a method of repairing a discrepancy in a casting , weldment or other workpiece comprised of a material characterized by thermoplastic properties includes excising the discrepancy by cutting , milling , or otherwise removing workpiece material surrounding the discrepancy so as to remove the discrepancy and form a cylindrical hole in the workpiece . the method further includes placing a cylindrical plug in the cylindrical hole and moving a friction stir welding tool around the circumference of the plug to weld the plug to the workpiece .