INFLUENCE OF WELDING AND HEAT TREATMENT ON ALUMINUM ALLOYS A Thesis Presented

124 Pages·2014·5.39 MB·English

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

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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.

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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.

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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
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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
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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
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