Laser Welding of Low Weldability Materials

Autogenous pulsed laser welding of aluminium alloy AA6082-T651

Rui Ravail Luz Rodrigues

Thesis to obtain the Master of Science Degree in

Materials Engineering

Supervisor: Prof. Maria Luísa Coutinho Gomes de Almeida Co-supervisor: Prof. Maria de Fátima Reis Vaz

Examination Committee

Chairperson: Prof. Maria Amélia Martins de Almeida Co-supervisor: Prof. Maria de Fátima Reis Vaz Members of the Committee: Prof. Eurico Gonçalves Assunção Prof. Inês da Fonseca Pestana Ascenso Pires

December 2015

Acknowledgements

It would not have been possible to realize this master thesis without the involvement of Prof. Dr. Maria Luísa Coutinho Gomes de Almeida hence I must express my deepest gratitude for her dedication and guidance throughout the semester. Her patience and encouragements kept me focused and helped me finish on time; I could not have imagined having a better advisor. I would also like to thank Prof. Dr Maria de Fátima Reis Vaz for her contribution and support during all my experimental work at Instituto Superior Técnico.

Then, I would like to thank Dr. Phill Carr for his hospitality and Eng. João Miguel Martins Silva for his unwavering support. Both were essential for the realization of this project. This was truly an edifying experience and I was really fortunate to have had this opportunity.

A final word to my family and close friends who were always present whenever needed.

Abstract

Autogenous laser welding of the AA6082-T651 aluminium alloy was investigated with 3 lasers, namely a pulsed laser of 300W, a disk laser of 4kW and a laser marker of 70W. First, the hot cracking susceptibility was studied with two conventional laser welding equipment, without using filler material or heat treatment. Additionally, an attempt was made to weld with a laser marking equipment. Most of the welds made were laser seam welds but, some continuous and laser spot welds were also tested. As each laser was operated with different parameters, more than 400 welds were obtained with a wide range of parameters. Selected welds were studied with visual inspection, dye penetrant inspection (DPI), optical microscopy and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). Results indicated that welds with pulse laser beam tend to develop hot cracking, due to the segregation of silicon-rich low melting point eutectics to the grain boundaries and the development of contraction stresses during solidification. On the other hand, positive results were found with continuous welding since this process results in much longer solidification times and lower stresses. Finally, promising results were obtained with the laser marking machine, which produced high aspect ratio laser seam welds without hot cracking and a penetration of 1 mm using 99.9% of overlap factor. These welds were crack free due to their small weld pool, avoiding segregation effects, and to the heat build-up of successive spots, what had similar effect to solidification in continuous welding.

Keywords:

 Aluminium alloy AA6082-T651  Autogenous laser welding  Hot cracking  Nanosecond pulse welding  Pulsed laser welding

III

Resumo

O objetivo deste trabalho foi estudar a viabilidade da soldadura laser sem material de adição ou tratamentos térmicos da liga de alumínio AA6082-T651, atendendo a sua suscetibilidade à fissuração a quente. A pesquisa foi efetuada com 3 lasers diferentes: um laser pulsado de 300W, um laser de disco de 4000W e um laser de marcação de 70W. Foram efetuadas maioritariamente soldaduras pulsadas em costura e adicionalmente soldaduras contínuas e por pontos. Mais de 400 soldaduras foram produzidas e testaram uma larga variedade de parâmetros de funcionamento. As soldaduras selecionadas foram analisadas por inspeção visual, DPI, microscopia ótica e SEM com EDS. Os resultados indicaram que a soldaduras pulsada tendem a desenvolver fissuração a quente devido à segregação de compostos eutécticos de baixo ponto de fusão, ricos em silício, para as zonas de limite de grão e ao desenvolvimento de tensões de contração durante a solidificação. Foram obtidos resultados positivos com soldadura contínua, visto que este processo resulta em maiores tempos de solidificação e menores tensões. Finalmente, foram obtidos resultados promissores com o equipamento de marcação laser que neste caso produziu soldaduras em costura, com elevado rácio de penetração/largura, sem fissuração a quente e com penetração de 1 mm usando 99.9% de fator de sobreposição. Estas soldaduras não evidenciaram fissuração, devido ao reduzido tamanho do banho de soldadura, que evitou os efeitos da segregação e à acumulação de calor dos sucessivos pulsos, produzindo efeito semelhante ao que ocorre na solidificação da soldadura continua.

Palavras-chaves:

 Liga de alumínio AA6082-T651  Soldadura laser autogénea  Fissuração a quente  Soldadura com pulsos de nano-segundos  Soldadura com laser pulsado

Table of Contents Acknowledgements ...... I Abstract...... III Resumo ...... IV Table of Contents ...... V List of Figures ...... VIII List of Tables ...... X Abbreviations ...... XI 1. Introduction ...... 1 1.1 Objectives and motivation ...... 1 1.2 Document structure ...... 1 2. Literature Review ...... 3 2.1 Aluminium ...... 3 2.1.1 Commercial alloys ...... 3 2.1.2 Common welding processes ...... 4 2.2 Laser welding...... 5 2.2.1 Laser physics ...... 5 2.2.2 Advantages of laser welding ...... 6 2.2.3 Laser types and parameters ...... 7 2.2.4 Welding modes ...... 7 2.2.4.1 Conduction mode welding ...... 8 2.2.4.2 Keyhole or deep penetration welding mode ...... 8 2.3 Weldability of aluminium alloys...... 8 2.3.1 Typical welding problems ...... 8 2.3.2 Hot cracking ...... 9 2.3.2.1 Metallurgical factors ...... 10 2.3.2.2 Mechanical factors ...... 12 2.3.2.3 Reducing hot cracking ...... 13 2.3.3 Hot cracking in laser welding ...... 14 2.4 Summary ...... 16 3. Experimental Procedure ...... 18 3.1 Base material characteristics ...... 18 3.1.1 Composition ...... 18 3.1.1.1 Alloying elements: silicon and magnesium ...... 18 3.1.1.2 Alloying element: manganese ...... 20 3.1.2 Temper ...... 20 3.1.2.1 Temper steps ...... 20 3.1.2.2 Precipitation hardening ...... 21 3.1.2.3 Ageing process ...... 22 3.1.3 Properties and applications ...... 23 3.2 Laser equipment and trials ...... 23 3.2.1 Technical data – AL 300 laser ...... 23

3.2.2 1st to 3rd trials – AL 300 laser ...... 24 3.2.2.1 1st trial ...... 25 3.2.2.2 2nd trial ...... 25 3.2.2.3 3rd trial ...... 26 3.2.3 Technical data – TruDisk 4002 laser ...... 26 3.2.4 4th to 6th trials – TruDisk 4002 laser ...... 27 3.2.4.1 4th trial ...... 28 3.2.4.2 5th trial ...... 28 3.2.4.3 6th trial ...... 29 3.2.5 Technical data – G4 Series Z Type laser ...... 29 3.2.6 7th trial – G4 Series Z Type laser ...... 30 4. Results and Analysis ...... 31 4.1 Penetration analysis ...... 31 4.1.1 Spot size of 0.2 mm ...... 31 4.1.2 Spot size of 0.3 mm ...... 33 4.1.3 Spot size of 0.4 mm ...... 34 4.1.4 Spot size of 0.5 mm ...... 36 4.1.5 Overall penetration results ...... 37 4.2 Hot cracking analysis ...... 38 4.2.1 Visual inspection 2nd trial ...... 39 4.2.1.1 Spot size 0.2 mm ...... 39 4.2.1.2 Spot size 0.3 mm ...... 40 4.2.1.3 Spot size 0.4 mm ...... 41 4.2.1.4 Spot size 0.5 mm ...... 42 4.2.2 Dye penetrant inspection 2nd trial ...... 43 4.2.2.1 Results of DPI ...... 43 4.2.2.2 Observations ...... 44 4.2.3 Visual inspection 3rd trial ...... 45 4.2.3.1 Spot size 0.2 mm ...... 45 4.2.3.2 Spot size 0.5 mm ...... 46 4.2.4 SEM examination...... 48 4.2.4.1 Welding speed 1 mm/s ...... 48 4.2.4.2 Welding speed 2.5 mm/s ...... 49 4.2.4.3 Welding speed 4 mm/s ...... 50 4.2.5 Overall results ...... 50 4.3 Chemical analyses ...... 51 4.3.1 Chemical compositions of the face of the welds ...... 51 4.3.1.1 Welding speed of 1 mm/s ...... 52 4.3.1.2 Welding speed of 2.5 mm/s ...... 53 4.3.1.3 Welding speed of 4 mm/s ...... 54 4.3.1.4 Prepared sample surface ...... 55 4.3.1.5 Face of the welds vs. prepared sample surface ...... 56

4.3.2 Chemical compositions of the fusion zone of the welds ...... 56 4.3.2.1 Penetration ...... 57 4.3.2.2 Welding speed of 1 mm/s – AL 300 laser ...... 58 4.3.2.3 Welding speed of 2.5 mm/s – AL 300 laser ...... 59 4.3.2.4 Welding speed of 4 mm/s – AL 300 laser ...... 60 4.3.2.5 Welding speed of 4.5 mm/s – TruDisk 4002 laser ...... 61 4.3.2.6 Material with cracks vs. material without cracks ...... 62 4.3.2.7 Final interpretation of the fusion zone ...... 64 4.3.3 Continuous and spot welds ...... 64 4.3.3.1 Continuous welds ...... 65 4.3.3.2 Spot welds ...... 66 4.4 Optical microscopy analysis ...... 68 4.4.1 Characterization of the welds ...... 68 4.4.2 Penetration results ...... 69 4.4.3 Research interpretation ...... 70 5. Conclusions ...... 72 References ...... 75 Annexe A – Bloc sample cutting and mounting ...... A - 1 Annexe B – Sample grinding, polishing and etching ...... B - 1 Annexe C – SEM with EDS equipment ...... C - 1 Annexe D – Pulsed shapes ...... D - 1 Annexe E – Dye penetrant inspection of spot size 0.3 and 0.4 mm ...... E - 1

VII

List of Figures Figure 1 Worldwide evolution of recycled and primary aluminium [2] ...... 3 Figure 2 Schematic of (a) absorption, (b) spontaneous emission, and (c) stimulated emission [10]...... 6 Figure 3 Difference between the output power of CW and PW [18] ...... 7 Figure 4 Effect of chemical composition of weld metal on relative crack susceptibility in various aluminium alloys [25], [36] ...... 11 Figure 5 Effect of composition on crack susceptibility [25] ...... 11 Figure 6 Guide to choose the filler metals for minimizing hot cracking in welds of high-strength aluminium alloys [25] ...... 13 Figure 7 Effects of alloying elements on cracking sensitivities [56] ...... 16 Figure 8 Al-Si-Mg alloys: (left) Al corner of ternary phase diagram; (right) Al-Mg2Si pseudo-binary section [60] ...... 19 Figure 9 Variation of main alloying elements in different Al-Mg-Si alloys [61] ...... 19 Figure 10 Quasi-binary section through the aluminium-rich corner of the ternary Al-Mg-Si phase diagram [61] ...... 21 Figure 11 The relationship between phases observed and ageing condition [63] ...... 22 Figure 12 Strength evolution during artificial (and natural) ageing [61] ...... 22 Figure 13 AL 300 laser with the AL-T 500 ...... 24 Figure 14 TruDisk 4002 laser with the KR16...... 27 Figure 15 G4 Series Z Type laser ...... 29 Figure 16 Penetration vs. peak power for spot size of 0.2 mm ...... 31 Figure 17 Refining of penetration vs. peak power for spot size of 0.2 mm ...... 32 Figure 18 Weld profiles for spot size of 0.2 mm ...... 32 Figure 19 Penetration vs. peak power for spot size of 0.3 mm ...... 33 Figure 20 Refining of Penetration vs. peak power for spot size of 0.3 mm ...... 33 Figure 21 Weld profiles for spot size of 0.3 mm ...... 34 Figure 22 Penetration vs. peak power for spot size of 0.4 mm ...... 34 Figure 23 Refining of penetration vs. peak power for spot size of 0.4 mm ...... 35 Figure 24 Weld profiles for spot size of 0.4 mm ...... 35 Figure 25 Penetration vs. peak power for spot size of 0.5 mm ...... 36 Figure 26 Refining of penetration vs. peak power for spot size of 0.5 mm ...... 36 Figure 27 Weld profiles for spot size of 0.5 mm ...... 37 Figure 28 – 2nd trial with spot size of 0.2 mm ...... 39 Figure 29 – 2nd trial with spot size of 0.3 mm ...... 40 Figure 30 – 2nd trial with spot size of 0.4 mm ...... 41 Figure 31 – 2nd trial with spot size of 0.5 mm ...... 42 Figure 32 – 2nd trial DPI results with spot size of 0.2 mm ...... 43 Figure 33 – 2nd trial DPI results with spot size of 0.5 mm ...... 44 Figure 34 – 3nd trial welds with spot size of 0.2 mm ...... 46 Figure 35 – 3rd trial welds with spot size of 0.5 mm ...... 47 Figure 36 SEM images of 2nd trial with welding speed of 1 mm/s ...... 48 Figure 37 SEM images of 2nd trial with welding speed of 2.5 mm/s ...... 49 Figure 38 SEM images of 2nd trial weld with welding speed of 4 mm/s ...... 50 Figure 39 Example of EDS spectrum obtained for the chemical analysis ...... 51 Figure 40 EDS locations of the face of the weld for 1 mm/s of welding speed ...... 52 Figure 41 EDS chemical compositions of the face of the weld for 1 mm/s of welding speed ...... 52 Figure 42 EDS locations of the face of the weld for 2.5 mm/s of welding speed ...... 53 Figure 43 EDS chemical compositions of the face of the weld for 2.5 mm/s of welding speed ...... 53 Figure 44 EDS locations of the face of the weld for 4 mm/s of welding speed ...... 54 Figure 45 EDS chemical compositions of the face of the weld for 4 mm/s of welding speed ...... 54 Figure 46 EDS locations of the prepared sample surface ...... 55 Figure 47 EDS chemical compositions of the prepared sample surface ...... 55 Figure 48 Mean chemical composition of the face of the welds vs. prepared sample surface ...... 56 Figure 49 SEM images of the fusion zone of the welds ...... 57 Figure 50 EDS locations of the fusion zone of the weld for 1 mm/s of welding speed ...... 58 Figure 51 EDS chemical compositions of the fusion zone of the weld for 1 mm/s of welding speed ... 58 Figure 52 EDS locations of the fusion zone of the weld for 2.5 mm/s of welding speed ...... 59 Figure 53 EDS chemical compositions of the fusion zone of the weld for 2.5 mm/s of welding speed 59 Figure 54 EDS locations of the fusion zone of the weld for 4 mm/s of welding speed ...... 60

VIII

Figure 55 EDS chemical compositions of the fusion zone of the weld for 4 mm/s of welding speed ... 60 Figure 56 EDS locations of the fusion zone of the weld for 4.5 mm/s of welding speed ...... 61 Figure 57 EDS chemical compositions of the fusion zone of the weld for 4.5 mm/s of welding speed 61 Figure 58 Mean chemical compositions of the locations with vs. without cracks...... 62 Figure 59 Mean chemical compositions of locations with cracks for each welding speed ...... 62 Figure 60 SEM images of the continuous weld with 2.8 kW of power ...... 65 Figure 61 SEM images of the continuous with 3.4 kW of power ...... 66 Figure 62 SEM images of spot welds with 0.38 mm of spot size and 1 ms of pulse duration without cracks at the face ...... 67 Figure 63 SEM images of spot welds with 0.38 mm of spot size and 1 ms of pulse duration with visible cracks at the face ...... 68 Figure 64 – 7th trial with spot size of 0.051 mm ...... 69 Figure 65 – 7th trial with pulse duration of 350 ns and 3.6 mm/s of welding speed ...... 70 Figure 66 Classical beam matter interaction [75] ...... 71

List of Tables Table 1 Composition series of wrought and cast aluminium alloys [1] ...... 4 Table 2 Typical welding problems in aluminium alloys [25] ...... 9 Table 3 Chemical composition of aluminium alloy 6082 [1], [57] ...... 18 Table 4 Physical properties for aluminium alloy 6082 [57] ...... 23 Table 5 Mechanical properties for aluminium alloy 6082 [57] ...... 23 Table 6 Summary of laser equipment and trials ...... 23 Table 7 Technical data of the AL 300 laser with the AL-T 500 [64], [65] ...... 24 Table 8 Technical data of the TruDisk 4002 laser with the KR16 [66]–[68] ...... 27 Table 9 Technical data of the G4 Series Z Type laser [69], [70] ...... 29 Table 10 Penetration results of initial welds plots ...... 37 Table 11 Penetration results of refined plots ...... 38 Table 12 Penetration, heat input and power density of the 4 welds...... 58 Table 13 Results of the visual inspection of the spot welds ...... 67 Table 14 Penetration results of the single pass welds ...... 70

Abbreviations

 AC Alternating Current  BTR Brittle Temperature Range  CW Continuous Wave  DPI Dye Penetrant Inspection  EBW Electron Beam Welding  EDS Energy Dispersive X-ray Spectroscopy  FZ Fusion Zone  GB Grain Boundary  GMAW Gas Metal Arc Welding  GP-zone Guinier-Preston zone  GTAW Gas Tungsten Arc Welding  HAZ Heat Affected Zone  MIG Metal Inert Gas  Nd:YAG Neodymium Yttrium Aluminium Garnet  PAW Plasma Arc Welding  PMZ Partially Melted Zone  PW Pulsed Wave  RSW Resistance Spot Welding  SEM Scanning Electron Microscope  SSSS Supersaturated Solid Solution  TIG Tungsten Inert Gas

XII

1. Introduction

1.1 Objectives and motivation Laser welding is becoming the main welding choice for many industries. Mass production and cutting edge industries like automotive and airspace evidence this trend. This technology offers important mechanical and economical advantages. However considering aluminium alloys, process parameters to assure adequate laser welding results have yet large room for improvement unlike in more conventional welding processes. Hence, one of the objectives of this thesis is to complement the existing knowledge about laser welding of low weldability aluminium alloys, precisely the AA6082- T651 alloy.

