Electroplasticity in metal alloys

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2017.03.20

Electroplasticity in metal alloys

- AZ91 magnesium alloy, AHSS-

2017

년도

1

학기 심화수업

Jeong Hye-Jin

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2

Effect of electric current during tensile test PART 1

Introduction Electrically-assisted Manufacturing (EAM)

 Issue in electrically-assisted manufacturing

 Application of EAM

CONCLUSION

PART 2 _ 980MPa grade Steel

Final Remark Conclusion  Total summary

 AZ91 Magnesium alloy

Effect of electric current during tensile test

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I. Introduction

High strength & Lightweight automobiles

Body parts mostly consists of high strength steel

&

lightweight metals

(Magnesium/Aluminum alloy)

 Body parts components

Low cold formability High spring back

25^oC 100^oC

150^oC

200^oC 300^oC

http://www.topspeed.com/cars/volvo/2009-volvo-xc60-ar52734/picture304392.html

https://www.esi-group.com/sites/default/files/software-services/1557/springback http://openi.nlm.nih.gov/legacy/detailedresult.php?img=3132537

 Limit properties of high strength & lightweight metals

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I. Introduction

Electrically-assisted Manufacturing (EAM)

After pulsing

Acceleration of Spheroidization [deformed pearlitic steel]

Edwin I et al., J. Mater. Res. (2010)

No current Current

0.00 0.06 0.12 0.18 0.24 0.30 0.36 -50

0 50 100 150 200 250 300

No current Pulsed current

Stress(MPa)

Strain

Penn State Univ. (2009) Decrease of Spring back Increase of Elongation

 Space saving

 Rapid heating

Advantages of EAM

 High energy efficiency

 No cost to maintain temperature of furnace/mold

 Enhancement of Formability  Microstructure control

: Metal forming by applying electric current during deformation Electrically-Assisted Manufacturing (EAM)

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I. Introduction

Application of EAM

0 5 10 15 20 25 30

-50 0 50 100 150 200 250

Nonpulsed Pulsed

Eng. stress (MPa)

Eng. strain (%) Pulsed

(e=5%) Nonpulsed (e=5%)

+Heat treatment Nonpulsed

(e=5%)

30 40 50 60 70 80

0.06 0.09 0.12 0.15 0.18 0.21 0.24 0.27

Nonpulsed (at e=5%) Heat treated: local point (at e=5%) Heat treated: gauge section (at e=5%) Pulsed (at e=5%)

2θ (deg.) FWHM (deg.) * Symbols : FWHM

Lines : Intensity

0 5000 10000 15000 20000 25000

Intensity (a.u.)

 AA5052 Al alloy

Non-pulsed Pulsed

 Electrically assisted annealing

Scripta Mater. 75, 2014

0 5 10 15 20 25

0 100 200 300 400

Solution treated

Nonpulsed Pulsed

Eng. stress (MPa)

Eng. strain (%)

 AA6061 alloy

Int. J. Plast., (in Press), 2016

 Electrically assisted Aging

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I. Introduction

Application of EAM

 AHSS (980 DP Steel)

The U-bending fixture

 Springback decreased by applying electric current

Int. J. Precis. Eng. Manuf. 15 (2014)

baseline

1.15 J/mm³ 0.736 J/mm³ 0.414 J/mm³

 Tungsten film

0 50 100 150 200 250 300 350 400 450 500

As-sprayed Induction heated Electro-treated

Microhardness (HV)

Conditions

38MPa Without 25MPa

900^oC

treatment 25MPa38MPa

1000^oC 1100^oC 38MPa 25MPa

Significantly improved hardness!

363HV

122HV

287HV

 Electrically assisted Densification

International Journal of Refractory Metals and Hard Materials (2016)

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II. Materials and Experimental method

Experimental Procedure

 Specimens

Composition Characteristic

AZ91 Mg alloy 9Al-1Zn YS 290MPa, El 7%

Complex Phase steel (DP 980 소둔) 0.07C-2.5Mn-0.96Cr-0.05Nb YS 849MPa, TS 1030MPa, El 9.3%

Sub size Standard size

G 25 50

W 6.25 12.5

L 100 200

[Tensile specimen (ASTM E8)]

 Instrumental set-up

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20μm

Phase Fraction (%) Mg₁₇Al₁₂ 21.1 ± 3.4

 EDS & SEM Analysis

 As-received specimen

More than 5 images in each specimen

Mg

₁₇

Al

₁₂

phase

Mg₁₇Al₁₂ Matrix

ED

Subsize

20μm

ED As-extruded

ED

20μm

 Property of AZ91

• β (Mg₁₇Al₁₂) phase - Intermetallic phase

- exist at α-Mg grain boundary - brittle β phase

- Melting point : 330 – 350 ^oC

 Low formability

β phase (Mg₁₇Al₁₂)

 Draw backs

Additional Heat treatment or

grain refinement process is required II. Materials and Experimental method

Experimental Procedure

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III. Results and Discussion

Magnesium alloy (AZ91)

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

0 100 200 300

400 Non-pulsed tensile test at 25^oC Non-pulsed tensile test at 70^oC Puled tensile test

E ng . st re ss (M P a)

Eng. strain

0.00 0.05 0.10 0.15 0.20 0.25 0.30

0 50 100 150 200 250 300 350 400

Puled tensile test

Non-pulsed tensile test at 70^oC

T em p er at u re

(o

C

)

Eng. Strain

Min. T : 70^oC

Property Non-pulsed at 25^oC Non-pulsed at 70^oC Pulsed

Yield stress 290 MPa 250 MPa -

Fracture elongation 7% 13.9% 28.9%

Why?

