Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
In the art of arc welding it has been found to be
necessary to take steps to prevent the inclusion of contaminants
such as water vapor in the weld~ In particular, it has been
observed that water vapor has a tendency to cause porosity in
the weld area. A popular method of prevention has been to sur-
round the welding tip with a shield of inert gas. In certain
environments, however, it was found that the width of shield
required resulted in a high volume rate of flow of the costly
inert gas, so various other schemes for increasing the isolation
of the weld from contaminants have been investigated. Among
these have been several double-shield systems, such as Wempe,
United States Patent No. 2,903,559, which employs shielding gases
other than air.
Summary of the Invention
The invention relates to the method of arc welding
that includes shielding the plasma "bell" formed between a weld-
ing cup and a workpiece by forming at least two concentric gas
shields by emitting from the welding cup gas streams which are
physically separated until they leave the welding cup, the inner
gas shield thereof comprising an inert gas. In a broad aspect,
the invention resides in forming an outer gas shield with a gas
stream in which at least half the gas is air separating the outer
gas shield from the inner gas shield at the exit of the welding
' cup, such that the outer gas shield is kept beyond the outside
of the plasma "bell" formed between the welding cup and the work-
piece during welding and causing the outer gas shield gas stream
, to exit the welding cup at a velocity approximately equal to the
'' inner gas shield gas stream velocity at the welding cup exit.
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Bri'ef Des'criptio'n'of''the~ Dr'awings
Figure 1 is a sectional view of an apparatus for
use with the method of this invention;
Figure 2 is a diagram of the gas flow from a single-
shield welding ~ystem:
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Figure 3 is a diagram of the gas ~low from a double-shield system
used according to the method of the present invention;
Figure 4 is a diagram of the gas flow from a double-shield system
used in accordance with an alternate embodiment of the method of the present
invention.
Detailed Description of the Preferred Embodiment
In Eigure 1, inner chamber 12 is formed by the inside surface of
- cylindrical wall 10. In a typical apparatus, the cross section of chamber
12 may have a radius of 5/16". An outer chamber 18 is formed by the outer
c~lindrical wP~l 20 and the outer surface of c~lindrical wall 10, and this
chamber is filled with a material 16 suitable for acting as a diffuser, such
as stainless steel wool. Outer wall 20 is penetrated by gas passage 26.
A typical radius of the cross section of chamber 18 would be 5/8". Cylin-
drical wall 10 terminates in separating member 22, which serves to separate
the outer shield from the inner shield and keep the outer shield beyond the
outside of the plasma "bell" that forms between welding tip 24 and the work-
piece during welding. A typical separating member could have a radius of
9/16".
During operation, the inner-shield gas, typically argon or another
inert gas, is introduced into inner chamber 12 by a method familiar to the
art. The gas travels down inner chamber 12 and out its exit, welding cup
25, shielding the welding tip 2l~, which might have a 3/16" radius. ~he outer-
shleld gas, which is claimed as being at least half a mixture of nitrogen and
oxygen and can therefore be ordinary air, is introduced from a suitable source
into gas passage 26, thr~ugh which it passes to outer chamber 18. The gas
entering chamber 18 has a horizontal bulk velocity because of its travel
along gas passage 26. ~his horizontal flow is broken up b~ stainless-steel-
wool diffuser 16, increasing the static air pressure in outer chamber 18,
which in turn causes a bulk velocity at exit 23 of chamber 18 due to the dif-
fusion which results from the static-pressure difference between chamber 18 and
the ambient atmosphere. An equilibrium is established between the volume of
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gas entering through gas passage 26 and that leaving exit 23, so the velocity
of the gas leaving exit 23 can be regulated by controlling the volume rate
of the gas entering outer chamber 18. With a welding-tip-to-workpiece dis-
tance of 1/2", the volume rate at which gas is supplied to inner chamber 12
is between 11 ft.3/hr. and 13 ft.3/hr. Between 15 ft.3/hr. and 20 ft.3/hr.
of gas is supplied to outer chamber 18. Taking into account the sizes of
exits 23 and 25, this gives a range of ratios of outer-shield gas velocity
to inner-shield gas velocity of 0.8 to 1.5, which is the range that produces
the best results. While the method of the present specification can be prac- -
ticed with outer-shield velocities which give velocity ratios outside this
range, significant deterioration in results can be expected outside a ratio
range of approximately 0.5 to 2.0, which translates to velocities in the
outer shield in the range of 100 ft./min. to 250 ft./~in.
