Reinforcing and Detailing of Thin Sheet Metal Using Wire Arc Additive Manufacturing as an Application in Facades 3658415398, 9783658415396

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Mechanik, Werkstoffe und Konstruktion im Bauwesen | Band 68

Christopher Borg Costanzi

Reinforcing and Detailing of Thin Sheet Metal Using Wire Arc Additive Manufacturing as an Application in Facades

Mechanik, Werkstoffe und Konstruktion im Bauwesen Band 68 Reihe herausgegeben von Ulrich Knaack, Institut für Statik und Konstruktion, Technische Universität Darmstadt, Darmstadt, Germany Jens Schneider, Institut für Statik und Konstruktion, Technische Universität Darmstadt, Darmstadt, Germany

Johann-Dietrich Wörner, Institut für Statik und Konstruktion, Technische Universität Darmstadt, Darmstadt, Germany Stefan Kolling, Fachbereich Maschinenbau & Energietechnik, Technische Hochschule Mittelhessen, Gießen, Germany

Institutsreihe zu Fortschritten bei Mechanik, Werkstoffen, Konstruktionen, Gebäudehüllen und Tragwerken. Das Institut für Statik und Konstruktion der TU Darmstadt sowie das Institut für Mechanik und Materialforschung der TH Mittelhessen in Gießen bündeln die Forschungs- und Lehraktivitäten in den Bereichen Mechanik, Werkstoffe im Bauwesen, Statik und Dynamik, Glasbau und Fassadentechnik, um einheitliche Grundlagen für werkstoffgerechtes Entwerfen und Konstruieren zu erreichen. Die Institute sind national und international sehr gut vernetzt und kooperieren bei grundlegenden theoretischen Arbeiten und angewandten Forschungsprojekten mit Partnern aus Wissenschaft, Industrie und Verwaltung. Die Forschungsaktivitäten finden sich im gesamten Ingenieurbereich wieder. Sie umfassen die Modellierung von Tragstrukturen zur Erfassung des statischen und dynamischen Verhaltens, die mechanische Modellierung und Computersimulation des Deformations-, Schädigungs- und Versagensverhaltens von Werkstoffen, Bauteilen und Tragstrukturen, die Entwicklung neuer Materialien, Produktionsverfahren und Gebäudetechnologien sowie deren Anwendung im Bauwesen unter Berücksichtigung sicherheitstheoretischer Überlegungen und der Energieeffizienz, konstruktive Aspekte des Umweltschutzes sowie numerische Simulationen von komplexen Stoßvorgängen und Kontaktproblemen in Statik und Dynamik.

Christopher Borg Costanzi

Reinforcing and Detailing of Thin Sheet Metal Using Wire Arc Additive Manufacturing as an Application in Facades

Christopher Borg Costanzi München, Germany

Vom Fachbereich 13 – Bau- und Umweltingenieurwissenschaften der Technischen Universität Darmstadt zur Erlangung des akademischen Grades eines Doktor-Ingenieurs (Dr.-Ing.) genehmigte Dissertation von Master of Science Christopher Borg Costanzi aus Malta 1. Gutachten: Prof. Dr.-Ing Ulrich Knaack 2. Gutachten: Prof. Dr.-Ing Oliver Tessmann Tag der Einreichung: 13.07.2022 Tag der mündlichen Prüfung: 12.09.2022 Darmstadt 2022

ISSN 2512-3238 ISSN 2512-3246 (electronic) Mechanik, Werkstoffe und Konstruktion im Bauwesen ISBN 978-3-658-41539-6 ISBN 978-3-658-41540-2 (eBook) https://doi.org/10.1007/978-3-658-41540-2 Springer Vieweg © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 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 translation, 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 Vieweg imprint is published by the registered company Springer Fachmedien Wiesbaden GmbH, part of Springer Nature. The registered company address is: Abraham-Lincoln-Str. 46, 65189 Wiesbaden, Germany

Zusammenfassung Plattenmaterial ist seit langem Teil der Erzählung, die die Entwicklung der Menschheit beschreibt. Die Festigkeit, die Leichtigkeit und die relative Einfachheit, mit der das Material geformt werden konnte, machten es zu einem attraktiven Material sowohl für die Verzierung von Schmuck als auch für den Schutz von Rüstungen (und Waffen) bei Auseinandersetzungen zwischen Zivilisationen. Seine Anwendung als Fassadenmaterial, in deren Kontext diese Arbeit steht, kam jedoch erst nach der industriellen Revolution richtig in Schwung. Die Entwicklung von Technologien wie der hydraulischen Presse und dem Bessemer-Verfahren - einer kostengünstigen Methode zur Massenproduktion von Stahl führte dazu, dass Metall in der Automobil- und schließlich auch in der Luft- und Raumfahrtindustrie, im Schiffsbau und im Fassadenbau immer häufiger eingesetzt wurde. Der kontinuierliche technologische Fortschritt führte unweigerlich zu neuen Innovationen, darunter Computer Aided Design (CAD), computergestützte Konstruktionswerkzeuge und neuartige Fertigungstechniken, einschließlich der additiven Fertigung (AM). Wir befinden uns heute in einer historischen Situation, in der mehrere parallele technologische Entwicklungen zusammenfallen, wodurch sich neue und aufregende Möglichkeiten ergeben, die Technologien noch weiter voranzutreiben. Im Bereich der Fassaden und des Bauwesens hat dies dazu geführt, dass immer mehr maßgeschneiderte, geometrisch komplexe Bauelemente entworfen und hergestellt werden, weg von der Massenproduktion von Bauelementen, die für die industrielle Revolution charakteristisch war. Dies führt natürlich zu einer zunehmenden Komplexität der Gebäudehüllen. Die in dieser Arbeit beschriebenen Forschungsarbeiten befassen sich mit der Nutzung der jüngsten technologischen Entwicklungen, insbesondere der additiven Fertigung von Metallen (AM) und der computergestützten Konstruktion (CAD), als Mittel zur Herstellung von Fassaden mit komplexer Geometrie, die aus dünnen Blechen bestehen. Das Projekt konzentriert sich insbesondere auf die Versteifung von 0,75 mm dickem Stahl - einem flexiblen und leicht verformbaren Material - und die Herstellung von Fassadendetails mit Hilfe des Wire Arc Additive Manufacturing (WAAM). Dies wird durch das robotergestützte Schweißen von Material direkt auf vorgeformte Bleche erreicht. Während sich die Forschung im Bereich Wire Arc Additive Manufacturing (WAAM) schon lange vor dem Beginn der Forschungsarbeiten entwickelt hat, steckte die Kombination mit dünnen Blechen noch in den Kinderschuhen. Der hohe Wärmeeintrag, der von Schweißgeräten erzeugt wird, und die geringe Steifigkeit des Blechmaterials führen zu zahlreichen Problemen, die es zu lösen gilt; das kritischste davon ist der Aufbau von inneren Eigenspannungen und die daraus resultierende Verformung des Materials. Dieses Problem wird noch deutlicher, wenn mehrere Lagen Schweißmaterial auf ein einziges

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Blech aufgebracht werden sollen. Ursprünglich wurde ein robotergesteuertes MIG/MAGSchweißverfahren eingesetzt. Aufgrund der hohen Wärmezufuhr, die mit diesem Verfahren verbunden ist, wurde jedoch das Fronius Cold Metal Transfer (CMT)Schweißverfahren eingesetzt. Das für die Automobilschweißindustrie entwickelte CMTVerfahren ermöglichte eine weitaus bessere Kontrolle über die einzelnen Schweißnähte, was schließlich zur Herstellung einer Reihe von metallbedruckten Prototypen führte. Um die Verformung während der Fertigung zu minimieren, werden die in der Automobil- und Schiffsbauindustrie bereits etablierten Schweißtechniken (intermittierendes Schweißen) mit digitalen Entwurfswerkzeugen kombiniert, um alternative Strategien für das Schweißen von Freiformblechen vorzuschlagen. Dem ging eine Schweißparameterstudie voraus, mit der ein akzeptables Schweißverfahren für den Druck auf das dünne Material ermittelt wurde. Die Studien wurden zunächst an ebenen Blechen durchgeführt, um die Wiederholbarkeit zu gewährleisten und externe Faktoren (wie z. B. das Schweißen an Schrägen) zu reduzieren, wobei durchgängig eine Elektrode mit 1 mm Durchmesser verwendet wurde. Sobald Parameter und Strategien gefunden waren, die für ebene Elemente akzeptable Ergebnisse lieferten, wurden sie auf vorgebogene Bleche übertragen. Dies war jedoch nur auf einfach gekrümmte Geometrien beschränkt. Dies eröffnete neue Probleme, insbesondere die Digitalisierung des Blechs, die erforderlich war, um genaue Schweißpfade entlang einer gebogenen Oberfläche zu erhalten, die Erzeugung von Werkzeugwegen sowie das Drucken mehrerer Materialschichten auf einer Schräge. Verschiedene Scantechniken wurden auf ihr Potenzial für die Erstellung digitaler Modelle von Blechen sowie für die Implementierung eines automatisierten Scan-to-Produktion-Workflows untersucht. Erste Studien mit Photogrammetrie, LIDAR und Structured Light Scanning waren nicht vielversprechend; die größten Probleme ergaben sich aus dem Reflexionsvermögen des Blechmaterials sowie dem mühsamen Prozess der Umwandlung von Punktwolken in brauchbare Daten. Daher wurde ein robotergestütztes Tastverfahren gewählt, das sowohl das Problem der Reflektivität als auch der Integration in einen Design-to-Produktion-Prozess löste. Insgesamt sollte diese Arbeit nicht alle Probleme im Zusammenhang mit dem Schweißen von Dünnblech lösen, sondern vielmehr als erste Iteration für zukünftige Forschungen zu diesem Thema dienen. Am Ende der Arbeit wird eine Zukunftsvision vorgestellt, sowohl im Hinblick auf Prozesse, die im Rahmen der Arbeit nicht gelöst werden konnten, als auch im Hinblick auf mögliche zukünftige Anwendungen.

Abstract (EN) Sheet metal has long been part of the narrative defining the progression of humankind. The strength, lightness and relative ease in which the material could be formed made it an attractive material for both the adorning of jewelry and providing protective armor (and weaponry) during clashes between civilizations. However, its application as a façade material, the context in which this thesis is placed, really took off after the industrial revolution. The development of technologies such as the hydraulic press and the Bessemer Process – an inexpensive method of mass-producing steel – meant that metal would eventually grow to be more widely adopted in the automotive and, eventually, aerospace, ship building and façade construction industry. The continued progression of technology inevitably led new innovations, amongst which were Computer Aided Design (CAD), computational design tools and novel fabrication techniques, including Additive Manufacturing (AM). We are now in a position in history where multiple parallel technological developments have coincided, giving rise to new and exciting possibilities to push technologies even further. In the context of facades and construction, this has resulted in the designing and manufacturing of more bespoke, geometrically complex building stock; shifting away from the mass-produced building stock elements which were characteristic of the industrial revolution. This, of course, leading to an increasing complexity of building skins. The research described in this thesis proposes the exploitation of recent technological developments, in particular metal Additive Manufacturing (AM) and Computer Aided Design (CAD), as a means of fabricating complex-geometry facades consisting of thin sheet metal. In particular, it focuses on stiffening freeform 0.75mm gauge steel - a flexible and easily-deformable material – and providing facade connection details by means of Wire Arc Additive Manufacturing (WAAM). This is achieved by robotically welding material directly onto pre-formed sheet metal. While research in Wire Arc Additive Manufacturing (WAAM) has been developing well before the initiation of this research, its combination with thin sheet metal was still in its relative infancy. The high heat input generated from welding devices, and the low stiffness of sheet material results in numerous issues to be solved; the most critical of which being the buildup of internal residual stresses and the resulting deformation of material. This issue becomes more pronounced when multiple layers of weld material are to be deposited onto a single sheet of metal. Initially, a robotically-controlled MIG/MAG welding process was adopted. However, due to the high heat input associated with the process, a Fronius Cold Metal Transfer (CMT) welding process was used. Developed for the automotive welding industry, CMT allowed for a far greater control on the individual welding seams

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Abstract (EN)

which, ultimately, resulted in a number of metal-printed prototypes being produced. In order to minimize the deformation during manufacturing, welding techniques already established in the automotive and ship-building industry (intermittent welding), are combined with digital design tools to propose alternative strategies for welding free-form sheet metal. This was preceded by a welding parameter study, used to determine an acceptable welding process for printing on the thin material. The studies were initially carried out on planar sheets of metal in order to allow for repeatability and reduce external factors (such as welding on slopes), using a 1mm diameter electrode throughout. Once parameters and strategies were found which gave acceptable results for planar elements, they were transferred to pre-bent sheets of metal. However, this was only restricted to single-curvature geometries. This opened up new problems, particularly the digitization of the sheet metal which was required to obtain accurate welding paths along a bent surface, generation of toolpaths as well as printing multiple layers of material on a slope. Different scanning techniques were explored for their potential in obtaining digital models of sheet metal as well as implementing an automated scan-to-production workflow. Initial studies using Photogrammetry, LIDAR and Structured Light Scanning were not promising; the largest issues arising from the reflectivity of the sheet material as well as the cumbersome process of converting point clouds into useable data. Therefore, a robotic touch sensing method was adopted, which solved both the issue of reflectivity and integration into a design-to-production process. Overall, the thesis wasn’t intended to solve all the issues relating to welding thin sheet metal, rather it is meant to serve as the first iteration into future research on the topic. The end of the thesis presents a future vision, both in terms of processes which were not able to be solved during the thesis as well as potential future applications in the built environment.

Preface Additive Manufacturing (AM) has had an exciting history of development; it is the cumulative knowledge arising from artists, inventors and engineers. Although at the time of writing we are witnessing the technology become more commonplace in the built environment, the first structures were actually 3d printed as far back as 1941, when William Urschel first experimented with the controlled deposition of cementitious material. Even more exciting are the early patent drawings by Ralph Baker who, in 1920, proposed the use of a welding device to build up three dimensional structures in metal. One can only speculate what these early innovators must think of AM today; from fully-printed steel bridges to printing of bio-composite materials, AM surely has had a profound impact on our built environment. Apart from AM, advances in computational design tools have also changed the way we design and construct buildings. Frank Gehry famously adopted the use of CATIA – a design modelling tool used by aerospace engineers – in order to realize his early visions of architecture that wasn’t bound by linear constraints, rather, he celebrated the curve. We are now in a position where architects, engineers and designers are able to merge these two worlds; to compliment the freedom of fabrication brought about by AM with the freedom of expression enabled through advanced CAD software packages. This thesis is positioned precisely at this intersection; serving to ask – we have the design tools; we have the fabrication technology; how do we combine the two for the benefit of the building industry? I began this journey with my master’s degree at TU Delft, questioning alternative methods for 3D-Printing concrete structures [1], and am continuing to do so with this research, speculating on how to use the new tools available at our disposal to generate novel applications in the built environment. The research is directed towards Wire Arc Additive Manufacturing (WAAM) – a metal welding AM process sketched out by Ralph Baker some 100 years ago. While at the time of writing, it is for the most part used to fabricate entire structures in metal, this thesis focuses on its combination with existing building stock. More specifically, it focuses on the use of the technology as a means of adding material to curved sheets of metal. During a WAAM process, a sacrificial base metal material is required in order to initiate a welding arc. After printing is completed, the object is generally cut away and either discarded or cleaned up and re-used. The research focuses on using thin sheet material as a non-sacrificial base material. In that respect, any printed additions would form part of a finalized part. The motivation in using thin sheet specifically is that, while it is certainly easy to form, it is also a material which lacks stiffness due to its thickness. In industry, this is usually accounted for by using elements such as stringers. However, in the context of free-

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form facades, where multiple parts must come together in three-dimensional space, AM offers potentials for printing stiffening elements directly onto a surface. The same problem may be contextualized to façade connections. In the majority of instances, façade details are resolved using standardized and mass-produced elements. However, when façade skins become more organic and geometrically-complex, so do the connections between the façade and sub-structure. This often leads to either using a combination of multiple standardized components which are fitted together, or completely customized detailing. It is in this small niche in facades, that is, where there is a need for both mass-production and mass-customization, that AM could prove to be a viable production technique.

Table of Contents 1. 1.1 1.2 1.3 1.4 1.5 1.6

2.

Introduction.......................................................................................................3 Wire Arc Additive Manufacturing and Mass Customization..........................................9 WAAM and complex-geometry facades .....................................................................14 Research Framework.................................................................................................17 Motivation ..................................................................................................................18 Research Questions ..................................................................................................19 Structure ....................................................................................................................20

Sheet Metal as a Material ...............................................................................23

2.1 Production of Sheet Metal..........................................................................................26

2.1.1

From raw material to a blank billet ..........................................................26

2.2 Shaping into thin sheet material .................................................................................28 2.3 Sheet metal facades ..................................................................................................29

2.3.1 2.3.2 2.3.3 2.3.4 2.3.5

Standing Seam and Snaplock Cladding ...................................................31 Shingle and Flat-lock facades ..................................................................33 Cassettes ..................................................................................................34 Alucobond Facades..................................................................................35 Custom/Bespoke Systems ........................................................................36

2.4 Sheet Metal forming Techniques ...............................................................................37

2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 2.4.6 2.4.7 2.4.8 2.4.9

3.

Hammering ..............................................................................................37 Sheet bending. .........................................................................................38 Sheet Spinning .........................................................................................40 Deep drawing ...........................................................................................40 Stretch Forming .......................................................................................42 Multiple Point Forming (MPF) ................................................................43 Hydro Forming ........................................................................................44 Incremental Sheet Forming (ISF) ............................................................44 Bead Rolling ............................................................................................45

Additive Manufacturing .................................................................................47

3.1 Additive Manufacturing Processes .............................................................................48

3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.1.6 3.1.7

Material Extrusion (ME)..........................................................................48 Vat Polymerization(VP) ..........................................................................49 Material Jetting (MJ) and Binder Jetting (BJ) .........................................50 Powder Bed Fusion (PBF) .......................................................................52 Directed Energy Deposition (DED) .........................................................52 Laminated Object Manufacturing (LOM)................................................53 Discussion ...............................................................................................54

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3.2 Additive Manufacturing of Metals ...............................................................................55

3.2.1 3.2.2 3.2.3 3.2.4

4. 4.1 4.2 4.3 4.4 4.5

5.

Powder Bed Fusion ..................................................................................56 Powder-Directed Energy Deposition (P-DED) ........................................57 Wire-Directed Energy Deposition (W-DED) ..........................................58 Discussion ................................................................................................59

Wire Arc Additive Manufacturing ................................................................61 WAAM in Construction ...............................................................................................63 WAAM Process ..........................................................................................................65 Heat Transfer Mechanisms ........................................................................................69 Cold Metal Transfer (CMT) Welding ..........................................................................72 Process Parameters ..................................................................................................74

4.5.1 4.5.2 4.5.3 4.5.4 4.5.5

Shielding gas............................................................................................78 Contact-tube-weld-distance (CTWD) ......................................................79 Travelling Speed (Ts) ..............................................................................81 Wire feed speed (Wfs) .............................................................................82 Combination of parameters......................................................................83

Welding distortion ins thin plates ..................................................................85

5.1 Discussion .................................................................................................................89 5.2 Mitigation Strategies ..................................................................................................91

5.2.1 5.2.2 5.2.3 5.2.4 5.2.5

Flame Straightening .................................................................................91 Mechanical Tensioning ............................................................................92 Thermal Tensioning .................................................................................92 Low Stress Non Distortion (LSND) Welding ..........................................94 Dynamically-Controlled Low Stress Non-Distortion (DC-LDNS) Welding.....................................................................................................95

5.3 Discussion .................................................................................................................96

6.

Proposed Hybrid WAAM and Thin Sheet Metal Welding..........................99

6.1 Scanning ..................................................................................................................101

6.1.1 6.1.2 6.1.3 6.1.4

Photogrammetry ....................................................................................102 3D Laser Scanning ................................................................................. 104 Structured Light Scanning (Microsoft Kinect) ...................................... 106 Tactile Sensing ...................................................................................... 112

6.2 Process Parameter Studies .....................................................................................119

6.2.1 6.2.2 6.2.3

Pre-Investigation .................................................................................... 120 Parameters on sheet metal thin sheet material ....................................... 123 Discussion .............................................................................................. 130

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6.3 Material Testing .......................................................................................................131

6.3.1 6.3.2

Welded sheet Tensile Testing ................................................................ 131 4 Point Bending Test ............................................................................. 136

6.4 Formulating Welding Strategies ...............................................................................141

6.4.1 6.4.2

Topology Optimization .......................................................................... 145 Proposing a curve-based approach ........................................................ 151

6.5 Preparing print paths................................................................................................158

6.5.1 6.5.2

Robotic slicing ....................................................................................... 158 Polynomial Regression and Parametric robotic programming .............. 162

6.6 Detailing ...................................................................................................................165

7.

Discussion ......................................................................................................173

8.

Summary and Future Outlook ....................................................................177

References ....................................................................................................................187

PART 1 Chapter 1 presents context to the research. A brief introduction to sheet metal and its application in facades is followed by a discussion on Wire Arc Additive Manufacturing and mass customization. The two are tied together by a brief discussion on Wire Arc Additive Manufacturing and complex geometry facades. A Research Framework is presented together with motivation, research questions and an overall structure of the thesis. Chapter 2 introduces Sheet metal as a building material; covering its production, methods of forming and application in facades Chapter 3 is based around Additive Manufacturing. First, an overview of the primary Additive Manufacturing technologies is presented. This is followed by an introduction to Additive Manufacturing methods specific to metallic materials and their comparison Chapter 4 is specific towards Wire Arc Additive Manufacturing, describing its application in construction as well as a deep-dive into the mechanisms behind the process. Chapter 5 presents literature on welding distortion in thin sheet metal

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Introduction

The use of sheet metal as an industrial material kicked off after the Industrial Revolution. While more primitive versions of it did exist well before this time, largely in the form of hammered artifacts [3], it was only after the adoption of industrial assembly processes, coupled with innovations such as the hydraulic press, sheet metal became more commonplace in industry. The lightweight characteristics and ease in which it could be formed made it an attractive material not only for the construction industry but also the shipbuilding, automotive and aerospace industries alike. One of the earliest, and perhaps most widely-known, uses of sheet metal in construction comes shortly after Jean Pierre Droz streamlined its production in 1783. When Frédéric Auguste Bartholdi and Gustav Eiffel conceived the construction structure of the Statue of Liberty, they did so using an underlying iron exoskeleton – the precursor to Eiffel’s later work – onto which around 300 pieces of 2.4mmthick copper sheets were attached (Fig 1.1). Being malleable, the copper was formed by a process of hammering the material against a mold until it formed the desired shape (Fig 1.2).

F i g 1 . 1 U n d e r l yi n g s u b s t r u c t u r e o f t h e S t a t u e o f L i b e r t y , N e w Y o r k . ( S o u r c e : C o u r t e s y o f Catherine Grisez)

© The Author(s), under exclusive license to Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2023 C. Borg Costanzi, Reinforcing and Detailing of Thin Sheet Metal Using Wire Arc Additive Manufacturing as an Application in Facades, Mechanik, Werkstoffe und Konstruktion im Bauwesen 68, https://doi.org/10.1007/978-3-658-41540-2_1

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1 Introduction

Fig 1.2 Detail of shaped copper sheet cladding for the Statue of Liberty, New York (Source: Courtesy of Catherine Grisez)

1 Introduction

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In order to connect each of the copper sheets to one another, the iron-exoskeleton and to provide rigidity against the flexible material, iron straps were also attached to the cladding material with rivets (Fig 1.3). The carving of wooden molds, hammering of copper, attachment of rivets and straps as well as fixation to a geometrically-complex iron substructure was no easy feat of engineering, even by today’s standards. Moreover, because each cladding panel was unique, this resulted in completely bespoke connection detailing throughout the entire structure, with every reinforcement strap manually crafted to not only follow the surface of the copper cladding but to line up with the underlying iron supports.

Fig 1.3 Principle of cladding structure used at the Statue of Liberty, New York

In 2000, Frank Gehry completed the construction of the Experience Music Project (EMP) Building in Seattle. The façade, which consisted of over 21,000 unique cladding panels in aluminum and stainless steel, was assembled using a process of Zahner Engineered Profile Panel (ZEPPs)[4] which was developed specifically for the project. The innovative process involved the cold-bending of the individual sheet material back to an underlying sub-structure[5], allowing for each individual panel to be shaped to a specific frame. However, similar to the cladding structure of the Lady Liberty, this did result in a large-number of individually-shaped frames to which the façade cladding had to be pulled back.

1 Introduction

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Fig 1.4 (left) View of the EMP Façade in Seattle (Source: Wikicommons). (right) Non-Uniform cladding details on the EMP Façade, Seattle (Source: G.Lee et al [5])

More recently, Zaha Hadid Architects, together with a consortium of façade planners and engineers, completed the Morpheus Hotel in Macao. The incredible feat of engineering, digital and on-site planning and construction, consisted of 150,000 unique cladding elements, each of which were shaped to fit to an underlying, free-form exoskeleton (Fig 1.5). This resulted in 100,000,000 unique parts[2] which had to not only be produced, but referenced , organized and communicated with both the design, fabrication and assembly consortia.

Fig 1.5 Hotel Cladding (Source: Wikicommons)

The project was realized due to the use of digital design tools, which created a streamlined workflow between the design, engineering and fabrication data. This was necessary as low-tolerance requirements of the façade cladding resulted in a multiple-step process of shaping, detailing and assembly. The curved panels were fabricated using a similar, multiple-step process adopted by Frank Gehry for the construction of the EMP building. Because each of the curved panels had a unique curvature and dimension, a corresponding unique mold was also required in order to bend the shape material.

1 Introduction

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This was achieved by laser-cutting sheet material which formed a temporary mold onto which the panels were spot welded. Once the panels were shaped, they were laser cut to their exact required dimensions, while still attached to the laser-cut mold. However, were the bent sheet material to be removed from the mold, the spring-back action would cause it to deform towards its initial flat state; an issue which was well outside the range of acceptable tolerances for the project. Therefore, the solution adopted was to attach stiffening stringers onto the backside of the still-held in place sheet before being demolded.

Fig 1.6 Buildup of typical cladding element used at the Morpheus Hotel, Macau

Fortunately-enough, contemporary fabrication techniques allow for a far more automated method of forming sheet metal – a response perhaps arising from the need for sculpting sheet metal in the context of automation, aerospace and shipping industries. The Dongaummum Design Plaza in Seoul, for example, consists of 45,000 unique aluminum sheets of cladding, having varying curvatures and dimensions. To solve this façade Multiple-Point Stretch Forming (MPF) – an adaptive molding process developed by the ship-building industry to produce curved sheet panels of various curvatures – was adopted[5]. Although both the Dongaummum Design Plaza and Morpheus Hotel project were realized by the same architects, MPSS wasn’t used in the Morpheus Hotel façade due to size limitations of the fabrication technique and the issues of springing back of the sheet panels.

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1 Introduction

F i g 1 . 7 D o n g a u m m u m D e s i g n P l a z a , S o e l . ( S o u r c e : W i k i c o mm o n s )

All the façade projects referenced above are feats of engineering in their own right. The EMP façade was carried out during a time well before parametric design tools were commonly available in architectural offices; the Dongaummun Façade was realized using adopted technology from the ship building industry and while the Morpheus Hotel Façade had a few more fabrication steps, it is a result of intensive use of digital design and fabrication tools. All the projects also open up a further question – in the context of complex-geometry facades, mass customization and mass production, how can digital fabrication tools be further exploited and integrated? This brings up the question of integrating Additive Manufacturing (AM) : A potentially wasteless method of fabrication which allows for creating objects with far less geometrical restrictions. Wire Arc Additive Manufacturing (WAAM), in particular, is an AM technique based on robotic welding. By precisely controlling the deposition of weld material, it allows for the building up of three-dimensional structures and a relatively quick rate. Therefore, given that digital design tools allow us to design with the freedom of geometry, shaping techniques allow for the sculpting of such forms and fabrication process, such as AM, allow for the freedom of fabrication, then what is the potential benefit of their combination? Therefore, the research presented in this thesis seeks to explore how the freedom of fabrication with WAAM may be used in combination with free-form sheet metal. In particular, how may it replace the use of hundreds of thousands of façade components and details in the context of free-form building skins.

1 Introduction

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1.1 Wire Arc Additive Manufacturing and Mass Customization Wire Arc Additive Manufacturing (WAAM) is an Additive Manufacturing (AM) process based on the controlled deposition of weld material. With the earliest concepts for this process patented as far back as the 1920’s (Fig 1.8)[6], it allows for the buildup of three-dimensional objects in metal material. To date, it has been used for the production of fully-printed metal components in industries including shipbuilding [7][8], the oil and gas industry [9] as well at mold making[10]. As is the case with other AM processes, WAAM offers the benefits near-net shape fabrication; in other words, only the material required for the production of components is required (although in some cases, post-processing steps would be required). This offers benefits over other fabrication processes for parts which either consist of multiple components assembled together or those which require a high degree of customization. With material deposition rates typically ranging between 1 – 9kg/hr. [11], WAAM allows for parts in metal to be fabricated relatively quickly.

