Note: Descriptions are shown in the official language in which they were submitted.
13230 ~7
The present invention relates to an improvement
in the resistance welding of galvanized, i.e. zinc or
zinc alloy coated, steel parts or sheets which is
achieved by coating to the welding electrode with a
resin binder containing a metal phosphide pigment, and
preferably a ferrophosphorus pigment.
This Application is a division of Canadian Patent
Application Serial No. 566,716, filed May 13, 1988.
The welding improvements realized by practicing
the present invention are improved weldability lobes
and dynamic resistance curves for better welding
control for resistance welding systems as well as
increased electrode life.
The use of galvanized steel sheets in the
automotive industry has become increasingly popular in
recent years due to the increase in concern for
corrosion protection for automobile body panels.
Corrosion problems are particularly severe in
environments where salt is used for preventing the
icing of snow on highway roads. Although efforts have
been made to enhance the corros:ion-resistance of steel
sheets, such as by using various chemical conversion
treatments and paint coatings, the corrosion
protection method of choice currently is galvanized
steel, with the galvanized coating formed by either
hot-dipping or electrodeposition.
1323~7
For zinc or zinc alloy coated sheet steels to
successfully substitute for uncoated sheet steels,
they must exhibit acceptable ~ormability and
weldability characteristics. As a qeneral rule,
coated steels have not demonstcat:ed properties as good
as their uncoated counterparts. Users of these
products are continually looking for new coated sheet
steels which provide the advantages of a coated steel,
but have weldability and formability characteristics
similar to uncoated steels.
132~Q~7
The most common method of ~oining steel sheets (particularly in
the ~utomotive and appliance industries) is resistance spot weld;ng.
Resistance spot welding is ideally suited for joining thin sheet
materials and is well adapted to mass production industries. In addi-
tion, operating costs for this process are relat;vely low. Resistance
spo~ welding has been used with uncoated steels quite successfully
since the 1930's.
Resistance spot welding is used to form joints between two
materials. The process uses a set of electrodes to apply pressure to
the weld area, to maintain the components in position, and to pass
current through the weld. As the current flo~s, joule heating of the
substrate occurs. Due in part to the cooling effects of the elec-
trodes, a molten nugget eventually develops at the weld centerline or
faying surface. On cooling, this nugget resolidifies and effects a
joining between the two materials.
As mentioned, resistance spot welding of uncoated steels has
historically been quite successful. However, the resistance spot
wel~abi1ity of coated sheet steels has not been as successful. The
problems can be best seen by reference to some typical measures of
spot weldability.
The ~eldability lobe is defined as the range of welding
conditions (weld current and weld time) over which weld nuggets of an
adequate size can be formed. This, in effect, defines a "window" of
acceptable welding conditions. When practical, weld nugget sizes
during lobe testing are estimated with a destructive test known as the
peel test. This test consists of welding two 1-1/4-inch by 4-inch
samples at two points, and destructively pulling apart the second of
the welds. The weld nugget will usually adhere to one of the two
sheets as a weld "button", and the size of this weld button can be
measured with a set of calipers. The weld button size is usually
considered a good measure of the nugget size. The limits of the
weldability lobe are defined by the welding conditions which produce a
minimum weld size on one side, and expulsion on ~2 oth~r (expulsion
occurs when l;qu;d metal is expelled from the weld during welding). A
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~323077
line representing a nominal button size (part way between the minimum
and expulsion) is also often included.
The weldability lobes are characterized by lobe position, lobe
width and the position of the nominal button line. See, generally, D.W.
Dickinson, We!dinc in the Automotive Industry, Report SG 8-15 of the
Committee of Sheet Steel Producers, the American Iron and Steel
Institute. The lobe position is defined as the average welding current of
the iobe. Though lobe position is not considered to be a critical
weldability parameter, higher welding currents do result in higher energy
lo costs, as well as a decrease in electrode life. More significant is the
width of the welability lobe defined as the difference in welding currents
be~ween minimum button and expulsion at a particular welding time. This
is a measure of a materials' '~lexibility" during spot welding. The position
of the nominal button line, although considered of lesser importance, is
also a measure of a materials' flexibility during spot welding. A central
position for this line indicates a button size with adequate current range
to both higher and lower currents.
Dynamic resistance is used as a measure of weld quality and is
defined as the resistance of the weld across the electrodes (as a function
of time) during welding. The dynamic resistance has been correlated to
weld development in uncoated steels, and successfully used as an input
signal for feedback control. Unfortunately, the results for zinc or zinc
alloy coated steels have not been as good. ~n particular, feedback
system~ have been largely unsuccessful in controlling weld development
2 5 in such coated steels which exhibit a featureless resistivity trace or curve.
