Research Status of Investment Casting Shell Performance Enhancement

Investment casting is a liquid metal forming process with little or no cutting. It is an excellent process technology in the casting industry and is widely used. It plays an irreplaceable role in the forming of high-precision and complex structure castings such as aircraft engine turbine blades, gas turbine blades, etc., especially in the forming of high melting temperature and high chemical activity metals. Investment casting also has broad application prospects in the research and production of biomedical materials and radar products. The organic combination of rapid prototyping and investment casting has greatly promoted the rapid development of investment casting products, improved production efficiency and manufacturing flexibility, and also complied with the rapid market demand of products.

With the continuous improvement of investment casting production technology, the requirements of "large, thin, precise and strong" are put forward for investment castings, and higher requirements are put forward for the performance of investment casting shells such as strength and permeability. The quality of investment casting shell directly affects the quality of castings. To obtain high-quality castings, high-quality shells must be manufactured. High-quality shells should meet a series of performance requirements such as strength, permeability and concession. Therefore, shell making is one of the key processes of investment casting technology. At the same time, the research on strengthening the comprehensive performance of the shell has also become a core topic in the field of investment casting. This paper discusses the strengthening of the comprehensive performance of the investment casting shell. By analyzing the process characteristics of the investment casting mold materials and shell manufacturing technology, it mainly introduces the research status of strengthening the comprehensive performance of the investment casting shell at home and abroad from the aspects of the strength and air permeability of the shell, which provides a certain reference for improving the comprehensive performance of the investment casting shell and improving the quality of the shell and castings in the future.

1 Optimizing the molding material to strengthen the investment casting shell

The investment casting mold has a solid shell and a multi-layer shell. Except for the gypsum mold, which uses a solid mold, a multi-layer shell is generally used. The shell is a multi-phase heterogeneous structure system from macro to micro. The coating is the basis of the shell structure. The main materials for making the shell are refractory materials (powder, aggregate) and binders. The process performance of the coating has a certain influence on the performance of the shell, and the performance of the coating is affected by different binders and refractory materials, so the shell is enhanced by optimizing the shell material.

1.1 The influence of refractory materials on the performance of the shell

In the production of investment casting, refractory materials account for about 90% of the total weight of the shell, and have a great influence on the comprehensive performance of the shell. After the shell is roasted, the binder and refractory particles are closely combined, so that the shell has a certain strength. Therefore, to analyze the influence of refractory materials on the comprehensive performance of the shell, it is necessary to comprehensively consider the physical composition of the refractory materials and their own strength, hardness and density.

1.1.1 Shell made of graphite

Graphite is an allotrope of carbon. Carbon refractory materials-artificial graphite are used for investment casting shell. Its main raw materials are petroleum coke and asphalt, which are pressed and calcined at a high temperature of 2600~3000℃. Artificial graphite has high refractoriness under vacuum, low thermal expansion coefficient, and low porosity. Its strength increases with increasing temperature. Using epoxy resin and phenolic resin graphite

investment casting process, large-sized fan hollow blades can be trial-produced by coating the shell with prepared coating and graphite sand of various particle sizes. It is found that the epoxy resin graphite shell making process has good performance, the shell has certain strength and toughness, and the surface quality is easy to ensure. The phenolic resin graphite shell has sufficient strength, good replicability, low cost, and no obvious softening phenomenon during firing, which is better than the epoxy resin graphite shell making process. Lirones[2] of Howmet Company in the United States

in his published patent introduced the process and use method of graphite melt casting shell. Artificial graphite is prepared with coating, coated and fired to form a shell with a certain thickness and strength, and has good yield. Therefore, the shell is made by using graphite sand or graphite slurry. At high temperature, graphite reacts with oxygen to form a reducing atmosphere to protect the high-temperature metal injected into the mold from oxidation. Graphite also has thermal insulation at high temperature, which can increase the filling capacity of the metal liquid into the thin-walled cavity of the mold shell, change the local heat dissipation capacity of the mold shell, and change the cooling and solidification sequence of the casting. Secondly, the oxidation of graphite during the roasting process of the mold shell is used to form uniformly dispersed micropores in the mold shell, thereby increasing the permeability of the mold shell.

1.1.2 Shell Making with Oxide Materials

Oxide ceramics are the most commonly used casting coating materials, which have great advantages. They have high melting points and good thermal insulation. They can reduce the cooling rate of the metal liquid and facilitate better filling. The oxide ceramics commonly used in investment casting are Y2O3, Al2O3, CaO, MgO, etc., which are used as casting materials for active metals. During the research process, the researchers selected reasonable oxide ceramic refractory materials in combination with different binders, and optimized the coating by changing the powder particle size or mixing the oxide binder with the oxide powder to prepare the coating, thereby improving its coating properties. The effects of three different particle sizes of Al2O3 powder on the rheology of silica sol coating and the room temperature and high temperature strength of ceramic shell were studied, and it was found that the shell made of 80 μm Al2O3 powder has good room temperature strength, but poor high temperature strength. Qi Xiumei studied the properties of three surface materials, including Y/Y (coating formed by yttria glue and yttria powder), ZY/ZY (coating formed by yttria-stabilized zirconia adhesive and yttria-stabilized zirconia powder), and TJ/ZC (coating formed by domestic adhesive and yttria powder stabilized by calcium oxide). Through TG/DTA test, shell strength test and thermal expansion performance test, the residual strength of TJ/ZC surface shell increases linearly with the increase of roasting temperature ts, and the residual strength is between 4.1 and 6.0 MPa. The surface material has high residual strength, strong ability to withstand the impact of molten metal, the surface shell is not easy to fall off, and the surface quality of the cast casting is good, indicating that TJ/ZC material is an ideal surface material for precision casting of titanium alloys.