The metal mold process is considered to be one of the most potential processes in titanium alloy casting production because of its low pollution in mold processing, can be used repeatedly, high production efficiency, low casting cost in mass production, stable casting size and high precision. However, due to the strong chilling effect of the metal mold process, the surface of the titanium alloy castings contains defects such as cold insulation and flow marks, and the short service life of the metal mold process limits the wide application of the process. The research shows that coating the inner cavity of the mold can effectively solve the above problems. Based on this, the coating preparation process and its effect on the surface quality of titanium alloy castings were studied by combining with the interfacial thermal conductivity characteristics of metal mold casting materials used in titanium alloy. Finally, the optimum casting process of titanium alloy mold was determined, which significantly improved the mass production of titanium alloy castings.

Titanium alloy is widely used in aerospace, petrochemical, biomedical and other fields because of its high specific strength, high specific stiffness, low density, good biocompatibility, excellent corrosion resistance and fracture toughness. Precision casting technology can realize the near-net forming of complex structures, which is the first choice of titanium alloy forming economically and efficiently.

However, in the actual casting process, because of the high chemical activity of titanium alloy in the high temperature melting state, higher requirements are put forward for the casting materials of titanium alloy. At present, the commonly used titanium alloy casting materials mainly include graphite, yttrium oxide and other rare earth oxide materials. The cost of the above-mentioned casting materials continues to rise in recent years, and the molding cost has accounted for more than 30% of the casting cost, resulting in a long-term high casting cost. Graphite type is easy to be damaged in the pouring process, and its service life is generally short, which is not conducive to the mass production of titanium castings. The process of ceramic investment mold is complicated, involving mold design, coating, drying and roasting of mold shell, which increases the production cycle of titanium castings to a certain extent. Therefore, how to select appropriate process methods and casting materials for different castings has become the key to high quality, low cost, high efficiency and mass production of titanium alloy casting.

Because of the characteristics of simple mold processing, small processing pollution, mold can be used repeatedly, high production efficiency, low casting cost, stable casting size and high precision in mass production, metal mold process has a broad application prospect in the field of non-complex titanium alloy products. The practical research of Pratt & Whitney Company shows that when titanium alloy is poured by metal mold process, the cost is reduced by about 40% compared with the investment ceramic process, and the comprehensive mechanical properties of the castings are better. At present, metal mold casting technology has been applied to manufacture the 4th and 5th stage high pressure compressor flame retardant titanium alloy guide blades of F119 engine. American EMTEC Institute has carried out the research on the metal mold casting process of titanium alloy exhaust valve. The metal mold and ceramic mold were respectively used to cast the samples, and the tensile strength and yield strength of the metal mold samples were relatively good. However, due to the low complexity of the metal casting process, defects such as flow marks, cold insulation and α brittle layer are easy to occur on the surface of the casting, thick and large titanium castings are easy to fuse and bond with the casting, and the low service life of the casting greatly restricts its wide use.

In order to improve the service life of metal mold and eliminate the defects of cold insulation, flow marks and cold layer on the surface of titanium castings caused by the chilling effect of metal mold, this paper carried out research from the aspects of the coating process design of metal mold and the surface quality analysis of casting, laying a foundation for high quality and low cost mass production of titanium alloy casting.

1. Test materials and methods

1.1 Selection of metal casting materials

At present, the metals that can be used for casting materials mainly include cast iron, cast steel, cast copper and some refractory metals, and their main physical properties are shown in Table 1.

Table 1 Main physical properties of several metal materials

The metal casting material suitable for titanium alloy should meet several requirements.

(1) With high melting point: high melting point of titanium alloy, in order to avoid melting deformation of the cast in the solidification process, resulting in the bond between the cast and titanium castings. Therefore, the casting material needs to have a high melting point.

(2) Proper thermal conductivity: Strong thermal conductivity of the cast is conducive to improving the liquid cooling ability of titanium, significantly refining the internal structure of titanium castings, and improving the mechanical properties of titanium castings. However, too high thermal conductivity is not conducive to the filling of titanium liquid, which is easy to produce defects such as cold isolation, flow marks and micro-cracks. Therefore, the thermal conductivity of the metal mold should be moderate.

(3) With high hardness, chamber (high) temperature strength, good mechanical processing and repair welding performance, good thermal fatigue resistance, good dimensional stability in the repeated hot and cold cycle.

