What is the Aluminum Extrusion Process?

What is Aluminum Extrusion？

Aluminum extrusion is a plastic deformation method where a preheated aluminum alloy billet (softened to 350-500℃) is placed into the container of an extrusion press. Pressure is applied, forcing the billet to flow out through the openings (dies) of the mold, compelling the aluminum billet to undergo directional plastic deformation. This process yields parts or semi-finished products with the desired cross-sectional shape, dimensions, and specific mechanical properties.

The extrusion is like squeezing toothpaste.

During this process, the aluminum alloy undergoes plastic flow and section reshaping:

• The metal particles inside the billet move along the direction of the die cavity.
• Grains are elongated and rearranged.
• The mechanical properties of the material can be enhanced through deformation strengthening.

For complex hollow profiles, such as the automotive crash beam shown in the figure, the aluminum slug is divided into several metal streams under pressure. These streams pass through the portholes and converge in the welding chamber. They are re-welded under conditions of high temperature, high pressure, and high vacuum, and finally flow out through the gap between the mandrel and the die opening. This forms tubular or hollow profiles that meet the required dimensions and performance. If bending is needed, a bending tool is added to the downstream equipment.

Extrusion Die

As product structure design engineers, although we do not design the aluminum extrusion dies, understanding the most basic die structure and the mechanism by which they form different extruded profiles is essential. This knowledge helps optimize designs to reduce die costs and improve the production efficiency of extruded parts.

What is Extrusion Die?

An extrusion die is fundamentally a thick circular steel disk containing one or more openings to form the desired profile. They are typically made of H13 tool steel and heat-treated to withstand the pressure and heat as the hot aluminum passes through the die. Although aluminum is a relatively soft metal, considerable pressure is required to push a solid aluminum ingot (billet) through a thin, porous aluminum extrusion die to form the required shape.

Die Classification

Based on the cross-sectional shape of the aluminum extruded part, the corresponding dies are categorized into 3 types:

1. Solid dies
2. Semi-hollow dies
3. Hollow dies

Among these, the hollow die has the most complex structure, is prone to wear and fracture, and has the highest cost.

Aluminum Extrusion Materials

Aluminum alloy materials suitable for extrusion include the 2XXX, 6XXX, and 7XXX series. The 6XXX series is the most widely used due to its high cost-effectiveness.

5XXX series aluminum: Mainly 5052. These features have outstanding corrosion resistance but lower strength. They are suitable for low-stress, high-corrosion-resistance applications, common in the construction industry.

6XXX series aluminum: Mainly 6061, 6063, 6005, and 6082. They contain Mg and Si. They exhibit good extrudability. After T6 heat treatment, they have moderate strength and a lower cost. This series is the most used extrusion material in New Energy Vehicles (NEVs).

Example 1: Motor casings are extruded from 6061 to incorporate complex cross-sections with integrated cooling channels. 6061 also offers better heat dissipation performance (thermal conductivity is 201 W/m·K.

7XXX series aluminum alloys: Mainly 7075. They offer extremely high strength. Extrusion is difficult for requiring higher pressure and more precise dies. They are suitable for parts with stringent strength requirements, such as the aluminum alloy extruded parts used in automobiles, shown in the figure.

Advantages of Aluminum Extrusion

High Dimensional Accuracy

Different aluminum alloy production processes yield varying levels of accuracy. For vacuum die casting structural parts, dimensional deviations are often significant due to shrinkage and deformation from heat treatment, sometimes reaching 1-3 mm. In contrast, the aluminum extrusion process generally results in higher cross-sectional dimensional accuracy, with dimensions that are more stable and easier to control.

With high-precision die processing and modification, aluminum extruded profiles for automotive use can fully meet the dimensional accuracy requirements of many automotive products. The dimensional accuracy of the extruded profile’s cross-sectional shape alone usually meets the product design requirements, with dimensional accuracy controllable within 1mm. Therefore, in terms of dimensional accuracy, aluminum extrusion is a mature, superior, and competitive process.

Fewer Manufacturing Steps

Because aluminum extruded profiles can have complex and diverse cross-sections, and wall thickness design offers a high degree of freedom, the rough parts from extrusion can achieve high dimensional accuracy. This minimizes the number of designed parts and the manufacturing steps from the extruded blank to the final finished part. The parts can be delivered for use solely through processes such as cutting, drilling, punching, machining, and bending. A heat treatment process can also be added if adjustments to mechanical performance are required.

