)

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Table 2.2 Comparison of properties of various types of aluminum alloys 5000 series alloya (2% Mg or less)

6000 series alloy

7000 series alloyb (ternary system)

7000 series alloyb (quaternary system)

Port hole extrusion (hollow material)

O

O

O

X

Weldability (joint efficiency)

100%

60–70%

80–90%

50–60%

Tensile strength

100–200 MPa

200–300 MPa

300–400 MPa

500–600 MPa

Corrosion resistance

O

O

Δ

X

a The

weldability and tensile strength of the 5000 series alloy are properties of O material b Ternary system: Cu content = 0.4 mass% or less, quaternary system: Cu content = 0.5 mass% or more (approximate values)

Fig. 2.22 Condition of deformation of test piece in axial crushing test of Al-2.0 Mg–Sc alloy H112 material (hollow extruded specimen with double-hollow cross section)

low Mg addition in the innovative Al–Mg–Sc alloy, SCC of cold-worked materials of this alloy is considered to be a negligible concern. Figure 2.23 shows the strength improvement effect by cold rolling for extruded materials of the Al-1.9 Mg–Sc alloy. The strength of the innovative Al–Mg–Sc alloys can also be improved greatly by cold working, and tensile strength of 300 MPa or higher can be achieved by cold rolling

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Fig. 2.23 Strength improvement effect by cold rolling of Al-1.9 Mg–Sc alloy extruded material (H112 material)

with a rolling reduction of 40% or more. The fact that these materials maintain the proof stress of 300 MPa or more of the 5000 series alloys with no fear of SCC can be considered a great advantage of these innovative Al–Mg–Sc alloys. However, tempering like H26 (work hardening + partial annealing) is recommended when these materials are to be used by press-forming or similar processes, as they have low ductility in the as-cold worked condition.

2.2.3.2

6000 Series Alloys

The 6000 series alloys are widely used in automotive parts because they have excellent hot workability and the specified strength can be obtained by press quenching in the extrusion process. Figure 2.24 shows the strength characteristics of the conventional 6000 series alloys. Heat-treatable materials such as 6061-T6 and 6082-T6 have the highest strength, and also have higher strength than the innovative Al–Mg–Sc alloy O material discussed above. Therefore, achieving higher strength by addition of scandium based on the 6000 series alloys was examined. The tensile properties of the developed innovative 6000 series alloy are shown in Fig. 2.25. In comparison with the conventional 6082-T6 material, the Sc-added alloy achieves approximately 20% higher strength. Although 6000 series alloys with addition of the proper amounts of transition elements such as Mn, Cr, Zr, etc. suppress recrystallization during extrusion and have a fibrous microstructure [36], coarse recrystallization in the surface layer sometimes occurs as a result of shear strain during extrusion. Therefore, when extruded 6000 series materials are to be used in impact absorbing members, there is concern that the coarse recrystallization layer on the surface may become an origin of cracking and cause a large drop in the impact absorbing capacity of the member. However, Sc addition in the innovative 6000 series alloy has a large recrystallization suppressing effect,

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Fig. 2.24 Tensile properties of conventional 6000 series alloys

Fig. 2.25 Tensile properties of innovative 6000 series alloy

and coarse recrystallization in the surface layer is suppressed, as shown in Fig. 2.26. For this reason, there is little risk of a decrease in impact absorbing capacity, and this material is considered to have high applicability to impact absorbing members.

2.2.3.3

Development of Laser Welding Technology

Laser welding with filler wire tests were carried out to develop a welding technology for automobile parts. The tensile properties of the base materials of the welding test materials are shown in Table 2.3. The conditions under which weld crack and

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Fig. 2.26 Cross-sectional microstructure of 6000 series alloys observed with optical microscope

lack of joint penetration do not occur in each material were confirmed, and strength evaluations were conducted after obtaining satisfactory welded materials. The results of the tensile test of welded materials are shown in Table 2.4. With the innovative 7000 series alloy, the strength of the bead portion was too low in comparison with the base material strength regardless of the welding method, so the bead portion fractured before the base material transitioned to plastic deformation, and the materials showed low joint efficiency of less than 60%. On the other hand, although some of the results for the innovative Al–Mg–Sc alloy were already shown in Fig. 2.21, with this alloy, joint efficiency was 100% with laser welding with filler wire using both a V-groove and an I-groove, and satisfactory weldability was obtained. However, joint efficiency decreased to 46% in wireless laser welding, indicating that supply of a filler metal is necessary for obtaining high weld strength. The comparison materials 6061-T6 and 7046-T5 showed the standard joint efficiency with laser welding with filler wire. Table 2.3 Base material tensile properties of laser welding test materials

Alloy

Tensile strength (MPa)

Yield strength (MPa)

Elongation (%)

Innovative 7000 series alloy

704

646

12

Innovative Al–Mg–Sc alloy

248

182

14

6061-T6

292

259

7

7046-T5

478

430

15

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Table 2.4 Results of tensile test of laser welding test materials (filler material: 5356 alloy) Welding method

Alloy

Tensile strength (MPa)

Joint efficiency (%)

Yield strength (MPa)

Elongation (%)

Break positiona

Laser welding with filler wire/I-groove

Innovative 7000 series alloy

408

58

–

0

W

Innovative Al–Mg–Sc alloy

248

100

175

12

B

Laser welding with filler wire/V-groove

Wobbling laser

a Break

6061-T6

206

70

148

4

H

7046-T5

421

88

327

3

B? H?

Innovative 7000 series alloy

298

42

–

0

W

Innovative Al–Mg–Sc alloy

247

100

177

11

B

6061-T6

198

68

148

3

H

7046-T5

389

81

315

2

W

Innovative 7000 series alloy

73

10

–

0.2

W

Innovative Al–Mg–Sc alloy

115

46

103

1

W

6061-T6

103

35

112

1

W

7046-T5

61

13

–

0

W

position B: Base material, H: heat affected zone (HAZ), W: weld bead

2.2.3.4

Prototyping of Automotive Parts

In the project, prototyping of lightweight automotive parts using the developed innovative alloys was also conducted. Part development was carried out with the innovative aluminum alloys targeting weight reduction of the front side member at the front of the auto body and the side sill inner at the bottom of the side body. The benchmark for both parts was the Tesla Model 3, and the reverse-engineering data were used for the designing. In the front side member, application of the innovative 6000 series aluminum alloy to the crash box (made from conventional 6000 series aluminum alloy) and the member body (high tensile strength steel) was examined, and for the side sill inner, a changeover from the conventional 6000 series alloy to the innovative 6000 series alloy was studied. Figure 2.27 shows a schematic diagram of the front side member part using the innovative aluminum alloy. A combination of an extruded shape of the innovative

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Fig. 2.27 Outline of front side member part applying innovative aluminum alloys

6000 series alloy and a pressed sheet-material of the innovative Al–Mg–Sc alloy was used in the member body, achieving a weight reduction of 48.6% in comparison with the benchmark, and a weight reduction of 19.8% was achieved in the crash box by using an extruded shape of the innovative 6000 series alloy, resulting in a total weight reduction of 44.6%. In addition, a weight reduction of 9% was also achieved in the side sill inner by reduction of the wall thickness using the innovative 6000 series alloy.

2.2.4 Development of Aircraft Materials As shown in Fig. 2.28 [37], the aim in the project was to increase strength by 25% or more against the benchmark 7150-T77511, which is currently the most widely used aluminum alloy for aircraft. Development was carried out with the final target of tensile strength of 750 MPa, proof stress of 700 MPa, total elongation (breaking elongation) of 12% and toughness at least equal to the level of the benchmark. This section introduces the following items that were carried out as part of this research.

2.2.4.1

Optimum Alloy Design

In this development, an Al–Zn–Mg–Cu–Zr system was used as the basic composition. The main alloying elements (Zn, Mg, Cu) were increased as close as possible to the

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Fig. 2.28 Target values of innovative aluminum alloys in the project [37]

solubility limit, and the final composition was decided by optimizing the balance of Mg and Cu addition to obtain high toughness, while continuing to consider SCC resistance. (In the following, this alloy is termed “innovative 7000 series alloy.”) Although the strength of developed alloy was slightly less than the target strength value of the innovative 7000 series alloy T6 material fabricated by extrusion, the project target was satisfied in the warm controlled-rolling process and forging process, which are described in the following Sects. 2.2.4.4 and 2.2.4.5.

2.2.4.2

Reduction of Hydrogen Gas in Billets

It is necessary to clarify the effects of hydrogen in aluminum alloys, as hydrogen causes decreased strength and fracture toughness values by forming pores in the material, and is also a cause of embrittlement. Figure 2.29 shows the effect of hydrogen on the Charpy impact value of the innovative 7000 series alloy T6 material. Although the hydrogen content in massproduced aluminum alloys is controlled to roughly about 0.2 ml/100 gAl, the Charpy impact value will decrease if the hydrogen content increases to 0.8 ml/100 gAl. On the other hand, since the Charpy impact value did not increase even when hydrogen was reduced to 0.05 ml/100 gAl by vacuum heating, the hydrogen content was considered not to be a problem so long as it does not exceed the mass-production level.

2.2.4.3

Refinement of Billet Microstructure by Electromagnetic Stirring Method

In this development, an electromagnetic stirring system was installed in the casting machine, and its effect on refinement of the grains and constituent particles in the developed alloy was confirmed. Figure 2.30 shows the microstructure refinement effect in the electromagnetic stirred 7150 alloy billets. Microstructural grains were refined by electromagnetic stirring with both air cooling and water cooling conditions [37]. In addition, while the grain refinement effect (suppression of feather crystals)

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Fig. 2.29 Effect of hydrogen content on Charpy impact property of innovative 7000 series alloy in T6 condition

Fig. 2.30 Results of grain refinement of 7150 alloy billet, a electromagnetic stirring device, b air cooling, without electromagnetic stirring, c water cooling, without electromagnetic stirring, d air cooling, with electromagnetic stirring (t = 0.5 s), e water cooling, with electromagnetic stirring (t = 0.5 s)

of electromagnetic stirring was confirmed for continuous casting billets, a refinement effect on constituent particles was not observed remarkably.

2.2.4.4

Development of Warm Controlled-Rolling Technology for Prevention of Hot Cracking

A warm controlled-rolling device was developed for the purpose of crystallographic texture control and formation of a stable sub-grain microstructure (fibrous microstructure) in the production of rolled sheets of high strength and high toughness aluminum alloys. The warm-rolled material produced using this device has a fibrous structure, and a strong accumulation to the Brass orientation can be confirmed in the crystallographic texture. As a result, the innovative 7000 alloy T6 material showed

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tensile strength of 750 MPa, proof stress of 700 MPa and elongation (breaking elongation) of 12% in small-scale trial materials, thus achieving the final target of the development. The developed alloy also showed good exfoliation corrosion resistance characteristics of ranks EA to EB in the exfoliation corrosion (EXCO) test.

2.2.4.5

Warm Forging Technology

The strength of the innovative 7000 alloy shows a tendency to increase as the billet reheating temperature, forging start temperature and forging reduction all increase. In the trial material, tensile strength of 766 MPa, proof stress of 726 MPa and elongation of 14% were obtained, achieving the final targets for the innovative 7000 alloy. It was also found that β fiber developed in the crystallographic texture, and the microstructure consisted of a fine-grains with a grain diameter of 2.8 µm and mainly large-angle grain boundaries.

2.2.4.6

Continuous Torsion Technology (RMA-CREO™)

To improve the material properties of strength and toughness, a small-scale continuous rotation evolutional control (CREO) equipment capable of processing round bars with a diameter of ϕ 50 mm was introduced in the first half of the project, followed by the introduction of a large-scale CREO equipment that can process the world’s largest ϕ 95 mm round bars in the second half, and its effects were confirmed. In the CREO processing (torsional processing), tension is applied to a workpiece with a round bar shape, while at the same time, torsional deformation is applied with local heating, and the entire workpiece is subjected to shear working by moving the heating/cooling unit at a constant speed [38]. In order to verify the effects of CREO processing, a process in which torsional processing of ϕ 95 mm billets is carried out, followed by homogenization, extrusion and final heat treatment was examined. However, almost no effect on tensile properties was obtained, as shown in Fig. 2.31. On the other hand, it is considered that crack propagation resistance was improved by torsional processing, as a crack propagation improvement effect could be observed in the weakest direction (T-L direction). This delay in crack propagation is a useful effect for aircraft.

2.2.4.7

Residual Stress Measurement Technology

A residual stress measurement device using the deep-hole drilling (DHD) method was introduced in order to evaluate the distribution of residual stress in the interior of materials, and a technology for measuring the distribution of internal stress with high accuracy was developed. The residual stresses measured by the DHD method and the neutron diffraction method were compared, and good agreement between the two was confirmed. Since the residual stress near the surface cannot be measured

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Fig. 2.31 Effect of continuous rotation evolutional control (CREO™) processing in extruded and T6 tempered innovative alloy

accurately by the DHD method, a hole-drilling device was also introduced, and a technique capable of accurate evaluation of residual stress from the material surface to a large depth was developed by using a combination of the hole-drilling method and the DHD method.

2.2.5 Conclusion In this theme, in the first half of the project, development of a high strength, high toughness aluminum alloy (7000 series alloy) and the related production process technologies were carried out, and in the second half, new 5000 series and 6000 series alloys for automotive parts were developed. In the development of automotive parts, Sc-added high strength-type innovative 5000 series and 6000 series alloys were successfully developed. Following this, new designs for automotive members using these alloys were developed and demonstrative manufacturing was conducted, confirming that large weight reductions are possible. Because these alloys contain expensive Sc, reduction of the cost of Sc will be essential for mass production. However, since Sc is now becoming progressively less expensive, we are continuing to study application of Sc to structural materials for various types of transportation equipment, including automobiles, railroad cars and others, in the future. In the development of the 7000 series alloy, together with optimization of the chemical composition, various process conditions were also optimized, including the development of a melting and casting technology (electromagnetic stirring casting), forging technology, torsional technology, controlled warm-rolling technology, heat treatment technology and hydrogen pore control technology. As a result, it was possible to complete the development of an innovative aluminum alloy (Al–10Zn– 2.5 Mg–1.5Cu–0.15Zr) which is greatly superior to the existing 7150-T77511 material. Future efforts will include the development of further technologies for scaling-up

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the production processes and transition to the stage of prototyping and evaluation of aircraft components using large-scale part materials for the purpose of achieving practical application.

2.3 Magnesium Alloys—Development of Innovative Magnesium Alloys for Railway Car and Automotive Structural Materials and Establishment of Application Technologies Takao Horiya, Yasumasa Chino

2.3.1 Introduction Because magnesium (Mg) has the lowest density among practical metals, displays excellent specific strength, and has also an abundant resource, it has attracted interest as a new structural material, corresponding to carbon fiber reinforced composites, and application to small interior parts of automotive parts, cases for laptop PCs and smartphones, and similar applications continues to expand, in the field of cast parts [39]. At the same time, examples of practical application of wrought Mg in these applications are also increasing [39]. On the other hand, Mg has a low ignition temperature in comparison with other metallic materials and cannot be used in parts with a risk of fire, and its balance of stiffness and mechanical properties is inferior to that of aluminum (Al) alloys, and these weak points have become an issue for expanding practical application. As other problems for expanding the range of applications of Mg alloys have low press-formability at around room temperature in comparison with Al alloys, as well as low corrosion resistance. Research and developments (R&Ds) to overcome these issues was carried out as part of the “Innovative Structural Materials R&D Project,” and R&Ds for application of wrought Mg alloys to the body structure of high-speed railway cars and automotive parts was conducted. This chapter introduces R&Ds in these two areas. For details of the outcomes of each R&Ds, the reader may refer to “Innovative Structural Materials and Multi-materials—Innovations in Materials, Joining and Design Technologies for Lightweight Transportation Equipment-” (in Japanese) published by Ohmsha Ltd. in June 2023 for details.

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2.3.2 Properties of Mg Alloys 2.3.2.1

Features of Flame-Retardant Mg Alloys

Flame-retardant Mg alloys are characterized by which the ignition temperature is dramatically increased by adding calcium (Ca) to a general Mg alloy (Mg–Al series alloy), as shown in Fig. 2.32 [40]. In these alloys, the ignition property is remarkably improved by using an element that has no problems in terms of resource supply stability, and the alloys have been continuously improved since they were first developed in the second half of the 1990s [41, 42]. Figure 2.33 shows a schematic diagram of the oxidation film on the molten metal surface of a general Mg alloy (Mg–Al system) and a flame-retardant Mg alloy (Mg–Al–Ca system). Because a thick, porous oxidation film tends to form on the molten metal surface of general Mg alloys and oxygen can easily react the molten Mg, ignition occurs simultaneously with melting. In contrast, a thin and dense oxidation film tends to form on the molten surface of flame-retardant Mg alloys, so oxygen penetration is inhibited and ignition is suppressed. In addition to general Mg alloys (AZ31, AZ61, AZ91) and current flame-retardant Mg alloys (AZX311, AZX611, AMX602), Fig. 2.32 also summarizes the ignition temperatures and Ca concentrations of the alloys developed in the project (AX41, AX92, AX81G, AX81S), which are described in the following [40]. In the current flame-retardant Mg alloys, 1–2 mass% of Ca is often added to a general Mg alloy (Mg–Al alloy). Focusing on the ignition temperature, it can be seen that the ignition temperature increases as the Ca concentration increases. The ignition temperature of Mg also shows a correlation with the concentration of Al, tending to increase

Fig. 2.32 Relationship of Ca concentration and ignition temperature of Mg alloys measured by differential thermal analyzer (DTA) [40]

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Fig. 2.33 Schematic diagram of oxide films on molten metal surface of general Mg alloy and flame-retardant Mg alloy

as the Al concentration increases. Details concerning the mechanical properties and reliability of the flame-retardant Mg alloys are described in the following section.

2.3.2.2

Plastic Formability of Wrought Mg Alloys

The lattice structure of Mg is a hexagonal close-packed (HCP) structure, and for this reason, press-formability of Mg alloys at room temperature is generally very low. At present, press-forming is normally performed at 250 °C or higher, because at around 250 °C or higher, non-basal slips become active. Thus, improvement of press formability at around room temperature is one of the major challenges for application of wrought Mg alloys to automotive parts. The temperature dependency of CRSS (critical resolved shear stresses: index of the activity of various types of slip systems) of the slip systems of Mg crystals is shown in Fig. 2.34 [43]. At room temperature, there is a large difference between the CRSS of the basal slip system and those of other slip systems, and mainly basal slip acts in deformation at room temperature. The CRSS of non-basal slip systems has a large temperature dependence and becomes on the same order with the CRSS of basal slip when the material is heated to 250 °C or higher. Therefore, the anisotropy of the slip systems is reduced and high elongation can be obtained. Texture formation is another factor for considering the plastic formability of Mg alloys. When Mg alloy sheets are produced by rolling, a basal texture is formed, in which the basal plane is aligned parallel to the rolling direction. Figure 2.35 shows the basal texture (and a schematic diagram) of a rolled general Mg alloy (AZ31) [44]. This figure shows a (0001) pole figure and its schematic diagram, and in the (0001) pole figure, contour lines of the pole density of the basal plane are included. “Pole density” is characterized by the pole density of the target basal plane defined as a multiple (m.r.d.: multiple of random distribution) of the average pole density of a basal plane with a random crystalline orientation. In general, the pole density

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Fig. 2.34 Temperature dependency of CRSS of representative slip systems of Mg [43]

(m.r.d.) of the basal plane of rolled general Mg alloys is at least more than 10, and as the pole density becomes stronger, the degree of orientation of the basal plane also becomes stronger. One { reason }for the formation of a strong basal texture by rolling is the generation of 1 0 1 2 twins during rolling [45]. If the texture shown in Fig. 2.35 is formed, deformation in the sheet thickness direction becomes limited, and ductility under biaxial tensile stress decreases, significantly. This is one of main factor for reduction of the room-temperature plastic formability of Mg alloy sheets. An effective approach for improving the room-temperature plastic formability of Mg alloys is a suppression of the formation of the basal texture, as discussed in detail in Sect. 2.3.4.1 below.

2.3.3 Development of Flame-Retardant Mg Alloy High-Speed Railway Car Body [40] One of the targets of the project is to develop a new flame-retardant Mg alloy, applicable to large-scale railway car bodies, and to achieve practical application with the aim of replacing the Al alloys used in the current high-speed railway car bodies. Since Mg alloys is more than 35% lighter than Al alloys and also has excellent specific strength, there is a high possibility, contributing to weight reduction of highspeed railway cars, in which weight reduction is highly required. However, Mg alloys have been never applied to large-scale railway car bodies, because Mg alloys have a lower ignition temperature than the other structural metals, and thus is inferior in terms of flame retardance. Therefore, the aim of the project is to develop flameretardant Mg alloys with improved flame retardance, and to establish the technologies for realizing practical application of Mg alloys by developing design, processing

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Fig. 2.35 Basal texture of rolled AZ31 alloy and its schematic diagram [44]

and welding technologies suitable for those Mg alloys, leading to application in high-speed railway as next-generation Shinkansen bullet trains. As described above, flame-retardant Mg alloys are characterized by which the ignition temperature is dramatically improved by adding Ca to a general Mg alloy (Mg–Al alloy; see Fig. 2.32). In the first half of the project, as shown in Fig. 2.36, four types of flame-retardant Mg alloys (AX41, AX81S, AX81G, AX92) with tensile properties comparable to those of general Al alloys (A6005C or A7204) were developed, and process technologies for production of large-scale rolled sheets and extrusions were established. In addition, welding technologies and surface treatment technologies suitable for these alloys were also developed. From the second half of the project, 5 m length railway car body with the same cross-sectional geometry as current high-speed railway car bodies were produced, using extrusions and rolled sheets of the developed alloys produced by commercial equipment, in order to accumulate technologies for construction of high speed railway car bodies. In the next section, the results of the projects are overviewed.

2.3.3.1

Development of High Performance Flame-Retardant Mg Alloys

Development of High-Speed Extrusion Mg Alloy (AX41) In “Development of Mg alloy for high-speed extrusion”, the target was an alloy with tensile strength of more than 270 MPa, elongation of more than 20%, an extrusion speed corresponding to that of the A6005C Al alloy, and flame retardance superior to that of the AZX311 alloy (Mg–3Al–1Zn–1Ca mass%). It is known that

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Fig. 2.36 Progress of R&Ds in the project

M–Al–Zn alloys with diluting the Al and Zn concentrations show high extrudability, but adequate strength cannot be obtained, because solution strengthening and age hardening response can not be expected. Recently, Nakata et al. clarified that a dilute Mg–Al–Ca–Mn alloy (Mg–0.3Al– 0.2Ca–0.5Mn alloy) has excellent extrudability, and high strength can also be obtained by forming Guinier Preston (G.P.) zones with a monoatomic layer thickness by age hardening for only several hours [46]. This alloy exhibits excellent extrudability and strength, comparable to the properties of A6005C Al alloy, but when application to the structural parts for railway car body is considered, some problems are still remained such as strength, corrosion resistance, flame retardance, etc. Therefore, in this R&Ds, the above-mentioned alloy was adopted as a basic alloy composition, and the alloy composition and processing technologies were optimized for the purpose of improving mechanical properties and flame retardance with maintaining high extrudability. At first, Al concentration was increased to 4%, in order to improve strength and corrosion resistance, and Ca concentration was increased to 1% in order to improve flame retardance and obtain high aging response. Mn concentration was also minimized for suppression of the formation of coarse Al-Mn intermetallic compounds. As a result, the composition of AX41 (Mg–4Al–1Ca– 0.2Mn) was finally deduced. This alloy successfully has an high extrudability of 20 m/min and a good balance of the targeted mechanical properties and corrosion resistance [47].

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High Strength Mg Alloys (AX81, AX92) In “Development of high strength Mg alloys (for extrusions and rolled sheets),” the targets were tensile strength of more than 360 MPa, elongation of more than 16% and flame retardance superior to that of the AZX 311 alloy. It is known that in cast billets of Mg–Al–Ca alloys, coarse precipitates of Al2 Ca are distributed continuously at around the grain boundaries. If the alloy ingot with the above distribution of Al2 Ca is extruded, the distribution of Al2 Ca precipitates deteriorates elongation, because the Al2 Ca precipitates aligns continuously parallel to the extrusion direction, providing a fracture propagation path. Furthermore, if a Mg–Al– Ca alloy with a Al high concentration (9%) is processed at a relatively low temperature (less than 300 °C), a fine Mg17 Al12 phase are precipitated, and these precipitates enable to strengthen the alloys without deteriorating elongation [48]. Therefore, in the first half of the R&Ds of the project, the effect of the alloy composition on the mechanical properties of Mg–Al–Ca system alloys was investigated systematically, and AX92 (Mg–9Al–1Zn–2Ca) was deduced as the alloy composition for extrusion, which can obtain the good balance in high strength and elongation. As the next step, the effort was focused on the development of a microstructure control for achieving even higher strength without deteriorating elongation. As a result, it was found that the Al2 Ca phase precipitates in cast billets of the AX92 alloy can be spheroidized by heat treatment at near the solidus temperature. It is noted that the Al2 Ca phase precipitates solution-treated at near solidus temperature exhibited more than 2 times hardness (before heat treatment: 300 Hv, after treatment: 665 Hv) [49]. Moreover, it is also revealed that the AX92 alloy is strengthened by the precipitation of fine Mg17 Al12 during extrusion, when the extrusion temperature is set to a relatively low temperature. Consequently, the significant improvement of elongation is attained by the reduction of the origin of the fracture initiation and fracture propagation path, and extrusion of AX92 alloy with the above-mentioned target values was successfully developed [47]. In the development of rolled sheets, a systematic search was carried out for optimization of alloy compositions for achieving high tensile strength (more than 360 MPa) and elongation (more than 16%), and as a result, the AX81G alloy (for thick and medium plates) and AX81S alloy (for thin sheets) with alloy composition of Mg-8Al-1Zn-1Ca were newly developed [47].

2.3.3.2

Development of Welding Technologies and Surface Treatment Technologies

Development of Welding Technologies for Flame-Retardant Mg Alloys In order to develop welding technologies for four kinds of the developed alloys, and the welding behavior and welding conditions of the representative welding processes (TIG (tungsten inert gas), MIG (metal inert gas) and FSW (friction stir welding)) were investigated and optimum conditions were extracted. Here, the appropriate range of

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Table 2.5 Fatigue strength obtained by plane bending fatigue test of AX92 extrusions (perpendicular to extrusion direction (ED)), MIG and TIG welded joint, and FSWed joint (N = 107 cycles) [51, 52] Stress ratio

Fatigue strength (MPa)/Joint efficiency (%) Extrusions (perpendicular to ED)

MIG welding

TIG welding

FSW

−1

142/–

74/52

47/33

140/99

0

111/–

40/36

30/27

85/77

welding conditions such as welding speed and welding current was extracted, and the appropriate range of welding conditions for achieving joint efficiency of more than 70% or higher was successfully extracted for all of the developed alloys. During the above investigations, it was clarified that there are some problems and features in welding processes by comparison with Al alloys. In the TIG and MIG welding processes, it was found that the oxide film on the welded joint can be removed mechanically with a wire brush, with the same manner as Al alloys, but it is essential to prevent oxidation of the filler material, because it causes arc break due to short-circuiting of the current from the contact tip, particularly with the MIG welding. In the case of FSW, it was found that fixture of the specimens by using a strong restraining jig is essential for achieving good joining, and delicate process control is needed, because the appropriate range of welding such as the welding speed and tool rotational speed is narrow compared with the Al alloys [47]. It is noted that a technology for in-situ measurement of defects in joints for long double-skin type hollow extrusion was also developed in the project by developing a wireless type acoustic emission (AE) continuous waveform measurement device, as a technique for nondestructive evaluation of the flame-retardant Mg alloy products [50]. Concerning to the design of high-speed railway car body, it is necessary to prepare a lot of fatigue data for various conditions (joint geometries, welding conditions, stress loading conditions, etc.) and to deduce the standard stress obtained by the fatigue strength. In the project, the fatigue characteristics of the developed alloys (not only extrusions and rolled sheets but also welded joints) were evaluated under experimental conditions similar to the actual conditions for high-speed railway cars. As an example, Table 2.5 shows the fatigue strength of the AX92 alloy extrusion, MIG welded joint, TIG welded joint and FSWed joint [51, 52]. In the both case of stress ratio of −1 and 0, the fatigue strength of the MIG joint showed a value of approximately 40–50% (joint efficiency) that of extrusions and rolled sheets, and the fatigue strength of the TIG weld joint showed a lower value than that of the MIG weld joint, while the FSWed joint showed a remarkably higher joint efficiency than the MIG or TIG weld joint. In comparison with the other structural metals, there is little available data on the reliability (fatigue and corrosion) of flame-retardant Mg alloys, so it is necessary to construct a database effectively by using the data obtained so far. Therefore, in this project, by using a computation module and machine learning module based on a physical model developed for steels materials by the SIP Project (Cross-Ministerial

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Strategic Innovation Promotion Project), computation modules for reliability (fatigue and corrosion, etc.) are being constructed for targeting welded joints, which are the weakest point in a railway car body structure [53]. For details concerning to the development of the computation modules for flameretardant Mg alloys, refer to Chap. 3, MI (Materials Integration).

Development of Surface Treatment Technologies for Flame-Retardant Mg Alloys Because the corrosion resistance of Mg alloys is generally inferior to that of Al alloys, corrosion tests conforming to the operating environment is indispensable for the practical application. However, there is almost no available data, particularly in connection with the corrosion resistance of flame-retardant Mg alloys. Therefore, atmospheric exposure tests (corresponding to JIS Z 22381) assuming the actual operating environment were carried out in the project in order to construct standards for evaluating corrosion resistance. Both direct exposure tests and sheltered exposure tests were conducted. In the former, the specimens are exposed directly to the atmospheric environment as a condition in which salt deposited on the specimen surface is washed off periodically by rain, and in the latter, the specimens are set in a sheltering roof so as to avoid direct exposure to sunlight, rain, wind, etc., as a condition in which salt accumulates and becomes more concentrated on the surface. In direct exposure tests of the extrusions and rolled sheets, until now, a tendency similar to the results of accelerated tests such as the salt spray test have been obtained, in which corrosion resistance improves as the Al concentration in the alloy increases. On the other hand, in sheltered exposure tests, it was found that there are some cases in which corrosion resistance deteriorates as the Al concentration increases [54]. In addition to the exposure tests, tests of welded joints and surface treated specimens are also being carried out, and the data are being analyzed and evaluated. In the project, in parallel to the construction of evaluation standards for corrosion resistance, surface treatment technologies applicable to large-scale railway car bodies are being developed. Concerning to chemical conversion treatment, a showertype chemical conversion method was developed, enabling conversion treatment without dipping the large-scale railway car body in the chemical agent. Concerning to painting, a room temperature drying-type paint (without baking process) suitable for the developed conversion treatment was also developed. These developed surface treatment processes are being applied in the trial production of partial body structures.

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Fig. 2.37 Appearance of double-skin structure panel and single-skin structure panel produced in the project

2.3.3.3

Trial Production of Large-Scale Railway Car Body Structure by Using Flame-Retardant Mg Alloys

Trial Production of Side Panels by Using Flame-Retardant Mg Alloys In 2016, side panels (part of the side of a high-speed railway car body structure which includes a window) were trial-produced by using the developed alloys for constructing the basic technologies for manufacturing large-scale products for high-speed railway car body, and technologies for assembly, formability, weldability, machinability, surface treatability, etc. were confirmed. Two types of the trial-produced panels imitating the high-speed railway car body structure including windows were produced, one being a double-skin structure panel (length 1380 mm × width 769 mm × height 43 mm) and the other a single-skin structure panel (length 1380 mm × width 790 mm × height 127 mm), as shown in Fig. 2.37. The double-skin structure panel was trial-produced by using a hollow extrusion (AX41 alloy) and jointed by MIG welding. In the case of single skin panel, an extrusion with ribs (AX92 alloy) was joined by the FSW and MIG welding. Side columns (AX81G and AX81S alloys) press-formed by rolled sheets were attached to the extrusion with ribs by TIG welding1 . Surface treatment was performed by using the shower-type chemical conversion treatment and painting by using the room temperature drying paint introduced in the previous section [47].

Trial Production of Mockup Body by Using Flame-Retardant Mg Alloys In FY 2017, a mockup body with a length of 1 m and with the same cross section as the current Shinkansen bullet train was trial-produced. The specifications for the

1

Side column: Indicates the main vertical columns of the body structure.

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Fig. 2.38 Appearance of the produced 1/1 scale mockup partial body (a), and its enlarged view (b)

Mg alloy double-skin extrusions used in the mockup body were set to the same as those of the Al alloy double-skin extrusions. In order to maintain the same stiffness with the Al alloy double-skin extrusions, two kinds of improvement were adopted. The first was a modification of the window size, in which the size of window frame was changed from 550 to 400 mm, which is approximately the same size as passenger airliner windows. The second was adoption of not only a truss cross section, but also a harmonica cross section, in the structure of the double-skin panel. Weight reduction and cost reduction could be achieved by using this harmonica cross section, because the rib size are relatively short and thereby high extrudability is obtained. For producing the mockup body, the extrusions were joined by using the MIG and TIG welding and FSW according to the appropriate locations. For surface treatment, chemical conversion treatment and painting were performed with the same methods as those used with the side panel. The appearance of the produced mockup body is shown in Fig. 2.38. The dimensions of the mockup body are height 2880 mm × width 3380 mm × length 1040 mm. The measured weight of the mockup body was 239 kg, while the nominal weight of the assumed Al body was 331 kg, demonstrating that it is possible to achieve a weight reduction of approximately 28% by using the developed Mg alloy and structural design [47, 55].

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Fig. 2.39 Appearance of produced mockup body structure with 5m length for cyclic pressure load test

Trial Production and Evaluation of Partial Railway Car Body for Cyclic Pressure Load Test by Using Flame-Retardant Mg Alloy From FY 2018, a larger partial railway car body (height 2.9 m × width 3.4 m × length 5.0 m) was trial-produced to verify fatigue properties under the cyclic pressure load, which is imposed on the body when Shinkansen bullet trains enter and exit tunnels. When designing the structure railway car body, it is assumed that the welded parts have the same fatigue strength as those of Al alloys, and it is designed so that it can tolerate 147000 cyclic load of 70 MPa, which is equivalent to the total cyclic load in an operation of Shinkansen bullet train for 20 years. In order to conduct a cyclic load test simulating deformation of the center part of the passenger cabin, an effective method was adopted for joining the both ends of the cap parts and evaluation part (center section). Figure 2.39 shows the appearance of the produced body for cyclic load test. Double-skin extrusions of the AX41 alloy were used for the roof and side panels of the body structure, and single-skin extrusions of the AX92 alloy were used for the floor panels and crossbeams. The total weight of the produced body structure (evaluation part) per unit of length was 192 kg/m. This value is approximately the same as in the mockup body produced in 2017, demonstrating that it is possible to obtain the required fatigue strength without increasing the weight of the body structure [56, 57]. Next, by using the produced body structure for cyclic load test, a 147000 cyclic load fatigue test equivalent to 20 years of tunnel entry/exit was conducted. As a result, more than six months of testing was successfully completed. Penetrant tests of the welded joint at the evaluation part (center part) revealed that there is no new cracks or growth at or near the welded joints before or after the fatigue test, demonstrating the high reliability of safety design of structures produced from flame-retardant Mg alloys [55–57].

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2.3.4 Development of Innovative Mg Alloys for Automotive Applications and Application Technologies As mentioned above, Mg is more than 30% lighter than Al and has superior specific strength, so it is likely to contribute to weight reduction in the automotive field, where weight reduction is essential. However, the Young’s modulus of Mg is 45 GPa, which is lower than that of steel (about 205 GPa) and Al (about 70 GPa), and there are concerns about maintaining stiffness compared to the other metals. On the other hand, the stiffness of parts is not simply proportional to Young’s modulus, but rather, is a function of the sheet thickness and other factors. In particular, since bending stiffness is proportional to the third power of sheet thickness [58], the weight can be reduced by about 60% although the plate thickness must be increased by 1.7 times compared to steel to ensure bending stiffness. Moreover, for Al, the plate thickness of Mg must be increased by a factor of 1.2, but the weight can be reduced by approximately 20% [59]. Particularly in the case of wrought Mg alloys, which are being studied for application to large-scale lightweight parts produced by press-forming, 1/1 scale practical application is expected, because wrought Mg alloys have excellent specific strength compared to cast Mg alloys. Therefore, from the second half of the project (2018~), we developed new alloys design and application technologies for significantly improving the sheet formability of Mg alloys, based on the alloy design and application technologies obtained in the development of railroad car bodies. Besides, we challenged development the technologies for the prototyping and performance evaluation of large-size automobile parts, and aimed to establish the basic technology for practical application. The front hood, which is one of the closures requiring high bending stiffness in standard automobiles, was selected for the application points of the wrought Mg alloys, because it has a particularly high weight per component. In the first half of R&Ds, efforts were focused on the development of new alloys for the significant reduction of press-forming temperature from around 250 °C (conventional) to 150 °C in order to attain press-forming of the front hood at near room temperature. In addition, a technology for simultaneous chemical conversion treatment of Mg and other metals was also developed, assuming that surface treatment of Mg parts was conducted on the conventional automobile surface treatment process. In the middle stage of R&Ds, a press-forming technology, adhesive technology and joining technology were also developed for the trial-production of front hoods by using the developed Mg alloys and surface treatment technology. In the second half of R&Ds, the developed technologies were assembled, and 1/1 scale front hoods were trial-produced and evaluated. An outline of the results of R&Ds is presented in the following. For details concerning to the trial-production, please refer to Volume 2 of “Innovative Structural Materials and Multi-materials—Innovations in Materials, Joining and Design Technologies for Lightweight Transportation Equipment-” (in Japanese) published by Ohmsha Ltd. in June 2023 for details.

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Development of Mg Alloy Sheets with High Press-Formability

One of the issues, which must be solved for applying Mg alloy sheets to front hoods, is the improvement of press formability. Mg has a hexagonal close packed (HCP) lattice structure, and the basal plane slip system acts preferentially at room temperature. However, the basal plane slip system activates only two independent slip systems, but activation of 5 independent slip systems is needed for deformation of complicated shape, which means that it is impossible to deform polycrystalline metals by only the basal slip system. In addition to the basal plane slip system, non-basal slip systems such as the second-order pyramidal slip system, etc. are needed for 5 independent slip systems, and then it becomes possible to deform complicated shape. However, as shown in Fig. 2.34, it is difficult to activate non-basal slip systems at around room temperature. It means that Mg intrinsically has large plastic anisotropy at around room temperature. Furthermore, the basal texture, which is characterized by alignment of the basal planes parallel to the sheet surface, tends to be formed during rolling process, and due to the formation of the basal texture, deformation in the sheet thickness direction becomes to be strictly restricted, because the slip directions of basal plane become parallel to the sheet surface. Thus, plastic anisotropy at the micro level also arises at the macro level, resulting in difficulties in press forming at near room temperature [60]. An effective method for improving the formability of Mg alloys at near room temperature is to suppress the formation of basal texture during rolling, as described above. Specifically, weakening the basal texture suppress the basal planes which are distributed parallel to the sheet plane, and can improve the formability by enhancing deformation in the thickness direction by activation of the basal planes. Figure 2.40 shows the relationship between the pole density of the basal plane and the Erichsen value, which is an index of formability, at room temperature. The pole density of the basal plane is expressed in terms of the multiple of random distribution (m.r.d.) of the basal plane, and is an index for the accumulation of the pole density of the basal plane, which are defined as a multiple of the average pole density of basal plane compared with a random crystal orientation. It is known that commercial (current) rolled Mg alloys show the m.r.d. of the basal plane of 10 or more [60], and they show the Erichsen value of 3–5, which is remarkably lower than those of Al alloys (8–10). On the other hand, if the pole density of the basal plane becomes lower than 5, room temperature formability significantly improved, the Erichsen value increases to 8.0 or higher. There are two techniques for weakening the pole density of the basal plane in rolled Mg alloy. The first is a technique of dilute addition of specific elements to a Mg–Zn alloy, and the second is a technique of optimization of the rolling process, such as a high temperature rolling method [61]. In the project, we focused on the first technique of dilute addition of specific elements because it is applicable independent of the rolling equipment. In the research of the dilute addition of special element, recently, it has been found that addition of a very small amount of specific elements (rare earth elements or Ca) to a Mg–Zn system alloy significantly suppresses the formation of the basal texture and promotes the formation of the TD-split texture, in which the basal plane

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Fig. 2.40 Relationship of basal plane pole density of rolled Mg alloy and Erichsen value

is tended to be inclined approximately 35° in the sheet width direction, resulting in the excellent room temperature formability [60, 61]. On the other hand, together with improvement of room temperature formability, weakening of the basal texture also results in a decrease in yield stress and strength [62]. Since in the present work, we focused on the R&Ds for improving the room temperature formability without deterioration of yield stress by microstructure control. Two techniques of “grain refinement” and “age-precipitation hardening” are raised for achieving high yield stress, maintaining improved room temperature formability by weakening the basal texture. “Grain refinement” is a method utilizing the fact that the yield stress of Mg alloys is more dependent on grain size than that of other metals, and that non-basal slip (prismatic slip) becomes more active with grain refinement, which improves ductility at the same time [63, 64]. Basal slip tends to expand into prismatic slip near the grain boundary (a few μm from the grain boundary), where stress concentration is more easily induced. Therefore, if the grain size is refined to less than 10 µm, prismatic slip is activated throughout the specimen and ductility is improved. “Age-precipitation strengthening” is a technique for strengthening by forming G.P. zones by short time age-hardening treatment, in the same way as in the dilute Mg-Al-Ca-Mn alloy (Mg–0.3Al–0.2Ca–0.5Mn alloy) [46, 65] which was utilized to the development of the Mg alloy for high speed extrusion. The above-mentioned two strengthening methods were studied in the project. In the trial-production of the 1/1 scale front hood in the final stage of the project, trial-production was carried out by using the former technique (grain refinement), which can be expected to achieve relatively high formability. The following section introduces the evaluation of the properties of the alloys applied to the prototype parts. The following introduces the evaluation of the properties of the newly developed alloy applied to the trial-production of the front-hood. Since it is now extremely difficult to press-form complex-shaped parts at room temperature with current technology, the target of the project was set to press forming

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at 150 °C, where the current press oils and adhesives are applicable. The following is a summary of the R&Ds in the project [66, 67]. In this R&Ds, the alloys were developed by using Mg-1.5%Zn as the basic alloy, with calcium (Ca) added as an element to weaken the basal texture, and aluminum (Al) and manganese (Mn) added in small amounts as elements to improve strength and corrosion resistance. The Al concentration strongly affects texture formation, and when a small amount of Al is added, a texture in which the basal plane is inclined approximately 30° with respect to TD (sheet transverse direction) similarly to Mg– Zn–Ca alloys are formed. However, if Al is added above a certain concentration, it forms a texture in which the basal plane is inclined toward the RD (rolling direction). This texture change is caused by the formation of the Al2 Ca phase in the matrix by addition of Al, which reduces the Ca concentration in the matrix [68]. In the project, we adopted a composition in which a sufficient amount of Ca is remained in the matrix, and the pole density of the basal plane in the texture is accumulated at a position inclined approximately 35° to the TD direction. Figure 2.41 shows the inverse pole figure maps and (0001) pole figures of a Mg– Zn–Al–Ca–Mn alloy with an optimized Al concentration and a Mg–Zn–Ca alloy without Al addition. The Mg–Zn–Al–Ca–Mn alloy has a grain size of 8.0 µm, and it shows relatively fine-grained compared to the Mg–Zn–Ca alloy (20.0 µm). On the other hand, both alloys showed random textures with weak poles at positions inclining from about 35° in TD direction. Since a Ca2 Mg6 Zn3 compound with a diameter of approximately 100 nm and spherical precipitates of Al–Mn compounds with a diameter of 10 nm were dispersed uniformly in the Mg–Zn–Al–Ca–Mn alloy, it could be suggested that fine dispersion of very small particles can be distributed while also maintaining a random texture by addition of trace amounts of Al and Mn to the Mg–Zn–Ca alloy. Figure 2.42 shows the results of a room-temperature Erichsen test and roomtemperature tensile test of the Mg–Zn–Al–Ca–Mn alloy (hereinafter, “developed alloy”) and a Mg–Zn–Ca alloy. The developed alloy shows substantially the same room-temperature Erichsen value as the Mg–Zn–Ca alloy, while its yield stress is approximately 30 MPa or higher than that of the Mg–Zn–Ca alloy. In the project, the formability of the developed alloy at 150 °C, which is assumed press-forming temperature of the Mg alloy front hood, was also evaluated. Figure 2.43 shows the forming limit diagram (FLD) of the developed alloy at 150 °C, and also shows the FLD for an Al alloy (A6022-T4) at room temperature. At 150 °C, the developed alloy exhibited the formability comparable to that of the Al alloy at room temperature.

2.3.4.2

Development of Simultaneous Conversion Treatment Technology for Mg Alloy and Al Alloy

For applying Mg alloy sheets to automotive parts, it is needed to configure those as “multi-material parts,” where they are joined or fastened to steel and/or Al parts. However, one of the challenges should be surface treatment of the multi-material

64 Fig. 2.41 (0001) pole figure and inverse pole map of developed alloy and reference alloy (Mg–Zn–Ca alloy)

Fig. 2.42 (Top) Results of room-temperature Erichsen test of developed alloy and reference alloy (Mg-Zn-Ca alloy) and (bottom) results of room-temperature tensile test (RD⊥LT) of same alloys

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Fig. 2.43 Forming limit diagram (FLD) of developed alloy (150 °C) and Al alloy (room temperature)

parts. In the surface treatment process of “body-in-white” (BIW), in order to eliminate “uneven coloring” on the body, the BIW must be assembled in advance and then chemical conversation and electrodeposition coating must be conducted. Hence, in order to assemble Mg parts into BIW, it is essential to develop techniques for simultaneously applying conversion treatment and electrodeposition coating to parts composed of Al, galvanized steel (GA), and steel. In this case, one particular challenge is the chemical conversion treatment technology. In the conventional “zinc phosphate coating treatment” adopted for steel BIW, Zn, Ni, Mn + Fe, and Al are contained as major components or impurities in the treatment solution. Hence, the utilization of phosphorus-zinc treatment inevitably results in the contamination of Fe and Ni, which promote corrosion of Mg. In the project, we focused on a zirconium (Zr) system conversion treatment, which has been used with multi-material BIW consisting of steel and Al in recent years. The main composition (and impurities) of the treatment solution for this chemical conversion treatment are Zr, Al, Zn, and Fe. In this chemical conversion process, Fe2+ is oxidized by dissolved oxygen and is precipitated as FePO4 , so that a small amount of Fe ions remain in the treatment solution even when an oxidation accelerator is employed together. On the other hand, Al and Zn do not promote corrosion of Mg even if they are contained in the treatment solution, and are considered applicable if the target of simultaneous chemical conversion treatment is limited to Mg, Al, and Zn(GA). Based on the above, in the project, optimization (or tuning) of the Zr-based chemical conversion process was carried out to enable simultaneous conversion treatment of Mg, Al and Zn (GA). Figure 2.44 shows the results of a JASO M609 combined cyclic corrosion test of the developed alloy treated by the Zr conversion treatment after tuning in the project, together with electrodeposition coating. In the test, specimens with pre-cut crosscuts were subjected to a 360-cycle test at the maximum, and the width of the bulging of the surface treatment arising from the crosscuts was measured at every 90-cycle interval. As a result, the width of the bulging on the developed Mg alloy was approximately the same as that on the GA steel sheet subjected to the surface treatment with the

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Fig. 2.44 Results of combined cyclic corrosion test (JASO M609) of developed alloy with Zr-based chemical conversion treatment

same manner, demonstrating that corrosion resistance comparable to the current GA steels could be given to the developed Mg alloys.

2.3.5 Conclusion As described above, it was possible to establish a press-forming technology for Mg alloy sheets at a temperature of 150 °C or less, which was the initial target of the project, and a simultaneous chemical conversion coating technology for Mg, Al, and Zn (GA). On the other hand, for full-scale practical application of Mg alloy front hoods, it is necessary to establish a technology to lower the press forming temperature of Mg to room temperature, similar to that of steel and Al, in order to reduce production costs. In addition, for surface treatment technology, it is essential to establish simultaneous conversion treatment technology including steel for commercial production. Therefore, it is necessary to further optimize the alloys composition, and further improve the die design to continuously develop the technology to produce pressformed parts with complex shapes even at room temperature. Regarding surface treatment technology, it is necessary to establish a simultaneous Mg, Al, and Fe chemical conversion technology in which the pH of the treatment solution is appropriately controlled and iron contamination can be suppressed even when chemical conversion of Mg and steel are simultaneously conducted. Although the applications of current wrought Mg alloy (press-formed products) are limited mainly to the housings of electrical appliances, in the future, through continued promotion of the above R&Ds and systematization of the technology, taking cost into account, it is expected that the application areas of wrought Mg alloy will gradually expand. Specifically, if the cost problem can be solved, application to small interior parts of automobiles (cases for electronic control units (ECU) and power control units (PCU), battery covers, etc.) is expected to be promoted. Moreover,

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if an infrastructure can be established in Japan to produce wide rolled sheets with more than 1.5 m width at low cost, the application of these sheets to large exterior parts such as front hoods, which were prototyped in the project, will be realized.

2.4 Titanium Alloys—Development of Technologies for Energy Saving in the Titanium Production Process and High Performance in Titanium Materials Takao Horiya

2.4.1 Introduction In comparison with steel, which is currently the mainstream material in automobiles, titanium (Ti) materials have excellent specific strength (strength/density) and corrosion resistance. If use of titanium materials in automotive applications is expanded, progress in automobile weight reduction and decreases in energy consumption and CO2 emissions are expected to be possible. Many automotive parts that utilize the distinctive features of titanium, such as light weight and high strength, high corrosion resistance and a low Young’s modulus, have been reported to date [69], but application has been limited to racecars and luxury vehicles (four- and two-wheeled). The largest reason for this is the remarkably high price of titanium materials in comparison with other structural materials. Metallic titanium raw materials (titanium sponge) are extremely expensive because the smelting process for obtaining metallic titanium is complex and requires a large amount of electric power due to the strong bonding force between Ti and O (oxygen). Ingots are then produced by a melting and casting process, and wrought titanium materials are obtained through hot- and cold-rolling processes. However, since surface oxidation occurs easily, adhesion between the titanium material and the rolling rolls also tends to occur easily, which decreases yield and productivity and is an obstacle to cost reduction. Moreover, in the production of aircraft components and similar products, the high cost of secondary processing of the material, such as machining and joining processes, is also a cause of the high price of titanium parts. At present, the applications of titanium materials are limited mainly to the aircraft sector, but because lightweight CFRP is coming into use as an airframe material in this field, and there is a possibility of problems such as galvanic corrosion with the aluminum materials which have mainly been used in conventional airframe materials, there is an increasingly strong tendency to use titanium as a substitute, and expanded demand for titanium materials is expected in the future. On the other hand, American and European makers continue to monopolize the aircraft manufacturing sector as in the past, as can be seen in Fig. 2.45 [70], suggesting

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Fig. 2.45 Production amount of aerospace industries of main countries (2020) [70]

that differentiation of aircraft components developed by Japanese makers, in terms of both cost and added value, will be a minimum requirement for full-scale entry of Japanese companies in the aircraft sector. Based on this background, the project set final targets of expansion of the applications of titanium materials and strengthening of Japan’s international competitiveness, and carried out development centering on the development of technologies for achieving a drastic reduction in the cost of titanium materials, focusing on the titanium smelting and melting processes and forging and rolling processes, which are the main causes of the high price of titanium. Although development was carried out for the nine technical items shown in Table 2.6, this section introduces four items (➃, ➄, ➆, ➈) which are expected to make particularly large contributions to weight reduction and cost reduction of transportation equipment, especially automobiles. (1) “Development of high efficiency production technology for titanium sheets” (➄ in Table 2.6) targets a large cost reduction by energy saving in the existing process. This development was studied preconditioned on the use of high quality, low impurity titanium sponge developed as a separate theme of the project (➀ in Table 2.6). In addition, achievement of a 20% improvement in the tensile strength/ductility balance of the current titanium sheet products was also targeted. (2) “Development of production process for high quality titanium foil” (➃ in Table 2.6), development of an innovative, low cost smelting technology as a substitute for the existing Kroll process was carried out, and the development of a production technology for high purity (low Fe (iron), low O (oxygen)) pure titanium foil from bivalent titanium ions in a molten salt system by electrodeposition technology, which is considered to have high potential for practical application, was studied. (3) “Development of impurity-tolerant high strength, high formability pure titanium sheet” (➆ in Table 2.6) targeted the development of a production technology for

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Table 2.6 Research and development items and targets in titanium field Process

R&D items

Smelting/ melting

➀ Development of high efficiency Establishment of production technology for production process for high low Fe, O, Cl titanium sponge with actual quality titanium sponge mill

Forging/ rolling

Secondary processing, etc.

