Achieving high shear strength and bonding accuracy for diffusion bonding joint of TC4 alloy at low temperature based on SMAT process

In order to lower the temperature of the diffusion bond (DB), Ti–6Al–4V alloy sheets with surfaces in different grain-size grades, micron (∼1.89 μm), submicron (∼500 nm), ultra-fine (134.4 nm), and nanoscale (28.4 nm), were fabricated through a different time of the surface mechanical attrition treatment (SMAT). It is confirmed that the quality of the DB joint increased with the grain size decreased on the bonding surface, and the mild SMAT process can lower the temperature of the DB process without deteriorating the microstructure and the mechanical strength. A remarkably high shear strength (∼852.2 MPa) joint, with a reduction in thickness approximating zero, of Ti–6Al–4V alloy was obtained at a low temperature of 800 °C under the 5 MPa for 1.5 h DB process after 3 h SMAT process, 15.24 % higher than the joint bonded in traditional method at 920 °C. The higher shear strength mainly contributed to the grain refinement, high dislocation density, and the introduced second phase on the interface. The nanograins layer promotes diffusion by decreasing the level of motivation of thermal diffusion, introducing the high gradient stored energy, providing more fast atomic diffusion channels, and lowering the resistance to deformation leading to more intimate contact.