Influence of Welding and Heat Treatment On

INFLUENCE OF WELDING AND HEAT TREATMENT ON

ALUMINUM ALLOYS

A Thesis Presented to The Graduate Faculty of The University of Akron

In Partial Fulfillment Of the Requirements for the Degree Masters of Science

Eric B. Hilty May, 2014

INFLUENCE OF WELDING AND HEAT TREATMENT ON

ALUMINUM ALLOYS

Eric B. Hilty Thesis

Approved: Accepted:

______Advisor Dean of the College Dr. Craig C. Menzemer Dr. George Haritos

______

Co-Advisor Dean of the Graduate School Dr. T.S. Srivatsan Dr. George Newkome

______Committee Member Date Dr. Anil K. Patnaik

______Department Chair Dr. Weislaw K. Binienda

ABSTRACT

The welding of structural materials, such as aluminum alloys 6063, 6061 and

6005A, does have an adverse influence on the microstructure and mechanical properties at locations immediately adjacent to the weld. The influence of heat input, due to welding and artificial aging, was investigated on aluminum alloy extrusions of 6063,

6061 and 6005A. Uniaxial tensile tests, in conjunction with scanning electron microscopy observations, were done on the: (i) as-provided alloy in the natural temper, (ii) the as- provided alloy artificially aged, (iii) the as-welded alloy in the natural temper, and (iv) the as-welded alloy subject to heat treatment. The welding process used was gas metal arc (GMAW) with spray transfer at approximately 140 - 220 amps of current at 22-26 volts. The artificial aging used was a precipitation heat treatment for 6 hours at 360oF.

The aluminum alloys of the 6XXX series contain magnesium (Mg) and silicone

(Si) and are responsive to temperature. Optical microscopy observations revealed the influence of artificial aging to cause change in both size and shape of the second-phase particles present and distributed through the microstructure. The temperature and time of exposure to heat treatment did cause the second-phase particles to both precipitate and migrate through the microstructure resulting in an observable change in strength of the material.

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Uniaxial tensile tests were conducted for desired specimen thicknesses for sake of comparison. Section 6.4.2-2 of the 2010 Aluminum Design manual discusses provisions for mechanical properties of welded and artificially aged aluminum light poles, fabricated from aluminum alloy 6063 and 6005A. A basis for these provisions was the result of older round – robin testing programs [2, 3]. However, results of the studies were never placed in the open literature. Hence, the focus of this study was to determine the expected mechanical properties of welded and artificially aged 6063, 6061 and 6005A aluminum alloys and publish the results. Tensile tests revealed the welded aluminum alloy to have lower strength, both yield and ultimate tensile strength, when compared to the as-received un-welded counterpart. The impact of post-weld heat treatment on tensile properties and resultant fracture behavior is presented and briefly discussed in light of intrinsic microstructural effects and nature of loading.

ACKNOWLEDGEMENTS

I would like extend abundance of thanks and appreciation to Mr. Stephen Gerbetz

(Engineering Technician Sr., Department of Mechanical Engineering) for his understanding and timely assistance with resolution heat treatment, and to Mr. David

McVaney (Engineering Technician Sr., Department of Civil Engineering) for much needed help in using the infrastructure (test machine) for purpose of mechanical testing.

The aluminum alloy used in this research study was provided by Hapco, Inc. I would like to thank Hapco for providing me with the materials and opportunity to carry out this research experiment.

The support provided by my advisor Dr. Craig Menzemer (Associate Dean,

Engineering Deans Office) has been a tremendous help throughout my graduate experience. With his advice and guidance, I have learned a tremendous amount of engineering and life skills that will help me throughout my career. In addition I would like to express my gratitude to my co-advisor Dr. T. S. Srivatsan (Professor, Department of Mechanical Engineering) for his resources and guidance in furthering my research study to a higher level of success.

With guidance from my parents Roger and Linda Hilty and all the faculty and professors it has been a great experience obtaining my graduate degree, and I would like to express my thanks and appreciation in return.

TABLE OF CONTENTS

Page

LIST OF TABLES..…………………………………………………………………….viii

LIST OF FIGURES………………………………………………………………………ix

CHAPTER

I. INTRODUCTION…...... 1

1.1 Background………………………………….…………………………….1

1.2 Research Significance……………………………………………………..1

1.3 Research Objective.…………...…………………………………………..3

II. LITERATURE REVIEW…………………………………………………………5

2.1 Background of Aluminum Alloys….……………………………………..5

2.2 6XXX Series Aluminum Alloys…………………………………………..6

2.3 Welding and Heat Treatments……...……………………………………..7

III. MATERIALS AND PROCEDURES ……………………………………………..9

3.1 Test Specimen Preparation………………………………………………..9

3.2 Heat Treatment…………………………………………………………...10

3.3 Mechanical Testing…………………………………………………...….11

3.4 Microstructure Characterization…………………………………………11

3.5 Failure-Fracture-Damage Analysis………………………………………12

IV. RESULTS AND DISCUSSION…………………………………………………13

4.1 Microstructure Analysis………………………………………………….13

4.2 Tensile Response and Properties………………………………………...25

4.2.1 Stress vs Strain: as-provided alloy……………………………….28

4.2.2 Stress vs Strain: as-welded alloy…………………………………31

4.2.3 Stress vs Strain: Solution Heat Treatment Comparison………….34

4.2.4 Unusual Tensile Strength Values………………………………...35

4.3 Tensile Fracture Behavior………………………………………………..38

4.3.1 As-Received Parent Metal……………………………………….38

4.3.2 As-Welded……………………………………………………….48

4.3.3 Mechanisms Governing Tensile Fracture………………………..58

4.3.4 Kinetics Governing Stress-Material Response…………………..60

V. SUMMARY OF CONLUSIONS………………………………………………..62

5.1 Conclusions………………………………………………………………62

REFERENCES…………………………………………………………………………..65

APPENDIX………………………………………………………………………………67

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LIST OF TABLES Table Page 1. Nominal Chemical Composition of 6XXX Series Aluminum Alloys [4]………...6

2. Aluminum Alloy 6005A Tensile Strength………………………….……………36

3. Aluminum Alloy 6061 Tensile Strength……………………………...….………37

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LIST OF FIGURES Figure Page 1. Weld Removal……………………………………………………………….……9

2. Final Tensile Shape……………………………………………………………..…9

3. Light optical micrographs of the as-welded aluminum alloy 6063-T4: (a) Base metal, (b) Region of the weld pool, and (c) At the boundary between the base metal and weld pool……………….….………………...... …………………..….15 4. Light optical micrographs of the post weld heat treated aluminum alloy 6063 (a) Distribution of both coarse and intermediate size second-phase particles in the base metal, (b) Distribution of second-phase particles in the region of the heat affected zone, (c) Fine recrystallized grains in the weld pool, and (d) Microstructure at the weld-pool-base metal interface………………………..….16 5. Light optical micrographs of the post weld heat treated aluminum alloy 6063: (a) Fine recrystallized grains at the region of the weld, (b) High magnification observation of (a) showing both size and morphology of the fine grains………..17 6. Light optical micrographs of the post weld heat treated aluminum alloy 6063: (a) boundary of the weld, and (b) at the toe of the weld pool……………………17 7. Light optical micrographs of aluminum alloy 6005A-T4 showing microstructure of the following: (a) Coarse and intermediate second phase particles in the base metal of the as-received or as-provided metal (b) High magnification observation of (a) (c) Distribution of intermetallic particles in the heat treated sample. (d) High magnification observation of (c)………………………………...…………19 8. Light optical micrograph of the base metal 6061 showing fine grains of varying size and shape: (a) Grain size and morphology in the weld pool in the as-received metal (b) High magnification observation of (a) (c) Weld pool in the as-received plus heat treated metal (d) High magnification observation of ( c)……….……..20 9. Light optical micrographs of AA6061 showing the following: (a) Microstructure at the weld-base metal interface of the as-received Aluminum alloy 6061-T4, (b) High magnification observation of (a), (c) Microstructure of the weld-base metal interface in the as-received plus heat treated aluminum alloy 6061, (d) High magnification observation of ( c)………………………………………….……..21 10. Light optical micrographs of AA6061 showing the following: (a) Distribution of intermetallic particles in the base metal adjacent to the weld bead, and (b) Microstructure of the weld pool of the heat treated alloy……………………...... 22

11. Light optical micrographs of aluminum alloy 6061 showing: (a) Weld bead-base metal interface of the as-received alloy, (b) High magnification observation of (a)………………………………………………………………………...... …….22 12. Light optical micrographs of aluminum alloy 6005A-T4 showing: (a) Coarse and intermediate second phase particles in the base metal of the as-received or as- provided metal (b) Region or location of the weld bead in the as-welded aluminum alloy 6005A-T4 (c) Microstructure at the interface of the weld bead and base metal, in as-welded metal. (d) Microstructure showing second phase particle distribution in the as-received metal that was subject to heat treatment………………………………………………….…………………...…24 13. Light optical micrograph of the base metal showing fine grains of varying size and shape of the heat treated aluminum alloy 6005A………………………....…25 14. AA6063 1/4” thick specimen, as-received vs as-received heat treated……..…...28

