Effect of an improved pin design on weld formability and mechanical properties of adjustable-gap bobbin-tool friction stir welded Al-Cu aluminum alloy joints

An improved tool was designed to enhance weld formation for adjustable-gap bobbin-tool friction stir welding. No macro defects are observed as the welding speed increases from 120 to 240 mm/min. Microstructure analysis shows the maximum density of geometrically-necessary dislocations reaches 2.52 ×10¹³m^-2 presenting at the stir zone (SZ). The Guinier-Preston-Bagaryatsky zones in heat affected zone (HAZ) and SZ are partially dissolved, and the relative volume fraction of S and θ phases in SZ is larger than that in HAZ. The hardness of the joints exhibits a W-shaped distribution. The minimum hardness is obtained at the HAZ. As the welding speed increases, the tensile strength initially increases up to 400 MPa at 180 mm/min, and then decreases. Poor mechanical properties at 60 mm/min and 300 mm/min are related to void defect.