The Diffusion Bonding of Dissimilar Titanium Alloy Powders using Field Assisted Sintering Technology

A novel processing method for diffusion bonding dissimilar titanium alloy powders, termed as FAST-DB, has been assessed to determine its viability as an industrial-scale process in the future. Small-scale 20 mm diameter trials were initially used to demonstrate the possibility of diffusion bonding CP-Ti, Ti-6-4, and Ti-5553 titanium alloy powders together. To control the bond interface a parametric study was undertaken by individually adjusting the dwell temperature, dwell time, and bond interface direction relative to the uni-axial compression direction. Large-scale 250 mm diameter trials were increased to up-scale the FAST-DB process so that the mechanical response under tensile and fatigue loading could be analysed. FAST-DB Ti-6-4 with CP-Ti combination compression samples were extracted from the billets and uni-axially compressed at elevated temperatures and at constant strain rates to characterise the bond interface evolution. Forging preforms were also extracted, which were forged into near-net shape (NNS) components using a comparatively uncontrolled process. FE modelling was used to predict the bond interface morphology and strain profiles for small-scale compressions and drop forged components. Initial small-scale trials showed that high integrity bonds could be produced as evidenced by an absence of porosity or cracking, and grains crossing the interface. The presence of Ti-5553 in FAST-DB combinations produced a high hardness interface, attributed to fine scale $\alpha$ formation. The dwell temperature, dwell time, and bond interface direction had small effects on the bond interface and bulk alloys. The mechanical response under tensile and fatigue loading was excellent, with failure occurring away from the bond interface. After controlled uni-axial compressions and relatively uncontrolled drop forging processing, the interface remained integral with no defects observed. A positive linear relationship was determined between the total strain and diffusion bonding width. The FE model for small-scale testing and drop forging showed good agreement with the experimental samples leading to promising results for the future. FAST-DB with a subsequent forging step has been demonstrated as a novel method to produce NNS components with multiple titanium alloys in different subcomponent regions. FAST-DB ultimately has the potential to improve the overall performance of titanium alloy components.