Effects of Cooling Process on Microstructure of As-cast AZ61 Magnesium Alloy

The liquid solidification process of AZ61 magnesium alloy was divided into two stages of high-temperature melt and solidification.The effects of different cooling rates of each stage on the cast microstructure and the variations of dendrite spacing on the microhardness were investigated.The results show that at the high-temperature melt stage,when the cooling rate increases from 0.65℃/s to 15.9℃/s,the dendrite microstructure is continuously refined and the size becomes more uniform.The primary dendrite is decreased from 230μm to 80μm and the secondary dendrite spacing is decreased from 12.8μm to 9.2μm.At the solidification stage,when the cooling rate increases from 7.8℃/s to 23.0℃/s,the primary dendrite spacing is decreased from 105μm to 73μm and the secondary dendrite spacing is decreased from 10.6μm to 8.8μm.In the two stages,the microhardness value presents increasing trend with the increase of cooling rate.By comparison,the change of cooling rate has a larger impact on the as-cast microstructure of high-temperature melt stage than that the solidification stage.