Microstructural development of electron beam processed Al-3Ti-1Sc alloy under different electron beam scanning speeds

A systematic study has been made of the microstructural development in as-cast 30 mm thick Al-3Ti-1Sc (wt%) plate samples that were processed by electron beam (EB) melting at different scanning speeds of 3, 5, 8, 12, 15 and 20 mm/s. The composition of the alloy is designed to capitalize on the potential for significantly extended solubility values of both Sc and Ti in Al when cooled at high cooling rates. The resulting microstructures are characterised using transmission electron microscopy (TEM) and other analytical means, assisted with Thermo-Calc predictions and Vicker's microhardness measurements. EB scanning speed plays a key role in determining the phase formation and microstructural development in the Al-3Ti-1Sc alloy when the beam is applied an accelerating voltage of 50 kV and a current of 30 μA. The major microstructural features in the re-solidified zone include (i) the formation of the primary tetragonal Al3(Ti,Sc) phases and their retention or subsequent transformation to stable cubic Al3(Ti,Sc) phases; (ii) the complete suppression of the primary tetragonal Al3(Ti,Sc) phases at the scanning speed of 20 mm/s; and (iii) the formation of cubic Al3(Ti,Sc) precipitates in the α(Al) matrix supersaturated with both Sc and Ti. These experimental findings are informative for both EB processing of Al-Ti-Sc alloys and the design of new Al alloys for additive manufacturing processes.