Friction stir spot welding (FSSW) is a variation of linear Friction Stir Welding (FSW), which was invented to compete with resistance spot welding (RSW) and riveting of lightweight alloys in the automobile, shipbuilding and aerospace industries. Recently, the application of FSSW has rapidly extended to a variety of metals and nonmetals. This article provides a comprehensive review of the recent progress on the process fundamentals, parameters optimization, microstructural evolution and mechanical properties, and relevant simulation and modeling of FSSW. The article also evaluates the energy generation, temperature distribution, plastic flow and joining mechanisms. The optimizations of tool design and welding parameters are obtained through experiments and modeling. Furthermore, a particular emphasis is given to microstructural characterization of the recovery, recrystallization and grain growth, and related annealing phenomena after in the welded alloys. The mechanisms of defect formation and liquidation cracking are discussed in detail. The mechanical properties, including hardness, static strength, fatigue performance and failure mechanisms and the relationship between mechanical properties and microstructures are also addressed along with residual stress and corrosion behavior.

The paper critically assesses the research progress towards aluminium–magnesium dissimilar friction stir welding (FSW). First, the theoretical requirements are explored through the understanding of joining mechanism and heat generation in aluminium–magnesium FSW. Next, the observed trends in microstructural characterisation and mechanical properties are analysed. Finally, the effects of welding parameters and how it influences process variables and materials responses are discussed in detail, and several suggestions are made based on these discussions.

In the present study, detailed microstructural characterisation of a friction-stir-welded joint between AA5059 alloy and high-density polyethylene was carried out using field-emission scanning and high-resolution-transmission electron microscopy analysis. The structural features indicate large numbers of macro-, micro- and nano-mechanical interlocks between the Al-fragments and melted/re-solidified polymer matrix at the stir zone of the joint, with a 30-nm thick semi-crystalline aluminium structure layer elevated in levels of O, and traces of C at the interface. An ultrafine-grained structure with an average cell size of <100 nm was formed for the embedded Al-alloy fragments in the polymer matrix, as a result of low-temperature severe plastic deformation during friction-stir welding process. The interfacial chemical reactions assisted by generation of nano-scale pores inside the metal surface at the interface and secondary Van der Waals bonding are suggested as the main joining mechanisms, leading to significant improvements in the mechanical properties.