冷却速率对A356铝合金显微组织和微观硬度的影响

Effects of Cooling Rate on Microstructure and Microhardness of A356 Alloy

研究了不同冷却速率对A356合金凝固组织和微观硬度的影响,以及T4热处理后,其显微组织和微观硬度的进一步演化.结果表明:提高冷却速率,凝固组织α?Al的二次枝晶臂间距(SDAS)和共晶Si的尺寸显著减小,微观硬度显著提高;8h的T4热处理后,相同冷却速率下的SDAS几乎没有变化,而较高冷却速率下的共晶Si更容易球化.冷却速率为9℃/s和0.15℃/s试样的微观硬度都得到进一步提高,冷却速率为167℃/s时,共晶Si在完全球化后进而长大粗化,试样的微观硬度反而降低.可见,当冷却速率较高时,细小的共晶Si仅需要相对较短的固溶时间就可以球化,达到最佳的力学性能.因此,通过控制冷却速率,可以细化晶粒,提高性能,还能缩短T4热处理的时间,节约生产成本.

Effects of different cooling rates on microstructure and microhardness of A356 alloy, before and after T4 treatment,were studied. The results show that for the as-cast alloy, the secondary dendrite arm spacing （SDAS） of primary a-A1 and the size of eutectic Si particles decrease obviously with increase of cooling rate, and thus resulting in increase of microhardness. Subjected to T4 treatment for 8 h, SDAS of a-A1 dendrite has been found to change slightly at the same cooling rates, but the eutectic Si particles at higher cooling rate are more readily modified. Microhardness of alloy at 9 ℃/s and 0.15 ℃/s has been further improved as compared to those of the as-cast alloy,but the microhardness at 167 ℃/s decreases as a result of coarsening of eutectic Si particles. It can he seen that when the cooling rate high, spheroidizing of tiny eutectic Si only need a relatively short period of solution time,which can lead to the best mechanical properties. Therefore, controlling cooling rate during solidification can refine the microstructures and further improve the mechanical properties, and also can shorten the T4 heat treatment time, and save the cost of production.