DESCRIPTION
RESISTANCE SPOT WELDING METHOD
Technical Field
[0001] The present invention relates to a resistance spot welding method.
Background Art
[0002] Automobile vehicle bodies are mainly assembled by joining pressed steel sheets
togetlier using spot welding. In spot welding used to assemble vehicle bodies, there are
demands both to secure a nugget diameter according to the sheet thickness, and to suppress
the o ecu rrence of sputter.
[0003] Sputter includes internal sputter (a phenomenon in which parent metal that has been
melted by welding is scattered from the superimposed faces of the steel sheets) and surface
sputter (a .phenomenon in which parent metal that has been melted by welding is scattered
from contact surfaces between the steel sheet and the electrodes). In both cases, the surface
quality of the vehicle body is impaired by the sputter scattering and adhering to the
automobile vehicle body. Sputter adhering to. moving parts of a welding robot is also a cause
of equipment malfunction. Needle shaped surface sputter remaining on the spot weld
surface causes damage to automobile wiring harnesses and the like, and therefore needs to be
ground olf with a grinder. There is accordingly a demand to avoid internal sputter and
surface sputter, as well as to secure a specific standard nugget diameter in order to secure the
required weld joint strength.
[0004] In vehicle body assembly, sometimes the nugget diameter is less than the standard
nugget diameter due to various disturbance factors such as electrode;wear, current forking to
an existing weld point, gaps between pressed components, and the like. It is accordingly
often necessary, when welding on a production line so as not to cause sputter to occur, to
make the appropriate current range 1.0 k A or greater, or 1:5 kA or greater in test sample level
•evaluation..;..
[00051 Recently, there is increasing use in automobile assembly of resistance spot welding
macliines that use an inverter direct current method in place of a single phase alternating
current method. Inverter direct current methods enable the transformer to be made small,
with the advantage that installation to a robot that only has a small carrying weight capacity is
possible, and so inverter direct current methods aye often used particularly on automated lines.
[0006] Inverter direct current methods have a high heat generation efficiency due to
imparling a continuous current, without switching the current ON and OIT as in
conventionally employed single phase alternating current methods. There arc, accordingly,
reports of forming nuggets of the standard nugget diameter or greater using a low current, and
. " 1 "'• • • ' • ••
of wider appropriate current ranges for than when using a single phase alternating current,
even in cases in wliich zinc plated thin soft steel sheet material is employed that does not
readily form nuggets.
[0007J In spot welding, as illustrated in Fig. 1, often resistance spot welding is employed in
automobiles using a single stage current pass in which electricity is only passed one time. Tn
Fig. 1,1 on the vertical axis is the weld current, and t on the horizontal axis is time (the same
applies to Fig. 2 to Fig. 7). However, the current value at which internal sputter is generated
by welding using a single stage current pass is low with high tensile steel sheets using an
inverter direct current method, and there is significant narrowing of the appropriate current
range.
[0008] Japanese Patent Application T.aid-Open (JP-A) No. 2010-188408 (sometimes
referred to below as "Document 1") describes a method to suppress the generation of sputter
in spot welding of high tensile steel sheets by, as illustrated in Fig. 2, employing a two stage
current pass method in wliich the main current pass is performed after the conformity has
been raised between contacting faces of steel sheets by using a preparatory current pass.
|0009] JP-A No. 2003-236674 (sometimes referred to below as "Document 2") describes a
nielhod to suppress the generation of sputter in spot welding of high tensile steel sheets by, as
illustrated in Fig. 3, employing a current pass method in which a preparatory current pass is
employed to raise the conformity between contacting faces of steel sheets, current passing is
then stopped, and then the main current pass is performed.
|0010] JP-A No. 2010-207909 (sometimes referred to below as "Document 3") describes
employing a current pass method in which, as illustrated in Fig. 4 and Fig. 5, a preparatory
current pass is employed to raise the conformity between contacting faces of steel sheets, the
current value is then lowered, and then the current value is raised again, and main current pass
is performed at a constant current, or a pulse main current pass is performed. The
suppression of sputtering from occurring thereby in spot welding of high tensile steel sheets is
described therein.
[001IJ '..IPrA No. 2006-181621 (sometimes referred to below as "Document 4") describes a
method to suppress sputtering from occurring in spot welding of high tensile steel sheets by
spot welding as the current value is increased, while repeatedly raising and lowering the
current, as illustrated in Fig. 6.
100.12j A document "ISO 18278-2 Resistance Welding and Weldability- Part 2 Alternative
procedure for the assessment of sheet steels for spot welding" (sometimes referred to below as
''Document 5") describes a spot welding method in which, as illustrated in Fig. 7, for steel
sheets of sheet thickness 1.5. mm or greater, six cycles (120 milliseconds) or more of current
; . . - : • • • : • . . • ' • : . 2 .
pass and two cycles (40 milliseconds) of rest are repeated three times or more.
SUMMARY OF INVENTION
Technical Problem
[0013] An object of the present invention is to provide a resistance spot welding method for
sheet assemblies of overlapped steel material including a high tensile steel sheet, the method
enabling a wide appropriate current range to be secured, even when using an inverter direct
current method.
Solution to Problem
[0014] As a specific example of such a method, the inventors have investigated employing
surface treated 1500 MPa grade hot stamp steel sheet in various sheet assemblies. As a
result, they have discovered that stable spot welding, in which internal sputtering and surface
sputtering is suppressed and having a wide appropriate current range, can be implemented by
combining a process in which a pulsating current is passed for a short period of time (plural
repetitions of current pass and current pass stop), followed by a continuous current pass
process.
[0.0J5] An aspect of the present invention provides a resistance spot welding method
including: a pulsation process of clamping a sheet assembly of two or more overlapped steel
sheets including at least one high tensile steel sheet using a pair of welding electrodes that arc
connected to a spot welding power source employing an inverter direct current method, and
performing plural repetitions of current passing and current pass stopping, while pressing the
steel sheets with the welding electrodes; and a continuous current pass process in which, after
the pulsation process, current is passed continuously for a longer period of time than a
maximum current pass time of the pulsation process, while pressing the steel sheets with the
•welding electrodes. /•.
