New Assembly Concepts and Technology for Metallic Structures of Next Generation Fuselage

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New Assembly Concepts and Technology for Metallic 
Structures of Next Generation Fuselage 
 
Gonçalo Filipe Pina Cipriano 
 
Thesis to obtain the Master of Science Degree in 
 Mechanical Engineering 
 
Supervisor: Prof. Maria Luísa Coutinho Gomes de Almeida 
 
Examination Committee 
Chairperson: Prof. Rui Manuel dos Santos Oliveira Baptista 
Supervisor: Prof. Maria Luísa Coutinho Gomes de Almeida 
Members of the Committee: Dr. Telmo Jorge Gomes dos Santos 
 
 
May 2014 
II 
 
 
III 
 
Agradecimentos 
 Em primeiro lugar, gostaria de expressar a minha gratidão à Prof.ª Dr.ª Luísa Coutinho, minha 
orientadora científica, pela oportunidade que me proporcionou, pela confiança em mim depositada e 
pelo apoio, atenção, motivação e inspiração que me deu, ao longo do desenvolvimento do meu trabalho 
no Helmholtz-Zentrum Geesthacht. 
 Ao Dr. Jorge dos Santos agradeço a oportunidade de trabalho que me foi atribuída, o 
supervisionamento e acompanhamento, que me permitiu não só, desenvolver o meu trabalho, como 
também crescer a nível profissional e pessoal, tendo sido para mim uma ótima experiência, integrar a 
excelente equipa de trabalho que dirige de uma forma tão dedicada. 
 Ao Engenheiro Jan Carstensen, pelo acompanhamento técnico, em tudo o que envolveu a 
realização do trabalho proposto, mas também pelo apoio pessoal, espírito de equipa que e amizade 
que demonstrou. 
 Ao Engenheiro Luciano Bergman, pela disponibilidade para partilhar o seu conhecimento e 
ajuda prestada, nas diversas etapas do meu trabalho. 
 Aos mestres José de Azevedo, André Abibe, Catarina Vidal, João Gandra e aos colegas e 
amigos, Camila Faria, Natascha Borba, Teresa Mendes, Mariana Gil, Sónia Cunha, Inês Soares, 
Sandra Gomes, Francisco Bandeira e João Caldeira, por terem em diversas alturas e circunstâncias, 
contribuído, de forma importante para o meu percurso académico e pessoal. 
 A minha família, em especial aos meus pais e ao meu irmão, pelo apoio incondicional que 
sempre demonstraram, que me ajudou muito a conseguir chegar até aqui. 
 Finalmente, a todos aqueles que, embora não tendo sido mencionados, tenham de uma forma 
ou de outra, contribuído para o meu percurso académico e pessoal. 
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Acknowledgements 
 Firstly, I would like to express my gratitude to Professor Dr. Luísa Coutinho, my scientific 
advisor, for the opportunity given to me, for trusting me and for the support, attention, motivation and 
encouragement provided, during the development of my work at Helmholtz-Zentrum Geesthacht. 
 To Dr. Jorge dos Santos, I phrase my appreciation, for the work opportunity created and for the 
supervising and support, not only on a professional basis, but also on a personal level, that allowed me 
to grow, both professional and personally, being a great experience for me, to have been part of the 
excellent work group, that he so delicately runs. 
 To Engineer Jan Carstensen, for all the technical support, whenever it was necessary 
throughout the development of the work. I also thank him for the team spirit demonstrated, and for 
personal support. 
 To Engineer Luciano Bergman, for the availability and willingness to share is knowledge, on 
various occasions and steps of my work. 
 To MSc José de Azevedo, André Abibe, Catarina Vidal, João Gandra and to colleagues and 
friends, Camila Faria, Natascha Borba, Teresa Mendes, Mariana Gil, Sónia Cunha, Inês Soares, Sandra 
Gomes, Francisco Bandeira e João Caldeira, for being there for me, on several occasions and 
circumstances, giving professional and personal support. 
 To my family, especially my parents and my brother, for the unconditional support always shown 
to me, which allowed me to be where I am today. 
 Finally, to everyone that, although not mentioned by name, have at some point and somehow, 
contributed to my academic path. 
 
VI 
 
 
VII 
 
Resumo 
 A cada vez mais importante procura por metodologias e processos que permitam o fabrico de 
estruturas integrais na indústria aeronáutica, cria a necessidade de substituição dos processos de 
ligação convencionais, como a rebitagem, por técnicas avançadas de soldadura. 
 O presente trabalho de investigação foi realizado como resposta às necessidades tecnológicas 
apresentadas por uma empresa do ramo aeronáutico, tendo como objetivo a integração da soldadura 
por fricção linear no seu processo produtivo. Foram então realizados cordões em chapas de alumínio, 
AA 20224-T351, com 2, 4 e 6 mm de espessura. 
 Numa primeira fase, foram utilizadas várias combinações de parâmetros do processo, com o 
objetivo de avaliar a influência que a quantidade de calor imposta ao material tem na qualidade 
mecânica da ligação, através de análises metalográficas e ensaios mecânicos. 
 Seguidamente, por forma a melhor compreender as propriedades locais das diferentes zonas 
dos cordões, com 6 mm de espessura, foram realizados microflat tensile tests. As temperaturas 
envolvidas no processo foram medidas utilizando termopares colocados ao longo do cordão, avaliando 
o clico térmico que o material experiencia durante o processo. 
 Foram ainda, realizados cordões com 2 mm de espessura, variando o alinhamento do eixo 
vertical da ferramenta com a linha de junta das chapas, procurando colmatar a falta de informação 
sobre a influência de uma possível baixa precisão no alinhamento, que pode ocorrer numa aplicação 
industrial, tendo assim em conta a rigidez estrutural do sistema de soldadura utilizado no processo 
produtivo. 
 Os resultados obtidos permitem sugerir uma gama de valores de weld pitch, que possibilitam a 
obtenção de ligações com boa qualidade, dentro dos requisitos da indústria aeronáutica. 
Palavras-chave 
Soldadura 
Fricção Linear 
AA 2024-T351 
Calor Imposto 
Weld Pitch 
Offset 
VIII 
 
 
IX 
 
Abstract 
 The ever more important demand for processes and proceedings, that allow the production of 
integral structures in the aeronautic industries, sets the need for the substitution of conventional joining 
techniques, such as riveting, for advance welding techniques on their productive process. 
 The present research work, was performed as a response to the technological demands of an 
aeronautic industry company, which aims to introduce Friction Stir Welding to their chain of production. 
For this work, friction stir welds were performed on AA 2024-T351 sheets of 2, 4 and 6 mm of thickness. 
 On a first stage, several different process parameter sets were used, with the objective of 
studying the influence of the heat input, on the quality of the weld, throughout metallographic and 
mechanical analyses. 
 Following this, in order to better understand the specific local mechanical properties of the 
different weld zones, on sheets of 6 mm, these were evaluated, through microflat tensile testing. The 
temperatures involved in the process were measured using several thermocouples, placed along the 
welding path, evaluating the thermal cycle that the material undergoes during the process. 
 On the last part, welds were performed on sheets with 2 mm of thickness, with six different 
horizontal offsets, between the tool vertical axis and the joint line, addressing the issue of lack of 
information on the influence of a possible poor accurate alignment likely to occur in industrial application, 
taking into account the stiffness of the FSW system used in the production process. 
 The results obtained, allowed for a suggestion of a weld pitch range that originate comparatively 
good quality welds, within the requirements of aeronautical industry. 
 