Targets

➁ Development of melting and deoxidation technology for low cost titanium raw materials

Development of melting and deoxidation process using low cost raw materials

➂ Development of continuous casting-thickness reduction process

Development of casting-thickness reduction technology and mass production technology

➃ Development of production process for high quality titanium foil

Development of production technology for A4 sized electrodeposition low pure Ti foil with low Fe, O contents

➄ Development of high efficiency Development of low cost pure Ti sheet production technology for production technology with 20% titanium sheets improvement of strength/ductility balance ➅ Development of low temperature, high speed super plastic titanium alloy thin strip

15% improvement of fatigue strength

➆ Development of impurity-tolerant high strength, high formability pure titanium sheet

20% improvement of tensile strength while maintaining same formability

➇ Development of high speed, high reduction foil rolling technology

Establishment of rolling conditions for 10 × increase in productivity

➈ Development of high High machinability and 20% improvement machinability titanium alloy plate of tensile strength

pure titanium sheets that achieves a 20% improvement in tensile strength, while maintaining the same formability, by optimizing the conditions of the titanium sheet rolling and annealing processes and the contents of Fe, O, C (carbon) and other impurities in the base material. (4) “Development of high machinability titanium alloy plate” (➈ in Table 2.6) aimed at the development of an alloy that can simultaneously achieve the contradictory aims of high machinability enabling a substantial reduction in the secondary processing cost of the conventional material Ti–6Al–4 V alloy, and weight reduction by improving tensile strength by 20%, and the establishment of a plate production process technology for the developed alloy. The main points of the development results are described below.

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2.4.2 Development Results 2.4.2.1

Development of High Efficiency Production Technology for Titanium Sheets

This theme targets the development of a process which makes it possible to produce sheets with the same quality and mechanical properties as the current sheet products while also achieving low cost by shortening the production process by omitting the melting and forging processes of the conventional process (Fig. 2.46) in order to reduce the production cost of titanium sheets. Concretely, as illustrated in Fig. 2.46, this is an innovative energy saving process in which briquettes (compressed compacts) of high-quality titanium sponge are vacuum-sealed in a titanium container (package), and sheets are then produced through direct hot rolling and cold-rolling of the package composed of briquettes and container. In the past, technologies for producing flat products by direct rolling of titanium sponge had been studied at the small-scale of laboratory test processes, but did not reach study at the industrial level. As reasons for this, the conventional melting process had the effects of removing chlorides and homogenizing the concentration distribution of Fe, O and other trace elements, but because the melting is omitted in the direct-rolling process, it was necessary to use homogenous, high quality titanium sponge with low concentrations of those elements, the vacuum sealed container production equipment using a large-scale welding device such as an electron beam welder was not sufficiently developed, etc. Because it is necessary to use high quality titanium sponge with low concentrations of Fe, O, Cl (chlorine) and other impurities, the material used in the proposed process is titanium sponge obtained in “High efficiency production process for high quality titanium sponge,” which is under development as a separate theme of the project (➀ in Table 2.6). As the reasons for this, if the conventional widely-used titanium sponge is used, the material has a high content of impurity Cl, which remains in the sponge, and pores that are not observed in the current sheet products produced by the

Fig. 2.46 (Top) Comparison of existing and new processes and (bottom) flow of new process

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Fig. 2.47 Tensile properties of cold-rolled and annealed titanium sheets

melting process form in the titanium sheet products, which causes deterioration of the mechanical properties of the cold-rolled sheets. As a result of the small-scale tests in the project, as shown in Fig. 2.47, it was found that a strength-ductility balance similar to that of the current sheet products can be obtained in most cases when the current titanium sponge is used, but in some specimens, necking did not occur and total elongation (elongation at fracture) or tensile strength decreased, and the elongation of the sheets decreased greatly as the Cl content of the titanium sponge increased, as can be seen in Fig. 2.48. Based on these results, in order to obtain tensile properties on the same level as the current sheet products, it was found that the concentration of Cl remaining during titanium sponge production must be held to not more than the certain specific value, and it is necessary to use homogenous titanium sponge with low contents of impurities. Next, to establish the scaled-up technology of this process, actual-size briquettes (compressed compacts) shown in Fig. 2.49 were prepared using the high-quality titanium sponge described above, vacuum-sealed in a large-scale titanium container, as shown in Fig. 2.50, and was then hot rolled to a thick plate (40 mm thickness), hotrolled sheets (5–6 mm in thickness) and a cold-rolled coil sheet (0.5 mm thickness × 255 mm width), as shown in Fig. 2.51. In implementing this process, studies for optimization of the container assembly conditions and the conditions of the hot rolling process and cold-rolling process were carried out. The tensile properties of the obtained pure titanium sheets are shown in Fig. 2.52. In comparison with the conventional material produced through the melting process, the results confirmed that the developed material displays similar tensile strength and elongation. It is also possible to produce high performance titanium sheet materials by using layered combinations of titanium materials with different compositions and titanium sponge. For example, a multilayered structure with titanium materials with different contents of O, which has a large effect on the tensile strength of pure titanium

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Fig. 2.48 Effect of chlorine (Cl) concentration in titanium sponge on tensile elongation

Fig. 2.49 Briquette (compressed compact)

materials, in the surface and inner layers, was studied, and it was found that the tensile strength and ductility balance can be improved by 30% or more in comparison with the current sheet products (Fig. 2.53).

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Fig. 2.50 Package of titanium container with a briquette (250 × 280 × 324 mm)

Fig. 2.51 Thick plate, hot-rolled sheets and cold-rolled coil obtained by new process (lab. scale)

2.4.2.2

Development of Production Process for High Quality Titanium Foil

In this theme, exploratory studies of the basic seed technologies for a new, low cost titanium smelting process as an alternative to the existing Kroll process were carried out in the first half of development, and in the second half, a production technology for electrodeposition titanium foil was studied with the aim of identifying the most promising seed technologies and practical application of the technology selected. Three basic seed technologies for the new smelting method were examined: (1) Electrolytic production technology for liquid titanium using a high temperature

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Fig. 2.52 Comparison of tensile properties of cold-rolled and annealed sheets (porosity < 0.14%)

Fig. 2.53 Relationship of tensile strength and elongation of multilayer rolled sheets (Ratio of high strength titanium: Condition A > Condition B > Condition C)

molten salt system, (2) Technology for production of titanium from high oxygen content titanium compounds and (3) Technology for electrodeposition of titanium from a molten salt system containing bivalent Ti ions. In narrowing down these basic seed technologies, the targets were purity of Fe ≤ 2 000 ppm, O ≤ 1 000 ppm, and a technology with the potential to reduce costs by 20% in comparison with the existing Kroll process. In the strict sense, one of these promising seed technologies, “Electrodeposition from a salt system containing bivalent Ti ions,” was an extension

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of the existing Kroll process, but because the electrodeposition product is a high purity titanium foil, it also has high potential for application to fuel cells for fuel cell vehicles and hydrogen production systems. The following introduces the main points of the development results. In this process, (1) titanium dichloride is produced from titanium tetrachloride, (2) titanium is deposited evenly on a cathode substrate by electrolytical dissolution from the metallic titanium anode into a molten salt containing the titanium dichloride, and (3) titanium foil or sheet materials are recovered by peeling the deposited titanium (foil) from the cathode substrate. First, “postage stamp” size and then “postcard” sized foils were electrodeposited in basic tests in the laboratory, suitable conditions for smooth electrodeposition were identified, and the conditions for peeling the titanium deposit from the substrate were studied. The results indicated that the current density and temperature of the electrolyte bath have large effects on the characteristics of the titanium foil, and it was found the purity of the obtained titanium foil can be improved to 2 000 ppm or less for Fe and 1 000 ppm or less for O by optimization of the conditions. A large-scale laboratory device was also prepared and used in optimization of the electrodeposition conditions, and finally, it was possible to produce a smooth, flat foil of A4 size of approximately 100 µm in thickness which was easy peelable from the electrode (Fig. 2.54). The O concentration of the obtained pure titanium foil was 180 ppm, or 1/4 that of the raw material (anode), while the Fe concentration was 0.9 ppm, or a reduction to 1/600 that of the raw material. In order to study the element technologies necessary for industrialization of this process, in the second half of the Project, a large-scale test equipment and a small-scale test equipment for the search for the proper conditions were introduced, and development was carried out targeting practical applications and productivity improvement. Specific items included an evaluation method for quantitative evaluation of peelability from the electrode using a peel testing machine, and productivity improvement by achieving a high current density in the salt bath by introducing an electrolyte circulating pump. In the future, identification of the issues for realizing a large-scale process and studies of appropriate countermeasures are planned to be conducted using this equipment. However, in order to improve productivity with the aim of mass production, cost reduction will be necessary based on continuing studies of (1) stable supply of a high density of Ti ions to the area near the electrode (⇒ optimization of the current pattern and electrolyte flow control) and (2) further scaling-up of the equipment, confirmation of product flatness and peelability, and improvement of yield.

2.4.2.3

Development of High Strength, High Formability Pure Titanium Sheet

The main impurities in industrial pure titanium sheets are O and Fe, and in particular, tensile strength tends to increase as the O content increases. Currently, mechanical properties such as tensile strength are generally controlled to the desired range by adjusting the contents of these components. Due to the low cost of titanium raw

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Fig. 2.54 Titanium foil obtained by electrodeposition in molten salt electrolyte method (Top: salt bath side surface, bottom: electrode side surface)

materials with high concentrations of impurity elements, including scrap and the like, there is a possibility that low cost can be achieved simultaneously with high strength in titanium materials if inexpensive raw materials of this type can be used. However, since the formability of pure titanium sheets generally deteriorates greatly as strength increases, higher costs in secondary processing of the titanium products are a concern. Therefore, the aims in the project were to derive a new microstructure control concept by satisfying both microstructure formation and grain refinement in order to achieve high strength and high formability, even when using the abovementioned low cost raw materials, and develop a process technology for realizing this concept. In the microstructure of industrial pure titanium sheets produced by the conventional process, the formation of a microstructure with strong anisotropy is unavoidable, and this becomes a limiting factor for improvement of formability. Therefore, in the project, we attempted to achieve high strength and high formability by microstructure control. To overcome this problem, we investigated the use of O, Fe, C and the influence of the heating process after the rolling process, particularly the effects of the

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Fig. 2.55 Strength-formability balance of developed material

heating temperature over a wide temperature range that included the high temperature region where the body centered cubic (bcc) crystal structure is stable, by experiments using the small test pieces. As a result, it was possible to obtain a balance of strength and formability which satisfies the target performance, as can be seen in Fig. 2.55. This result confirmed that it is possible to satisfy both the formation of the desired microstructure and grain refinement (i.e., suppression of grain coarsening) by selecting an appropriate combination of composition design and heat treatment processes. Thus, a prospect for scaling-up this process was obtained. Next, the optimal heat treatment pattern for reproducing the properties obtained at the small test piece scale was studied with an atmosphere-controlled furnace (Fig. 2.56) of the type that is assumed to be used in mass production. At the small test piece scale, heat treatment was performed in the phase transformation temperature region under an ordinary air atmosphere, followed by salt pickling treatment to remove the surface oxide layer as a post process. As a result of this study using A4 size specimens, heat treatment pattern conditions which can reproduce the properties obtained at the small test piece scale with an atmosphere-controlled heat treatment furnace were found.

2.4.2.4

Development of High Machinability Titanium Alloy Plate

For reduction of the cost of titanium components, it is important not only to reduce the cost of the material, but also to reduce the cost of secondary processing of that material, as mentioned previously. Since it is known that the cost of machining occupies a large part of secondary processing costs, particularly when producing aircraft components, the aim of the project was alloy development focusing on the machinability of titanium alloys.

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Fig. 2.56 Composition of atmosphere-controlled heat treatment furnace

In the first half of the project, first, a new alloy in which small amounts of Ni and Cu were added to a conventional Ti–531C alloy (Ti–4.5Al–2.5Cr–1.25Fe–0.1C) [71] as the base was trial-manufactured at the lab scale, and a titanium alloy with an improved balance of high strength and machinability was obtained by optimizing the heat treatment conditions. It was confirmed that this alloy has good hot forgeability and tensile strength of 1130 MPa, which is about 20% higher than that of the conventional annealed material. Moreover, in comparison with the conventional material and the base Ti–531C alloy, the resistance applied to the tool cutting edge is held to a low level, which reduces tool wear. In other words, it was found that there are cutting conditions under which an extended tool life can be expected, indicating that it was possible to achieve the goal of satisfying both high strength and machinability. Based on these laboratory test results, it was considered that one of the reasons why it was possible to satisfy both high strength and machinability with this alloy was because the hardness of the α phase and the β phase became similar as a result of the addition of small amounts of Cu and Ni, so the load could be dispersed comparatively uniformly and stress concentration at the α/β phase interface was reduced. Next, for verification of the possibility of large scale application by a mass production-equivalent process, a 1-ton ingot was melted in the mass productionequivalent process, and a billet with a diameter of approximately 180 mm, which is an intermediate material, was trial-manufactured by hot forging. In high strength α + β type titanium alloys such as Ti–64, forging is accompanied by a slight amount of surface cracking, but because of excellent hot forgeability of the developed material, forging was possible with virtually no surface layer cracking. Using this forged material, the mechanical properties necessary for application as a structural material for aircraft and the machinability and joining properties required in the fabrication of components were investigated. As a prototype with a simulated shape, assuming thick plates with a thickness of 2 inches (50.8 mm), which are used as materials for aircraft structural members, additional forging of the above-mentioned billet was carried out to obtain a thick plate with a thickness of 55 mm and width of 155 mm. Figure 2.57 shows the

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appearance of the prototype thick plate. Using this material, the tensile characteristics, fatigue strength, fracture toughness and other mechanical properties demanded in the airframes of aircraft were investigated. Figure 2.58 shows the balance of tensile strength and elongation of the developed material. The developed material has a good strength-ductility balance in comparison with the conventional material, and, the targeted 20% increase in the tensile strength of the conventional materials was obtained in the solution heat treated material, which was water-cooled after heat treatment, while maintaining substantially the same elongation as the conventional material. The machinability of the developed alloy in milling (milling cutter processing), turning, and drilling, which are the main forms of machining, was evaluated by the amount of tool wear. The results are shown in Fig. 2.59. With all three machining methods, the amount of tool wear was smaller than that of the conventional material, showing that processing conditions with good machinability were obtained. If at least appropriate cutting conditions are selected for these different cutting situations, the endurance life of the tool can be extended in comparison with the conventional material, and a guideline suggesting that low cost cutting is possible was obtained. In the project, a specimen with a component material shape simulating the actual

Fig. 2.57 Appearance of trial-manufactured thick plate of developed material

Fig. 2.58 Balance of tensile strength and elongation of developed material

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Fig. 2.59 Comparison of tool wear with conventional material and developed material in various cutting modes Fig. 2.60 Geometry of trial-manufactured simulated component

aircraft component shown in Fig. 2.60 was cut out, and a total evaluation of the machinability of the developed alloy was conducted. The results confirmed that the developed alloy has machinability equal or superior to that of the conventional material, and the possibility of reducing the cost of components, including forging costs, could be verified. While offering an excellent balance of strength and toughness in comparison with the conventional material, this alloy also has excellent forgeability, enabling near-net shape processing, and excellent machinability, confirming that a large reduction in machining costs in secondary processing is possible.

2.4.3 Conclusion Focusing on cost reduction, which is the most important issue for practical application of titanium materials, technology development of innovative smelting processes and manufacturing processes was carried out while bearing in mind practical application and mass production. To achieve innovative changes in the existing process, sufficient basic technical studies, and verification tests with scaled-up technologies over an extended time are necessary. However, it is thought that work on scaled-up technologies could be started in the project following laboratory-scale studies and a variety of issues on the possibility of practical application were confirmed. Based on the development results of the project, continuing developments by the individual

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companies are planned in the future with the aim of achieving practical application of more economical titanium components.

2.5 Carbon Fiber—Development of a New Precursor Polymer Not Requiring a Flame-resistant Treatment and Microwave Carbonization Technology Hiroaki Hatori, Ken-ichi Shida

2.5.1 Introduction Carbon fiber (CF) is a material that can meet various needs with its light weight and high strength properties. Japan currently accounts for approximately 70% of the global CF market. In the area of transportation, carbon fiber composite materials are already used to reduce fuel consumption for airplanes. It is expected to be used more extensively in new fields, such as automobiles, blades for wind power generation, and hydrogen tanks for fuel cell vehicles. The carbon fiber manufacturing technology completed in Japan was established as the “Shindo process” at the Osaka Industrial Research Institute, Agency of Industrial Science and Technology (current National Institute of Advanced Industrial Science and Technology) in 1959. The current carbon fiber manufacturing process (Shindo process), which performs flame-resistant treatment (oxidation) of the acrylic fiber at a high temperature in the air, has become problematic due to significant energy consumption and CO2 emissions at the manufacturing stage. While the carbon fiber is evaluated as having strength (specific strength) that is ten times greater than that of steel, the carbon dioxide emissions at the manufacturing stage are also evaluated as being ten times that of steel (22 kg–CO2 /kg for carbon fiber vs. approximately 2 kg–CO2 /kg for steel) [72]. Even if a significant energy-saving effect is created at the use phase, it can be assumed that Life Cycle Assessment (LCA) shows carbon fiber is disadvantageous to conventional steel and others in terms of carbon dioxide emissions. In addition, productivity cannot be currently improved to any great degree due to the limit of the heat removal efficiency of manufacturing equipment, which is also a significant challenge in carbon fiber manufacturing. Considering the increase in the need to reduce automobile weight, it is essential to appropriately respond to the foreseeable heavy demand for carbon fiber. To distribute carbon fiber to new fields, such as automobiles, the technology development of an innovative manufacturing process to halve energy and CO2 emissions at the carbon fiber manufacturing stage and significantly improve productivity and their evolution into industrial technologies are required. In this project, we carried out the design and synthesis and spinning technology development of a new PAN

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Fig. 2.61 Comparison of the current and the innovative carbon fiber manufacturing process

polymer (new carbon fiber precursor compound, not requiring a flame-resistant treatment) and the technical development for carbonization with microwave replacing the conventional high-temperature heat treatment furnace in the carbonization process after flame-resistant treatment (Fig. 2.61). Although new manufacturing technologies have been developed overseas for many years using natural materials, such as lignin, as the raw material to reduce the cost and environmental load, carbon fiber with the required performance has not yet been achieved, even at this point. Unless cost and CO2 emission reductions are achieved with the performance requirements for carbon fiber, it cannot be accepted as an industrial technology. Therefore, in this project, which carbon fiber manufacturers leading the global market participated in, we have conducted development aiming at the product performance in the market from the beginning. As a result, it produced scientifically excellent results, such as measuring/evaluating technologies to identify the performance mechanism of carbon fiber and assess the performance potential of newly developed carbon fiber.

2.5.2 Significance of the Development of the New Carbon Fiber Precursor Polyacrylonitrile (PAN) fiber or pitch fiber formed by melt spinning loses its fiber form if carbonization treatment is provided as it is. However, it changes into carbonized fiber while retaining its form by providing an air oxidation treatment called a flame-resistant or infusible treatment. In the Shindo process, an industrial manufacturing process for PAN-based carbon fiber, flame-resistant fiber is made with the chemical reaction shown in Fig. 2.62 before the carbonization process. PAN, an aliphatic polymer with carbon atoms connected in a straight chain, causes

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Fig. 2.62 Reaction in the flame-resistant process for PAN-based carbon fiber

an intramolecular cyclization reaction or aliphatic carbon chain oxidation by flameresistant treatment and generates an aromatic ladder structure. PAN, an aliphatic polymer, changes into high molecular weight compound with an aromatic structure in this phase. The flame-resistant fiber is comprised of a type of aromatic polymer. In addition, the flame-resistant process in PAN-based carbon fiber manufacturing is not only for retaining the fiber form, but also for controlling the anisotropic carbon texture after carbonization by controlling the structure at the molecular level. In other words, by uniaxially oriented molecular chains by stretching PAN fiber and combining it with an intramolecular cyclization reaction in the flame-resistant process, uniaxially orienting, where the ladder structure lines in the fiber axial direction, is generated. The molecular orientation generated in this manner succeeded in creating the hexagonal carbon structure after carbonization and is reflected in the anisotropic texture. On the other hand, if a heat generation reaction suddenly occurs in the process of PAN molecules generating a ladder structure, the reaction goes out of control due to self-heat generation by exothermic reaction, and the fiber form cannot be retained. Accordingly, being unable to increase the flame-resistant treatment speed beyond a certain level hinders the productivity improvement of the PAN-based carbon fiber. Also, the flame-resistant treatment requires a large furnace and consumes maximum energy in the manufacturing process. If polymer material with high aromaticity to be carbonized as a solid phase is formed fiber, the flame-resistant and infusible processes can be eliminated. However, maintaining the fiber form alone cannot provide carbon fiber with excellent mechanical properties. Orienting the hexagonal carbon layer assembly, a basic structural unit constituting carbon, in the fiber axial direction is essential. The organic substance with a high heat-resistant structure to be carbonized as a solid phase has a low solventsolubility for spinning. Therefore, the primary challenge is to control the trade-off relationship between spinnability and heat resistance in developing a new carbon fiber precursor that does not require flame-resistant treatment. In addition, whether the characteristics resulting from the chemical structure of polymer material and the molecular arrangement, especially the orientation of an aromatic segment, created by the external force added to the raw material fiber due to stretching and others, significantly influences the structure and physical properties of the obtained carbon fiber.

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Fig. 2.63 Mechanical characteristics of carbon fiber

Since the phenol resin fiber is carbonized in a solid phase, it is one of the essential materials in practical use as a precursor for fibrous activated carbon. However, since the carbon microstructure obtained from phenol resin fiber is isotropic, unlike the PAN-based carbon fiber, high mechanical characteristics do not appear (Fig. 2.63). Since the orientation of the aromatic components in the precursor structure significantly influences the subsequent development of anisotropic texture, it is vital in controlling the mechanical properties of the obtained carbon fiber.

2.5.3 Development of Solvent-soluble Flame-resistant Polymer [73] In this project, we searched, designed, and synthesized new high molecular compounds of a carbon fiber precursor that do not require a flame-resistant process. As a result, a new precursor compound with a flexible structure (Fig. 2.64), in which solubilizing agents are connected with oxidized polymer chains in a long-sleeved manner, was developed. Fig. 2.64 Structural model of solvent-soluble flame-resistant polymer

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Fig. 2.65 Position of innovative CF attained values of physical properties (compared with commercial products) [73]

The reaction conditions for the solvent-soluble flame-resistant polymer obtained in a liquid-phase reaction using PAN as the raw material were examined in detail. As a result, improvement of the flame resistance and spinnability was achieved. The quality of the precursor fiber obtained after spinning also improved. For the physical properties of carbon fiber after calcination, a strength of 2.1 GPa and elastic modulus of 237 GPa were achieved at the maximum for a diameter of 6.7 µm. Figure 2.65 clearly shows the position of the physical properties compared with commercial products.

2.5.4 Carbonization Technology with Microwaves Since the penetration rate of microwave ovens for ordinary households is 97.4% [74] as of 2004 in Japan, the heating technology with microwaves is well-known in Japan. It is broadly considered energy-saving heating technology and equipment because it allows cooking and heating in a short time and effectively. In the industrial area, it is used not only for heating, drying, and thawing foods, but also for drying timber and drying and sintering ceramic precursors. In the medical area, it is used for hyperthermic potentiation and others. One of the examination cases of microwave heating in carbon fiber manufacturing is heating with microwave-assisted plasma conducted by a group at the U.S. Oak Ridge National Laboratory. Polyacrylonitrile fiber and oxidized polyacrylonitrile fiber, which are precursors, are difficult to absorb in microwaves and require intense microwaves for their heating. The oxidation (flame-resistant) and carbonization reactions are heat generation reactions, and the fiber changes from dielectrics to electric conductors. Therefore, once it is heated, a runaway reaction easily occurs, and fiber is also easily severed due to unexpected discharge. Then, these problems were solved by using plasma excited with microwaves under reduced pressure [75, 76].

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Fig. 2.66 Heating the intermediate fiber with the cylindrical waveguide tube

In this way, although it was challenging to obtain carbon fiber by direct heating with microwave irradiation of the object with a significant change in the electromagnetic physical properties, in this project, we overcame the challenges by devising the design of a heating furnace depending on the change in the fiber’s physical properties [77]. At the beginning of the examination, the thermal runaway and unexpected discharge occurred, and continuous manufacturing equipment operations were challenging. However, in the project, we succeeded in obtaining carbon fiber continuously by using the TE10 -type slot antenna waveguide for heating the oxidized polyacrylonitrile fiber, dielectrics, by using the TM01 -type cylindrical waveguide tube (Fig. 2.66) for heating the intermediate fiber with carbonization progressed partway, by using the TE10n -type resonator in the case of requiring further heating for carbon fiber obtained, and by carrying out dielectric heating, electric conductive heating, and induced heating, respectively [78–81]. As mentioned above, although carbon fiber could be obtained continuously, the carbon fiber tensile strength obtained at the beginning was low. The causes of defect formation that hinder the manifestation of the tensile strength were estimated by detailed evaluation for structural change in graphite crystals in fiber, and tensile strength was improved by eliminating the causes one by one. The tensile physical properties of carbon fiber as of the end of the project are shown in Table 2.7. Carbon fiber reinforced composite material using polyamide-6 as the matrix and the obtained carbon fiber was prepared (Fig. 2.67), and the physical properties were evaluated. Although the tensile strength and tensile elastic modulus of the innovative carbon fiber were lower than that of commercially available carbon fiber [82, 83], the incidence rate of the composite material’s physical properties (the value of the composite material’s physical properties divided by the product of carbon fiber physical properties and the carbon fiber volume content in the composite material) was almost equivalent. Table 2.7 Carbon fiber physical properties obtained by microwave heating [83]

Number of filaments (K = 1000)

24 K

48 K

Strand tensile strength (GPa)

3.8

3.1

Strand elastic modulus (GPa)

223

220

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Fig. 2.67 Preparation of carbon fiber reinforced composite material [11]

As mentioned above, in the project, we succeeded in manufacturing carbon fiber with a tensile strength of 3.8 GPa and a tensile elastic modulus of 223 GPa by carbonizing the regular tow (24 K) by plasma-free microwave heating at atmospheric pressure using flame-resistant fiber derived from polyacrylonitrile-based precursor fiber. By evaluating the physical properties of the carbon fiber-reinforced composite material obtained from that carbon fiber, it was confirmed that it has the same level of incidence rate as that of commercially available general-purpose products. In addition, since the carbon fiber showing a tensile elastic modulus of 250 GPa was also successfully manufactured in the large tow (48 K) carbonization test, this process technology was confirmed to be applicable to the large tow.

2.5.5 Conclusion The new carbon fiber obtained in developing the new carbon fiber precursor compound and carbonization process technology with microwaves showed almost equivalent physical properties, such as tensile strength and bending strength, to those of commercially available carbon fiber as a target in the evaluation as a thermoplastic resin composite. In addition, this project also tackled solving the mechanism of the new carbon fiber’s physical property appearance and developing a new characterization method for carbon fiber, and obtained results with high novelty in terms of both science and engineering.

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2.6 Carbon Fiber Reinforced Plastic—Mass Production Technology for Thermoplastic CFRP Structure Shu Yamashita, Takashi Ishikawa

2.6.1 Introduction The needs for weight reduction of automobile body structures have increased in response to the requirements for global CO2 reduction, fuel consumption improvement, electric vehicle introduction and knowledge creation. The need for CFRP with excellent specific rigidity and specific strength for body weight reduction is a rapidly growing market.2 However, only extending the conventional thermosetting CFRP application technology has limitations for productivity and recycling efficiency, and the application to vehicles (mass production vehicles) is challenging. In response, although the EU has started developing the thermoplastic CFRP for aircraft to improve mass productivity and shock resistance, the resin used is extremely expensive, and its application range assumes its use in the condition with continuous fiber without severance of fiber. Therefore, applying the thermoplastic CFRP to structures with many complicated shapes, such as vehicles, is extremely limiting.

2.6.2 Objectives The aim of the investigation was to develop design and manufacturing technology for lightweight body structures with the non-continuous fiber thermoplastic CFRP, which uses carbon fiber cut by one millimeter to several-tens-of-millimeters to accommodate mass production vehicle productivity (cycle time of one minute), molding property for complicated shapes, and cost reduction. The technology capable of continuous molding within one minute, the cycle time required by a mass production vehicle, and its instrumentation, the design and evaluation technology, the technology to join the same and different kinds of materials, and recycling technology were developed.

2

In 2025, the industrial scale will be expanded to 284.5 billion yen with a breakdown of 194.9 billion yen for CFRP molded items, 62.6 billion yen for carbon fiber, and 27 billion yen for resin material (predicted by the Yano Research Institute Ltd. in 2014).

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2.6.3 Current Status of Materials (Technologies) in the Field CFRP is roughly divided into two types, thermosetting CFRP and thermoplastic CFRP, according to the difference in the matrix resin. Especially concerning structural materials, thermosetting CFRP was developed in advance for aircraft. To date, the thermosetting CFRP has been manufactured by methods, such as autoclave molding, which heats, pressurizes, and molds prepreg (the intermediate base material, the resin impregnated and semi-cured in carbon fiber) using an autoclave (pressure vessel), the RTM (Resin Transfer Molding) injecting liquid resin, and the VaRTM (Vacuum assisted Resin Transfer Molding). However, in recent years, the high cycle RTM molding and compression molding with the carbon SMC (Sheet Molding Compound) have attracted attention. On the other hand, various industries, such as metal press work and plastic molding, have entered the thermoplastic CFRP market because of the diverse applicability of the manufacturing method in addition to its excellent productivity, shock resistance properties, and recycling efficiency. Most technologies were developed to apply CFRP to small, mass-produced products. Although the carbon fiber used for the thermoplastic CFRP is basically the same material as that of the thermosetting CFRP, a surface treatment suitable for the matrix resin is applied. Although the matrix resin of the thermosetting CFRP used for aircraft is mainly epoxy resin, the matrix resin used for the thermoplastic CFRP has many types, and the matrix resin’s properties affect the thermoplastic CFRP significantly. These types of resin, which are super-engineering plastics such as PEEK (Poly Ether Ether Ketone), PEI (polyetherimide), and PPS (Polyphenylene Sulfide), have excellent mechanical properties and heat resistance (PEEK tensile strength of 98 MPa and continuous service temperature of around 250 °C). On the other hand, the high melt viscosity of thermoplastic CFRP makes it difficult to impregnate carbon fiber with resin. As a result, since molding at high temperature and pressure is required (PEEK molding temperature of 365 to 420 °C), a die with high rigidity and acid resistance and a heat source and a carrier device stable at high temperatures are required. Europe has led the development and mass production of thermoplastic CFRP for aircraft, and relatively much of it has been applied to aircraft parts. On the other hand, the development of thermoplastic CFRP in Japan mainly focuses on applications to small parts for vehicles for engineering plastics or generic plastics, and the development of high-performance thermoplastic CFRP manufacturing applicable to structural materials has only just started. The current status of the molding technology and applications of composite materials are shown in Fig. 2.68. Applications of CFRP to structural materials started for aircraft, where it is mainstream to mainly laminate a prepreg sheet with continuous fiber and mold impregnated with thermosetting resin by heating and pressing in an autoclave. While continuous fiber applications allow a design that takes advantage of CFRP’s elastic modulus and specific strength, there are issues such as molding time, molding property, and costs. After that, molding technologies were applied to

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Fig. 2.68 Molding technology and applications of composite materials

vehicles, such as RTM (Resin Transfer Molding), in which molding in a die, positioning textile base material (fabric and knit), injecting resin there, and molding are performed, and SMC (Sheet Molding Compound), in which molding the sheet-like intermediate base material consisting of non-continuous fiber and resin in a die is performed. However, since all of them have a limit to reduce the molding time (five minutes at the shortest) due to thermosetting resin, further improvement is required to apply them to mass production vehicle use, which requires short-time molding cycle performance (cycle time of one minute). On the other hand, in the thermoplastic CFRP using thermoplastic resin capable of short-time molding, the mainstream is injection molding, in which pellets consisting of short fiber less than 1 mm and resin are melt-kneaded and then injected into a die and molded. However, it is not suitable for molding for large parts in terms of die design. In this way, for the vehicle CFRP, there is a pressing need to develop noncontinuous fiber-reinforced thermoplastic CFRP with both moldability (cycle time, molding property, and cost) and mechanical properties required for structural material.

2.6.4 Summary of Research and Development Results Discontinuous carbon fiber and generic thermoplastic resin were used as the thermoplastic CFRP (hereinafter referred to as “CFRTP”), and the technology capable of continuous molding within a cycle time of one minute required by mass produced

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vehicles, technology to join the same and different kinds of materials, and recycling technology were developed. In addition, full-scale vehicle components using the project development technology were finally produced experimentally, and the technology for application was verified. The development was conducted at the two research bases of The University of Tokyo Centralized Laboratory (research representative, The University of Tokyo professor Jun Takahashi) and Nagoya University NCC Centralized Laboratory (research representative, Nagoya University Professor Takashi Ishikawa). The CFRTP molding processes at both research bases are shown in Fig. 2.69. The University of Tokyo Centralized Laboratory used polypropylene resin (hereinafter abbreviated as “PP”) as the thermoplastic resin, produced the intermediate base material (a molded precursor consisting of fiber and resin) consisting of discontinuous carbon fiber and resin, transported and placed it in a die with a material handling robot after preheating, and performed press molding. Nagoya University NCC Centralized Laboratory conducted the development, using the LFT-D (Long Fiber Thermoplastics-Direct) molding method, in which after kneading discontinuous carbon fiber and polyamide resin (hereinafter abbreviated as “PA”), the extruded raw material is transported and placed in a die with a material handling robot after preheating, and press molding is performed. Both bases developed the process for a molding cycle time within one minute and the technology to join the same and different kinds of materials, design and evaluation technology, and recycling technology as an elemental technology issue.

Fig. 2.69 CFRTP molding process in ISMA

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The University of Tokyo Centralized Laboratory and Nagoya University NCC Centralized Laboratory conducted research and development until the first term (FY 2017) and latter term (FY 2022), respectively, and TORAY, which participated in The University of Tokyo Centralized Laboratory in the first term, continued to conduct research focusing on practical applications (parts experimental production) in the latter term.

2.6.4.1

Summary of Research and Development at The University of Tokyo Centralized Laboratory

The University of Tokyo Centralized Laboratory conducted the development in cooperation while sharing information by setting up a WG for each elemental technology, such as an intermediate base material WG, a molding WG, a design and evaluation WG, and a joining WG, to build the short-cycle molding process. It developed the CTT (Carbon Fiber Tape Reinforced Thermoplastics) material, which is obtained by cutting unidirectional CFRTP tape material into a prescribed length and placing it quasi-isotropically in the plane as an intermediate base material, and the CMT (Carbon Fiber Mat Reinforced Thermoplastics) material consisting of non-woven fabric by carding and paper from a papermaking process. In the base material development, it also developed a recycling technology based on a superheated steam method with less deterioration on the fiber surface after recovery by conducting new technical development to recover carbon fiber (CF) from wasted CFRP. The results of the CTT material’s mechanical properties (elastic modulus, bending strength, and impact absorbing energy) for various tape lengths are shown in Fig. 2.70. It has a performance equivalent to or higher than the thermosetting CFRP used for aircraft. The results of the CF surface condition and interfacial shear strength recovered with the superheated steam method are shown in Fig. 2.71. A technology that removes resin from discarded CFRP with the one-process surface modification and treats the recycled CF surface to improve the adhesive property with resin was established. The figure shows that when N2 and CO2 are added to the atmosphere during the superheated steam treatment (SHS), the interfacial shear strength is improved by increasing the specific surface area and adding a functional group compared with the untreated atmosphere. The development target values were also achieved for all elemental technologies, and the high-cycle molding process within one minute was finally built, as shown in Fig. 2.72. The molding base material flowing into the line is heated to the melting point with an IR heater. After softening, it is charged into a pressing machine with a material handling robot and molded. Since charging to demolding is within one minute, the one-minute tact time high-cycle molding is achieved.

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Fig. 2.70 CTT material mechanical properties (dependency on tape length)

Fig. 2.71 CF recovery technology with the superheated steam method

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Fig. 2.72 High-cycle molding process

2.6.4.2

Summary of Research and Development at Nagoya University NCC Centralized Laboratory

The general plan of the technical development is shown in Fig. 2.73. In the project’s first term (Phases I and II), the establishment of the LFT-D molding line capable of molding large structural components in a short cycle was mainly developed. In the latter term (Phases III and IV), the development focused on the hybrid molding technology to reinforce LFT-D, which simultaneously molds flake sheet base materials with long fiber arranged randomly in the same matrix resin (PA), the recycling technology, and the improvement of LFT-D properties (strength values and their variance) for practical applications. Also, similarly to The University of Tokyo Centralized Laboratory, it conducted the development in cooperation while sharing information by setting up a WG for each elemental technology, such as a molding WG, a design and evaluation WG, and a recycling WG.

Fig. 2.73 General plan of the technical development

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Fig. 2.74 Outline of the LFT-D high-speed molding line

The outline of the LFT-D molding line is shown in Fig. 2.74. The line consists of the following: a kneading and extruding machine to melt and knead resin (with additives) and charge and knead fiber (with resin), an extruding raw material cutting device to cut extruded raw material by a certain quantity, a carrier device with heat insulation to transport molded raw material after cutting to a pressing machine in the heat insulation state for preheating, and a material handling robot to set the heat insulated molded raw material in the prescribed position in a pressing machine die. Here, the kneading/extruding process, which significantly improves the LFT-D properties, is outlined. As mentioned above, in the kneading/extruding machine, the mixture of thermoplastic resin raw material and additives is melted and kneaded first and then charged to a screw shaft. Here, continuous fiber or CF with a prescribed length is involved and charged. The CF is broken in the flow of the resin shear field generated by a screw and kneaded in the resin. Therefore, accurately controlling the fiber length is difficult, and the fiber length distribution is essentially generated. This fiber length and its distribution affect the resin dispersibility (fiber dispersibility) in a die during molding, which becomes a determining factor for the molded material properties and their variance. This time, the kneading/extruding process conditions were optimized for the mechanical property target of the LFT-D molded material. The mechanical property target was a tensile elastic modulus of 25 GPa, a tensile strength of 200 MPa, and the improvement of physical properties variance. The parameters finally affecting the fiber length and its distribution after kneading were narrowed down to the specification of the kneading part screw (shape and rotating speed of the screw) and the charging material condition.

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An LFT-D molding test was conducted based on the finally optimized kneading condition (such as screw specification) to evaluate the mechanical properties of the molded material. The properties of the CF original yarn used in the test are shown in Table 2.8. Also, a part of the X-ray CT image and fiber length distribution of the LFT-D material produced at the beginning of development and in FY 2018 and 2021 are shown in Fig. 2.75. The LFT-D material at the beginning of development indicates large beltlike fiber assemblies (fiber bundles) in the X-ray CT image, and it can be seen that the dispersion of fiber and resin is insufficient. Also, the fiber length distribution shows a broad distribution from a short fiber component to a long fiber component. A long fiber (about 10 mm or more) is likely to interfere with fiber dispersion because it is easily entangled. However, since the X-ray CT image of the LFT-D material produced in FY 2018 seldom shows a fiber bundle or lump, the CFRTP is recognized as being highly dispersed. On the other hand, concerning the fiber length distribution, long fiber disappears, only short fiber components exist, and the short fiber components’ distribution shifts to the short fiber side. This is caused by fiber cutting promoted during kneading. The LFT-D material in FY 2021 shows both long and short fibers are minor in the fiber length distribution. The decrease in the large belt-like fiber bundles is expected to increase tensile elastic modulus and tensile strength. In this way, since the fiber dispersibility and length distribution were improved by examining the kneading conditions, the improvement of tensile elastic modulus and tensile strength was confirmed. The mechanical property evaluation results (average values) of the LFT-D material molded under the optimized kneading condition are shown in Fig. 2.76. Both tensile strength and tensile elastic modulus could achieve the development target values. Also, variances of properties were significantly improved. Table 2.8 LFT-D molding test sample

CF original yarn (catalogue value)

Matrix resin

Standard material

High strength material

High elastic material

Tensile elastic modulus (GPa)

235

245

390

Tensile strength (MPa)

4900

5100

4800

Elongation (%)

2.1

2.1

1.23

Density (g/cm3 )

1.8

1.78

1.74

PA6

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Fig. 2.75 Change in X-ray CT image and fiber length distribution of LFT-D material associated with examination of kneading condition improvement

Fig. 2.76 Results of mechanical property evaluation of the LFT-D material molded under the optimized kneading condition

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2.6.5 Trial Production of Vehicle Part Components with Developed Materials Nagoya University NCC and TORAY experimentally produced and evaluated the vehicle components with a full-scale shape to demonstrate the developed technologies for practical applications. Nagoya University NCC experimentally produced and evaluated the floor panel using the hybrid molding technology integrating the developed LFT-D and T-RTM technologies. TORAY experimentally produced and evaluated the roof using the CFRTP/CFRP sandwich panel consisting of CFRP skin and CFRTP core materials.

2.6.5.1

Floor Panel

The outline of the floor panel trial production process is shown in Fig. 2.77. First, the preparatory molding for the crossbeam is performed using a flake sheet, which is obtained by cutting the CFRTP UD (one-direction) tape material produced using continuous pultrusion technology [84, 85] by the T-RTM (ε caprolactam high-speed in-situ polymerization in a die) molding. Next, this preliminarily molded base material is simultaneously molded with the LFT-D molded base material in a die, and a floor panel integrating the crossbeam and the panel is produced. At that time, a reinforcement rib for improving rigidity in the entire panel plane and a measure for shock load resistance (to prevent buckling) are also arranged on the back surface. The appearance of the experimentally produced floor panel is shown in Fig. 2.78. It was demonstrated that the structure with a complex shape consisting of the panel part, taking charge of rigidity, and the crossbeam part, mainly handling the collision load, could be molded integrally in a short tact time.

Fig. 2.77 Floor panel trial manufacturing process

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Fig. 2.78 Floor panel trial product

2.6.5.2

Roof

TORAY promoted the development of the CFRP/CFRTP sandwich panel for application to roof materials for vehicles as research for CFRP practical applications. The X-ray CT image of CFRTP, the core material of the sandwich panel, is shown in Fig. 2.79. This is the porous material of polypropylene (PP) material reinforced with CF short fiber, which was developed by TORAY while participating in The University of Tokyo Centralized Laboratory in the first term of the project. Since it has a structure in which PP firmly connects short fiber intersection points, it has a high rigidity despite the porous material. In addition, since the material expands by itself because of the CF’s resilience, it can be shaped just by press molding despite the forming material. The molding process of the CFRP/CFRTP sandwich panel is shown in Fig. 2.80. The skin material (CFRP) and the core material (CFRTP) are set in a die, and the adhesive sheet is set between the skin and core materials. They are pressed and heated by a pressing machine and molded. Then, the mold plate surface is set to the prescribed thickness, and cooling is performed. The core material expands by itself

Fig. 2.79 CFRTP X-ray CT image

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Fig. 2.80 Molding process of the CFRP/CFRTP sandwich panel Fig. 2.81 Roof trial product

during cooling and is firmly joined with the skin material through the adhesion layer, and shaping is performed along the die surface simultaneously. In the trial roof production, the optimum structural design by the CAE analysis using the practical vehicle model reverse data as the source was conducted. The trial product’s shape dimension (including skin and core materials) and the skin material composition were determined so that the rigidity inside and outside the plane was equivalent to the base model. The appearance of the roof trial product is shown in Fig. 2.81. Molding was performed without problems for both the roof’s curved surface followability and surface property. In addition, an outer surface indentation test in full scale, a falling weight test with a test piece, and a solar radiation evaluation under a solar radiation amount assuming Okinawa midsummer were conducted as property evaluations for practical applications. All of them met the roof specifications.

2.6.6 Conclusions Through this research and development program, mass production-level technologies have been demonstrated by developing new CFRP technologies capable of accommodating weight reduction and cost and experimentally producing practical vehicle-sized components. In addition, advanced technologies could be established in design and molding CAE to support design and manufacturing, non-destructive

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analysis device and evaluation systems, die technologies, molding equipment, and others. While some companies have already planned commercialization, such as the equipment introduction for practical applications and commercialization of these developed technologies, the following approach, including the recent movement of reducing environmental burden, is expected to disseminate and expand the technologies: • Establishing a continuous CFRP technology support system extending from cooperative areas to competing areas • Creating a database of obtained technical information in a convenient form and establishing a system to use it • Standardizing acquired technologies • Developing the foundations of low-carbonization environment-compatible type recycling technologies

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Chapter 3

Materials Integration—Data-Driven Approach to Materials Design Using Simulation and Database Manabu Enoki and Takao Horiya

Abstract Materials integration (MI) is an approach that links the four elements of materials, Processing, Structure, Properties and Performance, on a computer by integrating experiments, calculations, theory and data science. MI is a data-driven type of materials development, in which all calculations from the initial inputs of materials, processing and use conditions, to the final output, i.e., the performance (life prediction, failure probability, etc.) of the member of interest, are performed instantly and in an integrated manner by connecting multiple computational modules so that the output of one module becomes the input of the next module, and automating the exchange of that data, to realize substantial reductions in the cost and time required for materials development. Although the term “materials informatics” is sometimes used as a synonym for “materials integration,” much of the research classified as materials informatics gives the impression of a search for materials focused on their structures and properties. However, in materials integration, treatment that links the four above-mentioned elements of materials is essential. The technical overview of materials integration and efforts related to MI in the Project are described in detail in Chap. 3.

M. Enoki (B) The University of Tokyo, 7-3-1, Bunkyo-Ku, Hongo, Tokyo 113-8656, Japan e-mail: [email protected] T. Horiya Innovative Structural Materials Association, 1-9-4, Chiyoda-Ku, Yurakucho, Tokyo 100-0006, Japan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 T. Kishi (ed.), Innovative Structural Materials, Springer Series in Materials Science 336, https://doi.org/10.1007/978-981-99-3522-2_3

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3.1 Introduction 3.1.1 What is Materials Integration (MI) System? In general, it is not always easy to predict the performance used in material design because properties such as strength required for structural materials appear as macroscopic performance due to the complex involvement of phenomena occurring at various scales. Material development generally requires a lot of time to evaluate performance, which restricts the pace of material development. Recently, along with the deepening of theories on physical phenomena, remarkable improvements in computing power have made it possible to perform large-scale simulations of phenomena at various levels. In addition, the field of data science, which combines statistics, the theoretical basis for handling various types of data, and information engineering methods, the programming necessary for actual processing, has been attracting attention and is beginning to be actively used as an approach to solving various scientific and engineering problems. The method of solving problems, based on big data, which accumulates various types and huge amounts of information, and the results of analysis processed by algorism is called a data-driven approach. In the field of materials engineering, along with the model-driven approach that mainly uses simulation calculations, this data-driven approach has been vigorously studied in the fields of materials informatics and materials integration, and is attracting attention as a new method for materials development. Efficient and accurate performance prediction using this type of methodology will be key to maintaining a high level of competitiveness in the development of structural materials in the future. In developing such research methods, reliable experimental data that has been collected and accumulated over many years will become more important than ever. It is not always possible to obtain the desired answers using data science methods alone, and it is also important to have a system to utilize the knowledge of researchers and engineers with various past experiences as “learning data”. An approach to evaluate performance by dividing the entirety into the elements of Process-Structure–Property-Performance (PSPP), analyzing these linkages, and then sequentially connecting them is proposed to consider structural material performance, and its usefulness is commonly recognized [1]. Since the linkage mentioned above, in other words, the relationship between process, structure, property, and performance of materials has a very strong nonlinearity in structural materials, the method of dividing elements, grasping local problems, and advancing analysis is effective in experimentally verifying the validity of these analysis results. In each of the four elements of the process, structure, property, and performance of structural materials, the tool that can extract parameters for materials as required and is used for a target analysis is called a descriptor. Since the linkage for materials mentioned above generally has causality, the linkage is considered to be the correlation of an output descriptor to an input descriptor. This relationship of input and output is called a module. Although the phenomena on material performance appearance are complicated, it is possible to partially organize the phenomena in the

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individual elementary process and define them as the linkage of input and output descriptors, i.e., a module. The linkage organized in this way can be developed as a module allowing analysis on a computer by effectively using a theory, computational science, database, informatics, and others. In addition, in connection with the multiple modules, it is possible to realize a connection (workflow) to predict the final performance by entering a material condition, such as composition, a process condition of manufacture and fabrication, and a component service condition. A series of methods or systems developed using such an approach is called Materials Integration (hereinafter referred to as “MI”). In other words, MI is a comprehensive material technology tool, integrating all scientific technologies, such as theories, experiments, analyses, simulations, and databases, by using material science results and aiming to support material research and development from the engineering perspective. Generally, material informatics using information processing technology for material development is also called MI. This is an approach that explores the material satisfying the target performance by applying a data science approach, such as AI and machine learning, to the data created by experiments, simulations, and others. On the other hand, material integration aims to broadly solve engineering challenges by integrating theories, empirical rules, databases, and others. Both are approaches used to reduce costs for material development and expedite material development. They can target the performance under the environment in the practical use of every kind of material and component, such as metallic material, high polymer material, ceramics material, and composite material. For example, welding is one of the material processing technologies indispensable to forming many structures, and forming a weld with laser welding and arc welding includes major elements of material structure formation, such as melting, solidification, phase transformation, grain growth, precipitation, and recrystallization during heating and cooling thermal cycles. In addition, in predicting mechanical properties, it is required to consider the appearance of mechanical properties depending on microstructures in addition to a thermal conductivity analysis, a residual stress analysis, and others. Therefore, by developing a module capable of analyzing an individual elementary process and then defining the workflow connecting the module, predicting the structure after welding and the appearance of mechanical properties is enabled even for complicated phenomena such as welding. Therefore, the performance can be predicted with sufficient accuracy in multi-scale, complicated challenges in actual materials by combining module groups and workflow groups. This MI, which has two major features, i.e., the systematic approach to integrate experiments, calculations, and theories and the use of data science, ultimately aims to significantly reduce time and costs in material development. Also, MI can modularize simulations, empirical rules, theoretical formulas, databases, and others, execute the module by interconnecting them, and obtain performance prediction while delivering the result. It can optimize a module and workflow per the user’s needs and resources. The workflow may consider the way to connect multiple simulations, an estimate using data science, a substitution of a theoretical formula and an empirical formula for simulation. MI is expected to achieve high prediction accuracy by using a small

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amount of experimental data and the calculation results newly obtained by accumulating experiment and simulation results in the database. It is considered helpful in predicting properties and performance in the analytic manner of a forward problem (obtaining the output from the input) and in designing the optimum material from a target performance in the analytic manner of an inverse problem (estimating the input from the output).