15. AA6063 3/8” thick specimen, as-received vs as-received heat treated……....….28

16. AA6061 1/4” thick specimen, as-received vs as-received heat treated………….29

17. AA6061 3/8” thick specimen, as-received vs as-received heat treated………….29

18. AA6005A 1/4” thick specimen, as-received vs as-received heat treated………..30

19. AA6005A 1/8” thick specimen, as-received vs as-received heat treated………..30

20. AA6063 1/4” thick specimen, as-welded vs post weld heat treated……………..31

21. AA6063 3/8” thick specimen, as-welded vs post weld heat treated……………..31

22. AA6061 1/4” thick specimen, as-welded vs post weld heat treated……………..32

23. AA6061 3/8” thick specimen, as-welded vs post weld heat treated……………..32

24. AA6005A 1/4” thick specimen, as-welded vs post weld heat treated…………...33

25. AA6005A 1/8” thick specimen, as-welded vs post weld heat treated………...…33

26. AA6005A 1/8” thick specimen, as-received vs ARHT vs solution heat treatment with PHT………………………………………………………………………....34 27. AA6005A 1/8” thick specimen, as-welded vs PWHT vs solution heat treatment with PWHT………………………………………………………………………34 28. Scanning electron micrographs of the tensile fracture surface of as-received aluminum alloy 6063 in the T4 temper, showing: (a) Overall morphology of failure, (b) High magnification observation of (a) showing non-linear nature of macroscopic cracks, (c) Isolated pockets of striations on the transgranular fracture x

surface, (d) Observable population of voids of varying size intermingled with dimples…………………………………………………………………..……….39 29. Scanning electron micrographs of the tensile fracture surface of the heat treated aluminum alloy 6063-T4, showing: (a) Overall morphology of failure normal to far field stress axis, (b) High magnification observation of (a) showing population of voids of varying size intermingled with isolated microscopic cracks, (c) High magnification observation of (b) showing the nature and morphology of voids covering the transgranular fracture region and void coalescence to form microscopic crack, (d) Voids of varying size intermingled with dimples on overload fracture surface…………………………………………………………40 30. Scanning electron micrographs of the tensile fracture surface of as-received aluminum alloy 6061-T4, showing: (a) Overall morphology of failure, (b) High magnification observation of (a) showing non-linear nature of macroscopic crack surrounded by observable population of voids and dimples, (c) High magnification observation of (a) showing size and morphology of the voids, (d) The overload fracture surface, features give no indication of likely micro failure mechanism…………………………………………………………………...…..42 31. Scanning electron micrographs of the tensile fracture surface of as-received plus heat treated aluminum alloy 6061-T6, showing: (a) Overall morphology of failure showing an array of macroscopic and microscopic cracks, (b) High magnification observation of (a) showing non-linear nature of macroscopic crack surrounded by pockets of voids and dimples, reminiscent of highly localized ductile failure mechanism, (c) High magnification observation of (b) nature, morphology and overall distribution of the voids intermingled with highly shallow dimples, (d) A mixture of voids of varying size, shallow dimples adjacent to cracked grain boundary triple junction……………………………………………………….…43 32. Scanning electron micrographs of the tensile fracture surface of as-received plus heat treated aluminum alloy 6061-T6, showing elongated dimples indicative of shear and locally occurring ductile failure………………………………….……44 33. Scanning electron micrographs of the tensile fracture surface of as-received aluminum alloy 6005A in the T4 temper, showing: (a) Overall morphology of failure, (b) High magnification observation of (a) showing nonlinear nature of macroscopic crack surrounded by observable population of voids and dimples, (c) High magnification observation of (a) showing size and morphology of the voids, (d) The overload fracture surface, features give no indication of likely micro failure mechanism………………………………………………………………..46 34. Scanning electron micrographs of the tensile fracture surface of as-received plus heat treated aluminum alloy 6005A in the T4 temper, showing: (a) Overall morphology of failure showing an array of macroscopic and microscopic cracks, (b) High magnification observation of (a) showing non-linear nature of macroscopic crack surrounded by pockets of voids and dimples, reminiscent of highly localized ductile failure mechanism, (c) High magnification observation of (b) nature, morphology and overall distribution of the voids intermingled with

highly shallow dimples, (d) A mixture of voids of varying size, shallow dimples adjacent to cracked grain boundary triple junction…………………………...….47 35. Scanning electron micrographs of the tensile fracture surface of the as-welded aluminum alloy 6063-T4, showing: (a) Healthy population of voids of varying size on tensile fracture surface, (b) High magnification observation of (a) showing nature and morphology of the voids, (c) Healthy population of voids and dimples, (d) Microvoid growth during far field loading and coalescence to form a fine microscopic crack………………………………………………………………..49 36. Scanning electron micrographs of the tensile fracture surface of the as-welded and heat treated aluminum alloy 6063, showing: (a) Overall morphology of failure, (b) High magnification observation of (a) showing the nature, morphology and distribution of voids covering the transgranular fracture regions, (c) Voids and shallow dimples covering the overload fracture surface…………………………50 37. Scanning electron micrographs of the tensile fracture surface of as -welded aluminum alloy 6061 in the T4 temper, showing: (a) Overall morphology of failure showing an array of macroscopic and microscopic cracks, (b) High magnification observation of (a) showing non-linear nature of macroscopic crack surrounded by pockets of voids and dimples, reminiscent of highly localized ductile failure mechanism, ( c) High magnification observation of (b) nature, morphology and overall distribution of the voids intermingled with highly shallow dimples, (d) A mixture of voids of varying size, shallow dimples adjacent to cracked grain boundary triple junction……………………………………….….52 38. Scanning electron micrographs of the tensile fracture surface of post weld heat treated aluminum alloy 6061, showing: (a) Overall morphology of failure, (b) High magnification observation of (a) showing a healthy and observable population of voids and shallow dimples adjacent to the intergranular crack, (c) High magnification observation of (b) showing size, morphology and distribution of the voids, both macroscopic and fine microscopic, (d) The presence of voids and void coalescence to form macroscopic crack in the region immediately prior to overload……………………………………………………………………….53 39. Scanning electron micrographs of the tensile fracture surface of post weld heat treated aluminum alloy 6061, showing fine ripples or striations- like featuring reminiscent of locally occurring micro plastic deformation……………………..54 40. Scanning electron micrographs of the tensile fracture surface of as-welded plus heat treated aluminum alloy 6005A in the T4 temper, showing: (a) Overall morphology of failure showing an array of macroscopic and microscopic cracks, (b) High magnification observation of showing non-linear nature of macroscopic crack surrounded by pockets of voids and dimples, reminiscent of highly localized ductile failure mechanism, (c) High magnification observation of nature, morphology and overall distribution of the voids intermingled with highly shallow dimples, (d) A mixture of voids of varying size, shallow dimples adjacent to cracked grain boundary triple junction…………………………………………..56

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41. Scanning electron micrographs of the tensile fracture surface of aluminum alloy 6005A in the T4 temper and as-welded condition, showing: (a) Overall morphology of failure, (b) High magnification observation of (a) showing a healthy and observable population of voids and shallow dimples adjacent to the intergranular crack, (c) High magnification observation of (b) showing size, morphology and distribution of the voids, both macroscopic and fine microscopic, (d) The presence of voids and void coalescence to form macroscopic crack in the region immediately prior to overload……………………………………………57 42. Schematic showing the formation of void sheets between expanding or growing voids leading to void-void interactions and eventual coalescence………………60

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CHAPTER I

INTRODUCTION

1.1 Background

Structural aluminum alloys have been used in more applications in recent years because these materials are strong, lightweight and cost efficient. Many products are being fabricated from 6XXX series aluminum alloys due to their ability to be extruded, welded, and possess a natural resistance to corrosion [2, 7-10]. The use of these aluminum alloys does bring an aspect to engineering. This is because they are sensitive to temperature when subject to welding and heat treatment, and the resultant change in mechanical properties must be accounted for in engineering design. Understanding the weldability of these materials is important in order to make the most efficient use of the materials. It is uncommon for a material to be welded and have no effect on its microstructure and strength [1]. However, precipitation heat treatment can reduce the effects of welding on the parent metal.

1.2 Research Significance

One particular product of interest is welded aluminum light poles. Section 6.4.2-2 of the 2010 Aluminum Design manual discusses mechanical properties of welded and artificially aged aluminum light poles [1]

1) For lighting poles fabricated from 6005 aluminum that are less than or equal

to 0.25 in thick, welded in the T1 temper with 4043 filler and subsequently

artificially aged to the T5 temper after welding, mechanical properties of the

base metal within 1.0 in of the weld shall be taken as 85% of the un-welded

6005-T5.

2) For lighting poles fabricated from 6063 aluminum that are less than or equal

to 0.375 in thick, welded in the T4 temper and subsequently artificially aged

to the T6 temper after welding, mechanical properties of the base metal within

1.0 in of the weld shall be taken as 85% of the un-welded 6063-T6.

Basis for these provisions was the result of older round – robin testing programs [2, 3].

However, results of the studies were never placed in the open literature. Aluminum alloys 6063, 6061, and 6005A will be referred to throughout this manuscript as AA6063,

AA6061, and AA6005A.

The use of an appropriate precipitation heat treatment for extrusions of AA6063,

AA6061, and AA6005A will enable the alloys to achieve an optimum combination of mechanical properties. The precipitation heat treatment used was similar to that presented for MgSi-containing aluminum alloys; as specified in ASTM B918-01, which was 6 hours at 360o F [5]. The precipitation heat treatment process is especially important for welded aluminum alloys in an attempt to reduce or minimize residual stresses that tend to form during cooling of the weld while concurrently improving properties. Materials that are heated to the molten temperature experience changes in their microstructure. The subsequent rate at which the material cools has a direct influence on its microstructure and resultant mechanical properties, particularly strength

2 and ductility. An example of this is when steel is heated to its molten temperature of approximately 2550° F. When the steel is cooled fast, the micro carbon structure creates a microstructure that is hard, brittle and easily susceptible to cracking by hydrogen during solidification. When this occurs, the steel tends to become brittle and contains many points of “local” stress concentration that tend to promote fracture during loading.

Arc welding does require a great deal of heat input in an attempt to get the materials to fuse together. After the chosen material is welded, it gradually cools depending on the temperature of the room and/or the immediate surroundings. Welding of these alloys often causes an adverse effect on overall mechanical response of the material due to intrinsic metallurgical changes in the zones that are heat affected. The

AA6063 and AA6061 are commonly welded in the T4 temper, i.e. 6063-T4 and 6061-T4, and AA6005A is commonly welded in the T1 temper, i.e. 6005A-T1. Subsequent to welding these alloys are precipitation heat treated to get an alloy whose temper is comparable to the T5 and T6 temper, i.e. 6063-T6, 6061-T6, and 6005A-T5 [6].

1.3 Research Objective

Investigation of specific influence of heat treatment on the welded aluminum alloys is needed in order to have a better understanding of their structural behavior when subject to loading. The focus of this study was to determine the expected mechanical properties of welded and artificially aged AA6063, AA6061 and AA6005A and publish the results. Each alloy was tested in different thicknesses, with the prime objective of determining the results of post weld heat treatment (PWHT) tensile strength. AA6063,

AA6061 and AA6005A were tested in conjunction with light optical microscopy observations for the following:

(i) The as-provided alloy, i.e., 6063-T4, 6061-T4, and 6005A- T1

(ii) The as-provided alloy artificially aged, i.e., 6063-T6, 6061-T6, and 6005A-T5

(iii) The as-welded alloy welded in the natural temper, and

(iv) The welded alloy that was subject to post weld precipitation heat treatment.