 Pulsed tensile test

- Current density (ρ_i): 80A/mm², duration=0.5s. period=20s - Cross head speed : 1.0 mm/min

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0 5 10 15 20 25 30 0

100 200 300 400

Temperature (o C)

Eng. strain (%)

0 100 200 300 400

Nonpulsed at 25^oC Nonpulsed at 70^oC Pulsed

Eng. stress (MPa)

400

e=13%

0 50 100 150 200 250 300 350

0 100 200 300

400 Pulsed Tension (ρ_i=80A/mm², d=0.5s, p=20s, 13pulse) Induction Heat-Treatment

Temperature (o C)

Time (sec)

• Non-pulsed tension at 70^oC + Induction H-T

Eng. Strain : 13%

Non-pulsed tension at 70^oC

+ Induction H-T Pulsed tension As-received

III. Results and Discussion

Magnesium alloy (AZ91)

 EBSD Analysis : Inverse Pole Figure Map

ED ND

0 8

Min Max

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III. Results and Discussion

Magnesium alloy (AZ91)

Non-pulsed tension at 70^oC + Induction heat treatment

Pulsed tension ED

ND

As-received

 Mg

₁₇

Al

₁₂

Phase Fraction  Vickers Hardness Measurement

Mg17Al12 Matrix

 Electric current induced Dissolution of Mg

₁₇

Al

₁₂

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III. Results and Discussion

Magnesium alloy (AZ91)

Recrystallized grain : 0<GOS<2^o Recrystallized grain Un-recrystallized grain

Max.=2.3 Max.=2.1 Max.=1.9

50 60 70 80 90 100

Recrystallized area fraction(%)

0 5 10 15 20 25 30

Recrystallized grain size (µm)

Pulsed Tension

* Engineering Strain = 13%

Non-pulsed tension at 70^oC

+ IHT As-extruded

 EBSD Analysis : Grain Orientation Spread Map

+ Induction H-T Pulsed tension As-received

ED ND

0 8

Min Max

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0 2 4 6 8 10 12 0

200 400 600 800 1000 1200

1400

CP-O

En g. S tre ss ( M Pa )

Eng. Strain

Crosshead Speed : 1mm/min

 CP강 : 소둔재

소둔재 : YS 849MPa, TS 1030MPa, El 9.3%

 Composition

(wt.%) C Mn Cr Nb

0.07 2.5 0.96 0.05

• Optical image

13

III. Results and Discussion

980MPa grade steel

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 Pulsed tensile test

- Current density (ρ_o): 95A/mm², duration=0.1s. period=10s - Cross head speed : 1.0 mm/min

-

ρ

_o=current density based on

initial cross-sectional area

0 2 4 6 8 10 12 14

0 200 400 600 800 1000 1200 1400

Non-pulsed tension at RT Non-pulsed tension at 300ôC Non-pulsed tension at 200ôC Non-pulsed tension at 100ôC Pulsed tension

E n g . St re ss ( M Pa )

Eng. Strain (%)

0 50 100 150 200 250

0 100 200 300 400 500

Temperature of specimen during pulsed tension

T em per at ur e(

o

C )

Time(sec)

Engineering strain of 5.7%  Microstructural analysis

Property Non-pulsed Pulsed 100ôC 200ôC 300ôC UTS 1030 MPa 1107MPa 1012MPa 1023MPa 1060MPa

Fracture elongation 9.3% 12.1% 10% 9.6% 9.1%

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III. Results and Discussion

980MPa grade steel

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260 280 300 320 340 360

100^oC

85-331^oC

Non-pulsed tension

Pulsed tension 300^oC 200^oC

V ick er s H ar dn es s (HV

3

)

Eng. Strain : 5.7%

Eng. strain=5.7%

(Non-pulsed tension at high temperature)

Eng. strain=5.7%

(Non-pulsed tension) + Induction Eng. strain=5.7%

 Vickers Hardness Measurement

0 30 60 90 120 150

0 100 200 300 400

Pulsed tension

Induction heat treatment

T em per at ur e (

o

C )

Time (sec)

300^oC

200^oC

100^oC

 Temperature history

 From the Vickers hardness measurement, it can be observed that applying electric current can accelerate “aging effect” with a distinct effect from Joule heating

15

 Pulsed tensile test

- Current density (ρ_o): 95A/mm², duration=0.1s. period=10s - Cross head speed : 1.0 mm/min

-

ρ

_o=current density based on

initial cross-sectional area

Engineering strain of 5.7%  Microstructural analysis

III. Results and Discussion

980MPa grade steel

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 XRD Analysis : FWHM Value

METALLURGICAL AND MATERIALS TRANSACTIONS A, Vol. 47A, 2016—3109

15 20 25 30 35 40

0.10 0.15 0.20

0.25 Non-pulsed at RT Pulsed

Induction

Non-pulsed at 100^oC Non-pulsed at 300^oC

F W H M ( de g. )

2θ(deg.)

