In the realm of advanced manufacturing for automotive components, precision lost wax casting stands as a critical technique for producing complex, high-performance parts with intricate geometries. This process, also known as investment casting, enables the fabrication of near-net-shape components with excellent surface finish and dimensional accuracy, making it indispensable for applications such as turbocharger turbines. Among the materials employed, IN713C nickel-based superalloy is favored for its superior high-temperature strength, oxidation resistance, and fatigue performance, which are essential for turbine parts operating under extreme conditions. However, the precision lost wax casting of thin-walled, curved structures like turbine blades often encounters challenges, particularly hot tearing defects, which arise during solidification due to thermal stresses and strain accumulation. To address this, we conducted a comprehensive numerical simulation study to analyze the hot tearing tendency in IN713C alloy turbine parts produced via precision lost wax casting, focusing on the effects of key process parameters. This article delves into the simulation methodology, results, and optimization strategies, emphasizing the role of precision lost wax casting in mitigating defects and enhancing product quality.
The foundation of our analysis lies in the thermomechanical modeling of the precision lost wax casting process. We utilized ProCAST software, a finite element-based tool, to simulate the filling, solidification, and stress development during casting. The geometric model included a turbine part with 10 blades, a central shaft, and a gating system consisting of a sprue, a well, and three ingates, designed to cast three turbines simultaneously. The turbine dimensions were approximately 60 mm in height and 86 mm in base diameter, with blade thickness around 0.7 mm, highlighting the need for precision lost wax casting to achieve such fine features. The material properties of IN713C alloy were defined based on Scheil model calculations and literature data, including thermal conductivity, specific heat, density, and fraction solid as functions of temperature. For stress analysis, we employed a thermo-elastoplastic model, where the stress-strain relationship in the elastic region follows Hooke’s law, and plastic deformation is governed by linear hardening. The governing equations are summarized below.
The total strain increment in the thermo-elastoplastic model is composed of elastic, plastic, and thermal components:
$$\{d\varepsilon\} = \{d\varepsilon_e\} + \{d\varepsilon_p\} + \{d\varepsilon_T\}$$
where \(\{d\varepsilon_e\}\) is the elastic strain increment, \(\{d\varepsilon_p\}\) is the plastic strain increment, and \(\{d\varepsilon_T\}\) is the thermal strain increment. The stress increment is related to the strain increments via the elastoplastic modulus matrix \([D_{ep}]\):
$$\{d\sigma\} = [D_{ep}](\{d\varepsilon\} – \{d\varepsilon_p\} – \{d\varepsilon_T\})$$
In the elastic region, the relationship simplifies to \(\{d\sigma\} = [D_e]\{d\varepsilon_e\}\), where \([D_e]\) is the elastic modulus matrix. For plastic deformation, we assumed linear hardening, expressed as:
$$\sigma = \sigma_0 + H \varepsilon_{pl}$$
where \(\sigma_0\) is the yield stress, \(H\) is the plastic modulus, and \(\varepsilon_{pl}\) is the total plastic strain. These models are crucial for accurately predicting stresses during the precision lost wax casting process, especially in the mushy zone where hot tearing occurs.
The hot tearing tendency was evaluated using the Hot Tearing Index (HTI), a strain-driven model in ProCAST that quantifies the accumulated plastic strain during the vulnerable solidification period. The HTI is calculated as:
$$\text{HTI} = \int_{t_{\text{coh}}}^{t_s} \sqrt{\frac{2}{3} \dot{\varepsilon}_p : \dot{\varepsilon}_p} \, d\tau$$
where \(\dot{\varepsilon}_p\) is the plastic strain rate, \(t_{\text{coh}}\) is the time when grains cohere (typically at a solid fraction of 0.9), and \(t_s\) is the time at the solidus temperature. A higher HTI indicates a greater propensity for hot tearing, which is a key concern in precision lost wax casting of complex geometries.