Multiple tensile tests on each specimen were conducted in order to provide both substantial and valuable evidence of material behavior. The values of strength are compared with the typical values documented in the published literature in order to establish differences, if any, in both material and structural behavior. The main objective is to increase understanding of process-property relationship of welded and heat treated

6XXX series aluminum alloys.

CHAPTER II

LITERATURE REVIEW

2.1 The Background of Aluminum Alloys

Pure aluminum itself is not very strong, soft and ductile, and light in weight.

Mechanical and physical properties of pure aluminum can be changed by the addition of other elements such as copper, magnesium, and silicon. The addition of other elements is typically less than a few percent of the total solution. The benefit of producing these aluminum alloys is to enhance their properties and provide alloys with different properties useful for various products and applications.

The use of aluminum alloys in structural applications has primarily been to decrease weight and improve corrosion resistance [9]. The aerospace industry has used aluminum alloys in an attempt to design aircrafts that are faster and more fuel efficient.

The first use of aluminum alloys in airplanes was in the early 1900’s. At this time copper was being used to strengthen the alloy. The use of copper led to the development of the commonly used 2XXX series aluminum alloys (Al-Cu-Mg). However the weldability of these alloys was insufficient at the time and the alloys could not be used with confidence in structurally welded applications. The addition of magnesium and silicon led to the formation of 6XXX series aluminum alloys (Al-Si-Mg) [20]. Zinc was added to spark the creation of 7XXX series alloys (Al-Zn-Mg-Cu). All of these alloys are

5 precipitation heat treatable as a means to strengthen. Different heat treatments are used to change the alloy temper depending on the desired material properties needed.

2.2 6XXX Series Aluminum Alloys

Compared to pure aluminum, aluminum alloys contain solute additions that effect grain structures and the microstructures within the grains (Table 1). The occurrence of this allows the alloys to respond differently to working and heat treating. The properties of 6xxx Al-Mg-Si alloys have been known to be influenced by the precursor phases to the equilibrium Mg2Si (β) [19]. With copper in many 6xxx series alloys additional hardening phases appear [20]. The appearance of these phases makes the alloys temperature and time sensitive when aging. This can make predicting strength and material properties difficult. The 6XXX series alloys have better corrosion resistance and slightly higher strength compared to 2XXX series alloys. A major benefit to the 6xxx series alloys is the extrudability due to the addition of silicon.

Table 1: Nominal Chemical Composition of 6xxx's Series Aluminum Alloy [4] 6063 6005A 6061 Element % % % Al Max 97.5 96.65 - 98.95 95.8 - 98.6 Cr Max 0.1 0.0 - 0.3 0.04 - 0.35 Cu Max 0.1 0.0 - 0.3 0.15 - 0.4 Fe Max 0.35 0.0 - 0.35 Max 0.7 Mg 0.45 - 0.9 0.4 - 0.7 0.8 - 1.2 Mn Max 0.1 0.0 - 0.5 Max 0.15 Si 0.2 - 0.6 0.5 - 0.9 0.4 - 0.8 Ti Max 0.1 0.0 - 0.1 Max 0.15 Zn Max 0.1 0.0 - 0.2 Max 0.25 Other, each Max 0.05 Max 0.05 Max 0.05 Other, total Max 0.15 Max 0.15 Max 0.15

2.3 Welding and Heat Treatment

Welding of aluminum alloys has been known to be difficult and result in a decrease in strength near weld heat affected zones (HAZ). The most common process for welding aluminum alloys is gas metal arc welding (GMAW) with spray transfer. Spray transfer allows many small drops of filler metal to spray across the arc from the electrode wire to the base metal. This results in a larger weld puddle therefor limiting the possible weld positions to only lap joint and fillet welds. Shielding gas used for GMAW of aluminum is commonly argon and argon/helium mixture.

When welding metals it is important to prepare the base metal removing any contaminants before welding. Oxide layers on metals can contain many contaminates, that if not removed will result in poor weld quality and decreased material strength.

Aluminum oxide layers are often strong and must be removed using an abrasive technique such as a wire brush. The melting point of aluminum oxide is 3700o F, while the melting point of the base material underneath the oxide layer is 1200o F. If the material is not properly prepared before welding, particles from the oxide layer that have not melted will be deposited within the weld causing discontinuities in the material microstructure. Weld cracking may occur when welding aluminum due to the fact that aluminum dissipates heat quickly. Preheating can be used to prevent cracking and cold welds.

Aluminum alloys start as constituents in solution. Quenching is used in order to keep the elements in the solution from migrating to fast through the microstructure. After quenching, aging occurs over a period of a few days at room temperature until the alloy is in its natural temper. Following the natural aging of the alloy artificial aging is used to

7 obtain different temper designations, often referred to as precipitation heat treating. The material tempers are sensitive to time and temperature of artificial aging. The process of precipitation heat treating allows second phase particles to precipitate from the material matrix [18]. In most metals this process is a means of stress relieving and strengthening.

CHAPTER III

MATERIALS AND PROCEDURES

3.1 Test Specimen Preparation

The tensile test specimens were prepared consistent with ASTM standards for sheet tensile samples [5]. Each of the aluminum alloys tested were cut from extruded sections for sake of testing the alloys. The samples were first cut into one inch wide strips with the extrusion direction perpendicular to the width. Once all samples were cut, some were selected to be welded in order to obtain heat affected samples. After the welds were placed, the weld joint was machined off of the sample leaving only the parent metal with a heat affected zone. (See Figure 1) The machine used to remove the welds from the samples was a Bridgeport vertical hand mill. After welds were removed, each sample was then cut into its final tensile test shape using a Haas CNC router for accuracy. A desired quantity of welded and un-welded samples was then selected to be heat treated prior to testing.

Figure 1: Weld Removal Figure 2: Final Tensile Shape

3.2 Heat Treatment The heat treatment process was selected from ASTM B918-01 [5]. This precipitation heat treatment is often referred to as artificial aging (PHT or PWHT - Post

Weld Heat Treatment), and the treatment consisted of soaking the material at approximately 360° F for 6 hours and subsequently allowed to cool in the oven.

Aluminum alloys start with constituents in solution before aging. Artificial aging can then be used to precipitate second phase particles (Mg2Si) to change the aluminum alloy from a natural temper (i.e. 6063-T4) with a typical ultimate strength of 25.0 ksi, to a higher temper, such as 6063-T6 with a typical ultimate strength of 35 ksi. The un-welded materials used were artificially aged to a T6 temper for 6063 and 6061 parent metal tests and a T5 temper for the 6005A parent metal tests. The welded materials were artificially aged (PWHT) to obtain post weld heat treated samples with expected properties to be similar to that of 6063-T6, 6061-T6, and 6005A-T5.

A few selected samples of 6005A were re-solution heat treated prior to the precipitation heat treatment. The re-solution heat treatment (SHT – Solution Heat

Treatment) was selected from ASM Handbook, Volume 4, Heat Treating [6]. This treatment process was at 985o F for 1 hour and rapidly quenched in 60/40 (water/glycol) mixture. The purpose of this heat treatment was to obtain the aluminum alloy in solution and allow the second phase particles to precipitate naturally for the welded and un- welded samples. Following the solution heat treatment, the same samples were precipitation heat treated to obtain 6005A-T5. The benefit of this test was to compare strength properties of welded 6005A re-solution and precipitation heat treated samples to that of PWHT 6005A samples.

3.3 Mechanical Testing

Each test specimen was placed in a mechanical test machine (Model: Baldwin

Materials Testing Equipment: Warner & Swasey 300 HV-300.000 lb) and deformed to failure in uniaxial tension. A Class B extensometer was fixed at the gage section of each test specimen to obtain the axial strain during loading. The data from each tensile test was recorded on a PC-based Data Acquisition System and used to create the stress versus strain curve for the specific test specimen. The stress versus strain curves were used to both understand and compare the strength values and behavior of the aluminum alloy specimens. Multiple tensile tests were conducted on each specimen type, i.e. as-provided

AA6063-T4 (AR), as-provided artificially aged AA6063-T6 (ARHT), as-welded

AA6063-T4 (AW), and the welded AA6063-T4 subject to post weld heat treatment

(PWHT) with the objective of obtaining valuable physical evidence pertaining to behavior of the chosen alloy 6063, 6061 or 6005A. The values of strength were compared with the typical values in order to highlight any differences in material and structural behavior.

3.4 Microstructure Characterization

Light optical microscopy was used to examine the microstructure of each alloy- temper combination studied. Samples of 6063, 6061 and 6005A in the following conditions, i.e., as-provided AA6063-T4 (AR), as-provided artificially aged AA6063-T6

(ARHT), as-welded AA6063-T4 (AW), and the welded AA6063-T4 subject to post weld heat treatment (PWHT), were prepared very much in conformance with procedures followed for metallographic preparation of samples for purpose of observation in a light optical microscope. The ground and polished samples were etched using Keller’s reagent

(a solution mixture of hydrofluoric acid, concentrated nitric acid and distilled water).

The etched surface of each sample containing the weld region was observed in an optical microscope and photographed using a bright field illumination technique.

3.5 Failure-Fracture-Damage Analysis

Fracture surfaces of the deformed and failed test specimens were examined in a scanning electron microscope (SEM) so as to determine the macroscopic fracture mode, and to concurrently characterize the fine-scale features and topography of the fracture surface and thereby establish microscopic mechanisms governing fracture. The distinction between the macroscopic mode and microscopic fracture mechanisms is based entirely on the magnification level at which the observations are made. The macroscopic mode refers to the overall nature of failure, while the microscopic mechanism refers to the failure processes occurring at the ‘local’ level, such as: (i) microscopic void formation, (ii) their gradual growth under the influence of far-field stress and eventual coalescence, coupled with (iii) cracking. Samples, for observation in the scanning electron microscope (SEM), were obtained from the failed specimens by sectioning parallel to the fracture surface

CHAPTER IV

RESULTS AND DISCUSSION

4.1 Microstructure Analysis

Light optical micrographs taken over a range of low magnifications reveal the initial microstructure of the chosen alloy in the different conditions it was tested in this research study. This is shown in Figures 3-13. The microstructure of the as-welded

AA6063 in the T4 condition is shown in Figure 3. The base metal revealed a non- uniform dispersion of both large and intermediate-size intermetallic particles randomly distributed through the microstructure. No attempt was made in this research study to determine the actual chemical composition of these intermetallic particles, which primarily result from the presence of impurity elements iron and silicon. The result is the formation of a variety of Al-Fe and Al-Fe-Si intermetallic particles during solidification.

