| High entropy alloys have excellent properties such as high strength,high hardness,excellent thermal stability,fatigue resistance,corrosion resistance and oxidation resistance because of their single solid solution structure.It has important application potential in aerospace,mold,cutting tools,corrosion-resistant coatings,refractories and other industries.The traditional preparation technology of high entropy alloy mainly focuses on vacuum melting and casting,powder metallurgy,mechanical alloying and so on.The size of the alloy prepared by this kind of process is small and the shape is simple.There is no good solution to the problems of porosity,inclusion and composition segregation in the manufacture of industrial complex parts.Selective Laser Melting(SLM)is a metal increasing material manufacturing process with the highest forming accuracy and excellent mechanical properties at present.The manufacture of complex structure,integral structure and personalized structure metal parts by SLM has been widely used.However,the research of SLM forming high entropy alloy is still in the initial stage.CoCrFeNiMn high entropy alloy is the most mature alloy system with stable phase structure,high strength,hardness,good corrosion resistance,ductility and machinability.In this paper,CoCrFeNiMn high entropy alloying powder was used as the research material to study the SLM forming process window.the effect of process parameters on the phase composition,microstructure and mechanical properties of CoCrFeNiMn high entropy alloy formed by SLM was studied systematically.It lays a theoretical and technical foundation for SLM forming complex high entropy alloy parts.The main findings and conclusions are as follows:(1)The influence of laser process parameters on the surface quality of cladding channel and single layer was studied.The width of cladding channel increases with the increase of laser power or the decrease of scanning speed,and the height decreases with the increase of laser power or the decrease of scanning speed.In the range of scanning power 200W-250 W and scanning speed 1100 mm/s-1300 mm/s,the surface of the cladding channel is smooth and continuous spheroidizing,the width is about 100 ?m,and the height is about 26 ?m.The moderate temperature and solidification time of molten pool in the process range are beneficial to the spread and reduction of spheroidization of cladding path.The surface roughness of single layer increases with the increase of scanning spacing,which is mainly due to the separation of cladding channels and the increase of surface roughness.The surface quality of single layer is the best when the scanning spacing is 0.07 mm and the roughness Ra is 6.107 ?m.(2)The process window of SLM forming CoCrFeNiMn high entropy alloy was optimized.The highest density of the sample was 98.69%(7.95 ±0.028g/cm3)under the conditions of powder layer thickness of 30 ?m,laser scanning power of 225 W,scanning speed 1300 mm/s and scanning spacing 0.07 mm.The low density at low power is mainly due to the low energy density at low power,which can not fully melt the powder so that there are cracks,pores and unmelted metal particle defects in the alloy.The low density at high power is mainly due to the spheroidization,splash and surface warping caused by the high energy density at high power,which affects the density.(3)The results of XRD show that the structure of CoCrFeNiMn high entropy alloy formed by SLM is face-centered cubic(FCC)Fe2Ni2.In addition,a small amount of Cr-rich ? phase with body-centered cubic(BCC)is precipitated from some CoCrFeNiMn high entropy alloys.The phase transformation of the alloy is mainly caused by the high temperature of the molten pool.The microstructure is anisotropic in different forming directions: Columnar and cellular crystals are dominant along the laser scanning plane(XY plane).The length of columnar crystals is about 30 ?m,and the intergranular spacing is less than 1 ?m.In the direction of forming height(Z plane),the shape of "fish scale" is presented.The maximum length of columnar crystal is about 100 ?m.A single grain passes through the multilayer cladding channel and shows epitaxial growth characteristics.The EDS results show that the elements on each forming surface of the high entropy alloy are uniformly distributed and there is no obvious element segregation.(4)Under the optimized process parameters,the mechanical properties of CoCrFeNiMn high entropy alloy formed by SLM are better than those of as-cast CoCrFeNiMn high entropy alloy,which is close to that of forged alloy.The yield strength of 418.62 ±27.72 MPa was 126.52% higher than that of as-cast.The tensile strength of 641.65 ±5.76 MPa was 43.45% higher than that of as-cast alloy.The hardness of 284.07 ±5.33 HV is 49.51% higher than that of as-cast CoCrFeNiMn high entropy alloy.The elongation at break of 16.70 ±1.0% was significantly lower than that of as-cast CoCrFeNiMn high entropy alloy.The tensile fracture is characterized by brittle fracture and ductile fracture,mainly brittle fracture.The grain size of CoCrFeNiMn high entropy alloy is fine and compact and there is no composition segregation by SLM forming.Fine grain strengthening can improve the strength,hardness and other properties.Under the stress loading of holes,cracks and other defects in the alloy matrix,it is easy to expand the fracture to form a fluvial cleavage pattern,which leads to a low elongation at break. |
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