Oscillating arc welding

A welding process, such as a GMA welding process, employs a consumable electrode with the consumable electrode being oscillated back and forth over the surface of an object to which welding material is to be applied or across the gap between two pieces of metal to be welded together. At the ends of the swings of the consumable electrode an impact force is applied to the consumable electrode to interrupt its movement, thereby causing molten metal at the end of the consumable electrode from which the arc is struck to be impelled therefrom.

BACKGROUND OF THE INVENTION 
This invention relates to welding processes and apparatus that can be 
described generically as consumable electrode arc welding processes and 
apparatus. Specific examples of welding techniques that can be employed in 
practising this invention are GMA (gas metal arc) welding, submerged arc 
welding and flux cored electrode welding. 
The instant invention is particularly useful in the welding of pipelines, 
but its applications are by no means limited thereto. However, by way of 
example, the instant invention will be described herein in detail as it 
applies to pipeline welding, but this is not to be taken as limiting. 
In the welding of pipelines several different techniques using automatic 
welding equipment have been tried. Thus, the CRC process is a GMA welding 
process requiring special joint preparation including the formation of a 
chamfer on the inside edges of the pipe sections to be joined. In this 
process the root pass is accomplished internally by welding torches that 
are located inside the pipe sections. 
In the "ESSO" (trade mark) process the pipe sections to be joined are 
butted together and a saw blade that immediately precedes the welding 
torch cuts a 0.060" gap at the butt joint. A 0.040" consumable electrode 
is inserted into this gap. This electrode must be kept directly in the 
centre of the gap if consistent results are to be obtained. In practice 
this centering has been found to be so difficult to maintain that the 
"ESSO" process essentially is not used commercially. 
Although it is not used extensively in pipeline welding, the TIG (tungsten 
inert gas) welding process is used extensively in aerospace and nuclear 
welding, another application to which the instant invention may be 
applied. In the TIG process an arc is struck between a tungsten electrode 
and the base material being welded. The base material melts and a filler 
rod is fed into the pool of molten metal and itself melts. The TIG process 
is a relatively slow process (2" to 12" of weld per minute). The other 
technique commonly employed in welding pipelines is conventional welding 
using flux coated stick electrodes. Those skilled in the art are well 
aware that there are many problems inherent in stick electrode welding. 
During the GMA welding operation both large and small drops of metal melt 
off the welding electrode at random times. The occurrence of a large drop 
may interfere with the arc stability and thus a small defect may be 
created. When the arc is unstable it often causes one side of the article 
(base material) being welded to become hotter than the other, and it is 
generally accepted that poor heat distribution results in poor strength 
characteristic of the completed weldment. 
An important factor in welding is the penetration that is obtained. 
Penetration is a function of heat input, and if the arc is not stable, it 
is necessary to slow down the welding operation to achieve the required 
penetration. Because of this welders commonly slow down the travel speed 
and consequently put into the base material more heat than is required. 
Putting more less than is required into the base material is undesirable, 
however, since it degrades the metallurgical physical properties of the 
base material. 
SUMMARY OF THE INVENTION 
In accordance with this invention it has been discovered that many of the 
problems and disadvantages of prior art welding techniques can be overcome 
by oscillating or vibrating the consumable electrode in the gap between 
the sections being welded and causing the welding torch (or a component 
thereof associated with the electrode) to impact against stops at both 
limits of its travel, thereby causing molten metal from the consumable 
electrode to be impelled against the sidewalls defining the gap. 
It should be understood, however, that even though the following detailed 
description is in respect of a process where a weld is made in a gap, the 
invention is not so restricted. The process could be used, for example, in 
hard surfacing where the scraper blade of a bulldozer or the teeth of a 
power shovel, for example, have welding material deposited on the surface 
of the blade or tooth that digs into the ground. 
The process also could be used for sealing the fuel rod to the tube sheet 
of a nuclear reactor. In these cases the consumable electrode simply is 
vibrated over the surface or over the gap to be sealed and is caused to 
impact against stops at both limits of its travel. 
The process is advantageous in the aforementioned nuclear reactor 
application because the resulting weld will be spread out on the weldment 
surface and the penetration will not be excessively deep. Shallow 
penetration is desirable in this particular application because heat 
distortion and base material dilution must be avoided. 
