Control of liquidus isotherm shape during solidification of Ni-based superalloy of single crystal platforms

Abstract

The influence of internal thermal baffle between casting and central rod of the ceramic mould on the shape of liquidus isotherm was studied. Convex or nearly flat shape of solidification front was obtained after selection of suitable location of internal thermal baffle relative to the platform, based on measured and predicted temperature distribution. The results were compared with those acquired for castings manufactured with the use of standard mould. Application of internal thermal baffle brought about decrease of solidification front curvature, as well as transverse temperature gradient Gx=0.2°C/mm and maximum undercooling value ΔTm=16.5°C, which succeeded in proper growth of dendrites compared with Gx=2.8°C/mm and ΔTm=36.5°C in the mould without internal baffle. The convex shape of liquidus isotherm induced directional course of solidification process in the area, where casting cross-section changed significantly its dimensions, but it also led to excessive growth of shrinkage porosity on the platform surface. The increase of the distance between the platform and thermal baffle resulted in significant reduction of amount and size of pores along the length of the internal zone of platform.