1. Introduction
Automobile lightweighting has become a significant trend in the automotive industry. It not only improves vehicle performance but also contributes to energy conservation and emission reduction. The chassis is an essential part of a vehicle, and its lightweight design is crucial for achieving overall vehicle lightweighting goals. Cast aluminum alloy, with its excellent properties such as low density, high strength, and good corrosion resistance, has emerged as a promising material for automobile chassis lightweighting. This article comprehensively reviews the application of cast aluminum alloy in the lightweight of automobile chassis structural parts, including the current status, challenges, and future development trends.
2. Lightweighting of New Energy Vehicle Chassis
2.1 Research Objects
The chassis of automobiles, especially new energy vehicles, often uses a large number of high-strength steel, cast iron, and cast steel parts. To meet the lightweight requirements of chassis components while ensuring safety and reliability, lightweighting research mainly focuses on key parts such as subframes, control arms, steering knuckles, floor panels, battery boxes, and wheelhouse shock towers.
2.2 Significance of Lightweighting
Lightweighting of the chassis can improve vehicle handling, increase driving range, reduce energy consumption, and lower emissions. In the context of the growing demand for new energy vehicles, chassis lightweighting is of great practical significance for enhancing the performance and competitiveness of vehicles.
3. Casting Technologies for Chassis Lightweighting
3.1 Low Pressure Casting
Low pressure casting is widely used in the production of chassis structural parts, especially for hollow subframes. It offers advantages such as good casting quality, high dimensional accuracy, and the ability to produce complex shapes. The process involves injecting molten aluminum alloy into a mold under low pressure, ensuring a smooth filling and solidification process.
3.2 Vacuum Die Casting
Vacuum die casting is suitable for producing parts with high requirements for internal quality and mechanical properties, such as wheelhouse shock towers. By creating a vacuum in the die cavity, this process reduces porosity and improves the density and strength of the castings.
3.3 Extrusion Casting
Extrusion casting combines the advantages of casting and forging, resulting in castings with high mechanical properties. It is particularly suitable for producing parts with thick sections and complex shapes, such as control arms. The process involves extruding molten aluminum alloy into a preheated die under high pressure.
3.4 Gravity Casting
Gravity casting is a traditional casting method that is simple and cost-effective. It is mainly used for producing small and medium-sized chassis parts with relatively simple shapes.
4. Application of Cast Aluminum Alloy in Chassis Structural Parts
4.1 Subframes
Subframes are critical components for supporting the vehicle powertrain and suspension system. Cast aluminum alloy subframes can significantly reduce the weight of the vehicle while maintaining sufficient strength and stiffness. Different casting technologies and aluminum alloys are used for subframe production, depending on the specific requirements of the vehicle.
4.1.1 Front and Rear Subframes
The front and rear subframes can have hollow or solid structures. Hollow subframes are often produced by low pressure casting or vacuum die casting, while solid subframes may be fabricated using extrusion casting or other methods. The choice of casting process and alloy depends on factors such as vehicle load, NVH (Noise, Vibration, and Harshness) requirements, and cost considerations.
4.1.2 Aluminum Alloys for Subframes
Common aluminum alloys used for subframes include AlSi10MnMg, A356.2, JDA1b, JDA2b, C611, C611M, and AlSi9MnMoZr. These alloys offer a good combination of mechanical properties and casting characteristics, making them suitable for subframe applications. The chemical compositions and mechanical properties of these alloys are summarized in Tables 1 and 2.
