MECHANICAL PROPERTIES OF DISSIMILAR ALUMINUM-BASED ALLOY JOINTS BY MIG WELDING AHMAD DANIAL BIN ABDULLAH

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1 MECHANICAL PROPERTIES OF DISSIMILAR ALUMINUM-BASED ALLOY JOINTS BY MIG WELDING AHMAD DANIAL BIN ABDULLAH Report submitted in partial fulfillment of the requirements for the award of the degree of Bachelor of Mechanical Engineering with Mechanical Engineering Faculty of Mechanical Engineering UNIVERSITI MALAYSIA PAHANG JUNE 2012

2 vi ABSTRACT This thesis deals with the investigation of microstructure and mechanical properties of weld joint of AA5052-H32 and AA6061-T6 aluminum alloys by using MIG welding process. The objective of this thesis is to investigate the effect of parameters to the mechanical properties and microstructure of AA5052-H32 and AA6061-T6. The thesis describes the proper MIG welding process using automatic table in order to investigate the effect on microstructure and mechanical properties of weld joint of AA5052-H32 and AA6061-T6. The aluminum ER5356 was used as filler in this experiment. The studies of mechanical properties that are involved in this thesis consist of toughness, tensile strength of AA5052-H32 and AA6061-T6 weld joint before and after MIG welding process. Four different parameters were used in order to determine the correlation between mechanical properties and microstructure of the weld joint. As a result, it is observed that the current is the parameter which has the highest influence to the UTS and toughness and it is followed by torch angle, speed and lastly weld passes. The optimum parameter for the highest value of UTS and toughness is found; current=90a, torch angle=+15, speed=4mm/s and weld pass=1. The microstructure shows crack sensitivity and porosity which decreases the strength and toughness of weld joint. As for the recommendation, the other properties including hardness, corrosion resistance should be considered in order to optimally select a material for its specific application.

3 vii ABSTRAK Tesis ini membentangkan penyelidikan mikrostruktur dan ciri-ciri mekanikal logam kimpalan aluminium AA5052-H32 dan aluminium AA6061-T6 dengan menggunakan proses MIG welding. Objektif tesis ini ialah mengkaji kesan setiap parameter yang berlainan ke atas mikrostruktur dan ciri-ciri mekanikal logam kimpalan yang menggabungkan aluminium AA5052-H32 dan AA6061-T6. Selain itu, tesis ini juga menerangkan proses MIG welding yang betul dengan menggunakan meja automatik bagi menghasilkan logam kimpalan yang berkualiti. Antara skopnya ialah logam isian ER5356 digunakan untuk memastikan gabungan yang baik terhasil antara kedua-dua aluminium. Antara spesifikasi projek ini adalah merangkumi ciri-ciri mekanikal yang terdiri daripada kekerasan dan kekuatan tensil. Oleh yang demikian, empat jenis parameter proses telah ditetapkan bagi mengkaji dan memenuhi spesifikasi projek ini. Keputusan yang diperoleh membuktikan bahawa parameter yang berbeza mampu mempengaruhi cirri-ciri mekanikal dan mikrostruktur logam kimpalan tersebut. Dalam kajian dari segi mikrostrukturnya, ia membuktikan bahawa kehadiran kesan keretakan dan ruang-ruang udara member kesan ke atas kekuatan tensil dan kekerasan logam kimpalan yang terhasil. Secara konklusinya, kita perlu menjalankan kajian ke atas ciri mekanikal yang lain seperti ketahanan daripada karat dan kekuatan dimana ia dapat dihasilkan dalam kombinasi terbaik untuk kegunaan bidang kejuruteraan.

4 viii TABLE OF CONTENTS SUPERVISOR S DECLARATION STUDENT S DECLARATION DEDICATION ACKNOWLEDGEMENTS ABSTRACT ABSTRAK TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF ABBREVIATION Page ii iii iv v vi vii viii xi xiii xvi CHAPTER 1 INTRODUCTION 1.1 Background Studies Problem Statements Project Objectives Project Scopes Overview of Report 3 CHAPTER 2 LITERATURE REVIEW 2.1 Introduction Aluminum Alloys Types of Aluminum Alloys Intermetallic Compound (IMC) Aluminum AA6061-T6 and AA5052-H MIG Welding Process Equipment Welding Gun Power Supply Electrode Shielding gas 17

