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R A Y M O N D D E C K E R , S T E P H E N L E B E A U - T H I X O M A T / N A N O M A G
T R A C Y B E R M A N , W A Y N E J O N E S – T H E U N I V E R S I T Y O F M I C H I G A N
T O R I M I L L E R , T R E S A P O L L O C K - U N I V E R S I T Y O F C A L I F O R N I A S A N T A B A R B A R A
N I R M O S K O V I C H A N D B O R I S B R O N F I N - D E A D S E A M A G N E S I U M , L T D
Thermomechanical
Processing of Thixomolded Mg Alloys (TTMP)
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TTMP Outline
Concept of TTMP Thixomolder Schematic Predestined Thixomolded Microstructure TTMP Microstructure + Heat Treatment TTMP Alloys Cross Rolling Pressing Forging Wheels Fatigue and Creep Texture and Formability Commercial Orthopedic Knee Braces
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Concept of TTMP
Design Alloy With Depressed Eutectic Reaction
Thixomold (T) with Fast Cooling for Fine Grains
Thermomechanical Process (TMP) at High Strain & Strain Rate
Heat Treat for Finer Grains & For Low Texture
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480 Thixomolding Machines in 13 Countries Semi-Solid Molding with 5 to 15 % Solids
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Thixomolded Microstructure
Optical Micrograph Electron Micrograph
α
β- eut
α-eut
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Predestined Thixomolded Microstructure
Fine ɑ grains – 5 µm
β eutectic particles– 1 to 2 µm length
Supersaturated Matrix
Uniform Microstructure Across Part
Low Porosity
Freedom from Shot-well Flakes
Low Texture
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Thixomolded (T) Texture & Properties
YS = 135 MPa
Elongation = 10 %
Fatigue Strength = 50-70 MPa
Toughness = 16 Kq (MPam0.5)
Cl- Corrosion = 10% of Die Cast Alloy
Enhanced Hot Working
EBSD - Random Texture – MRD of 1.8, Grain Size = 5 µm
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Reduced Hot Cracking With Fine Grain Size
AZ61-Thixomolded + 3 passes of 50% each at 260°C – 1-2 µm G.S. No Edge Cracks
AZ31-TRC + 1 pass of 50% at 260°C, Edge Cracks Shear Bands of Fine DRX Grains in 84 µm G.S.
84 µm
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T + TMP Microstructure
Recrystallize to µm Sized Grains
Avoid Twinning and Shear Bands
Decorate Grain Boundaries with 1-2 µm β
Precipitate Additional β During Deformation
__________________________________
Thus Promoting Dynamic Recrystallization
Thus Weakening Texture by “β Disruption”
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Intragranular β Particles From TMP
β
Precipitated from Supersaturated Thixomolded Condition
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Effect of TMP+Anneal on Microstructure and Properties
EBSD - As Thixomolded EBSD - TTMP + Anneal
Grain Size – 5 microns Grain Size – 2 microns
Yield Strength – 135 MPa Yield Strength – 250 MPa
Fatique Strength – 50-70 MPa Fatigue Strength – 150 MPa
Toughness – 16 MPa m 0.5 Toughness – 28 MPa m0.5
Texture – MRD of 1.8 max. Texture – MRD of 3.5 max.
TMP+Anneal
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Effect of Heat Treatment
Completes Static Recrystallization
Minimizes Texture, for Formability w/o Rare Earths
By β Induced Large Orientation Spreads Within Individual Grains
Additional Age Hardening
Increases YS, UTS, Fatigue Strength and Toughness
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Mg alloys Thixomolded, Thixoblended and TTMP
Alloy Composition Thixomolded Thixoblended TTMP AM50L Mg-5Al-0.5Zn X X
AM60 Mg-6Al X X
AZ60L Mg-6Al-0.5Zn X X
AZ61 Mg-6Al-1Zn X X
AZ62 Mg 6Al-2Zn X X
AZ63 Mg 6Al-3Zn X X
AZ64 Mg 6Al-4Zn X X
AZ70-TH Mg-7Al-0.5Zn X X
AZ71 Mg-7Al-1Zn X
AZ81 Mg-7Al-1Zn X
AZ91D Mg-9Al-1Zn X X
AXJ810-TH Mg-8Al-1Ca-0.3 Sr X X
32 Alloys and Composites
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Alloys Composition Thixomolded Thixoblended TTMP ZA55 Mg 5Zn-5Al X X
ZA75 Mg-7Zn-5Al X X
ZA82 Mg-8Zn-2Al X X
ZA84 Mg-8Zn-4Al X X
ZA10/4 Mg-10Zn-4Al X X
AM-Lite Mg-12Zn-4Al X X
ZK60 Mg-6Zn X X
AXJ81E Mg-8Al-0.4Ca-1Sr-0.4RE X X
AJ52 Mg-5Al-2Sr X
AJ62 Mg-6Al-2Sr X
AJ62E Mg-6Al-2.3Sr-0.5RE X X
AJ72E Mg-7Al-2.6Sr-0.5 RE X X
AS41 Mg-4Al-2Si X
MRI 153 Mg-9Al-0.6Zn-0.9Ca-0.3Mn X
ZAC 8506 Mg-5Al-8Zn-0.6Ca X
AE42 Mg-4.3Al-0.3Zn-2.9RE X
Composites
Melram Composite Mg-6Zn-1.2Cu-0.8Mn+12% SiC X
AZ91 + Al2O3 X
AZ91 + SiC X
AZ91 + Hollow Fly Ash X
Mg alloys Thixomolded, Thixoblended and TTMP
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Effect of Initial Process & Al Content on TMP Properties
Alloy Process Al Zn Yield Strength, MPa Elongation, %
AZ31 Extrude + TMP 3 1 190 20
AZ31 TRC* + TMP 3 1 187 10
AZ50 TTMP 5 0.45 235 17
AZ60 TTMP 6 0.45 234 20
AZ61 TTMP 6 1 257 18