Advantageous Ellecls of Invention
[0016] The resistance spot welding method of the present invention enables a wide
appropriate current range to be secured, even in cases in which an inverter direct current
source is employed to perform spot welding of sheet assemblies of overlapped steel sheet
including a high tensile steel sheet.
RRTF.F DESCRIPTION 01' DRAWINGS
|0017] Fig. 1 is an explanatory diagram schematically illustrating a relationship between.
time and weld current in a single stage current pass method in which current pass is
performed only once, :
Fig. 2 is an explanatory diagram schematically illustrating a relationship between
time and weld current in a current pass method of Document I.
• • • 3 :.'
Fig. 3 is an explanatory diagram schematically illustrating a relationship between
time and weld current in a current pass method of Document 2.
Fig. 4 is an explanatory diagram schematically illustrating a relationship between
time and weld current in a current pass method of Document 3.
Fig. 5 is an explanatory diagram schematically illustrating a relationship between
time and weld current in a current pass method of Document 3.
Fig. 6 is an explanatory diagram schematically illustrating a relationship between
time and weld current in a current pass method of Document 4.
Fig. 7 is an explanatory diagram schematically illustrating a relationship between
time and weld current in a current pass method of Document 5.
Fig. 8 is a schematic diagram illustrating a resistance spot welding apparatus
according to an exemplary embodiment of the present invention.
Fig. 9 is an explanatory diagram schematically illustrating a relationship between
time ami weld current in a current pass method of a resistance spot welding method according
to an exemplary embodiment of the present invention.
Fig. 10A is an explanatory diagram schematically illustrating a relationship between
lime and weld current of a variation of a pulsation process current pass method in a resistance
spot welding method according to an exemplary embodiment of the present invention.
Fig. 1013 is an explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of a pulsation process current pass method in a resistance
spot welding method according to an exemplary embodiment of the present invention.
Fig. IOC!) is an explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of a pulsation process current pass method in a resistance
spot welding method according to an exemplary embodiment of the present invention.
Fig. 10D is an explanatory diagram schematically, illustrating a relationship between
time and weld current of a variation of a pulsation process current pass method in a resistance
spot welding method according to an exemplary embodiment of the present invention.
Fig. 10R is an explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of" a pulsalion'process current pass method in a resistance
spot welding method according to an exemplary embodiment of the present invention.
Fig. 10F is an explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of a pulsation process current pass method in a resistance
spot welding method according to an exemplary embodiment of the present invention.
Fig. 10G is an explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of a pulsation process current pass method in a resistance
A -•• •'.'.-•: ; - - : ' -:
spot welding method according to an exemplary embodiment of the present invention.
Fig. 10H is an explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of a pulsation process current pass method in a resistance
spot welding method according to an exemplary embodiment of the present invention.
Fig. llAis an explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of a continuous current pass method in a resistance spot
welding method according to an exemplary embodiment of the present invention.
Fig. 11B is an explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of a continuous current pass method in a resistance spot
welding method according to an exemplary embodiment of the present invention.
Fig. 11C is an explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of a continuous current pass method in a resistance spot
welding method according to an exemplary embodiment of the present invention.
Fig. 11D is an explanatory diagram schematically illustrating a relationship between
time and .weld current of a variation of a continuous current pass method in a resistance spot
welding method according to an exemplary embodiment of the present invention.
Fig. 11 Li is an explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of a continuous current pass method in a resistance spot
welding method according to an exemplary embodiment of the present invention.
Fig. 1 IF is an .explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of a continuous current pass method in a resistance spot
welding method according to an exemplary embodiment .of. the. present invention.
Fig. 11G is an explanatory diagram schematically illustrating a relationship between
time and weld current of a variation of a continuous current pass method in a resistance spot
welding method according to an exemplary embodiment of the present invention.
DESCRIPTION OF EMBODIMENTS
[0018] Explanation follows regarding a resistance spot welding method according to an
exemplary embodiment of the present invention. In Fig. 9, Fig. 10A to Fig. 10H. and Fig.
I1A to Fig. 11G, I on the vertical axis indicates the welding current, and t on the horizontal
axis indicates time. '•.:;':.
[00191 Recently, the usage is increasing of various high tensile steel sheets as automotive
materials, in order to achieve lighter weight vehicle bodies and to improve crash safety. The
application of hot stamping (a method in which, after heating steel sheet to a hardcnablc
temperature and converting into austenitc, pressing is performed in a mold at the same time as
cooling and quenching) is becoming widespread, and..most ultra-high strength pressed
• •• 5 . ' • .
components having a tensile strength of from 1200 to 2000 MPa are manufactured by hot
stamping.
[0020] Sometimes the surface of steel sheets used in hot stamping, rather than being
un-plated, is surface treated in order to prevent iron scale from being generated when heated
to a high temperature, such as by zinc-based-plating, aluminum-based-plating, or the like.
Hot stamped steel sheets are, in many cases, molded bodies that have been molded rather than
flat sheets, and in the present invention, "hot stamped steel sheets" includes molded bodies.
Moreover, sometimes in the following explanation hot stamped steel sheets obtained by hot
stamping zinc-based-plated steel sheets and aluminum-based-plated steel sheets are referred
to as "surface treated hot stamped steel sheets".
[0021] When hot stamped steel sheets are spot welded with a spot welding machine with an
inverter direct current source, in contrast to with soil steel sheets, sputter is generated at lower
current values than when a single phase alternating current source is employed, leading to the
phenomenon of narrowing appropriate current range. This phenomenon occurring, lor
example, in spot welding of aluminum plated hot stamped steel sheets, lias been reported in
•document "Schwcissen Sehneiden by LAURliNZ ct al, 64-10 (2012), 654-661) (sometimes
referred to below as "Document 6"), however there have not been any reports of fundamental
solutions thereto.
