Casting superalloys for structural applications

Abstract

Open air investment casting of superalloys for aerospace applications is a process that dates back about 80 years. The introduction of vacuum metallurgy 30 years later made the development and the use of higher strength and higher temperature resistant superalloys feasible since the very reactive alloying elements like aluminum, titanium and niobium that brought these propertied could be handled in the casting process without being oxidized. The development of the high strength weldable Alloy 718 (INCONEL 718 patented by INCO) 50 years ago enabled large size, complex geometry, structural castings to be produced efficiently and thus replacing previous weld assembly part designs made of less potent stainless steel grade alloys. With the introduction of hot isostatic pressing as a post processing step the quality of the castings could be significantly improved allowing for more critical aircraft engine applications. With increasing diameters there is a diminishing return in terms of capital investment, number of parts and capital amortization and today there is a reversed trend to build the very large diameter structures as assembly parts. Cast and wrought parts may be mixed in such structures for weight optimization. In this context the welding is the ultimately critical process not least when more temperature capable alloys are considered at the same time.