This article focuses on the simulation study of the filling and solidification process of K213 high-temperature alloy in the typical blade process of precision investment casting using MAGMA casting simulation software. It mainly analyzes the fluid velocity field distribution, solid fraction distribution during the solidification process, temperature field distribution, void distribution, etc. of the alloy melt during the filling process. The simulation results show that a small amount of solidification defects mainly concentrate on the surface of the blade root during the casting process, which has no impact on the overall performance, further indicating that the designed gating system can meet the production requirements of blade precision casting.
1. Introduction
In various fields such as aero-jet engines, marine gas turbines, and industrial gas turbines, high-temperature gases are commonly used to drive turbines. The blades of turbines, as important components of turbines, need to withstand the harsh environment of high temperature and hot corrosion (the working temperature of a typical gas turbine exceeds 500 °C). High-temperature alloys have good oxidation resistance, thermal stability, fatigue resistance, and good creep fracture resistance. Therefore, they have been widely used in this environment.
K213 is a kind of iron-nickel-chromium-based cast high-temperature alloy, in which Al and Ti are added to form the γ phase, achieving the precipitation strengthening effect. It uses W for solid solution strengthening and a small amount of B to strengthen the grain boundaries. This alloy has an excellent casting process and good comprehensive performance below 750 °C.
In recent years, high-temperature alloy components tend to be more precise and thin-walled, thus posing higher requirements for the precision casting process. Traditional experimental processes have problems such as high trial and error costs and long cycles, while computer-aided design can greatly reduce the trial and error costs and shorten the process design cycle. There have been precedents in China for using computers to simulate the casting solidification process of blades. For example, the Institute of Iron and Steel Research used computer simulation for the precision casting of high-temperature alloys, ultimately determining the optimal process plan, significantly reducing casting defects, and lowering production costs. Shenyang Aerospace University used finite element analysis to simulate the filling and solidification process of K438 alloy liquid in the investment casting process, determining the best gating system and process. Harbin Institute of Technology simulated the forming process of TiAl-based alloy blades in the suction casting process, analyzed the causes of blade defects during the filling and solidification process, and conducted relevant verifications. Foreign scholars have also combined high-throughput methods with numerical simulations and applied them to the optimization of precision casting process parameters to quickly and accurately optimize key process parameters and improve the comprehensive performance of castings.
Aiming at the problems of complex shapes of high-temperature alloy blades and prone to defects in the precision casting process, the authors of this article used MAGMA casting simulation software to simulate the filling and solidification process of K213 high-temperature alloy in the typical blade process of precision investment casting, mainly conducting simulation studies on the fluid velocity field distribution during the filling process, solid fraction distribution during the solidification process, temperature field distribution, void distribution, etc., and verifying the reliability of the designed gating system by determining the positions where casting pores are likely to occur, providing a reliable theoretical basis for on-site tests and production.
2. Experimental Method
The K213 cast high-temperature alloy master alloy was selected for the experiment, and its chemical composition is listed in Table 1. The melting point of the K213 alloy is 1324 to 1361 °C, the density is ρ = 8.14 g/cm³, and the expansion coefficient is 18.61 × 10^-6 °C^-1 when it is between 20 and 800 °C.
According to the requirements of a certain company, the simulated component was determined to be a certain type of turbine impeller blade, and the component model is shown in Figure 1.
The MAGMA casting simulation software was used to simulate the casting process of the K213 alloy blade, and the temperature field, defect formation, and distribution during the casting process were analyzed through the filling and solidification processes to provide correct process method guidance for actual production. Gravity casting was adopted, with a gravity acceleration of 9.8 m/s. The basic parameters include a weight factor of 0.8, a relaxation factor of 1.6, an initial time step of 0.0001 s, and the heat transfer coefficients between the casting and air, and between the casting and mold were both 0.023, with a casting temperature of 1550 °C.
The casting is a complex-shaped blade, and the SolidWorks three-dimensional solid modeling software was used for the modeling of the entire gating system. The top-pouring method was used to pour 4 alloy blades at once, with a single blade mass of 3 kg, and the gating and runner were used to feed the solidification process of the blades. The MAGMA casting simulation software was used for mesh division, and the mesh parameter was selected as 1 mm.
3. Results and Analysis
3.1 Solid Fraction during the Filling Process
Figures 2 and 3 show the solid fraction during the filling process, and the distribution of the solid fraction inside the blade and gating system can be observed through the longitudinal section in Figure 3. It can be seen from the figure that during the pouring process at the blade part, the alloy liquid is always in a liquid state, which can achieve a complete filling of the blade part. The filling is completed in 15 seconds, and solidification begins at the edge of the blade at 25 seconds. After the blade solidifies, the runner begins to solidify, and the final solidification part occurs at the center of the connection between the gate and the runner, and the final solidification area can be located at the connection between the gate and the runner.
