DETECTION OF ANOMALOUS MACHINING DAMAGES IN INCONEL 718 AND TI 6–4 BY EDDY CURRENT TECHNIQUES

This paper reports on an eddy current (EC) study aimed at detecting anomalous machining damages in Inconel 718 and Ti 6–4 samples, including (i) surface discontinuities such as re‐depositing of chips onto the machined surface, and (ii) microstructural damages manifested as a white surface layer and a subsurface layer of distorted grains, typically tens of microns thick. A series of pristine and machine‐damaged coupons were studied by EC scans using a differential probe operated at 2 MHz to detect discontinuous surface anomalies, and by swept high frequency EC (SHFEC) measurements from 0.5 MHz to 65.5 MHz using proprietary detection coils to detect surface microstructural damages. In general, the EC c‐scan data from machine‐damaged surfaces show spatial variations with larger standard deviations than those from the undamaged surfaces. In some cases, the c‐scan images exhibit characteristic bipolar indications in good spatial correlation with surface anomalies revealed by optical microscopy and laser profilometry. Results of the SHFEC measurements indicate a reduced near‐surface conductivity of the damaged surfaces compared to the undamaged surfaces.