Study On Microstructural Evolution And Mechanical Properties Of Mg-Sn-Al Alloy

The low density of magnesium alloys plays an important role in promoting the weight reduction of vehicles.However,the strength of magnesium alloys is low,so the internal law of homogenization and aging process of magnesium alloys,the interaction mechanism between alloys composition,microstructure,crystal defects and process parameters and precipitated phase,the structure and performance of magnesium alloys quantitative relationship,improving the strength of magnesium alloys and developing high-performance magnesium alloys materials have become an important research direction in the field of magnesium alloys.This thesis takes Mg-5.5Sn-4.5Al magnesium alloy as the research object to explore the effect of homogenization treatment on the structure and mechanical properties,study the microstructure and mechanical properties of the preset aging second phase under different extrusion ratios,observe the microstructure of the alloy under cold rolling and aging treatment process,and study the strengthening effect of work hardening and aging hardening on the alloy.The conclusions obtained are as follows:The phase of the as-cast Mg-5.5Sn-4.5Al magnesium alloy contains?-Mg,Mg₁₇Al₁₂ and Mg₂Sn.In the samples that were water-cooled after 420°C heated for 8hours,the Mg₁₇Al₁₂ phase was completely dissolved into the matrix,but the Mg₂Sn phase at the grain boundary failed to completely dissolve;and water-cooling after420°C heated for 24 hours,Mg₁₇Al₁₂ and Mg₂Sn were completely dissolved.The results of extrusion show that the alloy after homogenization will undergo dynamic precipitation of Mg₂Sn phase during extrusion,and the more complete the homogenization treatment,the more obvious the effect of dynamic precipitation of the second phase.This dynamically precipitated second phase will hinder the growth of recrystallized grains,while increasing the volume fraction of sub-grains and deformed grains,improving mechanical properties and improving the alloy's tensile-compression yield asymmetry.In the same initial state,the greater the extrusion ratio,the higher the degree of recrystallization.Under the same extrusion ratio,compared with the pre-homogenization treatment,the grains of the pre-aged alloy are finer and the second phase is more dispersed on the matrix.The extruded alloy was cold rolled at room temperature with various reductions,and the extruded and cold rolled alloys were aged at 200°C for 132 hours.The results show that after rolling,the alloy will enhance the intensity of basal texture,but will not change the texture type.Meanwhile,a large number of dislocations and twins are introduced via cold-rolling,which are conducive to refine the aging precipitation phase and increase the dispersion of the precipitation phase in the subsequent aging process.However,the strengthening effect of strain hardening caused by dislocations is stronger than that of ageing hardening.