Microstructure And Mechanical Properties Of CoCrFeNiMn_p/6061Al Composites Prepared By Friction Stir Processing

Aluminum matrix composites have attracted wide attention due to their excellent specific strength,high hardness and good wear resistance.In order to overcome the problem that the reinforced particles react with the aluminum matrix to form brittle intermetallic compounds due to melting in the process of preparing aluminum matrix composites by means of traditional methods.To avoid the defects of poor wettability,easy reaction and poor stability between the reinforced phase such as traditional metal particles,ceramic particles,metallic glass and the aluminum matrix.,friction stir processing（FSP）,as a new method to prepare aluminum matrix composites,is proposed in this work,and high entropy alloy（HEA）particles are used as reinforcing phase,which effectively improves the poor bonding ability between traditional reinforcing particles and aluminum matrix.In this work,Co Cr Fe Ni Mn_p/6061Al composites were prepared by underwater FSP,the hardness distribution,tensile and friction and wear performance of the composites were systematically studied,the microstructure of the composites was characterized by optical microscope,scanning electron microscope and electron backscatter diffraction,and the mechanical properties of the materials was evaluated by microhardness,transverse tension and wear experiments.In addition,the influences of different process conditions on the hardness distribution,tensile,and wear performance of composites were analyzed,based on the study of the effects of different rotation speed,different HEA particles size and“solution+aging”heat treatment on the microstructure evolution of composites.On the basis of the above,the structure-activity relationship between the microstructure and mechanical properties of the composites was established,and the mechanism of HEA enhancement relative to the mechanical properties of the composites was clarified.Dynamic recrystallization was prone to occur in the composites with added HEA particles,which further refined the grains during underwater FSP.Meanwhile,the HEA particles were uniformly distributed on the aluminum matrix due to the severe plastic flow of the material,and the HEA particles in the central area of the processing zone diffused between the HEA particles and the aluminum matrix,thereby forming a smooth bonding interface without the formation of new phases.When the rotation speed was 1000rpm and the HEA grains size was 5～10μm,the microstructure of the composites reached the best,which average grain size was refined to 2.35μm,and the recrystallized grains and high-angle grain boundaries were 72.1%and 60.9%,respectively.The results indicated that under the same process conditions,the mechanical properties of the composites were the best,Its ultimate tensile strength is 263 MPa,reaching 86.5%of the6061-T6 aluminum matrix（304 MPa）,which is 36.3%higher than that of the FSP-Al sample（FSP sample without HEA particles）（193 MPa）,and the elongation is 44%,which is 19.6%higher than that of the 6061-T6 aluminum matrix（36.8%）,reaching 80.4%of the FSP-Al sample（54.7%）.The hardness and tensile strength of composites had the same trend of change.During the underwater FSP process,the HEA particles were uniformly distributed and bonded well with the aluminum matrix interface.Meanwhile,the particle stimulated nucleation mechanism of the HEA particles promoted the dynamic recrystallization process,which was beneficial to grain refinement.The dispersion strengthening of HEA particles,the strengthening of load transfer between the HEA particles and the aluminum matrix interface,and the fine-grain strengthening of recrystallized grains have significantly improved the mechanical properties of the composites.The average grain size of the 10-AMCs sample（FSP rotation speed:1000 rpm,particle size:5～10μm）increased from 2.35μm to 2.54μm,and the fine-grained structured was still maintained,after the“solution+aging”heat treatment.During the heat treatment,diffusion occurred between the HEA particles and the aluminum matrix interface,resulting in the formation of an interfacial layer with a thickness of 200 nm,in which the new phase of BCC+FCC was formed,thus enhancing the interfacial bonding and improving the mechanical properties of the material.Moreover,the tensile strength of the T6-10-AMCs samples（10-AMCs sample after heat treatment）after heat treatment increased to 348 MPa,which was 32.3%higher than that before the heat treatment,while the elongation decreased to 31.5%,a decrease of 28.4%.The tensile strength was increased by 14.5%,and the elongation reached 85.6%of the 6061-T6 aluminum matrix.Furthermore,compared with aluminum matrix,the average friction coefficient of FSP-Al sample increased from 0.4491 to 0.5287,the average friction coefficient of 10-AMCs sample with addition of particles was 0.4855,the average friction coefficient of T6-10-AMCs sample after heat treatment decreased to 0.3188,and the wear rate reached the lowest(1.2×10^-5 g/（N·m）).The results showed that abrasive wear and a small amount of peeling wear occurred in the aluminum matrix,plastic deformation and adhesive wear occurred mainly in FSP-Al sample,adhesive wear,peeling wear and abrasive wear occurred in 10-AMCs sample,and abrasive wear occurred in T6-10-AMCs sample.