Abstract
This paper presents a numerical simulation of the investment casting process for automotive turbine parts made of IN713C alloy. By analyzing the turbine structure, solidification sequence, and stress distribution, the hot tearing tendency of the turbine was predicted. Additionally, the influence of different pouring process parameters on the hot tearing tendency was examined. The results indicate that hot tearing defects are concentrated at the edges of the turbine blades, and the simulated results align well with actual pouring specimens. The hot tearing tendency at the blade edges decreases with increasing mold shell temperature but increases first and then decreases with increasing pouring temperature.
1. Introduction
Turbocharging technology is widely used in the automotive industry to enhance engine power, reduce engine volume, and control CO2 emissions. The automotive turbine is a critical component in turbocharging technology. To improve turbine efficiency, turbines are designed to be lightweight, minimizing turbine lag. This results in a complex turbine structure with large blade curvature and a significant difference in cross-sectional area between the shaft and blades, which makes the turbine prone to hot tearing defects during the investment casting process.
2. Literature Review
Previous studies have investigated hot tearing in various alloy systems and casting processes. Cao et al. simulated the casting process of Ni3Al-based superalloy thin-walled components and determined the influence of filling sequence on hot tearing defects. By optimizing the pouring process, hot tearing defects were eliminated. Du et al. studied the effect of Ca/Al ratio on hot tearing susceptibility in Mg-Al-Ca alloy through numerical simulation and experimental analysis, finding that increasing the w(Ca)/w(Al) ratio reduces the alloy’s hot tearing sensitivity. He et al. conducted numerical simulations of the investment casting process for IN792 superalloy impellers, analyzing the influence of pouring speed, pouring temperature, and mold shell temperature on solidification defects. Hong et al. simulated the investment casting process of nickel-based superalloy K424 adjusting vanes and found that the stress at the crack location was high, and the solidification time was long. Bai et al. performed numerical simulations of the semi-continuous casting process of 7050 aluminum alloy and discovered that the tendency for hot cracking is highest during the initial casting stage, and cold cracks in the ingot are likely extensions of hot cracks.
3. Methodology
3.1 Stress Simulation Model
The ProCAST software was utilized to simulate the turbine casting process of IN713C alloy, observing the distribution of thermal stress and hot tearing tendency during solidification. This simulation provides insights into the hot cracking tendency at vulnerable locations on turbine blades under different pouring processes, offering a reference for predicting and optimizing hot crack defects in the turbine casting process of IN713C alloy.
3.2 Casting Gate System Design and Parameters
The research focuses on automotive turbines consisting of a turbine shaft at the center and ten turbine blades connected to the shaft, with significant blade curvature. The turbine is approximately 60 mm in height, with a maximum bottom diameter of about 86 mm, a blade wall thickness of approximately 0.7 mm, and a turbine shaft of approximately 28 mm. The gating system comprises one sprue, one runner, and three ingates, with each set pouring three turbines. The turbine axis forms a 130° angle with the sprue centerline.
| Component | Dimensions (mm) |
|---|---|
| Turbine height | 60 |
| Bottom diameter | 86 |
| Blade wall thickness | 0.7 |
| Turbine shaft | 28 |
3.3 Alloy Thermophysical Properties and Process Parameters
The alloy material is IN713C nickel-based superalloy, with the cast alloy matrix consisting of the γ-phase, and its primary strengthening phase being the γ’-phase. Additionally, the alloy contains a small amount of carbides and borides. IN713C exhibits good mechanical properties, oxidation resistance, and fatigue resistance at high temperatures, making it suitable for manufacturing aerospace components and turbocharger impeller wheels. The alloy composition is shown in Table 1. The alloy’s thermophysical properties are based on calculations using the Scheil model. The alloy’s elastoplastic properties are calculated using the JMatPro software within ProCAST and referenced from technical manuals. The alloy’s solidus and liquidus temperatures are 1,196°C and 1,345°C, respectively. Pouring temperatures were set at 1,400°C, 1,450°C, 1,500°C, and 1,550°C, and mold shell preheating temperatures were set at 800°C, 850°C, and 900°C, simulating the process from alloy pouring to complete solidification of the casting. The alloy material type was set to elastoplastic, and the mold shell material type was set to rigid. The heat transfer coefficient between the alloy and the mold shell is 900 W/(m2·K).
Table 1: The main composition of the IN713C alloy
| Element | Composition (%) |
|---|---|
| Ni | Balance |
| Cr | 13 |
| Co | 4.2 |
| Mo | 0.6 |
| Al | ≤1.8 |
| Ti | 0.1 |
| Nb+Ta | 0.1 |
| Zr | ≤0.05 |
| B | ≤0.006 |
| C | ≤0.12 |
| Si | ≤0.4 |
| Mn | ≤1.0 |
| Fe | Balance (remainder) |
4. Simulation Results and Analysis
4.1 Mold Filling Process and Temperature Field Analysis
Under various pouring conditions, the mold filling processes were consistent. Taking the pouring temperature of 1,450°C and mold shell temperature of 850°C as an example, the mold filling process. The metal billet is inductively heated within the sprue until it reaches the preset temperature. The molten metal then flows from the sprue into the runner, rapidly filling the mold cavity through the ingates. Due to the small size of the turbine casting and the large cross-sectional area of the gating system, the metal flow path is short, resulting in a short mold filling time of approximately 1 second.
The temperature field and solid fraction of the turbine at a pouring temperature of 1,450°C and a mold shell temperature of 850°C. it can be observed that the temperature gradually increases from the turbine blade tips towards the turbine center axis, with a significant temperature difference between the blade area and the center area. The solidification of the turbine blades begins earlier than the turbine center, progressing from the blades towards the turbine axis. When the turbine blades are halfway solidified, solidification in the turbine center axis has not yet started. The large difference in solidification intervals between the turbine blades and the turbine axis results in different thermal contraction rates, easily forming casting thermal stresses.
4.2 Stress and Hot Tearing Tendency Distribution
Hot cracks form when the alloy cools near the solidus temperature, where its strength and plasticity are low. When internal stresses exceed its strength limit, stress is released by forming cracks. Casting stresses consist of solid-state transformation stress, mechanical obstruction stress, and thermal stress. Since hot cracking occurs during solidification, the alloy has not undergone solid-state transformation, and transformation stress is not considered. Therefore, turbine stress is mainly composed of mechanical obstruction stress and thermal stress.
The hot tearing tendency index (HTI) in ProCAST software is a strain-driven model based on the total strain produced during solidification, which can simulate hot cracking conditions at different locations in the casting. This model calculates HTI based on the level of hot cracking sensitivity associated with the accumulated strain in the molten metal during solidification.