High Temperature Stability of Wrought and AM In718 Treated by Cyclic Laser Peening and Thermal Processing

Fuel efficiencies of jet engines and gas turbines are limited in part by thermally initiated fatigue and creep of hot section components.  Peening technologies have not been used because residual stresses they impart relax quickly at elevated (>600C) temperatures.  We present a novel technique for enhancing retention of stress and fatigue properties of nickel-based superalloys by thermal-mechanical treatment. The method is based on “cyclic” laser peening (LP) and annealing that produces a compressive stress distribution highly retained after exposure to high temperatures, even higher than half the melting temperature (Tm). Retention of compressive residual stresses in cyclically-laser peened additively and traditionally manufactured Inconel718 test samples was analyzed after thermal exposure at 760°C. The measurements indicate that over 50% of the residual stresses of cyclically treated samples survived after thermal exposure and directly contrast to equivalently exposed samples that were conventionally shot or laser peened and severely degraded in residual stress and fatigue performance. Fatigue life of cyclically treated samples after 600°C exposure for 350 hours was much higher than that for specimens treated with other methods. We hypothesize that the stability is attributed to the complex interaction of the dislocation substructures that are generated as a result of the laser shock peening. The dispersed-precipitated phases that form during the annealing steps are more resistant to thermal relaxation. While the mechanism behind these results is still under investigation, the results are of significant interest since they are in contrast with what is usually expected of peening at temperatures above 0.5Tm where the compressive residual stresses relax gradually, and yield strength begins to decrease dramatically as a result of increasing rate of solid-state diffusion. It is foreseen that this technique will offer solutions to some of the main concerns about AM parts such as fatigue and creep failures at high temperatures.