The Effects of Small Changes in Alloy Chemistry on the Solidification Behaviour of the Cobalt-Base Superalloy FSX-414

Abstract

Controlled casting experiments have confirmed the industrial experience that small changes in alloy chemistry dramatically affect the frequency and severity of microporosity and hot tear defects in investment castings made from the cobalt-based alloy FSX-414. A statistical model has been constructed from the defect characterization data and the differential thermal analysis data gathered from experimental test castings. Elements both within and outside of specification control were found to interact and affect FSX 414 solidification behaviour and therefore influence the formation of casting defects. The statistical model was subsequently used to create both optimized and unbalanced composition ranges for FSX-414. The predicted effects of the statistically-generated alloy chemistries were successfully validated using additional test castings. Thermal analyses of samples extracted from the test castings revealed fundamental differences in the solidification behaviour of the different alloys. The results of this investigation explain how small variations in alloy chemistry can yield significant changes in alloy solidification behaviour and therefore create a range of FSX 414 investment casting quality. A powerful statistical analysis technique has been used to create statistically valid relationships between alloy chemistry and casting quality, leading to a proposal for a streamlined and cost-effective method for optimizing equiaxed alloy chemistry.