Keywords 
Weld 
Friction Stir 
AA 2024-T351 
Heat Input 
Weld Pitch 
Offset 
X 
 
 
XI 
 
Contents 
AGRADECIMENTOS ......................................................................................................................................... III 
ACKNOWLEDGEMENTS ....................................................................................................................................V 
RESUMO......................................................................................................................................................... VII 
PALAVRAS-CHAVE .......................................................................................................................................... VII 
ABSTRACT ....................................................................................................................................................... IX 
KEYWORDS ..................................................................................................................................................... IX 
FIGURE LIST .................................................................................................................................................... XV 
TABLE LIST ..................................................................................................................................................... XIX 
ABBREVIATIONS ............................................................................................................................................ XXI 
SYMBOL LIST ............................................................................................................................................... XXIII 
1. INTRODUCTION ........................................................................................................................................ 1 
1.1 MOTIVATION ............................................................................................................................................ 1 
1.2 OBJECTIVES ............................................................................................................................................ 1 
1.3 STRUCTURE ............................................................................................................................................ 2 
2. ALUMINIUM ALLOYS ................................................................................................................................ 3 
2.1 WROUGHT ALUMINIUM ALLOYS .............................................................................................................. 3 
2.1.1 Classification ................................................................................................................................. 3 
2.1.2 Temper Treatments ...................................................................................................................... 3 
2.1.3 Aluminium Alloys in Aerospace Industries ................................................................................ 5 
2.1.4 Welding of Aluminium .................................................................................................................. 5 
3. FRICTION STIR WELDING........................................................................................................................... 7 
3.1 PROCESS DESCRIPTION ......................................................................................................................... 7 
3.2 PROCESS PARAMETERS ......................................................................................................................... 9 
3.3 HEAT TRANSFER IN FSW ..................................................................................................................... 11 
3.4 METALLOGRAPHIC CHARACTERIZATION OF THE WELDMENTS ............................................................ 11 
3.4.1 Stirred Zone or Nugget .............................................................................................................. 13 
3.4.2 Thermo-Mechanically Affected Zone ....................................................................................... 13 
3.4.3 Heat Affected Zone .................................................................................................................... 13 
3.4.4 Flow Arm and Shoulder Contact Area ..................................................................................... 13 
3.4.5 Typical Defects ........................................................................................................................... 14 
3.5 ADVANTAGES AND DISADVANTAGES OF FSW ...................................................................................... 15 
XII 
 
3.5.1 Advantages .................................................................................................................................. 15 
3.5.2 Disadvantages ............................................................................................................................ 16 
3.6 APPLICATIONS IN AEROSPACE INDUSTRIES ......................................................................................... 16 
4. EXPERIMENTAL PROCEDURES AND EQUIPMENT .................................................................................... 17 
4.1 BASE MATERIAL CHARACTERIZATION .................................................................................................. 17 
4.2 FSW SYSTEM ....................................................................................................................................... 18 
4.3 PARAMETERS ........................................................................................................................................ 19 
4.4 SPECIMENS ........................................................................................................................................... 20 
4.5 METALLOGRAPHIC ANALYSIS ................................................................................................................ 21 
4.6 MICHOHARDNESS EVALUATION ............................................................................................................ 23 
4.7 BENDING TEST ...................................................................................................................................... 24 
4.8 TENSILE TEST ....................................................................................................................................... 25 
4.9 MICROFLAT TENSILE TEST ................................................................................................................... 26 
5. RESULTS .................................................................................................................................................. 29 
5.1 WORK PACKAGE 1 – PROCESS DEVELOPMENT ................................................................................... 29 
5.1.1 Process Data ............................................................................................................................... 29 
5.1.2 Visual Characterization .............................................................................................................. 30 
5.1.3 Microstructural Analysis ............................................................................................................. 31 
5.1.4 Michrohardness Evaluation ....................................................................................................... 36 
5.1.5 Bending Test ............................................................................................................................... 38 
5.1.6 Tensile Test ................................................................................................................................. 40 
5.2 WORK PACKAGE 2 – DETERMINATION OF THE WELDED AREA’S LOCAL PROPERTIES ......................... 50 
5.2.1 Process Data .................................................................................................................................. 50 
5.2.2 Visual Characterization .............................................................................................................. 51 
5.2.3 Process Thermal Evaluation .....................................................................................................51 
5.2.4 Mechanical Testing Results ...................................................................................................... 53 
5.2.5 Microflat Tensile Test ................................................................................................................. 56 
5.3 WORK PACKAGE 3 – DETERMINATION OF THE INFLUENCE OF THE OFFSET ON THE QUALITY OF THE 
CONNECTION ..................................................................................................................................................... 60 
5.3.1 Microstructural Analysis ............................................................................................................. 60 
5.3.2 Michrohardness Evaluation ....................................................................................................... 62 
5.3.3 Bending Test ............................................................................................................................... 65 
5.3.4 Tensile Test ................................................................................................................................. 66 
6. CONCLUSIONS ........................................................................................................................................ 69 
7. REFERENCES ........................................................................................................................................... 71 
XIII 
 
8. ANNEX .................................................................................................................................................... 75 
A – FSW TOOL SCHEMATICS ........................................................................................................................... 75 
B – PROCESS DATA .......................................................................................................................................... 77 
C – MICROHARDNESS PROFILES ...................................................................................................................... 88 
 
 
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Figure List 
Figure 1 – FSW conventional process schematic representation. .......................................................... 8 
Figure 2 – FSW typical joint configurations: a) square butt; b) combined but and lap; c) single lap; d) 
multiple lap; e) 3 piece T butt; f) 2 piece T butt; g) edge butt and h) corner fillet weld. .......................... 8 
Figure 3 – FSW and processing tool probe configurations [14]. ........................................................... 10 
Figure 4 – FSW and processing tool probe configurations [14]. ........................................................... 10 
Figure 5 – FSW joint structure with indication of the different grain zones. .......................................... 12 
Figure 6 – A typical macrograph showing various microstructural zones in FSW of AA2024-T351 [20].
 ............................................................................................................................................................... 12 
Figure 7 – Typical macrograph of FSW, right side of the image shows AS of the weld. ...................... 14 
Figure 8 – FSW Portal Gantry System. ................................................................................................. 18 
Figure 9 – General cutting plan schematics, welding direction from the left to the right, keyhole at the 
end. ........................................................................................................................................................ 20 
Figure 10 – Schematic of a metallographic/hardness specimen........................................................... 21 
Figure 11 – Leica DM IRM optical microscope workstation. ................................................................. 22 
Figure 12 – Zwick/Roell ZHV michrohardness machine. ...................................................................... 23 
Figure 13 – Schematic of the microhardness indentation lines............................................................. 23 
Figure 14 – Zwick/Roell Universal Testing Machine. ............................................................................ 24 
Figure 15 – Three point bending test scheme. ...................................................................................... 24 
Figure 16 – Tensile test specimen, measurements in millimeters. ....................................................... 25 
Figure 17 – Microflat tensile test specimen. .......................................................................................... 26 
Figure 18 – Schematics of microflat tensile specimens’ cutting positions along the weld. ................... 26 
Figure 19 – Microflat tensile testing machine. ....................................................................................... 27 
Figure 20 – Microflat tensile testing machine detail. ............................................................................. 27 
Figure 21 – Welding data for S218. ....................................................................................................... 29 
Figure 22 – Macro and Micrographs of an initial test weld. ................................................................... 31 
Figure 23 – Macrographs of the welds with 2 mm of thickness. ........................................................... 32 
Figure 24 – Macrographs of the welds with 4 mm of thickness. ........................................................... 33 
Figure 25 – Macrographs of the welds with 6 mm of thickness. ........................................................... 34 
Figure 26 – Void defects in weld S215. ................................................................................................. 35 
XVI 
 
