Casting Considerations in Alloy Selection

Selection of cast superalloys is best conducted in conjunction with an investment casting foundry which can indicate the likelihood of success in casting a given design in the alloy selected. Figure 7 shows shapes and surface appearance of turbine blades produced to the three standard casting conditions (equiaxed, columnar grain, and single crystal). These parts also contain complex internal cooling passages that effect the necessary cooling required for airfoil operation in ultra-hot-gas streams.

https://cdn.lucideon.com/service-portfolios/engines_turbine_blades_interactive_sml1.jpg

Not all alloys are equally good for casting. Casting defects occur and vary with the alloy composition and casting practice. Oxide (dross) carried over from the master melt may cause surface and internal inclusion problems. Porosity is another major concern, especially with large castings for the cases of gas turbines. Porosity may be controlled by mold design and pouring modifications. Non-surface-connected porosity may be closed by hot isostatic pressing (HIP) of cast articles. Surface porosity in large castings may be repaired by welding. Ceramic inclusions that evidence themselves as surface pits or subsurface x-ray indications could result from the breakdown of the ceramic crucible used for melting, ceramic filter media, or the ceramic shell. Metallic inclusions can also result from the improper melting of alloy elements or segregation of the elements during casting. Other casting concerns have included intergranular attack (IGA) caused during chemical removal of molding materials, selectively located coarse grains in PC materials, misoriented grains, and spurious grains in directionally solidified (DS), materials, etc. Alloys may be modified in chemistry to optimize product yield but with a possible compromise in properties. Product yield is an important determinant of final component cost.