5 ix Operation Mechanical Testing Charpy Test Tensile Test Taguchi Approach 21 CHAPTER 3 METHODOLOGY 3.1 Introduction Flow Chart Flow Chart Description Design Selection Preparation of Specimen Preparation of Welds Welding Process Polishing Process Optical Investigation Mechanical Testing Analysis of Experimental Data 36 CHAPTER 4 RESULTS AND DISCUSSION 4.1 Introduction Mechanical Properties Tensile Strength Charpy Toughness Effect of Speed,Torch Angle Current, 43 Weld Pass to UTS and Toughness 4.3 Taguchi Method ANOVA Main Effect Plot Regression Analysis Contour Plot and Surface Plot for Charpy Toughness Contour Plot and Surface Plot for UTS Experimental Data and Predicted Data Comparison Microstructure Observation 63

6 x Microstructure Defect in Weld Joint 66 CHAPTER 5 CONCLUSION AND RECOMMENDATIONS 5.1 Introduction Conclusions Future Recommendations 70 REFERENCES 71 APPENDICES 73 A Gant Chart

7 xi LIST OF TABLES Table No. Title Page 2.1 Aluminium alloys in space frame car design in Europe and North America Classification of aluminum alloys Microstructure of AA6061-T6 and AA5052-H Composition of AA5052-H32 and AA6061-T Mechanical Properties of AA5052-H32 and AA6061-T Properties of AA5052-H32 and AA6061-T Example of process parameter: Injection moulding parameters and their levels Experimental plan using L9 orthogonal array Example of ANOVA: ANOVA table for bending test Welding Test Result Parameter of Experiment Design of experiment using Taguchi Method by using Mixed Level Design L18 (2 levels 1 columns + 3 levels 3 column) 3.4 Analysis of Variance (ANOVA) of UTS vs Speed; Welding pass; Current; Torch angle 4.1 Mechanical Properties of AA6061-T6 and AA5052-H32 Weld Metal (weld joint) ANOVA Rank of every parameter based on the Taguchi Analysis; Combination of UTS & Toughness vs Speed; Welding pass; Current; Torch angle Comparison of results with mean predicted value Factor levels for predictions based on Taguchi Method 51

8 xii 4.6 Predicted values of S/N ratio, Mean and Standard Deviation based Taguchi Method Predicted Data versus Experimental Data 60

9 xiii LIST OF FIGURES Figure No. Title Page 2.1 The eutectic particles of 6082 alloy Eutectic region of aluminum 6XXX Eutectic region in 5XXX Phase diagram of Aluminum 5XXX Phase diagram of 6XXX MIG Circuit diagram GMAW torch nozzle cutaway image GMAW weld area Charpy test Stress-strain curve Example for main effect graph Process flow chart of study (a) Weld specimen for Charpy s Test, (b) Weld specimen for 29 Tensile Test 3.3 a) Aluminum AA5052-H32 and b) AA6061-T6 sheet a) Power supply, b) Automatic Working Table Polisher-grinder Meserv for various size Optical Microscope a) Charpy s Test machine, b) Swinging Pendulum Charpy Test Specimen Tensile Test Machine Specimen of Tensile Test (ASTM E8M-04)

10 xiv 4.1 Graph UTS value versus experiment number Graph Stress versus Strain; a) Exp. 1 for minimum UTS, b) Exp for medium UTS, c) Exp. 6 for maximum UTS 4.3 Charpy value versus experiment number Weld toe angle weld toe of specimen: a) Experiment 1, b) Experiment 6 and c) 46 Experiment of optimize value parameter 4.6 Double weld passes of experiment 14 specimen Graph of Main Effect Plot for S/N Ratio Main Effect Plot for Mean Crack Profile of tensile specimen based on optimum parameter Crack Profile of Charpy s test specimen based on the optimum 53 parameter 4.11 (a) contour plot: Charpy s toughness vs current; speed, (b) 54 surface plot: Charpy s toughness vs current; speed 4.12 (a) contour plot: Charpy s toughness vs current; torch angle, (b) 55 surface plot: Charpy s toughness vs current; torch angle 4.13 (a) contour plot: Charpy s toughness vs torch angle; speed, (b) 56 surface plot: Charpy s toughness vs torch angle; speed 4.14 (a) contour plot: UTS vs current; speed, (b) surface plot: UTS vs 57 current; speed 4.15 (a) contour plot: UTS vs torch angle; current, (b) surface plot: 58 UTS vs torch angle; current 4.16 (a) contour plot: UTS vs torch angle, speed, (b) surface plot: 59