Figure 27 – Void defects in weld S217. ................................................................................................. 36 
Figure 28 – Void defects in weld S411. ................................................................................................. 36 
Figure 29 – Hardness profile of the S218 weld. .................................................................................... 37 
Figure 30 – Hardness profile of the S413 weld. .................................................................................... 37 
Figure 31 – Hardness profile of the S626 weld. .................................................................................... 37 
Figure 32 – Bending specimens, 2 mm of thickness. ............................................................................ 38 
Figure 33 – Bending specimens, 4 mm of thickness. ............................................................................ 39 
Figure 34 – Bending specimens, 6 mm of thickness. ............................................................................ 39 
Figure 35 – Specimen’s central surface coating for ARAMIS usage. ................................................... 40 
Figure 36 – Results for the 2 mm welds, increasing WP comparison diagrams. .................................. 41 
Figure 37 – Load stage prior to failure, ARAMIS, weld 214. ................................................................. 42 
Figure 38 – Load stage prior to failure, ARAMIS, weld 216. ................................................................. 43 
Figure 39 – Load stage after failure, ARAMIS, weld 216. ..................................................................... 43 
Figure 40 – Load stage prior to failure, ARAMIS, weld 218. ................................................................. 44 
Figure 41 – Load stage after failure, ARAMIS, weld 218. ..................................................................... 44 
Figure 42 – Results for the 4 mm welds, increasing WP comparison diagrams. .................................. 45 
Figure 43 – Results for the 6 mm welds, increasingWP comparison diagrams. .................................. 47 
Figure 44 – Load stage prior to failure, ARAMIS, weld S625. ............................................................... 48 
Figure 45 – Load stage after failure, ARAMIS, weld S625. ................................................................... 48 
Figure 46 – Load stage prior to failure, ARAMIS, weld S626. ............................................................... 49 
Figure 47 – Load stage after failure, ARAMIS, weld S626. ................................................................... 49 
Figure 48 – Welding data for S629. ....................................................................................................... 50 
Figure 49 – Surface of weld S629. ........................................................................................................ 51 
Figure 50 – Thermocouples positioning scheme. ................................................................................. 51 
Figure 51 –Thermal data of weld S629. ................................................................................................ 52 
Figure 52 – Hardness profile of the S629. ............................................................................................. 54 
Figure 53 – Load stage prior to failure, ARAMIS, weld 629. ................................................................. 55 
Figure 54 - Load stage after failure, ARAMIS, weld 626. ...................................................................... 55 
Figure 55 – Microflat tensile test results, along the thickness of weld S629, comparison diagrams. ... 56 
XVII 
 
Figure 56 – Microflat tensile test results, advancing vs retreating side, comparison diagrams. ........... 57 
Figure 57 – Microflat tensile test results, different temperatures comparison diagrams. ...................... 58 
Figure 58 – Macrographs of the welds with 2 mm of thickness and for the offset study. ..................... 61 
Figure 59 – Voids detected on weld S224. ............................................................................................ 61 
Figure 60 – Hardness profile of the S218. ............................................................................................. 62 
Figure 61 – Hardness profile of the S219. ............................................................................................. 62 
Figure 62 – Hardness profile of the S220. ............................................................................................. 62 
Figure 63 – Hardness profile of the S221. ............................................................................................. 63 
Figure 64 – Hardness profile of the S222. ............................................................................................. 63 
Figure 65 – Hardness profile of the S223. ............................................................................................. 63 
Figure 66 – Hardness profile of the S224. ............................................................................................. 64 
Figure 67 – Bending specimens, offset welds. ...................................................................................... 65 
Figure 68 – Tensile testing results from welds with offset tool positioning. .......................................... 67 
 
 
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XIX 
 
Table List 
Table 1 – Temper Treatments. ................................................................................................................ 3 
Table 2 – Temper treatments designations. ............................................................................................ 3 
Table 3 – Designations of strain hardened treatments. .......................................................................... 4 
Table 4 – Designations of heat treatments.............................................................................................. 4 
Table 5 – FSW heat input parameters relation. .................................................................................... 11 
Table 6 – Typical chemical composition of AA2024-T351 (weight %). ................................................. 17 
Table 7 – Mechanical properties of AA2024-T351. ............................................................................... 17 
Table 8 – Thermal properties of AA2024-T351. .................................................................................... 18 
Table 9 – FSW Tool Geometry Combinations....................................................................................... 19 
Table 10 – Welding Parameters, 2 mm of thickness. ............................................................................ 19 
Table 11 – Welding Parameters, 4 mm of thickness. ............................................................................ 20 
Table 12 – Welding Parameters, 6 mm of thickness. ............................................................................ 20 
Table 13 – Metallographic/hardness specimens’ useful length............................................................. 21 
Table 14 – Metallographic sample’s grinding and polishing procedures. ............................................. 22 
Table 15 – Metallographic Samples Etching. ........................................................................................ 22 
Table 16 – Indentations performed by the thickness of the sheets. ...................................................... 24 
Table 17 – Distance values on Figure 15. ............................................................................................. 25 
Table 18 – Heat Input for the 2 mm of thickness welds. ....................................................................... 30 
Table 19 – Heat Input for the 4 mm of thickness welds. ....................................................................... 30 
Table 20 – Heat Input for the 6 mm of thickness welds. ....................................................................... 30 
Table 21 – Bending Angles. .................................................................................................................. 39 
Table 22 – Tensile test results for 2 mm of thickness. .......................................................................... 41 
Table 23 – Fracture locations on the 2 mm welds. ............................................................................... 41 
Table 24 – Tensile test results for 4 mm of thickness. .......................................................................... 45 
Table 25 – Fracture locations on the 4 mm welds. ............................................................................... 46 
Table 26 – Tensile test results for 6 mm of thickness. .......................................................................... 46 
Table 27 – Fracture locations on the 6 mm welds. ............................................................................... 46 
Table 28 – Parameters of weld S629. ................................................................................................... 50 
XX 
 
Table 29 – Heat Input for weld S629. .................................................................................................... 50 
Table 30 – Maximum temperatures registered by each thermocouple. ................................................ 53 
Table 31 – Mechanical testing results for weld S629. ........................................................................... 53 
Table 32 – Microflat tensile test results, specimens from along the thickness of weld S629, at room 
temperature, T – top, M – middle, B – bottom. ...................................................................................... 56 
Table 33 – Microflat tensile test results, advancing vs retreating side of the weld, at room temperature, 
RS – retreating side, AS – advancing side. ...........................................................................................57 
Table 34 – Microflat tensile test results, tests performed at 100, 200, 300 and 400 degrees Celsius.. 58 
Table 35 – Microflat tensile test results, comparison with room temperature values, RT, in percentage.
 ............................................................................................................................................................... 59 
Table 36 – Welding parameters for welds with offset positioning. ........................................................ 60 
Table 37 – Bending Angles, offset welds. ............................................................................................. 65 
Table 38 – Tensile test results of the offset welds. ............................................................................... 66 
Table 39 – Fracture locations on the 4 mm welds. ............................................................................... 66 
 
 
XXI 
 
Abbreviations 
 
AA Aluminium Alloy 
AS Advancing Side 
ASTM American Society for Testing Materials 
FSW Friction Stir Welding 
HAZ Heat Affected Zone 
HI Heat Input 
HV Hardness Vickers 
HZG Helmholtz-Zentrum Geesthacht 
MIG Metal Inert Gas 
RS Retreating Side 
RT Room Temperature 
SZ Stirred Zone 
TIG Tungsten Inert Gas 
TMAZ Thermo-Mechanically Affected Zone 
TWI The Welding Institute 
UTS Ultimate Tensile Strength 
WP Weld Pitch 
YS Yield Strength 
 
 
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Symbol List 
 
α Tilt Angle 
Fx Tool Horizontal Force, Welding Direction 
Fy Tool Horizontal Force, Transverse Direction 
Fz Tool Downward Force 
M Tool Torque 
ƞ Process Efficiency 
P Power 
v Tool Transverse Speed 
ω Tool Rotational Speed 
 
 
 
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1 
 
1. Introduction 
1.1 Motivation 
 The present work was commissioned to the Helmholtz-Zentrum Geesthacht (HZG), by an 
aeronautic industry company and it falls under, the framework of the New Assembly Concepts and 
Technology for Metallic Structures of Next Generation Fuselage. 
 This research work aims for a better understanding of Friction Stir Welding (FSW), process and 
correlation, between the weld quality and the process heat input on the material being welded, in order 
to determine a range of values that meet the quality requirements. Evaluating the thermal cycle that the 
material undergoes during the process and the local properties of the weld, by means of microflat tensile 
testing, in order to better understand the specific mechanical properties of the different weld zones. 
 Also, although the use of this weld technique on aluminium alloys has been extensively studied, 
its use on industrial scale is still limited by the lack of knowledge, surrounding the process response to 
changes introduced by its application on industrial environment, specifically the need to take into 
account the stiffness of the FSW system used in the production process. The present work addresses 
this issue, by simulating a poor horizontal alignment of the process, assessing the consequences on the 
quality of the welds and setting limit values to that offset, in order to ensure a reliable implementation of 
FSW in industrial environment. 
 
