microstructure evolution of semi-solid magnesium alloy az91d under electric current
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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
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