Any silicon, which is not incorporated in the alpha-aluminum matrix or the Al-Fe-Si intermetallic phases, combines with magnesium to form Mg2Si during the later stages of the solidification process. The nature of intermetallic particles present in the final microstructure will thus be determined by not only the as-cast state, but also by the subsequent homogenization and thermo-mechanical processing given to the alloy [11].

The region of the weld revealed a fully recrystallized fine grain structure (Figure 3b).

A noticeable difference in microstructure was evident in the base metal and weld pool as evident on crossing the weld pool-base metal interface (Figure 3c).

The optical microstructure of the post weld heat treated AA6063 to the T6 temper is shown in Figure 4. The base metal revealed a random dispersion of both the coarse and intermediate size intermetallic particles, which result from the presence of the impurity elements iron and silicon (Figure 4a). At the region of the weld the microstructure was fully recrystallized with very fine recrystallized grains (Figure 4b).

A noticeable difference in microstructure of the two regions, i.e., base metal and weld pool is seen and is shown in Figure 4c and Figure 4d. The microstructure at the region of the weld is shown in Figure 5 at two different magnifications. The grains were fine in size and fully recrystallized (Figure 5a). Higher magnification reveals the random orientation of fine grains having well-defined grain boundaries. Microstructure of the post weld heat treated AA6063 did reveal observable differences at the boundary between the base metal and the weld pool (Figure 6a). In the base metal, i.e., AA6063, it was evident a healthy population of both the coarse and intermediate size intermetallic particles dispersed randomly through the microstructure. At the region of the interface between the base metal AA6063 and the weld pool isolated microscopic cracks were evident as shown in Figure 6b.

(a) (b)

20μm 20μm

(c)

20μm

Figure 3: Light optical micrographs of the as-welded aluminum alloy 6063-T4 showing: (a) base metal, (b) Region of the weld pool, and (c) At the boundary between the base metal and weld pool.

(a) (b)

50μm 50μm

20μm 20μm

Figure 4: Light optical micrographs of the post weld heat treated aluminum alloy, i.e.6063, showing: (a) Distribution of both coarse and intermediate size second-phase particles in the base metal, (b) Distribution of second-phase particles in the region of the heat affected zone, (c) Fine recrystallized grains in the weld pool, and (d) Microstructure at the weld-pool-base metal interface.

(a) (b)

25μm 20μm

Figure 5: Light optical micrographs of the post weld heat treated aluminum alloy, i.e., 6063, showing: (a) Fine recrystallized grains at the region of the weld, and (b) High magnification observation of (a) showing both size and morphology of the fine grains.

(a) (b) c) c)

40μm 20μm

Figure 6: Light optical micrographs of the post weld heat treated aluminum alloy, i.e., 6063, showing: (a) boundary of the weld, and (b) at the toe of the weld pool.

The as-provided, i.e., as-received, AA6061-T4 revealed a random distribution of both coarse and intermediate-size intermetallic particles (Figure 7a and Figure 7c).

Similar to AA6063, these intermetallic particles result from the presence of the residual elements, such as, iron and silicon. In an earlier study these particles have been identified to be the Al12Fe3Si and Al15 (FeMn) 3 Si and Al5FeSi [17, 18]. The iron-rich intermetallic particles range in size from 1 to 10 microns and are clearly responsible for the initiation of damage during plastic deformation [12, 13]. The dispersoids, which are manganese- rich particles in this alloy (Al9Mn3Si), help in controlling both grain size and grain growth during solidification. The as-provided AA6061 in the as-welded condition revealed very fine recrystallized grains at the region of the weld bead (Figure 8 b). A noticeable difference in microstructure between the two regions, i.e., weld bead and base metal, was clearly evident at the interface between the two regions (Figure 9). The as- provided alloy, i.e., AA6061-T4, that was artificially aged, or precipitation heat treated, to get the T6 temper revealed an observable volume fraction of both coarse and intermediate-size second-phase particles in the base metal as shown in Figure 10a. These particles were distributed randomly through the microstructure. The as-welded AA6061 that was subject to post-weld heat treatment revealed very well-defined grains that were

(a) small in size and of varying shape, and (b) distributed randomly through the microstructure of the base metal (Figure 10b). The microstructure at the interface of the weld bead and the base metal AA6061 is shown in Figure 11. Fine microscopic cracks initiated at the interface and propagated into the base metal.

(a) (b)

200 μm 500 μm

Figure 7: Light optical micrographs of aluminum alloy 6061-T4 showing microstructure of the following: (a) Coarse and intermediate second phase particles in the base metal of the as-received or as-provided metal, (b) High magnification observation of (a), (c) Distribution of intermetallic particles in the heat treated sample, and (d) High magnification observation of (c)

(a) (b)

200 μm 500 μm

Figure 8: Light optical micrograph of the weld pool showing fine grains of varying size and shape: (a) Grain size and morphology in the weld pool in the as- received metal (b) High magnification observation of (a), (c) Weld pool in the as-received plus heat treated metal, and (d) High magnification observation of (c)

(a) (b)

500 μm 200 μm

200 μm 500 μm

Figure 9: Light optical micrographs showing the following:(a) Microstructure at the weld-base metal interface of the as-received Aluminum alloy 6061-T4, (b) High magnification observation of (a), (c) Microstructure of the weld-base metal interface in the as-received plus heat treated aluminum alloy 6061, and (d) High magnification observation of ( c)

(a) (b)

200 μm 200 μm

Figure 10: Light optical micrographs of AA6061 showing the following: (a) Distribution of intermetallic particles in the base metal adjacent to the weld bead, and (b) Microstructure of the weld pool of the heat treated alloy

(a) (b)

200 μm 500 μm

Figure 11: Light optical micrographs of aluminum alloy 6061 showing: (a) Weld bead-base metal interface of the as-received alloy, and (b) High magnification observation of (a)

The as-provided AA6005A revealed a random distribution of both coarse and intermediate-size intermetallic particles similar to AA6063 and AA6061 (Figure 12a).

Residual elements iron and silicon cause the formation of these particles. The dispersoids, which are manganese-rich particles in this alloy, help in controlling both grain size and grain growth during solidification. The as-provided alloy in the as-welded condition revealed recrystallized grains having a very fine grain size at the region of the weld bead

(Figure 12-b). A noticeable difference in microstructure between the two regions, i.e., weld bead and base metal, was clearly evident at the interface between the two regions

(Figure 12-c). The as-provided alloy, i.e., AA6005A-T1, that was artificially aged, or precipitation heat treated, to get the T5 temper revealed an observable volume fraction of both coarse and intermediate-size second-phase particles in the base metal as shown in

Figure 12-d. These particles were distributed randomly through the microstructure. The as-welded alloy that was subject to post-weld heat treatment revealed very well-defined grains, small in size and of varying shape, distributed randomly through the microstructure of the base metal (Figure 13).

(a) (b)

50μm 50μm

Figure 12: Light optical micrographs of aluminum alloy 6005A-T4 showing:(a) Coarse and intermediate second phase particles in the base metal of the as- received or as-provided metal, (b) Region or location of the weld bead in the as-welded aluminum alloy 6005A-T4, (c) Microstructure at the interface of the weld bead and base metal, in as-welded metal, and (d) Microstructure showing second phase particle distribution in the as- received metal that was subject to heat treatment

20μm

Figure 13: Light optical micrograph of the base metal showing fine grains of varying size and shape of the heat treated aluminum alloy 6005A. 4.2 Tensile Response and Properties

It is important to understand that welding does exert an influence on mechanical properties of 6XXX se ries aluminum alloys. The influence differs depending on the alloy as well as the welding process used coupled with overall quality of the weld. The type of weld joint and thickness of the starting material influences the heat input and resultant strength. The results of an investigation on welded 6063, 6061 and 6005A aluminum alloys revealed precipitation heat treatment tends to increase strength of the materials.

Further, it was found that when these aluminum alloys were welded and not heat treated they experienced an actual decrease in strength as a consequence of welding. The reason for the observed decrease can be ascribed to changes in microstructure of the material as a direct consequence of heat input during welding. When the materials were welded, the rate of cooling depended upon the prevailing temperature in the room, approximately

72°F. The majority of the “as welded” samples broke at the region of the weld; usually on the side of the weld to which more heat was provided during welding. This is a

25 common occurrence in welded products primarily because the weld itself is stronger than the parent metal. However, the heat-affected zone (HAZ) immediately adjacent the weld bead tends to be lower strength.

A statistical analysis using the guidelines establish in the 2010 Aluminum Design

Manual [1] coupled with guaranteed minimum strengths was used to determine reasonable design minimum strength values for the post weld heat treated samples. The mechanical properties of the base metal, within 1.0 in of the weld for those test specimens welded in the as-received temper (6063-T4, 6063-T4, 6005A-T1) and subsequently subjected to post-weld heat treatment should be taken as a percentage of the guaranteed minimum un-welded strengths: (6063-T6, 6061-T6, 6005A-T5).

 1/4” 6063 – 93.4% ultimate and 95.7% yield

 3/8” 6063 – 95.0% ultimate and 79.7% yield

 1/4” 6061 – 97.9% ultimate and 98.5% yield

 3/8” 6061 – 74.5% ultimate and 60.1% yield

 1/8” 6005A – 92.6% ultimate and 82.5% yield

Using the results from tensile tests of post weld heat treated AA6063, AA6061, and AA6005A, recommendations for Section 6.4.2-2 of the Aluminum Design manual regarding mechanical properties of welded and artificially aged aluminum light poles would be as follows:

1) For lighting poles fabricated from 6005A aluminum that are less than or

equal to 0.125 in thick, welded in the T1 temper with 4043 filler and

subsequently artificially aged to the T5 temper after welding, mechanical

properties of the base metal within 1.0 in of the weld shall be taken as

80% of the un-welded 6005-T5.

2) For lighting poles fabricated from 6063 aluminum that are less than or

equal to 0.25 in thick, welded in the T4 temper and subsequently

artificially aged to the T6 temper after welding, mechanical properties of

the base metal within 1.0 in of the weld shall be taken as 90% of the un-

welded 6063-T6.

3) For lighting poles fabricated from 6063 aluminum that are 0.25 in thick

up to .375 in. thick, welded in the T4 temper with 4043 filler and

subsequently artificially aged to the T5 temper after welding, mechanical

properties of the base metal within 1.0 in of the weld shall be taken as

80% of the un-welded 6063-T6.