The process is advantageous for hard surfacing, again because of the 
relatively shallow penetration that will result. Thus, when the instant 
process is employed for hard surfacing, the hard surface of the blade or 
tooth will be composed primarily of hard weld material rather than 
primarily of an alloy of the base material and welding material, as would 
be the case where hard surfacing was achieved using a conventional MIG 
welding process. 
In accordance with one aspect of this invention there is provided in a 
process for making a weld in a gap between the walls of two pieces of 
metal to be welded together wherein a consumable electrode having a free 
end is moved through a welding torch and then into said gap, while, on the 
one hand said two pieces of metal and, on the other hand, said welding 
torch are moved relative to each other, thereby to move said welding torch 
along said gap or vice versa, sufficient potential is applied between said 
two pieces of metal and said consumable electrode to strike an arc 
therebetween which melts said consumable electrode and at least a part of 
said walls of said two pieces of metal, and said consumable electrode is 
oscillated back and forth across said gap, the improvement wherein the 
frequency of oscillation of said consumable electrode is at least 6 Hz, 
during the making of the weld said free end of said consumable electrode 
is maintained in said gap and at the ends of each half cycle of 
oscillation of said consumable electrode an impact force is applied to 
said consumable electrode to interrupt its movement towards the one wall 
of said metal pieces towards which said consumable electrode was moving 
before application of said impact force and immediately after the 
application of said impact force said consumable electrode is moved away 
from said one wall and towards the opposite wall of said metal pieces, 
thereby causing molten metal at said free end of said consumable electrode 
to be impelled against said one wall. 
In accordance with another aspect of this invention there is provided a 
welding torch including a housing, means for moving a consumable electrode 
through said housing along a predetermined path, meams for oscillating 
said consumable electrode back and forth across said predetermined path, 
and means for applying an impact force to said consumable electrode at the 
ends of each half cycle of oscillation of said consumable electrode to 
interrupt its movement, said welding torch including a pivotably mounted 
guide tube located within said housing, said consumable electrode passing 
through and contacting said guide tube, said means for moving said 
consumable electrode across said predetermined path comprising means for 
pivotably moving said guide tube, said means for pivotably moving said 
guide tube including electromagnets located on opposite sides of said 
guide tube, means associated with said guide tube adapted to be 
magnetically attracted to said electromagnetics when energized, and means 
for alternately energizing said electromagnets. 
The instant invention is not to be confused with the known technique, in 
manual or GMA welding, of slowly moving the stick electrode back and forth 
in the gap so as to fill the gap. The frequency of this operation is much 
lower than the frequency desired in the practice of the instant invention, 
and no impacting is involved. 
The aforementioned manual movement of the stick electrode has been 
automated, as is evident, for example, from U.S. Pat. No. 1,667,585 issued 
Apr. 24, 1928, V. J. Chapman, but the automated versions, like the manual 
version, are at low frequency and are devoid of impacting. In all cases 
the back and forth movement achieved in the prior art has been simply to 
fill the gap, whereas in the instant invention vibration and impacting are 
employed to achieve a unique weld with deposition of metal on the joint 
sidewalls and coalescence slightly behind.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown schematically a welding torch that can 
be used in the practice of this invention. The torch includes a rotatable 
reel 10 of electrode wire 11 that is fed via an electric drive motor 12 
through the torch and which emerges through the contact tip 13 of the 
torch. As is conventional MIG welding, although not shown in Figure, 
provision is made for supplying an inert gas mixture to the torch, the gas 
mixture flowing lout of the gas cup of the torch around electrode wire 11 
and serving to enhance the arc and prevent oxidation from taking place in 
the weld area. Means (not shown) are provided for controlling the speed of 
motor 12 so as to vary the speed of the electrode wire emerging from 
contact tip 13. 
The welding torch as just described is essentially conventional in nature. 
It is modified, however, by arranging the electrically conductive tube 14 
which guides electrode wire 11 between motor 12 and tip 13 so that it 
pivots about a horizontal axis rather than being stationary. The pivot 
pins for tube 14 are shown at 15 (FIG. 2). 