The dynamic resistance trade for uncoated steel, in contrast, exhibits a
characteristic "beta peak", followed by a resistance drop. It is the
presence of this "beta peak" which makes resistive feedback control
possible. See Dickinson, supra.
3 o When resistance welding uncoated steels, a single set of copper
welding electrodes can be expected to make approximately 50,000 welds.
When welding galvanized steels, however, the electrode life is reduced to
about 1000-2000 welds or less. Since the production line must be
,~ 4
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13~3(~7
stopped each time an electrode is replaced, at a
considerable expense to the user, the relatively
limited electrode life for galvanized steels repre-
sents a sig~ificant economic disadvantage.
The use of ferrophosphorus pigment for both
improved corrosion protection and weldability has
been suggested in the prior art. For instance, U~S.
Patent 3,884,705, issued May 20, 1975, and U.S.
Patent 4,119,763, issued October 10, 1978, both
disclose the use of coatings containing ferrophos-
phorus and zinc pigments, and a non-metallic cor-
rosion inhibitor such as zinc chromate, as a replace-
ment for zinc-rich coatings. These coatings also
contain a non-metallic corrosion inhibitor such as
zinc chromate. As contemplated in these Patents, the
ferrophosphorus pigment-containing coating is applied
to bare steel panels rather than to galvanized
sheets. The ferrophosphorus pigment used in such
applications is commercially available from the
Occidental Chemical Corporation under the Trade Mark
Ferrophos pigment.
A ferrophosphorus pigment dispersed in a resin
to bind adjacent steel plates to form a vibration-
damping composite suitable for resistance welding has
been described previously.
The use of a coating comprising a resin, ferro-
phosphorus powder and mica powder applied to a steel
sheet having a layer of fused aluminum or an
aluminum/zinc alloy has also been described pre-
viously, and such can be subjected to chemicalconversion, and is described as having excellent
weldability, processability and corrosion and heat
resistance.
.. . . . . . . .. ., - . .
132~7
The use of an iron layer containing less than
about 0.5 weight percent phosphorus applie~ to a
zinc/iron or zinc/nickel alloy electroplated steel
sheet for improved surface properties is described by
Honjo et al in Internal Journal of Materials_ and
Product Technoloqy, Vol. 1, No. 1, pp. 83-114 (1986).
It will be appreciated by those skilled in the
art that a continuing need exists for steel sheets
which possess the durability and corrosion resistance
of galvanized sheets but also possess the weldability
advantages of bare steel.
In accordance with the invention there is
provided a resi~tance welding electrode having
improved resistance welding characteristics, said
electrode having a coating comprising a binder and a
pigment, said pigment comprising at least one metal
phosphide selected from the group consisting of iron,
nickel, cobalt, tin, copper, titanium, manganese,
molybdenum, tungsten, vanadium, tantalum, and mixtures
thereof.
In accordance with another aspect of the
invention there is described a method for the
resistance welding of two zinc-coated steel surfaces
which comprises applying to the welding electrodes a
coating composition comprising a binder and a pigment,
said pigment consisting essentially of at least one
metal phosphide selected from the group consisting of
phosphides of iron, nickel, cobalt, tin, copper,
titanium, manganese, molybdenum, tungsten, vanadium,
tantalum and mixtures thereof, and welding said
surfaces.
There is also described herein a zinc or zinc
alloy coated steal sheet or part with improved
resistance welding characteristics has a top coating
of a binder and a pigmsnt consisting essential of at
least one metal phosphide selected from the group
--6--
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~323~77
consisting of phosphides of ixon, nickel, cobalt, tin,
copper, titanium, manganese, molybdenum, tungsten,
vanadium, tantalum and mixtures thereof. Preferably,
the metal phosphide is ferrophosphorus pigment having
a range of particle sizes of from about 0.1 to about
microns, and which is present in the coating
composition in amounts of from about 30~ to about 90%
by weight of non-volatile components.
The metal phosphide coat:irlcJ can also be applied
to a resistance electrode, fGr example, a copper
electrode, for improved weldability. An a~ditional
metal can be applied to the electrode surface prior to
coating the electrode with the metal phosphide,
.
~L323~77
the additional metal being selected from the group
consisting of iron, nickel, cobalt, silver, man-
ganese, vanadium, molybdenum and gold.