Fig 1.8 Ralph Baker’s patent for building up three dimensional structures in posited metal (Source: R. Baker[6])

de-

While the process is generally slower than other pre-established industrial processes used for the mass-production of repetitive, simple components, the benefits of WAAM may be exploited when there is a certain degree of mass-customization required. Standard channel profiles, for example, are

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1.1 Wire Arc Additive Manufacturing and Mass Customization

produced from continuous sheets of metal which are gradually shaped and cut to sizes of around 1 – 12m in length. Due to the efficient lead times, some manufacturers are able to produce similar components at rates up to 5,000 metric tons a month [12]

F i g 1 . 9 M a s s - p r o d u c e d s h e e t p r o f i l e s . ( S o u r ce : U . K n a a c k )

However, the quick lead times are possible only because of the high repeatability of components. In the case illustrated in Fig 1.9, the production of steel angles would require an entirely different setup of rollers as well as forming dye. The manufacturing of new components usually results in high setup costs – rollers have to be milled to shape, shaping dyes have to be produced from massive blocks of metal, which in turn results in high price-per-part, which is gradually reduced as the volume of production increases[13] Hopkinson et al [13] , Kirchheim et al [14] and Busachi et al [15] argue that the potentials of AM as a production process fall within the window, where a traditional fabrication technique would require a high initial startup cost, but the volume of production is too low for the price per part to be reduced significantly. Additive Manufacturing, on the other hand, tends to have lower initial startup costs than other production processes requiring large assembly lines, which do not vary with variation of parts being produced. Fig 1.10a and b show the comparison between two AM processes and their industrial counterparts. In (a) Fused Deposition Modelling and Stereolithography polymer printing processes are compared with injection molding – a production technique long established for the mass-production of repetitive parts. What is noticeable is that in the case of AM processes, the cost of production remains constant, regardless of the batch size, whereas molding only becomes viable after a certain production volume is reached [13]. Therefore, the production of a unique part would require a high-initial startup cost in the case of injection molding, with no change in cost with an AM process. They go on to argue that AM, in general, becomes competitive with conventional manufacturing processes when there is a high degree of complexity, which would otherwise result in multiple production steps in a conventional setup. Therefore, the values of AM become apparent when there is a tendency towards geometric complexity, a high degree of variation which would make conventional processes uneconomical due to startup costs with a low volume of repetition.

1 Introduction

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Fig 1.10 (a) Comparison of AM techniques with traditional Injection Molding (source: N.Hopkinson et al [13]) (b) Economical break-even point for AM (Source: Adapted from A.Kirchheim et al[14])

This brings forward the question to mass customization in façades. A typical medium and largescale façade project could consist of thousands, and in in some cases, millions [2] of components. When these components are geometrically equal and part of a standardized system, then conventional mass production techniques will always be favorable over AM. However, once a degree of customization is introduced then the question of adopting AM over conventional fabrications arises. In 2021, the researchers at TU Darmstadt, together with TU Ilmenau, Imagine Computation GmbH and Hoelscher Kleve GmbH presented a case for this. A 1250mm x 2300mm free-form glass-shingle façade was rationalized to be held up by 36, bespoke spider connections. Wire Arc Additive Manufacturing was adopted, due to the constant changing of (a) offset between the glass façade panel and concrete substructure, (b) the constant changing of the angle of incidence between the glass pane and the substructure and (c) the constant changing of relative angle between glass panels. As illustrated in Fig 1.11a initially an adaptation of standard extruded profiles was planned, but were eventually replaced by branching spider brackets (b) which were quicker to produce using WAAM.

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1.1 Wire Arc Additive Manufacturing and Mass Customization

Fig 1.11 (a,b) Mass customization of 3D printed façade connectors ( s o u r c e : B o r g Co s t a n z i e t a l [ 1 6 ] )

Using a combination of 3D scan data and 3D Modelling tools, each of the unique spider connections was parametrically designed to cater for the variations in geometry, and eventually printed at TU Ilmenau. Although the project did require several post-processing steps, in particular milling the ends of the connections to account for tolerances, and due to a number of issues there was no perfect fitting, the project illustrates the potentials adopting Additive Manufacturing as a means of mass customization in the façade building industry.

1 Introduction

F i g 1 . 1 2 W A A M- P r o d u c e d f a ç a d e c o n n e c t o r ( s o u r c e : B o r g C o s t a n z i e t a l [ 1 6 ] )

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1.2 WAAM and complex-geometry facades

1.2 WAAM and complex-geometry facades It has already been established that one of the major benefits for adopting Additive Manufacturing technologies are in scenarios calling for a low repeatability and high customization of parts. Therefore, herein lie the potentials solving complex-façade geometries which often rely on the use of mass-customization. Drawing back to the case of the EMP and Morpheus Hotel facades, where highly-customized components had to be manufactured and, in the case of the Morpheus Hotel, discarded after use, then it is interesting to speculate how AM could have served as an alternative fabrication technique. This was the motivating starting point of the thesis; how can the freedom of fabrication be used to produce better facades? Moreover, given the growing of digital design tools which made it possible for designers to conceive the complex forms in the first place then, how can these also be exploited to rethink how we detail and stiffen sheet metal facades? The research proposes combining WAAM directly with thin sheet metal – the thin sheet itself becoming the non-sacrificial element, and the WAAM-produced parts becoming the stiffener and the cladding connection (Fig 1.13).

Fig 1.13 Conceptual use for combining WAAM with thin sheet metal

1 Introduction

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The answer is not quite so straightforward. Firstly, the techniques used for fabrication facades are only viable because they fit within an industrialized mass-production framework. Therefore, were an alternative WAAM approach were to be integrated, it too would have to somehow form part of an automated and industrial process. Secondly, WAAM is a fabrication technique which is based on energy-intensive welding procedures. On the other hand, due to its low stiffness, thin sheet metal is notoriously problematic with heat-induced diction when welded. Therefore, apart from solving the issues of introducing a new fabrication technique into an already-established mass production process, importance must also be given to (a) controlling the deformation of sheet metal during welding – maintaining the same level of tolerances required by facades and (b) controlling the deformation to an extent that it is possible to build up useable, three-dimensional structures directly onto the sheet metal

F i g 1 . 1 4 W A A M- p r o d u c e d s t r i n g e r s f o r t h i n s h e e t m e t a l

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1.2 WAAM and complex-geometry facades

Fig 1.15 Lou Ruvo Brain Center Clinic, Las Vegas (Source : Wikicommons)

1 Introduction

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1.3 Research Framework The research scope of this thesis is exploring the use of Wire Arc Additive Manufacturing as a means of adding welded material directly onto the surface of free-form sheet metal. The intended applications are those of (a) providing local reinforcement and (b) providing built-up connection details for fixation to a facade’s substructure. The research presents the state of the art for Wire Arc Additive Manufacturing and the different welding processes available. Moreover, the different methods used for controlling heat-induced deformation in welded sheet metal are presented. Welding parameter studies and welding strategies are carried out to determine how to weld onto the thin material, without damaging it and reducing the heat-induced distortion as much as possible. The welding parameters are first determined on planar sheets of metal and, once acceptable welding processes are determined, they are applied to pre-bent sheet metal. In parallel to obtaining a welding process which allows for printing onto thin sheet material, a design-to-production workflow which could be implemented into a mass-production process is also speculated. This includes (a) exploring and proposing methods for automated design of structural elements (b) exploring and illustrating scanning techniques best suited for digitizing the process and (c) the automated generating and rationalization of welding paths to be communicated to a robotic welding device. In order to limit the scope of research, the process is carried out using 1mm diameter welding electrode, 0.75mm thick sheet metal, 82%Ar 18%CO2 Shielding Gas and is done on only planar and singly-curved sheets of metal.

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1.4 Motivation

1.4 Motivation Digital Design tools and Advanced Fabrication techniques have been in development for the last few decades. Architecture is at a point where the two are colliding more than ever, and opening up new possibilities in how we design our structures. This is resulting in more expressive, and geometrically-complex structures. However, it also results in more complicated construction sequences and waste of material resources. The motivation of this research is to explore how Digital Design tools and Advanced Fabrication techniques can be used to tackle complex geometry in the context of facades. Although steel is one of the most recycled construction waste streams, this still amounts to around 630 million tons a year [17]. Wire Arc Additive Manufacturing is proving to be an extremely promising fabrication technique in reducing waste streams by allowing for parts to be produced more efficiently, with less material. In most cases, the WAAM process utilizes a sacrificial base plate, which too has a number of use cycles before it must be scrapped. However, the ability to perform a WAAM process useable members, rather than sacrificial plates, allows for not only enhancing the function of the part, but also reduces the amount of waste generated in the process. This opens up the potentials for smarter components, as a result of combination of multiple technologies. In the context of complex geometry facades, where thousands of components have to be fit together as discrete elements, then the combination of two fabrication techniques could not only reduce the number of parts, simplify the design and assembly of a structure but reduce the amount of waste generated by maximizing the efficiency of the production process.

1 Introduction

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1.5 Research Questions “How can Wire Arc Additive Manufacturing (WAAM) be utilized for stiffening and building up connection details on thin sheet metal?” Further Sub questions formulated relating to particular thematic instances:

(1) On WAAM i. “ What distortion mechanisms are expected in welding thin sheet materials?” ii. “How can heat-induced distortion be controlled when welding on thin sheet material” iii. “What are the welding characteristics allowing for building up weld material on thin sheet metal?” iv. “What welding strategies may be adopted for reducing distortion in welded thin sheet metal?” (2) On Process i. “How can a Design-To-Production workflow (DTP) be established for welding thin sheet metal facades?” ii. “How can 3D Scan Data be best integrated into an automated DTP Workflow?” (3) On Design i. “How can material behavior and digital design tools be used to inform a design process? What are the Design Drivers?”

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1.6 Structure

1.6 Structure

1 Introduction

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The structure of the thesis composes of three main parts, each building upon one another. The first phase concerns background information on the topics of sheet metal fabrication, forming and typical facade typologies. In parallel, the theme of Additive Manufacturing (AM) is introduced, building onto Metal Additive Manufacturing and finally Wire Arc Additive Manufacturing. Finally, the topic of welding on thin sheet material is introduced; in particular, the means in which welding may cause distortion or failure in welded sheets, and what mitigation strategies have been researched thus far. The background research opens up the second phase of the thesis. This section concerns formulating a design strategy for welding onto planar and singly-curved sheets of metal using a Robotic Cold Metal Transfer (CMT) process. In this section, welding process parameters are investigated for which would allow for printing on both the planar and bent sheets of metal and is followed evaluating different welding path planning strategies. This is followed by a brief interlude of material testing. The information gathered from welding experiments is used to formulate a design-to-production process, in which digital design tools are used for proposing connection and stiffening weld layouts based on a Fabrication-Informed-Design strategy. The final part of the thesis illustrates the early prototypes developed for combining Wire Arc Additive Manufacturing with planar and bent sheet metal. This is followed by a future vision section and recommendations for future studies.

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Sheet metal as a material

The use of sheet metal material may be traced back quite considerably into our ancestry. In fact, the ancient Egyptians used metals such as zinc, copper and gold, to create weapons, and adorn their sculptures. This was largely due to the proximity of alloy ores along the Nile, with archeologists also uncovering ancient copper-working factories where scrap material extracted from mines were shaped to their intended function[3], as well as the ease in which the material could be shaped. Although one may only speculate, the general consensus on the method of shaping was that of hammering down ingots of metal material until a desired thickness was achieved, and subsequently cut into shape. The material had continued to be used through ancient history, both as a means of creating weapons and decorating sculptural pieces. However, it was Leonardo Da Vinci who provided the earliest sketches describing a rolling mill (Calandra o Laminatoio) which would later be used to roll sheets to uniform thicknesses. It is perhaps the Industrial revolution which really sparks off the use of sheet material; with the breaking away from manually-operated machines and the invention of steam engines and hydraulic presses, truly industrial production lines started to emerge. By 1770, Joseph Bramah had constructed the earliest known hydraulic press; a production process which is still referred to today. This eventually led to a snowball effect; as forming technologies continued to develop and new machineries conceived, so did the use of sheet metal material. Perhaps one of the largest early applications of the material being the Statue of Liberty in New York City. To construct Lady Liberty, Eiffel made use of some 300 pieces of 6mm sheets of copper which were gradually hammered d into shape against a mold, and later cut and stitched together with rivets (Fig 2.1a). A similar technique was adopted by Architect William Van Allen in the 1920s, who was tasked with designing the Chrysler Building for William Chrysler. Part of the building featured a pair of gargoyles, coming at the request of Chrysler to mirror the hood ornaments on his automotive company (Fig 2.1b,c)

F i g 2 . 1 ( a ) c o n st r u c t i o n o f t h e S t a t u e o f L i b e r t y c l a d d i n g ( b ) M o l d u s e d f o r h a m m e r i n g sheet copper (Source: courtesy of Catherine Grisez ) (c) Organic sheet metal used in the Chrysler Building, Chicago (source: Wikicommons)

The increased popularity of the sheet material during, and after, the industrial revolution had a further impact on the ship building industry. The advent of the steam engines not only resulted in locomotive technology, but also had the repercussion of phasing out wooden constructions with those made from stronger, and sturdier sheet metal. During the same period, the lightweight material was being applied to other industrial innovations. In 1915, Hugo Junkers, an aerospace engineer, © The Author(s), under exclusive license to Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2023 C. Borg Costanzi, Reinforcing and Detailing of Thin Sheet Metal Using Wire Arc Additive Manufacturing as an Application in Facades, Mechanik, Werkstoffe und Konstruktion im Bauwesen 68, https://doi.org/10.1007/978-3-658-41540-2_2

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2 Sheet metal as a material

constructed the world’s first fully-metal monoplane which was constructed out of an aluminum sheet fuselage which was shaped and tied back to its aluminum primary frame (Fig 2.2).

Fig 2.2 J1: The earliest fully-metal monoplane (Source: Wikicommons)

By the late 1900’s , Computer-Aided Design (CAD) was also becoming more available. Paralleled with the development of Computer-Aided Manufacturing (CAM), the fabrication, shaping and cutting of sheet metal became much more available. Architect Frank Gehry is one of the most widelyknown pioneers in making use of the early technologies. During the early 1990’s, his team adopted the use of CATIA – a software package which was until then exclusively used to design aerospace components – to design and prepare for the construction of the El Peix (Fig 2.2a)in Barcelona and the Weisman Art Museum Gallery in Minnesota (Fig 2.2b) which were clad with free-form sheets of perforated and brushed stainless steel, respectively.

Fig 2.3 (a) El Peix, Barcelona and (b) Weisman Art Museum (source: Wikicommons)

The continued advancement of manufacturing processes has allowed for sheet metal material to become a solid part of building cladding material libraries ranging from copper and brass, to zinc and titanium. Continued advancements in forming techniques have also allowed for facades to become more expressive, not only in material but in form.

2 Sheet metal as a material

F i g 2 . 4 ( a ) N I A B i r m i n g h a m c o p p e r , ( b ) C e n t r u m G a l e r i e D r e sd e n , ( c ) Z i e t a P a v i l i o n , Wroclaw (source: Wikicommons) (d) Guy’s Hospital, London

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2.1 Production of Sheet Metal

2.1 Production of Sheet Metal The different sheet metal production processes all begin in the same manner; a block of metal (billet) is heated and progressively rolled down to a desired thickness under immense pressure. However, this process can be performed at multiple temperatures, and it is this which distinguishes between cold-rolled and hot-rolled sheet metal. For a hot-rolled steel component, the rolling to the desired thickness takes place under high temperature, and is then left to cool. A cold-rolled sheet is essentially formed using the same steps as hot-rolled material, with the added production step of first cooling the material at an approximate dimension, and later rolling it to form the final thickness at room temperature. Both hot- and cold- rolled production process have their merits; the former resulting in cheaper material and a quicker and simpler manufacturing process. However, this comes at the expense of a loss in dimensional accuracy due to the shrinkage of metal as it cools down. Rolling sheet metal once it has cooled, accounts for the dimensional changes during shrinkage, and therefore sheet material produced by cold-rolling has a far greater dimensional accuracy but is generally more expensive and energy-intensive to produce.

2.1.1 From raw material to a blank billet The sheet metal production process begins with the melting and mixing of the primary raw materials. Creating metal from raw materials includes the use of limestone, iron ore and coal. Alternatively, scrap metal may also be re-melted and converted into new blocks of metal material. This has resulted in two main steel production steps – one in which a Blast Furnace is used to create metal from iron oxides, and an alternative method which utilizes an Electric Arc Furnace to re-melt scrap metal. When steel is made directly from raw materials, coal is crushed into a powder and heated to a temperature close to 10000C, reducing the amount volatile material (sulfur, oil, nitrogen, hydrogen and tar) present in it. The end product is referred to as coke, which is added , together with iron ore and limestone in a Blast Furnace at typical ratios of 4:2:1 or 8:4:1 (Iron : Coke : Limestone). The addition of coke acts a fuel to aid in the heating process as well as a means of reducing the presence of iron oxides. Limestone, on the other-hand is added to remove any remaining impurities in the molten material (Fig 2.5). After both the Blast Furnace and the Electric Arc Furnace processes are completed, the molten material is transferred to a ladle. In this step, alloys such as Nickel (Ni), Vanadium (V), Manganese(Mn) or Chromium (Cr)[18] are added which allows for the creation of different steel grades and performance values. The addition of Mi, for example, has the effect of increasing the strength of material while lowering ductility. Vanadium, is often added to increase the hardness and wear resistance of metals, while Copper aides in corrosion resistance yet exhibits large reductions in ductility[19].

2 Sheet metal as a material

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Fig 2.5 Typical Production process for sheet metal

Once the desired alloys (if any) are added, the molten material is cooled down and shaped into billets or slabs, which serve as blank material for the creation of further products. The most commonlyused method in industry is that of continuous casting. During this process, molten material is poured from the ladel through a copper die, solidifying the outer shell of the molten material. The semimolten material is then processed on an arrangement of horizontal rollers and further cooled by the spraying of water. At the end of this stage, the strands are cut to pre-determined lengths and prepared for post-processing. The blank strands resulting from continuous casting are used for shaping other steel products. In the case of sheet metal, the raw blocks of material have to be re-heated in order the malleable once again, and gradually rolled down to size under immense pressure and multiple processes.

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2.2 Shaping into thin sheet material

2.2 Shaping into thin sheet material Shaping the raw blank material into specific components always begins with a process of hot rolling. Blank material is reheated to a termperature above the recrystallization temperature. This is because when molten metal starts to cool, the once uniformly arranged crystals (dendrites) form into large, disorganized grain structures with gaps and no apparent alignment (Fig 2.6) [20], [21]

Fig 2.6 Sheet metal undergoes changes in grain structure after rolling. (Source: adapted from Musonda et al[20])

Therefore, heating the grains above the recrystallization allows for internal grain structure to be reformed as it is rolled under high compressive forces, resulting in smaller and more aligned grains while reducing the cross-sectional dimensions of the steel blank and increasing the mechanical properties. The rolling process is repeated through multiple sets of rollers who’s spacing is gradually reduced until the steel material is shaped to the rough dimensions required. The material is then left to slowly cool down to room temperature in order to avoid the buildup of internal stresses. The result is a material that has improved mechanical properties, due to the realignment of the internal grain structure. However, as the material is unconstrained during cooling, parts produced from hot-rolling tend to have poorer dimensional accuracy to cold-rolled steel counterparts. Cold rolling, on the other hand, starts off as a hot-rolling process, however after the parts have cooled down to room temperature, the material is rolled once again in order to achieve a higher dimensional accuracy. An added benefit of the cold-rolling process is that the shaping of the material is carried out between the yield point of the material and the ultimate strength, in a region referred to as the strain hardening zone, increasing the mechanical properties of the material. Due to the ability to achieve high dimensional accuracy [22], and superior surface finishing, cold rolled sheet metal is more commonly found in applications as facades, whereas hot-rolled steel is generally more reserved for applications not requiring high dimensional accuracy or surface finishing.

2 Sheet metal as a material

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2.3 Sheet metal facades Apart from serving as the first line of defense against the elements, a building’s façade also plays part in defining the architectural language of the building while transferring external loads to the underlying building structure, providing insulation against the heat ,cold and sound [23].

Fig 2.7 Principle of façade requirements (Source, Knaack [23])

There exist a multitude of façade types, ranging from massive and structural masonry solid-wall contractions which are built brick by brick, to light-weight, pre-fabricated systems which are attached to an underlying structural system[23]. The type of façade construction used is therefore not only determined by contextual requirements, such as prevailing environmental conditions, but is also determined by the primary material used as the building skin. Sheet metal does not have the same load-bearing capacity as concrete or brick, and is therefore not used in solid wall construction. Being a lightweight and durable material, it is more commonly applied as a form of expressive rainscreen cladding or shading devices. In principle, sheet metal building skins are positioned as the final outer layer to a building and either attached to secondary sub-structures, usually by means of horizontal and vertical rails, or directly onto a primary structure, such as a masonry wall (Fig 2.8)

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2.3 Sheet metal facades

Fig 2.8 (a) Principle of rainscreen façade buildup, in which the outer skin is separate to t h e p r i m a r y s t r u c t u r e (S o u r c e : K n a a c k [ 2 3 ] ) a n d ( b ) e x a m p l e o f s h e e t m e t a l a s a s h a d i n g d e vice, SEC Building Boston (Source: Courtesy of Amira El Bakry)

Given the wide range of possibilities for shaping and fixing sheet metal together as well their treatment, finishing and basic material, there also exist numerous sheet metal cladding systems, which will be presented in the next section.

2 Sheet metal as a material

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2.3.1 Standing Seam and Snap-lock Cladding Standing seam facades are widely recognizable by their characteristic horizontal or vertical folds. These façade cladding types are formed by joining adjacent sheets of metal by folding their edges into one another. Apart from achieving a distinguished aesthetic, the folding of sheet metal to form standing seams also increases the stiffness of the cladding element, allowing for parts to be produced even in the range of 3m in length.

Fig 2.9 Typical Standing Seam Detail and assembly

Fig 2.10 illustrates a typical standing seam connection where two façade panels are connected by means of folding with a connection clip. In this case, specialized equipment is needed to perform the pressing on site.

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2.3 Sheet metal facades

Fig 2.10 Typical standing seam connection using pressing: (a) an edge sheet (purple) is aligned with a pre-positioned connector (blue). (b) Adjacent sheet (green) is brought into position and aligned by the top edge of the connector. (c) and (d) the ends of all sheets are pressed and folded together to form a standing connection.

Fig 2.11 Typical standing seam connection types (a) Overlap joint (b) Single-Lock fastener (c) Double-lock fastener (d) Snap-connection with cover cap (e) Tee-connection (f) Snap-lock

Standing seam connections come in a variety of fastening mechanisms, though they are generally connected mechanically (Fig 2.11 a,b,c), snap-locks/clips (Fig 2.11d,f) or batten panels (Fig 2.11 e). The type of connection used depends on the requirements for water-tightness, façade loading and speed of installation. Mechanical seams, for example, require specialized equipment for bending profiles on site. However, these allow for very rigid and water-tight connections when using doubleseam edges. Snap-locks, on the other hand, allow for faster erection times with no need for specialized equipment.

2 Sheet metal as a material

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2.3.2 Shingle and Flat-lock facades Shingles / flat-lock systems are directly fixed to the surface of a façade’s substructure by means of rivets, clips and /or screws. These types of facades are characterized by the snapping and interlocking of adjacent panels, making them relatively easy to install onto a site, and are generally fixed to large underlying sub-structures, such as plywood.

Fig 2.12 A typical shingle façade is assembled using interlocking sheets which are fastened together by hidden clips

Shingles usually are produced as diamond, rectangular or square sections, depending on the type of wall they are being attached to. However, unlike the standing seam facades, shingles usually are produced to smaller sizes, around the dimensions of 300x300mm for rectangular/diamond panels, with rectangular shingles also being produced up to lengths of 1000mm.

Fig 2.13 Shingle type of facades require a flat surface for connecting clips (left) Standard shingle façade with overlapping horizontal joints. (right) Overlapping sheets are connected by means of hidden clips

2.3 Sheet metal facades

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2.3.3 Cassettes Cassette-type facades are distinguished form other metal facades by incorporating folded/beveled edges. The edges usually contain notches which are laser cut prior to folding, allowing for the façade to hang directly onto a sub-structure. The fixing mechanisms are usually be means horizontal or vertical clips. The modularity of cassette facades makes them attractive option when quick installation times are required, and a relatively repetitive cladding layout is required (Fig 2.14).

Fig 2.14 Cassette-type facades start off as flat sheets which are folded to form cassettes.

. Fig 2.15 a typical cassette façade, connected by means of vertical or horizontal clips as well as possibility of hanging by means of grooves cut into the sides of the cassette.

The examples shown in fig 2.15 illustrate the two most commonly-used cassette systems. Hanging cassettes (a) are directly fixed to the façade substructure by means of a cut-out edge detail which allows for fitting onto a pre-positioned hook. A hook-on / overlapping cassette (b) is fixed directly to the façade sub-structure by means of clips. The edge of the cassette, usually the top, is extended and detailed to allow for fitting directly into the clips.

2 Sheet metal as a material

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2.3.4 Alucobond Facades Alucobond is an aluminum composite façade panel, consisting of two sheets of aluminum which sandwich an inner core material. Due to the high versatility in performance and surface finishes, as well as the ability to be folded into three-dimensional shapes, it is widely adopted in lightweight façade cladding. The panels are fixed to an underlying sub-structure multiple ways, including hanging systems similar to cassette-types of facades or hidden clips/screws as with shingle facades.

Fig 2.16 Examples of alucabond fixation using glue, rivets and cleats

2.3 Sheet metal facades

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2.3.5 Custom Systems Especially in the context of free-form facades, standard façade systems do not provide adequate flexibility and therefore customized façade systems are required. The ZEPPS (Zahner Engineered Profile Panel System)from Zahner is one such case. Initially developed in response to the geometrically-complex façade used by Frank Gehry at the EMP building in Seattle, it has since grown to be commonly used in the context of free-form and sculptural façade systems. The ZEPPS façade consists of all the building skin requirements, such as waterproofing, membranes and insulation in a single element; essentially an integrated form of a cassette façade. The distinguishing feature of the façade cladding, however, is that it is specifically developed for free-form construction. In this case, the sheet cladding material is cut using automated CNC techniques and tied back to a pre-fabricated substructure, also produced using CNC processes.

Fig 2.17 Example of bespoke façade fixation (Source: Adapted from Zahner - ZEPPS)

2 Sheet metal as a material

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2.4 Sheet Metal forming Techniques A primary material used in aerospace and ship-building industry, methods for sheet-forming metals has been in development for decades. As demands for more complex and property-enhanced formed sheets increased, so did the methods for automation on an industrial level. This has resulted in multiple methods for forming sheet metals. In the majority of sheet forming processes, the deformed shape of the sheet is determined by a die. Most sheet metal forming techniques utilize the plastic deformation of sheet. This chapter will introduce some of the most commonly-used techniques. A problem with deforming sheet metal, however, is the spring-back due to elastic deformation. Because of this, sheet metal is often over-deformed in order to reach the target shape.

2.4.1 Hammering As its name implies, sheet metal hammering refers to the repeated hitting (or hammering) of malleable metals until it deforms to a desired shape. It is one of the earliest known techniques used to deform metals, dating back several thousands of years. When our ancient ancestors discovered the malleable properties exhibited by bronze and copper, hammering was used to gradually deform the metal into thin sheets. By hammering the sheet against concave-shaped tree stumps the sheet metal was able to be formed into the earliest vessel ornaments.

Fig 2.18 Metal hammering is one of the earliest methods used to deform

sh e e t m e t a l

Presently, adaptations of the ancient technique are still used today, largely in the automobile industry for the repair and fabrication of car and motorcycle body parts. However, this sheet forming technique relies heavily on skilled craftsmanship, making it a relatively laborious process. Therefore, sheet hammering is not so widely adopted in the context of facades cladding. Nonetheless, one of the most recognizable constructions adopting sheet hammering as a means of shaping cladding material is visible in the construction of the Statue of Liberty in New York City

2.4 Sheet Metal forming Techniques

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2.4.2 Sheet bending. This is a process in which sheet metal is deformed plastically through the application of force. The force causes the material to be stressed beyond its yield point; however, it is kept below its critical tensile strength to prevent tearing. The bending process allows for bending along a single deformation axis; however, multiple passes can be combined for create more complex shapes Fig 2.19

F i g 2 . 1 9 B e n d i n g a l l o w s f o r m u l t i p l e p a s s e s t o f o r m m e t a l i n t o m o r e c o m p l e x sh a p e s .

Fig 2.20 Sheet bending generally consists of a punch which shapes sheet metal against a dye

Sheet bending is a common process used to create elements in construction; U- and C-Channel profiles, for examples, are commonly produced by a series of right-angle bends to a flat sheet of metal. Several sheet metal façade connections also make use of this forming process. An edge-return is a feature commonly used in facades to hide a substructure; a façade panel (usually the last) is bent backwards into itself in order to close the gap between the outer skin of the façade and the supporting structure. In metal facades, this is achieved through a sheet-bending process. Several alucobond connections make use of sheet-punching and sheet bending as a means of securing to a façade substructure (Fig 2.21)

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Fig 2.21 typical Alucobond Façade detail which makes use of punching/laser cutting and sheet bending.

In this process, sheets of metal are progressively passed through a series of rollers which cause it to gradually deform into the desired shape. When more complex shapes are required, it is common to combine the roll-forming process with other sheet operations such as punching or sheering. This process is commonly used to create both open and closed profile sections (Fig 2.22) Roll forming is typically used for creating profiles such as façade sub-structures. In some instances, the sheet metal is pre-perforated before bending, allowing for light-weight profiles. These are typically used in building service engineering in elements such as cable trays. Several rollers can also be combined to produce corrugated profiles

Fig 2.22 A typical roll forming technique. A flat sheet of metal is gradually bent by rollers to make a channel section used in facades. (Image source: Ulrich Knaack)

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2.4.3 Sheet Spinning A Sheet Spinning process is used to form circular sheets of metal by rotating it and applying force to one face of the material by means of a mandrel. Sheets formed in these methods have one rotational axis of symmetry and is commonly used to form semi-dome, cylindrical or conical shapes with diameters up to 6meters long (Fig 2.23). Sheet spinning has many sub-processes such as Tube Spinning, where a sheet is formed internally into a cylindrical structure, or Shear Spinning; a variation of conventional spinning which also stretches thicker sheet metal along the mandrel. Although sheet spinning is mostly used for items such as Lighting fixtures or Bins, it has also been applied for the use in building construction. The façade for the Selfridge Store in Birmingham consisted of 15,000 Sheet metal disks attached to a substructure. These were produced by first pressing the disks into the desired shape and later forming an edge-flange by spinning the disk and folding the edges

Fig 2.23 A typical spin forming technique involves a solid mold spinning at high speed.

