A Study On The Microstructure And Properties Of AZ31 Magnesium Alloy Produced By Directional Solidification

Nowadays, research on high performance wrought magnesium alloy sheets has drawn focus from the material science realm. The formability in rolling process of magnesium alloy was restricted by microstructure and crystal orientation of the cast ingot. It is expected that developing new preparation of magnesium ingot could control the microstructure and crystal orientation effectively, thus improve the formability in rolling process.In this thesis, the author designed a set of directional solidification equipment and produced AZ31 ingot successfully. Through a serial of experiments such as examined microstructure, X-ray diffraction analyzing, room temperature compression test, high temperature compression test, hot rolling and tensile test; the effects to microstructure and crystal orientation of AZ31 ingot cause by directional solidification process were studied carefully. And the characteristics of room temperature and high temperature compression behavior as well as hot rolling behavior were also examined. The results showed:(1) AZ31 ingot produced by directional solidification had coarse column crystal, a little second spherical phase distributed equally in the matrix. Crystals grew regularly in some direction and showed an apparent [1010] orientation preference due to the effect of temperature gradient on the height direction. Such direction was parallel to the heat flow during the process.(2) The ingot produced by the method of 2-side heating showed anisotropy on mechanical properties ductility. The cross-side which was parallel to the heat flow and the heating surface had better strength and ductility compare to other directions.(3) Directional solidification could improve hot rolling formability of magnesium ingot effectively. The total deformation could reach 94% without cracking when hot rolling was held at 673K. At the time of deformation reached 40%, the appearance of the crystal changed greatly: the size refined, boundary turned clear, second phase disappeared and twin crystal structure appeared. The rolled sheet had good performance at room temperature tensile. And the strength of longitude had been improved while the strength of latitude had preserved, which diminished anisotropy caused by rolling.