3.2 Temperature Field Simulation Results and Analysis
Figures 4 and 5 show the temperature field distribution during the pouring process, and the temperature field distribution inside the blade and gating system can be observed through the longitudinal section in Figure 5. It can be seen from the figure that during the pouring process at the blade part, the alloy liquid is always in a high-temperature state, with a temperature of about 1500 °C. After 3 seconds, the filling of the blade part is completed, and the temperature begins to drop at the edge of the blade, with a minimum temperature of about 1400 °C, which is still higher than the alloy melting point, making it not easy to have defects. After 15 seconds of filling, the temperature in some areas at the edge of the blade drops to about 1230 °C. Combined with the analysis results of Figures 2 and 3, it can be known that solidification begins at the edge of the blade at 25 seconds, and after 15 minutes, the temperature at the center of the connection between the gate and the runner is still around 1300 °C, and it is always in a high-temperature state, while the temperature in other parts has dropped below the melting point. This indicates that the designed gate and runner can feed the blade part.
3.3 Fluid Velocity Field Simulation Results and Analysis
Figure 6 shows the simulation results of the velocity field during the filling process. It can be seen from the figure that when the filling of the blade part is completed at 3 seconds, there is still disturbance at the edge of the blade, and there is a small amount of air entrainment behavior in the two outermost blades. With the subsequent pouring process, this disturbance basically will not cause casting defects. When the alloy flow at the blade part is completely stationary at 4 seconds, and the casting is completed at 15 seconds, a significant reduction in the liquid level at the gate can be observed, indicating that the gate and runner have played a good feeding role.
3.4 Void Simulation Results and Analysis
Figure 7 shows the void simulation results during the pouring process. It can be seen that during the pouring process, voids will be formed in the center of the two outermost blades due to the disturbance of the pouring process, but with the progress of the pouring, under the action of the pressure of the molten metal, the voids continuously rise and finally concentrate at the gate and runner.
4. Conclusions
In this article, the MAGMA casting simulation was conducted on the fluid velocity field, solid fraction distribution during the solidification process, temperature field distribution, void distribution, etc. of the K213 high-temperature alloy in the filling and solidification process of precision investment casting. Through the analysis of the simulation results, the possible positions of the casting pores were obtained, and the reliability of the gating system was verified. The conclusions are as follows:
(1) During the pouring process at the blade part, the alloy liquid is always in a liquid state, which can achieve a complete filling of the blade part. After the blade solidifies, the runner begins to solidify, and the final solidification part occurs at the center of the connection between the gate and the runner.
(2) The simulation results of the temperature field show that after the filling is completed, the temperature at the edge of the blade begins to drop first, and the temperature shows a gradient increase from the top of the blade to the gate; the center position of the connection between the gate and the runner is always in a high-temperature state, and the temperature in other parts has dropped below the melting point.
(3) The simulation results of the flow field and void show that there is a small disturbance at the blade part in the initial stage of filling, the alloy flow at the blade part is completely stationary at 4 seconds; after the casting is completed, a significant reduction in the liquid level at the riser can be observed; the final solidification voids are concentrated at the gate and runner.
This article clarifies the evolution process of the flow field and temperature field of the K213 high-temperature alloy during the solidification process, accurately predicts the generation position and severity of casting defects. Therefore, it can provide a solid theoretical support for the design of the precision casting process of the K213 high-temperature alloy blade and has important reference significance for the design of the precision casting process of high-temperature alloys.
5. Literature Review
In order to further understand the research status and related technologies in the field of high-temperature alloy blade casting, a literature review was conducted. The following are some important research results:
5.1 Research on the Modeling and Simulation of Key Manufacturing Processes of High-Temperature Alloy Blades by Professor Xu Qingyan of Tsinghua University
The current research on the modeling and simulation of key manufacturing processes of high-temperature alloy blades has made significant progress and has become a research hotspot in the field of aerospace. Researchers are committed to establishing accurate models and conducting simulation studies for the manufacturing process of high-temperature alloy blades to guide the optimization of the manufacturing process and improve product quality.
In the aspect of material performance modeling, researchers use experimental testing and material analysis methods to collect and analyze data such as the thermodynamic properties, fracture mechanics properties, and heat conduction properties of high-temperature alloy materials. These data are used to establish corresponding material models, and numerical analysis methods are used to predict and evaluate the material performance.
For the manufacturing process modeling, researchers use numerical simulation methods such as computational fluid dynamics and finite element analysis to model and simulate the manufacturing process of high-temperature alloy blades. By establishing appropriate geometric models and operating parameter simulations, phenomena such as temperature distribution, metal flow, solidification process, and possible defects and deformations during the manufacturing process can be predicted.
In the aspect of casting simulation, researchers use computational fluid dynamics models to conduct detailed modeling of the casting process to simulate the flow and solidification behavior of the molten metal. This helps to analyze and optimize the casting process parameters, such as the pouring temperature, mold design, and casting rate, to obtain high-quality blade castings.
Another key research direction is heat treatment simulation, where researchers simulate the temperature change, phase transformation behavior, and microstructure evolution during the heat treatment process. Through numerical simulation, the phase transformation process and microstructure characteristics of the material can be predicted to guide the optimization of heat treatment parameters and material performance.
In summary, important progress has been made in the research on the modeling and simulation of key manufacturing processes of high-temperature alloy blades, and it has been widely applied in the aerospace field. However, further research and improvements are still needed to improve the accuracy and reliability of the modeling and simulation and solve the key problems in the manufacturing process to meet the needs of the manufacturing and application of high-temperature alloy blades.
