Research On Microstructure And Properties Of Al₂O₃/7075 Composite Fabricated By High-intensity Ultrasonic Treatment

Aluminum matrix composites are favored in the aerospace,automotive and other industries due to its superior properties such as light weight,high specific strength,specific stiffness,specific modulus,high wear resistance,low coefficient of thermal expansion.Among the three types of fiber,whisker and particle reinforced aluminum matrix composite,particle reinforced aluminum matrix composites have received extensive attention due to their advantages such as simple and economical manufacturing processes,isotropic characteristic and it could manufactured by traditional methods.The uniformly distributed particles in the matrix and the strong interface between them is the key to obtain high-performance particle-reinforced aluminum matrix composites.In this study,Al₂O₃/7075 composites were fabricated by high-intensity ultrasonic treatment and its microstructure and properties were studied.The results show that the introduction of Al₂O₃ reinforcement particles using the Al-10%Al₂O₃ intermediate composite powder helps to improve the wetting of the Al₂O₃ particles in matrix melt.Compared to the direct addition of Al₂O₃ particles to the melt,this method can obtain more uniformly particle-distributed Al₂O₃/7075composites with lower porosity.Compared to the mechanical stirring method,in the Al₂O₃/7075 composites prepared by high-intensity ultrasonic treatment,the Al₂O₃particles are more evenly distributed in the matrix.This phenomenon can be attributed to the acoustic cavitation and acoustic flow generated by high-intensity ultrasonic treatment in the melt.As the ultrasonic power increases,the distribution of Al₂O₃particles becomes more uniform.The addition of Al₂O₃ particles can refine the grains of the 7075 matrix,and the addition of Al₂O₃ particles makes the brittle eutectic phases distributed continuously at the grain boundaries become discontinuous.Al₂O₃particles are mainly distributed at the grain boundaries,and only a few are distributed within the grains.The grain refinement is attributed to the heterogeneous nucleation and particle pushing mechanisms during the solidification of the composite.The ultimate tensile strength and hardness of the composites increase with the addition of the Al₂O₃.When the Al₂O₃ content reached 2wt.%,the Al₂O₃/7075 composite exhibit highest ultimate tensile strength and hardness,and excessive addition of Al₂O₃ result in the deterioration of the mechanical properties of the composite material.The elongation of the composite material decreased slightly with the addition of the Al₂O₃.2wt.%Al₂O₃/7075 composite exhibits the best mechanical properties,compared to the 7075matrix,the ultimate tensile strength and hardness increased by24.7%and 17.5%,respectively.The increased mechanical properties of the composites can be attributed to fine-grained strengthening and thermal mismatch enhancement.The decrease in performance of 3wt.%Al₂O₃/7075 composite can be attributed to the increase of pores in the composite and the weakening of the interface between Al₂O₃ particles and the matrix.The wear volume of 7075 matrix and its composites increased with the increase of load;under the same load,the wear volume of composites was lower than that of the matrix.When the load reaches 45N or more,the wear volume of the 7075substrate and the 1.0wt.%Al₂O₃/7075 composites increased sharply,while the wear volume of the Al₂O₃/7075 composites with the Al₂O₃ content of 2.0wt.%and 3.0wt.%increased slightly.At low load,the instantaneous friction coefficient of the 7075matrix is more stable than that of the composite material during the steady wear stage;under high load,the instantaneous friction coefficient of the matrix and 1.0wt.%Al₂O₃/7075 composite material decreases with the prolonging time of wear,and tend to be achieve stable during the final stage of friction.The instantaneous friction coefficient of the 2.0wt.%and 3.0wt.%Al₂O₃/7075 composite fluctuated strongly during the final stage of friction.Under 60N high load,with the increase of friction time,a hard oxide film will form on the worn surface of the 7075 maxtrix,which is conducive to lubrication and reduce the subsequent wear rate;under high load of 60N,2.0wt.%and 3.0wt.%Al₂O₃/7075 composites exhibit excellent wear resistance.The as-cast microstructure of 7075 aluminum alloy is mainly composed ofα-Al and eutectic phases.Eutectic phase mainly consists of Mg（Zn,Cu,Al）₂ phase,followed by Al₇Cu₂Fe phase and Mg2Si phase.The optimum solid solution treatment process of 7075 aluminum alloy matrix and Al₂O₃/7075 composite material is460°C+6h.Under this condition,Mg（Zn,Cu,Al）₂ phase dissolves intoα-Al sufficiently and transforms into Al₂CuMg phase.The Al₇Cu₂Fe phase does not change significantly because of its high melting point.The hardness of the 7075 aluminum alloy matrix and Al₂O₃/7075 composite after solid solution treatment is 145.7HV and158.5HV,respectively.Increased by 48.3%and 37.2%,respectively;Al₂O₃/7075composite material to reach the aging peak time is shorter than the 7075 aluminum alloy.The aging peak hardness of 7075 aluminum alloy and Al₂O₃/7075 composite were 169.3HV and 176.7HV,respectively,increased by 16.5%and 11.5%compared to the un-aged specimens,respectively.Compared to the as-cast 7075 aluminum alloy,The hardness of Al₂O₃/7075 composite after T6 heat treatment increased by79.9%.