Turbine blades have complex geometries with free form surfaces. Manufacturing of such a complex geometries in Ni based super-alloys is typically done by the investment casting process. Blades have different thickness at the trailing and leading edges as well as sharp bends at the chord-tip shroud junction and sharp fins at the tip shroud. Shrinkage at the tip-shroud and cord junction is a common problem in casting of turbine blades, especially when produced in alloys with large freezing range. In this work, the effect of shell thickness on shrinkage porosity is evaluated. The test geometry used in this study is a thin-walled air-foil structure which is representative of a typical hot-gas-path rotating turbine component. The alloy used in study is a Ni-base super alloy with large solidification interval. Casting trials were performed to develop a relation between the mechanism driving formation of shrinkage and shell thickness.

It was observed that shell thickness is significant to achieve a steeper thermal gradient which is essential in order to minimize the width of the mushy zone. It was also observed that a slower cooling rate along with a steeper thermal gradient at the metal-mold interface not only helps to avoid shrinkage porosity but also increases fill-ability in thinner sections.

Keywords:

Investment casting, casting defects, shrinkage porosity, turbine blades and niyama criteria