In the field of lightweight structural materials, aluminum alloys have become a focal point of research and development due to their favorable properties. Among these, Al-Si alloys stand out as high-strength casting materials, offering excellent castability, good corrosion resistance, ductility, high specific strength, and specific stiffness. These attributes make them ideal for applications in aerospace, automotive, and other high-performance industries. As engineering structures evolve toward larger, more complex, and robust designs, the demand for high-strength, lightweight aluminum alloys has grown significantly. This study focuses on addressing the deformation challenges associated with large thin-walled Al-Si alloy casting parts, specifically a cylindrical shell structure with a height of 1,480 mm and a wall thickness of 8 mm. The internal cavity of this casting part lacks reinforcing ribs or protrusions, making it highly susceptible to thermal deformation during casting and heat treatment processes. Our research aims to develop and validate a process that mitigates such deformation, ensuring dimensional accuracy and internal quality of the casting parts.
The manufacturing of large thin-walled casting parts involves several critical steps, including process design, simulation, molding, pouring, and heat treatment. In this work, we employed a sand casting approach using resin sand molds, which provide rapid成型, high strength, and dimensional precision. Pouring was conducted via differential pressure casting, a method known for reducing porosity, improving surface quality, and enhancing feeding capability. To optimize the process, we utilized Novacast software for simulating the filling and solidification stages, allowing us to predict potential defects and refine the design before physical production. Additionally, 3DP sand printing technology was used to produce sand cores, ensuring complex internal geometries with high accuracy. Despite these advanced techniques, deformation remained a persistent issue, particularly during heat treatment, where thermal stresses could cause irreversible distortion, leading to out-of-spec dimensions and increased scrap rates. To tackle this, we modified the casting design by incorporating anti-deformation rings on the outer surface of the part, which acted as external fixtures to restrain radial distortion. This paper details our comprehensive investigation, from initial process design to final quality assessment, providing insights into deformation control strategies for Al-Si alloy casting parts.
The structural characteristics of the casting part pose significant challenges. The cylindrical shell has a uniform wall thickness of 8 mm, with end frames thickened to 40 mm for mounting purposes. The internal surface is non-machined, requiring high as-cast dimensional accuracy. Key technical specifications include mechanical properties such as room temperature tensile strength ≥290 MPa, yield strength ≥200 MPa, and elongation ≥3%, as well as a wall thickness tolerance within ±0.5 mm. Additionally, the casting part must be free from internal defects like shrinkage porosity, shrinkage cavities, sand inclusions, and slag inclusions. Meeting these requirements demands a meticulous process design, especially for large thin-walled casting parts where thermal gradients and residual stresses are pronounced.
Our process design began with the selection of the pouring method. Differential pressure casting was chosen for its ability to maintain a controlled atmosphere during pouring, reducing turbulence and promoting directional solidification. The setup involves pressurizing the mold cavity to create a counter-pressure that minimizes gas entrapment and improves metal density. For the pouring position, we adopted a bottom-gating vertical slot gating system, as illustrated in the design schematic. This configuration ensures平稳 filling, minimizes冲刷 and impact, and facilitates sequential solidification from the remote areas toward the feeders. The gating system design is critical for large thin-walled casting parts, as it affects both filling behavior and feeding efficiency.
The slot gating system was designed based on empirical formulas to determine the number of vertical risers, slot thickness, slot width, and riser diameter. These formulas are expressed as follows:
$$ n = (0.016 \sim 0.028) \frac{S}{\delta} $$
$$ a = (0.8 \sim 1.5) \delta $$
$$ b = \frac{1}{2} D + (15 \sim 35) \text{ mm} $$
$$ D = (4 \sim 6) a $$
Here, \( n \) represents the number of vertical risers, \( S \) is the outer perimeter of the casting part, \( \delta \) is the wall thickness of the casting part, \( a \) is the slot thickness, \( b \) is the slot width, and \( D \) is the riser diameter. For our casting part, with a diameter requiring extensive coverage, we set \( n = 8 \) to ensure adequate feeding and controlled filling across the entire circumference. The gating system layout includes multiple vertical risers connected via slots to the casting part, promoting even metal distribution and reducing thermal gradients. This design is particularly beneficial for large thin-walled casting parts, where rapid heat dissipation can lead to premature solidification and defects.
