advances in numerical modeling of manufacturing processes

RAJIV SHIVPURI : NUMERICAL MODELING OF MANUFACTURING PROCESSES
systematic analysis approach to the analysis of porosity
and its elimination using computational approaches.
5. FEM AND STATISTICAL METHODS
5.1 Cracking in Cold Extruded Parts: Forging
Industry 46-48
While cold forging certain automotive drive
components in industry, ‘End cracks’ were observed
to occur randomly. As the name suggests, they are
found on the front end of the extruded part. These
cracks are radial and propagate in the longitudinal
direction. They are often visible to the naked eye,
showing up after the extrusion stage or during
subsequent machining operation. An example of an
end crack can be seen in Fig. 18(a). These cracks
lead to significant economic losses as they increase
the scrap volume and the requirement for inspection
of each forged part once they are detected. Forging
companies often resort to expensive processes like
in-process annealing to reduce the probability of
cracking.
The center of an extruded product can develop cracks
(variously known as center-burst, center-cracking,
arrowhead-fracture, or chevron-cracking), as shown
in Fig. 18(b). These cracks are attributed to a state
of hydrostatic tensile stress (also called secondary
tensile stresses) at the centerline of the deformation
zone in the die. This situation is similar to the necked
region in a uniaxial tensile-test specimen. The
tendency for center cracking increases with increasing
die angles and levels of impurities, and decreases
with increasing extrusion ratio.
The ‘Counter Shaft’ part (Fig. 18(a)) was the focus
of this investigation. The billet material is 8620 steel.
The shaft is manufactured by first shearing a billet
from a rolled rod. Then three stages of extrusion
(high ratios) which is followed by one stage of
upsetting. In this particular family of parts, billets of
larger diameter had a greater propensity for end
cracking and the percentage of cracked parts reduced
greatly by annealing the billets after the shearing
process. Almost all cracks originated in the first
extrusion operation. There was a greater tendency to
crack when the burr formed in the shearing process
was large, or if the sheared billet profile was rather
uneven or oblique. Remedies tried in the past included
sawing the billets instead of shearing, using smaller
diameter billets in order to have reduced extrusion
ratios and annealing and stress relieving. None of
these remedies were able to deal with the cracking
problem effectively.
There are three types of issues that can be associated
with this problem:
Fig. 18: Examples of cracking during cold forging-extrusion: (a) End Cracks observed from the outside (b) Chevron internal
cracking
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