Mechanistic role of a Cu interlayer in dissimilar ultrasonic welding of Ti to low-carbon steel

Unlike conventional welding methods, dissimilar ultrasonic welding (USW) of steels to titanium alloys can avoid forming brittle intermetallic compounds (IMCs) and the associated loss of mechanical properties. However, insufficient formability hinders the proper bonding of these two materials at the interface. In order to solve this problem, a copper interlayer was used. St12/Ti lap joints were fabricated using a Cu interlayer under the following conditions: 7 bar pressure, 2 s welding time, and 1 kW welding power. The microstructural evolution and mechanical performance of the joints were investigated. The deformation induced by USW resulted in a bond density of 91.42 % through severe plastic deformation. Microstructural examinations revealed deformation of surface asperities at the St12/Cu interface, along with a similar deformation and the formation of IMCs at the Cu/Ti interface. Consequently, the bonding mechanism at the St12/Cu interface is mechanical interlocking, whereas it involves both interdiffusion and mechanical interlocking at the Cu/Ti interface. The IMC layer at the Cu/Ti interface is primarily Cu 2 Ti, with a maximum thickness of 11 μm. Diffusion analysis using Fick's second law revealed the enhancement of Ti interdiffusion into the Cu by thermo-mechanical coupling through localized heating and defect-assisted pathways. Electron backscatter diffraction revealed evidence of recrystallization and substructure formation in Cu and St12, whereas Ti experienced relatively low deformation. Lap shear testing showed strong bonding, with fracture occurring at the brittle Cu/Ti interface.