Microstructural evolution of pure copper during friction-stir welding

The microstructural evolution of pure copper during friction-stir welding was found to be principally influenced by welding temperature. At temperatures below ~0.5 T-m (where T-m is melting point), the microstructure was shown to be essentially determined by continuous recrystallization, leading to significant grain refinement and related material strengthening in the stir zone. In contrast, grain structure development at temperatures above ~0.5 T-m was dominated by discontinuous recrystallization producing a relatively coarse grain structure in the stir zone and giving rise to material softening.