Laser welding equipment is split in pulsed and continuous lasers and these types of equipment serve different purposes. Continuous lasers are widely used in the industry due to their very high productivity and the quality and strength of the welds it achieves. Initial investment cost is usually higher than for pulsed lasers but they require less maintenance and need fewer spare parts replacements which can balance its life cycle cost. Pulsed lasers, on the other hand, cannot compete with continuous lasers in terms of productivity but are particularly appropriate for micro-welding (precision welding) and other special applications including repairs of expensive mechanical parts like injection moulds or also for small series manufacturing. These two last examples are the core business of Carr’s Welding Technologies Ltd. which is a laser job shop located in Kettering (England) where most of the welding for this thesis was performed. This company owns both continuous and pulsed lasers and its request for this study was to investigate the possibility of laser welding AA6082- T651 aluminium alloy, using one of their pulsed lasers, specifically the AL 300 of Alpha Laser to obtain quality welds with 1 mm of penetration and more than 60% overlap factor.

The use of filler material, dedicated welding mounts or special heat treatments significantly increase the difficulty and cost of a welding procedure. Therefore, from a cost reduction perspective this research centred its tests on attempting to accomplish pulsed laser welding without heat treatment or filler material.

To resume, this thesis is focused on researching the possibility of pulsed laser welding the AA6082-T651 aluminium alloys without filler material, referred to as pulsed autogenous laser welding. It is known that the viability of this objective is conditioned by the high probability of occurrence of hot cracking. This has led to a complementary research using scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS) detector and other inspection methods to study the metallurgical causes of hot cracking when welding this aluminium alloy.

1.2 Document structure This document is organized in 5 chapters and several subsections. The 1st chapter, Introduction, presents the motivation behind this work and what outcomes were expected followed by a brief description of the chosen document structure of this study. The 2nd chapter, Literature Review, goes through the related work to outline the essential and current knowledge about aluminium, laser

1 welding and hot cracking. The information regarding aluminium and laser welding is introductory whereas emphasizes was given on the hot cracking explanation since it holds particular importance for this thesis analysis. The 3rd chapter, Experimental Procedure, begins with the specifications and characteristics of the AA6082-T651 aluminium alloy, highlighting the importance of composition and temper on this material. Subsequently the lasers and trials description starts by presenting and characterising each of the 3 laser equipment used, followed by the sequence of the respective trials. With the 3 distinct lasers used, 7 trials were made that summed a total of 412 welds. The 4th chapter, Results and Analysis, include results of the visual inspection, dye penetrant inspection (DPI), SEM images, EDS analysis and optical microscope images. In this chapter, these results were interpreted with references that support the analysis performed. Finally, the 5th chapter, Conclusions, resumes the central ideas that were stated and discussed in this thesis and suggests possible future work regarding autogenous pulsed laser welding of aluminium alloys.

2. Literature Review

2.1 Aluminium Since the end of the 19th century aluminium became an interesting option for many engineering applications. As it is the second most plentiful metallic element on earth, its extraction and production grew worldwide over the past 60 years (Figure 1). Nowadays, the aluminium industry is the largest non-ferrous metal industry in the world economy [1].

Figure 1 Worldwide evolution of recycled and primary aluminium [2]

Aluminium is a metal that offers a high level of versatility, with more than 300 alloy compositions developed. These alloys offer a wide range of physical and mechanical properties including low density, high specific strength, good corrosion resistance, good workability, high thermal and electrical conductivity, attractive appearance, and intrinsic recyclability [3]. Aluminium alloys can be found in many applications, namely in the markets of transportation (planes, trains, automobiles, bicycles), buildings and construction (doors, windows, frames, siding), packaging (cans, packaging foil), engineering applications and cables (heat sinks, electrical transmission lines, aluminium conductor steel-reinforced cables) [2], [4], [5].

2.1.1 Commercial alloys As mentioned, there are many aluminium alloys commercially available. As the major producing countries established their own classification, each alloy found itself to have multiple references. However, these classifications share the same structure. Consequently the explanation of a single one, namely the broadly recognized “Aluminium Association” system, reviews all others.

The Aluminium Association system divides aluminium alloys between wrought and cast alloy compositions. These different terminologies have been established to differentiate the respective alloys according to their composition. Subsequently, cast and wrought alloys are divided in 9 composition series (Table 1), each with a different selection of alloying elements. Finally, in terms of heat treatment a single terminology is applicable for wrought and cast alloys alike [1].

Table 1 Composition series of wrought and cast aluminium alloys [1] Wrought composition series Cast composition series

1xxx Controlled unalloyed (pure) compositions 1xx.x Controlled unalloyed (pure) compositions 2xxx Alloys in which copper is the principal alloying 2xx.x Alloys in which copper is the principal alloying element, though other elements, notably element, but other alloying elements may be magnesium, may be specified specified 3xxx Alloys in which manganese is the principal alloying 3xx.x Alloys in which silicon is the principal alloying element element, but other alloying elements such as copper and magnesium are specified 4xxx Alloys in which silicon is the principal alloying element 4xx.x Alloys in which silicon is the principal alloying element 5xxx Alloys in which magnesium is the principal alloying element 5xx.x Alloys in which magnesium is the principal alloying element 6xxx Alloys in which magnesium and silicon are principal alloying elements 6xx.x Unused 7xxx Alloys in which zinc is the principal alloying 7xx.x Alloys in which zinc is the principal alloying element, but other elements such as copper, element, but other alloying elements such as magnesium, chromium, and zirconium may be copper and magnesium may be specified specified 8xx.x Alloys in which tin is the principal alloying 8xxx Alloys including tin and some lithium compositions element characterizing miscellaneous compositions 9xx.x Unused 9xxx Reserved for future use

The two major categories of aluminium alloys, wrought and cast, are fundamentally different in terms of mechanical properties and so generally they cannot be used for the same purposes. Considering the applications of aluminium in the industry, cast alloys are less used than wrought alloys. Furthermore, the casting process has already been thoroughly investigated. As a result, more studies have been recently found on wrought aluminium alloys than on cast aluminium alloys and particular attention has been given to relatively new welding processes such as friction stir welding and laser welding.

2.1.2 Common welding processes While many welding processes have been industrially employed to weld both cast and wrought aluminium alloys such as arc, resistance, friction, electron beam, and laser welding [3], the most common welding processes for aluminium welding are [6]–[8]:

a) Gas-Tungsten Arc Welding (GTAW) or Tungsten Inert Gas (TIG)

This process creates an electric arc between a tungsten electrode and the workpiece to reach the necessary melting temperatures for welding. Inert gas protection is necessary and usually argon is used. Helium and mixtures of helium with argon can also be used [7].

b) Plasma Arc Welding (PAW)

This is an advanced version of the gas-tungsten arc welding process. It uses a specific torch with a constricted nozzle and 2 gas flows to form a concentrated plasma arc which delivers a more concentrated welding heat source than the gas-tungsten arc welding [6].

c) Gad Metal Arc Welding (GMAW) or Metal-Inert Gas (MIG)

Once more, this is an arc welding process. The main difference is that this process uses consumable metal electrodes (wire) with either inert or active gas protection to create an electric arc for welding [7].

d) Electron Beam Welding (EBW)

In this process welding is achieved by bombarding the workpiece with an intense beam of very highly accelerated electrons (0.3 to 0.7 of the speed of light). The impact of these electrons against the workpiece creates the necessary heat for welding. Usually vacuum is used but it is not an essential requirement [7].

e) Resistance Spot Welding (RSW)

This is a process where 2 (or more) metal sheets are welded together at particular spots with heat generated by electrical resistance. This process uses a pair (or more) of electrodes which deliver the necessary current to weld each spot and impose a clamping force on the sheets, to maintain a tight seal [7].

Aluminium can be welded with any other welding process, given that the necessary preparations are made and the adequate parameters are used. Nevertheless, due to its inherent characteristics when compared to all other processes, laser welding stands out as one of the most promising welding methods for aluminium alloys [3].

2.2 Laser welding "LASER" is an acronym for "Light Amplification by Stimulated Emission of Radiation" and a laser is a device which generates or amplifies light. The light of these devices, known as laser beam, has some unique properties specifically [9]:

 Monochromatic: has a single wavelength  Directional: shows low divergence  Intense: has a high density of photons  Coherent: has the same phase relationship

2.2.1 Laser physics The theoretical understanding of the process of generation or amplification of light was developed by Plank and Einstein in the beginning of the 20th century and is known as the quantum theory of light. This theory has multiple statements regarding the nature of light, one of which allowed the development of the laser devices. This concept relates to the generation of photons triggered by the transition of an atom or a molecule from an excited, more energetic, state (E2), to a lower and less energetic state (E1). To generate a sustainable laser beam emission, 3 different processes must happen simultaneously: stimulation absorption, spontaneous emission and stimulated emission (Figure 2) [9]–[13].

Figure 2 Schematic of (a) absorption, (b) spontaneous emission, and (c) stimulated emission [10]

The photons emitted by stimulated emission have the same energy, wavelength, phase and direction of the incident stimulating photons. These collective photons create a light emission called laser beam which has the properties mentioned above [9]–[13].

2.2.2 Advantages of laser welding Currently, lasers are used extensively in the processing industry, notably to join materials, which is one of the earliest recorded applications. Laser joining involves the use of a high power laser beam as heat source to fuse or join two solids. Joining can be made through different techniques which include welding, brazing, soldering and micro welding [14]. The use of a laser for welding offers certain advantages over the more conventional arc welding processes such as [7], [15]:

 High processing speeds with instant start and stop  High energy density output  Join difficult-to-weld materials (ex.: titanium or quartz)  Creates lower distortion thus the workpiece requires no fixation  No electrodes or filler materials are required but filler material is optional  Narrow and very precise welds can be made  Welds with little or no contamination can be produced  The heat-affected zone adjacent to the weld is very narrow

However, this welding method also has some unfavourable aspects such as [7]:

 Part fit-up and alignment are critical  Investment cost for laser welding is expensive

These disadvantages may have been a problem earlier, when this new technology was more expensive and less reliable. But today’s popularity of lasers, reflected by their growing use, indicates that this technology is becoming increasingly more profitable. Some industrial segments have already implemented this option, namely the automotive and, in particular, the aerospace industry [15]–[17] due to their necessity to join dissimilar materials with different section thickness, compositions, physical or chemical properties [14], [15].

2.2.3 Laser types and parameters There are two main types of lasers, pulsed wave (PW) lasers and continuous wave (CW) lasers. Both are used to weld aluminium alloys and the key difference between them is the power output. For the same the average power, pulsed laser are capable of delivering a high output power within a short time through what is called a pulse. Comparatively continuous laser is capable to maintain a continuous laser beam emission indefinitely but with a lower output (Figure 3) [18]. Generally CW lasers are used for high speed welding whereas PW lasers are used for precision welding [3]. Furthermore, the welds obtained, with one type of equipment or the other, are very different. Welding aluminium alloys with CW lasers produces welds free of porosity, cavity, or hot cracking and with good bead appearance, while the welds obtained with PW lasers may have worse bead appearances and can also have underfill, porosity, cavities, and cracks [19], [20]. Having a different laser output influences the welds obtained but also the process parameters that are used to define each type of laser. The parameters normally used in CW laser welding are welding speed, power and spot size (or beam diameter) while the parameters normally used in PW laser welding are pulse duration, pulse frequency, pulse shape, peak power spot size and welding speed [3], [18], [21], [22]. These differences of process parameters can be problematic when attempting to compare or replicate the results of a CW laser with a PW laser and vice versa [18].

Figure 3 Difference between the output power of CW and PW [18]

Besides these differences between types of lasers there are other process parameters which affect the quality, size and properties of the welds. Other important parameters include the welding speed for both continuous and seam welds and the welding time for seam welds. Additionally some parameters are related to the gas used to shield the welding process, namely the type of gas, the flow rate of gas, the angle of gas flow to the workpiece and the nozzle design [21].

2.2.4 Welding modes Laser welding has 2 principal modes (or regimes) which are conduction and keyhole (or deep penetration) mode but for some studies an additional mode can be defined between those two namely, transition mode [23]. For conduction and keyhole modes, it is generally accepted that below a

7 certain power density, around 106 W/cm2 for aluminium, the welding process is within conduction mode while above this value it is within keyhole mode. However the transition between modes is a complex subject which does not depends solely of the power density of the welding process. Therefore it is preferable to describe these modes with their respective characteristics [18].

2.2.4.1 Conduction mode welding Conduction mode is stable process with no vaporization which results in accurate control of the heat input [18]. In conduction mode welding, the surface of the material is heated above its melting point but below its vaporization temperature. In a similar way to conventional fusion welding processes, the fusion with laser occurs only by heat conduction and a semicircular weld bead with an aspect ratio of 1.2 or less is formed [3]. Typically this mode uses larger beams that have a good gap bridging ability. Consequently, there is no need for laser systems with high quality beams. Furthermore, the welds made in conduction, generally do not present porosity, cracks, undercuts and spatter. However, this process is slow and has a lower coupling efficiency which results in a lower productivity. Moreover it has a higher heat input which results in higher distortion [18]. Lastly, this welding mode is limited to materials with relatively thin thickness [3].

2.2.4.2 Keyhole or deep penetration welding mode Keyhole mode uses a higher power density to obtain partial vaporization of the material in order to reach much higher penetrations [3]. This welding mode is typically unstable and tends to produce welds with high level of porosity, high amount of spatter and loss of alloying elements. Additionally, the degradation of mechanical properties of the base material is possible. Unlike conduction, this mode has a low gap bridging capability and requires a laser system with a relatively good beam quality. On the other hand, this welding mode can make deep penetration, high aspect ratios welds using low heat input and low distortion. Finally, keyhole welding mode has a much higher productivity than conduction welding mode [18].

2.3 Weldability of aluminium alloys For conventional welding methods, the main factors that affect the welding of aluminium include parent metal, consumables, design, welding procedures, welding equipment and joint preparation. Since conventional welding processes like MIG, TIG or PAW are used regularly and have been studied extensively over the years, all the necessary information to make good quality welds is already detailed in different standards [24]. However, this is not yet the case of laser welding as information regarding difficult welding situations is still quite limited. Thus, much on-going research is focused on studying and improving the weldability of aluminium alloys using laser welding equipment.

2.3.1 Typical welding problems Aluminium alloys are used for numerous applications and are considered to have a good weldability. However, some alloys, under certain circumstances are vulnerable to the appearance of a number of defects. Typical problems of aluminium alloys include most notably porosity inside the weld, hot cracking at the fusion zone (FZ) or at the partially melted zone (PMZ), loss of ductility in the PMZ

8 and softening in the heat affected zone (HAZ) (Table 2). These problems can be more or less severe (or even eliminated) depending on the factors previously listed. Even so, as a general rule, some series of wrought aluminium alloys are particularly susceptible to some weld defects. Higher strength aluminium alloys like the 2xxx, 6xxx and 7xxx are examples of series which, mainly because of their composition, are particularly susceptible to have hot cracking [25]. Given the importance of hot cracking with these alloys the subsequent heading is entirely dedicated to its detailed explanation.

