The Semi-solid Microstructure And Properties Of Mg-6Zn-xZr Magnesium Alloys

Magnesium alloy has rather high specific strength, stiffness and low density, and has received great attention in automotive and aerospace fields in the view of weight loss and improvement of properties. In this study, more ideal casting processes were explored, and then Mg-6Zn-xZr (x=0,0.3,0.6,0.9wt.%) alloy has been prepared by permanent mold casting, and X-ray diffractometer, optical microscope, scanning electron microscope and universal mechanical testing machine have been used to analyze the effect of the content of Zr on the microstructure and mechanical properties of magnesium alloy. The effect of Zr content and isothermal heat treatment on the semi-solid structure has been also analyzed through preparing non-dendrite semi-solid structure by semi-solid isothermal heat treatment. And the effect of the variation of Zr content and aging time on the microstructure and mechanical properties has been investigated by the heat-treatment of alloy.By analyzing the microstructure of the as-cast Mg-6Zn-0.9Zr alloy that was smelted under different feeding temperatures and different pouring temperatures, it was found that the feeding temperature of760℃, and pouring temperature of710℃were more ideal technical parameters, and eutectic structure obtained under such condition was fine and uniform.Mg-6Zn alloy was mainly made up of a-Mg and MgZn phase, and Mg-6Zn-xZr alloy consists of a-Mg, MgZn and MgZn2. The tensile strength and vickers hardness of the as-cast Mg-6Zn alloy increased first but then decreased. When the content of Zr was0.6%, the maximum of the tensile strength and vickers hardness of the as-cast Mg-6Zn alloy was249Mpa、57.2HV respectively, and with the addition of Zr, the elongation of Mg-6Zn alloy decreased. The fracture mode of the as-cast Mg-6Zn alloy was cleavage fracture and the fracture modes of the tensile fracture changed from cleavage fracture to quasi-cleavage fracture with the addition of Zr.The process of forming non-dendrite structure of magnesium alloy through semi-solid isothermal heat treatment stages:the coarsening, separation and nodulizing of the dendrite and the merging and curing of the particulates. It was found that the semi-solid structure of Mg-6Z-0.6Zr alloy was rather well by analyzing the non-dendrite structure prepared under different holding time and temperatures, and when the holding temperature and time was610℃and60min respectively, semi-solid structure with a solid fraction of54.80%, average particle size of73.667μm, and shape factor as low as1.50can be obtained. The elongation of holding time and increase of holding temperature have both accelerated the evolution of non-dendrite structure, but if the holding time was too long, and the holding temperature was too high, the dendrite would grow and the structure would coarsen.After the heat-treatment of Mg-6Zn-xZr alloy, the grain structure decreased gradually with the increase of Zr content and when the content of Zr was0.9%, the structure of magnesium alloy reached the tiniest. With the elongation of aging time, the grain size of magnesium alloy with different constituent decreased first and then increased, and when the aging time was24hours, the structure was the tiniest, and the tensile strength and hardness of the alloy reached their maximum. The structure tended to grow with the increase of aging time, its tensile strength and hardness decreased, and the fracture mode of magnesium alloy after heat-treatment was cleavage fracture.