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EuroWire – May 2011
57
technical article
Analysis of surface flaws on
drawn wire and wire breaks
By Kazunari Yoshida, Tetsuo Shinohara, Tsutomu Yamashita and Atsuhiro Tanaka of Tokai University, Japan
Abstract:
Using experiments and finite element
method (FEM) analysis, this paper exa-
mines whether surface cracks on wire
rods grow or are removed during drawing.
The deformation behaviour of V-shaped,
concave, and U-shaped transversal cracks
were observed upon repeated drawing.
The authors clarified the conditions under
which these transversal cracks are removed
in order to achieve a surface condition
similar to that of the area without cracks.
1 Introduction
The diameter of wires and bars produced
by drawing varies from about 10μm to
100mm. For any diameter of wire, breaking
during drawing has a direct effect on
productivity. Also, flaws on the surface of
wires often cause fatigue breaking, the
deterioration of mechanical properties,
and loss of commodity value. This is
why surface flaws are the most common
complaint of users. Furthermore, finer
and longer wires have recently been
requested for use as the bonding wires of
semiconductors and in medical devices.
Therefore, wire breaking and surface flaws
are problems that must be solved.
Many researchers have investigated the
optimal die shape to reduce drawing force,
measures to prevent wires from internal
cracking (through the formation of central
burst defects or cupping), the optimal
lubricant, measures for reduction of
residual stress and so forth. Useful findings
have been obtained, but there have been
few studies of measures to prevent wire
breakage and on the effects of inclusions
and surface flaws on wire drawing
1, 4
.
In this study, the causes of wire breakages
in drawn wires of different diameters were
examined. Analyses of wires with surface
flaws and inclusions were performed
experimentally and by a finite element
method (FEM) to determine whether
surface flaws and inclusions cause wire
breakages or remain as surface flaws when
drawing is repeated.
2 Types of wire
breakage caused
by drawing
Recently, the occurrence of wire breaking
during drawing has greatly decreased
due to reduced particulate segregation
and fewer inclusions in bars and wires
brought about by effective material
cleaning and because of the improvement
of drawing conditions. However, the finer
the diameter of a wire, the higher the
frequency of occurrence of wire breakage
during drawing. There are several causes
of wire breaking (
Figure 1
). Two types of
breaking are caused by inclusions.
One occurs when inclusions are found on
the fracture surface, and the other, called
“hollow end” breaking, occurs when traces
of concave inclusions are found but the
inclusions themselves are not found.
Two types of breaking are caused by
foreign materials introduced during a
process other than the casting process.
One is called “contamination” breaking,
when foreign materials are found on the
fracture surface, and the other is called
“pressed hollow” breaking, when no
foreign materials are found. Additionally,
“split end” breaking is considered to be
caused by surface flaws, and “tension”
breaking is caused by applying a stress
stronger than the wire strength
5
.
▲
▲
Figure 1
:
Types of wire breaking
‘Inclusion’
‘Contamination’
‘Pressed hollow’
‘Hollow End’
‘Split End’
‘Tension’
▲
▲
Figure 2
:
Percentages of causes of wire breaking during drawing of superfine gold wires
Inclusion
Hollow end
Contamination
Pressed hollow
Split end
Tension
Others