Study On Microstructures And Mechanical Properties Of 6061-T6 Aluminum Alloy Joints Welded By Stationary Shoulder Friction Stir Welding Technology

5mm thick 6061-T6 aluminum alloy butt joints were successfully obtained by stationary shoulder friction stir welding process with the use of self-developed tool system. Based on experimental research, influences of the change of the welding process parameters on welding surfaces, microstructures, hardness profiles and tensile properties of the obtained joints were studied in detail. The results provided an important basis for further study of the subsequent SSFSW technology.The obtained 6061-T6 aluminum alloy joints by SSFSW process have smooth surfaces and nice-looking appearances. Compared with conventional FSW, SSFSW joints have nearly no flash defect and the phenomenon of the weld thickness reduction is almost eliminated. The SSFSW nugget zone has ??bowl-like?? shapes, which is quite different from that of conventional FSW joint.6061-T6 aluminum alloy SSFSW joint consists of four parts, that is welding nugget zone(WNZ), thermal mechanically affected zone(TMAZ), heat affected zone(HAZ) and base metal(BM). SSFSW joint has fairly narrow thermal mechanically affected zone(TMAZ) and heat affected zone(HAZ) and the microstructures of weld region are rather symmetrical and homogeneous. The grain size in the central part of WNZ center line is larger than that of the upper and lower part. The grain size of the upper part is a little smaller than that of the central part, but that of the lower part is rather fine. The grains near AS in the same thickness position of WNZ have obvious deformation and elongated shape, but the size and shape of those in the central part and near RS are similar, which present equiaxed shape and have no obvious deformation.At the same rotation speed, with the increase of welding speed, the interface between the TMAZ and WNZ increasingly blurs and the plastic deformation degree of materials becomes smaller. In addition, grains in WNZ become smaller. At the same welding speed, with the increase of rotation speed, grains in WNZ become larger.The hardness profiles of 6061-T6 aluminum alloy SSFSW joints present “W” shape. Hardness from near the WNZ center to HAZ on both sides is gradually reduced. When it reaches the minimum hardness of HAZ, then with the increase of distance from the centerline of the weld, hardness value gradually restores to the original level of BM. At the same rotation speed, with the increase of welding speed, the hardness value of WNZ basically shows an increasing trend and the width of softened region decreases. At the same welding speed, the WNZ hardnesses are higher at high tool rotation speeds than at low tool rotation speeds and the widths of softened region are also larger.Tensile strength of 6061-T6 aluminum alloy SSFSW joints shows an increasing trend with the increase of welding speed, while the change is not obvious with the increase of rotation speed, which remains in a relatively stable level. The elongation of joints with the change of welding speed and rotation speed is not regular, showing a trend of fluctuation. When the rotation speed is 1500 rpm and the welding speed is 300mm/min, the tensile strength reaches the maximum 256 MPa, which is up to 77.3% of the base metal.All the joints were fractured at the heat affected zone, which was the weakest region, on the AS or RS. The fracture mode belongs to the typical aggregation microporous fracture.