Muyiwa Olabode

WELDABILITY OF HIGH STRENGTH ALUMINIUM ALLOYS

Thesis for the degree of Doctor of Science (Technology) to be presented with due permission for public examination and criticism in lecture hall 1382 at Lappeenranta University of Technology, Lappeenranta, Finland on the 1st of December, 2015, at noon.

Acta Universitatis Lappeenrantaensis 666 Supervisors Professor Jukka Martikainen Laboratory of Welding Technology LUT School of Energy Systems Lappeenranta University of Technology Finland

Associate Professor Paul Kah Laboratory of Welding Technology LUT School of Energy Systems Lappeenranta University of Technology Finland

Reviewers Professor Leif Karlsson Department of Engineering Science University West Sweden

Professor Thomas Boellinghaus Department of Component Safety Federal Institute of Material Research and Testing Germany

Opponent Professor Leif Karlsson Department of Engineering Science University West Sweden

ISBN 978-952-265-865-4 ISBN 978-952-265-866-1 (PDF) ISSN-L 1456-4491 ISSN 1456-4491

Lappeenrannan teknillinen yliopisto Yliopistopaino 2015

Abstract

Muyiwa Olabode Weldability of high strength aluminium alloys Lappeenranta 2015 59 pages Acta Universitatis Lappeenrantaensis 666 Diss. Lappeenranta University of Technology ISBN 978-952-265-865-4, ISBN 978-952-265-866-1 (PDF), ISSN-L 1456-4491, ISSN 1456-4491

The need for reduced intrinsic weight of structures and vehicles in the transportation industry has made aluminium research of interest. Aluminium has properties that are favourable for structural engineering, including good strength-to-weight ratio, corrosion resistance and machinability. It can be easily recycled saving energy used in smelting as compared to steel. Its alloys can have ultimate tensile strength of up to 750 MPa, which is comparable to steel. Aluminium alloys are generally weldable, however welding of high strength alloys like the 7xxx series pose considerable challenges.

This paper presents research on the weldability of high strength aluminium alloys, principally the 7xxx series. The weldability with various weld processes including MIG, TIG, and FSW, is discussed in addition to consideration of joint types, weld defects and recommendations for minimizing or preventing weld defects.

Experimental research was carried out on 7025-T6 and AW-7020 alloys. Samples were welded, and weld cross sections utilized in weld metallurgy studies. Mechanical tests were carried out including hardness tests and tensile tests. In addition, testing was done for the presence of Al2O3 on exposed aluminium alloy.

It was observed that at constant weld heat input using a pulsed MIG system, the welding speed had little or no effect on the weld hardness. However, the grain size increased as the filler wire feed rate, welding current and welding speed increased. High heat input resulted in lower hardness of the weld profile. Weld preheating was detrimental to AW- 7020 welds; however, artificial aging was beneficial. Acceptable welds were attained with pulsed MIG without the removal of the Al2O3 layer prior to welding. The Al2O3 oxide layer was found to have different compositions in different aluminium alloys.

These findings contribute useful additional information to the knowledge base of aluminium welding. The application of the findings of this study in welding will help reduce weld cost and improve high strength aluminium structure productivity by removing the need for pre-weld cleaning. Better understanding of aluminium weld metallurgy equips weld engineers with information for better aluminium weld design.

Keywords: Aluminium alloys, aluminium welding processes, high strength aluminium, anodising, Al2O3, 7025-T6, AW-7020

Acknowledgements

I would like to express my appreciation to the many people that have in one way or the other helped me in the completion of this thesis. I gratefully acknowledge the efforts of Dr. Paul Kah for his input in the form of research methodology, article corrections, availability and readiness to guide. I thankfully acknowledge the efforts of Esa Hiltunen, Antti Heikkinen and Antti Kähkönen in carrying out laboratory weld experiments. I would also like to thank Dr. Liisa Puro and Toni Väkiparta for their assistance in carrying out O2 composition experiments on the Al2O3 layer. I would like to thank Peter Jones for the valuable input on the academic presentation of this thesis. I wish also to extend my thanks to Martin Kesse for all his support.

I would like to thank the pre-examiners of this work, Professor Leif Karlsson and Professor Thomas Boellinghaus for their valuable comments and suggestions that helped in improving the quality of this work.

I wish to acknowledge the encouragement of friends and families. My special thanks are extended to my families, the Olabodes, the Pöllänens and the Olamilehins for their encouragement and support. Additionally, my appreciation goes to Bidemi Orebiyi, Samuel Okunoye, Kevin Eyiowuawi, Edith Emenike, and others that have in one way or the other supported my journey.

My sincere appreciation is expressed to my immediate family, especially my wife, Olaitan Olabode, for all the encouragement, forbearance and understanding exercised during the course of this research. I feel blessed, thank you all.

Muyiwa Olabode

October 2015

Lappeenranta, Finland

Dedication

Dedicated to almighty God, The one who was, is, and is to come; allowing the acquisition of knowledge and giving the wisdom to know when, where and how to apply the acquired knowledge.

Contents

List of publications 11

Author's contribution 11

Nomenclature 13

1 Introduction 15 1.1 Research problem and research questions ...... 16 1.2 Scope and limitations of the study ...... 17 1.3 Contribution of the work ...... 18 1.4 Social and environmental impact ...... 18 1.5 Thesis outline ...... 18

2 State of the art of Al welding 19 2.1 Alloy designation ...... 19 2.2 HSA ...... 19 2.3 Weldability of HSA ...... 22 2.3.1 Joint types and process limitations ...... 22 2.3.2 Work preparation ...... 26 2.3.3 Welding defects in HSA ...... 30 2.4 Hybrid laser beam welding (HLBW) of HSA ...... 32 2.4.1 HLBW focusing head ...... 33 2.4.2 Challenges of HLBW of Al ...... 36

3 Experimental work 37 3.1 Welding metallurgy of HSA (7025-T6) ...... 37 3.2 Investigation of the Al2O3 layer in Al alloys ...... 39 3.3 Effect of Al2O3 layer on HSA (AW-7020) weld metallurgy ...... 39

4 Results 41 4.1 Findings on the welding metallurgy of HSA (7025-T6) ...... 42 4.2 Findings on the Al2O3 layer of Al alloys ...... 45 4.3 Findings on the effect of Al2O3 on HSA (AW-7020) weld metallurgy .. 45

5 Discussion 49 5.1 Welding metallurgy of HSA (7025-T6) ...... 49 5.2 Effect of Al2O3 on HSA (AW-7020) weld metallurgy ...... 50

6 Conclusions 53

7 Future work 55

References 56

11

List of publications

This thesis is based on the following papers. The rights have been granted by publishers to include the papers in the dissertation.

I. Olabode, M., Kah P., and Martikainen J. (2012). Experimental review on the welding metallurgy of HSA (7025-T6) alloy. The Paton Welding Journal, 4, pp.88-96. II. Olabode, M., Kah, P., and Martikainen, J. (2013). Aluminium alloys welding processes: Challenges, joint types and process selection. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 227(8), 1129-1137. III. Olabode, M., Kah, P., and Martikainen, J. (2015). Effect of Al2O3 film on the mechanical properties of a welded high-strength (AW-7020) aluminium alloy. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture. DOI: 10.1177/0954405415600678 IV. Olabode, M., Kah, P., and Salminen, A. (2015). Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys. Rev. Adv. Mater. Sci, 42, (2015) 6-19. Author's contribution

The candidate was the main author of all the publications attached to the doctoral thesis. The candidate generated the ideas and the conclusions presented in the publications. Revision was carried out together with the co-authors and reviewers as a joint effort. The contribution of the author to the publications was as summarized below:

I. Made the research design and experimental design, carried out the experiment, literature review and analysis, drew inferences and wrote the paper.

II. Made the research design, carried out the literature review and analysis, drew inferences and wrote the paper.

III. Made the research design and experimental design, carried out the literature review and analysis, drew inferences and wrote the paper.

IV. Made the research design, carried out the literature review and analysis, drew inferences and wrote the paper.

13

Nomenclature

Abbreviations Explanation 7025-T6 7025 high strength aluminium alloy, thermally treated AC Alternating current AW-7020 7020 high strength aluminium alloy BM Base material CZ Composite zone DC Direct current DCEN Direct current electrode negative EBW Electron beam welding EC Experiment conditions EDS Energy-dispersive x-ray spectroscopy FR Feed rate FSW Friction stir welding HAZ Heat affected zone HLBW Hybrid laser beam welding HSA High strength aluminium HV3 Hardness Vickers scale 3 I Current IR Infrared LBW Laser beam welding MIG Metal inert gas welding MZ Melt zone PAW Plasma arc welding PMZ Partially mixed zone S Speed SAW Submerged arc welding TIG Tungsten inert gas welding TZ Transition zone TZ-UMZ Transition zone to unmixed zone UHSA Ultra high strength aluminium UMZ Unmixed zone UTS Ultimate tensile strength UW Ultrasonic welding V Voltage WI Weld interface 14 Nomenclature

YS Yield strength

Elements and chemical compounds

Al Aluminium

Al2O3 Aluminium oxide

AlMg5 5xxx series aluminium magnesium alloy filler wire CdS Calcium sulphide

CrO3 Chromium trioxide Cu Copper Fe Iron GaAs Gallium arsenide Ge Germanium

H2SO4 Sulphuric acid

H3PO3 Phosphorous acid He Helium

HNO3 Nitric acid Li Lithium Mg Magnesium Mn Manganese NaOH Sodium hydroxide Ni Nickel

O2 Oxygen Si Silicon Ti Titanium Zn Zinc ZnS Zinc sulphide ZnSe Zinc selenite

Units of measurement

MPa Megapascals mm Millimetres s Seconds min Minutes V Volts I Amperes 15

1 Introduction

Lightweight welded metal structures are in increasing demand as a result of growing concerns regarding efficient energy use and sustainable development (Kopp and Beeh, 2010). Al has become, after steel, the second most used material in structural engineering (Ostermann, 2007, Schoer and für Schweisstechnik, 2002) due to its advantageous mechanical, chemical, thermal and electrical properties. These properties include its good strength-to-weight ratio, relatively good corrosion resistance (Ostermann, 2007), ease of machinability, high toughness, extreme low temperature capabilities, usability and recyclability.

Al is widely used in the transportation industry, particularly the automobile and aerospace industries, due to its relatively low density in comparison to steel, the lower dead-weight of Al constructions, and the resulting lower energy consumption with minimal compromise to load carrying capacity (Davis, 1999). About 50% of Al extrusions are used in the transportation industry (Cock, 1999). Other key industrial sectors in which Al use is widespread include the construction industry, and power production and power transmission (Vargel, 2004). In manufacturing industry, Al alloys are commonly used in pressure vessels and tanks because of their relatively high strength, good heat conductivity and beneficial properties at low temperatures.

Al alloys are categorised based on alloy composition and the manufacturing process. The alloys are classified into two types: cast alloys and wrought alloys, with each class comprising a series of alloys denoted as a range from 1xxx – 9xxx. This work considers the general weldability of wrought Al alloys and more detailed weldability of high strength Al (HSA) alloys, and uses 7025-T6 and AW-7020 alloys as case studies.

The use of Al parts is becoming increasingly common in automobile manufacture but brings some challenges, particularly additional costs resulting from the extra care needed when welding Al. Al and its alloys have properties that make welding challenging, such as the presence of the Al oxide (Al2O3) layer that appears when Al alloys are exposed to the atmosphere, high reflectivity and high heat conductivity (Sánchez-Amaya et al., 2012a, Sánchez-Amaya et al., 2012b). A number of different welding systems are applicable to welding of Al, for example, friction stir welding (FSW), laser beam welding (LBW), metal inert gas (MIG), tungsten inert gas (TIG), submerged arc welding (SAW), plasma arc welding (PAW), and hybrid laser beam welding (HLBW). Early efforts to weld Al (HSA) alloys found poor weldability, but more recent studies (Yeomans, 1990, Graeve and Hirsch, 2010, Dickerson and Irving, 1992) have indicated that this poor weldability was predominantly due to the presence of copper in the alloy. Recently, it has been found that new technologies like pulsed MIG welding, pulsed TIG welding and friction stir welding (FSW) can be more effective than conventional fusion methods. Welding defects commonly found in Al welds include porosity, incomplete fusion and hot cracking (ASM International Handbook Committee, 1993, Cary and Helzer, 2005). Research (Dickerson and Irving, 1992, Volpone and Mueller, 2008) has shown that in comparison to steel, greater care is 16 Introduction required when welding Al, especially control of weld heat input and pre-weld cleaning. In addition, there are limitations on the weld processes that are applicable. The TIG weld process has thus far been the most industrially accepted welding process for Al (Olsen, 2009).

The research approach of this study consists of literature review and experimental work. Experimental study of robotized pulsed MIG welded 7025-T6 and AW-7020 alloy is carried out and presented to provide a better understanding of HSA weld metallurgy. Definitions, properties, applications, weldability and welding defects of Al alloys are presented. Particular attention is given to the issue of Al2O3 and its effects on weldability. The Al2O3 layer and its chemical properties are studied and presented. The formation process and the composition of the two anodic layers are described. Properties like density, melting point and thermal conductivity are described, and the advantages and disadvantages of Al2O3 presented. Its formation can be controlled to gain structural advantages and improved characteristics, for example, by anodisation. Six basic joint types, and eight common weld processes are analysed. Process limitations, and associated welding challenges are studied, and their effects on selection of the optimal welding process considered. A study on HLBW optics applicable to Al, outlining the welding challenges in HLBW, is carried out and presented. New welding technologies for Al welding are studied (because newer technologies are expected to provide faster weld speed, cheaper welding cost and improved welding equipment efficiency).

The study presents no single optimum process for welding Al. However, FSW, pulsed MIG and HLBW produce better welds than TIG (Quintino et al., 2012). FSW is shown to be the presently most favourable process, as it brings important metallurgical advantages, for example, there is no solidification and liquation cracking, unlike with fusion welding (Mathers, 2002). It is found in this study that the grains appear to be reduced in size as the heat input decreases, and welding speed has no significant effect on the hardness across the weld if heat input is kept constant. The hardness of HSA joints is lower in the heat affected zone (HAZ) than in the parent metal.

This study is of value to practitioners and the scientific community as limited studies are currently available about the welding metallurgy of HSA alloys. Recommendations for selection of the optimal welding process for various Al welds can be based on knowledge gained in this study and understanding of the limitations of each weld process. As a contribution to scientific literature, this research provides valuable information that is of interest when seeking to achieve effective welding of Al alloys, specifically HSA alloys.

1.1 Research problem and research questions The motivation for this research can be found in the need for a light and strong material that can be effectively welded. This need is particularly acute in the transportation industry. From this starting point, a chain of questions was formed as follows: 17

1. What lighter material is commonly used in the transportation industry, especially aerospace, which has high strength comparable to mild steel? The answers of Al and Ti led to the next question:

2. Which of these two materials is weldable and which is cheaper? These questions were asked because welding is the most common joining procedure in industrial engineering and it is important that engineering solutions are economically viable, both as regards material costs and manufacturing costs. The welding costs should usually be as low as possible.

3. Since both materials are weldable and Al is considered to be the cheaper material, more specific questions were asked about Al. Thus, the next question was: does Al have other favourable properties for transportation industry uses like good corrosion resistance and high strength-to-weight ratio? Al clearly has these qualities, which led to the next question in the chain.

4. What groups or classes of Al alloys are most advantageous, and can the alloys be modified for structural advantage?

Further questions then are:

5. HSA alloys are favourable from a material properties perspective, but how can they be welded to obtain optimum weld metallurgy?

6. What are the welding challenges that need to be overcome, including the presence of

Al2O3, control of weld parameters and heat treatability?

Based on the above question chain, a niche arose that has been the subject of limited research: welding of HSA. The relative paucity of research on HSA may be due to the alloys being quite new, the cost of the material, and the strict weld requirements in demanding applications such as those found in the aerospace industry.

1.2 Scope and limitations of the study The scope of this study is focused on the weldability of HSA particularly the weld metallurgy. However the scope has the following limitations:

1. The literature review in this study is limited to an overview of Al alloys and their classification, in addition to discussion of their general weldability and applicable joint types. 2. The study on HLBW optics for Al alloys is limited to consideration of the optics, the welding head and challenges in HLBW of Al. 3. The experimental study is limited to the study of HSA weld metallurgy using a pulsed MIG robotic welding machine on 7025-T6 and AW-7020 samples. In addition, the experiment to investigate the effect of the Al2O3 layer on Al welds

18 Introduction

is limited to AW-7020 welds while the layer composition study is limited to 7025-T6, AW-7020 and 99.9% pure Al samples. 1.3 Contribution of the work As a contribution to the body of scientific knowledge, this work provides valuable information on HSA weldability and the effects of post-weld heat treatment, in addition to the effect of Al2O3 on HSA welds. The effects of pre-weld heat treatment and artificial aging are also determined. A further contribution is that the work provides information on the necessity or otherwise of Al2O3 removal before welding, and details under what conditions such Al2O3 removal is unnecessary. The results from the experiments contribute empirical data to Al welding knowledge, for example, hardness and tensile values for 7025-T6 and AW-7020. These data can be used by researchers as background information for further research on HSA weldability, and by welding engineers when designing welded HSA structures.

1.4 Social and environmental impact A key consideration in scientific research is the social and environmental impact of the knowledge gained. In the case of this study, improved understanding of the weldability of Al may generate a greater range of possible applications and thus enhance employment prospects in the welding industry. If HSA is able to replace mild steel in structures like motor vehicles, this in turn means lower deadweight, greater fuel economy, reduced pollution and cost savings for consumers. The ease of recycling Al allows high energy saving in the production process. When utilising welded materials in new application areas such as aerospace, knowledge of the material effects of welding techniques and welding parameters is clearly important, because of safety considerations, and contributes to reduced risk of structural failure. The issue of the necessity of pre-weld cleaning of joints has implications for manufacturing costs, productivity and efficiency, and, consequently, industrial competitivity, as welding costs are reduced. Efficient use of energy, particularly heat sources, reduces the amount of carbon emissions allowing for a cleaner environment.

1.5 Thesis outline This thesis comprises two parts: an overall summary of the research work is followed by reproductions of the published papers. The thesis includes both literature review and experimental work. Chapter 2 presents state-of-the-art information on welding of Al alloys on the basis of a review of the literature. An explanation of the terminology and classifications is presented, and HSA is defined. The chapter also presents key aspects of HLBW of HSA.

Chapter 3 presents the experiments that were carried out. The methodology is described, and experimental conditions and relevant weld parameters are presented. Chapter 4 presents the findings from the experiments. Observations and inferences are discussed in Chapter 5. The conclusions are presented in chapter 6, which also summarizes the study as a whole, and the first part of the thesis concludes with suggestions for further studies, given in chapter 7. 19

2 State of the art of Al welding

Trends in Al welding are discussed in this section, which covers material design, weldability, welding processes and improvements to HSA alloys. For clearer understanding, it is important first to present Al alloy designations and definitions of HSA.

2.1 Alloy designation Al alloys are grouped into cast and wrought alloys and the groups are identified with a four digit number system (e.g.7025-T6). Cast Al alloy designations are like those of wrought Al alloys except with a decimal between the third and fourth digit (e.g. 771.0- T71). The second part of the designation, i.e. the part following the 4-digit code indicating the class, denotes the temper and other fabrication treatments that have been carried out. For example, a T6 indicates that an alloy has been treated thermally and then artificially aged. (Maurice, 1997).

The alloy group is classified by the major alloying element, as shown in Table 1. The second digit denotes the alloy modification or the limits of impurity. ‘0’ in the second digit denotes an original alloy. Numbers 1 - 9 signify the different alloy modifications with slight variation in their compositions. In the 1xxx series, the second number denotes the modifications in impurity limits: ‘0’ implies that the alloy has a natural impurity limit, 1 - 9 imply that special control has been carried out on one or more impurities or alloying elements. The last two numbers represent the purity of the alloy (Campbell, 2006).

In the 1xxx series, the last two numbers signify the alloy’s level of purity. For example, 1070 or 1170 indicates that at least 99.70% Al is present in the alloy, 1050 or 1250 indicates that no less than 99.50% Al is present in the alloy, and 1100 or 1200 implies that at least 99.00% Al is present in the alloy. For all the other series of Al alloys (2xxx - 8xxx) the final two numbers have no special significance but are used to identify the different alloys in the group (Campbell, 2006, Kopeliovich, 2009).

2.2 HSA Al alloys with at least 300MPa yield strength are regarded as HSA. HSA alloys are generally in the 2xxx, 7xxx, and 8xxx series. HSA is not defined based on the series of the alloy. For example, two alloys within the same series can have significantly different yield strengths. For general purposes, however, an average range of the series yield strength is used to identify HSA alloys, as illustrated in Table 1, by the series average values.

20 State of the art of Al welding

Table 1 Wrought Al alloy classification (Kopeliovich, 2009, Matweb, 2010, Campbell, 2006) Alloying Tensile strength, Series average Series Percentages elements Yield range value 1xxx 99.0% minimum 10.0 - 165 MPa 94.4 MPa 2xxx Copper 1.9% - 6.8% 68.9 - 520 MPa 303 MPa 3xxx Manganese 0.3% - 1.5% 41.4 - 285 MPa 163 MPa 4xxx Silicon 3.6% - 13.5% 70.0 - 393 MPa 275 MPa 5xxx Magnesium 0.5% - 5.5% 40.0 - 435 MPa 194 MPa Magnesium and 0.4% - 1.5% 40.0 - 455 MPa 241 MPa 6xxx Silicon 0.2% - 1.7% 40.0 - 455 MPa 241 MPa 7xxx Zinc 1% - 8.2% 80.0 - 725 MPa 399 MPa 8xxx Others 110 - 515 MPa 365 MPa

Major characteristics of the 2xxx series include high strength (at both elevated and room temperatures), heat treatability and high tensile strength range of 68.9-520 MPa (Gilbert Kaufman, 2000, Matweb, 2010); some 2xxx alloys are weldable (Gilbert Kaufman, 2000). The chemical composition of 2xxx series alloys usually includes Cu and some other elements, like Mg, Mn and Si. 2xxx alloys (e.g. 2024 alloy) are used for high strength products such as those typically found in the aerospace industry, where they are expected to meet high engineering standards due to stringent safety requirements. 2xxx alloys are also used in the manufacture of truck bodies (e.g. 2014 alloy); and 2011, 2017, and 2117 alloys are extensively used for screw machine stock and fasteners. Under naturally aged T4 condition, 2xxx series alloys have similar mechanical properties to mild steel, with a proof strength of about 250 MPa and an ultimate tensile strength of around 400 MPa. They also have good ductility. When thermal treatment T6 is used, the proof strength rises to 375 MPa and the ultimate stress can reach 450 MPa. This, in turn, lowers ductility (John, 1999). Tempered alloys are generally painted or cladded to increase their corrosion resistance. They find application in parts such as internal railroad car structural members, tank trucks, structural beams of heavy dump and trailer trucks, booster rockets of space shuttles and fuel tanks (Gilbert Kaufman, 2000).

The 7xxx series comprises Al-Zn alloys. Mg is also present to control the ageing process. The alloy group possesses high strength in the high toughness versions. 7xxx alloys have poor corrosion resistance compared to, for example, the 5xxx series and are thus cladded in many applications. 7xxx alloys are heat treatable and can reach the 220 - 610 MPa ultimate tensile strength range. They are weldable with some welding processes, such as pulsed MIG. Some of the highest strength alloys in the 7xxx series have Cu in the alloy to increase the strength. However, these alloys are not commercially weldable. The weldability reduces as the Cu content increases (Yeomans, 1990, Graeve and Hirsch, 2010, Dickerson and Irving, 1992). In commercial applications, such alloys are commonly joined mechanically, e.g., by riveting. 2.2 HSA 21

Figure 1 Mechanical properties of aluminium alloys (Olabode et al., 2015b) 7xxx alloys are mainly used in components for which fracture resistance is a critical design consideration. A notable example is the Foresmo bridge in northern Norway, where Al-Mg 7xxx alloys are used in the girder system. 7xxx alloys are also found in structures in the aerospace industry, for example, they have been used in critical aircraft wing structures with integrally stiffened Al extrusions. Premium forged aircraft parts are made from 7175-T736 (T74) alloys (Gilbert Kaufman, 2000).

The 8xxx series have Al and other elements such as Fe, Ni, and Li. These elements provide the alloy with a specific property, e.g. Ni and Fe increase the yield strength in the alloy with almost no loss of electrical conductivity. The high strength members of the series mainly consist of alloys with Li and Cu. Li has lower density than Al and it has a relatively high solubility. A reduction in density of about 10% compared to other Al alloys is attainable. The 8xxx alloys have increased stiffness and are age-hardenable. Some of the series alloys are heat treatable. They have high conductivity, high strength (tensile strength range of 110 - 515 MPa (Matweb, 2010)) and high hardness. These alloys are used in the aerospace industry (8090, 8091). The Al-Ni-Fe alloy 8001 is found in nuclear power generation applications where resistance to aqueous corrosion at elevated temperatures and pressures is required. The alloy 8017 is used as electrical conductor (Gilbert Kaufman, 2000).

22 State of the art of Al welding

2.3 Weldability of HSA “Weldability is a measure of how easy it is to make a weld in a particular parent material, without cracks, with adequate mechanical properties for service, and resistance to service degradation. It varies with many factors” (TWI, 2015). Figure 1 is a reproduction from Publication I and Publication IV that presents the fundamental problem addressed by this research. Although the plots are not completely linear, there are correlations that can be made. It appears that with higher yield strength of Al alloys, the weldability, corrosion resistance, toughness, ductility, and fatigue is lower; while modulus of elasticity and density increases. Growing industrial need for Al alloys has resulted in considerable research on how to weld Al alloys, particularly newly- developed alloys. In conjunction with such research, the range of welding technologies and processes for utilization with Al alloys has increased. Based on previous studies, it can be stated that:

1. Within the scope of current manufacturing technology, 94% of Al alloys can be welded and over 50% have optimal weldability (Volpone and Mueller, 2008). 2. Industrially weldable thickness ranges from 0.l - 450 mm (the latter is a special case, attained using a single pass of EBW) (Volpone and Mueller, 2008). 3. High weld rates are attainable with lower thicknesses (0.8 - 3 mm), for example, laser butt-joint weld rates range from 5 to 3 m/min (Volpone and Mueller, 2008). 4. Weld heat input present in most fusion welding causes metallurgical problems. In concentrated energy processes the problems are reduced due to more controlled heat input and smaller HAZ. Few metallurgical problems are observed in FSW (Volpone and Mueller, 2008). 5. Conventional welding processes produce welds where the metallurgical properties of the weld zone deteriorate compared to the base metal. FSW produces welds that have minimal or even zero deterioration (Volpone and Mueller, 2008). 6. With the exception of FSW, fusion welding processes produce welds that have high tendency to suffer from porosity (Volpone and Mueller, 2008).

