The Semi-solid Microstructure And Properties Of Mg-7Zn-xV Magnesium Alloys

With the continuous development of society,magnesium aloys have received increasing attention due to their light weight,low density,and high specific strength.Magnesium aloys produced by general die casting methods have lower mechanical properties and hinder the application of magnesium aloys.Therefore,it is very necessary to develop a new type of magnesium aloy.The semi-solid forming process is one of the relatively new forming processes and can solve many problems of magnesium aloy products.Based on the Mg-7Zn binary aloy,the following studies were performed by adding a certain amount of V element and by means of optical microscopy,X-ray diffraction analysis,scanning electron microscopy.The microstructure and properties of the as-cast Mg-7Zn-x V magnesium aloy were studied.The results show that the non-dendritic structure of the as-cast aloy gradually refines as the amount of the appropriate amount of metal V increases.Its tensile properties and elongation showed a tendency of rising first and then decreasing.When the content of V is 0.1%and 0.3%,compared with Mg-7Zn binary aloy,the tensile strength and elongation are obviously increased.The tensile strength is 194MPa,192MPa,and the elongation is 12.70%and 12.10%respectively.When the content of V is 0.5%,the tensile strength and elongation begin to decrease.According to X-ray diffraction analysis,in the Mg-7Zn ternary aloy added with V,it is mainly composed of?-Mg matrix,Mg₇Zn₃ eutectic phase and Mg_102.08Zn_39.60 eutectic phase.The effect of heat treatment on the microstructure and properties of as-cast Mg-7Zn-0.1V magnesium aloys was studied.The results showed that the solid solution at340°C for 10 hours had the best effect of solid solution.A large number of eutectic phases were dissolved into matrix and the residual eutectic phases are uniformly and diffusely distributed after solution treatment at 340°C for 10h.On this basis,aging treatment was performed at 160°C.As the aging time prolonged,precipitation phases precipitated more and more.When aging at 160°C for 40h,the precipitated phases of the aloy were uniformly distributed and its tensile strength.Both microhardness and microhardness reached a maximum of 305MPa and 92.7VHN,respectively.Effects of remelting temperature and holding time on microstructural evolution of Mg-7Zn-x V magnesium aloy were investigated by semi-solid isothermal heat treatment.The results indicate that the holding temperature and holding time have significant effects on microstructural evolution of the Mg-7Zn-x V aloy in isothermal temperature process.By increasing the holding temperature or prolonging the holding time,the non-endritic structures in Mg-7Zn-x V can be transformed into smaller and more uniform spherical particles.When the holding temperature is too high or holding time is too long,the semi-solid particles will combine and grow up,the microstructural evolution mechanism is consistent with the rule of Ostwald ripening mechansim.The best parameters of isothermal heat treatment of Mg-7Zn-0.1V aloy are 600°C holding temperature and 40min holding time,obtaining the semi-solid microstructure which the average partial size is 46?m and the roundness is 1.28.The best parameters of isothermal heat treatment of Mg-7Zn-0.3V aloy are 605°C holding temperature and 40min holding time,obtaining the semi-solid microstructure which the average partial size is 48.5?m and the roundness is 1.26.The best parameters of isothermal heat treatment of Mg-7Zn-0.5V aloy are 605°C holding temperature and 30min holding time,obtaining the semi-solid microstructure which the average partial size is 47.2?m and the roundness is 1.26.