The document discusses friction stir welding (FSW), a solid-state welding process. FSW involves a rotating cylindrical shoulder tool that is plunged into two abutting metal pieces. Frictional heat and pressure generated by the tool's pin and shoulder forge the plasticized metal together. Key advantages of FSW include a solid-state process that avoids defects of fusion welding and the ability to join dissimilar metals. The document outlines the FSW process, microstructure analysis of welds, tool design considerations, applications, advantages, and challenges of the technique.