Study on metal mold for titanium alloy casting

Titanium alloys are widely used in aerospace, petrochemical, biomedical and other fields because of their high specific strength, high specific stiffness, low density, good biocompatibility, excellent corrosion resistance and fracture toughness. Precision casting technology can realize a near-net forming of complex structures, and is the first choice for economic and efficient forming of titanium alloys.

However, in the actual casting process, because of the characteristics of high chemical activity of titanium alloy in the high-temperature melting state, titanium alloy puts forward higher requirements for casting materials. At present, the commonly used titanium alloy casting materials mainly include graphite and yttrium oxide and other rare earth oxide materials, the cost of the above-mentioned casting materials has continued to rise in recent years, and the modeling cost has accounted for more than 30% of the casting cost, resulting in a long-term high casting cost. The graphite type is easy to be damaged during the pouring process, and the service life is generally short, which is not conducive to the mass production of titanium castings; The ceramic process of investment casting is complicated, involving mold design and multi-layer coating, drying and roasting of mold shell, which increases the production cycle of titanium castings to a certain extent. Therefore, how to choose the appropriate process method and casting material for different castings has become the key to high quality, low cost, high efficiency and mass production of titanium alloy casting.

Because of its simple mold processing, small processing pollution, mold can be used repeatedly, high production efficiency, low casting cost in mass production, stable casting size and high precision, the metal process has a broad application prospect in the field of non-complex titanium alloy products. Pratt & Whitney practical research shows that when the titanium alloy is cast by metal mold process, the cost is reduced by about 40% compared with the investment ceramic process, and the comprehensive mechanical properties of the casting are better. At present, the metal mold casting technology has been applied to manufacture the 4th and 5th high pressure compressor flame retardant titanium alloy vane of F119 engine. The metal casting process of titanium alloy exhaust valve was studied by EMTEC in the United States. The samples were cast with metal mold and ceramic mold respectively. The tensile strength and yield strength of the samples cast with metal mold were relatively good. However, the metal casting process has low complexity, the surface of the casting is easy to produce flow marks and cold insulation, α brittle layer and other defects, thick titanium castings are easy to fuse and bond with the cast, and the service life of the cast is low, which greatly limits its wide use.