| This paper focuses on researching the rule and mechanism for superheating and cooling process affecting the cast microstructure during the smelting and casting, in order to get ingot casting with fine and symmetrical microstructure. The different cast structure was obtained by controlling the temperature of melt and the rate of cooling during the melting and solidification of the conventional alloys of AZ31, AZ61 and AZ91. The effecs of superheating temperature and cooling rate on the microstructure and chemical composition distributing of elements Al and Zn was analyzed by OM(Optical Microscope), SEM(Scanning Electron Microscope), EDS(Energy Dispersive Spectrdmeter) and microhardness test. The main content of research includes the effects of overheating temperature on microstructure and microhardness of AZ series alloys with different Al content, and the effect of cooling process on the primary and secondary dendritic spacing and microhardness of the alloys.The resualt of superheating at temperature of 750℃,800℃,850℃,900℃and 950℃with the alloys of AZ31, AZ61 and AZ91 shows that, with the increase of Al content, the primary dendrite spacing and the secondary dendrite spacing decrease, the secondary dendrite become shorter and denser, the microhardness increases and became more uniform. The concentration of Al and Zn in the zone from the crystalline center to the gap increase along the radial direction. When the superheating temperature increases, the primary dendrite spacing and the secondary dendrite spacing decrease, the composition segregation weakens, and the microhardness increases. In order to refine the microstructure and to reduce the component analysis, the optimized superheating temperature of the AZ series magnesium is 900℃.The solidification structure of AZ61 magnesium alloy cooled with different rates separately in three solidifications stage of high-temperature melt stage, solidifying stage and solidified stage is investigated by comparing the dendritic spacing and microhardness. The result show that the cooling rate of AZ61 alloy in the high-temperature melt stage has more effect on solidification structure than other stages. The empirical equations about relationships of dendritic spacing and the cooling rate were regressed with mathmatical method, they areThe relationship between the primary dendritic spacing and the cooling rate at the solidifying stage of AZ61: y=121.04x-0.2618 The relationship between the secondary dendritic spacing and the cooling rate at the solidifying stage of AZ61: y=10.682x-0.0799The relationship between the primary dendritic spacing and the cooling rate at the stage solidified stage of AZ61: y=122.81x-0.2033The relationship between the secondary dendritic spacing and the cooling rate at the solidified stage of AZ61: y=9.8276x-0.1079During the researching on the effect of superheating temperature and the cooling history on the solidification structure of AZ series magnesium alloys, not only with the conventional method of comparing the dendrite morphology, dendrite arm spacing and microhardness, but also with a new method to compare the chemical composition difference in different dendritic character micro region, including primary dendritic matrix, pimary dendritic, secondary dendritic, secondary dendritic spacing and primary dendritic spacing. This new method was proved to be valuable for applying in research of solidification structure of AZ series magnesium alloys. |
