Extruded products constitute more than 50 % of the market for aluminium products in Europe of which the building industry consumes the majority. Aluminium extrusions are used in commercial and domestic buildings for window and door frame systems, prefabricated houses/building structures, roofing and exterior cladding, curtain walling, shop fronts, etc. Furthermore, extrusions are also used in transport for airframes, road and rail vehicles and in marine applications.
What is Extrusion?
The term extrusion is usually applied to both the process, and the product obtained, when a hot cylindrical billet of aluminium 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 for 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 for the purpose of producing extrusions with wide section sizes. Other presses are designed to push the die into the billet. This latter modification is usually termed "indirect" extrusion.
Figure 1. Schematic of the extrusion process.
The Versatility of the Extrusion Process
The versatility of the process in terms of both alloys available and shapes possible makes it one of the most valued assets in helping the aluminium producer supply users with solutions to their design requirements.
The Extrusion Process
The fundamental features of the process are as follows: A heated billet cut from DC cast log (or for small diameters from larger extruded bar) is located in a heated container, usually around 450°C - 500°C. At these temperatures the flow stress of the aluminium 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 of which is defined by the shape of the die.
Aluminium Alloys and Extrusion
All aluminium alloys can be extruded but some are less suitable than others, requiring higher pressures, allowing only low extrusion speeds and/or having less than acceptable surface finish and section complexity. The term ‘extrudability’ is used to embrace all of these issues with pure aluminium at one end of the scale and the strong aluminium/zinc/magnesium/copper alloys at the other end. The biggest share of the extrusion market is taken by the 6000, AlMgSi series. This group of alloys have an attractive combination of properties, relevant to both use and production and they have been subject to a great deal of R & D 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 but those containing the lower limits of magnesium and silicon e.g. 6060 and 6063 extrude at very high speeds - up to 100 m/min with good surface finish, anodising 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 diameter to 500 mm with length usually about 2-4 times the diameter and while most presses have cylindrical containers a few have rectangular ones for the production of wide shallow sections.
Design Aspects of Aluminium Extrusions
The ease with which aluminium alloys can be extruded to complex shapes makes valid the claim that it allows the designer to "put metal exactly where it is needed", 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 aluminium 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 aluminium beam by around 1,5 times the steel component it is intended to replace, to match the stiffness of the steel at half the weight. Also, at little added die cost, features can be introduced into the section shape which increase torsional stiffness and provide grooves for say fluid removal, service cables, anti-slip ridges etc. Such features in a steel beam would require joining and machining, thus adding to the cost and narrowing the gap between initial steel and aluminium costs.
Figure 2. Designing aluminium extrusions with improved stiffness.