Effects Of Tin, Calcium Alloying On Microstructure And Mechanical Properties On ZA62 Alloys

Magnesium alloys are widely used in the fields of automobiles, electronics, aerospace etc. However, the conventional magnesium alloys are characteristics of poor high temperature mechanical properties, which greatly restrict their applications. Therefore, developing novel low-cost heat-resistance magnesium alloys has become a hot topic recently.The Mg-Zn-Al alloy system, in which the high Zn but low Al content essentially eliminates theβphase with poor thermal stability has great potentials in developing high performance heat-resistance magnesium alloys. In this thesis, the effects of Sn and Ca on the microstructure, mechanical properties and corrosion resistance of ZA62 alloy as well as the influence of Sn and Ca alloying on the evolution of its microstructure and mechanical properties during the heat treatment were investigated. The experimental results were listed as follows.(1) The as-cast ZA62 alloy was mainly composed ofα-Mg and MgZn, the latter of which was the main strengthening phase at room temperature. A minor addition of Sn contributed to the refinement of the MgZn phase and the formation of dispersed Mg2Sn particles around the grain boundary. The Mg2Sn particles, with high melting point and high hardness, could pin the grain boundary effectively and prohibit the motion of dislocations, and hence caused the improvement in strength and plasticity. However, the addition of Sn more than 0.5mass% contributed to the precipitation of of the Mg2Sn particles and the MgZn phase, and the coarsening of the latter, which led to the reduction in the ultimate strength. The alloy designed as ZAS6205 had the best combination of mechanical properties. At room temperature, the ultimate strength, yield strength and elongation were 225MPa,118MPa and 8.9% respectively, while at 150℃the ultimate strength, yield strength and elongation were 212MPa, 113MPa and 12.2%.(2) A minor addition of Ca had no influence on the phase constitutions, but refined the grains and the divorced MgZn intermetallics. Therefore, the strength was greatly improved and the plasticity was maintained. At room temperature, the ultimate strength, yield strength and elongation of ZASX623005 were 255MPa,135MPa and 9%. The addition of Sn and Ca contributed to the improvement in the corrosion resistance of ZA62 alloy.