Evaluation of the Inertness of Investment Casting Molds Using Both Sessile Drop and Centrifugal Casting Methods
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TITANIUM aluminide alloys with an Al composition around 45 to 50 pct have been widely used in the automotive industry since the last century and are now being used for low pressure turbine blades for aerospace applications.[1] Compared with other titanium alloys, the benefits of using near c-TiAl alloy are that they have very good density-corrected rupture toughness, stiffness, and creep resistance at a working temperature around 973 K (700 C).[2] Because of the chemical heterogeneity and the physical properties of the c-TiAl alloys, many efforts have been made to introduce titanium aluminide into the market, but with limited success. The principle which holds back the ‘‘mass market’’ manufacture of TiAlbased components is that TiAl has very high chemical reactivity, high melting temperature, low ductility, and poor workability. Due to these problems, wrought methods to produce TiAl alloy components such as forging and rolling which are very prone to chemical and microstructural inhomogeneity are rarely used, while powder metallurgy is considered very expensive.[3] Investment casting, which can directly produce near netshaped components with a good surface finish and low production cost, is subject to growing interest.[4]
XU CHENG, Ph.D. Student, CHEN YUAN, Research Fellow, and NICK GREEN, Professor, are with School of Metallurgy and Materials, The University of Birmingham, Birmingham B15 2TT, U.K. Contact e-mail: [email protected] PAUL WITHEY, Professor, is with School of Metallurgy and Materials, The University of Birmingham, and also with Rolls-Royce plc, P.O. Box 31, Derby, DE 24 8BJ, U.K. Manuscript submitted February 27, 2012. Article published online October 10, 2012 888—VOLUME 44A, FEBRUARY 2013
In recent decades, yttria-based refractory materials have been used to make the face coat slurry for investment casting TiAl.[5] Yttria, as a rare earth metal oxide, has a very high chemical inertness, better than many other metal oxides such as Al2O3,[6] SiO2,[7] ZrO2,[8] and CaO[9] (against molten TiAl alloys). The reported interaction layer thickness between the metal and ceramic interface using an yttria face coat was less than 5 lm.[10–12] Because of the high melting temperature of yttria, around 2683 K (2410 C), it requires a high sintering temperature to achieve a dense structure [>1773 K (1500 C)]. The investment casting molds used in industry are normally sintered at temperatures around 1473 K (1200 C), which will result in many yttria inclusions at the casting surface and degrade the mechanical properties of the casting.[13] In order to enhance the sintering properties of yttria at low sintering temperatures, sintering additives such as B2O3,[14] TiO2,[15], Al2O3,[16] ZrO2,[17] and fluorides CaF2[18] or LiF[19] were selected and added into yttria. The addition of sintering aids can effectively influence powder sintering by promoting mass diffusion at lower sintering temperatures, which decrease the powder sintering temperature. But, these sintering additives may also decrease the chemical inertne
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