Table 2 Typical welding problems in aluminium alloys [25] Typical problems Alloy type Solutions  Surface scraping or milling Al-Li alloys (severe)  Thermovacuum treatment  Variable-polarity keyhole PAW  Thermovacuum treatment Porosity Powder-metallurgy alloys (severe)  Minimize powder oxidation and hydration during atomization and consolidation

Other types (less severe)  Clean workpiece and wire surface  Variable-polarity keyhole PAW  Use proper filler wires and dilution Higher-strength alloys Hot cracking in FZ (2xxx, 6xxx, 7xxx)  In autogenous GTAW, use arc oscillation or less susceptible alloys  Use low heat input Hot cracking and low Higher-strength alloys ductility in PMZ  Use proper filler wires  Low-frequency arc oscillation Work-hardened materials  Use low-heat input Softening in HAZ Heat-treatable alloys  Use low-heat input  Postweld heat treating

2.3.2 Hot cracking This section focuses on explaining the hot cracking of aluminium alloys while referring some studies that show evidence of the different causes of this problem. The studies made on laser welding are excluded since they are reviewed in the next section.

Definition

Hot cracking or solidification cracking is a weld-cracking failure mechanism. It usually occurs in the weld metal at elevated temperatures during cooling. Hot cracking also can be found in the HAZ, where it is known as liquation cracking [26].

Hot cracking occurs predominantly at the weld centreline or between columnar grains for the reason that the fracture path of a hot crack is intergranular [26]. There are several theories of solidification cracking which include the “strain theory”, the “brittleness temperature range theory”, Borland’s “generalized theory” and the “critical speed theory” [27]. All these theories accept that hot cracking is caused by the formation of a coherent interlocking solid network that is separated by almost continuous thin liquid films. This solid network ruptures because of the tensile stresses inherent to the solidification of the metal thus forming deep centreline cracks characteristic of this welding defect.

To understand the causes of hot cracking in aluminium alloys the simplest approach is to separate between the metallurgical factors and the mechanical factors [25], [28].

2.3.2.1 Metallurgical factors The metallurgical factors that have been known to affect the solidification cracking susceptibility of aluminium welds include [25], [28]:

a) Solidification temperature range

The solidification temperature range, also referred to as freezing temperature range, defines the temperature interval of a phase diagram where both liquid and solid coexist. It delimits the temperature interval during which the weld metal solidifies. A longer temperature range is more harmful to the solidification of the weld because it creates a larger region where solid and liquid coexist and allows more time for the liquid to spread and to form a detrimental thin liquid film that drastically weakens the resistance of the material to accommodate the stresses of contraction. Therefore, the hot cracking sensitivity of an alloy increases with increasing solidification temperature range. In aluminium alloys, eutectic reactions can occur during the terminal stage of solidification and extend the solidification temperature range [25]. Such undesirable aspect was observed multiple times with GTAW welding, spot GTAW and GMAW of AA6061, AA6082, AA6261 and AA6351 aluminium alloys using different filler metals. Multiple welds of these studies revealed that the base metal solidus temperature was below the weld metal solidus temperature. As a result the welds developed cracks in the HAZ [29]–[32].

b) Amount and distribution of liquid at the terminal stage of solidification

The crack sensitivity of aluminium and aluminium alloys is generally determined by the composition and so, the selection of the alloying elements and their respective percentages have a critical influence (Figure 4). As shown, with either pure aluminium or highly alloyed aluminium (i.e. more than 6 wt. %) the crack sensitivity is very low. This is because with pure aluminium there are no low melting-point eutectics that form at the grain boundaries whereas with highly alloyed aluminium there is abundant eutectic liquid between the grains that is able “heal” occasional cracks that appear. The main problem arises between those two compositions because the volume of liquid between grains is sufficient to create a thin, continuous grain boundary film that is insufficient for healing the cracks, making those alloys susceptible to solidification cracking. The formation of such thin films was confirmed with alternating current (AC) TIG welding of multiple alloys, including the ZL101, AA5083 and AA6082 [33]. Additionally, the mentioned lack of enough liquid was studied in castings of AlSi7MgCu-alloys [34] and again in castings of commercial alloys like the AA1050, AA3104, AA5182 and AA6111 and in Al-Si binary alloys [35].

Figure 4 Effect of chemical composition of weld metal on relative crack susceptibility in various aluminium alloys [25], [36]

Additionally, welds with fine equiaxed dendritic structure and with abundant liquid between grains deform more easily under stresses than welds with coarse columnar dendritic structure consequently they also have lower susceptibility to cracking (Figure 5) [25]. Hence, this is another example which shows that the liquid distribution and amount plays an essentially role in the cracking susceptibility of the welds.

(a) weld (b) crack susceptibility curve

(e) more solute (f) much more solute

Figure 5 Effect of composition on crack susceptibility [25]

c) Ductility of solidifying weld metal

During solidification the ductility of the weld metal is a concern. This is because in a determined temperature range, the solidifying weld metal has a much lower ductility than the weld pool and the completely solidified weld metal. This temperature range is called brittle temperature range (BTR) [25] and has been studied with 16 different commercial alloys welded with GTAW. In this study the solidification crack susceptibility was ranked according to the measure of this BTR [37]:

( ) * ( ) ( ) ( )+

Complementary, a study on GTAW of AA2024 aluminium sheets showed that solidification cracking was avoided by directing liquid nitrogen behind the weld pool increasing the cooling rate and therefore reducing the influence of the ductility factor [38].

d) Surface tension of grain boundary liquid

Surface tension essentially determines how the liquid wets the solid grains during the solidification. If the surface tension between the solid grains and the grain boundary liquid is very low, a liquid film will form between the grains causing a reduction of the strength of the solid network which increases the hot cracking susceptibility. On the other hand, if the surface tension is high, the liquid phase will be globular and will not wet the grain boundaries and as a result the cracking susceptibility will be lower [25]. Complementary, a study of 16 different commercial alloys welded with GTAW confirmed the importance of the surface tension on the crack susceptibility by observations of the dihedral angle of eutectic products in the grain boundary [37].

e) Grain structure of weld metal

As previously mentioned, fine equiaxed grains are less susceptible to solidification cracking than coarse columnar grains. This occurs because, unlike columnar grains, fine equiaxed grains can freely deform to accommodate contractions. Furthermore, during solidification of fine-grained materials the liquid at the grain boundary can more easily feed the incipient cracks which can effectively heal the welds. Finally, fine-grained materials offer more grain boundary area for the low melting-point segregates to be dispersed consequently they are less concentrated which is less detrimental to the cohesive strength of the solid network during solidification [25]. With the study of the 16 different commercial alloys welded with GTAW the mean grain size showed a weak correlation with the cracking susceptibility but, nonetheless, the crack susceptibility decreased with decreasing grain size [37]. Another study made about GTAW of 1050A-H14, AA6082-T6 and AA5083-H111 aluminium alloys used specific filler materials to reveal that the refinement of the microstructure prevented the formation of centreline solidification cracks. These studies corroborate that a smaller grain structure is beneficial to the weld metal [39].

2.3.2.2 Mechanical factors The more relevant mechanical factors to be taken into consideration in hot cracking are [25]:

a) Contraction stresses

The presence of stresses acting on adjacent grains during solidification is essential to the formation of cracks. Therefore, materials with high thermal contraction and high solidification shrinkage will be propitious to hot cracking. As aluminium alloys have high thermal expansion coefficients and high solidification shrinkage they tend to develop high levels of stress which explains their high cracking susceptibility, especially in alloys with wide solidification temperature ranges [25].

b) Degree of restraint

The degree of restraint of a workpiece is another mechanical factor that can cause hot cracking. For a specific joint design and material, the imposition of greater restraints will increase the hot cracking susceptibility [25] since it will difficult the accommodation of deformation which could otherwise decrease stresses in the workpiece.

2.3.2.3 Reducing hot cracking To reduce hot cracking in the aluminium alloys several aspects can be influenced, namely:

a) Control of weld metal composition

When welding aluminium alloys, it is desirable to have a weld metal composition that is far from the peak of the crack sensitivity curve (Figure 4) and to reach this desired weld metal composition, a filler metal of a proper composition (Figure 6) must be used and specific welding parameters must be selected in order to achieve the desired dilution ratio [28].

Figure 6 Guide to choose the filler metals for minimizing hot cracking in welds of high-strength aluminium alloys [25]

b) Control of solidification structure

As already discussed, hot cracking can be avoided by grain refining. This can be achieved through the use of small amounts of refining agents such as titanium and zirconium in the filler metal [25] or with the control of some aspect of the welding process such as, for example using magnetic arc oscillation in GTAW of AA6061 commercial aluminium sheets [40].

c) Use of favourable welding conditions

Favourable welding conditions can be obtained by either reducing strains or improving weld geometry. Reducing thermally induced strains can be achieved simply by using high-intensity heat sources, like electron or laser beams, which significantly reduces the distortion of the workpiece. Additionally, less joint restraint and proper preheating of the workpiece can help reduce strains therefore avoiding the hot cracking [25].

2.3.3 Hot cracking in laser welding To review the knowledge concerning the hot cracking of aluminium alloys with laser welding, a chronological approach was taken. Starting in 1988, Cieslak et al. investigated the autogenous laser welding of crack sensitive aluminium alloys like the AA6061-T6, AA5456-H116 and AA5086-H32 with different types of neodymium yttrium aluminium garnet (Nd:YAG) lasers and found that cracking was clearly process dependant (continuous vs. pulsed). With this research it became clear that, from a hot cracking perspective, pulsed welding is more detrimental than continuous welding. Furthermore, it was determined that for laser welding of Al-Mg-Si alloys, the data accumulated about hot cracking, when arc welding or in castings, were not adequate [41]. Hence, to fill this lack of reliable data concerning laser welding of aluminium alloys many recent studies were undertaken. These studies are subsequently referred, to list some alloys which are susceptible to hot crack and detail the parameters that were used to solve this problem.

In 2004, Hector et al. characterized the texture of continuous autogenous welds with Nd:YAG laser of AA5182-O and AA6111-T4. A close examination of the welds showed that grain boundary liquation occurred in the AA6111-T4 welds and caused the alloy to develop fine hot cracks [42]. In 2005, Cicalӑ et al. studied the influence of operating parameters in continuous welds (autogenous welds and welds with filler material) made with Nd:YAG lasers of AA6056-T4 aluminium alloy. This work found that the most influential factors in avoiding hot cracking were the welding speed, the fastening system and the wire parameters. In this work the best results were given by low welding speeds and a uniform compression fastening system. Furthermore, optimum values for wire feed rate and position were also determined [43].

Four years later, in 2008, Zhang et al. examined the effects of pulse shaping on the solidification susceptibility of pulsed autogenous laser welding made with a Nd:YAG laser in AA6061- T6 aluminium alloy. It was found that pulse shaping using a ramp-down gradient could eliminate the solidification cracking in this alloy [44]. Also in 2008, Chen et al. compared single and dual beam autogenous laser welds made with Nd:YAG and diode lasers of AA5052-H19 aluminium alloy. The results showed that unlike single-beam welds, which displayed some defects (namely hot cracking, voids and spatter) the dual-beam welds exhibited smooth surfaces with no evidence of the commonly observed aluminium weld defects such as hot cracking, porosity, spatter, and depletion of magnesium [45].

In 2009, Malek Ghaini et al. worked on pulsed autogenous laser welding with a Nd:YAG laser of AA2024-O aluminium alloy. This work investigated whether solidification cracks and liquation cracks acted independently or were related with each other in terms of initiation and propagation. It was found that two types of partially melted zones and fusion lines can be identified in the weld metal and these react differently depending on the energy of the pulses. It was also made clear that, in this case, the location of the cracks resulted of the opposing effects of crack healing through backfilling and crack propagation due to the mechanical stresses [46]. Also in 2009, Sánchez-Amaya et al. made continuous welds with a diode laser in AA5083-T0 and AA6082-T6 aluminium alloys under conduction

14 regime, in an attempt to increase the maximum penetration reached while maintaining good quality. It was found that cracks were due to the tensions formed during the solidification. Additionally, it was observed that the extension of the cracks diminished as the laser power increased and as the welding speed decreased [47].

A year later, in 2010, Chang et al. compared pulsed and continuous autogenous welds made with a Nd:YAG laser in dissimilar joints of AA6061-T651 with A3003-O. Similarly to the work of Cieslak et al. the results showed that the presence of defects was worse in the pulsed welds than in the continuous welds. Furthermore, adequate parameters of shielding gas and flow rate, focusing position and structure design were determined [48]. In 2010 as well, Katayama et al. studied the laser welding phenomena and the factors affecting weld penetration and welding defects in AA5083 aluminium alloys using a continuous CO2 laser, continuous YAG disk lasers and fibre lasers. It was found that cracks were easily formed at higher welding speeds in thicker plates [49].

In 2011, Pakdil et al. studied the microstructural and mechanical properties of continuous welds of AA6056-T6 aluminium alloy using a CO2 laser and an AlSi12 wire filler material. In this study, grain boundary liquation was detected in the FZ and in the HAZ although, there was no liquation cracking [50]. In 2011, similarly to the work of Cicalӑ et al. Silva located a processing window for continuous autogenous welding of AA6082-T651 aluminium alloy using a YAG disk laser. This investigation confirmed the importance of the welding speed and of the solidification period to avoid gas entrapment and hot cracking [6].

In 2013, Somonov et al. studied induction heating with the intent to prevent hot cracking during laser welding. Although no welds were actually made, computer modulation showed that prevention or reduction of the formation of hot cracks was theoretically possible by induction heating, thanks to the thermally induced compressive stress created in the weld area [51]. In 2014, Zhao et al. made pulsed autogenous welds of Al−Mn−Mg alloys with varying silicon contents. This study found that no cracking existed in the weld pool when silicon content was below 0.34 wt. % however, when the silicon content increased to 0.47 wt. % cracking happened in the weld pool, due to the distribution of liquid eutectic phases in the grain boundaries [52].

Finally, in the year 2015, Sheikhi et al. studied pulsed welding of AA2024 with a Nd:YAG laser and complemented the previous work of Malek Ghaini et al. by developing a prediction model. This model demonstrated that it is possible to avoid solidification cracking with proper control of the pulse ramp down shape [53]. Also in 2015, Witzendorff et al. studied the hot crack formation in pulsed laser welding of AA6082-T6 with a Nd:YAG laser. In this study, the hot cracking was evaluated using high- speed cameras that captured visible and infrared radiation assessing the strain rate, strain, and metallurgical outcome. It was found that different pulse shapes, within the conduction welding regime, reduced hot cracking. Alternatively, within the keyhole welding regime there was a high susceptibility to hot cracking regardless of parameters tested. Finally, it was found that extensive hydrogen diffusion at the solid–liquid interface promoted the crack initiation [54].

Considering the above mentioned studies, some essential notions can be made regarding aluminium laser welding. Although autogenous welding is a better choice for industrial applications [55], some aluminium alloys do not make good quality welds this way, which is the case of hot crack susceptible alloys. Such alloys include the AA2024 of the 2xxx series, the AA5456, AA5086, AA5182, AA5052 and AA5083 of the 5xxx series and the AA6061, AA6111, AA6056, AA6082 of the 6xxx series. With these alloys it might be necessary the use a filler material with proper dilution ratio to avoid the crack sensitive compositions (Figure 4). That is to say, for each of these series there is a minimum content of alloying elements to have good quality welds (Figure 7).

Figure 7 Effects of alloying elements on cracking sensitivities [56]

Concerning autogenous welds, the most influential choice to avoid hot cracks is the laser type. Continuous lasers are capable of providing longer solidification times to avoid hot cracks and are therefore a better choice. Pulsed lasers, on the other hand, typically make pulses with a few milliseconds and as a result the solidification times are smaller and the welds are more susceptible to hot cracking [41]. Furthermore, to avoid hot cracks with pulsed lasers conduction welding mode is preferred and in some cases pulse shaping is also recommended. Thus, if more penetration is required and the welding mode enters within the keyhole regime, a solution is yet to be found to avoid hot cracks [54]. Finally, for any welds a good joint design is an important consideration to reduce unnecessary stresses [55].

2.4 Summary This review has tried to clarify some important notions:

 Aluminium and aluminium alloys can be found in a wide variety of applications and can be welded with many different techniques, one of which is laser welding.  Although laser welding offers many potential advantages, as it is a relatively recent welding process, the existing data is still very limited.  Most aluminium alloys are easy to weld however, some of the 2xxx, 5xxx and 6xxx series alloys, can be susceptible to hot cracking, in this case they are consider low weldability alloys.  For the alloys susceptible to hot cracking, the simplest solution recommended when laser welding (or arc welding) is to use a filler material.

