Microstructures And Properties Of Laser Welded CoCrFeNi Base High Entropy Alloy Joints With Preplaced Powders

High entropy alloys are considered as one of the three theoretical breakthroughs in metal alloying in recent years.Due to its high entropy value,it has the characteristics of simple structure and excellent performance,and has become one of the hot research fields in new metal materials.The welding property has definite practical value for expanding the application of high entropy alloy.Especially laser welding,because of its concentrated heat input,the narrow heat affected zone,easy to get high quality weld.In this paper,CoCrFeNi high entropy alloy,which is the most common high entropy alloy,was selected as the base metal for welding,and CoCrFeNiMn high entropy alloy powder was used for laser welding of preplaced powder.In addition,Mn,Al and Mo elements were added to the base metal to adjust the composition and structure of the high entropy alloy,and the welding speed was adjusted to study the influence of preplaced powder on microstructures and properties of the welded joint of different high entropy alloy in CoCrFeNi base system.The main conclusions of this study are as follows:(1)The results of weld forming analysis showed that at the welding speed of 1 m/min to 3 m/min,the CoCrFeNi high entropy alloy sheet can be welded perfectly.However,due to the relatively low saturated vapor pressure of Al and Mn elements,as a result,the forming of CoCrFeNiMn high entropy alloy and AlxCoCrFeNi high entropy alloy is easily affected by the welding parameters,and the phenomenon of "negative residual height" tends to occur during low-speed welding.However,CoCrFeNiMox high entropy alloy has a good forming overall,without obvious defects.(2)The microstructure analysis of the weld shows that,compared with the base metal,the grain size in the weld of CoCrFeNi high entropy alloy is smaller,and the faster the welding speed is,the smaller the weld grain size is.In addition,the addition of preplaced powder can also improve the aspect ratio of the grains in the center of the weld to a certain extent,making the generated grains tend to be equiaxed.Elemental segregation appears to a certain extent in AlxCoCrF eNi and CoCrF eNiMox high entropy alloy welds.When the welding speed is faster,the weld cooling is faster,and the segregation phenomenon of high entropy CoCrFeNi alloy will be enhanced to varying degrees.(3)Under the premise of complete penetration of the weld,the smaller the welding heat input is,the better the mechanical properties of the weld will be.In CoCrFeNiMn high entropy alloy,the base metal has superior strength and elongation compared with the weld,and the smaller the welding speed is,the worse the mechanical properties of the weld will be.The weld has obvious secondary work hardening during the tensile process.Under the same welding parameters,adding preplaced powder can effectively improve the yield strength of the weld.In AlxCoCrFeNi high entropy alloy,the strength of the base material is higher than that of CoCrFeNi high entropy alloy,but the elongation decreases obviously because the presence of Al element promotes the lattice distortion,which is conducive to the formation of hard and brittle BCC phase.When the Al content is low(x=0.3),the yield strength of the welded joint with preplaced powder is significantly improved compared with that without preplaced powder.When the Al content is increased(x=0.6),the yield strength of the welded joint with preplaced powder can also be improved at a lower welding speed.In addition,the tensile strength of the two welds is similar.In CoCrFeNiMox high entropy alloy,when Mo content is less(x=0.1),the welded j oint has a higher tensile strength and elongation,but a lower yield strength.At a lower welding speed,the addition of preset powder has an obvious effect on improving the yield strength of the weld.However,when the content of Mo in the base metal increases(x=0.2),the mechanical properties of the weld are very close to the base metal at a higher welding speed,and the weld elongation of the preset powder laser welded at a welding speed of 3 m/min is slightly higher than that of the same base metal.(4)The main morphology of CoCrFeNi high entropy alloy base material and tensile fracture of the weld used in this experiment showed small dimples,and the dimples size was mainly determined by the welding speed.The larger the welding speed is,the smaller the welding heat input will be,the smaller the dimples size and the higher the density at the tensile fracture.