Microstructure and mechanical characterization of diffusion bonded CoCrCuFeNi HEA/TC4 joints using a Ni interlayer

Abstract

Abstract Aimed at the weak connection strength of CoCrCuFeNi high entropy alloy (HEA)/TC4 welding joint, Ni foil was selected as an intermedia layer for CoCrCuFeNi HEA/TC4 diffusion bonding, and sound joints were achieved. Concurrently, an investigation was conducted on the customary microstructure, the impact of temperature on both the microstructure and properties, as well as the interfacial formation mechanism of the joints. The results indicated that the CoCrCuFeNi HEA/TC4 diffusion bonded joint, with a Ni intermediate layer, exhibits a characteristic microstructure. This microstructure consists of several distinct layers, namely the HEA diffusion layer, Ni(s,s) solid solution layer, TiNi3 layer, TiNi layer, Ti2Ni layer, and TC4 diffusion layer. The formation of the interfacial layers primarily occurred through the process of mutual diffusion between elements and the Ni-Ti diffusion reaction. The highest shear strength of 230 MPa is observed in the joint formed at a temperature of 792 ℃. This joint exhibits an interfacial structure characterized by a multilayer composition consisting of 'HEA/HEA diffusion layer/Ni(s,s)/TiNi3/TiNi/TC4 diffusion layer/TC4'. The primary location of fracture initiation is observed within the TiNi3 layer, subsequently propagating through several interfacial layers. When the temperature of diffusion welding exceeds 842 ℃, it results in the formation of a continuous layer of Ti2Ni compound. This compound layer has the effect of reducing the shear strength of the joint. Consequently, the joint is prone to failure at this brittle layer once it is formed.