Development of Non-reactive Mold for Investment Casting of Ti-48Al-2Cr-2Nb Alloys

Abstract

The interfacial reactions between liquid Ti-48Al-2Cr-2Nb and various shell mold materials were investigated, and a cost-effective shell mold, containing a reacted component, was developed. Centrifugal investment casting was employed to supplement the low fluidity of the titanium aluminide alloy. The thickest part of the turbocharger was selected for analysis. The reaction-layer depth and the reacted product were evaluated using a micro-Vickers hardness tester and an electron microscope. The chemical composition and morphology of the molds were analyzed via X-ray diffraction and energy dispersive X-ray spectrometry. The TiAl alloys reacted with both colloidal silica and refractory materials in the alumina and zircon mold. However, only the colloidal silica reacted with TiAl in the yttria mold because of the high thermodynamic stability of the yttria refractory. The zircon mold showed the highest reactivity with TiAl because the zircon decomposed into ZrO2 and SiO2 at 1556 °C. The reactivity between TiAl and the mold decreased in the order of zircon > alumina > yttria ≥ rutile-containing alumina mold. The Ti5(Al, Si)3 or (Ti, Zr)5(Si, Al)3 phase was observed in the castings obtained using the conventional oxide mold. However, the reaction phase and layer were not observed in rutile-containing alumina mold. The molten TiAl penetrated the shell mold, especially in the colloidal silica site, although the penetration depth did not match the reaction-layer depth. The reaction and penetration of TiAl were controlled efficiently with the rutile-containing alumina mold.