Material Selection and Extrusion Process Control for Aluminum Extruded Side Sills in New Energy Vehicles

1. The Role of Aluminum Extruded Side Sills in Vehicle Safety

Aluminum extruded side sills are important structural components of automotive body-in-white systems. They play a critical role in protecting passengers during side-impact collisions. When a vehicle experiences an impact at a certain speed, the side sill undergoes controlled deformation to absorb collision energy and reduce the impact transmitted to the passenger compartment.

In new energy vehicles (NEVs), aluminum side sills also provide essential protection for the battery pack enclosure. During a side collision, excessive deformation of the vehicle floor or side structure may cause the battery enclosure to deform, potentially compressing internal battery modules and increasing the risk of damage, thermal runaway, or fire.

Therefore, the cross-sectional design of aluminum profiles, particularly the internal reinforcing ribs, together with material properties and manufacturing processes, plays a significant role in the crashworthiness of automotive structures.

2. Aluminum Alloy Composition and Material Selection

The chemical composition of aluminum alloys directly affects the mechanical properties and performance of extruded side sills.

Silicon (Si) and magnesium (Mg) are the primary strengthening elements in commonly used Al-Mg-Si alloys. Silicon contributes to alloy strengthening, but excessive silicon content may reduce ductility. Under certain loading and environmental conditions, insufficient elongation can increase the risk of premature cracking.

Magnesium is another essential strengthening element. It combines with silicon to form Mg₂Si precipitates, which contribute to the strength of heat-treatable aluminum alloys.

Copper (Cu) content also requires careful control because it can influence corrosion behavior and welding performance, including susceptibility to certain types of weld cracking.

During alloy design and casting, the proportions of these elements must be carefully balanced according to the required strength, ductility, corrosion resistance, weldability, and overall vehicle safety requirements.

3. Extrusion Process Control for Consistent Mechanical Properties

In aluminum extrusion, an aluminum billet is heated to a specified temperature and placed inside the extrusion container. Under the force generated by the extrusion press, the billet is pushed through a specially designed extrusion die. The aluminum flows through the die opening and forms a profile with the required cross-sectional geometry.

For automotive side sills, controlling the extrusion process is essential to achieving consistent mechanical properties and dimensional accuracy.

The first consideration is selecting an appropriate aluminum extrusion press tonnage based on the profile's cross-sectional dimensions, geometry, alloy characteristics, and extrusion requirements.

Other critical process parameters include:

• Aluminum billet heating temperature

• Extrusion die preheating and holding time

• Extrusion container temperature

• Ram speed and extrusion speed

• Exit temperature of the extruded profile

These parameters must be coordinated to maintain a consistent extrusion process. The extrusion speed should be optimized to achieve efficient production while ensuring that the profile reaches the required temperature conditions for subsequent solution treatment.

4. Cooling Process and Strength Development

After extrusion, rapid cooling is essential for retaining the required supersaturated solid solution and supporting the subsequent aging process in heat-treatable aluminum alloys.

The cooling process must reduce the profile temperature rapidly from above 500°C to near room temperature, with the actual cooling requirements depending on the alloy composition, profile dimensions, and heat-treatment specifications.

Common cooling methods include:

• Forced air cooling

• Water mist cooling

• Direct water quenching

For automotive side sills with large and complex cross-sections, water mist cooling can be selected to balance cooling intensity, mechanical properties, and dimensional stability.

Compared with more intensive direct water cooling, the cooling conditions of water mist can be adjusted to help control profile distortion and twisting while meeting the required mechanical property specifications. The final cooling method should be determined through process validation and mechanical testing.

5. Microstructure Optimization and Corrosion Resistance

In addition to alloy composition and extrusion parameters, microstructural control is another important factor affecting the performance of automotive aluminum profiles.

Microalloying and extrusion process optimization can help control the development of coarse-grain surface layers and improve microstructural uniformity. These factors are particularly important for components that must meet demanding mechanical and corrosion-resistance requirements.

Through coordinated control of alloy formulation, billet heating, extrusion speed, die temperature, and cooling conditions, manufacturers can improve the consistency of the final extruded profiles.

Corrosion testing and mechanical property evaluation are necessary to verify whether the selected material and manufacturing process meet the design requirements.

Conclusion

The performance of aluminum extruded side sills in new energy vehicles depends on more than the extrusion profile design. Alloy composition, extrusion process parameters, cooling methods, and microstructural characteristics all contribute to the final mechanical properties and crashworthiness of the component.

For aluminum extrusion manufacturers, precise process control and a suitable combination of material and equipment are essential for producing automotive profiles that meet demanding structural safety requirements.