The pigment can also include up to about 40~ by
weight of a metal additive selected from the group
consisting of tin, aluminum or lead. Thls metal
additive can be physically combined with the ferro-
phos in pigment form or deposited onto the surface of
the ferrophosphorus pigment. These additive metals
are used to increase the electrode life.
The use of a coating containing a ferrophospho-
rus pigment applied to the faying surfaces of a
galvanized steel sheet or part results in a sub-
stantial decrease in the welding current and an15 increase in the weldability lobe width as compared to
galvanized steel. If such a coating is applied to
the non-faying surfaces, or to the resistance welding
electrode, an increase in electrode life results. In
addition, the use of a coàting containing ferrophos-
phorus pigment results in a restoration of the
dynamic resistance beta peak to the dynamic
resistance trace.
13230 17
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The steel sheets or formed parts which are used in the present
invention contain a thin layer of zinc metal or a zinc alloy which is
direct contact with the steel surface. Typically, the zinc layer has
a thickness of about 0.5 mils. The steel substrate itself is gener-
ally about 30 mils thick. Thin steel sheets of this type are used
extensively in the automoti~e and appliance ;ndustries for forming
auto and appliance bodies. The zinc or zinc alloy coating or layer is
typically applied to the steel sheet using well-known techniques such
as hot-dip galvanizing, where the sheet is contacted wi~h molten zinc,
or electrogal~anizing, where zinc or a zinc alloy coating is applied
to the substrate by electrodeposition. This invention, however, does
not contemplate the further treatment of steel sheets having a layer
of highly electrically resistant material such as Zincrometal. A
steel sheet or part having this coating is characterized by a high~
surface resistance which results in the absence of a dynamic
resistance beta peak.
The metal phosphide pigment of the present invention comprises
particles having an average size within the range of from about 0.1 to
a~out 30 microns. Particles within the desired size ranges are suit-
ably obta;ned by pulverizing the metal phosphide using conventional
techniques. Suitable metal phosphides include phosphides of iron,
nickel~ cobalt, tin, copper, titanium, manganese, molybdenum, tung-
sten, vanadium, tantalum, as well as mixtures of these metal phos-
phides. The preferred metal phosphide is iron phosphide, which
includes various ratios of iron and phosphorus, and particularly
ferrophosphorus, which is an iron phosphide compound containing from
about 20~ to 28% of phosphorus and corresponding chemically to a
mixture of Fe2P and FeP. Ferrophosphorus is obtained as a by-product
in the commercial manufacture of elemental phosphorus by the electric
furnace reduction of phosphate ores, with the iron present in the
phosphate ores forming the ferrophosphorus. Ferrophosphorus typically
contains impurities, of which silicon and manganese are the major
impurities, typically being present in amounts of up to 5X by weight,
., . ~
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1323~77
an~ is further characterized as being electrically and thermally
conductive, brittle, and substantially unreactive in water, dilute
acidic or alkaline environments. A particularly suitable ferrophos-
phorus pigment is Ferrophos~ pigment which i~ manufactùred and sold by
the Occidental Chemical Corporation.
A coating composition containing the metal phosphide pigment of
the present invention may be formulated by admixing the metal phos-
phide particles with a suitable binder, also using conventional mixing
techniques. More specifically, when the metal phosphide of the pre-
sent invention is incorporated into a coating formulation, the binder
component of ~he formulation comprises 5g to 96~ by weight of non-
volatile components, and preferab1y from 10% to 70g by weight of the
non-volatile components. Various binder materlals, both organic and
inorganic, may be used, the choice of a particular binder being depen-
dent upon the characteristics which are desired for the particular
application. Typical binders include various synthetic resins, such
as epoxies, chlorinated rubber, polystyrene, polyvinyl acetate resins,
silicones, silanes, borates, silicates, acrylics, polyurethanes and
the like. In some applications, it may be desirable to apply a
coating which can be readily removed after the welding operation.
Typical binders of this type, i.e. those which are readily removable,
include, by way of ;llustration, carboxymethyl cellulose, ethyl
cellulose, polyvinyl alcohol, natural gums, etc. Other suitable
binders not specifically described herein will be readily apparent to
those skilled in the art.
The metal phosphide pigment can be preserlt in the coating in an
amount of from about 4~ to about 95X by weight of the total non-
volatile components in the coating, with amounts within the range of
from about 30~ to about 90g by weight being preferred. A portion of
the metal phosphide particles of the pigment can be replaced by other
metals such as tin, aluminum and lead. These additional metals can be
present in amounts of up to about 40X by total weight of the pigment,
and will typically have an average size within the range of from about
0.1 to about 30 microns. Alternatively, the additional metal can be
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1~23077
deposited directly onto the surface of the metal phosphide particles
using techniques whlch are well-known to those skilled in the art,
such as by physically grinding or blending mixtures of the metal
phosphide and added metal in the desired proportions, or by immers;on
coating, etc.
