Introduction

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.