5.2 Research on the Temperature Field Evolution and Grain Growth of the Spiral Selector in the Directional Solidification of a Large-Module by Researcher Li Jinguo of the Institute of Metal Research, Chinese Academy of Sciences
With the continuous improvement of the thrust-to-weight ratio and turbine front gas temperature of aero-engines, turbine blades need to serve under more stringent conditions for a long time, withstanding the interaction of complex thermal stress and centrifugal force. Single-crystal high-temperature alloys, which eliminate the grain boundaries – the weak structure at high temperatures, have high high-temperature strength, excellent creep and fatigue resistance, good oxidation resistance, hot corrosion resistance, high-temperature microstructure stability, and service reliability, and have become the preferred material for advanced engine turbine blades.
At present, the directional solidification technology is usually used to prepare single-crystal high-temperature alloy turbine blades. Since VERSnyder FL, etc. first proposed the exothermic agent method (EP method) and the power reduction method (PD method) in the mid-20th century, the main characteristics of these two directional solidification processes are to add an exothermic agent on the upper part of the casting or adjust the heater power, and use water cooling at the lower part to achieve unidirectional heat transfer. However, these methods cannot obtain a stable temperature gradient, have poor heat dissipation conditions, and cannot obtain well-structured directional column crystals, so they have not been widely used. In the early 1970s, Erickson GL, etc. creatively added a transmission system on the basis of water cooling and separated the heating zone from the cooling chamber by using a heat insulation baffle, thereby improving the temperature gradient (see Figure 1a, HRS method). Due to the use of water-cooled copper disks for cooling in the HRS method and the separation of the heating zone and the cooling chamber by the heat insulation baffle, the influence of the heating zone on the solidified alloy can be avoided, and a relatively high and stable temperature gradient can be obtained. The appearance and popularization of this method have enabled the large-scale application of directional solidification technology. Later, in the mid-1970s, based on the idea of the high-speed solidification method, researchers used liquid metal as the coolant (see Figure 1b, LMC method). At present, the HRS method and the LMC method are the two most common processes for preparing single-crystal high-temperature alloy turbine blades. However, compared with the liquid metal (Sn liquid or Al liquid) cooling of the LMC method, the HRS method with water-cooled copper disk cooling is more convenient to operate, does not pollute the alloy, and is more suitable for industrial production.
However, the traditional HRS process also has certain limitations. Firstly, in the directional solidification process, as the distance between the solid-liquid interface and the water-cooled disk increases, the dominant heat dissipation mode of the HRS process gradually changes from heat conduction to radiation heat dissipation, and the temperature gradient at the front of the solid-liquid interface drops rapidly from 30-50 K/cm to 10-20 K/cm. In addition, in the commonly used HRS single-crystal furnace, the heating and cooling speed near the furnace body is fast due to the inward radial radiation for heating and cooling the mold shell and the metal melt inside it (Figure 2), while the heating and cooling speed on the side away from the furnace body is slow, with a low temperature gradient, and there is a significant shadow effect. In addition, due to the limitation of the temperature gradient in the HRS directional solidification process, the size of the water-cooled crystallizer of the HRS single-crystal furnace in China can only reach Φ(100-300) mm, and the height of the heating zone of the furnace is 400-500 mm, resulting in a limited number of blades prepared per furnace, low production efficiency, and the inability to produce large-sized gas turbine blades, which seriously restricts the application and promotion of single-crystal blades and restricts the development of new hot-end components of aero-engines in China. Due to the limitations of the HRS method, there are various casting defects such as crystal selection failure, orientation deviation, small-angle grain boundaries, and hetero-grains in the preparation process of single-crystal turbine blades in China, and the qualified rate is less than 50%, which has become a major bottleneck plaguing the development of new hot-end components of aero-engines in China. Therefore, there is an urgent need to design a single-crystal production equipment with a higher temperature gradient, better stability, larger size, and higher production efficiency to solve the bottleneck problem in the production of single-crystal turbine blades.
Aiming at the existing weak links of the HRS directional solidification equipment, Researcher Li Jinguo’s team of the Institute of Metal Research, Chinese Academy of Sciences published an article on the “Temperature field evolution and grain growth of the spiral selector in the directional solidification of a large-module” in the 43rd volume, issue 01, 2023, of the “Special Casting and Non-Ferrous Alloys” journal. They designed a new type of efficient large-module single-crystal blade growth equipment by integrating a central heating system and a bottom water-cooling heat dissipation system on the basis of the original HRS single-crystal furnace to improve the heating and cooling method. Through commercial modeling software UG and finite element simulation software ProCAST, the temperature field evolution process and grain growth behavior of the new single-crystal furnace in the directional solidification process of the spiral selector were studied. The results show that the new equipment significantly improves the temperature gradient and its stability at the front of the solid-liquid interface, effectively solves the shadow effect problem in the growth process of single-crystal castings, reduces the casting defects of single-crystal blades, and improves the single-crystal quality and the qualified rate of blades.