100221 In particular, with surface treated hot stamped steel sheets, when spot welding is
performed with a spot welding machine with an inverter direct current source, internal sputter
and surface sputter are liable to occur, and the appropriate current range is significantly
narrowed. The nugget diameter that can be obtained without generating sputter therefore
also becomes smaller.
[0023] Although the causes of the above are not clear, the occurrence of internal sputter is
thought be as follows. With surface treated hot stamped steel sheets, due to an alloying
reaction occurring between the zinc based-plating skin or aluminum based-plating skin, and
the steel base material, a solid solution of intcr-mctal compound and base iron is formed on
the surface thereof, and there is also an oxide skin present on the outer surface that has a
principle component of a metal derived from.the. plating (for example, zinc in the case. of.
zinc-based-plating). This accordingly makes the resistance at the contact portions between
steel sheets higher in surface treated hot stamped steel sheets than in un-trcatcd steel sheets,
leading to a large amount of heat generation.
[00241 However, as alloying of the plating and the steel progresses during the hot stamp
process, the melting point at the vicinity of the surface becomes a high value near, to that of
iron, and so contact portions between the steel sheets arc less easily softened than the steel
6
sheets prior to heating that have plating skins present, suppressing the current pass path from
spreading out. In particular, due to the heat generation efficiency being higher in an inverter
direct current method due to the current being continuously input than with a single phase
alternating current, the formation of nuggets in the initial stage of a current pass is extremely
rapid. It is hypothesized that, for this reason, the growth of pressure welding portions at the
periphery of the nuggeis does not keep pace, so that the molten metal can no longer be
contained, and leading to the generation of internal sputter.
[0025] Regarding the causes of surface sputter generation, these are thought to be similar to
the causes of the occurrence of internal sputter described above. Moreover, in an inverter
direct current method, due to the current being continuously input, there is no current stop
time like that in a single phase alternating current, and a cooling effect from the electrodes is
clilliculi to obtain. It is accordingly hypothesized that the nuggets readily grow in the sheet
thickness direction, and the molten portion reaches the outermost layer of the steel sheet,
leading to surface sputter occurring.
[0026] With surface (reared hot stamped steel sheets, it is thought that internal sputter and
surface sputter are readily generated due to .the surface state described above, and the
appropriate current range is often less than 1 kA in cases in which the applied pressure is low.
However, in reality there lias been hardly any investigation into resistance spot welding
methods of sheet assemblies including a surface treated hot stamped steel sheet.
[00271 When the methods of Documents 1 to 5 arc applied to surface treated hot stamped
steel sheets, the following deficiencies arise.
100281 In the method of Document 1, which is-a resistance spot welding method for high
tensile steel sheets, due.to the current value that can be applied to surface treated hot stamped
steel sheets without sputter occurring in a preparatory current pass being low, the current pass
.path.spreads out at the interface between the steel sheets and the current density falls, and so
there is not a sufficient sputter generation suppressing effect. Thus cases have been found of
internal sputter and surface sputter occurring when the current value is raised for the main
current pass, and it has been difficult to secure a sufficient appropriate current range.
JO029J In the methods of Documents 2 and 3, similarly to the method of Document 1, the
current value that can be applied to surface treated hot stamped steel sheets without sputter
occurring in a preparatory current pass is low. The upper limit of the preparatory current
pass value is higher than that .of Document I, however cases have been (bund of internal
sputter occurring when the current value is raised for the main current pass, and it has been
tli Hicult to secure a sufficient appropriate current range.
10030] In the method described in Document 4, there is an ejflecl of widening the appropriate
current range for steel material up to 980 MPa grade tensile strength, however internal sputter
and surface sputter readily occur when the current is raised for the second and third time for
surface treated hot stamped steel sheets that have a higher strength, and this current pass
pattern is not suitable for welding surface treated hot stamped steel sheets.
[0031] In the current pass method described in Document 5, a current pass is six cycles (120
miliseconds) even in the shortest case. For surface treated hot stamped steel sheets, the
upper limit current cannot be raised by using this current pass method since internal sputter is
generated in a current pass time shorter than six cycles. The upper limit current value is
raised when the current pass time is made shorter in each pulse, however the lower limit
current value is also raised due to the drop in heat generation efficiency, and as a result, the
appropriate current range cannot be widened. This method is therefore also not appropriate.
[0032] However, in contrast thereto, in the resistance spot welding method of the present
exemplary embodiment, a wide appropriate current range is secured for high, tensile steel
sheets, including a surface treated hot stamped steel sheet, even when an inverter direct
current source is employed.
[0033] Explanation first follows regarding a welding machine employed in the resistance
spot welding method of the present exemplary embodiment.
|0034| As illustrated in Fig.'8, a.welding machine 10 includes electrodes 16, 18 that press
and pass a welding current through overlapped steel sheets 12, 14, a pressing mechanism 20
that applies a. specific .weld force to the electrodes 16, 18, a press controller 22 that controls
the weld force of the pressing mechanism 20, a welding power source 24 that applies current
to the electrodes 16,18, and a current controller 26 that controls the welding power source 24
and controls the current value applied to the electrodes 16,18.
[0035J A sheet assembly that is the target Tor the resistance spot welding method of the
present exemplary embodiment is two or more overlapped steel sheets, including at least one
sheet of high tensile steel sheets of 590 MPa grade or higher. Fig. 8 illustrates an overlapped
sheet assembly of.the two steel sheets 12, 14, however three or more sheets may be employed.
