Progress in weldability research of high entropy alloys

High entropy alloys usually show good weldability. The weldability problems of high entropy alloys are segregation, cracks, and hardening or softening of weld, etc. When an Al_xCoCrFeNi alloy is welded, Al and Ni will segregate to the interdendritic region in the weld, but the degree of segregation is less than that of the base metal. When an Al_xCoCrCu_xFeNi alloy or a CoCrCu_xFeNi alloy is welded, Cu tends to segregate to the interdendritic region in the weld. Increasing the cooling rate of the welding process, such as with laser welding, is conducive to reducing the segregation in the weld. The segregation in the weld and the heat affected zone, especially the segregation of Cu, will lead to the generation of hot cracks. Hot cracking is the main form of cracking in high entropy alloys joints. Welding will lead to changes in the hardness of the weld. The main factors affecting the hardness change are the grain sizes and the precipitations. With laser welding, if the base metal is cold rolled, the hardness of the weld will decrease. If the base metal is hot rolled and annealed or cast, the hardness of the weld will increase. With TIG welding, the hardness of the weld is usually lower than that of the base metal, unless the grain of the base metal is particularly coarse before welding. With friction stir welding, recrystallization and grain refinement occur in the stir zone, and the hardness of the stir zone will be significantly improved no matter the original base metal is cold rolled or cast.