1.2 Objectives 
 With the aim of answering to the requirements of a procedure development, for industrial 
implementation of this welding process, the following objectives were devised: 
 Process catalogue development on AA2024-T351 sheets, of 2, 4 and 6 mm, studying the 
relation between heat input and the quality of the weld, by performing mechanical and 
metallographic analyses. Establish a safe range of parameters to meet the quality requirements 
(Work Package 1); 
 Determine the local properties of the welded areas, through microflat tensile testing, accessing 
different results obtained from the retreating and the advancing sides of the weld. Verify the 
differences of heat input from one side to the other, by measuring the temperatures in the vicinity 
of the weld during the process (Work Package 2); 
 Assess the influence of a horizontal offset, between the tool vertical axis and the joint line, on 
the quality of the weld. Establish offset limit values, in order to meet weld quality requirements 
(Work Package 3). 
 
2 
 
1.3 Structure 
 The work done for this dissertation, is presented with the following structure: 
 Chapter I, establishes the motivation, as this work was proposed, to HZG, by the company, and 
lays the ground objectives, divided by the respective work packages. 
 Chapter II, presents the state of the art concerning this work. In this chapter is presented a brief 
explanation, about the material used for this work, AA 2024 – T351, defining aluminium and its alloys, 
the temper treatments used, the usage of this alloys in the aeronautic industries and their welding 
processes. 
 In chapter III, the FSW process is described, the work setup schematically represented, the 
parameters involved in the process, are stated and the heat transfer mechanism is described, 
establishing a way to relate the process parameters with the heat input on the weld. 
 Chapter IV, describes in detail the work done, by presenting the equipment used, to perform the 
welds and the parameter sets established. It also states the mechanical tests done, equipment, 
standards and procedures involved in those, in order to achieve and validate the objectives set. 
 Chapter V presents the results obtained for the present work. Divided in three major sections, 
each referring to one of the work packages mentioned above. On each section, the results from the 
mechanical tests are showed and discussed, as well as, data recorded during the weld. 
 Finally, in chapter VI, the conclusions drawn, from the results obtained by the mechanical tests 
conducted, are stated, in relation to the objectives of this work. 
 
3 
 
2. Aluminium Alloys 
2.1 Wrought Aluminium Alloys 
2.1.1 Classification 
 The aluminium alloys designated as wrought, are initially cast as ingots or billets and 
subsequently hot and/or, cold worked mechanically into the desired form, through rolling, extrusion, 
forming and forging processes. The classification system used, is based on a four digit nomenclature, 
the first indicates the series to which the alloy belongs, described in Table 1Error! Reference source 
not found., depending of the major alloying elements. The second digit, different from 0, indicates 
changes to the original alloy or limits to the impurities [1]. 
Series Major Alloying Element 
1xxx Aluminium, ≥ 99.0 % purity 
2xxx Copper 
3xxx Manganese 
4xxx Silicon 
5xxx Magnesium 
6xxx Magnesium and Silicon 
7xxx Zinc 
8xxx Other (mainly Lithium) 
9xxx Free Series 
Table 1 – Temper Treatments. 
 
2.1.2 Temper Treatments 
Depending on the alloy, two different types of treatments are applied, in order to improve its 
properties, heat treatment and plastic deformation. The 2, 6 and 7 series can undergo heat treatments, 
on the rest, plastic deformation is applied [1], [2]. Designations are showed in Table 2. 
 
Designation Description 
F As fabricated 
O Annealed 
H Strain hardened 
T Thermally Treated 
W As quenched condition, between solution heat treatment and aging 
Table 2 – Temper treatments designations. 
 
4 
 
 In more detail, treatments are described in the following tables: 
Designation 1st digit description 
H1x Strain hardened without thermal treatment 
H2x Strain hardened and partially annealed 
H3x Strain hardened and stabilized by low temperature heating 
Designation 2nd digit description 
Hx2 1/4 hard 
Hx4 1/2 hard 
Hx6 3/4 hard 
Hx8 full hard 
Hx9 extra hard 
Designation 3rd digit description 
Hxxy y – indicates a possible variant of the treatment 
Table 3 – Designations of strain hardened treatments. 
 
Designation Description 
T1 Cooled from an elevated temperature shaping process and naturally aged to a substantially stablecondition T2 Cooled from an elevated temperature shaping process, cold worked, and naturally aged to a 
substantially stable condition T3 Solution heat treated, cold worked, and naturally aged to a substantially stable condition 
T4 Solution heat treated, and naturally aged to a substantially stable condition 
T5 Cooled from an elevated temperature shaping process then artificially aged 
T6 Solution heat treated then artificially aged 
T7 Solution heat treated then overaged/stabilized 
T8 Solution heat treated, cold worked then, artificially aged 
T9 Solution heat treated, artificially aged then, cold worked 
T10 Cooled from an elevated temperature shaping process, cold worked, then artificially aged 
Table 4 – Designations of heat treatments. 
 
 The variations of the treatments in T3 throughout T7 alloys are designated by 1, 2 or 3 additional 
numbers, Txxx. 
 For the present work, AA 2024 – T351 was used, as so, it is important to describe the Tx51 
variation of the T3 heat treatment, since this will influence the properties of the material in the welded 
zone. This designation defines a solution heat treatment and stress relive by stretching, which creates 
a permanent deformation of 1 to 3%. 
 
5 
 
2.1.3 Aluminium Alloys in Aerospace Industries 
 The continuous increase of the demand for lighter materials and lighter joining solutions, in the 
aerospace industries, keeps pushing innovation and research forward, not only with new applications of 
new joining techniques, but also in the development of new processes. 
 The great concern with weight reduction, hence lower fuel cost, demands the usage of lighter 
structures, being the aluminium alloys chosen, for their good mechanical properties, for being light alloys 
and for having a good resistance to corrosion. 
 With the development of temper treatments by Alfred Wilm, in 1906, Berlin, alluminium alloys 
started to be applied in aerospace industries. Is work led to development of Duraluminium, which was 
soon after weightily applied in Germany, on the Junkers F-13 and on the Zeppelin, which had its first 
flight in 1910. 
 Today, the most used aluminium alloys in this industries are the 2, 6 and 7 thousand series, for 
their mechanical properties. 
 
2.1.4 Welding of Aluminium 
 Welding of the aluminium was only considered as a possible structural application, after the 
discovery of arc welding with shielding gas, Tungsten Inert Gas (TIG), in the 1940’s. Before that, the 
welds were done by oxi-fuel welding and shielded metal arc welding, but with poor results [2]. 
 The main obstacle to welding aluminium, was the presence of aluminium oxide (Al2O3), which 
would form on the surface of the weld during the process. As it as a melting temperature of 2037 ºC, it 
is not solvable in the liquid aluminium, with melting temperatures between 550 and 660 ºC. 
 With the emergence of the Metal Inert Gas (MIG) welding process, the welds performed 
acquired better mechanical resistance and the process gained versatility, allowing more welding 
positions and higher welding speeds. 
 The most common issues of welding aluminium by fusion, are the formation and retention of 
porosities, warping and also, the fact that the heat generated by the welding process, originates a great 
reduction on the mechanical properties in the Heat Affected Zone (HAZ) of the weld. 
 Recently, two processes are leading the innovation on the welding of aluminium, laser welding 
and FSW. The last, constituted a major improvement, as the process generates welds of high quality 
and repeatability, being it an automated process and depending very little on the environment. This 
allowed for a more broad range of applications for welded aluminium structures. 
 
6 
 
 
7 
 
3. Friction Stir Welding 
3.1 Process Description 
 
 Friction Stir Welding is a solid-state, autogenous, joining technique invented by The Welding 
Institute, (TWI), in 1991 [3]. This process allows the production of a weld, without the need for the 
materials to reach their melting temperature. Due to this fact, the welds produced by this process, when 
compared to conventional fusion welds, have less distortion. The changes in metallurgical and 
mechanical properties, are minimized and the residual stresses are reduced, due to a reduction of 
thermal contractions associated with solidification and cooling [4], [5], [6], [7], [8], [9], [10]. 
 This process represented a breakthrough in the metal joining technology field, due to its 
characteristics, it allows the welding of different types of metals, dissimilar welds, which were previously 
too hard or even impossible to weld by fusion welding techniques. 
 The conventional process concept, can be easily explained as demonstrated in Figure 1. The 
process consists on a nonconsumable rotating tool, is plunged into the joint line, formed by the edges 
of the pieces to join, then, a linear movement along this line is imposed, at the end, the tool is retracted, 
still rotating. The rotating tool, consists of concentric cylindrical shoulder and probe. The rotation of the 
tool and the downward force, pressing it against the material, promote, by friction, the heating and 
softening of the material, allowing it to be plastically deformed during the process. The material flow, 
resultant from the combination of the rotational and transverse speeds, moves the material from the 
front to the rear of the tool, as it passes by. The complexity of this flow depends on the probe geometry 
and profile. During the process, the material to be welded must be rigidly clamped together, to prevent 
a separation of the sheets as the tool moves along the joint line. 
 The fact that the material does not reach the melting temperature, during the process, prevents, 
especially in the 2xxx and 7xxx series aluminum, the formation of products resultant from brittle 
solidification [11]. 
 This can be considered a clean process, as it does not represent any major safety hazard, like 
other processes, with welding fumes or radiation. 
 