| Table 1: Chemical Compositions of Subframe Aluminum Alloys (mass fraction, %) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Alloy | Si | Fe | Cu | Mn | Mg | Sr | Ti | Cr | Zn | RE |
| AlSi10MnMg | 9.5 – 11.5 | ≤0.15 | ≤0.03 | 0.5 – 0.8 | 0.45 – 0.55 | 0.014 – 0.026 | 0.04 – 0.09 | ≤0.03 | ≤0.05 | |
| A356.2 | 6.5 – 7.5 | ≤0.12 | ≤0.05 | ≤0.02 | 0.25 – 0.45 | 0.1 – 0.15 | ≤0.05 | |||
| JDA1b | 8.0 – 10.5 | <0.15 | 0.1 – 1.5 | 0.5 – 0.8 | 0.1 – 0.5 | 0.02 – 0.03 | 0.1 – 0.2 | ≤0.25 | ||
| JDA2b | 2.0 – 3.6 | 0.5 – 0.8 | 6.0 – 8.0 | 0.15 – 0.2 | 0.1 – 0.2 | |||||
| C611 | 6.0 – 7.0 | ≤0.12 | ≤0.04 | ≤0.6 | 0.16 – 0.25 | ≤0.08 | ||||
| C611M | 6.665 | 0.094 | 0.002 | 0.524 | 0.296 | 0.02 | 0.005 | |||
| AlSi9MnMoZr | 9.5 – 10.5 | 0.15 | 0.02 – 0.05 | 0.35 – 0.6 | ≤0.08 | 0.012 – 0.02 | 0.04 – 0.15 | ≤0.05 | ≤0.05 |
| Table 2: Mechanical Properties of Subframe Aluminum Alloys | ||||
|---|---|---|---|---|
| Alloy | Heat Treatment | Tensile Strength /MPa | Yield Strength /MPa | Elongation (%) |
| AlSi10MnMg | T6 | 220 – 240 | 160 – 180 | 6 – 8 |
| A356.2 | T6 | ≥280 | ≥220 | ≥6 |
| JDA1b | F | 260 – 300 | ≥120 | 12 – 15 |
| JDA2b | F | 360 – 420 | 180 – 220 | 10 – 12 |
| C611 | F | ≥240 | ≥123 | ≥16.2 |
| C611M | F | ≥240 | ≥128 | ≥14 |
| AlSi9MnMoZr | F | ≥250 | ≥150 | ≥8 |
4.1.3 Case Studies of Subframe Casting
Several case studies demonstrate the successful application of cast aluminum alloy subframes. For example, Zhang Yunfeng et al. developed a large-size hollow subframe using low pressure casting. The subframe had dimensions of 1128.5 mm × 606.9 mm × 271.8 mm, was made of A356.0 alloy, and weighed approximately 16.25 kg. After T6 heat treatment, the subframe exhibited excellent mechanical properties, meeting the product requirements.
Another example is the production of the rear subframe of the Chery vehicle by Shi Jian et al. using low pressure casting with AlSi7Mg0.3 alloy. The casting had a complex structure with a large number of internal ribs and variable wall thicknesses. Through proper heat treatment, the mechanical properties of the subframe were significantly improved.
In addition, Sun Jue et al. produced the subframe of the Hongqi vehicle using extrusion casting. The casting had a symmetrical structure with complex end sections and a relatively simple middle section. After T6 heat treatment, the mechanical properties of the subframe met the design requirements.
4.2 Control Arms
Control arms play a crucial role in the vehicle suspension system, connecting the wheel hub to the vehicle chassis. Cast aluminum alloy control arms can offer weight reduction benefits while maintaining the required mechanical performance.
4.2.1 Casting Processes for Control Arms
Control arms can be produced by various casting processes, including gravity casting, low pressure casting, differential pressure casting, extrusion casting, and semi-solid casting. The choice of casting process depends on factors such as the complexity of the part geometry, required mechanical properties, and production volume.
4.2.2 Aluminum Alloys for Control Arms
Aluminum alloys commonly used for control arms include those with good casting fluidity and mechanical properties. The specific alloy selection depends on the design requirements of the control arm.
4.2.3 Case Studies of Control Arm Casting
Xu Shiwen et al. developed an aluminum alloy control arm using extrusion casting. The control arm had excellent mechanical properties, with a tensile strength of 293.2 MPa, a yield strength of 226.48 MPa, and an elongation of 9.01%. The study also investigated the effect of local extrusion on the elimination of shrinkage porosity in the thick mounting holes of the casting.
For hollow structure control arms, the sand core + low pressure casting process is often used, followed by T5 heat treatment to improve the mechanical properties. Examples include the control arms of the FAW Hongqi H9 and Deep Blue SL03/S7 vehicles.
4.3 Steering Knuckles
Steering knuckles are important components for connecting the steering system to the wheels. Aluminum alloy steering knuckles can contribute to weight reduction and improved vehicle handling.
4.3.1 Casting Processes for Steering Knuckles
Hollow steering knuckles are typically produced by low pressure casting, while solid steering knuckles can be fabricated using low pressure casting, extrusion casting, or gravity casting.
4.3.2 Current Status and Trends
Currently, mainstream mid-to-high-end vehicles use low pressure casting for aluminum alloy hollow steering knuckles. However, research on the use of extrusion casting for solid steering knuckles is ongoing, although large-scale application in vehicle production has not been reported yet. In addition, for economical vehicle models, sand-cast ductile iron steering knuckles are still widely used.
