Effect of constraint removal on single-crystal blade dimensions during investment casting

Abstract

Single-crystal turbine blades, manufactured via investment casting, are key components of high-performance aero engines and gas turbines. The final dimensions of the blades depend on the cumulative effect of heat shrinkage at different stages, including the superalloy, mold shell, and ceramic core, etc. To clarify the effect of these stages on the blade dimensions, we first use the heat transfer coefficient calculated from inverse heat-conduction analysis to simulate the directional solidification. By extracting and mapping the final physical field from the previous stage to the next stage, four successive constraint removal processes are performed at different time points. Subsequently, we establish a time-varying model for the entire process of directional solidification and constraint removal. The errors of the extraction/mapping are consistently less than 0.002 mm in all three directions, thus confirming the feasibility of data transfer. Secondly, we analyze the dimensional deviation patterns of the casting after sequentially removing the mold shell, process rib, grain selector, and ceramic core constraints, with emphasis on the dimensional accuracy of the airfoil while disregarding the dovetail generated from machining. The final average deviations of Section 1, Section 2, and Section 3 of the airfoil are 0.1896 mm, 0.1723 mm, and 0.1909 mm, and the maximum deviations occur consistently at the trailing edge near the tip. Finally, three section deviations are validated via measurements performed using a vacuum furnace and industrial computed tomography. The results show that the proposed method improves the dimensional accuracy of hollow turbine blades.