Ferrite (α) peak

• Annihilation of dislocations and fine precipitation occur simultaneously on aging

• The hardness and micro strain are increased by formation of precipitates which are coherent with matrix

• The coarsening of the precipitation leading to loss of coherency with the matrix  decrease of micro strain with increasing temperature

𝛽^∗ = 1

𝐷 + 2𝜀𝐾

𝛽 ∶ 𝐹𝐹𝐹𝐹 𝑊𝑊𝑊𝑊𝑊 𝑎𝑊 𝐻𝑎𝐹𝐻 𝑀𝑎𝑀𝑊𝑀𝐹𝑀 𝛽^∗ = 𝛽 cos 𝜃

λ , 𝐷 ∶ 𝑎𝑎𝑎𝑎𝑎𝑎𝑎 𝑎𝑎𝑎𝑊𝑔 𝑠𝑊𝑠𝑎 𝐾 =2 sin 𝜃

λ , λ: 𝑤𝑎𝑎𝑎𝐹𝑎𝑔𝑎𝑊𝑊 𝑜𝐻 𝑋𝑎𝑎𝑋 𝑏𝑎𝑎𝑀

 Williamson-Hall (WH) approach

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III. Results and Discussion

980MPa grade steel

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(Using Cu grid and replica film, specimens were prepared )

 TEM Analysis at engineering strain of 5.7%

Non-pulsed Tension

Pulsed Tension

NbC

200μm

(Nb,Ti)C

200μm

 Electric current can assisted “aging effect” during plastic deformation due to formation of Nano-sized precipitation such as NbC

0.4 μm 0.4 μm

III. Results and Discussion

980MPa grade steel

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Summary

IV. Conclusion

0 5 10 15 20 25 30

0 100 200 300 400

Temperature (o C)

Eng. strain (%)

0 100 200 300 400

Nonpulsed at 25^oC Nonpulsed at 70^oC Pulsed

Eng. stress (MPa)

400

 AZ91 Mg alloy

0 2 4 6 8 10 12 14

0 200 400 600 800 1000 1200 1400

Non-pulsed tension at RT Non-pulsed tension at 300ôC Non-pulsed tension at 200ôC Non-pulsed tension at 100ôC Pulsed tension

Eng. Stress (MPa)

Eng. Strain (%)

 Electric current assisted “dissolution effect”

during plastic deformation  Electric current assisted “aging effect”

during plastic deformation

 CP (DP980) steel

Applicability of EAM technique to Forming process ₁₈

Non-pulsed Pulsed

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III. Results and Discussion

Magnesium alloy (AZ91)

350, 400^oC 10min

T em p er at u re

Time

Heating rate :100^oC/min

cooling rate : quenching

Pressure

Vacuum chamber Graphite punch

Specimen Pulsed DC

Power supply

Pulsed DC Pressure Electric current

Thermocouple

【 Pulsed Electro-treatment Equipment】【 Schematic diagram of Condition】

15mm

1.2T Specimen Dimension

ED

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Initial

 Practical solution treatment condition : 380 ~ 420^oC _ 30min~ 1hrs

 Microstructure analysis : OM

Conventional

Heat-Treatment Electro-Treatment

350^oC, 10min

400^oC, 10min

 Fraction of Mg

₁₇

Al

₁₂

phase : Conventional heat-treatment > Electro-Treatment

Mg₁₇Al₁₂

III. Results and Discussion

Magnesium alloy (AZ91)

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III. Results and Discussion

Magnesium alloy (AZ91)

50 55 60 65 70 75

80

Heat-Treatment

Electro-Treatment

V ic ke rs H ar dne ss ( H v1)

Initial 350

^o

C 10min

400

^o

C 10min

The solution heat-treatment dissolves Mg

₁₇

Al

₁₂

phase to matrix

 This makes the hardness of specimen decreased!!

Vickers Hardness (HV1)

 Microstructure analysis : Vickers Hardness Measurement

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III. Results and Discussion

Magnesium alloy (AZ91)

2 4 6 8 10 12 14 16 18

20 Heat-Treatment

Electro-Treatment

Initial 350^oC 10min

400^oC 10min

Conventional

Heat-Treatment Electro-Treatment

350^oC, 10min

400^oC, 10min

20μm

Mg

₁₇

Al

₁₂

phase Fraction (%)

 it was observed that dissolution kinetic of Mg

₁₇

Al

₁₂

phase is accelerated by EAM

Mg₁₇Al₁₂ Matrix

Electroplasticity in metal alloys

2017.03.20