4) For lighting poles fabricated from 6061 aluminum that are less than or

equal to 0.25 in. thick, welded in the T4 temper and subsequently

artificially aged to the T6 temper after welding, mechanical properties of

the base metal within 1.0 in of the weld shall be taken as 95% of the un-

welded 6061-T6.

4.2.1 Stress vs Strain: as-provided alloy

45 ARHT 40

Stress Stress (ksi) 20

10 AR 5

0 0 2 4 6 8 10 12 14 16 Strain (%)

Figure 14: AA6063 1/4” thick specimen, as-received vs as-received heat treated

45 AR 40

30 ARHT 25

Stress Stress (ksi) 20

0 0 2 4 6 8 10 12 14 16 Strain (%)

Figure 15: AA6063 3/8” thick specimen, as-received vs as-received heat treated

45 ARHT 40

Stress Stress (ksi) 20 AR 15

0 0 2 4 6 8 10 12 14 16 Strain (%)

Figure 16: AA6061 1/4” thick specimen, as-received vs as-received heat treated

35 ARHT 30

Stress Stress (ksi) 20

15 AR 10

0 0 5 10 15 20 25 30 Strain (%)

Figure 17: AA6061 3/8” thick specimen, as-received vs as-received heat treated

35 ARHT

Stress Stress (ksi) 20

10 AR 5

0 0 2 4 6 8 10 12 14 16 Strain (%)

Figure 18: AA6005A 1/4” thick specimen, as-received vs as-received heat treated

35 ARHT 30

Stress Stress (ksi) 20

15 AR 10

0 0 5 10 15 20 25 30 Strain (%)

Figure 19: AA6005A 1/8” thick specimen, as-received vs as-received heat treated

4.2.2 Stress vs Strain: as-welded alloy

30 PWHT 25

Stress Stress (ksi) 20

10 AW 5

0 0 2 4 6 8 10 12 14 16 Strain (%)

Figure 20: AA6063 1/4” thick specimen, as-welded vs post weld heat treated

35 PWHT

Stress Stress (ksi) 20

15 AW

0 0 2 4 6 8 10 12 14 16 Strain (%)

Figure 21: AA6063 3/8” thick specimen, as-welded vs post weld heat treated

40 PWHT

Stress Stress (ksi) 20

15 AW 10

0 0 2 4 6 8 10 12 14 16 Strain (%)

Figure 22: AA6061 1/4” thick specimen, as-welded vs post weld heat treated

40 PWHT

Stress Stress (ksi) 20

15 AW 10

0 0 2 4 6 8 10 12 14 16 18 Strain (%)

Figure 23: AA6061 3/8” thick specimen, as-welded vs post weld heat treated

30 PWHT

Stress Stress (ksi) 20

10 AW 5

0 0 2 4 6 8 10 12 14 16 Strain (%)

Figure 24: AA6005A 1/4” thick specimen, as-welded vs post weld heat treated

30 PWHT

Stress Stress (ksi) 20

10 AW 5

0 0 2 4 6 8 10 12 14 16 Strain (%)

Figure 25: AA6005A 1/8” thick specimen, as-welded vs post weld heat treated

4.2.3 Stress vs Strain: Solution Heat Treatment Comparison

45 SHT+PHT 40

35 ARHT 30

Stress Stress (ksi) 20

15 AR 10

0 0 5 10 15 20 25 30 Strain (%)

Figure 26: AA6005A 1/8” thick specimen, as-received vs ARHT vs solution heat treatment with PHT

45 AW+SHT+PHT 40

30 PWHT 25

Stress Stress (ksi) 20

10 AW 5

0 0 5 10 15 20 25 30 Strain (%)

Figure 27: AA6005A 1/8” thick specimen, as-welded vs PWHT vs solution heat treatment with PWHT

4.2.4 Unusual Tensile Strength Values

During the testing of AA6061 and AA6005A, it was found that the tensile strengths of some samples were lower than the standard values for the as-received, as- received heat treated, as-welded, and post weld heat treated tests. There was no attempt to determine the exact chemical composition of the extruded alloys prior to testing. There was no evidence to explain the reason for the decreased material strength. Figures 28 and

29 show tensile yield and ultimate strengths for all the samples tested from AA6061 and

AA6005A.

Table 2: Aluminum Alloy 6005A Tensile Strength

Aluminum Alloy 6005A, 1/8" Tensile Strength Parent Metal Post Weld Heat Treat Parent Metal (AR) As Welded (AW) Typical Values (AR+PHT) (PWHT) Ultimate, Yield, Ultimate, Ultimate, Yield, Ultimate, Ultimate, Yield, Yield, ksi Yield, ksi ksi ksi ksi ksi ksi ksi ksi ksi 25.1 14.3 32.1 27.1 24.3 12.8 31.9 26.4 6005A-T1 25.5 14.1 32.5 27.4 22.9 12.9 33.6 28.2 NA NA 25.5 14.6 32.4 27.3 25.2 13.2 31.3 26.4 25.4 14.8 33.5 28.2 23.0 11.0 34.1 28.7 6005A-T5 26.9 11.5 34.6 31.5 27.2 12.5 35.0 29.9 42.0 38.0 26.5 11.1 34.5 29.1 26.3 12.8 35.2 30.0 27.2 11.0 34.4 29.0 26.2 11.9 32.6 27.4 27.3 11.1 34.5 29.4 24.2 11.2 34.5 29.2 25.1 11.2 34.1 28.8 24.2 10.0 29.9 24.1 29.8 24.4 30.8 25.9 39.9 32.1 37.8 31.9 39.2 34.5 Showing Material Strength values cut from 40.7 34.3 38.5 32.5 two different alloy extrusions 38.9 31.9 39.6 33.4 41.8 34.3 39.7 32.0 39.6 33.2 38.5 32.9 38.4 31.5

26.2 12.8 33.6 28.6 24.9 12.0 36.1 30.2

Table 3: Aluminum Alloy 6061 Tensile Strength

Aluminum Alloy 6061 - T4, 1/4" Tensile Strength Parent Metal Post Weld Heat Treat Parent Metal (AR) As Welded (AW) Typical Values (AR+PHT) (PWHT) Ultimate, Yield, Ultimate, Ultimate, Yield, Ultimate, Ultimate, Yield, Yield, ksi Yield, ksi ksi ksi ksi ksi ksi ksi ksi ksi 33.4 18.2 43.8 39.7 29.5 16.8 41.6 38.5 6061-T4 33.3 17.9 43.0 39.7 29.8 16.8 40.2 37.3 35.0 21.0 33.0 18.2 42.0 39.1 29.0 15.6 40.1 37.2 25.2 12.9 33.3 27.2 29.5 16.9 42.0 38.5 6061-T6 25.3 12.5 33.2 27.8 29.5 16.8 40.1 37.3 45.0 40.0 25.2 12.7 33.8 28.6 29.0 15.6 40.1 37.3 23.0 11.3 42.6 40.0 23.2 11.8 42.4 39.0 22.8 11.0 41.8 39.0 22.9 10.9 40.5 37.5 22.4 10.7 40.2 37.5 22.0 10.5 39.2 36.5 22.4 11.0 41.0 38.4 23.3 11.9 40.5 37.0 40.0 37.2 32.3 26.2 31.2 25.2 31.0 24.8 Showing Material Strength values cut from 30.6 24.1 28.8 23.9 two different alloy extrusions 29.0 24.0 31.9 25.7 33.3 27.1 34.5 29.3 32.3 26.5 29.2 15.4 38.2 33.7 25.6 13.4 37.1 33.0

4.3 Tensile Fracture Behavior

The fracture features on the surface of the deformed and failed test specimens are shown in Figures 28-41. Aluminum alloys 6063, 6061 and 6005A are tough alloys making linear elastic fracture mechanics little value in describing fracture conditions.

4.3.1 As-Received Parent Metal

Tensile fracture of the AA6063 in the as-received or as-provided condition, i.e., temper T4 (6063-T4), was essentially flat and normal to the far-field stress axis. At the higher magnifications the fracture surface was microscopically rough and revealed an array of macroscopic cracks running perpendicular to the far-field stress axis (Figure

28a). The highly non-linear nature of the macroscopic crack (Figure 28b) was interdispersed with pockets of transgranular regions. In the region immediately prior to overload the fracture surface revealed regions containing pockets of striations adjacent to the macroscopic cracks indicative of localized micro plastic deformation (Figure 28c).

The region of overload was covered with a noticeable population of voids of varying size intermingled with dimples, features that are clearly indicative of the “locally” occurring ductile failure mechanisms (Figure 28d).

Tensile fracture of the as-received AA6063 heat treated to the T6 temper is shown in Figure 29. Macroscopic fracture was normal to the far-field tensile stress axis and essentially flat (Figure 29a). Careful high magnification observation revealed an observable combination of microscopic cracks, voids of varying size intermingled with dimples (Figure 29b). These features are indicative of the occurrence of predominantly ductile and isolated brittle failure mechanisms at the fine microscopic level. The non- linear nature of the fine microscopic crack surrounded by a healthy dispersion of voids

38 and dimples is shown in Figure 29c. The region of overload revealed a healthy population of voids of varying size intermingled with pockets of shallow dimples indicative of “locally” occurring ductile failure mechanisms (Figure 29d).

(a) (b)

100μm 20μm

10μm 10μm

Figure 28: Scanning electron micrographs of the tensile fracture surface of as- received aluminum alloy 6063 in the T4 temper, showing: (a) Overall morphology of failure (b) High magnification observation of (a) showing non-linear nature of macroscopic cracks (c) Isolated pockets of striations on the transgranular fracture surface (d) Observable population of voids of varying size intermingled with dimples

(a) (b)

100μm 20μm

10μm 10μm

Figure 29: Scanning electron micrographs of the tensile fracture surface of the heat treated aluminum alloy 6063-T4, showing: (a) Overall morphology of failure normal to far field stress axis (b) High magnification observation of (a) showing population of voids of varying size intermingled with isolated microscopic cracks (c) High magnification observation of (b) showing the nature and morphology of voids covering the transgranular fracture region and void coalescence to form microscopic crack (d) Voids of varying size intermingled with dimples on overload fracture surface.

