Study On The Failure Mechanism And Improvement Methods Of Laser Melting Deposition Of FeCoCrNi High Entropy Alloy

FeCoCrNi high-entropy alloy is one of the most widely studied FCC phase high entropy alloys.FeCoCrNi has excellent plasticity and good toughness,meanwhile,it exhibits broad application prospects in aerospace industry.Traditional preparation methods are prone to produce metallurgical defects such as microcracks and segregation,and it is difficult to prepare materials with complex shapes.Additive manufacturing technology provides a new method for preparing high-entropy alloy components with complex shape and excellent performance due to its high free degree of manufacturing process and high dislocation density of its products.Laser melting deposition（LMD）technology has excellent process flexibility and high cooling rate,which can effectively regulate its microstructure and performances,thus,it is very suitable for the preparation of high-entropy alloys.Therefore,this paper mainly studies the grain growth mechanism and the weak area of mechanical properties of FeCoCrNi-LMD single wall,and improves the weak area of FeCoCrNi-LMD by changing the laser scanning strategy.The annihilation mechanism of the weak area of mechanical properties of FeCoCrNi-LMD was analyzed.In this study,the microstructure of FeCoCrNi-LMD deposited wall was systematically studied.The columnar grain growth mode was multi-layer continuous growth from the bottom to the top,and the top layer is equiaxed grain.As the heat accumulation increased,the grain growth rate raised,hence,the width of the columnar grain increased from the bottom to the top.Tensile stress was applied to the material along the direction perpendicular to the columnar grain growth to study the reasons of the weak zone appearing after plastic deformation.The results show that the maximum texture density increases when tensile stress is applied.During plastic deformation,dislocations accumulate at grain boundaries and stress concentration occurs at grain boundaries.In addition,the arrangement of atoms on both sides of the grain boundary is different,so cracks are prone to propagate along the grain boundary during stretching.In order to improve properties of the weak zone,FeCoCrNi bulk was produced using an interlayer orthogonal scanning strategy,which increased the competitive growth degree of columnar grains.The results showed that the change of scanning mode had led to the change of heat flow direction during the preparation process,which could increase the competitive growth degree of grains and decrease the anisotropy and grain size.Compared with the deposited wall,the deposited bulk obtained by orthogonal scanning between layers can not only effectively reduce the residual stress of the material,but also has more excellent mechanical properties(tensile strength increased by 145 MPa;microhardness increased by 30 HV_0.3).Then,the deposited wall and the deposited bulk were rolled to different degrees and the annihilation mechanism of the weak zone of FeCoCrNi-LMD after the scanning strategy was studied.The results showed that the dislocation density of the deposited bulk is smaller than that of the deposited wall at different degrees of deformation.And the texture strength of the deposited wall was much higher than that of the deposited bulk.Therefore,compared with the bulk,the FeCoCrNi-LMD deposited wall is difficult to coordinate deformation under yield stress,this can result in the cracks at the grain boundary.The improvement of the weak zone is mainly related to the decrease of texture and dislocation density.These findings in this study are expected to further enhance the application prospects of LMD technology in industry.