Investment casting is one of the most important and effectively used methods to produce metal components with high dimensional accuracy, excellent surface finish, and complex shapes [1–4]. Since the 1950s, investment casting technology has been widely used for manufacturing complex components in aviation, aerospace, automobile, etc. Especially in aviation, most hot end components such as turbine blade, turbine rotor blisk, impeller, casing have been produced by this process [5–8]. The traditional investment casting includes wax pattern pressing and assembly, ceramic shell preparation, alloy melt pouring, and post-treatment [9–12].
Intermediate casing is one of the largest titanium alloy componets in an aero-engine, characterized by a large, thin-walled and complex structure. Such component is ususally manufactured by investment casting. Nevertheless, the process of design and manufacture of metal mold for wax pattern has to spend a lot of time and cost. The ever-changing 3D printing technology has brought new opportunities for the casting of complex structural parts [14–18], among which selective laser sintering (SLS) technology is the most prominent [14]. With polystyrene (PS) resin as raw material, the investment casting pattern with complex structure can be manufactured by SLS equipment. The low-temperature wax infiltration process can greatly improve the surface finish and strength of the pattern, thus replacing the traditional wax pattern for the investment casting of complex metal components. In recent years, with the great improvement of its precision control level and worktable size, SLS can be used for the preparation of investment casting investment shell of large complex thin-walled structural parts. Such as intermediate casing to remove the shell manufacturing and wax pattern pressing links and greatly shorten the manufacturing cycle and cost of the intermediate casing.
At present, gravity casting and centrifugal casting are usually used in the titanium casting industry [19–21]. The flow velocity and superheat of the titanium alloy melt in the shell ceramic shell are the basic process factors that greatly influence the filling ability of the gravity casting titanium alloy melt. Due to the use of ceramic shell melting furnaces in the titanium casting industry, the improvement of metal superheat is limited, and the increase of molten metal head is limited, so gravity casting is only suitable for casting titanium alloy castings with a simple shape and large wall thickness. Because the solidification speed of titanium alloy is very fast in the pouring process, and the interaction between the titanium alloy melt and the shell ceramic shell [22–24], a large amount of gas appears in the casting forming process in the shrinkage of the casting. The advantage of centrifugal casting is that it can improve the filling ability of metal melt and compact the casting structure. At the same time, the gas and slag in the melt move to the inner cavity of the casting under the action of centrifugal force so that it can reduce the shrinkage cavity, porosity, slag inclusion, and other defects in the casting. It is suitable for large, complex thin-walled castings with shell wall thickness less than 4mm. Large titanium alloy castings easily produce metallurgical defects such as shrinkage porosity during centrifugal casting, directly related to the gating system and gating process. If the metallurgical defects of large titanium alloy castings are solved by trial and error, the time and cost will greatly increase. In recent 20 years, simulation technology in investment casting has greatly helped to shorten the casting time. Using ProCAST and other software to simulate the casting process can verify whether the setting of riser and pouring process parameters are reasonable to use the best investment casting process to produce qualified castings [8, 25–27].
The outer diameter of a certain type of titanium alloy intermediate casing developed in this paper is more than 1m, and its structure is very complex. Titanium alloy has the characteristics of high melting point, low casting superheat, and large shrinkage tendency. In the process of liquid forming, it is easy to have casting defects such as cold shut, under casting, and porosity, which significantly affect the service life of parts. At the same time, if the intermediate casing is trial produced for 3 ~ 4 rounds, the appropriate casting process can be determined, which will greatly increase the development cycle and cost. Therefore, this paper prepared a PS investment shell for titanium alloy intermediate casing precision casting based on 3D printing technology; combined with the use of ProCAST software, the centrifugal casting process was simulated, and the casting process of titanium alloy intermediate casing was verified.