Understanding the effects of interfacial micro-structures/-defects such as voids, grain boundaries (GB) and partial recrystallization (Rx) is critical to achieving superior mechanical properties for safety-critical parts. These features may appear during advanced manufacturing processes such as sintering, additive manufacturing (AM), and diffusion bonding (DB). However, the generations and evolutions of these micro-structures/-defects are simultaneous processes in manufacturing. Their complex interactions during deformation prevent the in-depth understanding of the effects of individual microstructures. Here, we offer the unrevealed micro-mechanisms of GB migration on local deformation by using integrated in-situ EBSD/FSE and crystal plasticity modelling. We demonstrated that the migrated GB does not change activated slip systems but can avoid the hardening around the interface voids caused by GB-multi-slip interaction induced geometrically necessary dislocation (GND), which alleviates stress concentrations. The stress mitigation caused by GB migration is almost the same with that by void closure under the exampled DB thermal mechanical process. This understanding sheds light on the mechanistic link between GND hardening, GB migration, and the corresponding tensile behaviours and opens a new avenue for achieving superior mechanical properties not only for DB parts, but also for metallic parts with micro-defects such as sintered and additive manufactured components.