Preliminary study of improving the speed and cost of diffusion bonding of metal sheets

Abstract

Diffusion bonding may be used, with superplastic forming, to create a great range of cellular structured materials, with great potential for material efficiency, multi-functionality and lightweighting. However, this potential is generally considered to be constrained by the slowness (hours) of and the overall cost of achieving such solid-state bonds. The work described is part of the basis for the development of a process that can produce good diffusion bonds, in the commonly used titanium alloy Ti64 (Ti6Al4V), in less than 60 s. And so it has the capability of increasing the productivity for diffusion bonding. Localised direct heating can induce high levels of stress, and these thermal stresses can accelerate the diffusion bonding process. The equipment used involved induction heating and required no large (and expensive) hot presses or pressure vessels. Hence, machinery capital cost may be reduced. A sequence of numerical models was found useful to understand the effects of the process parameters on diffusion bonding. Finite element modelling looked at accelerating the collapse of the voids, determined by the relative flatness and roughness of the metallic sheets at different spatial wavelengths. It appears that the localised heating drives the diffusion bonding process in inducing significant stress levels, into the material to be bonded, and thus accelerates the collapse of voids and the achievement of intimate contact. Raising the temperature as much as possible assists the actual diffusion processes required for bonding to occur and is an important aspect of accelerating the process. Therefore, the increase in the rate of bonding and the lower cost of the equipment open up the possibility of much more affordable diffusion bonding and the production of material efficient and functionally useful cellular materials.