Gas pocket-assisted underwater friction stir spot welding

The present study demonstrates the concept of gas pocket-assisted friction stir spot welding (GA-FSSW) as a new underwater wet welding (UWW) technique for the sub-sea industry. The stability of the gas pocket in GA-FSSW is controlled by a continuous high-velocity gas flow that drives out the seawater from the desired location of the workpiece. UWW challenges related to the presence of seawater in the operational area and the solidification defects caused by fusion welding are addressed with the GA-FSSW technique. The merit of the GA-FSSW has been confirmed by electron microscopy, which shows the absence of chlorine on the surface of the welded specimen in contrast to the conventional underwater friction stir spot welding (UFSSW). Microstructural changes in GA-FSSW associated with the mechanical and electrochemical properties are compared to those in UFSSW and conventional FSSW. The GA-FSSW shows a unique precipitation behavior with finer precipitates near the pinhole of the stir zone, achieving a higher surface microhardness region than UFSSW and FSSW conditions. The electrochemical polarization tests reveal that the GA-FSSW has a better corrosion resistance than UFSSW, FSSW, and the base material.