Characterization of near Surface Conductivity Profiles of Shot Peened and Laser Peened Inconel 718

This paper reports on comparison studies of near surface conductivity profiles of Inconel 718 sample plates that underwent two types of surface enhancement treatments, namely shot peening and laser peening.  Both treatments are used to introduce protective compressive residual stresses, but they are known to produce different surface conditions: shot peening introduces shallow compressive residual stresses and high percentages of cold work, while laser peening introduces much deeper compressive stresses at lower cold work percentages in the material.  The objective of this study is to characterize the near surface conductivity changes induced by the two peening processes, and to examine the correlations of the conductivity changes with the residual stress and cold work profiles. This aims to evaluate the feasibility of characterizing nondestructively the surface conditions of peened engine components, especially the residual stress profiles, by means of conductivity profiling using a swept frequency eddy current (SFEC) technique. By way of approach, Inconel 718 samples were solutionized, solutionized and aged, or direct-aged to produce three different types of microstructures. The samples were shot peened and laser peened at different intensities. SFEC measurements were conducted before and after peening.  Conductivity profiles obtained by model-based inversion will be presented for each of the two peening processes, and will be compared with the residual stress profiles measured by XRD in order to evaluate the ability of the SFEC technique to discriminate dissimilar material conditions and responses to the two peening processes.