Research On Microstructure And Properties Of Al_xCrCuFeNi₂ High Entropy Alloys Fabricated Via Selective Laser Melting

Selective laser melting(SLM) not only overcomes the limitations of traditional processing technology in the forming of complex parts,but also can obtain ideal alloy properties by highly controlling the microstructure.In order to solve the problem of high crack sensitivity of high entropy alloy formed by SLM,Al_xCrCuFeNi₂ dual-phase high entropy alloys will be fabricated in this paper,with emphasis on the controlling of microstructure and properties.The main research contents and conclusions are as follows:(1)The effect of Al content and laser volume energy density on the formability of Al_xCrCuFeNi₂ high entropy alloys were investigated.With the increase of Al content,the number of cracks in the alloy decreases first and then increases,and the formability of Al_0.75 high entropy alloy reaches the best.As the volume energy density increasing,the crack surface density of Al₀ and Al_1.0 high entropy alloys decreases first and then increases,while the crack surface density of Al_0.5 high entropy alloy decreases monotonically.The corresponding relative density changes tend to be opposite.In this paper,the optimal SLM process parameters are obtained as the laser power of 200 W,layer thickness of 0.02 mm,scanning rate of 200 mm/s,and scanning spacing of 0.10 mm.(2)The effect of Al content on the microstructure of Al_xCrCuFeNi₂ high entropy alloys were studied.With the increase of Al content,the coarse columnar grain with FCC single-phase structure in the Al₀ and Al_0.5 high entropy alloys is transformed into fine equiaxed grain with FCC+BCC(B2)dual-phase structure in the Al_0.75 and Al_1.0 high entropy alloys.A lamellar eutectic-like structure with alternately distributed FCC phase and BCC(B2)phase is extensively formed in the equiaxed grains.(3)Based on the observation and analysis of microstructure,the cracking mechanisms of Al_xCrCuFeNi₂ high entropy alloys formed by SLM were revealed.The root cause of intergranular hot cracks in Al₀ and Al_0.5 high entropy alloys is insufficient melt supply and low grain boundary strength at the long and narrow gaps between columnar FCC grains with high angle grain boundaries.The transgranular cold cracks in Al_1.0 high entropy alloy are caused by the brittle fracture of BCC equiaxed grains under the severe residual stress.The equiaxed grains composed of FCC+BCC(B2)lamellar eutectic-like structure can effectively eliminate both hot and cold cracks.(4)The effect of heat treatment on the microstructure and mechanical properties of Al_0.75 high entropy alloy formed with optimal SLM process parameters were studied.After heat-treated at 700°C,a large amount of short rod-shaped nano precipitates with B2structure and spherical nano particles with L1₂ structure are formed the alloy.Meanwhile,the microhardness of alloy reaches a maximum of 424 Hv.What's more,the optimal compression performance is obtained with a compression strength of 2056 MPa and compression strain of 30.63%,respectively.After aging treatment for 10 hours,the microhardness is furtherly increased to 466 Hv,the compression strength is increased up to 2341 MPa and the compression strain is 35.11%.The strengthening mechanisms of Al_0.75high entropy alloy after heat treatment are solid solution strengthening and precipitation strengthening.