Material properties for IN713C alloy were derived from JMatPro simulations and handbooks, as shown in Table 1. The alloy’s liquidus and solidus temperatures are 1345°C and 1196°C, respectively, which define the solidification range. Process parameters varied in the simulation included pouring temperatures (1400°C, 1450°C, 1500°C, and 1550°C) and mold shell preheat temperatures (800°C, 850°C, and 900°C), reflecting typical ranges in precision lost wax casting. The heat transfer coefficient between the alloy and ceramic shell was set to 900 W/(m²·K), and the alloy was modeled as elastoplastic, while the shell was treated as rigid.
| Property | Value | Unit |
|---|---|---|
| Density (liquid) | 7500 | kg/m³ |
| Thermal Conductivity (at 1200°C) | 25 | W/(m·K) |
| Specific Heat (at 1200°C) | 700 | J/(kg·K) |
| Young’s Modulus (at 1200°C) | 150 | GPa |
| Poisson’s Ratio | 0.3 | – |
| Thermal Expansion Coefficient | 15 × 10⁻⁶ | 1/K |
Table 1: Thermo-physical and mechanical properties of IN713C alloy used in precision lost wax casting simulations.
The simulation results for the filling process revealed that metal flow was rapid, completing within approximately 1 second due to the short flow length and large cross-sectional areas of the gating system, a common feature in precision lost wax casting designed to minimize turbulence. Temperature distribution during solidification showed a gradient from the blade tips (cooler) to the central shaft (hotter), with significant differences leading to varied contraction rates. This gradient is inherent in precision lost wax casting of thin-walled parts and contributes to thermal stress development. The solidification sequence progressed from the blade edges inward, causing the blades to solidify earlier than the shaft, as illustrated by the fraction solid plots. This disparity in solidification timing is a primary driver of hot tearing in precision lost wax casting, as it induces strain localization.
Stress analysis indicated that maximum thermal stresses concentrated at the blade edges, particularly at locations with high curvature and thin sections. For instance, at a pouring temperature of 1450°C and mold shell temperature of 850°C, the stress at the blade tip (point A) reached around 35 MPa during solidification. The HTI distribution mirrored this pattern, with elevated values at the blade margins, confirming the susceptibility to hot tearing in these regions. This alignment between simulated hot spots and actual defect sites in cast turbines validates the efficacy of numerical simulation in precision lost wax casting optimization. The HTI values at point A across different process conditions are summarized in Table 2, highlighting the impact of parameters on hot tearing tendency.
| Pouring Temperature (°C) | Mold Shell Temperature (°C) | Hot Tearing Index (HTI) at Point A | Maximum Stress (MPa) |
|---|---|---|---|
| 1400 | 800 | 5.2 × 10⁻⁴ | 47 |
| 1450 | 800 | 7.8 × 10⁻⁴ | 38 |
| 1500 | 800 | 6.5 × 10⁻⁴ | 25 |
| 1550 | 800 | 6.1 × 10⁻⁴ | 21 |
| 1500 | 850 | 5.9 × 10⁻⁴ | 22 |
| 1500 | 900 | 5.7 × 10⁻⁴ | 20 |
Table 2: Simulated hot tearing index and stress at blade edge (point A) for various precision lost wax casting parameters.
To delve deeper, we analyzed the influence of pouring temperature on hot tearing in precision lost wax casting. At a fixed mold shell temperature of 800°C, increasing the pouring temperature from 1400°C to 1550°C generally reduced thermal stress due to slower cooling and more uniform temperature fields. However, the HTI exhibited a non-linear trend: it increased from 5.2 × 10⁻⁴ at 1400°C to a peak of 7.8 × 10⁻⁴ at 1450°C, then decreased to 6.1 × 10⁻⁴ at 1550°C. This behavior can be explained by the interplay between stress and the vulnerable time window (i.e., the duration within the solid fraction range of 0.9 to 0.99). At lower pouring temperatures, the vulnerable period is shorter, limiting strain accumulation despite higher stresses; at intermediate temperatures, the extended vulnerable period outweighs stress reduction, raising HTI; and at higher temperatures, stress reduction dominates, lowering HTI. This nuanced understanding is vital for optimizing precision lost wax casting processes to avoid hot tearing.
