61 / 108
EuroWire – May 2011
59
technical article
The size was 0.53x0.27mm, and Di/Do was
about 0.2. It is presumed that the wire
did not break because of the low value of
Di/Do.
Figure 7
shows the results, gained
by FEM analysis, of drawing a wire with a
foreign material near the wire surface.
At the interface of the foreign material and
the wire, they are bonded mechanically.
Upon repeated drawing, stress acts on
the interface and causes separation at
the interface, generating an empty space.
Three dimensions FEM code MSC/Marc
Mentat 2008r1 was used in this study. The
results of FEM analysis are consistent with
the experimental results. Regardless of
whether the foreign material is located
in or on the wire, it does not undergo
deformation because of its hardness, even
if drawing is repeated. This leads to a high
value of Di/Do, increasing the drawing
stress and increasing the chance of wire
breakage.
4 Analysis of a wire
drawing with
surface cracks
Surface cracks develop on rods or wires
because of mishandling during casting,
hot rolling, drawing or transport, or
because of the improper winding of wires
6
.
The surface cracks that develop on wire
rods during upstroke rolling are classified
as shown
7
in
Table 2
; however, there is no
clear solution to this problem. In particular,
only a small number of studies have
reported on surface cracks formed during
drawing
8–11
.
In this study, wire rods that developed
circumferential cracks during casting
and rolling are used as mother wires
and drawn repeatedly. The growth and
removal of these cracks are examined in
the experiments and by FEM analysis.
Stainless steel (SUS304) rod wires were
mechanically scratched in the axial
direction using a lathe and analysed by
experiments and FEM. Rod wires that were
mechanically marked to form V-shaped,
concave, and U-shaped cracks in the
circumferential direction were used as
specimens.
FEM software, MSC/Marc Mentat 2008
R1, was used in this study.
Figures 8
and
9
and
Table 3
show the model used in the
FEM analysis, its material constant and
the parameters of the V-shaped crack,
respectively. Coefficient of friction(μ)
was set at 0.05. Moreover, the model was
assumed to be axis-symmetric in the FEM
analysis to save calculation time.
4.1 Comparison of results of experiment
and FEM analysis
A crack with a depth of h=0.8mm (8%)
was cut on an 8mm diameter wire and the
change in its shape was experimentally
and analytically examined after each
drawing pass. The initial crack on the
mother wire was asymmetrically V-shaped.
The shape of the initial crack was observed
using a microscope, and a mother wire
with a crack of the same shape was
modelled in the FEM analysis. It is clear
that the FEM analysis result agrees with
the experimental result. As shown in
Figure 10
, the crack appears to be removed
because side AB is pushed up into the
wire; however, side BC of the crack is tilted
so that it overhangs side AB, forming an
overlapping crack (defect).
Mechanical constants for gold
Young’s modulus
80GPa
Poisson’s ratio
0.44
Work-hardening curve
σ=475ε
0.07
Material condition for inclusion
Material
A1
2
O
3
, SUS304
Young’s modulus
300, 194GPa
Poisson’s ratio
0.23, 0.30
Yield stress
4.3, 0.205GPa
Die half angle, reduction
α
=7º, R/P=10%
Friction factor
0.05μm
▲
▲
Table 1
:
Condition of materials and drawing for FEM
▲
▲
Figure 5
:
Schematic diagram of wire breaking caused by foreign material
B) The case of a small
or soft inclusion
A) The case of a large
or hard inclusion
▼
▼
Figure 6
:
SEM image and componential analysis of wire with foreign material
C) Ni content
A) SEM image of the drawn wire
B) Fe content