Effect Of Vacuum Levitation Melting And Selective Laser Melting On Microstructure And Properties Of Al_0.3CoCrFeNi High Entropy Alloy

High entropy alloys have four core effects(high entropy effect,lattice distortion,slow diffusion,"cocktail"effect),which make them show higher strength,hardness,wear resistance and corrosion resistance compared with traditional alloys.The traditional preparation method of high entropy alloy is mainly vacuum melting.The sample shape prepared by this technology is relatively simple,which is prone to problems such as coarse grain size and element segregation.It is difficult to directly meet the needs of industrial parts with complex structure by post-processing technology to control the structure and properties of materials.In recent years,selective laser melting(SLM),as a new manufacturing technology,can quickly and directly form industrial parts with complex structure through three-dimensional construction of parts through computer-aided design(CAD),and has become one of the most potential industrial component forming methods for development and application.SLM samples have fine grains,anisotropic microstructure and high strength and hardness;However,there are unavoidable defects such as pores and cracks.In this experiment,Al_0.3CoCrFeNi high entropy alloy was used as the research material to study Al_0.3CoCrFeNi.The process window and optimal forming process parameters of Al_0.3CoCrFeNi high entropy alloy lay a theoretical and technical foundation for SLM forming complex high entropy alloy parts,and systematically analyzed the effect of traditional vacuum levitation melting(VLM)and advanced selective laser melting(SLM)on Al_0.3CoCrFeNi.The main conclusions are as follows:(1)Under the condition of fixed layer thickness of 35?m,the influence of laser power,scanning speed and scanning distance on the morphology,defect and roughness of monolayer forming sample was studied,and the optimized process parameters were obtained:The laser power is 180 W-300 W,the scanning speed is 200 mm/s-400 mm/s,and the scanning distance is 60?m-120?m.Then the bulk samples are formed through single factor experiment and orthogonal experiment,and then the density of the samples is counted and analyzed.It is concluded that the optimal process parameters are laser power180 W,scanning speed 300 mm/s and scanning spacing 120?m.The highest density of the sample is 98.31%.The internal defects of the above samples were scanned by industrial CT and microscopic pores were found.(2)In microstructure,the grains of the as-cast sample are equiaxed coarse-grained with an average grain size of 141?m.After deformation and heat treatment,the grains of the as-cast samples are mixed with coarse and fine grains,with an average grain size of 5.43?m and a large number of dispersed second phase particles.The microstructure of printed samples in different directions is anisotropic:the X-Y plane microstructure is composed of large equiaxed grain region with an average grain size of 7.42?m,and the internal substructure of equiaxed grain is cellular subgrain.The grain morphology of the sample shows fine cellular crystal and slender columnar crystal structure in the X-Z direction,and the average grain size is 14.89?m.(3)In terms of mechanical properties,the average hardness of the as-cast sample is 197.28HV,yield strength is 264.47MPa,tensile strength is 566.92MPa and elongation is 63.3%.The fracture mechanism of the sample is mainly dimple fracture,and cleavage fracture is supplemented.Under the joint action of fine grain strengthening and precipitated phase strengthening,the average hardness of recrystallizable samples reaches 226.54 HV,the yield strength increases by176.99 MPa,the tensile strength increases by 227.48 MPa,and the elongation decreases by 18.6%.The fracture mechanism of the samples is typical dimple fracture.The average hardness of printed samples is 230.44HV,yield strength is 511.04MPa,tensile strength is 604.5MPa,but elongation is only 14.0%.The high hardness and yield strength of the sample are attributed to the high dislocation density and fine grain size of the SLM molded sample,while the low elongation is the unavoidable effect of pores on the plasticity of the printed sample.(4)In terms of friction and wear properties,GCr15 steel,Al₂O₃ and Si C were selected as friction pairs.After grinding with different as cast,recrystallized and printed samples,it was found that the wear resistance trend of the three samples under different friction pairs was the same,but the wear mechanism was different:the as cast samples had the best wear resistance,and their wear mechanisms were abrasive wear and adhesive wear(grinding with GCr15 steel)Adhesive wear(against Al₂O₃),fatigue wear and oxidation wear(against Si C);The wear resistance of recrystallized samples is slightly lower,and the wear mechanisms are adhesive wear(against GCr15 steel),abrasive wear(against Al₂O₃),abrasive wear,oxidation wear and fatigue wear(against Si C);The wear resistance of printed samples is the lowest,and the wear mechanisms are abrasive wear dominated by furrow wear(against GCr15 steel),adhesive wear and abrasive wear(against Al₂O₃),oxidation wear and abrasive wear(against Si C).In the friction movement,the dispersed second phase particles in the recrystallized sample will increase the local contact stress between the grinding ball and the sample,which makes the sample more prone to damage,and the wear rate is higher than that of the as cast sample;There are pores in the printed sample,which seriously reduces the plastic deformation ability of the alloy,makes it easier to produce crushing damage,and the wear rate is relatively high?...