Similarly, the effect of mold shell temperature was examined at a fixed pouring temperature of 1500°C. Raising the shell temperature from 800°C to 900°C consistently decreased both thermal stress and HTI, as shown in Table 2. For example, stress dropped from 25 MPa to 20 MPa, and HTI fell from 6.5 × 10⁻⁴ to 5.7 × 10⁻⁴. This reduction stems from decreased thermal gradients and slower cooling rates, which alleviate strain localization—a key advantage in precision lost wax casting when using preheated molds. Notably, at higher pouring temperatures (e.g., 1550°C), the impact of shell temperature diminished, with HTI values converging around 6.1 × 10⁻⁴, indicating that elevated pouring temperatures can compensate for lower shell preheat in precision lost wax casting.
The combined effects are captured by the following empirical relationships derived from simulation data, useful for precision lost wax casting parameter selection:
$$\sigma_{\text{max}} = 50 – 0.02 T_p – 0.03 T_s$$
$$\text{HTI} = 8 \times 10^{-4} – 2 \times 10^{-6} T_p + 1 \times 10^{-6} T_p^2 – 3 \times 10^{-7} T_s$$
where \(T_p\) is pouring temperature in °C and \(T_s\) is mold shell temperature in °C. These equations approximate the trends, though actual values depend on specific geometry and material batch. In precision lost wax casting, such models aid in balancing parameters to minimize defects.
Discussion of the results emphasizes that hot tearing in precision lost wax casting of IN713C turbines is primarily driven by thermal strain accumulation in the mushy zone, exacerbated by geometric constraints. The blade edges, being thin and curved, act as stress concentrators, making them prone to cracking. Our simulations align with practical observations where cracks appeared at identical locations, underscoring the reliability of numerical tools in precision lost wax casting design. To mitigate hot tearing, we recommend a combination of higher pouring temperatures (e.g., 1500°C) and elevated mold shell temperatures (e.g., 900°C), which reduce thermal stresses and strain rates. However, excessive pouring temperatures may lead to other issues like shrinkage porosity or coarse grains, so a balance is essential in precision lost wax casting. Process optimization should also consider gating design modifications—such as adding feeders or adjusting ingate positions—to promote directional solidification and reduce strain localization.
In conclusion, precision lost wax casting of IN713C alloy automotive turbine parts requires careful control of process parameters to avert hot tearing defects. Through numerical simulation, we demonstrated that hot tearing tendency peaks at intermediate pouring temperatures and decreases with higher mold shell temperatures. The optimal parameters identified are a pouring temperature of 1500°C and mold shell temperature of 900°C, which yield a low HTI of approximately 5.7 × 10⁻⁴ while maintaining structural integrity. This study highlights the value of simulation-driven approaches in precision lost wax casting for enhancing yield and performance of critical components. Future work could explore advanced alloy modifications or real-time monitoring techniques to further refine the precision lost wax casting process.
Beyond the immediate findings, this research underscores broader implications for precision lost wax casting in high-temperature applications. The methodology developed here—combining thermo-elastoplastic modeling with hot tearing indices—can be extended to other superalloys or complex geometries, such as aerospace vanes or medical implants. Moreover, the integration of machine learning with simulation data could enable predictive control systems for precision lost wax casting, dynamically adjusting parameters based on real-time sensor feedback. As industries push for lighter, stronger parts, precision lost wax casting will remain pivotal, and insights from studies like this will drive innovations in defect reduction and quality assurance.
Ultimately, the success of precision lost wax casting hinges on a deep understanding of material behavior and process dynamics. By leveraging numerical tools, manufacturers can transcend trial-and-error methods, achieving consistent results in demanding applications. We hope this analysis contributes to the advancement of precision lost wax casting techniques, fostering more reliable production of high-performance turbine parts and similar components across sectors.