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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 ii 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. iii 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. iv 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. v 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 vi 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 vii 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 viii 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 ix 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
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