DESCRIPTION
Title of Invention: Spot Welding Electrode and Welding
Apparatus and Welding Method Using Same
Technical Field
[OOOl] The present invention relates to a spot welding
apparatus, more particularly relates to an apparatus for
resistance spot welding obtaining a weld having a stable
outside diameter.
Background Art
tO0021 "Spot welding" is the process of superposing
two welded members, bringing columnar electrodes into
contact with a weld location from the top and bottom, and
applying pressure while running current so as generate
Joule's heat by the resistance of the conduction path and
using t-h is to form a weld (below, "nugget").
-
100031 In recent years, to realize performance such as
compliance with safety standards for vehicle bodies and
reduction of the environmental load, high strength steel
sheet having a 1500 MPa or more tensile strength is used.
[00041 However, for example, NPLT 1 reports that the
tensile shear strength (TSS) of a spot welded joint tends
to increase along with the strength of the steel sheet,
but the cross tensile strength (CTS) of the spot welded
joint falls when the strength of the steel sheet becomes
780 MPa or more. In high strength steel sheet with a
strength of the steel sheet of 780 MPa or more,
improvement of the cross tensile strength (CTS) of the
spot welded joint is a critical issue. The spot welding
electrode according to the present invention does not
have to be limited to a steel sheet as the welded member,
but in this Description, the welded member will be
explained as a steel sheet.
[0005] It is known that the cross tensile strength
(CTS) of a spot welded joint is improved if the nugget
increases in outer circumferential length. R&D has been
conducted to increase nugget size. However, the problem
is faced that if trying to enlarge a nugget, the current
required for the spot welding also becomes larger and as
a result the spot welding apparatus also has to be made
larger.
[0006] Therefore, R&D is underway for realizing a
suitable cross tensile strength (CTS) of a spot welded
joint by using a suitable size spot welding apparatus.
[0007] PLT 1 discloses a welding electrode designed to
improve the cross tensile strength (CTS) of a spot welded
joint by making the nugget a donut shape and thereby
increasing the circumferential length without changing
the area. Specifically, it discloses a spot welding
electrode making the electrode a donut shape so as to
enable the formation of a donut-shaped (ring-shaped)
nugget and providing a cylindrical shaped hollow part so
that the center does not contact the steel sheet. -
[OOOS] FIG. 18 is a conceptual view of a spot welding
apparatus having conventional spot welding electrodes,
while FIG. 19 is a conceptual view of a conventional spot
welding electrode.
[0009] In FIG. 18, 140 indicates a spot welding
apparatus having conventional spot welding electrodes, 50
indicates steel sheets, and 100 indicates conventional
spot welding electrodes.
[OOlO] In FIG. 19, a contact-and-conduction surface
105 of a contact-and-conduction part 104 contacting and
conductive with the steel sheet is, for example, formed
as a donut shape (circular ring shape) projection. By the
projection contacting the steel sheet, a so-called donutshaped
nugget is formed. In the Description, an electrode
having a donut shaped contact-and-conduction part will be
called a "donut-shaped electrode".
[OOll] PLT 2 discloses a method of judging whether a
nugget is suitably formed so as to manage the quality of
spot welding. PLT 2 describes to perform spot welding
using for at least one welding electrode among the pair
of welding electrodes performing spot welding a welding
electrode with an electrode surface of a substantially
projecting curved surface and with a center of the
electrode surface formed with a recess set to a
predetermined depth whereby the weld formed in the metal
surface reaches the deepest part when spot welding forms
a good nugget and to judge that the spot welding zone is
suitably formed with a nugget based on the fact of the
resistance value at the time of welding becoming a
balanced state.
Citation List
Patent Literature
[00121 PLT 1. Japanese Patent Publication No. 2010-
131666A
PLT 2. Japanese Patent Publication No. 2007-253166A
PLT 3. Japanese Patent Publication No. 2010-247176A
Nonpatent Literature
-
[0013] NPLT 1. Seiji Furusako et al., "Recent Topics
in Bonding Technology for Car Bodies and Technology for
Dealing With Same - Part I", Nippon Steel Technical
Report, 2012, 193, pages 69 to 75
Summary of Invention
Technical Problem
[0014] In spot welding, ideally, the axis of the
facing electrodes (below, referred to as the "electrode
axis") is perpendicular to the steel sheet. Due to this,
the electrodes contacting the two sides of the steel
sheets are matched in position whereby an ideal nugget is
obtained. However, in practice, it is rare for the steel
sheets to be perpendicular to the electrode axis. When
the electrodes are point shaped, even if the steel sheets
and the electrode axis are not perpendicular, current
flows between the electrodes in point contact, so the
nugget shape is generally stably obtained. However, in
the case of donut-shaped electrodes, the electrodes
themselves have area, so if the steel sheets and the
electrode axis are not perpendicular, a state of "onesided
contact" occurs. In this case, the state of contact
of the electrodes at the two sides of the steel sheets is
not constant, so as a result the nugget shape is not
stable.
[00151 Normally, steel sheets have various shapes and
bends, so the electrode axis and steel sheets are not
necessarily perpendicular. Therefore, if using donutshaped
electrodes, such a "one-sided contact" state
frequently occurs. For this reason, often, even if using
donut-shaped electrodes, often a suitable donut-shaped
nugget cannot be obtained and a sufficient cross tensile
strength (CTS) cannot be obtained. As art for raising the
weldability even if the steel sheets are somewhat slanted
with respect to the electrodes, it is disclosed to add
head wagging mechanisms to the electrode holders for
multipoint simultaneous welding (PLT 3). However, the
electrodes themselves are point-shaped electrodes and not
donut-shaped electrodes (PLT 3 ) . If point-shaped
electrodes, they contact the steel sheets by point
contact, so current is conducted between the point-shaped
electrodes and the nugget never becomes unstable.