Mounted on a bracket 16 fastened to tube 14 are two fingers 17 made of 
iron. Located to one side of each finger 17 are electromagnets 18 that are 
supplied with essentially square wave pulses of voltage from a suitable 
power source (not shown), the output signal frequency of which is 
variable. When energized, electromagnets 18 attract the adjacent fingers 
17 causing tube 14 and electrode wire 11 to vibrate back and forth, as 
shown by the double-headed arrows, at a frequency determined by the 
frequency of the output signal of the power source. 
Fingers 17 are arranged so that they impact against their associated 
electromagnets, this impacting having been found to be important in 
achieving the desired results. 
If desired, motor 12 may be secured to fingers 17 and also may oscillate 
providing greater momentum and a more forceful impact. 
Referring now to FIG. 4, there are shown two sections 19 and 20 to be 
welded together. These two sections may be considered to be parts of two 
pipe sections. The sidewalls 21 and 22 of the two pipe sections each are 
bevelled at 6.degree. to the vertical and are separated from each other by 
0.125" at the point where they are spaced closest together. The 
aforementioned angles and spacings are not critical but have been found to 
provide good results where the sections to be joined are 1/2" thick. As 
opposed to the "ESSO" process, it is significant that in practising the 
instant invention minimum spacings between the two sections shown in FIG. 
4 have been varied from 0.060" to 0.160" with quite acceptable welds being 
obtained, provided that the amplitude of vibration of electrode wire 11 is 
appropriately adjusted, of course. In the interest of uniformity it is 
desired that the spacing between the two sections be the same at all 
points around the pipe sections. It is a feature of the instant invention, 
however, that considerable variations in the spacing can be tolerated, 
unlike the "ESSO" process. 
While a joint prepared as shown in FIG. 4 may be used in the practice of 
this invention, the preferred joint is shown in FIG. 9. In this joint 
sidewalls 21a and 22a (1/2" in thickness) are inclined at 7.degree. to be 
vertical, projections 24 each are 0.030" deep and extend outwardly from 
their associated sidewalls by 0.030" and the spacing between the facing 
ends of projections 24 is 0.050". However, successful welds have been 
accomplished with spacings as small as 0.030" using electrode wire of 0.9 
mm. and with spacings as high as 0.125". With larger spacings "suck back" 
may occur, however. If the spacing is increased or decreased, the 
amplitude of the oscillations and the wire speed likewise will have to be 
increased and decreased respectively. 
In practising the instant invention the torch either is mounted on 
equipment that moves the torch around the pipe sections, or the torch is 
held stationary and the pipe is rotated. In the embodiment of the 
invention shown in FIG. 2, a pipe band 30 that constitutes the track for a 
carriage 31 or "bug" is clamped to one of the pipe sections a uniform 
distance away from the gap 23 between the pipe sections. The two pipe 
sections 19 and 20 are held in end-to-end alignment and in closely spaced 
relationship by means of an internal clamp (not shown) that is of 
conventional design and pneumatically operated. 
Carriage 31 supports reel 10 and the welding torch, the latter being 
pivotably mounted on a horizontal shaft 32 to which it is capable of being 
fastened. 
Carriage 31 has wheels 33 that roll over track 30 and that are driven by an 
electric motor 34 via chains (not shown) and sprockets (not shown). 
Carriage 31 also has four legs 35 provided with rollers that engage the 
top, bottom and sides of pipe band 30 to hold carriage 31 on the pipe 
band. Carriages and pipe bands of the type shown are known in the art. 
It will be noted that adjusting bolt 35 permits the angle of the torch to 
be varied. It has been found that the longitudinal axis of the torch 
either should be on a radius of the pipe sections, or the contact tip 13 
should lead about 2.degree.-6.degree. in direction of travel of the torch. 
A rearward inclination tends to promote blow through. 
