Channelling Current in FAST Processed Titanium Alloys to Generate Uniquely Tailored Microstructures in a Single Step

The clear impact that intensifying the electric current density has on the sintering process has been observed previously in several works, through the addition of insulation, coatings, and across a range of material systems. In this thesis it is investigated, and demonstrated, that the impact of not only intensifying, but also spatially directing this current to generate tailored thermal distributions, has a significant and useful resulting impact on final microstructures. This is analysed using prior β grain size in super-transus Ti-6Al-4V sintered samples as a marker for thermal history, and TiB metal matrix composites where in situ reactions demonstrate the thermal gradients.
A finite element analysis (FEA) model was created using experimentally gathered data to examine, and predict any internal thermal profiles, proving to be spatially accurate and validated within 5% error. Using this, the electric flow was controlled through the precise addition of insulating boron nitride (BN) to the graphite foil surrounding the powder region, redistributing the current density, and thus the regions of highest resistive heating. Relative densities for 20 mm Ti 6Al-4V samples were demonstrated to vary by up to 15% and showed gradients matching projected thermal histories from the FEA model. MIPAR image analysis software was then utilised to examine the microstructure and provide a quantitative measure for further validation of the simulation and its predictive capacity. It was observed that the addition of these shaped insulation inserts resulted in temperature gradients over
1◦C / mm during sintering with a directly correlated impact on prior β grain size and material properties. This effect, up to 150 µm average grain diameter difference over 40 mm, was also determined to be more pronounced within a processing window for dwell times shorter than 5 mins due to temperature dependant resistive limitations with the insulating material and thermal equilibrium in smaller samples. Longer dwells have also been demonstrated for different material systems with similarly large thermal gradients through the generation of TiB needles in a thermally sensitive reaction. These needles were formed in large scale production of metal matrix Ti + TiB composites functionally graded 250 mm plates using FAST, a scale previously not seen, and the current control technique was applied to demonstrate the potential impact of thermal gradients at scale in a real world situation. The sintered composite plate then performed better than standard Ti-6Al-4V military standards highlighting the effectiveness of the FAST technique. The thermal sensitivity was then demonstrated, through an additional plate produced through the current control technique with an intentionally lower ballistic performance originating from the extreme differences in thermal gradients experienced during processing.