Page 385 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 15 3 Hot Extrusion
TABLE l5.l

Typical Extrusion Temperature Ranges for Various
Metals and Alloys

°C
Lead 200-250
Aluminum and its alloys 375-475
Copper and its alloys 650-975
Steels 875-1300
Refractory alloys 975-2200

Extrusion direction Extrusion direction

. 60°
"` " l-and ,r ttas-

(3) (D) (C)

FIGURE l5.7 Typical extrusion-die configurations: (a) die for nonferrous metals; (b) die for
ferrous metals; (c) die for a T-shaped extrusion made of hot-work die steel and used with
molten glass as a lubricant. Source: (c) Courtesy of LTV Steel Company.

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 af-
fect the flow pattern of the material. Their presence also results in an extruded prod-
uct 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 con-
tainer. As a result, a thin shell (skull) consisting mainly of the outer oxidized layer of
the billet is left in the container. The skull is removed later from the chamber.
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 man-
ner. Because of friction and the severity of deformation, thin-walled extrusions are
more difficult to produce than those with thick walls.