(4) Low cost of casting material and cost.

In summary, by comparing the physical properties of metal materials and considering the cost factors, cast steel is selected as the casting material, the specific material is 4Cr5MoV1Si die steel.

1.2 Metal casting coating process selection

The practice of metal mold casting based on other metals shows that coating on the inner cavity of the metal mold can effectively solve the casting defects caused by the casting process, and titanium alloy has high chemical activity under high temperature melting state and can react with almost all the casting materials. Therefore, coating a stable ceramic coating on the surface of the inner cavity of the titanium alloy cast is particularly important.

The main principle of selection of coating materials is: powder materials should have high refractoriness, thermal shock resistance and good thermal insulation or good thermal conductivity, and titanium liquid interface reaction is small, therefore, from the commonly used refractory oxide cast titanium selection of coating materials. The relationship between the binding free energy of refractory oxides and temperature is shown in Figure 1. It can be seen from the figure that the chemical stability of various oxide materials on molten titanium alloy is arranged in the order from low to high: SiO2, MgO, Al2O3, ZrO2, CaO, ThO2, Y2O3. Therefore, Y2O3 was selected as ceramic coating material in the experiment.

FIG. 1 Relation between the binding free energy of various oxides and temperature

The project selected two coating processes for metal type coating comparative test, namely manual brushing and plasma spraying. Manual brushing is a coating made by mixing yttrium oxide powder and binder, which is coated with a brush on the cast surface and roasted at high vacuum temperature to form a coating with certain bonding strength. Its advantage is that it is simple to operate, but the brush marks on the surface of the prepared coating are obvious. Plasma spraying is to spray yttrium oxide powder on metal surface in molten state under ionic arc heating by plasma spraying machine to form coating. In order to enhance bonding strength, Ni/Al alloy powder metal bonding layer is added between yttrium oxide coating and metal matrix. Compared with manual brushing, this process is more complicated, but the coating thickness is easy to control. And has a good surface finish, the experimental design of plasma spraying process is shown in Table 2. Titanium alloy samples were designed in the project, and the size of each sample was 80 mm×20 mm×20 mm. The coating was coated on the surface of the metal cast using the above two preparation processes (Figure 2). The coating thickness was about 0.3mm. Titanium alloy samples were cast in a vacuum consumable arc shell melting furnace at 150 r/min. The titanium alloy material was Ti-6Al-4V.

Table 2 Process parameters of plasma spraying cast titanium metal samples

After pouring, the samples were cut by electric spark wire cutting method, and then embedded in phenolic plastic powder and polished properly. During the analysis of the samples, the morphology of the cast surface and the surface surface of the samples were observed by scanning electron microscope, and the components were analyzed by EDS spectrometer, and the metal castings were poured.

2. Microstructure analysis of metal cast coating

FIG. 3 shows the longitudinal microstructure morphology of different coating processes on the metal casting surface, and FIG. 3a shows the coating prepared by manual brushing method. It can be seen that the boundary between the ceramic layer and the metal matrix is obvious, and there is local separation phenomenon, which is mainly because the coating prepared by manual brushing is combined by physical means. It does not match the metal matrix. The longitudinal contrast of the ceramic interior changes obviously, which is caused by the coating repeated brushing, brushing direction, solidification time and micro solid phase content is inconsistent. Pores and longitudinal microcracks are distributed in the ceramic layer. The formation of pores is mainly due to the fact that the environmental gases dissolved in the coating cannot be fully spilled out in time when the coating is brushed, and these gases remain in the dry coating layer like structure, forming a large number of pores. The longitudinal microcracks are caused by the ceramic coating releasing a large number of thermal stresses during the roasting process. Microscopic pores and cracks are the stress concentration areas in the coating, which are very easy to cause cracks and accelerate crack propagation, and become the diffusion channel of metal matrix elements, accelerate the chemical erosion of the coating and metal matrix, and then reduce the coating strength, accelerate the growth of oxides, and eventually lead to the failure of the coating. FIG. 3b shows the coating prepared by plasma spraying method. It can be seen that the interface between ceramic layer and bond layer, bond layer and metal matrix is well bonded, no obvious cracks exist, and the coating structure is relatively dense.

