Effect Of Electropulsing Treatment On Microstructure And Mechanical Properties Of AZ91 Magnesium Alloy And Its Composite

Magnesium alloys are considered as the lightest metal structure materials in practical applications and have been widely concerned in automobile,aviation,electronics and space.However,compared with engineering materials such as steel and aluminum alloys,the lower strength limits the popularization of magnesium alloy in industry.Therefore,it has a broad application prospect to develop a kind of exceptional mechanical properties for magnesium alloy as a structural material by optimizing the matrix structure and adding reinforcement phases.Recently,electrical pulse treatment(EPT)has been used as a new method of tissue optimization to improve the comprehensive performance of metal materials because of its advantages of short processing time and high input energy.In this paper,the effects of EPT and EPT+Si Cp on microstructure evolution of AZ91 magnesium alloy were studied by using as-cast AZ91magnesium alloy and Si Cp/AZ91 composites as raw materials.Firstly,AZ91alloy and composite after the solution treatment were extruded at 250?.After the extrusion,the microstructure changed from the as-received coarse grains to the deformed structure without obvious dynamic recrystallization.After the introduction of EPT with different parameters,the microstructure underwent static recrystallization and secondary phase precipitation in different degrees.The microstructure with bimodal structure and abundant Mg₁₇Al₁₂ precipitated phase were obtained in AZ91 alloy,whose yield strength and tensile strength increased from 110MPa and 195MPa to 463MPa and 527MPa respectively.When Si Cp reinforcement phase was added to AZ91 matrix,the mechanical properties of the extruded state were greatly improved with the yield strength increasing from 150MPa to 480MPa,the tensile strength increasing from210MPa to 556MPa and the elongation decreasing slightly.After EPT,the strength with uniform fine grain structure(with average grain size of 0.91?m)was reduced,and the yield strength and tensile strength were 380MPa and454MPa,respectively.The recrystallization of AZ91 alloy and its composite increased with the increase of duration,peak current and treatment time.The addition of Si Cp increased the dislocation density during extrusion and the athermal effect of EPT,and then promoted the static recrystallization of AZ91 matrix.The Mg₁₇Al₁₂ phase precipitation in the AZ91 alloy and Si Cp/AZ91 composite was promoted when treated by EPT.After the volume fraction of Mg₁₇Al₁₂ phase reached the maximum value,it was rapidly dissolved in matrix under EPT on the premise that the grains did not grow significantly.In this process,due to the relatively high deformation storage energy around Si Cp and the increment of the second phase nucleation rate result from Si Cp,the precipitation of the second phase around Si Cp was promoted.