With a wider separating member 22, the velocity ratios become less
critical. This, of course, alters the required velocity ratio somewhat from
that quoted above. Nevertheless, optimum results will still be found in the
range quoted. In addition, those skilled in the art will appreciate that
increased velocities are required when the tip-to-workpiece spacing increases.
The mechanism by which the method of the present invention works
has not been conclusively determined, but the following possible explanation
is offered. In a single-shield welding cup as shown in Figure 2, gas leaving
exit 25 is thought to flow in a path similar to that depicted by curved arrow
28. In the area indicated by arrow 30, air from the surrounding atmosphere
is drawn into the shield-gas flow with the result that ambient air is drawn
into plasma "bell" 32 if the shield diameter is not relatively large. In
such a situation it has been found that porosity occurs when the humidity
in the air reaching plasma bell 32 is at least at a -10C dew-point level.
;` According to the present invention, gas leaves inner chamber 18
of Figure 3 in the same manner as it leaves the single-shield welding cup
of Figure 2. As a result, there exists a tendency for a flow pattern to
result which is similar to that shown in Figure 2. However, the presence of
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an outer shield ha~ , ~ ~i~ i~ ~e ~n~ ~a~e~ ~y c~ p~esen~
invention is thought to have the effect of breaking up this pattern without
disturbine the plasma bell. Thus, since the tendency of the inner-shield
stream to draw air into the plasma bell is overcome by the outer stream,
the choice of gas for the outer stream is relatively unimportant, so even
ordinary air can be used in the outer stream. As will be appreciated by
those skilled in the art, the inner-shield gas velocity in area 34 must be
aa~usted for factors such as the area of the shield and the distance between
the welding tip and the workpiece; the precise optimum velocity relationship
for any welding setup can therefore only be achieved by ad~usting the outer-
shield velocity within a range dictated by the method of the present inven-
tion until the best results are observed.
The present invention may also be practiced by drawing air into,
rather than forcing air out of, outer chamber 18. Results have shown that
a greater velocity is required at entrance 23 when air is being drawn into,
rather than forced out of, chamber 18. Best results are obtained from the
apparatus shown when the velocity ratio is between 1.0 and 2.2 with an inert-
gas supply of between 15 and 20 ft.3/hr. Again, this supply rate may be in-
creased when tip-to-workpiece distance is increased, and the apparatus may
be operated outside this ratio range. Without increasing the spacing between
the inner and outer shields, however, a deterioration in results is encountered
outside the ra~io range o~ 0.7 to 2.8, which transl~tes to a velocity range
; o~ 150 ft./min. to 360 ft./min.
It i9 thought that the drawing of air into the welding cup works
for the same reason that forcing air out of the welding cup works. That is,
it overcomes the tendency of the inner-shield flow to cause air to be drawn
into the welding arc. In this case, as Figure 4 suggests, the circular flow
is diverted from its ordinary course by the ~low of air into the entrance 23
; corresponding to exit 23 in the previous embodiment.
The results of the present method with an inward-flowing outer
shield are not as good as those which result from an outward-flowing shield,
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but it is suggested that the inward-flowing shield may be preferred in
applications in which smoke is to be removed from the welding area.
While the invention has been described in con~unction with
speci~ic embodiments thereof, it is evident that many P~ternatives, modi-
~ications, and variations will be apparent to those skilled in the art in
light of the ~oregoing description. Accordingly, the invention is intended
to embrace all such alternatives, modi~ications, and variations as fall
within the broad scope o~ the appended claims.
What is claimed is:
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