2.3.1 Joint types and process limitations The joint-type affects the strength, functionality and applicable welding approach. Appropriate joint design is important from both a cost perspective and from the point of view of producing acceptable quality welds. Six common joints and the welding processes associated with their fabrication are considered in this research: butt joints, cruciform joints, T-joints, edge joints, lap joints and corner joints. The joint type, joint location, accessibility, weld processes and strength requirements determine the joint design. A desire for high weld deposition rate of weld metal makes flat or downward weld positions desirable and weld position is a factor that has to be considered when choosing joint designs. In the flat position, the weld pool is usually larger, which allows for a slower cooling and solidification rate. Trapped gases can escape from the weld pool due to the slower cooling rate. The flat position yields good quality welds at low weld cost. In addition, the welds are less prone to porosity and other weld defects, 2.3 Weldability of HSA 23 reducing overall welding costs. The throat thickness controls the static tensile strength of the welds and is designed to be able to carry the workload of the welded structure. A weld depth of 3 mm is attainable with conventional TIG and 6 mm with conventional MIG on plate welds. Bevelling is usually carried out when welding higher thicknesses. The bevelling can be single or double sided (Mathers, 2002).

A comparison table of the six joint types and welding process limitations as regards their use with Al alloys was presented in Table 2, reproduced from Publication II. It was concluded that butt and lap joints are the most applicable joint types, mainly due to the ease of fixing the workpiece, and cruciform joints the most problematical joint type. Not all types of welding are equally applicable to Al alloys, with each welding type having different limitations regarding its usability for Al alloy welding. Limitations for the different welding technologies considered in this work are given below.

Limitations in MIG welding of Al alloys include the limited weldable thickness of 25 mm when using Ar shielding gas and 75 mm with He (Mathers, 2002). The need for proper shielding of the weld metal limits the torch distance to a range of 10 to 19 mm. The demand for effective shielding also limits process flexibility, limiting the applicability of MIG welding in outdoor processes as air drafts can easily disperse the shielding gas. The heat radiation levels and arc intensity of MIG welding pose problems for operators.

A limitation in TIG welding of Al alloys is its shallower weld penetration compared to MIG welding. When gas shielding is used, the economically viable weld thickness limit is 10 mm for Ar shielding and 18 mm for He shielding (DCEN). The difficulty of obtaining adequate penetration in corners and affiliate roots also limits TIG use. Compared to MIG, it is expensive for welding thick sections, has lower filler and base metal tolerance and a lower metal deposition rate.

The PAW process has a plate thickness limitation of 6 mm to 60 mm for multiple pass plasma MIG and 2.5 mm to 60 mm for plasma TIG in a single pass. The equipment is relatively expensive in comparison with conventional TIG. The complexity of the torch architecture demands more maintenance and accurate electrode tip setback in reference to the orifice. Therefore it has limited operational acceptability.

FSW is claimed to be the best weld process for Al due to the minimal deterioration in the weld metallurgy caused by the process. Welds of 1 mm- 50 mm thickness can be achieved in a single pass. Available information on tool design, weld mechanical properties and process parameters is rather limited. Available information on specific alloys and thicknesses is even more limited. In comparison to LBW the productivity of FSW is low as fixing the workpiece requires a lot of heavy duty clamps because high downward forces are needed. The limited knowledge base also affects design guidelines for implementation. 24 State of the art of Al welding

LBW has a limitation of the energy conversion efficiency of the electrical energy to the laser beam, called the plug efficiency. The efficiency of lasers is generally from 10 – 30% but in fibre lasers, it can be up to 40%. LBW has low tolerance to gaps between the workpieces. High volume production is required to justify the high initial capital costs of LBW equipment. In critical applications, the process is sometimes economically viable at low production volumes.

RW is limited to 0.9 mm - 3.2 mm weld plate thicknesses. The required accessibility to both sides of the weld is a limitation, in addition to the low fatigue and tensile strength of RW welds in comparison to other welding processes. A further limitation is the limited number of possible joint designs and seam welds that can produce an unzipping effect. Upset thick sectioned welds are difficult to test for weld imperfection using non- destructive techniques.

EBW is limited by the need for the welding to be done in a chamber. There is a limit to the size of the workpiece that can be placed in the chamber. In addition, there is a time delay in vacuum welds. The weld cost is relatively high and the welds are prone to defects like cracking due to the rapid solidification rate. The x-rays produced in EBW are hazardous.

In UW, expensive high-powered transducers are needed for thick welds, which limit the acceptability of the process. The welding configuration range is also limited. In addition, vibration control strategies are needed to ensure quality welds across various geometries and thicknesses, which can be challenging. Weld process limitations are presented in a tabular form in Table 2, reproduced from Publication II. 2.3 Weldability of HSA 25

Table 2 Joint types and process limitations of aluminium alloys (Olabode et al., 2013) Processes MIG TIG PAW FSW LBW RW EBW UW Joints Butt joint (a)         Lap joint (b)         T-joint (c)         Edge joint (d)         Corner joint (e)         Cruciform (f)         Limitation Limitation Limitation Limitation Limitation Limitation Limitation Limitation

With argon, Limited to thin Limited to 6– 60mm Weldable thickness Limited Limited weld High cost of Expensive high weldable gauges of up to range. ranges from 1–50mm conversion thickness range equipment. powered thickness is 6mm thickness. (single pass). efficiency of (0.9–3.2 mm) transducers are limited to 25 Limited (shallower) Plasma TIG weld electrical power Work chamber needed to enable (a) mm, and with penetration into thicknesses range can Tool design, process to focused Lower tensile and size welding of thick helium, it is parent metal be less than 2.5– parameters, and infrared laser fatigue strength constraints. gauges, castings, limited to 75 compared to MIG. 16mm in a single mechanical properties beam also called compared to other extrusions, and pass. database is limited and fusion welding Time delay mm. wall plug when welding hydro-formed With argon only available for limited efficiency (about processes. components. Limited torch shielding gas, the Limited by the high alloys and thicknesses in a vacuum. (b) capital equipment and 10%–30% and Limited joint distance of 10– economical weld (up to 70 mm). up to 40% in High weld Alternative welding 19mm to thickness limit is material cost designs or configurations are compared to TIG. Limited to lower fibre lasers). configuration. preparation ensure properly 10– 18mm with costs. needed to weld a shielded weld helium (DCEN). productivity cases Limited fit up Seam welds can wide variety of Limited tolerance of compared to LBW. generate metal limits the process to joint tolerance. X-rays component (c) flexibility. Difficult to penetrate Precise fit up unzipping effect. produced geometries and into corners and gaps and Insufficient design misalignment. guidelines and limited (15% of material Limited operator joint configurations. Limited outdoor into the roots of fillet thickness) During welding application welds. education for acceptability of can be a health Vibration control Limited operator implementation. needed for butt the process because air acceptability of the and lap joints. risk. strategies are drafts can Limited by the lower because, in thick- needed to ensure (d) process due to the Exit hole left when tool is disperse the deposition rate, low Limited operator sectioned upset Rapid weld quality across tolerance on filler complex torch withdrawn. welds; there is solidification shielding gas. architecture that acceptability of a wide range of and base metal, Large down forces the process due lack of good non- rates can component Limited and cost for thick requires more destructive weld cause cracking maintenance and required with heavy duty to the large geometries and the operator sections compared clamping necessary to capital quality testing in some thickness of the acceptability of to MIG accurate set-back of high electrode materials. (e) the electrode tip with hold the plates together investment weld piece is the process needed, wear rate and limited. respect to the nozzle during welding. Can weld up to because of the therefore deterioration. orifice, which is 450mm thick relatively high Environmentally friendly requiring high levels of challenging. welding process In addition, it plates. volume requires access to radiated heat because fumes and production or and arc spatters are not both sides of the (f) critical joint. intensity. generated. applications to justify the expenditure.  FUNDAMENTALLY APPLICABLE  APPLICABLE WHEN MANIPULATED  NOT APPLICABLE  NO APPLICABILITY DATA YET 26 State of the art of Al welding

2.3.2 Work preparation Successful welding of HSA is very dependent on rigorous preparation work. The preparation work involves selection of a suitable welding process, correct storage and handling of the workpieces, appropriate workpiece preparation (like grinding off the Al2O3 layer, bevelling etc.) as well as application of acceptable joint design (Yeomans, 1990, Graeve and Hirsch, 2010, Dickerson and Irving, 1992).

The Al2O3 layer found on the surface of Al workpieces occurs because Al has a strong chemical affinity for O2 in air or moist environments. The thickness of the Al2O3 layer is dependent on the physiochemical conditions of the environment, and the thermal treatment or electrochemical treatment (anodisation) (Karambakhsh et al., 2010, ASM International Handbook Committee, 1994).

Figure 2 Schematic of aluminium oxide layer and the anodised surface showing the melting temperatures (Olabode et al., 2013)

In general welding practice, the Al2O3 layer is removed just before welding, by dry machining or pickling. The Al2O3 layer has a melting temperature of 2050°C, considerably above that of Al, which melts at about 660°C (as illustrated in Figure 2 from Publication II)(Mathers, 2002) The melting point difference is not a problem in high heat density welding processes but the significant mechanical strength of the layer can lead to Al2O3 remaining as a solid layer of fractured small particles when the surrounding metal is molten (Riveiro et al., 2010). Removal of the layer is therefore encouraged to avoid the possibility of the Al2O3 layer causing incomplete fusion.

In arc welding processes, the Al2O3 layer is an electrical insulator that inhibits arc initiation. Thick layers of Al2O3 in MIG process generate erratic electrical commutation in the contact tube of the gun thereby producing poor welds. Recommended removal methods are brushing with a stainless steel bristled brush and other mechanical processes like cutting or grinding using an Al2O3 grinding disk (Mathers, 2002). The Al2O3 layer regenerates immediately if scratched. This property is responsible for the corrosion resistance of Al (George and MacKenzie, 2003). Al2O3 is usually in hydrate form and is hygroscopic. In some welding processes, the layer is removed by the weld system. In UW, the vibration enhances removal of the oxide layer along with other contaminants due to the high frequency of the vibration (Baboi and Grewell, 2010). The layer is also removed by HLBW MIG welding systems, although cleaning by a mechanical process is still recommended (Olsen, 2009). Cathode etching is another 2.3 Weldability of HSA 27 27

means by which the Al2O3 layer is removed. In gas shielded arcs, it is chemically corroded thereby showing the microstructure (Novikov, 2003). It is beneficial to remove the Al2O3 layer because (ASM International Handbook Committee, 1993, Mathers, 2002) :

1. It minimises the possibility of hydrogen porosity in the weld 2. It provides better weld stability, especially in TIG welds 3. It ensures complete weld fusion. In TIG welding, chemical etching removing the layer is advantageous since the layer forms almost immediately after mechanical cleaning.

The Al2O3 layer can be formed and enhanced through a process called anodisation. Anodisation is an electrochemical process that converts the surface of metals into a durable, decorative and corrosion resistant anodic oxide finish (Thompson, 1999, Mukherjee, 2010). An even porous morphology of amorphous alumina (Keller et al., 1953) is formed in acidic and alkaline electrolytes. In the process, the layer is fully integrated into the Al substrate unlike plating or paint, so it does not chip off or peel. The layer can be processed by painting or sealing due to the ordered and porous structure (Thompson, 1999).

Anodisation is done to increase corrosion resistance, provide better appearance, increase resistance to fading, improve the bonding of adhesives, provide for paint adhesion, increase application abrasion resistance, increase lubricity, provide electrical insulation, permit investigation of surface flaws, provide for further plating, and to provide for the possibility of lithographic and photographic emulsion (Thompson, 1999, Mert et al., 2011, ASM International Handbook Committee, 1994). The structure of Al2O3 is double layered after anodization (the anodised layer and the oxide layer as in Figure 3 from Publication III) which makes welding challenging (Thompson, 1999).

Figure 3 Schematic diagram of a cross-section of a porous anodic film on aluminium showing the barrier, pore and other principal morphological features (Olabode et al., 2015b) 28 State of the art of Al welding

Shielding gas is generally needed to protect the weld from atmospheric interactions. Heated metal has an affinity to react with the atmosphere around its melting point. Al easily reacts with O2 at room temperatures. When selecting the shielding gas, the criteria that should be met are (Welding Journal, 2008, Boughton and Matani, 1967, Olson et al., 1993, Mathers, 2002):

1. The shielding gas must permit plasma arc generation and promote stable arc characteristics and mechanisms. 2. It should not degrade welding parameters like weld speed. 3. It should minimise the need for post weld cleaning. 4. It should protect the molten pool, the wire tip and welding head from oxidation. 5. It should enhance welds by providing better weld penetration and good bead appearance. 6. It should help to prevent undercuts. 7. It should not compromise the mechanical properties of the weld. 8. It should allow for smooth detachment of molten metal from the filler wire in addition to the provision of the required metal transfer mode.

Inert gases like He and Ar are common shielding gases used in Al welding. It is important to understand the characteristics of the gases to enhance the shielding gas selection process. Ar is a cheaper gas than He and it generates a stable weld arc, producing smooth welds. On the other hand, it is more prone to producing welds with porosity and lack of fusion due to the lower generated heat input and lower wire weld speeds. Black sooty deposits can be left on the weld surfaces, which need to be cleaned by brushing. He shielding gas produces hotter arcs, wider weld beads, and deeper penetration due to its 20% greater arc voltage compared to Ar. Therefore, arc positioning is less critical than when using He. Due to the slower cooling rate from the hotter weld pool, hydrogen diffuses better from the weld, thereby reducing porosity. He permits the attainment of weld speeds up to three times higher than when using Ar shielding gas. However, because of its cost and the degree of arc instability, He is mainly used in mechanised and automatic welding.

Ar is the recommended shielding gas for welding of HSA using a pulsed MIG process (Boughton and Matani, 1967, Yeomans, 1990). However Ar-He mixtures can also be used or He alone. Ar-He mixtures provide a compromise on the characteristics of both gases and enable improved weld productivity with higher weld speeds and more acceptable welds. A gas purity of over 99.998% at the weld torch is recommended and the moisture level must be kept low (-50°C less than 39 ppm H2O) (Mathers, 2002). Studies show that the shielding gas increases weld penetration by providing higher arc energy and metal deposition rate (Blewett, 1991, Yeomans, 1990). When the section is lower than 50 mm, He should be used (Mathers, 2002). Further details are presented in Table 3 and the effects of the shielding gases are presented in tabular form in Table 4.

2.3 Weldability of HSA 29 29

Table 3 MIG shielding gases for Al (Welding Journal, 2008) Metal transfer mode Shielding gas Characteristics Good cleaning action, less spatter and 100% Ar has the best metal transfer In addition to having good arc stability. In comparison to Ar it generates higher heat input with an improved Spray transfer 35% Ar - 65% He fusion characteristics for weld with minimal porosity This mixture requires the least 25% Ar - 75% He cleaning action, minimises porosity and produces the highest heat input. This mixture is fairly adequate for Short circuiting Ar and Ar + He sheet metal however Ar-He is preferred on thicker weld sections.

Table 4 Effect of shielding gas on Al welding (Kang et al., 2009, Hilton and Norrish, 1988, Matz and Wilhelm, 2011, Mathers, 2002, Campana et al., 2009, Campbell et al., 2012, Kah and Martikainen)

Relative effect (100% Ar as the reference) Shielding gas 100%Ar Ar+He 100% He

Gas flow Minimum Higher Highest

Arc voltage (MIG) Minimum Higher Highest

Arc (MIG) Minimum stability More unstable Most unstable

Weld seam width and Minimum width and Higher width Highest width depth standard depth Shorter depth Shortest depth

Weld seam appearance Minimum smoothness Smoother Smoothest

Minimum depth and Deeper and more Deepest and most Penetration roundness round round

Minimum welding Welding speed Higher attainability Highest attainability speed

Lack of fusion Standard Lower Lowest

Porosity Standard Lower Lowest

Pre-heating Standard Less needed Least needed

Heat production Minimum warmth Warmer workpiece Warmest workpiece

Cost of shielding gas Minimum price More expensive Most expensive

30 State of the art of Al welding

2.3.3 Welding defects in HSA The welding of Al requires strict control of heat input despite the fact that it has a lower melting point compared to steel. The welding of Al is critical because of the following considerations (Campbell, 2006, Olson et al., 1993):

1. Stable surface oxide needs to be eliminated before welding. 2. The presence of residual stresses can cause weld cracks as a result of the high thermal expansion coefficient of Al. 3. The high heat conductivity of Al means that more heat is required to attain welds; however, high heat input increases the possibility of distortion and cracking. 4. High shrinkage rates on solidification increase the incidence of cracking. 5. The high solubility of hydrogen in molten Al can cause porosity. 6. The general susceptibility of HSA series to weld cracking. Major welding defects in HSA series alloys include hot cracking, porosity, joint softening, non-recoverable post-weld ageing, poor weld zone ductility (HAZ degradation) and susceptibility of the joint to stress corrosion cracking. Further details on weld defects and remedies are presented in Table 5.

Table 5 Trouble Shooting Aluminium Welds (Renshaw, 2004, Ba Ruizhang, 2004, Olson et al., 1993, John, 1999, Joseph, 1993)

Defect Cause Remedy Oxide Poorly cleaned joints Ensure proper wire brushing of joints inclusions before and after each weld pass. The particles should be wiped off the surface.

The presence of an Al2O3 When possible, use fresh wire spool. layer on the filler rods of Clean wires and rods to remove oxides. the weld metal The presence of sharp Break sharp edges during weld corners on the joint's preparation. groove Weld porosity Insufficient weld shielding Eradicate draughts. Increase the amount of gas flow. Reduce the electrode extension.

Presence of dye penetrants Use solvent to clean the surfaces of the and lubricants workpiece. All lubricants should be removed from the weld area. High welding current Reduce the current according to the recommended welding procedure.

Impure or contaminated Inspect the gas hoses to ensure there shielding gas are no leakages, and make sure that there is no coolant leak on the torch. 2.3 Weldability of HSA 31 31

Replace the gas cylinders if possible.

Wrong torch angle or too Apply the correct torch angle and travel high travel speed speed based on the welding procedure.

Contaminated filler material Ensure that filler material is cleaned with solvent.

Moisture Before welding, heat and clean the surface of the workpiece.

Fusion zone Presence of hydrogen in Use pure He shielding gas, reduce porosity the base material sodium additives and amend degassing practices.

Cold cracking Presence of too rigid joint Preheat the workpiece and slacken the restraints clamps to reduce stress.

Hot cracking Excessive parent metal Reduce the weld current and increase dilution the filler wire deposition rate.

Too high interpass Reduce the weld current and introduce temperature pauses between each weld pass to increase cooling.

Undercutting Too high welding current Use the appropriate current.

Too high travel speed and Reduce the speed based on welding inadequate filler material procedure recommendations and use recommended filler material.

Too long arc length Use arc lengths based on recommendations.

Lack of Low welding current Use appropriate welding current based fusion on welding procedure recommendations.

High travel speed Reduce travel speed according to weld procedure recommendations.

Inadequate joint Improve joint preparation. preparation

Incorrect torch angle Use the correct torch angle based on the welding procedure.

Crater Inappropriate arc breaking Gradually reduce arc current, use ‘crater cracking fill’ control. ‘Back weld’ at least the last 25 mm of the bead.

Overlap Slow travel speed Increase travel speed according to welding procedure recommendations.

Insufficient weld current Increase the welding current. 32 State of the art of Al welding

Excessive filler material Reduce filler material.

Incorrect torch angle Use the correct torch angle based on the welding procedure.

Drop through Slow travel speed Increase travel speed.

Welding current too high Decrease the welding current.

Wide joint gap Reduce the joint gap and improve fit-up.

Too much heat build-up Reduce the interpass temperature.

2.4 Hybrid laser beam welding (HLBW) of HSA HLBW is a welding process that combines LBW with an arc welding process thereby utilizing the advantages of both processes. It has better weld bridgability, and higher weld speed and quality than LBW. HLBW beam delivery and focusing optics are presented in addition to the welding head configuration and the challenges of HLBW of Al.

The most commonly used HLBW system is laser hybrid MIG (Olsen, 2009). Absorption of the beam by Al depends on the wavelength of the laser beam. As presented in Figure 4 from Publication IV, due to the wave length of solid-state lasers, Nd: YAG and fibre lasers are the most common laser power sources used in hybrid MIG welding. Optics found in HLBW systems includes mirrors, lenses and fibre optics. In HLBW, the laser beam needs to be focused to achieve small spot diameter. The small spot diameter allows for higher beam density on the workpiece. The spot diameter is a function of the lens design and focal length. Beam transfer and focusing is achievable using diffractive optics, refractive optics or reflective optics.

Beam delivery optics before focusing utilize mirrors (for diffracting light). Mirrors can be planar or spherical in design. The mirror is firmly fixed to an adjustable screw with ease of accessibility for cleaning, inspection and replacement. The usability of conventional mirror delivery is limited by the rigidity of the mechanical mounting and the mirrors cannot move relatively to each other to avoid misalignment. Mirrors are limited in size therefore transferring beams over a long distance with high divergence can produce a beam diameter that is larger than the lens. Consequently there is a limit to the distance over which beams can be transferred via mirrors

The lens is a component in beam delivery and is used for converging or diverging light. The lens can be a simple one-element optic, generally with a focal length less than 254 mm, and can be an aspheric, plano-concave/convex or meniscus lens (Ready et al., 2001, American Welding Society et al., 2006). Compound optics can be used, where the lens is made of two or more separate lenses. 2.4 Hybrid laser beam welding (HLBW) of HSA 33 33

Fibre optics are another component in beam delivery and are used in Nd: YAG laser systems to deliver beams. Fibre optics are used due to the 1.06μm wavelength transferable over glass fibres. Fibre optics utilize the flexibility of glass fibre within the specified bend radii for a fibre bundle. They are attractive in comparison to conventional beam delivery due to the possibility of transporting beams over long distances of up to 50m and around curves (Bakken, 2001).

Figure 4 Absorption of laser wavelength by metals (Olabode et al., 2015b)

Focusing optics are common in low-power welding devices. Parabolic lenses are generally useful for focusing power above 1.5 kW in CO2 lasers. Due to the low cost and minimal spherical aberration attributes of f-numbers above five, lenses in focusing optics are usually plano-convex lenses. The f-number is derived by dividing the lens focal length by the beam diameter.

Laser protection lens is a sacrificial cheaper lens placed to prevent the debris having contact with the welding head lens. Protection lens is usually cheaper and easier to remove and replace compared to the welding head lens. It’s usually used in laser processes where the focal length is short or when the weld metal is volatile and contaminated; or when weld spatter can be generated and debris can attach itself to the welding head lens. Al is highly reflective to the laser beam wavelength, and reflected beams can damage laser optics

2.4.1 HLBW focusing head The performance of the beam delivery system determines the quality of laser beam welding. The beam delivery should be as simple and as small as possible, having neither

34 State of the art of Al welding actuators nor sensors, to allow for easy manipulation and integration with a robotic welding system. However, available technologies for laser welding heads have numerous advantages so consumers still tend to buy the technologies and thus the laser heads are becoming more and more complex. Common technologies in laser focusing heads include integrated actuators and sensors, closed loop systems, self-learning and self-adapting systems.

Combinations of laser beam and arc can take a number of different configurations that remarkably influence the weld performance. The principal classification criteria for laser beam and arc combinations are presented in Figure 5 from Publication IV (based on the heat source type), and Figure 6 from Publication IV (based on configuration). The choice of the secondary heat source can be either arcs with consumable electrodes or arcs with non-consumable electrodes. The former are selected when filler metal is required to solve specific weld problems, otherwise the latter are preferred. The arrangement plays an important role in the effectiveness and weld efficiency of the system and the quality of the welds. The heat sources can be arranged to have a common operation point (Figure 7 from Publication IV) or separate operation points (Figure 8 from Publication IV).

Heat sources for hybrid laser–arc welding

Primary heat sources Secondary heat sources

Arcs with consumable Arcs with non- electrodes consumable electrodes

Figure 5 Schematic of heat sources available for hybrid laser–arc combinations (Olabode et al., 2015b).

Geometrical arrangements for hybrid laser–arc welding

Common operation point Separate operation points

Parallel technique Serial technique

Figure 6 Geometrical arrangements for hybrid laser–arc welding (Olabode et al., 2015b). 2.4 Hybrid laser beam welding (HLBW) of HSA 35 35

(a) (b) Figure 7 Schematic diagrams of hybrid laser–arc welding with a common operation point replotted from (Olabode et al., 2015b)

Figure 8 Schematic diagram of hybrid laser–arc welding with separate operation points replotted from (Olabode et al., 2015b)

Separated operation point arrangements are defined to be serial technique or parallel technique or a combination of both. The serial technique is a configuration in which the primary and secondary heat sources have an acting point distance between them, known as the “working distance,” in a vertical or horizontal direction along the welding path. The arc source can lead or trail the laser beam.

36 State of the art of Al welding

Other HLBW configurations with more than two heat sources have been studied and are presented in Figure 9 from Publication IV.

Figure 9 Schematic diagrams of hybrid laser–arc processes with two secondary heat sources (Olabode et al., 2015b).