[0016] On the other hand, with donut-shaped
electrodes, the electrodes themselves are planar shapes,
so the state of contact with the welded members does not
become constant and the nugget shape becomes unstable.
Even if applying donut-shaped electrodes to PLT 3, it is
not possible to eliminate the "one-sided contact" at
multiple points simultaneously. Furthermore, in the case
of donut-shaped electrodes, sometimes the steel sheets
deform at the time of spot welding and contact the
recessed sunken parts at the electrode centers. In this
case, while donut-shaped electrodes, abnormal conduction
occurs where there is conduction from the electrode
centers as well and a donut-shaped nugget cannot be
obtained. That is, to obtain a stable donut-shaped
nugget, the "one-sided contact" due to the steel sheets
and electrode axis not being perpendicular and abnormal
conduction of the centers due to deformation of the steel
sheets become problems.
[0017] The present invention has as its object to
solve the above problem. That is, it has as its object to
stably obtain a nugget exhibiting a high CTS value even
when the steel sheets are not perpendicular to the axis
of the pair of facing spot electrodes (electrode axis) in
spot welding due to the donut-shaped electrodes or when
the steel sheets are not flat, but have somewhat of a
slant or bend. Furthermore, it has as its object to
obtain a spot welding electrode which is free of abnormal
conduction at the donut-shaped electrode center even if
the steel sheets deform at the time of spot welding and
which can give a stable nugget. Further, it has as its
object to provide a spot welded joint of a high strength
steel sheet with a tensile strength of 780 MPa or more
where a sufficient level of cross tensile strength (CTS) -
can be secured.
Solution to Problem
[00181 The inventors engaged in intensive R&D and as a
result discovered that by adding a movable mechanism to a
donut-shaped electrode itself, it is possible to
strikingly raise the ability to track a steel sheet and
make the entire surface of the donut shaped contact-andconduction
part contact the steel sheet. For example, by
configuring the electrode by an electrode tip and an
electrode support and making the electrode tip able to
swing'with'respect to the electrode support, the
electrode tip can contact a steel sheet by the entire
surface of the donut electrode while following the slant
of the steel sheet. At this time, the center lines of the
contact-and-conduction parts .of the two facing electrodes
(center lines of conduction) do not necessarily match
(FIG. 7), but it was discovered that even if offset by a
certain extent, a good donut-shaped nugget is obtained.
In experiments of the inventors, it was confirmed that a
donut-shaped nugget can be stably obtained even if the
steel sheets are slanted by about 20' from the reference
plane perpendicular to the electrode axis. Further, it
was confirmed that when the steel sheets are slanted 20°,
a CTS value in the present invention electrode higher
than the CTS value in a normal electrode (point-shaped
electrode) when the steel sheets are not slanted can be
secured. Further, the contact-and-conduction part is not
limited to a donut shape and may be any closed curve. In
particular, it may be a projecting polygonal shape or
regular polygonal shape closed curve. Below, in this
Description, these closed curve shapes will be referred
to all together as a "donut shape" in the explanation.
Furthermore, the inventors engaged in intensive studies
and as a result discovered that by placing an insulating
material at the center of a donut electrode, even if a
steel sheet deforms, it is possible to keep the welded
member from contacting the electrode center and prevent
abnormal conduction.
[00191 The present invention was made based on these
discoveries and has as its gist the following:
(1) A spot welding electrode having an electrode tip
having a donut shape contact-and-conduction part and an
electrode support supporting the electrode tip, in which
spot welding electrode, the electrode tip contacts the
electrode support to be able to move.
(2) The spot welding electrode according to (I), wherein
the electrode tip contacts the electrode support to be . . ,.
able to swing.
(3) The spot welding electrode according to ( 2 ) , wherein
a tail of the electrode tip and a head of the electrode
support forming contact parts of the electrode tip and
the electrode support are spherical crown shapes.
(4) The spot rs~elding electrode according to (3), wherein
radii of curvature of the tail of the electrode tip and
the head of the electrode support are 4 mm to 20 mrn and a
radius of curvature of the tail of the electrode tip is -
5% to +lo% of a radius of curvature of the head of the
electrode support.
(5) The spot welding electrode according to any one of
(1) to (4), wherein in the contact-and-conduction part,
an area of a contact-and-conduction surface contacting
and conductive with a welded member is 12 mm2 to 70 mm2
and a diameter D of a smallest circle surrounding the
contact-and-conduction surface is 8 rnm to 20 mm.
(6) The spot rvelding electrode according to any one of
(1) to ( 5 ) , wherein in the contact-and-conduction part, a
contact-and-conduction surface contacting and conductive
with a welded member is a closed curve of a circular
ring, elliptical ring, or n-sided polygon (1123) with a
0.3 mm to 2.5 mm width.
(7) The spot welding electrode according to any one of
(1) to (6), wherein the donut shape contact-andconduction
part has an insulating member placed at its
center. Alternatively, the spot welding electrode
according to any one of (1) to (6), wherein a part other
than the contact-and-conduction surface of the head of
the tip has an insulating member.
(8) The spot welding electrode according to any one of
(1) to ( 7 ) , wherein the metal used for the electrode tip
and the electrode support is copper or a copper alloy.
(9) The spot welding electrode according to any one of
(1) to (8) , further having means for holding the
electrode tip and the electrode support together (below,
referred to as "tip-support holding means").
(10) The spot welding electrode according to any one of
(1) to (9), wherein the tip-support holding means are
spring hooks.
(11) A spot welding apparatus for spot welding at least
two steel sheets, which spot welding apparatus has a spot
welding electrode according to any one of (1) to (10) as
at least one of the spot welding electrodes.
