Deformation processing in superplasticity regime-production of aircraft engine compressor discs out of titanium alloys

Superplastic deformation (SPD) that utilizes the inherent superplasticity in suitable alloys has been employed for developing compressor discs for a jet engine in a titanium alloy equivalent to IMI-685. Deformation in the SPD regime by appropriate modification of the microstructure of the workpiece has enabled forming to near-net shape at comparatively lower forming loads. The equipment used for the deformation processing includes an electric furnace with robotic handling of heated billets, a 2000 ton isothermal forging press with numerical controls for speed and load, and a cooling station for controlled cooling of forged discs. The starting microstructure of the forging stock consisting of mainly alpha laths within prior beta boundaries was refined to fine equiaxed alpha grains by appropriate deformation in the alpha + beta regime. The refined structure enabled isothermal forging in the alpha + beta phase field at low deformation loads. The effect of SPD and subsequent thermal treatment on the isotropy of properties in discs has been studied. The forging characteristics, and the microstructure and mechanical properties realized in the discs are presented. It is seen that SPD is a practical method which enables high quality items to be made that are unique in form and reliable in use. The process can be used in a serial production of parts of specified metallurgical properties. (C) 1998 Elsevier Science S.A. All rights reserved.