2.4.4 Deep drawing Deep Drawing is a form of stretching sheet metal into place. A tool is used to press down onto the sheet material, forming into a dye of the desired shape. Tensile forces cause the sheet to deform plastically into a cup shape. Although it is possible to form sheets having various cross-sectional dimensions, deep drawing is most commonly used for creating cylindrical and rectangular parts. Because this process has high tooling costs, deep drawing is most beneficial for when a high-number of repetitive elements are needed. However, when the batch sizes are large enough, deep drawing is one of the fasted methods for forming three-dimensional sheets since it relies on simple punching motions.

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Fig 2.24 A deep drawing process consists of a punch which is used to gradually shape and stretch sheet metal.

The façade of the Tripla Mall in Helsinki is one showcase of the deep drawing technology applied to architecture. Completed in 2018 by Scandinavian architects Soini & Horto, the façade consists of over 5,000 façade panels which were individually shaped using the deep drawing process. As seen in Fig 2.25 the perforated façade was produced using a geometrically-complex punching dye. This allowed for a complex façade screen to be fabricated quickly and efficiently. [24]

F i g 2 . 2 5 T r i p l a M a l l f a ç a d e , p r o d u c e d u s i n g a D e e p D r a w i n g p r o c e s s ( I m a g e co u r t e s y o f Stefan Ochsner, Soini Horto)

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2.4.5 Stretch Forming Stretch is a method of forming consists of a thin sheet metal simultaneously being stretched and bent over a die to form parts with significantly large radii. This operation is performed on a stretch press; the edges of the sheet are clamped and are pulled down onto a dye to form the sheet. This method of forming allows for larger sheet-sections to be formed with significantly larger sections than the other methods described. Stretch forming allows for both simple-curved surfaces and non-uniform cross sections to be realized with high accuracy and smooth finishing.

Fig 2.26 Stretch Forming involves stretching a thin sheet metal over a mold.

Variations of this process include Multiple-Point Stretch Forming (MPSF). In this setup, rather than using a massive dye, often consisting of wood or milled blocks of metal, an adaptable pin bed is used to form a surface across which sheet metal is stretched and deformed. MPSF is presenting itself as an attractive method of deforming thin sheet metal for the automobile and aerospace industry, as it allows for a single setup to be used to produce multiple geometric variations of thin sheet metal with minimal effort and material wastage.[25]

Fig 2.27 Multi-Point Sheet Forming is a sheet forming technique utilizing an adaptive formwork rather than a fixed dye

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2.4.6 Multiple Point Forming (MPF) Similar to MPSF, Multiple Point Forming also uses an adaptive dye to form different surfaces against metal is shaped. However, rather than stretching the sheet material across the adaptable formwork, the material is pressed between a negative and positive matrix of adjustable pins, which causes it to deform and take shape. The shape of the deformed sheet is determined by the punch elements, which can be individually adjusted in height. This allows for free-form curved sheet elements (with large bending radii) to be produced. Because of the adjustable matrix, a single machine can be used for an entire production line of panels with different shapes. This method of fabrication has already been applied to the construction industry. In 2014, Zaha Hadid architects used the MPF process for the façade of the Dongaummun Design Plaza in Seoul. The building skin consisted of 45,000 unique façade panels (Fig 2.28). Traditional forming techniques would have required 45,000 unique molds. MPF allowed for a drastic reduction in costs since a single machine could carry out all the fabrication

F i g 2 . 2 8 ( a ) A t yp i c a l M u l t i p l e P o i n t f o r m i n g s e t u p ( S o u r c e : M. L i [ 2 5 ] ) w h i c h w a s u s e d f o r the construction of the Dongaummun Design Plaza (source: Wikicommons)

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2.4.7 Hydro Forming Hydroforming is process which uses a non-compressible liquid as a means of applying high pressure to sheet material against a molding surface. The process is particularly useful in shaping thin and ductile materials, such as steel, copper and aluminum. Although the process is largely used in the automotive industry, where single molds can be used to mass-produce components, the technique has also found its way into façade applications. The earliest example of this is the façade cladding at the Science and Engineering complex (SEC) at Harvard University. Completed by Behnisch Architects in 2021, the façade consisted of sheets of stainless steel which were shaped under high pressure against concrete molds (Fig 2.29). This allowed for a façade cladding screen to be produced with high precision, as required in building facades.

F i g 2 . 2 9 T h i n - sh e e t p a n e l a f t e r h y d r o f o r m i n g a g a i n s t a c o n c r e t e m o u l d . ( S o u r c e : J a n i s Rozkalns/Behnisch Architekten) and (b) its installation on site

2.4.8 Incremental Sheet Forming (ISF) In an ISF process, metal sheets are deformed by means of incremental and localized deformations [26]. This is generally carried out by means of a tool attached to a CNC machine or robotic controller which gradually deforms a sheet of metal that is clamped around the edges. ISF comes in two main variations; Single Point Incremental Forming (SPIF) and Two-Point Incremental forming (TPIF). The underlying difference between the two techniques is in the number of tools used. In SPIF, a single tool is used to press against one face of sheet metal. In TPIF, two tools are used; one on either face of the sheet positioned mirrored to each other. These methods of forming allow for highlydetailed deformations shaped into sheets of metal (Fig 2.30)

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Fig 2.30 A typical Incremental Sheet Forming Process

As with other new sheet forming processes mentioned, ISF has found its way into architecture more recently[27]–[29]. Researchers at University College London used SPIF in combination with a robotic controller to deform and stiffen thin sheet metal [28]Researchers at KADK combined ISF with machine learning to produce a bridge structure consisting entirely out of formed sheet metal [29].

F i g 2 . 3 1 ( a ) I n cr e m e n t a l s h e e t m e t a l f o r m i n g i n c o p p e r . I m a g e : S t e v e D e m i c o l i , M a t t e r M a k e , M a l t a ( b ) R o b o t i c I n c r e me n t a l s h e e t f o r m i n g . I m a g e : C r i s t i n a G a r z a L a s i e r r a , U C L , London

2.4.9 Bead Rolling is a sheet-forming method in which forming tools are used to press grooves into sheet metal by means of bead rollers. A typical setup consists of positive and negative roller dyes (Fig 2.32) places on either side of a sheet of metal. As sheet metal is passed between the two dyes, it creases according to the shape of the dyes, forming an indentation in the material. It’s a method of forming which is commonly used to add stiffness to sheet metal parts by forming deep grooves into the material [30]. Other than stiffening large plates of material, commonly found in the automotive, oil and gas and aerospace industry, bead rolling is also used as a means of locally stiffening thin sheet metal prior to welding.

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Fig 2.32 A Bead Rolling process making uses of rolling dyes to deform sheet metal

A contemporary adaptation of bead rolling was further developed by researchers at Harvard University [31]. In this instance, a standard bead-rolling device was used in combination with a robotics arm, which allowed for more complex stiffening patterns to be creased on sheet material (Fig 2.33)

Fig 2.33 An example of robotic bead rolling, used to program complex stiffening patterns into a thin sheet of metal (Source: Friedman et al [31])

3

Additive Manufacturing

As its name implies, Additive Manufacturing is a process where materials are added to each other to form a three-dimensional object. This is a unique method of fabricating objects when compared to other commonly-used fabrication techniques. As illustrated in Fig 3.1 to fabricate an object using Subtractive Manufacturing, a blank of raw material (such as steel, block of stone etc.) which is larger than the intended object, is gradually reduced using processes such as milling. In the case of formative processes, one material is formed against a mold: be it concrete poured within a wooden formwork (concrete casting) or plastics formed against a metal mold under pressure (injection molding). These processes are often used in the context of mass-production of repetitive objects, as they generally allow for quick production rates with high quality finishing using a single mold.

F i g 3 . 1 S u b t r a ct i v e , F o r m a t i v e a n d A d d i t i v e M a n u f a c t u r i n g p r o c e s s e s

In an AM process, no blank material or molds are used. Rather, only the required material needed for the final geometry used (or in some cases, a temporarily printed support) as it is gradually deposited and bonded together in layers. A general AM process begins with a digital model of the object (Fig 3.2a), which is rationalized as horizontal cross-sections, spaced at a distance equal to the height of the printed material (slicing) (Fig 3.2b), with each slice referred to as a layer. Motion controllers, such as Computer Numerical Control (CNC) Devices or robotic arms, are used to precisely deposit material following the paths defined by each layer, gradually building up a threedimensional object (Fig 3.2c).The result is a manufacturing process which allows for complex threedimensional structures to be manufactured with relative speed and accuracy. © The Author(s), under exclusive license to Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2023 C. Borg Costanzi, Reinforcing and Detailing of Thin Sheet Metal Using Wire Arc Additive Manufacturing as an Application in Facades, Mechanik, Werkstoffe und Konstruktion im Bauwesen 68, https://doi.org/10.1007/978-3-658-41540-2_3

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Fig 3.2 An AM process begins with a digital model which is sliced into layers

3.1 Additive Manufacturing Processes The ability to build up layers of material has led to a diverse library of AM materials; including, but not limited to glass [32], [33], ceramics [34][35] and concrete[36] . The requirements for fusing different materials have resulted in numerous distinct AM processes. AM of glass, for example, requires the material to be heated until melting its point in order to achieve enough viscosity to be shaped. Concrete, on the other hand, is generally extruded through a nozzle, achieving viscosity through the addition of water and chemicals. Although there are numerous methods of classifying the various AM processes, a popular method, outlined by the ASTM Committee on Additive Manufacturing Technologies (F42), is to classify processes by the method in which material is fused together. In this regard, the ASTM committee classified AM into 7 distinct process classes, and are summarized as:

3.1.1 Material Extrusion (ME) As its name implies, Material Extrusion (ME) processes are characterized by the extrusion of material. This may be achieved by heating material up to its melting temperature, after which it is extruded through a nozzle and cooled down to form layers; as is the case with Fused Deposition Modelling (FMD) of plastics and glass material. Primarily consisting of thermo plastics, FDM AM has made its way as an application into the construction industry, with applications including from performance-driven facades (Fig 3.3a)[37] [38] , temporary formwork for concrete casting[39] and circular construction [40](Fig 3.3b). Direct Ink Writing (DIW), on the other hand is a ME process in which visco-elastic material, such as ceramics and cement, is deposited and subsequently hardened as layers are formed, with earliest examples of this dating back as far back as 1945[41], when William Urschell 3d printed the World’s first large-scale object in concrete. DIW is currently one of the most-commonly applied AM processes in construction, with large scale AM projects in cementitious[42],[1] and earthen[43], [44] materials already in the public domain.

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Fig 3.3 (a) Printed Insulated Glass Spacers (source:TU Darmstadt, BASF, Okalux) (b) R e c y c l e d P l a s t i c P r i n t i n g ( S o u r c e : T h e N e w R aw [ 4 0 ] ) a n d ( c ) D i r e c t I n k W r i t i n g o f cementitious material (source: Borg Costanzi et al [1])

3.1.2 Vat Polymerization (VP) Vat Polymerization (VP) is an additive process in which a vat of liquid photopolymer resins is selectively cured and hardened using a UV light source, and includes sub-processes such as Stereolithography (SL) and Direct Light Processing (DLP). In a typical VP process, a build platform is lowered into the vat of photopolymer resins and a curing light source is targeted at the surface of the vat, causing a thin layer of resin to harden. In general, the build platform is further lowered into the bath, and a new layer of resin is selectively-cured. The process of moving the build platform and curing layers of resin is repeated to build up a three-dimensional object.

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Fig 3.4 A typical Vat Photopolymerization process involves the curing of photocurable resins contained in a vat. The printed object is either gradually (a) submerged into the bath or (b) lifted out of it as the parts are cured.

Apart from being one of the earliest methods of Additive Manufacturing, objects produced in a VP process are generally of higher resolution than those in thermoplastic FDM printing, allowing for ultra-high-quality prints in a relatively quick process. The limitation with this technique, however, is that the size of the printed parts is heavily restricted by the size of the Vat. So far, applications of VP in construction have been rather limited, and is more commonly used for high-detailed and small-scale parts and rapid prototyping.

3.1.3 Material Jetting (MJ) and Binder Jetting (BJ) Material Jetting (MJ) and Binder Jetting (BJ) are two similar AM processes, distinguishable by the stock material used in the process. In a Binder Jetting process, a layer of powdered material is laid down onto a bed, in a similar manner as Powder Bed Fusion process. The underlying difference being that in Binder Jetting, particle material is solidified by means of a secondary binding chemical, such as resins, rather than a heat source. One of the earliest examples of Binder Jetting was demonstrated by Enrico Dini, who in the early 2000’s produced a full-scale, single-roomed dwelling as well as a large-scale sculptural element by selectively curing powdered magnesium oxide powder with magnesium chloride (Fig 3.5a). To date, Binder Jetting has been used with a wide library of materials including powdered stones, cement and ceramics [45].

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Fig 3.5 (a) A typical Binder Jetting Process and (b) A typical Material Jetting Process

Material Jetting, differs from Binder Jetting in that rather than using powder as a material source, liquid ink is deposited in layers, very much like traditional 2D Printer technology. With each layer deposition, the liquid ink, which is generally in the form of photo-curable polymers, is selectively hardened by means of UV-light curing. Objects produced using MJ are able to be produced with a very high resolution, with the possibility of combining multiple resin sources, allowing for singlyprinted parts to consist of multiple-materials with variable material characteristics (Fig 3.6 b).

F i g 3 . 6 ( a ) E n r i c o D i n i ’ s B i n d e r J e t t i n g p r i n t e r s ( S o u r c e : C o u r te s y o f E n r i c o D i n i ) a n d ( b ) M a t e r i a l J e t t i n g p a r t s , F O R M NE X T 2 0 2 1 , F r a n k f u r t

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3.1.4 Powder Bed Fusion (PBF) Powder Bed Fusion (PBF) is an AM process in which layers of powdered material as fine a 9µm in size, are spread over a horizontal bed. A heat source is used to locally-liquify particle material, usually metals or ceramics, causing it to melt and solidify once cooled, while the unheated particles remain unmelted. Once this is completed, a fresh layer of powdered material is spread over the entire bed, and particles in the newly-laid layer are selectively melted. This process of depositing a fine layer of material, followed by localized melting, is repeated for the entire height of the part to be printed. This results in a solidified object (caused by the selective melting of powdered material) and unmelted powder which is dusted away.

Fig 3.7 High resolution metal printed objects. (Source: Courtesy of PMD, TU Darmstadt)

Due to the use of fine material PBF is often used for the production of small-scale, highly-intricate objects in metals and ceramics (Fig 3.7). The process is described in further detail in section 3.2.1

3.1.5 Directed Energy Deposition (DED) With sub-processes including Laser Engineering Net Shape (LENS), Electron Beam Melting (DED), and Wire Arc Additive Manufacturing (WAAM)( Fig 3.8), Directed Energy Deposition is characterized by the extrusion of a wire, or spraying of powdered material feedstock, and subsequently melting it as it exists a nozzle. Depending on the process and material used, this generally done by means of lasers or a welding arc. Powder-Based DED processes also allow for high-resolution parts to be printed, as with powder-bed printing, with the benefit of not requiring an environmental enclosure, opening opportunities for larger scale objects to be produced. In general, a Directed Energy Deposition process utilizes metallic materials, and is further described in section 3.2.1

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Fig 3.8 Wire Arc Additive Manufacturing, a DED process based on wire-feed material

3.1.6 Laminated Object Manufacturing (LOM) The principle of the AM process is that layers of cut sheet material are bonded together in succession; be it adhesive-coated sheets of cardboard, plastic or wood that are bonded together, or metal sheet material which are ultrasonically-welded to one another. The earliest example of a LOM process dates as far back as 1892, where Blanther[46] produced topological contour relief maps by shaping sheets of wax material and stacking them to form a three-dimensional object. More recent applications of LOM are illustrated by Michael Hansmeyer, who used 2,700 sheets of laminated sheets of greyboard to produce 2.7m-high architectural columns in paper (Fig 3.9).

Fig 3.9 LOM in Paper. (Source: Courtesy of Michael Hansmeyer)

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3.1.7 Discussion The type of Additive Manufacturing process used is highly dependent on factors including budget, size and quality of parts to be produced as well as materials. Fused Deposition Modelling (FDM), for example, is widely used for producing parts quickly and cheaply in thermoplastics. In construction, FDM has already been applied in façade research [37] and as a means of circular construction through the use of recycled plastic materials [40]. More recently, Fused Deposition Modelling has also been used for novel AM materials, such as glass [32], [47]. Other material extrusion processes, such as robocasting, are currently some of the most applied Additive Manufacturing methods adopted due to its application in large-scale concrete [41], [42], [48] and ceramic additive manufacturing. Fig 3.9 is an image still extracted from BE-AM.de, an online database curated by the author illustrating the different AM materials used in construction projects (as at 2021). What is of significant noteworthiness is that cementitious materials (cement and concrete), and by extension, Material Extrusion processes, contribute to around 40% of all referenced projects. Plastics, also largely produced by material extrusion processes, account for 18.9% of referenced projects with metals contributing to 10.8% of the total. Bio-composite materials, which are largely produced using material extrusion processes thus far only account for 3.5% of referenced projects, however have also gained traction in research during the mid of 2021 and early 2022.

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3.2 Additive Manufacturing of Metals It has already been established that AM is becoming more commonplace in various industries and not just limited to construction. In 2020 the AMFG Autonomous Manufacturing published an annual report showcasing the current state of the global Additive Manufacturing landscape [49]. In the report, it was shown that the production of Metal Additive Manufacturing devices and Metal Materials had the most contribution to development.

Fig 3.10 Development of Metal Additive Materials and production of Metal 3D printers were dominant in the 2020 Additive Manufacturing Landscape. (Source: AMFG Landscape 2020 [49])

While most of the development was on high-resolution, small scale powder printing devices, the report also showed a development towards larger-scale metal printing processes. The possibilities of metal additive manufacturing currently range from nano-scale printed objects [50] to large-scale construction objects [51] Metal additive manufacturing primarily works by means of either fusing metal powder, a wire feedstock or a combination of the two. Each method has its own advantages and best-fit areas of application, and may be classified as: (a) Powder Bed Fusion (b) Powder Directed Energy Deposition (c) Wire Directed Energy Deposition

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3.2.1 Powder Bed Fusion As discussed briefly in 3.0, Powder Bed Fusion (PBF) refers to an AM process which uses a heat source (e.g., a Laser) to melt powdered materials together. In the context of metal AM, most metals, including steel, titanium and copper[52] are possible to be fabricated so long as they are available in powdered form. As illustrated in Fig 3.11 (left), metal powder contained inside a bed is gradually laid down in layers. With each layer, a heat source melts metal particles together, causing them to fuse Fig 3.11(right). Once all the particles for each layer are melted, a new layer of raw metal powder is passed over the completed part and the process is repeated. Depending on the size of the metal powder used, parts produced with PBF typically have layer thicknesses in the range of 20 -100 µm , allowing for high-resolution parts to be printed[53].

Fig 3.11 Principle of Powder Bed Additive Manufacturing systems. (left) Schematic of a t y p i c a l s e t u p a n d ( r i g h t ) s c h e ma t i c o f m e t a l p a r t i c l e s s e l e c t i v e l y f u s e d t o g e t h e r b y a h e a t source.

Once a part is completed, a post-processing step is carried out where metal particles which were not heated are cleaned away from the printed object. The unfused powder also serves as a temporary support material during printing, allowing for printing without support structures. The ability to print in high-resolution with intricate detail has opened up new possibilities in what is possible to fabricate in metal. One such being the use of structurally-efficient and lightweight lattice structures; an area of research which has gained quite some traction for producing material-efficient structural typologies. Adopting the use of generative design techniques, Materialise, Reinshaw and Altair[54] were able to produce a topology-optimized spiderglass bracket holder which was rationalized as a lattice structure (Fig 3.12a). The ability to small-scale structures at high resolution has resulted in PBF being applied across industrial sectors including the bio-medical industry for the printing of implants (Fig 3.12b) [55] as well as the aerospace industry for the production of light-weight aircraft and satellite components [56], [57].

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Fig 3.12 (a) Spiderbracket produced in PBF (Source: Materialize, Altair and Reinshaw [58]) and (b) High resolution lattices (Source: Kolken et al [55])

However, while PBF does allow for highly-intricate and optimized structures to be realized, parts produced with this AM process are limited by the dimensions of the process enclosure, which is required to create a barrier between the printed part and the surrounding environment. In this regard, the nature of PBF being a closed-system results in the applications for the metallic AM process to be to small-scale components.

3.2.2 Powder-Directed Energy Deposition (P-DED) A P-DED process is generally characterized by the use of a single nozzle which is used to shoot metal particulate material towards a substrate material. At the same time, a focused energy source is used to create a weld pool on the substrate material, which solidifies and forms a weld bead [52]. This process is repeated to build up three-dimensional objects. Unlike PBF AM, most P-DED systems do not require an enclosing space, and parts produced with P-DED are not necessarily restricted by geometrical dimensions, with some commercial systems able to work within enclosures of 3000x3000x3000mm in size[56]. Coupled with the ability to print using relatively coarser powders, P-DED processes allow for large-scale parts to be produced with fast deposition rates allowing for large-scale parts to be manufactured. The AM process begins with deposition of molten material onto a sacrificial substrate material, which is removed after welding. However, this also opens up the opportunities for depositing material onto substrate material which becomes part of a finished part. In this sense, P-DED has also found its use in applications of repairing and cladding of metal structures, apart from the AM of entire parts[59]. The ability to build up material with fast deposition rates has made P-DED also a viable AM method for producing parts, such as molds and tooling devices[59], in combination with subtractive processes to achieve high resolution surface finishes. Moreover, by locally varying the composition of the powdered material through pre-mixing, functionally-graded structures with variable stiffnesses have also been realized[60]. On the other hand, the downsides to P-DED include the lack of supporting structure, such as that used with PBF, results in fewer possibilities of producing intricately-detailed objects.

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Fig 3.13 (a) P-DED used to repair a component of a Titanium blade (Source : Nowotney e t a l [ 6 1 ] ) , ( b ) P - D E D u s e d w i t h m i l l i n g t o p r o d u c e t o o l i n g p a r t s ( s o u r c e : P a l č i č e t a l[ 6 2 ] ) a n d (c) use to produce Functionally-graded metal materials (source: Carrol et al [60])

3.2.3 Wire-Directed Energy Deposition (W-DED) A Wire-Directed Energy Deposition (W-DED) process is similar to P-DED in that a self-containing nozzle, or combination of nozzles, are used to expel material feedstock onto a metal substrate which is subsequently melted to form a weld bead and, eventually, three dimensional structures. As its name implies, a wire (or stick) electrode is extruded in the place of powdered material, and has similar benefits to P-DED processes, including the ability to weld onto pre-existing structures for the addition of detailing or connections [63], repair of damaged structures. Depending on the heat source used, W-DED comes in various unique processes, including laser-wire DED , electron-beam DED and Wire Arc Additive Manufacturing (WAAM). The use of a wire feed stock allows for WDED to achieve high deposition rates of material and the use of a self-contained nozzle places WDED as a prominent contender in applications of large-scale additive manufacturing, as was illustrated by the completion of a 12-meter-long bridge structure over the canals of Amsterdam[51]. As with other DED AM, subtractive processes may also be used in order to counteract the high surface roughness of the additive manufacturing process, allowing for not only the production of object in a large scale, but also the ability to achieve a high surface finish and level of detailing. This has made W-DED particularly useful in the context of shipbuilding and aerospace industries (Fig 3.14a) [7], [64]the oil and gas industry (Fig 3.14b) [51] as well as construction[51], [65]–[69] (Fig 3.14c).

Fig 3.14 W-DED used to produce (a) Ship Propellors (source: Ramlab [7]), (b) piping for the oil and gas industry (Source: MX3d[9]) and (c) fully-printed steel nodes (Source: D.Yili et al [69])

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3.2.4 Discussion The dominant distinguishing features between parts produced with metal AM include the maximum size of parts able to be produced (scale), the level of detail achievable which is affected by the minimum layer size (resolution) and the speed in molten material can be deposited (deposition rate). Powder Bed Systems are the most restricted when it comes to the maximum possible scale that a part can be produced in ; the environmental enclosure required for the process imposing this restriction. Nonetheless, large-volume PBF printers, such as the GE ATLAS (1100mm x 1100mm x 300mm build volume), already allow for relatively large parts to be produced. However, parts produced with P-DED and W-DED systems where no enclosing environment is required are capable of being produced in a far larger scale, and are generally restricted by the size and mobility of the motion controller. In 2016, researchers at Cranfield University illustrated the potentials of large-scale AM with W-DED [70], combining a moveable robotic arm on a travelling rail system to produce a 6-meter-long aluminum spar for the aerospace industry. Similarly, the researchers at Northwest Polytechnic University developed a large-scale Laser Additive Manufacturing cell and exploited the dimensional freedom P-DED to produce a 5-meter-long aircraft wing spar in Titanium[71]. With regards to differences in resolution of prints, Powder-based systems generally are capable of printing with fine material stock, allowing for high geometrical accuracy to be achieved, with layer thicknesses of the material typically falling in the range of 20-100 μm[53], Due to the use of a wireelectrode in W-DED, these processes tend to have far larger typical layer thicknesses (1-2mm[72] compared to P-DED, which allows for more-rapid deposition of material, however with the consequence of having a higher surface roughness. In this regard, post-processing steps (milling) would be required to achieve a high dimensional accuracy. The rate at which material is deposited is also quite varied between the different processes; with W-DED typically capable of depositing material at rates of 4-9kg/hr.[11] in contrast to the typical rates for P-DED(1kg/hr.). Due to the thickness of powdered material, PBF processes have a much lower deposition rate (ca. 50g/hour)[73] Therefore, the application of a metal-based AM process is highly dependent on the particular application; a highly-intricate, small scaled part (such as lattices or parts with internal voids) are more likely to be manufactured using PBF. On the other hand, large parts which require high turnover rates, such as those commonly found in construction, are more suited to be produced using a WDED manufacturing process.

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Wire Arc Additive Manufacturing

Wire Arc Additive Manufacturing (WAAM) is a Wire-Directed Energy Deposition method of Additive Manufacturing. In the broadest sense, a typical WAAM process consists of off-the-shelf welding equipment which is affixed to a motion controller (Fig 4.1) (generally, a robotic arm or gantry) which is used to consistently and accurately deposit weld material. The ability to use relatively lowcost stock material, achieve high rates of material deposition combined with relative geometric freedom, has resulted in WAAM being an attractive manufacturing process for the production of steel structures in construction.

Fig 4.1 A typical Wire Arc Additive Manufacturing setup. (Source: Cranfield University[70]

In order to initiate a welding process, a closed circuit is required. This is achieved by means of a welding arc, which is formed between the electrode tip of the welding device and a metal workpiece, both of which are connected to a common power supply.

Fig 4.2 Schematic of a welding circuit setup

© The Author(s), under exclusive license to Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2023 C. Borg Costanzi, Reinforcing and Detailing of Thin Sheet Metal Using Wire Arc Additive Manufacturing as an Application in Facades, Mechanik, Werkstoffe und Konstruktion im Bauwesen 68, https://doi.org/10.1007/978-3-658-41540-2_4

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As the voltage between the workpiece and a consumable welding electrode (a wire, in the case of WAAM) that are physically separated increases, an electric discharge occurs between the two parts. The discharge, referred to as an arc, is formed due to the breaking down of the insulating air gap between the electrode and workpiece, causing current to be discharged whilst converting electrical energy into heat energy. This, in turn, causes the heating up of both the parent material and wire electrode, causing the tip of the electrode to melt and deposit molten material (see section 4.3). When the deposition of material is synchronized with a controlled movement of the welding device, continuous lines of molten (and hardened) material called welding seams are formed. Because WAAM is a process which allows for the precise control of the movement and deposition of material, seams are able to be deposited in succession over one another (layers) to form three dimensional structures. Moreover, the precise control over both movement and deposition also allows for conventional welding techniques, such as spot welding, to be used to create three-dimensional structures (Fig 4.3).

Fig 4.3 The controlled deposition of molten weld material has allowed for novel metal structures to be formed with WAAM

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4.1 WAAM in Construction Although the basic description of the WAAM process dates back to Ralph Baker’s 1925 patent [6], the use of the technology in construction is still in its relative infancy, yet is rapidly being applied in research industry. One of the earliest examples of its application in the construction industries comes from Joris Laarman and MX3D who, in 2014, produced the first examples of large-scale lattice structures in WAAM [74]. Some 7 years later, the same group, installed the world’s first fully-printed pedestrian bridge over the canals of Amsterdam (Fig 4.4a) [51]. During this span, there have been considerable developments in the use of WAAM as a production process in construction [75]–[77]. LASIMM, a research consortium of industry and academia [78] propose the use of WAAM in combination with subtractive post-processing as a means of producing large-scale building structures, resulting in a 5meter-long cantilever structure optimized for structural performance [78]. Further contextual studies of large scale WAAM in construction include the first on-site printed bridge, completed by researchers at Darmstadt Technical University (Fig 4.4b) [67], half-scale diagrid column structures produced using a point-wise method of depositing metal material (Fig 4.4c) [79] as well as agent-based aggregate structures in the context of architectural structures (Fig 4.4d,e) [80].

