Process improvements of Ni-based superalloy investment castings have become the subject of intensive research efforts at the Foundry Institute of RWTH Aachen University. “Development of Heat Conductor Technique for SC Solidification of Turbine Blades” and “Investigation of Freckle Formation in DS Superalloys” are two recent topics of advanced research. Especially the investment casting of modern Ni-based superalloy castings with complex composition requires improvements of directional solidification in the Bridgman process. The narrow process parameter window requisite for high-quality production of complex castings then has to be optimized for each part geometry. For example, typical grain defects, such as stray grains and freckles, are not only dependent on the alloy composition, but also on the component′s unique geometry and arrangement in the Bridgman furnace. By application of a newly developed HC (Heat Conductor) technique, the solidification condition at the transition point improves significantly. Various casting processes were carried out using an industrial Vacuum Induction Melting Investment Casting (VIM-IC) system from ALD Vacuum Technologies GmbH. This furnace type perfectly met the requirements for high-quality production of Ni-based superalloys, and proved to be most suitable for rapid development and optimization of new Ni-based superalloy castings. The close relationship of RWTH with ALD was beneficial to this research and for other continuous improvements in casting and solidification processes. In particular, results of studies on freckle formation mechanism and stray grain reduction in Ni-based superalloys of complex composition and geometry will be highlighted. An outlook on development directions for future improvement of investment casting processes and equipment will be given.
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