The Aluminium Extrusion Process

What is an Aluminium Extrusion?

Aluminium extrusion is widely used in construction, automobile and aircraft manufacture, industrial machinery and consumer goods.

Extrusion is the process used to create lengths of aluminium of a fixed cross-sectional profile. Through extrusion, very complex cross-sections can be created with excellent strength and surface finish.

The process of aluminium extrusion works by forcing a block of metal, known as a billet, to pass through the die opening of smaller cross-sectional area than the billet itself.

The Extrusion Process

The extrusion process itself is a lot like the old Play-Doh Shape Maker. When you squeeze the Play-Doh through the press, the outcoming stream takes the shape of the selected mold. In the same way, the extruded aluminium takes the shape of the opening of the die. By using powerful hydraulic presses capable of exerting up to 15,000 tons of pressure, aluminium can be extruded into any shape you could imagine.

There are two main methods of aluminium extrusion:

• Direct
• Indirect

Direct extrusion

Direct extrusion is the most important and widespread method used in aluminium extrusion. In direct extrusion, the die head is held stationary and the aluminium billet is forced through the die using a moving ram.

Direct extrusion is commonly used in the manufacture of solid rods, bars, and hollow tubes. The design of the die can be modified to produce a wide variety of solid and hollow profiles.

Indirect extrusion

In direct extrusion, it is the billet that moves. In indirect extrusion, the aluminium billet is held stationary and the ram moves the die to exert pressure in the stationary billet. The advantage of keeping the billet stationary lies in keeping friction to a bare minimum.

Before extrusion starts, the cross-sectional shape is designed. The shape and features of the extruded metal are carefully calculated to maximize functionality, facilitate assembly, reduce weight, and minimize finishing costs. The unique characteristics of aluminium make it a cost-effective product with excellent functionality and a superb finish.

The Stages in the Aluminium Extrusion

1. A die is cast from the cross-section of the desired shape.
2. Aluminium billets are heated in a furnace until they reach 750-925ºF. At this point, the aluminium becomes a soft solid.
3. Lubrication, known as smut, is applied to the billet and ram. The smut is essential as it ensures the billet and ram do not stick together.
4. The ram applies pressure to the billet, pushing it through the die. During this process, liquid nitrogen is used to cool the die. The cooling prevents the formations of undesirable oxides, as well as prolonging the length of the die.
5. The extruded aluminium emerges from the die, taking on the same shape as the die opening. The extrusion is pulled onto a cooling table where it is exposed to air, water or a mixture of the two (depending on the final mechanical requirements of the metal).
6. Next a stretcher is applied. This corrects any twisting that may have occurred and straightens the metal. Hardness and strength are improved during the stretching process.
7. The extrusions – which may be 50 meters long – are fed to a saw conveyor and cut to the required profiles using a circular saw.
8. For some aluminium alloys, an artificial aging process is applied to achieve optimal strength. Artificial aging is accomplished via precipitation heat-treating in an aging oven. The aging process ensures the uniform precipitation of fine particles through the aluminium to enhance strength, hardness, and elasticity.
9. Finally, the finished profiles are taken to be finished or fabricated, ready to be shipped to the customer.

Benefits of Extruded Aluminium

Extruded aluminium has a number of advantages over other metals. There are metals which can match some of the desirable characteristics of aluminium, but no metal can match all these benefits at once.

The benefits of aluminium extrusions are many and include:

• Lightweight. Aluminium is around one-third of the weight of iron, steel, copper or brass. This makes it easier to handle and more economical to ship. For this reason, aluminium is widely used in automotive design, aerospace, and high-rise construction.
• Strong. The strength of aluminium is sufficient for all but the most demanding of applications. Aluminium is very suitable for cold environments, as it gets stronger as temperatures fall.
• Resistant to corrosion. Aluminium doesn’t rust. It is protected by a surface film of oxide, and this protection can be enhanced through anodizing.
• Excellent thermal conductivity.Aluminium conducts heat better than most metals. This makes it ideal for heat exchanger operations. Extrusion can be used to produce shapes optimal for thermal conduction.
• Affordable. The extrusion process is relatively inexpensive. Even prototypes can be made at a reasonable cost.