(12) A spot welding method for spot welding at least two
steel sheets, which spot welding method uses a spot
welding electrode according to any one of (1) to (11) as
at least one of the spot welding electrodes.
Advantageous Effects of Invention
[0020] By using the donut shape spot welding electrode
of the present invention, the spot welding electrodes and
the steel sheets contact each other planarly and a donutshaped
nugget can be stably obtained even when the steel
sheets are not perpendicular to but are slanted from the
pair of spot welding electrodes at the top and bottom or
when the steel sheets are not flat but are somewhat
slanted or bent. Due to this, the remarkable effect is
exhibited of improving the cross tensile strength (CTS)
of the spot welded joint at so-called high strength steel
sheets with a strength of the steel sheets of 780 MPa or
more.
Brief Description of Drawings
[00211 FIGS. 1A-1 to FIG. 1B are conceptual views
showing one embodiment of a spot welding electrode -
according to the present invention. They show the case
where the contact-and-conduction part is a split circular
ring shape and the electrode tip tail is a recessed
spherical crown. FIG. 1A-1 is a bird's eye view of the
electrode tip from the contact-and-conduction part side.
FIG. 1A-2 is a cross-sectional view along the electrode
axis of the electrode tip and a cross-sectional view
along A-A of FIG. 1A-1. FIG. 18 is a view showing an
electrode support.
FIGS. 2A-1 to FIG. 28 are conceptual vieris showing one
embodiment of a spot welding electrode according to the
present invention. They show the case where the contactand-
conduction part is formed by arc shapes and the
electrode tip tail is a projecting spherical crown. FIG.
2A-1 is a bird's eye view of the electrode tip from the
contact-and-conduction part side. FIG. 2A-2 is a crosssectional
view along the electrode axis of the electrode
tip and a cross-sectional view along B-B of FIG. 2A-1.
FIG. 2B is a view showing an electrode support.
FIGS. 3A-1 to FIG. 38 are conceptual views showing one
embodiment of a spot welding electrode according to the
present invention. They show the case where the contactand-
conduction part is formed by arc shapes and the
electrode tip tail is a recessed spherical crown Larger
than a half sphere. The explanations of F I G . 3A-1, F I G .
3A-2, and F I G . 3B are based on F I G S . 1A-1 to lB.
F I G S . 4A-1 to F I G . 4B are conceptual views showing one
embodiment of a spot welding eLectrode according to the
present invention. They show the case where the contactand-
conduction part is formed by arc shapes and the
electrode tip tail is a projecting spherical crown larger
than a half sphere. The explanations of F I G . 4A-1, F I G .
4A-2, and F I G . 4B are based on F I G S . 2A-1 to 28.
F I G . 5 is a view for explaining one embodiment of a spot
welding electrode according to the present invention
having a spring holding mechanism.
F I G . 6 is a view for explaining one embodiment of a spot
welding electrode according to the present invention
having a spring holding mechanism.
F I G . 7 is a view showing the concept when placing spot
welding electrodes according to the present invention
straddling slant steel sheets.
F I G S . 8A-1 to 8B are conceptual views showing one
embodiment of a spot welding electrode according to the
present invention. They show the case where the contactand-
conduction part is a circular ring shape and the
electrode tip tail is a recessed spherical crown. The
explanations of F I G . 8A-1, F I G . 8A-2, and F I G . 88 are
based on F I G S . 1A-1 to 1B.
F I G S . 9A-1 to 9B is conceptual views showing one
embodiment of a spot welding electrode according to the
present invention. They show the case where the contactand-
conduction part is a circular ring shape and the
electrode tip tail is a projecting spherical crown. The
explanations of F I G . 9A-1, F I G . 9A-2, and F I G . 9B are
based on F I G S . 1A-1 to 1B.
F I G S . 10A-1 to 10B are conceptual views showing one
embodiment of a spot welding electrode according to the
present invention. They show the case where the contactand-
conduction part is a hexagonal shape and the
electrode tip tail is a recessed spherical crown. The
explanations of F I G . 10A-I, F I G . 10A-2, and F I G . 10B are
based on F I G S . LA-1 to 1B.
F I G S . 11A-1 to 11B are conceptual views showing one
embodiment of a spot welding electrode according to the
present invention. They show the case where the contactand-
conduction part is a hexagonal shape and the
electrode tip tail is a recessed spherical crown. The
explanations of F I G . A - F I G . llA-2, and F I G . 11B are
based on F I G S . 1A-1 to 1B.
F I G S . 12A-1 to 12B are conceptual views showing one
embodiment of a spot welding electrode according to the
present invention. They show the case where the contactand-
conduction part is a split circular ring shape and
the electrode tip tail is a recessed spherical crown. The
explanations of F I G . 12A-1, F I G . 12A-2, and F I G . 12B are
based on F I G S . 1A-1 to 1B.
F I G S . 13A-1 to 13B are conceptual views showing one
embodiment of a spot welding electrode according to the
present invention. They show the case where the contactand-
conduction part is a split circular ring shape and
the electrode tip tail is a recessed spherical crown. The
explanations of F I G . 13A-1, F I G . 13A-2, and F I G . 13B are
based on F I G S . 1A-1 to 1B.
F I G S . '1K4 and 14B are vieris for explaining the concept of
an apparatus equipped with of spot welding electrodes
according to the present invention. F I G . 14A shows the
case of use of spot welding electrodes shown in F I G S . 1A-
1 to 1B at the top and bottom, while F I G . 14B explains
the case of use of a spot welding electrode shown in
F I G S . 1A-1 to 1B at one side and the case of use of a
conventional spot welding electrode at the other side.
