Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02610328 2007-12-06
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Title
Contact tip for arc welding.
Field of the invention
This invention relates to improved contact tips for use in Gas Metal Arc
Welding.
Background of the invention
In conventional gas metal arc welding operations, a wire of filler metal or
welding wire is
continuously fed and charged through a welding torch and contact tip to a
target metal work piece
where it is consumed and becomes the filler metal.
The contact tip has a double role; namely acting as a guide for the welding
wire, while also feeding
electric power to the welding wire. In order to supply welding current to the
wire, the contact tip is
put into contact with the welding wire. An electric arc is formed between the
charged end of the
welding wire and the oppositely charged metal work piece which provides heat
to form a weld
puddle.
The welding wire is unwound from a spool and automatically fed into the
welding zone as the
welding wire is consumed. Drive rollers are often used to feed the wire off
the spool and into the
welding torch. The welding wire has a cast that is formed when wound onto the
spool. The cast
helps to create electrical contact between the contact tip and the welding
wire.
The contact tip wire feed aperture is subject to wear damage by abrasion and
electrical erosion
caused by sliding friction between the charged wire feed aperture surface and
the moving welding
wire.
Contact tips are traditionally fabricated as cylindrical tubes made of pure
copper or high copper
content alloys because copper is a very good electrical conductor, is
relatively cheap, and is easy to
machine.
Copper materials have excellent electrical and thermal conductivity, are
readily fabricated, and have
good strength and fatigue characteristics. An important disadvantage of copper
is its low resistance
to surface oxidation and corrosion. Also, it is not regarded as having good
resistance to arcing,
welding and sticking, which are needed in contact applications.
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Silver alloy coatings are often used for elevated-temperature solid
lubrication. Silver deposits
prevent galling, especially during start-up. The silver serves both as a
corrosion deterrent and a dry
lubricant. Silver coating can be used up to 870 C, and thus is a good high-
temperature lubricant.
Silver, in pure or alloyed form, is the most widely used material for a
considerable range of
electrical contacts up to 600 A. Silver has the highest electrical and thermal
conductivity of all
metals at room temperature and, as result, will carry high currents without
excessive heating, even
when dimensions of the contacts are only moderate. For electrical contacts,
silver is used instead of
copper chiefly because of its resistance to oxidation in air. All non-noble
metals are subject to a
phenomenon called fretting corrosion. This fretting corrosion is an
accelerated corrosion that occurs
at the interface of contacting metals when they are subject to certain
relative motions.
During welding, the highest temperature of a welding gun is present at the tip
end of the contact tip.
This is due to the tip end being the closest portion of the contact tip to the
electrical arc and to the
welding puddle which emanates heat, and due to the heat generated by the
electrical current passing
through the contact tip to the welding wire.
Under normal service conditions, contact tips may be exposed to operating
temperatures well above
400 C. Operating temperature is critical in determining the performance of
contact tips. Higher
temperatures degrade the material properties and accelerate failure of contact
tips. In consequence of
the exposure to high operating temperatures, the charged wire feed aperture
surface, especially the
tip end, is so heavily worn that the tip is soon unable to conduct its double
role. In other words, the
tip fails to both guide the wire, resulting in misplaced welds, and to supply
welding current.
An ideal electrical contact material would have the following: high electrical
conductivity to
minimize heat generated; high thermal conductivity to dissipate both the
resistive and arc heat
developed; high reaction resistance to all environments used, in order to
avoid the formation of
insulating oxides, sulfides and other compounds; and finally, immunity to
arcing damage while
making and breaking electrical contact. Both the force required to close a
contact rnade of this
material and the electrical resistance between mating members would have to be
low. The melting
point of the material would have to be high enough to limit arc erosion and
metal transfer, but would
also have to be low enough to increase resistance to re-ignition in switching.
Also, the vapour
pressure would have to be low in order to minimize arc erosion and metal
transfer. Hardness would
have to be high enough to provide good wear resistance, while being ductile
enough to ensure ease
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of fabrication. Purity of the material would have to be maintainable at a
level that ensures consistent
performance of the product. Finally, the material would have to be available
at a low cost in any
desired form.
To achieve all these performance parameters in order to extend the contact tip
life, it is necessary to
use a mechanical mixture of materials and apply it at the charged wire feed
aperture surface,
specifically to the end of the contact tip, using a process that will not
change or destroy its
properties. This can be done using a new spray coating process called Cold Gas
Dynamic Spray.
Cold Gas Dynamic Spray or CGDS can be used to apply a wide variety of
metallic, dielectric,
metallic alloys, and mixed combinations to a variety of substrate materials.
CGDS is a relatively new coating process by which coatings can be produced
without significant
heating of the sprayed powder. In contrast to flame, arc, and plasma spraying
processes, in CGDS
there is no melting of particles prior to impact with the substrate. The
adhesion of particles in this
process is due solely to their kinetic energy upon impact. In this process,
very high particle
velocities are obtained by the acceleration of an expanding gas stream to
velocities in the range of
supersonic speed in a converging-diverging De Laval type nozzle. The gas and
particle temperatures
remain well below the melting temperature of the spray material.
