Page 392 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

3 2 Chapter 15 Metal Extrusion and Drawing Processes and Equipment

Surface Cracking. If extrusion temperature, friction, or speed is too high, surface
temperatures can rise significantly, which may cause surface cracking and tearing (Hr-
tree cracking or speed cracking). These cracks are intergranular (i.e., along the grain
boundaries; see Fig. 2.27) and usually are caused by hot shortness (Section 1.5.2).
These defects occur especially in aluminum, magnesium, and zinc alloys, although
they may also occur in high-temperature alloys. This situation can be avoided by
lowering the billet temperature and the extrusion speed.
Surface cracking also may occur at lower temperatures, where it has been
attributed to periodic sticking of the extruded product along the die land.
Because of the similarity in appearance to the surface of a bamboo stem, it is
known as a bamboo defect. When the product being extruded temporarily sticks
to the die land (see Fig. 15.7), the extrusion pressure increases rapidly. Shortly
thereafter, the product moves forward again, and the pressure is released. The
cycle is repeated continually, producing periodic circumferential cracks on the
surface.

Pipe. The type of metal-flow pattern in extrusion shown in Fig. 15.6c tends to
draw surface oxides and impurities toward the center of the billet-much like a fun-
nel. This defect is known as pipe defect, tailpipe, or fis/atailing. As much as one-third
of the length of the extruded product may contain this type of defect and thus has to
be cut off as scrap. Piping can be minimized by modifying the flow pattern to be
more uniform, such as by controlling friction and minimizing temperature gradi-
ents. Another method is to machine the billet’s surface prior to extrusion, so that
scale and surface impurities are removed. These impurities also can be removed by
the chemical etching of the surface oxides prior to extrusion.

Internal Cracking. The center of the extruded product can develop cracks, called
center cracking, center-burst, arrowhead fracture, or c/vez/ron cracking (Fig. 15 .17a).
These cracks are attributed to a state of hydrostatic tensile stress at the centerline in
the deformation zone in the die (Fig. 15.17b), a situation similar to the necked region
in a tensile-test specimen (see Fig. 2.23). These cracks also have been observed in tube
extrusion and in tube spinning (see Figs. 16.46b and c); they appear on the inside

- Die
Rigid product

Central burst
- Plastic
deformation zone
(H) (D)

FIGURE l5.l1 (a) Chevron cracking (central burst) in extruded round steel bars. Unless the
products are inspected, such internal defects may remain undetected and later cause failure of
the part in service. This defect can also develop in the drawing of rod, of wire, and of tubes.
(b) Schematic illustration of rigid and plastic zones in extrusion. The tendency toward
chevron cracking increases if the two plastic zones do not meet. Note that the plastic zone can
be made larger either by decreasing the die angle, by increasing the reduction in cross section,
or both. Source: After B. Avitzur.