BONDTEST · Development of an ultrasonic testing technique to characterise diffusion bonds in layered structures

Diffusion bonding offers a number of advantages to a wide range of industry sectors and applications, particularly where there is a need to join dissimilar materials. The main advantage of diffusion bonding is that, unlike welding, it is not a 'fusion' process – it is a solid state process and the parent materials to be joined do not need to be melted. This makes diffusion bonding an ideal solution for joining materials such as metals to ceramics and Aluminium, Titanium, Nickel, Steel etc.Other advantages include near-parent properties, minimal shape change to parts and no post-finishing. The key to successful diffusion bonding lies in choosing process conditions which are appropriate to the circumstances, because the joint needs to form successfully with both parent materials, whilst controlling deformation. Joint intimacy and cleanliness are also critical factors.A reliable NDE method is required for the inspection of the bonds. A non-satisfactory bonding may include voids that will have a negative effect of the integrity of the joint. The challenge in inspecting these bonds is that the joining process will create a multi-layered structure that will complicate the interpretation of the inspection data. In addition to this, the bonding layer will have an irregular surface making most inspection techniques unsuitable.It is anticipated that this issue can be solved by developing a phased array ultrasonic testing (PAUT) method and Full Matrix Capture (FMC), that combined with a material-based model of the multi-layered structure will give an assessment of the diffusion bonding. The model and the tailored inspection data will then be used in signal processing algorithms to extract features of the bonding layer. The following key stages are needed:• Phased array equipment• Inspection technique development• Material modelling• Signal processing• Full Matrix Capture (FMC) and focussing of the diagnostic region