Transcript Preparation of semi-solid slurry of AZ91D alloy in low

Microstructure Evolution of Semi-solid
Magnesium Alloy AZ91D Under
Electric Current
Y Yang, Q Zhou, J Tang, Z Hu
Institute of Metal Research
Chinese Academy of Sciences
2ed Sino-German Workshop on EPM, 16-19 Oct. 2005, Dresden, Germany
Outline
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Introduction
Experimental procedure
Results
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Dendritic growth
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Current pulse density
Current pulse duration
Discharging cycle
Treating time
Nondendritic particle size
Thermal fluctuation
Conclusions
Introduction
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Magnesium alloy offers numerous merits in
physical, mechanical and casting properties.
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The most lightest
structural alloy
Anti-vibration
Usages of magnesium alloy
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Magnesium alloy to near-netshape
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
density
time
duration
discharging cycle
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 ms
Discharging cycle: 0, 4, 6, 8, 10, 12 sec
Treating time: 0, 5, 10, 15, 20 min
Experimental Material
Commercial AZ91D alloy
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
Liquidus temperature: 595 oC
Solidus temperature: 470 oC
Effect of current pulse density on dendrite
growth
1 kA/cm2
4 kA/cm2
3 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
1.2 ms
1.0 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
10 sec
12 sec
(a) globular and cosh-shaped; (b, c) rosette-shaped; (d) dendritic structure.
Effect of treating time on dendrite growth
5min
15min
10min
20min
The shape of the primary grains from dendritic to rosette-shaped
then nondendritic with increasing treating time.
Non-dendritic particle size
-1
0
1
2
3
4
5
6
1600
600
600
500
Particle size (m)
1200
1000
800
600
500
400
400
300
300
400
200
200
200
0
-1
0
1
2
3
4
5
6
100
0.0
0.5
1.0
1.5
100
Current duration （ ms）
2
Peak current density (kA/cm )
1600
1600
1400
1400
1200
Particle size (m)
Particle Size(um)
1400
1200
1000
1000
800
800
600
600
400
400
200
200
0
-2
0
2
4
6
8
10 12 14 16 18 20 22 0
Treating time (min)
Distribution of non-dendritic particle size
30
AZ91D Alloy
Percent of particles, %
25
20
15
10
5
0
0
100
200
300
400
500
Particles sizes, m
The average size of the particles is about 150 m
18
o
The maximal temperature fluctuation( C)
Thermal fluctuation during solidification
16
14
12
10
8
6
4
2
0
1
2
3
4
5
6
2
Peak current density (kA/cm )
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Thermal history of specimen
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The temperature of the sample with
current pulse treatment is higher after
treating time exceeds 20 minutes due to
Joule heating.
Temperature fluctuation
The temperature fluctuation increases
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
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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.
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