 Pulsed laser welding is more susceptible to cause hot cracking than continuous laser welding. Even so, controlling pulse durations and pulse shapes can be enough to solve this problem.  Pulsed laser welding in keyhole mode is more susceptible to cause hot cracking than in conduction mode, hence no solution has been found to reach higher penetrations.

The last point of this enumeration is the central objective of this thesis. In other words, this investigation was focused on doing pulsed laser welding outside the conduction regime in order to produce welds with 1 mm of weld penetration without hot cracks, which was yet to be reported.

3. Experimental Procedure

Objective

This work attempted to pulsed laser weld the aluminium alloy AA6082-T651 in an industrial environment. The main difficulty of this work revolved around the hot cracking susceptibility of this alloy with the increased difficulty of using pulsed lasers instead of continuous lasers. Additionally, the complementary objective was to obtain a laser seam weld with a minimum of 1 mm of penetration and more than 60% of overlap and, if possible, to establish a processing interval that would show welds with minimal or no hot cracking.

In this chapter, first of all the aluminium alloy AA6082-T651 is presented. Then, since 3 different lasers were employed, the description of the different tests was divided according to each laser system and their respective trials.

3.1 Base material characteristics When studying the physical metallurgy of aluminium alloys it is worth to assess the effects of the composition, the mechanical working, and the heat treatment on the mechanical and physical properties of the alloy. Consequently the subsequent evaluation of the aluminium alloy AA6082-T651 starts with the description of its composition, follows with the description of its temper and finally indicates some of its general properties.

3.1.1 Composition The aluminium alloy AA6082 belongs to the 6xxx series of the wrought aluminium alloys and can be found commercially under multiple standard designations and specifications notably: AA6082, HE30, HP30, HS30, DIN 3.2315, EN AW-6082, ISO: AlSi1MgMn and A96082 [57]. For all these designations the recognized chemical composition is the identical [1], [57]:

Table 3 Chemical composition of aluminium alloy 6082 [1], [57] Composition, wt. % Unspecified other elements Si Fe Cu Mn Mg Cr Zn Ti Al min. Each Total 0.7-1.3 0-0.5 0-0.1 0.4-1 0.6-1.2 0-0.25 0-0.2 0-0.1 0.05 0.15 Balance

3.1.1.1 Alloying elements: silicon and magnesium The alloys of the 6xxx series contain primarily magnesium and silicon (up to 2 wt. % of each)

[58] in the proportions required for the formation of magnesium silicide: Mg2Si (atomic ratio of 2:1 and wt. % ratio of 1.73:1). The Mg2Si particles are formed as a result of a ternary peritectic reaction but instead of using a ternary system to study the average composition of these alloys, it is simpler to use the pseudo-binary Al-Mg2Si system (Figure 8). In view of this pseudo-binary system the main concept to understand is that the alloys of the 6xxx series precipitate Mg2Si in a given temperature range. Furthermore these precipitates tend to form particles that confer an increase in strength and so these alloys are heat treatable [1], [59].

Figure 8 Al-Si-Mg alloys: (left) Al corner of ternary phase diagram; (right) Al-Mg2Si pseudo-binary section [60]

The maximum solubility of Mg2Si is 1.85 wt. %. Moreover wrought aluminium alloys of the

6xxx series usually have compositions with approximately 1.2 wt. % Mg2Si [60] and can be divided between compositions in the following way. In the 1st group the sum of magnesium plus silicon is below 1.5 wt. %. In this group there are the 6060 and 6063 alloys. In the 2nd group the sum of magnesium plus silicon is equal or more than 1.5 wt. % and other additions such as 0.3 wt. % of Cu are possible. In this group there is the 6061 alloy. In the 3rd group the content of magnesium and silicon can vary and there is a substantial excess and silicon. In this group there are the alloys 6082 and 6005 (Figure 9) [1].

Figure 9 Variation of main alloying elements in different Al-Mg-Si alloys [61]

The alloys of the 3rd group, due to their excess of silicon, have typically a higher strength than other 6xxx alloys. For example, an alloy with 0.8 wt. % of Mg2Si, which has an excess of 0.2 wt. % of silicon has an increase in strength of about 70 MPa. However, this excess of silicon can bring

19 disadvantages, as in these alloys the silicon tends to segregate to the grain boundaries causing cracking. To counteract this problem, elements such as magnesium, chromium, or zirconium are generally added to the composition in order to prevent the recrystallization and control the grain structure during the heat treatment [1].

3.1.1.2 Alloying element: manganese In the aluminium alloy 6082 the manganese has two effects. It increases the strength of the alloy by being in solid solution or as a finely precipitated intermetallic phase or it prevents recrystallization and grain growth by precipitating in coarse dispersoids which increase the quench sensitivity of the alloy [1], [61].

3.1.2 Temper The aluminium alloy 6082 is commercially found with different tempers which the most commons are [57]:

 O – Annealed wrought alloy  T4 – Solution heat treated and naturally aged  T6 – Solution heat treated and artificially aged  T651 – Solution heat treated, stress relieved by stretching and then artificially aged

3.1.2.1 Temper steps The T651 temper that was made in this 6082 allows to obtain better mechanical properties by precipitation hardening. The T651 temper involves the subsequent steps (Figure 10) [60], [61]:

 Solution heat treatment consists of heating to an elevated temperature (T1≈530°C) and maintaining that temperature for a prescribed period of time (around 1 hour).  Quenching the alloy (in water) is the process of cooling fast enough to retain the same microstructure as during the solution heat treatment.  Stress relieving involves relieving the tensions created during quenching with stretching, compressing or a combination of both.

 Artificial aging entails maintaining the alloy at a lower temperature (T2≈175°C) for a prescribed period of time (for T6 it is usually about 24h).

Figure 10 Quasi-binary section through the aluminium-rich corner of the ternary Al-Mg-Si phase diagram [61]

The combination of solution heat treatment, quenching and aging (natural aging: T4 or artificially aging: T6) is called precipitation hardening. Each step has a different purpose:

 The solution heat treatment allows the alloying elements to form a solid solution with aluminium.  When the temperature is quickly reduced by quenching, a supersaturated solid solution is created.  Artificial aging is then used to precipitate (in a controlled way) the phases which cause strengthening of the alloy.

3.1.2.2 Precipitation hardening The generally accepted precipitation sequence which occurs during the precipitation hardening of the supersaturated Al-Mg-Si alloy is the following [62]:

( )

There are many precipitates that can form. The most effective hardening precipitates are the coherent needle like shape precipitates. Additionally the coarse rod-shaped precipitates can also make an important contribution to the increase in strength of the precipitation hardening. Finally, for a set composition, the precipitation sequence depends on the temperature and time used in the aging process (Figure 11) [61].

Figure 11 The relationship between phases observed and ageing condition [63]

3.1.2.3 Ageing process As it was above mentioned some precipitates contribute more than others for the final hardening effect. Therefore, depending on the density and size distribution of the hardening precipitates, the alloy has a peak value of strength and hardness that can be reached which is called the peak-aged condition. This condition is attained with the T6 heat treatment and contains a balance of the two metastable and precipitates (Figure 12). Resuming, the aging process causes precipitation within the grains which improves the mechanical properties of the alloy at expense of ductility [61].

Figure 12 Strength evolution during artificial (and natural) ageing [61]

3.1.3 Properties and applications The AA6082 is a relatively new alloy whose higher strength has replaced the AA6061 in many applications. This alloy has the following generic physical properties:

Table 4 Physical properties for aluminium alloy 6082 [57] Property Value Density 2700 g/cm3 Melting Point 555°C Modulus of Elasticity 70 GPa Electrical Resistivity 0.038x10-6 Ω.m Thermal Conductivity 180 W/m.K Thermal Expansion 24x10-6 K-1 The mechanical properties of AA6082 plates with a T651 temper and with a thickness between 6 to 12.5 mm are:

Table 5 Mechanical properties for aluminium alloy 6082 [57] Property Value Proof Stress 255 Min MPa Tensile Strength 300 Min MPa Elongation A50 mm 9 Min % Hardness Brinell 91 HB As a final note the aluminium alloy AA6082 is typically used in transport application and other high stress applications like, for instance trusses, bridges, cranes, ore skips, beer barrels and milk churn [57].

3.2 Laser equipment and trials The following section details the 3 laser equipment used and their respective trials. As mentioned before, the 412 welds made are divided in 7 trials which are presented in a chronological order (Table 6).

Table 6 Summary of laser equipment and trials Lasers Ownership Trials Nº of welds AL 300 Carrs Welding Technologies Ltd 1st to 3rd trial 372 TruDisk 4002 Carrs Welding Technologies Ltd 4th to 6th trial 16 G4 Series Z Type Welding Engineering and Laser Processing Centre 7th trial 24

3.2.1 Technical data – AL 300 laser The 1st laser employed for the tests the AL 300 of ALPHA LASER GmbH installed at Carrs Welding Technologies Ltd (located in Kettering, England). This 1st equipment was a typical Nd:YAG flash lamp pumped solid state laser with an emission wavelength of 1064 nm limited to pulsed laser welding. This laser was set up on top of a motorized workbench, the AL-T 500 from the same manufacturer (Figure 13).

Figure 13 AL 300 laser with the AL-T 500

The complete laser system featured the following technical data:

Table 7 Technical data of the AL 300 laser with the AL-T 500 [64], [65] Technical data of AL 300 Average power 300 W Peak power 9 kW Pulse energy 90 J Pulse duration 0.5 – 20 ms Pulse frequency Max. 100 Hz Welding spot diameter 0.2 – 2.0 mm Focusing optics 150 mm Pulse shape Adjustable power-shaping of a pulse Technical data of AL-T 500 Workpiece motion Motorized Welding speed Max. 25 mm/s

3.2.2 1st to 3rd trials – AL 300 laser All the tests made with this laser were conducted at Carrs Welding Technologies Ltd in an industrial environment. Therefore, due to the limited equipment and resources available at the company, these tests had to be conducted following the trial and error method. After each trial, the results were interpreted to select the most promising parameters for the next trial. Following this procedure a sequence of 3 trials were made with this 1st welding equipment.

The most critical problem to weld this alloy is its strong hot cracking susceptibility. This is typically solved either by pre and post heating, using adequate filler material or changing the welding parameters in order to control the heat input. While the effects of pre and post heating could not be tested, because there was no oven at the company and it was not possible to use another laser beam to pre or post heat by performing passes, the other two choices were still available [6]. The use of filler

24 material had already been verified and was common practice at these installations therefore this research focused on testing different welding parameters to attempt to obtain positive results. Consequently, the 3 trials made were focused on changing the welding parameters in an attempt to locate a viable processing window where hot cracking would not occur or be minimized.

Before starting making trials with different parameters it was necessary to establish the initials ones for this AL 300 laser. A common way of picking those parameters is choosing similar ones referred literature that showed positive results. For this laser, the subsequent values of pulse duration and shape, shielding gas and flow rate, laser inclination, pulse frequency and welding speed of the 1st trial were selected according to the parameters of similar studies [44], [54] and taking into account the company’s parameters in previous tests.

3.2.2.1 1st trial Experimental parameters:

 Pulse duration and shape: 4 ms with rectangular pulse shape

 Gas and flow rate: Argon with 15 L/min and Heliweld21 with 25 L/min  Laser inclination: 15°  Pulse frequency: 10 Hz  Welding speed: 2.5 mm/s  Spot sizes: 0.2, 0.3, 0.4 and 0.5 mm  Peak powers 1st step: 1.67, 2.15, 2.70, 3.28, 3.90, 4.60, 5.32 and 6.06 kW  Peak powers 2nd step: 3.28, 3.40, 3.53, 3.65, 3.78, 3.90, 4.04, 4.18, 4.32, 4.46 and 4.60 kW

This trial was made in 2 steps. In the 1st step, the peak powers tested were from 1.67 to 6.06 kW for all spot sizes with both shielding gases. Then, the peak powers closest to 1 mm penetration were refined only with argon. In the 2nd step, the peak powers tested were from 3.90 to 4.60 kW for spot size 0.2 and 0.5 mm and from 3.28 to 3.90 kW for spot size 0.3 and 0.4 mm. Testing spot sizes in conjunction with peak powers for both gases determined the best parameters to reach about 1 mm of penetration while assessing the superficial quality of the welds. Weld penetrations were measured and the quality of the welds was assessed visually. The 1st trial identified the following peak powers for the subsequent trial: 3.78, 3.90, 4.04, 4.18 and 4.32 kW.

3.2.2.2 2nd trial Experimental parameters:

 Pulse duration and shape: 4 ms with rectangular pulse shape  Gas and flow rate: Argon with 15 L/min  Laser inclination: 15°  Pulse frequency: 10 Hz  Welding speed: 1, 1.5, 2, 2.5, 3, 3.5 and 4 mm/s  Spot sizes: 0.2, 0.3, 0.4 and 0.5 mm

1 This is a shielding gas mixture of 75% helium and 25% argon.

 Peak powers: 3.78, 3.90, 4.04, 4.18 and 4.32 kW

The 2nd trial tested welding speeds from 1 to 4 mm/s, spot sizes of 0.2 to 0.5 mm and peak powers of 3.78 to 4.32 kW with argon in order to identify the influence of heat input and pulse overlap on the hot cracking susceptibility of the welds. For such evaluation all welds were photographed before and after being inspected with a liquid dye penetrant, which is capable of detecting small cracks on the welds. In this trial, higher welding speeds improved the weld quality thus 3, 3.5 and 4 mm/s were chosen for the next trial.

3.2.2.3 3rd trial Experimental parameters:

 Pulse duration and shape: 4 ms with rectangular pulse shape and 3 tailored pulse shapes  Gas and flow rate: Argon with 15 L/min  Laser inclination: 15°  Pulse frequency: 8, 10, 12, 14, 16 and 18 Hz  Welding speed: 3, 3.5 and 4 mm/s  Spot sizes: 0.2 and 0.5 mm  Peak power: 4.04 kW

The 3rd trial using argon gas and tested frequencies from 8 to 18 Hz, spot sizes of 0.2 and 0.5 mm, welding speeds of 3, 3.5 and 4 mm/s and 4 pulse shapes: 1st, 2nd, 3rd pulse shapes and rectangular pulse shape (Annexe D). These welding speeds with different frequencies and pulse shapes assessed, once more, the influence of heat input and pulse overlap on the hot cracking susceptibility of the welds. The welds were photographed, and then examined with a microscope to check for hot cracks. In this trial, it seemed that higher pulse frequencies produced better welds and so, pulse frequency around 16 Hz appeared adequate for the following trial with the 2nd laser.

3.2.3 Technical data – TruDisk 4002 laser The 2nd laser used was the TruDisk 4002 of TRUMPF GmbH + Co. KG also found at the same company (Carrs Welding Technologies Ltd). This 2nd equipment was a solid state laser which instead featured diode pumped Nd:YAG disks with an emission wavelength of 1030 nm capable of both continuous and pulsed laser welding. The focusing system of this laser was mounted on a robotic arm, the KR16 from Kuka that controls all movements, specifically the welding speed of the trials (Figure 14).

Figure 14 TruDisk 4002 laser with the KR16

The complete laser system featured the following technical data:

Table 8 Technical data of the TruDisk 4002 laser with the KR16 [66]–[68] Technical data of TruDisk 4002 Emission wavelength 1030 nm Laser Power 4 kW Beam Parameter Product (BPP) 8 mm x mrad Minimum diameter laser light cable 200 µm Technical data of KR16 Rated payload 16 Kg Horizontal distance (Lz) 150 mm Vertical distance (Lxy) 120 mm Interference radius 1611 mm Speed of motion of axis 156 °/s

3.2.4 4th to 6th trials – TruDisk 4002 laser For a second time, all tests made with this laser were conducted at Carrs Welding Technologies Ltd in an industrial environment with limited equipment and resources available and following the trial and error method.

As the TruDisk 4002 is designed to be used as a continuous laser it has completely different capabilities than the previous AL 300 laser. Namely, this disk laser is capable to generate very long (virtually unlimited) pulse durations and the software allows a much greater control of pulse shaping. Also, this laser presented more accurate focal positioning which allowed for precise spot sizes to be obtained. With these advantages in consideration 3 intrinsically different trials were made.

The AL 300 laser only tested seam welds to solve the hot cracking problem but due to the equipment malfunction it was necessary to change of laser equipment. Hence, the 4th trial (TruDisk 4002 laser) was focused on testing longer pulse durations with a pulse shape. Secondly, the 5th trial

27 confirmed that continuous welding with adequate welding parameters would not have hot cracking. Thirdly, the 6th trial tested single spot welds. To sum up, the 4th trial was similar to the trials of the previous laser but the following two tested different welding situations, specifically continuous and single spot welds.

