Investment Casting of Thin Sections in Superalloys

The ability to create highly complex components through investment casting of superalloys is extremely important for producing turbine blades. Often, the sharp corners in the castings will not fill completely, resulting in unintentionally rounded corner which must be machined to achieve the final, required dimensions. By better understanding the interactions between the liquid metal and the mold, the as-cast shape can be improved, producing higher quality castings. A 2D, analytical model was developed to predict the ability to fill a V-shaped channel lined with cylindrical particles. The model was found to accurately predict the ability to fill a channel based on the contact angle, particle radius, corner angle, and head height. The predictions from the analytical model were verified using small-scale experiments and the 3D surface modeling software, Surface Evolver. The head height and contact angle were found to have the biggest impact on the ability to fill, while the particle radius has a smaller effect on filling. Sessile drop experiments with pure nickel droplets on alumina substrates of varying roughness, similar to those found in industrial investment casting, in flowing argon found that the measured, or effective, contact angle can be altered by approximately 20° based on the substrate surface roughness, with surfaces that are more rough having higher effective contact angles. There is little benefit to decreasing the surface roughness below a -325 mesh particle size (?44 ?m), but increasing the surface roughness above a -325 mesh will negatively impact corner filling. The 2D analytical model was used to design a casting to measure the ability to fill fine features similar to those in turbine blades as a function of head height, feature size, and corner half-angle. The casting was produced from MAR M247 using conventional investment casting processes. The casting shows a dependence of the ability to fill sharp corners on head height, while the mold and wax pattern do not. This proves that the rounded corners found in castings can be due to the inability to fill the sharp corners in the mold