Effect of welding speed on mechanical properties of bobbin tool friction stir welded 6005A-T6 aluminum alloy joints

Effect of welding speed on mechanical properties of 6005A-T6 aluminum alloy joints welded by bobbin tool friction stir welding method was investigated under a constant rotation speed and heat input factor (Ratio of welding speed and rotation speed, WP). The results show that under a certain rotation speed, the tensile stress increased and then decreased with the increasing of welding speed. While under a certain WP, the tensile stress decreased continuously with the increasing of welding speed. Three typical fracture morphology were found in bobbin tool friction stir welded joints, namely, I type fracture at the heat affected zone (HAZ), Ⅱ type fracture at the weld nugget zone (WNZ) and Ⅲ type fracture at the thermal-mechanical affected zone (TMAZ). The macro morphology of I and Ⅱ type fracture were cup and cone morphology, serrated morphology, respectively. However, the microstructure indicated that both of them featured with toughness failure. Ⅲ type fracture was identified to be mixed fracture which consists of toughness failure and brittle failure. The tensile fracture position is not always occurred at the position with minimum microhardness. When the welding speed was lower than 1 000 mm/min, WP less than 1 is benefit to obtain joints with higher tensile strength and I type fracture. On the contrary, when welding speed was higher than 1 000 mm/min, WP more than 1 is benefit to obtain joints with higher tensile strength and I type fracture.