11 xv UTS vs torch angle; speed 4.17 Experimental UTS versus predicted UTS Experimental charpy toughness versus predicted charpy 62 toughness 4.19 Microstructure of; a) AA5052-H32 microstructure, b) AA T6 microstructure by magnification of 50X Porosity in Weld metal (WM) microstructure Heat affected zone (HAZ) and fusion zone (FZ) in 65 Microstructure 4.22 Fusion zone of 6061 welds Porosity in weld metal zone of 6061 welds Crack sensitivity in weld joint of 6061 microstructure 68

12 xvi LIST OF ABBREVIATIONS AA T6 H32 ASTM MIG TIG DOE UTS CTE AC DC SMAW DF SM MS F SS R SS E SS T Aluminum alloy sheet A type of heat treatment process A type of hardening process American Society for Testing and Materials Metal inert gas Tungsten inert gas Design of experiment Ultimate tensile strength Coefficient thermal expansion Alternating current Direct current Shielded metal arc welding Degree of freedom Sum of squares Mean square F-function Sum of square regression Sum of square error Sum of square total

13 1 CHAPTER 1 INTRODUCTION 1.1 BACKGROUND This thesis is about the investigation of weld joint (dissimilar Aluminum alloys) microstructures and mechanical properties using MIG welding process. Aluminum is the second important after steel usage because of its characteristic which is high strength stiffness to weight ratio, good corrosion resistance, good formability better conductor of heat and electricity. It has been the best candidate to replace heavier material like steel and copper in automobile because it has the recycling potential. The choice of material is influenced by the requirement to improve the economy of fuel and also the energy consumption. For example, automotive companies have been given mandate by US Government that they need to minimize vehicle exhaust emission, enhance fuel economy and improve occupant safety (W.S. Miller, 2000). In automotive industry, welding of dissimilar parts of different weld will be much needed because each part (inner and outer panel consist of different aluminum alloys) of car needs to be joined together. The increasing of aluminum and magnesium alloys is mainly caused by the rapid growth of application of material that has the light weight characteristic (D.- A.Wang, 2007). And for this study, MIG (metal inert gas) welding process is chosen to join the welds.the material that will be used is Aluminum alloy AA5052-H32 series and 6061-T6 series for the experiment because they are often used in the industry (W.S. Miller, 2000). While the microstructure and mechanical properties investigation consists of tensile strength, toughness and corrosion resistance of the weld before welding process and weld metal (WM) after MIG welding process.

14 2 1.2 PROBLEM STATEMENT Aluminum alloys may often be used to replace steel in many applications especially in automotive industry welding process (W.S. Miller, 2000). But, the problem is the high difference of thermal conductivity of different aluminum alloys using MIG or TIG will cause problems (Luijendijk, 2000). The lack of fusion of material or excessive melting of material that has lower thermal conductivity is caused by the larger thermal conductivity in arc that flow in material. The other problem in MIG welding between dissimilar weld relates to the transition zone between the metals and the intermetallic compounds (IMC) that produced in this transition zone. It is very important to investigate the phase diagram of the two metals for the fusion type welding process like MIG welding process. The dissimilar joints only can be made successfully if there is mutual solubility exist between both aluminum alloys. No solubility between the aluminum alloys can give the problem to the joint process. The intermetallic compound formed between dissimilar weld need to be investigated to check their crack sensitivity, ductility and susceptibility to corrosion, etc. The microstructure views need to be done observe the eutectic phase in the intermetallic compound. The other factor causes problem to the welding of dissimilar aluminum alloys relates to the thermal coefficient of thermal expansion (CTE) of both welds (Luijendijk, 2000). The widely difference between both thermal coefficient of thermal expansion can cause the internal stresses set up in the IMC zone during any temperature change of the welding process. The service failure may soon occur if there is extreme brittleness characteristic in intermetallic zone. The last factor is melting temperatures of the two aluminum alloys (Luijendijk, 2000). If there are differences in melting point, it will cause the other problem. This is of primary interest when a welding process utilizing heat is involved since one metal will be molten long before the other when subjected to the same heat source. The high heat input of welding will make the weld has advantage when welds of different melting temperature and thermal expansion to be joined.

15 3 1.3 OBJECTIVES The objectives of this project are: 1) To investigate the effect of parameters; torch angles, speeds, welding passes and currents to the mechanical properties of weld joints. 2) To predict the optimum parameters based on Taguchi methods analysis and verified with experimental 3) To study the different of mechanical properties and microstructure of weld joint (AA5052-H32 and 6061-T6 aluminum alloys) using the different welding parameters. 1.4 SCOPE OF PROJECT The scopes of the project are: 1) Welding process of different Aluminum alloys 2) Tensile and impact test investigation 3) Microstructure Analysis of weld joint (weld metal) 1.5 OVERVIEW OF REPORT Chapter 1 mainly briefs about the background of the project which involves the introduction, problem statements, objectives and scopes of the report. Chapter 2 basically describes more about the studies on microstructure, mechanical properties of aluminum alloy which has been done earlier by other scientists and engineers and Taguchi Method. Whereas Chapter 3 introduces the experimental procedure utilized to characterize the aluminum alloys studies the step by step process that will be done during this project and steps to perform Taguchi analysis with experimental values. Chapter 4 mainly discuss about the results obtained during the experiment. Lastly, Chapter 5 discuss about the conclusions that can be derived from this report and suggest few future recommendations.

