Microstructure and mechanical properties of laser welded TC4 titanium alloy/304 stainless steel joint with (CoCrFeNi)100-xCux high-entropy alloy interlayer

Abstract The feasibility of (CoCrFeNi)100-xCux high-entropy alloy filler metal for laser welding between TC4 titanium alloy and 304 stainless steel is investigated. Reliable metallurgical bonding was achieved between weld zone and 304 stainless steel. The weld zone consisted of ductile FCC solid solution phase and Cu-rich phase which segregated at the grain boundary of FCC phases. The Ti/Cu transition zone is composed of Ti-rich layer near TC4 titanium alloy substrate and Ti-depleted layer adjacent to the weld zone. Ti-rich IMCs and β-Ti phases formed in Ti-rich layer, and Ti(Fe,Co,Cr)2 and Ti(Fe,Ni)2 phases formed in Ti-depleted layer. With increase of Cu content in filler metal, the segregated Cu-rich phase in weld zone increased, and Ti(Fe,Co,Cr)2 and Ti(Fe,Ni)2 phases decreased along with the increase of (Fe,Cr)-rich phase and segregated Cu-rich phase. The tensile strength of the joint with CoCrFeNi filler metal was 158 MPa, and it decreased with increase of Cu in filler metal until the atomic percent of Cu reached 27.27 at. %, but raised to 161 MPa while using (CoCrFeNi)66.67Cu33.33 filler metal. All joints fractured through the transition zone, and the maximum microhardness of the transition zone reached 803 HV, higher than those of substrates and weld zone. The formation of Ti(Fe,Co,Cr)2 and Ti(Fe,Ni)2 phases resulted in the limited tensile strength and high microhardness in the transition zone.