Depending upon the particular binder which is selected, the
coating composition may also contain suitable solvents, curing agents,
suspending agents, plasticizers and the like. The selection of the
type and amounts of these other components will of course depend upon
the particular binder as well as the ultimate characteristics desired
for the coating and its use.
The formulated coating may be applied directly to the substrate
using any available technique such as, for example, spraying, brush-
ing, immersion, flowing or the like. If desirable, an intermediate
conversion coating can be applied to the substrate prior to the appli-
cation of the metal phosphide-containing coatlng. Typically, the
coating is applied to produce a film having a thickness within the
range of about 0.1 to 10 mils, although thicknesses which are outside
of this range may also be used to advantage.
In the present invention the coating containing the metal phosphide is
applied to the resistance electrode rather than or in addition
to application of the coating to the substrate. In this embodiment,
the resistance welding electrode can be first coated with a metal ~;
selected from the group consisting of iron, nickel, cobalt, tin, cop-
per, titanium, manganese, molybdenum, tungsten, vanadium, tantalum,
and mixtures thereof prior to application of the metal phosphide-
containing coating. Suitable methods of applying the coating to the ;~
resistance electrode include spraying, brushing, contact with an ex-
pendable ribbon containing the metal phosphide, and other methods as
will be readily appreciated by those skilled in the art.
The coating containing the metal phosphide can be applied to
either the faying or the non-faying surfaces of the steel sheet or
part, or both as desired. Application of the coating to only the
faying surfaces results in improvements in the welding lobe curve and
dynamic resistance curve, while application of the coating to the
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~32~7'7
non-faying surfaces results 1n improvements 1n electrode ll~e. The
presence of the ferrophosphorus-contain~ng coat~ng at the ~aylng
surfaces reduces shunt currents and, consequently, the temperature of
the electrode, increasing electrode life.
The following specific examples are provided as exemplary of
various embodiments of the present invention, but are not intended to
limit the full scope of the invention as defined by the appended
claims.
EXAMPLES 1-3
Lobe curves were generated using procedures established by Fisher
Body Specification MDS-247 for galvanized steel. Welding conditions
were as follows:
Welding Electrodes: RWMA Class II, 45 degree truncated
cone, 0.25 inch face diameter
Welding Force: 500 pounds
Weld Times: ll, 14, 16 and l9 cycles
Minimum Nugget Size: 0.16 inch
Nominal Nugget S;ze: 0.20 inch
This test consists of weld;ng two 1-1/4-inch by 4-~nch coupons 0.03
inches thick at two locations, and destructively pulling apart the
second weld. The d;ameter of the peeled weld nugget was measured to
determine the posit;on of the limit lines which comprise the weld-
ability lobe. The orientation of the coupons was such that the
coating on the top coupon was at the electrode-to-sheet interface, and
the coating on the bottom coupon was at the sheet-to-sheet interface.
Dynamic resistance traces were also obtained for welds made on
each of the materials to help interpret nugget development during spot
welding. These curves were also used to characterize each material's
suitability to feedback control. These were obtained both across the
welding tips and where necessary across the sheets.
-12-
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1~23Q77
The resistance welding character~stics of untreated coupons as
well as coupons having various zinc metal or zinc alloy coatings was
evaluated. The hot-dipped galvanized layer wclS applied in amounts of
from about O.g to about 1.25 oz. of metal per square foot, while the
Zincrometal coating was about 0.5 m~ls thic~. The types of coupons
evaluated were as follows:
EXAMPLE NO. COATING
1 None
2 Hot-dipped galvanized
3 Zincrometal
The weldability of the coupons of Examples 1-3 was evaluated, and
the results of the evaluation are shown in Table 1.
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~323~7
EXAMPLES 4-6
The metal phosphide-containing coating of the present invention :~
was evaluated using an epoxy ester binder containing 92X by weight of
a ferrophosphorus pigment, designated FERROPHOS~ HRS 2131 with a mean
particle size of 5 microns, which is manufactured and sold by the
Occidental Chemical Corporation.
This coat;ng was sprayed onto various substrate materials to a
thickness of 1 mil as follows:
EXAMPLE NO. SUBSTRATE
4 Bare Steel
Hot-dipped galvanized steel
6 Zincrometal coated steel
The weldability of the coupons of Examples 5-7 was evaluated, and
the results of the evaluation are shown in T~ble 2.
