Investigation on the Effects of Welding Speed on Bobbin Tool Friction Stir Welding of 2219 Aluminum Alloy

Abstract

The microstructure and mechanical properties of bobbin tool friction stir welded joints for 2219-T87 aluminum alloy using different welding speeds were investigated based on the experimental results together with a thermo-mechanical model developed. The stir zone created in the joint shows a dumbbell shape. As welding speed increases, both the width and grain size of stir zone decrease. The symmetrical hardness distribution forms across the joint thickness, while the asymmetric hardness is measured along the width of the stir zone. The hardness of the stir zone on the retreating side is lower than that on the advancing side. The hardness profile presents a W-shape, with the minimum values measured increasing with welding speed. The tensile residual stresses increase gradually towards the center of the joint from the outer surface, presenting a symmetrical feature as well. As welding speed increases, both the tensile strength and elongation show a maximum value 70% of joint efficiency. Tensile fracture occurs in the stir zone and shows a symmetrical surface with three different layer fracture modes identified. In layers I and III, the fracture surface presents 45° direction, while fracture propagates along the S-line in layer II.