<p>Elucidating the voids emerging mechanism of diffusion bonding (DB) joints during thermal deformation has a great significance for the manufacturing of titanium alloy DB components. The present study investigates the relationship between thermal deformation behavior, voids emerging and fracturing mechanism at different high temperatures by thermal tensile experiments and microstructure analysis at Ti6Al4V DB interface. The results reveal a strong correlation between fracture mode and deformation temperature. An increase in temperature and a decrease in strain rate collectively promote dynamic recrystallization and the alteration of the deformation mechanism during high-temperature deformation, where dislocation slip will transform into grain boundary migration gradually. Consequently, the non-uniform distribution of dislocations at the interface and within base material results in inconsistent dislocation slip at lower temperatures. At higher temperatures, the deformation mechanism changes, where grains at both locations grow concurrently. Deformation is more predominantly controlled by grain boundary migration, and the locations where the voids appear also change correspondingly. The mechanism of interfacial voids emerging is significantly influenced by the grain orientation difference between adjacent grains. Subsequently, the propagation of cracks inside the grain is also dependent on the grain orientation. It is easier for voids to emerge from α-Ti {0 0 0 1} orientation at the interface. The primary driving force for void nucleation arises predominantly from dislocation accumulation and stress concentration, which results from mismatched behavior between grains of differing orientations or phases. The present study offers potential prospects for guiding the post-DB deformation of critical aerospace components.</p>