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)
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
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
480 Thixomolding Machines in 13 Countries Semi-Solid Molding with 5 to 15 % Solids
Thixomolded Microstructure
Optical Micrograph Electron Micrograph
α
β- eut
α-eut
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
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
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
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
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Thus Promoting Dynamic Recrystallization
Thus Weakening Texture by “β Disruption”
Intragranular β Particles From TMP
β
Precipitated from Supersaturated Thixomolded Condition
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
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
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
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
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
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
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
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
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
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
Thixomolded & Forged Auto Wheel (Mazda)
T
TTMP
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
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
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
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
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
TTMP AM60 in DonJoy Orthopedic Knee Brace
World’s Lightest at 14 oz Cold Bent from TTMP Sheet
Laser Cut TTMP Sheet
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
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Result – Large Orientation Spreads in Finer Grains, Minimized Prismatic Slip
Result – Low r and ∆ r values, Improved Formability w/o RE
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
Acknowledgements
The Authors are pleased to honor support from:
National Science Foundation Project 0847198
BIRD Foundation Project 1243
State of Michigan Emerging Technology Fund