F I G S . 15A and 15B are views for explaining the concept of
an apparatus equipped with of spot welding electrodes
according to the present invention. FTG. 15A shows the
case of use of spot welding electrodes shown in F I G S . 2A-
1 to 2B at the top and bottom, while F I G . 15B explains
the case of use of a spot welding electrode shown in
F I G S . 2A-1 to 2B at one side and the case of use of a
conventional spot welding electrode at the other side.
F I G S . 16A and 16B are views for explaining an example of
use of the spot welding electrodes according to the
present invention. F I G . 16A shows the case of use of spot
welding electrodes shown in F I G S . 1A-1 to 1B at the top
and bottom, while F I G . 16B explains the case of use of a
spot welding electrode shown in F I G S . 1A-1 to 1B at one
side and the case of use of a conventional spot welding
electrode at the other side.
F I G S . 17A and 17B are views for explaining an example of
use of the spot welding electrodes according to the
present invention. F I G . 17A shows the case of use of spot -
welding electrodes shown in F I G S . 2A-1 to 2B at the top
and bottom, while F I G . 178 explains the case of use of a
spot welding electrode shown in F I G S . 2A-1 to 2B at one
side and the case of use of a conventional spot welding
electrode at the other side.
F I G . 18 is a view for explaining the concept of a spot
welding apparatus using conventional spot welding
electrodes.
F I G S . 19A and 19B are views for explaining the concept of
a conventional spot welding electrode. F I G . 19A is a view
when viewing the conventional spot welding electrode from
the head, while F I G . 19B shows a K-K cross-section in
F I G . 19A.
F I G . 20 is a view showing a relationship between a slant
angle of steel sheets and a CTS value after spot welding.
Description of Embodiments
[0022] Details of the present invention will be
explained below. As explained above, in this Description,
the welded members brill be explained as steel sheets.
100231 The spot welding electrode according to the
present invention has an electrode tip and an electrode
support supporting the same. At one end of the electrode
tip, there is a contact-and-conduction part for
contacting a steel sheet and causing electrical
conduction. The contact-and-conduction part of the
electrode according to the present invention has the
shape of a donut for obtaining a donut-shaped nugget.
Here, a "donut shape" indicates a continuous closed curve
having a width not 0 or a plurality of a group of arcs or
line segments having widths not 0 present on a closed
curve. Of course, it includes a circular ring shape of
consecutive arcs (total circumference), a group of arcs
present on the same circle (also called "split circular
ring"), and elliptical shapes and polygonal shapes (in
particular projecting polygonal shapes are preferred).
However, when split into a plurality of arcs or line
segments having widths not 0 on a closed curve, a smaller
-
number of splits is preferable, specifically four or less
splits are preferable.
LO0241 The electrode support is supported by an
electrode holder. The axis of the electrode support
matches with the axis of the spot welding electrode
(electrode axis). A pair of the spot welding electrodes
are placed facing each other across the steel sheets and
current is run across the two electrodes to heat the
sheets, so usually the pair of facing electrode axes
match.
roo251 The electrode tip is structured to be supported
at one end of the electrode support while able to move
with respect to the electrode support. The electrode tip
being "able to move" means the electrode tip can track a
steel sheet even if the steel sheet slants with respect
to the plane perpendicular to the electrode axis
(reference plane). Due to this, the entire surface of the
contact-and-conduction part can contact the steel sheet.
So long as structured to be able to track a slanted steel
sheet, the structure is not particularly limited. In
particular, if structured so that the electrode tip can
swing about the electrode axis, the electrode tip can
rotate about the electrode axis and can track even steel
sheet slanted with respect to the reference plane.
[00261 For example, the contact parts of the electrode
tip and electrode support, that is, the tail of the
electrode tip and the head of the electrode support, may
be made spherical crown shapes which fit with each other.
For example, as shown in F I G S . 1A-1 to F I G . lB, they may
be structured so that the tail of the electrode tip is a
recessed spherical crown shape, the head of the electrode
support is a projecting spherical crown shape, and the
two fit and slide with each other. According to this
structure, the electrode tip can swing in all directions
about the center of curvature of the spherical crown
forming the head of the electrode support and can track a
steel sheet slanted from the reference plane. F I G S . 2A-1
-
to F I G . 2B to F I G S . 4A-1 to F I G . 4B are examples of
application.
[0027] F I G . 7 shows the state where spot welding
electrodes shown in F I G S . 1A-1 to F I G . 1 B are made to
face each other and steel sheets slanted with respect to
a reference plane are placed between the electrodes. The
electrode tips track the slant 0 of the steel sheets and
swing by exactly 0 with respect to the electrode axes so
that the front surfaces of the contact-and-conduction
parts at first ends of the electrode tips contact the
...
steel sheets. At this time, as clear from F I G . 7 as well,
the center lines of the contact-and-conduction parts of
the facing electrodes (center lines of conduction) do not
match. For this reason, the stability of the nugget shape
changes depending on the slant angle 8 of the steel
sheets. The inventors investigated the slant angle of
steel sheets and the stability of the nugget shape by
experiments.
[0028] F I G . 20 shows the results of investigation of
the slant angle of the sheet sheets and stability of the
nugget shape using spot welding electrodes according to
the present invention, conventional donut-shaped
electrodes, and conventional point-shaped electrodes. For
the electrodes according to the present invention, No. 5
of Table 2 was used. For the conventional donut-shaped
electrodes, No. 2 of Table 2 was used. The conventional
point electrodes were generally commercially available
electrodes. The indicator of stability of the nugget
shape was the CTS value after spot welding.
