Non-linear Ultrasonic NDE of Titanium Diffusion Bonds

Diffusion bonding is an attractive solid-state welding technique that represents a valuable tool for reducing weight and improving performance in the aerospace industry. However, its full exploitation in titanium components is currently limited by a lack of robust NDE techniques capable of overcoming the crystallographic anisotropy of these important materials. Advanced ultrasonic techniques have been explored previously, but their sensitivity to imperfections is limited by the linear acoustic phenomena on which they depend. Non-linear ultrasonic methods have been shown to be significantly more sensitive than their linear counterparts to these types of imperfections, but suppressing extraneous contributions to the non-linear response of the interface is not trivial. An approach that succeeds in suppressing such contributions is presented here. The technique, which is based on the non-collinear mixing of ultrasonic waves to generate a spectrally, modally and spatially dissociable third wave, was used to reliably characterise a set of samples whose bond quality was indeterminable using conventional ultrasonic methods.