3.2.4.1 4th trial Experimental parameters:

 Pulse duration and shape: 20 ms with the 4th tailored pulse shape (Annexe D)  Gas and flow rate: Argon with 15 L/min  Laser inclination: 15°  Pulse frequency: 16.4 and 17 Hz  Welding speed: 4.5 mm/s  Spot size: 0.67 mm (-4 collimation)  Peak power: 3.1, 3.2, 3.3 and 3.4 kW

This 4th trial tested frequencies of 16.4 and 17 Hz with peak powers between 3.1 to 3.4 kW and revealed that the previous results using the AL 300 laser had inadequate pulse overlap. Additionally, the initial examination of the welds suggested that longer pulses with adequate pulse shapes reduced the hot cracking of the welds (but a later SEM examination refuted this observation). All welds were visually inspected and examined with a microscope. As this trial only tested 8 different combinations of parameters no improvements could be observed between the different welds and so, no parameters were selected.

3.2.4.2 5th trial Experimental parameters:

 Gas and flow rate: Argon with 12 L/min  Laser inclination: 10°  Welding speed: 25 mm/s  Spot size: 0.38 mm (-2 collimation)  Laser power: 2.8 and 3.4 kW

In the 5th trial, 2 continuous welds were made with a welding speed of 25 mm/s, spot size of 0.38 mm (-2 collimation) and laser powers of 2.8 and 3.4 kW. This trial confirmed that continuous welds can be made without occurrence of hot cracking using different laser powers and that, regarding hot cracking, welding speed is more influential than the size of the weld pools (which is essentially determined by the laser power). As this trial was not pulsed welding no parameters were selected.

3.2.4.3 6th trial Experimental parameters:

 Pulse duration and shape: 1 and 2 ms with rectangular pulse shape  Gas and flow rate: Argon with 12 L/min  Laser inclination: 10°  Spot sizes: 0.2, 0.38 and 0.67 mm (0, -2 and -4 collimation)  Peak power: 4 kW

In the 6th trial, single spot welds were made using pulse durations of 1 and 2 ms and spot sizes of 0.2, 0.38 and 0.68 mm. This trial tested if small pulse duration single spot welds with small fusion zones were able to accommodate the high shrinking tensions of a fast solidification. In this trial, it should be taken in consideration that since the surface preparation method was manual grinding the results lacked consistency.

3.2.5 Technical data – G4 Series Z Type laser The 3rd laser operated was the G4 Series Z Type of SPI Lasers UK Ltd made available by the Welding Engineering and Laser Processing Centre (located in Cranfield, England). This last equipment is a solid state diode pumped fibre laser with an emission wavelength between 1059 to 1065 nm capable of both continuous and pulsed laser welding (Figure 15).

Figure 15 G4 Series Z Type laser

This laser featured the following technical data:

Table 9 Technical data of the G4 Series Z Type laser [69], [70] Technical data of G4 Series Z Type Average power 70 W Maximum pulse energy 1 mJ Maximum peak power 13 kW Pulse duration 10 – 500 ns Emission wavelength 1059 – 1065 nm Beam quality factor (M2) 1.6 Full angle divergence 80 – 120 mrad Pulse frequency Max. 1 MHz

3.2.6 7th trial – G4 Series Z Type laser Experimental parameters:

 Pulse duration and shape: 240, 350, 520 ns with 3 pre-programed pulse shapes  Gas and flow rate: no shielding gas  Laser inclination: no fixed inclination  Pulse frequency: 70 kHz  Welding speed: 3.6, 10.5, 35, 35.7, 70, 105 mm/s  Spot size: 0.051, 0.1 and 0.15 mm  Peak power: 5, 7, 13 kW  Number of passes (or cycles): 1, 10 and 20

This 7th trial tested spot sizes of 0.051, 0.1, and 0.15 mm with welding speeds of 3.6 and 35.7, 35 and 70, 10.5 and 105 mm/s respectively and with 3 specific set of parameters of pulse duration, pulse shape and peak power. Each spot size was tested with 240 ns of pulse duration, pulse shape nº0 and 13 kW of peak power, then with 350 ns of pulse duration, pulse shape nº 32 and 7 kW of peak power and finally with 520 ns of pulse duration, pulse shape nº 36 and 5 kW of peak power (pulse shapes in Annexe D).

Furthermore, welds with multiple passes (or cycles) were also tested. The parameters used for the welds with 10 passes were:

 Pulse duration of 240 ns, pulse shape nº0, peak power of 13 kW with both welding speeds of 70 mm/s and 105 mm/s for spot size of 0.1 mm and 0.15 mm, respectively.  Welding speed of 35.7 mm/s with spot size of 0.051 mm and 2 different set of parameters, namely pulse duration of 240 ns, pulse shape nº0 and peak power of 13 kW and also pulse duration of 520 ns, pulse shape nº36 and peak power of 5 kW.

The parameters used for the welds with 20 passes were:

 Pulse duration of 240 ns, pulse shape nº0, peak power of 13 kW with both welding speeds of 70 mm/s and 105 mm/s for spot size of 0.1 mm and 0.15 mm, respectively.

The results of this equipment were unexpected. With a visual inspection it appeared that the laser marking equipment simply etched the surface of the samples. However, when assessing the penetration of the welds with spot size of 0.051 mm, it showed that positive weld profiles were obtained.

4. Results and Analysis

4.1 Penetration analysis This research began with the evaluation of the penetration results of the AL 300 laser. All the welds made during the 1st trial (AL 300 laser) were measured and registered, and then the resulting values of penetration vs. peak power were plotted. From these plots and for each spot size, the peak powers closest to the 1 mm penetration objective were pointed and selected. Those plots allowed as well, evidencing the transition from conduction to keyhole welding regimes. This transition is typically characterized by noticeable increase of penetration with peak power and modifications of the weld profile from a rounded to a pear shape.

The subsequent sections for each spot size respected the following order of presentation. For each spot size, the penetration measurements made with both argon and Heliweld2 gas were described. Then, the penetration measurements of the argon gas welds were measured in trimmed intervals to validate the previous results. Finally, relevant weld profiles were presented to illustrate the evolution of the weld profiles according to the welding regime.

4.1.1 Spot size of 0.2 mm The penetration achieved with spot size of 0.2 mm was slightly higher with argon than with Heliweld2. Also, with both gases a noticeable increase of the penetration was observed at 3.28 kW indicating that the transition of conduction to keyhole happened at this peak power. Secondly, the selected peak power to reach about 1 mm of penetration was initially 4.6 kW (Figure 16). However, refining the penetration measurements with argon between 3.9 and 4.6 kW showed that 4.32 kW was sufficient to reach the established objective of 1 mm penetration (Figure 17).

Argon Heliweld2 Polinomial (Argon) Potencial (Heliweld2)

2,5 2 2 y = 0,0843x - 0,2148x + 0,2884 R² = 0,9862 1,5 1 1,9218 0,5 y = 0,046x R² = 0,9441 Penetration (mm) 0 1,5 2,5 3,5 4,5 5,5 6,5 Peak power (kW)

Figure 16 Penetration vs. peak power for spot size of 0.2 mm

2 1,95 1,8 1,6 1,4 1,1 1,2 1,05 0,89 1 0,76 0,81 0,8

Penetration (mm) 0,6 0,4 3,8 4 4,2 4,4 4,6 4,8 Peak power (kW) Argon

Figure 17 Refining of penetration vs. peak power for spot size of 0.2 mm

On the other hand, with the observation of the weld profiles a distinctive alteration was identified. The weld changed from a rounded shape with low penetration at 2.7 kW to a pear shape with higher penetration at 3.28 kW which confirmed the transition of welding regime mentioned before (Figure 18). Identificar (transição entre regime identifier conduction e keyhole)

Conduction Keyhole

2.7 kW with Argon 3.28 kW with Argon

2.7 kW with Heliweld2 3.28 kW with Heliweld2 Figure 18 Weld profiles for spot size of 0.2 mm

4.1.2 Spot size of 0.3 mm The penetration achieved with a spot size of 0.3 mm with argon gas was very slightly higher than with Heliweld2 gas. Once again, with both gases a noticeable increase of the penetration started at 3.28 kW. It is noticeable that this spot size with 6.06 kW of peak power and argon gas reached the maximum penetration depth of all the measurements at 2.46 mm of penetration. Finally, the peak power closest to the 1 mm of penetration was about 3.9 kW for both gases (Figure 19). This was then approved by the refined penetration measurements with argon between 3.28 and 3.9 kW (Figure 20).

Argon Heliweld2 Polinomial (Argon) Polinomial (Heliweld2)

3 2,5 y = 0,0522x2 + 0,1719x - 0,3898 2 R² = 0,9633 1,5 1 2 0,5 y = 0,0219x + 0,3137x - 0,5678

Penetration (mm) R² = 0,967 0 1,5 2,5 3,5 4,5 5,5 6,5 Peak power (kW)

Figure 19 Penetration vs. peak power for spot size of 0.3 mm

2 1,8 1,6 1,4 1,2 1,09 1 0,87 0,73 0,78 0,8 0,59 0,59

Penetration (mm) 0,6 0,4 3,2 3,4 3,6 3,8 4 Peak power (kW) Argon

Figure 20 Refining of Penetration vs. peak power for spot size of 0.3 mm

Like previously, the weld profiles changed from a rounded profile to a pear profile when increasing the peak power from 2.7 to 3.28 kW. This modification confirmed that a transition of welding regimes occurred between these welds (Figure 21).

Conduction Keyhole

2.7 kW with Argon 3.28 kW with Argon

2.7 kW with Heliweld2 3.28 kW with Heliweld2 Figure 21 Weld profiles for spot size of 0.3 mm

4.1.3 Spot size of 0.4 mm The penetration achieved with a spot size of 0.4 mm with both gases was almost identical. Once more, the increase of penetration depth with both gases started at 3.28 kW. An almost ideal 1 mm penetration was reached with a peak power of 3.9 kW (Figure 22), and such penetration was confirmed by the refined penetration measurements with argon between 3.28 and 3.9 kW. Nevertheless, a weld made with argon and 3.78 kW reached 1.08 mm of penetration, hence this lower peak power was considered appropriate to accomplish the 1 mm objective (Figure 23).

Argon Heliweld2 Potencial (Argon) Polinomial (Heliweld2)

3 y = 0,0353x2,3625 2,5 R² = 0,9584 2 1,5 1 2 0,5 y = 0,0506x + 0,1354x - 0,3441

Penetration (mm) R² = 0,9594 0 1,5 2,5 3,5 4,5 5,5 6,5 Peak power (kW)

Figure 22 Penetration vs. peak power for spot size of 0.4 mm

2 1,8 1,6 1,4 1,08 1,2 1,01 1 0,8 0,59 0,63 0,44 0,51 Penetration (mm) 0,6 0,4 3,2 3,4 3,6 3,8 4 Peak power (kW) Argon

Figure 23 Refining of penetration vs. peak power for spot size of 0.4 mm

Yet again, the change of the weld profiles between 2.7 and 3.28 kW confirmed the presence of a transition of welding regimes between these welds (Figure 24).

Conduction Keyhole

2.7 kW with Argon 3.28 kW with Argon

2.7 kW with Heliweld2 3.28 kW with Heliweld2 Figure 24 Weld profiles for spot size of 0.4 mm

4.1.4 Spot size of 0.5 mm Finally, the penetration achieved with both gases for a spot size of 0.5 mm was almost identical. With this spot size, the increase of penetration started at 3.9 kW with both gases, indicating that the transition of welding regime occurred at this higher peak power. The peak power necessary to reach the 1 mm penetration objective was recognized between 3.9 and 4.6 kW (Figure 25). The refined penetration measurements with argon between 3.9 and 4.6 kW determined that 4.04 kW was sufficient to attain the 1 mm penetration (Figure 26).

Argon Heliweld2 Exponencial (Argon) Exponencial (Heliweld2)

3 2,5 y = 0,0403e0,7027x R² = 0,9322 2 1,5 1 0,5 y = 0,0455e0,6745x 0

Penetration (mm) R² = 0,9378 1,5 2,5 3,5 4,5 5,5 6,5 Peak power (kW)

Figure 25 Penetration vs. peak power for spot size of 0.5 mm

2 1,8 1,6 1,38 1,38 1,24 1,4 1,14 1,2 1,05 1 0,86 0,8

Penetration (mm) 0,6 0,4 3,8 4 4,2 4,4 4,6 4,8 Peak power (kW) Argon

Figure 26 Refining of penetration vs. peak power for spot size of 0.5 mm

In this spot size, the weld profiles changed between 3.28 and 3.9 kW confirming that, this time, the transition of welding regimes required a higher peak power (Figure 27).

Conduction Keyhole

3.28 kW with Argon 3.9 kW with Argon

3.28 kW with Heliweld2 3.9 kW with Heliweld2 Figure 27 Weld profiles for spot size of 0.5 mm

4.1.5 Overall penetration results To offer an overall perspective of the weld penetrations achieved in this trial, all the measurements were compiled in 2 separate tables (Table 10 and Table 11).

Table 10 Penetration results of initial welds plots Argon Heliweld2 (75% He+25% Ar) Spot size (mm) Spot size (mm) Pulse energy Peak power 0.2 0.3 0.4 0.5 0.2 0.3 0.4 0.5 6.67 J 1.67 kW 0.17 0.18 0.15 0.13 0.14 0.15 0.15 0.16 8.59 J 2.15 kW 0.22 0.21 0.19 0.19 0.19 0.19 0.20 0.20 10.80 J 2.70 kW 0.21 0.27 0.26 0.24 0.23 0.24 0.24 0.22 13.10 J 3.28 kW 0.66 0.50 0.53 0.27 0.42 0.65 0.43 0.30 15.60 J 3.90 kW 0.72 1.36 1.09 0.82 0.81 0.94 0.95 0.73 18.38 J 4.60 kW 1.04 1.52 1.67 1.63 1.18 1.57 1.63 1.65 21.26 J 5.32 kW 1.49 2.10 1.91 1.84 1.00 1.78 1.91 1.69 24.24 J 6.06 kW 2.12 2.46 2.10 2.00 1.29 2.02 2.18 2.12

Table 11 Penetration results of refined plots Argon

Spot size (mm) Pulse energy Peak power 0.2 0.3 0.4 0.5 13.10 J 3.28 kW - 0.59 0.44 - 13.60 J 3.40 kW - 0.59 0.51 - 14.10 J 3.53 kW - 0.73 0.59 - 14.60 J 3.65 kW - 0.87 0.63 - 15.10 J 3.78 kW - 0.78 1.08 - 15.60 J 3.90 kW 0.76 1.09 1.01 0.86 16.16 J 4.04 kW 0.89 - - 1.05 16.71 J 4.18 kW 0.81 - - 1.38 17.27 J 4.32 kW 1.05 - - 1.24 17.83 J 4.46 kW 1.10 - - 1.14 18.38 J 4.60 kW 1.95 - - 1.38

The refining penetration measurements were made solely with argon due to three simple reasons: the choice of shielding gas did not have any significant influence on the penetration results; the superficial quality of the welds was slightly better with argon gas, notably in terms of roughness, where it provided welds with smother surfaces; finally, argon gas was the cheapest option.

With all the previously results two conclusions were made regarding this analysis. The shielding gas did not influence relevantly the penetration depth achieved, so all subsequent trials used solely argon gas. To assure the 1 mm penetration objective, four different peak powers, one for each spot size, were selected. The interval of peak powers tested in the 2nd trial was from 3.78 to 4.32 kW.

4.2 Hot cracking analysis The 2nd and 3rd trial (AL 300 laser) attempted to solve the hot cracking problem using different welding speeds, pulse frequencies and pulse shapes while maintaining pulse duration of 4 ms. Since these trials were made in industrial environment providing limited characterisation equipment (sandpapers, chemical etchants and a microscope) the approach to follow could not rely on accurate characterisation of each weld. Therefore, numerous welds (i.e. 284 welds) were made to determine the best parameters that would minimize the hot cracking problem. As well, the limited time available did not permit all welds to be equally studied. Even though all welds were visually inspected and photographed but only a part of the results were subjected to complementary inspections, specifically the welds of the 2nd trial were inspected with dye penetrant and the welds of the 3rd trial were examined with the company’s optical microscope.

To sum up, this section studied the overall quality of the welds made in the 2nd and 3rd trial. First the results of the 2nd trial, which include visual and dye penetrant inspection (DPI), were reviewed. Then, the visual inspection of the welds of the 3rd trial was revised. Furthermore, 3 welds of the 2nd trial showing positive characteristics were examined with a scanning electron microscope (SEM), from a certified laboratory. The images obtained with this more powerful equipment offered unprecedented visual details of the welds to assess weld quality and crack location.

