Effect Of Pressure And Heat Treatment On The Structure And Properties Of Squeeze Casting Magnesium Alloy Bracket

AZ91D magnesium alloy is widely used in automobile manufacturing,aerospace and other fields due to its excellent casting properties,good comprehensive mechanical properties and heat treatment strengthening.However,AZ91D magnesium alloy castings have many casting defects under ordinary casting conditions,which is not convenient for subsequent heat treatment strengthening,which requires a more advanced forming process to produce magnesium alloy parts.Squeeze casting is an advanced material forming process that can perform near-net-shape forming of complex parts,which can effectively curb the formation of defects and improve the heat-treatability of magnesium alloy castings.Therefore,it makes an important and practical sense to study the influence of pressure and heat treatment on the structure and mechanical properties of squeeze cast AZ91D magnesium alloy brackets.The test material used in this papper is AZ91D,and the magnesium alloy brackets are produced by the squeeze casting with different pressure parameters,and then heat treatments are carried out on them.Vickers hardness tester and universal tensile testing machine were used to test the mechanical properties of the as-cast and the heat-treated(T4 and T6)AZ91D magnesium alloy bracket samples,and the use of optical metallographic microscope(OM)and scanning electron microscope(SEM)The microstructure,second phase and fracture morphology of squeeze casting AZ91D alloy under different conditions were observed.The effects of different pressures and heat treatment on the microstructure and mechanical properties of squeeze casting AZ91D magnesium alloy brackets were investigated.The test results show that with the increase of the extrusion pressure,the microstructure of the squeeze cast magnesium alloy bracket is significantly refined,and the number of second phases in the structure gradually decreases.At the same time,the alloy hardness,tensile strength and elongation have gradually increased.The hardness has increased from 64.0HV to 71.2HV,the tensile strength has increased from 161.3MPa to 180.2MPa,and the elongation has increased from 2.2%to 4.7%.Observation of fracture morphology found that with the increase of extrusion pressure,the number of dimples and tearing edges in the fracture increased significantly.The microstructure of magnesium alloys at different solid solution temperatures for 12 hours shows that as the solid solution temperature increases,the number of second phases in the structure decreases significantly.When the solution temperature reaches 430?,the solution effect is the best,indicating that 430?is the best solution temperature.The microhardness test of alloy samples at different solution temperatures found that as the solution temperature increased from 350°C to 430°C,the microhardness of the alloy samples gradually decreased,from 70.7HV to60.4HV.The tensile experiment on the alloy with a solid solution temperature of 430?found that its tensile strength is 163.0MPa and elongation is 5.3%.The fracture morphology of the sample with a solution time of 430?was observed,and it was found that there were more dimples,which indicated that the fracture mode was plastic fracture.The magnesium alloy structure was treated with different aging time at the same aging temperature,and the microstructure was observed.As the aging time continued to increase,the amount of Mg₁₇Al₁₂ in the structure gradually increased,and the degree of dispersion gradually increased.Extending to 40h,Mg₁₇Al₁₂ will appear obvious coarsening.The microhardness test and room temperature tensile performance test on the aged magnesium alloy samples show that with the extension of the aging time,the microstructure microhardness and tensile strength both show a change pattern of first increasing and then slightly decreasing.When the aging treatment was carried out to 15h,the Vickers hardness and tensile strength reached the maximum(84.8HV,196.0MPa);however,as the aging time continued to increase,the elongation of the sample gradually decreased to 4.3%.Observing its fracture morphology,it is found that with the progress of aging treatment,the characteristics of plastic fracture are gradually reduced,and the characteristics of brittle fracture gradually increase.It can be concluded that with the extension of the aging time,the strength of the squeeze cast AZ91D magnesium alloy continues to increase,and the plasticity continues to decrease.