Research Status of Investment Casting Shell Performance Strengthening

Abstract： Near net shape investment casting is a special casting method of less-cuttings, it is widely used in the complex structural parts of aerospace and automobile industry with bulk-production. In this paper, the research status of refractory material, binder and additives on shell-strengthen performance were summarized.

Investment casting is a kind of liquid metal forming process with little or no cutting, which is an excellent technology in the casting industry and is widely used. It plays an irreplaceable role in high-precision and complex structural castings such as aeroengine turbine blades, gas turbine blades, etc., especially in the forming of high melting temperature and high chemically active metals. Investment casting also has broad application prospects in the development and production of biomedical materials and radar products. The organic combination of rapid forming and investment casting technologies 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 process, the requirements of "large, thin, fine and strong" are put forward, and higher requirements are put forward for the properties of investment casting shell such as strength and permeability. The quality of investment casting shell directly affects the quality of castings, in order to obtain high-quality castings, high-quality shell must be manufactured, high-quality shell should meet a series of performance requirements such as strength, permeability and concessions. Therefore, shell making is one of the key processes of investment casting technology, and the research of shell comprehensive performance strengthening has become the core topic in the field of investment casting. In this paper, the strengthening of the comprehensive performance of investment casting shell is discussed. By analyzing the process characteristics of investment casting molding materials and shell manufacturing technology, the research status of strengthening the comprehensive performance of investment casting shell at home and abroad is introduced mainly from the aspects of the strength and permeability of the shell. In order to improve the comprehensive performance of investment casting shell in the future, Improve the quality of shell and castings to provide a certain reference.

1 Optimize molding materials to enhance investment casting shell

Investment casting mold has solid shell and multilayer shell, except gypsum mold using solid cast, generally use multilayer shell, shell from macro to micro is a heterogeneous structure system. Coating is the basis of shell structure, and the main materials for making shell are refractory materials (powder, aggregate) and binder. The process performance of the coating has a certain effect on the performance of the shell, and the performance of the coating is affected by different binders and refractories, so the shell is strengthened by optimizing the shell material.

1.1 Influence of refractory materials on shell performance

In investment casting production, refractory material accounts for about 90% of the total weight of mold shell, which has great influence on the comprehensive performance of mold shell. After roasting, the binder and the refractory particles are tightly bonded, so that the shell has a certain strength. Therefore, the analysis of the influence of refractory materials on the comprehensive performance of the shell needs to be comprehensively considered from the phase composition of the refractory material and its own strength, hardness and density.

1.1.1 Graphite shell

Graphite is an allotrope of carbon, carbon refractors-artificial graphite is used for investment casting shell, its main raw materials are petroleum coke and asphalt, calcined by pressing block and 2 600~3 000 ℃ high temperature. Artificial graphite has high refractoriness under vacuum, small thermal expansion coefficient, and low porosity, and its strength increases with the increase of temperature.

Using epoxy resin and phenolic resin two kinds of graphite investment casting technology, using the prepared paint and various particle size of graphite sand to coat the hanging shell, the large size of the fan blade can be produced. It is found that the epoxy resin graphite shell has good performance, the shell has certain strength and toughness, and the surface quality is easy to guarantee. The phenolic resin graphite shell has sufficient strength, good replication, low cost, and no obvious soft phenomenon during roasting, which is superior to the epoxy resin graphite shell process. Lirones of Howmet Company in the United States introduced the process and use of graphite melting model shell in his published patent, the artificial graphite preparation coating, by coating, roasting made with a certain thickness and strength of the shell, and has a good concession. Therefore, the use of graphite sand sand or graphite preparation slurry to make shells, at high temperatures, the reaction of graphite with oxygen to form a reducing atmosphere, to protect the high-temperature metal injected into the mold from oxidation. Graphite also has insulation at high temperature, which can increase the filling ability of liquid metal to the thin-wall 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 gas permeability of the shell is increased by using the oxidation of graphite during the roasting process to form uniform dispersed micropores in the shell.

1.1.2 Shell made of oxide materials

Oxide ceramics are the most commonly used casting coating materials, with great advantages, its high melting point, good insulation, can reduce the cooling rate of metal liquid, easy to better fill the mold, investment casting commonly used oxide ceramics Y2O3, Al2O3, CaO, MgO, etc., for active metal casting materials. In the study process, the researchers combined with different binders to select a reasonable oxide ceramic refractory, by changing the powder particle size or the oxide binder and oxide powder to prepare the coating, so as to optimize the coating, its coating performance has been improved. Yao Jiansheng et al. studied the effects of three different particle sizes of Al2O3 powder on the rheology of silica sol coating and the strength of ceramic shell at room temperature and high temperature, and found that the shell made of 80 μm Al2O3 powder had good strength at room temperature, but poor strength at high temperature. Meanwhile, Qi Xiu-mei et al. investigated the properties of three surface materials, Y/Y(coating formed by yttrium oxide glue and yttrium oxide powder), ZY/ZY (coating formed by yttrium oxide stabilized zirconia powder and yttrium oxide stabilized zirconia powder) and TJ/ZC(coating formed by domestic adhesive and yttrium oxide stabilized yttrium oxide powder) Through the 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 layer material has high residual strength, strong ability to withstand the impact of liquid metal, the surface layer shell is not easy to fall off, and the surface quality of the cast is good, indicating that TJ/ZC material is an ideal surface material for titanium alloy precision casting.