Figure 3. Metallographic structure of longitudinal section of metal casting mold with different processes

3. Analysis of interface reaction between metal mold and titanium alloy

In order to analyze the influence of coating metal profile technology on the surface quality of titanium samples, the contaminated layer on the surface of the samples was analyzed by scanning electron microscope line scanning, as shown in FIG. 4 and FIG. 5. As can be seen from the figure, the surface layer of artificially coated metal-cast titanium alloy samples showed a certain degree of diffusion of Y elements and O elements, and the diffusion thickness was about 10 μm. This was mainly due to the poor bonding and stability of the artificially coated coating, and the pores and micro-cracks were easy to react with the titanium liquid, leading to the contamination of the titanium liquid. However, the diffusion of Y and O elements did not appear on the surface of titanium alloy cast by plasma spraying. The above analysis shows that plasma Y2O3 coating can better prevent the chemical reaction between metal materials and molten titanium.

FIG. 4 Surface line scanning results of titanium alloy samples with manually coated metal mold casting

FIG. 5 Surface line scanning results of metal casting titanium alloy samples coated by plasma spraying

4. Trial production of typical titanium alloy castings

4.1 Metal casting preparation

In this experiment, the rudder shaft casting was cast using the above coating process, and the metal cast was divided into two pieces and assembled. The main material was 4Cr5MoV1Si hot working die steel. Since the structure of the casting was not too complicated and the size was not large, inserts, bases and guide rails were not used, and 45# steel bolts were used to fasten the mold. In order to enhance the binding strength between the metal base and the coating, after the metal type is treated with rust removal and oil removal, 16~24 mesh of brown corundum sand is blown for 5 min to ensure a good combination of coating and substrate. The centrifugal revolution of casting filling was 250r/min, and the quality analysis was carried out after mold cleaning, cutting and pouring system, sand blasting and HIP treatment.

Figure 6. Metal cast after coating

FIG. 7 shows the rudder shaft casting after pouring, and FIG. 8 shows the rudder shaft casting after sandblasting. As can be seen from the figure, there are a lot of flow marks and "titanium beans" on the surface of the artificially coated metal casting. The surface is uneven, and there are micro-cracks on some parts of the surface, and the surface quality is poor. The surface of the coated metal casting is smooth, and there are no obvious flow marks and cracks on the surface of the casting, indicating that the coating technology can obviously improve the surface finish of the casting.

FIG. 7 Casting after pouring

FIG. 8 Rudder shaft castings in sandblasted state

FIG. 9 Fluorescence comparison of rudder shaft castings

FIG. 9 is a comparison of fluorescent photographs of two rudder shaft castings. On the left is a manually coated metal cast casting, and on the right is a plasma coated metal cast casting. As can be seen from the figure, the casting on the left has obvious linear defects, which are micro-cracks on the surface. The casting on the right is of better surface quality and has no obvious defects.

After the final X-ray and fluorescent penetration inspection, the quality of the developed castings meets the technical requirements of the products, as shown in Table 3, and the surface quality is better than that of similar castings using machine graphite casting.

Table 3X Results of light detection and surface coloring inspection

The poured rudder shaft castings with cast samples were tested for mechanical properties and chemical composition. The results are shown in Table 4 and Table 5, and all meet the relevant requirements in GJB2896A-2007.

Table 4 Test results of chemical composition of the trial castings (ZTA15)

Table 5 Test results of mechanical properties of the trial castings (ZTA15)

5 Conclusion

(1) In this experiment, 4Cr5MoV1Si, a die steel, was used as the metal casting, and the micromorphology of the coating prepared by artificial brushing and plasma spraying was compared. The quality of the coating by artificial brushing was relatively poor, local separation occurred, and there were microscopic pores and cracks inside, while the quality of the coating by plasma spraying was better, the coating structure was relatively dense, and the bonding property was good.

(2) Interface reaction occurs on the surface of the artificially coated metal-cast titanium sample, and the coating material diffuses to the surface of the sample, and the surface roughness of the castings is low and the microcracks are serious. There is almost no interface reaction on the surface of plasma-coated metal-cast titanium samples, and the surface of the casting is smooth and flat, without obvious flow marks and cracks.

(3) The plasma coating process is ideal for preparing metal mold, which can reduce the cost and improve the surface quality of simple and small titanium castings in the mass production process.