Fatigue Crack Growth of Peened Friction Stir-Welded.pdfVIP

Fatigue Crack Growth of Peened Friction Stir-Welded 7075 Aluminum Alloy under Different Load Ratios Omar Hatamleh, Scott Forth, and Anthony P. Reynolds (Submitted April 8, 2008; in revised form March 16, 2009) The effect of peening on the fatigue crack growth performance of friction stir-welded 7075 aluminum alloy was investigated. The fatigue crack growth rates were assessed for laser- and shot-peening conditions at stress ratios (R) of 0.1 and 0.7. The surface and through thickness residual stress distributions were characterized for the different regions in the weld. The results indicate a significant reduction in fatigue crack growth rates using laser peening compared to shot peening and the as-welded condition. The effect of the compressive stresses obtained through laser peening was deemed responsible for increasing the resis- tance to fatigue crack growth of the welds. Keywords fatigue crack growth, friction stir welding, laser peening, shot peening, stress ratio, 7075 1. Introduction High-strength aluminum alloys, such as 7075-T7351, are commonly used in the aerospace industry because of their attractive properties. Unfortunately, these aluminum alloys are difficult to weld using conventional fusion welding, and therefore are mechanically joined at higher cost. The difficulty in welding aluminum structure is due to the mechanical properties of the welded joints being degraded because of the dendritic structure formed during the fusion welding process (Ref 1, 2). Friction stir welding (FSW) was invented by the Welding Institute in England (Ref 3) and is currently being targeted for structurally demanding applications. Since its invention in 1991, FSW has emerged as a promising solid state joining process capable of producing high-quality joints in high- strength aluminum alloys. This technique transforms metals into a plastic state at a temperature below the melting temperature of the material (Ref 4, 5), and then mechanically stirs the materials together und