Thixoforming The Microstructure And Properties Of Az91d Magnesium Alloy And Heat Treatment Process

AZ91D is the most popular Mg alloy due to its superior combination of castability, mechanical strength. But it is easy to produce porosity during die casting because its rang of solidify temperature is wide, so it is used in cast commonly. Thixoforming can eliminate air porosity produced during die casting, so the properties of thixoformed Mg alloys can be improved by heat treatment and some surface defects formed dujring heat treatment can be avoided. Thixoforming is a high-efficient, energy-save and advanced material processing technology because it combines the advantages of both casting and forging technoligies. In addition, the forming resistance force and forming temperature of thixoforming is obviously less than that of the solid state forming and casting, respectively, then the service life of mould is prolonged. Effects of process parameters and heat-treatment on microstructures and properties were studied in this paper.The results show that the.optimum process parameter is holding temperature at 585°C, holding time for 90min and mould temperature at 350°C. The filling property of alloy is excellent when mould temperature is 35o℃.It is unfavorable to fill mould when the temperature is too low. While holding temperature is at 575℃, the filling property reduces, because of high solid rate. It is difficult to obtain sound castings when the holding time is less than 90min. The oxidation is serious if the holding timen is too long.Ultimate tensile strength and yield strength of thixoformed AZ91D alloy are improved after solution treatment, but the hardness of the alloy decreases due to the formation of solid solution. The p-phase dissolved into the a-phase matrix to form solid solution being low micro-hardness. Thixoformed Mg alloys have remarkable aging harden characteristics. The strengthening phase is Mg₁₇Al₁₂.The aging temperature has obviously effect on the microstructure and properties of the alloy.The age time of reaching the best hardening result is ahead when age temperature is rising, while its best hardening result is reduced.During the ageing treatment, there are two types of precipitation: discontinuous and continuous, The discontinuous precipitation at grain boundary happen priorisly. Long age time and high age temperature is needed for continuous precipitation inside grain. There is only discontinuous precipitations when ageing temperature is 180°C; There are continuous precipitation in grain when the temperature is 200℃, 220℃, 250°C respectively. The time of starting precipitation is different for different treatment age temperature, and the proportion of continuous precipitation increases with the rising of age temperature. The aging role is very sensitive to solubility of solid solution. The aging role at 415°C-8h is better than that at 415°C-8h obviously. P-phase becomes finer after solid solution and aging precipitation. Then, the pin effect of the grain boundary can improve the mechanical properties of the alloy obviously. Ultimate tensile strength is 213Mpa after heat treatment at 415°C-24h-200°C-16h, being improved by nearly 51% than that of as-cast.