FEM analysis of surface flaw of wire during drawing - SIGMA-NOT

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HUTNIK - WIADOMOŚCI HUTNICZE
and measured values shows that for the axial component the
simulated stress based on the anisotropic model is very close to the
measured one, whereas, for the tangential component there is some
discrepancy between the calculated values from FE models and the
measured result. The reason is probably due to the yariances in the
measured peak positions caused by the curvature of the wire surface
when the ę angle was 45 and 90 degrees. It has to be pointed out that
it is of no use to take the shear stress components cr 13 and (723 into
account during the measurements. They are obliged to be zero at the
surface and because of the axial symmetry. Indeed, when these
components were considered as unknowns during the stress measurement, <7 13 = 5 (<5(cr13) = 1) MPa, <723 = —5(<5(<r 23 ) = 1)
MPa was found, i.e. they are nearly zero.
6. Conclusions. The finite-element method has been applied in
the simulation of wire drawing process to predict the distribution of
residual stresses in drawn wires. In the FE models, a good
description of the materiał behaviour is needed to achieve a high
level of accuracy. For textured materials, instead of isotropic yield
criterion, a texture based anisotropic yield locus can be incorporated
into the model to describe the anisotropic plastic behaviour of the
materiał. Good agreement has been reached between the calculated
axial residual stress and the result measured by X-ray diffraction on
the surface of the drawn wire.
7. Acknowledgements. The authors wish to acknowledge the
financial support of the Flemish Institute for the promotion of
scientific-technological research in industry (IWT).
Literaturę
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KAZUNARI YOSHIDA
UKD 621.778:519.6:620.191:669-426:669-428:621.753.5:616:669-124:629.1.01
Department of Precision Engineering, School of Engineering
Tokai Uniyersity
TETSUO SHINOHARA
Graduate Student, Graduate School of Engineering, Tokai University
FEM analysis of surface flaw of wire during drawing used
in spring of automobile and medical instrument
Analiza numeryczna metodą elementów skończonych wad
powierzchni drutów ciągnionych przeznaczonych na sprężyny
w przemyśle motoryzacyjnym i na narzędzia medyczne
The high surface ąuality of drawn wire and rods have been reąuired from the field of automobile, machinę and medical test. Using
three-dimensional FEA, this study analyzed wire breaks that occurred in the drawing fine wires containing inclusion and flaws on the wire
surface. The growth and disappearance mechanisms of flaws such as transversal cracks and scratches on a wire surface during wire drawing
were investigated.
Od drutów i prętów przeznaczonych do produkcji w przemyśle motoryzacyjnym i medycznym wymagana jest wysoka jakość powierzchni.
W pracy przedstawiono trój-osiowy model odkształcenia dala, analizowano pęknięcia występujące w cienkich drutach zawierających
wtrącenia oraz wady powierzchni. Badano wzrost i mechanizm zanikania pęknięć poprzecznych i rys powstających na powierzchni drutu
podczas procesu ciągnienia.
Key words: surface flaw, springs
Słowa kluczowe: wady powierzchni, sprężyny
1. Introduction. Technological advancement in the field of
semiconductors has been remarkable, and research on the production
of superfine wire of 10^30^m diameter is being actively carried
out. The minimum diameter and the use of some superfine wires on
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the market and in the laboratory in Japan are given in Table 1.
Market demands also include reduction of processing cost [1]. One
of the factors that directly influence the increase of processing cost is
wire breaks during the drawing process [2-18]. At the production
site, inclusions present in the wire, flaws on the wire surface, back
tension and inferior die shape have been noted to induce wire breaks,
however, several issues remain to be resolved [19]. In this study,
authors focused on wire-breaks caused by inclusions, flaws on the
wire surface and presence of yoids in the wire (Fig. 1).
Based on the results of previous studies [2,11,12,17], inclusions
are considered to comprise oxides and carbon compounds present
during casting, particles such as Fe and/or Cr compounds from
fractured and worn-out tools, and Si and Al oxides.
Using three-dimensional FEA, this study examined (1) the
influence of inclusions and voids when these are located away from
the center of the wire on wire breaks, and (2) growth of flaws on the
wire surface due to wire drawing.
2. Model and materiał properties used for FEA. The materiał
properties used in FEA for the drawing of superfine copper wire are
summarized in Table 2.
2.1. Drawing of a wire with inclusions or voids which are
located away from the center of the wire. The authors preyiously
reported the analytical results for an inclusion present at the center of
a wire (Fig. 2). Fig. 3 a shows an analytical model in which an
inclusion or void is located away from the center of the wire. The
mother wire is composed of copper and has a diameter d0 of 100 um.