3.1.2 Examples of MI Development to Date The conception and development/application of MI, despite different forms and names, have been actively promoted throughout the world. For example, in the U.S., a large budgetary measure was implemented under the Materials Genome Initiative (MGI) [2] concept. Integrated Computational Materials Engineering (ICME), which predicts the properties and performance of materials by combining theory, calculation, experimental data, and others in a government-industry-academia collaboration, is active. The Center for Hierarchical Materials Design (CHiMaD) [3], centered on the National Institute of Standards and Technology (NIST), Northwestern University, and the University of Chicago, supports material properties prediction and material development, focusing on thermodynamics and phase diagram calculation and adding kinetics simulation in response to individual needs, in its capacity as a consultant. The Center for PRedictive Integrated Structural Materials Science (PRISMS) [4] of the University of Michigan energetically analyzes the structure, strength, and fatigue of structural materials by developing and combining various tools to solve challenges of material multi-scale/multi-physics. Also, for material data, NIST advances the development of a usable, applicable, and open data curation system, a data registry allowing easy registration for that data, and a data repository to save and share data [5]. In Europe, centered on Max-Planck-Gesellschaft and RWTH Aachen University, the development of various ICME tools, such as the structure formation simulation with the phase field method and the crystal plasticity analysis DAMASK [6], have been advanced, and establishing the ICME platform linking these tools is underway. VTT in Finland is also developing a tool for a material design called Proper Tune [7] and is acting as a consultant for an individual challenge. Also, in Japan, under the “strategic innovation program (SIP) innovative structural material” of the Cabinet Office, which started in 2013, establishing the MI system was advanced in a government-industry-academia collaboration as an approach allowing the prediction of the structure and performance of structural material by integrating a theory, an experimental rule, numerical modeling, a database, machine learning, and others [8]. Then, the “Material revolution/inverse problem analysis with an integrated material development system” has continued to be advanced since 2018 [9], and the development to establish the MI system has thus progressed. This project, for example, concerning the performance, such as fatigue strength, creep strength, hydrogen embrittlement, and brittle fracture, aims to establish a performance prediction system to help with the development of actual structural material by developing

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a calculation module for a forward problem analysis covering all theories and experimental rules and a database module obtained from a performance data analysis. Also, a system integrating theories and empirical rules, numeric modeling, databases, and a data-driven approach is being established for structure prediction. In predicting the performance using a physical model, for the linkage with the nonlinearity of process, structure, property, and performance, the analysis with the data-driven approach using Bayes’ theorem is useful, for example. In addition, the material design with an inverse problem analysis by combining structure analysis, crystal plasticity analysis, and experimental fatigue results is being targeted for the linkage of structure and property. The SIP project establishes a platform combining such a database and a calculation module called MInt [10], and shortening of a structural material development period, cost reduction, optimizing process conditions of material manufacturing, use, and processing, and optimizing and improving reliability in material choice during structure design are expected (Fig. 3.1). The MInt is establishing a system centered on metallic material, which mainly targets steel material, aluminum alloy, titanium alloy, and nickel-based superalloy. Also, a platform for carbon fiber-reinforced plastic called CoSMIC [11] is being developed. Furthermore, the “Material Research by the Information Integration Initiative (MI2 )” based on the National Institute for Materials Science (NIMS) started in 2015 and is establishing a data platform (such as arranging a material database and a data computational science tool) and developing three kinds of functional materials (electricity storage, magnetic, and heat transfer/ thermoelectric materials) using a machine learning approach.

Fig. 3.1 Concept of materials integration with a linkage of process, structure, property, and performance (example of MInt)

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3.2 Purpose of MI Development in This Project The SIP project mentioned above has developed a calculation module to predict the joint fatigue life for steel materials and aluminum alloy welded joints. It applies a calculation method to conduct a forward analysis based on a physical model, establishes a model to reproduce the mechanical response under repeated loads based on material structure information, and predicts fatigue crack generation life and growth life with high accuracy [12]. The local repeated plastic deformation behavior is essential in the crack generation, and a prediction method using a model [13] based on a finite element method or dislocation theory is being developed. On the other hand, a method to predict unknown information is being considered for steel materials and aluminum alloys, using a database for mechanical and fatigue properties. Investigating the correlation between input conditions, such as chemical compositions, processing conditions, and inclusion dimensions, and a rotary bending fatigue strength for samples of a steel material welded joint, has revealed the following by using the methods, such as a linear regression and a neural network, and conducting an analysis that sets appropriate conditions. The input conditions that have a correlation with fatigue strength can be extracted, and a prediction with good accuracy is possible [14]. The features and purposes of the MI system development in this project, which started under such circumstances, are described below. A flame-retardant magnesium alloy with fire resistance, its weakness, improved while retaining the high specific strength of a generic magnesium alloy, is a promising material for use in the field requiring weight reduction of the body material of transportation equipment. This project is newly developing a flame-retardant magnesium alloy for the body structure of a high-speed vehicle and is advancing its practical applications. A fatigue life evaluation for a welded joint is essential for the following reasons: A fatigue property evaluation is required as a material reliability evaluation for practical applications, and a fatigue fracture tends to occur at a welded joint part in a welded structure. Therefore, the following was set as a target for a weld, which is the weakest part in a railway vehicle body structure: developing a calculation model capable of predicting a life from fatigue crack generation to fracture with a calculation model based on a physical model and considering even the cases with a complicated structure or a shape, such as a welding material. It was also a goal to establish a prediction system using a database as a complementary approach to predict reliability without having to perform costly, complicated calculations. Here, the reliability of flame-retardant magnesium alloy (fatigue, corrosion, and impact properties) data collected in developing the “innovative magnesium material” in this project was used, and a system to predict the long-term performance of flame-retardant magnesium alloy using machine learning was established. Specifically, the target was to develop in parallel a model to predict the reliability of flame-retardant magnesium alloy based on theoretical formulas or empirical formulas and a prediction model using machine learning based on a database, and establish a system to predict the performance (fatigue, corrosion, and impact properties).

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As mentioned above, the final purpose of the MI system development in this project is to establish a system capable of predicting the performance and life of magnesium material. The following was targeted: Calculation module groups are developed, a workflow connecting them is established and it is incorporated in a system to enable the prediction of the effect of material conditions and others on fatigue life. The results so far reveal that prediction with high accuracy is possible by appropriately setting material parameters, which are descriptors, based on the experimental results. The results obtained in this way are considered to be a useful tool for future magnesium material development.

3.3 Project Development Results (Development of MI Practical Use Technology for Performance and Life of Magnesium Material) 3.3.1 Development of Fatigue Life Prediction Module for Flame-Retardant Magnesium Alloy Welded Joint This research targeted the use of a calculation module based on a physical model developed for steel material in the SIP, establishing a model to reproduce a mechanical response under repeated loads at a flame-retardant magnesium alloy welded joint from the information of composition, structure, and residual stress as an initial condition, and the development of a module to predict the fatigue crack generation life and growth life with high accuracy. The module to predict the fatigue crack generation life and growth life of a flameretardant magnesium alloy welded joint consists of a welding simulation module with a thermal elasto-plasticity calculation, a fatigue crack generation simulation module, and a fatigue crack growth simulation module with a crystal plasticity calculation. Particularly for magnesium alloys, it is necessary to consider the effect of the plastic anisotropy resulting from the crystal structure (HCP structure) and precipitates existing inside a magnesium alloy on crack generation/growth properties. Also, in developing the calculation module, the calculation module prediction result should be able to be compared with an experimental result at any time, and the crack generation and growth behavior of a flame-retardant magnesium alloy joint should be evaluated in conjunction with the module development. For this reason, establishing the micro fatigue crack generation and growth life database required to verify the calculation prediction module mentioned above was advanced in parallel. The specific process to create a module to predict the fatigue performance of a typical metallic material using a physical model is described below. In many cases, the fatigue fracture mechanism is generally divided into a crack generation due to a shear mechanism on a slip system, a short crack growth, and a long crack growth. High-cycle fatigue of a typical structural material sometimes takes up as much as 90% of the total fatigue life, and the fatigue crack generation life strongly depends

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on the microstructure. Therefore, predicting the crack generation considering the microstructure is indispensable for life prediction. On the other hand, the crack stably grows in the long crack growth region, and the crack growth rate follows the Paris rule expressed by the stress intensity factor and does not significantly depend on the microstructure. Also, in the short crack growth region, a crack is divided into a physically small crack (smaller than 1 mm), a mechanically small crack (smaller than a plastic region size), a small crack in terms of the microstructure (smaller than or equivalent to a microstructure unit), and others, and the crack growth rate does not follow the Paris rule. However, since the crack growth length is small in this region, the contribution to the total fatigue life was considered to be minor. Then, first, the fatigue crack generation life was predicted by using the crystal plasticity finite element analysis considering the microstructure and calculating the shear plastic strain distribution of a slip system. Although various models are proposed for fatigue crack generation, the fatigue crack generation model (Tanaka-Mura model) [13], based on the behavior that dislocations are accumulated on main slip surfaces, was used. Considering that, as criteria for fatigue crack generation, a crack is generated when the energy accumulated on the slip band due to dislocation exceeds the surface energy, this model derives the number of the repeating cycles of fatigue crack generations. Based on this theory, the workflow predicting the fatigue life of a steel material welded joint [12] has been developed. This project introduced the constitutive equation for a crystal plasticity analysis considering a twinned crystal, essential in the magnesium alloy deformation, to be applied to welded joints of flame-retardant magnesium alloy and evaluated fatigue crack generation [15]. An example of the analysis result of the fatigue crack generation and growth of a flame-retardant magnesium alloy AX41 welded joint is shown below. The calculation flow is shown in Fig. 3.2. To compare experiments, a weld as is and a grinder-finished weld of a lap-welded joint were targeted. AX41 material properties depending on temperature were entered first, and the distribution of the residual stress introduced during welding was calculated by conducting a heat-transfer-elasto-plasticity analysis during welding. In the grinder-finished model, stress redistribution was considered by removing the finished part element. Subsequently, a stress–strain distribution under repeated loads was calculated by putting repeated loads on the finite element model, and a stress concentration zone was identified. For the stress concentration zone, a calculation considering plastic anisotropy was conducted with the crystal plasticity finite element method considering the creation and recovery of a twinned crystal. The crystal plasticity parameters used in the analysis were determined based on the experimental result of the AX41 base material. The number Ni of crack generation cycles was obtained by substituting the shear plastic strain obtained from a crystal plasticity analysis into the Tanaka-Mura model. The crack growth analysis shows the result of a growth life calculation using the Paris rule. The crack growth rate obtained from the high-cycle fatigue test result determined the Paris rule material constant. Then, the fatigue life was calculated by adding the crack fatigue life and the crack growth life. The calculation result is shown in Fig. 3.3. The fatigue life almost agreed with the experimental value. Similar to the case of the experiment, improvement of the fatigue life was confirmed for the grinder-finished material. Therefore,

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Fig. 3.2 Flow of fatigue life calculation of Mg alloy weld

this suggests that the relationship between the welded joint operation condition of a flame-retardant magnesium alloy and fatigue performance could be evaluated by using this calculation method. Also, the fatigue crack generation calculation result suggests that a fatigue crack was generated under a high-stress condition at the beginning of the test. This is a good way to evaluate the crack generation behavior, which is difficult to evaluate by an experiment alone. Also, by using this workflow, the effect of a weld toe shape or a weld defect on the fatigue property of flameretardant magnesium alloy can be quantitatively evaluated by virtually changing the weld toe shape (weld toe radius, flank angle, and excess metal height) or the weld defect shape (undercut open width/depth and the radius of curvature at the undercut bottom). Unlike conventional evaluations based on a weld quality grade, it can quantitatively estimate fatigue life. Therefore, it is expected to be effective in developing a policy for the inspection and repair in body structure design and operation.

3.3.2 Establishing Database Based on Fatigue Life Calculation for Welded Joint of Flame-Retardant Magnesium Alloy The effect of the weld shape on the fatigue of the welded joint of a flame-retardant magnesium alloy was evaluated below by creating a database based on the fatigue life calculation for a welded joint of flame-retardant magnesium alloy and conducting a database analysis based on the data science approach (Fig. 3.4). The fatigue life calculations mentioned above were repeated in predicting the fatigue performance of a flame-retardant magnesium alloy welded joint using a

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Fig. 3.3 Fatigue calculation and experimental results of a lap welding joint (with/without grinder finishing)

Fig. 3.4 Establishing weld fatigue property database using calculation workflow

fatigue calculation module. The results were put into a database. Welding was simply simulated first, and the weld residual stress distribution was calculated. The fatigue simulation calculated a macroscopic stress field when stress is repeatedly applied. Next, a stress concentration zone was identified based on the result in the previous step, and polycrystalline structures around there were restructured. Subsequently, a crystal plasticity analysis using a constitutive rule that considered twinned crystal generation/extinction was conducted, a plastic strain distribution was obtained, and the fatigue indicator parameter (FIP), an indicator of the crack generation life, was obtained with a crack generation model. Furthermore, the J integral was calculated for the model introducing a crack using a finite element method, and the crack growth

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life was calculated based on the Paris rule. The fatigue database (crack generation life, growth life, and entire generation life) creation was made possible by creating a model considering a weld toe radius, flank angle, and excess metal height and a model considering a flank angle, excess metal height, an undercut open width/ depth and a radius of curvature at the undercut bottom (Fig. 3.5) and by automatically conducting crack generation analyses many times while changing each value. The fatigue crack generation prediction obtained a regression formula to predict the fatigue crack generation life from the effective stress amplitude of a macro model without conducting a crystal plasticity generation life analysis for the model (model A) considering a weld toe shape and the model (model B) considering a weld defect (Fig. 3.6a). By using the obtained results, the effect of weld defects, such as a gap and overlap, on the fatigue life was evaluated. Figure 3.6b shows as an example of the calculation result of fatigue life for a lap-welded joint of flame-retardant magnesium alloy AX41 when changing the gap between base materials in various ways. By using a regression formula, the fatigue life under each condition could be predicted at a low calculation cost without having to directly conduct a crystal plasticity finite element analysis. Also, the obtained result did not conflict with the weld quality grade in the ISO standard (ISO 10042:2018) for an aluminum alloy joint. Since the weld quality grade for a magnesium alloy welded joint has not been established, it is significant that this approach can estimate the effect of each defect of a flame-retardant magnesium alloy welded joint on fatigue performance. In addition, by applying data science approaches, such as machine learning and the Markov Chain Monte Carlo (MCMC) method, to the obtained database, the prediction of the fatigue performance of a welded joint was attempted without conducting

Fig. 3.5 Example of calculation models for various welded joint shapes

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Fig. 3.6 Example of fatigue indicator calculation results for a Regression curve of fatigue crack generation prediction (Model A: model considering weld toe shape, Model B: model considering undercut), b the maximum efficient stress amplitude relating to gaps between base materials of lap welding joints and fatigue life

a crystal plasticity finite element analysis, requiring a high cost. In other words, sampling with the MCMC method from the probability distribution based on the prediction model with machine learning was conducted, and analysis was carried out using the approach to evaluate the relationship between an explanatory variable and an objective variable. An approach to visualize the effect of the weld toe shape and weld defect on fatigue life was used by artificially creating the data of the weld shape with a superior (or inferior) fatigue property in the MCMC [16]. In the model considering a weld toe radius, sampling was conducted centered on the value with a large (or small) inverse number of the fatigue indicator having a proportional connection with a crack generation life. The sampling result of the weld toe radius obtained is shown in Fig. 3.7. The figure suggests that when sampling large (small) inverse numbers of fatigue, large (small) weld toe radii are more sampled, and larger weld toe radii improve the crack generation life. Similar analysis showed in the model considering an undercut that an undercut depth and a radius of curvature were essential in crack generation life. In this way, it was demonstrated that the effects of material parameters on fatigue generation life could be extracted without an experiment. Since welded joints used in the field can include various defects, this approach, which visualizes their effects on fatigue life, is considered adequate to ensure the long-term reliability of a structure.

3.3.3 Magnesium Alloy Fatigue Strength Prediction Based on Literature Data Various research institutes have accumulated lots of fatigue properties data for material components and welded structures. Fatigue experimental data are generally organized in a relation between a stress range Δσ and a fracture cycle number N, and

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Fig. 3.7 Weld toe radius sampling result for fatigue indicator using MCMC

the data sets are linked with the information, such as material alloy composition/ defects and welding condition. A neural network model to predict fatigue strength using factors, such as material chemical composition and heat treatment condition and inclusion dimensions, based on the fatigue database has also been considered, and the establishment of a prediction model with good accuracy is confirmed by selecting an appropriate data set, the number of hidden layers, and an activation function [14]. In the magnesium alloy fatigue strength prediction based on literature data, experimental data, such as magnesium alloy chemical composition (Al, Zn, Mn, Ca, Fe, Ni, Cu, and Zr) and tensile strength, yield strength, fracture elongation, and fatigue strength (strength in the number of repetitions of 107 ), was collected from papers published in academic journals. A total of 113 pieces of literature published between 1984 and 2020 were used. Test pieces of a strongly processed member and those with a complex shape and multi-axial fatigue tests were eliminated to reduce data variance, and it was limited to axial fatigue tests for extruding material. The number of mechanical properties is the maximum for tensile strength, with 149 pieces. On the other hand, there was very little fatigue strength data available, with only 48 pieces. Then, predicting magnesium alloy fatigue strength with relatively less data by establishing a machine learning model for tensile strength with relatively more data and conducting its transfer learning were examined. The neural network structure (the number of layers, the number of elements in each layer, and an activation function) was determined with Bayes’ optimization. An approach combining Bayes’ estimation with the Levenberg–Marquardt (LM) method was used in transfer learning. In transfer learning, the weights w, except for the last hidden layer, were fixed at the same value as that of the learned tensile strength prediction model. Learning was conducted with the LM method using fatigue strength data. For comparison, learning in the same network structure using fatigue data alone without transfer learning was also attempted. The result is shown in Fig. 3.8. The prediction accuracy was low for direct learning due to its small number of data compared to the weight w and bias b. On the other hand, prediction accuracy was improved in the case of transfer learning

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Fig. 3.8 Comparison of fatigue strength prediction by neural network direct learning and transfer learning

with a learned model used. This result demonstrated that transfer learning also effectively predicted the magnesium alloy fatigue strength. In addition, the effect of each alloy composition on fatigue strength can be examined by conducting a virtual experiment of changing the chemical composition as input. These results are significant since fatigue strength can be quickly estimated when developing a new magnesium alloy.

3.3.4 Establishing Database for Flame-Retardant Magnesium Alloy Performance and Life As mentioned above, machine learning can predict magnesium alloy fatigue strength (base material). It was also possible to create a machine learning model based on strength results with many data to supplement the fatigue strength data amount and to improve the fatigue strength prediction accuracy by transfer learning. It is also possible to conduct a sensitivity analysis for specific parameters based on an established model. However, it is difficult to establish a model with good accuracy without a sufficient amount of data. Therefore, a database with sufficient quantity and quality is required. When extracting data from the literature, problems could occur, such as

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unequal conditions and the shortage of necessary data. Therefore, database reliability is an issue in machine learning analysis. As a result, in developing a prediction system using a database, research was advanced with the aim of establishing a system using the data reliability (fatigue, corrosion, and impact properties) of a flame-retardant magnesium alloy collected in this project and using machine learning approaches, such as linear regression and a neural network (NN) based on material alloy composition, processing conditions, inclusion dimensions, and others. Also, the database was developed so that the material manufacturing history and property evaluation approach could be traced back later to obtain a design guideline in a material process. As mentioned above, material integration considers a linkage model of process, structure, property, and performance to predict structural material performance. As a new database for MI, aware of this PSPP linkage model, the structure of a database capable of tracing back a process history for the material was proposed, and the database was arranged by actually incorporating data collected in this project and accumulated at each participating organization. First, a template was created for data entry, capable of linking the data reliability (mechanical fatigue, corrosion, and impact properties) of a flame-retardant magnesium alloy developed in collaboration with raw material suppliers, processing manufacturers, and vehicle manufacturers with compositions, structures, processes, and others (Fig. 3.9). At that time, aware of the linkage model of process, structure, property, and performance, establishing a database conforming to the MI system was targeted. The template for each field to collect data was arranged for a process, structure observation, property test, a performance test in addition to material composition in accordance with this linkage model and made it possible to trace back the process history taken by material as much as possible. Based on the PSPP model, templates of casting, extruding, rolling, heat treatment, Tungsten Inert Gas (TIG), Metal Inert Gas (MIG), and Friction Stir Welding (FSW) were established for a process, those of an optical microscope, Scanning Electron Microscope (SEM), X-ray, and Transmission Electron Microscope (TEM) were established for structure observation, those of hardness, tensile, compression, and impact (tensile and compression) were established for the property test, and those of fatigue, corrosion (exposure test), and corrosion (accelerated test) were established for the performance test. The templates covered generic items included in the observation and test data owned by organizations participating in the project. Although data was collected so that the templates were filled accordingly with as many items as possible, data was accepted even when all items were not filled in. Also, it became possible to add items for data with items not included in the templates as required so that original valuable data was not lost when incorporating them into a database. As a result, since all the pieces of process condition information affecting material have been obtained, a database was created to easily obtain strongly-influential factors by using process conditions and observation results as explanatory variables and material properties and performance as objective variables in machine learning. Materials are linked with process conditions (such as casting, extruding, and rolling), intermediate heat treatment conditions, and others using a composition as the only

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Fig. 3.9 Conceptual diagram of an integrated template for mechanical and fatigue properties and others of flame-retardant magnesium alloys

starting point. Also, tests such as fatigue are regarded as being equivalent to processes. As a result, material observation results after a fatigue test can be linked, for example. For welding materials, such a description states that the joining of multiple materials is possible considering dissimilar material welding and a welding rod. In this database, detailed documents on design and operation were also prepared. Also, it can address future expansion, and items of FSW and TEM were added later, for example.

3.3.5 Establishing Model Formula for Flame-Retardant Magnesium Alloy Performance and Life (Fatigue Property, Mechanical Property, and Corrosiveness) An example of establishing a model formula using the database created above is shown. This project enabled the establishment of a database with the following data linked in a flame-retardant magnesium alloy welded joint (MIG and TIG): [1] a welding process, such as welding condition (such as welding current and speed)/ base material composition, base material manufacturing condition, and filler material composition; [2] joint structure, such as grain size, solute element type and concentration, crystallized material distribution, texture, and defect distribution; [3] mechanical properties; and [4] fatigue properties. A prediction formula based on a theory and experience to predict mechanical properties was derived by using the database. The prediction result of fatigue properties with machine learning using a neural network is shown in Fig. 3.10. The obtained data had a significant error on the

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Fig. 3.10 Fatigue life prediction result of Mg alloy MIG welded joints predicted by machine learning

long-life side, which was estimated to be caused by using the data of a non-fractured sample for which fatigue tests were terminated 107 times. Also, for the relationship between a welding condition and joint strength, a model using machine learning was similarly derived [17]. The relationship between addition/heat input and strength predicted by machine learning is shown in Fig. 3.11. This confirmed that when the heat input increases, the graph’s peak, i.e., the optimum filler material with Al addition tends to shift in the lower direction. Also, while there is a peak around the filler material with the addition of 8.5 wt% aluminum for low heat input (1.63 kJ/cm), strength decreases as the addition of aluminum increases for high heat input (2.09 kJ/cm). In addition, the comparison of a predicted value by machine learning and an experimental value is shown in Fig. 3.12. Each broken line shows a prediction value by heat input. For the experimental values, the four points outlined in black mean the conditions used in learning, and the other ten points mean the conditions not used in learning. Based on the result, in MIG welding using AZX912 as the base material, the welding condition was predicted to be optimum in the case of the heat input of 1.63 kJ/cm and the filler material with an Al addition of 8.5 wt%. The tensile strength of the sample actually prepared in this condition was 254.6 MPa. This was the maximum efficiency among MIG welded joints using the AZX912 base material in this research. The experimental values showed that while it peaked at the aluminum addition of 8 wt% in low heat input, strength tended to decrease with an increase in the addition of aluminum in high heat input. Other experimental results confirmed this tendency, and the prediction with this model was demonstrated to be feasible. A model formula to predict the corrosion resistance of a base material of flameretardant magnesium alloy’s expansile material was also established. The result of the atmospheric corrosion test, which has few research cases and is practically significant, was adopted as a target. An approach was proposed to predict a corrosion rate based on a multiple regression analysis using alloy composition as an explanatory variable and corrosion weight loss as an objective variable. In addition, a multiple

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Fig. 3.11 Relationship between filler material Al addition/heat input and strength predicted by machine learning

Fig. 3.12 Prediction by machine learning and its verification result of the relationship of joint strength of an AZX912MIG welded joint and Al concentration in the filler material

regression analysis was conducted by adding exposure periods (one and three years) and installation sites (Choshi and Okinawa) to an explanatory variable [18]. Literature values [19] on the rolled material of a flame-retardant magnesium alloy were also used to enhance data reliability. There are two types of exposure tests, i.e., the direct exposure method, in which the tested surface was installed in the solar radiation direction to expose it to rain and wind, and the shielded exposure, in which it faced the ground. The result of the direct exposure test is shown in Fig. 3.13. The correlation coefficient R2 is 0.941, and the prediction accuracy was excellent. When it was applied to the shielded exposure test result, the correlation was low, as

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shown in Fig. 3.14. The reason for this is as follows: While corrosion weight loss expresses linearity to the Al concentration and exposure time, a major explanatory variable, under direct exposure, it does not express linearity to the Al concentration under shielded exposure. A similar nonlinearity phenomenon was also confirmed in a saltwater immersion test [20]. A corrosion product analysis confirmed that direct exposure material and shielded exposure material have different compositions and crystal structures [21]. Predicting a corrosion rate is traditionally considered difficult since the magnesium corrosion phenomenon complicatedly changes depending on the material, environment, and time. However, it should be noted that it showed linearity under certain conditions, as shown in Fig. 3.13. Incorporating an alloy composition, environmental factors, molding condition, and microstructure obtained from it into an explanatory variable and considering a corrosion mechanism are useful for improving the prediction accuracy. They are subjects for future examination. Fig. 3.13 Prediction of corrosion weight loss of magnesium alloy base material exposed to air by multiple regression analysis (direct exposure)

Fig. 3.14 Prediction of corrosion weight loss of magnesium alloy base material exposed to air by multiple regression analysis (shielded exposure)

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3.4 Conclusion—Platform Establishment and Future Prospects This project established a calculation workflow based on a theory, established various experiment databases, predicted the reliability (tensile, fatigue, corrosion, and impact properties) of a flame-retardant magnesium alloy by applying machine learning to the calculation and experimental data, and extracted descriptors contributing to the properties. The final targets are as follows: completing incorporation of the developed calculation workflow, obtained databases, and derived prediction formulas into the system and completing the comprehensive MI platform specialized in magnesium material. Then, challenges to using the platform were examined. First, since material data is highly confidential, it is essential to ensure its confidentiality. In other words, a company does not want to disclose the data it owns to other companies, but it does want to know as much of the other companies’ data as possible. Such a database structure whereby everyone can use disclosable data while non-disclosable data remains confidential was applied. In addition, remote use through the Internet is also required to secure convenience. This was then addressed by direct access of the MInt Internet. However, focusing on security, the database was arranged inside the firewall on the NIMS internal LAN, which was not directly accessible from the Internet. It was decided to use the “external resource utilization function” of the MInt system in the future to access the database from the workflow. A framework to effectively establish the database is also required. Then, the work extracting data for machine learning, manually conducted up to now, was scripted. The following was made possible: When executing the Python script, any local client can connect to the database and output the file for machine learning. This enabled output of the data for machine learning from the database including unpublished data. Since this script allows each participating organization to execute it in a subset database for the company, it also makes it easy for each participating organization to conduct machine learning using all database results in the future. On the other hand, it can be used safely via the Internet by also establishing a system remotely that makes the developed calculation module and database available. Also, for a calculation module, a manual usage is being established to make it easy to use. In addition, for the use of such an established database, the analysis using a machine learning tool was further examined, and access to the system using a normal Internet connection to improve convenience was made possible. The system development is underway to facilitate use of the calculation module and database developed above by incorporating the prediction module and database into the system. Enhancing these tools and expanding the database will help accelerate the research and development of magnesium alloys in the future (Fig. 3.15). The development period and cost reduction for structural materials are expected by establishing the developed MI system and predicting the performance. In addition, the process conditions for material manufacturing and fabrication will also be optimized. In order to optimize the material selection and further improve reliability

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Fig. 3.15 Using the MI platform as a tool to develop new Mg alloy components

in the structure design, not only the “performance prediction” by a forward problem analysis of predicting performance from a process, but also the advanced “material design” by an inverse problem analysis of designing a process from the performance will be unequivocally required. Organic integration of material knowledge and a data-scientific approach in a physical space and cyberspace is required to achieve this, and the promotion of further development is expected.

References 1. G.B. Olson, Science 277, 1237–1242 (1997) 2. Material Genome Initiative (MGI). https://www.mgi.gov/ 3. Center for Hierarchical Material Design (CHiMad). https://chimad.northwestern.edu/research/ index.html 4. Center for PRedictive Integrated Structural Materials Science (PRISMS). http://www.prismscenter.org/#/home 5. NIST Materials Data Curation System (MDCS). https://github.com/usnistgov/MDCS 6. DAMASK Crystal plasticity simulation package. https://damask.mpie.de/ 7. Computational material design—VTT ProperTunel. https://www.vttresearch.com/en/ourser vices/computational-material-design-vtt-propertune 8. Cross-ministerial Strategic Innovation Promotion Program (SIP) Structural Materials for Innovation—Materials integration. https://www.jst.go.jp/sip/k03/sm4i/project/project-d.html 9. Cross-ministerial Strategic Innovation Promotion Program (SIP) Materials Integration for Revolutionary Design System of Structural Materials. https://www.jst.go.jp/sip/p05/index.html 10. MInt (Materials Integration by Network Technology) system. https://www.nims.go.jp/MaDIS/ MIconso/MInt.html 11. Comprehensive System for Materials Integration of CFRP. http://www.cosmic.plum.mech.toh oku.ac.jp/

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12. 13. 14. 15. 16. 17.

T. Shiraiwa, F. Briffod, M. Enoki, Eng. Fract. Mech. 198, 158–170 (2018) K. Tanaka, T. Mura, J. Appl. Mech. 48, 97–103 (1981) T. Shiraiwa, Y. Miyazawa, M. Enoki, Mater. Trans. 60(2), 189–198 (2018) F. Briffod, T. Shiraiwa, M. Enoki, Mater. Sci. Eng. A 753, 79–90 (2019) R. Tamura et al., Sci. Technol. Adv. Mater. 21, 540–551 (2020) T. Takahata et al., in Summary of 2019 Fall Annual(165th) Meeting of the Japan Institute of Metals and Materials (2019), p. 134 I. Nakatsugawa et al., in Proceedings of the 138th Conference of the Japan Institute of Light Metals (2020), p. 55 A. Konno et al., in Proceedings of the 135th Conference of the Japan Institute of Light Metals (2018), p. 253 I. Nakatsugawa et al., J. Jpn. Inst. Light. Metals 70, 56–62 (2020) I. Nakatsugawa et al., in Proceedings of the 140th Conference of the Japan Institute of Light Metals (2021), p. 163

18. 19. 20. 21.

Chapter 4

Welding and Joining Yoshinori Hirata, Hidetoshi Fujii, Chiaki Sato, and Hisashi Serizawa

Abstract As mentioned in Chap. 2, the Project has been conducting materials development of ultra-high strength steel sheets of medium- and high-carbon steel, aluminum alloys, magnesium alloys, titanium alloys, CFRTP and other materials that will contribute to reduction of the car body weight. At the same time, it has been also developing welding and joining methods for ultra-high strength steel sheets and 3 combinations of dissimilar materials: steel to aluminum alloy, aluminum alloy to CFRTP, and steel to CFRTP. In this chapter, 6 types of joining processes, which are welding, brazing, friction joining, interface-melt joining, adhesive bonding and mechanical fastening, are described for multi-material structure car body consisting of the newly-developed materials and existing materials. The joint strengths of the feasible joining processes developed are mainly focused together with the target values for joining of ultra-high strength steels and dissimilar materials respectively.

4.1 Multi-material Joining Technologies—Overview of Joining Technologies Yoshinori Hirata

Y. Hirata (B) Osaka University, 2-1, Yamada-Oka, Suita 565-0871, Osaka, Japan e-mail: [email protected] H. Fujii · H. Serizawa Osaka University, 11-1, Mihogaoka, Ibaraki 567-0047, Osaka, Japan e-mail: [email protected] H. Serizawa e-mail: [email protected] C. Sato Tokyo Institute of Technology, G2-20, 4259, Nagatsuda, Midori-Ku, Yokohama 226-8503, Kanagawa, Japan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 T. Kishi (ed.), Innovative Structural Materials, Springer Series in Materials Science 336, https://doi.org/10.1007/978-981-99-3522-2_4

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4.1.1 Introduction The energy required to drive an automobile increases with increase of the car body weight. Therefore, efforts to reduce car body weight have already been made since the 1990s [1], and innovative light materials and their joining technologies have been developed together with car body design methods in order to save energy and reduce CO2 emissions. In this context, there has been a need to promote research and development of welding and joining technology from new perspectives, including joining technologies of dissimilar materials. In this section, it is described our efforts to develop joining technologies for multi-material car bodies to reduce car body weight, which is the subject of the “Research and Development of Innovative Structural Materials Project,” a project established by the Ministry of Economy, Trade and Industry. In this project, materials such as ultra-high-tensile-strength steel based on medium- and high-carbon steel, aluminum alloy, magnesium alloy, CFRTP (Carbon Fiber Reinforced Thermo-Plastics), and other materials have been developed to contribute to weight reduction, and a multi-material car body that employs the right materials in the right places is proposed, including existing materials along with these developed materials. In order to realize light car body structuring, joining technologies must be established to connect ultra-high-strength steel to each other, steel/ non-ferrous metals, and metal/resin & CFRTP. Although multi-material vehicles have already been put to practical use in luxury and sports cars [2], CO2 emissions, which have a significant impact on global warming, are emitted from mass-produced cars, which occupies an overwhelming percentage of the cars running around the world, and it is necessary to establish a weight reduction technology for them as soon as possible. Therefore, there is a need for a joining technology that can be applied as a mass-production technology with high productivity and low manufacturing cost, including facilities, equipment and running costs. However, resistance spot welding,1 which is widely used in car body welding, does not improve the point joint strength,2 even if the tensile strength of the steel is increased, as described in detail in the next chapter. In other words, it has been shown that, even if the strength of the base metal is increased, the joint limits the strength of the member of frame work and weight reduction cannot be achieved [3]. In addition, it has been shown that a hard and brittle intermetallic compound (hereinafter referred to as IMC) is formed at the joining interface in the diffusion bonding of cylindrical 1

Resistance spot welding: Used for spot joints of thin sheets of a few millimeters in car bodies, railroad bodies, and house construction. Metals such as steel sheets are overlapped and pressurized for several kN. At the same time, an electric current of several kA to 20 kA is passed between copper alloy electrodes for around 0.5 s, depending on the type of material and thickness of the sheet, to heat and melt the metal by resistance heat generation to make the joint. 2 Joint type: When joining technologies including welding, bonding, and mechanical fastening are adopted to assemble parts and products, the connecting shape (called a joint) varies depending on the load of the relevant parts to be subjected, construction method, productivity, and other factors. The joint types applicable to car bodies are as follows: lap joint, butt joint, flange joint, flare joint and T joint.

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rods of steel and aluminum alloy, and that the joining strength decreases as the IMC becomes thicker [4]. And in metal/resin or metal/CFRTP, there is the question of how molecular atoms bond between metal and organic materials in the first place and whether they function as a load-bearing joint, and there is the problem of how to respond to this question. In this project, the first step was to develop a process to secure the target strength of the joints. In the next stage, based on the performance required for the joints, assuming the location of application of the developed joining technology, issues such as environmental resistance and materials formability as well as joint strength were investigated, and the PDCA cycle was operated to improve the joint performance.

4.1.2 Car Body Weight Reduction and Multi-material Structure Figure 4.1 shows the evolution of car body weight reduction projects. For steel car bodies, the ULSAB (Ultra-Light Steel Auto Body) project, led by the International Iron and Steel Institute, was implemented from 1994 to 1998 [1]. As a result, it was demonstrated that a 25% lighter steel auto body was possible without increasing costs, while ensuring crash safety as well as body rigidity, which greatly affects vibration, noise, and handling stability. This was followed by the ULSAB-AVC project. In the 2000s, global warming and environmental issues came to the forefront, and the FSV (Future Steel Vehicle) project for PHEVs and EVs was implemented. Meanwhile, in the EU, the SuperLightCar project was implemented from 2005 to 2009, showing that the body weight of the then Volkswagen Golf V could be reduced by 39% by adopting a multi-material structure [5]. Subsequently, the ENLIGHT project for component weight reduction, including CFRP (here including both thermoplastic resin and thermosetting resin) and metal hybrids, the ALIVE project for mass-produced EVs, and the EPSILON project for small EV body design were conducted in parallel with these projects.

Fig. 4.1 Transition of car body weight reduction projects

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After the SuperLightCar project, automotive companies have been developing and commercializing multi-material cars. Table 4.1 summarizes the data published at the international conference “Joining in Car Body Engineering 2016” and the results of a survey by TWI (The Welding Institute: UK). Blank columns indicate data not published. The table shows vehicle price, material composition, and type of joining technology applied. For the point type joint, the number of joining points is shown; for the line type joints, the joint length is shown. In addition, Fig. 4.2 schematically shows various joining processes. It can be seen that aluminum alloys are used in large amounts in multi-material car bodies. In addition to resistance spot welding, laser welding, MIG/MAG arc welding (Gas Metal Arc welding), and other fusion joining processes, adhesive bonding is also applied to the assembly of these car bodies. In particular, it can be seen that the bonding length is longer than that of steel car bodies. In addition, mechanical fastening such as SPR rivets, FDS screws, clinching, and hemming are applied. Such luxury and sports cars can be made using a wide variety of joining processes, including some that are close to being handmade. Figure 4.3 shows information on car bodies presented in the International Conf. Euro Car Body over the past ten years (including car manufacturers in Japan, Europe, the U.S., and China). Thirty-two car models in the E segment (car body length: 4.7– 5 m) and J segment (SUV) are classified into steel car body (fifteen car models), steel-based MM (multi-material) car body (five models), aluminum-based MM car body (five models), and aluminum car body (seven models), showing the percentages of steel, aluminum, and resin/CFRP used (average values) [6]. The steel car body consists of more than 98% steel, while the steel-based MM car body consists of 85% steel, 13% aluminum, and 2% resin/CFRP. On the other hand, the aluminum-based MM car body is 57% aluminum and 43% steel, while the aluminum car body is 96% aluminum and about 4% steel. Figure 4.4 shows the average number of point joints and bonding lengths applied to each car body [6]. Resistance spot welding is overwhelmingly used for joining steel materials, with more than five thousands of point-joints applied to the steel car body and the steel-based MM car body. Resistance spot welding is also applied for joining aluminum to aluminum in aluminum-based MM car bodies and aluminum car bodies. In addition, mechanical fastening such as SPR rivets, FDS screws, clinching, etc. was adopted in a large percentage of aluminum-based MM car bodies and aluminum car bodies. The adhesive bonding length for the steel body is about 50 m; for the steelbased MM body, it is about 130 m; and for the aluminum-based MM body and the aluminum body, it is about 180 m. It is considered that for the multi-material car body with higher percentage of aluminum the longer bonding lengths need to secure sufficient rigidity. From the above, resistance spot welding, mechanical fastening, and adhesive bonding were adopted as the joining technologies together with GMA (gas metal arc) welding, laser welding and laser brazing applied to the body assembly of cars sold during the past decade.

98% 26% 2%

Steel (total %)

Ultra-high-tensile-strength steel (incl. Hot stamping steel)

Aluminum alloy

Material composition

Brazing

Laser brazing

Laser welding

Arc welding

Aluminum

Steel

Aluminum (MIG welding)

Steel (MAG welding)

Aluminum

3.2 m

9.1 m

5393 points

Steel

Welding

Resistance spot welding

445

Number of members

CFRP (incl. resin alone)

$15,000 to 19,000

Price

6.7 m

1.9 m

3.1 m

6283 points

483

22.4%

96%

$25,000 to 32,000

3.6 m

5.8 m

1m

312 points

4665 points

467

16%

15%

84%

$36,000 to 62,000

4%

18%

13%

78%

$79,000 to 93,000

Car D

15.7 m

3.0 m

3.1 m

3.7 m

2442 points

50.2%

8.3%

49.2%

$168,000 to 285,000

Car E

Multi-material car body Car C

Car A

Car B

Steel car body

Table 4.1 Joining technologies applied to steel and multi-material car bodies (until 2016)

(continued)

5.3 m

21.5 m

0.8 m

69.0%

31.0%

$35,000 to 45,000

Car F

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Mechanical fastening

Adhesive Bonding 40.2 m

Structural bonding

64.7 m

27.9 m

19.1 m

H (hemming)

26.7 m

172 points

21.5 m

97.2 m 1606 points 229 points

194 points

2827 points

168.7 m

208.2 m

Car F

56 points

114 points

163 m

Car E

Multi-material car body Car D

CJ (clinching)

30 points

55.1 m

94.3 m

Car C

FDS screw

SPR rivet

94.9 m

1.5 m

2.1 m 82.6 m

Arc brazing

Car B

Steel car body Car A

Bonding (total length)

Table 4.1 (continued)

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Fig. 4.2 Joining method and cross-sectional shape of joints Fig. 4.3 Material mix ratio in commercial car bodies during 2011–2021

Currently, along with the regulation of CO2 emissions per car, LCA (life cycle assessment), which evaluates total energy consumption and CO2 emissions, from the material manufacturing stage (such as steelmaking, refining, rolling of steel and aluminum, forming, parts processing, assembly, painting, and product completion at car/parts manufacturers) to driving in various environments by users, scrapping, and recycling, is now being highlighted in relation to the SDGs. It is expected to become an important indicator as one of the factors to judge product value in the near future, and the material mix may change significantly depending on future trends in manufacturing technology and domestic/international supply chains.

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Fig. 4.4 Average number of point joints and bonding lengths applied to car bodies

4.1.3 Overview of Development of Joining Technology Figure 4.5 shows the development of joining technology for multi-material structures. From the viewpoint of materials, there are two major categories of joining technology: joining technology for ultra-high-tensile-strength steels based on medium- and highcarbon steel, and joining technology for dissimilar materials. In the joining process, methods using various energy sources and joining mechanisms have been researched and developed to put into practical use, and they are classified into welding, brazing, friction joining, interface-melt joining, adhesive bonding, and mechanical fastening.3 3

Classification of joining methods The six types of joining methods are classified here based on the joining mechanism. [1] Welding The metals to be joined are heated and melted in a molten state. Then melted part is solidified and joined followed by cool down.

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From welding to friction joining, the joint is heated by resistance heating, laser, arc, or friction heat. In welding process, the joint is heated to above the melting point of the base material. In brazing process, the joint is heated to below the melting point of the base material but above the melting point of brazing material. And in friction joining process, the joint is heated to the mechanical softening temperature of the base metal, which is in a solid state but undergoes plastic flow. In interfacemelt joining process, laser or ultrasonic energy is focused on the joining interface and heated to cause local melting. Therefore, these processes are accompanied by temperature rise of the joint and are sometimes referred to as thermal processes. On the other hand, adhesive bonding and mechanical fastening are roomtemperature processes and do not cause material changes or thermal deformation due to temperature rise. In research and development of welding and joining technology in this project, thirteen companies, the National Institute of Advanced Industrial Science and Technology (AIST), the National Institute for Materials Science (NIMS), and Osaka University have taken charge of the development of joining technology for same kind of metals (mainly medium- and high-carbon steels) and dissimilar materials. In addition, universities and neutral research institutes have participated as subcontractors to conduct basic research that plays an important role in promoting the development of practical technologies. Figure 4.6 shows the joining technologies in practical use by domestic car manufacturers and those developed by welding machine manufacturers, including ISMA. Here, steel, aluminum, and composite materials are taken as structural materials. A circle is divided into six equal parts, each material is placed on each line segment, and the joining process between steel materials, for example, is plotted in the area between the steel materials. The inner pink colored concentric circle indicates practical technologies and the outer light green colored concentric circle indicates development technologies. Composite materials here are collectively referred to as CFRTP, CFRP, and GFRP (glass fiber-reinforced plastics). Specific details of the developed technologies are described in the next and subsequent sections. The development of [2] Brazing Brazing is performed by supplying a brazing material, which has a lower melting point than that of the materials to be joined, to the joining part, and melting the brazing material to join them. [3] Friction joining The temperature of the local area of the material is raised by frictional heat generated by rubbing the materials against each other or by frictional heat generated when a rotating tool or the like is pressed against the materials to be joined from the outside, and the both materials are joined in a solid-phase state with plastic flow. [4] Interface-melt joining Laser or ultrasonic waves are used to concentrate energy on the joining interface, causing melting in the sub-millimeter order. [5] Adhesive bonding This is performed by applying an adhesive to the materials to be bonded. [6] Mechanical fastening This is performed to mechanically join the materials to be fastened by using rivets, bolts, nuts, screws, etc.

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Fig. 4.5 Development of joining technology for multi-material structures

joining technologies is being conducted both inside and outside the ISMA project, and is being pursued with the aim of practical application.

Fig. 4.6 Joining technologies developed by the ISMA project

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Fig. 4.7 Development by the three-pronged approach of structural design, material technology, and joining technology

4.1.4 Conclusion In order to establish and put into practical use joining technology, it is necessary to ensure that the developed joining technology has a joint performance that is appropriate to the load and environment to which the joint is subjected. As shown in Fig. 4.7, this requires a three-pronged approach to development together with structural design and material technology. In the area of joining technology, the joint strength can be predicted based on multi-scale fracture mechanics, and the joint dimensions, shape, and interface properties, which govern the strength, can be predicted by process simulation, thereby optimizing the joining conditions. In practical use, we believe that car manufacturers will select joining methods based on cost, productivity, and LCA.

4.2 Joining Technologies for Medium–High Carbon Steels—Challenge of Joining Technologies for Medium–High Carbon Steels that Change the Conventional Concept of Welding Yoshinori Hirata, Hidetoshi Fujii

4.2.1 Introduction When the Project started, press hardening steel had been developed as a type of ultra-high strength steel sheet and was also being adopted by domestic automobile

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manufacturers in Japan. However, since this material required a heating furnace and quenching/tempering process, and productivity was low, ultra-high strength steel that could be press-formed at room temperature was desired. Against this background, the ISMA promoted the development of ultra-high strength materials of medium–high carbon steel with an increased carbon content to increase strength without addition of expensive alloying elements, and also developed joining technologies for those materials.

4.2.1.1

Background

As described in Sect. 4.1, the Ultra-Light Steel Auto Body (ULSAB) Project, led by the International Iron and Steel Institute (IISI), was carried out from 1994 to 1998 by 35 steel companies in 18 countries for the purpose of reducing car body weight. Based on this project, steel makers around the world have been promoting research and development of high strength steel materials for automobiles. Of course, together with steel development, the weldability of resistance spot welding, which is widely used as a car body welding technology, has also been studied. Figure 4.8 shows the relationship between the tensile strength of steel and joint strength by resistance spot welding [7]. The tensile shear strength test and cross tension strength test have conventionally been used to confirm the joint strength of car body joints. Joint strength is evaluated using TSS and CTS,4 which are the maximum loads in the respective strength tests. Figure 4.8 shows that TSS increases linearly with increasing tensile strength of steel up to about 800 MPa, but almost reaches saturation at about 1,000 MPa. On the other hand, the figure also shows that CTS peaks at around 700 MPa and decreases as the steel strength increases further. These results indicate that conventional resistance spot welding cannot be used to weld high strength steels. As described in Sect. 4.1, one of the objectives of the ISMA project is joining medium– high carbon steels more than 0.3 mass% carbon content without adding expensive rare metals. Then, there would be hard problems for making sound joint of medium–high carbon steels by conventional welding processes, including resistance spot welding. As shown in Fig. 4.9, in resistance spot welding of steel, when the C content in steel exceeds 0.1 mass% and the contents of phosphorus (P) and sulfur (S) increase, weld fracture occurs in welds, which cannot be used for practical purposes, even if their joint strength satisfies the required value [8]. In general, the carbon content of steels used for welded structures may not exceed 0.2% in consideration of weldability. In other words, as carbon content of the base material increases, weld cracking is more likely to occur. As the most widely used 4

TSS and CTS In resistance spot welding, an overlap joint as shown in Fig. 4.8 is applied, and the test methods of tensile shear testing and cross tensile testing are roughly shown in the figure to examine the joint strength. A tensile load is applied in the direction of the arrow, and the load value (unit: N) at which the base material or joint fractures is called tensile shear strength (TSS) and cross tensile strength (CTS), respectively.

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Fig. 4.8 Relationship between tensile strength of steel and joint strength by resistance spot welding [7]

Fig. 4.9 Influence of C content and P + 3S content on fracture mode of the joint by resistance spot welding [8]

welding technology, weld cracking tendency of arc welding processes has been investigated experimentally through y-type weld cracking tests of high tensile steels with various chemical compositions and plate thicknesses. The experimental data were statistically processed, and the weld cracking susceptibility index PC is expressed by (4.1) [9].

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PC = PC M +

t H + 60 600

(4.1)

where PCM : weld cracking susceptibility composition (mass%), H: diffusible hydrogen content of weld metal5 (cc/100 g), t: plate thickness (mm). PCM is expressed by the following equation: PC M = C +

Mn Cu Ni Cr Mo V Si + + + + + + + 5B 30 20 20 60 20 15 10

(4.2)

Equation (4.1) shows that weld cracking susceptibility is governed by the weld cracking susceptibility composition PCM , the diffusible hydrogen content and the plate thickness. The Japan Welding Engineering Society standard WES 3001 “Weldable high strength steel plates” recommends PCM ≤ 0.2 mass%. Accordingly, it can be predicted that application of conventional welding processes to medium–high carbon steels with carbon contents exceeding 0.3 mass% is difficult. Here, a typical example of high carbon steel welding is welding of rail steels with carbon contents of 0.5–0.7 mass%. Specifically, alminothermic welding, gas pressure welding, flash butt welding and enclosed arc welding have been applied. Gas pressure welding and flash butt welding are high temperature pressure welding methods, while alminothermic welding uses the chemical reaction heat between iron oxide and aluminum to adjust the chemical composition of the rail joint. Enclosed arc welding is a method of arc welding with performing preheating to 500 °C or higher. However, it would be difficult to apply these welding processes to the different application of the car body production line because of their low productivity and the large scale of the equipment.

