1. Introduction
TC4 alloy has been widely used in aerospace, marine engineering, and biomedical fields due to its excellent comprehensive mechanical properties and relatively low production cost. However, the presence of defects such as shrinkage porosity and stress concentration in TC4 alloy castings seriously deteriorates its serviceability and limits its large – scale application. Understanding the solidification characteristics of the alloy is crucial for effectively controlling these defects.
1.1 Research Background
Previous studies have investigated various factors affecting the casting process of TC4 alloy. For example, Suzuki et al. studied the effect of centrifugal speed on the filling behavior of TC4 alloy castings and found that increasing the centrifugal speed can improve the filling ability of the molten metal and inhibit the formation of shrinkage porosity. However, an excessively high centrifugal speed can cause the molten metal to break off and promote the formation of shrinkage porosity. Xiong C et al. explored the influence of filling pressure on the filling and solidification behaviors of titanium alloy in counter – gravity casting and found that increasing the filling pressure can enhance the fluidity of the molten metal, reduce heat loss, inhibit the formation of pre – solidified regions, and improve the feeding ability of the molten metal. But an overly high filling pressure is not conducive to the effective control of shrinkage porosity.
1.2 Research Significance
For aerospace vehicle components, most key parts have complex structures and varying cross – sectional areas, which pose many challenges in the actual casting process, such as difficult mold preparation, high requirements for wax patterns and shell molds, difficult metal core extraction, and easy occurrence of shrinkage porosity and stress concentration. Moreover, there are relatively few studies on the influence of the solidification characteristics of TC4 alloy variable cross – section castings. Therefore, this study aims to optimize the centrifugal investment casting process of TC4 alloy variable cross – section components, analyze the numerical simulation of the filling and solidification behaviors, and control the casting defects.
2. Experimental Methods
2.1 Numerical Simulation
- Modeling and Parameter Setting: The ProCAST software was used to analyze the centrifugal investment casting process of TC4 alloy variable cross – section components. The three – dimensional model of the component was established, and the casting and mold were meshed. The liquidus and solidus temperatures of TC4 alloy were set as 1660°C and 1604°C, respectively, and the thermal physical parameters of the ZrO₂ ceramic shell mold were also defined. The cooling method was air cooling, and the heat transfer coefficient between the casting and the shell mold was set as 800 W/(m·K).
- Orthogonal Experiment Design: Shrinkage porosity was selected as the target optimization object. A 4 – factor – 3 – level (L9(3⁴)) orthogonal experiment was designed to analyze the influence of pouring temperature, shell mold preheating temperature, pouring rate, and centrifugal speed on the volume and distribution of shrinkage porosity. The orthogonal experiment design is shown in Table 1.
| 序号 | A/°C | B/°C | C/(kg/s) | D/(r/min) | 方案 |
|---|---|---|---|---|---|
| 1 | 1680 | 300 | 3 | 350 | A1B1C1D1 |
| 2 | 1680 | 350 | 5 | 450 | A1B2C2D2 |
| 3 | 1680 | 400 | 7 | 550 | A1B3C3D3 |
| 4 | 1700 | 300 | 5 | 550 | A2B1C2D3 |
| 5 | 1700 | 350 | 7 | 350 | A2B2C3D1 |
| 6 | 1700 | 400 | 3 | 450 | A2B3C1D2 |
| 7 | 1750 | 300 | 7 | 450 | A3B1C3D2 |
| 8 | 1750 | 350 | 3 | 550 | A3B2C1D3 |
| 9 | 1750 | 400 | 5 | 350 | A3B3C2D1 |
2.2 Casting Quality Analysis
- Internal Quality Detection: An iDR450X – type X – ray non – destructive inspection device (X – ray Radiographic Inspection, XRI) was used to analyze the internal quality of the variable cross – section casting, along with a P17 – A automatic cleaner and Agfa C7 film. The working parameters included a focal length of 2000 mm, a tube voltage of 60 – 95 kV, a tube current of 15 mA, and an exposure time of 3 min.
