Page 387 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 15 3 Hot Extrusion

Poor Good
Sharp outside Knife edge
Come' Unba|an¢ed More balanced
Sharp inside die tongue diGi0f19U€
Comer Unbaianced 2; Sharp Cingers d

S9Cti0n Unbalanced
thickness voids Balanced
voids
(3) (D)

FIGURE l5.I0 Poor and good examples of cross sections to be extruded. Note the
importance of eliminating sharp corners and of keeping section thicknesses uniform.
Source: ].G. Bralla (ed.), Handbook of Product Design for Manufacturing. McGraw-Hill
Publishing Company, 1986. Used with permission.

the environment; otherwise they would develop oxides on the surfaces, thereby in-
hibiting 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.

Die Materials. Die materials for hot extrusion usually are hot-worked die steels
(Section 5.7). Coatings (such as partially stabilized zirconia) may be applied to the
dies to extend their life. Partially stabilized zirconia dies (Section 8.22) also are used
for hot extrusion of tubes and rods. However, they are not suitable for making dies
for extruding complex shapes, because of the severe stress gradients developed in the
die, which may lead to their premature failure.

Lubrication. Lubrication is important in hot extrusion because of its effects on
(a) material flow during extrusion, (b) surface finish and integrity, (c) product qual-
ity, and (d) extrusion forces. Glass (Section 8.4) is an excellent lubricant for steels,
stainless steels, and high-temperature metals and alloys. In a process developed in
the 19405 and known as the Séjournet process (after ]. Séjournet), a circular glass or
fiberglass pad is placed in the chamber at the die entrance. The hot billet conducts
heat to the pad, whereupon a thin layer of glass begins to melt and acts as a lubri-
cant at the die interface as the extrusion progresses. Before the billet is placed in the
chamber, its cylindrical surface is coated with a layer of powdered glass to develop a
thin glass lubricant layer at the billet-chamber interface.
For metals that have a tendency to stick to the container and the die, the bil-
let can be enclosed in a thin-walled container, or jacket, made of a softer and
lower strength metal, such as copper or mild steel. This procedure is called
jacketing or canning. In addition to acting as a low-friction interface, the jacket
prevents contamination of the billet by the environment. Also, if the billet mate-
rial is toxic or radioactive, the jacket prevents it from contaminating the environ-
ment. This technique also can be used for extruding reactive metal powders
(Chapter 17).