16 4 CHAPTER 2 LITERATURE REVIEW 2.1 INTRODUCTION In this chapter, it basically describes more about the studies on microstructure and mechanical properties of aluminum alloys which has been done earlier by other scientists and engineers. Therefore, it also discussed about the MIG welding process which has been used in this experiment. 2.2 ALUMINUM ALLOYS The typical alloying elements usually consist of copper, magnesium, manganese, silicon, zinc, etc. Aluminum alloy is classified into two principal namely casting alloys and wrought alloys that consist of the heat-treatable and non-heat-treatable. About 85% of aluminum used for wrought products. For example is foils, rolled plate and foils. Aluminum alloys are mainly used in the components and engineering structures that required the light weight and high corrosion resistance of material. Alloys composed of aluminum and magnesium have been used widely in aerospace manufacturing because of light weight features. Aluminum 5xxx is lighter than other aluminum alloys. It also less flammable than aluminum alloys that contained high percentage of magnesium. Outstanding bare metal corrosion of the 5xxx and 6xxx aluminum materials is the obvious and significance difference between aluminum and steel (W.S. Miller 2000). Table 2.1 shows the applications of aluminum 5xxx and 6xxx in automotive industries.

17 5 Table 2.1: Aluminium alloys in space frame car design in Europe and North America, Parts Europe North America Outer Panels 6061-T T4 Inner Panels 5051/5182/6181A 6111/2008/5182 Structure/Sheet 6XXX-T O Structure/Extrusion 6XXX 6XXX Source: W.S. Miller (2000) Types of Aluminum Alloys There are many types of aluminum alloys. It consists of heat treatable and nontreatable aluminum alloys. Table 2.2 shows the types of aluminum alloys and its classification.

18 6 Table 2.2: Classification of aluminum alloys Series Properties 1xxx It contains 99% of Aluminum Non-heat treatable 2xxx Aluminum Copper approximation 2-10%. Strength and allows precipitation hardening. Weld solidification cracking can occurs. 3xxx Aluminum Manganese. It increases strength. 4xxx Aluminum Silicon. It can reduce melting temperature and can be heat treated alloy when combined with magnesium. 5xxx Aluminum Magnesium. It increases strength. 6xxx Aluminum Magnesium plus Silicon. It creates a unique compound magnesium silicide Mg2Si and suitable for extrusion components. It has heat treat properties. 7xxx Aluminum Zinc. It provides a heat treatable aluminum alloy which has very high strength when zinc copper and magnesium is added. Stress corrosion cracking and some alloys can be weld using MIG and some cannot. Heat treatable Non-heat treatable Both heat treatable and none-heat treatable None-heat treatable Heat treatable Heat treatable Source: Electric, L (2009) a) None Heat Treatable Aluminum Alloys Cold working or strain hardening process can increase strength of this type of aluminum alloys. Mechanical deformation will occur in the aluminum structure in order to reach the desired strength and it will cause the increasing of resistance to strain producing both higher and lower ductility. Non-heat treatable alloys cannot get high strength characteristics of heat treatable precipitation-hardened alloys. The absence of precipitate-forming elements in the low-to-moderate strength becomes beneficial for a welding perspective. This is because many of alloy additions needed for heat treatable precipitation hardening, magnesium plus silicon or copper plus magnesium can lead to hot cracking during solidification in welding process.