2.4.2 Challenges of HLBW of Al Al alloys present challenges for HLBW optics. One of the challenges limiting utilization of such welding systems and optics is the high reflectivity of Al alloys, which limits the choice of laser beam source to Nd: YAG and fibre lasers. Secondly, the melt zone (MZ), and HAZ are larger in HLBW than in laser welding. The molten zone at the weld top is wider due to the presence of arc welding process (Page et al., 2002) and the large HAZ compromises the metallurgical properties of the weld. Thirdly, due to the wider weld pool and higher melt temperature in HLBW, difficulties arise in covering the weld pool with shielding gas, which can lead to contamination of the weld and porosity (Rasmussen and Dubourg, 2005). Fourthly, volatile elements in alloys can evaporate from the normally generated keyhole, resulting in poorer metallurgical properties of the weld and even porosity if gas bubbles are trapped in the weld. This problem can be mitigated by proper selection of filler material (Duley, 1999). In addition, volatile elements present in Al alloys can generate spatter during welding, which can adhere to the lens and damage it. A precaution is to use a protective lens. Fifthly, Al alloys have low surface tension, and they have poor ability for root-side melt pool support. This tends to cause difficulty in full penetration welding, specifically in thick butt welds (Andersen and Jensen, 2001). Finally, the existence of a large number of non- independent and interacting welding parameters compared to MIG or laser welding processes poses control challenges, in addition to the metallurgical challenges mentioned earlier. Therefore, HLBW of Al alloys is complicated to design and operate (Sepold et al., 2003). Rasmussen et al. (2005) show that successful welding of Al using HLBW demands clear understanding of the governing parameters and their effects and interactions (Rasmussen and Dubourg, 2005) to be able to maximise the advantage of HLBW as a robust industrial welding process (Sepold et al., 2003). 37

3 Experimental work

Three sets of experiments were carried out in this study. One focused on a study of the microstructure of a welded HSA. Another experimental focus was a study of the presence and composition of Al2O3 on Al alloys. The last experiment focused on analysis of the effect of the presence of Al2O3 on the mechanical properties of welded HSA alloys.

3.1 Welding metallurgy of HSA (7025-T6) Experimental work was carried out on Al alloy 7025-T6 using a robotised pulsed MIG welding machine. The setup is presented in Figure 10. The robot movement was programmed and some test sample welds were made, after which alloy 7025- T6 was welded. Different welds trials were made and the weld parameters were varied to study the effect of heat input and welding speed on the properties of the weld metal. A torch angle of 10° pushing in the weld direction was used to allow for purging of the weld area ahead of the arc. A 2 mm wire extension was used and the nozzle-to-workpiece distance (stick-out length) was 18 mm. The shielding gas used was 99.995 % Ar and the filler wire was 4043 Al.

Figure 10 7025-T6 weld setup Figure 11 AW-7020 weld setup The workpiece was a 5 mm thick plate with an area of 100 × 250 mm. The samples were bead-on-plate welds, so there was no bevelling. The joints were cleaned mechanically using a stainless steel bristle brush reserved for Al only. Experimental trials were performed, from which 6 different sample sets of 7025-T6 alloy were selected. The first three sample sets (A, B and C) had the same wire feed rate so as to study the effect of the welding speed (10, 20 and 30 mm/s respectively). The other three sample sets (D, E and F) had approximately the same heat input to investigate the effect of constant heat input on the weld. The pulse current frequency was approximately 250 Hz in each weld. For sample sets A - C, the wire feed rate was constant (10 m/min) and the heat input varied. Heat input Q for all samples was calculated as (Hirata, 2003): 38 Experimental work

푉 × 퐼 × 60 푄 = × 0.8 1000 × 푆 Where Q denotes heat input measured in kJ/mm; V is the voltage measured in volts; I denotes current measured in Amperes; and S is the welding speed measured in mm/min; 0.8 is the pulsed MIG process efficiency. The heat input for samples D - F was approximately constant and the feed rates were 10, 12 and 14 m/min, respectively. The weld parameters are presented Table 6. The filler wire used had a tensile strength of 165 MPa, yield strength of 55 MPa and an elongation of 18%. Shielding gas supplied through the weld torch protected the weld pool.

Table 6 Weld conditions for 7025-T6 experiment Welding conditions for 7025-T6 welding Weld type Bead-on-plate Base material 7025-T6, thickness 5 mm Filler material 4043 Al, 2 mm wire extension fro torch Shielding gas 99.995 % Ar Welding speed 7.5 mm/s

Nozzle distance 18 mm

Torch angle 10° to normal

Experiment specific parameter

Sample Weld speed Feed rate in Heat input Q Voltage (V) Current (A) in mm/min m/ min (kJ/mm) A 600 10 0.318 20.1 198 B 1500 10 0.127 19.4 205 C 1800 10 0.106 19.4 205 D 1200 10 0.160 19.8 202 E 1440 12 0.163 20.3 241 F 1728 14 0.158 20.5 278

The weld hardness test was carried out using HV3 scale on a hardness indenter machine. Indentations were made on the surface of the test piece by diamond indenters. The indenter was pyramid shaped (Figure 12) and a weight between 1 to 100 kg can be subjected on the indenter. In this research the indenter places pressure on the workpiece for about 10 s. The test was carried out with a 3 kg weight indentation of the diamond tool tip on the prepared weld cross-section. 3 kg was sufficient because Al is relatively soft and 3 kg is heavy enough to create indents. It is important that the weight is appropriate so that the material can resist the load to some extent. The indentations were done at about 1 mm from the weld surface in a row. The distance between each indentation was 0.7 mm. The shape of the indentation resembled a rhombus. The depth 3.2 Investigation of the Al2O3 layer in Al alloys 39 39 of the indents depended on the material’s hardness. Note that the longer the length of the diagonals appearing on the workpiece, the softer the material. The indenter footprint was measured with the aid of a microscope and the averages calculated. The averaged values were looked up from an HV3 table to determine the hardness values. The hardness values were then plotted on a graph against the distance of each indentation from the weld centreline.

Figure 12 Schematic of hardness testing indenter

3.2 Investigation of the Al2O3 layer in Al alloys Experiments were carried out to study the composition of the Al2O3 layer at different distances from the alloy surfaces. 99% pure Al alloy (1xxx series), and AW-7020 and 7025-T6 samples were pre-cleaned, and then exposed to the atmosphere for one hour. The samples were then tested for the presence of Al2O3 by placing them in an X-ray detector (Ultra Dry EDS). Each sample was tested at a depth of 0.2µm, 1.2µm, and 3.3µm. For each depth, four measurement spots of 0.2 by 0.5 mm were selected and the significant chemical contents analysed.

3.3 Effect of Al2O3 layer on HSA (AW-7020) weld metallurgy The purpose of this experiment was to study the effect of the presence of the Al2O3 layer on the mechanical properties of AW-7020. The experiment was carried out as butt welds of 2 samples each for the four weld experiment conditions (EC) 1 - 4 with the weld parameters presented in Table 7. A robotized pulsed MIG machine was used to weld the specimens and the weld setup is presented in Figure 11. A 5 mm AW-7020 plate was used as the workpiece. The air gap between the workpiece was 3 mm. Copper backing was used. Pure Ar (99.5%) shielding gas supplied at a flow rate of 15 l/min was used. A 1.2 mm diameter Elga AlMg5 filler material was supplied at 9 m/min. A nozzle distance of 15 mm and a welding speed of 7.5 mm/s were used. The weld torch was inclined at 15o to normal and the weld direction was such that the torch is pulling. An average voltage of 22.7 V and an average current of 140 were used in all the experiments.

40 Experimental work

Table 7 AW-7020 weld experiment parameters Welding conditions for AW-7020 welding Weld type Butt welding, I-groove, air gap 3 mm, against copper backing Base AW-7020, thickness 5 mm material Filler material Elga AlMg5, Ø1.2 mm Shielding gas Ar, flow rate 15 l/min Wire feed 9 m/min rate Welding 7.5 mm/s speed Nozzle 15 mm distance Torch angle 15° to normal Experiment specific parameter Experiment Voltage Current (A) Al O Pre- conditions (V) 2 3 Artificial ageing Averages thin film heating (EC) Averages 1 140 22.6 Present No No Yes 2 139 22.8 Present o No (130 C)

Yes (4800C/2 h + 3 140 22.7 Present No quenching in water, 900C/8 h + 1450C/15 h)

4 140 22.7 Absent No No

The test was carried out in a welding workshop in a controlled atmosphere and at room temperature. The samples for the four different EC were cut, welded, and examined. In EC 1, the weld was carried out without pre-weld cleaning of the Al2O3 in addition to the absence of weld heat treatment. In EC 2, the weld was conducted without removal of o Al2O3. However, the workpiece was preheated at a temperature of 130 C, which is within the recommended preheating temperature but close to the upper limit [30]. The oxide layer in EC 3 was not removed before the welding. No preheating was carried out but natural aging was conducted by post-weld heating at 480oC for 2 hours, followed by quenching in water at 90oC for 8 hours and, finally, reheating and maintaining the o workpiece heat at 145 C for 15 hours. The Al2O3 layer in EC 4 was removed and no preheating or artificial aging was carried out. In order to investigate the effect of Al2O3 on the mechanical properties, the samples were examined for ultimate yield strength, tensile strength, elongation, and hardness values. Macrographs were taken to evaluate the weld defects present, if any. 41

4 Results

This section presents the key findings of the research. For clarity in the result presentation, it is important to understand fusion weld region nomenclature. Figure 13 presents the weld regions comprising of the composite zone (CZ), transition zone (TZ), unmixed zone (UMZ), weld interface (WI) partially melted zone (PMZ), fusion boundary, “true” heat –affected zone (T-HAZ), and base metal (BM).

The CZ is a fusion weld region where the filler metal is mixed with the base metal as composite. The TZ is a region between the UMZ and CZ where there is compositional gradient from the BM to the CZ. The UMZ is the region where the melted and resolidified base metal is unmixed with the filler metal. The WI is an imaginary line that divides the UMZ and PMZ. The PMZ is a fusion weld region just before the T-HAZ where there is incomplete melting of the base metal. The fusion boundary is the region that consists of UMZ, WI, and PMZ. The T- HAZ is a region where there is no melting or liquation and the metallurgical interactions occur is the solid state. The PMZ and T- HAZ are considered as the HAZ. The BM is the region of the base metal unaffected by the weld heat input (Lippold, 2014).

(a) (b) Figure 13 Fusion weld zone nomenclature: (a) schematic diagram of fusion weld zone, (b) micrograph of 7025-T6 fusion weld It is sometimes challenging to distinct amongst the regions in micrographs, in this thesis TZ and UMZ have been combined as a single region and denoted as TZ-UMZ; and HAZ has been used to denote a combined region of PMZ and T-HAZ. The results of the experiments are analysed and presented in sections according to the three different experiments carried out. The findings presented are those relevant to the scope of this study. 42 Results

4.1 Findings on the welding metallurgy of HSA (7025-T6) The results of the experiment include the microscopic, macroscopic and hardness tests of 7025-T6 Al alloys. The results of hardness tests of samples A – C are presented in Figure 14 from Publication I. The plots for sample A, B and C are presented on the same graph providing for easier comparison. WI in the graph’s label represents the weld interface. Each indentation is represented by a point on the graph's curve. The weld regions are also displayed as CZ, TZ-UMZ, HAZ and base material (BM). Sample C has the lowest heat input of 0.106kJ/mm, sample B has a heat input of 0.127kJ/mm and sample A has the highest heat input of the three with 0.318kJ/mm. Sample B has about 120% heat input compared to C. In addition, sample A has 300 % more heat input than C. The feed rate is constant (Table 6). The observations from the hardness graph of Figure 14 are presented in Table 8.

Figure 14 Hardness testing of samples A, B and C with varying heat input. A 0.318kJ/mm, B 0.127kJ/mm and C 0.106kJ/mm (Olabode et al., 2012).

Table 8 Observations from hardness testing of samples A, B and C with varying heat input. Across CZ TZ-UMZ HAZ WI profile Highest C C C C C hardness Lowest hardness A A A A A Most uniform A C C C A and B

Shortest HAZ C Longest HAZ A Shortest WI from weld centre Longest WI from weld C A line centre line

4.1 Findings on the welding metallurgy of HSA (7025-T6) 43 43

Figure 15 Hardness testing of samples D, E and F with relatively constant heat input of about 0.16kJ/mm (Olabode et al., 2012). The hardness test results of samples D, E and F are presented as a combined graph in Figure 15 from Publication I. The heat input is relatively constant (Table 5). The labelling and description of the graph is the same as for samples A – C (Figure 14). The observations from the hardness graph of Figure 15 are presented in Table 9

Table 9 Observations from hardness testing of samples D, E and F with relatively constant heat input. Across CZ TZ-UMZ HAZ WI profile Highest E E E E F hardness Lowest hardness F F F F E Most uniform D D all D D

Shortest HAZ E Longest HAZ D Shortest WI from weld centre Longest WI from weld E F line centre line

The macrographs of the weld samples are presented in Table 10, where the weld penetration, weld width and WI can be seen for each sample. The macrograph shows the interactions across the weld at room temperature, providing an understanding of the interactions during the weld.

The micrographs of sample A - F are also presented in Table 10. The image of each sample shows the microstructure of the CZ, TZ-UMZ, HAZ and BM at 8x magnification. The grain transformation and transition in the WI is of paramount significance because with even grain transition, mechanical properties like hardness will also be uniform across the weld. The grain transformation can be seen in the micrographs.

44 Results Table 10 Weld appearance, macro-picture and micro-picture of 7025-T6 Al alloy. Fr is feed rate (m/min), Ws is welding speed (mm/s), Q is heat input (kJ/mm), V is voltage (V) and I is current (A).

Micro-picture Weld Weld appearance Macro-pictures CZ TZ-UMZ HAZ BM Data Fr:10 HAZ Ws:10 A Q:0.318 V:20.1 I:198

HAZ Fr:10 Ws:25 B Q:0.127 V:19.4 I:205

HAZ Fr:10 Ws:30 C Q:0.106 V:19.4 I:205 Fr:10 HAZ Ws:20 D Q:0.16 V:19.8 I:202

HAZ Fr:12 Ws:24 E Q:0.163 V:20.30 I:241

HAZ Fr:14 Ws:28.8 F Q:0.158 V:20.50 I:278 4.2 Findings on the Al2O3 layer of Al alloys 45 45

4.2 Findings on the Al2O3 layer of Al alloys Energy-dispersive x-ray spectroscopy (EDS) results are presented in Table 11. The measurement acceleration voltages 3 kV, 10 kV, and 20 kV represent the calculated depths of 0.2 µm, 1.2 µm, and 3.3 µm. In all the samples at 0.2 µm depth the presence of O2 is highest and lowest at 3.3 µm. The values for O2 content and other elements in AW-7020 and 7025-T6 are relatively close. This may be because they belong to the same alloy classification series; classification is based on the chemical composition of the alloy.

Table 11 Percentage weight composition weld samples Oxide layer formation period 1 hour (after cleaning) Measuring spot 0.5 x 0.2 mm Correction method Proza (Phi-Rho-Z) Take off angle 35.0 degrees

Measurement 3kV (0.2 µm) 10kV (1.2 µm) 20kV (3.3 µm) acceleration voltages

Al (Wt. Mg (Wt. Zn (Wt. Test depth from surface Material O (Wt. %) %) %) %) Al 99.90% 12.7 87.3 0.2 µm AW-7020 6.55 87.05 1.05 5.4 7025-T6 6.25 87.3 1.1 5.35

Al 99.90% 4.1 95.9 1.2µm AW-7020 1.525 92 1.2 5.3 7025-T6 1.55 91.95 1.175 5.325

Al 99.90% 2.75 97.3 3.3 µm AW-7020 1.2 93.1 1.2 4.6 7025-T6 1.075 93.35 1.15 4.4

4.3 Findings on the effect of Al2O3 on HSA (AW-7020) weld metallurgy The hardness test profile graph (Figure 16 from Publication III) presents the hardness values of the profile across the WI. The hardness profile shows how much the hardness in the CZ deviated from the BM and vice versa. The y-axis represents the hardness value (HV3) while the x-axis represents the distance in mm from a common reference in the BM to the weld centre. It is important to mention that 0 in the x-axis is located in the BM and the scale increases towards the CZ.

46 Results

The average hardness values are denoted by the nodes on the line graph. For good welds, the hardness from the BM to the weld centre line should have minimal fluctuation. The WI denotes the point at which the weld fusion line appears. It can be seen that the greatest hardness fluctuation is between the HAZ and the WI, which are usually the areas more prone to structural failure. EC 3 has the best hardness profile of the four ECs while EC 2 has the worst hardness profile, especially across the WI. The observations from the hardness graph (Figure 16) are presented in Table 12.

Figure 16 Hardness profile of welded AW-7020 (Olabode et al., 2015a)

Table 12 Observations from hardness testing of AW-7020 weld samples. Across CZ TZ-UMZ HAZ WI profile Highest EC 3 EC 3 EC 3 EC 3 and 4 EC 3 hardness Lowest EC 2 EC 1 EC 2 EC 4 EC 4 hardness Most EC 3 EC 1 EC 4 EC 2 EC 3 uniform

Shortest WI from weld centre line EC 3 Longest WI from weld centre line EC 2

Macrograph analysis of samples for each EC is presented in Table 13 from Publication III. A 10x objective lens was used and the interaction between the weld pool and the BM across the WI is presented. These images can show macro sized defects like porosity and cracks, if there are any. In addition, they also show the HAZ and the location of the WI. The macrograph samples also present the bead profile. It is important to mention that the indentations on the macrographs are made by the hardness-testing machine and the position of the indents from the plate surface is approximately the same for all the experiments. For all four EC it appears that there are 4.3 Findings on the effect of Al2O3 on HSA (AW-7020) weld metallurgy 47 47 neither cracks nor porosity on the macrographs, which suggests that the welds are acceptable.

Figure 17 from Publication III presents a comparison of the yield strength (YS) in Re/N/mm2, ultimate tensile strength (UTS) in Rm/N/mm2, and elongation at fraction in A/%. The y-axis is measured in units and the x-axis represents the averages of the four different EC and the control condition.

The tensile test measures the YS, which is the stress value at which the welded specimen begins to deform plastically and cannot return to its original position. It is used in this experiment to express the load bearing capacity of the weld before plastic deformation. Welds with higher yield strength, are more desirable. Based on the YS values, EC 1 is the best weld while EC 4 is the worst weld, which seems to be due to the removal of the Al2O3 layer from the latter, thereby increasing the amount of weld heat input. As seen in EC 2, preheating also seems to reduce the YS, while artificial aging in EC 3 appears to improve the YS.

Table 13 Macrographs of welded AW-7020 (Olabode et al., 2015a)

Experiment condition 1 Experiment condition 2

Experiment condition 3 Experiment condition 4

48 Results

UTS is used to present the maximum tensile loading the weld can be subjected to before failure. The higher the UTS, the better the weld is from the load bearing perspective. In these experiments, EC 3 produced the highest UTS value of 273 .55 Rm/ N/ mm2. This seems to be due to the effect of artificial aging. EC 1 has the next highest UTS value, probably due to the low heat input to the workpiece as a result of the presence of the Al2O3 layer. EC 2 has the next highest value, which suggests that workpiece preheating reduces UTS values. The lowest UTS value is in EC 4, which suggests that removal of the Al2O3 layer reduced the UTS

Figure 17 Tensile strength of welded AW-7020 (Olabode et al., 2015a) The elongation at fracture of AW-7020 welds expresses the proportional reduction of the cross sectional area of a tensile test piece at the plane of fracture, measured after fracture. It is expressed as a percentage reduction of area and it shows how brittle or ductile the weld specimen is. If the elongation is low, the weld piece will be brittle and therefore it can easily crack or break, for example, brittle ceramics have low elongation values and crack easily when subjected to tensile loading. On the other hand, if the elongation at fracture value is high, the specimen is ductile and can be plastically deformed. In many Al welds, it is desirable to have high elongation values. The best weld is usually case specific, based on the mechanical or metallurgical properties demanded by the application. For example, Al welds that are designed to carry torsion loads like shafts are supposed to be rigid with minimal elongation. On the other hand, structural Al beams are expected to have elongation so they do not break easily. EC 3 has the highest elongation values, which suggests that artificial aging increased the malleability of the workpiece. The next highest elongation value is in EC 4, which suggests that the absence of Al2O3 increases elongation in comparison to EC 1. 49

5 Discussion

The results show some interesting findings. 7025-T6 and AW-7020 alloys are weldable using pulsed MIG process with little or no weld defects. Acceptable welds were achieved with or without the removal of Al2O3 layer. This implies that with high energy density welding processes, the removal of Al2O3 before welding is not necessary. The Al2O3 layer composition varied in the tested alloys, suggesting that the base metal composition influences the Al2O3 composition. The discussions are presented in sections in the order in which the experiments and results were presented.

5.1 Welding metallurgy of HSA (7025-T6) Comparing samples A, B and C, it appears that with low heat input the grain sizes around the WI are small. As the heat input increases the grain size increases. The transition of cells at the WI from CZ to HAZ is smoother for higher heat input. Welding speed is inversely proportional to the heat input so when the welding speed increases the heat input reduces. It is also important to note that when there is high heat input there is higher cooling rate. The higher the cooling rate, the longer the available time for the cells to form and grow, as can be seen by comparing sample A to samples B and C. High heat input also causes wider HAZ, as seen in sample A, where the HAZ is about 10 mm long from the weld centre line.

Comparing samples A, B and C, Sample C has the highest hardness value of 113 HV just after the WI and the lowest value is found in sample A. With high heat input, wider weld beads are observed and the distance of the WI from the weld centre line is further. The weld penetration increases with higher heat input. Although the feed rate is constant, sample C appears to have distinct grain transition at the WI. This can be a failure point when the weld is loaded. Sample A shows that the grain sizes are bigger when there is longer solidification time or high cooling rate. This implies that high heat input allows for high hardness of the WI, which can be due to the solution hardening that occurs in 7xxx series during welding. More heat causes solutionizing thereby causing higher hardening through the solidification process. It is interesting to note that samples D, E and F have very close hardness in the WI due to the relatively constant heat input. In addition, the grain transition at the WI between the CZ and the HAZ is sharp in sample C compared to A and B. This can be a failure point as the cells are not adequately interlocked. Sample A confirms that the longer the solidification time, the bigger the size of the dendrite.

The hardness test also shows that there is a rapid increase in the hardness value around the WI. In samples D, E and F, it is observed that 7025-T6 shows a reduced hardness in the WI and it increases towards the BM. At the WI it is observed that the hardness value of sample F is higher than sample D, which is in turn higher than sample E. This implies that the lower the welding speed at constant heat input, the lower the hardness.

Comparing sample A and F (macrographs) it can be said that large heat input causes large weld beads, causing large distortion. Even with lower heat input it was observed 50 Discussion that faster welding speed allows for narrower weld seams. Compared to sample A and B, the grain growth in sample C is low, which suggests that the high heat conductivity of Al through heat sinks has an effect on weld microstructure especially when low heat input is used.

It can be observed from Table 10 that oxidation occurred on the surface of sample F. However sample F appears to be the best weld with narrow seem and narrow HAZ.

The Hall-Petch effect shows that strength and toughness increases as the grain sizes reduce. Sample C has the smallest grain size in the CZ. Thus it can be concluded that it has the highest strength and toughness. Sample F shows that complete weld penetration can be achieved with minimal heat input if other weld parameters are set correctly.

5.2 Effect of Al2O3 on HSA (AW-7020) weld metallurgy The effect of Al2O3 on the AW-7020 weld metallurgy is with reference to the hardness profile of EC 1 and EC 4. These two profiles are similar (Figure 16); however, the hardness values of EC 1 are higher than EC 4. The presence of the Al2O3 oxide layer in the weld process (EC 1) increased the YS by 20% and the UTS by 6% but reduced the elongation by 29% (compared to EC 4). This suggests that when the Al2O3 oxide layer was not removed before welding, improved hardness of the AW-7020 weld was attained. (It is important to note that there are no weld defects in the macrographs, like porosity due to oxide inclusion in the weld pool). The question therefore arises whether the higher strength could result from the reduced heat that gets into the weld pool due to the heat resistivity of the Al2O3 oxide layer; in addition to the suspected absence of chemical interaction between the molten pool and Al2O3 layer during welding (due to the welding technology and weld parameters). This issue can be clarified by further multiple experiments. However, it is important to mention that if there is a chemical reaction in which the Al2O3 layer is present in the weld (causing porosity) the mechanical properties will be lower.

The effect of pre-weld heat treatment on the AW-7020 weld appears to be that pre weld heat treatment is detrimental to the weld, as can be seen by comparing EC 2 to the other 3 ECs in Figure 16. The hardness profile across the weld in EC 2 is more uneven with sharp fluctuations in hardness values. For example, there is a sharp drop of the hardness value from 81.7 HV to 51.1 HV across the WI. This is usually a failure point of the weld piece. In EC 2, the WI is closer to the CZ (narrower HAZ), which is better when a narrower weld seam is desired. Preheating reduced the YS by 17% and UTS by 3% but increased the elongation by 17% (Figure 17).

The effect of artificial ageing on the AW-7020 weld appears to be that it relatively smoothens the hardness profile, in addition to increasing the hardness values in the HAZ, WI, and CZ. Comparing EC 1 and EC 3, the hardness value at the WI increased from 63.6HV to 79.3HV (Figure 16). Artificial aging reduced the yield strength by 8% but increased the UTS and elongation by 9% and 110% respectively (Figure 16). These results therefore suggest that artificial ageing improves the mechanical properties of 5.2 Effect of Al2O3 on HSA (AW-7020) weld metallurgy 51 51 welded AW-7020 provided there are no weld defects. Based on the macrographs, the welds in AW-7020 study appear to exhibit no defects (Table 13).

The necessity of pre-weld Al2O3 removal is examined in this study. Acceptable welds were achieved without pre-weld removal of the Al2O3 oxide layer (Table 13, EC 1 - 3) using a pulsed MIG welding process. This may be due to the absence of or low chemical interaction of Al2O3 with the weld pool as the EDS result shows that the O2 content of Al2O3 in AW-7020 is about 50% lower than in pure Al. Consequently, when using new welding technologies like pulsed MIG and friction stir welding (FSW) it may not be necessary to remove the naturally formed Al2O3 oxide layer before welding HSA alloys.

Good welds may have been attained due to a lower amount of O2 present on the surface of AW-7020 compared to pure Al. A lower amount of O2 can be considered as indicating that the Al2O3 layer is thinner, which might explain why HSA alloys have lower corrosion resistance in comparison to pure Al.

53

6 Conclusions

This research work studied Al alloys, with particular attention being given to HSA. A brief explanation of Al alloy series classification was presented. Possible joint configurations and welding process limitations, in addition to the preparation work required before welding, were also presented. Possible weld defects were discussed with recommendations on how to prevent or at least minimize the possibility of such defects occurring. It is not possible to propose an optimum weld process for all Al alloy structures as the optimum weld process is case specific and can be influenced by factors like joint design and joint accessibility.

