Friction Pull Plug Welding (FPPW) is the process necessary to plug the hole that is left behind as a friction stir weld (FSW) joint is completed and the pin tool of the welder retracts from the joint. FPPW involves a small, rotating part (plug) being spun and simultaneously pulled (forged) into a hole in a larger part. When the plug enters the hole, there is often chatter, and sometimes the machine stalls completely. NASA discovered that by optimizing the design of the pull plug, including angling the shoulder edge of the plug precisely, it makes contact with the hole in such a way that the chatter issue is improved. NASA has made the new design as an adaptation to make FPPW more practical and robust. The new plug has been used to make space-qualified parts at NASA, and the plug welds are as strong as initial welds.
The new friction pull plug design is optimized to reduce chatter that results as a fast-rotating plug enters the hole in the part. The plug design is based on a shank with multiple frusto-conical sections shown in the figure. The sections are carefully sized to ensure that the spinning plug contacts the edge of the hole at just the right position to minimize chatter. It keeps the machine from stalling when the plug enters the hole. This new design makes FPPW more practical, perhaps even as a future rivet replacement.
NASA has used the plug on a 0.5"-thick weld and on a 0.627"-thick weld. The plug helps make FPPW practical for 2015 and 2019 aluminum.