Microstructure Evolution of Semi-solid Magnesium Alloy AZ91D

Under Electric Current

Y Yang, Q Zhou, J Tang, Z Hu

Institute of Metal ResearchChinese Academy of Sciences

2ed Sino-German Workshop on EPM, 16-19 Oct. 2005, Dresden, Germany

Outline Introduction Experimental procedure Results

Dendritic growth Current pulse density Current pulse duration Discharging cycle Treating time

Nondendritic particle size Thermal fluctuation

Conclusions

Introduction

Magnesium alloy offers numerous merits in physical, mechanical and casting properties. The most lightest

structural alloy

Anti-vibration

Usages of magnesium alloy

Magnesium alloy to near-net shape will find widespread application in auto-, video-, computer- and communication- equipment, combined with the on-going ‘light-weighting’ of components.

Typical dendrites in magnesium AZ91D casting

Dendrites in AZ91D solidified with Low-Voltage Electric Current Pulses method (LVECP)

Experimental

Schematics of experimental setup

Pulse current density curve

duration

discharging cycle

density

time

Parameters: Current pulse density: 0, 1.3, 3, 4.3, 5.2 kA/cm2

Current pulse duration: 0, 0.6, 1.0, 1.2, 1.5 msDischarging cycle: 0, 4, 6, 8, 10, 12 secTreating time: 0, 5, 10, 15, 20 min

Experimental Material

Compositions of the AZ91D alloy (wt. ％ )

Al Zn Mn Be Si Cu Fe Ni Mg

9.03 0.64 0.33 0.0014 0.031 0.0049 0.0011 0.0003 Balance

Commercial AZ91D alloy

Liquidus temperature: 595 oC

Solidus temperature: 470 oC

Effect of current pulse density on dendrite growth

1 kA/cm2

3 kA/cm2

4 kA/cm2 5 kA/cm2

(a) Big dendrites; (b) small dendrites; (c) globular and cosh-shaped. (d) nondendritic, equiaxed paticles.

Effect of current duration on dendrite growth

0.6 ms 1.0 ms

1.2 ms 1.5 ms(a) Dendrites with long primary arms; (b, c) rosette-shaped. (d) globular and cosh-shaped.

Effect of discharging cycle on dendrite growth

6 sec 8 sec

12 sec

10 sec

(a) globular and cosh-shaped; (b, c) rosette-shaped; (d) dendritic structure.

Effect of treating time on dendrite growth

The shape of the primary grains from dendritic to rosette-shaped then nondendritic with increasing treating time.

5min 10min

15min 20min

Non-dendritic particle size

-1 0 1 2 3 4 5 60

200

400

600

800

1000

1200

1400

1600

Peak current density (kA/cm2)

Pa

rtic

le S

ize(um

)

-1 0 1 2 3 4 5 6

-2 0 2 4 6 8 10 12 14 16 18 20 220

200

400

600

800

1000

1200

1400

1600

Par

ticle

siz

e (

m)

Treating time (min)0

200

400

600

800

1000

1200

1400

1600

0.0 0.5 1.0 1.5100

200

300

400

500

600

Current duration ms（ ）

Pa

rtic

le s

ize

(m

)

100

200

300

400

500

600

4 6 8 10 12

140

150

160

170

180

190

200

210

220

Pa

rtic

le s

ize(m

)

Discharging cycle (s)

Distribution of non-dendritic particle size

0 100 200 300 400 5000

5

10

15

20

25

30

Particles sizes, m

Pe

rce

nt o

f p

art

icle

s, %

AZ91D Alloy

The average size of the particles is about 150 m

Thermal fluctuation during solidification

Thermal history of specimen

The temperature of the sample with current pulse treatment is higher after treating time exceeds 20 minutes due to Joule heating.

1 2 3 4 5 6

0

2

4

6

8

10

12

14

16

18

The

max

imal

tem

pera

ture

flu

ctua

tion(

o C)

Peak current density (kA/cm2)

Temperature fluctuation The temperature fluctuation increase

s as the current density increases. The maximal temperature fluctuation is about 16 oC.

Root remelting of dendrite arm

Schematic illustration of dendrite evolution

(a) initial dendritic(b) shrinkage of secondary arm roots(c) remelting and detaching of secondary arm roots (d) detaching finished

Conclusions

The morphology of primary phase is transited from dendritic to nondendritic, equiaxed particles by Low-voltage Electric Current Pulses during solidification of AZ91D alloy.

The particle size of AZ91D alloy decreases with increase of the current pulse density, discharging cycle, and treating time; but increases with increasing the current pulse duration.

Heat generation caused by Joule heating during discharge causes temperature fluctuation and decreases the cooling rate of solidification.

Electric current pulse restrains growth of the dendrites, makes dendrite arms remelted and attached during solidification, which leads to formation of nondendritic, equiaxed structure.

Thank you for your attention