Robotic friction stir welding of AA5754 aluminum alloy sheets at different initial temper states

Robotic friction stir welding experiments were performed on AA5754 aluminium alloy sheets, 2.5 mm in thickness, in two different temper states (H111 and O-annealed). A six axes robot with a hybrid structure, characterised by an arm with parallel kinematics and a roll-pitch-roll wrist with serial kinematics, was used. The effect of the process parameters on the macro- and micro-mechanical properties and microstructure of joints was widely analysed. It was shown that, under the same process condition, the mechanical properties of the joints are strongly influenced by the initial temper state of the alloy. In particular, as AA5754-H111 is welded, the ultimate tensile strength is not significantly affected by the process parameters whilst the ultimate elongation significantly depends on the welding speed. In AA5754-O, both ultimate values of tensile strength and elongation are affected by the welding speed whilst a negligible effect of the rotational speed can be observed. Irrespective of the welding parameters, the H111 temper state leads to mechanical properties higher than those given by the O-annealed state. An investigation has been also carried out in order to evaluate the micro-hardness profiles and microstructure of the FSWed joints in order to understand the mechanisms operating during robotic friction stir welding.