<p>Surface modification prior to diffusion bonding to enhance the atomic diffusion flux and to rupture the oxide layer is an area of wide interest. Friction stir processing (FSP) is utilized as a surface modification technique to facilitate the diffusion bonding of copper in an argon atmosphere. FSP results in significant grain refinement and accumulation of short circuit defects such as grain boundaries and dislocations. Diffusion bonding experiments were conducted by varying the bonding temperature and faying surface conditions. Grain growth of fine grains produced by FSP near the interface and grain boundary migration were observed to be the major mechanisms by which bonding has been achieved. The grain coarsening of the FSP processed base metal is restricted during thermal annealing owing to the thermal resistance provided by the high density of grain boundaries, and thus, a major drawback of conventional diffusion bonding is alleviated to a certain extent. The fine microstructure leads to micro-creep of asperities and plastic flow of the material in addition to enhanced atomic diffusion resulting in void closure and elimination of bond line. The microhardness survey shows enhanced microhardness at the joint interface owing to the plastic deformation and further grain refinement, which strengthens the joint.</p>