- Dimension Measurement: A Handy SCAN 700 handheld three – dimensional laser scanner was used to measure the dimensions of the casting. The working parameters included a measurement rate of 480000 s, a scanning area of 275 mm × 250 mm, a light source of 7 crossed laser lines, a resolution of 0.05 mm, a scanning accuracy of 0.03 mm, a reference distance of 300 mm, and a depth of field of 250 mm, along with Vxelements software.
2.3 Microscopic Structure Characterization
An Olympus GX71 optical microscope (Optical Microscopy, OM) and a Quanta 200FEG field emission scanning electron microscope (Scanning Electron Microscopy, SEM) were used to observe the microscopic structure of the TC4 alloy casting. Samples of 10 mm × 10 mm × 6 mm were prepared by wire cutting and then ground with SiC sandpaper of different grades and polished with diamond polishing paste until the surface was scratch – free. Finally, the observation surface was etched with Kroll reagent (3%HNO₃ + 5%HF + 92%H₂O).
2.4 Tensile Property Testing
An Instron 5569 electronic universal testing machine was used to test the room – temperature tensile properties of the variable cross – section casting. The tensile speed was 0.5 mm/min, and the sample specifications conformed to HB5143 – 1996 “Metal Room – Temperature Tensile Test Method”.
3. Results and Discussion
3.1 Process Optimization
- Orthogonal Experiment Results: The orthogonal experiment simulation results showed that a large number of shrinkage porosities were concentrated at the top of the casting, while there were fewer and more discretely distributed shrinkage porosities in other regions. The volume of shrinkage porosity in each scheme was calculated, and the intuitive analysis table of the orthogonal experiment was obtained. The results showed that the scheme A1B1C1D1 had the smallest shrinkage porosity volume (5.041 cm³) among the 9 groups of experiments, but further analysis of the K value was required to determine the optimal scheme.
- Factor Analysis and Optimal Scheme Determination:
- Pouring Temperature: When the pouring temperature increased from the 1st level to the 3rd level, the volume of shrinkage porosity increased. Therefore, 1680°C was selected as the pouring temperature.
- Shell Mold Preheating Temperature: As the shell mold preheating temperature increased from the 1st level to the 3rd level, the volume of shrinkage porosity decreased. Considering the possible occurrence of interface reactions at high temperatures, 400°C was selected as the shell mold preheating temperature.
- Pouring Rate: When the pouring rate increased from the 1st level to the 3rd level, the volume of shrinkage porosity increased. Thus, 3 kg/s was selected as the pouring rate.
- Centrifugal Speed: When the centrifugal speed increased from the 1st level to the 3rd level, the volume of shrinkage porosity first increased and then decreased. Considering the influence on the filling process and the formation of shrinkage porosity, 350 r/min was selected as the centrifugal speed.
The optimal centrifugal investment casting scheme for the variable cross – section casting was determined as A1B3C1D1 (pouring temperature: 1680°C; shell mold preheating temperature: 400°C; pouring rate: 3 kg/s; centrifugal speed: 350 r/min). The volume of shrinkage porosity in this scheme was 4.9326 cm³, which was smaller than that of the scheme A1B1C1D1, confirming the rationality of the optimization scheme. The influence degrees of the four factors on the shrinkage porosity of the casting were pouring temperature > pouring rate > shell mold preheating temperature > centrifugal speed.
3.2 Filling and Solidification Process
- Filling Process: The filling process of the variable cross – section casting (optimized scheme) was analyzed. At 2.37 s, the molten metal reached the casting. Then, under the action of centrifugal force, the molten metal preferentially flowed into the top (thicker region) of the casting. As the filling process continued, the molten metal gradually flowed into the middle and lower parts of the casting and filled from the outside to the inside. At 5.20 s, the casting was completely filled. The maximum flow velocity of the molten metal was 19.09 m/s, and the flow velocity in the top region was always higher than that in other regions, indicating that the filling stability in the top region was relatively low.
