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Refill Friction Stir Spot Welding (RFSSW) shows great potential to be a replacement for single-lap joining techniques such as riveting or resistance spot welding used in the aircraft industry. In this paper, the fatigue behaviour of RFSSW single-lap joints is analysed experimentally in lap-shear specimens of Alclad 7075-T6 aluminium alloy with different thicknesses, i.e., 0.8 mm and 1.6 mm. The joints were tested under low-cycle and high-cycle fatigue tests. Detailed observations of the fatigue fracture characteristics were conducted using a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS). The locations of fatigue failure across the weld, fatigue crack initiation, and propagation behaviour are discussed on the basis of the SEM analysis. The possibility of predicting the propagation of fatigue cracks in RFSSW joints is verified based on Paris’s law. Two fatigue failure modes are observed at different load levels, including shear fracture mode transverse crack growth at high stress-loading conditions and at low load levels, and destruction of the lower sheet due to stretching as a result of low stress-loading conditions. The analysis of SEM micrographs revealed that the presence of aluminium oxides aggravates the inhomogeneity of the material in the weld nugget around its periphery and is a source of crack nucleation. The results of the fatigue crack growth rate predicted by Paris’s law were in good agreement with the experimental results.

The dimensional accuracy and mechanical properties of metal components formed by the Single Point Incremental Forming (SPIF) process are greatly affected by the prevailing state of residual stress. An X-ray diffraction method has been applied to achieve an understanding of the residual stress formation caused by the SPIF process of deep drawing a quality steel sheet drawpiece. The test object for an analysis of residual stress distribution was a conical truncated drawpiece with a slope angle of 71° and base diameter of the cone of 65 mm. The forming process has been carried out on a 3-axis HAAS TM1P milling machine. Uniaxial tensile tests have been carried out in the universal tensile testing machine to characterize the material tested. It was found that the inner surface of the drawpiece revealed small linear grooves as a result of the interaction of the tool tip with the workpiece. By contrast, the outer surface was free of grooves which are a source of premature cracking. The stress profile exhibits a nonlinear distribution due to different strengthening of the material along the generating line of the truncated conical drawpiece. The SPIF parts experienced a maximum residual stress value of about 84.5 MPa.