Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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GAS-METAL-ARC WELDING CONTACT TIP
Cross Reference to Related Applicatioas
This application claims the benefit of U.S.
Provisional Application No. 60/089,752, filed June 18,
1998.
Field of the Invention
This invention relates to electric welding
torch tips and more particularly to a contact tip formed
of mixtures of copper and conductive-ceramic powders
using powder metallurgy processes which may have a non-
conductive ceramic insert therein through which a
continuous metal wire electrode is passed and charged
with enough current to become filler metal on a
workpiece.
Background of the Invention
In a conventional welding operation, a wire of
filler metal or welding wire is continuously fed and
charged through a welding torch contact tip having a
wire feed aperture with a receiving end through which
the filler wire enters the contact tip and a contact end
through which a short length of the filler wire projects
to be presented in a suitable position next to the weld
zone. An electric arc is formed between the charged end
of the wire and an oppositely charged workpiece which
provides heat to form a weld puddle. In many methods
and apparati, a welding machine further includes a
nozzle for blowing an inert or active gas over the weld
puddle to keep it under a controlled atmosphere. This
avoids unwanted reactions of the molten metal with the
surrounding air which result in poor weld quality.
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In welding methods of this general category,
the welding wire is unwound from a spool and
automatically fed into the welding assembly as the
welding wire is consumed. The welding wire has a cast,
which is arcuate in nature, as it is formed and wound on
the spool. It is desirable to maintain good electrical
continuity between the wire and the contact tip. Drive
rollers are often used to feed the wire off the spool
and into the welding torch. The wire must be moved
through the wire feed aperture smoothly, without
jerking, for accurate and high performance welding to be
satisfactorily achieved.
Welding methods of this type, measure process
efficiency by the percentage of arc-on time during
production. Manual, mechanized, automatic or robotic
systems using these welding methods often deliver
efficiencies of less than 500. The time during which a
laborer or machine is not welding is generally
attributable to operational difficulty of the welding
apparatus which is often related to the performance of
the contact tip.
Contact tips are traditionally fabricated as
cylindrical tubes made of pure copper or high copper
content alloys which have high electrical and thermal
conductivity. Under normal service conditions, contact
tips may be exposed to operating temperatures well above
400°C (752°F). Operating temperature is critical in
determining the performance of contact tips. Higher
temperatures degrade the material properties and
accelerate failure of contact tips. Heat is mainly
transferred by conduction from the contact tip to the
welding torch.
Under normal service conditions, contact tips
are subject to systematic accumulation of debris carried
by the wire surface into the contact tip wire feed
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aperture. These debris are burnt during the welding
operation leaving refractory non-conductive byproducts.
Contact tip wire feed apertures are fabricated with a
dimensional tolerance to minimize friction to the
passage of the welding wire. In such contact tips, true
electrical contact is only possible at discrete points
along the wire feed aperture, and depends on wire feed
aperture tolerance and the cast in the welding wire.
The presence of non-conductive material inside the wire
feed aperture causes electrical contact to become
unstable, which results in contact tip failure. Also,
the presence of additional material inside the wire feed
aperture increases the likelihood of wire choking due to
repetitive expansion and contraction cycles which may
prevent regular feeding causing bad weld starts or
burnback, a condition in which the wire melts to the end
of the contact tip.
In welding methods of this kind, there are
sporadic liquid metal bursts originated at regular
disruptions of the molten wire/puddle bridge or by
regular perturbations to the puddle surface. Bursts of
this nature result on liquid metal being regularly
expelled from the weld zone as miniature droplets or
spatter which binds and builds up on the contact end of
the contact tip on the inside of the wire feed aperture
around the outlet. Excessive spatter accumulation at
the front of the contact tip creates a metal bridge
between the moving welding wire and the contact tip
which eventually causes feeding fluctuations and may
bring the process to a halt. Also, spatter buildup in
and around the wire feed aperture outlet produces
choking of the filler wire leading to similar feeding
problems. Feeding instability leads to contact tip
failure by burnback or bad weld starts.
Furthermore, the contact wire feed aperture is
subj ect to wear damage by abrasion or electrical erosion
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caused by the sliding friction between the charged wire
feed aperture surface and the moving welding wire.
Excessive unidirectional wearing of the wire feed
aperture outlet or "keyholing" causes the wire to miss
the target seam resulting in misplaced welds. This is
critical to robotic welding in which program schedules
are not designed to compensate for such source of
variation.
It is known in the art relating to continuous
feed welding torches to use replaceable contact tips
that have an insert disposed in the contact tip distal
end proximate the workpiece. Typically these inserts
are of a harder material than the copper of the tip.
These inserts are claimed to extend the contact tip life
by reducing wear as the welding wire is fed through the
tip. However, these inserts, and other inserts so
positioned, inhibit current transfer close to the arc
where it is more efficient for arc stability and welding
soundness.
