Page 384 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

cut the round billet in half lengthwise and then mark one face with

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Metal Extrusion and Drawing Processes and Equipment
Chapter 15
364
A common technique for investigating the flow pattern is to
Dead
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which they are placed in the chamber together and are extruded.
The two pieces are then taken apart (by melting the braze, if used, in
IIIEIIIII 2009 =§§;‘ a square grid pattern. The two halves may be brazed together, after
::::::::r»~ ::::::f;¢ =/17a/A a furnace) and studied. Figure 15.6 shows typical flow patterns ob-
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(a) (b) (C) tained by this technique for the case of direct extrusion with square
dies (a 90° die angle). The flow pattern is a function of several vari-
ables, including friction.
FIGURE l5.6 Types of metal flow in extruding The conditions under which these different flow patterns
with square dies. (a) Flow pattern obtained at occur are described in the caption of Fig. 15.6. Note the dead-metal
low friction or in indirect extrusion. (b) Pattern zones in Figs. 15 .6b and c, where the metal at the corners essential-
obtained with high friction at the billet-chamber ly is stationary. This situation is similar to the stagnation of fluid
interfaces. (c) Pattern obtained at high friction or
flow in channels that have sharp angles or turns.
with cooling of the outer regions of the billet in
the chamber. This type of pattern, observed in
metals whose strength increases rapidly with Process Parameters. Because they have high ductility, wrought alu-
decreasing temperature, leads to a defect known minum, copper, and magnesium and their alloys, as well as steels and
as pipe (or extrusion) defect. stainless steels, are extruded with relative ease into numerous shapes.
Other metals (such as titanium and refractory metals) also can be ex-
truded, but only with some difficulty and considerable die wear.
In practice, extrusion ratios, R, usually range from about 10 to 100. They may
be higher for special applications (400 for softer nonferrous metals) or lower for less
ductile materials, although the ratio usually has to be at least 4 to deform the mate-
rial plastically through the bulk of the workpiece. Extruded products usually are less
than 7.5 m long because of the difficulty in handling greater lengths, but they can be
as long as 30 m. Ram speeds range up to 0.5 m/s. Generally, lower speeds are pre-
ferred for aluminum, magnesium, and copper, higher speeds for steels, titanium, and
refractory alloys. Dimensional tolerances in extrusion are usually in the range from
i0.25 to 2.5 mm, and they increase with increasing cross section.
Most extruded products-particularly those with small cross sections-
require straightening and twisting. This is accomplished typically by stretching and
twisting the extruded product, usually in a hydraulic stretcher equipped with jaws.
The presence of a die angle causes a small portion of the end of the billet to remain
in the chamber after the operation has been completed. This portion (called scrap or
the butt end) subsequently is removed by cutting off the extrusion at the die exit and
removing the scrap from the chamber. Alternatively, another billet or a graphite
block may be placed in the chamber to extrude the piece remaining from the previ-
ous extrusion.
In coaxial extrusion, or cladding, coaxial billets are extruded together-
provided that the strength and ductility of the two metals are compatible. An exam-
ple is copper clad with silver. Stepped extrusions are produced by extruding the
billet partially in one die and then in one or more larger dies (see also cold extrusion,
Section 15 .4). Lateral extrusion (Fig. 15.3c) is used for the sheathing of wire and the
coating of electric Wire with plastic.

l5.3 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 re-
quirements because of the high operating temperatures. For example, die wear can