On a macroscopic scale tensile fracture of the test sample taken from the as- received AA6061 was essentially normal to the far-field stress axis (Figure 30a). Overall morphology of fracture was rough at the fine microscopic level with the surface comprising of ductile voids and dimples and brittle macroscopic cracks. The macroscopic cracks were running parallel to the major stress axis (Figure 30b). Fine dimples of varying size and shape and intermingled with voids was found immediately adjacent to the intergranular fracture region (Figure 30c). The voids were microscopic in nature with little evidence of their growth and eventual coalescence during far-field loading. The overload fracture region was rough and devoid of features that would be indicative of purely ductile or brittle failure mechanisms (Figure 30d).

The overall morphology and features of the as-received AA6061 that was precipitation heat treated and then deformed to failure are shown in Figure 31. Overall morphology was also normal to the far-field stress axis with an array of macroscopic cracks, essentially co-planar in nature, and intermingled with fine microscopic cracks

(Figure 31a). High magnification observation of (a) revealed the non-linear nature of the macroscopic and fine microscopic cracks surrounded by pockets of voids and dimples; features reminiscent of locally occurring brittle and ductile failure mechanisms

(Figure 31b). Adjacent to the intergranular cracks was evident pockets containing shallow dimples of varying size (Figure 31c). This region of the fracture surface when observed at higher magnification revealed voids of varying size intermingled with shallow dimples (Figure 31d). A sizeable number of dimples on the fracture surface were elongated in shear providing concrete evidence of ductile failure processes occurring at the ‘local’ level (Figure 32).

(a) (b)

200μm 100μm

20μm 10μm

Figure 30: Scanning electron micrographs of the tensile fracture surface of as- received aluminum alloy 6061-T4, showing: (a) Overall morphology of failure, (b) High magnification observation of (a) showing non-linear nature of macroscopic crack surrounded by observable population of voids and dimples, (c) High magnification observation of (a) showing size and morphology of the voids, and (d) The overload fracture surface, features give no indication of likely micro failure mechanism.

(a) (b)

200μm 10μm

5 μm 4 μm

Figure 31: Scanning electron micrographs of the tensile fracture surface of as- received plus heat treated aluminum alloy 6061-T6, showing: (a) Overall morphology of failure showing an array of macroscopic and microscopic cracks, (b) High magnification observation of (a) showing non-linear nature of macroscopic crack surrounded by pockets of voids and dimples, reminiscent of highly localized ductile failure mechanism, (c) High magnification observation of (b) nature, morphology and overall distribution of the voids intermingled with highly shallow dimples, and (d) A mixture of voids of varying size, shallow dimples adjacent to cracked grain boundary triple junction

(a)

2 μm

Figure 32: Scanning electron micrographs of the tensile fracture surface of as- received plus heat treated aluminum alloy 6061-T6, showing elongated dimples indicative of shear and locally occurring ductile failure.

On a macroscopic scale tensile fracture of the test sample taken from the as- provided AA6005A was essentially normal to the far-field stress axis similar to AA6063 and AA6061 (Figure 33a). Overall morphology of fracture was rough at the fine microscopic level with the surface comprising of both ductile voids and dimples and brittle macroscopic cracks. The macroscopic cracks were running parallel to the major stress axis (Figure 33b). Fine dimples of varying size and shape and intermingled with voids was found immediately adjacent to the intergranular fracture region (Figure 33c).

The voids were microscopic in nature with little evidence of their growth and eventual coalescence during far-field loading. The overload fracture region was rough and devoid of features that would be indicative of purely ductile or brittle failure mechanisms (Figure

33d).

Samples of the as-received AA6005A that were precipitation heat treated and then deformed to failure the overall morphology and features are shown in Figure 34. Overall

44 morphology was normal to the far-field stress axis with an array of macroscopic cracks, essentially co-planar in nature, and intermingled with fine microscopic cracks (Figure

34a). High magnification observation of (a) reveals the non-linear nature of the macroscopic and fine microscopic cracks surrounded by pockets of voids and dimples; features reminiscent of locally occurring brittle and ductile failure mechanisms (Figure

34b). Adjacent to the intergranular cracks was evident pockets containing shallow dimples of varying size (Figure 34c). This region of the fracture surface when observed at higher magnification revealed voids of varying size intermingled with shallow dimples

(Figure 34d).

(a) (b)

20µm 20µm 20μm 20μm

10µm 5 µm

20μm 20μm Figure 33: Scanning electron micrographs of the tensile fracture surface of as- received aluminum alloy 6005 in the T4 temper, showing: (a) Overall morphology of failure, (b) High magnification observation of (a) showing nonlinear nature of macroscopic crack surrounded by observable population of voids and dimples, (c) High magnification observation of (a) showing size and morphology of the voids, and (d) The overload fracture surface, features give no indication of likely micro failure mechanism.

(a) (b)

50µm 10µm μm μm

10µm 5 µm 20μm 20μm Figure 34: Scanning electron micrographs of the tensile fracture surface of as- received plus heat treated aluminum alloy 6005 in the T4 temper, showing: (a) Overall morphology of failure showing an array of macroscopic and microscopic cracks, (b) High magnification observation of (a) showing non-linear nature of macroscopic crack surrounded by pockets of voids and dimples, reminiscent of highly localized ductile failure mechanism, (c) High magnification observation of (b) nature, morphology and overall distribution of the voids intermingled with highly shallow dimples., and (d) A mixture of voids of varying size, shallow dimples adjacent to cracked grain boundary triple junction.

4.3.2 As-Welded

Tensile fracture surface features of the AA6063 sample in the as-welded condition are shown in Figure 35. Overall fracture was at a slight inclination to the far- field stress axis and predominantly transgranular (Figure 35a). High magnification observation of (a) revealed a healthy dispersion of voids of varying size intermingled with dimples indicative of locally occurring ductile failure mechanisms (Figure 35b). At higher allowable magnifications of the SEM this region revealed an observable population of voids of varying size intermingled with dimples (Figure 35c). In the region of tensile overload were evident voids of varying size, void growth and eventual coalescence to form fine microscopic cracks surrounded by isolated pockets of shallow dimples (Figure 35d). These features are clearly indicative of the occurrence of both ductile and brittle failure mechanisms at the fine microscopic level.

Fracture features on the tensile sample of AA6063 that was heat treated following welding are shown in Figure 36. Overall morphology at low magnification revealed predominantly transgranular failure (Figure 36a) with the fracture surface covered with a healthy dispersion of voids of varying size and shallow dimples (Figure 36b). There was a distinct absence of macroscopic cracks and isolated fine microscopic cracks as a consequence of void growth and coalescence. In the region of overload was an observable dispersion of dimples, which were overall very shallow in nature (Figure 36c).

(a) (b)

100μm 20μm

10μm 10μm

Figure 35: Scanning electron micrographs of the tensile fracture surface of the as- welded aluminum alloy 6063-T4, showing: (a) Healthy population of voids of varying size on tensile fracture surface, (b) High magnification observation of (a) showing nature and morphology of the voids, (c) Healthy population of voids and dimples, and (d) Microvoid growth during far field loading and coalescence to form a fine microscopic crack.

(a) (b)

20μm 10μm

(c)

5μm

Figure 36: Scanning electron micrographs of the tensile fracture surface of the as- welded and heat treated aluminum alloy 6063, showing: (a) Overall morphology of failure, (b) High magnification observation of (a) showing the nature, morphology and distribution of voids covering the transgranular fracture regions, and (c) Voids and shallow dimples covering the overload fracture surface

Scanning electron microscopic observation of the as-welded sample of AA6061 is shown in Figure 37. Overall morphology was essentially flat and rough at the fine microscopic level (Figure 37a). At higher magnification the surface revealed a healthy dispersion of voids intermingled with dimples (Figure 37b). The exact morphology of the dimples and the nature of the voids are shown in Figure 37c. These fine features observed at the microscopic level are clearly indicative of the locally occurring ductile failure mechanisms. In the region approaching overload the fracture surface revealed features when observed at the higher allowable magnification of the SEM. Presence of a healthy population of voids of varying size intermingled with dimples along with fine microscopic cracks is indicative of both ductile and brittle failure mechanisms occurring at the fine microscopic level (Figure 37d).

Fracture behavior of the tensile sample of this aluminum alloy that was subject to heat treatment subsequent to welding is shown in Figure 38. Overall morphology was normal to the far-field stress axis and contained an array of both macroscopic cracks intermingled with fine microscopic cracks (Figure 38a). High magnification observation of the fracture surface revealed cracking along the grain boundaries with pockets of voids intermingled with dimples adjacent to the intergranular fracture regions (Figure 38b and

Figure 38c). At higher magnification the voids were observed to be of varying size and shape and inter-dispersed with shallow dimples (Figure 38d), features reminiscent of the locally occurring ductile failure mechanisms. The cracks propagated with ease along the grain boundary triple junctions. Coalescence of the fine microscopic voids initiated at both the coarse and intermediate-size second-phase particles dispersed through the microstructure results in dimples and fine microscopic cracks. At very high magnification the region on the fracture surface immediately prior to overload revealed fine striation-like features reminiscent of ‘locally’ occurring microplastic deformation

(Figure 39).

(a) (b)

200μm 10μm

5 μm 5 μm

Figure 37: Scanning electron micrographs of the tensile fracture surface of as-welded aluminum alloy 6061 in the T4 temper, showing: (a) Overall morphology of failure showing an array of macroscopic and microscopic cracks, (b) High magnification observation of (a) showing non-linear nature of macroscopic crack surrounded by pockets of voids and dimples, reminiscent of highly localized ductile failure mechanism, ( c) High magnification observation of (b) nature, morphology and overall distribution of the voids intermingled with highly shallow dimples, and (d) A mixture of voids of varying size, shallow dimples adjacent to cracked grain boundary triple junction.

(a) (b)

200μm 10μm

5 μm 5 μm

Figure 38: Scanning electron micrographs of the tensile fracture surface of post weld heat treated aluminum alloy 6061, showing: (a) Overall morphology of failure, (b) High magnification observation of (a) showing a healthy and observable population of voids and shallow dimples adjacent to the intergranular crack, (c) High magnification observation of (b) showing size, morphology and distribution of the voids, both macroscopic and fine microscopic, and (d) The presence of voids and void coalescence to form macroscopic crack in the region immediately prior to overload.

(a)

1 μm

Figure 39: Scanning electron micrographs of the tensile fracture surface of post weld heat treated aluminum alloy 6061, showing fine ripples or striations- like featuring reminiscent of locally occurring micro plastic deformation.

