Application of Nonlinear Ultrasonic Measurement for Quality Assurance of Diffusion Bonds of Gamma Titanium Aluminum Alloy and Steel

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 (c 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 c TiAl–steel bond with a combination of linear and nonlinear ultrasonic responses is proposed.

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