Recent advances in keyhole defects repairs via refilling friction stir spot welding

The goal of any welding processing is to produce a defect-free sample that will serve the intended purposes. The major setback in friction stir spot welding (FSSW) is the keyhole that always left at the centre of the spot after welding process and also the keyhole that left at the end of friction stir welding and processing (FSW/P). This keyhole has been classified as a defect in FSSW as well as FSW/P. In recent years, very few researchers have developed, characterised and validated some potential methods through research to eliminate this defect and make especially FSSW void free for use and hence reduce material loss during characterisation as a result of cut-off. In this review, the methods adopted in removing this keyhole defects through filling and refilling varying from convectional FSSW, friction forming process (FFP), swing FSSW, pinless shoulder methods, to using autoadjustable pin as well as friction plug welding have been summarised. It is expected that the literatures reviewed here will make meaningful contributions to the present and future researchers, manufacturers, scientists, engineers and the pilots of industries in the area of FSSW, FSW, FSP in order to have keyhole defects free during processing. In this review, refilling FSSW has proven to have better mechanical properties, better hardness and highest tensile strength as compare to others processes. On this note, the authors will recommend the use of refilling FSSW over the others even though, all reviewed processes has been established to have better mechanical properties.