8 
 
 The welding setup is shown below: 
 
Figure 1 – FSW conventional process schematic representation. 
 
 The pieces to be welded must be rigidly constrained, shown in a butt joint configuration on in 
the previous figure. The process can be performed using several different configurations [12], as the 
next figure illustrates: 
 
Figure 2 – FSW typical joint configurations: a) square butt; b) combined but and lap; c) single lap; d) 
multiple lap; e) 3 piece T butt; f) 2 piece T butt; g) edge butt and h) corner fillet weld. 
 
 Although these are the most convenient configurations, FSW can be also applied to 
circumferential, annular, non-linear and also to 3D welds, being any welding position possible. As one 
may infer, some configurations can pose a challenge, as to how the clamping system must be set, to 
guarantee the constraining of the pieces. 
 
9 
 
3.2 Process Parameters 
 
 The parameters involved in this process, responsible for the mechanical and metallurgical 
characteristics of the welds, are: 
 Tool rotational speed, ω – The tool rotation, begins before the plunging of the tool into the 
materials, being welded, and, stops only after the tool exits the material at the end of the weld. 
 Tool transverse speed, ν – This parameter defines the distance covered by the tool, along the 
weld line, per unit of time. 
 Tool downward force, Fz – Is the force responsible for guaranteeing the contact, between the 
shoulder of the tool and the surface of the material. By using force control of the process, 
keeping this force constant, allows for real time control of the vertical position of the tool. 
 Tilt angle, α – Is the angle between the tool axis and the workpiece surface. The use of a suitable 
angle, allows a more efficient movement of the stirred material, from the front of the back of the 
tool, italso ensures a good surface finish. 
 Type of control imposed during the process – Position or force control, the former being the 
most accurate, maintaining the downward force, Fz, constant. 
 Plunge speed –The plunging of the tool into the material being welded, can be done either with 
or without a previous performed hole, which is done in cases of great thicknesses or high 
hardness’s. 
 Tool rotation’s direction – Clockwise or counter clockwise. 
 Dwell time – Initial time immediately after the plunge of the probe, that the tool stays rotating 
with no linear velocity, doing so, to achieve a better temperature distribution in its vicinity. 
 Tool geometry – The tool geometry affects the heat generated and the plastic material flow 
during the weld [13]. 
 
 During the welding process, the plastic flow of stirred material is greatly influenced by the tool 
geometry, as it depends on the probe profile and the shoulder base, in contact with the weld surface, 
being these, responsible for the heat generation. The probe determines the amount of stirred material 
and direction of the flow. As for the shoulder, it keeps the material flow contained. So, it is important to 
briefly review, the most common configurations available. 
 The following image presents a wide range of probe configurations, distinguishing by type of 
outer shape and profile: 
10 
 
 
Figure 3 – FSW and processing tool probe configurations [14]. 
 
 As for the shoulder surface configuration, there are three major surface shapes: flat; concave 
and convex. Several different features can be used on the shoulder surface, as can be seen in Figure 
4. These features aim to improve friction with the surface of the material, generating the most part of the 
frictional eat. 
 
Figure 4 – FSW and processing tool probe configurations [14]. 
 
11 
 
3.3 Heat Transfer in FSW 
 In FSW, heat is generated by the friction, at the tool-workpiece interface and also, by the plastic 
shear deformation in the vicinity of the probe [15]. For a given material, tool geometry, downward force 
and clamping system, the temperatures involved in the process are governed mostly by the ratio 
between the transverse speed, ν, and the tool rotational speed, ω. This ratio will be designated by Weld 
Pitch (WP) and it gives an indication, on the amount of heat generated in the process, and on the plastic 
flow, of the material, in the vicinity of the probe. 
 If the WP is low, the stirring or plastic deformation per unit length is higher, compared to a high 
WP, leading this to an also higher heat input (HI) per unit length. 
 The welds can be classified, according to the heat generated, as follows in Table 5. 
Cold Hot 
ν ω ν ω 
HAZ TMAZ HAZ TMAZ 
Table 5 – FSW heat input parameters relation. 
 To estimate the heat input on the weld, a variety of models exist in literature, on the present 
work, the following equations are used [16]: 
 
𝑃 = ω ×
2𝜋
60
× 𝑀 + 𝐹𝑥 × 𝑣 ( 1 ) 
 
𝐻𝐼 = 
𝑃
𝑣
× 𝜂 ( 2 ) 
 
Where 𝑴 represents the torque applied, [Nm], 𝑭𝒙 is the force [N], in the direction of the transverse 
speed, and 𝜼 is the efficiency of the process, which can be estimated from the spindle efficiency. 
 Although the melting temperature is not reached, the material undergoes significant 
microstructural changes, such as reduction of grain size and boundary character, dissolution and 
coarsening of precipitates, redistribution of dispersions and texture modification [17]. 
3.4 Metallographic Characterization of the Weldments 
 The typical microstructure of FSW joints, presents several differences from the typical joint 
structures that result from conventional arc welding [18]. During the process, temperature and plastic 
deformation generate a significant microstructural change. These changes involve, grain size, grain 
boundary character, dissolution and coarsening of precipitates, breakup and redistribution of 
precipitates, as well as texture modification [19]. 
12 
 
 Taking in consideration the kind and size of the grains, ti is possible to distinguish five different 
zones, besides the base material, as shown in Figure 5, those areas being: 
 Stirred Zone (SZ) or nugget; 
 Thermo-Mechanically Affected Zone (TMAZ); 
 Heat Affected Zone (HAZ); 
 Flow arm; 
 Shoulder contact area. 
 
Figure 5 – FSW joint structure with indication of the different grain zones. 
 The grain differences can be better observed in 
Figure 6, it shows a micrographic image of the different areas of the weld, being also easier, to 
differentiate the advancing from the Retreating Side (RS) of the weld. The designation of Advancing 
Side (AS), refers to the one where the rotating and the transverse speeds, have the same direction. 
 
Figure 6 – A typical macrograph showing various microstructural zones in FSW of AA2024-T351 [20]. 
 
13 
 
 On the previous images, the area designated by A shows base material, B refers to the HAZ 
and C to the TMAZ. 
 
3.4.1 Stirred Zone or Nugget 
 In this area, the HI generated by the internal friction, from the plastic flow of material, and by the 
contact between the shoulder and the workpiece surface, promotes a recrystallization of the material, 
being this area also known as dynamic recrystallized zone, resulting in very fine and equiaxed grains. 
In cases of thermal treated alloys, it may occur precipitate coarsening or dissolution and even 
reprecipitation. These changes can lead to an increase in ductility and a reduction in hardness. The 
boundary between the recrystallized SZ and the TMAZ is relatively diffuse on the RS of the weld, but 
sharp on the advancing [19]. 
 In this area, it can also be seen, typical elliptical concentric structures designated as “onion 
rings”. Regarding the mechanical effects, on the material, during FSW, Biallas et al. [21] suggested that 
the process be considered like a simple milling process. Moreover, Sutton et al. [22] proposed the 
presence of a banded structure in the FSW joints, with different hardness values, latter, Chen and Cui 
[23] investigated the mechanism that generated the banded structure. 
 
3.4.2 Thermo-Mechanically Affected Zone 
 Sharing an interface with the SZ, the TMAZ undergoes relatively high temperatures and 
deformation, during the welding process, although these are not sufficient to promote new grain 
formation, the grains became elongated with second phases dispersed in the grain boundaries can be 
observed. 
 