Copper-carbon "alloys" have been suggested to
improve lubricity and reduce coefficient of expansion of
the contact tip wire feed aperture. This previous art
also refers to copper-carbon "alloys" that provide high
electrical conductivity while having low thermal
conductivity and high melting point. This art refers to
the fabrication of cylindrical contact tips by injection
molding or traditional machining.
Copper-carbon "alloys" are not possible since
carbon is immiscible in copper. However, copper and
carbon can be integrated into a single material by
powder metallurgy methods which involve pressure-die
molding followed by controlled sintering. Carbon comes
in many different crystallographic groups including
diamond, graphite, amorphous and buckyballs. Each one
of these groups exhibits contrasting mechanical and
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physical properties. Furthermore, within each group
specific mechanical and physical properties depend on
the level of impurities and orientation of the crystal
structure with respect to applied external forces.
5 Diamond exhibits the highest thermal conductivity and
hardness of any material while possessing the lowest
electrical conductivity of any material. Graphite
exhibit limited electrical and thermal conductivity
depending on crystal orientation. The good lubricity
ZO of graphite results from the hexagonal arrangement of
layer planes. Each plane is free to slip or slide past
one another. Amorphous carbons have smaller monoplanes
piled in turbostatic stacks which have a variety of
properties including small expansion coefficient with a
variety of thermal and electrical conductivities.
Buckyballs are special "soccer-ball" configurations of
carbon which exhibit special electrical and other
properties.
Powder metallurgy (P/M) materials have certain
percentage of undesirable porosity which is inherent to
the P/M process. A challenge of every P/M process is to
minimize porosity so that the density of the P/M
material is as close as possible to its theoretical
density.
Suamnary of the Invention
The present invention provides an improved
contact tip for gas-metal-arc welding wherein a
continuous wire of filler metal or welding wire is
passed through an electrically charged contact tip which
passes its charge to the welding wire while the
workpiece is oppositely charged.
As is hereinafter more fully described, the
contact tip that minimizes friction in its wire feed
aperture, display high thermal diffusivity, possess high
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electrical conductivity, provide permanent anti-spatter
protection, stabilizes the location of the current
transfer point (insert), minimizes operating
temperatures and eliminate excessive wearing of the wire
feed aperture.
More specifically, the present invention
provides a contact tip for use in a welding torch that
guides a welding wire toward a workpiece and transfers
welding current from a torch to the welding wire. The
contact tip comprises an electrically conductive
component including a mounting end and a distal contact
end extending along a longitudinal wire feed axis. The
conductive component includes a through-bore or wire
feed aperture that may have a large diameter portion
extending from a mounting, wire receiving, end and a
small diameter portion, sized for guiding the welding
wire and transferring the welding current, proximate to
the distal end and extending along the wire feed axis.
In one embodiment of the invention, an
elongated high temperature resistant, ceramic insert
including a corresponding through-bore sized for guiding
the welding wire, is coaxially mounted in the large
diameter portion of the electrically conductive
component. Therein, the insert through-bore and
conductive component small diameter portion define a
wire feed passageway for supporting the welding wire.
Preferably the small diameter portion of the
conductive component proximate its distal end extends
less than 100% the length of the through-bore extending
through the tip. The small diameter portion of the
conductive component exhibits a smaller tolerance
between wire size and bore size so that true electrical
contact is improved.
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One end of the insert through-bore may be
tapered. This tapered end is the end mounted adjacent
the conductive component mounting end to facilitate
introduction of welding wire through the insert portion
of the contact tip. Preferably the tapered end includes
a sidewall disposed generally at an angle of about 30
degrees to the longitudinal wire feed axis.
The front-end outside geometry of the tip
comprises an enlarged outside diameter at the front end
IO which enhances the ability of the tip to diffuse heat
thus lowering operating temperatures.
Another feature of the front-end geometry is
the lack of chamfer at the outlet part of the wire feed
aperture. A right angle corner edge provides enhanced
current transfer, lowers operating temperature and
prevents microspatter from entering the wire feed
aperture.
Another feature of the tip front-end geometry
comprises an enlarged radius of curvature and bulbous
shape which maximizes metal mass at the front end
lowering operating temperatures at the wire feed
aperture.
Another feature of the front-end may be a
coating of an extra-hard protective film of diamond-like
carbon which enhances the ability to reject spatter
buildup.
The mounting end of the contact tip includes
a connector that provides high heat transfer. In one
embodiment, the mounting end comprises a frusto-conical
surface and taper-lock thread arrangement which enhances
the ability of conducting heat transfer away from the
contact tip.
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In one embodiment of the invention, the
conductive component comprises copper or a high copper
content alloy. The invention also comprises a
conductive component made of a composite material
consisting of sintered powders of high purity copper and
conductive ceramic particles. Conductive ceramic
particles include special crystalline forms of carbon
including but not limited to graphite. Specially
oriented graphite particles in sufficient density
enhance anti-spatter properties of the material and
increase lubricity in the wire feed aperture. P/M
Copper-graphite composites exhibit lower electrical and
thermal conductivity than pure copper. Therefore
excessive amounts of graphite (above 200) may preclude
the principal function of the contact tip leading to
catastrophic failure. P/M Copper-graphite composites
must have a density higher than 80% of the ideal
densities of their solid counterparts.