Fig 4.4 (a) Fully-printed Steel bridge (Source: MX3d[51]), (b) On-site printed bridge at T U D a r m s t a d t ( S o u r c e : F e u c h t e t a l[ 6 7 ] ) , ( c ) F u l l y - P r i n t e d d i a g r i d s t r u c t u r e ( s o u r c e : c o p y right Laghi et al [79]), (d,e) WAAM-Produced sculpture (source: images courtesy of Prof. Roland Snooks, Dingwen Nic Bao (RMIT) [80])

Apart from the production of individual components, WAAM has also offered potential as a means of connecting steel elements. In most of the projects illustrated above, parts are produced on a sacrificial metal baseplate, required for initiating a welding process, which is removed and discarded after manufacturing. However, WAAM also offers the potential of being used in combination with standardized steel elements – which serve as a substitute for discarded material. The authors of the thesis, together with Technical University Ilmenau, Hoelscher Kleve GmbH and Imagine Computation GmbH, illustrated the potential for this in a 3D-Printed spider glass façade, by printing bespoke connection details directly onto sub-structure coupling plates, used to fix the details to a concrete wall Fig 4.5a) [16]. Similarly, researchers at ETH Zurich used a pointwise method of WAAM to connect and assemble standardized steel tubular elements Fig 4.5b) [68], allowing for complexgeometry spatial structures to be connected freely in three-dimensional space. Moreover, Feucht et al (Fig 4.5c) and Erven et al (Fig 4.5d) adopted the use of WAAM to produce connection and

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stiffening details for standard IPE beam sections. Combining WAAM with generative design processes, the researchers propose the use of WAAM as a means of not only achieving an automated fabrication process of steel connections, but also the optimization of the printed steel structures.[63], [66]

F i g 4 . 5 ( a ) B e sp o k e S p i d e r g l a s s c o n n e c t i o n s ( S o u r c e : [ 1 6 ] ) , ( b ) W A A M f o r t h e c o n n e c tion of steel elements (Source: Ines Ariza ([68]). Feucht et al (c) and Erven et al (d) use as a means of connecting elements (Source: Feucht et al and Erven et al([63], [66])

Utilizing the geometric freedom and the quick production cycle of the production process therefore opens up potentials for not only realizing fully-printed structures on a large scale, but also offers potentials for hybrid fabrication processes in which standardized, mass-produced building-stock elements are reinforced or detailed through the addition of WAAM material. The parallel development of computational design tools, which are often freely and widely-available to use, is aiding in formulating new and novel design paradigms. In the context of construction, we are therefore witnessing WAAM as a method not only as a means of complimenting the mass-production process, whose bottle neck lies in mass-customization, but also as a means of formulating new, performance-driven design solutions.

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4.2 WAAM Process Wire Arc Additive Manufacturing (WAAM) is a wire-based Additive Manufacturing process which falls under the category of Directed Energy Deposition (DED). Standard off-the-shelf welding devices are coupled with motion controllers to deposit molten weld material in a layerwise fashion. Although there have been considerable developments of WAAM since its description in 1925[6], with multiple variations in setups possible, a typical WAAM setup would consist of: a) b) c) d) e) f)

Gas Metal Arc Welding (GMAW) / Plasma Arc Welding (PAW) welding torch, which also includes material-feeding and shielding gas delivery Motion controller, usually consisting of robotic systems or computer numerically controlled (CNC) gantries. Motion controlling unit, used to communicate with the motion controller and receive data from the operator. Shielding Gas, which is delivered through the GMAW/PAW torch and is used to protect the welding arc during the production process Material delivery and welding parameter controller Base material on which the WAAM process beings.

Fig 4.6 Typical WAAM setup consisting of (a) Welding torch (b) Motion Controller (c) Motion Controller Unit (d) Shielding Gas (e) Wire feed delivery and welding control unit (f) Parent material

WAAM processes consist of Fusion-Arc Welding, and can be distinguished from one another by the means in which heat and material feed is delivered. The three most dominant WAAM processes

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can be categorized as Plasma Arc Welding (PAW), Metal Inert/Active Gas (MIG / MAG) welding and Tungsten Inert Gas (TIG) Welding, with the latter two processes falling under the fusion welding category of Gas Metal Arc Welding (GMAW) [81].

Fig 4.7 Overview of Fusion Welding Techniques (Source: Adapted from H.Arora et al [82])

A Metal Inert/Active Gas (MIG/MAG) welding process (Fig 4.8a), is characterized by the coaxial feeding of a wire electrode through a welding gun which also contains a delivery system for shielding gas[53]. The processes are distinguishable from one another by the type of shielding gas used. A MIG process utilizes a chemically inert shielding gas (e.g., mixtures of Argon, Helium or Nitrogen), whereas MAG utilizes a mixture of chemically Active Gasses (e.g. Mixtures of CO2 or Oxygen) and Inactive Gasses. The choice of gas used has a direct effect on the printed part; an active gas, for example, will partially break down during the heat-intensive welding process. This energy release of the active gas during the process has certain beneficial weld properties, such as deeper penetration and altering of the bead geometry, to be altered. Tungsten Inert Gas (TIG) welding, also commonly referred to Gas Tungsten Arc Welding (GTAW), distinguishable from MIG/MAG welding through the way in which material is fed. While MIG/MAG make use of a single welding torch containing material feed and gas supply, in a TIG process the material feed is provided externally to the welding torch in the form of a nonconsumeable tungsten electrode. Because of this, TIG setups require secondary equipment for feeding material in addition to the welding torch. Due to this, TIG falls short when compared to MIG/MAG in that path-planning could be required to take into consideration the orientation of the tungsten electrote relative to the direction of motion, although some market-available systems for providing co-axial TIG welding do already exist [83] Plasma Arc Welding (PAW) welding is similar to TIG in that a non-consumable tungsten electrode is used as a heating material, however it maintains the benefits of MIG/MAG welding in that this is fed co-axial to the nozzle direction. This WAAM process uses a highly-ionized gas mixture which results in an energy-intensive welding process when compared to TIG and MIG/MAG. However, the high density of energy concentration allows for higher penetration of weld beads into substrate material, as well as the production of a smaller heat-affected zone which allows for thinner-wall sections to be welded [81].

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Fig 4.8 Comparison between (a) MIG/MAG, (b) TIG and (c) PAW welding setups (Source: A d a p t e d f r o m D. D i n g e t a l [ 5 3 ] )

A 2018 comparison study between MIG/MAG, TIG and PAW [84] highlighted the fact that, although each of the WAAM processes have their distinct advantages and disadvantages, it is important to note that the use of a particular setup is highly situational and, in that sense, there is no WAAM process which is ununamously superior. A Fronius CMT process, which is a variant of MIG/MAG welding, was found to be most suitable for large stainless steel components with low / medium mechanical requirements, whereas both TIG and PAW were shown to be beneficial for use with small / medium steel and titanium parts with high requirements for mechanical performance. Of course, this is not to say that MIG/MAG isn’t applicabe for smaller-sized objects, especially when considering other factors such as the relatively cheaper cost and simplicity when compared to TIG/PAW. The research presented in this thesis makes use of a GMAW MIG welding process called Fronius Cold Metal Transfer Cyclestep (CMT Cyclestep). This is an adapted version of standard shortcircuiting MIG welding, which is fast being adopted in research due to the co-axial nature of welding, fast deposition rates using low welding temperatures in combination with stainless steel [84]–[86]. The next sections decribe in more detail the heat transfer mechanisms of a standard GMAW, distinguishing between short-circuited, globular, pulsed and spray transfer modes, together with typical process parameters which have to be taken into account to ensure a stable welding process.

4.2 WAAM Process

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Fig 4.9 GMAW welding setup at TU Darmstadt (2018)

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4.3 Heat Transfer Mechanisms During a typical GMAW process (Fig 4.10a), metals are heated to their melting point by means of an electric arc which is formed between a workpiece and an electrode wire. The electric arc causes the tip of the electrode to melt and form droplets at its tip. As the molten material grows, it detaches from the electrode – through gravitational force, surface tension and electromagnetic forces [87] (Fig 4.10b) – and forms a molten weld pool. Because the electrodes consist of uncoated material, a shielding gas is provided to protect the arc and molten material from atmospheric contaminants.

Fig 4.10 (a) Components of a GMAW arc and (b) Heat Transfer mechanisms from wire e l e c t r o d e t i p t o m e t a l s u b s t r a t e ( S o u r c e : A d a p t e d f r o m L . J o n e s e t a l[ 8 8 ] )

The transfer mechanisms of molted weld material are greatly affected by both the current (I) and voltage (V) used in the heating process, as well as the chemical composition of the shielding gas used to protect the welding arc [87]. Heat transfer mechanisms are generally categorized by the characteristics of the molten drops (size, frequency, shape) and welding parameters (current, voltage, weld diameter, and shielding gas). The heat transfer mechanisms can be categorized as short circuiting, globular, pulsed and spray transfer modes. The choice of transfer mechanism used depends on the intended application; a globular transfer mode, for example, is beneficial when high deposition rates are needed but, due to excessive splatter, is applicable where appearance or cleaning is not an issue. Short circuiting transfer, on the other hand uses comparatively lower welding amperes, making it ideal for thinner gauge materials. However, it does not have the same penetration as globular transfer when used with thicker materials. A globular transfer is characterized by the formation of large irregular balls of molten material as the electrode tip is heated up. Once a large enough ball is formed to overcome gravitational forces, it detaches from the unmelted electrode to form an uneven weld pool on the workpiece. During the process, high amperage and wire feed speeds are used, which allows for relatively fast movement speeds during welding with deep penetration. However, due to the energy-intensive welding process, a globular transfer mode is generally reserved for workpieces which are greater than 3mm in thickness. Moreover, globular transfer is a relatively aggressive and irregular welding mode, which often results in considerable amount of spatter material and, therefore, the need for cleaning of a workpiece after a welding job is complete [87], unless a submerged welding process is used. Due to

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the formation of large, irregular, drops of molten material which are susceptible to gravitational forces, a globular transfer mode is reserved for flat and horizontal work pieces.

F i g 4 . 1 1 D e p o s i t i o n o f d r o p l e t d u r i n g a g l o b u l a r t r a n s m i s s i o n mo d e ( S o u r c e : A d a p t e d from Silva et al [89])

Short circuiting is the coldest form of GMAW due to the use of very low welding voltages. In this method of welding, a droplet of molten material is formed around the tip of the wire electrode. However, unlike globular welding, the droplet does not transfer to the weld pool, rather it creates a bridge between the electrode tip and the base material. This, in turn, causes a short-circuit and temporarily extinguishes the welding arc. Surface tension causes the molten droplet to detach from the electrode, dropping onto the base material and re-igniting the arc. This process is repeated several times per second (2 – 200) and results in comparatively smaller, fast-solidifying weld pools. One of the greatest advantages of short circuit welding is the use of low voltages. While this results in lower penetration in thick materials, the relatively cold method of welding makes it ideal for applications involving thinner gauge materials[.37], [40], [41]. Moreover, because of the fast-setting nature of the weld droplets, short circuit welding is not constrained to horizontal and flat weld pieces, but is also applicable to welding vertically and in over-head welding [89]

Fig 4.12 Short circuiting transfer mechanisms (Source: Adapted from Silva et al [89])

Spray Arc was the earliest method of weld transfer used in GMAW. In this process, a continuous stream of molten droplets detaches from the electrode tip and are transferred, or sprayed, to form a weld pool. This is due to the use of a high welding voltage and current, which causes the tip of the welding electrode to become pointed at high temperatures. In turn, a stream of a large number of small-diameter welding droplets are formed and detach several hundreds of times per second in a globular fashion. However, this process differs from globular transfer in that the molten droplets are usually smaller than the diameter of the electrode, and also varies from short-circuiting due to the arc constantly being present. Because of the high currents and voltages used, spray arc welding is useful for providing high rates of deposition in thicker plate material, and not ideal for welding thin

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gauge metals. Similar to a globular transfer method, the spraying of molten material makes spray arc transfer only applicable for flat and horizontal welding situations.

Fig 4.13 Spray arc welding process with the (a) formation of an arc, followed by the (b)(c) a continuous stream of molten droplets (d) forming a weld pool (source: adapted from Silva et al [89])

Pulsed spray transfer is an adaptation of spray arc transfer which takes into account the benefits of near spatter-free welding. However, unlike conventional spray transfer, there is not a continuous stream of molten droplets being formed. Rather, the power source alternates between peak current/voltage to form a droplet, and a lower base current/voltage as transfer is being taken place. Unlike short circuit transfer, the background current/voltage ensures that the arc is always ignited which helps reduce the amount of spatter and also results in a lower heat input than globular and spray transfer mechanisms.

Fig 4.14 Stages of Pulsed Spray Transfer (source: adapted from Silva et al [89]

The use of a particular welding process is very much dependent on situational constraints. While a globular welding process is the more aggressive form of welding, it allows for a very good penetration in thick materials with high rates of deposition. A pulsed spray transfer, on the other hand, provides excellent weld quality and spatter-free welding, however, as with globular transfer, it is limited to welding on horizontal and flat metal base materials. A short-circuiting process, on the other hand, sacrifices a spatter-free welding for a very low welding heat input and a fast-solidifying weld bead, making it the ideal process for welding on non-horizontal base materials, as well as light gauge sheet materials[89], [90].

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4.4 Cold Metal Transfer (CMT) Welding

4.4 Cold Metal Transfer (CMT) Welding Cold Metal Transfer (CMT) is a modified GMAW welding process, developed by Fronius GmbH [86]. A CMT process makes use of the benefits of low-temperature welding associated with shortcircuiting heat transfer mechanisms, however modifications in the method in which droplets are transferred allows for a “colder” form of compared with other MIG/MAG processes [91][92]. Variations in the CMT process, such as CMT-Advanced and CMT-cycle step, allow for a high degree control and modification of the weld bead characteristics. This has made CMT particularly useful for applications where low heat input and high deposition rates are required, particularly for welding thin sheet material[93]. As described in section 4.3, a short-circuiting transfer mode begins with a wire electrode tip contacting a molten weld pool and extinguishing the arc (Fig 4.15a). In a CMT process, when the electrode makes contact with the weld pool and extinguishes the arc, the electrode is retracted and causes the droplet to transfer (Fig 4.15 b,c). Moreover, while a short-circuiting transfer mechanism results in a significant amount of current generated to cause detachment, the retraction of the wire during a CMT process causes the current dropping to a near-zero value [93]. After the transfer is complete, the arc is re-ignited and the process is repeated (Fig 4.15 d) with speeds up to 50-130Hz[86] resulting in a welding process with lesser heat input and better control on drop transfer when compared to standard MIG/MAG welding.

Fig 4.15 A CMT process begins with (a) ignition of an arc which is (b) extinguished a wire electrode makes contact. (c) the electrode is retracted after which (e) the droplet is transferred and the arc re-ignited. (Source: Adapted from Furukawa [93]) and wavefronts for current and voltages during a cycle (Source: Adapted from Fronius[94]

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During the recent years, several research initiatives have investigated the use of Cold Metal Transfer. For example, Almeida[95] undertook the task of analyzing and comparing the fundamental process parameters in both CMT and GMAW-P welding processes. Müller et al[96] illustrate the benefits of CMT and variants (Cold Metal Transfer) in being able to obtain reduce energy input when compared to standard GMAW welding. Several researchers have also studied the use of CMT in applications for joining dissimilar metals[97][98], including zinc-coated and hot-dip galvanized steel and aluminum[99], ; a task which is notoriously difficult with standard GMAW welding processes. The low-heat input and ability to weld thinner weld beads[95] has also resulted in researchers exploring applications of CMT as a means of cladding and repair of existing structures[91], [100]. In the field of welding thin metals, Talalaev et al[101] utilized a CMT process to successfully weld T-joints, butt-joints and corner joints on steel and aluminum material with thicknesses between 1.5-4mm. Furukawa [93] also adopted a CMT process to successfully weld fillet joints on 1.0mm thick sheets of aluminum, and 0.3 -2mm thick sheets of stainless steel. Grzybicki et al [102] also reported that CMT allowed for splatter-free joints produced on 0.4mm sheet metal, as an application in the automobile industry. Fronius have developed multiple variations of the basic CMT process; including CMT Pulse (CMTP), CMT-Advanced (CMT-ADV) and CMT Pulse Advanced (CMT-PADV)[94], each of which distinguishable by the process of droplet transfer mode. In a CMT Advanced process, for example, an alternating current reverses the polarity of the process in each CMT cycle, which results in an even lower heat input compared to a standard CMT process . A CMT-Pulsed process, on the other hand, is characterized by the introduction of an additional droplet in each CMT cycle[94]. While this allows for faster deposition rates, it also results in a higher amount of heat energy requiring minimal generation of thermal energy during the welding process. A variation which combines the benefits of a fast deposition rate in a CMT-P process and the reversal of polarity of a CMT-ADV process, a CMT-Pulsed Advanced is a CMT droplet transfer variant which allows for a higher deposition rate of weld material while also reducing the heat input. In 2014, Cong et al [103] compared the effects different CMT transfer modes had on the porosity of single and multiple-layer welded seams, concluding that the CMT-variants were superior to the standard process in achieving pore-free welds, with a CMT-PADV showing to be the most promising in terms of pore reduction, which was attributed to the lower level of heat input. A more recent variation of the CMT process is CMTCyclestep (CMT-CS), developed for Fronius for the joining of thin sheet materials in the automotive industry . As described earlier, a CMT process is characterized by cycles, a period during which short-circuiting occurs and droplets are transferred. A CMT-CS process further reduces the amount of heat inputted by firstly allowing for the precise control of how many droplets are formed in each welding cycle, and therefore giving control over the dimension of an individual spot weld. Moreover, a step is introduced; that is, a time interval between cycles where welding is paused (Fig 4.16).

4.5 Process Parameters

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F i g 4 . 1 6 C M T Cy c l e S t e p a l l o w s f o r t h e c o n t r o l o f i n d i v i d u a l s p o t w e l d s i z e s a s w e l l a s the spacing and therefore, overlap between welds.

The ability to introduce a pause between welding allows for a colder form of welding when compared to other CMT processes[86]. Moreover, the ability to introduce an interval between spot welds, which can be synchronized with parameters such as robot travelling speed and wire feed speeds, also gives greater control over the geometry and continuity of the weld bead, which has a direct impact on the size, widths and overlaps of welded layers.[104]

4.5 Process Parameters A welding process involves the fusion of two materials; the welding electrode and the parent material. The dilution of a weld bead refers to the change in chemical composition of the weld material caused by mixing with the parent material. The dilution of a weld bead (Fig 4.17a,b) is describes the ratio between the bead penetration area (Ap) and the bead reinforcement area; the area of weld material that propagates above the parent material (Ar) [105], and may be expressed as:

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% Dilution = 100 ·

Ap Ap + Ar

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(4.1)

Where: Ap is the Area of Penetration (mm2) Ar is the Bead Reinforcement Area (mm2) The percentage of the dilution is affected by factors including the type of welding process used, type and flow rate of shielding gas mixtures, wire feed rates (Wfs) and travel speeds (Ts) as well as the basic values for current (I) and voltage (V). The desirable values of dilution can vary from very low percentages (Fig 4.17c), for example in coating applications, to medium-high values where deep penetration of the bead is required. However, while a low dilution is associated with a lower heat input and less alteration of the weld material from its original composition, a too-low-a-dilution could also result in lower depths of penetration into the base material and therefore an unsecure connection between the weld and base material. A high rate of dilution, on the other hand, could result in a new material composition formed between the two materials which could result in unfavorable properties.

Fig 4.17 Different zones in the interface between a welded seam and substrate material

Second to the dilution of a weld beam is the so-called Heat Affected Zone (HAZ). This is the zone where the parent material undergoes changes in microstructure and grain size due to the exposure of high temperatures surrounding the weld seam (Fig 4.18a-d). The HAZ, which is identifiable by the discoloration of the surface of a parent material, is at its most intense at the area of welding (i.e., area of highest heat input) and gradually reduces as the heat dissipates through the parent material. As the degree of microstructural changes in the base material is dependent on the heat exposure, a HAZ is a heterogenous zone. This means that the mechanical properties of the parent material are affected differently throughout the section of the parent material. For example, the tempering process which occurs after the fusion zone, has been found to slightly increase the surface hardness and shear punch test strength values of the parent material, however also resulting in a reduction in the ductility and the materials resistance to cracking [106].

4.5 Process Parameters

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Fig 4.18The HAZ(a) may be visualized by different (b-d)color bands representing different states of change in the material

Limiting the size of the HAZ is therefore of quite significance. The primary factors in containing the spread of the HAZ are primarily the thermal diffusivity and thickness of the base material. However, a simplified expression for estimating the width of the heat-affected zone, given in equation 1.2, shows that the width is proportional to the heat input. Therefore, the choice of the welding heat transfer mechanisms and process parameters used has a significant impact on the changes in the microstructure of the parent material. Ignoring the effects of cooling, an estimated value of the width of a Heat Affected Zone on a plate with thickness (h), initial temperature (T0), and Net Heat Input per unit Length (Hnet) is given by [107]:

1 1 √2πe ∙ ρChY = + Tp − T0 Hnet Tm − T0 Where:

Tp T0 E ρ C H Y Hne Tm

= Maximum Temperature (0C) = Initial Plate Temperature (0C) = 2.718, Base of natural logarithms = Density of the base material (Kg/m3) = Specific Heat of parent material (J/Kg 0C) = Thickness of the workpiece (mm) = Distance from weld fusion boundary (mm) = Net heat input per unit length (J/mm) = Melting temperature of weld material (0C)

(4.2)

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The net heat input per unit length is a function of the welding process parameters; the welding current (I) and Voltage (V) and travel speed (Ts), and is given by:

Hnet = K ∙ Where:

K

U I

U∙I Ts ∙ 1000

= Thermal Efficiency of the welding process according to EN ISO 1011-1 1.0 for Submerged Arc Welding (SAW), 0.8 for Gas Metal Arc Welding (GMAW), 0.6 for Plasma Arc Welding (PAW) = Welding Voltage (V) = Welding Current (I)

Therefore, for a given parent material, the importance of welding process parameters become quickly apparent as these are what, ultimately, have a direct affect on the welding voltages and currents and,therefore, overall heat input into the material and size of the HAZ. The ability to rapidly deposited metal to form three dimensional structures is one of the advantages of WAAM over other metal AM processes. The speed at which the motion controller moves the welding torch, or travel speed (Ts), is one of the determining factors in how fast material is deposited. However, the rate of how fast material is extruded, or wire feed speed (Wfs), and melted imposes a certain limitation on the actual speed at which material deposition can take place. Therefore, balancing the travel speed and wire feed speed are just one of the many considerations which have to be taken into account during the WAAM process.

Fig 4.19 Overview of parameters affecting weld bead quality

(4.3)

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4.5.1 Shielding gas The primary use of a shielding gas in welding is to protect the molten weld pool from being contaminated from the surrounding atmosphere, which is primarily caused by the presence of nitrogen, oxygen and water vapor. If molten metal contacts the surrounding air, the chemical elements present in the air cause reactions in the weld; oxygen will result in oxidation of the molten material. Oxygen may also combine with carbon in steel, resulting in the formation of carbon monoxide (CO), which may get trapped in the molten material and cause pores. Exposure to large amounts of nitrogen in the air may also lead to the presence of pores, as illustrated in Fig 4.2. [108].

F i g 4 . 2 0 ( a ) A w e l d s e a m w h i ch u s e d a n 8 2 % A r 1 8 % C O 2 s h i e l d i n g g a s a n d ( b ) t h e s a m e process parameters without the presence of shielding gas causes a heavy presence of pores in the welded structure. Images taken with Keyence VHX 600digital microscope at the TU D a r m s t a d t I S M +D L a b

Apart from preventing contamination from the outside atmosphere, shielding gas also reacts with both the parent and filler materials, and can potentially alter the mechanical properties such as strength, toughness and corrosion resistance [109]. Moreover, shielding gas also directly alters the overall bead geometry; it’s profile, dilution as well as penetration with the parent material [109], [110] Shielding gasses can compose of different element compositions which, in turn, have different effects on the weld seam. Research into shielding gasses used in GMAW and WAAM shows that it is usually blends of Argon (AR), Carbon Dioxide (CO2), Hydrogen (H) , and Helium(He) are used, each of which sharing different properties of thermal conductivity, specific heat and reactivity [110][108]. Due to its cheapness, CO2 is different percentage of gas blends have significant effects on the weld properties. Kim et al [110] compared the effects of M21 (Ar + 18% CO2) and C1 (100% CO2), and show that for the same process parameters (Ts, Wfs, ESO), welds produced with an C1 gas exhibited higher and wider bead profiles, and considerable differences in the Ultimate Tensile Strength of the materials (1270MPA for M21, 1450 for C1), which was due to the presence of defects in the M21 welds. Similarly, Kah et al[109] reported that, for carbon steels, increasing CO2 in Ar/CO2 mixtures between 5 – 20% resulted in greater penetration as well as the ability to weld with faster Ts. Purwaningrum et al [111]. Liskevych et al [112] compared the geometry of weld beads of different CO2+Ar mixtures for GMAW processes, using the same mean process parameter settings. As with other findings, an increase in CO2 deteriorated the stability of the welding arc, resulting in excessive spatter and uneven beads. However, decreasing CO2 values resulted in lower penetration into the parent material.

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Fig 4.21 Effects on bead geometry and penetration using different CO2 and Ar shielding g a s m i x t u r e s ( S o u r c e : a d a p t e d f r o m , L i s k e v y c h e t a l [ 1 1 2 ] , T WI )

Essentially, the type of gas used is dependent on the intended application. For example, helium is a better conductor of heat than argon, and is therefore desirable when high amounts of heat input is needed, for example when welding thick parent material, or metals with high thermal conductivity, such as aluminum and copper, which rapidly conduct heat away from the weld area. Argon, on the other hand, has a lower heat conductivity, which makes it more suitable for applications such as welding thin sheet metal.

4.5.2 Contact-tube-weld-distance (CTWD) As already previously described, the quality of a welded structure is highly dependent on process parameters which, ultimately, reduce the amount of heat input during welding. One method in reducing the amount of heat input is by increasing the free-wire length (Lfw) , that is, the length of wire electrode that lies between a welding gun’s contact tip, and the formation of welding arc, also known as the electrical stickout (ESO). Alternatively, this can also be described by making use of the contact-tube-to-work-distance (CTWD), which is the distance between the welding gun tip and the workpiece (Fig 4.22a). As described by [113], by increasing the ESO and maintaining a constant welding voltage (U), the increased resistivity (R) of the electrode causes a reduction in welding current intensity (I) and, ultimately, lower heat input. In the context of WAAM, and particularly welding thin sheet metal, reducing the amount of heat generated during welding is essential to not only ensure a stable welding process, but also to reduce the amount of heat-induced distortion in thin sheet metal. Therefore, it might be easy to conclude that a high ESO will aid in achieving highquality welds. However, as illustrated in Fig 4.2,b , as the ESO increases, so does the eccentricity (e) of the electrode tip to the center-axis of the welding gun which results in an unstable welding process.

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Fig 4.22 The increasing of the CTWD results in resulting in eccentricities of the wire electrode (Source: adapted from Henckell et al [113])

Silva et al [114] illustrated the effects of CTWD through both practical experimentation and theoretical models with lengths varying between 5mm and 20mm. They found that the welding current values dropped, on average, at a rate of 6.5A/mm and 7.0 +- 1.4 A/mm for the theoretical and experimental models respectively. Henckell et al [113] elaborated further and showed that an increase in the CTWD also caused weld beads to gain higher profile heights, which is due to the formation of large globular weld material at the electrode tip (Fig 4.23)

Fig 4.23 Effects of increasing CTWD on the printing layer heights (Source: Henckel et al [113]

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4.5.3 Travelling Speed (Ts) As already mentioned, the travelling speed refers to how quickly a motion controller can move the welding gun in three-dimensional space, which has to be balanced with the rate at which material is extruded. The travel speed has an effect not only on the production value of WAAM, but also poses significant influences on the quality of a weld [115] as well as the penetration into the base material. For a constant Wfs, a fast travel speed will result in less heat input, which is beneficial in reducing the size of the HAZ and distortion, especially in the context of thin sheet material. However, if the heat input is too low, then the heat will not be sufficient to melt the base material and therefore could result in low penetration into the parent material.[116],[117]. For a constant Wfs, as the Ts decreases, the deposition per unit area of material increased. In effect, a too-slow welding speed could also result in too much material is deposited in given area, causing material to roll over rather than penetrate the base material. Such a phenomena was illustrated by Yehorov et al[115] who studied the effects of Ts on quality of WAAM-produced wall sections; reporting that, at a certain lower threshold of Ts, weld material tended to droop over the sides of the walls, reducing the quality of the printed surface. Furthermore, Adebayo et al[118] showed that , for a constant voltage and current, an increase in Ts altered the shape of the weld bead such that its width decreased due to a lower heat input per unit area available for melting the wire electrode. For a constant Wfs, one of the common side effects of a high travel speed is a phenomenon known as humping. This is a welding defect which is caused when the position of the welding arc lies in front of the molten weld pool ( Fig 4.245a-c), which is the case of fast travel speeds. The pressure from the arc causes molten material to flow backwards and solidify, causing peaks and valleys of solidified material [118]–[120]. This has an impact of the overall quality and stability of a weld seam and, in the context of WAAM in which multiple layers of material are deposited on one another, could have a carry-on affect through the printed structure.

Fig 4.24 (a,b) effects of Ts and the formation of backward pressure and humps (Source: A d a p t e d f r o m Ng u y e n e t a l [ 1 1 9 ] ) ( c ) I m a g e t a k e n w i t h K e y e n c e V H X 6 0 0 d i g i t a l m i c r o s c o p e a t t h e T U D a r ms t a d t I S M +D L a b )

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4.5.4 Wire feed speed (Wfs) The rate at which material is deposited goes hand-in-hand with how fast the welding torch is being moved; for a given desired thickness of weld, a high travel speed would naturally correspond to a higher amount of material being fed in order to maintain a constant area per unit length of material being deposited. As could be expected, Adebayo et al[118] illustrated that, for a constant Ts, a lowered Wfs results in beads being deposited with increased heights and widths, which is explained by an increased volume of material being melted per unit area. Xiong et al [121] also illustrated the effects of altering a Wfs when a constant Ts is maintained. They showed that an increase of Wfs was the main cause of an increased welding temperature, required to melt more material for the given time, having a directly-negative effect on the surface roughness and quality of welded wall structures. Therefore, finding the correct ratio between Wfs and Ts is of significant importance for producing welded structures of acceptable quality. Yehorov et al [115] showed that, when working within the upper and lower boundaries of Ts, the ratio between Ts and Wfs for maintaining a stable welding process holds a linear relationship. However, they also showed that, even if the Wfs/Ts ratio is maintained, there is an effect on the weld bead geometry when higher and lower values are used. An increase in both Wfs and Ts, for example, causes weld seams to become shallower and wider. They attributed this to a recorded increase of arc energy, which causes higher heat accumulation and an increase in the weld pool volume, as well as higher pressures which push down on the molten material, causing it to flatten. They concluded by noting that this essentially becomes a situational problem; when thin wall sections are required, then lower welding values would be required in order to achieve beads which are narrower in section. Conversely, if thicker walls are needed then, one could opt for either using lower welding values; which would require multiple passes per layer, but maintain a higher average layer height. Alternatively, one could opt for using higher values, which are associated with wider beads but lower height, resulting in more accumulated layers with less welding passes per layer (Fig 4.25).

