Centrifugal investment casting of Ti-47Al-6Nb exhaust valves

Abstract:

The shell mold filling process for high-Nb TiAl alloys exhausts valve castings by centrifugal investment casting is described in the paper. Compositions of the castings and shell mold are Ti-47Al-6Nb (at.%) and Zircon sand, respectively. As a comparison and verification of the simulation results, a series of shell mold filling experiments of centrifugal casting is carried out. Simulation results indicate that the shell mold filling time is about 3.8s; casting defects distribute along the central line of the exhaust valves, especially in the neck position. The comparison shows that the results of the simulation agree with those experiments. The powder/liquid ratio of 1.5:1 and mixing time of 30min is identified as the proper process for the inner layer slurry in the making shell mold process. There are no noticeable interfacial reactions between casting and shell mold using XRD analysis. The metallurgical analysis results reveal that the lamellar structure is the typical structure for the as-cast exhaust valves, with the grain size of 185-305μm.

Key words:exhaust valves; numerical simulation; macroscopic defects; microstructure