Sińce the inclusion is much harder than copper, the inclusion was
assumed to be a cemented carbide. In order to simplify the
calculation of FEA the inclusion was a rectangular parallelepiped
with the dimensions a = 50 um, b = 20 ^m and c — 20/im. The
position of this inclusion is represented by h0/r0 (ratio of distance
from the center of the wire to the center of the inclusion, h „, to radius
of the wire, r0). When h0/r0 = O, the inclusion exists at the center
of the wire, and when it is 0.8, the inclusion is tangential to the wire
surface. The influence of h0/r0 on wire breaks was examined by
changing h0/r0. Similar to the case of inclusions, an analysis was
carried out for a void located away from the center of the wire,
S. 137
having the same dimensions as the inclusion. The drawing conditions were half-die angle = 6 deg and reduction per pass
Re = 20 %.
2.2. Growth offlaw on wire surface by drawing. Fig. 3b shows
models of artificial transversal cracks used for experiments. For the
easy way to perform this experiment, the authors use the following
dimensions: diameter d = 10 mm, width in the axial direction
a = 2 mm, width in the circumferential direction b = 8.5 mm, and
depth c = 2.5 mm. The shape of the cracks and their effects on the
wires after multi-pass drawing were investigated.
3. Analysis results and discussion
3.1. Influence of inclusion and void located away from the
Fig. 2. Sphere inclusion
Rys. 2. Wtrącenie o kształcie kulistym
a)
Die
Flaw
25 um
Fig. 1. Wire breaks in ultra fine wire drawing and check mark on
wire surface
Rys. 1. Pęknięcia w procesie ciągnienia ultracienkich drutów i rysa
powstająca na powierzchni drutu
Fig. 3. Drawing model of 3-dimention
a - Inclusion or void which is not in center, b - Flaw on wire surface
Rys. 3. Trójosiowy model odkształcenia dala
a - Wtrącenie lub wada, która nie jest umieszczona w środku drutu, b — Wada na
powierzchni drutu
T a b l e 1. Minimum diameter of superfine wires in Japan
Tabela 1. Minimalne średnice supercienkich drutów produkowanych w Japonii
Materiał
Special mild steel
Diameter (ura)
Market
Use of the products
Laboratory
Mild carbon steel
-15
-100
High carbon steel
150-16
100-10
30-15
20-10
Screen mesh, mesh of the filter, wire ropę
100-50
10-5
Mesh of the filter, wire ropę, stiffening wire
Fishing linę, antenna
Stainless steel
Titanium
Shape-memory alloy
-40
-12
Fishing linę
-
Screw. pin, bolt, electronic wire
Amorphous
20-15
-10
30-10
Gold
50-15
-15
Copper
15-10
Aluminum
30-20
15-7
-20
Steel cord, sawing wire, cable, spring
Sensor, stiffening wire
Bonding wire
Electronic wire, bonding wire
Electronic wire, bonding wire
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HUTNIK - WIADOMOŚCI HUTNICZE
Drawing direction
T a b l e 2. Materiał property for FEA
Tablica 2. Własności materiału wykorzystane w procesie modelowania metodą elementów skończonych.
Materiał
Young's modulus E/MPa
Initialyield stress <ry/MPa
Fig. 4. Mesh deformation of drawn wire contains an inclusion
Rys. 4. Siatka odkształcenia opisującą proces ciągnienia drutu
zawierającego wtrącenie
90.0
88.0
-
2
- 1 0
1
Position of the axłal wire a/x
Fig. 5. Wire diameter changes during wire drawing with an
inclusion
Rys. 5. Zmiany średnicy drutu podczas procesu ciągnienia drutów
z wytrąceniami
(a) h 0 /r 0 =0
(b) Vr0=0.2
(c) h 0 /r 0 =0.4
(d) h0/r0=0.6
Fig. 6. Hydrostatic stress distributions during wire drawing with
an inclusion
Rys. 6. Rozkład naprężeń hydrostatycznych w procesie ciągnienia
drutów z wtrąceniami
b)
Inclusion
Void
Fig. 7. Wire misalignment during drawing with an inclusion and
a void and shape of drawn wire
a ~ Mesh deformation. b - Shape of drawn wire
Rys. 7. Niewspóiosiowość drutu podczas procesu ciągnienia
drutów z wtrąceniami i z wadą
a - Odkształcenie siatki, b - Ksztah ; u^nwnego drutu
Poisson's ratio v
Wire
Inclusion
Copper
106300
Cemented carbide
588400
228
0.34
1570
0.20
center of the wire. Using three-dimensional FEA, the influence of
inclusions which are located away from the center on wire breaks
was investigated keeping the size of the inclusion constant (Fig. 3a).
