1. 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

2. Outline Introduction Experimental procedure Results Dendritic growth Current pulse density Current pulse duration Discharging cycle Treating time Nondendritic particle size Thermal fluctuation Conclusions

3. Introduction Magnesium alloy offers numerous merits in physical, mechanical and casting properties. The most lightest structural alloy Anti-vibration

4. 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.

5. Typical dendrites in magnesium AZ91D casting

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

7. Experimental Schematics of experimental setup

8. Pulse current density curve duration discharging cycle density time Parameters: Current pulse density: 0, 1.3, 3, 4.3, 5.2 kA/cm 2 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

9. Experimental Material Compositions of the AZ91D alloy (wt. ％ ) AlZnMnBeSiCuFeNiMg 9.030.640.330.00140.0310.00490.00110.0003Balance Commercial AZ91D alloy Liquidus temperature: 595 o C Solidus temperature: 470 o C

10. Effect of current pulse density on dendrite growth 1 kA/cm 2 3 kA/cm 2 4 kA/cm 2 5 kA/cm 2 (a) Big dendrites; (b) small dendrites; (c) globular and cosh-shaped. (d) nondendritic, equiaxed paticles.

11. Effect of current duration on dendrite growth 0.6 ms1.0 ms 1.2 ms1.5 ms (a) Dendrites with long primary arms; (b, c) rosette-shaped. (d) globular and cosh-shaped.

12. Effect of discharging cycle on dendrite growth 6 sec8 sec 12 sec10 sec (a) globular and cosh-shaped; (b, c) rosette-shaped; (d) dendritic structure.

13. 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 15min20min

14. Non-dendritic particle size

15. Distribution of non-dendritic particle size The average size of the particles is about 150  m

16. 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. Temperature fluctuation The temperature fluctuation increases as the current density increases. The maximal temperature fluctuation is about 16 o C.

17. Root remelting of dendrite arm

18. 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

19. 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.

20. Thank you for your attention