3.4.3 Heat Affected Zone 
 Though this zone, common to all welding processes, shows no plastic deformation, it can 
undergo phase transformations, due to the heat coming from the SZ. Transformations like localized 
aging, precipitate coarsening, annealing phenomena and grain growth, depending on the material. 
 
3.4.4 Flow Arm and Shoulder Contact Area 
 The flow arm is a phenomenon caused by the trailing edge of the shoulder, which drags the 
softened material, in the top area of the joint, from the retreating to the AS of the weld, also promoting 
the non-symmetrical character of the joint. The shoulder contact area, is the shallow region over the SZ, 
14 
 
wider than the shoulder diameter, where recrystallized microstructure is determined by the contact, 
between the trailing part of the shoulder and the cooling of the weldment surface. 
 
3.4.5 Typical Defects 
 Several different flaws can be found in FSW welds which, depending on size and severity, can 
be considered unacceptable, therefore, considered as defects. With the use of a well-chosen parameter 
set, one can obtain defect free welds, excluding any parameter variations during the process. Also, the 
conditions of the workpieces must not change, considering material structures variations and clamping 
setup. 
 As for the types of defectson FSW weldments, six were identified [24]: void; worm hole; lack of 
penetration; faying surface flaw; joint line remnant, “entrapped oxide” or “lazy S”; and hook flaw. 
Considering the four most common defects, the following image correlates their relative position. 
 
Figure 7 – Typical macrograph of FSW, right side of the image shows AS of the weld. 
 
Defects: 
 Lack of penetration: occurs when the full thickness of the workpieces is not completely welded, 
remaining a part of the original, separated, plate surfaces, unbroken or undisturbed. Its primary 
causes are: the probe depth; too short; and the use of a too small downward force. 
 Alignment of particles or joint line remnant (Figure 7 defect 2): appearing on the lower center 
part of the weldments, occurs when the material is not actually welded, instead, it is bonded 
together, with no air remaining between the surfaces and offering some mechanical strength. 
This results from, an incorrectly broken and stirred fusion face creating a layer of oxides. 
 Voids: are volumetric flaws, resulting from a spatial absence of material, due to a discontinuous 
material flow. Usually aligned with the welding direction. 
 Alignment of particles or faying surface flaw (Figure 7 defect 4): is a surface breaking, on the 
upper part of the weldment, below the area in contact with the tool. This flaw may contain oxides. 
 
15 
 
3.5 Advantages and disadvantages of FSW 
3.5.1 Advantages 
 The FSW process, yields several advantages over conventional fusion welding techniques [25]. 
 Metallurgical Benefits: 
o Solid phase joining process; 
o No chemical effects or loss of alloying elements; 
o Low distortion of the workpieces, with less residual stresses compared to fusion 
welding; 
o Grain refinement and homogenization, resulting in a fine grain structure; 
o Possibility of working on thermal sensitive materials; 
o Absence of hot cracking. 
 Technical Benefits: 
o Good dimensional stability; 
o Good repeatability; 
o No surface special cleaning; 
o Easy process depth control; 
o One-step technique; 
o Easily automated process; 
o Possibility of performing dissimilar welds, joining different materials that couldn’t be 
fusion welded; 
o Any welding position is possible. 
 Environmental Benefits: 
o No shielding gas required; 
o No fumes produced; 
o Absence of grinding wastes; 
o Highly energy efficient process; 
o Reduced noise. 
 Economic Benefits: 
o Over twenty times less energy necessary, compared to laser welding; 
16 
 
o Great material usage efficiency, no need for consumable materials and great 
dimensionally precise welds; 
o Low energy consumption, since heat is generated by friction; 
o No special joint edge profiling needed, avoid preoperational preparations; 
o Automatizing the process reduces manpower related costs. 
 
3.5.2 Disadvantages 
 As every process, FSW also has some limitations and disadvantages, examples are [26]: 
 The workpieces to be welded, must be rigidly clamped against each other, always, and against 
a backing bar, if it is a full penetration weld, except in cases on which Bobbin-Tool is used; 
 The existence of a keyhole, at the end of the weld path, when the probe retracts, imposing in 
some cases run-on/run-off sheets. 
 Do to clamping, access and stiffness requirements, the portability of the process equipment is 
limited. 
 
3.6 Applications in Aerospace Industries 
 Being FSW already applied in several industries, such as maritime, automotive, aerospace and 
railway [27], [28], there is still a wide range of consumer appliances applications to explore, such as the 
recent use on the frame of the latest iMac by Apple, allowing a thickness reduction. 
 For the present work it is important to refer the applications in the aerospace industries, 
specifically aircraft production, being these on the vanguard of the investigation and development of the 
FSW processes. At present, FSW is already integrated in airplane production, both for military and 
civilian purposes, with the main objective of replacing several different types of fasteners. Applications 
such as weld fuselage skins, spars, window frames, ribs, frames and stringers are currently being 
studied or already applied by companies like Airbus, BAE, Boeing and Embraer [29]. These applications 
allow for the production of integral structures, considerably reducing overall weight. The application of 
FSW also presents fuselage structural safety benefits, as it can be used in the prevention of crack 
propagation, posing an obstacle to the dislocations phenomena, making it even more interesting to use, 
as in the stringer/frame connection in T-joint configurations. 
17 
 
4. Experimental Procedures and Equipment 
 
This chapter, describes the experimental procedures followed during the present work, as well as 
the equipment used. The work was carried out by performing friction stir welding, on sheets of aluminum 
alloy, AA2024-T351, with thicknesses of 2, 4 and 6 millimeters. The objective, as previously mentioned, 
was to answer the demands of an aeronautic company’s proposal, in order to complement results and 
knowledge of FSW process, to allow a better understanding and industrial implementation. Specimens 
were produced from the welds in order to characterize them, both metallurgical and mechanically. The 
mechanical properties were assessed by bending, microhardness and tensile testing, while the 
metallurgical were determined by macro and microscopic analyses. 
 
4.1 Base Material Characterization 
 As previously referred, the material to be welded, in the present work, is AA2024-T351, acquired 
and provided by HZG. The thicknesses of the AA sheets were defined by the initial proposal, being of 
2, 4 and 6 mm. The chemical composition of this material and its main properties are presented in the 
following tables [30]: 
Element Al Cr Cu Fe Mg Mn Si Ti Zn 
Weight 
% 
90.7-94.7 ≤0.10 3.8-4.9 ≤0.50 1.2-1.8 0.3-0.9 ≤0.50 ≤0.15 ≤0.25 
Table 6 – Typical chemical composition of AA2024-T351 (weight %). 
 
Mechanical Properties 
Density [kg/m3] 2780 
Hardness, Brinell 120 0.5 kgf on a 10 mm diameter ball 
Hardness, Vickers 137 Converted from Brinell Hardness Value 
Yield Tensile Strength [MPa] 324 
 
Ultimate Tensile Strength [MPa] 469 
Elongation at Break [%] 20 
Modulus of Elasticity [GPa] 73.1 
Poissons Ratio 0.33 
Shear Strength [MPa] 283 
Shear Modulus [MPa] 28 
Machinability [%] 70 AA scale, 0-100% 
Table 7 – Mechanical properties of AA2024-T351. 
 
18 
 
Thermal Properties 
Specific Heat Capacity [J/(kg.K)] 875 
Thermal Conductivity [W/(m.K)] 121 
Melting Point [ᵒC] 502 
Solidus Temperature [ᵒC] 502 
Liquidus Temperature [ᵒC] 638 
Table 8 – Thermal properties of AA2024-T351. 
 
4.2 FSW System 
 
 The welds performed for the work packages, mentioned above, were carried out, using the HZG 
FSW Portal Gantry System, Figure 8. This equipment allows, not only a live monitoring of the process 
parameters, such as, forces applied, probe rotational speed, torque and welding speed, but also the 
recording of all that data. The welds were performed using force control, by maintaining the downward 
force constant to control the depth of the probe penetration, in the workpieces, during the weld. 
 Sheets of 300x150 mm were cut, for all the thicknesses, clamped together on a butt joint 
configuration, with the joint line located over a steel backing bar. The backing bar worked as a fail-safe, 
so if any part of the weld got through the aluminium it would only damage the removable backing bar 
and not the work table of the FSW system. 
 