Scanning electron microscopic observation of the as-welded sample of AA6005A is shown in Figure 40. Overall morphology was essentially flat and rough at the fine microscopic level (Figure 40a). At higher magnification the surface revealed a healthy dispersion of voids intermingled with dimples (Figure 40b). The exact morphology of the dimples and the nature of the voids are shown in Figure 40c. These fine features observed at the microscopic level are clearly indicative of the locally occurring ductile failure mechanisms. In the region approaching overload the fracture surface revealed observable features when observed at the higher allowable magnification of the SEM.

Presence of a healthy population of voids of varying size intermingled with dimples along with fine microscopic cracks is indicative of both ductile and brittle failure mechanisms occurring at the fine microscopic level (Figure 40d).

Fracture behavior of the tensile sample of this aluminum alloy that was subject to heat treatment subsequent to welding is shown in Figure 41. Overall morphology was normal to the far-field stress axis and contained an array of both macroscopic cracks intermingled with fine microscopic cracks (Figure 41a). High magnification observation of the fracture surface revealed cracking along the grain boundaries with pockets of voids intermingled with dimples adjacent to the intergranular fracture regions (Figure 41b and

Figure 41c). At higher magnification the voids were observed to be of varying size and shape and inter-dispersed with shallow dimples (Figure 41d), features reminiscent of the locally occurring ductile failure mechanisms. The cracks propagated with ease along the grain boundary triple junctions. Coalescence of the fine microscopic voids initiated at both the coarse and intermediate-size second-phase particles dispersed through the microstructure results in dimples.

The coarse iron-rich and silicon-rich intermetallics along with other insoluble particles present coupled with the a microstructure that favors ‘localized’ inhomogeneous deformation due essentially to the presence of easily shearable matrix strengthening precipitates facilitates in the nucleation and coalescence of the voids, of varying size, to occur at low to moderate stress levels. In fact void nucleation at a coarse-second phase particle is favored to occur easily when the elastic energy in the particle exceeds the surface energy of the newly formed void surfaces.

(a) (b)

100µ 50µm m 20μm

20µm 10µm

20μm 20μm Figure 40: Scanning electron micrographs of the tensile fracture surface of as -welded plus heat treated aluminum alloy 6005A in the T4 temper, showing: (a) Overall morphology of failure showing an array of macroscopic and microscopic cracks, (b) High magnification observation of showing non-linear nature of macroscopic crack surrounded by pockets of voids and dimples, reminiscent of highly localized ductile failure mechanism, ( c) High magnification observation of nature, morphology and overall distribution of the voids intermingled with highly shallow dimples, and (d) A mixture of voids of varying size, shallow dimples adjacent to cracked grain boundary triple junction.

(a) (b)

50µm 20µm 20μm 20μm (c) (d)

10µm 5 µm 20μm 20μm

Figure 41: Scanning electron micrographs of the tensile fracture surface of aluminum alloy 6005A in the T4 temper and as-welded condition, showing: (a) Overall morphology of failure, (b) High magnification observation of (a) showing a healthy and observable population of voids and shallow dimples adjacent to the intergranular crack, (c) High magnification observation of (b) showing size, morphology and distribution of the voids, both macroscopic and fine microscopic, and (d) The presence of voids and void coalescence to form macroscopic crack in the region immediately prior to overload.

4.3.3 Mechanisms Governing Tensile Fracture

Formation of the macroscopic voids is favored to occur because of failure of the coarse second-phase particles dispersed through the microstructure by cracking.

Coalescence of the macroscopic voids occurred by the formation of void sheets and exacerbated by the intense localization of strain between the expanding and growing voids. This is shown in Figure 42. The highly localized deformation favors the formation of microscopic voids at the intermediate-size second phase particles upon reaching a critical value of strain. The presence of both coarse and intermediate-size second phase particles in the microstructure of AA6063, AA6061 and AA6005A may have little to no influence on strength but on account of their intrinsic brittleness they tend to either easily fracture by cracking during deformation thereby reducing the total energy that is required for rupture, or segregate from the aluminum alloy matrix when the local strain exceeds a critical value.

The transgranular regions did reveal pockets of highly deformed matrix reminiscent of localized plastic deformation, referred to as ‘microplasticity’, of the aluminum alloy matrix. The highly localized microplasticity is predominantly distributed through the microstructure of this alloy, presumably within the favorably oriented grains.

The observed distribution of microplastic deformation in the microstructure of AA6063,

AA6061 and AA6005A is dependent on the mutually interactive influences of the following [14, 15]:

(i) The orientation of grains (texture),

(ii) Elastic anisotropy and the concomitant stress concentration arising due to

contributions from both grain size and grain shape,

(iii) Presence and role of grain boundary triple junctions, and

(iv) The overall nature of loading.

The formation and presence of an observable population of voids, of varying size and shape, transforms this polycrystalline aluminum alloy, in the conditions it was tested or deformed in uniaxial tension, into a “composite:” with two populations of particles.

These can be considered to be the (a) grains, and (b) the void [a void being considered as a particle having essentially zero stiffness]. Since the voids, regardless of their size, are intrinsically softer than the grains in the aluminum alloy metal matrix, the local strain is exacerbated for the voids causing as a result an increase in their volume fraction. The presence of an observable population of voids, of varying size and shape, transforms the macroscopic mechanical response of the polycrystalline 6XXX series alloy through significant degradation in ductility. The presence of an observable amount of both coarse and intermediate size intermetallic particles coupled with an alloy microstructure that favors inhomogeneous deformation during loading facilitates the nucleation and coalescence of voids to occur at fairly low to moderate stress levels. In fact void nucleation at the second-phase particle is favored to occur when the elastic energy in the particle exceeds the surface energy of the newly formed void surfaces [16].

Figure 42: Schematic showing the formation of void sheets between expanding or growing voids leading to void-void interactions and eventual coalescence.

4.3.4 Kinetics Governing Stress-Material Response

It is important to understand the extrinsic influence of the heat that is both supplied and generated during the welding process on the mechanical properties of aluminum alloys belonging to the 6XXX series. Further, the extrinsic influence of heat, that is both supplied and generated during welding, can differ depending upon the exposure to the welding process and resultant quality of the weld. Also, the type of weld joint coupled with material thickness will tend to influence: (i) the heat input, (ii) heat build-up, and (iii) resultant strength of the weld.

Results of this investigation on welded 6xxx aluminum alloys showed heat treatment to increase strength of the chosen material. Also, it was found that when the welded aluminum alloy was not heat treated it experienced a decrease in strength as a consequence of welding. This has the tendency to favor premature failure during high rate of loading. Overall, it is important to understand the extent to which welding can affect the strength and ductility properties of the chosen aluminum alloy and the proper 60 loading that the welded aluminum alloy, and post-weld heat treated aluminum alloys can carry through the heat-affected zone (HAZ).

Strength, ductility, and overall fracture behavior are the three key properties which govern or determine the tensile response of a material. The grain size and distribution in the microstructure of these aluminum alloys did change as a consequence of the heat input during welding. The heat treatment enables the grains to return closer to their normal size and distribution. When the chosen aluminum alloy, is welded and not subject to post weld heat treatment, then the values of strength obtained are noticeably less than expected for the non-heat treated alloys. When comparing the strength values obtained from the tensile tests with the aluminum standards, the values for both the heat treated aluminum alloy and the welded counterpart were marginally higher than the standard values.

CHAPTER V

SUMMARY OF CONCLUSIONS

5.1 Conclusions

A study aimed at investigating the conjoint influence of welding and artificial aging on microstructural development, tensile behavior and fracture behavior of aluminum alloys 6063, 6061 and 6005A, provides the following findings:

1. Lap joint welds were deposited across some of the samples in order to examine

the effect of localized heating on the material. This welding process of gas metal

arc was chosen since it is one of the most commonly used in the industry for

aluminum alloy-related applications.

2. The base metal of each alloy revealed a non-uniform dispersion of both large and

intermediate-size intermetallic particles randomly distributed through the

microstructure. Presence of impurity elements iron and silicon result is the

formation of a variety of iron-rich and iron-silicon rich intermetallic particles

during solidification. Any silicon which is not incorporated in the alpha-

aluminum matrix or the Al-Fe-Si intermetallic phases combines with magnesium

to form Mg2Si during the later stages of the solidification process. The region of

the weld revealed a fully recrystallized fine grain structure.

3. Microstructure of the post weld heat treated alloys did reveal observable

differences at the boundary between the base metal and the weld pool. In the

base metal, it was evident that a healthy population of both the coarse and

intermediate size intermetallic particles dispersed randomly through the

microstructure. At the region of the interface between the base metal and the

weld pool observable microscopic cracks were evident.

4. The ultimate strength and yield strength of the post weld heat treated (PWHT)

samples increased when compared with the samples prepared from the as welded

aluminum alloy.

 The ¼ inch AA6063 PWHT samples showed a 66% (yield) and 36% (ultimate)

increase in strength over the as-welded condition.

 The 3/8 inch AA6063 PWHT samples showed a 17% (yield) and 9% (ultimate)

increase in strength over the as-welded condition.

 The ¼ inch AA6061 PWHT samples showed a 57% (yield) and 28% (ultimate)

increase in strength over the as-welded condition.

 The 3/8 inch AA6061 PWHT samples showed a 48% (yield) and 24% (ultimate)

increase in strength over the as-welded condition.

 The 1/8 inch AA6005A PWHT samples showed a 65% (yield) and 35% (ultimate)

increase in strength over the as-welded condition.

5. The re-solution heat treatment of AA6005A provided strength results comparable

to typical values of 6005A-T5. Tensile strength values of 38.7 ksi yield and 42.9

ksi ultimate were obtained from welded SHT + PHT tests. Typical values of

6005A-T5 are 38.0 ksi yield and 42.0 ksi ultimate.

6. (i) Tensile fracture of the 6063, 6061 and 6005A alloy in the as-received or as-

provided condition, was essentially flat and normal to the far-field stress axis.

(ii) The fracture surface of the three alloys was microscopically rough and

revealed an array of macroscopic cracks running perpendicular to the far-field

stress axis.

(iii) The highly non-linear nature of the macroscopic crack was interdispersed

with pockets of transgranular regions.

(iv) In the region immediately prior to overload the fracture surface revealed

regions containing pockets of ripples or striation-like features adjacent to the

macroscopic cracks indicative of localized microplastic deformation.