4.2.1.2

Outline of Joining Technologies with Target Values to Be Developed

As joining technologies for medium–high carbon steels, welding and friction joining were examined as possible candidates for development as extensions of conventional technologies. Although welding process has weldability issues as the problem to be solved, it has high productivity to be desired in car body manufacturing. And development of friction joining, which enables joining in a solid state, was also carried out. For practical application of ultra-high strength steel, which was under development as an innovative material in the beginning of the Project, it is the top priority to ensure sufficient joint strength in terms of the joint performance to be required for 5

Diffusible hydrogen content In welds of low-alloy steel and high-tensile steel, moisture in the atmosphere or crystalline water in the coating material of covered electrode may penetrate into the weld metal and cause cracking. In measurements of the hydrogen content, the amount of hydrogen that diffuses from the weld and is released from the bead surface, etc., is called the diffusible hydrogen content.

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Table 4.2 Target values for development of joining processes for medium–high carbon steels Item

Development of joining technologies for medium–high carbon steel to medium–high carbon steel

Development – Joint strength: For medium–high carbon steel with thickness of 1.4 mm and of spot tensile strength of 1.5 GPa or higher, average or higher tensile shear strength joining (TSS) and average or higher cross tensile strength (CTS) of JIS A class (JIS Z process 3140: 2017) Development – Joint strength: For joining of medium–high carbon steels with thickness of of line 1.4 mm and tensile strength of 1.5 GPa or higher, joint efficiency is 90% of the joining base material strength or higher process

the joining technologies applied to car body assembly. The target values are shown in Table 4.2.

4.2.2 Welding Process For spot-welded joints, two welding processes, which are resistance spot welding with external pressure and arc spot welding by gas shielded metal arc (GMA) welding, have been developed to meet the target values of JIS Z 3140 (Methods of inspection and acceptance levels for resistance spot welds). Figure 4.10 shows the principle of resistance spot welding with external pressure. In the development process, the aim was to increase the joint strength by increasing the nugget diameter (welded area). In order to increase the nugget diameter, it is necessary to increase the welding current, but there is usually a limit to the welding current because higher current values cause to generate spattering (molten metal ejection). Therefore, a coaxial pressure ring was attached to the outside of the electrode to pressurize the outside of the current-carrying area, thereby expanding the pressure range at the contact interface of the base materials. This suppressed spattering generation and enabled the formation of a large nugget diameter with a high current. Concretely, the project targets for the TSS of 0.4 C steel with a tensile strength of 1.2 GPa and 0.4 C steel with a tensile strength of 1.5 GPa were achieved. The target values of the project were revised because the revised edition of JIS Z 3140 was published in 2017, adding requirements for a cross tensile test for resistance spot welding of steel. Figure 4.11 shows the cross tensile strength for a resistance spot welded 1.5 GPa–0.4 C innovative steel sheet (thickness: 1.4 mm) and galvanized (GA) mild steel (thickness: 0.8 mm). The requirements for peel strength were satisfied by applying external pressure. In the GMA welding process, the cold metal transfer (CMT) welding method was developed in the 2000s, in which the feeding direction of the welding wire as the anode of arc discharge changes forward and reverse, as shown in Fig. 4.12, enabling extremely smooth short-circuiting welding without spatter. In addition, arc

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Fig. 4.10 Development of resistance spot welding with external pressure Fig. 4.11 Cross tensile strength by resistance spot welding of innovative steel sheets

spot welding, which was previously impossible, can now be performed, and the chemical composition of the weld metal can be controlled by adjusting the wire composition. As a result of this development, it became possible to modify the microstructure of the weld metal by the welding material [10]. As a result, the target value for TSS was met with 0.45 C steel. Fatigue strength was also 1 kN higher than that of conventional resistance spot welding at 106 cycles. Next, a laser welding process was developed as a line joining technology. The laser has a high power density and low heat input, resulting in low thermal deformation. Productivity is also high, as there is no need for strain correction so long as the accuracy of the temporary assembly is maintained. However, because of the small welding width, the joining area is smaller than that of arc welding. Figure 4.13 shows a cross section of a laser-welded joint of medium–high carbon steel. By setting the number of laser welds to three, it was possible to increase the junction area and improve the material quality of the stress concentration area [11].

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Fig. 4.12 Development of arc spot welding

The target values for 1.2 GPa–0.4 C steel and 1.5 GPa–0.4 C steel were met, and bond strength equivalent to the base material strength was achieved. Fig. 4.13 Development of multi-laser welding process

2-Laser Welding

3-Laser Welding

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4.2.3 Friction Joining Process Friction stir welding (FSW) was invented by The Welding Institute, U.K. (TWI) in 1991 and has since been applied to aluminum ships in Europe, railroad cars in Japan and the aerospace field in the United States. FSW does not require any filler materials and the use of inert gas as a shielding gas, thus enabling joining in a clean environment. FSW is a solid-phase joining method suitable for aluminum alloys which have a low melting point and excellent plastic flow. Then, it can also be used to join aluminum series 2000, some series 7000 and aluminum die castings, which could not be joined by arc welding or resistance spot welding, and therefore is being applied to an increasing number of aluminum structures. It has already been applied practically in the flooring of the Series 700 Shinkansen bullet trains, the outer panels of linear motor cars, road bridge floor panels and the fuel tanks of Japan’s H-IIB rocket. Although FSW is effective for joining aluminum alloys, the project challenged the possibility of applying FSW to steel, which has a higher melting point and strength than aluminum alloys. Figure 4.14 shows the relationship between joining process temperature and axial load of the tool, which was investigated experimentally [12]. The temperature of joining area (horizontal axis) indicates the temperature at which a sufficient plastic flow capacity can be secured to prevent joint defects. For aluminum alloys, a pressing force of about 5 kN at a process temperature around 500 °C is adequate. However, in the case of carbon steel, the process temperature must be increased to about 1,200 °C and an axial load of about 38 kN is required in order to cause a smooth plastic flow, which means that conventional FSW equipment cannot be used for joining. Since it is necessary to increase the capacity of the pressurizing equipment as well as the heat resistance of the tool material, a prototype of the joining equipment was developed. Fig. 4.14 Joining process temperature and tool axial load in FSW [12]

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In friction joining processes there are both of spot joining process and line joining process. Friction stir spot welding (FSSW) is a process in which a joining gun is held at the end of a robot arm, and is expected to replace the resistance spot welding process. Although FSSW has been used practically by Mazda, Toyota in car manufacturers in Japan, and other companies for joining aluminum materials, its applicability to high strength medium–high carbon steel was examined for the first time in the project. As shown in Fig. 4.15, when it is applied to high strength steel, the tool becomes red-hot color, resulting in increased oxidation and frictional wear. In the case of cemented carbide tools, the shoulder portion of the tool is oxidized, the tool wear becomes remarkable as the number of spots increases, and the tool becomes unusable after even 200 spots. Therefore, a highly durable film was coated to the tool surface by PVD to prevent from oxidation of the shoulder and wear of the probe. Figure 4.16 shows the appearance of each tool. In this theme, technology development was conducted with the aim of applying a heat-resistant and wear-resistant coating to cemented carbide tool materials, and optimization of tool cooling and joining conditions were also carried out in order to improve joint performance. As shown in Fig. 4.17, in these experiments, fully automatic operation using a transfer robot and FSSW robot was performed in all stages from specimen transfer and setting to FSSW joining operation and tensile shear testing. The damage appearance of the tool was checked and the joining conditions were optimized. As a result, it was possible to join 10,000 consecutive spots without damaging the tool while maintaining the JIS A-class joint strength for 1.5 GPa–0.45 C steel sheets. It may be noted that this FSSW robot was also demonstrated at an international exhibition.

Fig. 4.15 Application of FSSW process to medium–high carbon steel

Fig. 4.16 Tool appearances for medium–high carbon steel sheet

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Fig. 4.17 Automatic FSSW joining experiment and tool evaluation system

Next, the continuous line friction joining process will be described. Friction joining is a promising solid-phase joining method that avoids the risks of joining defects and microstructural degradation that can easily occur in welding process. In the project, two X-ray transmission devices were used to visualize the plastic flow during FSW joining process in three dimensions [13]. Based on the findings, it was studied that the steel is locally preheated using laser or high-frequency heating that quickly induces plastic flow and then, frictional heat generation and material stirring are effectively realized by tool rotation [14, 15] (Fig. 4.18). An experimental apparatus for an in-process preheating type single-sided FSW process was developed, and by using this method, the Project Phase 2 target (end of FY 2017) for joint efficiency of 70% of the base material strength or higher was achieved for medium–high carbon steel with a thickness of 2 mm and a strength of 1.2 GPa or higher. Furthermore, high-speed joining up to a joining speed of 2 m/min was also realized. Further development of in-process preheating and post-heating type FSW to achieve higher joint speeds was pursued, and a joining speed of up to 3 m/min Fig. 4.18 In-process preheating and post-heating FSW technique

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was achieved by using double-sided FSW [16]. To achieve a joining speed comparable to that of laser welding, a hybrid FSW with local high frequency preheating or laser preheating was developed, achieving a joining speed of 6 m/min (Fig. 4.19). Joining strength of 90% or higher of the base material strength of 1.4 mm thick, 1.5 GPa–0.4 C steel was achieved, thus satisfying the final target for joint strength. As the purpose of this theme is to join tailored blanks (semi-finished products) for application to actual products, a double-sided FSW process for differential thicknesses was established. As shown in Fig. 4.20a, double-sided FSW for differential thickness members such as combination of 1,180 MPa–1.2 mm thickness and 980 MPa–1.6 mm thickness materials was achieved by using a tool with taper and scroll (2018). From the cross section shown in Fig. 4.20b, it can be seen that both metals are sufficiently stirred and mixed by double-sided FSW. Fig. 4.19 From double-sided FSW process to hybrid FSW process

Fig. 4.20 Differential thickness joining technology using double-sided FSW

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In order to achieve the development of the FSW joining process shown above and identify issues for practical application, a prototype B-pillar outer was fabricated by tailor welded blank of different thicknesses and steel types [17]. Here, a 1.6 mm– 1.5 GPa steel sheet and a 1.6 mm–590 MPa steel sheet were joined by double-sided FSW, and four-step pressing and trimming were performed to produce the prototype shown in Fig. 4.21. Figure 4.22 shows the stress–strain diagrams of FSW butt joints. Tensile strength of 2 GPa and elongation at break of 20% were achieved by auto-tempering when appropriate amounts of Cr and Ti were added to medium–high carbon steel with carbon contents of 0.35 mass% C to 0.4 mass% C [18, 19]. For FSW joining of titanium (Ti) alloy as a hard-to-join material, development was carried out from the tool material itself, separately from the PVD/cemented carbide tool shown in Fig. 4.23 [20–24]. In order to provide an inexpensive tool based on a Co alloy, which has high durability at temperatures above 800 °C, a tool was prototyped by precision casting a master ingot of a Co alloy, as shown in Fig. 4.23. FSW joining of Ti alloy sheets (Ti–6Al–4 V) was performed, and joint strength of more than 90% of that of the base material was achieved at a depth of 10.8 mm [25]. Furthermore, to establish a FSW line joining process for curved members, prototype robot FSW equipment was developed, as shown in Fig. 4.24. The head section for inserting the tool into the member to be joined and the robot stiffness compensation system for correcting the tool position to follow the joining line were developed, and proper operation was verified. Next, the following describes the development of a linear friction process which achieves joining by rubbing the base materials against each other to generate frictional heat and induce plastic flow without using a rotating tool, as shown in Fig. 4.25. In linear friction joining, it is not necessary to consider the durability of the tool, as in conventional FSW. However, due to its principle, the entire joining surface is pressurized and rubbed, so it is necessary to grasp the object, which limits the size Fig. 4.21 B-pillar outer realized by tailor welded blank made by double-sided FSW joining process

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Fig. 4.22 Stress–strain diagram of FSW joints of medium–high carbon steel with addition of alloy elements

Fig. 4.23 Development of tool mass production process

and thickness of the sheets. With the current equipment, it is possible to perform experiments with steel sheets up to about 10 mm wide and 2 mm thick. As shown in joining process in Fig. 4.26, the temperature of the joining interface rises with the generation of frictional heat, causing plastic flow at the base material interface. Oxides and other harmful contaminants present at the interface are expelled by pressing force on the base material and are then removed. As a result, a pressure weld is formed, in which the clean interfaces of the two metals are metallurgically bonded to each other in a solid state at high temperature.

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Fig. 4.24 Appearance of robot FSW equipment Fig. 4.25 Principle of linear friction joining

The phase diagram of iron—carbon system is shown in Fig. 4.27. When the process temperature in linear friction joining is set at 723 °C (A1 transformation point) or higher, the microstructure of the joined interface becomes the austenite phase, which transforms during cooling followed by completion of joining. On the other hand, if the process temperature is set to below 723 °C (A1 point), the microstructure of the base metal and joining interface does not change during joining process, and then both materials are uniformly joined. The yield point of steel decrease with increase of the steel temperature. For example, the yield strength of mild steel SS400 drops to less than 1/2 at temperatures above 500 °C. Therefore, it is possible to control the joining interface structure by changing the process temperature in linear friction joining process. As the process temperature is increased, deformation resistance decreases, so the applied pressure can be reduced. However, when the process temperature is lower than the A1 transformation point, joining is possible by applying high pressure, which results in homogeneous joining between the two metals. Figure 4.28 shows the effect of the process temperature in linear friction joining on macrostructures and microstructure of the joint cross section. In Fig. 4.28a, the joining interface can be clearly distinguished under the high temperature joining condition. On the other hand, when joining is performed at a temperature lower

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Fig. 4.26 Temporal change in linear friction joining process Fig. 4.27 Phase diagram of carbon steel and process temperature range in linear friction joining

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Fig. 4.28 Effect of process temperature on cross-sectional macrostructure and interface microstructure in linear friction joining process

than the A1 transformation point, as shown in (b), the joining interface cannot be distinguished from the cross-sectional macrostructure. The microstructure in (c) also shows the disappearance of the original joining interface, and so-called seamless joining is achieved. Thus, linear friction joining makes it possible to unify the base materials by using the softening and transformation temperatures of the materials as parameters [26–30]. In this joining process, the target value was achieved using 1.5 GPa–0.45 C steel sheet (2 mm thickness).

4.2.4 Conclusion In joining of medium–high carbon steels, the technologies developed in the project have made it possible to fabricate sound joints without cracks or other defects in both welding process and friction joining process, and the target joint strength was

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achieved. However, assuming practical application, especially in the case of resistance spot welding, which is an extension of the conventional method, continuous research on microstructure softening/homogenization treatment by current supply as a post-weld heat treatment is considered necessary to ensure base material fracture. In the case of friction joining process, since tool durability issues can be overcome through joining of medium–high carbon steel and titanium alloys, joint performance can be sufficiently secured. This technique can also be applied to products other than car bodies. However, most of the current equipment and devices are too large for practical use. So, it will be necessary to develop compact equipment that can be used on factory lines or on-site, depending on the applicable parts. As for the base material itself, chemical compositions and mechanical properties of existing materials should be adjusted for friction welding process. The development of materials with high plastic fluidity suitable for friction joining process is also expected.

4.3 Joining Technologies of Dissimilar Materials – Toward Establishment of Production Process for Multi-material Structure Yoshinori Hirata

4.3.1 Introduction The SLC (Super-LIGHT-CAR) project proposed the multi-material concept that weight reduction can be achieved by combining various lightweight materials such as ultra-high strength steel, aluminum alloy, magnesium alloy, and CFRP. The SLC project did not develop such materials nor joining technologies, but instead used the existing materials and joining technologies available at the time (2005– 2009) to build a prototype car body using the VW Golf V as a benchmark, and showed that to achieve a weight reduction rate of 39%, the cost increase would be necessary by 7.8 euros for weight reduction of 1 kg [31]. The primary focus of the project was to ensure joint strength, and research and development have been promoted to develop joining methods that take productivity and cost into consideration.

4.3.1.1

Background

In the 2000s, mechanical fastening, which can join dissimilar materials such as steel plate/aluminum, aluminum/CFRP, and steel plate/CFRP without problems, was

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commonly used in Europe and the United States, and as shown in Sect. 4.1, mechanical fastening and adhesive joining have been applied for the past decade, including by domestic manufacturers. However, mechanical fastening methods increase weight and cost by attaching rivets and screws to car body. In addition, it is necessary to prepare rivets and screws of different sizes and shapes for each car body joining point, which generally lowers productivity. Therefore, from a medium- to long-term perspective, this project has pursued innovative joining technologies while taking into consideration joining processes that can reduce the cost of introducing new equipment and the running costs. Therefore, this section also refers to the applicability of Resistance Spot Welding, which is widely used in car production lines, to the joining of dissimilar materials.

4.3.1.2

Outline of Developing Technologies and Target Values

Figure 4.29 shows the joining processes of dissimilar materials developed in this project. Here, they are shown for the combination of steel plate and aluminum, aluminum and CFRTP, and steel plate and CFRTP. In welding process and brazing process, it is necessary to melt the base material and brazing material respectively. And resistance heating, laser, and electric arc are used as heat sources. In the case of friction joining process, the tool is often rotated to generate frictional heat, but there is also a linear friction joining process in which the base materials are rubbed against each other. In the interface-melt joining process, a laser is irradiated from the surface of the base material and the beam is focused at the joining interface to melt an extremely thin thickness, which has been developed for metal/CFRTP joining. As for adhesive bonding process, adhesives have been developed for joining structural materials. That is, the tensile strength, modulus of elasticity, and elongation of the adhesive itself, depending on the combination of materials, have been improved significantly compared to those of conventional adhesives. It can be said that the research and development of joining technologies of dissimilar materials have been accelerated thanks to the promotion of multi-materialization. In particular, curing speed, modulus of elasticity, and elongation have been significantly improved compared to the past. Since the product is intended to be applied to car bodies, methods that allow high productivity and low-cost construction as well as performance and quality of the joints are required. Since this project is aimed at practical application, we have set targets for productivity in terms of process time, etc. However, in the case of newly developed equipment, there are some aspects of cost that cannot be estimated, such as equipment introduction costs and running costs, so we are focusing mainly on the performance and quality of the joints, such as joint strength. The main issue is the performance and quality of the joints, such as joint strength, etc. The melting points and thermal conductivities of different metals, such as steel and aluminum, differ greatly, so it is essential to control the heat input distribution and its temporal change in the thermal process. Furthermore, because of the large

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Fig. 4.29 Joining technologies of dissimilar materials developed in this project

differences in thermal expansion and modulus of elasticity, temperature changes can cause thermal stress, which can lead to thermal deformation. In addition, metals with high ionization tendencies dissolve when in contact with electrolytic solutions such as water. Therefore, since a local battery is formed between materials with different ionization tendencies at dissimilar materials joint, galvanic corrosion occurs and corrosion prevention measures must be taken. CFRP also causes galvanic corrosion due to reinforcing carbon fiber included. For this reason, the target values shown in Table 4.3 have been established.

4.3.2 Joining Technologies of Dissimilar Materials for Aluminum/Steel The American Welding Society (AWS) has been studying the possibility of welding process between dissimilar metals for Ag, Al, Au, Be, Co, Cu, Fe, Mg, Mo, Nb, Ni, Pt, Re, Sn, Ta, Ti, W and Zr [32]. Specifically, based on the phase diagram, the feasibility of welding is determined by the nature of solid solution and intermetallic compounds formed in the weld zone. In addition, laser welding was performed on some combinations of dissimilar metals, and the results of examining the integrity of welds are summarized. The conclusion is that welding is not possible if hard and brittle intermetallic compounds are formed for the targeted dissimilar metal combinations.

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Table 4.3 Target Values for Dissimilar Material Joining Technique Item

Target Values

Development of joining technologies of steel/aluminum (spot joining process)

– Joint strength: TSS: JIS-A Class (JIS Z3140-2017) minimum or base material fracture, Average CTS of 1.5 kN or higher – Joining time: within 5 s per point – Establishment of evaluation and analysis method for thermal strain caused by thermal expansion difference between steel/aluminum – Establishment of anti-corrosion method against galvanic corrosion

Development of joining technologies of aluminum/CFRTP and other resin

– Joint strength: TSS: JIS-A Class (JIS Z3140-2017) minimum or base material fracture – Joining time: within 5 s per point – Establishment of joining technology for CFRTP with high melting point resin matrix such as PA and PPS – Establishment of anti-corrosion method against galvanic corrosion – Establishment of process monitoring method

Development of joining technologies of steel/CFRTP and other resin

– Joint strength at specimen level: Tensile Shear Strength of 15 MPa or higher – Joining strength of fabricated panels: Tensile Shear Strength of 20 MPa or higher – Fabrication of steel/CFRP composite panels – Establishment of evaluation method for joint corrosion by galvanic corrosion

Since dissimilar materials joining process for aluminum/steel is frequently used in multi-material car bodies, a reasonable direction for the development of joining technologies is that mechanical fastening and adhesive bonding processes that do not create intermetallic compounds should be applied as a method to obtain high technical reliability. In fact, there are actual applications as shown in Fig. 4.4 in Sect. 4.1 over the past 10 years. However, this project started to work on technological development aiming to realize practical application in 10–15 years, and the following steps have been taken to solve the problem of aluminum/steel combination. (a) Preliminary Study by Diffusion Bonding of Aluminum/Steel In this section, we describe our approach to establishing aluminum/steel joining process. Figure 4.30 shows the binary phase diagram of aluminum-iron. It can be inferred that Inter-Metallic Compound (IMC) such as FeAl2 , Fe2 Al5 , and FeAl3 are formed in welding and friction joining process including temperature rise. The mechanical properties of these IMCs are assumed to be hard and brittle, making it difficult to create a sound joint.

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Fig. 4.30 Aluminum-Iron phase diagram

Therefore, in order to understand the phenomena occurring at the aluminum alloy/ steel interface, we have studied the joint strength by diffusion bonding of aluminum 6061 and very low carbon steel in the solid state, as shown in Fig. 4.31 [33]. The bonding temperature was set at 500–600 °C and the bonding time was varied from 2.5 to 30 min to adjust the amount of diffusion at the aluminum/ultra-low carbon steel interface and to change the thickness of the reaction layer, mainly IMC, formed at the bonding interface. The figure shows that a reaction layer is formed at the interface between aluminum alloy A6061 and ultra-low carbon steel. Specifically, IMCs of FeAl3 with a hardness of 868 HV and Fe2 Al5 with a hardness of 1040 HV are formed, suggesting that the bonding interface is brittle. Next, the relationship between the reaction layer thickness and joint strength is shown in Fig. 4.32. It shows clearly that when the reaction layer exceeds 1 μm, the joint strength decreases significantly. Therefore, for practical application of joining for aluminum/steel by thermal processes such as resistance spot welding, appropriate joining conditions must be selected according to the type of base material and the thickness and distribution of IMC must be controlled. (b) Resistance Spot Welding If it becomes possible to use resistance spot welding to join dissimilar materials such as aluminum/steel, it will be the most desirable method because it does not require the introduction of new equipment and can be performed on the current production line. However, as mentioned above, the reaction layer at the joining interface is brittle and must be precisely controlled. Therefore, as shown in Fig. 4.33, the current waveform is set to multi-stage electric current-on. The steel plate section, which has high resistivity, is heated first. Once the electric current is not supplied, the heat diffusion from the steel section to the aluminum section occurs. Since the resistivity of steel increases with temperature,

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Fig. 4.31 Microstructure of interfacial reaction layer in aluminum/steel joint made by diffusion bonding

Fig. 4.32 Relationship between reaction layer thickness and tensile strength of aluminum/ultra-low carbon steel joints made by diffusion bonding

the second current-on selectively raises the temperature of the steel plate side. This temperature rise is then transmitted to the aluminum side by thermal conduction, resulting in melting and convection phenomena in the melted nugget. In other words, since an interface reaction layer is formed, it should be regulated by the current waveform [34].

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Fig. 4.33 Multi-stage current supplying method in resistance spot welding

Figure 4.34a shows a schematic diagram of resistance spot welding of aluminum/ steel. As shown in Fig. 4.34b, heat generation and melting occur at the steel plate section, and the reaction layer at the aluminum/steel plate joint interface is controlled to be thin. Specifically, Fig. 4.35 shows the effect of multi-stage current-on method on tensile shear strength. Then, the joint specimens were made for the two and three plates assemblies. The tensile shear strength was examined for single and multistage current-on method. In the two plates assembly of aluminum and 980GA steel, multi-stage current-on method satisfied the JIS A class for resistance spot welding of aluminum (1.2 mm thick). In addition, though in the single current supply it exhibited interface fracture, the aluminum base material fractured in multi-stage method. This was also effective in the three plates assembly aluminum (1.2 mm thick)/GA mild steel (0.8 mm thick)/1180 MPa steel (1.4 mm thick), achieving base material fracture by multi-stage method. Furthermore, when the resistance spot welding with external pressure shown in Fig. 4.10 in Sect. 4.2 was performed, it became clear that the pressure range became wider as shown in Fig. 4.36, and the deformation on the aluminum side was suppressed, thus securing the remaining thickness of aluminum. These phenomena can be predicted by simulation as shown in Fig. 4.36b. As a matter of course, to improve the prediction accuracy, it is necessary to input accurate values of material constants such as thermal dependence of electric resistivity, thermal conductivity, specific heat, plastic deformation of the material and so on for calculation. (c) Friction Stir Spot Welding (FSSW) A joining technology for aluminum/steel by FSSW, which has a lower temperature rise rate than resistance spot welding, has been developed [35]. The temperature rise due to frictional heat causes interdiffusion of aluminum and iron atoms at the joining interface, resulting in the formation of IMC. Figure 4.37 shows a cross section of the joint by FSSW process, with a rotating tool inserted from the aluminum side, and its structure of the interfacial reaction layer enlarged. In this example, since the interfacial reaction layer is about 5 μm, it is conceivable that, during the FSSW process, the zinc plating layer with a low melting point becomes liquid phase and complex phenomena occur at the joining interface.

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Fig. 4.34 Resistance spot welding of aluminum/steel

Therefore, in order to control the frictional heat and plastic flow that heats the joining interface, a multi-stage FSSW as shown in Fig. 4.38 has been developed [36]. By employing the multi-stage FSSW, as in resistance spot welding, it is possible to control the contact and heat input at the joining interface, then bring the zinc plating layer into a molten state to eliminate it from the joining area. For the reaction layer at the joining interface shown in Fig. 4.39, the Fe4 Al13 of IMC can be clearly identified and bonded consistently with the aluminum atomic arrangement. On the steel side, it was found that the structure was highly uneven, and stress concentration was likely to occur when a load was applied. This is believed to be related to the presence of a zinc plating layer on the steel side. It is necessary to investigate the interface structure without zinc plating in the future to clarify the

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Fig. 4.35 Effect of multi-stage energization in resistance spot welding of aluminum/steel plate

joining mechanism. In resistance spot welding, which is widely used for welding car bodies, the joint performances between steel plates and between aluminum alloy plates are specified in JIS Z3140 (Criteria for resistance spot welds). Although the standards based on the FSSW joining process have not yet been established. Then, JIS Z3140 was applied to the joints of dissimilar materials in order to judge the feasibility of the developed joining processes. In JIS Z3140, the joint strength required for point joints is dependent on the type of material to be joined (tensile strength of base material) and plate thickness. Therefore, when lap joint is applied to combination with materials of different tensile strengths and plate thicknesses in a plate assembly, the breaking load of each base material of the specimen is evaluated by multiplying the cross-sectional area of the specimen (plate width × plate thickness) by the tensile strength of the base material. Then the joint strength required is judged by the specified value based on the tensile strength and plate thickness of the base material, of which breaking load is lower side. In the aluminum/steel assembly, there is a high possibility that the joint will fail on the aluminum base metal side because the tensile strength of aluminum is lower than that of steel and the plate thickness in the area where it is expected to be applied is not significantly different from that of steel plate. Based on the experimental results, it was found almost joints were not fractured in IMC/Fe, and therefore, the factors governing the fracture of dissimilar material joints can be divided into three types of ductile fracture of aluminum, brittle fracture of IMC, and delamination of aluminum/IMC. Then, multi-scale fracture mechanics model (Fig. 4.40) in which the weakest part fractures due to stress and strain flow when the load is applied was constructed, and estimated the suitable joining conditions under which both TSS and CTS are maximized and the failure mode is plug fracture (base material fracture) [37]. Figure 4.41 shows an example of a simulated tensile shear test of an aluminum/steel FSSW joint.

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Fig. 4.36 External pressure resistance spot welding of 3 plates assembly of Al (1.2 t)/GA mild steel (0.8 t)/1180 MPa steel (1.4 t)

(d) Linear Friction Welding (LFW) Linear Friction Welding is a pressure welding process that uses the frictional heat generated by sliding at the interface and the applied pressure to contact fresh metals together by promoting the plastic flow with ejection of contaminated matters at the rubbing interface, requiring the specified pressure that matches the softening temperature depending on the combination of materials [38]. As shown in Fig. 4.42, in the case of aluminum alloy and carbon steel, if nickel is inserted as an insert material, the softening temperatures of the aluminum-nickel and nickel-carbon steel combinations match respectively, and an aluminum-nickel-carbon steel dissimilar material joining can be achieved. In resistance spot welding, the current supply is limited, depending on the power supply capacity. To generate heat in the joint by electric current conduction, it is necessary to supply the Joule’s heating ρ j 2 [W/m3 ] required, taking into

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Fig. 4.37 Cross section of macro- and micro-structures at the joining interface by FSSW for aluminum/steel Fig. 4.38 Temporal change of pressurization for multi-stage FSSW

account the heat conduction loss. Here, ρ: resistivity [Ω − m] and j: current density [A/m2 ]. Therefore, in resistance spot welding, the electrode diameter and electrode tip curvature limit the range of applied pressure and the heating area. In other words, there is a limit to expand the nugget diameter (joining area). On the other hand, in FSSW, the friction stirring area can be increased by increasing the diameter of the rotary tool, so the joining area can be adjusted by tool selection. However, it is assumed that the joining gun is capable of securing the tool rotation torque. Therefore, FSSW and LFW have the advantage that the required joining strength can be easily secured. However, many car manufacturers will need to invest in this system as a new equipment, and we believe that the decision on its

164 Fig. 4.39 Lattice image of the reaction layer at the joining interface

Fig. 4.40 Mechanical model of the joining interface of aluminum/steel FSSW

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Fig. 4.41 Simulation of tensile shear test of aluminum/steel FSSW joint

Fig. 4.42 Dissimilar materials joining by center-driven Linear Friction Welding

practical use will be made according to the future technical requirements for material mixes and dissimilar material joints. (e) Element Arc Spot Welding (EASW)

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As shown in Fig. 4.43, the developed process [39] involves only a steel element fitted into a hole in aluminum alloy sheet and joining process is performed by arc welding which melts and joins lower steel plates with steel element. The aluminum and steel plate are joined by shrinkage forces in the direction of the plate thickness. Therefore, an interfacial reaction layer is not formed between aluminum and steel, which has been a problem with conventional methods. However, the lower steel plate melts by arc welding, mixing and diluting with the welding material. Therefore, it is necessary to consider the chemical composition of the welding material (wire) and apply it according to the steel side material. Figure 4.44 shows that for joining dissimilar materials of innovative steel plate and aluminum 6022-T4, increasing the nickel content of the welding wire improves the cross tensile strength [40]. (f) Laser Brazing and Arc Brazing In brazing process, the brazing material, which has a lower melting point than the base material is melted and simultaneously infiltrated into the gap at the joining interface. The brazing process is used to form a metallic bond between aluminum and brazing material, as well as between steel and brazing material. Therefore, the brazing material contains chemical components that not only form a reaction layer with the aluminum surface, but also react with steel surface. The temperature distribution of the base metal and the selection of brazing material components are important because the wettability between the molten brazing material and the solid metal Fig. 4.43 Element Arc Spot Welding (EASW)

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Fig. 4.44 Effect of wire material on TSS and CTS of Innovative steel/A6022-T4 joined by EASW

greatly affects the smooth penetration of the molten brazing material into the gap at the joining interface. As shown in Fig. 4.45, the developed laser brazing method uses two heating lasers, one of which preheats the base metal to increase the wettability of the brazing material and enables high-speed joining [41]. For arc brazing process, base metal serves as a cathode of electric arc discharge, generating a large number of cathode spots which remove oxide layer. So, stable brazing process was achieved by controlling the arc length, wire feed, and current waveform so that the cathode spots are distributed over the entire base metal surface.

4.3.3 Joining Technologies of Dissimilar Materials of Metal/CFRTP Welding technology have been evolving as a joining technology for steel structures such as ships, automobiles, railway cars, various equipment in power, oil and chemical plants, line pipes, bridges, and buildings. The tensile strength, chemical composition, and thickness of the steel materials used in these structures vary depending on shape and size of the product, load and operation environment. Although all technologies are steel welding, specific welding process is used differently from the viewpoint of quality assurance and productivity. As a matter of course, metal/glass and metal/ ceramics joining technologies have also been addressed for a long time. Systematic accumulation of knowledge and methodologies began around 1980s with the development of ceramic turbo-chargers, and the application of brazing process and shrinkfit process to the joining of silicon nitride turbine and metal shaft was considered, and thermal stresses were quantitatively analyzed together with the microstructure of the joints. As well known, engineering plastics and other resins as the light weight material have improved in strength. Especially CFRP and CFRTP, which are representative

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Fig. 4.45 Dual laser brazing process

of high-strength materials, have come to be used in aircraft. Since many car makers including BMW have promoted practical use of adhesive bonding process for metal/ CFRP and metal/CFRTP, various joining methods have been developed along with improvements in adhesive bonding process. (a) Types of Chemical Bonding and Their Characteristics Joining principle can be broadly classified into chemical bonding between atoms and mechanical fastening. In the former, joining is achieved by atomic attraction through chemical reaction, while in the latter, as seen in rivets and bolts/nuts, both materials are joined by shear and clamping forces between the fastening material and base material. Table 4.4 shows the types of chemical bonding and dissociation energies (bonding energies) that act on the atoms and molecules that make up a material. Metallic bonding, covalent bonding, and ionic bonding have dissociation energies, which mean the energy required to pull one atom away from a substance, is 1 to 10 eV, i.e., 1 to 10 × 1.6 × 10−19 J.

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Table 4.4 Types of chemical bonding and dissociation energies Chemical bonding Example

Dissociation energies (eV)

Applications in joining technology

Metallic bonding

1.2–3.6

Welding of metals

Covalent bonding H2 O molecules, O2 molecules, Ceramics

0.65–7.3

Joining ceramics, Joining glass, glass to metal, ceramics to metal

Ionic bonding

5.8–10.9

Joining of ceramics to metal, and glass

Hydrogen binding Ice, found in many organic binding

0.13–0.43

Adhesive bonding

Van der Waals binding

−0.02

Adhesive bonding

Fe, Al and Cu, etc.

NaCl, Salts, Ceramics

Individual Ar (Noble gas binding), Organic crystals

Take iron (Fe) specifically. The dissociation energy required to heat solid iron at room-temperature to gas via melting and evaporation is 4.8 eV. Since the density is 7,874 kg/m3 and the mass per mole is 55.84 g, the number of atoms per unit volume of Fe is 8.4 × 1027 atoms/m3 . Therefore, the number of atoms per unit area of iron is roughly (8.4 × 1027 atoms/m3 )2/3 = 4.1 × 1018 atoms/m2 , requiring 3.1 J/m2 of energy to dissociate the atoms present in 1 m2 . This means that 3.1 J of energy is required to create a new surface of 1 m2 . When the fracture of solid iron is assumed that the atom plane moves to the interatomic distance (around 0.25 nm) in the ideal atomic structure without any defects, the interface strength is 3.1(J/m2 )/0.25(nm) = 12 GPa. From the viewpoint of joining technology, this means that if there is no contamination such as microstructural disturbances or adsorption at the interface, a strong bonding can be achieved. When applying organic materials to structures that are subject to large loads, it is necessary to use hydrogen binding and van der Waals binding as the joining principles. However, Table 4.4 shows that the dissociation energy of a single atom is 1/ 10–1/100 of metallic bonding, and furthermore, the number of atoms per unit area is much smaller than that of metals. For example, the compositional formula (monomer) of PP (polypropylene) shown in Fig. 4.46a is C3 H6 . As a polymer compound, PP is composed of many molecules of this compositional formula with chained molecules of different lengths. As shown in Fig. 4.46b, PP exists as an aggregate of polymers of different lengths, and there is a mixture of high density regions and low density regions within the material. In addition, each polymer chain is polarized by differences in electronegativity, and polymer chains are acted each other by weak electric forces [42]. A single chain of polymer is consisted from covalently bonded atoms, and the electronegativity shown in Table 4.5 indicates that, for example, in the bond between hydrogen (H) and carbon (C), the electrons on the hydrogen side

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shift slightly to the carbon side within the material, and the polarity is positive on the hydrogen side surface and negative on the carbon side surface [43]. In light of the above, the joining of metals to resins and CFRPs requires the application of joining technologies including adhesive bonding that have the effect of promoting chemical bonding such as hydrogen binding on the surface of the base material. Adhesive bonding process is described in Sect. 4.4 of this volume, so it should be referred to this section. (b) Friction Welding Friction welding process has been applied to the joining of dissimilar materials such as aluminum/CFRTP. Figure 4.47 shows the schematic diagram of FSSW process of aluminum/CFRTP and the cross section [44, 45]. The tool is inserted into the metal part to generate frictional heat, which raises the temperature of the CFRTP matrix of thermoplastic. Table 4.6 shows the melting points of some thermoplastic materials. For example, polyamide 6 (PA6) has a melting point of 225 °C and may join to metals such as aluminum. Fig. 4.46 Chemical formula and polymer structure of polypropylene (PP)

Table 4.5 Electronegativities by Pauling

H

C

N

O

2.2

2.25

3.04

3.44

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Fig. 4.47 Dissimilar material joining process by FSSW of Al/CFRTP

Table 4.6 Examples of melting points of thermoplastic materials

Thermoplastics

Melting point

Polyethylene (PE)

141 °C

Polypropylene (PP)

160 °C

Polyamide 6 (PA6)

225 °C

Polyamide 6 (PA66)

265 °C

Poly phenylene sulfide (PPS)

290 °C

Poly ether ether ketone (PEEK)

343 °C

However, as mentioned above, the joining between the metal and the resin is not achieved only by melting the thermoplastic matrix. As shown in Fig. 4.48, when the matrix resin is polypropylene (PP), the joining cannot be achieved even if the aluminum surface is etched and the molten resin is allowed to penetrate into the roughened concave areas to expect an anchor effect.6 It indicates the need for chemical bonding with the aluminum surface by adding functional groups to the resin PP surface. Although some types of resin have functional groups such as –COOH and 6

Anchor effect: The surface of the base material is treated to create complex irregularities to help the adhesive penetration into the indentations on the surface of the base material, and the shear force working between the base material and the adhesive is used to develop joint strength. However, the influence of anchor effect on joint strength cannot be expressed only by the roughness of the surface irregularities. It has been reported that adhesive bonding strength varies significantly depending on the size, length and distribution of cavities inside just below the surface, but no quantitative evaluation method has been established at present.

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Fig. 4.48 Effect of pretreatment of surface to be joined on FSSW of Al/ CFRTP

–OH groups, it is important to perform surface treatment before joining in order to chemically bond melting plastic with the metal surface. Figure 4.49 shows a cross-sectional microstructure of aluminum/CFRTP (matrix PA6) by the multi-stage FSSW process (Fig. 4.10); carbon fibers are observed on the CFRTP side, and it can be seen that matrix PA6 are infiltrated into complex morphology of the gap formed at the interface of Al/CFRTP. This structure is considered to be well joined for Aluminum to CFRTP. Next, the effect of joining pretreatment on joint strength was investigated. As shown in Fig. 4.50, a large tensile shear strength (TSS) of 7 kN is obtained with the rough surface treatment alone. Next, when the silane coupling treatment (chemical bonding force applied) is applied while the aluminum surface is left flat, the TSS strength exceeds 3 kN. Therefore, both of these pre-treatments were used together, then the joint strength increased considerably to 8.5 kN for TSS and 1.7 kN for CTS, indicating a synergistic effect. Although the joint quality can be expected to improve if the joint pretreatment process is increased, considering productivity and cost, the joint pretreatment method should be determined according to the loading conditions at the car component where this joining process is applied.

Fig. 4.49 Cross-sectional microstructure of the interface of multi-stage FSSW process of Al/ CFRTP

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Fig. 4.50 Effect of surface treatment before joining on the strength of FSSW joints of Al/CFRTP (Matrix PA6)

In this project, Friction Lap Joining (FLJ) of aluminum and CFRTP, shown in Fig. 4.51, has been developed [46–48]. Similar to the FSW process, FLJ is a continuous joining method using friction heating and pressure. Figure 4.52 shows the results of an experiment that investigated the effect of joining speed on tensile shear strength. This experiment also shows that silane-coupling treatment of the aluminum surface significantly improves the joint strength. Silane-coupling treatment procedures vary depending on the type of metal or thermal plastics. Usually, OH groups are present on the oxide film on the metal surface, and as shown in Fig. 4.53, the metal surface is modified by reacting these OH groups with the OH groups of the silane-coupling agent. We believe that as a result, the metal surface is covered with R groups (alkyl groups), which in this case promotes chemical bonding with PA6 (polyamide) for the matrix of CFRTP and leads to improving joint strength. (c) Laser Interface-Melt Joining This process was developed for joining steel to CFRTP. At the beginning of the development, the goal of practical application was to replace the steel used in the Fig. 4.51 Schematic diagram of friction lap joining for Al/CFRTP

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Fig. 4.52 Effect of silane-coupling treatment on joint strength of Al/CFRTP

Fig. 4.53 Modification of metal surface by silane coupling treatment

upper deck of a ship with CFRP in order to reduce the weight of the upper deck. The development of this method was undertaken as a method that does not require a rotating tool or other mechanical operating apparatuses, as is the case with friction joining, and that can be achieved with relatively low laser output power. Since the mode and focal point of the laser can be freely adjusted by optics, as shown in Fig. 4.54, we think that the method can be applied to a variety of product sites if the holding down restraint between steel and CFRTP can be precisely and easily performed in the manufacturing process. The relationship between laser power and tensile shear strength is shown in Fig. 4.55. When the laser output power is low, lack of fusion occurs at the joining interface and the joining strength decreases. On the other hand, if the laser output power is increased, excessive heat input will result in melt down, so it is necessary to select an appropriate laser output power. In this experiment, pretreatment before

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Fig. 4.54 Schematic diagram of laser interface-melt joining process

welding is not mentioned, and this will be an issue to be addressed in the future, depending on the component where this method is applied. (d) Adhesive Bonding Figure 4.56 shows the effect of adhesive thickness on tensile shear strength in adhesive bonding of steel to CFRTP (matrix PA6) [49]. It has been clarified that the strength of the welded joint becomes closer to that of the base metal as the thickness of the weld metal becomes thinner [50], and the same trend is observed in adhesive bonding process. Therefore, in adhesive bonding process, which is surface joining, one challenge in terms of production technology is to apply the adhesive uniformly in assembling car body during the manufacturing process and to maintain a constant distance between the joining surfaces of the base materials. Therefore, in bonding process, in addition to storage of adhesive in a manner to prevent deterioration, it is important to ensure a joining environment with controlled temperature and humidity to prevent adhesive performance changes due to exposure to the atmosphere during bonding. Furthermore, a device that can automatically adjust the coating amount is required so that the adhesive can be applied at a constant thickness during bonding. In addition, it is necessary to perform in-process monitoring of various process parameters in order to establish the quality control method. There Fig. 4.55 Relationship between laser output and tensile shear strength

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Fig. 4.56 Effect of adhesive thickness on tensile shear strength of steel/CFRTP joint

are measurement of the gap between base materials, control methods for bonding temperature and applied pressure, and confirmation of robustness for practical use. Therefore, automated joining system was demonstrated for quality control of the dissimilar materials joining process shown in Fig. 4.57, and it is working on quality assurance and joining automation for practical use.

Fig. 4.57 Automated equipment of joining process of dissimilar materials

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Fig. 4.58 Application of coating sealer for corrosion prevention of dissimilar material joints

4.3.4 Anticorrosion Technology When the contact area of dissimilar materials is exposed to a wet environment, galvanic corrosion occurs, which significantly affects the durability of car body. Here, we describe the concept of corrosion prevention method for mechanical fastening, which is already in practical use. As shown in Fig. 4.58a, when fixing CFRP, holes are drilled and rivets, etc. are used to join the CFRP in advance. While doing this operation, gaps are created between base materials. And another gaps exist between base material and rivets. Therefore, it is necessary to cover the surface of the base material with an insulator to prevent corrosion current from flowing even if moisture enters. Basically, the assembled component side that may be exposed to moisture is completely prevented from moisture infiltration by coating sealer.7 On the other hand, as shown in Fig. 4.58b, when the aluminum/steel plate is completely integrated with SPR rivets, corrosion protection is considered by blocking moisture penetration with coating sealer. Therefore, the performance of sealer can be considered to govern the durability of joint, and it is necessary to study the deterioration of sealer. As a degradation mechanism, it is believed that heat and water have a significant effect on the decrease of plasticizers.8 Therefore, in order to establish an accelerated degradation method, using the amount of plasticizer in the sealer component 7

Sealer: To prevent moisture and dust from penetrating through the joints of constituent materials of car body, including joints, and to ensure rustproofing of edges of steel plates, it is applied manually or robotically in the painting process and hardened in a baking furnace. 8 Plasticizers: A substance added to a polymer material to lower its viscosity, glass transition temperature, modulus of elasticity, etc., and to give the material flexibility. Plasticizing polymers causes a decrease in the cohesive force of polymer chains, etc., and increases flexibility, which allows the sealer to fit in the joint surface and seams, contributing to waterproof and dustproof performance.

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as an index for considering a sealer degradation criterion, the correlation between the amount of plasticizer and degradation in the operating environment is being investigated.

4.3.5 Thermal Strain of Dissimilar Material Joints and Evaluation Analysis Table 4.7 shows examples of linear expansion coefficients of steel material, aluminum alloy, resin, and CFRP. The expansion coefficient of resin alone is larger than that of steel material and aluminum. As for CFRP including CFRTP, the expansion coefficient is much smaller, in some cases negative, due to the influence of carbon fiber. Figure 4.59 shows an example of joining a steel plate to an aluminum alloy. For example, when joining at room temperature, as the temperature of the joint increases, the aluminum alloy has a greater expansion coefficient than the steel plate. Because the length of the joining area is constant, shear stress is generated at the interface of adhesive. As a matter of course, since both metals are constrained by each other at the joint, the product of the strain and modulus of elasticity causes the thermal stress. If the base material thickness is thin and the base material itself is not restrained, thermal deformation will occur. By the way, weld-bond process, which is a combination of resistance spot welding and adhesive bonding, is widely applied to steel car bodies in order to increase body rigidity. In the case of multi-material car bodies, adhesives play an important role in order to control galvanic corrosion in addition to ensuring car body rigidity. Table 4.7 Examples of linear expansion coefficients of body materials

Fig. 4.59 Thermal stresses at joint between aluminum and steel plates

Material

Linear expansion coefficients (× 10–6 /K)

CFRP

−1.0–1.7

PA6 resin alone

80–83

PP resin alone

100–120

Aluminum alloy

23.0–23.5

Steel

11.8–12.1

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Adhesive bonding process is applied during the assembly of car body in manufacturing stage. Then, after going through a curing process at 170–200 °C for 30 min in a paint-drying oven, the adhesive bonding process is completed. Actually, the commercially available car undergoes temperature changes in various driving environments, and thermal stress is generated in the joining area due to the difference in linear expansion coefficients of different materials. In order to quantify the history of thermal strain generated in the joining area of dissimilar materials, we simulated the change in thermal strain due to temperature changes in members bonded with aluminum/steel plate as shown in Fig. 4.60 and compared it with experimental results. As shown in Fig. 4.60b, by considering the curing process, the error from the experimental value was improved from 35% of the conventional model to 13%. This indicates that the use of CAE can expect to improve accuracy by applying counter-strain or pre-strain in terms of production process. Note that in this project, development of adhesives is also underway, and it has become possible to reduce thermal stress due to the viscoelasticity of the cured adhesive. Fig. 4.60 Accuracy of prediction of calculation of thermal strain of dissimilar material joints

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4.3.6 Conclusion In this section, we took up steel materials, aluminum alloys, and CFRTP as car body component materials, and described our research and development efforts to establish joining technologies that can secure the necessary joint strength for car body structures. As joining principles, joining technologies such as Welding process, Brazing process, Friction joining process, Interface-melt joining process, Adhesive bonding process, and Mechanical fastening process have been developed, and the characteristics of each are summarized. Future issues include the need to ensure joint performance according to the load and environment to which the joint is subjected, and to optimize the joining conditions by establishing process simulations that can predict not only joint strength based on multi-scale fracture mechanics but also the dimensions, shapes, and interface properties of the joint that govern strength. It is necessary to optimize the joining conditions. Joining technologies of dissimilar materials are evolving on a daily basis, with many stakeholders involved in its development, including not only the organizations participating in the project but also material manufacturers, welding machine manufacturers, and neutral organizations. This means that, based on the information disseminated through the annual meetings to report the ISMA achievement results, etc., the industries involved in Japan are moving in the direction of achieving practical application by continuously improving joining performance, reducing costs, and further increasing productivity in the future. However, at present, there is little discussion of technological development that takes LCA into account, and we believe that the selection of joining processes will be made based on LCA evaluations as well.

4.4 Adhesive Technologies—Development of Innovative Adhesives and Establishment of Strength Design Methods and Durability Prediction Methods by Elucidating Interfacial Adhesion Mechanisms Chiaki Sato

4.4.1 Introduction Adhesives are materials that are used to bond objects together, and have a long history. For example, asphalt was used as an adhesive in ancient civilizations, and starch for attaching paper and animal glue for wood were introduced to Japan around the seventh century. Adhesives derived from natural materials such as lacquer have

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also been used in Japan for a long time. However, rapid expansion of the use of adhesive bonding began in the twentieth century, when synthetic polymers appeared and began to be used as adhesives, so its history is relatively short. Because synthetic polymers have superior characteristics not found in naturally derived resins, such as extremely high material strength and chemical stability, it has become possible to achieve adhesive bonding that is incomparably stronger and more durable than in the past. For example, in the early days of aircraft, most of the wood and fabric used as structural members were joined by adhesives, and adhesives have continued to be used even after structural members were replaced by metals. Along with the evolution of aircraft, stronger adhesives have also evolved. In recent years, application of adhesive bonding to automobile structures has also begun. For example, the windshield and rear window are now bonded to the vehicle body. Adhesives are also used in combination with spot welding in the main structural parts of the vehicle body in a method called “weld bonding.” Resistance spot welding has been widely used in assembling steel auto bodies, but because the distance between welding points was as wide as 30–40 mm, this method had the problems of low rigidity, vibration and noise when the components were joined solely by welding. These weaknesses have been overcome by combining adhesive bonding and resistance spot welding. In this way, adhesive bonding has already become an important bonding method for current vehicles. However, from the viewpoint of weight reduction, it is expected that conventional body materials will be replaced with lightweight materials such as aluminum alloys and CFRPs, further increasing the importance of adhesive bonding. Adhesive bonding, which is suitable for bonding dissimilar materials, will be the most promising and indispensable bonding method, especially in multi-material structures that “apply the right materials to the right parts” to optimize weight reduction and cost. For this reason, the application of adhesives is attracting a great deal of attention as a technique for joining automobile body components. Against this background, the theme of this project, “Development of Adhesive Technologies for Structural Materials,” was launched, and adhesive technologies for joining the body of automobiles and the peripheral technologies were developed. In addition to the development of adhesives, analytical techniques for understanding adhesion phenomena more deeply, strength design techniques for joints and durability evaluation techniques were also developed in this theme, and research on pretreatment technologies for adhesion surfaces and inspection technologies for adhesive joints were also conducted.

