Microstructures And Mechanical Properties Of SLMed FeCoCrNiMo_0.2 High Entropy Alloy

3d transition group high entropy alloy has good strength,excellent toughness and thermodynamic structure stability,is a type of basic structural materials with strong plasticity.Selective laser melting（SLM）technology can realize the near net shape of complex parts,and its rapid heating and cooling rate can obtain a non-equilibrium solidification structure different from the traditional methods（casting and melting）,significantly improve the mechanical properties of the alloy.In this study,FeCoCrNiMo_0.2high entropy alloy was prepared by SLM,and the effects of process parameters on microstructure and mechanical properties of high entropy alloy were investigated,the influence of process parameters on the size of cellular structure was studied by simulation,and the control method of cellular structure size is put forward.The quantitative relationship between cellular structure and yield strength was revealed,the corresponding relationship between process parameters,microstructure and mechanical properties was established,and obtained the strengthening and toughening mechanism under the optimal mechanical properties.The main research contents and conclusions are as follows:（1）The microstructure of SLMed FeCoCrNiMo_0.2 high entropy alloy was studied,and found that the alloy has a multi-layer structure of molten pool,columnar structure,cellular structure,and maintained a single-phase FCC structure,there was obvious Mo segregation at the cellular structure boundary.When the energy density was in the range of 37 J/mm³-150J/mm³,the depth of molten pool was about 44μm-310μm,the grain size of columnar structure was about 10μm-20μm,the size of cellular structure was about 0.36μm-0.99μm。（2）The effect of process parameters on the size of cellular structure was studied,and found that the cell size increased from 0.36μm to 0.99μm with the increase of energy density from 37 J/mm³ to 150 J/mm³.The relationship curve between energy density and temperature gradient,cooling rate in SLM process was obtained by simulation method.The size of cellular structure was calculated according to the change of cooling rate.It was found that the size of cellular structure calculated by simulation is essentially in agreement with the actual value,indicating that the change of cellular structure size was mainly related to temperature gradient and cooling rate,and proposed the control method of cellular structure.（3）The effects of process parameters on the mechanical properties of the alloy were studied,and found that the strength of alloy increased first and then decreased with the increase of energy density.When the energy density was 46 J/mm³,the comprehensive mechanical properties of the alloy were the best,and the yield strength,tensile strength and elongation could reach 707 MPa,947 MPa and 34%respectively.Compared with the alloy prepared by traditional melting and casting,the strength is increased by about 60%.（4）The strengthening and toughening mechanism of SLMed FeCoCrNiMo_0.2high entropy alloy was studied,and found that the high strength of the alloy comes from the multi-layer structure containing ultra-fine nano cellular structure and the segregation of Mo at the cellular boundary.The good plasticity of the alloy is due to the interaction between cellular structure and deformation twins.