Upscale of a Novel Powder Interlayer Bonding Repair Process for Complex Titanium 6Al-4V Parts

Abstract

A novel Powder Interlayer Bonding (PIB) process has been developed at Swansea University, and has been demonstrated to show excellent capability of joining various metallic materials commonly used in aerospace gas turbine engines. The most recent PIB studies have focussed on the bonding of Titanium 6Al-4V, with a promising 90% strength retention having been demonstrated. This has led to proposals for PIB to be explored as a potential repair process within the gas turbine industry, for a range of components manufactured from Titanium 6Al-4V. Example components identified as potential repair applications for PIB include large single piece components such as bladed discs (BLISKs), as well as thin wall case structures for which there are currently very limited options for repair. While early feasibility studies have shown great promise for PIB with respect to filling these voids in repair capability, they have typically involved bonding of small-scale flat coupons which do not bear any resemblance to the shape and size of these components. The purpose of this EngD project was therefore to adapt the PIB principles, producing a machine capable of joining coupons of upscaled size and geometric complexity. It was hoped that this would help “bridge the gap” between small-scale feasibility trials and industry adaptations of the technique. The PIB principles developed at Swansea University have been successfully implemented in a new bespoke joining machine which remains in-situ at Swansea University. Small-scale bonds produced using identical parameter sets were first compared using both legacy and newly developed apparatus to validate the bespoke machine. The newly developed system was shown to produce bonds of equivalent quality with respect to upset of the fusion zone, and key powder consolidation characteristics. Moreover, additional benefits were realised in improved control, reliability and repeatability of bonding using the newly developed system. Attention was then turned to investigation of coupon interface geometry effects on key PIB bonding outputs, with a total of four interface geometries tested. Aggressively curved surfaces were shown to reduce fusion zone upset at the expense of consistent powder collapse across the interface, while mild surface changes were observed to promote powder consolidation at the expense of some dimensional loss. This led to the down selection of a 50:1 curved interface geometry which appeared to demonstrate adequate process capability for the proposed application, while also balancing coupon deformation with effective powder consolidation behaviour. Upscaled coupon testing focussed on the isolation of optimised parameter sets for bonding coupons of differing cross sections in a flat “plate and boss” interface geometry. Optimised parameters were shown to produce bonds featuring a high level of microstructural retention in combination with effective pore closure and acceptable fusion zone upset. Read across of this parameter set was successfully implemented on similarly sized coupons, mated with the down selected 50:1 curved interface geometry, in addition to a further trial involving oversized flat near-net shape BLISK aerofoil coupons. The success of these trials demonstrates the capability of PIB with respect to bonding geometries representative of the components requiring repair in the proposed applications. Finally, potential avenues for investigation which could benefit the continued adaptation of PIB for these applications were highlighted.