[0029] As clear from FIG. 20 as well, if there is no
steel sheet slant (8=0), by using donut-shaped electrodes,
the CTS value becomes about 1.8 times that of the
conventional point electrodes and therefore the effect of
the donut shape was verified. However, along with the
increase of the steel sheet slant, the donut-shaped
electrodes become the one-sided contact state resulting
in point contact, so the CTS value rapidly falls and
becomes equal to that of conventional point-shaped
electrodes at a 5' or so slant.
[00301 As opposed to this, the electrodes according to
the present invention are not affected much at all by the
steel sheet slant if about 5O. As the steel sheet slant
increases, the conduction offset increases, so the CTS
value falls. However, this fall is slight. Even with a
steel sheet slant angle of 20' (8=20°), the value only
falls about 17%. An approximately five-fold CTS value was
obtained compared with point-shaped electrodes or
conventional donut-shaped electrodes.
[0031] Here, the geometric relationship of the
electrodes according to the present invention will be
verified. Here, the length between centerlines of the
facing contact-and-conduction parts (conduction offset)
is made "L", the radius of a contact-and-conduction part
(donut shape) is made "A", the minimurn thickness of an
electrode tip (distance between apex of spherical crown
part and tip of contact-and-conduction part) is made "h",
the steel sheet thickness is made "t" , and the radius of
curvature of the spherical crown of the head of an
electrode support is made "a". The steel sheets for spot
welding are the same in thickness. Two are superposed for
welding. In this case, the following relationship stands
geometrically.
L= (2a+2h+2t) tan6
Unless the projections of the front and back contact-andconduction
parts on the steel sheets at least overlap,
the nugget shape will become unstable. Therefore, the
following relationship is necessary.
2A>L
That is, to obtain a nugget shape, the following
relationship must be satisfied:
A> (a+h+t) tan6
_For example, in the No. 5 electrode of Table 2 used in
the experiments, if a=8 mm, h=3 mm, A=6 mm, and t=2.6 mm,
then tan6<0.44. That is, it is learned that if 0<23.8O,
the nugget shape is stable and a high CTS is obtained.
While depending also on the electrode or steel sheet
thickness or other shape conditions as explained above,
it is learned that if a spot welding electrode according
to the present invention, with at least a steel sheet
slant angle of up to about loo, the nugget shape
stabilizes and a wide circumferential length nugget is
obtained, so a high CTS value can be secured.
LOO321 Next, measures for suppressing abnormal
conduction due to deformation of the steel sheets during
the spot welding will be explained. Even if using the
electrodes according to the present invention, as the
steel sheet thickness becomes thinner, deformation of the
steel sheets at the time of spot welding is unavoidable.
For this reason, contact with the center (recessed sunken
part) of a contact-and-conduction part of the donut shape
becomes easy. Furthermore, if a donut shape contact-andconduction
part increases in diameter, contact becomes
further easier. If the steel sheets contact the center of
a donut shape contact-and-conduction part and current
ends up running through even the center (abnormal
conduction), the nugget shape does not become stable
leading to a fall in the CTS value. In fact, even during
a spot welding experiment of thin-gauge steel sheets, the
above-mentioned abnormal conduction occurs with a
probability of several percent. This abnormal conduction
causes a drop in the reliability of the spot welding. The
inventors solved this problem by placing an insulating
member at the center of the donut shape contact-andconduction
part (recessed sunken part) so that conduction
does not occur even if a steel sheet deforms. Here, t,he
insulating member may be a material with an internal
resistance of 105/C2m or more. This is because if less
than 105/C2m, with the voltage used in spot welding,
insulation breakdown is liable to occur and conduction to
result. The thickness of the insulating member should be
0.1 mm or more. If the thickness is less than 0.1 mm, the
loss due to the heat at the time of welding is liable to
cause consumption of the insulating substance.
[0033] Next, an electrode according to the present
invention will be explained using examples of
embodiment$.
First Embodiment
A first embodiment of the present invention is a metal
welding use electrode having an electrode tip and an
electrode support where the tail of the electrode tip is
recessed.
LO0341 FIGS. 1A-1 to FIG. 1B and FIGS. 3A-1 to FIG. 38
show a spot welding electrode with a tail of the
electrode tip of a recessed shape. FIGS. 1A-1 to FIG. 1B
show a spot welding electrode with a recessed tail of the
electrode tip where the tail 13 of the electrode tip and
the head 23 of the electrode support mechanically contact
and with a sliding surface of a spherical crown shape
smaller than the radius. FIGS. 3A-1 to FIG. 38 show-aspot
welding electrode with a recessed tail of the
electrode type of a spherical crown shape where the tail
13 of the electrode tip and the head 23 of the electrode
support mechanically contact and the sliding surface is
larger than the radius. The only point of difference is
the sliding area by which the tail 13 of the electrode
tip and the head 23 of the electrode support mechanically
contact. Therefore, to avoid overlapping explanations,
below, the explanation will be given in accordance with
FIGS. 1A-1 to FIG. 1B.
LO0351 Support
FIG. 1B shows an electrode support, where 23 is the head
of the electrode support and 22 is the tail of the
support.
100361 (a) Form and function
The function of the head 23 of the electrode support is
to support the electrode tip 1 from the tail 13 of the
electrode tip. The electrode support 2 has a bulletshaped
form. The head of the electrode support has a
projecting spherical crown shape with a radius of
curvature of 4 to 20 mm (in the figure, a semispherical
shape). The remaining part is cylindrical. The head 23 of
the electrode support contacts and supports the tail 13
of the electrode tip I. The reason why the head 23 of the
electrode support is made a projecting shape with a
radius of curvature of 4 to 20 mm is that if the radius
of curvature is less than 4 mm), 1) the region where the
head 23 of the electrode support and the tail 13 of the
electrode tip electrically contact becomes narrower and
they are liable to fuse together and 2) the head 23 of
the electrode support and the tail 13 of the electrode
tip cannot withstand the load by which the electrode is
pressed. Further, if the radius of curvature is over 20
mm, the displacement of the electrode tip 1 becomes
larger and control of displacement of the electrode tip 1
becomes difficult. While not shown, the tail 22 of the
electrode support is connected with an external power
supply.
