DIE TECHNOLOGY FOR INVESTMENT CASTING OF AEROSPACE TURBINE BLADES AND VANES

摘要

Gas Turbines in Aerospace applications operate at extreme conditions of temperatures and stresses. The Blade/Vane components of these turbines have sculpted and twisted aerofoil geometry and are made using Nickel based super alloys through the investment casting process. In order to achieve near-melting point operating temperatures for maximizing turbine efficiency and thrust, these components are made hollow having geometries of equal intricacy, with added complications of innumerable fine features such as turbulators, tie pins etc. Design and development of an injection molding wax pattern die is the first critical step in establishing investment casting process. Advanced CAD/CAM techniques are used for developing a process warpage/shrinkage- compensated three dimensional CAD model of the blade/vane component, which is split along parting lines keeping feature reproducibility and extraction of pattern in mind. Various CAD models of die inserts are developed based on the finalized die configuration and machining programs are generated on CAD/CAM systems and processed on 3-axis CNC milling machines. Each and every die insert thus machined is inspected using 3D Coordinate Measuring Machines (CMM ) and matched against its CAD model and re-machined/polished. Die assembly is the final critical step, wherein all the die parts are assembled to a common reference and qualified through CMM inspection. The die must also incorporate ceramic core locating and locking features in order to get a hollow casting, maintaining wall thicknesses of the order of 0.5 mm on either side of the pattern. Design and development of wax pattern dies is a well established process in DMRL, where in highly complicated dies for a variety of components have been successfully developed and proved. Based on this expertise of over a decade, DMRL has forayed into development of ceramic core dies in recent times. Ceramic core dies present an entirely different set of challenges for die technologist, which include machining of hardened blocks of 55-60 HRc, development of a series of EDM electrodes and control of spark erosion machine parameters, superior surface finish requirements, reproduction of fine features & fillets of casting internal geometry of the order of 0.25-0.50 mm, etc. DMRL has successfully developed dies for highly twisted and thin aerofoil ceramic cores and is in an advanced stage of establishing this crucial technology. The present paper deals in depth the path traversed by DMRL in establishing die technology of wax pattern as well as ceramic core components.