Ordinarily, in automobile vehicle body assembly resistance spot welding is performed ("or
sheet assemblies of two or three overlapped steel sheets. :
[0036] There is no' particular limitation to the .type of high tensile steel sheet, and, for
example, precipitation hardened steels, DP steels, transformation induced plasticity (TRIP)
steels, hot stamped steel sheets, and the like, that have a tensile strength of 590 MPa or greater,
are applicable as the high tensile steel sheet. The resistance spot welding method of the
present exemplary 'embodiment is applicable to a sheet assembly including a high tensile steel
sheet having a tensile strength of 980 MPa or higher. In particular, application is preferably
" ' 8 '.:;'.:.:•.;'
made to a sheet assembly including a high tensile steel sheet having a tensile strength of 1200
MPa or higher, and application is more preferably made to a sheet assembly including a high
tensile steel sheet having a tensile strength of 1500 MPa or higher.
[0037] The steel sheets included in the sheet assembly may be cold-rolled steel sheets, or
may be hot-rolled steel sheets. The steel sheets may be un-treated steel sheet or plated steel
sheet, and there is no particular limitation to the type of plating. The resistance spot welding
method of the present exemplary embodiment is applicable to various high tensile steel sheets,
however, it is particularly applicable to surface treated hot stamped steel sheets.
[0038] There are no particular limitations to the sheet thickness of the high tensile steel
sheets. For example, the sheet thickness of steel sheets employed in automotive components
or vehicle bodies is from 0.6 to 3.2 mm, and the resistance spot welding method of the present
exemplary embodiment is adequate for application across this range.
[0039j i'he welding machine 10 is a spot welding machine including the welding power
source 24 using an inverter direct current method. In cases in which welding is performed
on sheet assemblies including high tensile steel sheets such as hot stamped steel sheets,
internal /sputter and surface .sputter, is liable be generated at a lower current value in the
Avoiding power source 24 using an inverter direct current method than with a welding power
source using a single phase alternating current method. The resistance spot welding method
of the present exemplary embodiment is applied to the welding machine 10 that employs the
spot welding power source using such an inverter direct current method.
[0040] The pressing mechanism 20 for the electrodes 16, 18 of the welding machine 10 may
be one that presses using a servomotor, or may be one.that, presses using air. The shape of
the gun employed may also be a static-type, a C-typc, or an X-typc. There are no particular
limitations to the weld force applied during welding, however, the weld force is preferably
controlled by the press controller 22 to from 200 to 600 kgf. During spot welding, a
constant weld force may be applied, or the weld force may be changed for each of the
processes, described later.
[00411 There are no particular limitations to the elcelrodes 16, 18 and. for example, dome
radius (OR) type cleetiodcs with an outer-circumferential radius of curvature from 6 to 8 ..mm
may be employed. A most typical example is -DR-type electrodes having an outer
-circumferential radius of 6. mm, and a a lip radius of curvature of 40 mm. The electrode
substance may be chromium copper,. or. alumina dispersion strengthened copper, and is
preferably alumina dispersion strengthened copper from the perspective of preventing
welding to the workpiccc and surface sputter.
[0042] Next, explanation follows regarding a resistance spot welding method performed
:.. '9
using the welding machine 10.
[0043] Under control by the press controller 22, the electrodes 16, 18 clamp the sheet
assembly of the overlapped steel sheet 12 and steel sheet 14 with a specific weld force, and a
weld current pass from the welding power source 24 is passed tliiough the steel sheets 12, 14
via the electrodes 16,18 using a current pass method controlled by the current controller 26.
[0044] In the current pass method, as illustrated in Fig. 9, first, a pulse wave cunent pass of
current value 10 and a current pass time of tO is performed three times (see pulses PI to P3 in
Fig. 9). When this is being performed, a stop time tl in which current is not passed is
constant between each pulse. After elapse of a stop time t2, in which current is not passed
after the final pulse P3 in the pulsation process (sometimes referred to below as "final stop
time"), a continuous current pass process is then performed, as described later.
[0045] The pulsation process is from the rise of the first pulse PI, to the end of the final stop
timet2. ..'
[0046] "Pulse" in the present exemplary embodiment includes sloped and saw-tooth shaped
pulses illustrated in the examples of variations described later (see Fig. 10D and Fig. 10E).
[0047] When the final stop time 12 has elapsed after the end of current passing in the final
pulse P3 of the pulsation process, a continuous current is passed from the electrodes 16, 18
through the steel sheets 12, 14 at a current value Tl that is lower than the current .value 10 of
the pulses PI to P3, and over a current pass time t3 longer than the (maximum) current pass
time 10 of each of the pulses P1 to P3, and a specific nugget 28 is formed at the interlace
between the steel sheets 12, 14.
|0048| In the pulsation process of the present exemplary embodiment, the pulses PI to P3
correspond to "current passing in the pulsatiou process" of the present invention. The range
of the stop times tl and the range of the final .slop.time. 12 in the pulsation process of the
present exemplary embodiment each correspond to "current pass stopping of the pulsation'
process" of the present invention. Moreover, the range of the stop time tl between the pulse
PI and P2, and P2 and P3, in the present exemplary embodiment correspond to the "current
pass stop" between adjacent current passes in the present invention, and the range of the final
•'slop lime 12 of the present exemplary embodiment corresponds to the "final current pass stop"
of the present invention.
[0049] Moreover, the continuous current pass process is performed after the pulsation
process of the present invention, as in the present exemplary embodiment, however, the final
current pass slop of the pulsation process is always positioned prior to the continuous current
pass process. :
100501 Performing resistance spot welding with such a current pass method obtains the
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following advantageous effects.
[0051] In the pulsation process, the current pass time, the stop time, and the number of
pulses is adjusted according to the type of material, sheet thickness, and sheet assembly. The
resistance spot welding method of the present exemplary embodiment enables the conformity
between the contact faces of the steel sheets to be raised in a short period of time by first
providing the pulsation process.
