Extrusion of Aluminium Alloys

Although extrusion is a modern process (rolling and forging being much older) it precedes the development of aluminium which was only commercially available following the invention in 1886, concurrently by Hall and Heroult, of the electrolytic process to extract the metal from bauxite. Among the industrial methods by which aluminium billets can be transformed to exceedingly complex shapes, extrusion has no rival and has firmly established itself as a major industrial process. The process converts a cast billet of solid metal into a continuous length of generally uniform cross-section by forcing it to flow through a die which is shaped to produce the required form of product. Generally it is a hot working operation, the metal being heated to give it a suitable flow stress (i.e. degree of softness and ductility); but it can also, in some instances, be carried out in the cold. In the modern process, cast billets of cylindrical shape, loaded into a composite cylinder (the container), are extruded through the die under pressure exerted by a ram, actuated hydraulically.

It has long been established that the type of failure exhibited in materials is a function, not only of the magnitude of the applied stress or the basic material characteristics, but also of the way in which they are applied. Thus for annealed EC grade aluminium we might expect an elongation of ~30% if tested in tension, of ~400% if cold rolled and several 1000% if extruded. It was Von Karman [1] who first demonstrated the effect of hydrostatic stresses on the mode of yielding when he demonstrated the ductility of marble under the application of high compressive hydrostatic stresses and this has since been confirmed by observing other more typical cases such as notched bars where the ductility appears to be reduced in the presence of high tensile hydrostatic stresses. In both rolling and extrusion the stresses are in each case predominantly compressive and in extrusion are compressive in all three directions.

Wrought alloys are divided into seven major classes according to their principal alloy elements. Each class demonstrates a different type of microstructure because of these alloy differences. Typical microstructural features are described in this chapter for each class and show how microstructure progressively develops from the as-cast ingot to the final wrought form (although this is treated in more detail in Chapter 4). Furthermore, alloy classes can be divided into two categories according to whether they are strengthened by work hardening only or by heat treatment (precipitation hardening). The former applies to 1XXX, 3XXX, 4XXX and 5XXX alloys, while the latter applies to 2XXX, 6XXX and 7XXX alloys.We shall also consider some of the more recently developed Al-Li alloys.

The extrusion process is complex, involving interaction between the process variables and the material’s high-temperature properties. Theoretically, the process variables which may be controlled are the extrusion ratio R, the ram speed V _R, and the extrusion temperature T. However, the extrusion ratio is generally fixed by customer specification so that the temperature and the speed become the only controllable factors.

The object of this chapter is to consider the conditions required for practical homogenization and its effect on extrusion processing parameters and, where appropriate, investigate how property development is influenced by homogenization. The examples are drawn from more recent work for aluminium alloys in each of the alloy series. In sections 5.1–5.8 we shall, in the main, discuss metallurgy and the homogenization requirements leaving the extrusion considerations for discussion under the heading of ‘limit diagrams’. It is normal to relate alloy extrusion to maximum extrudate speeds but the reader will be aware from the discussion on shape considerations in Chapter 4 that it is, in general, meaningless to suggest a maximum speed without taking into account the extrusion ratio, the shape factor, metallurgical factors (i.e. the retention of the press effect in 6XXX alloys or the structural factors required to produce acceptable fracture toughness in 2XXX or 7XXX alloys) and geometrical specifications of the product. Therefore we shall show these effects by means of limit diagrams and, at the conclusion of that consideration, will be able to grade the alloys in terms of extrudability.

This section presents only the major considerations of the metallurgy of 6XXX alloys. Following sections investigate some of the principal specific alloys.

In the preceding discussion we have investigated the physics and mechanics of the extrusion process and we will consider what is required in die design to ensure a product of the correct dimensions. Meeting these criteria should produce the correct product at the correct cost.