In modern manufacturing, the demand for cast components with localized enhanced properties, such as high hardness, wear resistance, and strength, has led to the adoption of advanced casting techniques. Among these, bimetal casting combined with the lost foam casting (EPC) process offers a promising solution. This study explores the application of bimetal lost foam casting to produce machine tool castings, focusing on a stamping die component as a case study. The lost foam casting process, characterized by its use of expendable foam patterns, provides unique advantages in achieving complex geometries and controlled metal deposition. By integrating bimetal pouring strategies, we can tailor the microstructure and mechanical properties of specific regions within a single casting, thereby optimizing performance while reducing costs. The EPC method, with its simplified gating and molding approach, facilitates the precise control required for bimetal applications, making it an ideal choice for industrial-scale production.
The fundamental principle of bimetal lost foam casting involves sequentially pouring two different metal alloys into a single mold cavity to create a composite structure. This process leverages the inherent benefits of lost foam casting, such as reduced turbulence and improved surface finish, while enabling the incorporation of alloying elements in targeted areas. In this work, we detail the entire process chain, from pattern preparation to final heat treatment, emphasizing the critical parameters that influence the quality and performance of bimetal EPC castings. Through experimental validation, we demonstrate that this approach can achieve significant improvements in hardness and tensile strength, meeting the rigorous requirements of machine tool applications. The following sections provide a comprehensive analysis of the materials, methods, and results, supported by quantitative data and theoretical insights.
Materials and Pattern Preparation in Lost Foam Casting
The success of the bimetal lost foam casting process hinges on the selection of appropriate materials for the foam pattern, coating, and molding sand. For the expendable pattern, we utilized expanded polystyrene (EPS) sheets with a density of 17 kg/m³. This low-density EPS ensures minimal gaseous residues during decomposition, reducing the risk of defects in the final casting. The foam patterns were meticulously machined and assembled through adhesive bonding to replicate the complex geometry of the stamping die, which featured a lower working section with thicker walls (70–75 mm) and an upper section with thinner walls (30–35 mm). The average mass of the dried foam pattern was measured at 2.05 kg, highlighting the lightweight nature of EPC patterns.
To enhance the stability and surface quality of the mold, a water-based graphite coating was applied using a combination of flow coating and manual brushing techniques. This coating serves multiple purposes: it prevents sand erosion, facilitates gas permeability, and improves the metallurgical integrity of the cast metal. The coating was dried in a controlled environment to eliminate moisture, which is critical for avoiding steam-related defects during pouring. For the mold material, we selected cold-cured furan resin sand with natural silica sand as the base aggregate (50/70 mesh size). This choice ensures high dimensional accuracy and adequate collapsibility post-casting. The linear shrinkage allowance for the casting was set at 1%, accounting for the contraction of the metal during solidification.
The design of the gating system is a pivotal aspect of lost foam casting, particularly for bimetal applications. We implemented an open gating system with specific cross-sectional ratios to control the flow and mixing of the two metal streams. The dimensions and configuration of the gating system are summarized in Table 1, which outlines the parameters for both the first and second pours. This design minimizes turbulence and promotes a layered solidification structure, essential for achieving distinct property zones in the bimetal casting.
| Component | Dimensions (mm) | Quantity | Metal Mass (kg) | Pouring Temperature (°C) |
|---|---|---|---|---|
| First Pour (Lower Section) | 15 × 65 (Ingate) | 5 | 300 | 1360 |
| 60 × 80 (Runner) | 1 | |||
| Ø60 × 600 (Sprue) | 1 | |||
| Second Pour (Upper Section) | 15 × 65 (Ingate) | 4 | 600 | 1380 |
| 60 × 80 (Runner) | 1 | |||
| Ø60 × 600 (Sprue) | 1 |
The gating ratio for the system was designed as \( F_{\text{sprue}} : F_{\text{runner}} : F_{\text{ingate}} = 1 : (1.25 \text{ to } 1.5) : (2 \text{ to } 2.5) \), where \( F \) represents the cross-sectional area. This ratio ensures a balanced flow distribution, reducing the likelihood of cold shuts or misruns. The sprue components were constructed using ceramic tubes to withstand the thermal shock and erosive forces of the molten metal. Additionally, test blocks with dimensions of 40 mm × 40 mm were integrated into the gating system at positions A, B, and C to monitor the compositional gradient and mechanical properties across the casting. These blocks featured transverse channels of 15 mm × 30 mm for sampling.