[0052] In particular, in cases in which surface treated hot stamped steel sheets covered with
a high electrical resistance skin, such as of zinc oxide, the high melting point oxide layer can
be effectively removed from the outside of the weld portion due to being able to induce
vibration in the contact faces by thermal expansion and contraction due to repetitions of
current pass and current pass stop. Moreover, an electrode cooling effect can be made to
work sufficiently due to repetitions of current pass and slop in.the pulsating current pass,
enabling a rapid rise in the temperature of lhc„nugget 28 to be suppressed, and thereby
enabling the advantageous effect pi' raising the conformity between the contact laces of the
steel sheets. 12, 14 in a short period of time, while suppressing the generation of internal
sputter and surface sputter.
j00531 The current pass time K) of each of the pulses PI to P3 in the pulsation process is
preferably 10 milliseconds to 60 milliseconds. When the current pass lime tO is less than 10
milliseconds, the heating lime is short, and there is insufficient heat generation .at.the contact
faces pl'.lhe steel sheets 12, 14. When the current pass lime 10 exceeds 60 milliseconds, the
heating time is too long, and there is a concern of raising the probability of surface sputter and
internal sputter being generated. ..• The current pass timetO is more preferably 15 milliseconds
or longer. The current pass time 10 is more preferably 45 milliseconds or shorter, with 25
milliseconds or shorter being even more preferable.
[0054] The current value 10 of the weld current in the pulsation process is preferably from
7.0 to 14.0 kA. Normally when the current pass time is increased in pulsation, this results in
sputter being generated at low current values, and so preferably the current value is
appropriately adjusted in the pulsating current pass so as to be in a range of 7.0 to 14.0 kA in
consideration of the balance with the current pass time.
[0055] The slop times tl in the pulsation process, excluding the final stop time 12, are
preferably each from 10 milliseconds to 60 milliseconds each time. When the stop times tl
arc less than 10 milliseconds, Ihe slops arc short, and cooling of the steel sheets 12, 14 is
'insufficient, leading to a concern regarding internal sputter and surface sputter generation.
However, when the stop times 11 exceed 60 milliseconds, there is too great a cooling effect
from the electrodes 16, 18, leading to a concern regarding a drop in the formation amount of
the nugget 28 in the subsequent continuous current pass process. The stop times tl are
preferably 15 milliseconds or longer. The stop times tl are more preferably 45 milliseconds
or shorter, and are even more preferably 25 milliseconds or shorter,
[0056] The final stop time t2 in the pulsation process is preferably from 10 milliseconds to
120 milliseconds. When the final stop time t2 is less than 10 milliseconds, there is
insufficient cooling of the nugget 28, such that sputter is generated at a low current value
during the continuous current pass process. However, when the final stop time t2 exceeds
120 milliseconds, the nugget 28 is cooled too much, raising the current value in the
continuous current pass to obtain the standard nugget diameter giving a specific joint strength,
and resulting in a narrow appropriate current range. The final stop time t2 is more
preferably 15 milliseconds or longer. The final stop time t2 is more preferably 100
milliseconds or shorter, and is even more preferably 60 milliseconds or shorter.
• 10057] By providing the pulsation process as a preparatory current pass in this manner,
current pass can be repeated, while interspersing the cooling effect of the electrodes 16, 18.
Accordingly, while suppressing generation of sputter due to rapid growth of the nugget 28. at
the contact faces of the steel sheets 12,14, the conformity of the high tensile steel sheet lo the
other steel sheet can be promoted, enabling an increase in the current paih at the interfaces
between the steel sheets to be achieved.
[00581 Thus by inserting the continuous current pass process aller the pulsation process, a
drop in current density due to an increase in current path between the steel sheets 12, 14 is
suppressed, and generation of sputter is suppressed by suppressing the rise in temperature at
the contact portions between the steel sheets, even in cases in which the continuous current
pass is for a longer period of time than the {maximum) current pass lime 11 of the respective
pulses Tl to P3 in the pulsation process. Namely, the current value at which sputter is
generated is raised. J0059] Moreover, after growth of the nugget 28 has started in the pulsation process, by
passing current in the continuous current pass process for the current pass time 13 that is
longer than the current pass time tl of each of the pulses in the pulsation process, the nugget
28 grows to the specific standard nugget diameter at. a.lower current value than when a single
current pass is employed (sec lug. 1). :
J0060] As a result, in the resistance spot welding method of the present exemplary
embodiment, the appropriate current range is increased in the continuous current pass process.
[0061] Reference here lo the appropriate current range indicates a range defined by a lower
limit of a current value that produces a standard nugget diameter of Aim {wherein t is the
sheet thickness (mm), also referred to below as "4Vt") capable of obtaining a specific weld
• ' • ' 1 2 • . ' • . • • •
strength, and with an upper limit of the maximum current value at which sputter (expulsions)
are not generated. The sheet thickness t is the thickness (mm) of one steel sheet out of the
two steel sheets in which the nugget is formed. In cases in which the thicknesses of the two
steel sheets are different from each other, the sheet thickness t is that of the thinner steel sheet.
Moreover, in cases in which there are three or more overlapped steel sheets, the sheet
tliickness t is the thinner of the steel sheets from out of two steel sheets in which the nugget
diameter is measured.
[0062] Thus employing the resistance spot welding method according to the present
exemplary embodiment enables stable resistance spot welding to be performed, even for steel
sheets including a surface treated hot stamped steel sheet that is liable to sputter generation.
[0063] The resistance spot welding method according to the present exemplary embodiment
enables the quality of the external appearance of a product to be raised by suppressing sputter
generation. Moreover, the operational availability of robots can be raised due to being able
to prevent sputter from adhering to movable parts of a welding robot. An improvement in
productivity can also be achieved due to being able to omit post-processing that accompanies
sputter generation, such as burr removal.
[006*1] The number of pulses (current passes) in the pulsation process is at least two or more.
This is because an advantageous effect of suppressing sputter generation is not obtainable
unless there are two or more pulses when employing a surface treated hot stamped steel sheet.