In another embodiment of the invention, the
conductive component may comprise a cylindrical insert
made of a ceramic material including, but not limited
to, aluminum oxide, boron carbide, silicon carbide,
silicon oxide, aluminum nitride, zirconium oxide, boron
nitride or any mixture of these substances. The ceramic
insert limits current transfer to the front end of the
contact tip which minimizes burnback occurrences.
These and other features and advantages of the
invention will be more fully understood from the
following detailed description of the invention taken
together with the accompanying drawings.
Brief Description of the Drawincrs
In the drawings:
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FIG. 1 is a sectional view of a welding torch
contact tip constructed in accordance with the present
invention; and
FIG. 2 is a sectional view of a welding torch
contact tip constructed in accordance with the present
invention and including a ceramic insert at a welding
wire receiving end.
Detailed Description of the Invention
Referring now to FIGS. 1-2 in detail, numeral
10 generally indicates a contact tip for use in a
continuous feed welding torch, not shown. Contact tip
10 feedingly guides a welding wire, as is known, toward
a workpiece and transfers welding current from the torch
to the wire.
Referring to FIGS. 1-2, the contact tip 10
includes an electrically conductive component 12
including a mounting end 14 and a distal contact end 16
extending along a longitudinal wire feed axis 18. The
conductive component 12 includes a through-bore 20
having a large diameter portion 22 extending from the
mounting end 14 and a small diameter portion 24 sized
for guiding welding wire, proximate the distal end 16
and extending along the wire feed axis . Mounting end 14
includes a connector that provides high heat transfer.
In the embodiments shown, mounting end 14 is a frusto-
conical surface and taper-lock thread arrangement for
attaching the tip 10 to a cooperating surface on a
welding torch.
Outlet 30 of the small diameter portion 24
contains no chamfer and the edge around the outlet and
forms generally a right angle with no metal debris or
inconsistent sharp edges. As illustrated, the radius of
curvature of the front end 32 or contact end of the tip
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is increased and has a bulbous shape which lowers
operating temperatures around the outlet 30 of the small
diameter portion 24.
Where applicable, an elongated high
5 temperature resistant ceramic insert 26 is coaxially
mounted in the large diameter 22 of the conductive
component 12. Insert 26 includes a through-bore 28
corresponding to the small diameter portion 24 and is
sized for guiding welding wire therethrough. The insert
10 through-bore 28 and conductive component small diameter
portion 24 define the wire feed passageway for
supporting the welding wire.
In the embodiment illustrated, the conductive
component 12 comprises copper, a high copper content
alloy, or sintered mixtures of copper and conductive
ceramic particles including composite materials made of
copper and certain forms of graphite with final density
higher than 80s. The ceramic insert 26 comprises
ceramic material including but not limited to one of
aluminum oxide, boron carbide, silicon carbide, silicon
oxide, aluminum nitride, zirconium oxide, boron nitride
or any mixture of these substances.
With continuing reference to FIG. 1, the small
diameter portion of the conductive component 12
proximate the conductive component distal end 16 has a
length in the range of 100% the length of the contact
tip wire teed aperture. It is at this portion 24 that
the welding current is transferred from the welding
torch to the welding wire.
In FIG. 2, the small diameter portion 24 of
the conductive component 12 proximate the conductive
component distal end 16 has a length of 50s the length
of the contact tip through-bore. The limited length of
the small diameter portion 24 provides for better arc
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stability during the transfer of the welding current
from the torch to the wire and thereby keeps the tip 10
cleaner and reduces the occurrence of burnback. The
ceramic insert 26 confines the arcing to the small
diameter portion 24 of the conductive component 12.
Preferably, the insert through-bore 28 is
tapered at one end 31 adjacent the conductive component
or contact tip mounting end to facilitate the feed of
welding wire into the insert portion of the tip 10.
Preferably the tapered end 31 is defined by a sidewall
disposed generally at an angle of about 30 degrees to
the longitudinal wire feed axis.
In the embodiment illustrated, an optional
diamond-like thin coating 34 of carbon has been applied
to further enhance anti-spatter properties.
Although the invention has been described by
reference to specific embodiments, it should be
understood that numerous changes may be made within the
spirit and scope of the inventive concepts described.
Accordingly, it is intended that the invention not be
limited to the described embodiment, but that it have
the full scope defined by the language of the following
claims.
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Reference Numerals
10. contact tip 54.
12. conductive component 56.
14. mounting end 58.
16. distal contact end 60.
17. 62.
18. wire feed axis 64.
20. through-bore 66.
22. large diameter 68.
24. small diam. portion 70.
26. ceramic insert 72.
28. through-bore 74.
30. outlet 76.
32. front outside corner 78.
34. 80.
36. 82.
38. 84.
40. 86.
42. 88.
44. 90.
46. 92.
48. 94.
50. 96.
52. 98.
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