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Technological developments in the area of functionally graded multi-material manufacture are poised to disrupt the aerospace industry, providing the means for step-change improvements in performance through tailored component design. However, the challenges faced during the downstream processing, i.e., machining of such functionally graded multi-materials are unclear. In this study, the challenges involved when face-turning billets consisting of multiple alloys are assessed. To achieve this, a cylindrical billet consisting of Ti-64, Ti-6242, Ti-5553 and Beta C alloys was manufactured from powder feedstock using field-assisted sintering technique (FAST) and termed MulTi-FAST billets. A detailed study of the structural integrity during machining at the diffusion bond interfaces of multiple titanium alloy bond pairings in the MulTi-FAST billet was conducted. The machining forces were measured during face-turning to investigate the impact and behaviour of different alloy pairings during a continuous machining operation. The results showed the significant differences in force machining response, surface topography and the type of surface damage was dependent on the direction the titanium alloy graded pairings were machined in. In terms of subsurface microstructural damage, regardless of the machining direction, no critical damage was found in the vicinity of the bonded alloys. The findings provide an insight into the deformation characteristics and challenges faced in the machining of functionally graded components with multiple titanium alloys.

Titanium alloys have a high strength-to-weight ratio, fatigue performance and excellent corrosion resistance, and therefore are widely used in the aerospace sector due to their ability to withstand severe mechanical and thermal stresses in service. There are numerous cases where it would be desirable to use different titanium alloys in defined subcomponent regions to improve performance and efficiency. Conventional processing routes do not permit components to be produced with multiple titanium alloys and thus, design efficiency and optimization of component properties is compromised or over-engineered. In this study, a hybrid solid-state consolidation route is presented whereby field assisted sintering technology (FAST) is exploited to diffusion bond (DB) dissimilar titanium alloy powders in defined regions—a process termed FAST-DB. Titanium alloy powders Ti-6Al-4V (Ti-64) and Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) were bonded using FAST in order to study the tensile deformation behavior and strain localization across a dissimilar alloy solid-state bond. FAST-DB was carried out at the sub- and super- beta transus temperatures of both alloys to generate dissimilar microstructure morphologies across the bond. In all cases, diffusion bonds showed excellent structural integrity with no defects and a smooth hardness profile across the bond. The deformation characteristics of the bonds was studied using two different tensile test approaches. The first approach used ASTM standard specimens to measure the mechanical properties of FAST-DB samples and study the location of the tensile failure. The second approach used a microtester and optical Digital Image Correlation to capture the grain interaction in the bond region under tensile loading. The work demonstrated that the diffusion bond remains intact and that tensile failure occurs in Ti-64 (i.e. the lower strength alloy) and is independent of the grain crystal orientation. The results from this study will provide materials engineers confidence in nesting FAST-DB technology in future near net shape manufacturing routes.