Hot Extrusion

For metals and alloys that do not have sufficient ductility at room temperature, or in order to reduce the forces required, extrusion is carried out at elevated temperatures (Table 15 .1). As in all other hot-working operations, hot extrusion has special requirements because of the high operating temperatures. For example, die wear can 

     

   

be excessive, and cooling of the surfaces of the hot billet (in the cooler chamber) and the die can result in highly nonuniform deformation (Fig. 15 .6c). To reduce cooling of the billet and to prolong die life, extrusion dies may be preheated, as is done in hot-forging operations.

Because the billet is hot, it develops an oxide film, unless it is heated in an inert-atmosphere furnace. Oxide films can be abrasive (see Section 33.2) and can affect the flow pattern of the material. Their presence also results in an extruded product that may be unacceptable when good surface finish is important. In order to avoid the formation of oxide films on the hot extruded product, the dummy block placed ahead of the ram (Fig. 15 .1) is made a little smaller in diameter than the container. As a result, a thin shell (skull) consisting mainly of the outer oxidized layer of the billet is left in the container.  

Die Design. Die design requires considerable experience, as can be appreciated by reviewing Fig. 15.7. Square dies (shear dies) are used in extruding nonferrous metals, especially aluminum. Square dies develop dead-metal zones, which in turn form a “die angle” (see Figs. 15 .6b and c) along which the material flows in the deformation zone. The dead-metal zones produce extrusions with bright finishes because of the burnishing that takes places as the material flows past the “die angle” surface.

Tubing is extruded from a solid or hollow billet (Fig. 15.8). Wall thickness is usually limited to 1 mm for aluminum, 3 mm for carbon steels, and 5 mm for stainless steels. When solid billets are used, the ram is fitted with a mandrel that pierces a hole into the billet. Billets with a previously pierced hole also may be extruded in this manner. Because of friction and the severity of deformation, thin-walled extrusions are more difficult to produce than those with thick walls.

 

 

Hollow cross sections (Fig. 15 .9a) can be extruded by welding-chamber methods and using various dies known as a porthole die, spider die, and bridge die (Figs. 15.9b to d). During extrusion, the metal divides and flows around the supports for the internal mandrel into strands. (This condition is much like that of air flowing around a moving car and rejoining downstream or water flowing around large rocks in a river and rejoining.) The strands then become rewelded under the high pressure in the Welding chamber before they exit through the die.The rewelded surfaces have good strength because they have not been exposed to

 

the environment; otherwise they would develop oxides on the surfaces, thereby inhibiting good welding. However, the welding-chamber process is suitable only for aluminum and some of its alloys, because of their capacity for developing a strong weld under pressure (as is described in Section 312). Lubricants, of course, cannot be used, because they prevent rewelding of the metal in the die.

Some guidelines for proper die design in extrusion are illustrated in Fig. 15.10. Note the (a) importance of symmetry of cross section, (b) avoidance of sharp corners,and (c) avoidance of extreme changes in die dimensions within the cross section