Combined Cold Expansion and Friction Stir Processing of Fastener Holes in Aluminum Alloy Al-2014-T6

Abstract

Cold expansion is used to induce compressive stress zone, by inserting a tapered mandrel/pin or ball into an undersized fastener hole in order to improve fatigue life. In this paper, combined cold expansion and friction stir processing of fastener holes in aluminium alloy Al-2014-T6 is presented wherein the cold expansion tool rotates and thus friction stirs the cylindrical surface of a fastener hole while expanding it. The residual stress, surface roughness and hardness of wall of fastener holes are measured and SEM images are captured. The combined cold expansion method is evaluated for three conditions at tool and fastener hole interface, namely wet (i.e., in presence of metal working fluid), solid lubricant (i.e., in presence of Al2O3 nanoparticles), and dry (i.e., without any external medium). The conventional cold expansion process is compared with combined cold expansion and friction stir processing. The results indicate efficacy of using nanoparticles as a medium in combined cold expansion and surface processing of Al-2014-T6 aluminium alloy to improve surface integrity and fatigue life.