Melting and Pouring Strategy for Bimetal EPC
The bimetal lost foam casting process requires precise control over melting and pouring operations to ensure proper alloy distribution and interfacial bonding. We employed two medium-frequency induction furnaces with capacities of 1.5 tons and 4 tons for melting the respective metal alloys. The first pour consisted of HT300 alloyed with chromium (Cr) and molybdenum (Mo), aimed at enhancing the hardness and wear resistance of the lower working surface. The second pour used standard HT300 iron for the upper section, providing a balance of strength and toughness. Both melts were treated with Si-Ba inoculant to refine the graphite morphology and improve mechanical properties.
Chemical composition analysis was conducted using a光谱分析仪 (spectrometer) after inoculation, and the results are presented in Table 2. The first pour exhibited higher concentrations of Cr and Mo, which are critical for forming carbides and enhancing hardenability. The pouring temperatures were carefully selected to promote sequential solidification: 1360°C for the first pour and 1380°C for the second. This temperature differential, combined with the mass ratio of 1:2 (first to second pour), facilitates controlled mixing at the interface while maintaining distinct property zones.
| Pour Sequence | Furnace Capacity | Tap Temperature (°C) | C (%) | Si (%) | Mn (%) | P (%) | S (%) | Cr (%) | Mo (%) |
|---|---|---|---|---|---|---|---|---|---|
| First Pour | 1.5 t | 1480 | 2.95 | 1.70 | 0.81 | 0.05 | 0.07 | 1.12 | 1.15 |
| Second Pour | 4 t | 1500 | 3.10 | 1.75 | 0.89 | 0.045 | 0.07 | 0.045 | 0.001 |
The pouring sequence was meticulously timed to achieve optimal bimetal integration. The first pour was initiated through sprue 1, and approximately 2–3 seconds before its completion, the second pour was started through sprue 2. This overlap ensures a brief period of simultaneous pouring, which promotes metallurgical bonding at the interface without excessive dilution. The relationship between pouring time and interfacial quality can be described by the equation:
$$ t_{\text{overlap}} = \frac{V_{\text{interface}}}{A \cdot v} $$
where \( t_{\text{overlap}} \) is the overlap time (2–3 s), \( V_{\text{interface}} \) is the volume of the mixing zone, \( A \) is the cross-sectional area of the ingates, and \( v \) is the flow velocity of the molten metal. After pouring, residual metal was retained in the ladles to prevent slag inclusion. The entire process underscores the adaptability of lost foam casting for complex bimetal applications, as the foam pattern’s decomposition allows for smooth metal advancement and reduced turbulence compared to conventional sand casting.
Quality Evaluation and Mechanical Properties
Post-casting, the bimetal lost foam casting was subjected to thorough inspection and heat treatment to evaluate its dimensional accuracy, appearance, and mechanical performance. The casting exhibited no visible defects such as cold shuts or cracks at the bimetal interface, and the surface coloration was uniform, indicating successful integration of the two alloys. The net mass of the casting was 805 kg, conforming to the design specifications. Following stress-relief annealing, hardness measurements were taken on the lower and upper working surfaces using the Brinell scale (HBS). The results revealed an average hardness of 195 HBS for the lower section and 170 HBS for the upper section, meeting the target range of 190–220 HBS for the wear-resistant zone.
Tensile tests were conducted on specimens cast from a mixture of the first and second pour metals, and the average tensile strength exceeded 300 MPa, satisfying the requirement for machine tool components. To assess the distribution of alloying elements, samples were extracted from positions A, B, and C within the test blocks, and their Cr and Mo contents were analyzed, as shown in Table 3. The data indicate a gradient in composition, with the highest concentrations in the lower region (position A), gradually decreasing toward the upper region (position C). This gradient aligns with the intended design, where the lower section is enriched with Cr and Mo for enhanced wear resistance.
| Sampling Position | Cr Content (%) | Mo Content (%) |
|---|---|---|
| A (Lower) | 0.76 | 0.93 |
| B (Middle) | 0.25 | 0.27 |
| C (Upper) | 0.17 | 0.15 |
The mechanical properties can be correlated with the microstructure using the relationship for hardness in cast iron:
$$ \text{HBS} = k \cdot \sqrt{\text{Carbide Volume Fraction}} + b $$
where \( k \) and \( b \) are material constants. The higher carbide volume in the lower section, due to Cr and Mo additions, results in increased hardness. Additionally, the tensile strength \( \sigma_t \) can be expressed as a function of graphite morphology and matrix strength:
$$ \sigma_t = \sigma_0 + \Delta \sigma_{\text{graphite}} + \Delta \sigma_{\text{matrix}} $$
where \( \sigma_0 \) is the base strength, and the increments account for graphite distribution and pearlite content. The successful implementation of bimetal lost foam casting in this study highlights the process’s capability to achieve graded properties in a single casting, reducing the need for secondary operations like surface hardening.