-15-
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132~077
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1323~77
The ab~ve-described examples demonstrate the tmprovement 1n
resistance welding character;st;cs which is obta;ned following the
procedure of the present invention. The welding current requ~red on
the galvanized sheets decreased substantially and the lobe w;dth
increased sign;ficantly following application of the metal phosphide-
containing coating. In addition, the presence of a beta peak was also
detected after application of the metal phosphide-containing coating.
When a high electrically resistant layer is present, such as for a
Zincrometal coating, the dynamic resistance beta peak does not occur,
thus adversely affecting dynam;c res;stance feedback control.
EXAMPLES 7-24
Ferrophos-contain;ng coatings were applied to bare steel test
panels, hot-dipped galvanized steel test panels, and hot-dipped
galvanized steel test panels that had a conversion coating. The
conversion coatings employed were Bonderite*37, a zinc phosphate
coating, and Bonderite 1303, a complex ox;de coating. The binder was
an epoxy ester resin, and the coating was spray-applied to a th;ckness
of 0.4 mils on both s;des of the test coupon.
The coa~ed test panels were subjected to res;stance weldlng tests
to determine whether they would weld. The welding condit1ons used
were similar to those of Example l, and the results are summarized in
Table 3.
* Trade Mark
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1323~7
TABLE 3
Amount of
Surface Ferrophos
No. Treatment ~ Weldable
7 None None (Control, no binder) Yes
8 None 0 (Binder only) No
9 None 45 Yes
None 61 Yes
11 None 76 Yes
12 None 86 Yes
13 Bonderite 37 None ~Control, no binder) No
14 Bonderite 37 0 (Binder only) No
Bonder;te 37 45 No
16 Bonderite 37 61 No
17 Bonderite 37 76 Yes
18 Bonderite 37 86 Yes
19 Bonderite 1303 None (Control, no binder) Yes
Bonderite 1303 0 (Binder only) Yes
21 Bonderite 1303 45 Yes
22 Bonderite 1303 61 Yes
23 Bonderite 1303 76 Yes
24 Bonderite 1303 86 Yes
-18-
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~32~077
The results shown ~n Table 3 demonstrate the effectiYeness in
terms of weldability of Ferrophos-conta~ning coatings applied to
galvan;zed steel with or without conversion coatings at various
pigment to binder loadings.
EXAMPLE 25
A Ferrophos-containlng coating having a removable gelatin-based
binder and containing about 86X by weight of pigment was applied to
hot-dipped galvanized steel test panels to a thickness of 0.5 to 1.0
mils. After drylng, the panels were resistance welded. The welding
lo proceeded normally, and after cooling the panels were subjected to an
aqueous wash which effectively removed the relnaining coating, leaving
the surface su;table for subsequent finishing.
EXAMPLES 2G-30
Ferrophos-containing coatings were prepared to evaluate the use
of tinl lead and aluminum in combination with the Ferrophos pigment.
The Ferrophos pigment used was Ferrophos grade HRS 2132, having an
average particle size of about 3.0 microns and available ~rom
Occidental Chemical Corporation.
Three pigment compositions were prepared by grinding 15 grams of
either the tin, lead or aluminum powder with 500 grams of Ferrophos in
a ball mill for 16 hours. The tin powder used was MD 301, available
from Alcoa Aluminum Co., the lead powder used was obtained from Fisher
Scientific Co., and the aluminum powder used was from Matheson,
Coleman and Bell. A fourth pigment used was 15 grams of tin powder
which was added to 500 grams of Ferrophos without grinding. A control
pigment was used with Ferrophos alonè, subject to the same milling,
for comparison purposes.
Coatings were prepared using 200 grams of each pigment listed
above, 30 grams of an epoxy ester resin (Reichhold Epotuf 38-4071), 2
grams of fumed silica (Cab-O-Sil~, 1 gram of hydrophobic fumed silica
--19--
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132~Q77
(Aerosil R972), and 0.1 gram of cobalt naphthenate. The solvent used
was xylene.
The coatings were spray-applied to 4"x12" hot-dipped galvanized
test panels, aged, and subjected to testing. Test strips for each
coating were all successfully resistance welded.
Although the present invention has been described with respect to
several illustrative embodiments, it should not be interpreted as
being so limited. As will be evident to those skilled in the art,
other substitutions and equivalents are possible without departing
from the spirit of the invention or the scope of the claims.
* Trade Mark
-20-
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