The number of pulses is more preferably three or more. Generally, the number of pulses
should be increased as the total sheet thickness of the sheet assembly increases. However,
due to a tendency for saturation of the advantageous effect.when there arc more than nine
pulses, thenumber of pulses is preferably nine limes or less.
[0065] In cases in which application is made to a surface treated hot stamped steel sheet that
is liable to sputter generation, in the pulsation process, for example, preferably current passes
at from. 7.5 kA to 12 kA for 16,6 milliseconds (one cycle at 60 Hz) to 20 milliseconds (one
cycle at 50 Hz) and stops are performed repeatedly from three to seven times.
[0066] The resistance spot welding method of the present exemplary embodiment includes
the continuous current pass process after the pulsation process. If only the pulsation process
is employed, even if the current path can be increased, there is only a small effect on
increasing the nugget diameter; however, by providing the continuous current pass process
after the pulsation process, heat generation is promoted id the interlace between the steel
sheets 12, 14, enabling the nugget 28 to be formed at sufficient size without causing internal
sputter and surface sputter to be generated.
|0067] In the continuous current pass process, the current pass time t3 is preferably a
• • ' ' 1 3 •:•-'
continuous current pass performed for from 100 milliseconds to 500 milliseconds. When the
current pass time t3 in the continuous current pass process is less than 100 milliseconds, the
time is insufficient to enlarge the nugget 28 and an effect is not obtained. When the current
pass time t3 exceeds 500 milliseconds, there is saturation in the effect to enlarge the nugget 28,
leading to an increase in the takt time. The current pass time t3 in the continuous current
pass process is more preferably 120 milliseconds or longer, and is more preferably 400
milliseconds or shorter.
[0068] The current value 11 in the continuous current pass process is preferably from 5.0 kA
to 12.0 kA. The current value II in the continuous current pass process is preferably the
maximum current value 10 in the pulsation process or lower. Making the current value II in
the continuous current pass process lower than the maximum current value TO in the pulsation
process is done in order to suppress (he generation of sputter. In a continuous weld process,
the currenl value does not necessarily need to be constant, and a change may be made to the
.current value during the continuous weld process, and an upslopc or a down slope of from 16
milliseconds to 60 milliseconds may he included.
[00691 When using a high tensile steel sheet, such as a hot stamped steel sheet, a further
single current pass or pulsating cunent pass.may be performed after the continuous current
pass process, in order to control the cooling process and l.o raise the toughness of the nugget
28 (see Fig. 11F, Fig. .1.1 G). Performing another current pass after the continuous currenl
pass process has the advantages of raising the tougliness of the nugget 28 by alleviating
solidification segregation of phosphorous in the nugget 28, and by transforming the nugget 28
into a tempered marlensile structure, enabling the spot weld joint strength to.be raised.
[0070 j In the resistance spot welding method according to the present exemplary
embodiment, a further holding process of pressing by the electrodes 16, 18 without current
flowing may be provided after the pulsation process and the continuous current pass process
described above have finished. Provision of the holding process enables solidification
fracture within the nugget 28 to be suppressed. There are no particular limitations to the
holding time when the holding process is provided; however, since the takt time increases
vvhen the holding time is too long, the holding time.is preferably 300 milliseconds or shorter.
(0071 j The current value 10, the current pass time tO, and (he stop time 11 for each of the
pulses PI to P3 in.the pulsation process may be constant, or they may vary for each pulse.
[0072j Namely, when resistance spot welding is performed on steel sheets in a two-sheet
assembly, there is no limitation to a configuration in which, as in the present exemplary
embodiment, the current pass time for each of (he pulses in the pulsation process and the
current pass slop time between each of the pulses is constant, and the current value of each of
• • ' • • • ' T ' l •
the pulses is constant.
[0073] For example, as illustrated in Fig. 10A, control may be performed such that the
current value rises after the first pulse PI on progression through the pulses P2, P3.
Moreover, as illustrated in Fig. 10D, the rising portion of the first pulse PI may be sloped.
Moreover, as illustrated in Fig. 10E, the rising sides of each of the pulses PI to P3 may be
sloped so as to give a saw-tooth shape. Moreover, as illustrated in Fig. 10G, a first stop time
t i l between the first pulse PI and the second pulse P2 may be configured alone so as to be
longer than another stop time t l 2 , increasing the electrode cooling effect on the first pulse PI
compared to that on the other pulse P2.
[0074] Employing such a current pass method in the pulsation process suppresses a rapid
growth of the nugget 28 in the pulsation process (suppresses a rapid temperature rise in this
portion), enabling the generation ol* internal sputter and surface sputter to be suppressed.
100751 Moreover, for example, as illustrated in Fig. 10H, a configuration may be considered
in which the final stop lime t2 (the current pass stop time between the final pulse P3 and the
continuous current pass process) is shorter than in (he other examples (see Fig. 10A to Fig.
10G). By making the final stop time t2 shorter in this manner, excessive cooling of the steel
sheets 12,14 by the electrodes 16, 18 is suppressed, enabling the electrical energy imparted to
the steel sheets (for example, the current pass time and the current value) during the
continuous current pass to be suppressed.
[00761 Moreover, in cases in which resistance spot welding is performed to a three-sheet
assembly of steel sheets hi which a thin sheet is overlapped on the outside of two thick sheets,
the following variation of the current pass method.may be considered in the pulsation process.
10077] For example, as illustrated in Fig. 10B, control may be performed so as to lower the
current value from the first pulse PI on progression toward the final pulse P3. Moreover, as
illustrated in Fig. 10C,'the fii'st pulse PI alone may have a current pass with a higher current
value that those of the other pulses P2, P3. Moreover, as illustrated in Fig, 101% the first
pidse PI alone.may have a current pass time longer than current pass time of the other pulses
P2, P3. •:. ;••:•;••
[0078] In this maimer, ihe electrical energy imparted by (he first pulse PI to the .three-sheet
assembly of steel sheets is a higher electrical energy that that imparled in the other pulses P2,
P3. Due to imparting high electrical energy at the point in tunc .when, .there is a high contact
resistance between the thin sheet and the thick sheet, this thereby enables a high temperature
to be achieved in the thin sheet and the thick sheet, and a nugget to be grown therebetween.
