Increased Ʃ3 boundaries: Effects of friction stir and post heating on pure copper

This research explored the mechanisms of increased electrical conductivity in pure copper (Cu) which was first joined with aluminum via micro friction stir blind riveted and then exposed to post heating. The increased Ʃ3 boundaries with reduced random grain boundaries are the main reason contributing for this improvement. In this research, microstructural characterization, resistance measurement, and hardness testing were performed at the stir zone (SZ) and thermomechanical affected zone (TMAZ) of Cu in as-fabricated and heat-treated joints. A ratio between the fraction of Ʃ3 boundaries to total fraction of Ʃ9 and Ʃ27 boundaries was introduced to confirm that the increases of Ʃ3 boundaries in TMAZ at 300 °C were caused by the twinning mechanism whereas were through the regeneration mechanism in the SZ (heated at both 300 °C and 500 °C) and TMAZ (heated at 500 °C). Although electrical conductivity was improved by this grain boundary engineering approach, the high-temperature heating caused a dramatic reduction in microhardness due to the substantial strain relief and grain growth.