Excellent Thermal Conductivity

Aluminum is widely used in automobiles for heat transfer in cooling and heating applications. Aluminum alloys exhibit superior thermal conductivity per unit weight compared to most other metal materials. Examples include heat sinks in electric drive axle controllers, cooling modules at the front end of vehicles, and chilled water pipes for air conditioning.

Aluminum extruded profiles can be formed into hollow shapes (tubes) to avoid or reduce the risk of liquid leakage. Cooling water channels for motor casings, water pipe connectors, and water cooling lines for battery packs all utilize aluminum extruded profiles. Furthermore, for areas with Electromagnetic Compatibility (EMC) requirements, seamless aluminum profiles can provide excellent shielding.

Easy Connection and Assembly

Aluminum alloy parts can be connected using a variety of forms, such as welding, Friction Stir Welding (FSW), mechanical connection (including FDS and SPR), adhesive bonding, embedding, and snap-fitting. These methods are all suitable for connecting aluminum profiles to other materials. For assembly, fixing with bolts to other parts offers rich and flexible options.

Short Production Cycle

Aluminum extruded parts offer a significant advantage in short development cycles, whether for low-volume production in “mule” vehicles and soft-tooling parts, or for high-volume, hard-tooling parts.

• The cycle from mold opening to part production for simple aluminum extruded profiles is only 1 to 2 weeks.
• Even for some large automotive aluminum extruded profiles with complex cross-sections, such as motor casings and sill beams, the mold opening cycle is only about 4 weeks.
• In contrast, the mold opening cycles for casting and stamping parts typically range from 4 to 16 weeks.
• Although rapid prototyping via 3D printing and metal powder sintering has shorter cycles for single-part production, they are more expensive. They sometimes cannot meet the requirements for part quantity or performance.

Low Die Cost

The cost of a set of extrusion dies for aluminum profiles ranges from a few thousand to tens of thousands of dollars. This is significantly cheaper compared to the hundreds of thousands to millions of dollars required for casting and stamping dies. For OEMs, adopting extruded parts saves a substantial portion of the one-time investment cost, reduces the cost of subsequent design changes, and increases design flexibility.

Challenges of the Aluminum Extrusion

Die Design and Manufacturing

For complex cross-section dies, the metal flow rate must be calculated precisely to avoid localized flow that is either too fast (leading to cracking, such as in thin walls) or too slow (resulting in insufficient filling). The die processing precision needs to reach 0.01mm, which results in high costs.

Dimensional Accuracy

Due to thermal expansion and contraction of the material, as well as fluctuations in the extrusion speed, parts are susceptible to dimensional deviations (e.g., straightness errors exceeding 0.5mm/m). This necessitates correction through stretching and straightening, and subsequent machining.

Surface Quality

Defects such as scratches, residual oxide scale, and air bubbles may occur. Strict control over the surface quality of the billet and the smoothness (roughness) of the die (Ra less than 0.8µm) is required.

Extrusion of Large-Sized Parts

Parts such as NEV battery pack casings (with lengths exceeding 2m) require large-tonnage extrusion presses (more than 5000 tons) and are prone to deformation due to uneven metal flow.

Design Features of Aluminum Extruded Parts

Extruded Part Dimensions

The size of an extruded part is evaluated by the Circumscribing Circle Diameter (CCD). As illustrated in the figure, the goal should be to minimize the CCD.

Avoid Asymmetrical Structures

The cross-section should be symmetrical and simple. Asymmetrical and unbalanced cross-sectional shapes increase the complexity of extrusion production and are prone to quality issues:

• Difficulty in guaranteeing dimensional accuracy.
• Difficulty in guaranteeing flatness.
• Parts warp at the center.
• Low production efficiency.
• Dies are prone to wear during mass production.

Wall Thickness

The wall thickness of an extruded part is related to the material, shape, and its Circumscribing Circle Diameter (CCD). The extrusion shape also influences the wall thickness design. As shown in the figure (red for tubular structures, blue for non-tubular structures) [Image showing relationship between CCD and optimal wall thickness, the most suitable wall thickness increases as the CCD increases.

Matching

Assembly tolerances with other parts (such as battery modules) should be 0.1mm. This prevents assembly difficulties caused by fluctuations in extruded dimensions. At the same time, the thickness dimension tolerance of the part should account for performance requirements, retaining a design margin.