Material reductions, weight savings, design optimisation, and a reduction in the environmental impact can be achieved by improving the performance of near-net shape (NNS) titanium alloy components. The method demonstrated in this paper is to use a solid-state approach, which includes diffusion bonding discrete layers of dissimilar titanium alloy powders (CP-Ti, Ti-6Al-4V and Ti-5Al-5Mo-5V-3Cr) using field-assisted sintering technology (FAST), followed by subsequent forging steps. This article demonstrates the hybrid process route, firstly through small-scale uni-axial compression tests and secondly through closed-die forging of dissimilar titanium alloy FAST preforms into an NNS (near-net shape) component. In order to characterise and simulate the underlying forging behaviour of dissimilar alloy combinations, uni-axial compression tests of FAST cylindrical samples provided flow stress behaviour and the effect of differing alloy volume fractions on the resistance to deformation and hot working behaviour. Despite the mismatch in the magnitude of flow stress between alloys, excellent structural bond integrity is maintained throughout. This is also reflected in the comparatively uncontrolled closed-die forging of the NNS demonstrator components. Microstructural analysis across the dissimilar diffusion bond line was undertaken in the components and finite element modelling software reliably predicts the strain distribution and bond line flow behaviour during the multi-step forging process.
Titanium alloys are used extensively in the aerospace sector due to the good combination of high strength-to-weight ratio and corrosive resistance. Many aerospace components are exposed to extreme service stress states and temperatures, which in some applications could compromise the component’s performance if a single titanium alloy is used. A potential solution to this issue could be the combination of dissimilar titanium alloys in subcomponent regions, achieved through consolidating powders via field assisted sintering technology (FAST-DB) and subsequent hot forging (FAST-forge). In this paper, near net shape titanium-titanium alloy demonstrator components are produced from oversized AM powders in just two hybrid solid-state steps; FAST-DB and hot forging.
The conventional approach when engineering components manufactured from titanium is to design the thermomechanical processing to develop an optimal microstructure in a single alloy. However, this conventional approach can lead to unnecessary over-engineering of components, particularly when only a specific subcomponent region is under demanding service stresses and environments. One approach being developed to join multiple alloys in a single component and enhance engineering performance and efficiency is FAST-DB—whereby multiple alloys in powder form are diffusion bonded (DB) using field-assisted sintering technology (FAST). But the joining of multiple alloys using conventional welding and joining techniques can generate high residual stress in the bond region that can affect the mechanical performance of the components. In this study, the residual stress distribution across dissimilar titanium alloy diffusion bonds, processed from powder using FAST, were measured using X-Ray diffraction and the Contour method. The measurements show low residual stress in the bulk material processed with FAST as well as in the diffusion bond region. In addition, FAST-DB preforms subsequently hot forged into different near-net shapes were also analyzed to understand how the residual stress in the bond region is affected by a subsequent processing. Overall, no sharp transitions in residual stress was observed between the dissimilar alloys. This study reinforces confidence in the solid-state FAST process for manufacturing next generation components from multiple titanium alloy powders.
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