Page 389 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 15.4 Cold Extrusion 369
of the metal, and .9 is the true strain that the piece undergoes based on its orig- Pungh
(15.2)
mm
where AO is the cross-sectional area of the blank, Yavg is the average flow stress
The force, F, in cold extrusion may be estimated from the formula
£€
=:-“r”
F = 1100A0Yavge,
inal and final cross-sectional area; i.e., ln(A(,/Af). For example, assume that a "M “' W k _
or plece
(3)
round slug 10 mm in diameter and made of a metal with Yavg = 350 MPa is
reduced to a final diameter of 7 mm by cold extrusion. Then the force would be if Pungh
ii 1
;
F = 1100(¢f)<1o2/4)(35o)[1n(1o/7)2} = 21.6 MN. »--1 “"
i i'
Cold extrusion has the following advantages over hot extrusion:

_ _ _ Punch
° Improved mechanical properties resulting from work hardening, provided (b)
that the heat generated by plastic deformation and friction does not recrys-
tallize the extruded metal.
° Good control of dimensional tolerances, reducing the need for subsequent FIGURE l5.l2 Two examples
of cold extrusion. Thin arrows
machining or finishing operations.
indicate the direction of metal
° Improved surface finish, due partly to the absence of an oxide film and provided
flow during extrusion.
that lubrication is effective.
° Production rates and costs that are competitive with those of other methods
of producing the same part, such as machining. Some machines are capable of
producing more than 2000 parts per hour.

The magnitude of the stresses on the tooling in cold extrusion, however, is very
high (especially with steel and specialty-alloy workpieces), being on the order of the
hardness of the workpiece material. The punch hardness usually ranges between
60 and 65 HRC and the die hardness between 58 and 62 I-IRC. Punches are a criti-
cal component, as they must possess not only sufficient strength, but also sufficient
toughness and resistance to wear and fatigue failure.
Lubrication is critical, especially with steels, because of the possibility of sticking
(seizure) between the workpiece and the tooling (in the case of lubricant breakdown).
The most effective means of lubrication is the application of a phosphate-conversion
coating on the workpiece, followed by a coating of soap or wax, as described in
Section 34.10.
Tooling design and the selection of appropriate tool and die materials are cru-
cial to the success of cold extrusion. Also important are the selection and control of
the workpiece material with regard to its quality, the accuracy of the slug dimen-
sions, and its surface condition. Several specialty alloys have been developed (partic-
ularly for critical applications requiring high performance) that are suitable for a
variety of cold-extrusion and cold-forming operations with good properties, dimen-
sional tolerances, and at low cost.

EXAMPLE l5.3 Cold-extruded Part

A typical cold-extruded product, similar to the metal shown in Fig. 15.12 but with a blind hole. Then the
component of an automotive spark plug, is shown in material at the bottom of the blind hole is punched
Fig. 15.13. First, a slug is sheared off the end of a out, producing the small slug shown. Note the respec-
round rod (Fig. 15.13, left). It then is cold extruded tive diameters of the slug and the hole at the bottom
(Fig. 15.13, middle) in an operation similar to those of the sectioned part.