Microstructural Alterations Associated With Friction Drilling of Steel, Aluminum, and Titanium Scott F. Miller, Peter J. Blau, and Albert J. Shih (Submitted May 26, 2005) Friction drilling, also called thermal drilling, is a novel sheet metal hole-making process. The process involves forcing a rotating, pointed tool through a sheet metal workpiece. Grain boundaries form a continuous network throughout the metal, and because of the disrupted structure at the boundary, the metal often acts differently at the boundary locations. Grain boundaries aside, each grain in a pure metal has the same crystalline structure as any other grain, assuming the temperature is the same.