Heterogeneity of mechanical properties of friction stir welded joints of 1050-H24 aluminum alloy

Abstract

Many studies on the microstructural characteristics and mechanical properties of joints have indicated that a softened region occurs in the friction stir welded joints of heat-treatable aluminum alloys [1–5] and strainhardened aluminum alloys [6–10]. In order to solve the joint softening problems of heat-treatable aluminum alloys, some researchers adopted postweld aging treatment techniques and obtained better compensating results [1–3]. Up to now, however, no better method has been found to improve the deterioration in mechanical properties of friction stir welded joints of the strainhardened aluminum alloys. In principle, any joint can be considered to be composed of finite thin-layers in the direction of thickness, and any thin-layer is different from the other ones in mechanical properties because they experience different thermo-mechanical actions during friction stir welding (FSW). If the mechanical properties of some weak thin-layers of the joint can be improved, the mechanical properties of the entire joint will be raised. However, the real difference in mechanical properties between thinner-layers is still unknown. This letter aims to demonstrate this difference so as to obtain useful information for improving the mechanical properties of the joints of strain-hardened aluminum alloys. The base material used in this study was 1050H24 aluminum alloy which was strain-hardened and then partially annealed. The dimensions of the rectangular welding samples were 300 mm long, 80 mm wide and 5 mm thick. The samples were longitudinally butt-welded using an FSW machine. The designated welding tool size and welding parameters are listed in Table I. For the sake of convenience, the revolutionary pitch is expressed as d in this letter. After welding, the joints were cross-sectioned perpendicular to the welding direction for metallographic analyses and mechanical property tests using an electricaldischarge cutting machine. The cross-sections of the metallographic specimens were polished with alumina suspension, etched with Keller’s reagent, and observed by optical microscopy. In order to examine the heterogeneity of the mechanical properties of the joints, each tensile specimen was prepared in two steps. First, the joints were transversely cut into primary specimens of 100 mm long, 12.5 mm wide and 5 mm thick. Finally, every primary specimen was cut perpendicular to the direction of thickness into three tensile specimens of 1.4 mm thick, and they were marked by upper, middle and lower parts of the joint. Prior to tensile tests, Vickers hardness profiles were measured under the load of 0.98 N for 10 s along the centerlines of the tensile specimens using an automatic micro-hardness tester, and the Vickers indents with a spacing of 1 mm were used to determine the fracture locations of the component parts of the joints. The tensile tests were carried out at room temperature at a crosshead speed of 1 mm/min using a screw-driven test machine, and the tensile properties of each component part were evaluated through three tensile specimens. Fig. 1 shows the tensile test results of the different component parts of the joints. The ultimate strength of the upper part of the same joint is the highest, while that of the middle part is the lowest. As the revolutionary pitch increases, the ultimate strength of the upper part increases, while that of the middle or lower part increases to the maximum at the revolutionary pitch of 0.27 mm/r (see Fig. 1a). In the three component parts of the same joint, the middle part always possesses the lowest proof strength. When the revolutionary pitch is smaller than 0.27 mm/r, the proof strength of the lower part is higher than that of the upper part; when the revolutionary pitch is greater than 0.27 mm/r, the proof strength of the lower part becomes lower than that of the upper part (see Fig. 1b). The elongation of the upper part is always the highest, while that of the lower part is always the lowest. As the revolutionary pitch increases, the elongation of the upper part increases and that of the middle or lower part decreases (see Fig. 1c). These results clearly indicate that the different component parts of the joint possess different mechanical properties, and the welding parameters have different effects on the mechanical properties of these different component parts. In the three component parts of the joint, the middle part is weakest and the upper part is strongest in mechanical properties. This is a very interesting and useful information for improving the mechanical properties of the entire joint of 1050-H24 aluminum alloy because a two-side welding process can be adopted to make one joint possess two “upper parts”.