The electrode wire 11 is inserted into the gap 23 between sidewalls 21 and 
22, power is supplied via tube 14 to the electrode wire 11 from a 
conventional source causing an arc to be struck, the electrode wire 11 is 
fed at a controlled rate by motor 12 and the torch is moved relative to 
the pipe sections (or vice versa) along gap 23. While this is taking 
place, energization of electromagnets 18 causes electrode wire 11 to 
vibrate back and forth across gap 23. The amplitude of the oscillations of 
the wire are adjusted so as to avoid having the wire strike sidewalls 21 
or 22. However, there is impact between fingers 17 and electromagnets 18. 
The effect of this has been observed using high speed photography which 
shows that the molten metal at the end of the electrode wire 11 is thrown 
off the wire and impelled against the sidewalls 21 and 22. At the same 
time a pool of molten metal is formed between the two sections behind the 
arc in the lower part of gap 23. The resulting weld has the configuration 
shown in FIG. 5, whereas prior art techniques tend to produce a root pass 
weld having the confirguration shown in FIG. 6. The latter configuration 
is undesirable because of the doming effect. Thus, on the second or hot 
pass the arc will strike to the point of the root pass weld that is 
closest to the electrode, namely to the dome, resulting in accentuation of 
the dome and lack of filling immediately adjacent the sidewalls of the 
sections 19 and 20. The resulting lack of fusion to the sidewalls is 
called "wagon tracks" in the art and is undesirable. The result is even 
more accentuated when one is welding between 3 o'clock and 9 o'clock and 
is known as "roping". 
A weld made in accordance with the instant invention does not have the 
characteristic dome of a prior art weld. Thus "roping" and the formation 
of "wagon tracks" on the fill pass are avoided. 
On the other hand, with a root pass weld of the type produced by the 
instant invention there is a tendency for a crack to appear along the 
centre of the weld as it solidifies. This can be overcome by making the 
hot pass follow closely behind the root pass before the latter has time to 
cool and form a crack. In practice, this is achieved by mounting a second 
torch only several inches behind the torch that makes the root pass. With 
1/2" material, the hot pass will be followed by a third and final pass 
called the cap pass, the resulting weld being as shown in FIG. 7. While 
vibration may be employed on the hot and cap passes and on any fill passes 
therebetween, it has been found not to be necessary. If it is employed, it 
can be at a lower frequency than the lowest frequency that is acceptable 
for the root pass. 
As an example of the instant invention, a joint was prepared as indicated 
in FIG. 4 (6.degree. angles and 0.125" spacing) between two steel pipes 
each having a wall thickness of 1/2". For the root pass power was supplied 
to the electrode wire 11 from a Hobart (trade name) Model M400 power 
supply made by Hobart Bros. Company, Troy, Ohio, U.S.A. 230 amps were 
supplied at 23 arc volts. The wire was fed at 650" per minute and the 
linear speed of the torch was 20" per minute. Hobart HB 18 0.35" wire with 
1/2" stick-out (static set) was employed. The inert gas mixture used was 5 
cu. ft./hr. of CO.sub.2 and 35 cu. ft./hr. of argon. The wire was 
oscillated at a frequency of 19.4 Hz producing 38.8 impacts per second. 
The welding operation was observed using a high speed camera. Pictures were 
taken at 4000 frames per second. As indicated previously, the vibrating 
electrode "painted" the sidewalls of the two sections being joined with 
molten metal, and three different modes of welding could be observed, 
namely dip transfer, drop transfer and spray transfer, the latter two 
being predominant. An entirely satisfactory root pass weld of the type 
shown in FIG. 5 was obtained. 
As a matter of interest, while one would expect that the electrode would 
move simply from one side of the gap to the other, such was not the case 
in this particular situation. After the electrode did move from one side 
of the gap to the other, it rebounded then moved towards and away from 
that one side from three to four times before moving back to the other 
side where the action was repeated. The additional oscillations 
super-imposed on the main oscillation were observed to be about one third 
to one half the amplitude of the main oscillation. 
Welding of the root pass was carried out following the procedure set out in 
the preceding example, but the frequency of vibration of the electrode 
wire was varied. Under the particular conditions noted in the example, it 
was not possible to produce a satisfactory weld at a frequency of 
vibration less than 6 Hz, although this is not to say that under different 
conditions lower frequencies could not be employed. At low frequencies 
there is a tendency for the molten metal simply to fall through or be 
blown out of the gap 23. 
