friction plug welding usage is advantageous for friction stir welding hole close - outs and weld repairs in 2195 al — cu — li fusion or friction stir welds . current fusion welding methods of al — cu — li have produced welds containing varied defects . these areas are found by non - destructive examination both after welding and after proof testing . current techniques for repairing typically small defects weaken the weldment , rely heavily on welders &# 39 ; skill , and are costly . friction plug welding repairs increase strength , ductility and resistance to cracking over initial weld quality , without requiring much time or operator skill . friction plug welding while pulling the plug is advantageous because all hardware for performing the weld can be placed on one side of the workpiece .