Development of mullite-alumina ceramic shells for precision investment casting of single-crystal high-pressure turbine blades

Abstract

Alumina-mullite ceramic shell moulds were developed to cast single-crystal hollow high-pressure turbine blades (SX-HPTB). Mullite and fine alumina powders were used as fillers. Two colloidal silica binders were used to bind the filler particles. One colloidal silica binder (binder A) had no polymer and the second colloidal silica binder (binder B) had 8 wt % polymer. Eight slurry compositions with fine alumina substitution (0, 10, 30 and 50 wt %) were prepared. Silica cores were used to achieve hollow castings. The effect of alumina substitution and binders on the slurry characteristics, flexural strength of green and fired samples, air permeability and self-load sag resistance was studied. It was observed that with an increase in alumina percentage, the test specimens showed increased viscosity, pick-up weight, density, green strength and fired residual strength. In addition, the slurries prepared from binder B possessed larger values of viscosity and pick-up weight compared to the slurries prepared from binder A. The residual strength of fired test specimens containing binder A was higher than those containing binder B. The test specimens with 30 wt % fine alumina substitution showed the highest self-load sag resistance w.r.t. each test temperature. Also, the air permeability of the fired samples was higher than the green (de-waxed) samples. The casting of hollow HPTB components was performed at two temperatures (1525 °C and 1550 °C) using nickel-based superalloy CMSX4. The HPTB components, cast at 1550 oC from ceramic shell mould having binder B and 30 wt % fine alumina substitution, resulted in an acceptable aerofoil profile and surface roughness.