Effects Of Heat Treatment On Microstructure Of Mg-7Zn-1Cu-xV Magnesium Alloy

Magnesium alloys have lively performance and are easy to oxidize and burn at high temperature,while the forming temperature of semi-solid forming technology is low,which can effectively solve this problem.The common method for preparing magnesium alloy parts is die casting,but the properties of magnesium alloy formed by completely liquid die casting cannot be improved by heat treatment.Semi-solid formed parts can be heat-treated,but the grade alloy used in semi-solid forming has poor heat treatment strengthening effect,which is not conducive to fully utilizing the advantages of semi-solid formed parts.Therefore,it is necessary to develop new magnesium alloys suitable for semi-solid forming.Mg-Zn-Cu series alloy has better heat treatment strengthening effect,but the semi-solid structure and heat treatment of this series alloy are rarely studied.Therefore,in this paper,the trace V element was added to Mg-7Zn-1Cu alloy,and the alloy was studied by means of OM,XRD,SEM,EDS,TEM and hardness tester.The study on the as-cast Mg-7Zn-1Cu-x V alloy found that the microstructure and properties of the alloy changed after adding the element V.When the amount of V was0.3 wt.%,the microstructure refinement effect was obvious and th e hardness was the maximum,58.6 HV.The as-cast microstructure of the alloy was mainly composed of?-Mg,MgZn₂,CuZn,and CuMgZn phases,and no new phase was generated after adding the V element.The effect of V content on the microstructure of non-dendritic Mg-7Zn-1Cu alloy was studied by isothermal heat treatment.The results indicated that when the V content was 0.3 wt.%,the alloy non-dendritic structure was significantly refined.After isothermal heat treatment of Mg-7Zn-1Cu-0.3V alloy at 580°C for 35 min,an ideal semi-solid structure was obtained.Under this process,the average size and shape factor of the solid particles in the structure reached the minimum,which were33.25?m and 1.33,the solid phase rate was 45%.The solid phase in the semi-solid structure of alloy was mainly composed of?₁-Mg and?₂-Mg,and the liquid phase was mainly composed of liquid film,liquid molten pool and small droplets.During the whole isothermal heat treatment process,the evolution of semi-solid microstructure mainly experienced three stages:initial coarsening,tissue separation and spheroidization,particle combination and ripening.The best solution treatment process for as-cast Mg-7Zn-1Cu-0.3V alloy was solution at 390°C for 24 h,followed by artificial aging at 160°C.After aging 20 h,it reached the peak-aged state.The precipitation phase in the alloy structure was the most and the dispersion was uniform,the hardness of the alloy was 88.0 HV.The structure of peak-aged alloy was composed of?-Mg,MgZn₂,CuZn,and CuMgZn phase.The main strengthening phases in the TEM structure of alloy included lath-shaped?₁'phase(?{0001}_Mg basic plane)and plate-shaped?₂'phase(?{0001}_Mg basic plane).The optimal solution and aging treatment process of the non-dendritic Mg-7Zn-1Cu-0.3V alloy was the same as that of the as-cast alloy.In the early stage of aging treatment,G.P.zone and a small amount of transition phase?₁'and?₂'appeared in the TEM structure of the non-dendritic alloy.The main strengthening phases in non-dendritic alloys under peak-aged were lath-shaped?₁'phase and plate-shaped?₂'phase,which were the same as those of the as-cast alloy,but with more quantities,more uniform distribution,and better strengthening effect.