AZ62 TTMP 6 2 255 17
AZ70 TTMP 7 0.45 246 15
AZ91D TTMP 9 1 256-273 0-3
* Twin-roll Cast
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Effect of Al/Zn Ratio on TTMP Properties
Alloy Al Zn YS, MPa UTS, MPa Elong, %
AZ62 6 2 255 329 17
AZ55 5 5 203 332 10
ZA64 4 6 249 349 12
ZA75 5 7 225 353 10
ZA82 2 8 226 334 6
ZA84 4 8 307 361 6
ZA10/4 4 10 340 399 2
AM-Lite 4 12 310 351 2
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Effect of TTMP & Heat Treatment on AM60
Condition YS, MPa UTS, MPa Elong., %
As-Thixomolded 135 240 10
TTMP+Heat Treatment A 328 371 10
TTMP+Heat Treatment B 244 312 21
700 nm Grain Size
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Effect of TTMP & Heat Treatment on AZ61
Condition YS, MPa UTS, MPa Elong., %
As-Thixomolded 140 240 9
As-TTMP 305 362 6
TTMP+Heat Treatment C 326 378 8
TTMP+Heat Treatment D 343 380 8
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Directional Properties of Cross Rolled AM60
Condition Test Direction [°] YS [MPa] UTS [MPa] Elong.[%]
As Thixomolded (T) 0,45,90 135 240 10
As TTMP 0 305 323 13
As TTMP 45 302 324 14
As TTMP 90 306 325 15
TTMP + Heat Treat B 0 266 304 15
TTMP + Heat Treat B 45 262 302 15
TTMP + Heat Treat B 90 263 301 15
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Effect of TTMP Pressing on Properties of AZ70-TH
Condition YS, MPa UTS, MPa Elong. %
As-Thixomolded 140 273 15
TTMP Hot Pressed 248 401 16
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Thixomolded & Forged Auto Wheel (Mazda)
T
TTMP
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Effect of Process on Fatigue Strength of AM60
Process Fatigue Limit,
MPa at 106 cycles
Fracture Toughness,
KQ (MPam0.5 )
Die Cast* 20-40 -
As-Thixomolded 50-75 16
TTMP 100-125 30
TTMP+Anneal 125-150 28
* Commercial Cross Beams
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Texture (MRD) and Langford (r) Values for
TTMP AZ61 and AZ31 Sheet
Condition Texture, MRD r (Stretching) ∆r (Earing)
AZ61 - TTMP 4.8 1.44
AZ61 – TTMP
+ Anneal
3.5 1.12 0.18
AZ31 - Sheet 8-10 3 0.3
Low r and ∆r predict improved formability
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Chemical Compositions of Dead Sea
Magnesium Alloys for Thixomolding
Alloy Al Zn Mn Ca Sr Si Cu Ni Fe Be
AZ70L-TH 7.4 0.47 0.20 - - 0.003 0.001 0.001 0.003 0.001
AXJ810-TH 8.0 - 0.20 1.02 0.31 0.01 0.001 0.001 0.003 -
Low Mn for Ductility Ca for Restraint of Grain Growth Ca & Sr for Grain Boundary Segregation & Creep Resistance
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Room Temperature Tensile Properties of TTMP
DSM Alloys
Alloy Condition YS, MPa UTS, MPa El,%
AZ70-TH TTMP+Anneal 246 336 15
AXJ810-TH TTMP+Anneal 252 330 10
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High Temperature Properties of DSM AXJ810-TH
Property Value
Tensile Yield Strength at 150°C 115-125 MPA
Ultimate Tensile Strength at 150°C 150-170 MPA
Elongation at 150°C 15-20 %
Creep Strain at 150°C, 50 MPa, 200 hr 0.15 %
Creep Strain at 150°C, 70 MPa, 200 hr 0.35 %
Creep Strain at 175°C, 50 MPa, 200 hr 0.39
Minimum Creep Rate at 150°C, 50 MPa, 200 hr 9.8 x 1010 s-1
Minimum Creep Rate at 150°C, 70 MPa, 200 hr 16.4 x 1010 s-1
Minimum Creep Rate at 175°C, 50 MPa, 200 hr 15.7 x 1010 s-1
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TTMP AM60 in DonJoy Orthopedic Knee Brace
World’s Lightest at 14 oz Cold Bent from TTMP Sheet
Laser Cut TTMP Sheet
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Microstructural Mechanisms Tracy Berman, Tori Miller, Wayne Jones, Tresa Pollock
Thixomolded Fine Grains Minimize Twinning During TMP
β Eutectic Particles Stimulate Nucleation (PSN) of Randomized Grain Orientation During TMP
β Eutectic Particles Restrain Grain Growth During TMP and Heat Treatment
________________________________________________
Result – Large Orientation Spreads in Finer Grains, Minimized Prismatic Slip
Result – Low r and ∆ r values, Improved Formability w/o RE
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Conclusions
Thixomolding’s fine grain size, low porosity, homogeniety predestine effective TMP
TMP refines grains, thus imparting increased tensile and fatigue strength and toughness
In moderating texture, TTMP is a substitute for Rare Earth alloying elements in Mg
In AM and AZ Mg alloys, 6-7 % Al is optimum for strength and ductility at room temperature
Ca and Sr additions retard grain growth and alloy the grain boundaries to increase creep strength
Applications in Biomedical ex-fix and Sports Equipment
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Acknowledgements
The Authors are pleased to honor support from:
National Science Foundation Project 0847198
BIRD Foundation Project 1243
State of Michigan Emerging Technology Fund