Fig 4.25 Changes in Wfs and the effects of weld bead dimensions. (Source: Adapted from Yehorov et al [115])

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4.5.5 Combination of parameters Given that both the Travel Speed (Ts) and Wire Feed Speed (Wfs) have a significant effect on the weld bead geometry, it comes to no surprise that the ratio between the two (Wfs/Ts) also plays a role in determining the geometric qualities of a welded seam. In comprehensive research outlining process parameter characterization for WAAM, Almedia illustrates that the ratio between Wfs and Ts has significant effects on factors including the fusion between the weld bead and the parent material, the width and height of a weld seam, and therefore also the effective width of the weld material[95]. The research showed that, for a GMAW-P welding process, an increase in the Wfs/Ts ratio also tends to lead towards increases in bead geometry widths whilst also resulting in higher penetration depths into the parent material. In the case of CMT processes, an increase in Wfs/Ts had the effect of increasing the contact angles, that is, the angle with which a weld bead contacts the parent material. The ratio between Wfs/Ts also plays a significant role in the ability to achieve smooth welded surfaces, with higher ratios resulting in a rougher printed surface (increased surface waviness)[95]

Fig 4.26 Effects of increasing the Wfs/Ts ratios on contact angles (Source, Adapted from Almedia)

Almedia[95] further illustrates the complex relationships between process parameters including the CTWD, Wfs, Ts and their ratios have on factors such as the overall weld bead geometry and its interaction with parent material, the ability to build up layers of weld material as well as the overall surface quality, and proposes mathematical predictive models for determining expected causes and effects which may occur between the interaction of said parameters. In turn, he illustrates how, for prescribed geometric requirements, the predictive models may be utilized to determine regions for which the complex interaction of process-parameters are ideal. (Fig 4.27)

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Fig 4.27 Almedia illustrates the possibility to determine a region of optimized welding process parameters which would give the most acceptable results (source: Almedia[95])

5

Welding distortion in thin plates

Parts produced with WAAM are particularly susceptible to thermally-induced distortion. During a welding process, the material immediately surrounding the welding arc is heated up close to its melting point and cooled down, resulting in non-uniform heat buildup in the plates. The degree in which heat travels through the surrounding material is controlled by its thermal expansion coefficient, which restrains the size of the weld zone. This results in localized heating and cooling of the base material, as well as localized buildup of stresses and distortion. The degree of distortion is influenced by factors which include the material properties of both substrate and weld materials, the welding process parameters which directly affect the heat input, heating and cooling before, during and after welding, as well as the degree to which substrate material is clamped [122],[123],[124]. Specimens which are restrained during the welding process, for example, experience a gradual buildup of internal residual stresses which may result in out-of plane distortion after the removal of the restraining mechanisms [125], [126], [123]. This local buildup of residual forces can be simplified and represented as a stretched rubber band that is introduced onto a member Fig 5.1, with the stiffness of the plate, and the degree in which the rubber band is stretched (degree of residual forces) determining the amount of distortion that the member undergoes.

Fig 5.1 Analogy of a rubber band introducing tensile and compressive forces into a welded plate element (Source: adapted from [127])

A 3-bar analogy is often adopted to explain the formation of residual stresses inside thin sheet metal under welding. Consider 3 bars of equal lengths connected by a common edge, with the two outer bars being of greater width than the center one (Fig 5.2). At room temperature (a) all bars are of equal length and have no internal forces induced. Consider (b) where the middle bar is heated (as is the case during welding), causing expansion of the bar. Because the 3-bars are connected to a common connection, the expansion of the center strip causes the side strips to stretch and be put under © The Author(s), under exclusive license to Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2023 C. Borg Costanzi, Reinforcing and Detailing of Thin Sheet Metal Using Wire Arc Additive Manufacturing as an Application in Facades, Mechanik, Werkstoffe und Konstruktion im Bauwesen 68, https://doi.org/10.1007/978-3-658-41540-2_5

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tension. The expanding middle strip, on the other hand, is resisted by the fixed edges and therefore undergoes compression. As the system is cooled (c) the center strip shrinks and can be considered to be put under tension, pulling with it the side strips, causing them to be put into compression. With this analogy, it can therefore be expected that during welding, the area surrounding the weld will be put into tension whereas the surrounding material will be put into compression[128]

F i g 5 . 2 3 - b a r a n a l o g y i l l u s t r a t i n g b u i l d u p o f s tr e s s e s i n w e l d e d p l a t e s ( a ) b a r s o f e q u a l length at room temperature with (b) welded center plate expanding during heating and (c) compressing upon cooling (Source: Adopted from Deo [128])

Heat-Induced distortions in plates may result in multiple modes of failure. Masubuchi [129] classifies three principal modes of failure in welded plates as: • • • • • •

Transverse Shrinkage perpendicular to a weld line Longitudinal Shrinkage parallel to a weld line Longitudinal Distortion which is in-plane through the weld line and perpendicular to the plate Angular Distortion in the plane of a plate as a result of thermal expansion Angular Change as a result of non-uniform thermal expansion throughout the thickness of the plate Buckling Distortion due to thermal compressive stresses

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Fig 5.3 Expected modes of distortion in welded plates (Source: K. Masubuchi [129])

These distortions are generally a result of the contraction of weld material as it cools down. One of the factors which determines the type of distortion to be expected is the thickness of the substrate material. Angular distortion, for example, is caused by different rates of thermal expansion and shrinkage occurring throughout the thickness of a plate[126]. As the plate thickness decreases, so do the effects of angular distortion due to a decrease in temperature gradients throughout the thickness of the plate. Subsequently, plates which are thin relative to their size, are not as susceptible to angular distortions, and are primarily affected by buckling distortion [130], which arises from inplane compressive stresses.Buckling distortion occurs when the critical buckling stress level (σcr) of the plate is exceeded by the compressive stresses caused by weld shrinkage. In order to mitigate the effects of buckling, it must be ensured that the induced compressive stresses are either reduced to a level below the critical buckling stress, or redistributed throughout the transverse section of the plate. Fig 5.4, [126] illustrates how residual stresses typically propagate along a plate section, with a high tensile stress occurring in the area adjacent to welding and compressive stress occurring along the plate edges, with curve 1 illustrating compressive stresses exceeding the critical buckling stress level.

Fig 5.4 In order to avoid the buildup of residual stresses which exceed the critical buckling stresses (σcr ,curve 1) which result in buckling distortion, then stresses must (a) either be reduced below σcr or (b) redistributed along the cross section of the plate. (Source: van der Aa[126] )

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Strategies for ensuring that the critical buckling stress is not exceeded include reducing the welding stress such that the critical stress level is not exceeded (Fig 5.4,a). One such method in achieving this is the use of welding processes which utilize low amounts of heat energy [86]. Alternatively, redistributing the welding stresses throughout the plate also allow for avoiding exceeding the critical stress level. The geometrical properties of the plate also play a part in controlling buckling deformation. A stiffer plate material will allow for a higher critical buckling stress level. Soffel [131] illustrated the effects substrate thickness had on the overall deformation in plates varying between 2mm to 6mm in thickness. While this is expected, it may be explained by estimating the critical buckling stress induced in a simplified rectangular plate subjected to uniform shrinkage effects, as the Euler Equation used to determine critical buckling load in beams subject to uniform compressive load [126]. The critical load (Fcr) and stress (σcr) for a rectangular section with a length a, width b, thickness h and Young’s Modulus E, are described in (1.2) [132]: Fcr = −

Ebh3 π2 Fcr Eh2 π2 , σ = = − cr 12a2 bh 12a2

(5.1)

Where:

Fcr E h b a

= = = = =

The Critical Buckling Load Young’s Modulus of the Material Thickness of material in consideration Width of the material in consideration Length of the material in consideration

Consider two rectangular plates which undergo a uniform compressive stress and have a length (a) of 200mm, width (b) of 50mm, thicknesses (h) of 1mm and 4mm, and Young’s Modulus (E) of 210Mpa. Using equation (1.2), the 2mm and 6mm plates are expected to reach σcr at 17.3MPa and 155.5MPa respectively. While it becomes quickly apparent that, for a simplified rectangular substrate, the thinner the material becomes, the more susceptible a substrate is to distortion, Corbin [133] points out that thicker materials also exhibit a reduction of through-heating within the crosssection of the plate, further noting that thin sections, on the other hand, are more susceptible to a reduction of yield strength around the weld area which compromises its stiffness.

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5.1 Discussion It quickly becomes apparent that, for thin sheet materials in particular, the effects of distortion are quite prominent and need to be taken into consideration. As Van der Aa[126] points out, distortion can occur both during and after the welding process. If a workpiece is unrestrained and free to move during a welding process, then compressive forces caused by the shrinking of material as it cools will likely cause the material to buckle. Fig 5.5 illustrates this with a sheet of metal which is minimally restrained and therefore undergoes high amounts of deformation during welding.

Fig 5.5 Minimally-restrained thin sheet metal undergoing large amounts of buckling distortion.

On the other hand, if a plate if sully restrained from moving during a welding process, then the effects of distortion will be reduced. However, it will also result in a buildup of residual stresses in the plate which, unless minimized, will also result in deformation after the plate material is freed from its restraint.

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Fig 5.6 The degree of restraint has a direct effect on the amount of deformation exhibited by a welded plate as well as the buildup of residual stresses (Source: Adapted from Van der Aa[126])

Therefore, factors which affect the behavior of the sheet material against welding are primarily: (a) The stiffness of the plate being welded and is therefore directly related to its cross-sectional dimensions (b) The amount of heat introduced during the welding process, which is therefore related to external factors, such as cooling or pe-heating of the plate, as well as the welding currents and voltages used during welding. (c) The degree of restraint used during the welding process

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5.2 Mitigation Strategies Throughout the years, multiple strategies for reducing buckling in plates have been studied. These include pre-treatment of plates such as heating and cooling[131], [133], the use of mechanical and thermal stretching during and after welding [134], [135], as well as the development of low-heat input welding processes with reduced heat input per unit length of weld [86]. Additional strategies, such as intermittent welding [136], the degree of fixation during welding [125], [126] or controlling interpass temperatures [122] have all been shown to be useful in controlling the deformation.

5.2.1 Flame Straightening One of the earliest methods adopted for mitigating the effects of deformation in welded steel elements is the use of Flame straightening. As weld material cools down, it contracts and often results in distortion in the weld piece. During flame straightening process, deformed material is locally reheated until plastic deformation occurs, usually adjacent or opposite to a weld, in order to induce counteractive distortion upon cooling [136]. When weld parts are restrained from expansion during heating, an upsetting of the material occurs, which undergoes localized shrinkage during cooling[137].

Fig 5.7 Principle of flame straightening, causing already-welded parts to distort towards their initial shape prior to welding.

However, because flame straightening relies on locally-restraining the material during heating, which is generally achieved by the stiffness of the welded part or clamping, the method requires considerable attention when used for thin-sheet materials. This is due to the thin sheet material having inadequate stiffness to resist out of plane displacements during the heating and cooling process [138] . For this reason, thin sheets which are flame-straightened rely on the use specialized clamping devices. Due to degree of craftsmanship flame straightening relies on, it has traditionally been reserved for its use in workshops or on-site repair of bridges, although considerable efforts have been undertaken in the prediction and effects of flame straightening[137], [139]

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5.2.2 Mechanical Tensioning Referring to the 3-bar analogy described in Fig 5.8, as the system expands, the bars are put into compressive and tensile forces. In mechanical stretching, as the plate is heated up and expands, it is also mechanically-stretched in order to accommodate for the shrinkage and expansion of the material during the heating process. However, this is done with sufficient force to ensure that the compressive forces generated in the center bar during stretching fall below the yield stress of the material [128], so as to ensure that the material is able to return back to its original position after cooling. While this method does allow for the control of distortion in thin plates, it relies on industrial equipment to stretch the material in a laborious process and is therefore not so commonly-utilized today

5.2.3 Thermal Tensioning Thermal tensioning builds upon the same process used in mechanical stretching – that is, to accommodate for the expansion of plate material as it is heated up. However, rather than using mechanical fixings to stretch the material in a laborious process, localized heating and cooling is used to expand and contract the material as it is heated and allows for a far less laborious process to be used when compared to mechanical tensioning.

F i g 5 . 8 c o m p a r i s o n b e t w e e n m e c h a n i s m s i n Me c h a n i c a l a n d T h e r m a l T e n s i o n i n g (Source: Adapted from Deo [128])

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In this method, temperature gradients are introduced along the length of a plate by local-linear heating and cooling of the areas parallel to the weld seam, before welding begins. The temperature gradients result in tensile stresses within the plate, essentially pre-stressing it such that plastic strains during welding are prevented[126], [138].

Fig 5.9 (a) Temperature gradients produced during thermal tensioning and setup proposed by Deo [128] (Source: Adapted from Deo)

In 2003, Deo et al [128] used this method to weld stiffeners onto 3.2mm thick sheet material. What was found is that while side heating (thermal tensioning) did contribute to the reduction of buckling distortion in the plates, they underwent angular distortion modes of failure (Fig 5.10). However, they further concluded that the use of mechanical restraints during the welding process mitigated the problems of angular distortion.

F i g 5 . 1 0 T h e e f f e c t s o f t h e r m a l t e n s i o n i n g o n 3 . 2 m m t h i c k p l a t e s ( s o u r c e : D eo [ 1 2 8 ] )

During the 1970’s, Burak [140] used mathematical analysis which approximated the propagation of stresses in 4mm aluminum plates which were linearly restrained parallel to the welding seam and subjected to localized heating and cooling. These early experiments were further developed by Guan [141], [142] who illustrated that while this method of thermal tensioning was applicable for plates over 4mm in thickness, thinner plates were still subject to out of plane displacement due to the need for excessive restraint during the welding process.

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5.2.4 Low Stress Non-Distortion (LSND) Welding Driven by the need for controlling distortion in thin plate members, which were especially common in the aerospace and shipbuilding industry, the thermal tensioning methods described in 5.2.3 were further developed for use in thin sheet metal. The earliest attempts for achieving this was driven by Guan who built upon the work by Burak [140]. In order to reduce distortions which were inevitable in thin plates, Guan first introduced a second linear restraint, P2 (Fig 5.11), which was also placed parallel to a welding seam. Using a process which he termed “Low Stress Non-Distortion” welding, a Temperature profile (T), is introduced into the plate before welding. This was achieved by applying cooling along the full length of the welding seam and heating at a prescribed distance away from the welding zone. This results in a thermal stress field (σ) that is characterized by tensile stresses around the weld zone, and compression at a distance from the weld. The introduction of linear restraints, P1,P2, are used to stiffen the thin sheet during this process in order to mitigate out of plane distortions.

Fig 5.11 Low Stress Non Distortion involves inducing a temperature profile within the welded plate in order to counteract the effects of distortion. The process, however, relies on the use of clamping to avoid distortion during welding (source: Q.Guan et al [138])

The early experiments with LSND proved to be positive in eliminating heat-induced distortions in thin plate materials. Fig 5.12 illustrates the results presented by Guan in which 1000mm x 1.6mm thick aluminum and steel plates were welded using conventional GMAW (with optimized parameters) and LSND. In both cases, it was reported that while the plates welded using conventional GMAW all experienced severe buckling, those welded using LSND were as flat as before welding [138] due to residual stresses being controlled to a level below the critical buckling σcr of the plate.

Fig 5.12 Comparison of plates welded with and without LSND (source: Q.Guan [138])

However, while this was proven to be beneficial in controlling deformation in thin plates, the process was limited by the “Static” nature of the setup; relying on the use of costly and impractical fixed heating and cooling sources.

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5.2.5 Dynamically-Controlled Low Stress Non-Distortion (DCLDNS) Welding Further developing on the work of static LSND which relied on pre-setting a temperature profile prior to welding of plates, Guan[138] introduced a dynamically-controlled method of welding in which adjustment of temperature profiles are carried out during the welding process itself. This was achieved by introducing attaching a liquid-cooling device to the nozzle of the welding gun, which locally cooled a just-solidified weld bead. In order to avoid interference with the welding arc, the cooling device is provided with a vacuum pump in order to draw away any vaporized coolant. Throughout the years, multiple researchers experimented with the type of coolant used as well as it’s positioning relative to the weld seam. Gabzdyl et al [143], for example, studied the use of directed CO2 jets 1.6mm 304L steel, whereas Van der Aa[126] made use of an enclosed, trailing C02 Snow with a glass-wool shielding as a cooling source for 1.5mm thick AISI 316L steel. Mitigating the need for evacuation of confined cooling jets and evacuation systems, researchers at TWI [144] utilized localized underside cooling of the plate, rather than directly cooling the weld bead.

F i g 5 . 1 3 D y n a mi c a l l y - c o o l e d L S N D w i t h C O 2 s n o w ( s o u r c e : V a n d e r A a [ 1 2 6 ] )

Although the application of LSND in industrial application is still being researched, research on an industrial application is being developed. O’Brien et al [145] and TWI [146] showcase the use of C02-cooled LSND with a patented pressurized, flexible cooling seal and extraction system, intended to be applicable to a number of different joints and difficult situations.

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5.3 Discussion As Masubuchi describes, there are expected modes of failure which excessive heat input may cause, the most prominent of which being buckling distortion in thin plates due to the critical buckling stresses of the material being exceeded. The heat induced distortion in thin plates may be dealt with in numerous ways. Firstly, reducing the heat input during the welding process, which can be accounted for by controlling the welding process parameters. The case studies presented above utilize sheet material which, in some cases exceeds 3mm in thickness. However, when welding process control is complimented with heat mitigated strategies, then the ability to control distortion in thin sheet material seems promising. While processes such as Dynamically-Controlled Low Stress NonDistortion Welding (DC-LSND) are still mostly applied in the context of research, some initiatives, such as the MALCO project by TWI Ltd[146] show promising results of a LSND process as an industrial application.

PART 2 The second part of this thesis builds upon the literature described in Part 1, and proposes a framework for combining Wire Arc Additive Manufacturing with thin sheet metal as a means of reinforcing and detailing the material for the use in façade cladding. The section describes the process, from determining acceptable welding parameters to the studies of various 3D scanning techniques used to imply a semi-automated design to production process.

Section 6 presents a general workflow and general steps required for welding on free-form thin sheet metal Section 6.1 introduces and compares various techniques used to create digital references of arbitrary bent sheet metal – a step required to automatically generate robotic movement trajectories. Section 6.2 Describes the determination of preliminary process parameters which would allow for welding on the thin sheet material without causing blowouts and minimizing deformation. This is followed by material testing on welded sheet metal Section 6.3 Presents and proposes alternative design strategies which could be beneficial in the context of welding thin sheet metal as a façade application. This is initially carried out on planar sheets of metal Section 6.4 Presents the combination of the findings from the previous sections, and illustrates a number of prototypes on planar and bent sheet metal.

6 Proposed Hybrid WAAM and Thin Sheet Metal Welding This research proposes a novel method for utilizing robotic welding (WAAM) to weld directly onto thin sheets of metal. By building up multiple layers of welded material, stiffening elements and façade connection details are added directly onto planar and pre-bent sheets of metal (Fig 6.1).

F i g 6 . 1 C o n c e p t u a l a p p l i c a t i o n f o r c o m b i n i n g WA A M o n f r e e - f o r m s h e e t m e t a l

While it is envisaged that the process would eventually be integrated into a fully-automated design to production process, combining sheet-forming techniques such as Multiple Point Forming (MPF) (See section 2.4.6), the process described in the research is simplified to the case of planar and singly-curved sheets of metal. This is done in order to provide a narrow framework within which the study is carried out. One of the areas in which Wire Arc Additive Manufacturing presents itself as a competitive alternative to traditional fabrication techniques is that of Mass Customization (See Section 1). This is largely due to the low startup and running costs of WAAM systems, together with the ability to fabricate objects with geometric variations without additional costs. In the context of free-form facades, where one may very well may be dealing with thousands of unique parts ([2]), the ability to produce different parts without changes in setup becomes quite attractive. However, the ability to fabricate is only one aspect that has to be dealt with. Just as important is the ability to digitally generate the potentially thousands of parts which all must be prepared for printing and communicated with the welding device. Therefore, formulating a so-called design-to-production workflow which allows for a semi-automated generation of objects, their analysis and sending to printing plays a significant role in © The Author(s), under exclusive license to Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2023 C. Borg Costanzi, Reinforcing and Detailing of Thin Sheet Metal Using Wire Arc Additive Manufacturing as an Application in Facades, Mechanik, Werkstoffe und Konstruktion im Bauwesen 68, https://doi.org/10.1007/978-3-658-41540-2_6

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establishing an efficient process. In this research, it is largely achieved by adopting parametric design tools (namely, Rhinoceros 3D and Grasshopper).

Fig 6.2 Proposed workflow in which (a) an arbitrary sheet surface is (b) prepared for printing. (c) 3d scanning is used to create a digital model of the surface and used to (d) upd a t e w e l d i n g p a t h s b e f o r e ( e ) b e i n g s e n t f o r we l d i n g .

The proposed workflow consists of 5 main steps, namely:

(a) An arbitrary sheet of metal is formed. In future applications, it is envisaged that MPF, or similar sheet forming techniques, are adopted. In the research, this is limited to planar and singly-curved sheets of metal (b) Rib layouts and connection details are digitally generated (c) 3D scanning is used to create a digital representation of the physical sheet material. (d) Based on the 3D scan data, the planned model is updated and the welding paths are adjusted. The printing path data is also generated in this stage (e) The information is communicated with a robotic welding device and the parts are produced. The individual steps are complimented with additional factors, including material testing and the determination of welding process parameters, both of which are used to develop strategies for generating reinforcement patterns described in part (b).

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6.1 Scanning As explained in Section 4.5, one of the factors which determines a stable welding process is maintaining a constant Contact tube to Work Distance (CTWD). When welding on horizontal, planar sheets of metal, a constant CTWD is maintained by maintaining a constant height on the motion controller (Fig 6.3,a) . However, maintaining a constant CTWD along a free-form surface is not as straight-forward. Careful path planning is needed in order to ensure that the motion controller follows the contour of the surface while maintaining a consistent CTWD. Moreover, information regarding the tangency of the surface along the printing path is also required in order to orient the welding gun perpendicular to it (Fig 6.3b) Therefore, a three-dimensional model is required when welding along curved surfaces. This is so that pre-defined welding paths can be updated to match the actual state of the curved sheet metal, as well as extracting information for consistently orienting the welding gun perpendicular to the face of the sheet. For this reason, a number of 3D scanning techniques are explored and evaluated in terms of their resolution, potential for automation as well as potential for integration with a parametric-design environment, which would allow for the automated generation of welding paths. Initially, three scanning methods; Photogrammetry, LIDAR Scanning and Structured Light Scanning techniques are explored.

F i g 6 . 3 C T W D me a s u r e d a s t h e p e r p e n d i c u l a r d i s t a n c e b e t w e e n t o o l t i p a n d s u r f a c e

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6.1.1 Photogrammetry When photographs are taken, three-dimensional objects in the physical world are converted into flat, two-dimensional images. This is because a photograph loses information about the depth of the object, and therefore converts it into a two-dimensional image. The process of Photogrammetry, in essence, reverses this. By combining multiple, overlapping photographs together, information about the depth of a physical object is reconstructed to form a three-dimensional object. This is done through the use of triangulation, whereby information about the position of a camera, its orientation relative to an object and intersection of lines in three-dimensional space is used to reconstruct multiple points at a time [147]. The resulting pointcloud is then used to construct a three-dimensional mesh using specialized software (Fig 6.4). Photogrammetry was used to recreate a 3D model of a sheet of arbitrarily-bent sheet metal as illustrated in Fig 6.4a. This was achieved by taking 50 photographs of the object at different radial positions, while maintaining a relatively constant distance between the camera (a Nikon D3000 Digital SLR) and the object. Autodesk Recap, specialized software used for processing images for photogrammetry, was used to reconstruct a three-dimensional object from the photographs. The result was a three-dimensional mesh which could be exported to grasshopper 3D / Rhinoceros for further processing, particularly performing target/actual comparisons, updating intended print paths and generating information for robot motion trajectories.

Fig 6.4 Schematic of Photogrammetry

Photogrammetry was used to recreate a 3D model of a sheet of arbitrarily-bent sheet metal as illustrated in, Fig 6.4 . This was achieved by taking 50 photographs of the object at different radial positions, while maintaining a relatively constant distance between the camera (a Nikon D3000 Digital SLR) and the object. Autodesk Recap, specialized software used for processing images for photogrammetry, was used to reconstruct a three-dimensional object from the photographs. The result was a three-dimensional mesh which could be exported to grasshopper 3D / Rhinoceros for further processing, particularly performing target/actual comparisons, updating intended print paths and generating information for robot motion trajectories.

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Fig 6.5 (a) sample of photographs used for photogrammetry and (b,c) resulting mesh surface

As shown in Fig 6.5b, the result was a high-resolution mesh. However, the result also illustrates a number of issues with using photogrammetry. Due to the reflectivity of the metal surface, and therefore, inconsistent area of overlap between subsequent images, defects were quite common in the reconstructed mesh (Fig 6.5 b and c). Attempts were made to overcome this by using markers placed at regular intervals on the surface (Fig 6.5b) as well as spraying the surface with a non-reflective coating. While these strategies did aid in reducing the amounts of defects, they were not ideal as they relied on changing the surface of the metal material which could be problematic when welding. Apart from the issues of reflectivity, the mesh reconstructed using photogrammetry includes area outside of the object that is intended to be converted into a three-dimensional model. In other words, excessive background information is present in all of the meshes (Fig 6.5 b and c). Therefore, a degree of manual post-processing was also needed to isolate only the intended scan object for further processing, which is also carried out in specialized software.

Fig 6.5 Different co-ordinate systems for robotic device and scanning

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Furthermore, photogrammetry, as well as most scanning techniques, introduces the issue of orienting global co-ordinate system of the scanned model to match the co-ordinate system used by the welding device. The motion controller references three-dimensional space using predefined co-ordinate systems. A Base Frame, for example, specifies the origin of the co-ordinate system at the base of the device. A User Frame can also be manually specified to shift the origin of the co-ordinate system to a particular position. The co-ordinate system used during the photogrammetry process, on the other hand, is not readily referenced to the co-ordinate systems defined by the robotic arm. While techniques such as the inclusion of fixed markers during the scanning process (reference) may be used to orient to the origin of the co-ordinate system of the scanned model with the robotic arm, efforts to achieve this were cumbersome and also relied on multiple manual interventions.

6.1.2 3D Laser Scanning Further to photogrammetry, 3D Laser scanning was also evaluated as a potential means for integration in the intended design workflow. A Faro Focus 3D, provided by the institute for Geodetic Measuring Systems and Sensors (GMSS) at TU Darmstadt was used to scan pre-bent sheet metal as shown in Fig 6.6. The basic scanning principle works by emitting a rapidly-pulsing or continuous laser beam multiple times a second. As beams are being emitted, a rotating mirror causes the beams to scatter around a vertical axis, with the scanning head simultaneously rotating around its horizontal axis.

Fig 6.6 basic principle of a LIDAR scanner in which (a) a mirror is used to reflect a laser in a (b) vertical and (c) horizontal sweeping motion to capture a pointcloud of data

The result, is a continuous sweeping of beams over a given area which lies within the field of view of the scanning device. As beams hit objects, some of their energy bounce back and are detected by the scanner. The time it takes for a particular beam to be emitted and bounce back is stored and used to calculate the distance from the scanner to the surface causing the beam to bounce, and converted into a single point. This process is repeated to create pointcloud consisting of millions of points that are converted into meshes using specialized software, such as Cloud Compare.

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Fig 6.7 Setup used for LIDAR scanning at TU Darmstadt, with the help of Dr. Florian Schill, GMSS.

However, as with photogrammetry, the issue of reflective surfaces – which are particularly common in sheet metal – still presents an issue with laser scanning and, therefore, brings up the issues of surface treatment already mentioned in the previous section. Moreover, while laser scanning does allow for high accuracy (0.1 – 0.9mm uncertainty over 10 meters [148]), this method of scanning also results in excessive amounts of pointcloud data; essentially an entire enclosure is 3D scanned even if the object of interest is small. Therefore, this method of scanning is also seen as undesirable as being implemented in the intended design workflow, largely due to the large amounts of manual post-processing required to obtain the desired scan information.

Fig 6.8 Results of LIDAR scanning is a high-density pointcloud with unnecessary information which has to be manually post-processed.