4.4 Wheelhouse Shock Towers
Wheelhouse shock towers are responsible for supporting the vehicle suspension and absorbing shocks. Cast aluminum alloy wheelhouse shock towers can effectively reduce the weight of the vehicle.
4.4.1 Casting Process and Parameters
The wheelhouse shock towers are mainly produced by vacuum high-pressure casting. Taking the shock tower of the Xiaopeng G6 as an example, the casting has a contour size of 532 mm × 365 mm × 299 mm, an average wall thickness of 3 mm, and a weight of 3.6 kg. The casting is produced using an IDR 3500 T die-casting machine with specific process parameters, such as a die-casting temperature of 680 – 700 °C and a mold temperature controlled above 200 °C. After T6 heat treatment, the casting exhibits excellent mechanical properties.
4.4.2 Weight Reduction and Performance Improvement
Compared to the traditional steel structure, the aluminum alloy wheelhouse shock tower can achieve a significant weight reduction of approximately 35%. This not only reduces the overall weight of the vehicle but also improves its handling and fuel efficiency.
4.5 Front and Rear Floor Panels
The front and rear floor panels are large structural components of the vehicle chassis. The application of cast aluminum alloy in these parts requires the use of large-tonnage vacuum die-casting machines and heat-treatable aluminum alloys.
4.5.1 Challenges in Casting Large Floor Panels
Casting large floor panels presents challenges such as ensuring uniform filling of the molten metal, controlling porosity, and achieving high dimensional accuracy. Tesla has been at the forefront of this technology, but even with continuous process optimization, the initial qualification rate was only 30%, and it has now reached 90% – 95% after two years of debugging.
4.5.2 Examples and Advantages
Domestic companies such as Guangdong Wencan and Guangdong Hongtu have also made significant progress in this area. The rear aluminum floor panel of the Zeekr 009, produced using a 7200 T die-casting machine, is the largest integrated die-cast aluminum floor panel in the world. It has eliminated nearly 800 welding points, reduced the number of parts by more than 80, and achieved a 16% weight reduction. This not only simplifies the manufacturing process but also improves the structural integrity and performance of the vehicle.
The integrated die-casting of the front and rear floor panels has become a development trend in the automotive industry. It can simplify the production process, reduce costs, and improve the overall performance of the vehicle. However, further research and development are needed to overcome the remaining challenges and optimize the casting process.
5. Challenges and Future Trends
5.1 Challenges in Cast Aluminum Alloy Application
5.1.1 Sand Core Technology
For hollow structural parts, the strength and collapsibility of the sand core are crucial. Currently, achieving a balance between high sand core strength and good collapsibility is a challenge. The use of 3D printed sand cores has limitations in terms of collapsibility and is not suitable for mass production. Therefore, the development of core materials that can meet both strength and collapsibility requirements is essential.
5.1.2 Heat Treatment
Proper heat treatment is necessary to improve the mechanical properties of cast aluminum alloy parts. However, the heat treatment process needs to be carefully controlled to ensure uniformity and avoid distortion or cracking of the parts.
5.1.3 Cost and Scale Effect
The cost of cast aluminum alloy parts is relatively high compared to traditional materials. To achieve widespread application, it is necessary to reduce costs through technological innovation and scale production.
5.2 Future Trends
5.2.1 Development of New Alloys
The development of new aluminum alloys with higher strength, better formability, and improved corrosion resistance will further expand the application scope of cast aluminum alloy in automobile chassis lightweighting.
5.2.2 Optimization of Casting Processes
Continuous optimization of casting processes, such as improving filling efficiency, reducing porosity, and enhancing dimensional accuracy, will improve the quality and performance of castings.
5.2.3 Integration and Modularization
The trend towards integration and modularization of automobile chassis components will drive the development of large-scale integrated die-casting technology. This will not only simplify the manufacturing process but also improve the structural integrity and performance of the vehicle.
5.2.4 Digitalization and Intelligent Manufacturing
The application of digitalization and intelligent manufacturing technologies in the casting industry will enable real-time monitoring and control of the casting process, improve production efficiency, and reduce costs.
6. Conclusion
Cast aluminum alloy has significant potential in the lightweighting of automobile chassis structural parts. Through continuous research and development of casting technologies and alloys, as well as the optimization of manufacturing processes, the application of cast aluminum alloy in the automotive industry will continue to expand. However, challenges such as sand core technology, heat treatment, cost, and scale effect need to be addressed. With the development of new alloys, optimization of casting processes, integration and modularization, and digitalization and intelligent manufacturing, the future of cast aluminum alloy in automobile chassis lightweighting looks promising.