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4.4.2 Basics of Adhesion and Technology Development in the Research 4.4.2.1

Principle of Adhesion

Although many hypotheses have been proposed over the years concerning the principle of adhesion, that is, why two objects can be joined with an adhesive, few examples have clarified the mechanism of adhesion, and many discussions have been limited to the realm of speculation. However, due to recent advances in various analytical methods, the phenomena that occur at adhesive interfaces are gradually being understood, and it is becoming possible to analyze “buried interfaces” [51]. In this theme as well, research and development examined analytical methods for adhesive interfaces. As a result, interesting new knowledge has been obtained. Adhesion involves a series of events: a solid adherend and a liquid adhesive come into contact with each other, the adhesive spreads over the surface of the adherend to form an interface, then bonding strength develops as the adhesive solidifies. Since this process refers to a series of flows that emerge, the “wetting” generated by some type of interaction between the adhesive molecules and the adherend molecules is important. The main interactions are known to be chemical bonding forces and intermolecular forces, and in addition, the “anchor effect,” in which the adhesive fits into unevenness of the surface of the adherend, causing a kind of mechanical fastening, is also considered to be a major mechanism of adhesion (Fig. 4.61). In this theme, various state-of-the-art analytical approaches were used to verify these hypotheses. In terms of strength, the highest interfacial bond strength is obtained when chemical bonds, especially covalent bonds, occur between the adhesive and the adherend, but there are few examples of adhesion by this mechanism. Intermolecular forces are roughly divided into hydrogen bonds and van der Waals forces. When a highly electronegative atom such as oxygen bonds covalently to hydrogen, the hydrogen becomes positive and interacts with surrounding negatively charged atoms. This is called a hydrogen bond. There are cases where they interact within a single molecule and cases where they interact with different molecules, and the latter corresponds to intermolecular forces. The van der Waals forces are classified into three types: orientation force, which is the interaction between polar molecules; inductive force, which is the interaction of a polar molecule and a molecule that is induced to become polar by it; and dispersion force (London forces) caused by the instantaneous charge bias between non-polar molecules. In terms of bonding strength, the magnitude relationship is orientation force > induced force > dispersion force. In particular, the bonding strength of the orientation force is quite large, approaching that of hydrogen bonding. In many cases, the bonding strength of adhesives is generated by hydrogen bonding and orientation force, and sufficient strength is obtained for practical use. On the other hand, high-strength bonding cannot be expected with induced force or dispersion force. The reason why there is a large difference in strength when different

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Fig. 4.61 Basic mechanisms of adhesion phenomenon

adherends are joined with the same adhesive is partly because different types of intermolecular force are generated by various combinations of the adhesive and adherend. Furthermore, this difference is largely due to the properties of the molecules present on the surface of the adherend rather than the properties of the adhesive. In this theme, changes in the adhesion mechanism were clarified by observing the interface between a plastic material and the adhesive with a transmission electron microscope [52–54]. The target plastic material was polypropylene (PP), and observation was carried out for cases such as surfaces subjected to flame treatment,9 to plasma treatment10 or use of an adhesive mixed with an alkylborane amine complex. As shown in Fig. 4.62, in the case of flame treatment, only a flat structure exists at the adhesion interface and no anchoring effect is observed, whereas plasma treatment creates a nanometer-sized uneven structure which the adhesive enters, and thus has an anchor effect. When using an adhesive mixed with an alkylborane amine complex, it was found that there is a high possibility that covalent bonds are formed

9

Flame treatment: A surface treatment method for improving wettability and adhesion by exposing the material surface to a flame for a short period of time. 10 Plasma treatment: A surface treatment method for improving wettability and adhesion by removing contaminants and introducing functional groups by irradiating the material surface with plasma.

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Fig. 4.62 Differences in interface between polypropylene and adhesive, (left) alkylborane amine complex, (center) plasma treatment, (right) flame treatment

by graft polymerization11 between the adhesive and the adherend. Bonding strength was also highest with the alkylborane amine complex, and strength decreased in the order of plasma treatment and flame treatment. As these results suggest, this study clarified the fact that the difference in the nanometer-scale interface, which cannot be distinguished by the naked eye, greatly changes bonding strength. Surface treatment is effective for improving adhesion even when bonding metals. In addition to removing stains and the fragile oxide film that exist on the metal surface, many treatment methods are also used to form a porous surface in the hope of increasing the anchoring effect. However, because many of the formed pores are only tens of nanometers in size, it has been questioned for many years whether the polymers actually enter the pores. Therefore, in this theme, a method for observing the entry of resin into micropores at the interface between an aluminum alloy and epoxy resin was established by devising a sample preparation method using a scanning transmission electron microscope [55–59]. At the interface with the aluminum alloy, from which organic contaminants were simply removed by chloroform washing, it became clear that the resin does in fact enter the micropores, suggesting that there is a high possibility that the anchor effect is achieved (Fig. 4.63). Compared to only washing, the bonding strength was approximately doubled by high-temperature steam treatment, and when the interface was observed, it was found that micropores had increased near the interface, and resin had also permeated into those pores. Based on these facts, it was presumed that an increase in the anchoring effect contributed to the improvement in strength. Even when chemical surface treatment was applied to create micropores with a three-dimensional structure on the aluminum surface, it was found that the resin also permeated into the interior of the micropores. Thus, the anchor effect has a great influence on the strength of adhesion to metal surfaces.

11

Graft polymerization: Polymerization (grafting) of graft chains (branch polymers) onto a polymer base material (trunk polymer).

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Fig. 4.63 Observation of anchor effect due to micropores on surface of aluminum adherend and permeation of adhesive

4.4.2.2

Types and Characteristics of Adhesives

The structural adhesives currently used in automobiles can be broadly classified into two types, epoxy-based adhesives and polyurethane-based adhesives. Epoxybased adhesives have high bonding strength, heat resistance and chemical stability, whereas polyurethane-based adhesives are characterized by high flexibility and have the ability to fill large gaps. Each adhesive also has its own drawbacks, as epoxy-based adhesives often require heating for curing and have low impact resistance since they are often brittle materials, while polyurethane adhesives have low bonding strength and many types of adherends require primer treatment.12 Focusing on where each type of adhesive is applied in a vehicle body, epoxybased adhesives are often used in joints of steel structures in combination with spot welding, that is, in joining by weld bonding. Because an epoxy-based adhesive is applied to the steel members, and the members and then overlapped and joined by spot welding from the outside, it is possible to proceed to the next process before the adhesive cures. The adhesive is then cured together with paint curing in the paint baking process. Polyurethane-based adhesives are used to bond window glass to the vehicle body and to bond plastic parts to the vehicle body structure. Depending on the adherend, a glass primer is required for glass, and a steel primer is required for steel. The primer is first applied to the material, and then a polyurethane adhesive is applied on the primer for bonding. Although there are still few examples of application of acrylic adhesives and modified silicone-based adhesives to vehicle body structures in Japan, their practical use is increasing in other countries. Acrylic adhesives are characterized by high bonding strength and peel strength, as well as quick curing at room temperature. However, as one problem of this type of adhesive, it is difficult to predict joint durability due 12

Primer treatment: A surface treatment method that aims to improve adhesion by thinly applying a primer that has an affinity for both the adhesive and the adherend.

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to its low creep strength. Modified silicone-based adhesives provide good adhesion to many materials regardless of the type of adherend, and are flexible and have high impact absorption. Application of these adhesives to vehicle bodies is still in its early stages but is expected to increase in the future.

4.4.2.3

Strength and Durability of Adhesive Joints

Since stress concentration is likely to occur at the edge of an adhesive joint, it is necessary to design the strength of the joint with sufficiently considering this point. For example, an overlapping bonded joint where two plates are joined together has high stress concentrations at both ends. Therefore, when performing strength design, it is essential to identify the stress distribution by a highly accurate stress analysis. Many stress distribution identification methods were proposed in the past, but most were calculation methods for simple joint shapes and were not suitable for stress analysis of joints with complex shapes, such as those in auto bodies. Therefore, it is necessary to perform simulations, including use of the finite element method, for stress analysis of joint shapes that are close to those of the actual structures. The breaking strength of adhesive joints can be obtained by applying the strength criteria of adhesive joints to the stress distribution results obtained by simulation. However, it is necessary to set appropriate strength criteria according to use conditions, since strength criteria change depending on the type of load, such as static load, impact load or fatigue load. In addition to these types of loading, in the case of adhesive bonding, the effects of temperature and creep are also very important, and how these effects are incorporated into the strength criteria becomes important. There are many items to consider when evaluating the strength of adhesive joints, and the interfacial strength changes depending on the combination of adhesive and adherend, so it is necessary to set the strength criteria again for each combination. These various complications are factors that have obstructed the widespread adoption of adhesive bonding. Since adhesives are polymer materials, it is much influenced by temperature and creep. Compared to metal materials, the upper temperature limit for use is extremely low and the creep property is also low, so it is necessary to design the joints taking into account these features, even in the automotive use environment. Polymer materials have a temperature–time conversion rule in which they behave similarly at high temperatures and low speeds and at low temperatures and high speeds; that is, strength tends to decrease at high temperatures and low speeds, and when a creep load is applied during heating, the load is doubled and strength drops sharply. Therefore, even with epoxy-based adhesives that have excellent heat resistance, caution is necessary when a continuous load is applied at high temperatures of 100 °C or higher. Adhesives are also highly polar polymer materials, and one factor that causes various problems is that they easily absorb moisture from the surrounding environment. Adhesive layers that contain water tend to have lower heat resistance and cause accelerated creep failure. Since the adhesive interface is also susceptible to deterioration due to water absorption, when considering the durability of adhesive joints,

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it is necessary to take into account both the relationship of durability with temperature and creep and the relationship with moisture. In this theme, a new method for predicting strength based on the results of accelerated tests for water absorption and adhesive joint deterioration was established.

4.4.2.4

Problems of Joining Dissimilar Materials

In order to realize a multi-material structure, it is necessary to join dissimilar materials. However, in dissimilar material joining, distinct problems must be considered different from joining the same kind of materials. Representative issues include thermal deformation and thermal stress at joints due to differences in the physical properties of the materials and galvanic corrosion caused by the electric potential difference between the materials.

Thermal Deformation and Thermal Stress When bonding two materials with different linear expansion coefficients, if the temperature changes after bonding are complete, this temperature difference causes a difference in thermal expansion between the adherends (thermal mismatch), resulting in thermal deformation and thermal stress. These phenomena are sometimes so large compared to the joint strength that they cannot be ignored, and may cause large residual deformation or even destruction of the joint. When considering thermal deformation and thermal stress, it is important to know the curing temperature of the adhesive. Temperature differences occur when an adhesive with a room temperature curing temperature is used in an environment with a temperature other than room temperature. For example, when the adhesive is used in an 80 °C environment, the temperature change from room temperature (23 °C) is 57 °C. However, with thermal-curing adhesives, temperature differences occur under almost all use conditions. For example, when the adhesive is cured at 180 °C, there is a temperature difference of 157 °C if it is used at room temperature, which is greater than the difference when a room-temperature-curing adhesive is used in a high temperature environment. In general, thermal-curing adhesives are more susceptible to thermal deformation and thermal stress than room-temperature curing adhesives because the thermal-curing temperature is often much higher than the expected operating temperature. Figure 4.64 shows an example of a thermal deformation calculation using simple material mechanics. Here, the case of bonding two adherends having different linear expansion coefficients is considered. It is assumed that the adherends have sufficiently high stiffness, whereas the adhesive has relatively low stiffness and can be flexibly deformed. Furthermore, it is assumed that each adherend is thermally deformed individually, and the difference (thermal mismatch) is absorbed by the adhesive layer through shear deformation. For example, if a 1 m long steel and aluminum alloy are bonded with a 1 mm thick adhesive layer and then subjected to a temperature change

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Fig. 4.64 Example of simple analysis for thermal stress in dissimilar material adhesive joints

of 100 °C, a thermal mismatch of approximately 0.5 mm will occur at both ends. When this is divided by the thickness of the adhesive layer, the shear strain γ of the adhesive layer is 50%, and when converted to vertical strain ε, ε = 25%. This value is extremely large and exceeds the upper limit of deformation of general high-strength adhesives (such as epoxy adhesives). Therefore, in order to prevent the joint from breaking, it is necessary to develop an adhesive with a larger deformation capacity than conventional adhesives. With room-temperature curing adhesives, issues other than thermal deformation and thermal stress are more important. For example, adhesives that cure near room temperature often have lower strength than thermal-curing adhesives and take longer to cure, so the development of a new adhesive with combination of high joint strength, sufficient durability and rapid curing is required.

Galvanic Corrosion When two substances with different ionization tendencies are brought into contact, an electric current may flow at the interface and cause galvanic corrosion. Adhesive joints of dissimilar materials are subject to this phenomenon, which is especially noticeable when moisture permeates the joint. Since the adhesive itself can be regarded as an insulator, adhesives tend to prevent galvanic corrosion, and from this viewpoint, the application of adhesives is preferable. However, when electrical contact exists between the adherends, or when welding or mechanical fastening is used together with an adhesive, an electric current due to galvanic corrosion

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flows even in the case of adhesive bonding. Since this galvanic cannot be prevented completely, sufficient protection against galvanic corrosion must be considered even when using adhesives. In order to prevent galvanic corrosion, it is necessary to prevent intrusion of water into joints, so sealing of the joint ends is important.

4.4.2.5

Inspection Technology

Because adhesive technology is used to join members and materials, it is classified as a “joining technology” in terms of the quality management system (QMS). Although there are some differences depending on the type of product and members, the technology and processes for making joints fall within the same framework as other joining methods (joints produced by welding are called “weld joints” and those produced by adhesion are called “adhesive joints”). Adhesive joining processes correspond to “special process” as defined in the ISO’s international quality standard ISO9000f. Specifically, since it is not possible to inspect the joints of production, it is necessary to “build in quality” when performing adhesive joining. Therefore, in addition to quality management based on the knowledge, skills and experience of engineers and technicians, it is also necessary to ensure the reliability of joints through non-destructive inspections, for example, by ultrasonic inspection. In adhesion joining, adhesion process management is extremely important and difficult. For example, if even a small amount of oil or contaminant remains on the adhesive surface, bonding strength will be greatly reduced, but this cannot be confirmed visually. Therefore, it is important to establish a technology for surface analysis before bonding. On the other hand, in non-destructive inspection after bonding, it is difficult to detect defects in the adhesive layer and interfaces at high speed and with high accuracy. Against this background, advanced inspection technology is also required for adhesive bonding, but at present, almost no inspection methods fully meet these requirements. For this reason as well, adhesive bonding has not yet been established as an industrial process.

4.4.3 Representative Research and Development Results Against the background outlined above, the following technologies were developed under the Project theme “Development of Adhesive Technology for Structural Materials.”

4.4.3.1

Development of Adhesives

In this theme, two types of adhesives were developed [60] because two bonding processes are conceivable, depending on the relationship with the painting process. One type assumes post-bonding painting (in-process painting), in which the adhesive

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Fig. 4.65 Development of adhesives suitable for post-bonding painting

is first applied to the parts, which are then overlapped, and the adhesive is cured by heating together with paint baking, while the other assumes post-painting bonding (out-process painting), in which the parts themselves are painted first and are then bonded with an adhesive at near room temperature. Both adhesive processes consider the production site, but post-bonding painting is mainly intended for joining of metal materials such as steel and aluminum, while the post-painting bonding is mainly used for joining of dissimilar materials, plastics and CFRPs. Because extremely large thermal stress and thermal deformation occur when joining dissimilar materials by post-bonding painting, an adhesive for thermal stress relaxation with excellent elongation as a one-liquid thermal-curing epoxy-based adhesive (Fig. 4.65) was developed, and another one-liquid thermal-curing epoxybased adhesive with extremely high heat resistance was also developed, although its elongation property is not suitable for joining dissimilar materials. In post-painting bonding, an adhesive that cures quickly at near room temperature and has high chemical stability is required. From this viewpoint, two types of adhesives are being developed, namely, a polyurethane-based adhesive and a modified silicone-based adhesive (Fig. 4.66). Extremely superior adhesives of both types have already been developed, and sample shipments have begun.

4.4.3.2

Strength and Durability Prediction

Simulation techniques such as the finite element method (FEM) are necessary for strength prediction of adhesive joints with complicated shapes. The material parameters required for analysis include the elastic modulus and tensile strengths of both

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Fig. 4.66 Development of adhesives suitable for post-painting bonding

adhesive and adherend, and the fracture toughness of the adhesive layer. In recent years, the method of analyzing the adhesive layer by the cohesive force model (Cohesive Zone Model: CZM) has been widely adopted, and this method is also used in this theme. In the cohesive force model, the stress-displacement diagram of the adhesive joint is given, as shown in Fig. 4.67, and the dynamic state until the adhesive layer breaks can be expressed. Here, the slope of the rising part is obtained from the elastic modulus of the adhesive, the peak stress (σ0 ) is obtained from the material strength or the bonding strength of the adhesive, and the triangular area bounded by the stressdisplacement diagram and the x-axis is obtained from the critical energy release rate (GC ), which is the fracture toughness value. The elastic modulus and strength are determined by material tests of the adhesive itself or strength tests using bonded specimens, and the critical energy release rate is determined by fracture mechanics methods such as the Double Cantilever Beam (DCB) test. These tests have already been standardized, and reasonable parameters can be obtained by conducting tests according to the standards. By inputting these values into the FEM analysis, it is possible to perform a crack propagation analysis until the joint fractures and calculate the strength of structures with adhesive joints, as well as how they break down, on a personal computer. Thus, the current level of technology related to adhesive joints is such that an environment in which strength prediction can be performed relatively easily is being established, but no method has been established for predicting durability. Therefore, one aim of this theme was to establish a durability prediction method for adhesive joints. Assuming moisture, temperature and repeated loads (cyclic loading) as the main factors that affect the durability of adhesive joints, the process by which

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Fig. 4.67 Stress-displacement diagram in Cohesive Zone Model (CZM)

the deterioration develops was clarified experimentally, and a durability prediction method was established by constructing a model from the deterioration level.

Prediction of Water Absorption Degradation and Durability of Adhesive Joints Himuro et al. conducted an experiment on deterioration of adhesive joints due to water absorption using a one-liquid thermal-curing epoxy-based structural adhesive [61, 62]. By investigating the change over time in the Fourier transform infrared spectroscopy (FT-IR) spectrum of the adhesive layer after absorbing water, it was found that hydrolysis occurred between the urethane component and the epoxy resin in the adhesive, clarifying the fact that breakage of the cross-linked structure due to infiltration of water led to deterioration of bonding strength. Since the rate of interfacial destruction increases with water absorption, it is assumed that water preferentially diffuses near the adhesive interface, breaking the bond between the adherend and the adhesive [62]. Water absorption deterioration of adhesives occurs only in the range where water has infiltrated. Therefore, when evaluating the deterioration status with adhesive joints, both the deterioration of the adhesive itself due to water absorption and the deterioration of the adhesive layer contribute to deterioration of bonding strength. However, it is difficult to discuss the effects of these two factors separately because the diffusion of water changes depending on conditions in the use environment, such as temperature and humidity, and the shape of the joint. Water absorption is relatively slow in adhesive joints due to the limited entry route, so an evaluation requires considerable time. Therefore, in this theme, a high speed, high accuracy method for evaluating only the water absorption deterioration of the adhesive was developed. Specifically, as shown on the left in Fig. 4.68, an Open Face adhesive specimen was prepared by applying the adhesive to the adherend, curing the adhesive, and immersing the specimen in water. In comparison with the conventional Closed Face specimen, use of the Open Face specimen greatly reduced the time required for water to infiltrate the adhesive layer. Since the Closed Face specimen adheres

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Fig. 4.68 Bonding strength evaluation using Open Face specimen, (left) appearance of Open Face specimen, (center) water infiltration paths into specimen, (right) SAICAS test

to the adherend on both sides of the adhesive layer, water infiltrates only from the side. Therefore, in addition to a slow diffusion rate, the conventional method also has the problem that the condition of water absorption deterioration differs depending on the location. In contrast, since the Open Face specimen adheres to the adherend only on one side of the adhesive layer, water infiltrates also from the other side of the adherend (Fig. 4.68, center). In the Open Face specimen, water infiltrates almost uniformly throughout the adhesive layer. Thus, in addition to shortening the water absorption time, the entire adhesive layer deteriorates uniformly, and it is not necessary to consider the diffusion state. Therefore, by using this method, it is possible to evaluate the deterioration of the adhesive or bonding interface after water infiltration for a certain time, and since the diffusion of water into the adhesive layer can be analyzed by using the diffusion coefficient, it is possible to predict the strength reduction in the adhesively bonded state by combining the two. The temperature dependence and time dependence of changes in bonding strength were evaluated by immersing Open Face specimens at different temperatures and measuring the bonding strength for each fixed immersion time [63]. A device called SAICAS (Surface And Interfacial Cutting Analysis System) was used for the strength evaluation (Fig. 4.68, right). This is a method for examining the strength and deterioration of the adhesive and the adhesive interface by cutting the adhesive layer obliquely from the upper surface side, and is suitable for a relative examination of strength reduction. However, as a remaining problem, the correlation between the SAICAS results and the bonding strength of actual adhesive specimens was not clear. Therefore, the correlation with the bonding strength of re-bonded specimens was investigated by performing a tensile shear strength test using re-bonded specimens obtained by bonding the Open Face specimens after immersion deterioration with a different adhesive [64] (Fig. 4.69). The results of the two tests showed a strong correlation, demonstrating the usefulness of the SAICAS test. Although it is clear from the left side of Fig. 4.70 that the strength of the adhesive joint decreases as the temperature increases and as the immersion time increases, this approach is not directly applicable to general environments, in which temperature changes occur during exposure. Therefore, the results were arranged with an Arrhenius plot, and the rate of decrease in strength with respect to an arbitrary temperature and immersion time was obtained (Fig. 4.70, right). The reason for the discontinuity on the

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Fig. 4.69 Comparison of bonding strength evaluation results by SAICAS and re-bonded specimen test

Fig. 4.70 Effect of immersion time and immersion temperature on water absorption deterioration of adhesive joints, (left) Residual strength at each temperature, (right) Arrhenius plot

left and right sides of the glass transition temperature (Tg ) is that the diffusion rate and deterioration mechanism change before and after this temperature. Based on the above, a method for predicting the actual residual strength with the immersion times at each temperature was established.

Prediction of Fatigue Strength of Adhesive Joints Since moving objects such as automobiles are constantly exposed to vibration, fatigue durability is also important when applying adhesive bonding to automobile bodies. There are two evaluation methods for predicting fatigue life: Obtaining the fatigue limit13 from the relationship between the stress amplitude and the number of loading 13

Fatigue limit: The maximum value of stress amplitude at which an object does not break under repeated cyclic loading applied a large number of times. Experimentally, the fatigue limit refers to the stress amplitude at which fracture does not occur after 106 –107 loading cycles.

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cycles (SN diagram), and predicting the amount of crack growth by applying the Paris law based on the relationship between the fatigue crack growth rate and fracture toughness. The latter destructive method is used for aircraft structures and other objects to which damage tolerance standards can be applied. However, damage tolerance standards cannot be applied to automotive structures, partly because periodic destructive inspections cannot be implemented. Since evaluation based on stress standards using the SN diagram is considered to be appropriate, this evaluation method was also adopted for this theme, and a prediction method for the fatigue properties of the adhesive was examined. Figure 4.71 shows the SN diagram of a simple lap joint in which an aluminum alloy is joined with a one-liquid thermal-curing epoxy-based adhesive. As the fatigue loading condition, a sinusoidal stress load with a stress ratio of R = −1 and a repetition frequency of f = 10 Hz was adopted. Here, the stress ratio R is expressed as R = / σmin σmax , where σmin is the minimum stress and σmax is the maximum stress. The SN diagrams of metal materials often show a linear decrease in load when the horizontal axis is plotted logarithmically, and from this figure, it can be understood that adhesive joints show the same tendency. Since the stress amplitude was about 5–7 MPa and no fracture occurred after 106 cycles of loading, the adhesive joint also has a fatigue limit. The bond strength is temperature dependent, and the strength of adhesive joints in static tests often decreases with increasing temperature. The fatigue test also revealed that the fatigue limit decreased as the temperature increased (Fig. 4.72). The degree to which the fatigue limit (τw0 ) decreased relative / to static strength (τB ) is called the strength reduction rate and is expressed as τw0 τB With this adhesive, the strength reduction rate was within the range of 20–30% under each temperature condition tested here, and no significant temperature dependence was observed. This ratio is generally considered to be about 50% for general metal materials, and adhesive joints experience a greater decrease in strength due to fatigue than metal materials. Therefore, if a fatigue source exists in the practical environment, the allowable stress must be considered when doing structural designs. On the other hand, since the strength reduction rate is almost constant regardless of the temperature, it is possible to predict the temperature dependence of the fatigue limit of adhesive joints from the temperature dependence of static strength. The adhesive used in this study was a general one-liquid thermal-curing type epoxy-based adhesive. Because epoxy-based adhesives for automotive structures also have a similar composition, this relationship is expected to be valid for many adhesives used in auto body structures [65]. Adhesives are polymeric materials and often exhibit viscoelastic properties. Therefore, creep deformation is likely to occur in the high temperature range, and also occurs in some cases even in the room temperature range. This means the material may break if a constant load is applied continuously, even if the load is lower than its static strength. Because repetitive loading is not an exception to this, the average / stress (σmax − σmin ) 2 and stress amplitude are important under loading conditions where creep and fatigue coexist (−1 < R ≤ 1). Since the average stress increases as R approaches 1, the fatigue limit of adhesive joints is greatly affected by the stress ratio. Figure 4.73 shows the SN diagram obtained by changing the stress ratio. Although the stress ratio has an effect, the trends are common and that effect was

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number of cycles Fig. 4.71 SN diagram of adhesive joint at various temperatures

Fig. 4.72 Comparison of static strength and fatigue limit of adhesive joint

found to be predictable. Figure 4.74 shows the endurance limit curve. The slope for the adhesive joint (solid line) used in this study is larger than the slope of the modified Goodman diagram (broken line) used for metallic materials, etc., presumably because adhesives are affected by viscoelasticity while metal materials are not. This

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Fig. 4.73 Effect of stress ratio on SN diagram of adhesive joint

Fig. 4.74 Endurance limit curve (mean stress-shear stress amplitude relationship)

difference also requires attention in the structural design process [66]. With the adhesive used in this study, it was possible to determine the safe design area, although the area was smaller than that given by the modified Goodman’s law (Fig. 4.74).

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Inspection Technologies

When conducting inspections with the aim of ensuring the soundness of adhesive bonding, it is necessary to conduct a surface inspection to check the properties of the adherend surface before bonding and a non-destructive inspection to check for existence of defects after bonding. As a pre-bonding surface inspection, wettability tests such as contact angle measurement are generally performed. However, this involves problems, e.g., it is not possible to perform bonding work until the liquid used in the measurement has dried. Thus, it is not realistic to apply this method to 100% inspections such as those in the automobile manufacturing process, since shortening of the process time is always required. In this theme, an inspection technology using the LIBS (Laser-Induced Breakdown Spectroscopy) method was developed as a high-speed, non-contact inspection technology. This equipment performs elemental analysis by laser beam irradiation of the adherend surface and detection by a spectrometer of the emitted light of plasma from the measurement point, and thus enables rapid, noncontact inspection of large surface areas. Figure 4.75 shows the LIBS inspection method, and Fig. 4.76 shows the rate of decrease in adhesive strength and the signal intensity of Si detected by LIBS when a small amount of silicone oil was applied as a contaminant to the aluminum alloy (adherend). As this method is capable of detecting contaminants on the adherend that reduce adhesive strength by about 5%, its sensitivity may be sufficient for practical purposes [67, 68]. Ultrasonic flaw detection is a method for detecting defects after bonding, but the conventional method has not been used at mass production sites from the viewpoints of labor and inspection time. Therefore, in this theme, a laser ultrasonic flaw detection technology that can inspect large areas at high speed was developed. In this method, the object to be inspected is irradiated with pulsed laser beam to excite ultrasonic waves that propagate inside the object, and the presence or absence of internal defects is determined by detecting these ultrasonic waves with a sensor attached to the

Fig. 4.75 Overview of LIBS system

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Fig. 4.76 Example of contaminant (Si oil) detection by LIBS. (Left) Relationship between tensile shear strength and contaminant concentration, (right) Relationship between LIBS detection intensity and contaminant concentration

inspection object. Figure 4.77 shows an overview of the developed laser ultrasonic flaw detector and its inspection results. Since the detected ultrasonic waveforms were complex, depending on the type of material and the shape of the object being inspected, it was extremely difficult to check for defects directly from this waveform information. Therefore, a method that enable easy checking for internal defects by visualizing the propagation of ultrasonic waves was also developed. Specifically, a galvano scanner is used to irradiate a large number of laser spots at high speed, and the amplitude of the ultrasonic signal excited at each laser irradiation point is modulated in brightness to display images in chronological order. The results obtained by this method appear as if the inspector is viewing at a propagation image of the ultrasonic wave generated from the sensor. As shown on the right in Fig. 4.77, defects can be detected by identifying ultrasonic wave disturbances due to internal defects (in this case, reflected waves due to voids) from the visualized ultrasonic wave images. The device developed for this theme is capable of detecting internal defects with a diameter of about 1 μm that exist in an adhesive joint between CFRP and aluminum, providing sufficient resolution for practical use [69]. However, this is not a complete non-contact measurement method, as a sensor must be attached to the measurement target to detect the ultrasonic waves, so problems remained in applying it to actual vehicle body manufacturing lines. To realize complete non-contact detection, a new system that uses a laser vibrometer instead of a sensor was developed to measure ultrasonic vibration and detect defects (Fig. 4.78). This device irradiates a laser beam for excitation to scan the bonded part, and measures the vibration at a predetermined point with a laser beam for vibration detection. Sufficient accuracy for defect detection has been obtained with this device, and it can be applied to non-contact, high-speed inspection of large-area joints [70].

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Fig. 4.77 Laser ultrasonic inspection system consisting of excitation laser and contact ultrasonic sensor. (Left) Overview of system, (right) Inspection results [top: Water immersion ultrasonic testing, bottom: Developed product]

Fig. 4.78 Laser ultrasonic inspection system consisting of excitation laser and laser beam for vibration detection. (Left) Overview of system, (right) Inspection results

4.4.4 Social Implementation and Future Prospects of Bonding 4.4.4.1

Assumed Multi-materialization

In considering the application of adhesive bonding, it is important to determine what types of materials are to be used as the adherends, since there are many options and scenarios for multi-materialization of auto body structures. At present, however, it is difficult to narrow the objects to be considered for application. Nevertheless, in view of the balance with cost, it is expected to start with the adoption of high tensile strength steel materials, followed by partial application of aluminum materials, then progress to the full application of aluminum materials and later application of composite materials, etc. At the same time, plastic materials will be applied to parts with minor strength requirements, and the proportion of plastics will gradually increase. From

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this viewpoint, it may be reasonable to begin adhesive development with a strong adhesive for steel, then develop a flexible adhesive with good elongation, adhesion with both steel and aluminum and thermal stress reduction performance, and finally, an adhesive for bonding composite materials, etc. For multi-materialization, the basic concept is to use the optimum adhesive in the optimum part. However, instead of adopting this kind of complex evolutionary approach, in this theme, adhesives and bonding technologies were developed focusing on the final multi-material structure. As a result, although we succeeded in developing adhesives that are ahead of the times, it is likely that full-scale application at manufacturing sites will take time. At this point, some have already begun to be applied to vehicle bodies, and it is thought that the scope of application will gradually expand in the future.

4.4.4.2

Adhesives as Means for Bonding Dissimilar Materials

The use of adhesive bonding as a means of joining dissimilar materials is considered extremely promising because adhesives can be applied to a wider variety of materials than welding. Thus, adhesive bonding is expected to become a major joining method in the future. At present, most adhesives are used in combination with other bonding methods, and there are few examples of automotive structures being assembled using only adhesives. This is based on the idea that dual joints are more reasonable at present, and not because there are problems with adhesive bonding itself. Since it will be more advantageous from the cost perspective to use adhesive bonding alone, even where combination bonding is currently used, the number of joints produced by welding and mechanical fastening will be reduced. From this viewpoint, it will be important to develop adhesives that can achieve sufficient durability with adhesive bonding alone, together with design methods for joints and methods for predicting joint durability.

4.4.4.3

Improving Reliability of Adhesive Joints

Adhesive bonding is a very reliable joining method when used within a range of empirically known applications. Conversely, when used in an environment that exceeds past experience, various problems tend to occur. In spite of this, it is also true that there are demands for reduction of the bonding area and use at higher temperatures, and breakthroughs to meet these demands are needed. As long as adhesives are applied to a large area and used at a low temperature, delamination is extremely unlikely, but if the adhesive area is not sufficient or use in a high temperature environment is expected, it will be difficult to ensure reliability. In particular, failure of adhesive joints due to high temperature creep has not been fully elucidated scientifically. Therefore, it is important to accelerate and strengthen research on use in harsh environments in order to further improve the reliability of adhesive joints and develop new applications.

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4.4.5 Conclusion Innovative research and technological developments were carried out on a wide range of implementations in the field of adhesive bonding. As a result, we succeeded in elucidating the bonding mechanism, which was not known in the past, and developing new adhesives with performance that greatly exceeds that of the existing adhesives, and also established methods for designing joints and predicting durability. We believe that these technologies are applicable to the multi-material structure auto bodies currently envisioned and can sufficiently meet those requirements. However, in the future, adhesive bonding is expected to be applied not only to auto bodies, but also to the parts installed inside them, such as batteries, motors, fuel cells and power modules, and as a result, the use of adhesive bonding will continue to increase. Since application to unexplored fields will present new challenges in terms of material development, reliability, inspection technology, etc., in the future, it is expected that more focused research and development in the adhesive bonding field will be necessary in order to comprehensively solve various problems in various applications.

4.5 Joint Performance Database—Creating Database of Joint Technical Integration System Using Machine Learning Technology and Corrosion Fatigue Properties of Dissimilar Material Joints Considering Practical Use Environment Hisashi Serizawa

4.5.1 Introduction To design and manufacture a car body structure actively using various innovative materials with excellent specific strength and rigidity developed for the radical weight reduction, the appropriate replacement of innovative materials utilizing their properties into various components is indispensable. For this reason, reliable dissimilar material joining technology is required for components or parts of the car body consisting of various innovative materials and existing materials. In car body design, it is necessary to refer mainly mechanical properties of materials to be adopted and joint performances of various materials combination. So, databases have been constructed in car manufacturers, separately. Previous databases for material properties and joint performance considered only data measured by reliable experiments, and most of them accumulated only measured data. Generally, database users are designer and production engineers of car body structures and they use it only to the limited extent of searching a database consisting

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of experimentally measured data, which are internally accumulated in each company. They have been used it to select the material or joint performance closest to the property required for the designed component. On the other hand, when designer and/ or production engineers judge that it is difficult to realize the designed components with use of existing material or joint performance, they want to search the possibility of material improvement and/or the mechanical properties of newly developed material. Especially they want to predict the characteristics of the new material through their databases and the joint performance of new combination of the materials. Ideally, the material maker can easily and flexibly develop and commercialize the material having properties corresponding to designed components considering cost and LCA. Actually, it is difficult to obtain joint performance data of the same kind of material using new material, and available data is limited to academic data published in papers and others in most cases. Databases of such academic data are created as a paper list but are for the most part not organized as material property and joint performance. For example, overseas, Prof. Bhadeshia, University of Cambridge, has published predicted data of material properties and joint performance, including a program and focusing on his research results as the Material Algorithms Project (MAP) Data Library [71]. In Japan, the Research and Services Division of the Materials Data and Integrated System, National Institute for Materials Science plays a leading role in actively publishing material properties and joint performance as a database, using information science, including artificial intelligence (AI), in addition to experimentally measured data [72–75]. However, joint performance data is only for the same kinds of material joints, and no data on dissimilar material joint performance has been published as a database. One of the reasons for dissimilar material joint performance is not published as a database is that joint performance is significantly affected by joint fabrication conditions. In most cases of the same kinds of material joint fabrication, welding is used. However, when using welding for dissimilar metallic material joint production, hard and brittle intermetallic compounds are often generated, and joint performance is deteriorated [76]. For this reason, various joining processes, such as a solid-phase bonding process other than welding and mechanical fastening process, are applied to restrict or control intermetallic compound generation [77–79]. Also, in dissimilar material joining, an intermediate layer, the third material, can be inserted [76, 77]. In addition, in joining metallic material to non-metallic material, an adhesive is often used [80]. Therefore, creating a database of joint production methods is considered necessary to create a database of dissimilar material joint performance. However, since database creation itself is difficult, it cannot be addressed in most cases, and it is a problem to be solved before publication. In addition, when joining dissimilar materials, in most cases, potential difference is generated both between dissimilar metallic materials, and between a metallic material and non-metallic material. Therefore, if resin material, one of the non-metallic materials, is completely nonconductive, there will be no factors generating a potential difference on the metallic material. However, a potential difference is generated if resin material contains materials with electric conductivity, such as carbon fiber

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and carbon nanofiber, to improve mechanical properties. As a result, galvanic corrosion inevitably occurs. Therefore, for fatigue properties, one of the performances of dissimilar material joints with a view to long-term use, it is essential to understand properties considering performance deterioration due to galvanic corrosion. In particular, in recent years, as shown in Table 4.8, the average use period of vehicles has approximately doubled in the past 40 years [81], and understanding fatigue properties, i.e., the corrosion fatigue property, has become one of the crucial performances for dissimilar material joints. For the most part, the corrosion fatigue property has been experimentally evaluated as an essential property for the material itself for application to ships and ocean structures [82, 83]. For example, as shown in Fig. 4.79, similarly to a tensile test or a fatigue test, the corrosion fatigue property is determined by immersing only the machined part of a cylindrical bar with only the center part machined thinner in seawater, corresponding to the corrosion environment, and applying a rotary fatigue load at the ends. A rotating ring R is adhered to the rotary test piece and rotates with the test piece. Corrosive-liquid sealing is maintained by a sponge packing P placed between the rotating ring and the partition wall of a lower corrosion tank C1. The mechanism prevents the corrosive-liquid from dispersing by covering the lower corrosive-liquid tank with an upper corrosive-liquid tank C2. In addition, the mechanism supports the lower corrosive-liquid tank with a spring S and moves up and down to prevent the lower corrosive-liquid tank from being damaged due to a test piece fracture. Furthermore, as shown in Fig. 4.80, a test to evaluate the fatigue crack growth under the corrosive-liquid environment is also conducted by using a standard CT test piece S (compact test piece) and immersing the point beyond the fatigue crack tip in the corrosive -liquid. The mechanism places the test piece S between the two corrosive-liquid tanks from both sides through the sponge packing P. On the other hand, for bonded joints, a test to evaluate the corrosion fatigue property is also conducted by applying a fatigue load to a vertical sheet, for example, under the environment of immersing the sheet and weld of a weld-jointed T-shaped joint in the corrosive liquid. However, for a thin sheet lap joint commonly used in transportation equipment, such as vehicles, no corrosion fatigue test method has been standardized yet, and only a fatigue test for a test piece corroded under the accelerated corrosion environment is conducted for the most part. In recent years, Thierry et al., The French Corrosion Institute, fabricated a fatigue test machine less than one meter in Table 4.8 Average use period of vehicles (years) [81] Average age in the first user Passenger

Truck

Average age to scrap Bus

Passenger

Truck

Bus

1981

4.33

4.36

5.55

8.7

8.24

9.95

1991

4.54

5.25

6.45

9.17

9.26

11.86

2001

6.04

7.48

8.64

10.4

10.68

13.72

2011

7.74

10.04

10.78

12.43

13.04

17.37

2021

8.84

11.53

12.07

13.87

15.73

18.38

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full length, installed the fatigue test machine body in the combined cycle corrosion test equipment for accelerated corrosion, and reported the result of evaluating the corrosion fatigue property for a lap joint fabricated with a resistance spot welding method [84]. However, research on a dissimilar material joint has not been reported to date. When evaluating the corrosion fatigue property of dissimilar material joints, the effect of galvanic corrosion cannot be disregarded. In other words, when creating a database of the corrosion fatigue property of dissimilar material joints for practical use with a view to their future practical applications, the performance of such joints fabricated by weld bonding combined with adhesive material is essential. For this reason, at the Innovative Structural Materials Association (ISMA), we have constructed the joint performance database for dissimilar material joints fabricated using innovative materials of the innovative steel sheet having a tensile strength

Fig. 4.79 Corrosive-liquid tank for rotary bending fatigue test [82]

Fig. 4.80 Sandwich-type corrosive-liquid tank [82]

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of 1.5 GPa-grade, innovative aluminum material with high ductility, and advanced carbon fiber-reinforced composite material, as one of the guides for use of the developed innovative materials in future multi-material car bodies. The joint used for creating the database is a lap joint commonly used in vehicles. The database has been created by fabricating dissimilar material joints using two different kinds of bonding methods respectively for three kinds of innovative materials and by experimentally measuring the tensile shear strength and shear fatigue properties. Also, to obtain the corrosion fatigue property, the database has been constructed by combining an innovative adhesive material also newly produced, fabricating a lap-dissimilar material welded/bonded joint (weld-bonded joint) using innovative material, using a tripletype fatigue test machine (Fig. 4.81) [85] installable in the combined cycle corrosion test equipment newly manufactured, and experimentally measuring the corrosion fatigue property. In addition, based on the data experimentally obtained, we have proceeded with preparations to publish the performance database for multi-material dissimilar material joints by using the latest machine learning technology, developing the bonding technology integration system to predict the performance of a dissimilar material joint fabricated under an unknown bonding condition, and using the interface to be constructed.

Fig. 4.81 Appearance photograph of triple-type corrosion fatigue test machine [85]

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4.5.2 Science and Technology of the Field 4.5.2.1

Joint Performance Database (Utilizing Machine Learning Functions)

Before the computer was developed, past records were recorded using paper as a medium in various fields. In other words, “old knowledge” was stored in paper documents. The mid-1960s, when the computer, developed as an electronic calculator, was expanded, and a record of electronic data using a magnetic disk was enabled, is considered to be the starting point of the database concept. However, from the late 1970s to the early 1980s, until the so-called personal computer was born, the primary purpose of a database of electronic data was to record past knowledge as electronic data and prevent the loss of knowledge at large companies, research institutes, and other organizations. It was not until access to accumulated electronic data, i.e., its use was available, that the use as a database virtually started [86]. However, when it first started to be used as a database, the computer’s calculation capability, including the input/output of electronic data, was insufficient. Accordingly, the system developer determined the electronic data accumulation method, and users were required to use it following the means determined by the developer. Also, when use of the personal computer began spreading, the difference between the data recorded on spreadsheet software (professionally, data accumulated on a spreadsheet), such as Microsoft Excel, and that on a database was unclear for many people. The main reason for this is that when a person uses data accumulated by another because a file summarizing data was copied for use through recording media, such as a floppy disk, it is rare that many users have access to the accumulated data simultaneously. Except for those places where a network was available, such as large companies and research institutes, most people were unaware of the difference between a spreadsheet and a database. The rapid spread of the Internet after the mid-1990s [87] can be attributed to many people understanding the difference between a spreadsheet and a database. As mentioned above, up to that time, it was rare for many people to use the data accumulated by one person simultaneously. When the use of the Internet first started, a system allowing many people to use data stored on a large recording medium through the Internet was constructed. Since a few users will try to simultaneously access the established system, a file recording the data is required assuming that a few users will simultaneously use it. This is a significant difference between a spreadsheet and a computer system. In other words, since a file stored as a database is designed initially assuming that a few people will simultaneously use it, it can prevent access problems and damages [86]. The primary purpose of database users is to obtain necessary data through searching. Database users were initially restricted in that they could not search the data unless they had the same software as the software that had created the file storing the electronic data. For that reason, databases and software capable of high-speed or ambiguous searching were increasingly used. On the other hand, to prevent the

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software from hindering and making unavailable even the database in which important data was stored, the SQL standard, which is a language to search data and obtain necessary data with a common standard, regardless of the database/software, was published as a unified standard in 1986. In the following year, the language specification was standardized (ISO 9075:1987) by the International Organization for Standardization (ISO) and subsequently has also been expanded to improve convenience. The SQL standard establishment, Internet spread, and World Wide Web (WWW) have changed not only database users, but also its utilization. Although the utilization of electronic data has depended on individual users to date, a technology called Knowledge Management was proposed as a technology to use electronic data, i.e., digitized information. Although the definition of Knowledge Management is unclear, it is considered to be, for example, using the knowledge accumulated by an individual employee in daily work in a company/enterprise as a knowledge asset, not just knowledge, and making it a motive force to create a new value. An intelligent database (Fig. 4.82) consisting of a three-tier structural system, adding an application server with a business logic that considers how to transmit important data to the demand from users, between the database server storing the data and important data by using Knowledge Management technology and the presentation server, creates a window of information transmission [88]. Most databases available through the Internet are currently intelligent databases, the essential part of which is an application server with business logic. The performance database for multi-material dissimilar material joints using innovative materials, established by ISMA, accumulates reliable data experimentally measured in the database server. Multi-material car body designers, who are database users, expect to be able to search their targeted joint performance as simply

Fig. 4.82 Conceptual scheme of intelligent database [88]

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as possible. The database has been constructed to make it possible to transmit the performance of dissimilar material joints to be fabricated as the search result when selecting two kinds of materials containing innovative material and the joint method and condition to join the innovative material, to respond to the expectations above. Since reliable data experimentally measured is limited, the performance of a joint fabricated under a condition not experimentally measured needs to be predicted to find the joint performance targeted by designers. According to conventional views, the method to predict the joint performance under an unknown condition by the least-squares approximation from joint conditions changing two or more kinds of joints is considered. However, since the effect of the joint condition on the joint performance does not necessarily show a linear dependency, a prediction method responding to various changes is essential. Information science, including artificial intelligence (AI), in the early 2010s, when this project started, considered that neural network technology was useful as a technology to predict non-linear phenomena [89]. Although multilayer artificial neural network technology, which expanded the neural network, was regarded promising as deep learning, it required a dedicated, expensive computer [90]. However, the dramatic development and price reduction of the Graphics Processing Unit (GPU) and the open-sourcing of a machine learning technology program, including deep learning, have created the applicability to an application server in an intelligence server [91, 92]. Then, in this project, we will develop a bonding technology intelligent system predicting dissimilar material joint performance using the latest machine learning technology and introduces it into an application server. The joint performance database applicable to the multi-material car body design has been constructed.

4.5.2.2

Corrosion Fatigue Property

Long-term utilization is required for transportation equipment, including vehicles. In particular, in the 2000s, it took ten or more years for a vehicle to reach the scrapping stage. For that reason, the corrosion property has become one of the essential elements in the performance of not only materials, but also various joints used in vehicles, and corrosion property evaluation methods have been developed as regulations in each country [81]. Based on an actual corrosion mechanism over ten or more years, these evaluation methods evaluate the corrosion property under an accelerated corrosion environment in a short time. However, the corrosion mechanism during practical vehicle traveling has not been clarified yet. The accelerated corrosion evaluation methods specified in each country have not been unified, as shown in Table 4.9. JASO M609, which is a standard for accelerated corrosion and was standardized in 1987, is an evaluation method under a stringent corrosion environment compared with European and U.S. standards, with an extremely high Salt Spray Test (SST) frequency to simulate corrosion and high-concentration salt water. For that reason, ISMA has attached a lap-bonded joint fabricated from the same kind of material and dissimilar material to a practical traveling vehicle, evaluated the amount of corrosion, and evaluated it under the accelerated corrosion environment specified

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in each country. The result with the JASO method, specified in Japan, significantly deviated from the practical vehicle traveling result. The results with the SAE method of the U.S. and the European VDA method were closer to but still deviated from the practical vehicle traveling result. Therefore, ISMA proposed a new accelerated corrosion environmental condition, under which a joint performance evaluation is essential. The dissimilar material joints using innovative material are essential to use the innovative materials developed by ISMA for a radical weight reduction in future car bodies, i.e., an innovative steel sheet with a tensile strength of 1.5 GPa-grade, innovative aluminum material with high ductility, and advanced carbon fiber-reinforced composite material. Galvanic corrosion occurs in not only the dissimilar material joints of the innovative steel sheet with a tensile strength of 1.5 GPa-grade and the innovative aluminum material with high ductility, but also in the dissimilar material joints of advanced carbon fiber-reinforced composite material since such material contains electrically conductive carbon fiber. In addition, producing a dissimilar material joint was difficult, and producing a weld-bonded joint combining adhesive material was initially considered necessary. For that reason, ISMA has aimed to control galvanic corrosion and has also developed innovative insulating adhesive material. ISMA initially planned to create a database of the corrosion property of an innovative material weld-bonded (welded/bonded) joint under the accelerated corrosion environment based on the JASO method, as the corrosion property of a dissimilar material joint fabricated using innovative material. Table 4.9 Standard for vehicle corrosion in each country Standard No

Salt solution type

Salt supply method

Salt water spray time (SST)

SST ratio

Year of the standard

Japan

JASO M609

5 mass% NaCl

Spray (SST)

1 time (2 h)/cycle 25 (8 h)

1987

U.S.

SAE J2334 Manual

0.5% NaCl + 0.1% CaCl2 + 0.075% NaHCO3

Immersion, spray (SST), shower

1 time (15 min)/ cycle (day)

0.7

1988

SAE J2334 Auto

0.5% NaCl + 0.1% CaCl2 + 0.075% NaHCO3

Immersion, spray (SST), shower

1 time (15 min)/ cycle (day)

1.0

Germany

VDA-233–102

1 mass% NaCl

Spray (SST)

3 times (3 h)/ cycle (week)

5.4

2013

Sweden

ACT1 (ISO 16701)

1 mass% NaCl

Shower

2 times (15–15–15 min)/ cycle (week)

0.8

2003

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On the other hand, in 2016, Thierry et al., the French Corrosion Institute, simultaneously conducted a fatigue test under the accelerated corrosion environment based on the VDA method using a lap joint fabricated with the resistance spot welding method, compared the joint performance with a fatigue test that had been previously conducted after corrosion under an accelerated corrosion condition, and showed that a difference appeared between them [84]. Therefore, obtaining the corrosion fatigue property for dissimilar material welded/bonded joints using innovative material is required to use the innovative materials developed by ISMA. It was also required to create a database for the corrosion fatigue property in addition to the shear fatigue property, which is essential for a multi-material car body design. The snow-melting salt-type galvanic corrosion in cold regions was evaluated under the accelerated corrosion environment based on the practical vehicle traveling result. However, the equipment capable of implementing a combined cycle corrosion test at low temperatures and installing a fatigue test machine less than one meter in total length could not be used in Japan within this project period. Therefore, ISMA newly proposed the accelerated corrosion environmental condition for medium–high temperature regions and advanced the creation of a database by conducting fatigue tests under the accelerated corrosion environment in medium–high temperature regions and experimentally obtaining the joint corrosion fatigue property. Furthermore, ISMA actually conducted corrosion fatigue tests in salt damage districts, medium–high temperature regions, and corrosion fatigue tests under room temperature and in the air and advanced examination of the guide for the corrosion fatigue property measurement method based on the comparison with the corrosion fatigue property under the new accelerated corrosion environment.