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6.1.3 Structured Light Scanning (Microsoft Kinect) Both the photogrammetry and laser scanning techniques explored had clear advantages in that pointclouds with high accuracy could be obtained when the metal surfaces are treated. However, both of the methods are time consuming and require post-processing efforts to remove unnecessary data as well as orienting the co-ordinate systems of the scanned 3d model with that of the welding robot. Moreover, in both cases, external software is required for not only generating a 3D mesh from pointcloud data, but also for the post-processing steps. Structured Light Scanning is a cost-effective method of targeted scanning of small-scale objects [149]. In this method, a pre-defined pattern is projected onto a target object, causing the pattern to deform. A camera, at a calibrated distance away from the projector, observes the distorted pattern from which depths of the patterns are calculated (Fig 6.9a)

Fig 6.9 Principle behind structured light scanning used on the xbox kinect

Microsoft Kinect makes use of the structured light principles; combining a Near Infrared (NIR) projector with an RGB and monochromatic NIR camera to calculate the depths of projected light [95], (Fig 6.9,b). Using a pattern of fixed dots to illuminate the target object, the distortions are coupled with triangulation techniques in order to compute the distance between the projected pattern as seen by the cameras, and the emitter source [96]. This process allows for x,y,z co-ordinates, and therefore meshes, to be generated in real-time. The result is a far quicker method of scanning when compared to photogrammetry and laser scanning, making SL techniques quite popular for low-cost method for quality control.[150] The Kinect scanner was evaluated at the Digital Design Unit, Darmstadt. An arbitrarily-deformed sheet of metal with no additional surface coatings was fitted with printed scanning markers. A Microsoft Kinect V1 was scanned by manually moving around the target, maintaining a relatively constant distance between scanner and target (Fig 6.10a,b). The scanning device was connected to Scannect 3D by Occipatal [151], which allowed for a 3D model to be generated live during the scanning process (Fig 6.10a,c)

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F i g 6 . 1 0 S e t u p o f s t r u c t u r e d l i g h t s c a n n i n g u s e d a t T U D a r m s ta d t

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While the scanning process was significantly faster that photogrammetry and laser scanning, with the 3d model being generated instantaneously, the surface quality of the final mesh was inferior to that of the other scanning methods, with excessive amounts of noise being consistently present in the scanning attempts. Nonetheless, one of benefit of utilizing a Kinect scanner is that numerous software packages [152], [153] allow for synchronization with parametric modelling tools, in particular Rhinoceros 3D and Grasshopper. This allows for a single software package to be used to generate the reinforcing and connection details, analyze 3D scan data and update the relevant motion paths to match it, and upload robot motion commands. (Fig 6.11. Fig 6.12, Fig 6.13)

Fig 6.11 Schematic for utilizing Structured light scanning together with robotic welding

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Fig 6.12 Initial setup for Structured Light Scanning used at the Digital Design Unit, TU Darmstadt

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Fig 6.13 Initial setup for Structured Light Scanning used at TU Darmstadt (Digital Design Unit)

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Comparing the different scanning methods, it quickly becomes apparent that certain trade-offs have to be established. While photogrammetry and Lidar scanning offer the best results in terms of scan resolution, this comes at the price of potential for automation due to the need for external software to process pointclouds and remove unnecessary scan data. The SL scanning with a Kinect on the other hand, offered potentials for targeted scanning with the ability to directly generate the scanned mesh within the same parametric environment for robotic control and generating welding paths. However, this came at the price of accuracy which is critical for maintaining a constant CTWD during welding. While researchers have shown that it is possible to increase the accuracy of such scanners[154], [155], a further remaining issue is that the resulting data from scanning is in the form of a mesh. This is because when a mesh is constructed, it actually consists of multiple planar faces which, together, estimate a curved object (Fig 6.14) Therefore, for surfaces which have a high degree of curvature, in order to extrapolate accurate weld paths, meshes have to consist of a high number of points and planar mesh faces (polygons)

Fig 6.14 Scanning using Structured Light results in a mesh-type geometry. In order to o b t a i n a s m o o t h s u r f a c e f r o m wh i c h m o t i o n p a t h s a r e g e n e r a t e d , a h i g h - r e s o l u t i o n m e s h i s required

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6.1.4 Tactile Sensing Tactile sensing is commonly used in robotics, where information about the surrounding environment is measured through the physical interaction with it, one of the most commonly used applications being the touchscreen on mobile devices. Fronius TouchSense is a feature used in robotic welding which is commonly used to obtain geometrical information about a workpiece before welding. In this process, the wire electrode used for welding is used as a means of detecting contact between itself and a workpiece. This is achieved by activating a Touch Sensing signal and a low current (Ca. 3Amps) and Voltage (Ca. 70 Volts) is passed through the wire electrode [156]. With the signal activated, a welding gun is directed towards a workpiece, at a reduced speed, until contact between the wire electrode and the workpiece is made (Fig 6.15 a,b). The contact between the two causes a short-circuiting and during this time, an output signal is sent to the welding device/robot to stop movement and begin retracting after a delayed period of time (Fig 6.15 c,d).

F i g 6 . 1 5 P r o c e ss o f F r o n i u s T o u c h S e n s e ( S o u r c e : a d a p t e d f r o m F r o n i u s G m b H [ 1 5 6 ] )

Fronius TouchSense is commonly used to automatically calculate weld seam paths. For example, if two plates at a certain angle are to be welded together, as illustrated in Fig 6.16, then the process is used to record the position of two sample points (Touch Points 1 and 2). The position of the two points is then used to calculate the angle between two plates, and the welding seam path automatically calculated.

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Fig 6.16 Touch Sense used to automatically calculate welding paths (source: adapted from [156])

The ability to record position information about a workpiece has allowed TouchSense to be a beneficial tool in the context of quality control of WAAM-Produced objects. In 2019, researchers at the Technical University Darmstadt used TouchSense to record and analyze the layer heights of welded wall structures and compare their values with expected heights [157]. Similarly, Feucht et al [158] used this process to detect the flanges of standard IPE sections onto which stiffeners were printed, whereas Waldschmitt et al [158] adopted TouchSense to systematically record the layer positions of a 3D printed column structure and adapt the CTWD accordingly. Touch Sensing was used in this research to store x,y,z co-ordinate values of a pre-defined grid of points lying on planar and freeform sheet metal. The provision of a pointcloud with regular spacing and oriented along a common axis allows for Non-Uniform Rational B-Spline (NURBS) curves to be generated using parametric software (Fig 6.17) which can then be used to generate NURBS Surfaces.

F i g 6 . 1 7 P r o p o s e d w o r k f l o w f or r e c o n s t r u c t i n g s u r f a c e s w i t h r e c o r d e d c o - o r d i n a t e s

The use of surface over mesh geometry is also beneficial when it comes to extrapolating welding paths from scan data. As mentioned previously, a mesh typically consists of planar faces which, when joined together, form a geometry; a mesh geometry which is curved is approximated by planar faces and are not truly curved. The welding paths which are used communicated to the robotic

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controller are generated defined by curves which lie on the mesh, and the orientation of the welding device is defined by the perpendicular vectors at specific points along the mesh. When the resolution of a mesh is not high enough, or contains errors during the scanning process (such as those arising from reflectivity), then the resulting orientation vectors are also incorrect.

Fig 6.18 Difference between number of vertices generated in a surface reconstruction and mesh construction

Before using TouchSense on curved objects, initial tests were first carried out on flat, horizontallyclamped sheets of metal. In this instance, a robotic arm was programmed to move towards a uniform grid of pre-defined points of known x- and y-co-ordinates and for each point the z value was stored. This was done in order to see the accuracy of the process on a sheet of metal of known geometry, as well as to develop a parametric workflow in which recorded points are used to automatically reconstruct a surface. As illustrated in Fig 6.19, the TouchSense process consists of 4 main steps. First, the robotic arm moves the welding device towards a pre-programmed point, and stops at a certain distance above said point (safety distance). At this point, the robot speed is reduced (2% of maximum speed), and the TouchSense function is activated. The gun is then directed vertically towards the target point until contact is made with the plate and the corresponding z-co-ordinate is stored. The robot arm then guides the welding device away and the process is repeated for the remaining grid of points.

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Fig 6.19 General setup used for recording points on a thin sheet surface

Fig 6.19 and Fig 6.20a shows a setup used to determine the accuracy of using TouchSense to create a digital model of a horizontal plate of known dimensions. A 200x100mm plate was clamped along the long edges and the robot arm was programmed to move towards a grid of 5x8 Points. As recommended by Schneider et al [159], a Stickout of 14mm and reduced movement speed of 2% during touch sensing was used.

Fig 6.20 General setup used for recording points on a thin sheet surface

Initially, each plate was scanned three times in a successive fashion. In some instances, there was no electrical signal sent to the robot and the welding gun continued moving vertically into the plate (Fig 6.20 b, c), causing the stickout to reduce and the recorded Z-value to be false. This error is most likely to occur when the surface of the plate is not cleaned and, in these cases, the plate was cleaned, points are discarded and recalculated. The graph illustrated in Fig 6.21 show that, while there is an overall displacement in measured points to the order of approx. 1mm over the entire length of the plate, this could also arise from external factors, such as the plate not being completely horizontal. Comparing the discrepancies between measurements taken across the same point, the average deviation was averaged to around 0.324mm.

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Fig 6.21 Overview of Touch Sensing sample points

The same method was then applied to an arbitrary, pre-bent sheet of metal. In order to maintain an efficient distribution of sample points, the regular-spaced grid used in planar sheets was discarded. Instead, the grid of points was generated such that they were distributed along the surface in accordance to the degree of curvature of the surface (Fig 6.22a). This was done in order to reduce the total number of sample points needed to reconstruct a surface, and to have them targeted towards the areas where most changes in geometry , and therefore, where most detail is needed.

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Fig 6.22 General process used for reconstructing a nurbs surface from recorded points

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As illustrated Fig 6.22b, the scanning was carried out in two steps. While it is imagined that in future applications, a more exact description of the bent sheet is known, during this thesis, the geometry was approximated by knowing the span of the sheet and the maximum height of the profile. From this approximated geometry profile, a low-density pointcloud is generated. Gasshopper3D is used to analyze the curvature of the profile, and distribute the pointcloud such that areas with higher analyzed curvature are populate with more points. These co-ordinates are then communicated to a TouchSense routine, which has a high tolerance for the movement boundary (20mm); the welding gun will move to an approximate Z position and move down a maximum height of 20mm until hitting the sheet metal. This allows for a degree of tolerance in the approximated model. Once the first TouchSense routine is carried, out, the recorded points are re-uploaded to a parametric script which uses the new points to rebuild a more precise NURBS curve profile. This curve is reevaluated for its curvature, a higher-density pointcloud is added and distributed once again according to the curvature of the rebuilt NURBS curve. The TouchSense routine is carried for a second time. Using a parametric Grassshopper3D script, the new pointcloud is processed into a new NURBS curve network of a higher accuracy, and later converted into a surface geometry. The surface geometry is crucial for updating weld-path information which allows the welding gun to follow the face of the surface in a consistent manner. While this process does involve multiple steps, some of which involve the manual uploading of pointcloud information and re-evaluating within a parametric environment, this was partially due to the limitations of the hardware used at the time. At the time of this research, the robotic controller used (Comau Smart NM 16-3) was not setup to send / receive external digital signals, which would allow for direct communication between the robotic arm and parametric design environment during the TouchSense process, mitigating the need for manual intervention. Nonetheless, while Touch Sensing did not exhibit the same degree of accuracy as LIDAR scanning, the benefits of near-automated scanning, the ability to have a targeted distribution of pointcloud density without having to rely on surface treatment (due to reflections) far outweigh this. Moreover, as the Touch Sensing is carried out using the same device that is used for welding, the issues of orienting the scanned geometry with the actual geometry, as was exhibited in all the other aforementioned scanning techniques, are mitigated. Instead, a single global co-ordinate system is therefore used for both the scanning and welding process.

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6.2 Process Parameter Studies Controlling the heat input during the WAAM process is essential to obtain good quality welds. This is particularly true in the context of welding thin sheet metal, where the substrate material with relatively low stiffness is also susceptible to deformation during welding . The deformation may in turn lead to variations in parameters, such as in CTWD, which will cause a knock-on effect to cause failure during welding. Apart from factors such as the provision of external cooling , the type of shielding gas used or even degree of restraint provided, the welding characteristics play an important role in the quality of a welded bead which ultimately affect the amount of heat induced during the welding process (see 4.5). As illustrated in Fig 6.23, the combination of improper clamping with a high heat input quickly leads to the buckling of thin sheet material, causing a progressively unstable welding process.

Fig 6.23 High distortion from (a) unrestrained plate and (b) high heat input from welding parameters

Therefore, in order to understand the effects, namely of varying critical parameters of Travel speed (Ts), Wire Feed Speed (Wfs) and welding characteristics pre-investigation studies were carried out on thicker substrate material which would remain uninfluenced by the deformation. This allowed for multiple parameters to be studied on a single base plate, serving as a benchmark before performing further studies on individual thin sheet plates (Fig 6.24).

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Fig 6.24 Welding parameter investigation starting from an 8mm plate

6.2.1 Pre-Investigation A 6-axis Comau Smart NM 16-3.1 robotic arm, fitted with a Fronius CMT Advanced welding source was used at the laboratory of the Institute for Structural Mechanics and Design (ISM+D) and the Institute for Steel Construction and Mechanics of Materials (IFSW) at Darmstadt Technical University. Cooling of samples was provided by means of a compressed air source, and were affixed to a mass block of steel by means of bolting (Fig 6.25).

Fig 6.25 setup used at TU Darmstadt for pre-investigations

For the welding of both the 8mm steel plate and thin sheet metal, an 82% Argon 18% C02 Ferroline C18 shielding gas and a WDI Weko ø1mm welding electrode were used. For welding on the 8mm base plate, visual inspection of the widths of welds was carried out to determine which parameters would be transferred to the thinner sheet material. In a fist series of tests, weld seams 70mm in length spaced at 10mm intervals were produced. Robot Travel Speeds (Ts) ranging between 0.2 and 0.45m/min were varied against a gradually increasing CMT cycles (10 – 50) with a constant Interval Break (Tint) of 150ms and Wire Feed Speed (Wfs) of 3.0m/min. A contact tube to working distance was maintained at 14mm. Fig 6.26 illustrates two exemplar cases where (a) a Ts of 0.35m/min and

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Tint of 150ms (the pause time between each cycle) is maintained while increasing the number of steps in the CMT Cyclestep process and (b) the same parameters maintained for a Ts of 0.29m/min. The average heights (Havg) and Widths( Wavg) were measured at 4 points across the weld seam using measurement calipers. As illustrated in (a), increasing the number of cycles alone – that is, the number of weld beads in a spot weld an interval is activated – leads to the increase of width and height of the weld seams as more material is extruded. A process parameter using 40 cycles, for example, lead to an average height and width of 1.81mm and 3.24mm respectively, whereas reducing the number of cycles to 10 decreased these values to 1.25mm and 2.18mm, respectively. Repeating the process for a slower Ts, whilst maintaining the same parameter values, lead to a further increase of overall bead geometry dimensions. A Ts of 0.29m/min with 40 cycles resulted in a Havg and Wavg of 3.418mm and 2.14mm, respectively. Comparing these values to the corresponding Ts of 0.29m/min, there is a roughly 5% decrease in bead geometry width.

Fig 6.26 Effects of increasing number of cycles

As the sheet material is considerably thinner than the weld seams resulting from parameters lying in the range of the above-mentioned Ts and Cycles, the Ts was increased and Cycles decreased. This was so as to strive for thinner wall sections by varying the geometry of the bead. Moreover, g the Cycle Step Interval (Tint) – the time between each welding cycle where the device is deactivated – was also gradually decreased in order to maintain an overlap between spot welds. Fig 6.27 illustrates how, for constant welding parameters, the increase of Tint results in discontinuities in the weld. Therefore, either decreasing the Ts or Tint is also critical in maintaining a consistent weld seam.

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Fig 6.27 Effects of increasing Tint

The Travel Speed Ts, was increased between the range of 0.55 - 0.71m/min, maintaining a Wfs of 1.5m/min. This was in order to alter the Wfs/Ts ration which would be expected to result in narrower weld bead geometry [95]. The number of CMT Cycles was also reduced, initially to 10 cycles and subsequently between 2 – 4 cycles, to reduce the size of each spot weld. Correspondingly, the Tint values were also gradually reduced (10ms to 2ms) in order to achieve narrower beads. Fig 6.28 is an exemplar showcase of weld seams produced with a Ts of 0.65m/min and 2 Cycles. The Tint was gradually reduced between 8ms and 4ms. This gradual reduction of Tint, and therefore increasing of overlap between weld seams, had an effect on the weld seam geometry. A Tint value of 8ms resulted in the smallest weld, with a Wavg of 1.53mm and Havg of 1.16mm. However, the weld seam was also quite unstable, with frequent breaks in the seam, possibly due to not high enough of energy used to properly cause fusion with the thicker base plate material. Reducing the Tint to 6ms resulted in weld seams of an average width of 1.56mm and height of 1.18mm. The weld seams also appeared to be more continuous, although did also exhibit instances of failure in the seam. Welds which were produced with a Tint of 4ms showed the most promising visual quality. Although this did result in slightly wider welds of 1.24mm, the seams were more consistent

F i g 6 . 2 8 T h i n - wa l l e d w e l d s a c h i e v e d b y b a l a n c i n g W f s , T i n t a n d T s

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Seams which were produced faster than 0.75m/min (whilst maintaining the same Wfs) did not produce satisfactory results. Even with a low Tint, the welds tended to produce excessive amounts of spatter and discontinuity in the weld seam. As outlined in 4.5, an excessive travelling speed (with a low Wfs) is likely to result in backward pressure to act upon the weld pool, causing a backward flow of molten weld material and forming humps. Nonetheless, the key values, particularly the use of a low number of cycles with a corresponding Tint and Wfs/Ts ratio served as the starting point for welding on thinner sheet material.

6.2.2 Parameters on sheet metal thin sheet material Maintaining the same wire electrode, shielding gas and welding process, the pre-parameter studies referenced earlier, served as a starting point for welding onto thinner sheet material. The setup used is illustrated in Fig 6.29 where (a) sheets are restrained along the short edge and (b) sheets restrained along the long edge. A gap, approximately 15mm in height, was left underneath the sheets in each setup, which allowed for the provision of underside cooling, which was used whenever welding was carried out.

Fig 6.29 Setup for welding on thin sheet metal

While the setup used in (a) was beneficial as it allowed for sheet metal to be bent along its long edge in future studies, it did not provide adequate restraint against deformation during the welding process. As [125], [126] point out, low levels of restraining result in highest amounts distortion during printing and low residual stresses, whereas the inverse is true for maximum clamping, where deformation generally occurs after the removal of clamps. Moreover, plates which were welded with restraint along the short edge were more prone to vibrating during the welding process, which often led to failure of the weld seams (Fig 6.30).

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Fig 6.30 Unproperly-clamped plates were suspectable to vibration during welding which c a u s e d s e a m s to d i v e r g e f r o m a n o t h e r w i s e s t r a i g h t p a t h

Directly transferring the welding parameters found in the pre-parameter studies did result in excessive deformation and frequent blowouts (Fig 6.31) when (a) a large number of cycles, (b) low Tint and (c) High Wfs, and therefore higher Current and Voltage values, combined with a low movement speed. However, this is expected as heat input is proportional to current and voltage values and inversely proportionate to movement speed, expressed as: Q = K.

U∗I V

∗ 10−3 [KJ/mm]

(6.1)

Where: Q is heat input. (KJ/mm) K is the thermal efficiency coefficient of the welding process (0.8 for GMAW) U is the welding voltage (V) I is the welding current (A) V is the welding (robot) travelling speed (m/min)

Fig 6.31 (a) High heat input resulted frequent blowouts which were eventually controlled (b) by altering the welding parameters

Therefore, the process of parameter finding on thin sheet metal was repeated, using a lower benchmark of cycles in the welding job. Evaluating the results was done by means of (a) visual inspection of the seam (b) measurement of basic bead geometry (Wavg, Havg) and (c) the average spread of

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the Heat Affected Zone (HAZ) on the underside of the plates (HZavg). In the case where small bead widths were achieved, the HAZ was unclear on the topside of the plate (Fig 6.32a), and therefore the measurement was discarded. Moreover, the HAZ is characterized by a diffused band of colors with no clear boundary. With this regard, the measurements used to calculate the average HAZ width were taken at the interface between tempering zones, indicated by white and black color bands at the HAZ extremities (Fig 6.32a,b). Although this did not give an exact measurement, as diffusion between the bands is also evident especially in the case of wider HAZ spreads, it did serve as a reliable technique in evaluating the relative spread of the HAZ for different welding parameters.

Fig 6.32 Effects of increased number of cycles on the growth of the weld quality and g r o w t h o f H e a t A f f e c t Z o n e ( H AZ )

Pre-investigations using 6 cycles and Ts of 0.65m/min were initially carried out with a Wfs varying in steps of 0.5m/mm between 5 6m/mm and a maintained Tint of 10s. Plates were welded with seams of 120mm in length, on a plate 200mm x 100m x 0.75mm (length, width, thickness). This allowed for seams onto be welded along the central line of axis of the plates, with a 50mm offset from the longitudinal edges. It immediately became apparent that the combination of number of cycles and increasing Wfs added to poor quality welds. As illustrated Fig 6.33(a-c), the increasing of the Wfs for the same welding parameters had the effect of (a) widening of the average joint width, as would be expected with increasing Wfs/Ts [95] . This is due to the increased current and voltage (power) input, which is proportional to the Heat input. The increasing heat energy is illustrated by a gradual growing of Heat Affected Zone bands, with plates welded using a Wfs of 5m/min resulting in a HZavg of 13.95mm, whereas a higher Wfs of 6m/min resulted in higher spread of the HAZ (HZavg of 22.01mm) as well as signs of penetration through the plate.

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Fig 6.33 Effects on weld quality and HAZ with Wfs

Therefore, in order to reduce the amount of heat input into the plate, firstly the number of cycles was reduced, as well as the Wfs/Ts ratio. Plates were welded with speeds varying between 0.6 – 0.64m/min, Wfs ranging between 1 – 3m/min and Cycles ranging from 4 – 2. This was so as to reduce the Wfs/Ts, which is inversely proportional to the heat input as well as the width of the bead geometry [95]. Fig 6.34 shows the results of plates welded with a Ts of 0.6m/mm, Tint of 20ms, and Wfs varying between 1 -3m/min, each welded with 4 cycles. While there was still an increase in HAZ spread with the increase of Wfs/Ts, this was not as pronounced as with the previous parameter settings with High Wfs and cycles. The lower amount of welding cycles also results in smaller spot weld sizes, and therefore also aiding in reducing the amount of heat input into the plate. The same trend of bead geometry change was also apparent. A lower Wfs of 1m/mm produced seams which were around 2.28mm in width whereas a Wfs of 3m/min produced a seam with 2.39mm in average width. This further falls in line with the expected increase of bead width with an increased Wfs/Ts.

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Fig 6.34 Effects on the spread of HAZ by varying the Wfs

Varying the Tint values also had considerable effect on the joint geometry. Fig 6.35 show images taken with a Keyence VHX 600 digital microscope, provided by the ISM+D laboratory. What can be noted is that, for a given Ts (0.6m/min), Wfs (2m/min) and number of cycles (4 cycles), increasing the Tint results in a progressively larger spacing between spot welds, eventually becoming fully detached once the Tint values exceeded 50ms.

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Fig 6.35 effects on altering Tint values

Maintaining the same welding parameters and reducing the number of cycles to 3 led to a further reduction in the overall average bead geometry size. This is also expected as, reducing the cycle number also results in a reduction of the amount of material extruded per spot weld. A Tint of 10ms (3cycles) resulted in a weld seam approximately 2.32mm in width, whereas a Tint of 10ms (4cycles) produced seams approximately 2.41mm in width. As also expected, the gradual increasing of Tint even with two welding cycles led to the gradual decreasing in overlap, and therefore, continuity, of weld seams (Fig 6.36).

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Fig 6.36 Effects of Tint on the weld bead geometry

Joints produced with 2 welding cycles, and the same welding parameters referenced above, produced the thinnest welding beads, as would be expected with lesser material being extruded in each spotweld. Fig 6.37 shows the differences in bead geometries for (a) 2 cycles, (b) 4 cycles and (c) 15 cycles with a constant travelling speed. Using a 2-cycle process with Ts of 4ms produced welds with widths averaging 1.18mm and heights of 0.6mm, and a HAZ spread of around 3.21mm, due to a relatively low heat input. Processes with 15cycles, Wfs of 3m/min on the other hand produced welds with widths of 3.48mm in average dimension, with a HAZ spread on the baseplate averaging 13.95mm.

F i g 6 . 3 7 T h i n - wa l l e d w e l d s w e r e p o s s i b l e w i t h l o w c y c l e s ; h o w e v e r , t h e q u a l i t y w a s a l s o inconsistent compared to those with 4 cycles

6.2 Process Parameter Studies

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6.2.3 Discussion Bending distortion was also present in all samples after unclamping, even with the presence of underside water cooling. However, this only occurred after the removal of clamps and did not affect the weld during the printing process. Multiple researchers have proposed setups for reducing deformation in thin sheet metal by means of locally-heating thin sheets with a travelling heat source [138], [143], as well as Low Stress No Distortion (LSND) welding [126]; a method in which CO2 snow is used to directly cool a weld seam after deposition. Moreover, the electrode diameter used was a 1mm gauge, it is recommended that smaller gauge electrodes which require fewer working currents and voltages, and therefore heat input, are used in further research.

Fig 6.38 Effects on bead geometry and dilution by different gas mixtures (a) 82%Ar 18%C02 and (b) 100% Ar gas

Attempts to use a lower number of cycles also did not produce satisfactory results. This was largely due to a failure of arc initiation, which caused the welding process to stop. Therefore, the welding characteristics used in further parts of the research were based on process parameters with movement speeds of 0.6m/min, Wfs of 1-2m/min, Tint between 10-20ms and 2-3 cycles. The parameters also indicate a good connection between the weld material and the base plate, as was noted by 4-point bending test results (tests 6.3.2). Analysis of weld beads produced with the parameters also indicate a good adhesion with the base material, with a dilution of approximately 10% (4.5), (Fig 6.38). Fig 6.38b illustrates the effects of different gas mixtures on the bead geometry. A 100% Ar gas mixture is illustrated, with a noticeable increase in weld bead height and reduction in width. Moreover, the penetration depth is higher than the 82%Ar12%C02 mix, with a dilution of approximately 11%. Further studies were not carried out on the particular mixture, however the ability to weld bead geometries with higher heights could prove beneficial in providing stiffness of the plate during printing through a higher moment of inertia. Moreover, the higher height profile would result in heat being drawn away from the plate in fewer layers than an 82%Ar,12%C02 mix.

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6.3 Material Testing Material testing was carried out on welded sheets of metal, to gain understanding of the effects an increasing HAZ had on the performance of the sheet material. Moreover, 4-point bending was carried out to gather information regarding the connection between a welded stringer and the baseplate. Due to the inability to buildup wall structures of sufficient height with the welding parameters used on the sheet material, from which dog-bone structures are to be milled, tensile specimens are not presented.

6.3.1 Welded sheet Tensile Testing In order to understand the effects welding had on the mechanical properties of the parent material, sheets of metal were welded with two parallel lines of varying distance and a third series using process parameters which promoted extended spread of the HAZ (Fig 6.39). A 200 mm x 18 mm strip of metal was cut from the area between the two welded lines and loaded under tension in a Zwick Roel Z050. This was done in order to study how the effects of welding changed with respect to the spread of the heat affect zone around the weld seam. As discussed in 4.5, the area immediately surrounding a weld seam, visually indicated by a Heat Affected Zone (HAZ) undergoes changes in its microstructure, with a grain growth region forming immediately around the weld, followed by a recrystallization and tempering zone[160], [161].

Fig 6.39 Preperation of plate strips (a) Welded plate (b) strip cut between welds, (c) plate srayed for detection by DIC camera and (d) speciments with higher-heat input welding parameters and spread of HAZ

The specimens were first cut from 1000 mm x 600 mm, and 1000 mm x 200 mm sheets of metal, with dimensions of 200 mm x 100 mm before being welded and material specifications listed in Table 1. With each sheet, 5 test specimens were also cut in order to maintain control and account

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for any changes in the material properties of the sheet metal. The plates were cut such that testing is carried out such that loading occurs parallel to the roll direction of the sheet material (Fig 6.40), with control specimens for both parallel and perpendicular roll directions.

Fig 6.40 Specimens were cut from the same sheet. Specimens were cut in the direction of rolling, which were indicated by marks on the raw steel.

After welding, the 200 mm x 18 mm strip used for testing was cut. Rather than using CNC-Milling or laser cutting, the sheets were cut using a set of table Guillotine Shears. This was so as to avoid any changes in the properties of the test specimen during the cutting process due to heat generated. Moreover, as sheets were not completely flat due to the welding process, using a guillotine allowed for a repeatable cutting process using the facilities available at the time. After cutting, the specimens were sprayed with a matte-white lacquer and a subsequent black speckle paint layer. Each specimen was loaded in tension using a Zwick Roel Z050, at a rate of 3N/s. A Digital Imaging Correlation (DIC) camera was used in parallel to the testing in order to account for any discrepancies in values obtained from the Zwick device.

F i g 6 . 4 1 T e n s i l e T e s t s e t u p a t t h e I S M +D l a b o r a t o r y , T U D a r m s t a d t .

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Table 6.1: Material Specification Specification

133

Value

Unit

S235 JR Ungalvanized Structural Steel

Material Specification Strength Class Nominal Thickness Specified Yield Strength Specified elongation at rapture Chemical Composition

DIN EN 10025-2

-

A

-

0.75

mm

≥ 235

MPa

≥ 18 in roll direction

%

C(0.17%), Mn(1.40%), P(0.035%), N(0.012%), Cu(0.55%), CE(0.35%)

-

Although specimens were rectangular, only a small percentage failed outside the middle 1/3 during testing, and were discarded from the results. As illustrated in Fig 6.42, there was no significant changes in the Yield Strength and Ultimate Strength of the material. However, there was a significant change in the ductility of the material, with welded sheet metal exhibiting around a 6% reduction in elongation when compared to unwelded specimens.

