Study On The Properties Of AZ91D/SiC_P Composites Prepared By Vacuum Infiltration

Particle reinforced magnesium matrix composites have high specific strength and specific stiffness,excellent wear resistance,corrosion resistance and vibration damping performance,as well as good thermophysical properties,so it has great potential for future development.However,many existing methods for preparing magnesium matrix composites have their own problems,which cannot be used in large-scale industrial production.Among them,the composite material prepared by vacuum pressure infiltration method has a compact structure,the reinforcement is evenly distributed in the magnesium matrix,the interface is well combined,and the near-net formed composite material product can be obtained,which has unique advantages over other preparation methods.At present,the research on particle reinforced magnesium matrix composites prepared by vacuum pressure infiltration method often focuses on one of the properties.However,there are few systematic studies on its comprehensive performance.Therefore,AZ91D magnesium alloy was used as the matrix alloy and SiC particles as the reinforcement in this study and AZ91D/SiC_P magnesium matrix composites were prepared by vacuum pressure infiltration method.The aging behavior of AZ91D/SiC_P magnesium matrix composites was investigated,analyzed the evolution law of aging precipitated phase and determined the optimum heat treatment process of composite materials.Finally,the mechanical property at different temperatures,wear behavior and coefficient of thermal expansion of the composites are systematically investigated.The main research results are as follows:(1)In this paper,AZ91D/SiC_P magnesium matrix composites were successfully prepared by vacuum pressure infiltration.It was found that the prepared composite has compact structure,uniform SiC particle distribution,good interface bonding,and no macroscopic defects or undesirable phases through SEM observation and XRD analysis.(2)Brinell hardness tests were performed on aged AZ91D magnesium alloy and AZ91D/SiC_P magnesium matrix composites.The results show that the brinell hardness of magnesium alloy and its composites increases first and then decreases with the increase of aging time.The aging time for magnesium alloy to reach the maximum hardness value is 36h,while the aging time for composite materials to reach the maximum hardness value is 18h.This indicates that the addition of SiC particles accelerates the aging kinetics of matrix.Therefore,the optimal heat treatment process for composite materials:415°C solution for 24h+175°C ageing18h.(3)The microstructure evolution of aging precipitated phase of as-cast AZ91D/SiC_P magnesium matrix composites are as follows:At the beginning of aging,the precipitated phase grew up near the nucleation of SiC particles for discontinuous interface precipitation.With the increase of aging time,the interface precipitated phase along the SiC particles precipitated out in a mesh,and continuously extended to the grain interior.The morphology of the precipitated phase grew from granule to needle and then to strip.At the later stage of aging,the precipitated phase covers almost the entire magnesium matrix.Moreover,the precipitate size near SiC particles was larger than that in other regions,and the morphology was rod-like.(4)The high-temperature deformation behavior of AZ91D magnesium alloy and AZ91D/SiC_P magnesium matrix composites was studied by compression tests at different temperatures.Both the peak stress and the strain corresponding to the peak stress of magnesium alloy and its composites decrease with the increase of compression deformation temperature.And the composite has higher yield strength and ultimate compression strength at all test temperatures.(5)The wear rate and specific wear rate of the composites are obviously lower than magnesium alloys,and the addition of SiC particles improves the wear resistance of the composites.The as-cast composite decreases the wear rates of AZ91by about 70-95%and in the case of the heat treatment composite wear rates decreases by about 88-95%.However,the anti-attrition effect of SiC particles reduced with the increase of load.Abrasive wear is dominant mechanism observed by SEM analysis and the delamination and adhesive wear existed in the severe wear.The directivity of wear surface of composite materials is not obvious.When the applied load is 10N,the SiC particles in the microstructure of the composites are not damaged,and the wear surface is relatively flat.When the applied load increases to20N,SiC particles with fracture appeared in the composites.(6)The thermal expansion coefficient of composite materials is obviously lower than magnesium alloy.The thermal expansion coefficient of the best heat treated composites is higher than as-cast composites,but still lower than magnesium alloys.(7)Hardness,wear resistance and high temperature compression strength of as-cast composites were improved after the optimum heat treatment.Compared with the as-cast composites,the brinell hardness of the heat treatment composites increased by 25%,the wear rate decreased by 8.7%to 60%,and the compressive strength at high temperature increased by 15%.This is due to the second phase nucleates and grows on the dislocation after aging,resulting in the dislocation was pinned and improved the deformation resistance of the composite.