Extrusion is among the most widely used of the aluminum forming processes, affording designers almost unlimited creative latitude to design profiles to meet specific needs. Extruded products constitute a large share of the market for aluminium products 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 mass transport for airframes, road and rail vehicles and in marine applications.
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 that precisely matches the profile of the shape specified by the designer (forward or direct extrusion). 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.
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 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.
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.
Billet is the starting stock for the extrusion operation. Extrusion billet may be a solid or hollow form, commonly cylindrical, and is the length charged into the extrusion press container. It is usually a cast product but may be a wrought product or powder compact. Often it is cut from a longer length of alloyed aluminum, known as a log.
Alloys are metals composed of more than one metallic element. Aluminum extrusion alloys contain small amounts (usually less than five percent) of elements such as copper, manganese, silicon, magnesium, or zinc. These alloying elements enhance the natural properties of aluminum and influence the extrusion process.
Billet length varies according to a number of factors, including the desired length of the finished profile, the extrusion ratio, the length of the runout, and the requirements of the extrusion press. Standard lengths may run from about 660 mm up to 1,830 mm. The outside diameter may range from 76 mm to 838 mm; 155 mm to 228 mm diameters are the most common.
Text: European Aluminium Association, Aluminum Extruders Council, Aluminum Association
Photos: Automation Supplies Ltd, Advanced Aluminium Design Ltd , Dubal