Interface Grain Boundary (IGB) migration is a critical phenomenon in solid-state bonding, yet its direct effect on joint mechanical performance remains unclear due to its coexistence with processes such as dynamic recrystallization and oxide evolution. This study isolates the influence of IGB migration by minimizing other contributing factors. Forge welding and diffusion bonding were applied to high-purity nickel to fabricate joints with and without IGB migration while maintaining void- and impurity-free interfaces. Microstructural evolution was characterized using multiple analytical techniques. Furthermore, a Crystal Plasticity Finite Element (CPFE) model, combined with in situ tensile testing, was employed to clarify deformation mechanisms near the bond line. Results reveal that, in the absence of oxides and voids, IGB migration exerts minimal influence on overall mechanical behavior. Both experimental observations and CPFE simulations indicate that local stress concentrations are alleviated through grain orientation differences and reorientation during deformation, thereby reducing the impact of unmigrated boundaries. A four-grain CPFE model further illustrates the synergistic effects of IGB migration and grain reorientation when examined separately. These findings enhance understanding of the intrinsic role of IGB migration and provide guidance for designing high-performance bonded materials.