Research On Process And Mechanism Of Selr-reacting Friction Stir Welding For 6061 Aluminum Alloy

Self-reacting friction stir welding, as a new type of friction stir welding, not only retains the advantages of solid-state joining, but also solves problems of conventional friction stir welding like inefficiency, high demand for fixtures, root defect of joint. So it has a high value of research and application, and gets more and more attention. This paper designed the rotational tool, and self-reacting friction stir welding on 5mm thick 6061-T6 Al alloy was carried out. The influence of rotational speed, welding speed and shoulders’ spacing on microstructure and properties of joints had been analyzed. The temperature field of self-reacting friction stir welding had been simulated and demonstrated by experiments.The welding experimental results showed that, within certain extent, welded joints had a good appearance, and no defects on weld surface; dumbbell-shaped cross section of joint were divided into four different areas: nugget zone, thermo-mechanically affected zone, heat-affected zone and base metal, among which, nugget zone contained top zone, middle zone, bottom zone and band pattern; the micro-hardness of welded joints show a “W” distribution, base metal had the highest micro-hardness, heat-affected zone had the lowest, while nugget zone had a higher one than it, and thermo-mechanically affected zone was between them, there was no significantly different along the thickness direction; with the decrease of rotational speed, and the increase of welding speed and shoulders’ space, the softened zone of joints was smaller, and the lowest micro-hardness increased; when the heat input was low, defects made a poor tensile property, which showed a mixed fracture; when rotational speed was 400r/min, welding speed was 350mm/min, shoulders’ spacing 4.9mm, tensile strength and elongation of welded joint had a maximum level of 221 MPa and 9.4%, which were 75% and 72% of base metal, respectively; with the increase of heat input, tensile property deteriorated; joints had no root defects which appeared in conventional friction stir welding, excepting the joints with defects, all welded joints showed a good 180 degree bend properties; both nugget zone and heat-affected zone had a higher impact energy than base metal, and it changed with different parameters.The simulated results showed that, the peak temperature of self-reacting friction stir welding had been consistently lower than melting point of base metal, which kept consistency with solid-state joining; with the increase of welding speed, decrease of rotational speed, heat input and the peak temperature decreased; temperature field in cross section of self-reacting friction stir welding joint showed a dumbbell-shape, which kept consistency with cross section of welded joint; the simulated results agreed well with the experiment.