Table 6.2 : Material Testing results for welded sheets Specification

S235JR-A, Salzgitter GmbH 00 roll direction 900 roll directions 10mm offset 5mm offset

Yield Strength, Re N/mm2 ≥ 235

Ultimate Strength, Fu N/mm2 360 - 510

Elongation % ≥16

380.25 ± 4.4

386.15 ± 4.3

24.71 ± 3.6

379.33 ± 3.2

388.6 ± 2.15

20.4 ± 0.5

275.85 ± 5.29

382.5 ± 8.17

19.17 ± 0.25

278.65 ± 3.86

378.625 ± 3.75

18.4 ± 0.24

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6.3 Material Testing

F i g 6 . 4 2 T e n s i l e T e s t R e s u l t s fo r w e l d e d S - 2 3 5 - J R s p e c i m e n s

F i g 6 . 4 3 T e n s i l e T e s t R e s u l t s fo r g a l v a n i z e d s t e e l s p e c i m e n s

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When the same test was repeated on welded galvanized steel, the same tendency towards a reduction in ductility was also exhibited as the specimens were affected by the Heat Affected Zone through closer spacing of weld (Fig 6.42). Although this research didn’t delve deeply into the causes of the loss in ductility of the material caused by welding, a possible explanation could be due to the heating and subsequent rapid cooling of the material. Heat treatment processes, that is, the heating or cooling of metals, are often adopted to alter the mechanical characteristics of metals. Annealing, for example, is used to increase the ductility of metal. This is achieved by first heating a metal to a temperature in which internal stresses are released, followed by heating the metal above its recrystallization phase. When this temperature is exceeded, yet remains under the melting point of the metal, new grains are formed within the material which, when cooled, exhibit an increase in size (grain growth), and promote an increased ductility in the material [162]. An important, factor, however, is the rate at which material is cooled after heating; in general, a long cooling period will promote increases in ductility and reduction in hardness of the material, while a rapidly-cooled metal will become harder yet more brittle through a process known as quenching. This is due to the formation of martensite which are very hard and brittle crystalline structures[163], [164]. When the sheet metal was welded, the area surrounding the weld was immediately cooled by sprayed air and water vapor; in other words, the material was heated and underwent a form of quenching. Therefore, one possible explanation for the reduction in ductility next to the weld specimen could be due to the formation of martensite. Abdunllah et al [164] also point out that, as the formation of martensite is largely dependent on the rate of cooling (quenching), thin-sheet metals content is also particularly susceptible to this due to the rapid-cooling of thinner materials. However, the results show only an indication that there is a reduction in ductility; the changing of the hardness of the material around the weld (as would be expected during quenching), for example, was not evaluated. A second point of note is orientation of the sheet metal grain structure. The sheet metal used was cold-rolled steel- this production process orients the grain structure of the material towards the direction of rolling. As exhibited in Fig 6.43, when specimens are tested parallel to the rolling direction, results show ductility characteristics[165]–[167]. During the welding process, however, the area within the heat affected zone undergoes a degree of grain growth and refinement. Therefore, one may expect that a loss of grain-orientation may also lead to a reduction in the ductility of the material. However, at the point in research, this is purely speculative and further research is needed for clarification.

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136

6.3.2 4 Point Bending Test 4 Point bending tests were also carried out on specimens with ribs of varying heights printed onto 0.75mm sheet metal of 150 x 50mm in size (Fig 6.44).

F i g 6 . 4 4 4 p o i n t b e n d i n g t e s t s tr i n g e r s p e c i m e n

A Zwick Roel Z050 was used to load the specimens at a rate of 3N/s. The specimen was placed with the unwelded face of the sheet facing downwards, this was so as to prevent the specimen from slipping during testing, as illustrated in Fig 6.45. In order to induce tensile forces into the top edge of printed section of the specimen, the support location of the 4-point bending.

F i g 6 . 4 5 4 p o i n t b e n d i n g s e t u p , a t t h e I S M +D L a b o r a t o r y a v a i l a b l e a t T U D a r m s t a d t .

A significant factor in the performance of the specimens was the quality of the weld. Fig 6.46 shows two specimens with different moments of inertia, and welding quality. A first sample consisting of a 10mm rib (S_10) which contained no visual imperfections other than the start and end during welding and an approx.. 6.5mm (S_6.5) rib produced which contained imperfections.

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Fig 6.46 Results from specimens with 10mm and 6.5mm heigh sections

Assuming the plate and welded stringer acts as an isolated T-Section, then the effective width of its flange, bf is given as the lesser of:

bf = bw +

lo b

lo

+4

; bf = b

Where: bf = Effective width of the flange lo = Span of between two points of 0 moment (120mm) bw = The breadth of the web (100mm)

(6.2)

6.3 Material Testing

138

b = The width of the flange (1.4mm) Table 6.3: Section Properties Specimen S6.5

Specimen S10

B (mm)

24.50

24.50

H (mm)

6.50

10.00

h (mm)

0.75

0.75

b (mm)

1.40

1.40

Iyy (mm4)

920.6

921.4

Y (mm)

5.67

8.05

Assuming the plate acts as an inverted beam then the moments of inertia for welded sections Iyy10 and Iyy6.5 1.4mm in thickness are 920.6mm4 and 921.4mm4 respectively. The S_10 specimen is subject to a maximum imposed load of 1205N, whereas the S_6.5 specimen undergoes a maximum imposed load of 519.4N. The corresponding maximum Bending stress is given by:

σmax = −

M∗y Iyy

(6.3)

Where: σmax = the maximum bending stress (MPa) M = the maximum bending moment, acting in the center span of the beam (KNmm) Y = the vertical distance from the Neutral Axis Iyy = is the moment of inertia for the printed section, assumed to be acting as an upstand, rectangular beam section (mm4)

Therefore, substituting values for S_10 and S_6.5 having the geometrical properties and bending moment listed above, the corresponding maximum stress imposed in the section is 212.5MPa (S_10) and 235MPa (S_6.5) respectively. Fig 6.47 illustrates the two samples after testing. The S_10 sample which did not contain any visual welding imperfections within the area of interest did no exhibit any failure or cracking within the weld, nor any delamination from the thin sheet plate. In fact, the sheet metal immediately in the area of maximum moment / shear showed signs of buckling. The S_6.5 sample which did have welding imperfections, showed failure within the weld as well as signs of delamination (Fig 6.47c). This is despite the imposed bending stress for both specimens being subject to stresses well below the specified Ultimate Tensile Strength of the material (WDI G3Sil, 520MPa). A further possible explanation for the failure could be the buildup of stress concentrations around the humps and valleys in the weld seam – as illustrated below, failure occurs at the valley of one of the welding seams.

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Fig 6.47 results of 4-point testing on two specimens. (a,b) showing good connection between the baseplate and the welded geometry, with not visible signs of cracking of the weld and (c) specimen with a failure in the weld.

The 4-point bending did give some insights into the performance of the welded plate elements. Particularly, it highlighted the effects of welding imperfections and therefore, the need for process control during welding. While the S_10 rib did indicate a good bond between the weld and the plate, as no visual cracks were present, the specimen with improper welding did fail unexpectedly. This could be due to a multitude of reasons, including, for example, an inhomogeneous weld. In order to further showcase this, a plate was welded with multiple, single-layer welding lines – having both desirable welding properties (Fig 6.48a,c) and welded without the presence of gas so as to promote the presence of pores within the weld ( Fig 6.48b,d).

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6.3 Material Testing

F i g 6 . 4 8 D i f f e r e n c e i n w e l d s e a m c o n s i s t e n c y ( a , c ) w i t h n o c a vi t i e s p r i o r t o l o a d i n g a n d (b,d) with voids prior to loading

As the welds were welded on the same plates used for testing, they underwent the same loading conditions. However, it is quite apparent that a homogenous weld has superior bonding to the plate as no surface cracks were visible, whereas the seams welded with the inclusion of pores did indeed crack at point of maximum moment/shear, inhomogeneous welds can therefore be expected to behave as a more brittle material. However, assuming that a weld is homogenous, opportunities for improving the performance of stiffened elements also becomes more apparent.

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6.4 Formulating Welding Strategies Among the strategies commonly adopted by shipbuilding and metal working for reducing the deformation in sheet metal is the use of intermittent and backstep welding. In backstep welding, the welding line is broken up into multiple steps. An example illustrated in Fig 6.49a, the general progression of the weld moving from right to left is broken up into smaller welding segments, and welded in the opposite direction. As weld segments are deposited, weld material contributes to the stiffness of the plate and thus its resistance to distortion [168]. Intermittent welding is a process of using shorter and spaced welding seams, rather than continuous ones, which reduces the amount of weld material being used and therefore also the amount of heat input. These two strategies therefore offer quite some potential in reducing distortion in plates, simply by controlling the progression and/or length of welding seams used.

Fig 6.49 Principle for backstep welding strategies where (a) a seam is broken up into smaller segments, which are progressively welded in the opposite direction of travel and (b) intermittent welding where weld seams are broken up into spaced segments.

In order to illustrate the effects welding length had on the overall distortion of a welded plate, sheet metal specimens were welded with varying welding lengths. Two sets of samples were prepared; those welded on 0.75mm thick material and a second welded on 1.5mm thick sheet metal. The plate specimens, were fully clamped along their long edges and welded using constant welding characteristics and movement speed (Fig 6.50a). The distortion was evaluated by measuring the global displacement of the welded plate when placed horizontally (Fig 6.50b,c). This method of evaluation was also used by other researchers [126] in order to gain an indication on the relationship between welded seam and distortion in thin sheet material.

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6.4 Formulating Welding Strategies

Fig 6.50 Setup used to determine the effects of welding length on the global distortion on thin sheet metal

In both thicknesses of material, the primary distortion was that of longitudinal distortion, that is, distortion occurred along the welding axis. However, for the 0.75mm specimens, the effects of transverse distortion were also more pronounced (Fig 6.51a,b). This is likely due to the thinner sheet material having a lowers stiffness than the 1.5mm sheet metal and therefore, a lower resistance to the effects of heat-induced distortion.

Fig 6.51 (a) Transverse distortion mostly evident when welding close to the edge of a s h e e t m e t a l a n d ( b ) l o n g i t u d i n a l d i s t o r t i o n w a s t h e m o s t c o m m on f o r m o f d i s t o r t i o n f o r l o n g e r seams

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Moreover, the 1.5mm sheet specimens had a tendency to have out-of-plane distortion which was convex in nature, relative to the face of the welded sheet, whereas the distortion for the 0.75mm specimens were almost always concave in nature (Fig 6.52. Although it is not known why, it could be due to external factors such as clamps not being completely plane with one another, causing prebending in one direction which did not have a significant-enough effect on the thicker sheet material.

Fig 6.52 underside of welded plate with reduced HAZ

The effects of welding length and the extent of distortion was quite apparent in both sets of specimens; a longer weld results in a larger heat input introduced into the specimens and therefore a higher degree of distortion. However, while longer seams do increase the overall distortion, the weld material also adds a degree of stiffness to the plate. As illustrated in Fig 6.53 the rate of distortion does not seem to be linear as the length of the weld increases. Since specimens are cooled down before removing the clamp, it could be probable that the welded material also contributes to the stiffness of the plate. However, this is merely speculative and further research should be conducted for longer weld seams.

Fig 6.53 Increase of global distortion with the increase of weld seam length

Nonetheless, it can be concluded that shorter welding lengths do result in less deformation in the sheet material. Moreover, there are indications that while the welded material is the primary cause of distortion in the plate, it may also contribute to the stiffness of the plates after welding. With this in mind, different welding strategies are proposed, whereby the lengths of the welding seams are minimized with the intent to also stiffen the plate as the welding sequence progresses.

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6.4 Formulating Welding Strategies

Fig 6.54 Development of welding strategies to imply spacing between welds as well as reduced segments lengths.

Fig 6.54a,b illustrates a welding sequence for a typical welding pattern in which welds are deposited orthogonal to one another, with the intention of providing stiffness to a larger area of the plate to counteract distortion. In this instance, the weld seam, A-A, is broken up into shorter segments. The weld seams 1 – 5 are first welded in the sequence such that opposing welds are sequentially welded, with cooling between welding each seam. This is to distribute the heat being input into the plate and to provide stiffness to the plate before welding seams 6 – 9. The breaking up of welding seams into smaller segments led to the development of alternative welding sequences and patterns.Fig 6.55a illustrates a single, continuous weld seam with a length equal to the entire length of the plate. Fig 6.55b shows the use of intermittent welding as a method of generating curtailment. The single weld seam is broken up into shorter, spaced welds which have a shifting overlapping pattern to form curtailment. Fig 6.55 c - e illustrate the use of smaller welding lengths to provide a stiffening grid at angles of 300,600 and 900 respectively. In these cases, each diagonal is printed as a single segment, using a welding sequence illustrated in Fig 6.55. This allows for shorter welding lengths to be printed, which is beneficial for minimizing the deformation, controlling the buildup of heat in the plate by staggering the order in which seams are welded as well as using backstep welding by alternating the direction of welding. However, the introduction of cross-pattern led to issues of excessive amount of material at the intersection of welding lines in order to overcome this, the generating of print paths should include the stopping of welding seams before overlap occurs (Fig 6.55f)

Fig 6.55 Development of alternative print path strategies (a) a continuous path (b) curtailment with parallel paths (c-e) cross-diagonal printing paths.

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The ability to vary the length of a welding seam therefore results in a number of potential welding strategies for stiffening and detailing thin sheet metal. Although tests were only carried out on smallscale samples, with a maximum length not exceeding 300mm, the use of shorter weld seams / intermittent welding strategies could aid in allowing for larger details to be printed on the sheet metal. Fig 6.56 shows the use of different cross-patterns for (a,b) stiffening sheet metal in order to weld larger stringer elements and (c,d) using the same cross-pattern for providing smaller details, such as façade fixation points. The issues of overlapping weld seams (Fig 6.56f) quickly become apparent, particularly in Fig 6.56b and c, where the inhomogeneous weld in the cross-pattern caused failures in subsequent layers of weld material, particularly at the point of intersecting weld seams. Therefore, when multiple layers of material are to be welded, the cross-patterns should either be omitted or shortened (Fig 6.56d) at the location where the printed detail is to be included.

Fig 6.56 Buildup of weld layers in combination with welding strategies

6.4.1 Topology Optimization The ability to additively manufacture geometrically-complex structures with relatively low material expenditure has allowed for novel form-finding techniques to be adopted, such as Topology Optimization (TO). This is a structural form-finding process in which material distribution for a given design space is optimized to match criteria, such as maximized stiffness or minimal mass, subject to prescribed loading conditions. The availability of multiple commercial and non-commercial TO software packages [169]–[172], as well as the organic nature of forms, has allowed TO to be widely adopted in the context of Additive Manufacturing [173]–[176]. Although different software packages may be based on different TO algorithms, the general workflow for utilizing TO in additive manufacturing is illustrated in Fig 6.57.

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6.4 Formulating Welding Strategies

F i g 6 . 5 7 G e n e r a l p r o c e s s u s e d i n T o p o l o g y O p t i m i z a t i o n i n d i f f e r e n t c o m m e r ci a l l y - a v a i l a ble software packages.

An initial design space – that is, the volume in which TO is to be performed, is characterized with a certain resolution of Finite Element Meshes. The design space is prescribed loading and support boundary conditions. The resolution, and therefore size, of the FE meshes has a direct impact on the speed in which TO is carried out as well as the level of detail achieved. A coarse mesh will result in a faster processing time – ideal in the context of mass-production, however, it may also result in large sections being generated. Once a TO mesh is defined, Finite Element Analysis is performed on the volume to assign a rejection criterion to each mesh depending on their contribution to the overall structure. Mathematical algorithms are used to progressively remove inefficient FE-Meshes, and therefore, material, from the design space. This process is iterated until an objective function, such as the reduction of initial structural mass to a defined % whilst also minimizing deflection or strain energy (compliance), is satisfied, or the maximum defined iterations is reached. The result of these steps is a mesh which is defined by the efficient distribution of material. When used for additive manufacturing, the end results are often not suitable for printing – for example, the meshes may be open, or course, depending on the resolution of the initial density model. Therefore, post-processing steps are often incorporated into the TO process to either refine the mesh or completely reconstruct it to be suitable for additive manufacturing. While TO is certainly beneficial for

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generating structurally-efficient forms suitable for additive manufacturing, it is a time and energyintensive process which often requires multiple post-processing steps. In order to determine the suitability of TO as a means of reinforcing thin-sheet metal with WAAM in facades (i.e., mass-production), different software packages were evaluated to illustrate what a workflow would have to consist of. This was carried out using both non-commercial [170], [177]commercial [171]software tools. Non-commercial software packages were used as they are widely available within the CAD / algorithmic modelling tools Rhinoceros3D[178] and Grasshopper[179], and therefore offered potentials for being implemented within the design-to-production workflow which was established with these tools (Fig 6.58).

Fig 6.58 Integrating TO into the desired workflow would require multiple steps

A 300 x 150 x 0.75mm volume is created to represent an arbitrary sheet metal element supported by 4 points and loaded with an arbitrary load of 50Nmm-2 across its entire surface (Fig 6.59). Two free-to-use grasshopper plugins, TopOpt and Millipede were used to perform topology optimization on the digital model in order to determine what a workflow would look like and therefore, achieving a fully-optimized design was not the scope. In the TO process, the volume to be optimized must first be defined by a volume of FEA meshes, the size of which has a direct impact on the level of detail of the final structure, at the expense of the processing time. As shown in Fig 6.59, decreasing the size of the FEA mesh (increasing resolution) results in finer TO structures, with far greater processing time. Due the WAAM process uses welds of 1-2mm in thickness, however, the FEA mesh should also be of a similar size. This results in a relatively long processing time to generate the topology mesh. in the example illustrated below, a mesh with 1.5mm resolution required 822.54 seconds to generate a single result. In the context of facades, in which it is imagined that thousands unique results are to be generated, it quickly becomes apparent that the benefits of being able to quickly iterate through designs and generate details with parametric design tools could potentially be outweighed by long processing times.

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Fig 6.59 Alternative strategies for using TO based on a 3D mesh and 2D image map

Moreover, the TO tools used in grasshopper were not as versatile as other commercially-available, non-parametric tools, and did not allow for imposing geometric constrains, such as minimizing overhangs or fabrication. This is also quite an important feature, as it was envisaged that printed structures would consist of solid, continuous material in order to allow for a stable welding process and straight-forward motion paths (Fig 6.60b)The geometry generated using 3D meshes resulted in very geometrically-complex structures, as illustrated below. Therefore, while the geometry may be structurally sound, it requires very complex path planning and post-processing.

Fig 6.60 TO results based from (top) meshing and (bottom) refinement and extrusion of 2D image map

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Therefore, a second process is explored – rather than using a direct topology optimization method in which a reduced-mass 3D mesh model is output, the same structural setup is analyzed as a shell in Grasshopper3d. As done previously, the shell is defined by an FE-Mesh with a certain resolution, which has a direct impact on the processing time and the level of detail achievable. However, because the analysis is performed on a 2D-element, the processing time is far more rapid. The output results of the analysis are a series of colorized 2D Meshes. The colored meshes (Fig 6.61) indicate the distribution of a stiffness factor; an optimization-calculated factor which indicates the relative importance of the element in the overall system. In this example, a white mesh indicates an area where relatively high stiffness (and therefore, material) is factored and black where the least amount of material contributes to stiffness.

F i g 6 . 6 1 T O i ma g e m a p u s e d t o d i s t r i b u t e a n d a p p l y s t i f f n e s s f a c t o r s

Using the same parametric design tools, each individual mesh is evaluated for its assigned stiffness factor, and assigned a thickness based its value. This process allows for a 2D-Mesh to be automatically extruded into a three-dimensional mesh, using a 2D color map as a reference to what extent this happens. This mesh thickening step results in material which is continuous, unlike that using a direct topology optimization method – making it far more ideal in later steps of converting the mass into printing paths. Conventional slicing tools available within Grasshopper are then used to create contours of the mesh which are then converted into print paths sent to the robotic welding device. While this allows for a fully-parametric workflow, which is essential in the context of mass customization, it comes at the expense that the structure itself is not full optimized. The colored mesh only indicates the relative distribution of material and it is the user who must prescribe the relative thickness of material that must be assigned within the range of values; integrating engineering judgement to properly assign relative material thickness values.

Fig 6.62 Creation of contours based on distributing thickness from 2D Image map

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Fig 6.63 Plates welded using TO did not give satisfactory results, largely because it wasn’t possible to control the spacing of seams and their individual lengths as well as the thickness of members (without resorting to high-resolution, and time-consuming meshing)

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Welding the structure onto sheet metal presented many problems. As discussed in Section 6.4, the length and spacing of the weld seams had a direct effect on how much the sheet material deforms. Because the welding paths from the TO process resulted in sections with a thickness greater than the width of a welding seam, multiple welding passes were needed. This resulted in multiple welding lines being printed parallel to one another , causing significant deformation during printing. This resulted in an unstable welding process, as illustrated by the multiple discontinuities in the welding seam. Moreover, because there was no real control over the outcome of what the TO process outputs, there was also no control in defining minimum spacings between weld seams as well as minimum weld lengths. Moreover, when the same process was repeated for non-planar elements, which is the intention for this research, it quickly became apparent that alternative strategies for generating rib layouts would be required.

6.4.2 Proposing a curve-based approach While topology optimization is certainly a form-finding process that is complimentary to additive manufacturing, it is also quite apparent that this process does not fit into the context mass production and reinforcing/detailing thin sheet metal. Firstly, the reliance of multiple processing steps, makes this approach rather time intensive – the benefits of structural performance are perhaps lost to the sheer amount of time that would be required to generate hundreds, if not thousands, of solutions, as would be expected in a typical façade project. Secondly, the inability to control aspects of the geometry, such as lengths and spacing of individual sections, make this approach rather unresponsive. Therefore, an approach which is directly based applying curves is perhaps more suitable. This would not only give more control on the generation of ribs and details, but would also allow for adaptability for welding on planar and free-form surfaces.

Fig 6.64 curve-based approach allowing for quick adaptability to both planar and curved surfaces

One of the benefits of adopting parametric design tools is that a single software package can be used to analyze a façade, generate rib layout patterns, perform structural analysis and generate motion commands uploaded to a robotic device. Although many structural analysis plugins for grasshopper do exist[170], [180], [181], the chosen analysis plugin used was the parametric engineering plugin, Karamba3D[182]. Developed by Bollinger + Grohmann GmbH. Karamba3D is often adopted for industrial engineering scenarios, and is therefore already widely-adopted established outside of academia[183]–[185]. As illustrated in adopting a curve-based approach (rather than using solids) opens up opportunities for using relatively simple design variables (in the image below, the variation of rib spacing, number and height, which are all defined as curves), which may be evaluated using

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said structural analysis tools and automatically converted into robotic motion control commands. While, perhaps, the outcomes are not as structurally-optimized as topology optimization, the ability to vary fabrications aspect such as spacing of ribs (and therefore, establishing a minimum distance between print paths to reduce deformation) or the maximum length of a printed segment (which also affects distortion) allows for a design-to-production process which takes into consideration the limitations of the fabrication process itself.

F i g 6 . 6 5 P a r a me t r i c E n g i n e e r i n g S o f t w a r e , s u c h a s K a r a m b a 3 d , a l l o w s f o r d e s i g n s t r a t e g y i t e r a t i o n s t o b e q u i c k l y g e n e r a t e d , a n a l y ze d a n d p r e p a r e d f o r p r i n t i n g .

Fig 6.65 illustrates this approach for determining the rib layout for an arbitrary plate supported at a short edge, and loaded as a cantilever. Input design variables, namely having a single-layer square grid to provide stiffness after printing as discussed in 6.4, with a minimum spacing of 15mm (arbitrarily assigned), as well as the possibility to vary the height of the ribs to match prescribed loading conditions, allowed for a number of solutions to be generated from the parametric design inputs. Karamba 3d was used to evaluate solutions by changing the design variables, outputting a library of solutions. In the example illustrated in Fig 6.66, a degree of curtailment was applied (closer spacing of Y1 – Y5, versus Y6 and Y7), which matched more closely the expected bending moment induced in the plate. Moreover, the solution also resulted in a tapered central rib being produced, with the variable layer number, Δz, gradually decreasing as moment also decreased along the length of the plate. While the solution exhibited in this example is far from optimal – several assumptions were made, including neglecting the change in material properties after welding and ignoring welding imperfections, it does illustrate the potentials of generating structural welding solutions in a fully parametric environment, with a higher degree of control over fabrication-driven constraints.

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Fig 6.66 A Curve based approach opens up potentials for a fully-parametric workflow

The use of computational design tools to directly control the individual spacings, heights and lengths of printed segments opens up exciting design strategies for using WAAM to reinforce sheet metal. One promising design methodology is the use of force-flow vectors as a means of indicating optimal placement of material [31], [186]–[189], with one of the earliest examples of this illustrated by Hans-Dieter Hecker who used isostatic curves to generate positioning of ribs for the Freiburg University auditorium roof structure.Tam et al[190] proposed the orientating of printed plastic material along major lines of forces, known as principal stress lines, as a means of generating 2.5d printed grid shell topologies. By orienting material along the principal stress lines; pairs of orthogonal curves which conceptualize trajectories of internal forces representing idealized paths of material continuity [191], the researchers proposed a framework for Stressline Additive Manufacturing (SLAM), in which parametric design tools (Karamba 3D) are used to generate force-driven printing paths It is not the intent of this research to fully develop a SLAM approach for WAAM, rather it is intended to give light to the possible tools which are available to researcher’s disposure. Therefore, while there is much to be developed in the use of stress trajectories as a means of generating print paths; particularly creating a streamlined workflow between generating the stress trajectories, refining them and preparing for printing during post-processing, the topic will only be touched upon briefly as a means of demonstrating potential future implementation. This was done utilizing the same Karamba3D software mentioned earlier, which allows for FEA analysis of plate structures. The process for extracting the isostatic curves for a plate supported by predefined supports and undergoing a uniform load is begins with prescribing an initial point (seed), from each of which Finite Element Analysis is used to determine the principal stress directions for that given point, from which a curve is drawn along the stress vectors, representing one iteration [191]. The end of the curve, which has a prescribed length, represents the next point of analysis where the stress vectors are computed. This iterative process is repeated until the design space boundaries are met. The resulting networks of curves are representatives of the flow of forces through an element – offering potentials as acting as an estimated guide of there to place material efficiently (Fig 6.67).

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Fig 6.67 a random seed point (a) is generated on a design space from which orthogonal and principal stress trajectories are given (b). The increasing of seed points (c) eventually leading to a stress field which is then refined (e) with a smaller number of seed points.

To illustrate this, an arbitrary steel plate, 50 x 150mm in length with a thickness of 0.75mm is chosen, and loaded centrally with a constant load as illustrated in Fig 6.67a. As illustrated in Fig 6.67b, taking a random seed point, the principal stress vectors propagate as an iterative process described earlier. Adding further seed points (Fig 6.67 c), results in a larger curve network of stress trajectories being formed. As can be quickly be concluded, the resulting stress pattern is highly dependent on the positioning and number of arbitrary seed points. Populating with a high density of seed points (Fig 6.67 d) also results in a higher density curve network. In this respect, it is difficult to quantify which stress curve to take as a printing curve. However, generating a high density of trajectory curves gives the opportunity to filter out and select curves based on constraints such as spacing . This is particularly beneficial for combining in WAAM, as it has already been established that certain minimum spacing between print paths is beneficial for controlling distortion. Therefore, imposing a gird allows for selecting curves which lie on the pre-established grid, which has a pre-established minimum spacing, in order to only select trajectory curves which, satisfy the spacing criteria as well as printing lengths which can be segmented by intersecting curves. As a result, the printing pattern is not optimized in the sense of structural performance, rather are an estimate, or guide, to generating print paths which follow the expected flow of forces in the plate. Furthermore, because the entire process is based on using curves, the same process described earlier – that is, distributing thicknesses of members according to loading condition may also be applied to the filtered curves. Therefor the resulting process, which has to be restated, is an approximation, begins with the generating of a force-based rib pattern generation, followed by structural analysis to determine the distribution of rib heights (Number of layers printed) which are subsequently processed for manufacturing by firstly creating a printing path order logic to distribute the order in which segments are printed and subsequently generating robot movement code.

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Fig 6.68 Using a curve-based approach allows for the quick placement into structural analysis software (Karamba 3D) and generation of print path patterns

Fig 6.68 a-d shows the results of two exemplar cases; firstly, a rectangular steel profile (a,b) which has a low seeding population after filtering. This results in a curve network which is more evenly distributed around the plate and, therefore, more in line with the goal of maintaining a certain spacing between welds. However, a degree of local distortion is still present, particularly where the unavoidable situation of curved being close to one another coincide. Therefore, future research should also focus on readjusting, or rebuilding, selected curves to always ensure a minimal spacing – in the process used, it was the seed point which determined the spacing of the curves and therefore does not assure that this minimal distance is always respected. (Fig 6.68c,d) shows the same process used with a much higher seeding density, resulting in a much more closely-packed curve network. This resulted in excessive deformation when printed on 0.75mm sheet metal and was in fact replaced with 1.2mm thick sheet metal for this exemplar case. While for simple geometry, such as rectangular sections, this is perhaps an over-engineering process, as classical methods can still be used, the process is particularly useful for more free-form and geometrical complex situations as illustrated in (Fig 6.68e,f), where the solutions for placing material are perhaps not as straight forward to determine. In the context of mass customization of free-form architecture, where cladding elements are not only non-planar but non-rectangular in dimension (Fig 6.68d), then such force-following design tools, or rather, guidelines, begin to open up potential for use.

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Fig 6.69 Prototypes on (a,b) rectangular sheet metal using continuous welding and (c,d) r e s u l t s u s i n g i n c r e a s e d T i n t v a l u e s a n d ( e , f ) g e o m e t r i c a l l y - c o mp l e x s h e e t m e t a l

The benefit of using such an approach – that is, a completely curved-based (and therefore, nonsolid) approach to generating print paths is that conventional slicing isn’t needed. Moreover, the strategies developed for planar surfaces were directly applicable to non-planar surfaces. Combining the use of touch sensing, which is discussed in section 6.1.4, printing tool paths are able to be directly generated from the same curve-based model used for structural analysis. However, it is not to say that utilizing a force-based curve generation method is the solution, since it is quite a simplification and ignores the effects of welding on material property changes, rather, it should serve as motivation to further developing alternative curve-based methods of generating welding paths without the need of conventional slicing.

Fig 6.70 Transferring the strategies of 2D plates to curved places using parametric design tools

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6.5 Preparing print paths Apart from the semi-automated methods of scanning and generation of form, the ability to quickly prepare printing paths for communication with the robotic welding device is also paramount. In the next section, two strategies are presented.

