Linear and non-linear ultrasonic NDE of titanium diffusion bonds

Abstract

Diffusion bonding is an attractive solid-state welding technique that promises weight reduction and improved performance in the aerospace industry. However, its adoption in fracture critical titanium components has been limited by the complications that macroscopic anisotropy introduces to typical ultrasonic NDE. Two strands of ultrasonic NDE, linear and non-linear acoustics, have been studied with the aim of overcoming these complications.

A promising linear technique that uses the phase of reflected diffusion-bond signals to extract otherwise hidden interface information was selected for further development. The principal parameters that affect the phase analysis of ultrasonic signals were investigated and their optimisation resulted in up to an order of magnitude improvement in phase measurement reliability, even at low signal-to-noise ratios. The application of these optimised parameters without a priori knowledge of the signal arrival time was illustrated, and the sensitivity of the approach to ambient temperature and annealing effects was also explored. The original technique was susceptible to measurement error and proved impractical for typical aerospace component geometries, but these shortcomings have been overcome by the improvements and adaptations proposed here. However, it was shown that the efficacy of the technique depends on the relative acoustic impedances of the bonded media and, coupled with the sensitivity limit intrinsic to linear acoustic methods, this dependence acted to curtail the benefits of the approach and prompted the exploration of alternative techniques.

Non-linear ultrasonic methods are significantly more sensitive than their linear counterparts to the imperfections likely to be present at diffusion-bonded interfaces, but suppressing extraneous contributions to the non-linear response of the interface is not trivial. An approach that succeeds in suppressing such contributions was studied and developed 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 linear ultrasonic methods. Application of the technique to diffusion-bonded titanium aerospace components has been demonstrated and a significant improvement in ultrasonic NDE capability was achieved.