.[0079 j Similarly, explanation follows regarding variations in the current pass method of the
continuous current pass process of the resistance spot welding method of Ihe present
• - ' 1 5 •
exemplary embodiment, with reference to Fig. 11A to Fig. 11G.
[0080] For example, as illustrated in Fig. 11 A, by sloping the rise of the current waveform in
the continuous current pass process, or, as illustrated in Fig. HE, by setting a current value for
the first half of the continuous current pass process lower than the current value of the latter
half, a rapid rise in the temperature of the nugget 28 can be suppressed at the time when the
continuous current pass is started, enabling the generation of internal sputter and surface
sputter to be suppressed.
[0081] Moreover, there are configurations in which, as illustrated in Fig. 11B, the fall of the
current waveform of the continuous current pass process is sloped, or in which, as illustrated
in Fig. 11C, the current value of the latter half of the continuous current pass process is set
lower than the current value of the first half. Adopting such an approach enables the
strength of the weld joint to be raised, by gradually cooling after welding so as to change the
characteristics of the melal structure of the weld portion.
•|0082] Moreover, for example, as illustrated in Fig. 11F and Fig. IK), there are also
configurations in which, after the continuous current pass process, a single current pass is
performed, or pulsating current pass is performed. This thereby improves the metal structure
: of the weld portion, and raises the strength of the weld joint.
[0083] Moreover,"as'illustrated in Fig. 11D, nugget growth is promoted between a thin sheet
and a thick sheet by passing current through a sheet assembly of three overlapped sheets,
these being a thin sheet, a thick sheet, and a thick sheet, with a high current value initially in
the continuous current pass process.
Examples . . .
[0084] '•Explanation follows regarding examples; '-however, the present invention is not
limited to these examples. :::
[0085] Examples I
[0086] The welding machine employee! in the present examples is an inverter direct current
spot welding machine (hat uses a servo press method, and includes DR-type electrodes
(alumina dispersion strengthened copper) having an outer-circumferential radius of curvature
of 6 mm, and a tip radius of curvature of 40 mm. The material to be welded is two
overlapped sheets of-aluminum-plated 1500 MPa'grade, hot stamped sleel sheets (the plating
amount prior to hot stamping was 40 g/m2 pel" side, and the heating conditions are heating
inside a gas furnace at 900°C for 4 minutes) with a sheet thickness of 1.2 mm and a size of 30
mm* 100 mm.
[0087 j The welding methods arc listed in Table 1. Although Test Nos. 6, 7 include a
preparatory-current pass prior to the continuous; current pass process, they are tesls in which a
16
two stage current pass is performed without a stop time between the preparatory current pass
and the continuous current pass processes. Test No. 8 is configured with a current pass stop
time (34 milliseconds) between the preparatory current pass and the continuous current pass
process. In both the examples of the present invention and the comparative examples, the
weld force was a constant value (300 kgf) in the pulsation process or the preparatory current
pass, and in the continuous current pass process.
[0088] In each of the tests, spot welding was performed by varying the current value of the
continuous current pass process while keeping the conditions of the pulsation process or the
preparatory current pass constant, the minimum current value (4Vt current) is found at which
the nugget diameter, this being the determinant of weld joint strength, reaches 4Vt (wherein t
is the sheet thickness in mm) = 4.3 mm or greater, and the maximum current value (maximum
•sputter-less current) is found at which sputter (internal sputter and surface sputter) is not
generated. The range from the 4Vt current value to the maximum sputter-less current value,
namely the range of current values for the continuous current pass process such that sputter is
not generated, while still forming a nugget that imparts a specific strength to the weld joint, is
the appropriate current range. 'The lest results arc illustrated in Table 1.
[0089]; Regarding the nugget diameter, after spot welding, destructive testing was performed
using a chisel, and the fracture was measure with Vernier calipers. The presence or absence
of the occurrence of sputter was confirmed by eye during spot welding.
.[0090] .Table 1 V
lest
No.
1
2
3
'1
5
6
Pulsation Process
Welti
current
(kA)
8.5
JJ"~~
7.5
7.5 ,;•.
Pielimin
Current
pass
time
(msec)
17
17
34
17 /
- :. . •'
ary curren
Current
stop
lime -
other
tllHIl
final
(msec)
17
17
34
17
" • ' • • • •
Final
current
stop
lime
(msec)
17
60
34
51
-'. .'.
Number
of
pulses
(limes)
3
3 . , ' . . ' ••
3
9 ;
- • •
tot"5.0kA for 83 msec
Continuous
current
pass
process
Current
pass time
(msec)
133
133
133
133
250
133
Test Results (current values of
continuous current pass process)
• . ' : ' . « . . . . ' . • .
4Wt
current
(kA)
5.5
5.5
5.8
4.6
5.0
5.8
Maximum
sputter-less
current
(kA)
8.0
8.2
8.5
7.8
5 . 5 ;•••.
6.2
Appropriale
current
range
(kA)
2.5
2.7
2.7
32~ •"~
0.5
Comment
Examples of
present
invention
Comparative
lixamples
17
7
8
Preliminary current of 5.0kA for 83msec
Preliminary current of 5.0kA for 83msec +
34msec stop
250
250
5.2
5.5
6.0
6.5
0.8
1.0
[0091] As illustrated in Table 1, the appropriate current ranges of the Test Nos. 1 to 4 of the
examples of the present invention that include the pulsation process are three times wider or
more than those of the Test Nos. 5 to 7 of the comparative examples that either do not have a
preparatory current pass, or do have a preparatory current pass but do not have a stop time
provided between the preparatory current pass and the continuous current pass. Moreover, it
was confirmed that the appropriate current ranges of the Test Nos. 1 to 4 of the examples of
the present invention that include the pulsation process (plural repetitions of current passing
and slopping) arc two times wider or more Ihan those of the comparative example Test No. 8
that has a stop tune provided between the preparatory current pass and the continuous current
pass.
