Experimental characterization of ceramic shells for investment casting of reactive alloys

The investment casting of reactive Ti and TiAl alloys requires the use of selected ceramics in the face-coat layer to prevent the reaction between the cast metal and ceramic shell, avoiding the formation of a hard layer at the metallic components surface. This work aims to study the influence of ceramic shells composition in some of its characteristics such as flexural strength, friability and dimensional accuracy. The microstructure of the shells was evaluated by SEM. Changes in the face-coat and back-up ceramic shells composition determines the ceramic shell strength to withstand the casting stage with adequate mould permeability and thermal conductivity, and a compromise resistance for knock-out. All the non-conventional ceramic shell systems with interest for reactive alloys, based on fumed alumina binder and alumina sand for the back-ups, present higher dimensional stability (low shrinkage or expansion) compared with traditional systems based on colloidal silica binder and zircon and aluminosilicates back- ups. In this work, better mechanical strength and lower friability were obtained with non-conventional face-coats of alumina and polymer binders, both with yttria flour and stucco, followed by alumina back-ups. Selecting the right ceramic shell composition, it is possible to achieve adequate properties for casting titanium alloys.