4.2.1 Visual inspection 2nd trial The core parameter tested in the 2nd trial was welding speed. First and foremost, the welds of the 2nd trial had weld defects notably undercuts, burned edges and blowholes. These welding defects appeared somewhat irregularly across the welds therefore, for each spot size, we identified the welds with the best quality.

The results are presented in figures with an identical configuration. Each figure includes all welds made with a determined spot size. Also, each figure has 5 images, viz. one image for each peak power (from 3.78 to 4.32 kW), and every image has 7 welds, viz. one weld for each welding speed (4, 3.5, 3, 2.5, 2, 1.5, and 1 mm/s).

4.2.1.1 Spot size 0.2 mm Using spot size of 0.2 mm, it was noted that with welding speed of 1 mm/s and peak power of 4.32 kW there were problems of burned edges coupled with undercuts. All lower welding speeds and peak powers revealed positive results with just occasionally some blowholes. Due to its good weld quality, the spot size of 0.2 mm was selected for the 3rd trial (Figure 28).

3.78 kW 3.90 kW 4.04 kW

4.18 kW 4.32 kW  left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s  welds start at the top Figure 28 – 2nd trial with spot size of 0.2 mm

4.2.1.2 Spot size 0.3 mm Using spot size of 0.3 mm it was noted that with welding speeds of 1 and 1.5 mm/s in all peak powers, burned edges and undercuts persistently appeared across the welds. Also, blowholes randomly appearing in the welds and created small localized burned undercut sections, making this spot size an inadequate choice for the 3rd trial (Figure 29).

3.78 kW 3.90 kW 4.04 kW

4.18 kW 4.32 kW  left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s  welds start at the top Figure 29 – 2nd trial with spot size of 0.3 mm

4.2.1.3 Spot size 0.4 mm The welds with spot size of 0.4 mm also presented a bad quality as those with spot size of 0.3 mm. Welding speeds of 1 and 1.5 mm/s at 4.04, 4.18 and 4.32 kW peak powers showed serious burned edges and undercuts. Apart from welding speeds of 3.5 and 4 mm/s at 3.78 kW of peak power, severe blowholes randomly occur throughout the other welds, which forced the exclusion of this spot size as well for the 3rd trial (Figure 30).

3.78 kW 3.90 kW 4.04 kW

4.18 kW 4.32 kW  left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s  welds start at the top Figure 30 – 2nd trial with spot size of 0.4 mm

4.2.1.4 Spot size 0.5 mm For this last spot size of 0.5 mm, the welds with 3.78 kW of peak power displayed a silver glow suggesting that the laser beam barely penetrated the aluminium oxide layer. These welds brought suspicion on the integrity of the laser components resulting in the inspection of the laser pumping cavity, which confirmed that the equipment was not working properly as the pumping flash lamp was partially damaged. At this point it is important to mention that certainly a significant part of the tests made so far were influenced by the use of a flash lamp that was not in good condition. Even so, with welding speeds of 1 and 1.5 mm/s and for all peak powers, except the 3.78 kW, the welds exhibited undercuts and burned edges. Also, with peak powers of 4.18 and 4.32 kW the welds had more blowholes. As the overall quality of the welds was slightly better with spot size of 0.5 mm than with 0.3 and 0.4 mm and the doubts about the influence of the condition of the flash lamp, this spot size was selected for the 3rd trial (Figure 31).

3.78 kW 3.90 kW 4.04 kW

4.18 kW 4.32 kW  left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s  welds start at the top Figure 31 – 2nd trial with spot size of 0.5 mm

4.2.2 Dye penetrant inspection 2nd trial The dye penetrant inspection (DPI) had two purposes which complemented the previous observations. This inspection assessed the gravity of the hot cracking problem of this alloy. Additionally it helped to select the peak powers, spot sizes and welding speeds for the 3rd trial. DPI is a procedure with low reproducibility that offers a qualitative analysis of the hot cracking problem. This inspection begins by presenting all the image results and then performs the analysis.

Although the DPI was performed for the 4 spot sizes (0.2, 0.3, 0.4, and 0.5 mm), a preliminary observation has determined that only the results of the spot sizes 0.2 and 0.5 mm are presented and analysed, due the manifest bad quality observed on the welds of the 0.3 and 0.4 mm spot size (Annexe E). Consequently these spot sizes were not selected for the 3rd trial.

4.2.2.1 Results of DPI The dye penetrant results are presented in 2 figures with the same configuration as in the visual inspection. Each of the following figures includes the welds for a single spot size. Also, each figure has 5 images, viz. one image for each peak power (from 3.78 to 4.32 kW), and every image has 7 welds, viz. one weld for each welding speed (4, 3.5, 3, 2.5, 2, 1.5, and 1 mm/s).

3.78 kW 3.90 kW 4.04 kW

4.18 kW 4.32 kW  left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s  welds start at the top Figure 32 – 2nd trial DPI results with spot size of 0.2 mm

3.78 kW 3.90 kW 4.04 kW

4.18 kW 4.32 kW  left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s  welds start at the top Figure 33 – 2nd trial DPI results with spot size of 0.5 mm

4.2.2.2 Observations All the welds inspected with the dye penetrant revealed the presence of cracks which confirmed the high hot cracking susceptibility of this AA6082-T651 alloy (Figure 32 and Figure 33). This inspection also proved that these variations of parameters were insufficient to solve the hot cracking problem. However, some welds showed better results than others thus some progress was made.

Comparing the dye penetrant results of spot sizes 0.2 and 0.5 mm we noted that the influence of the peak power in the hot cracking problem was unclear (Figure 32 and Figure 33) and for that reason the intermediate value, 4.04 kW was chosen for the 3rd trial.

Finally, at all peak powers in both spot sizes, it appeared that less hot cracking occurred with welding speed of 3, 3.5 and 4 mm/s therefore these welding speeds were selected for the 3rd trial (Figure 32 and Figure 33).

4.2.3 Visual inspection 3rd trial In this 3rd trial new samples were tested using the parameters selected by the inspections of the 1st and 2nd trial. It should be noted that a new pumping flash lamp was used consequently any comparison with previous trials is not reliable. The core parameters tested in the 3rd trial were frequencies and pulse shapes. Again, the welds of the 3rd trial occasionally displayed weld defects and so this inspection started by identifying the welds with the best quality for each spot size.

The results were once more grouped by spot sizes (i.e. 0.2 and 0.5 mm), thus just two figures include all welds made in this trial. Each figure has 12 images. There are 4 different pulse shapes (i.e. no shape + 3 pulse shapes), each pulse shape has 3 images, viz. one for each welding speed (3, 3.5 and 4 mm/s), and every image has 7 welds, viz. one weld for each frequency (18, 16, 14, 12, 10 and 8 Hz).

4.2.3.1 Spot size 0.2 mm The evaluation for this spot size is presented grouped by frequencies (Figure 34).

 At frequencies of 8 and 10 Hz: the welds made with all welding speeds and all pulse shapes presented undercuts and the surfaces were unexpectedly rougher than in the previous trials.

 At frequency of 12 Hz: the welds made with welding speeds of 3 and 3.5 mm/s and rectangular pulse shape, as well as with welding speeds of 4 mm/s and the 3rd pulse shape presented similar defects characteristics at the beginning of the welds (from the top to almost the middle) but then the process stabilized and these defects presented a positive improvement.

 At frequencies of 14, 16 and 18 Hz: the welds made with all welding speeds and all pulse shapes presented positive results.

No shape

3 mm/s 3.5 mm/s 4 mm/s

1st shape

3 mm/s 3.5 mm/s 4 mm/s

2nd shape

3 mm/s 3.5 mm/s 4 mm/s

3rd shape

3 mm/s 3.5 mm/s 4 mm/s  left to right: frequencies of 18, 16, 14, 12, 10 and 8 Hz  welds start at the top Figure 34 – 3nd trial welds with spot size of 0.2 mm

Globally no improvement of the quality of the welds could be noticed with the use of a particular pulse shape. Finally, all these welds were observed on the company’s (Carrs Welding Technologies Ltd) optical microscope. This examination confirmed that all welds presented hot cracks.

4.2.3.2 Spot size 0.5 mm The evaluation for this spot size presented worse results than the previous one (Figure 34). All the welds made with rectangular pulse shape presented more defects, particularly burned edges. At frequencies 14, 16 and 18 Hz the welds of the 3rd pulse had also serious burned edges. Comparatively

46 the best results were obtained with the 1st and 2nd pulse shapes, though these welds revealed weld defects randomly in all frequencies and welding speeds. Therefore, no pattern could be clearly identified except that, unlike the previous trial, the frequencies of 14, 16 and 18 Hz did not present a better weld quality (Figure 35).

No shape

3 mm/s 3.5 mm/s2 4 mm/s

1st shape

3 mm/s 3.5 mm/s 4 mm/s

2nd shape

3 mm/s 3.5 mm/s 4 mm/s

3rd shape

3 mm/s 3.5 mm/s 4 mm/s  left to right: frequencies of 18, 16, 14, 12, 10 and 8 Hz  welds start at the top Figure 35 – 3rd trial welds with spot size of 0.5 mm

2 Exclude the weld with a cross mark at the top.

Lastly, these welds were also examined on the company’s optical microscope and confirmed that all welds had hot cracks. To resume, this trial showed that neither pulse shape nor frequency could solve or improve the hot cracking susceptibility of the alloy.

4.2.4 SEM examination The welds of the 3rd trial were observed with the company’s (Carrs Welding Technologies Ltd) optical microscope. With this procedure some differences were noted between the cracks, specifically some cracks spread at the centred of the weld, some started at the centre but deviated to the sides and others appeared at the edges of the welds. Consequently, to illustrate these different cracking behaviours detailed images were taken with a SEM equipment of the MicroLab (at Instituto Superior Técnico of Lisbon, Portugal).

The welds of the 2nd trial with 0.2 mm of spot size, 4.32 kW of peak power and welding speeds of 1, 2.5 and 4 mm/s were chosen because they had fewer defects than the others and revealed promising results with the DPI. The SEM used was a JEOL JSM 7001F (Annexe C).

For each weld, 2 images with different amplifications of the same location were presented. From the 50x magnification we perceived the general crack location and from the 200x magnification we assessed the severity of the crack (depth and width).

4.2.4.1 Welding speed 1 mm/s For a welding speed of 1 mm/s the images revealed lots of cracks on the weld and showed they appeared not only in the centre but also closer to the edges (Figure 36).

It is known that the cooling stresses concentrate at the centre of the welds consequently cracks tend to be located at the centre. However, low welding speed increased the heat input and the solidification time, which allows more time for low strength precipitates and impurities to segregate to different locations, probably explaining the spread like distribution of cracks across the welds.

200x

Figure 36 SEM images of 2nd trial with welding speed of 1 mm/s

4.2.4.2 Welding speed 2.5 mm/s For a welding speed of 2.5 mm/s fewer cracks were present but the weld displayed a continuous and significant centreline crack.

This aspect suggests that the problem with this welding speed is related to the stress accumulation at the middle of the weld during solidification of the melt pool and cooling of the material (Figure 37).

200x

Figure 37 SEM images of 2nd trial with welding speed of 2.5 mm/s

4.2.4.3 Welding speed 4 mm/s For a welding speed of 4 mm/s the weld had fewer cracks than for a welding speed of 1 mm/s but with a similar pattern as they appeared also in the centreline and on the sides of the weld, although these cracks were narrower.

Higher welding speed lowered the heat input and reduced the appearance of cracks in the material. Still, this was insufficient to avoid hot cracking at locations favouring the segregation of lower strength precipitates and impurities. At these locations the increase of stress combined with the precipitation of lower strength material caused cracks to appear (Figure 38).

200x

Figure 38 SEM images of 2nd trial weld with welding speed of 4 mm/s

4.2.5 Overall results Many different parameters were tested during the 3 trials of this AL 300 laser, including spot sizes, peak powers, welding speeds, frequencies, pulse shapes and shielding gases. The purpose of this approach, which had already been successful with another aluminium alloy 3 , was to select adequate parameters which would produce welds with no hot cracking and overall good quality. Unfortunately, using the AL 300 laser to weld AA6082-T651 aluminium alloy without hot cracking was unsuccessful but some parameters provided decent weld quality.

On a different note, the burned pumping flash lamp compromised the accuracy of the peak powers mentioned in the 1st and 2nd trial. Nevertheless, the hot cracking problem could still be studied since the cracks of welds allowed different interpretations of cracking propagation. Besides, the results were inconsistent and consequently the laser was subjected to further maintenance which identified that one of the mirrors of the optical system was misaligned. Because of this, the spot sizes mentioned were also incorrect and so, the rest of the trials were made with 2 other lasers (TruDisk 4002 and G4 Series Z Type).

3 This procedure was previously carried out with success by Carrs Welding Technologies Ltd with an AA3003 alloy.

4.3 Chemical analyses This section presents the results of the analyses performed on the trials of the AL 300 laser and on the trials of the TruDisk 4002 laser.

With the AL 300 laser the pulse duration was kept constant at 4 ms and a large number of parameters were tested (Figure 39). A significant number of samples of those tests were analysed using SEM with EDS. Since all tests at this pulse duration revealed hot cracks, the most logical change in other to solve the hot cracking problem in subsequent trials, with the TruDisk 4002 laser, was to test different pulse durations. As a result, both longer and smaller pulse durations, namely 1 ms, 2 ms and 20 ms were tested. Due to the limited availability of this 2nd laser (TruDisk 4002) and of material samples, it was not possible to test as many different parameters as it was done with the AL 300 laser. Therefore, the approach for the analysis of the trials of 2nd laser (TruDisk 4002) was focused in search of any positive results, using a SEM with EDS on the welds available. To conclude, some of the welds of the 1st laser (AL 300) and some of the 2nd laser (TruDisk 4002) were examined with a SEM with EDS in order to identify possible causes of the hot cracking.

Figure 39 Example of EDS spectrum obtained for the chemical analysis

To resume, the SEM images and the EDS chemical compositions at multiple locations in the welds are presented in this section:

 First, we determined the chemical composition of the face of the welds of the 2nd trial (AL 300 laser) and we compare it with a prepared sample surface.

 Secondly, we studied the images and chemical compositions of the fusion zone of the 2nd trial welds (AL 300 laser) and of the 4th trial welds (TruDisk 4002 laser).

 Finally, we reviewed the SEM images of the continuous welds and the spot welds made in the 5th and 6th trial (TruDisk 4002 laser), respectively.

4.3.1 Chemical compositions of the face of the welds The face of the welds of the 2nd trial (AL 300 laser) with spot size of 0.2 mm, peak power of 4.32 kW and 1, 2.5 and 4 mm/s of welding speed were chemically analysed at 6 different locations.

Specifically, 3 locations with cracks and 3 locations without cracks were examined to identify differences of chemical composition. And so, the exact locations analysed on each figure are marked by pink rectangles followed by their respective chemical compositions.

4.3.1.1 Welding speed of 1 mm/s Location 1, 2 and 3 had no cracks while locations 4, 5 and 6 had cracks (Figure 40).

Figure 40 EDS locations of the face of the weld for 1 mm/s of welding speed

The analysis discovered mostly aluminium with small, varied amounts of oxygen, magnesium, and silicon. As it can be observed, no clear pattern was found between the chemical compositions with and without cracks (Figure 41).

100 94,34 93,46 93,86 93,72 91,84 90 85,95 80 70

60 O 50 Mg Wt.% 40

30 Al

20 Si

10,37

6,34

4,57

4,2

4,09 3,85

10 2,31

1,37

1,32

1,16

1,13

1,09

0,97

0,96

0,87

0,82

0,76 0,68 0 Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4 Spectrum 5 Spectrum 6 Locations

Figure 41 EDS chemical compositions of the face of the weld for 1 mm/s of welding speed

4.3.1.2 Welding speed of 2.5 mm/s Location 1, 2 and 3 had no cracks while locations 4, 5 and 6 had cracks (Figure 42).

Figure 42 EDS locations of the face of the weld for 2.5 mm/s of welding speed

This second analysis revealed the same elements with similar amounts. Correspondingly, no clear pattern could be identified between the chemical compositions with and without cracks (Figure 43).

97,85 95,7 100 91,28 87,58 90,58 90 78,4 80 70

60 O 50

Mg Wt.% 40

30 Al

19,29

20 Si

10,33

6,87

6,74

10 2,73

1,33 1,33 1,34

1,21

1,1

1,07

1,01

0,98

0,94

0,86

0,82

0,62 0 0 Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4 Spectrum 5 Spectrum 6 Locations

Figure 43 EDS chemical compositions of the face of the weld for 2.5 mm/s of welding speed

4.3.1.3 Welding speed of 4 mm/s Likewise, location 1, 2 and 3 had no cracks and locations 5, 6 and 7 had cracks (Figure 44).