HBLW was also studied. The optics and welding head configuration play an important role in the quality of the welds produced. The challenges that occur when using HBLW to weld Al were presented. Experiments were carried out to investigate the weld metallurgy of HSA (using 7025-T6), the presence and composition of the Al2O3 layer (using 99.9% pure Al, AW-7020 and 7025-T6), and the effect of the Al2O3 layer on HSA weld metallurgy (using AW-7020).

The study showed that with 7025-T6 Al alloys the grain size reduces as the heat input reduces. The transition of cells from the CZ to HAZ is smoother with higher heat input. At constant heat input the grain size increases when feed rate, weld speed and current increase while the hardness remains relatively constant. When heat input is high, the HAZ is wider, nucleation is lower, and the grains around the WI are coarser.

In 7025-T6 Al, it was found that the higher the heat input, the wider the weld bead, the further away is the WI from the weld centre line, and the deeper the weld penetration. In addition, the dendrites became larger with longer solidification time. It was noted that a high cooling rate allows for epitaxial cell formation. The 7025-T6 alloy, like other HSA alloys, experiences HAZ softening but can be restored by weld post heat treatment.

Study was carried out to investigate the effect of the Al2O3 layer on the weld metallurgy of HSA (AW-7020). The structural formation of the Al2O3 layer was briefly explained; the O2 composition of the Al2O3 layer varies depending on the class of Al alloy. The characteristics and properties of the Al2O3 layer were discussed, and the study presented how the Al2O3 structure can be modified for structural advantage. Based on literature review and experimental study, the following conclusions can be drawn:

1. Pre-weld heat treatment of the AW-7020 alloy is detrimental to the mechanical properties of the weld.

2. Artificial aging of AW-7020 welds improves mechanical properties, including hardness, tensile strength, and ultimate yield strength. Therefore, it is suggested that post weld heat treatment is advantageous in high strength Al alloys. 54 Conclusions

3. Acceptable welds are attainable without pre-weld cleaning of the Al2O3 layer. It is therefore suggested that removal of the Al2O3 layer is not necessary when using new welding technologies like the pulsed MIG process on HSA alloys.

4. The presence of the Al2O3 layer is not detrimental to the mechanical properties of HSA welds if there is no chemical interaction between the weld pool and the Al2O3 layer, and if there are no Al2O3 particle inclusion in the weld pool. This suggests that new weld technology preventing Al2O3 chemical interactions during welding that can cause weld porosity and other weld defects is advantageous.

5. The O2 composition of Al2O3 varies across the different classes of wrought Al alloys. This suggests that the thickness of the Al2O3 layer is not the same for all Al alloys but similar in each Al alloy class. In addition it suggests that the composition is also dependent on the chemical composition of the parent metal. 55

7 Future work

Many questions in the area of HSA remain unanswered. Some issues requiring further study are listed below in the form of questions (Q) followed by suggestions (S) of how the questions could be addressed.

Q1: Do all HSA alloys have the same hardness pattern?

S1: Welding experiments should be carried out on other HSA alloys and the hardness results correlated.

Q2: Do all HSA have the same tensile test pattern across welds?

S2: Welding experiments should be carried out on other HSA alloys and the tensile test results correlated.

Q3: How does filler material influence the weld?

S3: Various applicable Al filler wire should be used in welding of HSA and the weld metallurgy studied to analyse their effects.

Q4: Will changing pulsed MIG weld parameters improve HSA welds?

S4: Welding experiments in which parameters like current and voltage are varied should be carried out on HSA to study the effect of weld parameters on weld metallurgy.

Q5: The O2 composition of Al2O3 layer appears to vary in different Al alloys. Does the structure also vary?

S5: Structural analysis of the Al2O3 should be carried out for various Al alloy series

56

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Errata for Publications The errors in the attached published journals are presented for Publications I and III in this section.

Publications I: 1. The fusion weld regions referred to as unmixed zone (UZ) should be composite zone (CZ) as illustrated in the Figure I below. 2. The fusion weld region referred to as partially melted zone (PMZ) should be a joint region of the transition zone (TZ) and the unmixed zone (UZ) as illustrated in the Figure I below. The replacement for PMZ should be TZ-UMZ.

(a) (b)

Figure I Fusion weld zone nomenclature: (a) schematic diagram of fusion weld zone, (b) micrograph of 7025-T6 fusion weld

3. The observed errors and corrections in Publication I are presented in Table I.

Table I Errors in Publication I

Location of error Error Correction Page 26, column 2, paragraph 5 Unmixed zone (UZ) Composition zone (CZ) Transition zone to unmixed zone Page 26, column 2, paragraph 5 Partially melted zone (PMZ) (TZ-UMZ) Page 28, column 1, paragraph 2 UZ CZ Page 28, column 2, paragraph 2 UZ CZ Page 29, column 1, paragraph 2 UZ CZ Page 29, column 1, paragraph 3 UZ CZ Page 29, column 1, paragraph 4 PMZ TZ-UMZ Page 29, column 2, paragraph 1 UZ CZ Page 30, column 1, paragraph 4 UZ CZ Page 30, column 2, paragraph 1 UZ CZ Publication I, Figures 4-11 Label: UZ Label: CZ Publication I, Figures 4-11 Label: PMZ Label: TZ-UMZ Publications III 1. The fusion weld regions referred to as unmixed zone (UMZ) should be composite zone (CZ) as illustrated in the Figure I. 2. The observed errors and corrections in Publication III are presented in Table II.

Table II Errors in Publication III

Location of error Error Correction Page 7, column 1, paragraph 1 Unmixed zone (UMZ) Composition zone (CZ) Page 7, column 1, paragraph 1 UMZ CZ Page 8, column 1, paragraph 2 UMZ CZ Page 8, column 1, paragraph 3 UMZ CZ Publication III, Figure 5. Label: Weld Interface Label: TZ-UMZ

Publication I

Experimental review on the welding metallurgy of HSA (7025-T6) alloy.

Olabode, M., Kah P., and Martikainen J. (2012). The Paton Welding Journal, 4, pp.20-30. ISSN: 0957-798X (print)

© PWI, International Association «Welding», 2012

EXPERIMENTAL REVIEW OF THE WELDING METALLURGY OF HIGH-STRENGTH ALUMINIUM ALLOY 7025-T6

M. OLABODE, P. KAH and J. MARTIKAINEN Lappeenranta University of Technology, Lappeenranta, Finland

In this review, various aspects such as designations, definitions, applications, properties and weldability of high-strength aluminium alloys are presented. The effect of heat input on microstructure and hardness of the 7025-T6 alloy welded joints is studied. It is shown that at constant heat input the welding speed had no effect on the weld hardness. Thus, limiting heat input in welds on high-strength aluminium alloys is important to preserve their mechanical properties.

Keywords: high-strength aluminium alloys, alloy 7025-T6, four digit number system. Cast alloy designations are pulsed MIG welding, heat input, Vickers hardness, welding metallurgy similar to those of wrought alloys but with a decimal between the third and fourth digit (123.0). The second Light welded metal structures are in high demand, part of the designation is the temper which accounts and the market keeps growing along with societal for the fabrication process. When the second part needs. The diversification of aluminium structures also starts with T, e.g. T6, it means that the alloy was continues to grow. Welding is an important process thermally treated. The numbers show the type of the in producing these structures. The fusion welding of treatment and other consequent mechanical treat- high-strength aluminium alloys (HSA) using pulsed ment, namely T6 shows that the alloy is solution heat- MIG method involves heat input and is, thus, chal- treated and artificially aged [5]. In alloy designations lenging but accomplishable if proper care is taken to F denotes as fabricated and O – annealed. An addi- understand the nature and behaviour of HSA being tional suffix indicates the specific heat treatment. H welded. A number of studies [1—3] have shown that denotes strain-hardened (cold-worked) and it is al- earlier technologies available for welding HSA present ways followed by at least two digits to identify the poor weldability due to the presence of copper in the level of cold-working and other heat treatments that alloy. However, new technologies like pulsed MIG have been carried out to attain the required mechanical welding, pulsed TIG welding and friction stir welding properties. W denotes solution heat-treated, it is fol- (FSW) can be effectively to compared with conven- lowed by a time indicating the natural ageing period, tional fusion methods. FSW proved to be presently e.g. W 1 h. T denotes thermally treated and is always the most acceptable process as it allows obtaining followed by one or more numbers to identify the spe- important metallurgical advantages, for example, no cific heat treatment [4]. solidification and liquation cracking, compared with The full designation therefore has two parts which fusion welding [4]. Based on literature review, this specify the chemistry and the fabrication history, e.g. paper outlines the definitions, properties, applica- in 7025-T6, 7025 specifies the chemistry while T6 – tions, weldability, welding defects of HSA and studies the fabrication. Aluminium is classified based on the their weldability with a focus on the effect of heat chemical composition. The classification is mainly in input on welding metallurgy using the pulsed MIG two categories based on the type of production which process. This study adopts both a literature review of are wrought aluminium alloys (fabricated alloys) and HSA and an experimental study of 7025-T6 alloy cast aluminium alloys. Others can be categorised on welded by robotised pulsed MIG method. In addition, the basis of strain hardening possibility or heat treat- the effect of heat input and welding speed as welding ment [6]. The wrought aluminium category is large parameters on welding metallurgy of HSA are pre- because aluminium can be formed to shapes by virtu- sented. It was found that the grains reduce in size as ally any known process including extruding, drawing, heat input decreases, and welding speed had no effect forging, rolling etc. Wrought alloys need to be ductile on the hardness across the weld if heat input was kept to aid fabrication, whereas cast aluminium alloys need constant. The hardness of HSA joints lower in the to be fluid in nature to aid castability [7]. Cast alu- HAZ than in the parent metal. This study is of sig- minium alloys are identified with four digits in their nificance as there are limited studies available about classification. A decimal point separates the third and the welding metallurgy of the 7025-T6 alloy. fourth digit. The first digit indicates the alloy group Alloy designation. Aluminium alloys are grouped which is based on the major alloying element (Ta- into cast and wrought ones and are identified with a ble 1) [8]. The next two digits denote the aluminium

© M. OLABODE, P. KAH and J. MARTIKAINEN, 2012 alloy itself or the purity of the alloy. In lxx.x series 20 4/2012 Table 1. Cast aluminium alloy classification [6—9]

Series Alloying elements Content, % Tensile strength, MPa Series average value, MPa

1хх.х Al Min 99.0 2хх.х Cu 4.0—4.6 145—476 302 3хх.х Si 5—17 159—359 249 With added Cu and/or Mg 5—17 159—359 249 4хх.х Si 5—12 131—296 187 5хх.х Mg 4—10 138—331 232 7хх.х Zi 6.2—7.5 241 241 8хх.х Sn — 138—221 163 9хх.х Others — — — alloys, these two digits denote the purity in percent- In the 1xxx series the last two numbers signify the ages. For example, 150.0 show the minimum 99.5 % alloy level of purity. For example, 1070 or 1170 im- purity of the aluminium alloy. In the groups 2xx.x— plies that at least 99.7 % Al is present in the alloy, 9xx.x series, the two digits signify the different alloys 1050 or 1250 – no less than 99.5 % Al, and 1100 or present in the group. The last digit signifies how the 1200 – at least 99.0 % Al. For all the other series of product is formed. For example, 0 denotes casting, aluminium alloys (2xxx—8xxx) the last two numbers and 1 or 2 – ingot based on what chemical compo- have no special significance but are used to identify sition the ingot has. alloys in the group [6, 8]. Further modifications from the original cast alu- High-strength and ultra high-strength aluminium minium alloy groups are identified by adding a serial alloys. Aluminium alloys with at least 300 MPa yield letter in front of the numerical denotations. The serial strength are regarded to be HSA, whereas ultra high- letters are assigned in alphabetical order starting with strength aluminium alloys (UHSA) are those with A but omitting I, O, Q and X [8]. X is left out with yield strength of 400 MPa or more. HSA and UHSA experimental alloys. are generally included in the 2xxx, 7xxx, and 8xxx Wrought alloys are given four digits. The first one series. There are no strict rules as to what series HSA represents the alloy group which is based on the major and UHSA belong to. For example, two alloys can alloying element (Table 2). The second digit tells how have significantly different yield strengths within the the alloy has been modified or the limits of impurity. same series. To be exact, the HSA and UHSA can be 0 in the second digit denotes an original alloy. Num- classified only specifically to certain alloys in the se- bers 1—9 signify the different alloy modifications with ries. For generality purpose, however, an average slight variation in their compositions. In the 1xxx range of the series yield strength is used to identify series the second number denotes the modifications in HSA and UHSA (see Table 2). impurity limits: 0 implies that the alloy has a natural Properties and applications of HSA and UHSA impurity limit, 1—9 imply that special control has been Series. The 2xxx series includes the Al—Cu alloys. carried out on one or more impurities or alloying ele- The major characteristics of the 2xxx series are heat ment. The last two numbers represent the purity of treatability, high strength both at room and elevated the alloy [6]. temperatures, and high tensile strength range of 68.9— 520 MPa [9, 10]. The alloys can be joined mechani- Table 2. Wrought aluminium alloy classification [6, 8, 9]

Series Alloying elements Content, % Tensile strength, MPa Series average value, MPa

1ххх Al Min 99.0 10.0—165 94.4 2ххх Cu 1.9—6.8 68.9—520 303 3ххх Mn 0.3—1.5 41.4—285 163 4ххх Si 3.6—13.5 70.0—393 275 5ххх Mg 0.5—5.5 40.0—435 194 6ххх Mg and Si 0.4—1.5 40.0—435 241 Si 0.2—1.7 40.0—435 241 7ххх Zn 1.0—8.2 80.0—725 399 8ххх Others — 110—515 365

4/2012 21 Figure 1. Mechanical properties of aluminium alloys cally while some are weldable [11]. The chemical com- The 7xxx alloys are mainly used when fracture position is usually copper and some other possible critical design concepts are important, e.g. the elements, like magnesium, manganese and silicon. Foresmo Bridge in northern Norway. Al—Mg alloys They comprise high strength products that are usually are used for building the girders system. Another main typical of the aviation industry (2024 alloy). In the application is in the aircraft industry [10]. They have industry they are expected to meet high engineering poor corrosion resistance compared to, for example, standards due to high safety requirements. These re- the 5xxx series and are thus clad in many applications. quirements make the 2xxx series expensive. However, They are used for critical aircraft wing structures of the alloys are also used in the manufacture of truck integrally stiffened aluminium extrusions and long- bodies (2014 alloy); 2011, 2017 and 2117 alloys are length drill pipes, and premium forged aircraft parts extensively used for fasteners and screw machine are made from 7175-T736 (T74) alloy [10]. stock. Under naturally aged T4 condition, the 2xxx The 8xxx series includes alloys with aluminium series alloys have similar mechanical properties as mild and other elements such as iron, nickel and lithium steel, with a proof strength of about 250 MPa and an (not presented in Table 2). These elements provide a ultimate tensile strength of around 400 MPa. They specific property to the alloy, e.g. nickel and iron also have good ductility. When T6 conditioning is yield strength to the alloy with almost no loss to used, the proof strength gets up to 375 MPa and the electrical conductivity [10]. The high strength mem- ultimate tensile strength can get up to 450 MPa. This, bers of the series mainly consist of lithium and copper. in turn, lowers ductility [11]. Moreover, they are gen- The lithium proportion is higher than that of copper. erally painted or clad to increase their corrosion re- The relatively recently developed Al—Li alloys 8090, sistance. Succinctly, the 2xxx series alloys are used 8091 and 8093 are also included in the series. Lithium for the construction of aircraft internal and external has lower density than aluminium and relatively high structures, internal railroad car structural members, solubility. Thus, it can be alloyed with aluminium in structural beams of heavy dump and tank trucks and sufficient quantities. A significant reduction in density trailer trucks, and the fuel tanks and booster rockets (usually about 10 % less than other aluminium alloys) of space shuttles [10]. is attainable. The resulting alloys have increased stiff- The 7xxx series includes the Al—Zn alloys with ness, and they also respond to age-hardening. Some magnesium to control the ageing process. The alloy of the series alloys are heat treatable [12]. They are group possesses very high strength in the high tough- therefore referred to as special alloys and have high ness versions. They are also heat treatable with an conductivity, strength (tensile strength of 110— ultimate tensile strength range of 220—610 MPa. They 515 MPa [9]) and hardness. These alloys are used in can be mechanically joined and, with selected welding the aviation industries (8090, 8091). The Al—Ni—Fe method like pulsed MIG process, they are weldable. alloy 8001 is used in nuclear power generation for Some 7xxx alloys content copper to yield the highest applications demanding resistance to aqueous corro- strength in the series. However, these alloys are not sion at elevated temperatures and pressures. The alloy commercially weldable (Figure 1). The weldability 8017 is used for electrical conduction [10]. reduces as the copper content increases [1—3]. Thus, Weldability of high-strength aluminium alloys. in commercial applications they are mechanically The increasing industrial need for aluminium alloys joined, e.g. by riveting. has resulted in profuse research on how to weld the 22 4/2012 new alloys. There are more ranges of applicable weld- • traditional methods give inferior mechanical ing processes available on the market. Based on studies properties with respect to those of the corresponding it can be stated that: base materials. The decrease varies from 20 to 35 % • within the scope of manufacturing technology, and is highly influenced by the metallurgical state of 94 % of alloys can be welded and over 50 % have the base material. Particularly, an insignificant or optimal weldability; even zero reduction is only found with the FSW proc- • industrially weldable thickness range is 0.1— ess, which is, at the same time, the only welding 450 mm (the latter, exceptional case, in a single pass process offering fatigue characteristics of butt joints by means of electron beam welding (EBW)); that are entirely comparable to the base metal in the • high welding speeds are attainable with reduced as-welded condition; thicknesses (0.8—3.0 mm), for example, the laser • generally all fusion welding methods, with the welding of butt joints, varying between 5 and exception of FSW, give welds affected by widespread 3 m/min; porosity; • metallurgical problems caused by welding heat • generally, and considering similar sized welding input are present with all fusion methods, but reduced equipment, laser and FSW technologies involve up to in the concentrated energy processes, where heat input 10 times higher investments than traditional technolo- is more precise and hence the HAZ is less extensive. gies, but the level of productivity is decidedly supe- FSW produces a low level of metallurgical distur- rior. Currently, large scale of the aluminium alloy bance; structural components welded by FSW have at least • with concentrated energy processes, the presence 10 % lower costs compared to those welded by MIG of the Al2O3 film on the surfaces undergoing welding process [13]. does not compromise the quality of the weld. How- Work preparation. The successful welding of HSA ever, pre-weld cleaning is encouraged; is very dependent on the work preparation due to the • both EBW and FSW can be conducted without extra consideration necessary for welding aluminium the use of gas to protect the weld pool from oxidation; compared to steel. It depends on using a suitable weld-

Table 3. Work preparation guide [4, 9, 14, 15]

Consideration Precautions

Stress in weld Avoid sudden changes in thickness as they act as stress raisers in the weld. It is better to taper a section in the joint if it is to be joined with a thinner section Ensure a good fit-up prior to welding. Aluminium is intolerant of poor fit-up and joints should have the smallest gap possible to allow the penetration of the filler into the joint. In a general fit-up, gaps of more than 1.5 mm are not acceptable. Larger gaps are easy to fill in steel but will introduce excessive stresses in aluminium due to thermal contraction. This will compromise the life of the weld Ensure a good alignment of the joint prior to welding. A misaligned weld will introduce bending stresses, which will also shorten the life of the weld Make sure that the joint preparation is suitable for the thickness of the material and complies with the drawing Conditions for good quality Make sure that the ambient conditions are suitable for welding. Aluminium is very sensitive to welds hydrogen contamination, so that any moisture will result in defective welds due to porosity. Welding outdoors is particularly risky as condensation can form on the joint during cold weather or the component may be left out in the rain. If welding is to be carried out during humid periods, moderate preheating may be usefully applied to prevent hydrogen porosity. Even if the joint is dry, the risk of draughts destroying the gas shield must be considered. Welding of aluminium is best carried out in a dedicated warm, dry, draught-free area indoors Pre-weld cleaning of joint Aluminium is very intolerant of contamination in the joint. Cleaning should start with a wipe by a clean cloth soaked in a solvent such as acetone to remove oil and grease from the joint area and 25 mm over both sides of the joint. All aluminium products have a very thin layer of oxide on the surface. This melts at about 2060 °C [4, 14] compared with 660 °C [9] for pure aluminium. This oxide must be removed after degreasing and before welding by mechanical cleaning with a stainless steel wire brush, which is reserved for aluminium use only. A grinding disk must not be used as these are made from corundum (aluminium oxide) and will deposit particles in the surface. This is precisely the material that cleaning intends to remove. The weld should preferably be made immediately after cleaning, but welding within 3 h of cleaning is acceptable Suitability of welding Welding is normally carried out using argon or mixture of argon and helium, and the purity of consumables these gases is important. A minimum purity of 99.995 % is required. Wire for MIG welding is normally supplied clean enough and it is sufficient to always ensure that the spool is preferably removed from the welding machine and placed in a clean plastic bag overnight or at least covered to keep it clean

4/2012 23 Table 4. Shielding gases for MIG welding of aluminium [16]

Metal transfer mode Shielding gas Characteristics

Spray 100 % Ar Best metal transfer and arc stability, least spatter, good cleaning action

Ar + 65 % He Higher heat input than in 100 % Ar, improved fusion characteristics on thicker material, minimised porosity Ar + 75 % He Highest heat input, minimised porosity, least cleaning action

Short circuiting Ar or Ar + He Ar satisfactory on sheet metal, Ar + He preferred for thicker base materials

ing process, storage, handling and workpiece prepa- rate of about 20 l/min. A mixture of argon and helium ration as well as applying a practically acceptable can also be used and even helium alone. Helium in- joint design [1]. creases weld penetration, offers higher arc energy and, The workpiece to be joined with the pulsed MIG thus, an increased deposition rate [1, 19]. When the process involves joint preparation which is imperative section is lower than 50 mm, helium should be used to ensure quality welds. Based on the thickness of the [4]. More details can be seen in Table 4. workpiece, the joints need to be bevelled and in some Welding defects in HSA and UHSA. The welding cases a root back-up must be applied. It is important of aluminium is rather critical despite the fact that it to clean the joint surface to remove the thin oxide has a lower melting point compared to steel. The weld- layer (Al2O3). The removal can be done by mechanical ing of aluminium is critical because of the following abrasion processes like brushing with stainless steel considerations [6, 18]: brushes or by chemical etching. The Al2O3 layer re- • stable surface oxide needs to be eliminated before generates itself when scratched. It is responsible for welding; the corrosion resistance in aluminium alloys [14] and • presence of residual stresses causes weld cracking also for the arc instability problem because it is elec- due to the high thermal expansion coefficient of alu- trically non-conductive. Al2O3 is hygroscopic and it minium; is usually found hydrated. The melting temperature • high heat conductivity of aluminium implies that is 2060 °C [4, 14] which is high when compared to great heat is required to achieve welds, whereas high the melting temperature range of 476—657 °C of the heat input increases the possibility of distortion and 7xxx series alloys [9]. A work preparation guide is cracking; presented in Table 3. • high shrinkage rates on solidification enhance Shielding gas. The primary function of shielding cracking; gas is to protect the weld metal from the atmosphere • high solubility of hydrogen in molten aluminium because heated metal (around the melting point) usu- causes porosity; ally exhibits a tendency to react with the atmosphere • general susceptibility of the 2xxx, 7xxx and 8xxx to form oxides and nitrides. For aluminium it easily series to weld cracking. reacts with oxygen at room temperatures. In selecting Applicable major welding defects in HSA series the shielding gas, the criteria that should be met are include hot cracking, porosity, joint softening, not as follows [4, 16—18]: recoverable on post-weld ageing, poor weld zone duc- • gas must be able to generate plasma and stable tility (HAZ degradation) and the susceptibility of the arc mechanism and characteristics; joint to stress corrosion cracking (Table 5). • it should provide smooth detachment of molten Experimental set-up. The experiment was carried metal from the wire and fulfil the desired mode of out using a robotised pulsed MIG welding machine. metal transfer; The schematics of the MIG welding process are pre- • it should protect the welding head (in the arc sented in Figure 2. immediate vicinity), molten pool and wire tip from The robot movement was programmed and some oxidation; test sample welds were made, after which alloy 7025- • it should help to attain good penetration and T6 was welded. Many different welds were made, and good bead profile; the weld parameters were varied to study the effect • it should not be detrimental to the welding speed of heat input on properties of the weld metal. Fur- of the process; thermore, the effect of the welding speed was studied. • it should prevent undercutting tendencies; The MIG torch used was Fronius Robacta 5000 • it should limit post-weld cleaning; 360 (max 500 A). The torch was connected to the • it should not be detrimental to the weld metal Motorman (EA1900N) robot. The robot has 6 axes mechanical properties. and can attain an accuracy of up to ±0.06 mm. A torch The recommended shielding gas for pulsed MIG angle of 10° pushing weld direction was used to allow welding of 7xxx aluminium is argon [1, 17] at flow for the purging of the weld area ahead of the arc. The 24 4/2012 Table 5. Defects in aluminium welds and their prevention [11, 15, 18, 20]

Defect Cause Remedy

Oxide inclusions Insufficient cleaning of the joint Thoroughly wire brush before welding and after each pass, then wipe clean Oxide layer on welding wire or filler Clean wire and rods by abrading with stainless steel wool or rods «Scotchbrite» Use fresh spool of wire Sharp edges on the joint groove Break sharp edges in weld preparation

Porosity in weld Inadequate shielding Increase gas flow Eliminate draughts Reduce electrode extension Dye penetrants, lubricants Remove any defects fully Clean surfaces with a solvent Keep lubricants away from the weld area Welding current too high Reduce current and refer to the weld procedure Contaminated shielding gas Check gas hoses for loose connections or damage Check torch coolant to ensure no leaks Replace gas cylinders Incorrect torch angle Use correct angle and refer to the weld procedure Travel speed too high Apply correct speed and refer to the weld procedure Contaminated wire or rods Clean wire or rods with solvent Moisture Preheat and clean the surface

Porosity in fusion zone Hydrogen in the base metal Improve the degassing practice Reduce sodium additions Apply 100 % He shielding

Cold cracking High joint restraint Slacken holding clamps Preheating

Hot cracking Excessive dilution by parent Reduce welding current Add more filler wire Interpass temperature too high Reduce welding current Cool between passes and sequence welds