[0037] (b) Metal material
The metal material forming the electrode support is
required to be high in electrical conductivity and high
in strength, so pure Cu or Cu alloy is preferable. A Cu-
Cr alloy is also preferable.
[0038] Tip
[00391 (a) Form and function
The head 11 of the electrode tip has a contact-andconduction
part 14. This supplies current to the steel
sheets. In the contact-and-conduction part 14, the part
contacting and conductive with a steel sheet will be
called the "contact-and-conduction surface 15". The tail
13 of the electrode tip is supported by the head 23 of
the electrode support. Current is supplied from the
electrode support 2. The tail 13 of the electrode tip has
- ~
a recess with a radius of curvature of -5% to +lo% of the
radius of curvature of the head 23 of the electrode
support of the support 2. If the radius of curvature is
over lo%, the region where the tail 13 of the electrode
tip and the head 23 of the. electrode support electrically
contact becomes smaller and they are liable to end up
fusing together due to the concentration of current. On
the other hand, if less than -5%, the tail 13 of the
electrode tip and the head 23 of the electrode support
become large in contact resistance at their centers and
the flow of current becomes harder. Even if the electrode
axes of the facing spot electrodes are offset, the
electrode tip can slide on the head 23 of the electrode
support and therefore the contact-and-conduction surface
can evenly contact the steel sheet. In this way, the
current supplied from the contact-and-conduction surface
15 can be used to stably form a nugget with a long outer
circumference at the steel sheet.
[0040] (b) Metal material
The material of the metal forming the electrode tip 1 is
required to be high in electrical conductivity and high
in strength, so is preferably pure Cu or Cu alloy. A Cu-
Cr alloy is preferable.
[004l] Contact-and-conduction Surface Etc.
(a) Shape
The total S of the area of the contact-and-conduction
surface of the contact-and-conduction part 14 positioned
at the head 11 of the electrode tip (below, referred to
as the "area of the contacting and conduction surface")
should be 12 mm2 to 70 mm2. If the area of the contactand-
conduction surface is less than 12 mm2, the current
density at the contact-and-conduction part becomes high
and the generated heat causes the wear of the electrode
to become greater. On the other hand, if over 70 mm2, the
current density falls and the heat density required for
melting (amount of heat generated per unit volume of the
steel material) can no longer be obtained. The area of
the contact-and-conduction surface is preferably 20 mm2 to
60 mm2, more preferably 30 mm2 to 40 mm2. The shape of the
contact-and-conduction surface 15 is preferably a circle
or ellipse such as shown in F I G S . 8A-1 to 8B or a closed
curve such as an n-sided polygon (n23) such as shown in
F I G S . 10A-1 to 10B.
[0042] (b) Diameter of smallest surrounding circle
The diameter D of the smallest circle surrounding all of
the contact-and-conduction surface (below, referred to as
the "smallest surrounding circle") is an important
element. F I G S . 1A-1 to F I G . 1B shows the case where the
contact-and-conduction part 14 is scattered in island
shapes. The circle drawn by the broken line 16 is the
smallest surrounding circle. The diameter D of the
smallest surrounding circle should be 8 mm to 20 mm. If
the diameter of the smallest surrounding circle is 8 mrn
or less, a sufficiently large nugget cannot be formed. On
the other hand, if over 20 mm, the weld area of the steel
sheet ends up becoming larger, which is inefficient. The
diameter of the smallest surrounding circle is preferably
10 nun to 16 mm, more preferably 12 mm to 15 mm.
[0043] (c) Insulating substance
When the contact-and-conduction surface 15 draws a closed
curve, the space inside the closed curve may have a
ceramic or other insulating member with a high heat
resistance other than air inside it. By having the
insulating member, it is possible to prevent deformation
of a steel sheet at the time of spot welding from causing
the steel sheet to contact the recess of the closed curve
side (also referred to as the "center") and from causing
abnormal conduction. Furthermore, by the center having
the insulating member, the contact-and-conduction surface
15 and the center become the same plane, so the shock
when the contact-and-conduction surface 15 contacts the
steel sheet is eased. Using the electrode of FIGS. 8A-1
to 88 as an example, an example of placement of the
insulating member 32 at the inside of the contact-andconduction
part 14 will be shown.
100441 Holding Springs
FIG. 5 is a view for explaining a spot welding electrode
equipped with holding springs according to the present
invention. In FIG. 5, 19 indicates spring hooks for
holding the electrode tip, 29 indicate spring hooks for
holding the electrode support, and 39 indicates holding
springs. As shown in FIG. 5, the holding springs 39
enable the electrode tip 1 to be pushed against the
electrode support 2.
[0045] Second Embodiment
The second embodiment of the present invention is a metal
welding electrode having an electrode tip and an
electrode support wherein the tail of the electrode tip
is a projecting type.
roo461 FIGS. 2A-1 to FIG. 2B and FIGS. 4A-1 to FIG. 4B
show a spot welding electrode with a recessed tail of the
electrode tip. The electrode of FIGS. 2A-1 to FIG. 2B
differs from the electrode of FIGS. 1A-1 to FIG. 1B in
that the tail of the electrode tip is recessed, but the
--other requirements and the substantive functions are the
same as the electrode of FIGS. 1A-1 to FIG. 1B. In the
same way, the electrode of FIGS. 4A-1 to FIG. 4B differs
from the electrode of FIGS. 3A-1 to FIG. 3B in that the
tail of the electrode tip is recessed. The other
requirements and substantive functions are the same as
the electrode of FIGS. 4A-1 to FIG. 4B. Here, overlapping
explanations are avoided, but the electrode of the first
embodiment and the electrode of the second embodiment
have substantially the same functions.