Figure 44 EDS locations of the face of the weld for 4 mm/s of welding speed

Once more, the chemical composition revealed the same elements with similar amounts and no distinguishable pattern (Figure 45).

100 98,46 97,32 93,34 94,74 93,92 95,11 90 80 70

60 O 50 Mg Wt.% 40

30 Al

20 Si

4,74

4,31

3,29

10 3,09

1,44

1,24

1,08

1,06

1,03

0,96

0,91

0,89 0,89

0,81

0,73

0,65

0 0 0 Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 5 Spectrum 6 Spectrum 7 Locations

Figure 45 EDS chemical compositions of the face of the weld for 4 mm/s of welding speed

4.3.1.4 Prepared sample surface Following the identical procedure the prepared sample surface was also chemically characterized. This time, 2 locations were selected (Figure 46).

Figure 46 EDS locations of the prepared sample surface

The chemical composition of both locations were closely matched indicating that the surface, after preparation was homogeneous (Figure 47).

100 90,89 91,42 90 80 70

60 O 50 Mg Wt.% 40

30 Al

20 Si

6,45

10 5,54

1,84

1,67

1,2 0,99 0 Spectrum 1 Spectrum 2 Locations

Figure 47 EDS chemical compositions of the prepared sample surface

4.3.1.5 Face of the welds vs. prepared sample surface With these EDS chemical compositions the following comparison was made between the face of the welds, regardless of the location (that is with or without cracks) and the prepared sample surfaces. This comparison was based on the mean wt. % of oxygen, magnesium and silicon (Figure 48).

7,00 6,00 6,00 5,27 5,00 4,00 3,00 1,76 O 2,00 1,05 1,04 1,10 Mg

Weight% (%) 1,00 0,00 Si Face of the Prepared sample welds surface Locations

Figure 48 Mean chemical composition of the face of the welds vs. prepared sample surface

The comparison indicated that almost no vaporization took place since the amount of magnesium was nearly the same with an average of 1.05 wt. % on the face of the weld and 1.10 wt. % on the prepared sample surface which is a decrease of about 4.5%. The amount of oxygen was also similar with a decrease close to 12.2% which attested that the gas protection was adequate. Lastly, the silicon content variation was more noticeable because it decreased significantly, about 40.9%. Since silicon was not lost by evaporation it is believed that the silicon segregated to the fusion zone of the welds.

4.3.2 Chemical compositions of the fusion zone of the welds The fusion zone of the welds of the 2nd trial (AL 300 laser) with 1, 2.5 and 4 mm/s of welding speed, and of the 4th trial (TruDisk 4002 laser) with 3.4 kW, 16.4 Hz and 4.5 mm/s were inspected with the SEM and chemically analysed with EDS. Those welds were cut, mounted, contrasted (Annexe A and B) and examined to study their chemical composition, more precisely the surfaces of the cracks and the surrounding material without cracks. For each weld 2 different cracks in the fusion zone were selected and for each crack 2 locations were selected. Specifically, it was analysed 1 location with a crack and 1 location without any crack. It was also assessed the penetration of these welds through the images taken with the SEM. So, first these images and respective penetrations are presented, and then the EDS locations and respective chemical compositions are detailed.

4.3.2.1 Penetration The following SEM images were taken with a low magnification of 100x to allow the full penetration to be measured (Figure 49).

1 mm/s (AL 300 laser) 2.5 mm/s (AL 300 laser)

4 mm/s (AL 300 laser) 4.5 mm/s (TruDisk 4002 laser) Figure 49 SEM images of the fusion zone of the welds

The penetrations were all similar although different heat inputs and different power densities were used. Heat input and power density was calculated with the following equations:

[71]

[17]

First, the use of different heat inputs for the welds of the AL 300 laser did not influence the penetration at all, which indicates that penetration is not a function of heat input [17]. Secondly, it is known that the power density of these welds was lower than the estimated values because the laser was defective and compromised both peak powers and spot sizes. As a result, the TruDisk 4002 laser, which was in proper condition, reached an equivalent penetration with a power density much lower (Table 12).

Table 12 Penetration, heat input and power density of the 4 welds Penetration Heat input Power density Laser Spot size Welding speed 2 (mm) (J/mm) (MW/mm ) 1 mm/s 0.72 172.8 AL 300 0.2 mm 2.5 mm/s 0.75 69.1 13.75 4 mm/s 0.77 43.2 TruDisk 4002 0.67 mm 4.5 mm/s 0.74 128.2 0.96

4.3.2.2 Welding speed of 1 mm/s – AL 300 laser There are 2 images of cracks: in the 1st crack, locations 1 and 2 are respectively with and without crack; in the 2nd crack, locations 4 and 5 are respectively with and without crack (Figure 50).

1st crack at 3500x 2nd crack at 2500x Figure 50 EDS locations of the fusion zone of the weld for 1 mm/s of welding speed

As expected, the EDS revealed that the fusion zone was mainly aluminium and small amounts of oxygen, magnesium and silicon. Interestingly, there was a clear difference between the chemical compositions in the locations with and without cracks, specifically without cracks the material was 100 wt. % of aluminium, while with cracks the other elements mentioned were also present (Figure 51).

100 93,89 100 97,07 100 90 80 70

60 O 50 Mg Wt.% 40 30 Al 20 Si 10 2,75 1,02 2,34 0 0 0 0 1,45 1,48 0 0 0 0 Spectrum 1 Spectrum 2 Spectrum 4 Spectrum 5 Locations

Figure 51 EDS chemical compositions of the fusion zone of the weld for 1 mm/s of welding speed

4.3.2.3 Welding speed of 2.5 mm/s – AL 300 laser There are 2 images of cracks: in the 1st crack, locations 3 and 5 are respectively with and without crack; in the 2nd crack, locations 1 and 3 are respectively with and without crack (Figure 52).

1st crack at 2500x 2nd crack at 2500x Figure 52 EDS locations of the fusion zone of the weld for 2.5 mm/s of welding speed

Like previously, this EDS analysis found the same elements and the same pattern between locations with and without cracks. Also, it was noted that the amounts of oxygen and silicon present at the locations with cracks increased when compared to the welding speed of 1 mm/s (Figure 53).

100 100 100 92,08 87,11 90 80 70

60 O 50 Mg Wt.% 40 30 Al 20 Si 7,45 10 4,68 4,34 1,1 2,15 0 0 0 1,1 0 0 0 0 Spectrum 3 Spectrum 5 Spectrum 1 Spectrum 3 Locations

Figure 53 EDS chemical compositions of the fusion zone of the weld for 2.5 mm/s of welding speed

4.3.2.4 Welding speed of 4 mm/s – AL 300 laser There are 2 images of cracks: in the 1st crack, locations 3 and 1 are respectively with and without crack; in the 2nd crack, locations 2 and 3 are respectively with and without crack (Figure 54).

1st crack at 2500x 2nd crack at 2500x Figure 54 EDS locations of the fusion zone of the weld for 4 mm/s of welding speed

This EDS indicated similar results to the previous welding speeds of 1 and 2.5 mm/s. Namely, the amounts of oxygen, magnesium and silicon increased in comparison to both 1 and 2.5 mm/s of welding speed and, except the presence of a small amount of magnesium on the location 3 of the 2nd crack, the pattern for this welding speed was identical (Figure 55).

100 100 98,83 87,31 90 80 76,35 70

60 O 50 Mg Wt.% 40 30 Al 20 11,3 Si 10,07 8 10 3,44 2,27 0 0 0 1,25 0 1,17 0 0 Spectrum 3 Spectrum 1 Spectrum 2 Spectrum 3 Locations

Figure 55 EDS chemical compositions of the fusion zone of the weld for 4 mm/s of welding speed

4.3.2.5 Welding speed of 4.5 mm/s – TruDisk 4002 laser There are 2 images of cracks: in the 1st crack, locations 1 and 2 are respectively with and without crack; in the 2nd crack, locations 1 and 2 are respectively with and without crack (Figure 56).

1st crack at 3500x 2nd crack at 3500x Figure 56 EDS locations of the fusion zone of the weld for 4.5 mm/s of welding speed

Finally, the EDS chemical analysis of the TruDisk 4002 laser weld was consistent with the pattern formerly observed. To be specific, this EDS showed, once more differences of chemical composition between location with and without cracks. Oxygen and silicon are present at the location with cracks, but unlike the pattern for welding speeds of 1 and 2.5 mm/s it was noticed that there is magnesium in the locations without cracks (Figure 57).

100 98,5 98,75 89,12 90 83,9 80 70

60 O 50 Mg Wt.% 40 30 Al Si 20 10,49 10 5,87 3,67 4,13 1,34 0 1,5 0 1,49 0 1,25 0 0 Spectrum 1 Spectrum 2 Spectrum 1 Spectrum 2 Locations

Figure 57 EDS chemical compositions of the fusion zone of the weld for 4.5 mm/s of welding speed

4.3.2.6 Material with cracks vs. material without cracks The welds of the 2nd trial (AL 300 laser) and of the 4th trial (TruDisk 4002 laser) were different in almost every parameter, including frequencies, peak powers, pulse duration, pulse shape, spot sizes and welding speeds. Due to the malfunction of the 1st laser, both reference peak powers and spot sizes were compromised and as a result, any analysis using these metrics is not reliable. Therefore, this research could not study the influence of heat input and power density on composition which is usually the standard. So it was necessary to focus on comparing chemical compositions between the locations with and without cracks or between different welding speeds. To sum up, with all the EDS results of the fusion zone the mean chemical composition in wt. % was calculated for locations with and without cracks (Figure 58). Additionally, the evolution of mean wt. % of oxygen, magnesium and silicon in the locations with cracks for each welding speed was also determined (Figure 59).

99,51 100,00 88,35

80,00 60,00 O 40,00 Mg

Meanwt.% 20,00 6,32 Al 1,38 3,95 0 0,49 0 0,00 Si With crack Without crack Locations

Figure 58 Mean chemical compositions of the locations with vs. without cracks

With the mean chemical composition of both locations with and without cracks it is clear that, in average, oxygen, magnesium and silicon are present in substantial amounts in the cracks with an average of 6.32 wt. % of oxygen, 1.38 wt. % of magnesium and 3.95 wt. % of silicon. On the other hand, the locations without cracks are free of those elements, except of magnesium but in smaller amounts with only 0.49 wt. %. These results suggest that predominantly oxygen and silicon are responsible for, or in correlation to the hot cracking of the welds.

10,00 9,65 10,00 8,18 8,00 8,00

6,76 6,07 6,00 6,00 3,90 O 4,00 3,25 4,00 Mg Meanwt% 1,91 1,76 2,00 1,38 1,24 1,10 2,00 1,42 Si

0,00 0,00 1 mm/s 2,5 mm/s 4 mm/s 4,5 mm/s Welding speed

AL 300 laser TruDisk 4002 laser Figure 59 Mean chemical compositions of locations with cracks for each welding speed

First, it was noted that increasing the welding speed also increased the amounts of oxygen and silicon at the locations with cracks. It is known that unlike magnesium and silicon, which can only come from the weld itself, oxygen may come from the various surface oxides, as for instance Al2O3,

MgO, Mg(OH)2 [72], [73], or directly from the atmosphere when gas protection is ineffective. On this point, the results of the characterization of the face of the welds pointed towards oxygen coming from the surface oxides. On the other hand, faster welding speeds lead to lower heat inputs and faster solidification times. Additionally, with faster welding speeds, elements that are present at the surface and that can vaporize, like oxygen, are more susceptible to be trapped inside the fusion zone. Hence, the presence of oxygen in the welds can be related to the solidification times in the following manner: a lower welding speed increases the solidification time which allows more oxygen to escape from the metal weld pool and less oxygen to be left inside the FZ and segregate to the locations causing cracks. This explains why higher concentrations of oxygen were found with faster welding speeds.

The silicon and magnesium come from the weld itself, mainly from the inorganic compound

Mg2Si, which precipitates during the precipitation hardening treatment and contributes to the mechanical properties of the alloy [63]. The chemical composition of the locations without cracks indicated 99.51 wt. % of aluminium. This suggests that the silicon and some of the magnesium detected at the locations with cracks came from the surrounding weld metal causing the surrounding material to be almost depleted of any other element besides aluminium. Interestingly, the amount of silicon increased significantly with increasing welding speeds but the amounts of magnesium did not follow the same pattern. This increase of silicon content with welding speed can be related to the solidification time. Lower welding speeds lead to higher heat inputs and longer solidification times. These longer solidification times allow more of the silicon to precipitate into the bulk of the material. Hence, forming these precipitates decreased the amount of silicon found in solid solution at the surface of the cracks with these analyses.

Overall, the results of the chemical compositions of the fusion zone suggest that the hot cracking problem arises from the depletion of hardening elements from the neighbouring metal to the grain boundary location through segregation during solidification. The amount of silicon found at the crack locations was irregular, with results going from 1.45 wt. % to 10.07 wt. % and was overall high with an average of 3.95 wt. %. In contrast, at the location without cracks there was no silicon detected. Thus complete depletion of silicon around the cracks weakened the strength of the material in these locations while other locations, probably the grain boundaries, accumulated high amounts of silicon, became brittle and cracked with the solidification stresses. On the other hand, the magnesium at the locations with cracks was roughly the same for all welding speeds with amounts going from 1.02 wt. % to 2.27 wt. % and an average of 1.38 wt. %. Additionally magnesium was also found at the location without cracks with amounts up to 1.5 wt. %. Thus, unlike with silicon, it is difficult to relate hot cracking with the amount of magnesium and further analysis is necessary to clear this possibility.

In conclusion, this analysis shows that the hot cracking is partially caused by the presence and accumulation of oxygen and silicon at specific locations which are likely the grain boundaries. It was also shown that oxygen must come from surface of the material whereas silicon is one of the main

63 alloying elements of this aluminium alloy. Consequently, while oxygen can be reduced or even completely eliminated with proper surface preparation the same cannot be done with silicon.

4.3.2.7 Final interpretation of the fusion zone This chemical analysis showed that there is a difference in the compositions of the locations with cracks and the locations without cracks, namely the location with cracks have higher contents of silicon and oxygen. It is known that there are many factors affecting the hot cracking susceptibility of an alloy and that one of these factors is the possibility to form low melting point eutectics during welding and solidification. Considering the possible causes of hot cracking in literature for the aluminium alloys of the 6xxx series, it is believed that this chemical analysis found evidence that the hot cracking of this alloy can be explained by the formation of silicon rich low melting point eutectics during welding and solidification. These low melting point eutectics belong to the following Al-Mg-Si alloy systems [44]:

 Al-Mg2Si;  Al-Si;

 Al-Mg2Si-Fe-Mg3Si6Al8-Si

 Al-Mg2Si(CrFe)4Si4Al13-Si;

 Al-CuAl2-Mg2Si,

 AlCu2Mg8Si6Al5-CuAl2-Si

These different systems have low melting point eutectics with melting temperatures ranging from 514 to 595°C. In comparison, the melting point of the aluminium alloy AA6082 is about 660°C [44]. Therefore, with this difference of temperatures it may have caused a liquid film to be formed resulting in the hot cracking of the welds.

4.3.3 Continuous and spot welds Lastly, on one hand, we confirmed that using continuous welding (5th trial) with adequate welding speeds was able to solve the hot cracking in the welds [6]. On the other hand we tested if single spot welds (6th trial) with shorter pulse durations could also solve hot cracking due to their much smaller weld pools and faster solidification times which, in theory, reduces the possibility to accumulate of oxygen and silicon in stress sensitive locations. These 5th and 6th trials (TruDisk 4002 laser) were studied with the SEM without EDS because there were no cracks to analyse (5th trial) and it was not feasible on the available time schedule (6th trial).

4.3.3.1 Continuous welds In this 5th trial, it was confirmed that continuous welds with adequate parameters had no hot cracking. The penetration of the welds was about 2.30 mm with 2.8 kW of power (Figure 60) and 2.73 mm with 3.4 kW of power (Figure 61).

Fusion zone (30x) Face of the weld (30x)

Intersection of the fusion zone with the heat affected zone (400x) Figure 60 SEM images of the continuous weld with 2.8 kW of power

Fusion zone (30x) Face of the weld (30x)

Intersection of the fusion zone with the heat affected zone (400x) Figure 61 SEM images of the continuous with 3.4 kW of power

These 2 welds had different powers and the same welding speeds. This shows the importance of adequate cooling rate for the welds to accommodate the stresses of the solidification. Unfortunately, it was not possible to recreate similar conditions with pulse welding, even with pulses durations of 20 ms, because each spot tends to solidify during off times.

