Achieving good strength-ductility synergy of friction stir welded Cu joint by using large load with extremely low welding speed and rotation rate

Cu plates with 2 mm in thickness were joined by large load friction stir welding with extremely low welding speed and rotation rate. Heat-affect zone was successfully eliminated for the improved thermal cycle. The grain refinement in the stir zone is attributed to the combination of continuous dynamic recrystallization, discontinuous dynamic recrystallization, dynamic recovery as well as twinning-induce dynamic recrystallization. The stir zone exhibited ultra-refined grains with low dislocation density and a high fraction of high angle boundaries. Abundant twin boundaries were introduced into the stir zone as well. As a result, the stir zone showed a superior strength-ductility synergy with a strength of 548 MPa and a uniform elongation of 36%. This work also provides an effective strategy to enhance the strength of friction stir welded Cu joint without ductility loss.