4.5.3 Representative Research and Development Results 4.5.3.1

Joint Performance Database (Using Machine Learning Functions)

This project has experimentally measured the tensile shear strength, shear fatigue property, and corrosion fatigue property of dissimilar material joints using developed innovative materials and has created a database. We have proceeded with the construction of a database for the performance of a joint that is also applicable to the multi-material car body design by developing a bonding technology intelligent system to predict the dissimilar material joint performance using the latest machine learning technology. When using the latest machine learning technology, using the data as much as possible improves the prediction accuracy of such technology. For that reason, several types of lap dissimilar material joints were fabricated using a laboratory cold rolled steel sheet (ultra-high strength steel) with a tensile strength of 1.5 GPa-grade, 5083-O material of 5000-series aluminum alloy, and the carbon fiber reinforced composite material (CFRTP sheet) made by injection molding TORAY INDUSTRIES-made carbon fiber reinforced thermoplastic

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resin pellet TLP1060, which are similar to commercially available innovative materials, and the performance has been confirmed by measurement. The manufacturing methods and bonding parameters of the lap dissimilar material joints fabricated using commercially available materials are shown in Table 4.10. In this project, we produced dissimilar material joints with different total heat inputs by changing only one parameter for each bonding method. For the laser welding/bonding method, the total heat inputs were limited to two kinds since the width of the parameter that enabled production of a sound dissimilar material joint was narrow. The dissimilar material joints were fabricated by changing the total heat input to three kinds for other bonding methods. The bonding condition displayed as “medium” in the variation range in Table 4.3 corresponds to the optimum condition in each bonding method. The lap dissimilar material joint to measure tensile shear strength is a bonded joint 210 mm in length fabricated by bonding a test piece 125 mm in length and 40 mm in width with an overlapping margin of 40 mm. The static tensile shear test was conducted at room temperature and in the air based on JIS Z3136. The tensile test was conducted with a length of the specimen between grips (distance between clamps) of 100 mm and under the condition of a test speed of 5.0 mm/min. The result of the tensile shear test of lap dissimilar material joints using commercially available materials, arranged by heat input (low, medium, and high in Table 4.10), is shown in Fig. 4.83 [87]. Generally, since higher heat input during bonding broadens the bonding area, the tensile shear strength is considered to be larger. However, the obtained result does not necessarily show a positive dependency on the heat input. For that reason, although neural network technology is considered necessary as a machine learning technology, the joint performance for which a database is created is not only tensile shear strength, and the dissimilar material joint fabrication conditions determining the joint performance are diverse. As a result, an Table 4.10 Fabrication methods and bonding parameters of the lap dissimilar material joints using commercially available materials Types of lap dissimilar material joints

Bonding method

Parameter

Variation range

Ultrahigh strength steel/ 5083-O material

Resistance spot welding method (RSW)

Welding current

Low, medium, and high

The refill friction stir spot welding method (RFSSW)

Rotation speed

Low, medium, and high

Friction stir spot welding (FSSW)

Tool insertion speed

Low, medium, and high

Laser welding/bonding method (LIAPW)

Laser output

Low and medium

Friction stir spot welding (FSSW)

Tool insertion speed

Low, medium, and high

Laser welding/bonding method (LIAPW)

Scanning speed

Medium, and high

Ultrahigh strength steel/ CFRTP sheet

5083-O material/CFRTP sheet

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Fig. 4.83 Result of static tensile shear tests for dissimilar material joints using commercially available materials [94]

algorithm was required to predict the relationship between multi-variable input and output. Then, the applicability of the deep neural network (Keras in Tensorflow) [92] published by Google as an open source, was examined. For the experimental data on lap dissimilar material joints using commercially available materials, the experimental data was stored as a database, and a user interface to facilitate management was developed. At the same time, the development of a bonding technology integration system using the deep neural network was advanced. The examination using the experimental data of the lap dissimilar material joints using commercially available materials showed that the algorithm using only the bonded joint fabrication condition as input data lacked the amount of learning required of a machine learning function. Then, for the image on the surface of the lap-joint part of the fabricated bonded joint, an algorithm used as input data into the deep neural network has been developed, and prediction accuracy improvement has been addressed.

4.5.3.2

Corrosion Fatigue Property

In creating a database for the corrosion fatigue property, dissimilar material welded/ bonded joints have been fabricated using commercially available materials, the corrosion fatigue property has been obtained, and the prediction accuracy for the corrosion fatigue property of dissimilar material welded/bonded joints using innovative materials has been improved. The fabrication methods and bonding parameters of the lap dissimilar material welded/bonded joints fabricated using commercially available materials and innovative adhesive materials are shown in Table 4.11. Similarly to the

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case of tensile shear strength, dissimilar material welded/bonded joints with different total heat inputs were fabricated by changing only one parameter for each bonding method. The lap dissimilar material joint to measure the corrosion fatigue property is a bonded joint 260 mm in length fabricated by bonding a test piece 150 mm in length and 40 mm in width with an overlapping margin of 40 mm. The corrosion fatigue tests were conducted using a fatigue test machine installable in a combined cycle corrosion test equipment capable of simulating the accelerated corrosion environment and under the accelerated corrosion condition assuming a medium–high temperature region with a dry–wet/saltwater process using mixed salt water of NaCl with a concentration of 0.5% and CaCl of 0.1% (Table 4.12) [85]. The length of the specimen between grips (distance between clamps) was set at 160 mm, and the fatigue loads were applied by repeating a tensile load and unweighting at a frequency of 0.5 Hz. When the welded/bonded joint had not yet fractured at the stage where the maximum number of repetitions reached 1.0 × 106 times, the test was terminated as not fractured. The results of corrosion fatigue tests of the 5083-O material/CFRTP sheet manufactured with a laser welding/bonding method, lap-dissimilar material welded/ bonded joints of commercially available materials, and innovative adhesive material are shown in Fig. 4.84 [85]. The test results open-marked in the figure mean those for which chemical treatment and electrodeposition coating were not conducted after fabricating dissimilar material welded/bonded joints. The trend that smaller maximum applied load, repeated numbers up to fracture increased, was recognized. Table 4.11 The fabrication methods and bonding parameters of the lap dissimilar material welded/ bonded joints using commercially available materials Types of lap dissimilar material welded/bonded joints

Bonding method

Innovative adhesive materials (epoxy based)

Parameter

Variation range

Ultrahigh strength steel/5083-O material

RSW

High-temperature curing type

Welding current

Low, medium, and high

Rotation speed

Low, medium, and high

Insertion speed

Low, medium, and high

LIAPW

Scanning speed

Low and medium

FSSW

Insertion speed

Low, medium, and high

LIAPW

Laser output

Low and medium

RFSSW

Ultrahigh strength steel/CFRTP sheet

5083-O material/CFRTP sheet

FSSW

Low-temperature curing type

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Table 4.12 Accelerated corrosion condition assuming a medium–high temperature region

The result showed that the corrosion fatigue property was determined by a crack growth phenomenon. Also, for the dissimilar material welded/bonded joint fabricated with the laser welding/bonding method and innovative material, innovative adhesive material was applied on the outer edge portion of the overlapped part, and hardening treatment was applied after fabricating a bonded joint using the laser welding/bonding method. As a result, the bonding interface of the innovative adhesive material and the component on the outer edge portion generated by a shear fatigue load was corroded, a crack progressed, and a fracture occurred. Also, the reason why the effect of chemical and electrodeposition treatment was remarkable, as shown in Fig. 4.84, is as follows: When chemical and electrodeposition treatment was not conducted, the bonded interface over the overlapped portion generated by a laser welding/bonding process was clearly affected by the accelerated corrosion environment, and deterioration on the bonded interface was accelerated.

4.5.4 Social Implementation and Future Prospects 4.5.4.1

Joint Performance Database (Using Machine Learning Functions)

The joint performance database constructed in this research aims to be commercialized in the form of publishing through the “Osaka University Joining Technology Hub,” for which the data activity hub plan in this project is advancing. However, since a system needs to be newly constructed to specify the range of use for the database according to the user’s fee, the operation start is still two years away. Specifically, a user classification plan is established in the first half of the first year, and a certification system corresponding to the classification is established in the second half. The linkage of the joint performance database and the certification system, established in the first year, is established in the first half of the second year, and the joint performance database capable of providing information corresponding to the user’s fee is established in the second year while advancing a trial operation.

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Fig. 4.84 Corrosion fatigue property of the 5083-O material/CFRTP sheet dissimilar material welded/bonded joints fabricated using a laser welding/bonding method and innovative adhesive material [85]

The joint performance database established in this research covers dissimilar material joints fabricated using multi-material bonding technology, which is expected to significantly contribute to the use of innovative materials and to have significant technical and social effects. In addition, since it has an algorithm to predict the performance of the joint fabricated under unknown bonding conditions, in the new multi-material car body design using innovative material, as shown in Fig. 4.85 [81], it enables a car body design that is not bound by the existing concept and is expected to contribute to developing new human resources in car body design. For example, when optimizing a car body structure using a mathematical analysis model, a multi-material car body designer first determines the properties required for each component. However, since using the property with low feasibility as input

Fig. 4.85 Conceptual scheme of multi-material car body design using dissimilar material joint performance database [81]

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data for a mathematical analysis model is simply implausible (fantastic story), the performance of a joint with high feasibility is searched by the database established in this research. Through a search, the joint performance with high feasibility is determined by changing the joint thickness and major bonding conditions based on the conditions experimentally measured. Then, the aim is to optimize the multimaterial car body structure using the determined joint performance. However, it is almost impossible to determine the optimum structure primarily. Therefore, the joint performance with high feasibility is repeatedly searched using the established joint performance database. The final multi-material structure is determined by repeating the search on the joint performance database and optimization of the multi-material car body structure.

4.5.4.2

Corrosion Fatigue Property

The result [84] achieved by Thierry et al., the French Corrosion Institute, in 2016 is the first result of the corrosion fatigue property evaluation for the thin sheet lap dissimilar material welded/bonded joint used for transportation equipment, including vehicles. The corrosion fatigue property evaluation of the lap dissimilar material welded/bonded joint conducted in this research is considered to be the first significantly essential data for multi-material car body design in the world. For this reason, the aim is to expand the corrosion fatigue property evaluation method established in this project to the corrosion fatigue property evaluation of the same kinds of material joints and dissimilar material joints. Also, for further important corrosion fatigue properties, obtaining the corrosion fatigue property is targeted for not only medium–high temperature regions conducted in this research, but also cold regions. Fatigue tests under an accelerated corrosion environment in cold regions are also planned using equipment capable of conducting a combined cycle corrosion test at low temperatures and installing a fatigue test machine less than one meter in total length. Then, through a comparative review of the corrosion fatigue property obtained in medium–high temperature regions and cold regions, actual fatigue test results in salt damage regions and cold regions, practical vehicle traveling test results, and others, a corrosion fatigue property evaluation test method for thin sheet lap joints will be standardized for the first time in Japan. Furthermore, a standardized evaluation test method is proposed overseas as well as global standardization through various ways such as proposing a standard dissimilar material welded/bonded joint.

4.5.5 Conclusion The dissimilar material joint performance database established in this research is essential to use ISMA-developed innovative materials in the future multi-material car body design and is expected to be a useful database. Also, for the established bonding

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technology intelligence system using a machine learning function, the accuracy of predicting the performance of a dissimilar material joint fabricated under unknown bonding conditions improves as data for machine learning increases. Furthermore, a similar algorithm is applicable to not only dissimilar material joints using innovative materials, but also other joints. In other words, the established bonding technology intelligent system is expected to be used to predict the performance of various joints. A multi-material car body designer who desires to use this system is expected to conduct machine learning using the data retained by an individual as basic data and construct an individual joint database. Also, for the corrosion fatigue property evaluation test, developed in this research and one of the essential joint performances, the corrosion treatment test method under the accelerated corrosion environment responding to both medium–high temperature regions and cold regions will be established for application to not only dissimilar material joints, but also to the same kinds of material joints. The aim is for it to be established as one of the essential joint performance evaluation test methods to design and manufacture multi-material car bodies.

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Chapter 5

Analysis and Evaluation Sakae Fujita, Shusaku Takagi, and Yoshio Akimune

Abstract The knowledge on steel corrosion, galvanic corrosion of dissimilar materials, hydrogen embrittlement of steel, and nondestructive testing techniques in automobiles is summarized, and an overview of the issues and research results related to them in this project is given. Chemical and physical stresses during vehicle use cause various degradations. In particular, corrosion and hydrogen embrittlement are difficult to evaluate. The outlines of the developments on this project including electrochemical measurement techniques on the surfaces of microstructures of 1.5 GPa high strength steel containing high carbon, galvanic corrosion of multi-material automobile bodies, standard test methods for evaluating hydrogen embrittlement, and analysis techniques for crack propagation morphology were presented. Nondestructive testing is positioned to eliminate defective products in the process and to estimate the remaining life of the product. In the production process, it is desirable to be able to eliminate defective products online or in-line without affecting the product, and it is necessary to introduce equipment that can identify defective parts nondestructively without relying on human labor. In addition, to estimate the remaining life expectancy, studies to correlate various measurements with life expectancy are needed, and it is expected that many producers will cooperate in this research in many cases.

S. Fujita (B) · Y. Akimune Innovative Structural Materials Association, 1-9-4, Chiyoda-Ku, YurakuchoTokyo 100-0006, Japan e-mail: [email protected]; [email protected] Y. Akimune e-mail: [email protected] S. Takagi JFE Steel Corporation, Kawasakidori 1Chome, Mizushima 712-8511, Kurashiki, Japan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 T. Kishi (ed.), Innovative Structural Materials, Springer Series in Materials Science 336, https://doi.org/10.1007/978-981-99-3522-2_5

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5.1 Corrosion—Issues in Automotive Anti-Corrosion Design and Evaluation Methods Sakae Fujita

5.1.1 Introduction Because means of transportation, such as railways, automobiles, ships and aircraft, are closely related to our everyday lives, even slight damage of the materials that comprise them can lead to a serious accident resulting in death or injury. For this reason, detailed preliminary countermeasures against damage are implemented to prevent in advance accidents involving the means of transportation. Corrosion is a chemical reaction between a material and the reactant used (water, oxygen, hydrogen, chemical substances). Because the use environments of transportation equipment are not uniform, corrosion is considered to be the most difficult problem for countermeasures. From the second half of the twentieth century, corrosion databases for the product production process and final use fields increased, and the relationship between corrosion environments and corrosion protection life is gradually becoming clear. On the other hand, from the beginning of the twenty-first century, global warming, which had long been a concern, entered a new phase and intensified at a rapid pace. As part of the formation of a “sustainable society,” weight reduction of transportation equipment became an important achievement target, and a changeover from mild steel, which had conventionally been the basic material of transportation equipment, to multi-material design using combinations of the optimum materials, including ultra-high strength steel and stainless steel, titanium alloys, aluminum alloys, magnesium alloys and carbon fiber reinforced plastic (CFRP). To address this current reality, the project was launched in 2013 with the aim of leading the world in achieving drastic weight reductions. In addition to developing various types of innovative materials for multi-material design, the project has also conducted research on welding and joining technologies, CAE, nondestructive inspection, durability performance (joint strength, fatigue, corrosion fatigue, corrosion, hydrogen embrittlement) for implementing those material in automobile bodies. In automobiles, painting and Zn coating have an extremely large corrosion protection effect, and no large change in this is expected in the future. On the other hand, in order to form a sustainable society, reduction of painting and Zn-based coatings and improvement of the corrosion resistance of the substrate steel are also important challenges for development. However, almost no research has been done on the corrosion resistance of the substrate steel in the corrosion protection life of automobiles, and in particular, knowledge related to medium- and high-carbon steels is negligible. This section first summarizes the conventional knowledge concerning the corrosion protection life of automobiles, and then presents an overview of the results of

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corrosion analyses of the microstructures of medium- and high-carbon steels with C contents of 0.3% or more which were carried out as part of the project.

5.1.2 Conventional Knowledge of Corrosion Protection Life of Automobile Corrosion 5.1.2.1

Corrosion of Transportation Equipment

The transportation field (transportation industry) can be divided into land transportation (railways, automobiles), marine transportation (ships) and air transportation (aircraft) [1]. The corrosion factors are basically water, oxygen and salt, but also include sulfur dioxide, hydrogen sulfide and ammonia, depending on the part. The following describes the corrosion parts and corrosion factors of the means of transportation mentioned in “Rust in Everyday Life and its Countermeasures” [1], which was edited by the National Institute of Resources, Science and Technology Agency. (1) Railways In above ground equipment, corrosion includes the problems of corrosion of the wiring, retaining fixtures, etc. that make up the overhead power system, stress corrosion cracking of wiring, connection terminals, etc. by hard steel for use in signaling equipment, corrosion, corrosion fatigue, etc. of rails and fasteners, and the problem of fretting corrosion, etc. in the axles of rolling stock. It is well known that direct current (DC) leakage from rails cause corrosion damage in nearby underground piping and other facilities by “stray current.” Corrosion is also the main cause that determines the life of railway cars. (2) Automobiles The main causes of corrosion of motor vehicles are airborne salts from coastal zones and deicing salts spread on roads. In the front edge of the hood, damage of the paint film and resulting cosmetic corrosion occur easily when this part is hit by a flying stone. Door seams and other lap joints are prone to perforation corrosion from inside panels. The fuel tank is also an important component for safety measures, so close attention is paid to anti-corrosion countermeasures. In addition, Battery Electric Vehicles (BEV) and Hybrids Electric Vehicles (HEV) have become increasingly popular in recent years, and protective cases for batteries and electrical equipment control devices will be positioned as important components for corrosion protection countermeasures, since even minute corrosion can hinder vehicle travel and lead to a serious accident. (3) Ships The corrosion problems of ships are classified as problems of the hull and superstructure, which are in contact with the severe marine corrosion environment, and corrosion of internal parts which are in a wet condition. In the superstructure, corro-

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sion prevention of the exterior of the hull centers on painting. On the other hand, in submerged parts, electrolytic protection is used in combination with painting. (4) Aircraft Because many types of metals are used together in the airframes of aircraft, these structures are susceptible to contact corrosion of dissimilar metals (galvanic corrosion), for example, between the aluminum alloy which is the main component material and fasteners made of different metals. Although corrosion also occurs in condensation-prone parts in the interior of the airframe, parts exposed to service water and the like, these problems can be handled by painting, sealing, drain methods, etc. In addition, aircraft in which substantial weight reduction is achieved by applying carbon fiber reinforced plastics (CFRP) have come into use in recent years, and countermeasures are taken to prevent galvanic corrosion between aluminum-based alloys and CFRP.

5.1.2.2

Anti-corrosion Targets for Automobiles

In snowy regions, deicing salts are spread on roads in wintertime to secure safe travel by automobiles. The amount of deicing salt spread on roads increased rapidly in North America and Northern Europe since the 1960s, and in the 1970s, automotive corrosion caused by deicing salt became apparent and was taken up as a social problem. The deicing salts spread to enable drivers to drive automobiles safely caused corrosion of the automobile body, and ironically, this hindered safe travel of automobiles. Therefore, anti-corrosion quality targets for automobile bodies were announced in Canada, Northern Europe and the United States, including the Canada Code (1981, no perforation corrosion: 200,000 km or 5 years, no cosmetic corrosion: 60,000 km or 1.5 years), the Nordic Code (1983, no perforation corrosion: 6 years, no cosmetic corrosion: 3 years) and the voluntary 10-5-2-1 target (1989, no perforation corrosion: 10 years, no cosmetic corrosion: 5 years, no engine room rust: 2 years, no rust of undercarriage parts: 1 year) of the Big 3 (GM, Ford, Chrysler) in the United States. Although this voluntary target did not have legally binding force, for many years it was positioned as an anti-corrosion quality guideline of auto makers worldwide, and even today, it is firmly maintained as a voluntary anti-corrosion target in the United States. About 10 years thereafter, the German auto makers announced an anti-corrosion target of no perforation corrosion for 12 years, and at present, this 12- year anti-corrosion target is still firmly maintained as an anti-corrosion target for vehicles in Europe.

5.1.2.3

Parts Requiring Corrosion Protection

Figure 5.1 [2] shows the parts of the automotive body where corrosion is severe. The following 3 types of parts are severe corrosion parts

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Fig. 5.1 Corrosion parts of automotive body [2]

(1) Parts where water or chlorides easily stagnate: Interior of lap joints of steel sheet materials, interior of hemming joints (2) Parts where the paint film is easily damaged by flying stones, etc.: Outer surface side of outer panels (3) Parts where the material edge is easily exposed: Cut edge of materials Crevices in lap joints are parts that are difficult to completely cover by chemical conversion treatment, electrodeposition coating and subsequent painting, and water and chlorides also stagnate easily in these crevices. For this reason, these are parts where perforation corrosion becomes a problem. “Perforation corrosion” is a type of corrosion in which the material corrodes from the interior of the crevice and finally penetrates the full thickness and reached the outer side. It is classified as so-called “bare metal corrosion in a crevice environment.” The outer surface of automotive outer panels is a part where underfilm corrosion becomes a problem. In underfilm corrosion, the substrate metal under the paint film corrodes from a part where the paint film is damaged, the paint swells, and red rust occurs. It is classified as “cosmetic corrosion” (called “chipping rust” in Fig. 5.1). The material edge is a part where the paint tends to contract during paint baking, exposing the edge, and the edge material then corrodes in the edge part. It is classified as “cut edge corrosion” (called “edge rust” in Fig. 5.1). In comparison with cosmetic corrosion and edge corrosion, which can be repaired comparatively easily by painting, perforation corrosion is extremely difficult to repair because it has already reached the outer side of the outer panel from the inner side when it is discovered from the external appearance. Therefore, perforation corrosion is positioned as the most important issue for corrosion protection of the automobile body.

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Fig. 5.2 Process of progress of corrosion until perforation corrosion of hot-dip Zn coated steel sheets [3]

5.1.2.4

Corrosion Mechanism and Life of Perforation Corrosion of Automotive Coated Steel Sheets

Figure 5.2 [3] shows a model in which the corrosion period until perforation corrosion occurs in hot-dip galvanized sheets (hereinafter, GI) is divided into 4 stages based on results of measurements of the perforation depth of auto bodies in areas of North America where deicing salt is spread. Figure 5.3 [4] shows an outline of the corrosion mechanism in each of these corrosion processes. These processes are divided into a period (τ1 ) in which the Zn coating film covering the entire surface of the substrate steel sheet corrodes, a period (τ2 ) in which the coating partially disappears and the Zn coating film provides sacrificial corrosion protection for the substrate steel, a period (τ3 ) in which the sacrificial corrosion protection effect of the coating film is lost, but corrosion of the substrate steel is suppressed by Zn corrosion products, and a period (τ4 ) in which the corrosion suppression effect of the Zn corrosion products is lost and the substrate steel corrodes until perforation of the steel occurs.

5.1.3 Corrosion of Substrate Steel (τ 4 ) 5.1.3.1

Positioning of Corrosion Rate of Corrosion Issue (τ4 )

The corrosion rates of the Zn coating and substrate steel sheet (mild steel) inside a lapped steel sheet portion such as those shown in the above Fig. 5.2 is calculated as follows.

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Fig. 5.3 Corrosion model of Zn-coated steel sheet [4]

Zn coating corrosion rate (τ1 to τ3 ) ( / )/ V(coating) = 120 g m2 9 years / ⚌1.5 × 10−6 m year

(5.1)

Corrosion rate of substrate steel sheet / V(steel) = 0.75 mm 5 year ⚌1.8−4 m/year

(5.2)

Figure 5.4 is a diagram of the corrosion rates (mm/year) of the above-mentioned lapped steel sheets portions shown in “Introduction to Material Environmental Science” (Japan Society of Corrosion Engineering, Eds.), Table 8.6 Representative corrosion rates [5]. The corrosion rate of lap steel sheets portions of automotive steel sheets is close to the rate in seawater. It can also be understood that the corrosion rate of the Zn-coating is positioned lower than the corrosion rate of the Zn-coating in the atmosphere.

5.1.3.2

Iron Rust Formation Mechanism in τ4

The corrosion mechanism of cyclic drying and wetting of iron and steel in atmospheric corrosion was proposed for the first time in the world by Evans [6]. In the Evans model, corrosion of carbon steel progresses by an oxidation–reduction reaction of rust by this cyclic drying and wetting. For easier understanding of the model, the reaction equations according to Misawa [7] are shown in (5.3)–(5.5).

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Fig. 5.4 Corrosion of zinc and steel at inner side of automotive lapped steel sheets portions. Source “Introduction to Material Environmental Science” (Japan Society of Corrosion Engineering, Eds.), Table 8.6 Representative corrosion rates [6]

[Wetting process: Anodic dissolution of Fe and reduction reaction of rust] Fe → Fe2+ + 2e

(5.3)

Fe2+ + 8FeOOH + 2e → 3Fe3 O4 + 4H2 O

(5.4)

[Drying process: Air oxidation reaction of rust] 3Fe3 O4 + 3/4O2 + 9/2H2 O → 9FeOOH

(5.5)

That is, when Fe corrodes in the atmosphere, iron oxyhydroxide (FeOOH) forms in the outer layer, and magnetite (Fe3 O4 ) forms in the under layer. However, in the wetting process, the iron oxyhydroxide that formed in the outer layer becomes an oxidant and changes to magnetite, and anodic dissolution of Fe proceeds. In the drying process, the magnetite of the under layer is oxidized by oxygen and changes to iron oxyhydroxide. Subsequently, research was carried out on the characteristics of various types of iron rust [7–17]. Figure 5.5 shows the formation process (Misawa diagram) of atmospheric corrosion (iron oxides) [17]. The main crystalline Fe rust components which

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Fig. 5.5 Formation process of atmosphere corrosion (iron oxides) (Misawa diagram) [8]

forms in natural environments such as the atmosphere are the three iron oxyhydroxides γ -FeOOH (lepidocrocite), α-FeOOH (goethite) and β-FeOOH (akageneite), and the iron oxide Fe3 O4 (magnetite). In the atmosphere, α-FeOOH (goethite) is considered to be the most stable corrosion product in terms of energy. Using an X-ray internal standard method, Fujita et al. analyzed the composition of iron rust that formed on automobile bodies recovered in North America after use for 5–10 years. Based on the Evans model, a classification of the rust composition detected in the North America automobile bodies in a ternary system (I: “α-FeOOH,” II: “Fe3 O4 + γ -FeOOH,” and III: “Amorphous rust + β-FeOOH”) that takes into account the oxidation–reduction reaction of Fe shown in Fig. 5.6 [18] was devised, and the formation process of iron rust in automotive corrosion was classified based on this. That is, in the rust detected in the North American auto bodies, α-FeOOH can be treated as the final iron hydroxide, which is both thermodynamically and electrochemically stable in the atmosphere environment. This α-FeOOH is classified separately from the other types of iron rust. In contrast, because γ -FeOOH is easily reduced to Fe3 O4 in the stage of the wetting process, γ -FeOOH and Fe3 O4 are classified in the same group. In comparison with these types of iron rust, β-FeOOH is a substance that easily changes to α-FeOOH or Fe3 O4 , but because only a very small amount was found in the analysis results of the North American auto bodies, it was classified as “Other rust” together with amorphous rust. In parts where Zn-coated steel sheets are applied, “amorphous” iron rust forms in the initial period, corrosion of the substrate steel sheet is suppressed by the above-mentioned mechanism in which the Zn corrosion products suppress the oxidation–reduction reaction of iron rust, and under the condition in which Zn and iron rust coexist, β-FeOOH forms, after which γ -FeOOH disappears. Finally, corrosion reaches the corrosion process of substrate

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Fig. 5.6 Corrosion processes of cosmetic corrosion and perforation corrosion observed from changes in iron rust composition [18]

steel and transitions to the distinctive compositions of corrosion of lapped steel sheets portions and outside portions.

5.1.3.3

Steel Factors Affecting the Corrosion Rate of τ4

Influence of Chemical Components in Steel If the corrosion rate differs greatly, the influence of the chemical components in the steel and their corrosion protection mechanisms will also be different. The corrosion rate of the steel (mild steel) used in automobiles, approximately 0.15 mm/year, is comparatively close to the rate in the splash zone of offshore steel structures (0.3 mm– 0.5 mm/year). During the 1960 and 1970s, Japanese steel makers actively developed high corrosion resistance, low-alloy steels for offshore steel structures covering the range of corrosion environments from the ocean atmospheric zone to the submerged zone. Offshore steel structures are classified as the atmospheric zone, splash zone, tidal zone and submerged zone, and the effects of chemical components on the corrosion resistance of low-alloys steels is different in each of these environments. As components with those effects, steels with compound addition of C, Si, Mn, P, S, Ni, Cr, Ni, Mo, Sn, Al and other chemical components were developed and also manufactured with actual equipment, but none showed stable corrosion resistance in various marine environments. Therefore, complete corrosion protection that displays stable, long-term corrosion resistance, employing multilayer corrosion protection or an extra heavy coating system in which the steel structure is covered with a thick coating material, was proposed in the form of corrosion protection guidelines for offshore steel structures [19]. Because low-alloy type seawater corrosion-resistant steels were no longer adopted as corrosion-resistant materials for offshore steel structures, almost no research on low-alloy steels for use in seawater was carried out after that time. However, a commentary [20] on the thinking concerning the component systems in the development of seawater corrosion-resistant steels has been published, and interested readers may refer to that document.

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Where automobiles are concerned, no reports have examined the effects of chemical components on corrosion resistance in actual corrosion environments. Nakayama et al. analyzed the effects of chemical components on the corrosion resistance of carbon steel ( 0 is a parameter called a regularization coefficient that decides the degree of regularization; further, this parameter makes it possible to qualitatively set the geometric complexity of an optimal structure. And then, fictitious time t should be introduced, and with the assumption that fluctuation of the level set function in virtual time is proportional to the gradient of the regularized target generic function, the level set function, which is a design variable, should be updated by introducing a reaction–diffusion equation. ) ( , ∂φ = −K F − τ ∇ 2 φ ∂r ,

(6.6)

Here, K is a constant, and F is design sensitivity of F related to a level set function ϕ(x), for which a derivative function of topology is used in this case. As an optimal structure can be obtained in a small number of iterations or a few dozen iterations in this update procedure, it can be done at a higher convergence than general mathematical optimization methods and thus it is extremely practical. Figure 6.10 shows optimal structures when using the level set function expressed as the formula (6.4). Optimal structures were obtained under optimization conditions whose setting was the same as in Fig. 6.8a. This figure indicates that first an extremely clear optimal structure without grayscale parts was obtained, which is different from conventional methods. In addition, a simple optimal structure can be obtained by increasing the effect of all the fluctuation of the level set function that is a regularization item based on a diffusion term, while a complicated optimal structure can be obtained by decreasing the effect. Needless to say, a complicated optimal structure has higher stiffness, but in most cases it is difficult to manufacture such a structure. In that case, a simple structure may be proposed as an alternative solution although performance is reduced. These are significant advantages that are not found in conventional methods. Here, a method based on this level set method is used for weight reduction and performance improvement on the basis of multi-materialization of automobile body structure.

Fig. 6.10 Topology optimization by level set method

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Fig. 6.11 Case of applying surface force to a design region

In structure design, the most basic and common goal is to maintain the shape and preserve structure stability without deformity to the utmost extent against a given load. To achieve this goal, securement of rigidity is absolutely imperative. Now, as shown in Fig. 6.11, focus is placed on the case where surface force t is applied to the boundary Γt while completely fixing the boundary Γu of the design region Ωd indicating the target structure to be obtained. Incidentally, it is assumed that there is no change in the locations of the boundaries Γu and Γt in the process of optimization. In this case, rigidity of the boundary Γu to which the surface force t has been applied can be evaluated with mean compliance lm.c. of the following formula: ( lm.c. = l(u) =

t·udΓ

(6.7)

Γt

The mean compliance in the above formula indicates the average compliance in the boundary Γu , and by minimizing l m.c. rigidity can be maximized. In general topology optimization, formulation is performed so as to minimize mean compliance lm.c. under volume restriction. However, to achieve weightreduction of an automobile body structure it is more effective to minimize total weight as an objective function. As properties necessary for an automobile body structure, rigidity, strength, vibration characteristics related to ride quality, collision characteristics, etc. should be considered. Strength and collision characteristics cannot undergo quantitative evaluation unless details of an automobile body structure are decided. Therefore, rigidity and vibration characteristics should be evaluated in topology optimization at a conceptual design phase. Here, for the creation of an automobile body structure and an outline layout as the primary objective, restrictions should be set so that mean compliance becomes no less than the required lower limit with rigidity as an assessment measure. In other words, the optimization problem should be formulated as follows with the mean compliance lm. c. j when applying j surface force t j to the boundaries Γt ( j = 1, . . . , m) at m numbers of locations and j the upper limit value l0 of the required mean compliance:

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( minimize f =

χΩ dΩ

(6.8)

j lm.c. ≤ l0 for j = 1, . . . ,m

(6.9)

design variables

D

Subject to j

Moreover, it is necessary to take into consideration the manufacturing costs of automobile body structures. Nonetheless, as it is difficult to evaluate actual manufacturing costs in detail at the time of a conceptional phase, the following two cases were studied in this research. (1) Material cost: calculate and evaluate material costs in reference to costs per weight of each material (2) Joint cost: set costs per length and area of joint surface of materials and evaluate based on length and area of the joint surface of materials. In the case of (2), for example, of using two types of materials, joint costs can be expressed in the following formula if the Multi-Material Level Set (MMLS) method, shown in formula (4.1) in Chap. 4 of Volume 2 of “Innovative Structural Materials and Multi-materials—Innovations in Materials, Joining and Design Technologies for Lightweight Transportation Equipment -” (in Japanese) published by Ohmsha Ltd. in June 2023, is applied. Cw =

( C(χ1 (1 − χ2 ) + χ1 χ2 )dΩ

(6.10)

D

where, ( Xi =

1 if φi ≥ 0 0 if φi < 0

for i = 1, 2

(6.11)

6.2.3 Conclusion This chapter described the basic concept and types of topology optimization. Topology optimization was first handled by a homogenization design method where design space was made originally based on the homogenization method, but there was a problematic issue of creating an optimal structure with many grayscale parts, so the industry did not promote its use. In contrast, a density method was used by many industries through commercial software because it could create practical optimal structures with fewer grayscale parts compared with a homogenization

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design method. However, even when a density method was used, the existence of grayscale parts could not be avoided, and thus performing consistent work to create a CAD model based on topology optimization was contentious. To essentially address this problematic issue, a method to express shapes based on a level set method was proposed, so that optimal structures with clear boundaries without grayscale parts could be obtained, resulting in the achievement of consistent processes to make CAD models. In addition, it becomes possible to express boundaries between various materials even when deployed to multi-material structure design due to this merit of having clear boundaries, and thus it is an extremely useful method. Therefore, this project promoted deployment of this topology optimization method based on a level set method to multi-material automobile body design by expanding this method so as to enable multi-material expression.

6.3 Multi-material Design—Significant Weight Reduction Through Optimization of Material Arrangement and Shape Shu Yamashita

6.3.1 Introduction The multi-material design technology intended to reduce the weight while maintaining characteristics equivalent to those of the existing product (or while seeking to improve the performance) by arranging multiple materials in the right places is globally unprecedented as a design technology for structures of not only automobiles, but also of others. The trend toward the use of multi-material members for structures is broadly seen in products such as automobiles and aircraft. The major challenge for these products is to achieve both the product weight reduction and product performance improvement. However, the existing design and development using a single material has already reached its limit, and further weight reduction and performance improvement would require a breakthrough to overcome such limitations. The use of multi-material members is the most promising method, and will make it possible to achieve truly significant weight reduction and performance improvement. Furthermore, the use of multi-material members can provide a new added value to structures. For example, if two materials with different thermal expansion coefficients are appropriately arranged, it will provide a singular property in which a structural member shrinks when heated [29]. In addition, with the compliant mechanism,1 1

Compliant mechanism: A concept of forming a movable part by taking advantage of the elasticity of the material.

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which is a mechanism taking advantage of flexibility, an actuator that has a certain function can be designed through use of multiple materials including a material that has an energy conversion function [30]. As described above, the use of multi-material members for structures is expected to make achievements such as weight reduction, performance improvement and function provision, and bring about innovation to the structural design in the future. In this project, we developed various methods, such as topology optimization that enables multiple materials to be handled (multi-material topology optimization), which was developed as a measure to create a lightweight and high-performance body structure made of multiple materials, development of a design tool based on the topology optimization method, conversion of sheet combination considered as necessary to gain a realistic body structure based on the lightweight body structure obtained through topology optimization, and change of the shape through collision analysis.

6.3.2 Multi-material Automotive Body Design Currently, structural design of automotive bodies is conducted given the balance between rigidity design, which attaches importance mainly to driving operability and ride quality, and strength design, which attaches importance to collision safety, based on the design policy of each automobile company. Also in this project, as shown in Fig. 6.12, topology optimization is first conducted with the aim of achieving weight reduction with rigidity equivalence to that of a body-in-white (BIW) of the benchmark vehicle based on a similar design policy, and sheet assembly conversion with the obtained result is conducted. Subsequently, collision analysis for a BIW with converted sheet assembly is conducted, and changes such as shape change are made with the aim of securing collision safety equivalent to that of the benchmark vehicle. This routine is repeated until the desired collision safety is gained, and finally a multi-material lightweight vehicle body structure that has both rigidity and collision safety is acquired. Figure 6.13 shows the developed design tool. Having a pre- and post-processing function in front of and behind a solver that executes optimization (multi-material topology optimization), the tool can also be linked with external nonlinear analysis software. As shown in Fig. 6.13, spFrame, which is the design platform, enables consistent setting of a design region (pre-processing), topology optimization, display of the optimal structure (post-processing) and creation of a CAD model with the optimal structure. In addition, this system can also be linked to other general-purpose software through the CAD model of the optimal structure, and can also be used to evaluate the performance of the optimal structure. Figure 6.14a shows a reverse-engineered BIW model of the benchmark vehicle. In addition, Fig. 6.14b shows a BIW model in which the surface of a multi-material structure obtained through multi-material topology optimization for four materials

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Fig. 6.12 Multi-material vehicle body design process

Fig. 6.13 Topology optimization design tool

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Fig. 6.14 BIW model of the benchmark (a) and BIW model of multi-material (b)

(aluminum, magnesium, steel and CFRTP) is extracted, with the aim of achieving the rigidity of this BIW model. Figure 6.15a shows a BIW model in which a collision-resistant structure is adopted and the material distribution is adjusted in order to control the stress propagation caused by collision during a collision analysis, which is the next analysis step, for the multi-material BIW model shown in Fig. 6.14b. Further, Fig. 6.15b shows a BIW model in which the sheet thickness is set through dimensional optimization in order to secure collision safety performance equivalent to that of the benchmark BIW model. The basic idea of securing collision safety performance can be summarized in three points: (1) maintaining a robust passenger space, (2) flexibly confining the passengers in the space and (3) reducing the reaction force (evenly) and absorbing energy until the vehicle halts. The purpose of this is to minimize the damage to the passengers by realizing an automobile structure with high collision safety performance. That is to say, ideally speaking the load applied to the passengers is minimized and the survival space of the passengers is not deformed until the vehicle halts. For this project, an evaluation through frontal collision, offset deformable barrier (ODB) collision and side collision was conducted as a collision safety evaluation. As shown in Fig. 6.15a, reinforcing parts in which 1.5 GPa ultra-super high tensile strength steel, high strength aluminum (5000-based, 6000-based and 7000-based), fire-resistant magnesium and CFRTP, which are materials developed by the ISMA,

Fig. 6.15 BIW model (a), to which collision-resistant parts are added, and BIW model (b), in which the sheet thickness is optimized

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are adopted and added to the BIW model shown in Fig. 6.14b. In addition, as shown in Fig. 6.15b, collision safety performance was evaluated through a collision analysis as shown in Fig. 6.16, for the lightest model in which the sheet thickness is optimized. Collision safety characteristics equivalent to that of the benchmark vehicle were obtained. Figure 6.17 shows the BIW section of the lightest model in which materials developed by the ISMA are used. Weight reduction with the whole BIW by 41.4% from the benchmark vehicle was achieved. Further, a 50% weight reduction, which is the project’s target, was almost achieved, in conjunction with the weight reduction through replacement of the body panel material.

Fig. 6.16 Result of safety analysis and evaluation for multi-material BIW model using materials developed by ISMA Fig. 6.17 Lightest model (BIW section) using materials developed by ISMA

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6.3.3 Conclusion The above summarizes an outline of the approach for a method for reducing the weight of the vehicle body through use of multi-material members. By conducting the development of a design tool that uses a multi-material topology optimization method as a solver, conversion of the topology optimization result to an actual structure (thin-sheet structure), and design changes such as a shape change that also takes into account collision safety in combination, we were able to propose a multi-material lightweight vehicle body that has both good ride quality and collision safety. With this design approach, the use of multi-material members makes it possible to gain a specific structural design idea that can achieve both weight reduction and performance improvement. In addition, this method can be applied also to the design of various structures such as aircraft, and as a result, it is expected that structural weight reduction and performance improvement will be achieved.

6.4 Application of Topology Optimization to 3D Additive Manufacturing—Further Weight Reduction by Application of 3D Additive Manufacturing Akihiko Chiba, Shu Yamashita, Takao Horiya

6.4.1 Introduction As described in Sect. 6.3, in order to propose a multi-material weight reduced car body, we are studying the optimization of mainly a body-in-white (BIW) using a topology optimization method. An actual car body structure, however, consists of not only panel assembly structural members, but also casting structural members having three-dimensional complex shapes, and if an optimal structure can also be applied to these members, a greater weight reduction effect can be expected. 3D additive manufacturing (3DAM) technology is a forming method best suited to embodying structures optimized by topology optimization and recently, technologies for multimaterialization in 3DAM have also been developed. We developed a technology that allows a structure optimized by topology optimization to be directly net shaped by a moldless forming process aiming at achieving further weight reduction by multi-materialization.

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6.4.2 Overview of Metal Additive Manufacturing Technologies Metal additive manufacturing technologies are broadly classified as those using alloy powder and those using alloy wire. For metal additive manufacturing technologies using alloy powder, a wide variety of manufacturing methods have been developed. Figure 6.18 shows the metal additive manufacturing technologies using alloy powder classified by the manufacturing method. The methods include: the Powder Bed Fusion (PBF) method in which an alloy powder bed is formed and the powder bed is selectively fused using laser or electron beams as the heat source; the Directed Energy Deposition (DED) method in which alloy powder is charged into a laser heat source by carrier gas and is fused and deposited; and the Bound Metal Deposition (BMD) method in which alloy powder is kneaded with thermoplastic resin which serves as a binder to produce filaments, a green compact is produced from the filaments according to the procedure of the Fused Deposition Modeling (FDM) and a sintered compact is produced through the same process as the MIM method. In the above Powder Bed Technology, the method using a 3D printer ink jet printer is a binder jetting method, which uses a process based on the sintering of alloy powder. On the other hand, the Laser or Electron Beam Melting method and the DED method use a process of fusing and solidifying alloy powder, in which thin layers of metal powder are melted and laminated one by one to produce machine parts, etc. Especially, in the PBF-based Electron Beam Melting, PBF-EBM (hereinafter referred to as EBM) and PBF-based Laser Beam Melting, PBF-LBM (hereinafter referred to as LBM), when the irradiance conditions of the heat source (energy density, scanning speed, scanning interval, etc.) and the scanning pattern are optimized, the temperature gradient (temperature field), solidification rate (composition field), heat flux (temperature field), concentrated solute distribution (composition field), melt flow rate (temperature field/composition field), etc. formed at the solid–liquid interface of the melt pool can be changed in a wide range. Not only has the possibility of metal

Fig. 6.18 Various manufacturing methods in metal additive manufacturing technologies using alloy powder

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structure control such as single crystallization and polycrystallization [31, 32] of metal structures received attention, but the potential as new solidification processes is noteworthy which may allow the realization of “segregation-free supersaturated solid solution” [33, 34] by solute trapping (an absolute stability condition with which a solid–liquid interface becomes a stable smooth interface) [35] appearing as a result of ultrafast solidification obtained by high-speed scanning of the heat source.

6.4.3 Efforts for Multi-Materialization In the case, for example, where two kinds of metal materials, steel (Fe–C-based alloy) and Al alloy (Al–Si–based), are used to manufacture a multi-material component by welding technology, an Fe–Al intermetallic compound phase which causes an embrittlement of the interface between Fe and Al is coarsely formed. Therefore, practical application of multi-materialization by welding is difficult due to the embrittlement of the interface. Also, in the manufacturing of components by the metal additive manufacturing technologies, which were classified in the previous section, monolithic alloy powder is used in most cases, but an example has been reported where multi-materialization of components was attempted with the in-process change of powder to be supplied by the DED method [36]. However, in the DED method, the process conditions such as the moving velocity of a heat source are the same as those in welding, and the problem of formation of an embrittled phase at the interface in multi-materialization by welding has not been solved yet. In the metal additive manufacturing technologies, the DED method enables multi-material manufacturing, but it is considered to be unsuitable for manufacturing of a multi-material component having an interface bond strength equal to or higher than the strength of the base material. In contrast, as described in the previous section, in the EBM and SLM, the heat source is scanned at high speed and the solidification rate can be increased to about 1 m/s2) . This corresponds to the solidification rate with which an absolute stability condition where the solid–liquid interface of the melt pool becomes a stable smooth interface is achieved and is a solidification condition where no local equilibrium at the solid–liquid interface is established and no solute partitioning occurs, and under the condition, solute trapping can be expected to appear. Therefore, in the case where the above-described steel (Fe–C-based alloy) material and Al alloy (Al–Sibased) are used to manufacture a component by EBM or SLM, the scanning speed of the heat source is set at 1 m/s or higher, which is required for establishment of the absolute stability condition to prevent an Fe–Al intermetallic compound phase which causes embrittlement from being formed at the interface between the different alloys. Furthermore, it is necessary to study optimization of manufacturing process parameters and optimization of powder materials, such as the use of powder favorable to an increase in the solidification rate through shape control of the melt pool [34, 35], and the efforts in this study provide pioneering research and development case studies that have global reach.

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Study of Possibility of Multi-Materialization by SLM

Concerning the characteristics of the solidification process of the melt pool in EBM or SLM as described in the previous section, this section considers how they affect multi-material manufacturing and explains solute trapping. Figure 6.19 shows a schematic diagram of the melt pool in EBM (SLM) (upper figure) and a schematic diagram of the central cross section along the scanning direction (x direction) of the same melt pool (lower figure). The shape is typical of a melt pool formed by a high-scanning speed heat source (electron beam or laser). The temperature of the solid–liquid interface (solid line) which is a fusion boundary does not reach agreement with the liquidus line (dotted line) on the equilibrium diagram, from the bottom toward the top of the melt pool, due to the supercooling ΔT at the cell/dendrite tip which solidifies and grows. Solidification by EBM or SLM is epitaxial growth from the heat-affected zone of the base material which starts at the fusion boundary and is unidirectional solidification in which the main solidification process is crystal growth requiring no nucleation. It is also a solidification process basically with “constrained growth” where there is a positive temperature gradient because the heat source is in front of solid growth, by which crystal growth is constrained. This type of solidification process is very similar to the solidification of the melt pool in weld solidification, and the concept in weld solidification is helpful to understand the relation between the moving velocity of the heat source and the solidification rate [37]. From Fig. 6.19, when the angle formed by the heat source scanning direction and the growth direction of the solidifying particle is θ , the growth rate V s of the columnar crystals which grow from the fusion boundary toward the melt pool is Vs = Vb cos θ

(6.12)

where V b is the scanning speed of the heat source. As is clear from Fig. 6.19, at the start of the solidification at the fusion boundary (that is, at the bottom of the melt pool), θ is the largest and the solidification rate V s is the lowest. As the growth direction of solidification becomes closer to the moving direction of the heat source, θ gets closer to zero and V s becomes higher and reaches maximum at the outermost surface of the center of the melt pool. The relationship between the solidification rate V s of the melt pool and the scanning speed of the heat source V b is shown in the lower right of Fig. 6.19. As shown, the solidification rate V s of the melt pool in EBM (SLM) can be increased by an increase in the scanning speed of the heat source V b according to the (6.1). Therefore, the fusion-solidification process in SLM (EBM) is expected to be applied as a rapid solidification process at the solidification rate of 1 m/s or higher at which solute trapping described in the previous section appears, which could not be achieved by the existing processes such as welding. However, since the generation of latent heat associated with solidification causes an increase of the temperature of the solid–liquid interface, the movement of the solid–liquid interface

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Fig. 6.19 Schematic diagram of melt pool in EBM and SLM (central longitudinal section) Relationship between scanning speed of heat source, Vb , and moving rate of solid–liquid interface (solidification rate), Vs

will be interrupted unless the latent heat is released to the low-temperature solid phase side. Accordingly, the solidification rate V s simply cannot follow the scanning speed of the heat source V b . Therefore, it is considered that the upper limit of the solidification rate V s that is increased by an increase in the scanning speed of the heat source V b is restricted by the heat conductivity specific to a material, but EBM (SLM) is expected to be used as a solidification process which produces a solute trapping effect through optimization of the scanning method of the heat source, shape control of the melt pool, optimization of the characteristics of metal powder to be used, etc. Based on the above consideration, this paper studies the effects of the shape and particle shape of metal powder to be used as well as optimization of manufacturing process parameters, especially, interface reaction phase formation behaviors caused by an increase in the scanning speed of the heat source.

6.4.3.2

Basics of Study on Solidification for Multi-Materialization

As described above, non-equilibrium solidification in EBM and LBM is expected to be used as a fusion-solidification process for finely dispersing an intermetallic compound phase crystallized in front of the solid–liquid interface or suppressing crystallization itself. For the achievement of multi-materialization with high strength and high ductility, it is necessary to suppress formation of an intermetallic compound phase. To that end, it is important to study how to control the concentration (segregation) of solute elements formed at the solid–liquid interface in the solidification process from the aspect of solidification engineering.