Welding of the root pass was carried out following the procedure set out in 
the preceding example but at higher frequencies of vibration. Entirely 
suitably welds were produced at 4 Hz. However, at about 114 Hz. it was 
found that the arc climbed up in the gap leaving a recess at the bottom of 
the weld, as shown in FIG. 8. Such a notch or recess is objectionable in a 
pipeline because it will create turbulance in the oil or gas and will 
result in undesirable physical and metallurgical properties such as stress 
corrosion. It is also unacceptable in pressure vessels because the notch 
will create a stress riser where the vessel may fail. This is not to say, 
however, that under different conditions frequencies of the magnitude 
indicated or higher could not be employed. Also in applications where the 
notching effect is not objectionable, high frequencies can be used. Higher 
frequencies might be employed, for example, in processes where the root 
pass is performed by internal welding. 
The power source used in the preceding example was one that is normally 
used in standard commercial welding processes and automatic MIG welding 
and is a high reactance power supply. Power supplies used with automatic 
welding equipment normally have a much lower reactance. Notwithstanding 
the fact that the welding operation carried out in the example was 
automatic, the use of a high reactance type power supply, at least on the 
root pass, providing a "soft" arc was found to be very desirable. On the 
fill passes, on the other hand, it was found preferable to use a 
conventional power supply for automatic welding equipment to provide 
adequate penetration into the preceding weld pass. 
The importance of impact was demonstrated by constructing and operating a 
welding gun that was oscillated by a rotating cam, the gun being spring 
biased against the cam. With such an arrangement, no impact took place, 
and at frequencies in excess of 6 Hz unsatisfactory results were obtained, 
the weld burning through. 
Under conditions set forth in the specific example noted hereinbefore the 
arc voltage was found to be important in achieving the desired results. 
Thus, it was found that if the arc voltage was reduced to 21 or below, 
there was a tendency for the molten metal to blow out of the gap 23, while 
if it was increased to 25 or above, the tendency was for the arc to climb 
in the gap 23 and for there to be a lack of penetration. This not to say, 
however, that an arc voltage of about 23 volts is essential to the instant 
invention. The arc voltage varies with the wire size, the distance to the 
workpiece and the type of inert gas mixture employed. It also varies with 
the degree of electrode stick-out (the length of wire that protrudes from 
the contact tip), so under different conditions than those described in 
the example herein, different arc voltages may give the best results. For 
example, in the submerged arc process a heavier wire would be used and a 
higher arc voltage would be anticipated. 
As far as the inert gas mixture is concerned, for the root and hot passes a 
mixture of 95% argon (by volume) and 5% CO.sub.2 preferably is employed, 
but wide variations are possible with excellent results still being 
obtained. Thus ratios of 80/20 have been successfully employed. The use of 
CO.sub.2 alone should be avoided, because this leads to the spray transfer 
mode never being achieved, with inadequate penetration and lack of fusion 
resulting. On the other hand, the use of argon alone produces too hot an 
arc and blow through can result. 
On any fill pass and on the cap pass a gas mixture that keeps down the 
spatter is desired, e.g., argon to CO.sub.2 in an 80/20 ratio. 
As another example of the instant invention, a joint between steel pipes of 
1/2" thickness was prepared as shown in FIG. 9 and as described 
hereinbefore in the description of FIG. 9, the spacing between projections 
24 being 0.050". For the root pass power was supplied to the electrode 
wire 11 from a Hobart (trade mark) Model MC500 power supply 230 amps were 
supplied at 23 arc volts. The wire, Union (trade mark) K5 Ni wire (0.9 
mm), was fed at 650" per minute, and the linear speed of the torch was 30" 
per minute. 1/2" stick-out (static set) was used. The inert gas mixture 
was 60 cu. ft./hr. of a 95/5 ratio of argon to CO.sub.2. The wire was 
oscillated at a preferred frequency of 30 Hz producing 60 impacts per 
second. An entirely satisfactory root pass weld of the type shown in FIG. 
10 was obtained. 
While a preferred embodiment of this invention has been described herein, 
those skilled in the art will appreciate that changes and modifications 
can be made therein without departing from the spirit and scope of this 
invention as defined in the appended claims.