Bonding copper in the solid state requires either a high temperature, large deformation, or high vacuum. In the present study, copper was butt-bonded using a K-shaped groove at 298 K under no bonding pressure through copper electrodeposition. The initial gap between the faying surfaces of the copper rods were gradually filled with the electrodeposition of copper from the center to the periphery. No significant defects were observed in the bond layer, and a high joint strength of approximately 240 MPa was obtained. There were three regions in the bond layer: fine columnar grain region, ultrafine grain region, and recrystallized grain region. The size of the ultrafine grains was several tens of nanometers, and the microhardness was larger than that of the base metal. As electrodeposition progressed, there were insufficient additives for electrodeposition near the center of the bond layer, and recrystallization occurred owing to self-annealing.

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