High Strength Cast Aluminum Alloy Development

146 Pages·2013·4.56 MB·English

by  Edward A Druschitz

HIGH STRENGTH CAST ALUMINUM ALLOY DEVELOPMENT

by

Edward A. Druschitz

ROBIN D. FOLEY, COMMITTEE CHAIR
GREGG M. JANOWSKI
JAMES B. ANDREWS
DERRICK R. DEAN
GREGORY B. THOMPSON

A DISSERTATION

Submitted to the graduate faculty of The University of Alabama at Birmingham,
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy

BIRMINGHAM, ALABAMA

2013
Copyright by
Edward A. Druschitz
2013
CAST HIGH STRENGTH ALUMINUM ALLOY DEVELOPMENT
EDWARD A. DRUSCHITZ

MATERIALS ENGINEERING
ABSTRACT
The goal of this research was to understand how chemistry and processing affect
the resulting microstructure and mechanical properties of high strength cast aluminum
alloys. Two alloy systems were investigated including the Al-Cu-Ag and the Al-Zn-Mg-
Cu systems. Processing variables included solidification under pressure (SUP) and heat
treatment.
This research determined the range in properties that can be achieved in BAC
100TM (Al-Cu micro-alloyed with Ag, Mn, Zr, and V) and generated sufficient property
data for design purposes. Tensile, stress corrosion cracking, and fatigue testing were
performed. CuAl and Al-Cu-Fe-Mn intermetallics were identified as the ductility
2
limiting flaws. A solution treatment of 75 hours or longer was needed to dissolve most of
the intermetallic CuAl . The Al-Cu-Fe-Mn intermetallic was unaffected by heat
2
treatment. These results indicate that faster cooling rates, a reduction in copper
concentration and a reduction in iron concentration might increase the ductility of the
alloy by decreasing the size and amount of the intermetallics that form during
solidification.
Six experimental Al-Zn-Mg-Cu series alloys were produced. Zinc concentrations
of 8 and 12wt% and Zn/Mg ratios of 1.5 to 5.5 were tested. Copper was held constant at
0.9%. Heat treating of the alloys was optimized for maximum hardness. Al-Zn-Mg-Cu
samples were solution treated at 441°C (826°F) for 4 hours before ramping to 460°C
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(860°F) for 75 hours and then aged at 120°C (248°F) for 75 hours. X-ray diffraction
showed that the age hardening precipitates in most of these alloys was the T phase
(Mg Zn Al ). Tensile testing of the alloys showed that the best mechanical
32 31.9 17.1
properties were obtained in the lowest alloy condition. Chilled Al-8.2Zn-1.4Mg-0.9Cu
solidified under pressure resulted in an alloy with a yield strength of 468MPa (68ksi),
tensile strength of 525MPa (76ksi) and an elongation of 9%.

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DEDICATION
First and foremost, I wish to dedicate this to my wife Amy Lauren Druschitz.
Without you I would not have completed the amazing feat of earning my doctorate, nor
would I have found my inspiration in life. You mean more to me than I could have ever
imagined, and without you I would not be where I am today. It is because of you I was
able to successfully manage the ups and downs, twists and turns in life that have brought
me to this point. I love you and look forward to spending the rest of our lives together.
Also, to my parents, Alan and Lori, who have always believed in me and pushed
me to catch my dreams. To my sister, Laurel and her husband Chase, who have always
supported me and been there for me when I needed someone to talk to.

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ACKNOWLEDGEMENTS
As I near the completion of this part of my life, I would like to acknowledge those
who have helped me during this journey. First, I would like to thank Dr. Foley for being
a better adviser then I deserved, you helped propel me forward in my research and were
always there for me when I needed advice.
To the casting group at UAB, John Griffin, Ricardo Aristizabal, Jeff Hamby,
Santosh Ghanti, Joe Jablonsky, and all the undergraduate assistants during my tenure.
Without all of you, none of this would have been possible, we were a great team. It has
been an honor to know all of you, and I consider all of you lifelong friends.
To my committee members: Dr. J. Barry Andrews, Dr. Derrick Dean, Dr. Gregg
Janowski, Dr. Gregory Thompson, and Mr. Tom Prucha. Thank you so much for your
invaluable input, advice, inspiration, and guidance.
I would also like to thank the unsung heroes of the materials engineering
department: Mrs. Cynthia Barham, Mrs. Robin Mize, and Mr. Vernon Merchant for all of
their support during my time here.