Figure 8 – FSW Portal Gantry System. 
 The FSW tool used, consisted of a probe and a tool holder, the last providing the shoulder 
contact area and both made of HOTVAR. The setting varied with the different thicknesses of the sheets, 
as shown on the following table:19 
 
Tool Ø [mm] Profile 
Probe 5 6 Conical Threaded Triflat 
Shoulder 13 15 Flat + Scrolled 
Sheet Thickness [mm] 2 4 6 
Table 9 – FSW Tool Geometry Combinations. 
 
The tool schematics can be found in annex A – FSW Tool Schematics. 
 
4.3 Parameters 
 
 The welding parameters used to perform the welds for this study, were based on the results 
obtained from previous experiments, performed at HZG, and data available from literature. 
 Taking into account, the proposal objective of studying welds with several different heat inputs, 
parameters were set, for all the thicknesses, in order to produce welds with at least four different levels 
of heat input (HI), varying mainly the rotational speed and feed rate, welding speed. There was no tilt 
angle applied to the tool, on any of the welds performed. The length of the probe to penetrate into the 
workpieces, was set to less 0.2 mm than the thicknesses of the sheets to be welded. 
 The identification code of the welds, for example, S406, designates the thickness of the sheets, 
with the first digit, 4, meaning four millimeters, and the set of parameters used, with the last two digits. 
 The parameters used for each thickness were as follows: 
 
Weld Welding Speed 
[mm/min] 
Rotational 
Speed [rpm] 
Force [kN] WP [mm/rev] 
S214 300 1200 4 0.25 
S215 600 1200 5 0.50 
S216 300 2000 4 0.15 
S217 600 2000 4 0.30 
S218 400 1600 4 0.25 
Table 10 – Welding Parameters, 2 mm of thickness. 
 
20 
 
Weld 
Welding Speed 
[mm/min] 
Rotational 
Speed [rpm] 
Force [kN] WP [mm/rev] 
S406 200 800 8.5 0.25 
S407 400 800 9 0.50 
S408 400 900 9 0.44 
S409 480 1000 9 0.48 
S410 200 1200 9 0.17 
S411 400 1200 8 0.33 
S412 200 1500 9 0.13 
S413 480 900 9 0.53 
Table 11 – Welding Parameters, 4 mm of thickness. 
 
Weld 
Welding Speed 
[mm/min] 
Rotational 
Speed [rpm] 
Force [kN] WP [mm/rev] 
S625 180 800 14 0.23 
S626 360 800 15 0.45 
S627 180 1200 15 0.15 
S628 360 1200 14 0.30 
S629 240 1000 14 0.24 
Table 12 – Welding Parameters, 6 mm of thickness. 
4.4 Specimens 
 In order to perform the several analyses, one weld of each parameter set was sent to the HZG 
workshop, where the specimens needed were cut, following the general cutting plan presented below, 
where, from the left to the middle, specimens can be seen can be seen as follows: 1x 
metallographic/hardness; 2x bending test; and 2x tensile test. 
 
Figure 9 – General cutting plan schematics, welding direction from the left to the right, keyhole at the 
end. 
 
21 
 
4.5 Metallographic Analysis 
 
 The metallographic analysis was performed through the use of an optical microscope. With this 
analysis, the quality of the welds was determined, by visually evaluating the weldment and its vicinity, 
in order to determine if flaws or defects were present, and correlate their existence, to the parameters 
used and to the results obtained by mechanical testing. The macrographic analysis, also allows for a 
geometrical characterization of the weldment, differentiating the advancing from the retreating side. With 
micrographic analysis, the different regions/zones, of the welded joint structure, can be distinguished, 
establishing the relative positions of their interfaces by grain characteristics. 
 This evaluation was carried out, in accordance with the ASTM E3 – 01, Standard Guide for 
Preparation of Metallographic Specimens. 
 As a productive approach, the specimens used for the macro and microstructural analysis, were 
the same as for the microhardness, having this in mind, the specimens geometry was set as the following 
figure shows: 
 
 
Figure 10 – Schematic of a metallographic/hardness specimen. 
 The used length for the tests, value of d, of the specimen with the weld on the middle, varies 
with the thickness of the AA sheets, assuming the values of: 
AA sheet thickness [mm] d [mm] 
2 28 
4 32 
6 32 
Table 13 – Metallographic/hardness specimens’ useful length. 
 
 The metallographic samples were prepared according to the following procedure: 
 
22 
 
 Step 
1/4 2/4 3/4 4/4 
Surface 
Type SiC-Paper, 
#320 
MD-Largo MD-Dac MD-Chem 
Speed [rpm] 300 150 150 150 
Abrasive Type - DiaPro Largo 
9 µm 
DiaPro Dac 
3 µm 
OP-S, 0.4 µm 
Lubricant Type Water - 
Holder 
(6 specimens) 
Force [N] 30/180 35/210 25/150 15/90 
Speed [rpm] 110 110 110 110 
Time [min] 1 5 3 1 
Table 14 – Metallographic sample’s grinding and polishing procedures. 
 
 After the samples were polished, they were chemically etched as follows: 
 
Etching 
Etching performed with Kroll reagent: 
92 ml H2O + 6 ml HNO3 + 2 ml HF 
Table 15 – Metallographic Samples Etching. 
 
 The equipment used, to analyze and document the samples, was an optical microscope, Leica 
DFC 295, connected to a digital imaging system, Figure 11. 
 
 
Figure 11 – Leica DM IRM optical microscope workstation. 
 
23 
 
4.6 Michohardness Evaluation 
 
 The michrohardness measurements, Vickers Hardness, were done in accordance with ASTM 
E384 – 10´1, Standard Test Method for Knoop and Vickers Hardness of Materials, by applying a 0.2 kgf 
load, for an indentation time of 10 seconds. The equipment used was a Zwick/Roell ZHV microhardness 
machine, Figure 12. 
 The approach taken on the measurements, varied with the thickness of the sheets, for the 2 
mm, one line of indentations was performed in the middle of the thickness, along the width of the 
samples. On the 4 mm sheets, three lines of indentations were performed, the top one at 1 mm from the 
top surface of the weld, the second in the middle and the third, at 1 mm from the lower surface of the 
weld. The 6 mm also undertook three lines of indentations, one at the top, 1.5 mm from the top surface, 
one in the middle and one at 1.5 mm from the lower surface, schematically represented in Figure 13. 
 
 
Figure 12 – Zwick/Roell ZHV michrohardness machine. 
 
 Once more the actual length tested varies with the thickness, as shown in Table 13. 
 
Figure 13 – Schematic of the microhardness indentation lines. 
 
 The indentations performed are summarized in the following table: 
 
24 
 
 
Sheets Thickness [mm] 
2 4 6 
Distance between indentations [mm] 0.4 0.4 0.4 
Number of indentations 70 80 80 
Indentation line length [mm] 28 32 32 
Number of lines 1 3 3 
Table 16 – Indentations performed by the thickness of the sheets. 
 
4.7 Bending Test 
 
 Bending tests were performed on all of the welds, in order to establish an initial comparison 
between the different parameters sets used. These tests were carried out using a screw-driven 
Zwick/Roell universal testing machine, Figure 14, with a load capacity of 100 kN. The welds undertook 
a three-point root bending test, in accordance with the ASTM E190 – 92 (Reapproved 2008), Standard 
Test Method for Guided Bend Test for Ductility of Welds. 
 
Figure 14 – Zwick/Roell Universal Testing Machine. 
 
Figure 15 – Three point bending test scheme. 
25 
 
 Being the values related to thickness as follows: 
Thickness [mm] B [mm] C [mm] E [mm] 
t 2t 6t + 3.2 19.05 
Table 17 – Distance values on Figure 15. 
 
4.8 Tensile Test 
 
 In accordance with the previously presented cutting plan, four tensile test specimens were 
made, for every different weld. The specimens, as can be seen in Figure 9, have the weld path in their 
middle. The tests were carried out in accordance with the ASTM E8M – 09, Standard Test Methods for 
Tension Testing of Metallic Materials. The tensile specimen geometry is shown in more detail in Figure 
16. 
 The equipment used to perform the tensile testing of the specimens, was the same screw-driven 
Zwick/Roell testing machine previously mentioned, shown in Figure 14. 
 The tests were carried out at room temperature, with a crosshead speed of 1 mm/min. The 
displacement was recorded by a MTS extensometer, with a gauge length of 50 mm. 
 
