The casting of titanium and titanium alloy is usually made of gray cast iron (htl50 or HT200), cast steel (zg200-400), cast copper (pure copper or brass) and a very small amount of refractory metal (such as molybdenum). Except for refractory metals, the melting points of these materials are all lower than that of titanium. They can be used as molds for casting titanium alloys because:
(1) the thermal conductivity of these materials is higher than that of titanium, and the heat generated by the pouring of titanium liquid into the mold is transferred rapidly;
(2) the thermal conductivity of titanium is low. After the titanium liquid scratched the mold and contacted with the mold surface, it solidified immediately to form a thin shell, which can effectively increase the temperature gradient of holding mold.
The above metal mold materials have their own advantages and disadvantages:
(1) cast iron has good rigidity, easy processing, low price and long service life. It is stable for molten titanium at low temperature. At the same time, because of the flake graphite structure in it, titanium liquid has good wettability, which is conducive to the filling of titanium liquid and the forming of castings. Its disadvantages are high coefficient of linear expansion and low melting point. Therefore, it is usually only suitable for the outer mold of casting, not for the core.
(2) cast steel is similar to cast iron, with higher strength and durability than cast iron, but its deformation is slightly larger than cast iron.
(3) the thermal conductivity of cast copper is very high, but its strength is low and its rigidity is poor. It can only be used to make small castings with high price.
(4) refractory metal is resistant to high temperature, high hardness, wear-resistant, but very brittle, expensive, and poor processability. Molybdenum is mainly used as die-casting punch.
The key of the metal mold casting lies in the design of the mold, which is directly related to the quality and dimensional accuracy of the casting, the life of the mold, the processing and manufacturing cost of the mold. In addition to the design principle of common metal mold, the characteristics of titanium and titanium alloy casting must also be considered in the design, which can be summarized as follows:
(1) bottom injection shall be adopted as far as possible;
(2) open pouring system shall be adopted;
(3) centrifugal casting shall be adopted as far as possible, and the structure and fixing mode of metal mold shall be designed according to the requirements of centrifugal casting.
In order to reduce the cost of the metal mold and improve the service life of the metal mold, the metal mold and the machined graphite mold can be used together. Graphite inlaid block can be used for mold cavity which is not easy to be lifted and damaged, and machined graphite can be used for all pouring systems. Titanium alloy artificial joint mold is a combination of metal mold and machined graphite. The machined graphite parts must be baked at a higher temperature (no more than 400 ℃) before pouring, preferably in vacuum at 950-1000 ℃ for 1.0-2.0h to remove the adsorbed gas.