Microstructure And Mechanical Properties Of Al_0.5CoCrFeNi High Entropy Alloy Fabricated By Selective Laser Melting

The multi-principal design concept and the inherent high entropy effect,cocktail effect,hysteresis diffusion effect,and lattice distortion effect give the high entropy alloy excellent comprehensive performance.Such as high strength,high hardness,good corrosion resistance,high temperature oxidation resistance,etc.,which makes it become a research hot spot in the field of metallic materials in recent years.At present,the preparation of high entropy alloys is mainly based on traditional arc melting.However,the traditional preparation method has limitations such as composition bias and limited shape and size of formed parts,which restrict the preparation and industrialization of complex parts of high entropy alloys.Additive manufacturing technology has become one of the main methods for the preparation of complex metal parts due to its"discrete-stacking"forming characteristics.Selective laser melting is considered to have great potential for the preparation of high entropy alloys due to the advantages of fine structure,high precision and good overall mechanical properties of the formed parts.In this paper,the effects of laser power and scanning speed on the forming quality of high entropy alloy were investigated using Al_0.5CoCrFeNi high entropy alloy powder as raw material.The effect of process parameters on the densities and porosities of high entropy alloys was analyzed by measuring the forming quality in terms of densities and porosities.The forming process window of Al_0.5CoCrFeNi high entropy alloy was determined by establishing the relationship between bulk energy density,dense density and porosity.The microstructure and mechanical properties of the formed high entropy alloy within the process window were further analyzed to optimize the forming process parameters of the Al_0.5CoCrFeNi high entropy alloy.The heat treatment and electropulsing methods were used to post-treat the high entropy alloys prepared by selective laser melting,respectively.The microstructure and mechanical properties of the high entropy alloys post treated by the two methods were characterized and tested,and the mechanism of the post treatment methods on the microstructure and mechanical properties of the high entropy alloys was analyzed.The results show that the forming quality of selective laser melting of Al_0.5CoCrFeNi high entropy alloy is related to the bulk energy density.When the energy density is low,the defects of selective laser melting of Al_0.5CoCrFeNi high entropy alloy are mainly irregularly shaped holes.As the energy density increases,the powder melting degree increases,the spheroidization effect decreases,the number of irregularly shaped holes decreases,and the density of the formed high entropy alloy increases.When the energy density is too high,the evaporation of low melting point metal elements inside the high entropy alloy also leads to an increase in porosity.The defects inside the selective laser melting Al_0.5CoCrFeNi high entropy alloy at this stage are mainly round holes.The porosity of the formed high entropy alloy is less than 1%when the energy density is in the process window range of 46.29-104.16 J/mm³.The formed high entropy alloy has the highest densities of 7.67 g/cm³ at an energy density of 83.33 J/mm³.The formed high entropy alloy with epitaxial growth of columnar grains along the deposition direction in the process window exhibits a single FCC structure and achieves good metallurgical bonding.When the laser power is 200 W and the scanning speed is 0.8 m/s,the comprehensive mechanical properties of the selective laser melting Al_0.5CoCrFeNi high entropy alloy are better,and the tensile strength of the high entropy alloy is 856 MPa and the elongation is 20%at this time.The results of heat treatment showed that BCC andσ-phase precipitates were precipitated inside the high entropy alloy after heat treatment.With the increase of heat treatment temperature,the dislocation density of the high entropy alloy decreases and the content of BCC precipitated phase decreases and increases in size.Theσ-phase and dislocation network disappeared after heat treatment at 900°C.The high entropy alloy was partially recrystallized after heat treatment at 1100°C.The high entropy alloy was completely recrystallized and the grains grew coarsely after heat treatment at 1400°C.The high entropy alloy reached a tensile strength of 1419 MPa after heat treatment at 800°C for 4 h under the dual strengthening of dislocation network and precipitation phase.The weakening of the precipitation strengthening effect and recrystallization result in an elongation of 24%for the high entropy alloy after heat treatment at 1100°C.During the heat treatment,the dislocation network boundary can be used as a nucleation site for the precipitation phase and promote precipitation phase precipitation.After heat treatment at 800°C,the precipitated phase grows with longer holding time,the dislocation network boundary fractures,and the high entropy alloy softens.Studies of electropulsing have shown that electroplasticity significantly enhances the elongation of high entropy alloys.When the pulse voltage is 30 V,60 V,90 V and the pulse time is 10 s,the columnar crystal width of the high entropy alloy increases and the substructure size decreases with increasing pulse voltage.The strength of the high-entropy alloy increases and the elongation decreases.Electropulsing 90 V,20 s,the internal substructure of the high entropy alloy disappeared and BCC precipitation was precipitated.By extending the pulse time to 30 s,the size of BCC precipitation increases,while recrystallization occurs inside the high entropy alloy.Compared with the printed-state high entropy alloy,the pulsed current of 90 V and the percentage of grains that revert and recrystallize in the high-entropy alloy are enhanced by 45.68%and 14.24%,respectively,after 30 s.This leads to a decrease in the dislocation density from 0.84×10¹⁴m^-2 to 0.45×10¹⁴m^-2.Meanwhile,recrystallization weakens the weaving of the printed-state high entropy alloy in the<001>//X direction.