Friction Boring Process for Aluminum Alloys

Author:  Murray W Mahoney, Camarillo, CA
Source:  US Patent 5,725,698
Doc ID:  1998020
Year of Publication:  1998
Abstract:  
A friction boring process creates a corrosion resistant fine grain microstructure in the wall surfaces of holes bored in aluminum alloy materials. A rotating tool is inserted directly into the aluminum material, or into a pre-drilled pilot hole, at a sufficient rotational velocity and feed rate to cause working that extends beyond the diameter of the tool, frictional heating, and extraction of aluminum material by metal deformation rather than cutting action as with a conventional drill bit. Burring, smoothing, and otherwise removing aluminum material extracted from the hole may be performed by a finishing segment that limits insertion depth of the tool. Frictional heating generates a temperature sufficient for rapid recrystallization of the remaining worked metal to form a fine grain microstructure to a depth of about 2.5 mm in the hole surfaces. Corrosion protection is retained even if some fine grain material is removed during a subsequent reaming operation. Friction boring is fast, suitable for a wide variety of aluminum alloy compositions, and easily adaptable to initial fabrication of aluminum components or to field repair of assembled structures such as on aging aircraft. The process creates a fine grain corrosion and fatigue resistant surface microstructure in aluminum alloy holes without the use of peening, heat treatments, or environmentally objectionable chemicals and coatings.