Page 383 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 15.2 The Extrusion Process 363

Chamber Billet Die

Pressure Af Relief angle

FIGURE I5.4 Process variables in direct extrusion. The die angle, reduction in cross
section, extrusion speed, billet temperature, and lubrication all affect the extrusion
pressure.

Extrusion Force. The force required for extrusion
depends on (a) the strength of the billet material, (b) the Ta 6) QQ, Qi
. 5 <97 6 o
extrusion ratio, (c) the friction between the billet and the g eq,” ’)@@ O50 3;
chamber and die surfaces, and (d) process variables, such je’ 400 _ /f’/7; QQ, O6 Q
as the temperature of the billet and the speed of extrusion. .E ®\ GS/
The extrusion force, F, can be estimated from the formula .3 “FOG
§
A
Q
_
see/G0
/1
C
P _ AO/Q ln(Af>, (154) zoo _ C0/:pe ®°`~s\ Cob, `/`O
where le is the extrusion constant (which is determined dj QU,”
experimentally) and AO and Af are the billet and extruded 0 ,/"U/77 (_
product areas, respectively. The /2 value in Eq. (15.1) is 400 500 300 1000 1200 1400
thus a measure of the strength of the material being Temperature (OC)
extruded and the frictional conditions. Figure 15 .5 gives
the le values of several metals for a range of extrusion tem-
peratures.
FIGURE l5.5 Extrusion constant /2 for various metals at
different temperatures. Source: After P. Loevvenstein.

EXAMPLE I5.| Calculation of Force in Hot Extrusion

A round billet made of 70-30 brass is extruded at a at the given extrusion temperature. Thus,
temperature of 675°C. The billet diameter is 125 mm,
and the diameter of the extrusion is 50 mm. Calculate _”(125)2 7,.(125)2
the extrusion force required. F = -Z-- (250) ln = 4

Solution The extrusion force is calculated using ,_ 5_6 MN_
Eq. (15 .1), in which the extrusion constant, lc, is ob-
tained from Fi. 15.5. For 70-30 brass, le = 250 MPa (See Section 15.6 for capacities of extrusion presses.)

Metal Flow in Extrusion. The metal flow pattern in extrusion, as in other forming
processes, is important because of its influence on the quality and the mechanical
properties of the extruded product. The material flows longitudinally, much like in-
compressible fluid flows in a channel; thus, extruded products have an elongated
grain structure (preferred orientation). Section 15.5 describes how improper metal
flow during extrusion can produce various defects in the extruded product.