Fig. 4 shows mesh deformation of a wire, in which an inclusion is
positioned at h0/r0 = 0.6, after one-pass drawing.
Similar to the case of two-dimensional FEA, when a wire is
drawn which has an inclusion located away from the center, necking
readily occurs in front of the inclusion. Fig. 5 shows the change in the
diameter of the drawn wire (dn = \rnl + |r„ 2 |). The abscissa in
Fig. 4 indicates the position in the drawing direction; it is O at the
center of the inclusion, and 0.5 and —0.5 at the front and back ends
of the inclusion, respectively. When a wire with this type of
inclusion is drawn, necking occurs in front of the inclusion and
bending occurs throughout the wire.
The degrees of necking and bending increase as the inclusion
approaches the surface of the wire. It is considered that when the
inclusion reaches the surface of the wire, the wire is subjected to
stress concentration due to process deformation, leading to an
increased degree of necking.
The hydrostatic stress distribution of the wire as the inclusion
passes through the die is shown in Fig. 6. High tension is generated in
front of and behind the inclusion. The closer the inclusion is to the
surface of the wire, the higher the compressive stress. As indicated in
Fig. 6, the authors find that when an inclusion is present in a wire, the
wire is subjected to non-uniform processing. Hence. the behaviour
and bending of a wire with an inclusion or void during drawing were
examined. Fig. 7 shows a schematic diagram of the introduction of
the wire into a die and bending of the wire after processing, for
h0/r„ = 0.6. As shown in the figurę, when an inclusion or void
passes through a die, the back of the wire is introduced into the die
with a certain angle of deviation. When the wire contains an
inclusion, the back of the wire is deflected to the side which contains
the inclusion; on the other hand, when the wire contains a void, the
back of the wire is deflected to the side which does not contain the
void. The closer the inclusion or void is to the surface of the wire, the
greater the amount of deflection. In addition, bending occurs in the
wire after drawing. When the inclusion or void is located away from
the center, the processing condition of the wire in the radial direction
is non-uniform. Accordingly, deflection at the back of the wire
arises, as well as non-uniform processing and residual stress in the
drawn wire, leading to bending of the wire.
3.2. Growth of transversal crack in wire surface by repeated
drawing. The authors investigated the growth process of flaws by
repeated drawing, assuming the presence of a transversal crack on
the surface of a wire rod. First a 2-mm-deep traversal crack was
formed on an elemental wire rod (q> 10 mm) and wire drawing with
a die half-angle a of 6 deg and a one-pass reduction R/P of 20 % was
repeated. Fig. 8 shows a schematic of the growth process of
transversal cracks. Even when the total reduction Rt approaches
90 %, the linear scratch and satin-finished surface concavity remain;
this fact suggests that once a deep crack is developed, it does not
disappear easily under generał drawing conditions.
Next, the growth of the flaw was obseryed on the axial cross
section (Fig. 9). Fig. 9 shows micrographs of the wire rod after 2, 4,
8 and 15 passes of drawing. The corner of the flaw (circled in Fig.
9b) is subjected to shearing deformation as explained above and
slanting of the flaw is induced. After repeated drawing, the length of
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Transversal
crack
Drawing
direction
S. 139
ly drawn, the flaw grows into a defect shape which is similar
to that of a check mark.
Acknowledgnient. This work has been supported by Japan
Society for the Promotion of Science (No. 15360393). We sincerely
thank Mr. K. Maeda at MSC Software Japan Co., Ltd.
References
Rt = 90%
Fig. 8. Schematic diagram of growth process of surface crack
by repeated drawing
Rys. 8. Schematyczny diagram procesu wzrostu defektów powierzchniowych w kolejnych ciągach
(d)<|>1.7mm ISpass. Rt=97.1°/
Fig. 9. Micrographs of flaw in cross section of wire after 2 to 15
passed of drawing
Rys. 9. Mikrografia płynięcia na przekroju poprzecznym drutu po
2 i 15 ciągach
the oyerlap flaw increases. The oyerlap flaw rolls over the wire
surface which may lead to peeling of the wire subjected to 15
drawing passes.
4. Conclusions. The influence of inclusions and voids located
away from the center of the wire on wire breaks, and the growth of
a flaw on the wire surface, were analyzed by FEA. Authors reached
the following conclusions:
a. When a wire is drawn which has an inclusion located away
from the center, necking readily occurs in front of the
inclusion.
b. When an inclusion or voids in the wire passes through a die,
the back of the wire is introduced into the die with a certain
angle of deviation. The front of the wire is constricted, and
bending of the wire occurs after drawing.
c. The growth mechanism of surface flaw such as transversal
cracks during drawing was clarified by experiments and
three-dimensional FEA. When a wire with a flaw is repeated-
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