[0047] Spot Welding Apparatus
FIGS. 14A and 14B are conventional views of a spot
welding apparatus using electrodes of a first embodiment,
while FIGS. 15A and 15B are conceptual views of a spot
welding apparatus using electrodes of the second
embodiment. As shown in FIG. 14.4 and FIG. 15A, sometimes
the electrode according to the present invention is
applied for both of the facing electrodes, while as shown
in FIG. 148 and FIG. 158, sometimes it is applied for
just one. Even if the electrode holder itself deforms and
the axes of the facing two electrodes cross at a slant,
by applying the electrode according to the present
invention to one electrode, the two electrodes can
suitably contact the steel sheets. As shown in FIGS. 14A
and 14B and FIGS. 15A and 15B, by applying the spot
welding electrode according to the present invention for
at least one of the two facing electrodes, it is possible
to obtaYn a stable nugget even with a donut-shaped
electrode.
[0048l Spot Welding Method
FIGS. 16A and 16B are conceptual views of a spot welding
method using an electrode of the first embodiment, while
FIGS. 17A and 17B are conceptual views using an electrode
of the second embodiment. As shown in FIG. 16A and FIG.
17A, sometimes the electrode according to the present
invention is applied for both of the facing electrodes,
while as shown in FIG. 168 and FIG. 17B, sometimes it is
applied for just one. As shown in-FIG. 16 and FIG. 17, by
using the spot welding electrode according to the present
invention for at least one of the two facing electrodes,
it is possible to obtain a stable nugget even with a
donut-shaped electrode.
Examples
[00491 Spot welding was performed by the conditions
shown in Table 1 and Table 2. For comparison, the steel
sheet slant angles were changed to 0°, 5", lo0, and 20' to
measure the CTS. The CTS was tested in accordance with
JIS Z 3137 (1999).
[OOSO] The "steel sheet slant angle", as shown in FIG.
7, means the angle 0 formed by the plane perpendicular to
the electrode axis (center axes of facing electrodes) and
a steel sheet when using an electrode of the present
invention.
[00511 The contact-and-conduction part was based on a
donut shape (circular ring shape shown in FIGS. 8A-1 to
8B and FIGS. 9A-1 to 9B) made a split circular ring
(shape of split circular ring shown in FIGS. 12A-1 to 12B
and FIGS. 13A-1 to 138) or hexagonal ring (hexagonal
shape shown in FIGS. 10A-1 to 108 and FIGS. 11A-1 to
11B). No. 1 and No. 2 are comparative examples. In the
conventional spot welding electrodes, a point-shaped
electrode (disk shaped contact-and-conduction part) was
made No. 1, while a donut shape (circular ring shape)
electrode was made No. 2.
100521 As invention examples and comparative examples
of No. 3 to No. 9, No. 14 to No. 29, No. 34, and No. 35,
spot welding electrodes of the present invention shown in
FIGS. 8A-1 to 8B with contact-and-conduction surfaces of
donut shapes (circular ring shapes) and recessed tails of
electrode tips (first embodiment) were used.
roo531 Note that, in the invention examples of No. 34
and No. 35, the shape when viewing the contact-andconduction
surface 15 from right above the tip 1 is a
closed curve. At the space at the inside of the sealed
curve, there is an insulating member. In the case of the
invention example of No. 34, mica was attached, while in
the case of the invention example of No. 35, silicon
nitride was attached.
[00541 The steel sheets and welding conditions used
for the examples are shown in Table 1. Further, the
dimensions of the electrodes used in the examples are
shown in Table 2.
[00551 The results are shown in Table 3. In the
evaluation, the CTS [kN]=7.1 at the time of a steel sheet
slant angle 0' in the point-shaped electrode usually used
in No. 1 is made the base. The change with respect to
this was defined as the "base ratio [ % I " . Further, in
each test electrode, the ratio between the CTS value at
the time of a slant angle O0 and the CTS values at the
time of the slant angles So, lo0, and 20° xias made the
"CTS ratio (%) ". If the CTS-ratio is 60% or more, the
result was evaluated as passing ("good") while otherwise
it was evaluated as failing ("poor").
[0056] Case of Invention Examples
Nos. 4 to 8, 11, 12, 15 to 17, 20 to 24, and 26 to 35 are
invention examples. According to Table 3, it was
confirmed that if applying the spot electrode of the
present invention (recessed shape of tail of electrode
tip), even if the steel sheet slant angle is 20' or so,
deterioration of the CTS can be prevented.
[0057] Case of Comparative Examples
Nos. 1, 2, 3, 9, 10, 13, 14, 18, 19, and 25 are
comparative examples.
[0058] According to Table 3, Nos. 14 and 18 are
examples where the sliding parts fused together thereby
making welding impossible, No. 19 is an example where the
contact parts melted thereby making welding impossible,
and No. 25 is an example where a nugget could not be
formed. Further, it was confirmed that even if not able
to be welded, if the slant angle becomes 20" or so, the
CTS greatly deteriorates from the base.
[00591 Furthermore, these results are graphed in FIG.
20. While explained above, as clear from FIG. 20, when
there is no steel sheet slant (8=0), by making the
electrode donut-shaped, the CTS value becomes about 1.8
times that of a conventional point electrode, so the
effect of the donut shape is verified. However, along
with an increase of the steel sheet slant, the donutshaped
electrode ends up one-sided in contact resulting
in point contact, so the CTS value rapidly falls and
becomes equal to that of a conventional point type
electrode with a slant of about 5".