4.3.3.2 Spot welds In this 6th trial, spot welds were produced with 1 and 2 ms of pulse duration and 3 spot sizes. All 6 parameters were repeated 10 times each, making a total of 60 spot welds. Unfortunately all the parameters tested revealed spot welds both with and without cracks consequently this trial was inconclusive (Table 13).

Table 13 Results of the visual inspection of the spot welds Pulse duration Spot size Nº. of spot welds with cracks 0.2 mm 7 1 ms 0.38 mm 3 0.67 mm 10 0.2 mm 5 2 ms 0.38 mm 6 0.67 mm 9 It seems probable that some spots did not have cracks because the reduced pulse duration limited the size of the fusion zone therefore avoiding the typical stress and segregation issues. It also seems likely that very little penetration was achieved however this was not verified as the heterogeneity of these results did not justify further investigation. Such heterogeneous results can be caused by exterior factors like the superficial defects of the material or an uneven sample preparation. It is worth to note that due to the discrete nature of spot welds, these factors greatly influenced the quality of each spot weld.

The best parameters, that is to say the spot weld parameters with fewer cracks, used 0.38 mm of spot size and 1 ms pulse duration and revealed 3 out of 10 spots with cracks (Figure 62 and Figure 63). This result seemed promising as each spot weld was a bit different however no clear pattern was identified to distinguish those with or without cracks.

 Magnification 40x Figure 62 SEM images of spot welds with 0.38 mm of spot size and 1 ms of pulse duration without cracks at the face

 Magnification 40x Figure 63 SEM images of spot welds with 0.38 mm of spot size and 1 ms of pulse duration with visible cracks at the face

4.4 Optical microscopy analysis The spot welds with short pulse durations of 1 and 2 ms made in 6th trial (TruDisk 4002 laser) revealed that some spots had no visible hot cracking. This directed this study to further reducing the pulse duration to 240, 350 and 520 ns in the 7th trial with the G4 series Z Type laser. Considering this laser is designed for laser marking and micro-machining applications, no comparable research and no reference values were found on laser welding aluminium alloys with such unconventional equipment. Furthermore, having in mind that this last trial is somewhat innovative, the choice of parameters was intended to test the full processing interval of the laser. So, this 7th trial tested wide-ranging parameters and made a brief overview to support future investigation.

The initial visual inspection of the samples was misleading since these tests resembled simple laser markings and for that reason, at a first approach, penetrations were evaluated with an optical microscope. This analysis with the optical microscope revealed that proper welds with very high aspect ratios were made. Hence, in this section first the images taken with the optical microscope are presented, then the results of the weld penetrations are assessed and finally it is proposed an explanation for the causes of this unexpected but favourable situation.

4.4.1 Characterization of the welds In the 7th trial only the tests made with spot size of 0.051 mm were selected. For each spot size, tests with single and multiple passes were made. It is worth to note that tests made with multiple passes had a black oxidized protruded surface which is uncharacteristic of the welding process. These tests revealed that, regarding laser welding of aluminium alloys, the use of multiple passes was not a good option for two reasons: the first reason was that the material evaporated and the laser left a deep hole instead of a weld and the second reason was that multiple passes did not increase penetration.

On the other hand, the tests of spot size 0.051 mm, with single passes showed that pulsed laser welding was successfully achieved independently of the others parameters tested and revealed no cracks. Additionally, it was noted that these welds had a high aspect ratio and that some porosity and holes occasionally appeared though their general appearance suggested that good quality welds

68 were made when compared to previous pulsed welds of the AL 300 and the TruDisk 4002 lasers (Figure 64).

240 ns with 35.7 mm/s 240 ns with 3.6 mm/s

Single pass

350 ns with 35.7 mm/s 350 ns with 3.6 mm/s

520 ns with 35.7 mm/s 520 ns with 3.6 mm/s

Multiple passes (10 cycles)

240 ns with 35.7 mm/s 520 ns with 35.7 mm/s Figure 64 – 7th trial with spot size of 0.051 mm

4.4.2 Penetration results As stated above, the penetration results revealed welds with high aspect ratios. For instance, the weld with pulse duration of 350 ns and 3.6 mm/s of welding speed had an aspect ratio of 17.3:1

(Figure 65) which suggests that the welding process was within the keyhole regime. However with pulse durations between 240 and 520 ns the resulting keyhole is naturally instable which explains the occasional presence of porosity and holes in those welds. The penetration results also indicated that lower welding speeds achieved much higher penetration. Although the overlap factor is known to influence the penetration of aluminium welds, an increase of 74.3% of weld penetration by increasing the overlap factor of 1% (from 99% to 99.9%) was unexpected (Table 14).

Figure 65 – 7th trial with pulse duration of 350 ns and 3.6 mm/s of welding speed

Every test has multiple weld lines (4 lines drawing a rectangle) so 2 penetration measurements were made for the single pass welds and are detailed as follows (Table 14).

Table 14 Penetration results of the single pass welds Penetration Penetration Penetration increase Tests with overlap Tests with overlap from 99 to 99.9% factor 99% factor 0.999% 240 ns 0.38 mm 240 ns 0.84 mm 119.9% with 35.7 mm/s 0.38 mm with 3.6 mm/s 0.85 mm 127.0% 350 ns 0.61 mm 350 ns 1.00 mm 64.5% with 35.7 mm/s 0.55 mm with 3.6 mm/s 0.98 mm 75.9% 520 ns 0.63 mm 520 ns 0.96 mm 51.4% with 35.7 mm/s 0.45 mm with 3.6 mm/s 0.61 mm 35.3% Average 0.50 mm Average 0.87 mm 74.3%

4.4.3 Research interpretation First, it is considered the fact that proper welding was achieved. It is known that this type of equipment is usually intended to perform laser ablation which is the removal of material from a substrate by direct absorption of laser energy [74]. In ablation with pulsed laser radiation, depending

70 on the pulse duration, different beam-matter interaction mechanisms become dominant. With microsecond or nanosecond laser pulses the ablation process is dominated by heat conduction, melting, evaporation and plasma formation (Figure 66). In the ablation processes involving nanosecond lasers, the absorbed laser energy first heats the target surface to the melting point, and then to the vaporization temperature [75]. Moreover, metals require much more energy to vaporize than to melt [76], therefore it was reasonable to assume that adequate control of the laser parameters could allow to make welds with melting of the surface and minimal vaporization. In other words, choosing parameters that would increase the surface temperature while keep it close to the vaporization temperature would perform laser welding very close to the keyhole regime.

With the spot size of 0.051 mm, the use of a high power density of 49 J/cm2 (far above the typical ablation threshold of metals that is from 1 to 10 J/cm2) combined with long pulse durations of 240 to 520 ns and pulse frequency of 70 kHz caused the ablation process to be inefficient for metal surface vaporization but quite efficient in accumulating heat and in melting the metal through joule heating [77]. Hence, these conditions resulted in welds in the keyhole regime featuring high aspect ratios.

Figure 66 Classical beam matter interaction [75]

Remarkably, no hot cracks could be spotted in any of the welds. Considering that even at the kilohertz repetition rate, the coupled laser energy is not dissipated until the next laser pulse arrives [77] the accumulation effect at 70 kHz heated every spot weld multiple times, creating a linearly temperature decrease that reduced the solidification tensions. Additionally these high aspect ratio welds had very small melt pools consequently the segregation of critical elements such as oxygen or silicon was much reduced, compared to normal conduction and keyhole laser welds. It is this combination of soft temperature decrease and small high aspect ratios welds that made possible to form autogenous pulsed seam welds of AA6082-T651 without hot cracks.

Finally, it was noticed that the weld penetration increased significantly with the overlap factor which allowed the welds to reach the objective of 1 mm at 99.9% of overlap. The influence of overlapping pulsed laser processing on the melting ratio has already been investigated [77]–[79] thus these result confirmed previous studies. Finally, it must be noted that the achievement of the main objective of this investigation was successful, specifically the autogenous laser spot welding of AA6082-T651 featuring 1 mm of weld penetration, an overlap factor over 60% of and no hot cracks.

5. Conclusions

This work investigated autogenous laser welding of AA6082-T651 aluminium alloy. For such purpose, 3 completely different lasers were used, namely a conventional pulse laser welding equipment of 300W, a high power continuous laser welding equipment of 4000W and a laser marking equipment of 70W. In order to widen the field of investigation of this research every laser tried a different range of parameters and so the results were analysed independently.

AL 300 laser

This equipment tested different pulse shapes, gas compositions and flow rates, pulse frequencies, welding speeds, spot sizes and peak power while keeping the pulse duration constant at 4 ms. The 372 welds produced had hot cracks and the equipment experienced technical problems which made any tests with longer pulse durations impractical. From the 3 trials of this laser the main conclusion is that typical pulsed laser welding equipment operates with particularly unfavourable pulse durations for autogenous welding of crack sensitive alloys, such as the AA6082-T651 aluminium alloy of this work. The problem of the pulse duration was confirmed with multiple chemical analyses which showed that at the millisecond pulse range, the metal weld pools created are large enough to allow segregation of lower melting point eutectics at the grain boundaries. This segregation coupled with the high solidification stresses, typical of the pulsed laser welding process, resulted invariably in hot cracking of the welds.

TruDisk 4002 laser

This equipment did continuous welding, pulsed seam welding and spot welding. A total of 16 welds tested different pulse durations and shapes, pulse frequencies, welding speeds, spot sizes and peak powers. From the 3 trials of this laser different conclusions were found regarding the weldability of the AA6082-T651 aluminium alloy. The welds made using long pulses of 20 ms presented superficial improvements with some partially healed locations. However, SEM images and EDS chemical analysis revealed that the welds had hot cracks due to similar reasons to the AL 300 welds. The continuous welds made using a welding speed of 25 mm/s showed no signs of hot cracking which confirmed that continuous lasers were a viable option for autogenous laser welding. The spot welds results were unclear as, for every set of parameters tested, part of the spots had hot cracks. Therefore, with both the TruDisk 4002 and the AL 300 the joint conclusion is that autogenous laser welding using pulses with durations of milliseconds is inadequate for welding the AA6082-T651 aluminium alloy. This difference of results was due to the control of the solidification time which is achieved differently with each type of equipment. Pulse lasers can use longer pulse durations and pulse shapes to create a smoother solidification profile of each spot which diminishes the solidification stresses resulting in less hot cracking. However, pulse durations of 20 ms or shorter do not decrease sufficiently the stresses of the solidification after each pulse. Continuous lasers on the other hand, use welding speed to control the solidification time thus achieving much slower solidification which ensures the development of less stresses, resulting in crack free welds.

Experimental parameters without cracks:

 Single pass  Continuous welding  Gas and flow rate: Argon with 12 L/min  Laser inclination: 10°  Welding speed: 25 mm/s  Spot size: 0.38 mm (-2 collimation)  Laser power: 2.8 kW and 3.4 kW

G4 Series Z Type laser

This laser marker made 24 different welds and tried different pulse durations and shapes, welding speeds, spot sizes, peak powers and number of passes. The results of this laser were unexpected and very clear. Using multiple passes caused the laser to remove material through ablation, which is the intended purpose of this laser marking machine. However, using a single pass produced high aspect ratio welds. Moreover, these welds revealed no hot cracks and one of the welds achieved the 1 mm of penetration objective initially stated. This result has two explanations. First, the parameters used were inadequate to achieve ablation therefore the irradiated material heated and melted, resulting in welds instead of ablation marks. Secondly, the laser marker operated with a frequency of 70 kHz consequently, each successive pulse was fired before achieving the complete cooling of the previous spot. This caused heat to accumulate creating a weld with a similar thermal profile to continuous welding. Furthermore, it is remarkable that no hot cracks were found. Such result was probably due to the small metal weld pool of these high aspect ratio welds, which limited the segregation of low melting point eutectics, in addition to longer cooling times and lower stresses provided by the continuous heat build-up.

Experimental parameters without cracks and 1 mm of weld penetration:

 Single pass  Pulsed welding  Pulse duration and shape: 350 ns with the pre-programed pulse shapes nº32 (Annexe D)  Pulse frequency: 70 kHz  Welding speed: 3.6 mm/s  Spot size: 0.051 mm  Peak power: 7 kW

Suggestion for future work

As a final point, very positive results were achieved with the laser marking equipment thus it would be interesting for future work, to make further research with this type of equipment. Accordingly, frequencies in the kilohertz range with different welding parameters should be studied to determine weld penetration, aspect ratio and overall weld quality. Additionally, applying laser markers to practical (industrial) issues would probably open a new field of possibilities for welding low weldability alloys, including the AA6082-T651 alloy of this thesis.

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Annexe A – Bloc sample cutting and mounting

First, the selected weld had to be cut from their respective bloc samples. Once these welds were separated they were cut transversely to the welding direction with a certain angle, using a liquid cooled circular saw:

Welds placed with angle to be cut by the liquid cooled circular saw

The purpose of this angle was to make the crack surfaces more accessible to the EDS signal. The pieces obtained had about 20° of inclination:

2nd trial 4th trial  spot size of 0.2 mm  peak power of 3.4 kW  peak power of 4.32 kW  frequency of 16.4 Hz  welding speed of 1, 2.5 and 4 mm/s  welding speed of 4.5 mm/s

Welds after cutting process to make the angled surface

These pieces were then placed in moulds where resin was poured:

2nd trial 3rd trial

Mounted samples inside their moulds during the 12h curing period of the resin

Mounting the pieces in resin provides easier and homogenous grinding and polishing of the samples, especially in the later stages of the polishing process when cloth and colloidal solutions are used to do the fine polishing.

A - 1

Annexe B – Sample grinding, polishing and etching

It is important to prepare the sample surfaces before applying the etching agent otherwise it will not react appropriately. For this work, the grinding steps chosen were found in the characterization guide of the Laboratório de Caracterização de Materiais (at Instituto Superior Técnico of Lisbon, Portugal). The following grades of sandpapers were used:

 230, 320, 600, 800, 1000 and 2400-grit, for 1 to 2 min each  4000-grit, for 5 to 10 min to remove all the scratches of the 2400-grit

Then, the samples were immersed in ethanol and placed in an ultrasonic cleaner for 10 min. Afterwards they were polished with the following particle sizes and respective cloths:

 Cloth ADR II using diamond particles of 3 µm  Cloth HSB using diamond particles of 1 µm (this step is optional)  Cloth SUPRA using SPM (OPS) suspension and applying distilled water for lubrication

Before etching, the samples were, once more cleaned of any residue with the same process of immersion in ethanol and ultrasonic cleaning for 10 min.

Finally, the polished sample surfaces were chemical etched with a Keller’s reagent which was prepared with the following list of components:

Keller's reagent Component Quantity Distilled water 190 mL H2O Nitric acid 5 mL HNO3 Hydrochloric acid 3 mL HCL Hydrofluoric acid 2 mL HF Multiple etching times were tested: 20, 30, 35 and 40 s. The etching time selected was 30 s because longer etching times revealed some darken regions which are evidence of over-etching. The etching was halted by rinsing the samples with distilled water and drying them using ethanol and a hair drier.

As final note, the last polishing cloth and etching steps were made 20 min prior to the SEM analysis to decrease the possibility of atmospheric or handling contamination of the samples.

B - 1

Annexe C – SEM with EDS equipment

As mentioned, a number of welds were analysed with a scanning electron microscope (SEM) specifically the JEOL JSM 7001F. This equipment took detailed images and made multiple chemical analyses of different locations in the samples using an acceleration voltage of 15 kV. Additionally, the different compositions of the welds were determined by energy dispersive X-ray spectroscopy (EDS) using backscattered secondary electrons.

SEM with EDS device JEOL JSM 7001F

C - 1

Annexe D – Pulsed shapes

 rectangular pulse shape (rectangular form):

 1st pulse shape, decreasing power after 80% of the pulse duration:

 2nd pulse shape, decreasing power after 60% of the pulse duration:

 3rd pulse shape, decreasing power after 40% of the pulse duration:

D - 1

 4th pulse shape, with the pulse divided in welding section (4 ms) and cooling section (16 ms); the cooling section starts at 65% of output power and decreases linearly until reaching a minimum output power of 40 W:

 pre-programed pulse shape nº 0 (WF0):

 pre-programed pulse shapes nº 32 and nº 36 (WF32; WF36):

D - 2

Annexe E – Dye penetrant inspection of spot size 0.3 and 0.4 mm

3.78 kW 3.90 kW 4.04 kW

4.18 kW 4.32 kW

 left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s  welds start at the top 2nd trial DPI results with spot size of 0.3 mm

3.78 kW 3.90 kW 4.04kW

4.18 kW 4.32 kW

 left to right: welding speeds of 4, 3.5, 3, 2.5, 2, 1.5 and 1 mm/s  welds start at the top 2nd trial DPI results with spot size of 0.4 mm

E - 1