6.5.1 Robotic slicing Further building on the curve-based approach to generate print paths is the method in which the paths themselves are prepared for sending to the robotic welding device. Industrial robotic devices used for Additive Manufacturing rely on the use of co-ordinates which are described relative to a co-ordinate system (Base Co-Ordinate, User Frame, Workpiece Frame etc) and Euler Angles, which describe the pitch, yaw and raw orientation of the end-effector (welding gun) (Fig 6.71). Generally speaking, the motion co-ordinates are obtained by deconstructing the x,y,z co-ordinates (and respective Euler angles) from points lying on a print path curve, and the robot moves linearly through the points defined along said curve (Fig 6.71b). However, due to the linear interpolation between points, print paths which are of particularly high changes in curvature are further subdivided into a dense point network. As illustrated in Fig 6.71b and c, this often results in a high density of points through which the robot interpolates linearly through, subsequently increasing the length of code which has to be transmitted to the robot controller – which is often limited in number. One benefit of using curves from the start of the design process is that they may be evaluated for their curvature and, using parametric design tools [192], distribute the interpolation points such that they coincide with areas of high change in curvature and less towards direct linear paths (Fig 6.71e).

Fig 6.71 General setup for generating print paths (a) using defined co-ordinate systems (b-d) curves are divided into points which are (e) distributed by curvature

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This approach of distributing interpolation points by curvature was particularly useful during touchsensing routines . In order to efficiently distribute scanning points, as well as for preparing code for the force-driven curves on planar and non-planar surfaces described in 6.4.2. In the case of printing on pre-bent sheet metal, dividing a print path by its curvature also allowed for a more accurate motion along the surface and maintaining a constant CTWD. In order to extract the Euler Angles, that is, the pitch, yaw and roll of the welding gun such that it remains perpendicular to the surface it is printing along, the use of curves rather than solids played yet another important role, in particular, when printing on curved surfaces and therefore having varying Euler angles. This was achieved by first obtaining a scanned instance of the bent plate in the form of a surface (refer to 6.1.4), and the intended print paths parametrically adjusted to fit it. A future process would have to include the updating of the geometry in its entire context. Fig 6.72s2s illustrates the entire process on an arbitrary, free-form sheet of metal, representing façade cladding. (a) a NURBS surface I s constructed from touch-sensed points, (b) projecting and adjusting the intended print paths to match the scanned surface, (c) dividing each segment curve into points, such that they are distributed by curvature and their x,y,z co-ordinates recorded along with their respective Euler angles (d), calculated as deconstructed U,V,W Vectors of planes perpendicular to the scanned surface at each co-ordinate point.

Fig 6.72 Overview of converting curves to useable data for printing

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Fig 6.73 Final welded prototype

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Fig 6.74 Final welded prototype

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6.5.2 Polynomial Regression and Parametric robotic programming An alternative strategy for processing curves for printing is the use of mathematical functions, rather than extracting individual co-ordinates along a curve. Parametric Robotic Programming (PRP) is a mathematical function-based method of generating co-ordinates, proposed by researchers at TU Darmstadt [193]. Rather than using a list of every single co-ordinate which the robot must pass through, PRP makes use of loops to iteratively calculate and generate co-ordinates from mathematical functions. One of the major benefits of this is that the number of co-ordinates generated is greatly reduced. The printed structure illustrated in Fig 6.75a was generated using such a method. The structure, which was designed to be 520mm in height and consisting of an ellipsoid with varying cross-section with a major and minor axis ranging between 145mm and 47mm, and 32mm and 100mm respectively, was fabricated in 400 individual layers. Using a standard method of slicing, that is, dividing each layer (curve) into a number of points such that the robotic arm can move linearly through them, would have required between 20 – 36 points per layer (depending on dimension) resulting in around 14,000 unique co-ordinates (Fig 6.75b,c). In the context of automation, this is quite undesirable, as programs would have to be divided into smaller files containing a limited number of points, each uploaded separately. Using PRP allowed for the co-ordinates to be generated using mathematical functions; in this case, the point on an ellipse in terms of their sine and cosine values (Fig 6.75d)

F i g 6 . 7 5 T h e u s e o f P a r a m e t r i c R o b o t i c P r o g r a m m i n g a t T U Da r m s t a d t ( a - d ) ( S o u r c e : Waldschmitt et al [194]) and (e-f) in combination with polynomial regression (Source: Feucht et al [193])

The same strategy was also used for generating code for the bridge structure printed on site by the author and researchers of TU Darmstadt [193] (Fig 6.75e). The bridge, which was form-found using mesh relaxation and therefore not constructed directly using mathematical functions (as was the case with the ellipsoid), was rationalized as 100 isoparametric (iso-) curves which followed the surface of the geometry (Fig 6.75e). However, as the iso-curves were arbitrary and not defined by readilyknown mathematical functions, polynomial regression was used to extract the polynomial equation of each curve. Polynomial regression is used in data analysis for finding mathematical equation

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describing the best-fitting curve passing through a certain point dataset[195]. In order to obtain the point dataset through which regression is performed, the three-dimensional isocurve is projected onto its respective XZ and YZ plane, and subsequently divided into points (Fig 6.76). A polynomial regression tool, available on Grasshopper 3D – the parametric software used throughout the thesis – was used to calculate the polynomial function from the division points using the general equation: (𝐹𝐹𝐹𝐹𝑟𝑟 𝑖𝑖 = 1 𝑡𝑡𝑡𝑡 𝑛𝑛),

𝑦𝑦𝑖𝑖 = 𝑎𝑎1 + 𝑎𝑎 2 𝑥𝑥𝑖𝑖 + 𝑎𝑎3 𝑥𝑥𝑖𝑖2 + 𝑎𝑎4 𝑥𝑥𝑖𝑖3 + ⋯ + 𝑎𝑎 𝑛𝑛 𝑥𝑥𝑖𝑖𝑛𝑛−1

(6.4)

Extracting the general equation for each curve through regression allowed for the entire bridge structure to be defined by 100 polynomial functions which were calculate the x and y co-ordinates for corresponding layer (z). While this process was particularly useful for minimizing the number of co-ordinates needed for large-scale fabrication, which typically would require thousands of individual co-ordinates, and therefore, thousands of lines of code, the same process can also be implemented into the workflow of this thesis. Although ultimately the workflow used was based on that described in 6.5, largely due to the fact that the number of points used was already minimized, the integration of PRP into mass-production of welded sheet elements is still viable. In this regard, there would be some slight alterations to the workflow outlined in 6.5, largely in the stage after print paths are defined, and is recommended for future development. Taking the exemplar case presented an individual printing curve is projected along its YZ and XZ plane (Fig 6.76). This is due to the polynomial regression function in Grasshopper ([196], [197]) only being capable of processing curves which are bound to a plane.

Fig 6.76 The use of polynomial regression allows for print paths to be defined by the u s e r a s m a t h e ma t i c a l f u n c t i o n s r a t h e r t h a n i n d i v i d u a l c o - o r d i n t a t e s .

The reason for utilizing YZ and YX planes in this case is that solutions for X and Z are found in terms of a common axis (Y); using curve projections along an XZ plane will result in a curve projection which is self-intersection and therefore has no solution. For each projection, with Fig 6.76bd illustrating the Y-Z projection, the curve is divided by a number of points along its length (domain). The points, which serve as sampling for polynomial regression to be performed, are once again distributed by curvature as outlined in 6.5. This is done so as to distribute points as efficiently as possible to approximate the projected curve. The polynomial regression tool creates solutions for specific, pre-defined degrees – that is, the maximum order to which the function is expressed.

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Polynomial functions with one degree, for example will result in linear equations (y = ax + b) and will therefore result in large deviations from curves with high curvature. Increasing the degree of the polynomial function results in better-fitting curves, however results in longer equations which must be calculated. Therefore, the degree of the polynomial regression is increased, the maximum deviation from the original curve is recorded. Once a solution is found which satisfies maximum deviation from the original curve with minimal degree, the process is repeated for the remainder of curves. In order to generate the co-ordinates from the found polynomial functions, a loop must be created within the robotic programming language – PDL language in the case of the robotic system used in this thesis. The loop is used to cycle through a predefined number of points which lie within the domain of the polynomial function. In the example illustrated above, the curve has a span of 300mm. Assuming that points will be calculated every 10mm, and therefore be defined by 30 points, with the polynomial regression functions expressed below with degree 2, then one possible code structure is:

Polynomial function for: F(Y),z = 5.15563 + 0.53772*y – 0.0018*y2 F(Y),x = 1.91694 + 0.47975*y – 0.0016*y2 Point_no_max = 30 For point_no :=1 TO point_no_max DO Point[point_no].Z = (5.15563) + (0.53772)* [point_no] - (0.0018)* [point_no]**2 Point[point_no].X = (1.91694) + (0.47975)* [point_no] - (0.0016)* [point_no]**2 Point[point_no].Y = [point_no]*10 ENDFOR Fig 6.77 co-ordinates for X and Z are calculated separately with respect to the same value of Y, resulting in x,y,z co-ordinates required for motion control. At the time of writing, however, solutions were only found for non-intersecting curves; curves which have two solutions for a particular point (such as those which turn back on themselves) or those with very sharp kinks did not produce satisfactory results using polynomial regression. This was one of the reasons for opting for the more conventional co-ordinate generation methods in this thesis since it gave more freedom in dealing with more complex curves.

Fig 6.77 (a) a 3D-Curve is projected onto horizontal planes from which (b) polynomial regression is used to extract mathematical functions.

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6.6 Detailing The thesis lies in the context of providing both stiffening reinforcement patterns and connection details for façade cladding. These two aspects have been treated independently during the research; largely due to the broad nature of the topic. While sheet metal facades may be formed and connected to their underlying substructure using multiple methods (Section 2.3), the research contextualized the problem to cladding elements which are (a) formed using processes similar to Multiple Point Forming (MPF) and fastened using cleat-type connections. This due to MPF presented as an adaptive process of automated forming techniques used for mas-customization of bent sheet metal in ship-building [198], automation[199] and construction [200]. Moreover, the use of similar adaptable molding techniques has also been proven to fit within fully-parametric workflows using the same software presented in this thesis[201]. With regards to detail type, cleats were taken as they are presented to be the simplest in terms geometric complexity, are applicable to both planar and freeform façade types (Fig 6.78) [2]and were therefore deemed an acceptable starting point into the topic.

Fig 6.78 Complex facades, such as the Morpheus Hotel, requiring a large number (hundreds of thousands) of connecting details (Source:wikicommons¸ adapted from from Mcneel [2])

Using the same curve-based approached described in 6.4.2 and 6.5, typical cleat-type connections are invisaged to be printed directly onto planar and pre-bent sheet metal (Fig 6.79). Moreover, due to issues such as overhangs and tolerances, fastening details – particularly, openings – are considered to be part of a post-processing step in which a milling device is used to perform subtractice manufacturing on an already-printed part. This would allow for not only a more precise end connection, but also reduces the complexity of printing. As a result, however, this step would rely on the use of a secondary robotic device, or interchangeable toolhead, which performs the operation after printing. This, however, was out of scope of the thesis and remains speculative.

Fig 6.79 Schematic of a cleat-type of façade connecting to a printed counterpart on sheet metal

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In order to show the potential of providing a buildup of details onto sheet metal, a number of prototypes are produced. It must be reiterated that the parts produced are examplary; as the field of printing multiple layers on thin substructures, such as sheet metal, is still relatively unexplored and therefore the parts presented are merely meant to illustrate the potentials of such printed parts. The setup and printing strategy used to print all parts were identical; under-plate cooling by means of compressed air and water was provided constantly during the printing process of the layers. This was done to prevent a buildup of heat in the plate and to minimize the spread of the heat affected zone (5). Air cooling was provided at each interlayer pass – once one layer was printed, the welding device was paused for 30seconds and the cooling was activated directly onto the printed part. Once the cooling period passed, the welding routine continued and the process was repeated until the part was complete. Moreover, in order to have an even distribution of start and end seams – areas at the beginning and end of the weld which have a higher heat intensity and resulting in more material deposition, the polarity of the welding curves were switched with each subsequent layer printed (Fig 6.80).

Fig 6.80 (a) welding strategy for building up multiple layers on thin sheet metal resultig in (b) wall-like structures serving as connection details.

Fig 6.81 illustrates a scaled connection piece with an average wall thickness of 1.58mm, 60mm in length at its base and 10mm in height. Prior to welding the connection, a single extended welding line was printed in order to locally stiffen the material before building up further layers. As outlined in Fig 6.81, each layer was printed with varying welding direction and cooled in between each welding pass, allowing for the build up of multiple layers. While the print itself is not adequate in scale to serve as a cleat connection, it served as the first iteration into developing larger connection details.

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Fig 6.81 The building up of layers begins with laying down extended lines to provide local stiffness in the plate

In order to print larger connection details, the same strategy was adopted, however multiple locallyreinforcing welds were first laid down in order to increase the stiffness around the connection area to aid in reducing distortion during printing. With this strategy, it was possible to build up larger details, 30mm in height, directly onto the sheet material. Although, in all instances, distortion was present in the plate during printing despite the use of cooling and path planning. However, this was to be expected, especially for the buildup of larger and localized details without the use of external heat-mitigating strategies; particularly those outlined in section 5.2.4 and 5.2.5.

Fig 6.82 Alternative strategies include the printing of multiple parallel lines to increase the stiffness of the plate around the area being welded

In an attempt to produce larger details of greater height that are more relatable to a building scale, the time between cooling between each layer was increased to 60 seconds. Moreover, further cooling was also provided at the top surface of the print metal since the weld quality itself did not exhibit much surface waviness in the print, and it was the plate which was undergoing distortion. While it was possible to build up weld layers with a satisfactory weld quality, distortion was still present in the plate, particularly after clamping. This largely occurred at the extremes of the plate, which was left unreinforced in the samples illustrated in Fig 6.83. This reiterates the requirement for use of external heat mitigation strategies (particularly, Low-Stress Low Distortion Welding, outlined in 5.2.5.).

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Fig 6.83 A typical connection detail was possible to be printed, however issues of deformation are still prominent in the plate.

In attempts to provide further stiffness around the detail during printing, two strategies were explored. Firstly, an underlying grid is applied to provide stiffness to the sheet metal to overcome longitudinal distortion. This strategy, however, did not provide any noticeable difference in reducing distortion. Moreover, the issue of longitudinal distortion at the short edge of the plates still remained (Fig 6.84).

Fig 6.84 Strategies for including cross-directional stiffening patterns to stiffen the plate prior to welding

6 Proposed Hybrid WAAM and Thin Sheet Metal Welding

169

A second strategy explored was the provision of stiffening webs (Fig 6.85) This was done to provide localized reinforcement for restraining deformation during printing, as well as to increase the structural performance of the piece when acting as a connection detail. However, due to improper path planning, fully-printed details weren’t possible to manufacture. This particularly due to the overlapping of printed layers orthogonal to one another, which caused discontinuity in the welded layer heights, ultimately resulting in failure of the welds.

F i g 6 . 8 5 H o w e ve r , t h e m u l t i p l e i n t e r s e c t i n g l i n e s o f t e n l e d t o f a i l u r e o f t h e w e l d d u e t o an uneven material deposition at intersection points

The same process was also applied to adding detailing elements to bent sheets of metal. The strategy outlined in 6.5, that is, reconstructing a digital model of a pre-bent sheet of metal by means of robotic touch sensing and refitting planned print paths to the surface, was also used to weld connections. The building up of multiple layers on a curved surface consisted of offsetting each layer curve in the direction perpendicular to the scanned surface, which ensured that the offset remained constant along the curve. Moreover, this allowed for the orientation vectors which defined the orientation of the welding gun to remain constant, relative to each point on each layer (Fig 6.86)

Fig 6.86 (a) Print paths were obtained as lines perpendicular to a welding surface from which (b) they are divided by curvature for an efficient distribution of points and also obtaining Euler Angle values used for the robotic welding device.

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Attempts for printing multiple layers on a slope were cumbersome. Even with cooling and slow welding speeds, the constant variation of the angle of printing which was coupled with a constant movement speed and welding parameters resulted in inconsistent quality of seams along the length. While this was not noticeable for the first printed layer, as more welded layers were deposited, the greater were the effects of an inconsistent welding process. Further research is therefore recommended on this theme; that is, the ability to adapt welding characteristics (or robotic movement speed) during the printing process which are responsive to the variation of welding inclinations. Nonetheless, exemplar details were possible to print, albeit in a small scale. The ability to print larger connection details on non-planar sheet metal was, at the time of writing, not achieved – largely due to the weld seams failing at an early stage and having a negative carry-on-effect to subsequent layers Fig 6.87 shows just this, where failure was a result of both the weld failing due to changing inclination as well as deformation of the thin sheet metal during printing due to excessive heat input, causing a change in the CTWD and therefore failures in the weld seam.

6 Proposed Hybrid WAAM and Thin Sheet Metal Welding

171

Fig 6.87 Welding device followed the sheet metal with a perpendicular orientation, using information obtained from a touch-sensing process and a digitally-rebuilt surface.

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6.6 Detailing

Fig 6.88 Building up of layers on a curved surface was possible to produce connections, however the constantly-changing inclination of the weld led to failures.

7

Discussion

The work presented in this section presented a generalized workflow for the generation of stiffening devices to be used in combination with planar and singly-curved sheet metal, as well as the building up of 2.5D connection details in metal. Referring back to Fig 6.3, in which a semi-automated design to production workflow is required, the following points are to be made: a)

With regards to 3D scanning: Although the conventional scanning techniques tested allowed for the generating of high-resolution digital representations of a bent sheet material, there existed many problems with both the surface reflectivity of the material as well as the numerous post-processing steps required to obtain a useable model. Moreover, the resulting mesh geometry also made analysis for obtaining robotic trajectories, as well as varying co-ordinate systems made such processes less than ideal. This was due to the geometry consisting of thousands of planar mesh faces. The use of robotic touch sensing allowed for the extraction of a targeted pointcloud, from which it was possible to directly reconstruct a surface model. This was beneficial in that the issues of surface reflectivity, excessive data and post processing were all solved. The resulting surface model, which shared a co-ordinate system with the welding device, could be directly analyzed, and welding paths generated.

b)

The use of a curve-based approach, rather than a conventional AM process of slicing solid geometry proved beneficial on multiple parts. Firstly, by generating geometry directly from curves, it is possible to control aspects, such as the spacing and maximum lengths of welding paths. This opens up the possibilities to elaborate on existing design strategies, such as the use of isostatic curves, to automatically generate curve-based reinforcement patterns.

c)

The process illustrated was carried out only on singly-curved sheets. Therein still exist a number of issues yet to be solved, particularly the need for adaptive process parameters to deal with the constantly-varying printing angles of inclination when printing along a curved surface. It is envisaged that, once an adaptive welding process is developed, then the entire design-to-production process may be applied to more arbitrary and free-form surfaces.

© The Author(s), under exclusive license to Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2023 C. Borg Costanzi, Reinforcing and Detailing of Thin Sheet Metal Using Wire Arc Additive Manufacturing as an Application in Facades, Mechanik, Werkstoffe und Konstruktion im Bauwesen 68, https://doi.org/10.1007/978-3-658-41540-2_7

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PART 3

8

Summary and Future Outlook

This thesis was aimed at exploring the potentials of combining Wire Arc Additive Manufacturing (WAAM) as a means of stiffening and adding connection details to thin sheet metal as an application in free-form metal facades. Sparked by the motivation to reduce the number of components which are usually synonymous with such construction and developing an integrated design-to-production process involving the generating of details, using 3D scan data to analyze non-planar sheet metal and adapt details accordingly, and finally generating and communicating welding paths to a robotic welding device. Starting off with 0.75mm thick planar sheet metal, welding parameters which would allow for depositing molten metal material on such a thin substrate are explored, with the ultimate aim of, firstly avoiding complete blowout-failures of the metal and secondly, mitigating, to as a great extent as possible the inevitable deformation in the thin substrate. Once acceptable parameters are found, different path-planning strategies are explored, involving the use of intermittent welding strategies to further mitigate these problems. Parametric design tools are then exploited to investigate and suggest how the basic parameters and strategies could be exploited to develop new strategies for welding thin sheet metal. In parallel, the building up three-dimensional details in molten material are also explored, intended to open up the possibility to directly print façade connection details onto the substrate material. After studies in planar sheet material are concluded, the same process was applied to pre-bent sheet metal. In order to maintain a narrow framework to work in, only singlycurved sheet elements are explored, using a single welding electrode type and diameter as well as gas mixture. The end results are a series of welded prototypes showcasing the potentials in how these technologies could be used

Fig 8.1 Detail produced on thin sheet metal

© The Author(s), under exclusive license to Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2023 C. Borg Costanzi, Reinforcing and Detailing of Thin Sheet Metal Using Wire Arc Additive Manufacturing as an Application in Facades, Mechanik, Werkstoffe und Konstruktion im Bauwesen 68, https://doi.org/10.1007/978-3-658-41540-2_8

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A roadmap of potential applications was drawn up towards the end of the research (Fig 8.2), which defines the process of reinforcing and detailing sheet metal in three distinct categories. Firstly, the development of digital design strategies (Digitales Design) – which are informed by the limitations of the welding process (such as, minimizing welding lengths, maximizing spacing and so on). Next is the Manufacturing process, which is defined by the study of different surface typologies (planar, singly-curved and free-form), and environmental conditions (methods of fixation for the different surface types, use of cooling, heating and other residual stress mitigation strategies). Finally, the application proposes 3 distinct directions in which the welded sheet material is utilized. Firstly, as was proposed in the research, the welded plate is directly applied as a façade cladding or structural component without further processing. (In order for this to be utilized, the complete removal deformation still needs to be addressed). Secondly, rather than mitigating the deformation induced by welding, creating prediction models of the deformation of welded sheet metal to purposely-deform and shape sheet material through welding. A third direction is to introduce a postprocessing forming step, such as pressing, to shape already-welded sheet metal into a desired shape.

Fig 8.2 Overview of welding strategies

8 Summary and Future Outlook

179

More specific future applications which are deemed most promising and of interest to the author are summarized as: (a) Combination with Existing Technologies for reduced waste façade construction An area of interest is the integration with other, already-existing technologies for the realization of a more productive production line. One such cases of this is the use of existing adaptable formwork techniques to bend sheet metal into shape without the need for additional sacrificial material. This brings to light once again the comparison case study projects, particularly the Morpheus Hotel Façade, in which each unique sheet metal cladding element required its own, single-use laser cut mold to shape it[2]. Comparing this with the Dongaummun Design Plaza façade, which was completed by the same architects, an adaptable formwork (Multiple Point Forming - MPF) (Fig 8.3a) was used to form the uniquely-shaped sheet metal façade panels without the need for waste material. While the use of MPF is still not so commonplace in architecture, with the only known instance being the Dongaummun project, it has been already adopted in shipbuilding and aerospace applications. Moreover, similar adaptable processes, namely Multiple Point Stretch Forming (Fig 8.3b) [25], [202], [203] are also under research as industrial applications in architecture, ship building and aerospace applications.

Fig 8.3Potentials for combining with pre-existing adaptive forming technologies, pictured – Multiple-Point Stretch Forming (Source Castaneda et al:[203])

In this regard, it is not inconceivable that Wire Arc Additive Manufacturing could be used in combination with the same, or similar adaptive forming techniques for a low-waste free form façade production line. A conceptual schematic of how this process could look like is illustrated in Fig 8.4

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Fig 8.4 Schematic for potential application in facades

8 Summary and Future Outlook

181

(b) Use with sheet-based composites One of the greatest challenged faced in this research was the deformation of the thin sheet material. Apart from utilizing thicker material, a recommended line of future research is to utilize WAAM with stiffer sheet-based components. Alucobond, for example, is a metal composite panel which consists of two outer aluminum faces that are pressed together with an inner core, providing an overall stiffer material which is welded. (a) Cross-Industrial Applications. The research has always presented itself in the context of construction. While there are cases for free-form sheet metal facades, this is a relatively small application of area when compared to other industries, such as ship-building, aerospace and automation. In these industries, the use of curved sheet material is far more common, be it in the application of airplane fuselages, ship hulls or car bodies. In fact, in the aerospace industry such an application already exists. In 2019, Reinshaw and Stelia unveiled a conceptual outline for a partially-printed aluminum airplane fuselage. Conventional aerospace fuselages consist of stringers, acting as stiffeners, onto which the airplane’s skin is attached. The researchers of the DEFACTO project (Development de la Fabrication Additive pour Composant TOpologique) proposed the use of WAAM to directly manufacture stringers onto aluminum plates, reducing the number of bolts and rivets required to fix the panel to an underlying fuselage. In this sense, a proof of concept is already demonstrated for the case of aerospace engineering, which further opens up the case for the shipping and automotive industry applications. Reflection and Recommendations The concept of welding connection and stiffening elements onto thin sheet metal is certainly an interesting one. It was quite surprising that multiple layers of weld material were successfully deAlthough this was not posited on such a thin substrate without causing extreme distortion. completely resolved, the preliminary results give some exciting outlooks into how the technology could progress further. A number of major issues are still present and, as yet, have not been solved within the framework of the thesis. Although, it must be reiterated that this was not the intention of the thesis ; it was not intended to solve every single issue, rather it is meant to serve as a first iteration of what would, hopefully, continue to develop in future research. The major points which have to be pointed out are: (a) Deformation was still present in sheet metal. Initially, this was occurring during the welding process itself, however after the use of cooling, refining of parameters, developing welding strategies and utilizing clamping, the main causes of problems arose due to residual stresses which were not released after clamping. Presently, a number interesting solutions exist – Low Stress Non-Distortion Welding (LSND) [126] is an area of research which has gained interest in welding thin sheet metal and has been proven to be a viable solution to mitigating deformation due to residual stresses. The MALCO project [146][145] is a research project investigating the industrial application of a LSND process and, as illustrated in Fig 8.5, it serves to aid in reducing heat-induced welding distortion. This would be viable over other distortion mitigating strategies adopted in welding sheet metal, particularly that of pre-heating sheet material to reduce the temperature differences

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8 Summary and Future Outlook

during welding. This is due to the need for clamping and restraining devices which would be required during the preheating of the place [130]

F i g 8 . 5 L S N D co o l i n g s t r a t e g i e s h a v e a l r e a d y b e e n s h o w n t o r e d u c e t h e d e f o r m a t i o n i n sheet metal and offer an attractive solution for future research (Source: [145])

(b) Furthermore, research into different welding gas mixtures would aid in reducing the amount of heat input energy during a welding process and, therefore, the amount of heatinduced welding deformation. The research made use of an 82%Ar 18%CO2 Shielding Gas. While it was possible to control the deformation using the particular gas mixture, researchers at TU Dresden suggest that a 100% CO2 gas mixture emits less radiation into a shielding gas nozzle when compared to a 100% Ar gas[109](Fig 8.6).

Fig 8.6 Using a 100% CO2 gas mixture is known to have a lower heat input, which could be beneficial for controlling distortion in sheet metal (Sources: Kah et al [109])

8 Summary and Future Outlook

183

Although the research was primarily carried out using an 82%Ar18%CO2 mixture, the effects of shielding gas mixtures is also briefly explored. In the case of CO2 gas mixtures, weld beads showed a tendency to be shorter in height and broader in width when compared to an Ar Gas mixture. The higher weld bead geometry of an Ar gas mix, on the other hand, could also be beneficial in reducing the heat induced into thin sheet metal while printing. Firstly, because the distance between the hot welding gun tip is more rapidly increased due to larger layer heights, as well as conducting heat through the weld bead, rather than the sheet plate.

F i g 8 . 7 D i f f e r e n c e b e t w e e n C 0 2 a n d 1 0 0 % A r g a s m i x t u r e i n we l d b e a d h e i g h t

In order to better understand the heat flow through the welded plate, more information regarding the welding process should be recorded. For example, the use of thermal imaging during the welding process will give a better understanding into the heat flow through a plate. Fig 8.8b shows and image taken from a Flir C3-X Thermal camera with a maximum heat-reading range of 300oC, which is too low-a range to record the entire spectrum of welding temperatures (operating at the order of 1500 o +). C

Fig 8.8 Recording of in-process data (a,b) Heat propagation and (c) Current, Voltage values

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Fig 8.8c shows a waveform recorded using the Fronius Xplorer device. Although at the time of writing the setup wasn’t yet properly calibrated, the live recording of parameters, coupled with thermal imaging of the welding process, would allow for far greater understanding of the state of the sheet metal during welding. (c) The recorded information during the welding process would be useful in developing more accurate, predictive models for welding on thin sheet. This would allow for integrating requirements (such as minimum spacing and lengths between welds) into a design process. Moreover, it could open up alternative tracks of research, such as the deliberate and controlled deformation of sheet metal through welding

Fig 8.9 Prediction models for deformation induced in thin sheet metal could allow for the intentional weld-induced deformation of the material. Images produced using Ansys Transient Thermal

8 Summary and Future Outlook

185

(d) Detail and reinforcement sections The research was focused on a a simple, specific type of cleat connection-type of façade fixations. Moreover, the prototypes presented also represent connections which are printed along a single, upward axis without any inclinations. In reality, however, the benefits of using directly-welded connections would be the ability to connect components at multiple angles, placing all the geometric complexity into the sheet metal component which would be resolved to fit to standardized substructure details. Therefore, it is recommended to continue research into welded connections which are not oriented perpendicular to the base surface, as well as the possible combination of connection details (Fig 8.10a). Moreover, the stringers used in the research were of rectangular cross section. However, in order to increase the stiffness of the stringer devices, a recommended area of research is that of cross-sections with better-performing moments of Inertia (Fig 8.10b). Unfortunately, during the timeframe of the research such connections weren’t developed to an acceptable standard and therefore remains unexplored.

Fig 8.10 (a) future studies into welding parameters for printing inclinations and (b) gene r a t i n g m o r e s t r u c t u r a l l y - e f f i c i e n t s t r i n g e r t y p e s s u c h a s i n v e r te d- T s e c t i o n s .

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