APPLICATION OF NONLINEAR ULTRASONIC MEASUREMENT FOR QUALITY ASSURANCE OF DIFFUSION BONDS OF GAMMA TITANIUM ALUMINUM ALLOY AND STEEL

摘要

A new nonlinear C-scan acoustic microscope (NCAM) has been developed and applied to inspect diffusion bonds of gamma titanium aluminum alloy-chromium molybdenum steel (γ TiAl-steel). The large mismatch of acoustic impedance gives strong ultrasonic reflection and masks a small signal from imperfections at the interface. However, NCAM detects indications of minute cracks or gaps, which are rarely observed by a conventional pulse-echo inspection method. Application of high-power ultrasound, with amplitude ten times greater than the conventional method, results in generation of second harmonic signals. All A-scope signals were stored in a computer, and frequency components were analyzed by fast Fourier transformation (FFT). As a result, no indication of fundamental frequency component was observed on a C-scan image, whereas clear indications of the second harmonic components were observed on the C-scan image. The minute cracks or gaps were confirmed at the indicated area by micrographic analysis following the inspection. Moreover, a quality assurance process for the γ TiAl-steel bond with a combination of linear and nonlinear ultrasonic responses is proposed.