Undercutting Welding current too high Reduce current Travel speed too high and Reduce speed and refer to the weld procedure, add more filler insufficient filler metal metal Arc length too long Reduce arc length

Lacks of fusion Welding current too low Increase current and refer to the weld procedure Travel speed too high Reduce travel speed, and refer to the weld procedure Poor joint preparation Improve joint preparation Incorrect torch angle Apply correct torch angle, and refer to the weld procedure

Crater cracking Improper breaking of arc Reduce arc current gradually Use «Crater fill» control if available. «Back weld» over last 25 mm of the bead

Overlap Slow travel speed Increase speed and refer to the weld procedure Welding current too low Increase current Too much filler metal Reduce filler metal addition Incorrect torch angle Change torch angle

Drop through Slow travel speed Increase travel speed Welding current too high Decrease welding current Joint gap too wide Reduce gap and improve the fit-up Too much heat built up in part Reduce interpass temperature

4/2012 25 Figure 3. Hardness testing on a weld sample yield stress of 55 MPa, tensile strength of 165 MPa and an elongation of 18 %. The shielding gas was 99.995 % argon and it was supplied through the MIG torch to protect the weld pool from the atmosphere, because heated metal (around the melting point) usu- ally exhibits a tendency to react with the atmosphere to form oxides and nitrides. For aluminium it easily Figure 2. Schematics of MIG welding process: 1 – power source; reacts with oxygen at room temperatures. The recom- 2 – shielding gas; 3 – MIG torch; 4 – filler wire; 5 – aluminium mended shielding gas for pulsed MIG welding 7xxx workpiece series aluminium is argon [17]. filler wire extension was 2 mm, and the nozzle-to- The hardness testing experiment of the welds was workpiece distance (stick-out length) was 18 mm. The done on a Vickers hardness machine. The test method shielding gas used was 99.995 % argon and the filler involved the indentation of the test workpiece with wire was 4043 aluminium. The workpiece was a 5 mm a diamond indenter in the form of right pyramid with thick plate with an area of 100 × 250 mm. The samples a square base and angle of 136° between opposite faces; were bead-on-plate welds, so there was no bevelling. subjected to a weight of 1—100 kg. The full load was However, the joints were cleaned mechanically by normally applied for 10—15 s. The two diagonals of using a stainless steel bristle brush reserved for alu- the indentation made on the surface of the material minium only. after the removal of the load were measured using a Many experimental trials were performed, for microscope and their averages calculated [22]. which 6 different samples of 7025-T6 alloy were se- This test was carried out by a 3 kg weight inden- lected. The first three samples (A, B and C) had the tation of the diamond tool tip on the prepared weld same feed rate so as to investigate the effect of the cross-section. The weight can be varied for different welding speed (10, 20 and 30 mm/s). The other three materials, but 3 kg was sufficient because aluminium samples (D, E and F) had approximately the same is relatively soft and 3 kg is enough to create an in- heat input to investigate the effect of constant heat dentation. Moreover, it is important that the weight input on the weld. The pulse current frequency was is low enough for the aluminium test piece to resist approximately 250 Hz in each weld. it. The indentations were done at about 1 mm from For samples A—C, the feed rate was constant at the weld surface in a row (Figure 3). 10 m/min, and the heat input varied. Heat input Q The distance between each indentation was for all samples was calculated as [21] 0.7 mm. The shape of the indentation resembled a rhombus. The depth of the indents depended on the VI⋅60 Q = ⋅ 0.8, (1) material hardness. The dimension of the diagonals of 1000S an indentation was measured and the average value where Q is the heat input, kJ/mm; V is the voltage, from the diagonals was looked up from the hardness V; I is the current, A; S is the welding speed, table of HV3 to determine the hardness value. The mm/min; 0.8 is the efficiency of the pulsed MIG values were then plotted against the distance of each process. indentation from the weld centreline. For samples D—F, heat input was approximately Results and discussions. Effect of heat input on constant and the feed rates were selected as 10, 12 HSA. Micro- and macrostructure, as well as weld ap- and 14 m/min, respectively. pearance on samples A—C, are presented in Figures The base material was a 5 mm thick 7025-T6 plate, 4—6. The picture of each sample shows the microstruc- and the welding wire was ER 4043 (Table 6). The ture using an ×8 magnification lens for analysing the typical mechanical properties of the wire include the unmixed zone (UZ), partially melted zone (PMZ),

Table 6. Chemical composition of base metal and filler wire used, wt.%

Other Metal Al Be Cr Cu Fe Mg Mn Si Ti Zn Total each

7025 91.5 — 0.30 0.10 0.40 1.50 0.60 0.30 0.10 5.0 0.05 0.15 ER 4043 — 0.0001 — 0.01 0.20 0.01 0.01 4.80 0.02 0.01 — —

26 4/2012 Figure 4. Experimental results for sample A welded at vw.f = 10 m/min, vw = 10 mm/s, Q = 0.318 J/mm, U = 20.1 V and I = 198 A

Figure 5. Experimental results for sample B at vw.f = 10 m/min, vw = 25 mm/s, Q = 0.127 J/mm, U = 19.4 V and I = 205 A

Figure 6. Experimental results for sample C at vw.f = 10 m/min, vw = 30 mm/s, Q = 0.106 J/mm, U = 19.4 V and I = 205 A 4/2012 27 Figure 7. Experimental results for sample D at vw.f = 10 m/min, vw = 20 mm/s, Q = 0.16 J/mm, U = 19.8 V and I = 202 A heat-affected zone (HAZ) and base metal (BM). The the three samples (see Figure 4). Thus, it can be said transition around the weld interface is of great sig- that the higher the heat input, the wider the HAZ. nificance. The picture shows how the grains have been The grains of UZ in sample C compared to B and transformed, from which inferences can be made as A are very fine, which shows that low heat input in to the mechanical properties of the weld samples. A and B is insufficient to melt the pool and penetrate Comparing samples A, B and C (see Figures 4—6), the weld. The high heat input and high welding speed it can be seen that the grain sizes around the weld caused high heat energy on the weld in sample C, interface are small when heat input is low, and vice which makes the weld bead large with a wider root. versa. Furthermore, the transition flow of cells at the Sample C has fine grains compared to B and A, interface as it moves from the UZ to the HAZ is which shows that with high heat input and welding smoother with higher heat input where the grain sizes speed there is higher nucleation. In sample C, the are bigger. With lower heat input as in sample C (see grain growth is low compared to A and B because Figure 6) the transition is not as smooth, so the in- aluminium dissipates heat relatively fast through heat terface is distinct. Heat input is inversely related to sinks; low heat input means that the high conductivity the welding speed. When the welding speed increases, of aluminium strongly affects the weld microstructure heat input reduces. The higher the heat input, the (sample C cools fast). higher the cooling rate. A high cooling rate allows By comparing the results from samples D, E and epitaxial growth to occur and also for the cells to F presented in Figures 7—9 it can be noted that keeping grow large, as seen by comparing sample A to samples the heat input relatively constant but varying the B and C. In sample A, the HAZ is about 17 mm from welding speed causes changes in the microstructure. the weld centreline, which is the greatest distance of As the welding speed and the wire feed rate increase,

Figure 8. Experimental results for sample E at vw.f = 12 m/min, vw = 24 mm/s, Q = 0.163 J/mm, U = 20.3 V and I = 241 A 28 4/2012 Figure 9. Experimental results for sample F at vw.f = 14 m/min, vw = 28.8 mm/s, Q = 0.158 J/mm, U = 20.5 V and I = 278 A also the grain sizes increase. Furthermore, the in- microstructure. Sample C has the smallest grain size creased welding speed gives lower nucleation and in the UZ. Thus, it can be concluded that it has the coarser transitions of grains around the weld interface, highest strength and toughness as the Hall—Petch ef- which is similar to the effect of heat input in 7025-T6 fect predicts that both strength and toughness increase aluminium welds. as the grain sizes reduce [24, 25]. Sample F shows Samples D, E and F indicate that the higher the that complete weld penetration can be achieved with wire feed rate, the deeper the penetration. Sample C minimal heat input if other weld data are set correctly. has a constant feed rate with A and B but the grain Weld defects such as porosity and oxidation were transition at the weld interface between the UZ and found on the welds. Porosity could be due to gas the HAZ is very sharp. This may be a possible failure entrapment during welding, whereas oxidation could point as the cells are not as interlocked as in sample be due to poor shielding gas covering (the weld pool B. Sample A shows that the longer the solidification has contact with atmospheric air). time, the bigger the size of the dendrite [23]. Hardness of HSA welded joints (7025-T6). The The grains are equiaxed with dendrites within the hardness tests of samples A—C are presented in Fi- grains. Fine grain sizes appear when heat input is low, gure 10, where the plots for samples A, B and C are and coarse grain sizes when heat input is high. For combined on the same graph. The vertical line, la- example, the UZ in Figure 8 has fine grains due to belled WI, denotes the weld interface. The points on the low heat input of 0.163 kJ/mm, whereas the UZ the graph curve indicate the distance of each inden- in Figure 4 has coarse grains due to high heat input tation point from the weld centreline on the horizontal of 0.318 kJ/mm. The grain size variations in the UZ axis and the hardness value when traced on the vertical in Figures 4—9 are mainly due to the amount of heat axis. The graph also shows the weld zones, HAZ and input, since high heat input means a high cooling rate. BM. Sample C has the lowest heat input of A faster welding speed allows narrow welds even 0.106 kJ/mm resulting in a high hardness profile, with lower heat input (comparing samples A—F). Sam- sample B – relatively higher heat input of ple F seems to be the best weld with a narrow bead, 0.127 kJ/mm resulting in a lower hardness profile narrow HAZ and complete penetration. On the other than sample C, and sample A – the highest heat hand, oxidation occurred on the surface. At a constant input of 0.318 kJ/mm resulting in the lowest hardness welding speed, high heat input increases the weld profile. bead size and HAZ size. The PMZ shows epitaxial growth, which indicates that new grains had nucleated on the heterogeneous sites at the weld interface. There is a random orientation between the base metal grains and weld grains. As can be seen from samples A—F, since the ratio of 7025-T6 alloy temperature gradient G to the growth rate R decreases from the weld interface towards the centre line, the solidification modes have changed from planar to cellular, to columnar dendrite and equiaxed dendrite across the weld interface . The ratio Figure 10. Hardness distribution for samples welded with varying G/R determines the solidification modes found in the heat input: 0.106 (C), 0.127 (B), 0.318 (A) kJ/mm 4/2012 29 2. In 7025-T6 aluminium alloy, high heat input results in a low hardness profile but the hardness of the UZ is the same in all the selected samples. The higher the heat input, the wider the weld bead, the further away is the weld interface and the deeper the weld penetration. The longer the solidification time, the larger the dendrites and a high cooling rate allows for epitaxial cell formation. The 7025-T6 alloy, like other high-strength aluminium alloys, experiences Figure 11. Hardness distribution for samples welded with relatively HAZ softening but can be restored by postweld heat constant heat input of about 0.16 kJ/mm treatment.

Sample C also has the highest hardness at the WI, 1. Yeomans, S.R. (1990) Successful welding of aluminium and thus, implying that high heat input allows for high its alloys. Australian Welding J., 35(4), 20—24. hardness of the WI, due to solution hardening during 2. Graeve, I.D., Hirsch, J. (2010) 7xxx series alloys. http://aluminium.matter.org.uk/content/html/eng/def welding. High heat input causes solubility and thereby ault.asp?catid=214&pageid=2144417086 higher hardening through the solidification process. 3. Dickerson, P.B., Irving, B. (1992) Welding aluminium: It’s not as difficult as it sounds. Welding J., 71(4), 45—50. It can also be said that the higher the heat input, the 4. Mathers, G. (2002) The welding of aluminium and its al- wider the weld bead and the further away from the loys. Boca Raton: CRC Press; Woodhead Publ. 5. Maurice, S. (1997) Aluminum structures: Handbook of weld centreline is the WI. The hardness test also shows structural engineering. 2nd ed. CRC Press. this with relatively constant heat input. The hardness 6. Campbell, F.C. (2006) Manufacturing technology for aero- pattern of samples D, E and F are similar, but E space structural materials. Amsterdam; San Diego: Elsevier. 7. (2008) ASM Handbook. Vol. 15: Casting. Materials Park: exhibits small variation. The hardness around 3 mm ASM Int. away from the weld centreline shows a rapid increase 8. Kopeliovich, D. (2009) Classification of aluminum alloys. In: Substances and technology. in the value from the previous point (around 2 mm 9. (2010) MatWeb – The Online Materials Information Re- from the weld centreline). This is due to the closeness source. 10. Kaufman, G.J. (2000) Applications for aluminum alloys and of the WI. From samples D, E and F it can be seen tempers. ASM Int. that for 7025-T6 weld, hardness reduces in the weld 11. John, D. (1999) Heat-treatable alloys. In: Aluminium de- zone and increases towards the base material. The sign and construction. New York: Taylor & Francis, 301. 12. Aluminum alloys and temper designations 101. Dayco Ind., hardness graph presents half of the symmetric welds. 1—5. At the WI it can be said that the hardness values of 13. Volpone, L.M., Mueller, S. (2008) Joints in light alloys to- day: the boundaries of possibility. Welding Int., 22(9), D, E and F samples are relatively identical. This im- 597—609. plies that at constant heat input, the hardness profile 14. George, E.T., MacKenzie, D.S. (2003) Handbook of alumi- num: Physical metallurgy and processes. New York: Marcel of 7025-T6 aluminium alloy remains the same. Dekker. The hardness tests of samples D, E and F, presented 15. Renshaw, M. (2004) The welding of aluminium castings. In: Aluminium – light strong and beautiful. A.F.o.S. Africa, in Figure 11, show that the hardness profiles for the 11—13. three samples are relatively similar. The WI range is 16. (2008) Choosing shielding gases for gas metal-arc welding. within 0.5 mm as a result of a relatively constant heat Welding J., 87(4), 32—34. 17. Boughton, P., Matani, T.M. (1967) Two years of pulsed arc input. The labelling and description of the graph is welding. Welding and Metal Fabr., Oct., 410—420. the same as for samples A—C. 18. Olson, D.L. (1993) Welding, brazing, and soldering: ASM handbook. Metals Park: ASM Int. 19. Blewett, R.V. (1991) Welding aluminium and its alloys. CONCLUSIONS Welding and Metal Fabr., Oct., 5. 20. Ba Ruizhang, G.S. (2004) Welding of aluminum-lithium al- 1. The study showed that in 7025-T6 aluminium alloys loy with a high power continuous wave Nd:YAG laser. IIW the grain size reduces as the heat input reduces. The Doc. IV-866—04. 21. Hirata, Y. (2003) Pulsed arc welding. Welding Int., 17(2), transition of cells from the UZ to HAZ is smoother 98—115. with higher heat input. At constant heat input the 22. Chandler, H. (1999) Hardness testing. Materials Park: ASM Int. grain size increases when wire feed rate, welding speed 23. Kou, S. (2003) Welding metallurgy. Hoboken: Wiley-Intersci. and current increase simultaneously but the hardness 24. Sato, Y.S., Urata, M., Kokawa, H. et al. (2003) Hall— remains relatively constant. When heat input is high, Petch relationship in friction stir welds of equal channel an- gular-pressed aluminium alloys. Materials Sci. and Eng. A, the HAZ is wider, nucleation is lower, and the grains 354(1/2), 298—305. around the weld interface are coarser. 25. Vander Voort, G.F., (2004) Metallography and microstruc- tures. Materials Park: ASM Int.

30 4/2012

Publication II

Aluminium alloys welding processes: Challenges, joint types and process selection.

Olabode, M., Kah, P., and Martikainen, J. (2013). Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 227(8), 1129-1137. DOI: 10.1177/0954405413484015

© Sage publications, 2013

Review Article

Proc IMechE Part B: J Engineering Manufacture 227(8) 1129–1137 Aluminium alloys welding processes: Ó IMechE 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav Challenges, joint types and process DOI: 10.1177/0954405413484015 selection pib.sagepub.com

Muyiwa Olabode, Paul Kah and Jukka Martikainen

Abstract Aluminium and its alloys have gained increasing importance in structural engineering due to advantageous properties such as light weight, ease of machining and corrosion resistance. This article presents surface-related challenges facing aluminium welding, specifically weld process limitations and joint limitations. The methodological approach is a critical review of published literature and results based on eight industrial welding processes for aluminium and six joint types. It is shown that challenges such as heat input control, hot cracking, porosity and weldable thickness vary with the process used and that there is no optimal general weld process for all aluminium alloys and thicknesses. A selection table is pre- sented to assist in selection of the optimal process for specific applications. This study illustrates that knowledge of weld limitations is valuable in selection of appropriate weld processes.

Keywords Aluminium alloys, aluminium oxide, shielding gases, anodising, aluminium welding process selection

Date received: 17 September 2012; accepted: 4 March 2013

Introduction Researches7,8 have shown that welding aluminium demands greater caution compared with steel, particu- Aluminium and its alloys are widely used in welding larly as regards the amount of heat input and pre-weld industries due to economic advantages such as light cleaning, and that acceptable weld processes for alumi- weight, good corrosion resistance, high toughness, nium joints are limited because the weldable thickness extreme temperature capabilities and easy recyclabil- 1 varies considerably with the different welding processes. ity. Aluminium alloys are used for construction of air- It is therefore of interest to study the limitations facing planes, cars, rail coaches and marine transports. aluminium welding, particularly joint- and process- Aluminium alloys are used in manufacture of tanks specific limitations. and pressure vessels because of their high specific The aim of this article is to present a comprehensive strength, good heat conductivity and beneficial proper- 2 guide to understanding aluminium-welding challenges. ties at low temperatures. Aluminium is the second In the field of aluminium welding, there are eight most used metal after iron and steel in the industry; for industrially common welding processes and six basic example, aluminium is the second most used material joint types that have been analysed. For comparison taking about 15% of total body weight of average cars 3 purposes, a table is designed that shows the influence of and about 34% in Audi A2. There are comprehensive joint and process limitations on optimum welding pro- reviews on the uses and applications of aluminium and 4,5 cess selection. The remainder of this article is divided its alloys. Welding is a means of joining metals by into two main parts, which are surface-related welding creating coalescence due to heat. The work piece is challenges and joint types and process limitations. melted at the joint point (weld pool) that solidifies on cooling. Welding of aluminium alloys is important for fabricating structural constructions and mechanical Lappeenranta University of Technology, Lappeenranta, Finland fabrications like aircrafts. However, welding has prob- lems and can be challenging. Welding defects common Corresponding author: Muyiwa Olabode, Lappeenranta University of Technology, Skinnarilankatu to aluminium include porosity, hot cracking, incom- 34, 53850 Lappeenranta, Finland. 2,6 plete fusion and so on. Email: [email protected] 1130 Proc IMechE Part B: J Engineering Manufacture 227(8)

Evaluation of the findings shows that there is no sin- the removal of the oxide layer just before welding is gular optimum process for welding aluminium. important. However, understanding of the limitations of individ- The aluminium oxide layer is, furthermore, an elec- ual welding processes helps in selection of the optimal trical insulator, and the layer may sometimes be thick process for specific aluminium weld applications. enough to prevent arc initiation. In MIG processes, a thick oxide layer can produce erratic electrical com- mutation in the gun’s contact tube, resulting in poor Surface-related welding considerations welds. A clean, smooth and protected surface is important in It is thus evident that aluminium oxide has to be pre-weld aluminium structures to ensure good alumi- removed before welding because it compromises the nium weldments except in high energy density welding quality of the weld. Generally, the oxide removal can processes like hybrid laser beam welding (LBW) (using be done by mechanical processes like brushing with a pulsed metal inert gas (MIG)).9 It is therefore impor- stainless steel brush, cutting with a saw or grinding with 9 tant to understand different surface-related phenomena semi-flexible aluminium oxide grinding discs. Some and their effect on the weldability of the work piece. In welding processes enhance additional oxide removal addition, knowledge of preventative measures ensuring processes, for example, in ultrasound metal welding the attainment of acceptable welds, despite any adverse processes (UW), oxides and contaminates are removed surface effects, is also important. by high-frequency motion, thus providing metal–metal contact and allowing for the work pieces to bond prop- erly.17 In hybrid laser MIG-welding of aluminium Presence of aluminium oxide surface alloys, the MIG-welding process has a cleaning effect Oxide formation in aluminium occurs due to the strong that removes the aluminium oxide layer. However, it is chemical affinity of aluminium for oxygen on exposure recommended that pre-weld cleaning of the weld sur- face should be carried out preferably by pickling or dry to air. The aluminium oxide thickness increases as a 18 result of thermal treatment, moist storage conditions machining. In gas-shielded arc welding, aluminium and electrochemical treatment (anodising).10–14 It is oxide removal from the weld pool can be done by cath- ode etching (which is controlled chemical surface corro- also important to note that Al2O3 melts at about 19 2050 °C, while aluminium alloys melts at about 660 °C9 sion done to reveal the details of the microstructure). (as illustrated in Figure 1). Therefore, the layer is A direct current passes through the electrode connected removed by pickling or dry machining just before weld. to the positive pole of the power source. There is thus a However, the difference in melting point is not a prob- flow of electrons from the work piece to the electrode lem during the processing by means of high energy den- and the ions flow in the opposite direction, bombarding sity welding processes; it can also be an advantage, for the work piece surface. The aluminium oxide film is example, the presence of oxide layer during laser weld- broken and dispersed by the ion bombardment, thereby ing increases the absorptivity of aluminium and its allowing the flowing weld metal to fuse with the parent alloys to laser radiation.15,16 It should be noted, that a metal. It is advantageous to remove the aluminium 2,9 main challenge in applying most joining technologies to oxide layer before welding because aluminium is its tendency to form a thick, coherent oxide layer. This oxide layer has a melting temperature 1. It significantly reduces the amount of hydrogen much higher than that of aluminium itself; moreover, it porosity in the weld. has a significant mechanical strength. Therefore, this 2. It helps to improve the stability of the weld process oxide layer can remain as a solid film (or fractured in especially in tungsten inert gas welding (TIG). small particles) due to the flow of the molten material,16 3. It allows for complete fusion of the weld. Cathode even when the surrounding metal is molten. This can cleaning is important in TIG process as the oxide result in severe incomplete fusion defects. Therefore, starts to form immediately after wire brushing.

Figure 1. Schematic of aluminium showing its oxide layer and the anodised surface. Olabode et al. 1131

Table 1. Chemical treatments for cleaning and oxide removal.9

Solution Concentration Temp (°C) Procedure Container material Purpose

Nitric acid 50% water 18–24 Immerse 15 min Stainless steel Removal of thin oxide 50% HNO3 Rinse in cold water film for fusion welding (technical Rinse in hot water grade) Dry Sodium hydroxide 5% NaOH in 70 Immerse for 10–60 s Mild steel Removal of thick oxide followed water 18–24 Rinse in cold water Stainless steel film for all welding and by nitric acid Concentrated Immerse 30 s brazing operations HNO3 Rinse in cold water Rinse in hot water Dry Sulphuric chromic 5LH2SO4 70–80 Dip for 2–3 min Antimonial Removal of films and acid 1.4 kg CrO3 Rinse in cold water lead lined steel stains from heat treating 40 L water Rinse in hot water tank and oxide coatings Dry Phosphoric chromic 1.98 L of 75% 95 Dip for 5–10 min Stainless steel Removal of anodic acid H3PO3 Rinse in cold water coatings 0.65 kg of CrO3 Rinse in hot water 45 L of water Dry

Aluminium oxide can also be removed by chemical welding aluminium and its alloys are inert gases such etching or pickling. Table 1 presents chemical treat- as argon and helium. ments for oxide layer removal.9 Argon is used as a shielding gas for manual and One of the causes of the oxide layer is from anodisa- automatic welding. Argon is cheaper than helium, and tion, which is an electrochemical process by which a the use of argon produces a more stable arc and metal surface is converted into a decorative, durable, smoother welds. However, argon gives lower heat input corrosion resistant anodic oxide finish.20,21 Anodisation and lower attainable welding speed, and therefore there utilises the unique ability of amorphous alumina to build is the possibility of a lack of fusion and porosity in up an even porous morphology22 formed in alkaline and thick sections. In addition, use of argon can result in a acidic electrolytes. During anodising, aluminium oxide is black sooty deposit on weld surfaces, although this can not applied like paint or plating. Rather, it is integrated be wire brushed away. It has been observed that with fully with the underlying aluminium substrate. helium shielding gas, the arc voltage is increased by Therefore, it cannot peel or chip off. The anodic oxide 20%, resulting in a higher, hotter arc, deeper penetra- structure is highly ordered and porous, thereby allowing tion and wider weld beads. This implies that the criti- for further processing like sealing and colouring.20 cality of arc positioning (aids avoidance of missed edge The reasons for the utilisation of anodisation are to and insufficient penetration defects) is lower with increase corrosion resistance and ensure the metal sur- helium. There is a reduction in the level of porosity face is fade proof for up to 50 years,23 to improve dec- when helium shielding gas is used because the weld orative appearance, to increase abrasion resistance and pool is hotter and there is slower cooling, which allows paint adhesion, to improve adhesive bonding and lubri- hydrogen to diffuse from the weld pool. Due to the city, to provide unique decorative colours or electrical higher heat produced, the use of helium allows that insulation, to permit subsequent plating, to detect sur- welding speeds up to three times higher than with face flaws, to increase emissivity and to permit applica- argon. The high cost of helium and the inherent arc tion of photographic and lithographic emulsions.14,20,24 instability mean, however, that helium is used mainly Anodising of aluminium alloys is generally advanta- in mechanised and automatic welding processes.9 geous. However, it poses challenges for aluminium It is common practice to use a mixture of helium and welding because the arc cleaning effect of the AC cur- argon as it provides a compromise on the advantages of rent cannot remove the double layer (the anodised layer each gas. Common combinations are 50% or 75% of and oxide layer as in Figure 1). Before welding, the helium in argon, which allow for better productivity by anodised surface needs to be removed.20 increasing the welding speed and provide a wider toler- ance for acceptable welds. The purity of the shielding gas is of importance. At the torch, not at the cylinder Shielding gas selection regulator, a minimum purity requirement of 99.998% Shielding gas protects the molten weld pool from the and low moisture levels of less than 250 °C (less than 9 atmosphere, which is important because aluminium has 39 parts per million (ppm) H2O) are expected. a tendency to react with atmospheric air to form oxide Generally, the shielding gas should be selected with the and nitrides. The shielding gases commonly used in following considerations.2,9,25,26 1132 Proc IMechE Part B: J Engineering Manufacture 227(8)

Table 2. MIG shielding gases for aluminium.26

Metal transfer mode Shielding gas Characteristics

Spray transfer 100% Argon Best metal transfer and arc stability, least spatter, and good cleaning action. 35% Argon–65% Helium Higher heat input than 100% argon; improved fusion characteristics on thicker material; minimises porosity. 25% Argon–75% Helium Highest heat input; minimises porosity; least cleaning action Short circuiting Argon or Argon + Helium Argon satisfactory on sheet metal; argon–helium preferred for thicker base material.

