Effect of the welding speed on the microstructure and the mechanical properties of robotic friction stir welded AA7B04 aluminum alloy

For weld defects such as holes, debasement of the joint properties and large deformations easily appear in fusion welding of the AA7B04 aluminum alloy, and the use of mechanical connections such as riveting increases the weight of the connector. Herein, a 2-mm AA7B04 aluminum alloy was friction stir welded using an industrial KUKA Titan Robot, with a friction stir welding end effector mounted onto the robot. The F_x, F_y, and F_z forces of the tool during welding were recorded, and the resultant mechanical strength and microstructure of the joints were studied. The results show that welding speed imposes a great influence on F_z. The microstructure and mechanical properties at different welding speeds of a friction stir welded joint of an AA7B04 aluminum alloy sheet were investigated via optical microscope observation, transmission electron microscope observation, a tensile test, a three-point bending test, and a hardness test. The results show that the maximum tensile strength of the joint is 447 MPa for a welding speed of 100 mm·min^-1, equivalent to 80% of the parent metal. No crack is observed in the joints when bending at 180°. W-shape hardness distribution is observed on the cross-section of all joints; moreover, owing to the lowest hardness in the heat affected zone and the junction of the welding area, different welding speeds lead to different welding thermal cycles, and the hardness of joint increases with the increase in the welding speed. Dynamic recrystallization occurs in the nugget zone, and fine equiaxial grains are produced. The grains of the advancing and retreating sides of the thermo-mechanically affected zone are obviously deformed. The η' phase can be observed in the heat affected zone of the advancing side, and η' phase particles can also be observed in the heat affected zone of the retreating side. Owing to the higher temperature, the larger η phase can also be found in the heat affected zone of the retreating side.