[0060] As opposed to this, the electrode according to
the present invention is not affected much at all by the
steel sheet slant at 5' or so. As the steel sheet slant
increases, the offset increases during conduction, so
the CTS value falls. However, the drop is slight. Even
with a steel sheet slant angle of 20' (8=20°), it only
falls about 17%. A CTS value of about 5 times a point
electrode or conventional donut-shaped electrode was
obtained. Above, the present invention was explained, but
the present invention is not limited to the above
examples. If satisfying the requirements of the present
invention, the effects can be obtained.
100611 Table 1
Steel sheet strength
Sheet thickness
Pressing force
Current value
Conduction time
980 MPa class
1.2 mm
3 kN
8 kA
300 ms
COO621 Table 2
I
N
Cn
I
23
24
25
26
Inv.
ex.
Inv.
ex.
Comp.
ex.
Inv.
ex.
27 Inv. Projecting shape of 8 8.05 0.6 37.68 0 . 5
u h e a d of support
Projecting shape of
head of support
Projecting shape of
head of support
Projecting shape of8
head of support
Projecting shape of8
head of support
-
28
29
30
31
32
8
8
33 Inv. Projecting shape of8 8.05 0.6 Circular ring 12
ex. head of support (inside silicon
nitride)
Inv.
ex.
Inv.
ex.
Inv.
ex.
Inv.
ex.
Inv.
ex.
8.05
8.05
8.05
8.05
-
Projecting shape of
head of support
Projecting shape of
head of support
Projecting shape of
head of support
Projecting shape of
head of support
Projecting shape of
head of support
0.6
0.6
0.6
0.6
8
8
8
8
8
12
12
12
9
8.05
8.05
8.05
8.05
8.05
2
2.2
2.4
1
0.6
0.6
0.6
0.6
0.6
62.80
67.70
72.35
25.12
--
FIG. 10
Hexagonal ring
Circular ring
(inside mica)
0.5
0.5
0.5
0.5 --
17
20
12
12
12
1
1
1
1
1
50.24
59.66
32.81
28.58
34.54
0.5
0.5
0.5 1
0.5
0.5
I
P2
m
I

Industrial Applicability
[00641 The present invention can be utilized for a
spot welding electrode used in the past. For this reason,
the effect can be enjoyed by application to a
conventional apparatus without requiring massive capital
investment.
Reference Signs List
[0065] 1. electrode tip
2. electrode support
11. head of electrode tip
13. tail of electrode tip
14. contact-and-conduction part
15. contact-and-conduction surface
16. smallest surrounding circle
19. spring hook for holding electrode tip
22. tail of electrode support
23. head of electrode support -
29. spring hook for holding support
31. electrode axis
32. insulating member
39. holding spring
50. steel sheet (welded member)
100. conventional spot welding electrode
101. head of conventional spot welding electrode
102. tail of conventional spot welding electrode
104. contact-and-conduction part of conventional spot
welding electrode
105. cont'act-and-conduction surface of conventional spot
welding electrode
131. center axis of conventional spot welding electrode
140. conventional spot welding apparatus

CLAIMS
Claim 1.
A spot welding electrode having an electrode tip
having a donut shape contact-and-conduction part and an
electrode support supporting the electrode tip, in which
spot welding electrode, said electrode tip contacts said
electrode support to be able to move.
Claim 2.
The spot welding electrode according to claim 1,
wherein said electrode tip contacts said electrode
support to be able to swing.
Claim 3.
The spot welding electrode according to claim 2,
wherein a tail of said electrode tip and a head of said
electrode support forming contact parts of said electrode
tip and said electrode support are spherical crown
shapes.
Claim 4.
The spot welding electrode according to claim 3,
wherein radii of curvature of said tail of said electrode
tip and said head of said electrode support are 4 mm to
20 mm and a radius of curvature of said tail of said
electrode tip is -5% to +lo% of a radius of curvature of
said head of the electrode support.
Claim 5.
The spot welding electrode according to any one of
claims 1 to 4, wherein in said contact-and-conduction
part, an area of a contact-and-conduction surface
contacting and conductive with a welded member is 12 mmz
to 70 rnm2 and a diameter D of a smallest circle
surrounding said contact-and-conduction surface is 8 mm
to 20 mm.
Claim 6.
The spot welding electrode according to any one of
claims 1 to 5, wherein in said contact-and-conduction
part, a contact-and-conduction surface contacting and
conductive with a welded member is a closed curve of a
circular ring, elliptical ring, or n-sided polygon (1123)
with a 0.3 rnrn to 2.5 mmwidth.
Claim 7.
The spot welding electrode according to any one of
claims 1 to 6, wherein said donut shape contact-andconduction
part has an insulating member placed at its
center.
Claim 8.
The spot welding electrode according to any one of
claims 1 to 7, wherein the metal used for said electrode
tip and said electrode support is copper or a copper
alloy.
Claim 9.
The spot welding electrode according to any one of
claims 1 to 8, further having means for holding said
electrode tip and said electrode support together.
Claim 10.
The spot welding electrode according to any one of
claims 1 to 9, wherein said means for holding said
electrode tip and said electrode support together are
spring hooks.
Claim 11.
A spot welding apparatus for spot welding at least
two steel sheets, which spot welding apparatus has a spot
welding electrode according to any one of claims 1 to 10
as at least one of the spot welding electrodes.
Claim 12.
A'spot welding method for spot welding at least two
steel sheets, which spot welding method uses a spot
welding electrode according to any one of claims 1 to 11
as at least one of the spot welding- electrodes.