Table 3. Effect of shielding gas on aluminium welding.9,29–34

Shielding gas Relative effect (100% argon as the reference) 100% Ar Ar + He 100% He

Gas flow Nominal Higher Highest Arc voltage (MIG) Nominal Higher Highest Arc (MIG) Nominal stability More unstable Most unstable Weld seam width and depth Nominal width and depth Higher width Highest width Shorter depth Shortest depth Weld seam appearance Nominal smoothness Smoother Smoothest Penetration Nominal depth and roundness Deeper and more round Deepest and most round Welding speed Nominal welding speed Higher attainability Highest attainability Lack of fusion Nominal Lower Lowest Porosity Nominal Lower Lowest Pre-heating Nominal Less needed Least needed Heat production Nominal warmth Warmer work piece Warmest work piece Cost of shielding gas Nominal price More expensive Most expensive

MIG: metal inert gas welding.

1. The gas must be able to generate plasma and a sta- shielding gas.29 In addition, the alternating shielding ble arc mechanism and characteristics. gases reduces weld porosity.30–32 2. It should provide smooth detachment of molten metal from the wire and fulfil the desired mode of metal transfer. Joint types and process limitations 3. It should protect the welding head (in the arc’s This article considers eight industrially accepted welding immediate vicinity), molten pool and wire tip from processes and six joint types. Joint design is important oxidation. because it costs money to buy weld metal. The fillet throat, 4. It should help to attain good penetration and good weld accessibility and the functionality of the welded work weld bead profile. piece are taken into consideration in this design. The six 5. It should not affect the welding speed of the joints considered are butt, T-joint, corner, cruciform, edge process. and lap joint (see Table 4), which are derived from the 6. It should prevent undercutting tendencies. three basic welding joints (fillet, lap and butt joints). Joint 7. It should limit the need for post-weld cleaning. designs are based on the strength requirements, the alloys 8. It should not be detrimental to the weld metal to be joined, the thickness of the material, the joint type mechanical properties. and location, weld accessibility and the welding process. Before choosing the joint design, it is important to note The recommended shielding gas for welding alumi- that welding in the flat or downward position is preferable nium using pulsed MIG is argon (99.998%)25,27 at a in all arc-welding processes, as there is the easier possibility flow rate of about 20 L/min.27 A mixture of argon and of depositing high-quality weld metal at a high deposition helium can also be used and even helium alone. Helium rate in a flat position. Additionally, the weld pool is larger, increases the weld penetration and offers higher arc allowing for a slower cooling and solidification rate, which energy and thus increased deposition rate,27,28 and it enhances the escape of trapped gases in the weld pool. The should be used when the section is greater than 50 mm.9 flat position reduces weld porosity, reduces weld cost, and More details can be seen in Table 2, which presents gives the best weld metal quality compared with other MIG shielding gases for aluminium, and Table 3 pre- positions. The static tensile strength of the weld is deter- sents the effects of shielding gases on aluminium weld- mined by the throat thickness, which must be designed to ing. Studies have shown that welding of aluminium can ensure that it can carry the workload for which the weld is be improved (arc stability) by oxygen doping of inert designed. Conventional TIG and MIG processes produce lbd tal. et Olabode Table 4. Joint types and process limitations of aluminium alloys.2,8,9,17,18,35–52

Processes MIG TIG PAW FSW LBW RW EBW UW

Joints

Butt joint (a)  Lap joint (b)  T-joint (c)  Edge joint (d)  Corner joint (e)  Cruciform (f)  

Limitation Limitation Limitation Limitation Limitation Limitation Limitation Limitation

With argon, weldable Limited to thin gauges of Plasma MIG weld Weldable thickness Limited conversion Limited weld thickness High cost of Expensive high thickness is limited to up to 6 mm thickness. thicknesses limited to 6– ranges from 1–50 mm efficiency of electrical range (0.9–3.2 mm) equipment. powered transducers (a) 25 mm, and with helium, Limited (shallower) 60 mm range. (single pass). power to focused are needed to enable it is limited to 75 mm. penetration into parent infrared laser beam also Lower tensile and fatigue Work chamber size welding of thick gauges, metal compared to MIG. Plasma TIG weld Tool design, process called wall plug efficiency strength compared to constraints. castings, extrusions, Limited torch distance of thicknesses range can be parameters, and (about 10%–30% and up other fusion welding and hydro-formed 10–19 mm to ensure With argon shielding gas, less than 2.5–16 mm in a mechanical properties to 40% in fibre lasers). processes. Time delay when components. properly shielded weld the economical weld single pass. database is limited and welding in a vacuum. (b) metal limits flexibility. thickness limit is 10– only available for limited Limited fit up tolerance. Limited joint designs or Alternative welding 18 mm with helium Limited by the high alloys and thicknesses Precise fit up (15% of configuration. Seam High weld preparation configurations are Limited outdoor (DCEN). capital equipment and (up to 70 mm). material thickness) welds can generate costs. needed to weld a wide application because air material cost compared needed for butt and lap unzipping effect. variety of component drafts can disperse the Difficult to penetrate to TIG. Limited to lower joints. X-rays produced geometries and joint shielding gas. into corners and into the productivity cases Limited operator during welding can be a configurations. (c) roots of fillet welds. Limited tolerance of the compared to LBW. Limited operator acceptability of the health risk. Limited operator process to joint gaps and acceptability of the process because, in Vibration control acceptability of the Limited by the lower misalignment. Insufficient design process due to the large thick-sectioned upset Rapid solidification strategies are needed process because of the deposition rate, low guidelines and limited capital investment welds; there is lack of rates can cause to ensure weld quality relatively high levels of tolerance on filler and Limited operator education for needed, therefore good non-destructive cracking in some across a wide range of radiated heat and arc base metal, and cost for acceptability of the implementation. requiring high volume weld quality testing high materials. component geometries intensity. thick sections compared process due to the production or critical electrode wear rate and and the thickness of (d) to MIG. complex torch Exit hole left when tool applications to justify the deterioration. Can weld up to the weld piece is architecture that is withdrawn. expenditure. 450 mm thick plates. limited. requires more In addition, it requires maintenance and Large down forces access to both sides of accurate set-back of the required with heavy duty the joint. electrode tip with clamping necessary to (e) respect to the nozzle hold the plates together  orifice, which is during welding. challenging. Environmentally friendly welding process because fumes and spatters are not generated. (f)  DCEN: direct current electrode negative; EBW: electron beam welding; FSW: friction stir welding; LBW: laser beam welding; MIG: metal inert gas welding; PAW: plasma arc welding; RW: resistance welding; TIG, tungsten inert gas welding; UW: ultrasonic welding. 1133 1134 Proc IMechE Part B: J Engineering Manufacture 227(8)

Table 5. Weld process selection (the highest factor summation is the best of the processes considered).

Selection factors Process A (TIG) Process B (FSW) Process C (PAW) Process D (MIG)

Quality of the welded joint Imp. Ad. I. Fac. Imp. Ad. I. Fac. Imp. Ad. I. Fac. Imp. Ad. I. Fac. Strength 3 2 6 3 2 6 – – – 3 2 6 Elongation 2 2 4 2 3 6 2 2 4 2 3 6 Chemical stability 2 2 4 2 3 6 2 3 6 2 3 6 Weld defects 2 3 6 2 1 2 2 1 2 2 1 2 Penetration 1 3 3 1 3 3 1 3 3 1 3 3 Distortion 1 1 1 1 2 2 1 2 2 1 2 2 Suitability for use Welding thin sheet (\1mm)224 236 236 224 Sheet welding (.3mm)111 122 122 133 Welding Al-Mg alloys 1 2 2 1 2 2 1 2 2 1 2 2 Overhead welding 1 1 1 1 3 3 – – – – – – Variable material thickness 2 1 2 2 1 2 2 2 4 2 2 4 Variable welding speed 1 1 1 1 2 2 1 3 3 1 2 2 Welding of castings 2 2 4 2 3 6 2 2 4 2 2 4 Joining cast to wrought alloys 1 3 3 1 3 3 1 1 1 1 2 2 Repair welds on castings 2 3 6 2 3 6 2 1 2 2 2 4 Suitability for automation With filler 1 1 1 1 3 3 1 2 2 1 3 3 Without filler 2 3 6 2 1 2 2 1 2 2 1 2 Butt welding \3mm2 12 2 24 2 24 2 24 .3mm 1 22 1 11 1 33 1 11 Suitability for joint type Butt joint 1 2 2 1 1 1 1 1 1 – – – Lap joint 1 3 3 1 3 3 1 1 1 1 1 1 Economic aspects Equipment costs 3 2 6 3 3 9 3 2 6 3 1 3 Maintenance costs 3 2 6 3 2 6 3 3 9 3 2 6 Labour costs 1 3 3 1 2 2 1 3 3 1 3 3 Welder’s training time 1 1 1 1 1 1 1 1 1 1 3 3 P Process rating ( ) 80897376

Imp.: importance level; Ad.: advantage level; I. Fac.: impact factor; EBW: electron beam welding; FSW: friction stir welding; LBW: laser beam welding; MIG: metal inert gas welding; PAW: plasma arc welding; RW: resistance welding; TIG, tungsten inert gas welding; UW: ultrasonic welding. weld metal on the surface of a plate during bead-on-plate considered provides better process selection and thus a weldstoadepthof3mmforTIGand6mmforMIG. better evaluation. Therefore, to attain complete penetration for welds over It is important to point out that the scaling is subject 3 mm (MIG) and 6 mm (TIG), there is the need for bevel- to the designer’s discretion and not completely objective. ling on butt joints, for example. The bevel can be single or The welding designer determines the importance level of double sided.9 the selected aluminium-welding project by answering a As presented in Table 4, eight considered welding question like ‘how important is’ strength, elongation, processes are correlated with their applicability on six chemical stability, etc., to the finished product. The different welding joints. Butt and lap joints are applica- designer defines the importance level on a scale of 1–3 ble to all the selected weld processes. Cruciform joints (1= least, 2 = moderate and 3 = high). In a similar have the least applicability across the processes, which fashion, the advantageous level is determined by answer- is due to limited fixturing possibility during welding. ing a question like ‘how advantageous is’ the selected Table 4 provides additional information on the viabi- welding process to the selected consideration. The lity of six joint types on the eighth selected welding pro- importance level is multiplied by the advantage level cesses by presenting the process-specific limitations. and the result is called an impact factor. The impact fac- An application of this review article is to use the pre- tor is summed up for each selected welding process, and stated information to influence the selection case- the welding process with the highest impact factor sum- specific optimum welding process. It can be challenging mation is selected as the optimal welding process. to determine an appropriate welding process to be used for aluminium. However, the challenge can be simpli- fied by considering various comparison selection factors Case study as presented in Table 5. The solution to the challenge is A welding process for high-strength aluminium for aero- case specific. An understanding of the selection factors space is to be selected. The available welding processes Olabode et al. 1135 are as presented in Table 5. A blank table is constructed detrimental when welding anodised aluminium as the and the considered welding processes are selected and anodised layer has to be cleaned before welding. The filled into the table. melting point of aluminium alloys is generally around The selection factors under consideration are as pre- 660 °C and the melting point of aluminium oxide is sented in Table 5, which are categorised under quality 2050 °C. It is therefore recommended that the aluminium of the weld joint, suitability for use, suitability of fillers, oxide layer or the anodised layer be removed, mechani- joint suitability and economics. Therefore, at this stage cally or chemically, just before welding. in the design, the processes row and the selection factor Aluminium alloys have high chemical affinity; there- column are filled in the table. fore only inert gases can be used as shielding gases dur- As the designer, the importance level is determined ing welding. Argon and helium gases are used in and designed on a scale of 1–3, and using a scale of five aluminium welding to protect the weld pool. The pres- is also applicable, but the calculation becomes more ence of helium increases the arc heat input and there- complex. Choosing a scale of 1–3 (1 = low, 2 = moder- fore allows for deeper penetration compared with ate and 3 = high), a number is assigned to the consid- argon gas, but on the other hand, helium is more ered selection factor. Therefore, at this stage, the expensive than argon. A mixture of helium and argon importance level of the selection factor under consider- is sometimes used to improve weldability of some alu- ation is filled into the ‘Imp.’ column (Table 5). It is minium alloys. A wider range of shielding gases would important to note that the number is the same across increase the manipulation possibility for aluminium row (all processes) because the importance of a selec- alloy welding, but currently argon and helium are the tion factor is independent of the process. only gases used. The advantage level is determined and designed by The industrial welding processes considered in this the designer on the same scaling used for importance work include MIG, TIG, plasma arc welding (PAW), level. If the scaling used in importance level is five, the FSW, LBW, resistance welding (RW), electron beam scaling of five should be used. In this case, a scaling of welding (EBW) and UW. The weldable thickness is a 1–3 is used where 1 = low, 2 = moderate and 3 = high. limitation in all the processes; the highest weldable At this stage, the entire advantageous level column on thickness of up to 70 mm is achieved with EBW. FSW Table 5 is filled for all the considered selection factors produces the best weld because the mechanical property into the ‘Ad.’ column. deterioration is minimal, and the process is friendly as The calculation for the impact factor and the process no fumes or spatters are produced during welding. rating is carried out. The impact factor for each consid- The joint configurations considered include the butt ered selection factor is derived by multiplying the joint, lap joint, T-joint, edge joint, corner joints and importance level column of each process by advanta- cruciform joint. The butt joint and lap joint are appli- geous level column of each process. The derived value cable to all the considered welding processes. The pos- is filled into the ‘I. Fac.’ (impact factor) column of sibility of using different joint orientations with the Table 5. The process rating (welding process) is derived considered welding processes depends on the manipula- by the summation of all the impact values column of tion of the work piece (fixturing). each process. Therefore, the process rating row is filled Although FSW produces the best weld for alumi- in Table 5. nium alloys, the optimal welding process is case spe- The optimum weld process is the process with the cific. The designed table for weld process selection highest process rating, which in this case study is pro- provides information on how to select the optimal cess C friction stir welding (FSW). process based on case-specific considerations for alu- minium alloys.

Conclusion Declaration of conflicting interests This article examined the surface-related challenges, joint types and limitations of aluminium alloys with the The authors declare that there are no conflicts of focus on providing a guide on how to select an optimal interest. welding process. Aluminium and its alloys have welding challenges, which include the presence of aluminium Funding oxide on surfaces, welding of anodised aluminium and limited shielding gas options. The aluminium oxide sur- This research received no specific grant from any fund- face is formed when aluminium is exposed to an atmo- ing agency in the public, commercial or not-for-profit sphere containing oxygen, and the aluminium oxide has sectors. to be cleaned away from the surface before welding because its causes weld defects like porosity. The chemical affinity of aluminium for oxygen is uti- References lised for anodising aluminium alloys and then painting 1. Anderson T. Aluminum’s role in welded fabrications. to improve corrosion resistance. However, it can be Weld J 2009; 88: 26–30. 1136 Proc IMechE Part B: J Engineering Manufacture 227(8)

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Publication III

Effect of Al2O3 film on the mechanical properties of a welded high-strength (AW 7020) aluminium alloy.

Olabode, M., Kah, P., and Martikainen, J. (2015). Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture. DOI: 10.1177/0954405415600678

© Sage publications, 2015

Original Article

Proc IMechE Part B: J Engineering Manufacture 1–10 Effect of Al2O3 film on the mechanical Ó IMechE 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav properties of a welded high-strength DOI: 10.1177/0954405415600678 (AW 7020) aluminium alloy pib.sagepub.com

Muyiwa Olabode, Paul Kah, Esa Hiltunen and Jukka Martikainen

Abstract The use in motor vehicles of lightweight metals such as aluminium and titanium provides a high strength-to-weight ratio, thereby lowering overall weight and reducing energy consumption and CO2 emissions. Aluminium alloys have thus become an important structural material especially high strength and ultra-high strength alloys such as AW 7020. Many studies have shown that the presence of an aluminium oxide (Al2O3) thin film formed naturally on aluminium alloys is detrimental to welding. This article further investigates the specific effect of the Al2O3 thin film on welding AW 7020 alloy. An analytical experiment of welded AW 7020 alloy using a pulsed metal inert gas (MIG) robotic weld machine is carried out. Four specimens were cut, butt welded, and examined. The weld parameters included pre-weld cleaning of the Al2O3, pre-, and post-weld heat treatment. Al2O3 was removed by wire brushing; preheating was conducted at a temperature of 130 °C; and natural ageing was conducted by post-weld heating at 480 °C for 2 h, followed by quenching in water at 90 °C for 8 h, reheated, and sustained at 145 °C for 15 h. The result shows that the presence of Al2O3 layer appears not to be detrimental to the weld with new welding technologies, therefore suggesting that it is not necessary to grind off the Al2O3 layer before welding. This finding implies that welding costs can be lowered and weld quality improved when new welding technologies are applied in the welding of high-strength aluminium alloys.

Keywords Al2O3, AW 7020, high-strength aluminium, pulse MIG welding, mechanical properties

Date received: 23 April 2014; accepted: 13 July 2015

Introduction chemical properties. It describes the formation process and the composition of the two anodic layer films. The use of lightweight metals in industrial applications Properties such as density, melting point, and thermal has gained importance recently as a means of achieving conductivity are presented. The advantages, disadvan- a greener environment with low pollution. For exam- 1,2 tages, and applications of Al2O3 are also presented. Its ple, studies show that the use of lightweight material formation can be controlled to gain structural advan- in the construction of car bodies reduces weight, fuel tages and improved characteristics, for example, by consumption, and CO2 emissions. Welding of alumi- adonisation. nium is considered challenging due to the inherent The purpose or this research is to study the effect of properties of aluminium alloys such as the high heat Al O on the mechanical properties of AW 7020. New conductivity of aluminium alloys and the presence of 2 3 welding technologies for aluminium welding are studied an aluminium oxide (Al O ) film that appears when the 2 3 (because newer technologies are expected to provide alloy is exposed to the atmosphere (which is detrimen- faster weld speed, cheaper welding cost, and improved tal to welding).3,4 welding equipment efficiency), with focus on the effect Researches have shown that the presence of Al O is 2 3 of Al O on the processes and how the welding process detrimental to the welded piece and it also presents 2 3 challenges for the welding process. The information gap of how detrimental is the Al2O3 to AW 7020 weld if Lappeenranta University of Technology, Lappeenranta, Finland new welding technologies that can prevent oxide inclu- Corresponding author: sion are used exist. Muyiwa Olabode, Lappeenranta University of Technology, Skinnarilankatu This article discusses the Al2O3 layer formed on alu- 34, 53850 Lappeenranta, Finland. minium alloys exposed to air or moisture and its Email: [email protected]

Downloaded from pib.sagepub.com by guest on September 1, 2015 2 Proc IMechE Part B: J Engineering Manufacture is used to produce acceptable welds despite the pres- 21,675 kJ. This is a very high oxidation reaction energy ence of Al2O3 film. and explains why aluminium has high affinity towards The experiments are carried out on an AW 7020 oxygen.1 alloy using a robotised pulse MIG machine. The test The closest layer to the aluminium alloy is called the pieces were cut across the weld, etched, and tested for barrier layer and is illustrated in Figure 2. This layer hardness and tensile strength. As a contribution to the has dielectric properties2 and forms as soon as the alu- literature, this research provides facts on the effect of minium alloy is exposed to an oxidising media. This Al2O3 on AW 7020. layer consists of cells and pores that are generated due to reaction with the external environment. Attaining the final thickness can take several weeks or even Al2O3 layer of exposed aluminium months, depending on the physicochemical conditions 3 Aluminium is resistant to corrosion because alumi- of the environment. This layer is less compacted than nium, like all other passive metals, is covered with a the barrier layer, and because of the presence of the continuous and uniform natural oxide film on exposure pores, it reacts with the external environment during to an environment containing oxygen (as illustrated in transformation. Figure 1). The film is formed spontaneously in oxidis- ing media according to the following reaction Al2O3 characteristics and properties 3 Amorphous alumina, chemically called Al O , forms at 2Al + O2 ! Al2O3 2 3 2 a temperature range of less than 50 °Cto60°C and has a density of 3.40. Further principal properties are pre- sented in Table 1. It is important to note that there is a Formation process difference in the density of the aluminium alloy and the

Al2O3 layer forms immediately, within 1 ms or even Al2O3, so the Al2O3 film is under compression. This dif- 1 less. Al2O3 is a natural, non-uniform, thin, and non- ference is responsible for the ability of the Al2O3 film to coherent colourless oxide film made up of two superim- resist deformation without breaking and the excellent 4 posed layers of a thickness in the range of 4–10 nm. resistance during forming operations. The mechanical The oxidation reaction has a free energy of reaction and structural characteristics of Al2O3 layer are depen- dent on the oxygen partial pressure.5 It is should be noted that the composition of the oxide film depends on the chemical composition of the aluminium alloy. Therefore, the thin film oxide layer is not the same for all classes of aluminium alloys.6

Al2O3 advantages, disadvantages, and applications

Figure 1. Schematic of aluminium (melting temperature of The Al2O3 layer brings both advantages and disadvan- 660 °C) and its oxide layer (melting temperature of 2050 °C). tages to the use of aluminium in structures. It is

Figure 2. Schematic diagram of a cross section of a porous anodic film on aluminium showing the barrier, pore, and other principal morphological features.

Downloaded from pib.sagepub.com by guest on September 1, 2015 Olabode et al. 3

advantageous as it is responsible for the corrosion resis- that natural self-occurring Al2O3 is very thin (0.01 mm); tance of aluminium alloys as presented in Table 2. anodising produces a higher thickness range (12– 14 Al2O3 allows for increased surface treatability of alumi- 25 mm). The advantages of anodising include fade nium alloys using procedures such as anodising and resistance of structural aluminium alloys up to 15 painting. The presence of Al2O3 can cause weld defects 50 years, corrosion resistance, abrasion resistance, such as incomplete weld fusion and weld porosity. electrical insulation, unique decorative colours, adhe- Incomplete fusion describes a weld that does not com- sive bonding, decorative appearance, paint adhesion, pletely merge or mix and porosity describes a weld in improved lubricity, permission of subsequent plating, which gas bubbles are present in the weld. increased emissivity, surface flaws detection, and photographic and lithographic emulsions application possibility.16–18 Al2O3 modification for structural advantage by The three main anodising processes are chromic ano- anodising dising (in which the agent is chromic acid), sulphuric anodising and sometime referred to as mild adonisation The thickness of the Al O layer can be varied to gain 2 3 (in which the active agent is sulphuric acid), and hard structural advantages by anodising. Anodising is a con- anodising (in which the agent is sulphuric acid, alone or trolled corrosion process of aluminium alloys in alka- in combination with additives).19 line and acidic electrolytes to attain a uniform continuous protective oxide film.13 It employs the unique ability of amorphous alumina to build up an New welding technology for aluminium even porous morphology. It is important to mention alloys

Table 1. Principal properties of Al2O3. Newer welding technologies are expected to provide better and cheaper welding processes, which make its Property Value important to be studied. Resistance welding (RW) can Melting point 2054 6 6 °C be used in the form of seam welds or spot welds. The Boiling point 3530 °C fusion occurs due to the heat created by a flowing cur- Linear expansion coefficient at 25 °C 7.1 3 1026 K rent through a resistance device for a given period of Thermal conductivity at 25 °C 0.46 J/cm/s/K time while the materials to be welded are pressure Specific heat at 25 °C 0.753 J/g/K pressed against each other.20 The presence of Al O on Dielectric constant at 25 °C 10.6 2 3 Electrical resistivity at 14 °C1019 O/cm the pre-weld surface influences the total resistance across the weld electrodes. RW is thus a surface critical

Table 2. Advantages, disadvantages, and applications of Al2O3.

Advantages Disadvantages Application

1. It is responsible for the corrosion 1. The melting point of Al2O3 1. Al2O3 is used as insulators, an ion resistance of aluminium alloys. (approximately 2050 °C) is higher barrier and a protective layer in 9 2. Al2O3 allows for increased than that of AW 7020 thin film industries. surface treatability of aluminium (approximately 660 °C), which 2. The Al2O3 layer is used in the alloys with procedures such as implies that a higher heat density manufacture of marine vehicles anodising and painting. than welding heat density is requiring high corrosion 3. Al2O3 made through ALD is very needed to break the Al2O3 resistance. thin and consistent. It has structure. 3. Due to its high, wear resistance, excellently controlled thickness 2. It has a significant mechanical Al2O3 is used in protection and composition of the film at an strength. During welding, the against friction and corrosion in atomic level.7,8 ALD has excellent oxide layer can therefore remain optical applications. dielectric properties, thermal and as a solid film (or fracture into 4. The Al2O3 layer is used in micro- chemical stability, and good small particles) due to the flow of electromechanical systems as a adhesion to various surfaces and the molten material,7 even when dielectric layer to prevent is therefore used for silicon the surrounding metal is molten. electrical short-circuit5 and as microelectronics.9 This can result in severe catalyst.12 4. The presence of an oxide layer incomplete fusion defects. during laser welding increases the 3. Due to its higher weight absorptivity of aluminium and its compared to aluminium, Al2O3 alloys to laser radiation.10,11 can lead to weld inclusion. 4. Its hygroscopic properties make it bind to moisture, which leads to the formation of pores in the welds.7

ALD: atomic layer deposition.

