The effects of laser peening and shot peening on high cycle fatigue in 7050-T7451 aluminum alloy

The high cycle fatigue performance of 7050-T7451 aluminum was investigated for untreated as-machined, laser peened, and shot peened conditions. Constant amplitude, smooth (Kt = 1) fatigue tests were conducted in four-point bending at a stress ratio of R = 0.1. Results show that laser peening induces a layer of compressive residual stress more than three times deeper than for shot peening. Both treatments significantly increase fatigue performance. At a moderate level of stress, peened specimens outlast as-machined specimens, by a factor of 7.9 for laser peening and 2.9 for shot peening. At higher stress, life improvements are lower, a factor of 3.3 for laser peening and 2.1 for shot peening. At a 100,000-cycle lifetime, fatigue strength of laser peened specimens is 41% higher than as-machined specimens and the fatigue strength of shot peened specimens 30% higher than as-machined. A form of pitting was noted on the laser peened surfaces and follow-on tests assessed the effects of the pitting on fatigue performance. Results indicate that the pitting does not significantly influence fatigue performance.