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TABLE OF CONTENTS
ABSTRACT .........................................................................................................................III
DEDICATION ..................................................................................................................... V
ACKNOWLEDGEMENTS ................................................................................................. VI
1. INTRODUCTION .......................................................................................................1
2. BACKGROUND AND LITERATURE REVIEW .........................................................2
2.1 Cast and wrought properties in aluminum alloys .......................................... 2
2.2 Alloying effects in cast aluminum alloys ........................................................ 4
2.3 Phase stability and heat treatment ................................................................. 6
2.3.1 Solution treating ................................................................................... 9
2.3.2 Aging ................................................................................................... 10
2.4 Al-Zn-Mg-Cu chemistries............................................................................. 11
2.5 Hydrogen, porosity, and the effect of solidification under pressure
(SUP)............................................................................................................ 13
3. SPECIFIC AIMS .......................................................................................................18
3.1 Objective 1: Improve the mechanical properties of BAC 100TM ................. 18
3.2 Objective 2: Determine chemistry, processing and heat treatment
effects on microstructure and properties of cast Al-Zn-Mg-Cu Alloys. ....... 18
3.3 Objective 3: Demonstrate that Al-Zn-Mg-Cu alloys can be cast to
near-net-shape using best practices. ............................................................ 19
4. EXPERIMENTAL METHODS ..................................................................................20
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4.1 Casting ......................................................................................................... 20
4.1.1 BAC 100TM .......................................................................................... 25
4.1.2 Al-Zn-Mg-Cu alloys ............................................................................ 25
4.2 Heat treating ................................................................................................ 26
4.2.1 BAC 100TM .......................................................................................... 26
4.2.2 Al-Zn-Mg-Cu alloys ............................................................................ 27
4.3 Testing .......................................................................................................... 27
4.3.1 Aging study ......................................................................................... 27
4.3.2 Tensile testing ..................................................................................... 28
4.3.3 Direct tension stress corrosion cracking ............................................ 29
4.3.4 Fatigue testing .................................................................................... 30
4.4 Characterization .......................................................................................... 30
4.4.1 Microstructural analysis ..................................................................... 30
4.4.2 Intermetallic measurement ................................................................. 31
4.4.3 Stereo microscope analysis ................................................................. 31
4.4.4 Scanning electron microscopy (SEM)................................................. 31
4.4.5 X-Ray diffraction................................................................................. 31
4.4.6 Differential scanning calorimetery ..................................................... 32
5. BAC 100TM: A NEW HIGH STRENGTH, HIGH TOUGHNESS, CAST
ALUMINUM ALLOY ................................................................................................33
6. HIGH STRENGTH CAST ALUMINUM: SOLUTION TREATING AND
AGING STUDY .........................................................................................................66
7. HIGH STRENGTH CAST ALUMINUM: TENSILE PROPERTIES..........................99
8. OVERALL SUMMARY ............................................................................................126
9. GENERAL REFERENCES......................................................................................130

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LIST OF TABLES

Table Page
INTRODUCTION AND LITERATURE REVIEW

1. Composition ranges (wt%) of aluminum casting and wrought alloys [1]. .................... 3
2. Aluminum alloys and their mechanical properties (for cast alloys, properties
are from separately cast test bars) and liquidus and solidus [1]. .................................... 4
3. Chemistries of BAC 100TM heats produced at UAB (values in wt%) ......................... 25
4. Target chemistries for each of 6 alloys. ....................................................................... 26

BAC 100TM: A NEW HIGH STRENGTH, HIGH TOUGHNESS, CAST ALUMINUM
ALLOY

1. Nominal chemistry ranges (wt%) for 201 [2], A206 [2] and BAC 100TM [1] .............. 36
2. Typical tensile properties for 201 [3] and A206 [4] ..................................................... 37
3. Chemistries for BAC 100TM heats produced at UAB (values in wt%) ......................... 39
4. Tensile properties for high strength seat frame castings produced at UAB,
HIPed at Bodycote and heat treated at UAB ................................................................ 44
5. Tensile properties for high toughness seat frame castings produced at UAB,
HIPed at Bodycote and heat treated at UAB ................................................................ 45
6. Tensile properties for high toughness separately cast tensile bar produced at a
commercial foundry, HIPed at Bodycote and heat treated at UAB ............................. 46
7. Results of direct stress corrosion cracking tests ........................................................... 58
8. Fatigue properties for high toughness heat treatment ................................................... 59

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HIGH STRENGTH CAST ALUMINUM: SOLUTION TREATING AND AGING
STUDY

1. Typical Al-Zn-Mg-Cu Alloys and their Mechanical Properties [2]. ............................ 69
2. Target chemistries for each of 6 pours and which two alloys were chosen for air
and SUP pouring. ......................................................................................................... 71
3. Actual chemical compositions for each alloy determined using spectrometer
analysis. ........................................................................................................................ 75
4. Thermal analysis results for each alloy. ........................................................................ 77
5. DSC results for “solution treating study” of as-cast samples with peak locations
in Celsius. ..................................................................................................................... 94
6. DSC results for “aging study” of previously solution treated samples with the
peak location in Celsius. ............................................................................................... 95

HIGH STRENGTH CAST ALUMINUM: TENSILE PROPERTIES

1. Target chemistries for each of 6 alloys. ...................................................................... 103
2. Actual chemical compositions for each alloy. ............................................................ 108
3. Average mechanical properties of Al-Zn-Mg-Cu tensile bars .................................... 110

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