To validate the process design, we conducted simulation studies using Novacast software. The casting part and gating system were meshed, and parameters such as pouring temperature (690°C ± 5°C), material properties of Al-Si alloy, and boundary conditions were input. The simulation captured the filling and solidification processes in detail. As shown in the results, the metal flow remained平稳 throughout filling, with no short shots or excessive turbulence. Cold irons were strategically placed between adjacent feeders to enhance directional solidification, ensuring that the casting part solidified from the farthest points toward the risers. This arrangement minimizes shrinkage defects in the thin-walled sections. The simulation also predicted areas of potential micro-shrinkage, primarily in the thicker regions like the end frames and feeder junctions, but no macro-shrinkage cavities were indicated. Based on this, we proceeded to physical production with confidence that the process would yield sound casting parts.
The molding process involved preparing resin sand molds using traditional pattern equipment. The sand was evenly compacted to ensure uniform density, followed by a natural drying period of 49–50 hours. To enhance mold strength, the dried molds were baked at 500°C ± 5°C for 4–5 hours. After baking, a refractory coating was applied to the mold surfaces to improve surface finish and prevent metal penetration. The molds were then assembled, ready for pouring. For the cores, we utilized 3DP sand printing, which allowed for precise internal geometries without the need for complex core boxes. This technology is advantageous for producing intricate cores for large thin-walled casting parts, reducing lead time and improving dimensional accuracy.
Melting and pouring were carried out in a vacuum differential pressure furnace. The Al-Si alloy was melted under controlled atmosphere to minimize oxidation and gas absorption. After reaching the desired temperature, the metal was poured into the mold under differential pressure conditions. The pressure was maintained for 180 seconds after pouring to ensure complete solidification under pressure, enhancing densification and reducing porosity. This step is crucial for achieving high-integrity casting parts, especially for thin-walled sections where solidification shrinkage is a concern.
Despite these precautions, initial heat treatment trials revealed significant deformation. The casting parts underwent T5 heat treatment (solution treatment followed by artificial aging), which induced distortion due to thermal stresses. Measurements using 3D scanning showed that over 90% of dimensions met CT9级 standards per GB/T6414 before heat treatment, but afterward, the internal cavity deviated beyond tolerances. Machining the outer surface resulted in wall thickness variations from 2.2 mm to 5.7 mm, failing the ±0.5 mm requirement. This deformation is primarily attributed to the固溶 stage, where elevated temperatures increase material plasticity, and gravitational forces cause permanent distortion in unsupported thin-walled sections. For large thin-walled casting parts, this is a common issue due to their high aspect ratio and low stiffness at elevated temperatures.
To address this, we modified the casting design by adding four external anti-deformation rings on the cylindrical surface. These rings, connected to the gating system, act as structural reinforcements to resist radial pulling and twisting during heat treatment. The modified design is shown in the schematic, where the rings are integrated into the casting part as part of the gating network. However, this introduced new challenges, as the ring areas became thick sections prone to shrinkage. To mitigate this, we placed internal chills in the cavity near the rings to promote sequential solidification and prevent defects. We re-ran the Novacast simulation for the modified design, confirming that no shrinkage cavities formed and micro-shrinkage was limited to non-critical areas like the rings and feeders. The simulation assured us that the modified process would produce casting parts with acceptable internal quality.
After implementing the anti-deformation rings, we produced a new batch of casting parts. Post-heat treatment measurements showed remarkable improvement: the internal cavity deformation was controlled within ±0.5 mm, and wall thickness consistency met specifications. This demonstrates the effectiveness of external fixturing in managing distortion for large thin-walled casting parts. The rings were later removed during machining, leaving the final part with the desired dimensions. Below is a table summarizing the key process parameters and their impact on deformation control for Al-Si alloy casting parts:
| Process Parameter | Initial Value | Optimized Value | Effect on Casting Parts |
|---|---|---|---|
| Pouring Temperature | 690°C ± 10°C | 690°C ± 5°C | Improved fluidity, reduced premature solidification in thin walls |
| Gating System | Conventional top pouring | Bottom-gating slot system with 8 risers | 平稳 filling, better feeding, minimized turbulence |
| Mold Material | Ordinary sand | Resin sand with baking | Higher strength, better dimensional accuracy for casting parts |
| Heat Treatment Cycle | Standard T5 | T5 with anti-deformation rings | Reduced distortion from >2 mm to <0.5 mm in casting parts |
| Simulation Software | Not used | Novacast for filling/solidification analysis | Predict defects, optimize design before production of casting parts |
The quality of the final casting parts was rigorously evaluated. After shot blasting, the surfaces appeared smooth and free from defects like pits, flashes, or burrs. Non-destructive testing included X-ray inspection and fluorescent penetrant testing, both confirming the absence of cracks, shrinkage porosity, or slag inclusions. The casting parts met Class I requirements according to HB 963-2005 standards. Mechanical properties were assessed using test bars extracted from the casting parts. The results, as shown in the table below, exceed the technical specifications, demonstrating the efficacy of our process for producing high-performance Al-Si alloy casting parts.
