Study On Friction Stir Welding Of 2024 Aluminum Alloy

Friction stir welding is a novel joining technology. During friction stir welding materials can be joined together without melting; therefore defects associated with fusion such as porosity and solidification cracking can be avoided and high quality joints can be produced efficiently. Furthermore, under proper processing parameters, joints with low-distortion and low residual stresses can be made. With these advantages, soon after its invention friction stir welding was widely applied in many industries such as aerospace and automotive in U.S.A, shipbuilding in north Europe and high-speed train in Japan. In other fields such as electric power, architecture, energy etc., friction stir welding also shows great application potential.2024 aluminum alloy has high strength and good mechanical working properties; and has been widely used in many industries, such as aerospace, automotive and shipbuilding. However, due to its hot crack sensitivity, 2024 aluminum alloy is often classed as hard-to-weld material by traditional fusion welding. Since friction stir welding is a solid joining technology, it's suitable for welding 2024 aluminum alloys. Therefore 2024 aluminum alloy was chosen for this study.In this paper, with an original designed tool and selected welding parameters, 2024-T4 aluminum alloy sheets were friction stir welded successfully. A series of weld experiments were performed; the microstructure and the mechanical properties of the joints were analyzed. The influences of the welding parameters upon the quality of the joints were studied. The optimized welding parameters window was obtained in which high quality joints can be made and the tensile strength of the optimized joints were more than 80% of that of parent metal.The residual stresses of FSW joints have been investigated by using the Hole-drilling method. The results shown that high longitudinal tensile residual stresses were asymmetrically distributed at the different sides of the weld line: high at the advancing side and relatively low at the retreating side; outside the tool shoulder direct affected zone the longitudinal residual stresses decreased rapidly and became compressive residual stresses away from the weld line; the peak of the longitudinal residual stresses was lower than that of fusion welded joints. In this study, it also found that the peak of the longitudinal tensile residual stresses increased with higher welding speed.Based on these experiments, a finite element model was developed for friction stir welding, which contains both the effects of the thermal load and the tool mechanical loads. According to the proposed model, the torque of the tool, the power input together with the temperature field, and the residual stresses and distortion could be predicted. The results shown that,due to the effects of the pressure of the tool shoulder the longitudinal tensile residual stresses become smaller; The differences in friction forces between the advancing side and the retreating side are responsible for the asymmetric distribution of the residual stresses.