Development of Non-reactive Mold for Investment Casting of Ti-48Al-2Cr-2Nb Alloys
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GAMMA titanium aluminide (c-TiAl) has a good potential for replacing Ni-based superalloys owing to its low density and excellent high-temperature properties, such as corrosion resistance and structural stability.[1] Therefore, titanium aluminide is suitable for automotive turbochargers because the weight saving of the turbocharger reduces the turbo-lag.[2–4] The c-TiAl turbocharger exhibits the maximum response time of the turbocharger acceleration that is 26 pct faster than that of the Inconel 713C turbocharger.[3] However, parts made of the c-TiAl alloy are difficult to manufacture because of its high melting temperature, low fluidity, and workability.[3, 5] Many studies have been performed for improving the castability by adding an alloying element or applying special casting technology.[6–9] In particular, the c-TiAl turbocharger part is extremely SEUL LEE, MYEONG-SOO SHIN, and YOUNG-JIG KIM are with the School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 16419, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted June 14, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
difficult to manufacture owing to its complex shape and thin section. Therefore, we apply centrifugal investment casting to complement the low fluidity of c-TiAl. Investment casting not only has a high dimensional accuracy for complex shapes but also is cost-effective, providing a smooth surface.[10] However, titanium and titanium alloys have a high reactivity with oxygen, nitrogen, etc., and a reaction layer is formed on the casting surface, resulting from the reaction between the titanium alloy and the oxide mold.[1,11] This reaction layer degrades the mechanical properties of castings, especially the ductility and fatigue property, by acting as a microcrack initiation site in the service environment.[12] Therefore, the reaction layer must be removed via chemical milling before use. However, the milling process is expensive and limits the dimensional tolerance. From this viewpoint, studies have been conducted on reaction-layer control using various mold materials.[1,13,14] The zircon is widely used refractory in investment casting because of its excellent thermo-physical properties.[15] Alumina is more refractory than silica and is less reactive to many metals than siliceous refractories.[16] However, the c-TiAl alloy has a high reactivity with zircon and alumina molds and easily forms a reaction layer. Jia [13] reported that the depth of the reaction layer in Ti-46Al alloys was 80 lm when an
alumina mold was used. It is well known that a yttria mold having high chemical inertness can reduce the reaction layer. A reaction layer of 25 lm is formed by the reaction between Ti-48Al and a yttria mold.[13] Although the yttria mold hardly reacts with the c-TiAl alloy, it is difficult to use widely because of its high cost, short slurry lifetime, and poor sintering hardness.[14,17] Additionally, with the use of colloidal silica as a binder, the yttria mold
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