| Sample Number | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|
| 1 | 314 | 224 | 3.4 |
| 2 | 308 | 220 | 3.1 |
| 3 | 322 | 225 | 3.5 |
| 4 | 315 | 221 | 4.1 |
| 5 | 318 | 219 | 3.1 |
| 6 | 302 | 227 | 3.9 |
| Standard Requirement | ≥290 | ≥200 | ≥3 |
From a theoretical perspective, the deformation in large thin-walled casting parts can be modeled using thermal stress analysis. The distortion during heat treatment is driven by the relaxation of residual stresses and gravitational effects. The total strain \( \epsilon_{\text{total}} \) can be expressed as:
$$ \epsilon_{\text{total}} = \epsilon_{\text{thermal}} + \epsilon_{\text{mechanical}} + \epsilon_{\text{creep}} $$
where \( \epsilon_{\text{thermal}} \) is due to thermal expansion mismatch, \( \epsilon_{\text{mechanical}} \) from external loads like gravity, and \( \epsilon_{\text{creep}} \) from time-dependent deformation at high temperatures. For our casting parts, the primary contributor is \( \epsilon_{\text{mechanical}} \) during the solution treatment, where the material’s yield strength drops significantly. The anti-deformation rings introduce a constraining force that counteracts this, effectively reducing the net strain. This can be approximated by a simple beam bending model for the cylindrical shell:
$$ \delta = \frac{F L^3}{3 E I} $$
Here, \( \delta \) is the deflection, \( F \) is the applied force (e.g., gravitational load), \( L \) is the unsupported length, \( E \) is Young’s modulus, and \( I \) is the moment of inertia. By adding rings, we increase \( I \) and reduce \( L \), thereby minimizing \( \delta \). This principle underpins our deformation control strategy for casting parts.
In addition to mechanical modeling, the solidification behavior plays a key role in the initial stress state of casting parts. The Novacast simulation helped us visualize temperature gradients and cooling rates, which are critical for predicting shrinkage and hot tearing. The software solves the heat transfer equation:
$$ \rho C_p \frac{\partial T}{\partial t} = \nabla \cdot (k \nabla T) + Q $$
where \( \rho \) is density, \( C_p \) is specific heat, \( T \) is temperature, \( t \) is time, \( k \) is thermal conductivity, and \( Q \) is latent heat release. By optimizing the gating and chilling design, we achieved a favorable gradient that promoted directional solidification, reducing residual stresses in the as-cast state. This pre-conditioning is essential for subsequent heat treatment stability of casting parts.
Our research also highlights the importance of integrated process chain for large thin-walled casting parts. From simulation to 3DP sand printing to differential pressure casting, each step must be协同 to achieve quality outcomes. For instance, the use of 3DP for cores allowed for complex internal geometries without compromising accuracy, which is vital for thin-walled sections where core shifts can cause wall thickness variations. Similarly, the differential pressure casting provided the necessary environment for dense, pore-free casting parts, especially in thin walls where porosity is more detrimental.
Looking forward, there are several avenues for further improving deformation resistance in Al-Si alloy casting parts. One potential direction is the development of advanced heat treatment fixtures that can be integrated into the casting design, such as sacrificial supports that are removed post-treatment. Another is the exploration of alloy modifications to enhance high-temperature strength, reducing creep during solution treatment. Additionally, machine learning algorithms could be coupled with simulation software to optimize process parameters dynamically, further reducing trial-and-error costs for casting parts production.
In conclusion, our study successfully addressed the deformation challenges in large thin-walled Al-Si alloy casting parts through a combination of process design, simulation, and innovative fixturing. The key findings are: First, Novacast software proved effective in simulating filling and solidification, accurately predicting defect-prone areas and guiding design modifications for casting parts. Second, the addition of external anti-deformation rings significantly reduced heat treatment distortion, limiting dimensional variations to within ±0.5 mm for the internal cavity of casting parts. Third, comprehensive quality assessments confirmed that the casting parts met all internal, mechanical, and dimensional requirements, validating our approach. This research provides a practical framework for deformation control in similar large thin-walled casting parts, contributing to the advancement of lightweight alloy manufacturing. As industries continue to demand larger and more precise components, such strategies will be essential for producing high-quality casting parts efficiently and cost-effectively.