Study Of Stationary Shoulder Friction Stir Welding Of Aluminum Alloy

Stationary Shoulder Friction Stir Welding(SSFSW), proposed by The Welding Institute, is a novel adaptable variant of conventional friction stir welding technology. It produces smooth welds with little flash and thickness reduction using low welding load and creates homogenous heat input through the thickness direction, leading to small distortion and low residual stress. Thus, it has a promising potential in the welding of titanium and aluminum alloy joints with various thickness and complex configuration. However, up to now, limit open literatures have been published on the microstructures and mechanical properties of SSFSW joint. It is of great significance to systematically investigate the welding tool, joint configuration, process, microstructure, defects and mechanical properties of SSFSW joint.In this research, SSFSW welding tools for butt joint and T-joint were developed and defect-free joints of 6061-T6, 7075-T651 and 5A03-H112 aluminum alloys were obtained. Based on the study of welding parameters, the metallurgy characteristics, microstructures, defects, mechanical properties, inhomogeneities, tear toughness and fatigue properties were investigated. The main conclusions are listed as follows:1. SSFSW welding tools were manufactured on the basis of the investigation of tool shape, dimension and fit tolerance of the stationary shoulder and the welding tool. Defect-free butt joints of 5 mm thickness aluminum alloys were successfully obtained. Defect-free additive and non-additive T-joints were made consequently. This provides the basis of the investigation of welding parameters, joint microstructure and mechanical properties.2. The welding parameters, weld formability and defects of SSFSW joint were studied in detail. Results showed that material properties are the key factor affecting the weldability in SSFSW. The optimal processing window are 750~2000 rpm & 50~400 mm/min, 750~1500 rpm & 100~200 mm/min, and 1500 rpm & 30~50 mm/min for 5 mm thick 6061-T6, 5A03-H112 and 7075-T651 aluminum butt joints, respectively. The optimal welding parameter for 6061-T4 non-additive T-joint is 1500 rpm & 100 mm/min. Defectfree butt joints exhibit smooth weld surface with little thickness reduction and distortion. The fillet radii of additive T-joint improved the geometry of the transitional zone of the skin and the stringer. Due to the pressing effect of the stationary shoulder, defects are rare in SSFSW joint with smooth surface. Besides, surface groove defect is the major defect of SSFSW joint made using improper welding parameters.3. The macro-morphology of the transverse section and the microstructures of SSFSW butt joints were analyzed. SSFSW butt joints consist of nugget zone(NZ), heat affected zone(HAZ), and thermally affected zone(TMAZ). The NZ of 6061-T6 and 5A03-H112 butt joints show a cone shape, which is similar to the shape of the probe; while, the 7075-T651 butt joint shows a drum-shaped NZ. Narrow TMAZ and HAZ in SSFSW butt joints lie adjacent to NZ and TMAZ, respectively. Non-additive and additive T-joints both show umbrella-shaped NZ with TMAZ and HAZ locating adjacent to NZ and TMAZ, respectively.4. The configuration of welding tool, the rotational speed and the welding speed are the predominating aspects that determine the mechanical properties of the joints of different aluminum alloys. Under optimal welding parameter, the joint efficiency for 6061-T6 and 7075-T651 butt joints is 78% and 69%, respectively. The weak position locates in HAZ with the highest hardness values reaching 78% and 69% of that of BM. The welding speed shows an evident effect on the mechanical properties under various welding parameters. Welding parameter makes little effect on the mechanical properties of 5A03-H112 aluminum joint with the joint efficiency reaching 99.6% under optimal welding parameter. Screw-type probe effectively enhances the material flow and reduces the defects in non-additive T-joint. Under optimal welding parameter, the joint efficiency of non-additive and additive T-joints are 68% & 78% for the skin and 83% & 96% for the stringer.5. Material property is the key factor that results in the evident inhomogeneity in microstructures and mechanical properties for different aluminum alloy joints. Little inhomogeneity in mechanical properties was observed in 5A03-H112 butt joints. The hardness and mechanical properties of 5A03-H112 SSFSW joint are similar to those of BM. While, great inhomogeneity in microstructure and mechanical property were observed for heat treatment strengthened aluminum alloy 6061-T6 and 7075-T651. The inhomogeneity in microstructure is mainly characterized by the refinement of grain structure and the breaking of the constituent particles in NZ, and the precipitate evolution. The hardness scatter range of the low-hardness region are 50~68% and 64~69% of that of BM for 6061-T6 and 7075-T651 joint, respectively. The scatter ranges of ultimate tensile strength and yield strength at different positions of 6061-T6 butt joint made by various welding parameters are 34.8~86.2 MPa and 61.9~113.3 MPa, respectively.6. Material property is the key factor that affecting the tear toughness of SSFSW butt joint of various aluminum alloys. For heat treatment strengthened aluminum alloy joints, the crack initiation resistance toughness of NZ and HAZ can reach 91~92% and 62~68% of that of BM, respectively, illustrating that HAZ exhibits evidently lower crack initiation toughness compared to BM. For strain strengthened 5A03 aluminum, the crack initiation resistance toughness of NZ and HAZ are both 131% higher than that of BM, illustrating that the crack initiation resistance of the welded zone are higher than that of BM.7. The fatigue property of non-additive and additive T-joints was investigated in depth. The fatigue property of non-additive T-joint is lower than that of additive T-joint, but evidently higher than that of butt joints made by conventional FSW and T-joint made by TIG. At the confidence interval of 90% and the survival rate of 97.5%, the fatigue strength of non-additive and additive T-joints at the fatigue life of 2 × 106 are 101.0 MPa and 101.4 MPa, which are 361% and 362% of the fatigue strength recommended by IIW for arc welding of T-joint. The fracture location in fatigue test is determined by the stress concentration and the fatigue property is influenced by the welding parameter. The welding parameter makes evident effect on the ultra-fine grain structure on the surface of the non-additive T-joint and consequently influence the fatigue property of the joint.