- Solidification Process: The solidification process of the casting was also analyzed. When the entire casting system was completely filled, the top and bottom had already solidified 明显. This was because the front end of the molten metal contacted the low – temperature shell mold during the filling process, resulting in severe heat dissipation of the molten metal. At the same time, the molten metal in the region far from the pouring system was difficult to obtain temperature compensation from the inner runner, so these regions solidified first. The solidification time in the middle region of the casting was 明显 later than that at both ends of the casting. The main reason for this was that the high – temperature molten metal in the runner compensated the temperature of the middle region. At 49.74 s, the casting was completely solidified. The complete solidification order of the casting system was bottom → top → middle → runner, which could effectively ensure the solidification feeding of the runner to the casting and inhibit the formation of shrinkage porosity.
3.3 Casting Defects Analysis
- Shrinkage Porosity:
- Distribution and Cause Analysis: Using the optimized scheme A1B3C1D1, a large number of shrinkage porosities were concentrated at the top of the casting, and a small number of shrinkage porosities were discretely distributed in the middle or bottom. The formation of isolated liquid phase regions was the main cause of shrinkage porosity. In the top region, the effective cross – sectional area was larger, resulting in difficult feeding. At the same time, the poor filling stability in this region also contributed to the formation of shrinkage porosity. For the discrete shrinkage porosities in the middle or bottom, in addition to the influence of isolated liquid phases, the entrainment of gas during the filling process might also be an important factor.
- Microstructure and Alloy Characteristics: During the initial solidification of the casting, columnar crystals nucleated and grew along the vertical mold wall. As the solidification process progressed, columnar crystals fully developed, and the cooling rate in the center of the casting gradually decreased, providing favorable conditions for the nucleation of equiaxed crystals. When a large number of columnar crystals and a small number of equiaxed crystals formed a grain framework, isolated liquid phase regions were formed. The TC4 alloy in this study exhibited the characteristics of a narrow crystallization temperature range alloy, and the flow of the molten metal stopped showing the characteristics of a narrow crystallization temperature range alloy.
- Stress Concentration and Deformation:
- Stress Concentration: The effective stress simulation results of the casting using the optimized scheme A1B3C1D1 showed that stress concentration mainly occurred at the connection between the inner runner and the casting (regions I and II, with a maximum effective stress value of 414.0 MPa), while the effective stress of the casting itself was at a relatively low level. The large structural change at the connection between the inner runner and the casting led to a high level of stress concentration during solidification shrinkage.
- Deformation: The casting had a relatively small degree of contraction after complete solidification, with a maximum contraction amount of only 0.4734 cm, which was consistent with the stress concentration simulation results. The casting had no obvious solidification deformation and a relatively low stress concentration level, but the stress concentration at the connection between the inner runner and the casting needed to be fully considered during the casting process to avoid crack formation.
3.4 Casting Preparation and Quality Analysis
- Casting Appearance and Defect Detection: The ZrO₂ ceramic shell mold and the TC4 alloy variable cross – section casting prepared using the optimal process parameters obtained from numerical simulation were observed. The casting had a complete overall structure, without obvious defects such as insufficient pouring, cracks, flash, burrs, and large – size deformation, confirming the rationality of the centrifugal investment casting process used in this study. The internal defects of the casting were detected using X – ray non – destructive testing equipment. The results showed that there was no shrinkage porosity in the casting, which was slightly different from the numerical simulation results but also confirmed the rationality of the numerical simulation optimization scheme. To further verify the reliability of this result, the casting was anatomized. The results showed that there were no shrinkage porosity and other defects inside the casting, indicating that the optimized casting process used in this study could effectively prepare high – quality TC4 alloy complex components and provide a theoretical basis and technical guidance for the preparation of similar titanium alloy components in the future.
- Dimension Analysis: The dimensions of four castings were analyzed using a handheld three – dimensional laser scanner. The comparison between the scanning results and the simulation results showed a high degree of agreement. The actual dimensions were only slightly larger than the target dimensions, indicating that only a small amount of machining was required in the future. In other words, the casting had no obvious solidification shrinkage or large – scale deformation, and the characteristic dimensions could meet the design requirements well.