Experimental and Finite Element Simulation Study of Thermal Relaxation of Residual Stresses in Laser Shock Peened IN718 SPF Superalloy

Abstract

An integrated experimental and modeling/simulation approach was developed to investigate and secure a quantified knowledge of the impact of high temperature exposures on the stability of residual stresses in a laser shock peened (LSP) high temperature aero-engine alloy, IN718 SPF (super-plastically formed). Single dimple LSP and overlap LSP treatments were carried out utilizing a Nd:Glass laser (λ = 1.052 μm), and subsequent heat treatments on the LSP-treated coupons were conducted at different temperatures between 550 and 700 °C. A 3-D nonlinear finite element (FE) computational model and the rate-dependent Johnson-Cook material model were calibrated using the experimental results of residual stress from the single dimple LSP and thermal relaxation treatments, and were further extended to the overlap LSP treatment case. Both experimental and FE simulations show that: a) a high level of compressive residual stress (~700 MPa at surface) and residual stress depth (~0.4–0.6 mm) were achieved following LSP, and b) the overlap LSP treatment gave higher residual stress and greater depth. The magnitudes of the initial residual stress (and plastic strain), heating temperature and exposure time were identified as the key parameters controlling the thermal relaxation behavior. The stress relaxation mainly occurs initially before 20 min exposure and the extent of relaxation increases with an increase in temperature and a higher magnitude of the initial as-peened residual stress. In addition, in regions deeper than ~300 μm or after initial thermal exposure where the residual stress was lower than ~300 MPa, stress relaxation was found to be negligible. Kinetic analysis of the experimental thermal relaxation data based on Zener-Wert-Avrami model gave an activation enthalpy of 2.87 to 3.77 eV, which is near that reported in the literatures for volume and/or substitutional solute diffusion in Nickel. These results suggest that thermal relaxation of the LSP-induced residual stress occurs by a creep-like mechanism involving recovery, rearrangement and annihilation of dislocations by climb.