Study On Microstructure Stability And Mechanical Properties Of Magnesium Alloys With Sn And Ca Additions

High temperature magnesium alloys have find widespread application respects in the fields of automotive, aircraft, aerospace and national defense equipment due to the excellent combination of low density, high specific strength, high specific modulus, shock absorption and electromagnetic shielding. So, it is of great importance to develop novel elevated temperature magniesum alloy with low cost and high performance.Mg-Zn-Sn-Al alloys have great potentials in developing high performance heat-resisting magnesium alloys, in which the high content of Zn and low content of Al essentially eliminate theβphase which has poor thermal stability. In this thesis, Sn and Ca were selected to improve the high temperature mechanical properties of ZA62 alloy. The influence of addition of Sn, Ca and extrusion temperature and heat treatment processing on the microstructural stability and mechanical properties of ZA62 magnesium alloys were investigated. The conclusions were drawn as follows.(1) The ZA62 alloys with Sn addition was mainly composed ofα-Mg solution,divorced eutectic MgZn and Mg2Sn particles, The microstructure of which after annealing for 360h at 150℃,200℃coarsened little compared with that annealed for 24h, but the precipitated granular phase obviously increased around grain boundary and within grain. And the precipitation near grain boundary became much more when annealing at 200℃than at 150℃,it was demonstrated that higher temperature,more precipitated phase; The hardness of ZA62 alloys with Sn addition became stable after long-term annealing at 150℃for 192h, however when annealing at 200℃,the hardness descended gradually except ZA62 alloy with 0.5% Sn.It was proved that trace Sn addition had a little effect on the microstrural stability of magnesium alloys.(2) There was no difference between ZA62 alloy with Sn,Ca addition and ZA62 alloy with Sn addition in phase composition after annealing for 360h. But the precipitation of the former around grain boundary became finer and more dispersed and the precipitation within grain increased compared with the latter. It could be deduced that Ca addition could refine the precipitation and accelerate the nucleation within grain. The amount of precipitation near grain boundary largly increased after annealing for 360h at 200℃than for 24h.During long-term annealing, the hardness curve of ZA62 alloy with Sn,Ca addition appeared two peak points, the first peak value was smaller than ZA62 alloy with Sn addition and the hardness kept constant after reaching the second hardness peak.It was shown that Ca could quick up the precipitation process of MgZn and Mg2Sn and stabilize high temperature microstructure of ZA62 alloy with Sn addition.(3) The dynamic crystallization of ZA62 alloy with Sn,Ca addition took place after extrusion at 330℃, and within the extruded alloy granular phase of MgZn and Mg2Sn precipitated on lath-shaped MgZn phase, overlapped with a certain angle,which could hinder the grain coarsening,boundary migration and dislocation glide.The mechanical properties of extruded ZA62 alloy with Sn,Ca addition has been eventually improved: the tensile strength was 377.5MPa, yield strength 244.4MPa and elongation percentage was 7.5. The precipitated phase in the extruded alloy after solution+aging heat treatment increased and became more dispersed and the yield strength reached 172.5MPa.