Figure 16 – Tensile test specimen,measurements in millimeters. 
 
26 
 
4.9 Microflat Tensile Test 
 
 Microflat tensile tests were performed on specimens taken from the welds, in order to locally 
evaluate their properties. The objective of these tests, was to establish a correlation between variations 
in the results and the different areas of the weld, from where the specimens were taken [31]. The tests 
were conducted, at room temperature, with specimens taken along the thickness, from the middle, from 
the advancing and from the retreating side of the weld. The last were centered at 5.5 millimeters from 
the middle of the weld. The cutting positions can be schematically seen in Figure 18. 
 Tests were also performed at 100, 200, 300 and 400 ᵒC, with specimens taken from the middle 
of the weld. 
 The geometry of these test specimens is presented in the figure below: 
 
Figure 17 – Microflat tensile test specimen. 
 
 
Figure 18 – Schematics of microflat tensile specimens’ cutting positions along the weld. 
 
 Tests were performed using a Zwick/Roell Z005 testing machine, with a load capacity of 2.5 kN, 
seen in Figure 19. For the tests at elevated temperatures, the specimens were heated by a setup, which 
can also be seen in the image. The measuring system for the temperature, consisted in an optimal 
measurement on one side of the specimen and a thermocouple on the other, having a maximum error 
of 5 ºC. 
 
27 
 
 
Figure 19 – Microflat tensile testing machine. 
 
 The specimen set up can be seen in more detail in the following image: 
 
 
Figure 20 – Microflat tensile testing machine detail. 
 
28 
 
 
29 
 
5. Results 
 
5.1 Work Package 1 – Process Development 
5.1.1 Process Data 
 A detailed analysis of the torque and forces, involved on each weld, can be performed, by 
compiling the data exported from the Gantry welding system. 
 An example of the data collected is presented in Figure 21, for the weld S218. 
 
Figure 21 – Welding data for S218. 
 As it can be seen, after the initial period of the process, the force and torque values stabilize. In 
the case presented, weld S218, the torque varies between 2 and 4.5 Nm, the force along the welding 
direction, Fx, varies between 400 and 800 N. 
 For the 4 mm of thickness welds, only on three conditions it was possible to record the data, 
due to temporary unavailability of the data acquisition system. The remaining process data collected 
relative to the welds performed, can be found in annex B – Process Data. 
 The following tables compile all the data for the welds performed, allowing for a better 
understanding of the average forces and heat input (HI) involved in the process. 
 
30 
 
Weld 
Welding 
Speed 
[mm/min] 
Rotational 
Speed [rpm] 
Fz 
[kN] 
WP 
[mm/rev] 
Torque 
[Nm] 
Fx 
[N] 
|Fy| 
[N] 
P 
[W] 
HI 
[J/mm] 
S214 300 1200 4 0.25 4.8 523 394 605 109 
S215 600 1200 5 0.50 6.6 925 787 838 75 
S216 300 2000 4 0.15 2.8 654 300 589 106 
S217 600 2000 4 0.30 2.4 779 485 510 46 
Table 18 – Heat Input for the 2 mm of thickness welds. 
 
Weld 
Welding 
Speed 
[mm/min] 
Rotational 
Speed [rpm] 
Fz 
[kN] 
WP 
[mm/rev] 
Torque 
[Nm] 
Fx 
[N] 
|Fy| 
[N] 
P 
[W] 
HI 
[J/mm] 
S410 200 1200 9 0.17 8.9 1409 947 1122 303 
S411 400 1200 8 0.33 9.5 1825 1576 1205 162 
S412 200 1500 9 0.13 6.9 1526 717 1088 294 
Table 19 – Heat Input for the 4 mm of thickness welds. 
 
Weld 
Welding 
Speed 
[mm/min] 
Rotational 
Speed [rpm] 
Fz 
[kN] 
WP 
[mm/rev] 
Torque 
[Nm] 
Fx 
[N] 
|Fy| 
[N] 
P 
[W] 
HI 
[J/mm] 
S625 180 800 14 0.23 19.7 2140 1220 1656 497 
S626 360 800 15 0.45 24 3484 1907 2031 305 
S627 180 1200 15 0.15 13.7 3268 1240 1731 519 
S628 360 1200 14 0.30 15.6 3935 2191 1983 297 
S629 240 1000 14 0.24 17.9 2596 1732 1884 424 
Table 20 – Heat Input for the 6 mm of thickness welds. 
 
5.1.2 Visual Characterization 
 
 As a first step of assessing the quality of the welds, detailed visual observation was done for 
each weldment. These observations aimed for, a qualitative evaluation of the surfaces resultant from 
the welding process, allowing the detection of any major surface flaw, which could indicate immediately 
an incorrect setting of parameters. 
 The only weld that did not present a normal, regular and with no flash welding surface, was weld 
S626, where there was a continuous small flash formation along the weld path, not easily removable. 
There were no visual defects detected on other welds. 
 
31 
 
5.1.3 Microstructural Analysis 
 
 It has been proven that, although the macro scale properties off the welds, performed by FSW, 
are very consistent, there are regular variations, due to the band structure character of the weldments. 
This segregated banded structure consists of, alternating hard particle-rich and hard particle-poor 
regions. This structure affects the macroscopic fracture process and is, directly related to the welding 
pitch of the process [11], [22]. 
 This analysis aims to relate the metallurgical characteristics of the weld structure with the 
parameters used. The quality of the weld is determined, by the presence, or not, of flaws, which 
depending on the size, can be considered defects. 
 In Figure 22, macro and micrographs, show the typical structure found in these welds, 
presenting the different grain zones and a defect, on the bottom surface of the weldment, where part of 
the material got attached to the backing bar, being this considered a major defect. 
 
 
Figure 22 – Macro and Micrographs of an initial test weld. 
 
 The macrographs of the welds performed, are shown from Figure 23 until Figure 25, according 
to the thickness of the sheets, in which, the AS of the weld is shown on right side of the image. 
 
32 
 
 2 mm 
S214 
 
S215 
 
S216 
 
S217 
 
S218 
 
Figure 23 – Macrographs of the welds with 2 mm of thickness. 
 4 mm 
S406 
 
S407 
 
33 
 
S408 
 
S409 
 
S410 
 
S411 
 
S412 
 
S413 
 
Figure 24 – Macrographs of the welds with 4 mm of thickness. 
 
34 
 
 6 mm 
 
S625 
 
S626 
 
 
S627 
 
S628 
 
S629 
 
Figure 25 – Macrographs of the welds with 6 mm of thickness. 
35 
 
 As expected, on all the welds, it was easy to identify the different regions of the structure, related 
to the significant differences of grain type and configuration. In the center of the stir zone, nugget, the 
grain was more refined, with small precipitates and homogeneously distributed. On every weld it was 
noticeable an onion ring banded structure. The TMAZ, presents a highly deformed grain structure, due 
to the temperatures and the plastic material flow. On the HAZ, it was possible to detect a grain growth 
in comparison with the base material. This zone is where the lower hardness values are expected, 
resulting from localized differences in material aging and precipitate coarsening. 
 After analyzing the images, defects were observed on welds S215, S217 and S411. All defects 
present on the stir zone, were voids, located on the AS, close to the frontier with the TMAZ of the weld. 
This defect is created by the lack of a more homogenous material flow, consequence of a high thermal 
gradient, present in the material along the thickness, originating different material flow speeds, hence a 
non-continuous flow. 
 All the defects detected, can be explained by insufficient heat input and low downward force, in 
relation to the high WP used. The defects detected are shown in more detail from Figure 26 until Figure 
28. 
 
Figure 26 – Void defects in weld S215. 
 Voids can be easily seen in the detailed view, with a higher resolution. 
 
 
 
36 
 
 
Figure 27 – Void defects in weld S217. 
 
 
Figure 28 – Void defects in weld S411. 
 
5.1.4 Michrohardness Evaluation 
 
 The Vickers hardness profiles were done as described at section 4.6, in order to evaluate the 
relative mechanical resistance along the transverse