Mechanical property and microstructural characterization of vacuum die cast superalloy materials

Abstract

Application of the vacuum die casting process to high strength, high volume fraction nickel base superalloys and a high usage cobalt base alloy produced material with a novel fine grain, cast equiaxed microstructure. The fine grain structure was retained after HIP (Hot Isostatic Pressing) processing and subsequent heat treatment. Mechanical property testing showed that the fine grain structure resulted in increased strength and reduced stress rupture properties for materials typically produced via conventional investment casting. It should be noted that some of the investment cast alloys showed significant apparent hot tearing when processed through the die casting process due to the high cooling rates observed in the die casting process. The high volume fraction wrought disk/shaft alloy (Gatorized Waspaloy ref. 1) was also processed through the die casting process. A reasonably fine grain structure was achieved, however it was coarser than what is typically observed for the wrought form of the alloy. As would be expected from the coarser grain size, the die cast material exhibited lower strength and improved stress rupture capability relative to the wrought form of the alloy. No attempt was made to optimize the various alloy compositions for improved processing or mechanical property behavior using the die casting process or the die casting process parameters.