Filling system for investment cast Ni-base turbine blades

In investment foundries, it is usual to pay relatively little attention to the design of the filling system of a casting. Vacuum-cast turbine blades represent a typical case, where the mould filling process is poorly controlled, the mould cavities themselves often being top-filled. The consequential surface turbulence causes the entrapment of oxide film on the liquid surface, despite the use of vacuum melting and pouring. In this work, bottom filling principles were used to minimize turbulence. X-ray radiographic video was used to observe and compare the filling of both bottom- and top-filled moulds. Ni-based alloy investment-cast blades melted and cast in vacuum clearly validated the bottom filling design. Compared to conventional top filling in vacuum, dye penetrant indications were reduced by approximately a factor of 10, and hot cracking appeared to be eliminated.