Downloaded from pib.sagepub.com by guest on September 1, 2015 4 Proc IMechE Part B: J Engineering Manufacture process; it is therefore recommended to remove the electrode positive (EP) to electrode negative (EN). oxide before welding. With this technology, there is now the possibility of Friction stir welding (FSW) is a mechanical solid- adjusting the EP and EN independently to enable good state welding process that softens the material to be cleaning action (EP) and good penetration (EN).28 welded by the heat generated by friction between a Another modification to PAW is plasma MIG welding, rotating tool and the workpiece. There is no need to which uses a coaxial MIG welding torch. Plasma MIG clean off the oxide layer prior to welding. However, welding reduces spatter and fume formation and flaws due to oxide inclusion can occur if the tool improves the weld bead appearance (the weld bead is shoulder selected is oversize thereby sweeping surface flatter with deeper penetration in Al–Mg weld if the oxide into the weld. The amount of oxide inclusion can plasma current is increased). With plasma MIG weld- 29 be reduced by increasing the weld speed, resulting in ing, pre-weld Al2O3 layer removal is important. low oxide layer disruption per millimetre.21 Another new modification is laser-assisted FSW (in which the laser is mainly used for preheating) which has the addi- Experimental procedure tional advantage of using more simple and inexpensive The purpose of this experiment is to study the effect of machines, in addition to the reduction in tool wear and 22 the presence Al O of layer on the mechanical proper- higher attainable welding speeds. 2 3 Low-energy arc (MIG) welding methods such as cold ties of AW 7020. The experiment was carried out as metal arc transfer (CMT) are a recent development of the butt welds of two samples each for the four weld experi- MIG process. Low-energy arc welding uses the wire feed ment conditions (ECs) 1–4 with the weld parameters system to control the weld process. The wire is fed into the presented in Table 3. A robotised pulsed MIG machine weld pool until the short circuit occurs, after which the was used to weld the specimen. The weld set-up is pre- feeddirectionisreversedandthefeedwireiswithdrawn. sented in Figure 3, respectively. A 4-mm AW 7020 plate Thewirefeedisthenfedforwardagainandtheprocess was used as the workpiece. The air gap between the begins anew.23 The process utilises high-speed digital con- workpiece was 3 mm. A copper backing was used. Pure trol systems to control the arc length, metal transfer, and argon (99.5%) shielding gas supplied at a flow rate of thermal input on the workpiece. When integrated with 15 L/min was used. A 1.2-mm-diameter Elga AlMg5 fil- ler material was supplied at 9 m/min. A nozzle distance pulsed MIG, it produces even better welds and the Al2O3 film is decomposed by the pulsed MIG process.24 of 15 mm and a welding speed of 7.5 mm/s were used. Laser welding employs the use of laser beams as a The weld torch was inclined at 15° to normal and the heat source for welding aluminium. Newer technologi- weld direction was such that the torch is pulling. An cal modifications involve the use of an active flux that average current of 140 A and an average voltage of improves the mechanical properties and appearance of 22.7 V were used in all the experiments. the aluminium weld.23,25 In addition, dual beam lasers The test was carried out in the welding workshop, in are used, producing better weld quality (deep penetra- a well-controlled atmosphere and at room temperature. tion, surface smoothness, and high strength) compared The samples for the four different EC were cut, welded, to single beam laser welds.26 and examined. In EC 1, the weld was carried out with- Hybrid laser welding involves the use of a conven- out pre-weld cleaning of the Al2O3 in the absence of tional arc welding process in combination with laser pre- and post-weld heat treatment. In EC 2, the weld welding. The process utilises the advantages of both laser was conducted without the removal of Al2O3. However, and arc [Metal inert gas (MIG) or Tungsten inert gas the workpiece was preheated at a temperature of 130 °C (TIG)] welding such as high process speed, low heat within the recommended preheating temperature and 30 input, low thermal distortion, good gap bridging ability, close to the upper limit. The oxide layer in EC 3 was and good process stability23,27 with high-precision weld- not removed before the welding. No preheating was ing. It is important to mention that in TIG welding pro- carried out but natural ageing was conducted by post- cess Al2O3 film decomposition occurs by cathode weld heating at 480 °C for 2 h, followed by quenching etching, however, it is still important with hybrid laser in water at 90 °C for 8 h, and finally, reheating and TIG welding processes to remove the Al2O3 layer just maintaining the workpiece heat at 145 °C for 15 h. The before welding. Al2O3 layer in EC 4 was removed and no preheating or Plasma arc welding (PAW) is a high-power-density artificial ageing was carried out. In order to investigate weld method for aluminium that is advantageous for the effect of Al2O3 on the mechanical properties, the making deep welds. In addition to the general informa- samples were examined for ultimate yield strength tion in scientific and technical articles, aluminium welds (YS), tensile strength, elongation, and hardness values. are stabilised using direct current (DC) power and neg- Macrographs were taken to evaluate the degree of weld ative polarity; research has shown that stability can defects present, if any. also be achieved using the alternating current (AC) Further experiments were carried out on the welded 23 power source. Variable polarity plasma arc welding sample to study the composition of Al2O3 layer at dif- (VPPA) is a relatively new technology that uses ferent distances from the alloy surfaces. A sample of advanced power supply to generate rapid switches from over 99% pure aluminium alloy (1xxx series), AW

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Table 3. AW 7020 weld experiment parameters.

Welding conditions for AW 7020 welding Weld type Butt welding, I-groove, air gap 3 mm, against copper backing Base material AW 7020, thickness 5 mm Filler material Elga AlMg5, ; 1.2 mm Shielding gas Ar, flow rate 15 L/min Wire feed rate 9 m/min Welding speed 7.5 mm/s Nozzle distance 15 mm Torch angle 15° to normal

Experiment-specific parameter

ECs Current (A) Voltage (V) Al2O3 thin film Preheating Artificial ageing averages averages

1 140 22.6 Present No No 2 139 22.8 Present Yes (130 °C) No 3 140 22.7 Present No Yes (480 °C/2 h + quenching in water, 90 °C/8 h + 145 °C/15 h) 4 140 22.7 Absent No No

ECs: experiment conditions.

and 3.3 mm. For each depth, four measurement spots of 0.2 3 0.5 mm were selected and each significant chemical content was analysed; the results presented in Table 4 are based on the averages.

Result The mechanical properties of the welds were observed and the results are presented. The result consists of ten- sile test (that provides information on the YS, ultimate tensile strength (UTS), and elongation at fracture), hardness test, and macrograph examination.

Tensile tests The tensile test bar graph in Figure 4 presents a com- parison of the YS (Re/N/mm2), UTS (Rm/N/mm2), and elongation at fraction in A/%. The y-axis is mea- sured in units and the x-axis represents the averages of the four different ECs and the control condition. The YS values represent the amount of force the welded AW 7020 can resist before plastic deformation. The UTS value shows the amount of force needed to break the weld, and the elongation shows how far the weld will stretch before breaking. The tensile test measures the YS, which is the stress value at which welded specimen begins to deform plas- Figure 3. AW 7020 weld set-up. tically and cannot return to its original position. It is used in this experiment to express the load bearing capacity of the weld just before plastic deformation. The higher the YS, the more desirable is the weld. 7020, and 7025-T6 were placed to a thermo scientific Based on the YS values, EC 1 is the best weld while EC ultra dry Silicon Drift Detector (SDD) energy- 4 is the worst weld, which is due to the effect of remov- dispersive X-ray spectroscopy (EDS). Three weld sam- ing the Al2O3 layer thereby increasing the amount of ples were pre-cleaned and then exposed to atmosphere weld heat input. As seen in EC 2, preheating also seems for 1 h. The samples were then tested for the presence to reduce the YS while artificial ageing appears to of Al2O3. Each sample was tested at a depth of 0.2, 1.2, improve the YS in EC 3.

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Table 4. Percentage weight composition weld samples.

Oxide layer formation period 1 h (after cleaning) Measuring spot 0.5 3 0.2 mm Correction method Proza (Phi-Rho-Z) Take off angle 35.0° Measurement acceleration voltages 3 kV (0.2 mm) 10 kV (1.2 mm) 20 kV (3.3 mm)

Test depth from surface Material O (wt%) Al (wt%) Mg (wt%) Zn (wt%)

0.2 mm Al 99.90% 12.7 87.3 AW 7020 6.55 87.05 1.05 5.4 7025-T6 6.25 87.3 1.1 5.35 1.2 mm Al 99.90% 4.1 95.9 AW 7020 1.525 92 1.2 5.3 7025-T6 1.55 91.95 1.175 5.325 3.3 mm Al 99.90% 2.75 97.3 AW 7020 1.2 93.1 1.2 4.6 7025-T6 1.075 93.35 1.15 4.4

Figure 4. Tensile strength of welded AW 7020.

The UTS is used to present the maximum tensile piece will be brittle, and therefore, it can easily crack or loading the weld can be subjected to before failure. break, for example, brittle ceramic cracks easily when The higher the UTS, the better the weld is from the subjected to tensile loading. On the other hand, if the load bearing perspective. In these experiments, EC 3 elongation value is high, the specimen is ductile and produced the highest UTS value of 273.55 Rm/N/ can be plastically deformed. In many aluminium welds, mm2. This seems to be due to the effect of artificial it is desirable to have high elongation values. The best ageing. EC 1 has the next high, which seems to be weld is usually case specific based on the mechanical or due to the low heat input to the workpiece due to the metallurgical properties of the weld demanded by the presence of Al2O3. EC 2 has the next high value, application. For example, aluminium welds that are which suggests that workpiece preheating reduces the designed to carry torsion loads such as shafts are sup- UTS values. The least UTS value is in EC 4 which posed to be rigid with minimal elongation. On the other suggests that the removal of Al2O3 layer reduced the hand, structural aluminium beams are expected to have UTS. elongation so they do not break suddenly. EC 3 has the Elongation at fracture expresses the ratio as a per- highest elongation values which suggest that artificial centage of the final length to the original length to ageing increased the malleability of the workpiece. The which the area of the specimen stretches just before fail- next high elongation value is in EC 4 which suggests ure. The elongation shows how brittle or ductile the that the absence of Al2O3 increases elongation in com- weld specimen is. If the elongation is low, the weld parison to EC 1.

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Figure 5. Hardness profile of welded AW 7020.

Hardness test as porosity and cracks, if there are any. In addition, The hardness profile graph (Figure 5) presents the hard- they also show the HAZ and the location of the WI from the weld centre line. The macrograph samples also ness values of the profile across the weld interface (WI). present the bead profile. It is important to mention that The hardness profile shows how much the hardness the indentations in Figure 6 are made by the hardness deviated from the base material (BM) into the unmixed testing machine and the position of the indents from zone (UMZ) and vice versa. The y-axis represents the the plate surface is approximately the same for all the hardness value (HV3) while the x-axis represents the experiments. In the four ECs, it appears that there are distance in millimetre from a common reference in the no cracks or porosity on the macro scale which suggest BM to the weld centre. It is important to mention that that the welds are acceptable. 0inthex-axis is located in the BM and the scale increases towards the UMZ. The hardness test is done using a diamond tip inden- EDS ter to create indentations on the weld cross section. The The EDS result is presented in Table 4 showing the test indenter has two diagonals, which are measured and parameters. The measurement acceleration voltages 3, used to determine the hardness value on the HV3 scale. 10, and 20 kV represent the calculated depths of 0.2, The indenter carries a load that is enough to create an 1.2, and 3.3 mm. At 0.2 mm depth, the presence of oxy- indentation on the alloy. The hardness of the material gen is highest in all the samples and lowest at 3.3 mm. determines the material resistance against the indenter. The oxygen content in addition to the other elements in This implies that the softer the material (aluminium AW 7020 and 7025-T6 are relatively close. This may be alloy), the deeper the indents and the longer the diago- due to the same alloy series they belong to in the classi- nals of the indents. The depression caused by an inden- fication. The classification is based on the chemical ter can be seen in Figure 6. composition of the alloy. In Figure 5, the average hardness values are denoted by the nodes on the line graph. The hardness from the BM to the weld centre line should have minimal fluc- Discussion tuation. The WI denotes the point at which the weld The effect of Al O on the AW 7020 weld is based on fusion line appears. It can be seen that the greatest 2 3 the hardness profile of EC 1 and EC 4, which are simi- hardness fluctuation is around the heat-affected zone lar (Figure 5). However, the hardness values of EC 1 (HAZ) and the WI, which are usually the areas more are higher than EC 4. The presence of Al O layer in prone to structural failure. EC 3 (Figure 5) has the best 2 3 the weld process (EC 1) increased the YS by 20% and hardness profile of the four ECs while EC 2 has the the UTS by 6% but reduced the elongation by 29% worst hardness profile, especially across the WI. (compared to EC 4). This result shows that when Al2O3 layer is not removed before welding, improved hardness of AW 7020 weld was attained (it is important to note Macrograph analysis that there are no weld defects such as porosity due to The macrograph samples of each EC are presented in oxide inclusion in the weld pool). The question there- Figure 6 using 10 3 objective lens to present the inter- fore arises whether the higher strength could result from action between the weld pool and the BM across the the reduced heat that gets into the weld pool due to the WI. These pictures are used to present weld defects such heat resistivity of the Al2O3 layer in addition to the

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Figure 6. Macrographs of welded AW 7020.

suspected absence of chemical interaction of Al2O3 defects. Based on the macrographs, the welds in this layer during the weld (due to the welding technology study appear to exhibit no defects (Figure 6) and weld parameters). This can be clarified by further The necessity of pre-weld Al2O3 removal is examined multiple experiments. However, it is important to men- in this study. Acceptable welds were achieved without tion that if there is a chemical reaction in which Al2O3 pre-weld removal of the Al2O3 layer (Figure 6, EC 1-3) layer is present in the weld (causing porosity), the using a pulsed MIG welding process. This may be due mechanical properties will be lower. to the low chemical interaction of Al2O3 with the weld The effect of pre-weld heat treatment on the AW pool as the EDS result shows that the oxygen content 7020 weld is detrimental to the weld comparing EC 2 of Al2O3 in AW 7020 is about 50% lower than in pure to the other three in Figure 5. The hardness profile aluminium. It therefore suggests that with new welding across the weld in EC 2 is more uneven with sharp fluc- technologies such as pulsed MIG and FSW, it is not tuations in hardness values. For example, there is a necessary to remove naturally formed Al2O3 layer sharp decrease in the hardness value from 81.7 to before welding high-strength aluminium (HSA) alloys. 51.1 HV across the WI. This is usually a crack failure Good welds may have been attained due to the lower point in the weld piece. In EC 2, the WI is closer to the amount of oxygen present on the surface of AW 7020 UMZ (narrower HAZ) which is better when narrower compared to pure aluminium. This suggests why HSA weld seam is desired; however, it reduced the hardness alloys have lower corrosion resistance in comparison to values. Preheating reduced the YS by 17% and UTS by pure aluminium. It is important to mention that based 3% but increased the elongation by 17% (Figure 4). on the literature review, the effect of Al2O3 in alumi- The effect of artificial ageing on the AW 7020 weld nium welding seemed to have been active in the 1940s is that artificial ageing relatively smoothens the hard- until 1960. There appears to be a break in the interest ness profile, in addition to increasing the hardness val- for this research, as it seems to have then picked up ues in the HAZ, WI, and UMZ. Comparing EC 1 and again from 1990 until date, which suggest that there EC 3, the hardness value at the WI increased from 63.6 was a lost in interest during the 1970s and 1980s. to79.3 HV (Figure 5). Artificial ageing reduced the YS by 8% but increased the UTS and elongation by 9% Conclusion and 110%, respectively (Figure 4). It therefore suggests that artificial ageing improves the mechanical proper- This study was carried out to investigate the effect of ties of welded AW 7020 provided there are no weld the Al2O3 film on the mechanical properties of HSA

Downloaded from pib.sagepub.com by guest on September 1, 2015 Olabode et al. 9 alloys using pulsed MIG-welded AW 7020 as a case 3. Hunter M and Fowle P. Natural and thermally formed study. The structural formation of the Al2O3 layer was oxide films on aluminum. J Electrochem Soc 1956; 103: briefly explained; the structure varies depending on the 482–485. class of aluminium alloy. The characteristics and prop- 4. Dunlop H and Benmalek M. Role and characterization of surfaces in the aluminium industry. J Phys IV 1997; erties of the Al2O3 layer were discussed, and the study presented how the Al O structure can be modified for 7(C6): 6–163. 2 3 5. Peng WW, Roy P, Favaro L, et al. Experimental and ab structural advantage. A brief description of new weld- initio study of vibrational modes of stressed alumina ing technologies used for aluminium alloys was pre- films formed by oxidation of aluminium alloys under sented. In addition, weld defects in aluminium welds different atmospheres. 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Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys.

Olabode, M., Kah, P., and Salminen, A. (2015). Rev. Adv. Mater. Sci, 42, 6-9.

©2015 Advanced Study Center Co.Ltd.

6Rev.Adv. Mater. Sci. 42 (2015) 6-19 M. Olabode, P. Kah and A. Salminen

OVERVIEW OF LASER SYSTEMS AND OPTICS APPLICABLE TO HYBRID LASER WELDING OF ALUMINIUM ALLOYS

Muyiwa Olabode, Paul Kah and Antti Salminen

Lappeenranta University of Technology, Lappeenranta, Finland Received: January 27, 2015

Abstract. The need for green and sustainable energy is continually on the rise. The use of light weight yet load bearing materials like aluminium has become important as structural materials. Aluminium can be fabricated by welding which is challenging compared to steel due to the presence of aluminium oxide coating and high conductivity of aluminium. The objective of this paper is to present an overview of the optics and laser systems applicable to hybrid laser welding of aluminium. This article is a critical review on aluminium alloys and their weld defects including hot cracking, porosity and heat affected zone (HAZ) degradation. Furthermore, the effect of the properties of aluminium in fusion welding, hybrid laser welding optics and the challenges alu- minium presents to hybrid laser welding are also studied. It is observed that aluminium limited the selection of hybrid laser welding system and optics. The configuration of the welding head is critical to the effectiveness and efficiency of the welding system. The required weld properties influence possible optimization of hybrid laser welding. This article can be used by welders and welding engineers for hybrid laser welding of aluminium in addition to understanding how viable is hybrid laser welding of aluminium.

1. INTRODUCTION makes welding challenging [7,8]. There are differ- ent welding systems applicable to aluminium weld- The need for lightweight metal for construction and ing like laser bean welding (LBW), friction stir weld- fabrication is on the increase due to the advantages ing (FSW), metal inert gas (MIG), tungsten inert of sustainable energy and economy [1]. Aluminium gas (TIG), hybrid laser beam welding (HLBW), is the second most used structural material after plasma arc welding (PAW), submerged arc welding steel [2,3]. The increased rate is due to advanta- (SAW), and others. TIG weld process had been the geous properties of aluminium such as its light- most industrially accepted welding process for alu- weight to strength ratio, relative corrosion resistance minium [9]. Studies have shown that FSW, pulsed [2], ease of machinability.They are used in the trans- MIG and HLBW produce better welds than TIG [10]. portation industry, due to its relative low density in This paper focuses on hybrid laser welding optics comparison to steel, the lower dead-weight of con- applicable to aluminium. It further presents the chal- struction and low energy consumption with minimal lenges of aluminium alloy welding in HLBW. compromise to load carrying capacity [4]. About 50% of aluminium extrusions are used in the trans- portation industry [5]. Other sectors include con- 2. ALUMINIUM ALLOYS struction and power transmission [6]. Aluminium and its alloys are grouped into cast alu- Aluminium and its alloys have their disadvan- minium and wrought aluminium alloys [11,12]. The tages like high reflectivity and high conductivity that wrought alloys are usually used in fabrication be- Corresponding author: Muyiwa Olabode, e-mail: [email protected]

© 2015 Advanced Study Center Co. Ltd. Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys 7

Fig. 1. Woodward diagram showing general relationships between some properties of aluminium alloys. cause of its high strength compared to cast alloys to weld (due to the chemical properties). Further [13]. This paper focuses on wrought alloys. The relationship between the properties of aluminium wrought aluminium alloys are grouped into series alloys is presented in Fig. 1. based on the chemical composition. They are de- noted by 4 digits where the first denotes the char- 3. COMMON ALUMINIUM WELDING acteristic alloying element. They range from 1xxx DEFECTS to 9xxx series. For example, 99% pure aluminium belongs to 1xxx series while high strength aluminium Welding of aluminium is rather critical despite the (HSA) alloy like 7025 belongs to the 7xxx series. fact that it has lower melting point compared to steel. Aluminium alloys weigh about 1/3 of copper and Criticality of welding aluminium is due to the: iron at equal volume. It is slightly heavier than mag- 1. Presence of a stable surface oxide formed on nesium and slightly lighter than titanium and it is a exposure to oxygen relatively weak metal. Alloying of aluminium can be 2. Presence of residual stresses that causes weld done to attain high strength. Aluminium is resistant cracks due to aluminium’s high thermal expansion to corrosion due to the formation of its thin oxide coefficient. layer on exposure to moisture. Aluminium conducts 3. High heat conductivity of aluminium that implies electricity, heat and reflects light and it is easy to that high heat input is required for achieving sound fabricate. welds. High heat input on the other hand, increases HSA alloys like the 2xxx, 7xxx, and 8xxx are the possibility of distortion and cracking. becoming of high industrial interest because their 4. High shrinkage rates on solidification, that en- yield strength is comparable to mild steel. However, hance cracking. the higher the yield strength the more difficult it is 5. High solubility of hydrogen in molten aluminium which causes porosity. 8 M. Olabode, P. Kah and A. Salminen

Fig. 2. Relative crack sensitivity ratings of selected aluminium (base alloy/filler alloy), redesigned from [15].

6. General susceptibility of aluminium to weld crack- ties, the degree of homogenisation, and the alloy ing [14,15] as presented in Fig. 2. content.

3.1. Hot cracking 3.2. Porosity The crack in aluminium welding occurs during weld In high temperatures, during arc welding processes, metal solidification. It mechanically involves the aluminium approaches its boiling point on the weld splitting apart of liquid film because of the stresses pool surface. In this situation, aluminium undergoes and the strain that spring up due to solidification two order magnitude changes of hydrogen solubil- shrinkage and thermal contractions. The liquid film ity. The change occurs when it cools from initial is related to the low melting eutectics. In situations high temperature to the onset of solidification; the where the difference between an alloy’s liquid film activeness of hydrogen to aluminium is due to the and the lowest meeting eutectic is large, the large temperature in the melted weld pool. Dissolution of solidification range makes the liquid film shrink hydrogen in aluminium is based on the high tem- more. In addition, it is more demanding to feed perature equilibrium and the fast mixing of the pool shrinkage over large distances. When the base of (due to the electromagnetic forces). The weld pool the dendrites solidifies fully and the shrinkage is therefore has high gas content relative to the sur- culminated, feeding inter dendrites liquid to the face temperature [14]. This effect is vivid in alu- shrinkage is then critical [14]. minium because the arc weld region is under super The loss of properties due to hot cracking in an high heat and the pores can be supersaturated such aluminium welded joint is due to the failure in the that gas pore formation is possible without the aid liquated region of the HAZ. The cracking suscepti- of solidification. When the weld starts to cool, there bility is based on the alloying elements. When the is not enough time for the entrapped gas to move to parent alloy adjacent to the fusion zone experiences the liquid’s surface, and escape from the weld pool. high heating rates the phenomenon of non-equilib- The entrapped gases are the pores in aluminium rium melting arises. Micro-cracks can also arise in welds [14-16]. The source of porosity is usually due the liquated regions in the presence of hydrogen to the entrapment of various gases in the weld, the and/or sufficient strain. In additions, a change in type of filler wire used, and the weld pools cooling composition of the weld regions, toughness can be rate. There are numerous possibilities of gas enter- seriously impaired following ageing. Precautions can ing the weld pool (shielding gas, air product of tur- be taken to control liquation and liquation cracking bulent arc action and even dissolved hydrogen). by controlling the grain size, the residual impuri- Hydrogen or water is the source of porosity. Hydro- gen is the typical source of porosity in aluminium Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys 9

Table 1. Welding defects and remedies, modified from [17].