L0092] Examples 2
[0093] The welding machine employed in .the present examples is the same as that of
Examples 1. •'•'.The material to be welded was three overlapped sheets, these being a
GA-pIated 270 MPa grade steel sheet with a sheet thickness of 0.7 mm aud a size of 30 mm x.
100 mm, a GA-plated 1500 MPa grade hot stamped steel sheet with a sheet thickness of 1,2
mm (the plating amount prior to hot stamping was 55 g/m per side, and the heating
conditions were the same as those of Examples I), and an unplated 440 MPa grade steel sheet
with, a sheet thickness of 1.4 nun. The welding methods are listed in Tabic 2. The current
pass methods of the comparative examples are similar to those of Examples 1. In both the
examples of the present invention and the comparative examples, .the..weld..force'was. a
constant value (300 kgI).in the pulsation process or the preparatory current pass, and in the
continuous current pass process.
[0094] Testing and evaluation of the lest results were performed similarly to in Ihe Examples
• i . : ' • ; , • • . ' • •
[0095] The 4;vt current value was determined in the following manner, due to the thickness
being different in Die three sheets. Namely, the minimum current value at which the nugget
diameters at the respective interfaces between the steel sheets respectively satisfied 4^t
(wherein I is the sheet thickness in mm of the thin plate side at an overlapped face) is the 4vl
current value. Specifically, for Ihe nugget al the interface between Ihe steel sheet of sheet
thickness 0.7 mm and (he steel sheet of sheet thickness 1.2 mm, the nugget diameter is 4Vt
when it is 4 x (0.7)'^ — 3.4 mm, For the nugget at the interface between the steel sheet of
" 18 :
sheet thickness 1.2 mm and the steel sheet of sheet thickness 1.4 mm, the nugget diameter is
4Vt when it is 4 x (0.7)l/2 (= 4,4 mm) or greater. Thus the 4Vt current value is the minimum
current value at which the nugget diameters at both the interfaces are respectively 4Vt or
greater.
[0096] The test results are illustrated in Table 2,
[0097] Table 2
Test
No.
1
2
3
4
5
6
Pulsation Process
Weld
current
(kA)
12
12
12
- '••
Current
pass
time
(msec)
17
17 r
17
Current
stop
l i m e •-.
other
than
final
(msec)
17
17;
17
- ':
Final
current
stop
lime
(msec)
*7
34 ..:•;
17
- ;'
Number
of
pulses
(limes)
3
3 • . . • • • . ' . - . •.
3 . . • • • . . ' • . •
.Preliminary current of 5.0kA for 83msee
Preliminary, current, of 5.0kA for 83msec l
34msec stop
Continuous
current
pass
process
Current
pass time
(msec)
282
282 ; •
•5Q0,.;:
282
282
282
Test Results (current values of
continuous current pass process)
4Vt
current
(kA)
5,8
6.0
5.6
5.8
5.8
6.0
Maximum
sputter-less
current
(kA)
8.6
8.8
8.8
6.6
6.6
7,0
Appropriate
current'
range
( k A )
2.8,-.;---.:
::-:
2.8
3.2. .
0.8
0.8
1.0
Comment
Examples of
present
invention
Comparative
Examples
' [0098] As illustrated ill Table 2, with the .material lo be welded of tlircc overlapped sheets
including hot stamped material too, it was confinncd that, similarly lo in the Examples'.1,-the
appropriate current ranges of the Test Nos. 1 to 3 of the present invention that include the
pulsation process are almost three times wider (at 2.0 kA or more) than those of the Test Nos.
4 to 6 of comparative examples Ihat cither do not have a preparatory current pass, do have a
preparatory current pass but do not have a stop time provided between the preparatory current
pass and the continuous ..current pass, or do have a stop time provided between the preparatory
current pass and the continuous current pass.
[0099] Examples 3. •'•
{0100] The welding machine employed in the present examples is an inverter direct current
spot welding machine that uses an air press method, and includes DR-type electrodes
(alumina dispersion strengthened copper) having an ouler-circumfeicntial radius of curvature
: • • ' . ' • • • • • 1 9 •:
of 6 mm, and a tip radius of curvature of 40 mm. The material to be welded was two
overlapped sheets of fiirnace heated ZnO skin-treated Al-plated 1500 MPa grade hot stamped
steel sheets having a sheet thickness of 1.6 mm and a size of 30 mm x 100 mm. The
welding methods are listed in Table 3. The current pass methods in the comparative
examples are similar to those in Examples 1. The weld force in the examples of the present
invention and the comparative examples was a constant value (350 kgf) in the pulsation
process or the preparatory current pass, and in the continuous current pass process.
[0101] Testing and evaluation of the test results were performed similarly to in the Examples
1.
[0102] Test results are illustrated in Table 3.
[0103] Table 3
Test
No.
1
2
3
.4 .
5
6
7
8
Pulsation Process
Weld
current
(kA)
7.5
8.5
9.5
8.5
-
-
Current
pass
time
(msec)
17
17
17
17
-
-
Current
stop
time -
other
than
final
(msec)
17
17
17
17
-
-
Final
current
stop
time
(msec)
50
50
50
50
-
-
Number
of
pulses
(times)
7
7
7
9
-
-
Preliminary current of 5.0kA for 83msec
Preliminary current of 5.0kA for 83msec +
34msec stop
Continuous
current
pass
process
Current
pass time
(msec)
333
333
333
250
333
600
333
333
Test Results (current values of
continuous current pass process)