Titanium alloy casting of the modeling process is mainly metal, machining graphite, metal surface, ceramic shell, oxide ceramic shell.
Metal type in the field of titanium alloy casting, metal materials used for casting are mainly copper, steel, cast iron, tungsten, molybdenum, and graphite processing together collectively referred to as hard mold system. Due to the difficulty of the classification of the process, this method is difficult to produce complex shapes of titanium castings, and most of the use of precise castings only.
Machined graphite type of high strength, the concession is not good, liquid titanium to produce chill, the titanium casting surface cracks and cold separation, high production costs, long production cycle. Graphite porosity larger, easy to absorb moisture, so the machine must be used before processing graphite degreasing, degassing treatment, or titanium surface oxidation serious. Casting size is relatively large, thick wall (5mm), the shape is simple, the required number of only one or several pieces. Choose machined graphite.
Metal tiles ceramic shell with refractory metal tungsten powder as a refractory material, metal tungsten high melting point, contact with titanium liquid chemical stability, but tungsten powder should have a high purity, impurity content can not exceed the prescribed standard, otherwise Will affect the quality of titanium casting. Tungsten surface melting mold shell must be solvent dewaxing, and a special dewaxing tank, the human health has a lot of damage. But also pollute the environment. Tungsten surface type shells are fired at high temperature in a reducing atmosphere
, the dewaxing of the shell in the appearance of the mold as is difficult to burn, in the pouring is easy to react with the liquid titanium, in the casting appearance of the formation of pores. Paint slurry process performance is poor, poor suspension, coating slurry life is short, tough to save, expensive.
The oxide ceramic shell is an inert oxide as a shell-type shell refractory. The order of the various oxide materials according to their chemical stability to the molten titanium alloy is this : SiO2, MgO, Al2O3, CaO, ZrO2, Y2O3, ThO2. ThO2 has been essentially free of radioactivity. CO is easy to absorb moisture, so it hinders its application. Now, the material used for the investment casting shell layer and the adjacent layer is mainly Y2O3, ZrO2.
The ZrO2, which has not been stabilized, can not be used as a casting material for cast titanium because it undergoes allotrope transformation, monoclinic at room temperature, tetragonal crystals at high temperature, and higher temperature is converted into cubic crystals, Crystal into a tetragonal crystal, accompanied by about 9% of the volume changes, so that the shell cracking. Usually in the ZrO2 by adding 4% to 8% of the CaO, after high temperature fusing or calcination can be stable ZrO2 solid solution (also useful Y2O3 stability), most of the industrial use of fused ZrO2.
Y2O3, like ZrO2, must be subjected to extreme temperature stabilization before it can be used as titanium alloy molding materials. Y2O3 ceramic shell with low thermal conductivity, high strength, pouring out the titanium casting surface quality is good, but Y2O3 price is more expensive, the source is obscure.