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In equilibrium solidification, crystallization of the solid phase occurs, which is predicted from the equilibrium diagram, while in rapid solidification, the crystallization phase and the solidification structure which appear vary widely from low to rapid solidification rates. Therefore, as a problem with “primary phase selection” where a phase is selected depending on the solidification rate, there is a possibility that a metastable phase may appear, which cannot be predicted only from the concept of equilibrium theory. The primary phase selection in the case of high-speed welding or surface melting treatment by laser or electron beam is based on the solidification interface temperature and a phase having a higher interface temperature grows first [8]. In multi-material manufacturing with Fe/Al which is the subject of this study, for example, α-Al dendrite, Al3 Fe and Al6 Fe phases which appear on the Al-rich side of the Fe–Al equilibrium diagram become competing phases, and α-Fe dendrite, γ -Fe dendrite, DO3 –Fe3 Al, B2–FeAl, etc. which appear on the Fe-rich side also become competing phases. It is considered that the interface temperature TI (= TL − ΔT; TL is liquidus temperature, ΔT is supercooling temperature) is obtained for each phase and a phase having the highest interface temperature grows preferentially. Therefore, for the Fe-Al material, it is necessary to calculate the interface temperature of each phase dependent on the composition, compare the relationship between the solidification rate and the interface temperature (Interface Response Function; IRF) for each phase and obtain IRFs for the Al3 Fe and Al6 Fe phases appearing on the Al-rich side and DO3 –Fe3 Al, B2–FeAl, etc. appearing on the Fe-rich side. For Al–1.2%Fe and Al–4%Fe alloy, the IRFs have been obtained and it has been shown that as the solidification rate increases, the interface temperature of α-Al dendrite becomes higher, and α-Al dendrite preferentially grows as a solidified phase [39]. As described above, when the solidification rate increases, the growth of the brittle intermetallic compound phase is suppressed and α-Al dendrite with ductility grows as supersaturated solid solution. It is expected that when the manufacturing parameters such as heat source power, scanning speed and scanning interval are optimized so that the solidification rate at which the brittle intermetallic compound phases do not preferentially grow is reached at the solid–liquid interface, crystallization of intermetallic compound phases which is an obstacle to multi-materialization can be prevented. That is, the important point is to know how the interface temperature and concentration at the tip of a solid phase based on which a phase selection is conducted are determined by the solidification conditions at that time, i.e. initial composition, solidification rate and temperature gradient and this is a key requirement for optimization of the manufacturing process for multi-material manufacturing using EBM and SLM. Therefore, it is necessary to obtain the IRF for each phase that appears on the equilibrium diagram. The dendrite tip temperature required for obtaining the IRF based on which a phase selection is conducted is described as follows. (1) IRF of single-phase alloy Single-phase alloys do not have phase selection phenomenon, but it is considered that at a given temperature gradient (G), the interface morphology transits with increasing growth velocity in the sequence: planar (smooth interface) → cellular → cellular/

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dendrite → dendrite → refined cellular → banding → planar (smooth interface). For different growth morphologies, the liquid–solid interface temperature can be described with different models. (1) When a single phase grows as a smooth planar front, the interface growth temperature is [8].

T p = Tm +

Cl∗ m V

( −

Rg T f ΔS f

)

/ V V0

(6.13)

where Tm is the melting temperature of the pure metal, Cl∗ (=C0 /kV ) is the composition of the liquid at the interface, ΔS f is the molar entropy of fusion, V 0 is a constant in the order of the velocity of sound, mv is the non-equilibrium liquidus slope dependent on the solidification rate and V is the solidification rate. For steady solidification, Cl∗ =C0 / kV, C0 is the initial alloy composition and kV is the non-equilibrium distribution coefficient dependent on the solidification rate. (2) When a single phase grows as a cellular and dendrite front, the interface growth temperature is given by the KGT model for G = 0 as ( Td0 = Tm − Γ K + Cl∗ m V −

Rg Tm ΔS f

) V

V0

(6.14)

where Γ is the Gibbs–Thomson coefficient, K is the curvature of the dendrite tip (K = 2/R, and R is the radius of the dendrite tip). =

) [( ]1/2 Γ/σ ∗ /(m V ζC G C − G)

{ ( [ ])} m e 1 − k V 1 − ln kkV mV = (1 − k) ) ([ ]1/2 ζC = 1 − 2k V / 1 + (2π/Pc)2 − 1 + 2k V (1 − k V )V Cl∗ D ( ) k + a0DV ) kV = ( 1 + a0DV

Gc = −

Cl∗ =

C0 (1 − (1 − kv )Iv (PC ))

(6.15)

(6.16) (6.17) (6.18)

(6.19) (6.20)

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Pc =

RV 2D

(6.21)

where s* = 1/4p2, zc is the stability parameter, GC is the solution gradient of the dendrite tip, C1 is the liquid composition of the dendrite tip, Iv[Pc] is the Ivantsov function of Peclet, and Pc is the Peclet number. When G /= 0, the growth temperature of cellular/dendrite is given as Tc/D (V ) = Td0 − ΔTC ΔTC =

G DL V

(6.22) (6.23)

where ΔT C is the tip supercooling of cells with low growth velocity. The full interface response function (IRF) for single phase growth in a positive temperature gradient is: ( ) IRF(V) = max T p(V ), Tc/ d (V )

(6.24)

The IRF reflects the influence of growth velocity on the solidification interface temperature at a given composition and temperature gradient.

6.4.4 Efforts in This Project In 3DAM technologies, the powder bed technology is the most widespread and research and development of use of the technology in Selective Laser Melting (SLM) and Electron Beam Melting (EBM) have been actively conducted. In this project, we planned to trial-manufacture a full-scale sample of a suspension tower as an automobile component in the final stage and adopted SLM which currently allows manufacturing of a relatively large structural member. First, we selected metal powder for SLM. Figure 6.20 shows the processes for manufacturing base powder for SLM and the shapes of powder obtained. Among them, the Plasma Rotating Electrode Process (PREP) powder was judged to be suitable for additive manufacturing of structural materials because it can produce gas pore-free particles. Gas pores in powder particles are formed by a mixture of atomized gas in powder manufacturing and are often caused internal defects. The gas pore-free particles have high sphericity, are superior in particle size distribution control and can produce a homogeneous surface layer. For aluminum powder, the disc atomized (DAT) powder was adopted, which has characteristics equivalent to PREP powder and can form a defect-free and homogeneous surface layer.

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Fig. 6.20 Shapes of metal powder produced by various methods in metal additive manufacturing

For trial manufacture and evaluation of a multi-material automobile component (suspension tower), a small-sized element sample consisting of S25C and Al–3%Si– 1%Mn (hereinafter, referred to as Al3Si1Mn) for evaluation was manufactured by SLM and the mechanical properties of the interface and the structure of the interface reaction phase were analyzed. Figure 6.21 shows the elemental mapping of the Al3Si1Mn/S25C interface by the high-resolution TEM (STEM). In the interface field of S25C and Al3Si1Mn, the Al-rich field (vermilion) and the Fe-rich field (green) form a linear boundary (smooth interface). In the vicinity of the interface on the Fe-rich side is Fe solid solution (δ ferrite) in which Al is in a solid solution state at low concentration, while in the vicinity of the interface on the Al-rich side, a cellular solidification structure of the B2 (FeAl) phase which grows from the smooth interface as a starting point is formed and continues to the eutectic structure of the Al solid solution having Fe in a slightly solid solution state and the θ –FeAl3 phase. General fusion-solidification processes such as weld solidification are nonequilibrium fusion-solidification processes by which local equilibrium is established at the solid/liquid phase interface, and redistribution of solute elements occurs at the solid/liquid phase interface, resulting in a difference in the concentration of solute elements in the solid phase and the liquid phase. Normally, in an alloy having the distribution coefficient of solute elements of 1 or less, a larger quantity of solute elements in the solid phase is distributed to the liquid phase side, resulting in an increase in the concentration of solute elements at the interface on the liquid phase side, which becomes a factor of formation of coarse intermetallic compounds. However, when the moving of the solid/liquid phase interface, that is, solidification is promoted while no local equilibrium is established at the solid/liquid phase interface that is, redistribution of solute elements is suppressed (which is called “solute trapping”) by control of the laser scanning speed v, it may be possible to prevent the

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Fig. 6.21 Elemental mapping of the Al3Si1Mn/S25C interface by the high-resolution TEM

formation of a locally high-concentration solute element field, that is, to suppress the formation of intermetallic compounds. The study result confirmed that at the S25C/Al3Si1Mn sample interface, formation of an intermetallic compound phase (interface reaction phase) of the coarse η–Fe2 Al5 phase and θ –FeAl3 phase, which can be observed by the existing joining technologies such as welding, was suppressed under the conditions where the scanning speed increased, resulting in an increase in the bonded interface strength and have the joint strength equivalent to the yield stress of the Al3Si1Mn base material. Since an increase in the scanning speed has the effect of increasing the solidification rate, it is notable that the increase in the S25C/Al3Si1Mn joint strength is a characteristic of the metal additive manufacturing processes such as SLM and EBM by which the solidification rate is increased to the level where the solute trapping effect is exerted. Figure 6.22 shows the appearance of an actual vehicle suspension tower and its topology optimization model. We finally entered the CAD data of the optimization model into the additive manufacturing device and trial-manufactured an actual-size suspension tower. Figure 6.23 shows the appearance of the prototype. We have studied and accumulated the findings obtained through the formation of various models since last year and we successfully formed an actual-size multi-material suspension tower by 3D additive manufacturing, for the first time in the world, under the optimal conditions while suppressing the residual thermal stress at the interface of different materials.

6.4.5 Conclusion As an attempt to reduce the weight of an automobile component by multi-material topology optimization, we evaluated the applicability of 3DAM technologies.

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Fig. 6.22 Appearance of actual vehicle suspension tower (a) and its topology optimization model (b)

Fig. 6.23 Actual-size suspension tower trial-manufactured by 3D additive manufacturing. Front (a) and back (b)

Through the trial-manufacturing of a topology optimization model of an actual automobile component (suspension tower) using 3DAM technology, we obtained highly valuable findings, for example, that by rapid solidification in 3DAM technology, formation of an embrittlement layer (intermetallic compound) at the bonding interface between different materials can be suppressed. That is, it was found that in the metal additive manufacturing processes such as SLM and EBM, by which the solidification rate is increased to the level where the solute trapping effect is exerted, formation of a brittle and coarse Fe-Al intermetallic compound phase that is formed in weld solidification of Fe/Al can be suppressed and also by reduction of the residual thermal stress at the interface of different materials, the joint strength for a structural material can be secured. We could embody a weight reduced structure by multi-material topology optimization using 3DAM technology for the first time in the world. We demonstrated

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that in the future, various multi-material light weight structures could be realized through the combination of topology optimization methods and 3DAM technologies.

References 1. J.R. Lemon, S.K. Tolnai, A.K. Kolsterman, R. Wilhelm (eds.), CAD-Fachgespräch, GI—10. Jahrestagung, Saarbrücken, vol. 30 (1980), pp. 161–183 2. For reference, see https://www.altairjp.co.jp/hypermesh/ 3. For reference, see https://beta-cae.jp/products/ansa/ 4. For reference, see https://www.mscsoftware.com/ja/product/msc-nastran 5. For reference, see https://www.ansys.com/ja-jp 6. For reference, see https://www.3ds.com/ja/products-services/simulia/products/abaqus/ 7. For reference, see https://www.mscsoftware.com/ja/product/marc 8. For reference, see https://www.jsol-cae.com/product/struct/lsdyna/ 9. For reference, see https://1dcae.jp/about/ 10. H. Nishigaki, S. Nishiwaki, T. Amago, Y. Kojima, N. Kikuchi, SAE Technical Paper, No. 2001-01-0768 (2001) 11. For reference, see https://smbiz.asahi.com/article/14448065 12. S. Nishiwaki, K. Izui, N. Kikuchi, Topology Optimization (computational dynamics lecture course) (Maruzen Publishing Co., Ltd., 2013) 13. S. Nishiwaki, SIMULATION 39(2), 117–122 (2020) 14. M.P. Bendsøe, N. Kikuchi, Comput. Methods Appl. Mech. Eng. 71, 891–909 (1988) 15. F. Murat, L. Tartar, Research Notes in Mathematics 127, 1–8 (1985) 16. K. Lurie, A. Cherkaev, A. Fedorov, J. Optim. Theory Appl. 37–4, 499–522 (1982) 17. C. Jog, R. Haber, M. Bendsøe, Int. J. Numer. Meth. Eng. 37–8, 1323–1350 (1994) 18. M. Bendsøe, Struct. Multidiscip. Optim. 1–4, 193–202 (1989) 19. N. Olhoff, On optimum design of structures and materials. Meccanica 31–2, 143–161 (1996) 20. K. Suzuki, N. Kikuchi, Comput. Methods Appl. Mech. Eng. 93–3, 291–318 (1991) 21. Z. Hashin, S. Shtrikman, J. Mech. Phys. Solids 11–2, 127–140 (1963) 22. R. Yang, C. Chuang, Comput. Struct. 52–2, 265–275 (1994) 23. M. Bendsøe, O. Sigmund, Arch. Appl. Mech. 69–9, 635–654 (1999) 24. A. Kawamoto, T. Matsumori, S. Yamasaki, T. Nomura, T., T. Kondoh, S. Nishiwaki, Struct. Multidiscip. Optim. 44–1, 19–24 (2011) 25. J. Guest, J. Pr´evost, T. Belytschko, Int. J. Numer. Methods Eng. 61–2, 238–254 (2004) 26. A. Takezawa, S. Nishiwaki, M. Kitamura, J. Comput. Phys. 229–7, 2697–2718 (2010) 27. J.S. Choi, T. Yamada, K. Izui, S. Nishiwaki, J. Yoo, Comput. Methods Appl. Mech. Eng. 200-29-32, 2407–2420 (2011) 28. T. Yamada, K. Izui, S. Nishiwaki, A. Takezawa, Comput. Methods Appl. Mech. Eng. 199–45, 2876 (2010) 29. A. Takezawa, M. Kobayashi, Compos. Part B 131, 21–29 (2017) 30. R.C. Carbonari, E.C.N. Silva, S. Nishiwaki, Smart Mater. Struct. 14(6), 1431–1447 (2005) 31. Y. Lei, K. Aoyagi, A. Chiba, Acta Mater. 227, 117717 (2022) 32. Y. Zhao, K. Aoyagi, K. Yamanaka, A. Chiba, Mater. Des. 221, 110927 (2022) 33. Y. Zhao, K. Aoyagi, Y. Daino, K. Yamanaka, A. Chiba, Addit. Manuf. 34, 101277 (2020) 34. Y. Zhao, H. Bian, H. Wang, K. Aoyagi, K. Yamanaka, A. Chiba, Mater. Des. 221, 110915 (2022) 35. M.J. Aziz, J. Appl. Phys. 53(2), 1158 (1982) 36. A. Bandyopadhyay, B. Heer, Mater. Sci. Eng. R 129, 1–16 (2018)

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37. M. Gaumann, W. Kurz, Mathematical Modelling of Weld Phenomena, vol 4, ed. by H. Cerjak (The Institute of Mater, London, 1999), pp. 125–136. 38. T. Umeda, T. Okane, W. Kurz, Acta. Mater. 44, 4209–4216 (1996) 39. P. Gigien, A. Zryd, W. Kurz, Acta. Mater. 43, 3477–3487 (1995)

Chapter 7

Prototyping of Multi-material Parts—Efforts to Realize Practical Application of Innovative Materials and Technologies Koji Chiba

Abstract To confirm the potential for practical application of the innovative materials and innovative joining technologies developed in this project, we examined the performance of the requirements for practical application, such as formability, joining, coating, and rust prevention, through trial production of parts. Specifically, innovative steel sheet was used for the A-pillar, tailored blank fabricated using FSW for the B-pillar outer and inner panels, innovative aluminum for the side member and sill reinforcement member, innovative magnesium for the hood, CFRP/CFRTP panels for the roof, and LFT-D material for the floor, prototypes and evaluations were conducted by applying aluminum and CFRTP dissimilar material joining to the doors. Crash analysis at the actual vehicle level was conducted, and it was confirmed that the performance was equivalent to that of the base vehicle. In summary, the results of the evaluation of the component prototypes indicated that the innovative materials and innovative joining technology have high potential for practical application.

7.1 Introduction ISMA conducted part prototyping and evaluated the required characteristics in order to verify the level of the potentials of innovative materials and related technologies, which we have developed in the past ten years, for their practical application. Figure 7.1 and 7.2 show the covered parts. Specifically, application of the innovative

K. Chiba (B) Innovative Structural Materials Association, 1-9-4Chiyoda-Ku, YurakuchoTokyo 100-0006, Japan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 T. Kishi (ed.), Innovative Structural Materials, Springer Series in Materials Science 336, https://doi.org/10.1007/978-981-99-3522-2_7

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Fig. 7.1 Innovative materials/technologies and applicable components

steel sheet to the pillars,1 the TWB2 member manufactured by FSW3 to the B-pillar, innovative aluminum alloy to the front side member4 and side sill,5 innovative magnesium alloy to the hood,6 LFT-D to the floor, CFRP/CFRTP composite panel to the roof, and dissimilar material joining to the multi-material door was studied.

7.1.1 Meaning of the Prototyping Currently, car manufacturers are making cars while conducting simulations. However, the ideal solution with a simulation can be obtained only under ideal conditions. In reality, there are no ideal conditions under which any item can be fabricated, 1

Pillar: The pillars are the frame members of an automobile, and run in the vertical direction in the vehicle. A pillar besides the driver is called the A-pillar or front pillar, and the pillar next to it is called the B-pillar or center pillar. 2 TWB: An abbreviation of Tailor Weld Blank: A blank that is made through welding using a laser or the like with material and thickness combined. 3 FSW: An abbreviation of Friction Stir Welding: One of solid-phase bonding in which a rotating tool with protrusions is pressed against the joint to induce plastic flow, and bonded together as the tool travels. 4 Front side member: A member that is located in the front of the vehicle and runs in the front-back direction to support the engine, etc., and serves to absorb the energy generated in a frontal collision. 5 Side sill: A part that is located at the bottom of the right and left doors and runs in the vehicle’s front-back direction, and serves to absorb the energy generated in a side collision. 6 Hood: Also called a bonnet, this part is located in the front of the vehicle, and covers the engine room.

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Fig. 7.2 Multi-material car body structure (Car body framework)

and thus a component different from the analysis result is produced (Fig. 7.3). In simulations, actual conditions accumulated so far are incorporated to increase accuracy. However, for innovative materials and technologies, actual conditions are unknown, and therefore, prototyping is essential in that it can clarify the actual conditions to increase the accuracy of the simulation in the part deployment in the future, and it can identify events that would not occur in testing using a test piece under the JIS standard. For example, in press forming of the innovative steel sheet, matters such as the understanding of the deformation of the press machine and the mold, effect of the position where the blank is placed, and the coefficient of friction between the mold and the material are involved. For joining, prototyping will enable us to understand the resistance between the electrode and the sheet and between the sheets, and understand the interface (presence/absence and amount of clearance) between adjacent parts. For performance, the boundary conditions (e.g., condition for part securing) vary between the simulation and the experiment in many cases, and a different result may be obtained as a result. Therefore, conducting an experiment and evaluation using a prototype part is essential.

7.1.2 Part Prototyping in Overseas Projects Overseas projects similar to this project include: (i) “ENLIGHT” project conducted in Europe with the aim of developing a technology for an advanced lightweight material as the structural member of electric vehicles, (ii) “Epsilon” project for developing a small electric vehicle, and (iii) “ALIVE” project intended to develop a mass-produced

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Fig. 7.3 Forming accuracy results of actual part and CAE

electric vehicle using a multi-material body. The parts prototyped in the individual projects are as follows. (i) “ENLIGHT” project (Fig. 7.4): a floor made of CFRP, a bumper system made of Al,7 a front member made of CFRP and Al, and a control arm made of CFRP.8 (ii) “Epsilon” project: an engine compartment9 section was prototyped and a collision analysis and experimental evaluation. (iii) “ALIVE” project: rear outer and inner parts of a front side member made of PHS1.8GPa,10 an A-pillar lower outer panel made of DP980MPa,11 a front suspension strut12 made of DP780MPa,13 a roof made of CFRP, an engine compartment, an undercover,14 and a frame15 all made of Al. The collision experiment of the engine compartment in the “Epsilon” project seems to be the only case where the performance of any of those parts is evaluated, 7

Bumper system: A part that is located at the front and rear ends of the vehicle, and absorbs the energy generated in a minor collision. It consists of a beam, and a stay that supports the beam. 8 Control arm: Being one of the suspension parts, it is a suspension link connected with a hinge between the chassis and an upright suspension or a hub that supports the wheel. 9 Engine compartment: A region where the engine is installed in the front section of the vehicle. 10 PHS 1.8 GPa: A hot stamped material with a tensile strength of 1.8GPa. 11 DP980MPa: Dual Phase Steel: A two-phase steel material made of ferrite and martensite, with a tensile strength of 980 MPa. 12 Front suspension strut: A body parts that supports the front coil or shock absorber. 13 DP780MPa: Dual Phase Steel: A tensile strength of 780 MPa. 14 Undercover: A part that is located on the floor of the body to prevent interference with anything on the road and control the air flow. 15 Frame: A member that supports the engine and the suspension, and on which the cabin is installed.

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Fig. 7.4 Prototype parts for “ENLIGHT” projects

and none of the prototyped parts have been evaluated for their performance. In that sense, evaluation of the prototyped parts in various items is essential.

7.2 Organization The organization is as shown in Fig. 7.5 Kobe Steel, Ltd. applied the innovative steel sheet to the A-pillar. For the application of TWB manufactured by FSW to the B-pillar, JFE Steel Corporation manufactured the materials and TWB, and ISMA studied turning it into a part. IHI Corporation and National Institute for Materials Science (NIMS) fabricated a simulated B-pillar that is composed of a steel sheet and CFRTP bonded together. UACJ Corporation studied application of the innovative aluminum to the front side member and the side sill. NIPPON KINZOKU CO., LTD. and Toyota Customizing & Development (TCD) applied the innovative magnesium to the hood. Toray Industries Inc. studied application of the CFRP/CFRTP composite panel to the roof. Nagoya University’s National Composites Center (NCC) studied application of LFT-D to the floor.

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Fig. 7.5 Organization

7.3 Evaluation Items As the characteristics required for practical application, five characteristics: material property, formability, joining, coating and rustproofing, and performance (weight saving) were evaluated for each component as shown in Table 7.1. The following shows the specific evaluation items for each component.

7.3.1 Study of Application of the Innovative Steel Sheet to the A-pillar An analysis and an experiment were conducted in FLD (forming limit diagram)16 and spherical stretch forming17 for formability; adaptability (weldability) to spot welding, FSW, and laser welding for joining; hydrogen embrittlement for coating and rustproofing; and bending strength for part performance in performance. Further, in the study at an actual-car level, full-wrap frontal collision analysis18 and offset collision analysis19 were conducted.

16

FLD: An abbreviation of Forming Limit Diagram. It is a drawing of a press molding with the minimum logarithmic strain and the maximum logarithmic strain at each part being measured, and plotted in a coordinate with the minimum logarithmic strain (ε 2) on the horizontal axis and the maximum logarithmic strain (ε 1) on the vertical axis. 17 Spherical stretch forming: A method of checking the height limit below which a plate formed using a spherical punch is not broken. 18 Full-wrap frontal collision: An experiment in which the vehicle collides with a rigid wall at a velocity of 56 km/h. 19 Offset collision: An experiment in which the vehicle’s 40% offset part collides with a barrier at a velocity of 64 km/h.

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Table 7.1 Necessary characteristics of innovative materials /technologies and their practical application Innovative material/ technology

Applied part

Forming

Joining

Painting and Rust Prevention

Performance

Light weight

Innovative Steel

A-Pillar DOOR beam

FLC, Molding CAE, slidability

Laser, RSW, adhesive

Hydrogen embrittlement

Bending strength (frontal impact)

32%

TWB made by FSW

B-Pillar

Bending, Ball head tension

Strength, speed

Conversion treatment, ED, Hydrogen ingress

Bending – strength (side impact)

Steel/ CFRTP

Simulated B-Pillar

–

Laser, adhesive

Galvanic corrosion thermal stress

Bending 30% strength (side impact)

Innovative aluminum

Frontal side Extrusion MBR/Sill

RSW, FSW

–

Crushing Property/ Bending strength

60%

Innovative magnesium

HOOD DOOR Beam

150 °C temp./RT forming

Dissimilar joint, adhesive

Conversion treatment, Galvanic corrosion

Pedestrian protection , bending strength

45%

Al/CFRTP dissimilar joining

DOOR

CFRTP forming

Dissimilar by FSW

Thermal stress

Side impact

45%

CFRP/ CFRTP panel

ROOF

Composite panel forming

Dissimilar by adhesive

Galvanic corrosion

Internal rigidity External rigidity

50%

LFT-D

FLOOR

LFT-D/ T-RIM Forming

Dissimilar by adhesive

–

Side impact Pole impact

38%

7.3.2 Study of Application of the TWB Manufactured by FSW to the B-pillar An analysis and an experiment were conducted for bendability and spherical stretch forming for formability; welding speed and strength for joining; hydrogen diffusion quantity in the weld and electrodeposition20 adhesion in the FSW region for

20

Electrodeposition: Providing rustproof coating by applying electricity in order to apply the coating evenly and uniformly with the aim of preventing the steel sheet from rusting and the coating from peeling off due to flying stones.

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rustproofing; and bending strength of the part alone for performance. At an actualcar level, Japan New Car Assessment Program (JNCAP) side collision analysis21 and Insurance Institute for Highway Safty (IIHS) side collision analysis22 were conducted.

7.3.3 Matters of Study with a Simulated Pillar to Which Dissimilar Material Joining Between a Steel Sheet and CFRP is Applied The matters studied were laser and adhesion for joining; GC (galvanic corrosion) resistance and thermal stress resistance for coating and rustproofing; and bending strength for performance.

7.3.4 Study of Application of the Innovative Aluminum to the Front Side Member and the Side Sill The matters studied were crushing characteristics, in particular no dispersal and achieving crushing reaction force, for performance.

7.3.5 Study of Application of the Innovative Magnesium to the Hood An evaluation was conducted for non-anisotropy23 for material; formability at normal temperature, comparison with aluminum in FLD, and spherical stretch forming for formability; dissimilar material joining, in particular GC resistance, for joining; simultaneous chemical convertibility24 with steel, aluminum, and magnesium for coating and rustproofing; and pedestrian protection performance25 for performance.

21

JNCAP side collision: Side collision with a moving deformable barrier at a velocity of 55 km/h. IIHS side collision: Side collision with a moving SUV deformable barrier at a velocity of 50 km/h. 23 Anisotropy for material: A characteristic in which the material’s property differs between the rolling direction and the vertical direction. 24 Chemical convertibility: Surface preparation intended to facilitate the adhesion between the metal and the electrodeposition. Formerly, zinc phosphate treatment was the mainstay. But currently, Zr treatment is the mainstay approach. 25 Pedestrian protection performance: An experiment in which collision between a vehicle and a pedestrian is assumed, and the values of damage to the head and legs of the pedestrian are evaluated. 22

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7.3.6 Dissimilar Material Joining Between CFRTP and Aluminum in the Multi-material Door The matters studied were FSSW joining26 between CFRTP and aluminum for joining; GC resistance and thermal deformation for coating and rustproofing; and side collision performance for performance.

7.3.7 Study of Application of the CFRP/CFRTP Composite Panel to the Roof An evaluation was conducted for dissimilar material joining using an adhesive for joining; GC resistance for coating and rustproofing; and in-plane rigidity, which would affect the body rigidity, out-of-plane rigidity, transmitted sound, and impact for performance.

7.3.8 Matters Concerning Study of a Floor Made of LFT-D The matters studied were tensile modulus of elasticity and strength and bending rigidity and strength for material characteristics; minimum sheet thickness for formability; thermal fusion joining for joining; GC resistance and thermal deformation resistance for coating and rustproofing; and in-plane rigidity and pole side collision performance27 for performance. The important issues identified in the evaluations for each component as shown above are as follows: Sect. 7.3.1 hydrogen embrittlement for the innovative steel sheet, Sect. 7.3.3 thermal deformation for the joining between a steel sheet and CFRP, Sect. 7.3.4 crushing characteristics, which are required at a high level for the front side member and the side sill, for the innovative aluminum alloy, Sect. 7.3.5 formability at normal temperature28 and simultaneous chemical convertibility with steel and aluminum for the innovative magnesium alloy, Sect. 7.3.6 thermal deformation, etc. for the multi-material door, Sect. 7.3.7 securing of in-plane rigidity, which would affect the body rigidity for the sandwich roof made of CFRP and CFRTP, and Sect. 7.3.8 side impact and pole side collision resistance for LFT-D. The following explains an outline of the results of studying the application of the individual materials and technologies. Details of the results are described in 26

FSSW: An abbreviation of Friction Stir Spot Welding. A method in which the junction of the workpieces is bonded with frictional heat generated by a rotating tool, and the workpieces are welded together at one spot without having to move the tool. 27 Pole side collision: Side collision with a static pole at an angle of 90° and a velocity of 32 km/h. 28 Anisotropy of the yield stress of magnesium: Yield stress during tension and the yield stress during compression differ; in the case of magnesium, the yield stress during compression is smaller.

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the Volume 2 of “Innovative Structural Materials and Multi-materials—Innovations in Materials, Joining and Design Technologies for Lightweight Transportation Equipment-” (in Japanese) published by Ohmsha Ltd. in June 2023.

7.4 Outline of the Result of Evaluating the Innovative Materials and Technologies 7.4.1 Study of Application of the Innovative Steel Sheet to the A-pillar There was no issue posing a problem in any of the items in formability, joining, and coating and rustproofing. In hydrogen embrittlement in particular, the innovative steel sheet demonstrated a performance equivalent to or higher than a hot stamped member. For performance, the innovative steel sheet achieved the target value in bending strength for part performance with almost the same reaction force in both the analysis and the experiment. Further, in the actual-car level evaluation, the steel sheet achieved a performance equivalent to the A-pillar of Tesla model 3, which was used as the base vehicle, in the results of full-wrap frontal collision analysis and offset collision analysis. (See Figs. 7.6 and 7.7).

7.4.2 Study of Application of the TWB Manufactured by FSW to the B-pillar The TWB demonstrated performance equivalent to that of a laser-welding part in both formability and joining. In joining, the TWB secured a sufficient nugget diameter even if the conditions with a spot-welded member of the same grade fluctuated by ±10%. For rustproofing, electrodeposition adhesion was achieved by removing the burr from the edge of FSW. For bending strength of the part alone in performance, the results of analysis and experiment were almost consistent to achieve the target reaction force. Furthermore, the joint point input was also below the joint strength in analysis, and there was no delamination in the experiment.. The results of the JNCAP side collision analysis and IIHS side collision analysis with the actual car show that the performance was equivalent to the base vehicle and posed no problem in application. (See Figs. 7.8 and 7.9).

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Fig. 7.6 Vehicle crash analysis of A-pillar made of innovative steel (full-wrap crash analysis)

7.4.3 Study of Application of Dissimilar Material Joining Between a Steel Sheet and CFRP to a Simulated Pillar For joining, the materials demonstrated sufficient strength in laser application under appropriate conditions (heat input, welding pressure), and in adhesion. For coating and rustproofing, the materials achieved GC resistance with GF (glass fiber) cloth placed on the surface. For performance, the 3-point bending strength increased to approximately 109% in laser application and to approximately 140% in adhesive application.

7.4.4 Study of Application of the Innovative Aluminum to the Front Side Member and the Side Sill In performance, the crushing characteristics with the component alone were in bellows deformation mode. The result of analysis of the characteristics of the side member alone shows that both the reaction force and the deformation mode were

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Fig. 7.7 Vehicle crash analysis of A-pillar made of innovative steel (offset crash analysis)

Fig. 7.8 Crash analysis of TWB B-pillar by FSW (JNCAP crash analysis)

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Fig. 7.9 Actual vehicle crash analysis of TWB B-pillar by FSW (IIHS crash analysis)

equivalent to that of the base, and the result of analysis of the characteristics of the side sill alone also showed that the reaction force was equivalent.

7.4.5 Study of Application of the Innovative Magnesium to the Hood The result of the evaluation shows that the material has anisotropy in material characteristics. For formability, warm forming at 150 °C was necessary, and it was difficult to secure part accuracy due to thermal deformation. Heating was necessary also in hemming, and the deformation due to this is also a problem. For joining, it is necessary to have a method for inserting an adhesive and sealing the edge as a measure against GC, and to provide surface treatment for the respective regions. For, simultaneous chemical conversion with steel, aluminum, and magnesium can be achieved by controlling the pH value at greater than 4.5 in Zr treatment. For performance, the panel rigidity is lower than aluminum, and in pedestrian protection performance, the deformation is large, and the possibility of interference with parts in the engine compartment is expected to be large.

7.4.6 Application of Dissimilar Material Joining Between CFRTP and Aluminum with the Multi-material Door The evaluation results were as follows. For joining, the strength is increased with aluminum pretreatment and weld bonding. In coating and rustproofing, adhesive insertion and edge sealing were performed as a measure against GC. It became possible to evaluate the endurance reliability of the seal using the change in the amount of the plasticizer as an index. For performance, the crack problem was

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Fig. 7.10 Multi-material door structure and side impact test results

dissolved by changing the door inner structure from the integrated CFRTP structure, specifically changing the periphery to an aluminum part, and the middle section to a CFRTP parts, and joining them by FSW and an adhesive (Fig. 7.10). In addition, in the application of the innovative steel sheet to the door beam, the achieved result exceeds the target. And in the application of the innovative magnesium, cracking occurring on the compressed side was prevented by controlling the deposit through cross section expansion (a problem found in layout) and aging, and optimizing the control of crystal miniaturization. For thermal deformation, the result with CAE and that with a test piece part composed of an aluminum sheet and a CFRTP part joined together were almost consistent. Utilizing this, it was verified that multi-material doors are practically feasible.

7.4.7 Application of the CFRP/CFRTP Composite Panel to the Roof For joining, there was no problem in dissimilar material bonding using an adhesive. For coating and rustproofing, an adhesive was inserted as a measure against GC and there was no problem. As for performance, the results were almost equal in in-plane rigidity, which would affect the body rigidity. In out-of-plane rigidity, the achieved result almost doubled the existing value. For transmitted sound, the approach is considerably effective in the vehicle exterior noise range with rain sound and wind sound. For impact performance, ductile fracture resulted.

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7.4.8 Floor Made of LFT-D For material characteristics, the floor achieved the target values for tensile modulus of elasticity and strength. For formability, the product area and the sheet thickness are proportionate, 2.1 mm at 1 m2 , and 2.6 mm at 1.5 m2 . For joining, thermal fusion bonding was adopted, and for strength, the matrix strength and melting area are adopted. For coating and rustproofing, GFRP was arranged on the surface as a measure against GC, and there was no problem. In pole side collision performance, the cross member with a thickness of 10 mm achieved crushing strength exceeding the target value of 600 kN. As for weight saving, weight reduction of approximately 36% was achieved with the floor and the cross member.

7.5 Social Implementation and Future Prospects The remaining issues for further practical application are as follows: (1) massproduction of the material of the innovative steel sheet, (2) life extension and cost reduction for tools for the TWB by FSW, (3) a non-destructive inspection method for the dissimilar material joining between a steel sheet and CFRP, (4) cost reduction for the innovative aluminum, (5) conversion to isotropic material, formability at normal temperature, pedestrian protection performance, and material procurement for the innovative magnesium, (6) thermal deformation and cost reduction for the multi-material door, (7) edge treatment and cost reduction for the CFRP/CFRTP composite panel, and (8) a method for utilizing recycled carbon fiber (rCf) for LFT-D. The innovative steel sheet, the innovative aluminum, and the CFRP/CFRTP composite panel are components that are very close to social implementation. Those requiring other efforts are the TWB by FSW, the dissimilar material joining between a steel sheet and CFRP, and the multi-material door. Components considered to require further continuation of material and technological development are the innovative magnesium and the LFT-D. Concerning future prospects, the innovative steel sheet is promising since a mass production trial has been conducted. The innovative aluminum has a problem with the Sc cost, but has high potential for practical application since it is likely that the cost will be reduced due to an increase in the amount of the resource excavated in the future. The dissimilar material joining between a steel sheet and CFRP also has high potential since a technology developed with a non-destructive inspection technology for the adhesive can be diverted.

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7.6 Conclusion This chapter explained the study of the practical application of the innovative materials and technologies as summarized below. In developing an innovative material or technology, it is important to identify the needs of automobile manufacturers that will use them, and determine not only the material’s characteristics, but also whether or not it is possible to make a part using it, or in other words, to determine whether or not it is possible to ensure shaping or joining, secure rustproofing performance, and present rigidity or strength performance. And therefore, it is necessary to set a development goal while taking these viewpoints into account. In the future, parts prototyping will be required for the study of practical application of the new materials and the new joining technologies. In recent years, process analysis technologies are advancing, enabling the analysis of press forming and analysis of spot welding and FSW bonding processes, and it has become possible to efficiently pursue member prototyping for new materials. Furthermore, it will become possible to evaluate the practical application of new materials with digitalized member prototyping thanks to further increase in the accuracy of process analysis technologies.

Chapter 8

Recycling and Lifecycle Assessment Toshiyuki Seko, Shu Yamashita, Ken-ichi Shida, and Ichiro Daigo

Abstract Recycling and Life Cycle Assessment (LCA) are important issues for future materials development. Here, in the narrow sense, “recycling” means materials recycling, and is a recycling method in which waste is reused as the raw material for new products. Development of recycling technologies for aluminum alloys and CFRP products was carried out as part of the Project. In substitution of automotive materials for auto body weight reduction, it is necessary to consider the effects on society as a whole. In the Project, we created an LCA model with a system boundary extended spatially and temporally, which can evaluate the environmental, social and economic impacts in Japanese society as a whole up to the year 2050, together with the necessary database, and developed an evaluation tool that demonstrates the model and its database. Technology development related to recycling and LCA is described in detail in this Chap. 8, “Recycling and Life Cycle Assessment (LCA).”

8.1 Recycle—Technical Development for Material Circulation Toshiyuki Seko, Shu Yamashita, Ken-ichi Shida

T. Seko (B) · S. Yamashita · K. Shida Innovative Structural Materials Association, 1-9-4, Chiyoda-Ku, Yurakucho, Tokyo 100-0006, Japan e-mail: [email protected] S. Yamashita e-mail: [email protected] K. Shida e-mail: [email protected] I. Daigo The University of Tokyo, 4-6-1, Meguro-Ku, Komaba, Tokyo 153-8904, Japan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 T. Kishi (ed.), Innovative Structural Materials, Springer Series in Materials Science 336, https://doi.org/10.1007/978-981-99-3522-2_8

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8.1.1 Introduction Promoting the creation of a recycling society is required based on the Sustainable Development Goals (SDGs), and significant elements for creating such a society include resource circulation by recycling. On the other hand, recycling is expected to not only increase the circulation of resources significantly, but also to restrict CO2 emissions and reduce costs. We developed an innovative material for the structural material of transportation equipment (mainly automobiles) and focused on the material recycling of the developed material. Especially since in the material for weight reduction, such as aluminum and carbon fiber, CO2 emissions during manufacturing tended to increase compared with steel material, we aimed to achieve CO2 emissions reduction by developing the recycling technique. In the aluminum recycling development, we developed a new manufacturing process technique, which upgraded low-grade aluminum scrap, difficult to recycle, to high-purity aluminum. Also, in CFRP recycling, we developed a process capable of efficiently recovering recycled carbon fiber (CF) from CFRP mill ends and scraps and a quality evaluation method to expand the use of recycled carbon fiber.

8.1.2 Recycling Technique for Aluminum and CFRTP 8.1.2.1

Aluminum Recycling Technique

Background and Objectives Because of their low melting points, aluminum and its alloys are suitable metal materials for melting and recycling. In Japan, approximately 4 million tons of aluminum products are manufactured annually, 1.8 million tons of which are manufactured from primary aluminum, while the remnants are manufactured from recycled base metal. While the recycled base metal manufactured in Japan is approximately 1.3 million tons, the imported base metal is approximately 0.8 million tons. The energy consumption for the recycled aluminum is as much as 3 to 5% that of the primary aluminum, significantly contributing to saving energy. However, in the outlook for the future, the production forecasting for aluminum raw materials expects that the demand for the wrought material manufactured using high-purity aluminum will increase rapidly. The mainstream of the current aluminum recycling is cascade recycling, which involves a decrease in the purity of aluminum. However, development of an aluminum high-upgrade technique allowing purification of aluminum is required, not cascade recycling. The aluminum high-upgrade technique, an extension of the aluminum electrorefining process using molten salt or ionic liquid, is the same in that the purpose is increasing the purity of aluminum. Against this background, the knowledge obtained

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from the aluminum electrorefining process is significantly important information to establish this technique, but very few examples of such studies have been found. As shown in Table 8.1, research on the electrorefining of scrap aluminum using molten salt was performed by Ishikawa et al. [1] and Schwarz et al. [2] in the 1990s and showed that aluminum with a purity of 99.8% or higher could be obtained at a very high current density. On the other hand, since the temperature of the electrolyte is high at about 1,000 K, it seems a little difficult to implement this technique in present society, where processes under mild conditions are considered essential. Meanwhile, in the 2000s, Reddy et al. reported the following using an AlCl3 -based ionic liquid as an electrolyte: Electrorefining of aluminum was made possible more simply and at a relatively lower temperature (353–413 K) than the process using the molten salt, as mentioned above, scrap aluminum could be electrorefined to a purity of about 98% [3], and the electric power consumption rate was 3.2–6.7 × 103 kWh t−1 , and others (Table 8.1) [3–5]. Since the energy required for manufacturing the primary aluminum using the Hall-Héroult process1 is 13–15 × 103 kWh t−1 [6], the energy consumption is expected to be significantly reduced by using scrap aluminum as the raw material. However, the current density is as much as one-tenth that of the Hall-Héroult process, and productivity is a significant problem. Also, there is little information about the effects of the prolonged operation on obtained aluminum and an electrolyte. Overcoming the challenges mentioned above is required to enhance the aluminum electrorefining process using the AlCl3 -based ionic liquid for aluminum high-upgrade technique capable of continuous production of primary aluminum. Thus, industry and academia are working in cooperation on developing aluminum high-upgrade technique capable of application to aluminum scrap and purification of aluminum. The high-upgrade recycling flow in an automobile, a target for this project, is shown in Fig. 8.1 [7]. Also, investigating electrolyte and developing high-speed electrodeposition technique are essential in developing the high-upgrade technique.

Electrolyte for High-Upgrade Technique The principle of aluminum (Al) electrorefining by the ionic liquid method used in investigating electrolyte is shown in Fig. 8.2 [8]. In Fig. 8.2, the principle of using a diaphragm set in the ionic liquid and between electrodes, removing impurity component dissolved from an Al anode, and depositing Al at a cathode was used. As the electrolyte with low environmental burden and low cost, an electrolyte used lower temperature than 423 K and high ionic conductance was selected. Since the Al deposition potential is considerably less noble2 than the hydrogen-generating 1

The Hall–Héroult process, the only refining method practically applied, is a representative of molten salt electrolysis, which obtains the target material by electrolyzing molten raw material. However, enormous energy is required to extract metallic aluminum from alumina. 2 While a metal with a relatively low standard electrode potential is called a “less-noble” metal, a metal with a relatively high standard electrode potential is called a “noble” metal.

Al2020

23.0SiC–5.78Si–2.31Cu–1.05Fe–0.77Zn–0.38 Mg–0.10Ni–66.22Al

25.07Si–0.345 Mg–0.130Ti–0.08Fe–0.03Zn–0.02Cu–0.008Mn–0.007Ni–0.002Cr–7.425Al

66.7–33.3 mol% AlCl3–[BuMeIm]Cl

66.7–33.3 mol% AlCl3–[BuMeIm]Cl

62.3–37.3 mol% AlCl3–[BuMeIm]Cl

–

98.15

–

99.96 99.86

4.7Si–4.1Cu–0.9 Mg–0.6Zn–0.3Fe–89.0Al

–

4.6 ~ 6.3 3.2 ~ 6.7 4.70 ~ 5.05

376 ± 2 363 ± 3

–

353 ~ 413

973

1023

2

1

Literature

0.02 ~ 0.12 5

0.02 ~ 0.04 4

0.02 ~ 0.08 3

~ 0.5

~1

Purity/ Electrolyte Energy Current wt% temperature/ consumption/ density/ K 10–3 KWht−1 Acm−2

48.1–39.4–5.0–7.5 mol% 1Fe–0.2Si–0.3 Mg–39.6Cu–58.9Al

Anode alloy composite/wt%

44.4–55.6 mol% Lil–NaCl with 3.9 M AlCl3

Electrolyte

Table 8.1 Summary of research on aluminum electrorefining

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Fig. 8.1 Flow of high-upgrade recycling in automobiles [7]

Fig. 8.2 Principle of aluminum electrorefining by the ionic liquid method [8]

potential, aqueous electrolytes cannot be used. Therefore, non-aqueous electrolytes are used. However, volatile organic compound electrolytes are seldom selected in terms of reducing the environmental burden at present. As a result, an inorganic ionic liquid (salt in a liquid state at temperatures exceeding 373 K) with an AlCl3 system as one of the components, organic-based ionic liquid (salt in a liquid state at temperatures below 373 K), and Deep Eutectic Solvent (obtained by mixing organic compound and salt) are selected. These electrolytes are similar in having excellent thermal stability and allowing electrodeposition of Al with high coulombic efficiency. Each has their advantages and disadvantages, and the three types of electrolytes were investigated in parallel to select the optimal one. The basic characteristics of the three electrolytes are shown in Fig. 8.3 [9]. As one of the basic characteristics, the ability of each electrolyte to remove metal elements was investigated. As one of the results, by using an organic-based ionic liquid as an electrolyte, the energy consumption calculated from a cell voltage and a current efficiency changed between 1,530 and 1,780 kWht−1 , and high-purity Al could be obtained at an energy consumption about 10–15% that of 13,000–16,000 kWht−1 , required in the primary aluminum by the Hall-Héroult process.

Fig. 8.3 Basic characteristics of the three types of electrolytes [9]

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High-Speed Electrodeposition Technique In developing the high-speed electrodeposition technique, the methods to improve the electrolysis method (such as bath temperature, current waveform, and stirring method) when using a high-purity (more than 99.9 mass%) Al anode and the electrodeposition rate (surface and thickness directions) by optimizing additives were investigated, by using an organic-based ionic liquid. Based on the investigation results, electrodepositions of Al were performed with targets of an operation temperature (electrolyte temperature) of lower than 150 °C, a maximum current density of 100 mA cm−2 , a current efficiency of more than 90%, and a cell voltage (electrolysis potential) of lower than 2.0 V. The result clarified the following: Since the effect of the additives might decrease over time at an electrolyte temperature of 90 °C, the electrolyte temperature should be 70 °C to perform electrodeposition at the current density of 100 mA cm−2 .

8.1.2.2

CFRTP Recycling Technology

Background and Objectives While CO2 emissions reduction has become a global challenge, carbon fiber reinforced plastic (CFRP), which is expected to significantly reduce weight because of its characteristics of high specific rigidity and specific strength, is now being used in aircraft, vehicles, and others. On the other hand, most of its process mill ends and scraps undergo landfill disposal, and no recycling system for CFRP cyclic use has been established yet. Although Europe, mainly Airbus, has started establishing technologies and supply chains to recycle CFRP, there is no standard approach to evaluate scrap CFRP, and an effective cyclic use system is currently not established. However, the EU aims at landfill zero by 2025 for the process scraps and spent products of carbon fiber and CFRP, and the development of practical CFRP recycling technologies is expected. In the vehicle field, reducing energy consumption during traveling by reducing body weight thanks to CFRP is expected, which is associated with reducing all energy consumption (lifecycle energy) required from vehicle manufacturing to disposal. On the other hand, carbon fiber used in CFRP consumes enormous energy during manufacturing compared with steel (48 MJ/kg for steel and 234 MJ/kg for carbon fiber) [10]. Under these circumstances, establishing the recycling technologies for process scraps and spent products of carbon fiber and CFRP is expected to significantly contribute to reducing the energy consumption rate during the manufacturing of carbon fiber and others and the lifecycle energy. Considering the current situation, this project planned to establish the CFRP recycling technology to recover the carbon fiber from process mill ends and scraps and evaluate the recovered carbon fiber.

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Table 8.2 Comparison of carbon fiber recovery technologies

Current Status of CFRP Recycling Technology The technologies to recycle the carbon fiber (CF) from CFRP are roughly divided into the thermal decomposition method, superheated steam method, normal pressure dissolution method, and subcritical/supercritical fluid method at present. The competent organizations and other features of each recovery technology are shown in Table 8.2. The thermal decomposition method, which thermally decomposes plastic by heating CFRP and recovers carbon fiber, has been developed by many universities, companies, and research institutes and is the process closest to practical applications. The superheated steam method, which thermally decomposes CFRP by using superheated steam, can overheat by the progress of local oxidation in thermally decomposing CFRP in the air atmosphere. However, it can heat a sample uniformly by introducing 100% steam with a high thermal conductivity into a reaction system and prevent deterioration of carbon fiber physical properties due to overheating. Also, as mentioned later, it can apply the treatment to improve the adhesion between fiber and resin by adding N2 or CO2 to the treatment atmosphere after the treatment. The subcritical/supercritical fluid method decomposes high polymers into low molecules by using a unique reaction field in the solvent and cleaving a specific connection. Although a significant amount of research to liquefy CFRP by using subcritical/supercritical solvent and recover carbon fiber has been reported so far

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[11, 12], further reduction of the reaction temperature and pressure is considered to be required for its practical applications. CFRP industrially uses epoxy resin with a basic structure having bisphenol A polymerized with an acid anhydride and includes an ester bond in a molecule. In the normal pressure dissolution method, by using as a solvent the benzyl alcohol in which potassium tripolyphosphate is added as a catalyst, the ester bond is cleaved by an ester exchange reaction with solvent, epoxy resin is liquefied, and carbon fiber can be recovered from CFRP under a relatively mild condition of normal pressure and approximately 200 °C [13]. It must have bonds that are easily cleaved, such as an ester bond, in plastic, and the types of plastic allowing solubilization are limited.

Recovery Technology for Recycled Carbon Fiber Carbon fiber recovery conditions were optimized by applying the superheated steam method as the carbon fiber recovery technology from CFRP and combining the primary treatment (process to remove most of the resin) using superheated steam and the secondary treatment (process to remove resin residue after the superheated steam treatment with oxygen added) for aircraft CFRP scraps, hydrogen tank scraps, and molding remnants/mill ends. By applying the superheated steam method, developed by the Japan Fine Ceramics Center and Takasago Industry Co., Ltd., a carbon fiber recycling process to recover carbon fiber from aircraft and vehicle CFRP scraps and molding remnants and mill ends during LFT-D molding has been developed by the Tokai National Higher Education and Research System, Nagoya University National Composites Center Japan (Nagoya University NCC). The interfacial shear strength between fiber and epoxy resin after the superheated steam treatment is shown in Fig. 8.4 [14]. The result shows that the superheated steam method provided higher strength under appropriate treatment temperatures than the commercially available sizing fiber,3 for which surface treatment was applied by adding N2 to the treatment atmosphere. In other words, it is possible that a technology that removes resin from discarded CFRP with the one-process surface modification and treats the recycled CF surface to improve the adhesive property with resin will be established. The outline of the carbon fiber recycling process applying this superheated steam treatment is shown in Fig. 8.5. First, procured CFRP scraps or molding remnants and mill ends are separated and dismantled; subsequently, they are crushed into size to make it possible to conduct the superheated steam treatment efficiently, and then the superheated steam treatment is conducted. In this regard, the treatment is divided into the primary process, removing most of the resin, and the secondary process, removing the resin residue. For the recovered carbon fiber,

3

Sizing agent is applied to the carbon fiber surface to prevent the fiber from damage and improve adhesion with the matrix resin.

