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