19 7 b) Heat treatable aluminum alloys Different from non-heat treatable alloys, heat treatable aluminum alloy achieve their optimum mechanical properties by thermal controlled heat treatment. The 2xxx, 6xxx and 7xxx series are heat treatable aluminum alloys where 4xxx series consist of heat treatable and non-heat treatable alloys. It tends to undergo hot cracking. Heat treatable alloys get their mechanical properties by solution heat treatment, thermal treatment and artificial aging are the most common methods Intermetallic Compound (IMC) Intermetallic compound in aluminum AA5052-H32 and AA6061-T6 need to be observed before the experiment is performed. For example for this observation, aluminum 6082 is chosen to be example for this study (G. Mrowka-Nowotnlk, 2007). There are three types of eutectic particles in the intermetallic compounds that is consist of α, β and the Mg 2 Si as shown in the Figure 2.1(a) and 2.1(b). (a)

20 8 (b) Figure 2.1: The eutectic particles of 6082 alloy in: a) ternary eutectic, b) the quaternary eutectic Source: G. Mrowka-Nowotnlk 2007 Mg 2 Si appeared in this microstructure view of 6082 because Si is the important element in the aluminum 6xxx as AA6061-T6. This is shown in Figure 2.2. Figure 2.2: Eutectic region of aluminum 6xxx Source: Donald R. Askeland 2002

21 9 The Mg 2 Si possibly not appear in the aluminum 5xxx series as AA5052-H32 because Si is not the major element. This is shown in Figure 2.3. Figure 2.3: Eutectic region in 5xxx Source: Donald R. Askeland Aluminum AA6061-T6 and AA5052-H32 There are a few things that must be reviewed in both aluminums which is chemical composition, mechanical properties, thermal properties, etc. This is to make sure the factor that will give the problem to the welding process of both materials Composition of AA6061-T6 and AA5052-H32 The microstructure of the aluminum AA6061-T6 and AA5052-H32 is shown in the Table 2.3. It seems that there is a difference in the grain size of both alloys. The grain size may be one of a factor that can be considered can affect the mechanical properties of aluminum alloys.

22 10 Table 2.3: Microstructure of AA6061-T6 and AA5052-H32 Aluminum Alloys Micrograph (room temp.) AA5052- H32 AA6061- T6 Source: S. Mahabunphachai (2010) The composition of both alloys is shown in the Table 2.4. ER5356 filler composition also put in the table to compare its composition with both AA5052-H32 and AA6061-T6. As mentioned before (refer to Table 2.2), the AA5052-H32 has more magnesium composition than AA6061-T6. But the ER5356 contains highest magnesium composition compared to both alloys. AA6061-T6 has the highest composition of Silicon compared to AA5052-H32 and ER5356. The composition of both aluminums also must be considered to make sure that the continuity can be success in the weld materials.

23 11 Table 2.4: Composition of AA5052-H32 and AA6061-T6 Alloys AA5052- H32 Elements and weight percentage (wt%) Al Cr Cu Fe Mg Mn Si Ti Zn Be Other Max Max 2.2- Max Max - Max - Max AA6061- T Max Max Max 0.15 Max Max 0.15 ER Max Max 0.1 Max Max 0.15 Source: Metal Handbook 10 th (1990) The ultimate tensile strength (UTS) of AA6061-T6 is higher than AA5052-H32. This can be seen in the Table 2.5. The table shows the other mechanical properties like hardness, fatigue strength of AA5052-H32, AA6061-T6 and also the filler used in this welding. Table 2.5: Mechanical Properties of AA5052-H32 and AA6061-T6 Mechanical Properties AA6061-T6 AA5052- H32 Filler 5356 Hardness, Brinell Hardness, Vickers Tensile Strength, Ultimate (MPa) Tensile Strength, Yield (MPa) Modulus of Elasticity (GPa) Poissons Ratio Fatigue Strength (MPa) Machinability 50% 30% - Shear Modulus (GPa) Shear Strength (MPa) Source: Metal Handbook 10 th (1990) The other important mechanical property that must be considered is melting point because it will cause the problem to the weld dilution if the both weld has the big difference in boiling point. It seems like there is no big difference between AA5052- H32 and AA6061-T6 in boiling point as shown in the Table 2.6. This means that the

24 12 joining process of these two different welds can be successful because they will melt at the almost same temperature. The CTE values are also not very different in both materials as shown in the table. The thermal stress will not be the problem to welding process in this study. Table 2.6: Thermal Properties of AA5052-H32 and AA6061-T6 Thermal Properties AA6061-T6 AA5052-H32 ER 5356 CTE, linear (µm/m- C) C Specific Heat Capacity (J/g- C) Thermal Conductivity (W/m-K) 23.8 µm/m C 24.1 µm/m C Melting Point ( C) Solidus ( C) Liquidus ( C) Source: Metal Handbook 10 th (1990) Phase diagram of AA5052-H32 and AA6061-T6 Figure 2.4 shows the phase diagram of aluminum 5xxx series and the eutectic particles α, β and (α + β) in the aluminum 5xxx series. Figure 2.5 shows the eutectic region in the aluminum 6xxx series and the eutectic particles α, β and (α + β) in the aluminum 6xxx series.