Various innovations for welding torch contact tips are present in prior art,
including contact tips
with an insert shown in U.S. Pat. Nos.: 5,101,093 and 4,937,428, and in
Canadian Pat. Nos.:
1,182,871 and 2,233,662. Typically these inserts are ceramic or of a
conductive material harder than
the copper, which claim to extend the contact tip life by reducing wear or
temperature of the contact
tip as the welding wire is fed through it. However, these inserts are much
less conductive and inhibit
current transfer close to the arc, and the cost of manufacturing contact tips
with these inserts is high.
Summary of the invention
This invention relates to electric welding torch tips for use on a consumable
electrode type welder
having a wire feed aperture for a welding wire in the center thereof,
comprising a tiip body with a tip
end that is permanently built up on it via the Cold Gas Dynamic Spray process.
Cold Gas Dynamic
Spray or CGDS is a spray deposition technique wherein the deposits form due to
high-velocity
impacts of solid particles. This process produces a high density coating with
fairly high bonding.
CGDS coatings exhibit phase purity, while inclusions such as pores, and oxides
are minimal or even
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absent. This leads to excellent thermal and electrical conductivity with
excellent wear resistance
values of sprayed coatings.
The tip body is composed of copper or a copper alloy material. The tip end is
made of a material or
a mixture of materials such as silver, silver alloy, copper, copper alloy,
aluminum, nickel,
chromium, tungsten with aluminum oxide, zirconium oxide, metal carbides and
their combinations.
Brief description of the drawings
Fig.l is a sectional view of a known welding contact tip;
Figs. 2, 3, 4, 5 are sectional views of welding contact tips embodying the
present invention.
Detailed description of the preferred embodiments
As shown in Fig.l, a cross-section of a conventional welding contact tip
comprises a tip body 1 with
an axial centered wire passage 10 to feed the welding wire through it.
Figs. 2, 3, 4, and 5, show cross-sections of a welding contact tip according
to the present invention
comprising a tip body 1 and a tip end 2. The welding contact tip is fabricated
by building up the tip
end 2 over the tip body 1 by spraying it on layer by layer using the Cold Gas
Dynamic Spray
process, in such a way that the two parts are integrated with each other.
The tip end 2 is sprayed over a variety of prepared shapes 21, 31, 41, and 51
of the tip body 1 as
shown in Figs. 2, 3, 4, and 5. The most economical one is the tip end shown in
Fig.3 and Fig.5 with
shape 31 and 51 accordingly on the tip body 1.
According to the present invention, the tip body 1 is made of copper or a
copper alloy, and the tip
end 2 which is subjected to serious wearing by abrasion and electrical erosion
during welding is
made of a heat- and wear-resistant conductive material or mixture of materials
suc:h as silver, silver
alloy, copper, copper alloy, aluminum, nickel, tungsten with aluminum oxide,
zirconium oxide,
metal carbides and their combinations.
The wire passage 10 in the tip body 1 has a diameter 1.05 - 1.35 times the
welding wire diameter,
the wire passage 10 in the tip end 2 has a diameter 1.05 - 1.20 times the
welding wire diameter. If
the contact tip is manufactured using an extruded tube with an existing wire
passage 10, this wire
passage 10 has to be temporarily closed at the tip end 2 before applying the
CGDS spray - possibly
during preparation of the shapes 21, 31,41, or 51. To create a smooth coating
it is necessary to
prepare a uniform surface without holes or dents. For better bonding strength
before applying a
coating, shapes 21, 31, 41, and 51 can be sandblasted with grit 80 silicone
carbide or aluminum
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oxide. After spraying the tip end 2, a machining operation is required to
create or rework the
welding wire passage 10 and create the final cut for shapes 21, 31, 41 and 51
with smooth surfaces.
The thickness of the sprayed tip end 2 after final machining should be at
least 2 times the size of the
welding wire, while being no less than 1 mm thick.
A variety of powder mixtures was used to extend the contact tip life as well
as to obtain low
manufacturing cost of such contact tips. The best results were obtained when
tip end 2 was sprayed
with mixture # 1- silver powder - size 1-5 m with aluminum oxide powders mesh
-200 +325 at a
volume ratio of 3:1; mixture #2 - silver powder - size 1-5 m with copper
powder -- size -325 mesh
and aluminum oxide powders -200 +325 mesh at a volume ratio of 2:1:1; and
mixture #3 - silver
powder - size 1-5 m with tungsten powder - size 5-10 m and aluminum oxide
powders -200 +325
mesh at a volume ratio of 2:1:1. In all these cases, the tip end 2 composed of
the silver powder
mixture has excellent non-stick properties when faced with welding splatter.
Another advantage to
using these powder mixtures is an economical one. Since silver critical impact
velocities for 25 m
particles are as low as -350-375m/s, and copper and tungsten critical impact
velocities for the same
sized particles are -475-500m/s and -480-650m/s respectively, the low pressure
portable CGDS
equipment can be used for the spraying process. Other advantages to using
metal powder mixtures
with brittle materials such as aluminum oxide, zirconium oxide, and metal
carbides in the CGDS
process include very good deposition efficiency, strength of adhesion, as well
as wear resistance of
the coating.
While other tested powder compositions had good results, the above presented
materials were
superior.