Problem Causes Solutions

Porosity Turbulence of weld pool Increase welding current to stabilise transfer of metal droplets. Hydrogen from hydrated Keep wire covered. Store wire in a low humidity oxide film or oil on wire, base chamber at a constant temperature. Clean base metal metal, drive rolls and liner. of oil and oxide immediately prior to welding. Wet or contaminated Reject bottles above -57 °C dew point. Increase flow shielding gas or inadequate rate. Shield from air currents.Use higher welding flow. Fast cooling rate of current and/or a slower speed. Preheat base metal. weld pool Weld Cracking Improper choice of aluminium Select welding wires with lower melting and welding wire or rod. solidification temperatures. Critical weld pool Avoid weld pool chemistry of 0.5 to 2.0% silicon and chemistry range 1.0 to 3.0% magnesium. Avoid Mg-Si eutectic problems (5xxx welded with 4xxx). Inadequate edge preparation Reduce base metal dilution of weld through increased or spacing bevel angle and spacing. Incorrect weld technique Clamp to minimise stress. Narrow heat zone by increased traverse speed. Produce convex rather than concave bead.Minimise super-heated molten metal, to control grain size. Proper weld size (not too small). Preheat base metal. HAZ degradation Excessive exposure of Control the heat input and keep it minimal by workpiece to heat input controlling the current. Heat sinks can also be used to hasten the heat dissipation after welding. Optimizations that yield narrow weld seams should be used. welding; other sources include oxygen, and other caused by modification of the microstructure by gases in the surrounding air [14]. devoted temperature. The nature of the HAZ is de- Porosity affects the mechanical properties of pendent on the diffusion in the region and the heat aluminium welds. The degrading effect on the ten- input. Due to the thermal dependency of the metal- sile strength and ductility depends on the size and lurgical transmogrification, the degradation depends distribution of the pores. Elongation drops immedi- on the type of welding process and parameters. ately as porosity level increases, tensile ductility Preheating parent metal before weld and using high drops by as much as 50% from its highest level heat input increases the HAZ region and the degra- when the porosity is about 4% of the volume. At dation level. The HAZ degradation may be limited same porosity level, tensile strength is observed to by using multi pass welding, avoiding preheating, be very tolerant and yield strength is slightly reduced and by controlling the inter pass temperature [14]. [14]. In 7xxx series, zinc vapour is formed at the A summary of welding defects and remedies faying surface during welding which generates gas applicable to aluminium are presented in Table 1. inclusion (porosity). aluminium has melting tempera- ture of 560 °C and high boiling temperature of 2050 4. EFFECTS OF THE BASIC °C and (compared to 420 °C and 907 °C for zinc); PROPERTIES OF ALUMINIUM IN thus cleaning zinc in the weld region mechanically FUSION WELDING or using arc heat to volatize zinc ahead of the pool helps to reduce the possibility of porosity. An understanding of the peculiarities associated with aluminium fusion welding is important as the 3.3. Heat affected zone degradation physical and chemical properties influence the weld [18,19]. Some of the properties considered include The HAZ is created beside the fusion zone and it the high heat conductivity of aluminium, which is results in the degradation of the parent materials approximately three times the heat conductivity of 10 M. Olabode, P. Kah and A. Salminen

Fig. 3. Absorption of laser wavelength by metals, redesigned from [54]. steel [2]. This implies that high energy density weld- cesses. The hydrogen gas is usually segregated ing systems like MIG, plasma and laser welding as regular spherical pores of typical diameter of 5 systems are applicable. With high energy density, to 10 ฀m [2]. They can act as crack initiation in the there is a lower loss of strength in the HAZ and less weld and lowering the dynamics and static strength distortion. Another property is the extent of expan- of the weld [24]. The sources of hydrogen in alu- sion which is about twice for low alloyed steel [20]. minium fusion welds include humidity and organic On exposure to oxygen sources like air and water, contamination on the filler material and base metal the surface that becomes coated with a thin layer surfaces, hydrogen content of the base material and of naturally formed, chemically stable and thermally filler material, incomplete gas feeding of the weld. It stable nonconductive aluminium oxide (Al2O3)[21], is important to suppress the level of porosity in the melts at about 2050 °C while aluminium alloys melts weld so that the mechanical properties of the weld at about 560 °C [11]. This oxide layer has a melting do not deteriorate drastically. Finally, the high temperature much higher than that of aluminium it- reflectivity of aluminium to wavelengths limits the self; moreover, it has a significant mechanical laser beam welding science that can be used [25]. strength. Therefore, this oxide layer can remain as a solid film (or fractured in small particles due to the 5. HYBRID LASER WELDING OF flow of the molten material [22]). This can result in ALUMINIUM ALLOYS severe incomplete fusion defects. It is recommended that the layer is removed by pickling or dry machin- Aluminium alloys can be welded by most welding ing just before weld. It is important to state that the processes [2,26]. However, for fully automated sys- difference in melting point is not a problem during tems, the common ones are MIG, TIG, LBW, and the processing by means of high energy density HLBW. Plasma MIG and other electron beam weld- welding processes; for example, the presence of ing processes are however applicable with limita- oxides during laser processing increases the ab- tions and therefore restricted to welding of special sorptivity of aluminium alloys to the laser radiation products [26]. Newer technological developments [22,23]. It should be noted, that the main challenge on the MIG process like cold arc [27] or cold metal in applying most joining technologies to aluminium transfer welding (CMT) [28] are also applicable and is its tendency to form a thick, coherent oxide layer. are growing in the industry. The most commonly Another important property is solubility of hy- used hybrid welding system is laser hybrid MIG [9] drogen in aluminium. Hydrogen has high solubility The usability of hybrid laser welding systems in molten aluminium as opposed to the solid alu- have been presented by Bagger and Olsen [29] then minium. The solubility is reduced to one twentieth by Rasmussen and Dubourg [30]. Moreover, in the 1980s and early 1990s CO lasers were the only of the solidification range in fusion welding pro- 2 Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys 11 ones with sufficient power for aluminium welding. 5.1.1. Beam delivery optics before

Therefore, CO2 lasers were the most investigated focusing [25,31,32]. Today many more investigations with the solid-state lasers, about 80% of the laser hybrid Most laser welding system consists of components welding processes investigated are carried out on like mirrors (for diffracting light). Mirrors can be pla- solid state lasers like Nd:YAG lasers [33-35], high nar or spherical in design. The mirror is fixed to a power-fibre laser [36-40].As stated by Ueyoma [41] firmly adjustable screw with the ease of accessibil- and researched by Wang et al. [42], defocused high- ity for cleaning, inspection, and replacement. power diode laser beam can be used. There is lim- The usability of conventional mirror delivery is ited research on the use of disk lasers as laser limited by the rigidity of the mechanical mounting sources in hybrid welding of aluminium alloys [9]. and they cannot move relatively to each other to More than 80% of recorded research has been avoid misalignment. The mirror is limited in size carried out using MIG power source especially therefore transferring beams over a long distance pulsed MIG [43-45]. TIG power sources have also with high divergence can produce a beam diameter been used but usually for basic investigations on that is larger than the lens. However, the beam can interactions and parameter effects [25,46-50] In be tailored with lenses in the beam path to prevent Ueyama research, AC MIG arc was applied in com- this phenomenon (this is called a relay system). bination with high-power diode laser [41,51]. In some Mirror as reflective optics are usually found in gas other experiment plasma arc has been used as a laser systems. The mirrors are generally made from laser source [32,47,52]. bear metal or polished metal (usually molybdenum The absorption of beam by aluminium depends or copper) to improve the reflectivity.A material like on the wavelength of the laser beam. As presented gold can also be used for coating the surface of in Fig. 3, due to the wave length of solid-state la- metal mirrors to produce high reflectivity.Metal made sers Nd: YAG and fibre lasers are common laser mirrors are less prone to damage compared to power sources used in hybrid MIG welding. Com- lenses because they can be easily cooled by pass- pared to CO lasers, Nd:YAG and fibre lasers have ing water through the inner walls of the mirror thereby 2 resulting in higher repeatability than transmissive approximately double the wavelengths of CO2 laser [25]. This advantageously minimises the keyhole lens. Usually, high-powered lasers use all reflective welding intensity needed. The delivery of fibre and water-cooled optical components ruggedly built to Nd:YAG lasers can be done using fibre optics which survive in industrial environment and to require mini- increases the process flexibility [53] and the pos- mal maintenance. The mirrors can be as simple as sibility of having a robust welding system. In addi- having one to having ten mirrors. tion, in Nd: YAG and fibre laser, shielding of the Retaining rings and springs are used to keep laser beam by the arc plasma and laser induced the mirror in place thereby sustaining consistent pressure and limiting movement. The mirror mount- metal vapour is not expected as compared to CO2 systems. ing plate must be flat to avoid pressure that can Based on the amount of the research available, force the mirror to warp causing beam distortion and it can be stated that Nd: YAG laser with MIG is the difficulty in focusing the beam. Dielectric coatings most usable state of the art hybrid welding process are used on mirrors to eliminate phase shifting. This for aluminium alloys. In addition, the Nd: YAG la- coating can be easily damaged during cleaning so sers can be replaced with solid-state lasers like the mirrors should be cleaned using acetone and lens fibre laser. tissue. Cleaning is important to prevent build up and contamination that can result in heat absorption that will distort and destroy the mirror. In some special 5.1. Hybrid laser welding optics cases, the mirror’s dielectric coatings are multi-lay- Optics found in hybrid laser welding systems appli- ered to rotate the polarization of the laser beam by cable to aluminium welding includes mirrors, lenses 90°. This is common for circular polarization needed and fibre optics. In hybrid laser welding, laser beam for bidirectional welding and cutting so that beams needs to be focused to achieve small spot diam- can create consistent kerf width in all travel direc- eter. The small spot diameter allows for higher beam tions. These mirrors are referred to as quarter-wave density on the workpiece. The spot diameter is a phase retarders. function of the lens design and focal length. Beam In some cases, the mirrors are coated so that it transfer and focusing is achievable using diffractive can absorb one component of linear polarization and optics, refractive optics or reflective optics. reflect the orthogonal component. These are called 12 M. Olabode, P. Kah and A. Salminen anti back reflection mirror and are used for beam sharing with fibre optics is less complex and easily delivery along with phase retarders to absorb re- achievable than with mirrors. They degrade beam flected energy that can otherwise travel back to the quality with larger focal spot sizes compared to laser resonator and damage it [54]. mirror delivery. The usability of fibre beam delivery Mirror can also be adaptive designed spherical has therefore been limited to most welding systems or flat but can change surface curvature based on where the focal size needed is larger than 100 ฀m the input signal. Adaptive mirror is necessary in la- [56]. It is important to state that fibre optics are not ser material processing for controlling raw beam effective for transmitting ultraviolent (UV) wavelength propagation through the guide and beam delivery and can be destroyed by CO2 lasers. Fibre optics system. The principle of adaptive mirror operation is common in diode lasers and Nd: YAG. Plastic is that it compensates the axial shift of the focal material are also used in place of glass for fibre position that had been caused by the thermal load optics however, they are used for visible wavelength on the optical components. Therefore, the focal po- lasers. Plastics are not effective in Nd: YAG due to sition is kept constant or changed to a desired po- losses in transmission and lower damage thresh- sition. In aluminium laser welds of components olds. where ‘’flying optics” is used, the distance between Beam delivery optics before focusing include the laser source and the welding head changes (de- bending mirrors (e.g.CO2), beam splitter (CO2, Nd: pending on the shape of work weld piece), adaptive YAG), optical fibre (Nd: YAG, Diode lasers, Disk optics is therefore adequate [55]. and fibre laser), circular polariser and collimator. Lens is another component for beam delivery Laser beams are delivered to the workpiece by turn- usually for converging or diverging light. The lens ing mirror system in CO2 lasers and Nd: YAG la- can be a simple one-element optic generally with a sers. For accurate repeatability of laser welds, it is focal length less than 254mm. They can be as- important that the laser optics is firm and rigid, as pheric, Plano-concave/convex or meniscus [56,57]. misalignment and vibration are not desired. How- Lenses can be compound, where the lens is made ever if the laser optics is rigid then the workpiece of two or more separate lenses that fit together to will need to be moved around during welding. This reduce spherical aberration common in a simple becomes impracticable when welding large work single lens. Aspheric simple one component lens pieces. For such fixed beam systems, the floor is made to reduce spherical aberration. This is space for the machine must at a minimal be four achieved by turning the lens with a diamond tool on times larger than the largest work piece for which it the lathe to a certain calculated aspheric curve. It is is designed for. On the other hand, moving optics important to note that glass is generally the mate- will save floor space but high care must be put into rial used for lenses in the visible spectrum. How- controllig beam divergence, rigidity, and alignment. ever, glass in the infrared (IR) region does not trans- Nd: YAG laser heads are small thereby allowing it mit. The lenses made to transmit in the IR region to be mounted on moving axis with limited deterio- are called IR lenses. IR lenses can be made from ration to it focus, and therefore more flexible than germanium(Ge), silicon (Si), zinc selenite (ZnSe), CO2 laser heads that are large and usually installed zinc sulphide (ZnS), and gallium arsenide (GaAs). to operate stationary. Other materials like diamond and calcium sulphide Laser applications that are categorised as 1 kW (CdS) and sapphire are less common [58]. or less use transmitting optics for beam focusing in Fibre optics is another component for beam welding. The beam transfer can be achieved by con- delivery used in Nd: YAG to deliver beams due to ventional mirror, fibre optics, or a combination of both. the 1.06 ฀m wavelength transferable over glass fi- Up-collimator or beam extenders are used to re- bres. Fibre optics utilizes the flexibility of glass fi- duce beam divergence by increasing the beam di- bre within the specified bend radii for fibre bundle ameter.Laser beam divergence along with the choice (100-200 mm). They are attractive in comparison to of focus lens determine the focal spot size, research conventional beam delivery especially due to the study [9] has shown that, beam divergence can be possibility of transporting beams over long distances improved by a factor of two with half times smaller of up to 50 m and around curves [59]. In addition, focal size (using proper focus system) compared to the optics is compact and easier to move around a system without collimator by doubling the beam particularly useful in robotic welding. A highly con- diameter. The usability of collimator is usually lim- sistent focal spot size is achievable with fibre op- ited to low power CO2 lasers and most Nd: YAG tics compared to mirror. Time sharing and energy lasers to extend beam diameter from 6 - 10 mm to Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys 13

Table 2. Lens shape choices for Nd:YAG and CO2 The presence of foreign particle on the lens can lasers at various f-numbers, modified from [60]. reduce transmission; create localized absorption on the surface of the lens thereby destroying the lens surface or any coating on it. Lenses can be very f-number CO2 Nd:YAG range expensive and in such cases where an expensive lens life can be drastically reduced, a sacrificial 4+ plano-convex plano-convex cheap optic is placed in front of it as a window or a 3 to 4 meniscus plano-convex cover slide to protect the expensive lens. For ex- 2 to 3 diffractive-convex doublet ample, Nd: YAG and Nd: glass lasers use the pro- < 2 triplet triplet tective optics due to the low cost of the cover slide.

It is less common in CO2 lasers.

12 - 25 mm. CO2 lasers of above 500W usually do not need collimator because of the raw beam diam- 5.1.3. Hybrid laser focusing head eter and its low beam divergence. The performance of beam delivery system deter- mines the quality of the laser beam processing. It 5.1.2. Focusing optics is desired to be simple and as small as possible having neither actuators nor sensors to allow for easy Focusing optics is common in low-power welding manipulation and integration on to a robotic welding devices. Parabolic lenses are generally useful for system. However, the available technology for laser focusing power above 1.5 kW of CO lasers. Due to 2 welding head is attractive that consumers still tend the low cost and minimal spherical aberration at- to buy the technology thus the laser heads are be- tributes of f-numbers above five, lenses are usually coming more and more complex. Common tech- Plano-convex lenses. The f-number is derived by nologies in laser focusing heads include the pres- dividing the lens focal length with the beam diam- ence of integrated actuators and sensors, closed eter. When the f-number is less than four, complex loop systems, self-learning and self-adapting sys- optical lenses compared to Plano-convex lenses are tems. used. The thumb rule is that the higher the f-num- The combination of laser beam and arc can be ber the higher the problem of spherical aberration of varying configurations which remarkably influence [60]. Aguide to selecting the best lens is presented the weld performance. It is important to mention that in Table 2. in hybrid laser welding, the primary heat source is Laser protection is used in laser processes laser while the secondary can be any arc process. where the focal length is short or when the weld Laser assisted arc welding is the vice versa [9]. metal is volatile and contaminated; or when weld The welding head is based on the heat source spatters can be generated. Debris can attach itself type and relative position of the heat source to one to the welding head lens. Aluminium highly reflects another [61]. The principal classification criteria are laser beam wavelength, and the reflected beam can presented in Fig. 4 (based on the heat source type) damage laser optics. The solution adopted gener- and Fig. 5 (based on the configuration). The choice ally to solve this is to change to a laser with differ- of the secondary heat source can be either arcs ent wave length, paint or etch the surface of the with consumable electrodes or arc with non-con- workpiece to reduce reflectivity, or to use keyhole sumable electrodes. The earlier is selected due to welding where the energy density of the spot diam- the necessity of filler metal to solve specific weld eter is great enough to overcome reflectivity; in ad- problems otherwise, the latter is preferred. dition to using a cheaper protective lens.

Fig. 4. Schematic presentation of heat sources available for hybrid laser–arc combinations. 14 M. Olabode, P. Kah and A. Salminen

Fig. 5. Geometrical arrangements for hybrid laser–arc welding.

(a) (b)

Fig. 6. Schematic diagrams of hybrid laser–arc welding with a common operation point, redesigned from [9].

Fig. 7. Schematic diagram of hybrid laser–arc welding with separate operation points, redesigned from [9].

The arrangement plays important role for the ef- point, the arc root and laser beam spot centre are fectiveness and efficiency of the weld system as in the same surface location of the workpiece. Many well as the welds. The heat source can be arranged hybrid laser-arc configurations use arc welding torch to have a common (Fig. 6) or separated (Fig. 7) inclined to the laser beam along the weld direction operation point as illustrated. In common operation (Fig. 6a) or across (Fig. 6b). The position of the arc torch affects the focal point position. Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys 15

Table 3. Configuration advantages of conventional hybrid laser welding.

Laser leading configuration Arc leading configuration

Useful in aluminium welds because it helps remove oxide layer Generates deeper weld penetration[9] before arc welding [63] Creates superior beam appearance because the laser as it gas Allows for weld preheating does not affect the molten pool created by arc heat source[64] Improves the homogeneity of the weld metal[64] Requires less heat input per volume of weld metal (J/mm3)[64]. Better stability in terms of current and voltage measurement [65]

Beyer et al. (1994) reported according the con- different region as opposed to serial configuration figuration in Fig. 6a that laser power was respon- where the thermal load spread to the same region. sible for attainable weld penetration depth and the It is important to state that in many cases it is diffi- arc parameters were responsible for the adjustment cult to distinguish between a parallel and a serial of the weld seam width [62]. The same research technique as they are usually applied simulta- group used the setup in Fig. 6b for tailored blank of neously. In Seyffahrt et al. (1994) a separate opera- two different plate thicknesses. The result showed tion point configuration was carried out with the pur- that the configuration (1) reduced the need for edge pose of increasing weldable metal sheet thickness. preparation (2) increased molten material volume Laser heat source welded the root and the top layer and (3) generated a smooth zone transition between was welded by the arc source Fig. 7 [66]. the plates because the arc burns the thicker plate’s Other hybrid laser welding configuration with edge therefore improving weld appearance. more than two heat sources have been experimented A separated operation point arrangement can be and presented in Fig. 8. Winderlich (2003) used the of serial technique or parallel technique or a combi- configuration where a CO2 laser beam and TIG arc nation of both. The serial technique is a configura- touch acted on the same side while the second TIG tion in which the primary and secondary heat source torch acted on the opposite side of the weld. The has an acting point distance known as “working dis- configuration provided attainable notch-free weld tance” between them in vertical or horizontal direc- seam useful in dynamic loading while improving fa- tion along path. The arc source can lead or trail the tigue resistance [67]. laser beam. Leading arc source allows for preheat- Another configuration referred to as Hydra (hy- ing thereby increasing laser heat source efficiency brid welding with double rapid arc) is presented in due to a reduction in heat loses by conduction. It Fig. 8b. It was initially experimented by Dilthey and also increases the weld seam quality because of Keller (2001) using CO2 laser and MIG heat sources. more stabilized keyhole. Leading arc generates The configuration increased the possible deposition deeper welds because at close working distances, rate and thus increasing attainable welding speed the laser beam strikes the deepest part of the weld and reducing thermal load, in comparison to con- pool surface suppressed by the arc forces. To at- ventional hybrid laser welding configuration [68]. As tain deepest weld penetration, the focal point must illustrated, the working distance of the leading arc be set to hit at the lowest weld pool surface. can allow for preheating while the trailing arc can Trailing arc source with short working distance provide heat treatment. Wieschemann (2001) shows provides stability and efficiency due to the common that two leading arc configuration provides optimum phase plasma interaction between the heat sources gap bridgability [69]. and also due to the thermal impact on the weld. Another configuration discovered by Stauter With greater working distances, trailing arc source (2007) is presented in Fig. 8c where the second arc can act as heat treatment for the weld which is torch is a tandem having two consumable electrodes. favourable in HSA welds for the improvement of joint The electrodes depositions are controlled by two properties. A summary of the principal advantages separate power sources. This configuration increases of a leading and training arc is presented in Table 3. deposition rate and productivity and the cooling rate A parallel technique is a configuration where there is easily optimized by varying the work distance is a displacement between the laser focal point and between the conventional hybrid configuration and the arc acting point. The thermal load spreads in the tandem torch (working distance between torch 1 and torch 2). 16 M. Olabode, P. Kah and A. Salminen

Fig. 8. Schematic diagrams of hybrid laser–arc processes with two secondary heat sources, redesigned from [9].

5.2. Challenges for aluminium welding using a hybrid laser beam welding demands that, a clear understanding of the governing parameters, Aluminium alloys presents challenges for hybrid the effects and their interactions are understood [30] laser welding optics. One of the challenges limiting to be able to maximise the advantage of hybrid la- the welding system and the optics is; the high ser beam welding as a robust industrial welding pro- reflectivity of aluminium alloy that limits the choice cess [71]. of laser beam source for example to Nd: YAG and fibre laser. Firstly, the melts zone (MZ), and HAZ are larger 6. CONCLUSIONS in hybrid welding, than in laser welding. The molten Aluminium alloys have become an important struc- zone at the weld top is wider due to the welding arc tural material and have found applications from gen- process [70]. This compromises the metallurgical eral kitchen utensils to aerospace vehicles. They properties of the weld. Secondly, due to the wider are grouped into cast and wrought aluminium al- weld pool and high melt temperature in HLBW diffi- loys. The wrought aluminium alloys are sub grouped culty arises in covering the weld pool, which can mainly into seven. Pure aluminium is weak, light lead to contamination of the weld, and porosity [30]. and corrosion resistant. It conducts electricity, heat, Thirdly, alloys with volatile elements like 5xxx se- reflects light and easy to fabricate. When alloyed, it ries can evaporate from the normally generated key- can attain strength comparable to mild steel. How- hole thus resulting in lower metallurgical properties ever, some of its properties are detrimental to its of the weld and even porosity if the gas bubbles are fabrication. For example, the high strength alloys trapped in the weld. This can be improved by proper have poor weldability. They are relatively prone to selection of filler material [53]. In addition, volatile weld defects due to for example, it’s self-forming elements present in aluminium alloys can generate Al2O3 oxide layer, hydrogen solubility in molten alu- spatters during welding that can adhere to the lens minium, high shrinkage rate on solidification. and damage the lens. A precaution is to use a pro- The properties of aluminium have affects on fu- tective lens. Fourthly, aluminium alloys have low sion welding. For example, the high heat conductiv- surface tension; they have poor ability for root-side ity implies that high heat energydensities are needed melt pool support. This tends to cause difficulty in to weld the alloy that in turn increases HAZ degra- full penetration welding specifically in thick butt dation. The presence of Al2O3 although can be bro- welds [45]. Finally, the presence of high number of ken with high heat energy density welding process welding parameters that is non-independent of each during welding, it has a significant mechanical other in interaction compared to MIG or laser weld- strength that it can remain solid even when the sur- ing process, in addition to the metallurgical chal- rounding metal is molten which can result into in- lenges in aluminium fusion welds. Therefore hybrid complete fusion. In addition, the high solubility of laser beam welding of aluminium alloys are compli- hydrogen in molten aluminium makes HLBW of alu- cated to design and operate [71]. Rasmussen et al. minium prone to porosity although with proper know (2005) shows that successful welding of aluminium how, it can be minimized. Overview of laser systems and optics applicable to hybrid laser welding of aluminium alloys 17

HLBW systems are used for welding aluminium [4] J.R. Davis, Corrosion of aluminum and and the most commonly used is hybrid MIG weld- aluminum alloys (ASM International, OH, ing. The optics are used as resonator optics, beam 1999). delivery optics and processing optics. The optics [5] T. Cock, Aluminium - a light metal (European include output windows, fold mirror, rear mirror, beam Aluminium Association, 1999). splitters, optical fibre, circular polarizers, collima- [6] C. Vargel, Corrosion of aluminium (Elsevier, tor, scanning optics, and other special optics. Amsterdam- Boston, 2004). The usability of HLBW system is mainly limited [7] J.M. Sánchez-Amaya, Z. Boukha, M.R. in aluminium alloys due to the limited absorption of Amaya-Vázquez and F.J. Botana // Welding laser wavelength by aluminium. Therefore, the com- Journal 91 (2012) 155. monly used laser power sources are fibre laser and [8] J.M. Sánchez-Amaya, Z. Boukha, M.R. Nd: YAG. The focusing optics used is selected with Amaya-Vázquez, L. González-Rovira and F.J. reference to the f - number with the aim of avoiding Botana // Aluminium Alloys Materials Science spherical aberration. A rule for selecting optics is Forum 713 (2012) 7. based on the fact that the higher the f - number, the [9] F.O. Olsen, Hybrid laser-arc welding higher the problem of spherical aberration. Beam (Woodhead Publishing, Cambridge, 2009). delivery can be done using mirror optic or fibre op- [10] L. Quintino, R. Miranda, U. Dilthey, tics; but mirrors are limited due to the need of a D. Iordachescu, M. Banasik and S. Stano, rigid mechanical mounting and the difficulty of trans- In: Structural Connections for Lightweight ferring beams over long distances. On the other Metallic Structures (Springer, Berlin, 2012), hand, fibre optics is limited by the bend radius and p. 33. beam quality degradation. HLBW focusing heads [11] G. Mathers, The welding of aluminium and its are desired to be simple so that it is easy to inte- alloys (Woodhead Publishing Cambridge, grate. However due to the numerous advantages the England, 2002). available technologies, they have only become more [12] R.D. Joseph, Aluminum and aluminum alloys complex. Some of them have mechanical moving (J. R. Davis & Associates, ASM International, parts to allow for more manipulation in attaining 1993). closed loop, self-learning and self-adapting systems. [13] S.R. Yeomans // Aust. Weld. J. 35 (1990) The choice heat source and their configuration plays 20. important role for the effectiveness and efficiency of [14] F.C. Campbell, Manufacturing technology for HLBW. The challenges faced in HLBW of aluminium aerospace structural materials (Elsevier, alloys are HAZ degradation, possibility of contami- Amsterdam -San Diego, 2006). nated weld pool due to the presence of a wider weld [15] ASM International Handbook Committee, pool compared to LBW, the presence of a volatile ASM handbook. Volume 6: Welding, element in the alloy like zinc causing porosity and brazing, and soldering (ASM International, degradation of metallurgical properties. In addition, Ohio, USA, 1993). the presence of low surface tension that makes full [16] G.S. Ba Ruizhang, Welding of Aluminum- penetration welding difficult in thick butt welds. Fi- Lithium Alloy with a High Power Continuous nally, there is the presence of a high number of in- Wave Nd3+:YAG Laser 2004; IIW Doc. terdependent welding parameter, in addition to the IV-866-04 (accessed 2012). metallurgical challenges that are present in alu- [17] Welding consumables pocket guide, ed by minium fusion welds. P. Cigweld (SPW GROUP PTY LTD, Preston, Victoria, Austalia, 2008). REFERENCES [18] W. Chang, In: International Welding/Joining Conference (Korea 2012), p. 79. [1] G. Kopp and E. Beeh // Materials Science [19] J. Enz, S. Riekehr, V. Ventzke and Forum 638 (2010) 437. N. Kashaev // Physics Procedia 39 (2012) [2] F. Ostermann, Anwendungstechnologie 51. aluminium (Springer Verlag, 2007). [20] G. Schulze, H. Krafka and P. Neumann, [3] H. Schoe, Schweißen und Hartlöten von Schweißtechnik (VDI Verlag, Düsseldorf, Aluminiumwerkstoffen (Verlag für Schweissen 1996). und verwandte, Verfahren DVS-Verlag GmbH, [21] M. Schütze, D. Wieser and R. Bender, 2002). Corrosion resistance of aluminium and 18 M. Olabode, P. Kah and A. Salminen

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