Binder is one of the important materials of investment casting shell, it affects the performance of shell, casting quality, but also affects the production cycle and cost. At present, the binders widely used in the domestic investment casting industry are water glass, silica sol and composite binder. The composite shell is the improvement or optimization of water glass shell, which is superior to the former in casting surface quality, yield and repair rate, but it is still different from the silica sol shell. In addition to the short production cycle and slightly lower shell cost, the quality and stability of its casting is not as good as that of silica sol shell, so silica sol is an ideal binder with good shell performance and high quality, and has been widely used in the investment casting industry.

1.2.1 Water glass shell making

Water glass binder has the advantages of low cost, short production cycle, excellent peeling property and high permeability, but the low strength of water glass shell making leads to the serious phenomenon of drum shell and gunfire. The main factors affecting the high temperature strength of the shell and the coating process performance are the type of refractory powder, particle size distribution and the modulus and density of water glass. Low modulus can improve the powder liquid ratio of the coating, but the coating property is poor, which is not conducive to the formation of the dense structure of the sand completely covered by the coating, and the formation of silica gel is small, so the high temperature strength and normal temperature strength of the shell are reduced. However, the high modulus increases the viscosity of the coating, reduces the powder to liquid ratio, and deteriorates the process performance of the coating. By adding rare earth oxide La2O3 and other additives to the silicate quartz shell, Zhang et al. conducted experiments on the high temperature strength, residual strength and permeability of the silicate quartz shell, and found that the addition of an appropriate amount of rare earth oxide La2O3 could significantly improve the high temperature strength of the silicate quartz shell, and had little effect on the permeability of the silicate shell.

1.2.2 Preparation of silica sol shell

Silica sol is easy to be prepared into high-quality coating with high powder to liquid ratio, good coating stability, and the shell does not need chemical hardening and has good high temperature strength and high temperature deformation resistance. It is especially suitable for the production of stainless steel and high alloy steel castings, but the wet strength of the shell is low and the shell making cycle is long. Compared with ordinary silica sol, the enhanced quick-drying silica sol developed by Lv Zhigang et al. [10,11] can quickly establish the wet strength of the shell and increase the final wet strength of the shell. However, silica sol coating has poor wettability to wax mold, needs to add wetting agent and defoamer, has slow drying speed and low wet strength. Konrad et al. [12] used silica sol and ZrSiO4 as the surface layer of superalloy IN738LC alloy investment casting shell, and the back layer was prepared with polyalusalusite and silica sol. By measuring the transient temperature of the shell, the heat transfer coefficient of the shell was calculated to be between 300 and 660 W/(m2·K). The drying time is a key process in the preparation of silica sol shell. The popularization of fast drying silica sol will greatly shorten the drying time of silica sol shell. Because with the drying of the shell, the silica sol content increases, the collision probability of colloidal particles increases, and the refractory particles are firmly bonded together to enhance the strength of the shell.

1.2.3 Composite shell

The single use of sodium silicate, silica sol, ethyl silicate binder has become more and more difficult to meet the performance of investment casting shell production, so the composite shell manufacturing precision castings has been studied by the majority of scholars. Zhou Zeheng et al. [14] prepared a mixed binder with silica sol and ethyl silicate. The high temperature strength of the shell made by the mixed binder with silica sol as the main body was about 7.3MPa and the normal temperature strength was about 3.5MPa, which was higher than that of the shell made by the binder with ethyl silicate as the main body. The shell quality has been improved by using the composite process of silica sol-ethyl silicate alternately, and the surface quality and dimensional accuracy of the stainless steel precision castings produced have been greatly improved [15]. At the same time, the ethyl silicate-silica sol composite shell process can achieve near-net investment casting, and the resulting casting accuracy can reach CT5 level, and the surface roughness can reach Ra0.8~ 1.6μm. Although the process of alternately hardening with ethyl silicate and silica sol can improve the high-temperature strength and deformation resistance of the shell, the service life of the composite shell is shortened because of the mutual pollution of the two coatings. Therefore, Yang Shizhou et al. [16] studied the preparation process of a high-performance investment casting composite shell, in which ethyl silicate coating was used for the 1-3 layers and silica sol coating was used for the back layer, which improved the utilization rate of the coating and was conducive to the reduction of production costs. Since 1983, Wuhan Precision Foundry has carried out experiments on the ethyl silicate-sodium silicate composite shell, and under the cooperation of Tsinghua University, gradually explored a new composite shell technology, which only uses ethyl silicate coating on the surface layer and sodium silicate coating on the back layer, thus improving the strength of the shell and improving the surface quality of investment castings. Reduced casting scrap rate [17,18]. In order to solve the problem that the shaped shell made of zirconium sol binder cannot be dewaxed with hot water and the interface reaction between silica sol binder and titanium alloy is relatively large, Li Jianwei [19] developed a composite binder based on the characteristics of the two binder. The experiment proved that it has good wettability for wax mold, good chemical stability for titanium liquid, and can be dewaxed with hot water, which is harmless to human body. And more environmentally friendly.

