Extruded products constitute more than 20.8% of the market for aluminum products in the USA, of which the building industry consumes the majority. Aluminum extrusions are used in commercial and domestic buildings for window and door frame systems, prefabricated houses and buildings, roofing and exterior cladding, curtain walling, shop fronts, and so on. Furthermore, extrusions are also used in transport for airframes, road and rail vehicles and marine applications.
What is Extrusion?
The term extrusion is usually applied to both the process and the product obtained. In the extrusion process, a hot cylindrical billet of aluminum is pushed through a shaped die (forward or direct extrusion, see Figure 1). The resulting section can be used in long lengths or cut into short parts for use in structures, vehicles or components. Also, extrusions are used as the starting stock for drawn-rod, cold-extruded and forged products.
While the majority of the many hundreds of extrusion presses used throughout the world are covered by the simple description given above, it should be noted that some presses accommodate rectangular shaped billets. This is to produce extrusions with wide section sizes. Other presses are designed to push the die into the billet, this is usually called indirect extrusion.
Figure 1. Schematic of the extrusion process.
The Versatility of the Extrusion Process
The versatility of the extrusion process in terms of both alloys available and shapes possible makes it one of the most valued assets in helping the aluminum producer supply users with solutions to their design requirements.
The Extrusion Process
The extrusion process is as follows. A heated billet cut from DC (Direct Chill semi-cast) log or, for small diameters, from a larger extruded bar is located in a heated container, usually between 450°C and 500°C. At these temperatures, the flow stress of the aluminum alloys is very low and by applying pressure by means of a ram to one end of the billet the metal flows through the steel die, located at the other end of the container to produce a section. The cross sectional shape is defined by the shape of the die.
Aluminum Alloys and Extrusion
All aluminum alloys can be extruded but some are less suitable than others, requiring higher pressures. This allows only low extrusion speeds or a less than acceptable surface finish and section complexity. The term extrudability is used to embrace all of these issues with pure aluminum at one end of the scale and the strong aluminum and zinc, magnesium or copper alloys at the other end.
The biggest share of the extrusion market is taken by 6000, AlMgSi series alloys. This group of alloys has an attractive combination of properties relevant to both use and production, and they have been subject to a great deal of industrial research and development in many countries. The result is a set of materials ranging in strength from 150 MPa to 350 MPa, all with good toughness and formability. They can be extruded with ease and their overall extrudability is good. However, those containing the lower limits of magnesium and silicon, for example, 6060 and 6063, extrude at very high speeds – up to 100 m/min with good surface finish, anodizing capability and maximum complexity of section shape combined with minimum section thickness.
Press load capacities range from a few hundred tonnes to as high as 20,000 tonnes, although the majority range between 1,000 and 3,000 tonnes. Billet sizes cover the range from 50 mm to 500 mm in diameter, with length usually about 2-4 times the diameter. While most presses have cylindrical containers a few have rectangular ones for the production of wide shallow sections.
Design Aspects of Aluminum Extrusions
The ease with which aluminum alloys can be extruded to form complex shapes makes valid the claim that it allows the designer to put metal exactly where it is needed. This is a requirement of particular importance with a relatively expensive material. Furthermore, this flexibility in design makes it easy, in most cases, to overcome the fact that aluminum and its alloys have only 1/3 the modulus of elasticity of steel (Figure 2).
Since stiffness is dependent not only on modulus but also on section geometry it is possible, by deepening an aluminum beam by around 1.5 times the steel component it is intended to replace, thus matching the stiffness of the steel at half the weight. Also, at little added die cost, features can be introduced into the section shape which increases torsional stiffness and provides grooves for fluid removal, service cables, anti-slip ridges and so on. Such features in a steel beam would require joining and machining, thus adding to the cost and narrowing the gap between initial steel and aluminum costs.
Figure 2. Designing aluminum extrusions with improved stiffness.
Source: European Aluminium Association
For more information on this source please visit European Aluminium Association.