Fundamental Principles of Bimetal Selection
Selecting compatible metal pairs is crucial for the success of bimetal lost foam casting. The primary criteria include similarity in solidification behavior, metallurgical structure, and thermal contraction coefficients. For instance, pairing gray iron with alloyed iron, as in this study, ensures coherent solidification with minimal interfacial stresses. In contrast, combining metals with disparate solidification ranges, such as carbon steel and gray iron, can lead to defects like hot tearing or poor bonding due to differences in phase transformation.
The solidification process in bimetal EPC involves sequential nucleation and growth, which can be modeled using the Fourier number for heat transfer:
$$ Fo = \frac{\alpha t}{L^2} $$
where \( \alpha \) is the thermal diffusivity, \( t \) is time, and \( L \) is the characteristic length. Metals with similar \( Fo \) values will solidify in a coordinated manner, reducing the risk of shrinkage voids. Additionally, the equilibrium phase diagram can guide alloy selection; for example, the Fe-C-Si system for gray irons allows for predictable graphite formation. The concentration gradient of alloying elements across the interface can be described by Fick’s second law:
$$ \frac{\partial C}{\partial t} = D \frac{\partial^2 C}{\partial x^2} $$
where \( C \) is the concentration, \( t \) is time, \( D \) is the diffusion coefficient, and \( x \) is the distance. In this study, the Cr and Mo gradients (Table 3) follow this diffusion-controlled profile, ensuring a gradual transition that mitigates stress concentration.
Practical considerations for bimetal lost foam casting also include the pouring technique. The two metals should be melted separately to prevent premature mixing, and the gating system must be designed to minimize turbulent interaction. The brief overlap in pouring times (1–3 seconds) is critical; too short an overlap may result in a weak interface, while too long can cause excessive dilution. The ratio of precious alloy addition, as seen with Cr and Mo, should be optimized based on the desired property gradient and economic factors. For example, the relative content of Cr at position B was 32% of that at A, indicating controlled diffusion and efficient use of resources.
Advantages of Lost Foam Casting for Bimetal Applications
The lost foam casting process offers distinct benefits for bimetal casting, primarily due to its unique mold-filling characteristics. In conventional sand casting, metal flow is often turbulent, leading to oxide formation and erratic mixing. However, in EPC, the metal advances radially through the decomposing foam pattern, resulting in a smoother, layered filling that is ideal for bimetal deposition. This behavior can be approximated by the equation for flow front velocity:
$$ v = \frac{dP}{dx} \cdot \frac{K}{\mu} $$
where \( dP/dx \) is the pressure gradient, \( K \) is the permeability of the coating, and \( \mu \) is the dynamic viscosity. The controlled velocity in lost foam casting reduces entrapment of gases and slag, enhancing the integrity of the bimetal interface.
Moreover, the slower cooling rate in EPC molds, compared to green sand molds, promotes a more uniform solidification structure, which is beneficial for stress relief in bimetal castings. The thermal profile can be modeled using the heat conduction equation:
$$ \frac{\partial T}{\partial t} = \kappa \nabla^2 T $$
where \( T \) is temperature, \( t \) is time, and \( \kappa \) is the thermal diffusivity. The extended solidification time in EPC allows for better atomic diffusion at the interface, improving bond strength. Additionally, the simplicity of gating design in lost foam casting reduces the complexity of orchestrating bimetal pours, as seen in this study with the open gating system. This efficiency translates to lower production costs and shorter lead times, making EPC a viable option for high-value components like machine tools.
The integration of bimetal technology with lost foam casting also aligns with sustainability goals, as it minimizes material waste by depositing high-alloy metals only where needed. For instance, in this work, the use of Cr and Mo was localized to the lower section, reducing overall consumption without compromising performance. The EPC process itself is environmentally friendly, as the foam pattern vaporizes completely, leaving no residual waste in the mold.
Conclusion
The bimetal lost foam casting process demonstrated in this study effectively produces machine tool castings with graded mechanical properties, leveraging the advantages of both techniques. By using EPS patterns, water-based coatings, and furan resin sand, we achieved a sound casting with no interfacial defects and desirable hardness and strength characteristics. The pouring strategy, involving sequential pours with controlled overlap, ensured proper alloy distribution and bonding. The principles of bimetal selection, based on solidification behavior and diffusion kinetics, provide a framework for extending this approach to other alloy systems. The lost foam casting method, with its inherent control over metal flow and solidification, proves to be highly suitable for bimetal applications, offering a cost-effective and efficient alternative to traditional methods. Future work could explore the application of this process to other components, such as gears or rolls, further expanding the scope of bimetal EPC in industrial manufacturing.