REFERENCES

[1] 2010 Aluminum Design Manual, The Aluminum Association, Washington, D.C., 2010

[2] Willard, J.P., ALCOA Green Letter: Four Extrusion Alloys 6061, 6063, 6351, 6005, Aluminum Company of America, PA, USA, 1971.

[3] R.C. Minor, Pole Stub Test Reports, Letter to J.R. Meadler, Nov. 30, 1970

[4] 2009 Aluminum Standards and Data, The Aluminum Association, Washington, D.C., 2009

[5] ASTM B918-01 Standard Practice for Heat Treatment of Wrought Aluminum Alloys, DOI: 10.1520/B0918-01

[6] ASM Materials Handbook, Vol. 4 and Vol. 6, ASM International, Materials Park, Ohio, USA, 1991.

[7] Ding Xian-fei, Sun Jing, Ying Jia, Zhang Wei-dong, Ma Ji-jun, Wang Li-chen. Influences of aging temperature and time on microstructure and mechanical properties of 6005A aluminum alloy extrusions. Transactions of Nonferrous Metals Society of China, 2012, s14-s20

[8] I.J. Polmear: Light Alloys-Metallurgy of the Light Metals, Third Edition, Arnold Publishers, London, New York, Sydney, 1995

[9] Lucas, G., Aluminum Structural Applications, Advanced Materials and Processes, 1996, 149, pp. 29-30.

[10] Barbosa, G, Dille J., Delplancke, J.L., Rebello, J.M., ansd Acselrad, O.: A microstructural study of flash welded and aged 6062 and 6013 aluminum alloys, Materials Characterization, 2006, Vol. 57, 187-192.

[11] Hsu, C., O’Reilly, K.A.Q., Cantor, B., Hamerton, R.: Non-equilibrium reactions in 6XXX series alloys, Materials Science and Engineering, 2001, Vol. 304-306, pp. 119-124

[12] C. Gallais, A. Simar, D. Fabreque, A. Denquin, G. Lapasset, B. de Meester, Y. Brechet, T. Pardoen: Metallurgical and Materials Transactions, Vol. 38 No., 5, 2007, pp. 964-981

[13] D. Lassance, D. Fabregue, F. Delannay, T. Pardoen: Progress in Materials Science, Vol. 52, 2007, pp. 62-129.

[14] Sanders, R.E., Baumann, S.F., Stumpf, H.C. in Aluminum Alloys: Contemporary Research and Applications [editors: A.K. Vasudevan and R.D. Doherty), Treatise in Materials Science and Technology, Vol. 31, Academic Press, New York City, 1989, pp. 65-90.

[15] Menzemer, C.C., Srivatsan T.S., The Quasi static fracture behavior of aluminum alloy 5083, Materials Letters, Vol. 38, 1999, pp. 317-320.

[16] Van Stone, R.H., Cox, T.B., Low, J.R., and Psioda, J.A.: International Materials Reviews, Vol. 30, 1985, pp. 157-187.

[17] Aluminium Properties and Physical Metallurgy (edited by: J.E. Hatch), ASM Materials Park, Ohio, USA, 1984.

[18] M. Warmuzek, G. Mrowkaand J. Sieniawski: “Influence of Heat Treatment on Precipitation of Intermetallic Phases in Commercial AlMnFeSi Alloy, Journal of Materials Processing Technology, Vol. 157-158, 2004, pp. 624-632

[19] D.J. Chakrabarti, Alcoa Technical Center, Alcoa Center, PA 15069, USA, Precipitation in Al-Mg-Si-Cu Alloys and the Role of the Q Phase and its Precursors, The Minerals, Metals, and Materials Society, 1998

[20] G. Mroxka-Nowotnik, J. Sieniawski, M. Wierzbinska, Analysis of intermetallic particles in AlSi1MgMn aluminum alloy, Journal of Achievements in materials and manufacturing engineering, Vol. 20, Issues 1-2, 2007

APPENDIX

6063-T4, As Received 0.25" Thick, 0.5" Width 25000

20000

15000

Stress (psi) 10000

5000

0 0 0.05 0.1 0.15 0.2 Strain

6063-T6, As Received 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 35000

30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 Strain

6063-T4, As Welded 0.25" Thick, 0.5" Width 25000

20000

15000

Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 Strain

6063-T4, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 35000

30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 Strain

6063-T4, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 35000

30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 Strain

6063-T4, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F)

35000

30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 Strain

6063-T6, As Received 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 50000 45000 40000 35000 30000 25000

Stress (psi) 20000 15000 10000 5000 0 0 0.02 0.04 0.06 0.08 Strain

6063-T6, As Received 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 50000 45000 40000 35000 30000 25000

Stress (psi) 20000 15000 10000 5000 0 0 0.02 0.04 0.06 0.08 Strain

6063-T6, As Received 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 50000 45000 40000 35000 30000 25000

Stress (psi) 20000 15000 10000 5000 0 0 0.02 0.04 0.06 0.08 Strain

6063-T6, As Received 0.375" Thick, 0.5" Width 50000 45000 40000 35000 30000 25000

Stress (psi) 20000 15000 10000 5000 0 0 0.02 0.04 0.06 0.08 Strain

6063-T6, As Received 0.375" Thick, 0.5" Width 50000

45000

40000

35000

30000

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6063-T6, As Received 0.375" Thick, 0.5" Width 50000

45000

40000

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6063-T6, As Welded 0.375" Thick, 0.5" Width 40000

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6063-T6, As Welded 0.375" Thick, 0.5" Width 40000

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6063-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 50000 45000 40000 35000 30000 25000

Stress (psi) 20000 15000 10000 5000 0 0 0.005 0.01 0.015 0.02 0.025 Strain

6063-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 50000 45000 40000 35000 30000 25000

Stress (psi) 20000 15000 10000 5000 0 0 0.01 0.02 0.03 0.04 Strain

6063-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 50000 45000 40000 35000 30000 25000

Stress (psi) 20000 15000 10000 5000 0 0 0.01 0.02 0.03 0.04 0.05 Strain

6063-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 50000 45000 40000 35000 30000 25000

Stress (psi) 20000 15000 10000 5000 0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 Strain

6063-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 50000 45000 40000 35000 30000 25000

Stress (psi) 20000 15000 10000 5000 0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 Strain

6063-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 50000 45000 40000 35000 30000 25000

Stress (psi) 20000 15000 10000 5000 0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 Strain

6061-T6, As Received 0.25" Thick, 0.5" Width 40000

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6061-T6, As Received 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 50000 45000 40000 35000 30000 25000 20000 Stress (psi) 15000 10000 5000 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Strain

6061-T6, As Welded 0.25" Thick, 0.5" Width 35000

30000

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10000

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0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 Strain

6061-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 45000 40000 35000 30000 25000 20000 Stress (psi) 15000 10000 5000 0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Strain

6061-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 45000 40000 35000 30000 25000 Series1 20000 Series2 Stress (psi) 15000 Series3 10000 Series4 5000 0 0 0.02 0.04 0.06 0.08 0.1 0.12 Strain

6061-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 45000 40000 35000 30000 25000 20000 Stress (psi) 15000 10000 5000 0 0 0.02 0.04 0.06 0.08 0.1 0.12 Strain

6061-T6, As Received 0.375" Thick, 0.5" Width 30000

25000

20000

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6061-T6, As Received 0.375" Thick, 0.5" Width 30000

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6061-T6, As Received 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

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6061-T6, As Received 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

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6061-T6, As Welded 0.375" Thick, 0.5" Width 30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 Strain

6061-T6, As Welded 0.375" Thick, 0.5" Width 30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Strain

6061-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

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6061-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

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0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Strain

6061-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 35000

30000

25000

20000

15000 Stress (psi) 10000

5000

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6061-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

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6061-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 35000

30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Strain

6061-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

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Stress (psi) 15000

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0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Strain

6061-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 35000

30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Strain

6061-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

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5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Strain

6061-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 35000

30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 Strain

6061-T6, As Welded 0.375" Thick, 0.5" Width 40000 Precipitation Heat Treat (6 Hrs, 360o F)

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.02 0.04 0.06 0.08 Strain

6061-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 Strain

6061-T6, As Welded 0.375" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

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0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Strain

6005A-T6, As Received 0.25" Thick, 0.5" Width 25000

20000

15000

Stress (psi) 10000

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6005A-T6, As Received 0.25" Thick, 0.5" Width 30000

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6005A-T6, As Received 0.25" Thick, 0.5" Width 25000

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6005A-T6, As Received 0.25" Thick, 0.5" Width 30000

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6005A-T6, As Received 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

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Stress (psi) 15000

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5000

0 0 0.02 0.04 0.06 0.08 0.1 Strain

6005A-T6, As Received 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 Strain

6005A-T6, As Received 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 Strain

6005A-T6, As Received 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 35000

30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width 25000

20000

15000

Stress (psi) 10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width 25000

20000

15000

Stress (psi) 10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width 25000

20000

15000

Stress (psi) 10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 Strain

6005A-T6, As Welded 0.25" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 Strain

6005A-T6, As Received 0.125" Thick, 0.5" Width 30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Strain

100

6005A-T6, As Received 0.125" Thick, 0.5" Width 30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Strain

6005A-T6, As Received 0.125" Thick, 0.5" Width 30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 Strain

101

6005A-T6, As Received 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.02 0.04 0.06 0.08 Strain

6005A-T6, As Received 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.02 0.04 0.06 0.08 Strain

102

6005A-T6, As Received 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Strain

6005A-T6, As Received 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.02 0.04 0.06 0.08 Strain

103

6005A-T6, As Welded 0.125" Thick, 0.5" Width 30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.02 0.04 0.06 0.08 Strain

6005A-T6, As Welded 0.125" Thick, 0.5" Width 25000

20000

15000

Stress (psi) 10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Strain

104

6005A-T6, As Welded 0.125" Thick, 0.5" Width 30000

25000

20000

15000 Stress (psi) 10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 Strain

6005A-T6, As Welded 0.125" Thick, 0.5" Width 25000

20000

15000

Stress (psi) 10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 0.06 Strain

105

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 Strain

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 Strain

106

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 Strain

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 Strain

107

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 Strain

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 Strain

108

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 Strain

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 Strain

109

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 Strain

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 Strain

110

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 Strain

6005A-T6, As Welded 0.125" Thick, 0.5" Width Precipitation Heat Treat (6 Hrs, 360o F) 40000

35000

30000

25000

20000

Stress (psi) 15000

10000

5000

0 0 0.01 0.02 0.03 0.04 0.05 Strain

111