2 Additives to strengthen investment casting shell

Some additives are added to the coating to strengthen the properties of the shell, thereby improving the quality of the casting. In investment casting, the amount of mineralizing agent added and the concentration of silica sol have a significant impact on the strength and chemical stability of ceramic shell. With the increase of mineralization dose, the content of SiO2 participating in secondary mullitization increases, and the formation of mullite structural phase at high temperature increases, which is conducive to strengthening the connection strength between corundum sand and thus improving the strength of ceramic shell [20]. The polymer is added to the quick-drying silica sol to strengthen the shell, which has a series of products in foreign countries, so that the drying time between the shells is greatly shortened, the wet strength and high temperature strength of the shell are high, the residual strength is low, and the shell is easy to peel. Fiber reinforced fast drying silica sol shell, which has strong adhesion to wax mold and sand particle, good permeability of shell, and short interlayer drying time [21-23].

2.1 Effects of composite additives, acicular carbon and pulverized coal on shell properties

The polymer film formed by the polymer molecules in the composite additive has a physical adhesion effect on the silica sol particles. In the drying process of silica sol, with the evaporation of water, the molecular SiO2 in the colloidal molecular group accumulates, making the sol become a gel, and the binding effect of the polymer molecules on the silica sol particles accelerates the aggregation process. The critical concentration of silica sol is reached faster. Yao Zhuo [24] referred to the production principle of quick-drying silica sol, added polyenol polymer and dimethylsilicone oil latex as composite additives into the coating to make shells, and found that the complete drying time of the shell test block was reduced from 7 h to 3 h, greatly improving the drying rate and shortening the production cycle. At the same time, the normal temperature strength of the shell is further improved and the residual strength is reduced. After roasting, the dense surface of the shell has small dispersing pores formed by the burning of composite additives, which improves the permeability of the shell. Aiming at the air permeability of the high-temperature ceramic shell of single crystal blade and the casting back slurry, acicular carbon was added as a filler to improve the high-temperature ceramic shell of single crystal blade [25]. Due to the volatilization of acicular carbon, many small pore structures appear in the shell, which significantly improves the permeability of the shell. The addition of acicular carbon can also greatly improve the coating performance and collapsibility of the coating, and can maintain sufficient strength in the process of drawing single crystal blades, which is a high temperature ceramic shell with excellent performance. The addition of fly ash as the material of the back shell can not only reduce the production cost of the investment casting industry, but also reduce the environmental pollution. Adding a small amount of fly ash has little effect on the bending strength of the shell, and the content of fly ash is between 2% and 4% [26].

2.2 Influence of fiber on shell properties

Fiber has the characteristics of high strength and light weight, and the shell is strengthened by fiber in investment casting to obtain the shell with high performance and high quality. Q. B. iu et al. [27] added fiber into the refractory powder and added it into the shell by means of sand scattering. After measuring the strength of the shell, it was found that the fracture toughness of the shell increased with the increase of the fiber content, and the addition of fiber could reduce the cracking of the shell. C. uan et al. [28] added nylon fiber with length of 1~1.5 mm to the shell to strengthen the shell properties, and found that the addition of fiber can significantly increase the thickness of each layer of shell and reduce the time required for shell making, while the wet strength of the shell has little change. At the same time, after calcination of the shell, the nylon organic fiber is burned away, leaving many micropores in the shell, which improves the permeability of the shell. P.c. Metity et al. [29] enhanced the strength of the shell by adding ceramic fibers to the shell, and the ceramic fibers would not be lost during high-temperature roasting, but would remain in the shell to increase the high-temperature strength of the shell. With the increase of the amount of ceramic fibers, the strength of the shell would increase.

Fiber reinforced investment casting shell, the presence of fiber is conducive to the water transfer from inside to outside, the shell drying speed is accelerated; The fiber forms a three-dimensional network structure in the shell, which can increase the wet strength of the shell when dewaxing, and the organic fiber is burned off during the shell roasting

3 Conclusion

(1) With the development of the aerospace industry, there is a high demand and demand for investment casting parts. Refractory powder accounts for 90% of the total weight of the mold shell and has a significant impact on its performance. Therefore, low-cost and stable refractory powder is a research focus. Research and optimize the characteristics of refractory materials based on the different layers of different types of shells, in order to improve the performance of the shells.

(2) Composite shells can meet the performance requirements for manufacturing general investment precision castings, but it is difficult to meet the technical requirements or control the manufacturing process when manufacturing high melting point and high activity high-temperature alloy investment castings for aviation, aerospace, and other industries. The silica sol shell, due to its high shell quality, stable casting quality, and low repair rate, has still been widely used in China's precision casting industry and is also a future development direction.

(3) The use of fiber-reinforced investment casting for shell has become possible, but the influence of fiber characteristics on investment casting slurry is particularly important. The length, particle size, and ratio of fibers need a lot of experimental and mechanistic research to optimize the optimal fiber characteristics and make the shell performance more stable.