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Fig. 8.4 Carbon fiber-resin interfacial shear strength after superheated steam treatment [14]

physical properties (mechanical characteristics, fiber surface properties, fiber interface structure, adhesion, and impurities/residues screening) were evaluated and fed back to the superheated steam treatment conditions to obtain optimum conditions. The applicability of the recovered carbon fiber to the LFT-D molding, developed by Nagoya University NCC, was examined. Since constant feeding of bulky recycled carbon fiber into a kneading/extruding machine was a technical challenge, the weight feeder (equipment to constantly feed a certain amount of material in a certain time) and the side feeder (equipment to forcibly supply the material fed by the weight feeder to the kneading/extruding machine) were improved. As a result, the stable transfer and feeding of recycled carbon fiber was enabled, as were the subsequent production and molding of the extruded raw material. The generic technology for the process applied to LFT-D molding of recycled carbon fiber was established using the above.

Evaluation Technology for Recycled Carbon Fiber Establishing a base capable of objectively evaluating the quality of recycled carbon fiber is required to promote the use of recovered carbon fiber (recycled carbon fiber). However, since the existing carbon fiber-related evaluation method assumed the new carbon fiber’s properties, it was insufficient to evaluate the quality of recycled carbon fiber. Then, in this project, the National Institute of Advanced Industrial Science and Technology (AIST) developed the technology to evaluate [1] the mechanical

Fig. 8.5 Carbon fiber (CF) recycling process in ISMA

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Fig. 8.6 Fiber bundle test piece

characteristics of recycled carbon fiber and [2] the number of various impurities contained in recycled carbon fiber. In strength tests for carbon fiber, which is brittle and has a broad distribution of fiber strength, Weibull parameter evaluations based on many single-fiber tensile tests have been conducted. However, since the approach is significantly time-consuming, there is a concern about the amount of labor required for the recycled carbon fiber, for which significant variance is anticipated. Then, this time, we developed a tensile test of a fiber bundle with an aligned orientation as a brief fiber strength tests using a number of fibers. The developed fiber bundle test piece is shown in Fig. 8.6 [5]. Although recycled carbon fiber with a certain length often gets tangled, a fiber bundle with an aligned fiber orientation can be produced by combing and removing unnecessary fiber. The elongation of a carbon fiber bundle was extracted from the displacement measured during a tensile test. The equation to express a stress–strain curve converted to strain and behavior is shown in Fig. 8.7 [15]. In a tensile test, the following behavior can be seen: Stress increases linearly at the beginning of a test, and subsequently, stress decreases gradually. The behavior corresponds to the following phenomenon: All fibers increase in length due to the force simultaneously applied at the beginning of a test, and subsequently, fibers gradually fracture by order of strength. Therefore, the mechanical characteristics of the recycled carbon fiber can be evaluated by analyzing the data appropriately. The fiber characteristics could be successfully extracted with good accuracy by the test piece preparation method, the improvement of the measuring system’s rigidity, the compliance correction method to separate the carbon fiber and system displacements, the establishment of the analytical method, and others. The fiber’s elastic modulus can be evaluated from the inclination of linear stress increase at the beginning of a test. Also, the curve after the stress peak is considered to reflect the distribution of fiber strength. In analyzing the strength distribution, attention was paid to the fact that some stress remains after all fibers fracture (stress component of approximately 80 MPa after the

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Fig. 8.7 Fiber bundle stress–strain diagram and proposed equation to express tensile behavior [15]

strain of 0.025 in the example in Fig. 8.7). This was considered as the force resulting from friction generated between surfaces in fractured fibers transferring relatively. Then, the equation in Fig. 8.7 was proposed as an analysis equation considering this factor. The following four parameters are variables: E, fiber elastic modulus; m, Weibull shape parameter; σ0 , Weibull scale parameter; and T, friction parameter. The range of all experimental data is well-fitted, and this equation can determine four parameters with good accuracy. This approach corresponds to conducting tests for several thousands of fibers in one tensile test and significantly improved the efficiency of the fiber mechanical characteristics evaluation.

8.1.3 Representative Research and Development Results 8.1.3.1

Aluminum Recycling

In the recycling technique for aluminum scraps, concerning the “high-upgrade technology” significantly improving aluminum purity, three types of electrolytic solutions were developed, and the optimal electrolytic solution was selected. In addition, by developing high-speed electrodeposition technology and establishing the generic technology for high-upgrade technology, we are optimistic about future technologies.

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CFRP Recycling

The recovery treatment technology for recycled carbon fiber with excellent mechanical characteristics was established for the CFRP scraps for aircraft and vehicles by optimizing the treatment conditions with a connection of the primary process, using superheated steam and removing most of the resin, and the secondary process, removing the resin residue. In addition, a system to constantly feed bulky recycled carbon fiber into an LFT-D molding kneading/extruding machine was developed, and a generic technology for the LFT-D molding process using recycled carbon fiber was established. Furthermore, in the technology to evaluate recycled carbon fiber, a tensile test for a fiber bundle with an aligned direction was developed. Based on the tensile test result, a stress–strain curve of the fiber bundle and the analytical equation to show the behavior were proposed. As a result, the method to evaluate the mechanical characteristics of recycled carbon fiber was established.

8.1.4 Implementation in Society and Vision of the Future 8.1.4.1

Aluminum Recycling

Several challenges must be overcome to implement the developed aluminum highupgrade technology in society. One of the challenges is atmosphere control during aluminum electrorefining. Generally, electrorefining is performed under an atmosphere of high-concentration argon or nitrogen. However, an alternative method must be urgently established since the process complicates work and causes an increase in cost. In addition, improving the electrodeposition rate of aluminum is also an essential challenge. Research and development have been performed for each challenge, and high-upgrade technique has been steadily developed for societal implementation.

8.1.4.2

CFRP Recycling

Future challenges include establishing quality assurance technology (material traceability and standardization) for recycled carbon fiber. Although this project targeted scraps with a clear source, building a system to categorize and grasp the type and characteristics of scraps is required to use the recycled CF recovered from various scraps. Also, a clear solution and a stable supply of scraps are required to conduct business steadily, and establishing a supply chain from upstream to downstream is essential.

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8.1.5 Conclusion The use of materials to reduce weight is expected to increase. Under such a situation, the following is expected: High-upgrade technique for aluminum recycling and the recovery and evaluation technologies for carbon fiber will be developed, and further introduction of materials for weight reduction will lead to the reduction of CO2 emissions and contribute to carbon neutrality by 2050.

8.2 LCA—New Lifecycle Assessment (LCA) Method for Evaluating the Effects of Alternatives to New Materials on the Environment, Economy, and Society Ichiro Daigo, Ken-ichi Shida

8.2.1 Introduction The life cycle assessment (LCA) has been developed to assess the impacts of targeted products or services on the environment from cradle to grave [16, 17]. A U.S. beverage manufacturer conducted the first LCA study to compare beverage containers made from different raw materials and select a container with less impact on the environment in 1969 [18]. The ISO standard was established in the 1990s, and the LCA has become known as a technique to assess the environmental impacts of a product throughout its lifecycles. However, when using it to select materials, the result of the comparative assessment of different materials is not conclusive even with current knowledge [19]. The schematic diagram of the product lifecycle is shown in Fig. 8.8. While the “grave” of products is the end-of-life product treatment (waste management) where functions of products are lost, materials to be recycled are recovered as secondary resources (such as metal scrap) in the waste management and used as raw materials for the following products. The “grave” of materials includes the dispersion into the environment due to wear and corrosion during use, the landfill disposal of ash as the result of incineration of unrecovered end-of-life products, and the direct landfill disposal of mixed waste. Materials with high recycling efficiency go through multiple product “graves” before arriving at the “grave” of materials [20, 21]. Generally, although the “products and services” mentioned above also include materials as one of the products, this assessment considering material recycling efficiency is recognized as a challenge in the LCA [22, 23]. One of the LCA models for vehicles is the GREET (The Greenhouse gases, Regulated Emissions, and Energy use in Technologies) model, provided by the Argonne National Laboratory under the U.S. Department of Energy [24]. Based on this model, the LCA tool capable of assessing energy consumption and environmental impacts

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Fig. 8.8 Schematic diagram of a process flow throughout a product lifecycle, an example of cold rolled steel sheet production, and major input/output materials in a blast furnace process

in the combination of various vehicles and fuels is published. This tool is wellprepared; in particular, the environmental impacts following the lifecycle until fuel combustion are classified by the manufacturing technology and fuel transport route, and data is updated as required. Also, another LCA model for vehicles is the UCSB (University of California, Santa Barbara) Automotive Energy & GHG (greenhouse gases) model provided by World Auto Steel [25]. The UCSB model, which assesses the change in energy consumption and GHG emissions in the vehicle lifecycle by substituting materials to reduce weight, incorporates an LCA tool. Various substitute materials and weights can be selected, and the amount of change by lightweight can be obtained for each life stage, such as the manufacturing, use, and waste stage, as a result. In conducting LCA for substituting materials for one unit of the vehicle, a conventional LCA is useful. However, since vehicles account for a large percentage of the demander’s use of various materials based on their magnitude of production, the impact of both materials before and after substitution on the resource circulation cannot be disregarded if the new material is incorporated in many vehicles. In addition, since the impact on resource circulation by applying the materials seems to occur after the elapse of a vehicle’s service life (approximately 14 years on average), the time gap must also be considered. Furthermore, the following is assumed: Vehicle electrification will be promoted, and the power supply configuration of electricity

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consumed by battery electric vehicles (BEV) will be significantly decarbonized. Therefore, the effects of the weight-reducing materials on reducing environmental impacts during traveling have a considerable influence. Considering the magnitude of the impact of such material substitution on the whole of society, the change in vehicle power trains by 2050, and electricity decarbonization, there are concerns that the traditional LCA assessment results for one vehicle do not necessarily agree with the change in the environmental impacts on the whole of society considering the future in some cases. This project, targeting innovative structural materials for assessment, requires an assessment tool that targets all vehicles, not one vehicle, and considers the future change with time. Therefore, the aim of this research is to develop an LCA assessment model capable of assessing the environmental impacts on the whole of society by 2050 with space and time extended and an assessment tool which the model incorporates.

8.2.2 Inventory Data Required for LCA The LCA describes the “goal and scope definition” as the first of four procedures consisting of the LCA in ISO [26]. The purpose of the LCA is defined, and the system boundary is set in accordance with it [17]. Here, the system boundary is set in various ways depending on the purpose. For example, the schematic diagram in Fig. 8.8 may be set as from resource extraction to waste disposal or from resource extraction to material production. Next, a life cycle inventory analysis is performed as the second procedure. As the example of steel production shows in Fig. 8.8, the material production has various detailed processes. The figure shows the major materials fed in a blast furnace and the products. In the lifecycle inventory analysis, for the processes included in the system boundary, the information (input/output data) of materials and energy fed in each process and materials and energy produced or discharged is referenced, and the value multiplied by the activity of each process, depending on the assessment functional unit,4 is added. For example, when calculating the environmental burdens associated in setting the functional unit of 1 kg of a cold rolled steel sheet in Japan in 2022, the environmental burdens for each upstream process required for 1 kg of production from the cold rolling process are tallied. Here, can an LCA performer conduct the inventory analysis without collecting data for all these calculations? For LCA, representative process input/output databases (inventory DBs) were established in some countries as soon as the method was developed. Similarly, in Japan, the National Institute for Resources and Environment (current National Institute of Advanced Industrial Science and Technology) has maintained and updated the database since establishing [26] the first inventory 4

Functional unit: When conducting the LCA, the functions of products and services for assessment should be identified in setting the objective and scope. The environmental burden as an assessment result is analyzed by the unit quantity of the identified function. This unit quantity of a function is called a functional unit, which should be expressed in a physical quantity and others.

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DB along with the LCA software in 1994. Currently, it is provided as Inventory DB for Environmental Analysis (IDEA) v3.2.0 [27], arranging approximately 4,700 process data and one of the inventory DBs with the largest number of processes in the world. The process data creation method of these inventory DBs, called a process approach, assuming a plant implements the process, identifies, by measurement or stoichiometry, all items brought in from an arrival gate (including electricity, gas, and water with a separate infrastructure) and carried out from a shipping gate (including discharges as sewage or gas with a separate infrastructure), regardless of valuable or inverse onerous contracts, and creates input/output data. For this reason, the input/ output data of this unit process is also called gate-to-gate (G to G). In the LCA, these process data are calculated, going back to the upstream of a supply chain. For example, when performing a calculation for a vehicle, if it is known that 1,000 kg of steel material is purchased and used, the input/output data of the unit process to roll 1 kg of steel material alone is insufficient, and performing a calculation that goes back to iron ore mining is required. Such data prepared as the input/output from the cradle to the gate, tracing a supply chain back to resource extraction, is called the cradle-to-gate (C to G) inventory data. For example, as a part of the output, CO2 emissions are often referenced. The induced amount of specific environmental burden materials in this C to G is sometimes called a CO2 emission factor. IDEA organizes what such C to G data has already been calculated for each item. If the process data is sufficient with a consistent relationship between them, it is easily calculated since a retrogressive calculation can be performed using LCA software. In Japan, in addition to IDEA, the LCA Japan Forum summarizes inventory data provided by each industry association, and inventory data for approximately 550 items is currently published [28]. However, there is also the input–output analysis approach, and the Embodied Energy and Emission Intensity Data for Japan Using Input–Output Tables, 3EID) [29] is renowned in Japan. The input–output analysis provides C to G inventory data, for which environmental burden materials induced by the final demand for each unit of classifications based on the input–output table were calculated through the input– output analysis using the Leontief inverse matrix. Since it is based on the input–output table, in intermediate trades between industries, not only materials directly charged to products, but also the costs required for mechanical equipment, i.e., the environmental burden due to equipment production required for production, are tallied. Generally, in the process-based inventory data, the environmental burdens on equipment used in production, considering it as one unit of products, for instance, assuming maintenance by 10,000 units of production, only one 10,000th of burdens on maintenance are tallied. Therefore, they are considered negligible and tallied in most cases. As a result, the input–output approach is excellent in comprehensibility. On the other hand, since the input–output analysis approach has limitations in dividing classifications in the input–output table, the analysis for a more detailed division is difficult. In this respect, the process-base inventory data has an advantage in that it allows a detailed division by arranging the process data. Co-products are one of the challenges generated in the material production process. This is a process in which products from the process are not limited to

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one type, with multiple products or main products and multiple products produced. It is difficult for each database to describe co-products as they are included in the arrangement of process data. For example, when assuming that product A, one of the co-products of A and B in process α, is used in process β and when going back from process β, tallying up all process α burdens means that extra co-products of B are produced. Even if process α cannot actually produce only product A or B, the process data for producing one of the co-products should be virtually arranged to assess the environmental burden. This isolation of input and output between coproducts is called an allocation. The inventory DB accommodates the process data after input and output are allocated between multiple products (co-products) based on a particular allocation method. For example, in electrolytic copper manufacturing by an electrolysis process in IDEA v3.2.0 [27] (product code: 231112000pJPN), four of “electrolytic copper, copper slime, nickel sulfate, and copper sulfate” are co-products. For example, the electrolytic copper process data specifies it as data after allocation with the allocation criteria. There are also regional differences in the database. It is easy to imagine that there is a difference between the greenhouse gas emissions associated with 1 kWh of electricity in Japan and the emissions in China since the grid mix of power generation differs. Besides electricity, the following can happen: The upstream processes for aluminum raw material differ significantly between Japan, which imports allprimary aluminum, and countries producing it by hydraulic power generation in their countries; the magnesium raw material upstream process significantly differs between the country using mainly the Pidgeon method and the country mainly using the electrolysis method; and transport distances are different even when importing the same item from the same country. Therefore, using the inventory DB depending on an assessment target is desirable. Supply chain globalization is advancing, and there is also the movement of the Global LCA Data network (GLAD), associated with the inventory DB internationally, mainly by the Life Cycle Initiative of the United Nations Environmental Programme (UNEP) [30]. In this research, the power grid mix in an import country significantly affects the result for the aluminum raw materials, whose base metal is produced overseas with a large amount of power consumption. The environmental impact factor of power should be established for each import country to consider the impact. In addition, nearly half of the vehicles produced in Japan, assessment targets in this project, are exported. It applies to the fuel and power consumed at that time. In addition, process data changes with time. Several factors can be considered for this, which include the process efficiency improvement by technical development and the change in the ratio in the case of having provided the same items in different production processes. Particularly, the latter can change in a short period, and electricity is an easy example to understand. Although a change in a short period due to a natural disaster or a significant change of market cannot be predicted, it is desirable in this project to consider a huge change by 2050. Particularly important are decarbonization of the production processes for assessment target materials, fuel conversion associated with the change in vehicle power trains, and electricity decarbonization. The inventory DB for an assessment over such a feature period has not

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been established, and a future inventory DB to cope with multiple scenarios has been arranged in this project.

8.2.3 Lifecycle Inventory Analysis for Materials Finally, the inventory analysis methodology challenges are mentioned separately from the data arrangement. While for the co-products mentioned above, the challenge is allocation where products with multiple functions come from the same process; for the secondary resources recovered from end-of-life products, functions are considered to be provided from the system boundary (the entire product cycle is considered here) of the assessment target. For this reason, since other functions are also provided from the system boundary in addition to the main function provided by a product set as a functional unit, a model to deal with this multi-functionality is required. Also, since the secondary resources recovered from end-of-life products are consumed as raw materials during raw material production, they also relate to the input of the raw material production process. Before explaining the specific methodology of a model relating to the multi-functionality of the secondary resources, it should be understood that the LCA has two different approaches, i.e., the attributional LCA and consequential LCA. Data and the modeling method for multi-functionality vary depending on the applied approach. The broadest possible definitions accepted in recent years are introduced for these two approaches. The attributional LCA (ALCA) is a method to visualize the environmental impact attributed to the current lifecycle of an assessment target product [31]. For this reason, the physical input/output flow associated with the environment for the lifecycle of the assessment target product is described [32], and the environmental impact attributed to the assessment target product is quantified [33]. This is a method by which the portion attributed to the assessment target product is separated from the global environmental burden [33]. For this reason, the system boundary is the product life cycle (or a part of it) and the assessment target’s average geographical and temporal data. The allocated inventory data is used for a co-production process, and all inputs involved in the system boundary are traced back to the upstream processes of the input from the natural sphere, such as resource extraction, and the input and output are tallied. The consequential LCA (CLCA) is a method that quantifies the change in the environmental impact when a change is applied to the current lifecycle of the assessment target product [32]. It describes a flow associated with the environment changed by possible decision-making [34] and quantifies the environmental impact that changes in the lifecycle of the assessment target product [33]. This can also be understood as a method that defines the amount of change of the environmental burden of the entire earth affected by the production, use, and waste of the assessment target product [33]. Therefore, all affected processes should be accommodated in the assessment system boundary, and preventing allocation by expanding the system is desirable for co-production. Also, the data reflecting the impact assumed due to a change should be used for the inventory data used in the calculation. For example,

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linking the average process data with an increase in one unit of raw material consumption leads to the inclusion of the secondary resource in some parts of the raw material. However, when the supply of secondary resources is restricted with an increase in consumption, it is practical to use the production process from the primary resources as the inventory data for the activity of the changed portion. In this way, the CLCA enables various types of modeling in marginal analysis and system extensiveness for used data [35]. However, if obtaining data to quantify is difficult despite system extensivity being desirable, a substitute method for system extensivity has to be selected. The calculation method, including modeling, can be decided depending on the purpose, target, and availability of data. The ALCA often calculates the past and present, and the CLCA often calculates a future change. However, the following are also being discussed: both can apply to the past, present, and future, they can apply to an average analysis and marginal analysis, and they can apply to products and decision-making [35]. Since the view and methodology of the ALCA and CLCA are under discussion in the field, this issue is beyond the scope of this paper. Since this project aims at assessing the impact of material substitution, a consequential approach is essential. On the other hand, applying the attributional approach, the generic product LCA does not consider the multi-functionality due to the recovered secondary resources (metal scraps). Also, for the other secondary resource consumption, it does not trace back up to the process supplying the secondary resources, in the C to G calculation up to raw material production and handles the secondary resources as available without environmental burden. Also, the process to recover and supply the secondary resources is recognized as a process that belongs to the production process used for the previous products, not the production process for the products using the secondary resources. In other words, the product lifecycle, a secondary resource supply process, and the product lifecycle for assessment, a consumption process, are completely separated, and it is called the cutoff approach in discussions of secondary resource handling. By modeling, it is also possible to consider recycling multi-functionality in the attributional approach. As shown in Fig. 8.9, it is an approach that expands the assessment system boundary to the “material lifecycle” involving multiple product lifecycles and divides the environmental burden on material production, recycling, and waste of the entire system, and is called the partitioning approach [22]. For simplicity’s sake, this explanation disregards that the amount of material is depleted due to a process yield for each recycling. It assumes including the material production from natural resources with one-time material recycling, the material production from the secondary resources with N– 1-times material recycling, and one-time final disposal. Assuming the amount of induced environmental burden of each process for one unit X pr , X re , and X w , the amount of induced environmental burden in the material lifecycle is X pr + (N–1) X re + X w . In addition, the burden for one-time use can be quantified by assuming that the function performed by the material at each time of a product lifecycle varies and making allocations based on the function size [22]. However, since the lifecycle up to the final dispersion of future target raw material must be anticipated in the partitioning approach, it was difficult to use it in an actual quantitative assessment, and previous case studies considering a material’s recycling effect have seldom

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used it [36]. The UCSB model mentioned above, the fourth edition, described that the multi-step recycling approach based on the partitioning approach and the consequential system expansion approach based on the avoided burden approach mentioned below could be selected. However, considering the assessment based on the partitioning approach may be defective in its assumption, the fifth edition limited it to the assessment based on the avoided burden approach [25]. It is considered that the uncertainty was recognized as a concern since the lifecycle of the material to be considered in the partitioning approach is prolonged. This project applied the consequential approach and considered the effects of the recovery and consumption of secondary resources. Raw material recycling is conducted by recovering secondary resources from the waste management process for end-of-life products and producing materials using secondary resources. Here, the recovery and material production processes are part of different product lifecycles. The processes of these different product systems distinguished with solid and broken lines are shown in Fig. 8.10a. Since the consequential approach quantifies the change due to being recycled, the cases where recycling is not conducted are assumed, and the changes in various processes due to recycling are identified. In the process assumed where recycling is not conducted, as shown in Fig. 8.10b, it is assumed that end-of-life products are treated, involved raw materials are finally disposed of, raw materials are separately produced from natural resources, and two product systems are independent. Here, comparing when recycling is conducted and identifying the change due to being recycled, the final disposal process and a series of processes from natural resource extraction to material production are avoided, and a material production process using secondary resources is added. The change in environmental effects with the amount of the added process deducted from those avoided processes (generally, the amount of reduction) is called the recycling “avoided burden [23].” The avoided burden is quantified by comparing it with the assumption that recycling is not conducted. Therefore, since recycling is conducted, it means that

Fig. 8.9 Schematic diagram of a material lifecycle involving multiple product lifecycles. For simplicity’s sake, this figure assumes that this material is used without a loss in all processes N times in total

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two product systems already have the avoided burden. The cut-off approach can be briefly explained as follows: While the previous product system (product system on the supply side) has the additional burden due to the avoided burden of final disposal and recycling, the following product system (product system on the demand side) has the avoided burden of a series of processes in the material produced from natural resources. The environmental burden associated with the use of secondary resources is generally estimated as zero, which is because the effect of avoiding production from natural resources is obtained without decision when using secondary resources. Subsequently, to quantify the avoided burden, the flow and the process’s amount of induced environmental burden material are required for producing Y kg of material. Assuming 1 kg of secondary resources are recycled with a yield Y of the material production process, Y kg of materials is provided. The flow and the amount of induced environmental burden materials are shown in Fig. 8.10. Setting the environmental burdens associated with the process producing 1 kg of materials using secondary resources as X re , the burdens in this process can be described as Y*X re . The environmental burdens assuming recycling was not conducted in producing the same Y kg of materials, adding it to the final disposal process X w and setting the induced amount of 1 kg production from natural resource extraction to material production (C to G) as X pr , is Y*X pr . Using these, the avoided burden of the environmental burden due to one unit recycling, LCI avoid , is expressed by Formula 8.1. Here, for metal scraps, even if they are not recycled, since the final disposal of intentionally discarding them is difficult to assume, the dispersion process is considered to be practical. Since dispersion is considered not to add any additional environmental burden, X w can be disregarded, and Formula 8.2 is applied to metal. ) ( LC Iavoid = X w + Y X pr − X r e

Fig. 8.10 Process flow diagram of a when recycled and b when not recycle

(8.1)

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) ( metal LC Iavoid = Y X pr − X r e

(8.2)

The approach using the avoided burden defined based on the consequential approach is called the “avoided burden approach”. As shown in the schematic illustration in Fig. 8.11, the avoided burdens that could be quantified earlier are allocated between the product system before recycling of supplied secondary resources and the product system after recycling of consumed (demanded) secondary resources. Recycling is understandably conducted when there are supply and demand of secondary resources. Here, when recycling one unit additionally, since secondary resources are generated, its supply can be restricted. Similarly, demand can also be restricted due to the insufficient demand for materials using secondary resources. Considering the two conditions as restrictions on the supply and demand side, respectively, the avoided burdens are allocated to the product system on the recycling restriction side in the avoided burden allocation [22, 23]. This is considered as follows: Although recycling was already conducted if additional recycling is restricted, one additional unit of supply or demand on the restricted side formed the additional recycling, compared with where one unit of recycling was not conducted. In other words, the decision-making on the product system on the restriction side contributed to promotion of recycling, which led to avoiding the environmental burden in the entire system boundary. The method to allocating the avoided burdens of secondary resources on the demand side is called the waste mining (WM) method, and the method to allocating the avoided burdens of secondary resources on the supply side is called the end-oflife recycling (EoLR) method. When it is not determined as either, they are allocated to both the supply and demand sides by 50% each, which is called the 50:50 method [22]. For steel, the life cycle inventory analysis method, considering recycling effects,

Fig. 8.11 Flow diagram of three product lifecycles, i.e., a product lifecycle before supplying secondary resources to current products, the product lifecycle in scope, and the lifecycle of the following products consuming (demanding) secondary resources recovered from the current products

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is standardized [37, 38]. Here, the EoLR method is applied based on the closedloop recycling of steel [39, 40]. Equation 8.3 is established by adding the burden associated with scrap consumption to the amount of induced environmental burden in the real process and subtracting the avoided burdens associated with scrap recovery. A standardized method can be expressed in the steel industry by substituting (8.2) for (8.4) in this. In Japan, the inventory data conforming to this standard is arranged, which is available from the Japan Iron and Steel Federation [41]. This example assumes the following: The recycling in consuming scraps and the recycling in recovering scrap are recognized as the same recycling. As a result, the same allocation method was applied. However, the same allocation method is not necessarily applied. metal metal X LC I = X + S · LC Iavoid − R R · LC Iavoid ) ( = X − (R R − S)Y X pr − X r e

(8.3)

) ( ) ( X LC I = X + S · Y X pr − X r e − R R · Y X pr − X r e ( ) = X − (R R − S)Y X pr − X r e

(8.4)

S Scrap ratio during production. RR Scrap recovery ratio from end-of-life products. X The amount of induced environmental burden in the real process. Greenhouse gas emissions through a lifecycle are called the carbon footprint. The environmental information about products through their lifecycles in other various environmental effect regions is called the product environmental footprint (PEF), which is an alternative name for a product LCA result. Figures published as the product environmental footprint are sometimes compared with those of product footprints of similar systems by consumers. For this reason, in the product environmental footprint program, a product category rule (PCR) is prepared for each type of product, and calculation results conforming to the PCR are published as the footprint [42, 43]. In Europe, the Product Environmental Footprint Category Rules Guidance (PEFCR) is published as a guide to develop the product category rules for the product environmental footprint [44]. This guide provides the circular footprint formula (CFF) as a calculation formula considered by the avoided burden approach and the allocation factors to each material used for the product in scope [42, 44]. The allocation factor for the CFF is called the A factor. The values of 0.2–0.8 are recommended, and either a value of 0.2, 0.5, or 0.8 is set [42, 44]. The correspondence to the methods above means the WM method for the A of 1, the 50:50 method for the A of 0.5, and the EoLR method for the A of 0. In most cases, the A factors of the CFF are set as the same A factor for each material, regardless of the product type. The A factor is set as 0.2 for almost all metallic materials. This reasoning for this is as follows: Although the standard for the steel recycling effect [37, 38] is similar to applying the EoLR method since the A was set as more than 0.2 as a principle in the guideline, it is set as 0.2, not 0.

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A logical assessment method for the recycling effect has been developed, and some standards already incorporate the recycling effect. However, there are no logical determination criteria with which to select a recycling effect allocation method or decide the allocation factor, which remains a challenge. As mentioned above, determining which of the supply side or the demand side restricts recycling is required to select the allocation factor. The methods referring to the ratio of a secondary resource price and a natural resource price or the elasticity of a secondary resource price have been proposed so far [22, 23, 45]. However, a determination can rapidly change depending on the assessment time in a method based on an economic value. Accordingly, the results of comparing environmental effects between materials can be reversed depending on the assessment time. Therefore, developing a methodology in which the difference in selecting the allocation factor is difficult to grasp is expected depending on the assessment time. Then, this research has developed a method to select an allocation method for the recycling effect based on technical information [46]. A method to determine recycling restriction by physical quantity, one of the developed elements, has been developed by estimating the supply potential and demand potential of secondary resources based on the material flow analysis (MFA) [47, 48]. In addition, based on the relationship [49] between the material’s chemical composition and material characteristics, such as whether a problem with material characteristics is caused by impurity components possibly contained in secondary resources, by determining whether avoiding it on the supply or demand side is technically possible on the basis of materials science knowledge, a method to select allocation factors has been established.

8.2.4 Representative Research and Development Results This research set an assessment target for all vehicles produced in Japan, which are targets for the innovative structural material assessment, not one vehicle, and has established an assessment method considering a future change with time. Accordingly, arranging data for the assessment was required, and the database was also established. This research has also developed an LCA assessment model capable of assessing the environmental impacts on the whole of society by 2050 with space and time extended and an assessment tool with the necessary database incorporated. The schematic diagram of the entire structure of the developed LCA assessment model is shown in Fig. 8.12. The β version was distributed to the ISMA project members as an Excel calculation tool in 2021, and the release candidate (RC) version as a tool capable of calculating on the web in 2022. An example of the operation screen of the web version assessment tool is shown in Fig. 8.13. The features of the method and data incorporated in this tool and coping with the challenges in the various assessments mentioned above involve the future social scenarios and future inventory data responding to the scenario, the technical scenarios for material production and inventory data responding to it, the future scenario of the

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Fig. 8.12 Schematic diagram of the developed LCA assessment model

Fig. 8.13 Material selection screen of the assessment target in the Web version assessment tool

change in vehicle power trains, overseas inventory data, the material flow analysis (MFA) for the recycling effect, and the establishment of the method to select an allocation method for the recycling effect and the collection of the material knowledge for it.

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8.2.5 Implementation in Society and Vision of the Future This research has developed a method to assess the change in environmental effects of material substitution for vehicle weight reduction. In vehicles, especially following the electrification of power trains, since only traditional fuel consumption and emission regulation cannot assess the major environmental effects, the assessment through the product lifecycle is desired. Such a context, as the PEFCR in Europe introduced in this paper, could also be spread to products other than vehicles. In this trend, to select a material to be used for a long time, depending on its use, the current assessment, which complies only with the current situation, is insufficient, and it is desirable to introduce an assessment method that also considers recycling efficiency in a material assessment. Since the current assessment as is leads to selecting the raw material with poor recycling efficiency but only minor environmental effects during production based on the current technologies, there are concerns that the proper materials will not be selected so that the amount of environmental burden of the whole of society is reduced, from a long-term perspective. The method developed in this research is one of the fair methods applicable to various materials. Although additional information is required in the assessment, these research results are expected to spread and become a part of the future product life cycle assessment standard. Since materials with high recycling efficiency are likely to be selected by applying this method, it is expected that society will change so that materials advancing decarbonization and contributing to the circular economy are selected. This assessment method and tool have been developed based on the current knowledge. However, following the change of awareness of environmental problems in society, the change in vehicle power trains, the trend of power decarbonization, and the development situation of decarbonization technologies may differ from the current forecast. Therefore, the enormous inventory data and social and technical scenarios possessed by this assessment tool in the background are considered to require maintenance, such as data updating.

8.2.6 Conclusion Although the GREET and UCSB models are propagated as a vehicle LCA model, the method developed in this research to assess the change in environmental effects through a product lifecycle by material substitution for vehicle weight reduction succeeded in developing the only method in the world to assess the material substitution effect considering even future situations. Since the method alone was difficult to apply due to the enormous amount of data required for assessment, an assessment tool in which the developed assessment method was incorporated was also successfully developed in addition to the assessment method. The LCA is an assessment method and a communication tool that also uses the result. The assessment results in

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the whole of society obtained by the developed method are expected to help advance discussions involving all stakeholders toward vehicle decarbonization.

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24. M. Wang, A. Elgowainy, U. Lee, A. Bafana, S. Banerjee, P.T. Benavides, P. Bobba, A. Burnham, H. Cai, U.R. GracidaAlvarez, T.R. Hawkins, R.K. Iyer, J.C. Kelly, T. Kim, K. Kingsbury, H. Kwon, Y. Li, X. Liu, Z. Lu, L. Ou, N. Siddique, P. Sun, P. Vyawahare, O. Winjobi, M. Wu, H. Xu, E. Yoo, G.G. Zaimes, G. Zang, Greenhouse gases, Regulated Emissions, and Energy use in Technologies Model® (2021.Net). Computer Software. USDOE Office of Energy Efficiency and Renewable Energy (EERE) (2021). Web. https://doi.org/10.11578/GREET-Net-2021/dc. 20210903.1 25. R. Geyer, User Guide for the University of California at Santa Barbara (UCSB) Automotive Energy & GHG Model 5. (2017). Accessed from https://www.worldautosteel.org/downloads/ ucsb-model-5-user-guide/. Accessed 31 Aug 2021 26. M. Kobayashi, A. Inaba, T. Nakayama, J. Jpn Inst. Energy 73(12), 1075 (1994) 27. AIST (National Institute of Advanced Industrial Science and Technology): Inventory Database for Environmental Analysis (IDEA) version 3.1.0. (2021). Accessed from https://riss.aist.go. jp/lca-consortium/. Accessed 15 May 2022 28. LCA Society of Japan: JLCA database. https://lcaforum.org/database/ 29. K. Nansai, Y. Moriguchi, S. Tohno: Embodied Energy and Emission Intensity Data for Japan Using Input-Output Tables, 3EID. https://www.cger.nies.go.jp/publications/report/d031/jpn/ index_j.htm 30. UNEP Life Cycle Initiative: Global LCA Data network, G. https://www.lifecycleinitiative.org/ resources-2/global-lca-data-network-glad-2/ 31. Y. Kikuchi, J. Life Cycle Assess. Jpn. 18, 11–20 (2022). https://doi.org/10.3370/lca.18.11 32. G. Finnveden, M.Z. Hauschild, T. Ekvall, J. Guinée, R. Heijungs, S. Hellweg, A. Koehler, D. Pennington, S. Suh, J. Environ. Manag. 91, 1–21 (2009). https://doi.org/10.1016/j.jenvman. 2009.06.018 33. T. Ekvall, Attributional and consequential life cycle assessment, in Sustainability Assessment at the 21st century [Internet], eds. by M.J. Bastante-Ceca, J.L. Fuentes-Bargues, L. Hufnagel, F. Mihai, C. Iatu (IntechOpen, London, UK, 2019). https://doi.org/10.5772/intechopen.89202. Accessed 25 Apr 2022 34. M.A. Curran, M. Mann, G. Norris, J. Clean. Prod. 13, 853–862 (2005). https://doi.org/10.1016/ j.jclepro.2002.03.001 35. T. Schaubroeck, S. Schaubroeck, R. Heijungs, A. Zamagni, M. Brandão, E. Benetto, Sustainability 13, 7386 (2021). https://doi.org/10.3390/su13137386 36. J. Liu, I. Daigo, D. Panasiuk, P. Dunuwila, K. Hamada, T. Hoshino, J. Clean. Prod. 349, 131317 (2022). https://doi.org/10.1016/j.jclepro.2022.131317 37. ISO 20915:2018, Life cycle inventory calculation methodology for steel products 38. JISQ 20915:2019, Life cycle inventory calculation methodology for steel products (in Japanese) 39. C. Broadbent, Int. J. Life Cycle Assess. 21, 1658–1665 (2016). https://doi.org/10.1007/s11 367-016-1081-1 40. World Steel Association: Methodology report–Life cycle inventory study for steel products (Belgium, 2011), p. 48 41. The Japan Iron and Steel Federation: Life Cycle Thinking. https://www.jisf.or.jp/business/lca/ lct/index.html (in Japanese). Accessed 15 May 2022 42. European Commission: Results and deliverables of the Environmental Footprint pilot phase. https://ec.europa.eu/environment/eussd/smgp/PEFCR_OEFSR_en.htm. Accessed 15 May 2022 43. CFP program: The list of certified CFP-PCR. https://www.cfp-japan.jp/calculate/authorize/ pcr.php (in Japanese). Accessed 15 May 2022 44. European Commission: Product Environmental Footprint Category Rules Guidance version 6.3 (2018). https://ec.europa.eu/environment/eussd/smgp/pdf/PEFCR_guidance_v6.3. pdf. Accessed 15 May 2022 45. T. Ekvall, Resour. Conserv. Recycl. 29, 91–109 (2000). https://doi.org/10.1016/S0921-344 9(99)00057-9 46. I. Daigo, K. Hamada, K. Takeyama, P. Dunuwila, D. Panasiuk, T. Hoshino, Conditions for selecting an allocation approach of the avoided burden by material recycling in life cycle

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Chapter 9

Review and Future Development Yoshio Akimune

Abstract In this chapter, we will discuss the management of projects that will lead to future prospects, including methods for evaluating the progress of project tasks and methods for accumulating, managing, and utilizing project data and other information that will lead to future development. The evaluation method is the use of the Technology Readiness Level (TRL), and the use of data is the establishment of center functions that connect technical research institutes in each field.

9.1 Evaluation of the Project from the Viewpoint of TRL Yoshio Akimune

9.1.1 Overview of TRL TRL is a method for estimating the degree of maturity of element technologies in a development program, i.e., the “technology readiness level.” It is a methodology which is determined by a Technical Readiness Assessment (TRA) of the concept, technical requirements and proven technological capabilities of the program. TRL is evaluated in 9 levels from TRL 1 to TRL 9, where TRL 9 is the most mature level and TRL 1 is the embryotic level. Evaluation by TRL was proposed in a NASA space development project [1], and in Japan as well, evaluations are carried out using parameters aligned with the directions of projects in Japan carried out by Cabinet Office and the Ministry of Economy, Trade and Industry (METI), which are responsible for National Projects. Although there are several evaluation parameters that could be used in this Project, here, the outcomes of the this Project will be examined using the criteria proposed by Japan’s Council for Science and Technology and Innovation [2] and the concepts used in the “bridging the gap between innovative Y. Akimune (B) Innovative Structural Materials Association, 1-9-4Chiyoda-Ku, YurakuchoTokyo 100-0006, Japan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 T. Kishi (ed.), Innovative Structural Materials, Springer Series in Materials Science 336, https://doi.org/10.1007/978-981-99-3522-2_9

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Fig. 9.1 Evaluation of maturity of technologies by TRL (Technology Readiness Level)

technological seeds and commercialization” [3] (social implementation of research outcomes) of the National Institute of Advanced Industrial Science and Technology (AIST). The concept of TRL is shown in Fig. 9.1.

9.1.2 TRL Evaluation of the Project by Field of Technology Figure 9.2 shows the technology level at the end of this Project, TRL, the manner of proceeding after the Project ends and the technology targets for 2030, which is the target period of the Project, for each of the fields of technology in the Project. Looking at the technologies by technical field, at the end of the Project, “Structural Design” had achieved TRL 5, that is, the stage where the topological optimization technique has been established and can be proposed. With further research, it is considered that this technology can achieve a level in 2030 where it is possible to propose guidelines for design methods taking into account the material composition, multi-material panel combinations and actual joining methods. As mentioned in Sect. 9.5, the research center in charge of this technology is Kyoto University. In “Recycling/LCA,” technology for recycling from aluminum scraps is at TRL 4, and continuing steady acceleration of research leading to mass production is necessary, for example, improvement of the deposition rate. Recycling of carbon fiber was evaluated as TRL 6, but development of the next stage, e.g., a process for recovery from sources other than scrap, is considered important. Although “LCA” has also reached TRL 6 and the basic computational flow and software based on it have been completed, it is necessary to acquire various types of data from actual production processes, take those data back to the companies concerned from the Project, and verify the developed LCA tools by using actual data. As the research center for LCA technologies is the National Institute of Advanced Industrial Science and Technology (AIST), data collection and improvement of the software are considered to be possible.

Fig. 9.2 Status of technologies at end of this Project, expected TRL, manner of proceeding after Project end and technology targets for 2030 for each field of the Project

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In “Welding and Joining Technologies field,” introduction of state-of-the-art equipment in order to develop the basic joining technologies began in the first half of the Project, and development of the technologies necessary for achieving the target of manufacturing actual components was carried out in the second half. At the end of the Project, this technology had reached TRL 6. It has become possible to provide a range of alternatives for welding methods and adhesives for the automobile manufacturing process, and a direction aiming at application to actual processes, actual components and actual automobiles is foreseen. The research center for welding and joining technologies is Osaka University, and the center for adhesive bonding is AIST. In “Measurement and Evaluation field,” measurement by neutron beam and other quantum beam techniques is evaluated as TRL 6, and reliability of iron and steel products is evaluated as TRL 5. A two measurement port for a neutron beam device was completed, and use as an evaluation center is expected to be possible in the future, with AIST as the research center. In reliability of iron and steel products, it has become possible to grasp crack propagation behavior, and the establishment of an evaluation method tied to actual phenomena such as corrosion and stress corrosion cracking is expected. The National Institute of Materials Research (NIMS) will be the research center. In “Iron and Steel Materials,” the target value (strength: 1.5 GPa and elongation: 20%) was achieved from the first half period. After the end of the Project, guidelines for a high strength material manufacturing process at lower cost than commercial products will be presented, aiming at the establishment of a mass production/ material flow that takes recycling and export and import into account. In “Nonferrous Metals,” guidelines for a manufacturing process for high strength aluminum materials at lower cost than commercial products were completed. For magnesium materials, it has become possible to tie strength and other properties to the production process by Materials Informatics (MI). Magnesium is evaluated as TRL 5. The future directions for aluminum materials are confirmation of mass producibility with actual equipment and construction of low cost and decarbonization technologies. In the case of magnesium materials, study of anticorrosion guidelines and the material flow are desired for realizing mass production. The research center for nonferrous metals will be AIST. The center for “CFRTP” is the Nagoya University National Composites Center Japan (Nagoya University NCC). As a positive outlook has been obtained for trialmanufacture of parts with large surface areas, uniform performance, etc., this technology is evaluated as TRL 5. The direction for the future is development of technologies leading to low cost and uniform performance even when manufacturing complex shapes. “Carbon Fiber field” is evaluated as TRL 4 because this technology is still in the stage of demonstration of basic unit processes (direct flame-retardant spinning process, microwave carbonization technology). The Innovative Structural Materials Association (ISMA) has allowed the participating companies to take sample materials back, and it is assumed that this will lead to practical application through efforts to achieve low cost and low CO2 by continuous production in a future National Project.

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9.1.3 Recommendations for the Future The research outcomes in each field of the ISMA Project were evaluated in terms of their Technology Readiness Level (TRL). Many companies and universities participated in each theme, and evaluations were carried out considering the final targets (TRL; 9 levels) in the respective fields. There are many complex elements when using this technique to compare progress between fields, including differences in the targets from the standpoint of production of materials, components and products, and in the promotion of the Project, differences in approaches to technology development, such as collaboration in specific processes and competition between companies in production processes. Therefore, we recommend setting clear evaluation criteria for processes and the process as a whole, and carrying out repeated evaluations each year from the start of the Project.

9.2 Future Development by Formation of Centers by Research Field Yoshio Akimune

9.2.1 Introduction This Project conducted research in the fields of lightweight materials technologies (lightweight metal materials, carbon fiber reinforced plastic (CFRP)), dissimilar material adhesion and welding and joining technologies, material structural analysis and evaluation technologies, and material and product LCA technologies. To ensure the continuing development of these fields after the end of the Project, it will be important to promote the use of the project data and the development of technologies obtained in the respective fields, together with other related technology development. Therefore, preparations were made to construct “Field Centers” (research bases by field of research) and a “Multimaterial Collaborative Research Hub” that will enable collection management of the project data, etc. and its utilization for the future. In the Project, efforts were made to increase common core technologies toward the creation of Field Centers, including the construction of a large number of devices in the common core technology area. Much knowledge was acquired by using those devices, and rules were created to make it possible to use these facilities at each Center. Since it is also possible to verify the effectiveness of technologies by use in actual company products and create supplementary research by using these facilities, regardless of whether the user is a Project participant or not, it is expected that this can be used synergistically in future operation as Technical Field Centers.

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9.2.2 Features of Technical Field Centers In achieving lighter weight, longer life and greater strength and toughness in structural materials, it is indispensable to establish the reliability of the individual materials, centering on lightweight materials, and effective evaluation technologies, and to actively promote Multimaterialization of components by combining the optimum materials in the optimum parts. In the Project, technology development contributing to these aims was promoted in a unified manner. As described in Sect. 9.1, results showing large improvement in TRL were achieved in all fields, and the research organizations that will become the Centers for the development of those technologies in the future were also introduced. In addition, a new mechanism for accumulating and utilizing data, etc. was also established in order to effectively utilize the precious data, etc. created through these technology development efforts even after the end of this Project. Concretely, this means the construction of “Technical Field Centers” for each research field and a “Multimaterials Collaborative Research Hub” which will make it possible to accumulate, manage and utilize data, etc. even after the end of the Project.

9.2.3 Requirements for Technical Field Centers [4] The following may be mentioned as the functions which Technical Field Centers should possess in order to ensure that the outcomes achieved by the activities of the Centers can be utilized and applied widely and continuously. However, the functions mentioned here are after all only secondary functions for wide, continuous use of the outcomes of Center activities. . Promotion of the use and application of research outcomes of the Center and Center resources . Long-term organization, maintenance and management of Center resources . Collaboration and integration with other fields . Securing and training human resources . Establishment of financial soundness (income/cost balance) For a Technical Field Center to progress and develop while continuing to fulfill its social contribution responsibilities in a sustainable manner, it is necessary to regard the operation of the Center as one business, formulate a management strategy, and renew that strategy daily in response to environmental changes. However, in the initial stage, it will be necessary to provide the expected functions one by one. For this, it is assumed that the Center must unavoidably work toward a stable structure in the initial stage by assigning an order of priority to the above-mentioned concern issues. Specifically, the first priorities are to begin from “Establishment of financial soundness” and “Long-term improvement, maintenance and management of Center resources,” followed by “Securing and training human resources” and “Promotion

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of the use and application of research outcomes of the Center and Center resources,” and finally “Collaboration and integration with other fields.” By addressing these five concern issues in this order, it will be possible to construct a “Multimaterials Collaborative Research Hub” that can continuously accumulate, manage, analyze and utilize data, create reliable material design tools, develop evaluation/analysis techniques, and carry out international standardization even after the conclusion of the ISMA Project.

9.2.4 Construction of Technical Field Centers by Public Organization Project Participants [5, 6] Preparations will be made aiming at the construction of a “Multimaterials Collaborative Research Hub” that can accumulate, manage, analyze and utilize data even after the conclusion of the Project through collaboration by the respective Centers which are promoting technology development in the areas studied by this Project, i.e., lightweight materials (lightweight metal materials, carbon fiber and carbon fiber reinforced plastic (CFRP), etc.), adhesive bonding, joining technologies for dissimilar materials, material structural analysis and evaluation technologies, material and product LCA technologies, etc. The outcomes, etc. of this research and development will be actively dispatched through research presentations, lectures, contribution of papers, etc. (Fig. 9.3). For the purpose of establishing and improving joint facilities that will enable data creation and use, a common infrastructure for creating high quality data and data structures will be developed, and data-specialist human resources and engineers will be trained and maintained. To form a core center and network for material data, formation of a Technical Field Center that accumulates and manages data and

Fig. 9.3 System of field centers and multimaterials hub

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Fig. 9.4 Functions of centers

data structures and collaboration between Centers will be promoted. This Center is also expected to be given functions for utilizing the knowledge possessed by the Centers to respond to new needs from companies and research institutes other than the actual implementing organizations (Project participants), and discovering new research fields based on those needs (Fig. 9.4). The main organizations of the Centers to be constructed in each field and the Multimaterials Collaborative Research Hub are as follows. Fundamentals of Structural Design Technology Center (Kyoto University) Welding and Joining Technology Center (Osaka University) Steel Reliability Evaluation Center (NIMS) CFRP Reliability Evaluation Center (Nagoya University National Composites Center (NCC)) Nonferrous Metals Reliability Evaluation Center (AIST) Adhesive Technology Center (AIST Tsukuba) Quantum Beam Measurement Center (AIST) LCA Evaluation Technology Center (AIST) Multimaterials Collaborative Research Hub (AIST). Among these, the AIST Multimaterials Reliability Center will be established for the fields of Nonferrous Metals Reliability Evaluation, Adhesive Technology, Quantum Beam Measurement, and LCA Evaluation Technology.

9.2.5 Future Outlook When field centers for science and technology remain in the future after the completion of a National Project, the groups of devices created in that project were excellent in comparison with those that exist in other research centers around the world, and the researchers and the form/way of conducting research were fully committed to excellence. In this Project, many groups of equipment were created in the first half

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of the Project, and materials research and data accumulation using those equipment were carried out in the second half. Since the equipment, researchers and state of research in the ISMA Project were all top-level, the Technical Field Centers are also expected to be able to contribute to research in the future after the completion of the Project.

References 1. J.C. Mankins, Technology Readiness Levels A White Paper (Office of Space Access and Technology, NASA, 1995) 2. Distributed document 8, Council for Science, Technology and Innovation 4th Basic Plan Expert Panel (2015) 3. Ministry of Economy, Trade and Industry, Review of the initial plan on the organization and general tasks of the National Institute of Advanced Industrial Science and Technology at the end of mid-target period (2014) 4. Trend survey of research center activities on multimaterial reliability design technology report, Nippon Steel & Sumikin Research Institute Corporation, vol. 2 (2019). pp.7–45 5. New Innovative Structural Materials Research and Development: Presentation outline document on FY2021 Annual Debriefing Session, 42 Trend survey research on material/joining technology/ III. Canter for data utilization (tentative) plan (2022), pp. 92–93 6. New Innovative Structural Materials Research and Development: Presentation outline document on FY2021 Annual Debriefing Session, 42 Trend survey research on material/joining technology/ III. Canter for data utilization (tentative) plan/1) AIST multimaterial reliability center (2022), pp. 94–95