Note: Descriptions are shown in the official language in which they were submitted.
10~
PHN 8342
The invention relates to a method of welding in
a thermally ionized gas, a MIG-arc being maintained between
a continuously-fed consumable electrode and a workpiece in a
gas plasma which is produced by a plasma arc which is main-
tained in a gas flow between a non-consumable plasma electrode
and a second electrode.
A method of this kind, known as the plasma-MIG-
welding process9 is already known from our Canadian Patent
963,098 which issued on February 18, 1975. According to this
known method, the welding current is fed via a contact tube
to the consumable electrode upstream of the nozzle, the part
of the consumable electrode which is present at the level of
the plasma electrode then being current-carrying, so that
electromagnetic interaction occurs between the gas plasma and
the current-carrying consumable electrode. This interaction
may give rise to instability of the plasma arc; which in-
stability may have two aspects, i.e.:
a) Contraction of the gas plasma around the consumable elec-
trode, so that the voltage across the plasma arc is increased.
This is a detrimental effect, because generally a less steep
part of the drooping voltage characteristic of the plasma
power supply source is used for the plasma arc and this makes
the continued maintenance of the plasma arc less reliable.
b) Climbing of the contracted gas plasma along the consumable
electrode which, in the case of positive polarity of the con-
sumable electrode (relative to the workpiece), causes
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4.3.1977
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additional and undesired heat development and may cause and
initiate the formation of a cathode (relative to the plasma
electrode) on the consumable electrode, so that the plasma
arc is not directed to the workpiece but rather to the
nearer consumable electrode. The latter is a very undesirable
- phenomenon, because the welding process is affected and
usually a di~urbance occurs in the continuity of the
process.
According to the known method it has been
possible to maintain the stability of the plasma arc in the
case of negative polarity, notably if the cathode formation
on the consumable electrode is stimulated by the addition
.
of some oxygen or C02 to the plasma gas, but stability is
not obtained if the potentials of the contact tube and
the plasma electrode are not adapted to each other within
comparatively narrow limits. Complete or partial current
transfer from the plasma electrode to the consumable
electrode instead of to the workpiece again readily occurs,
which is undesirable and which usually leads to disturb-
ances because the plasma arc, driven by electromagnetic
forces, travels further upwards between the plasma electrode
and the contact tube, or even so far that these parts are
destroyed so that leakage usually occurs.
According to the known method, in which
current is fed to the consumable elec~rode via a contact
tube, this tube is usually situated above the nozzle outside
the range of action of the arc plasma and usually upstream
of the end of the plasma electrode. A problem then occurs
in that the extension of the consumable electrode, i.e.
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PHN ~342
4.3.1977
~ 6~49
the current-carrying portion thereof between the contact
tube and its free end, is comparatively large, which is
detrimental in certain cases.
The invention has for its object at least to
mitigate these drawbacks, to improve the plasma-MIG-welding
; proce~s, and to enlarge its range of applications.
This object is achieved in accordance with
the invention in that the welding current is fed to the
consumable electrode downstream of the plasma electrode.
Surprisingly, it has been found that the
fact that the current feed to the consumable electrode
takes place in the gas plasma itself has not detrimental
effect on the welding process. On the contrary, in the
case o~ positive as well as in the case of negative
polarity of the plasma electrode and of the consumable
electrode, the advantages obtained are evident. When the
step in accordance with the invention is taken, the part
of the consumable electrode at the level of the plasma
electrode is not current-carrying, so instability of the
plasma arc due to magnetic interaction between the gas -
plasma and the consumable electrode is prevented. Conse-
quently, the plasma electrode as well as the guide for
the consumable electrode can be arranged nearer to the
nozzle and the extension, .e. the current-carrying portion
of the consumable electrode can be reduced.
- A further advantage is very evident in the
case of welding with negative polarity of the plasma
electrode and the consumable electrode. Even if a dis-
charge should occur between the plasma electrode and the
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P~IN 8342
4.3.1977
lQ~69~9
consumable electrodeS this discharge would not tend to travel
upwards. On the contrary, the discharge will rather travel
in the direction of the workpiece. A partial or complete
discharge between the plasma electrode and the consumable
electrode then even occurs in a stable manner should the
welding circumstances change, accidentally or intentionally,
so that the plasma arc between the plasma electrode and t
the workpiece cannot be maintained or only with great
difficulty, whilst the discharge between consumable
electrode and workpiece is maintained. This may occur, for
example, if the working distance between welding torch and
. workpiece is iricreased beyond given limits. When the
normal working distance is restored again, the plasma arc
will be spontaneously struck again towards the workpiece.
This is a major advantage which is of essential importance
if the welding torch is held by hand. This is possible
because the anode formation of the plasma arc on the
consumable electrode or on the workpiece is very smooth;
when use is made of the method in accordance with the
invention, full advantage can be taken of this aspect,
because the discharge between plasma electrode and consumable
electrode is then stable. A major advantage consists in
the fact that the plasma arc cannot be extinguished during
welding by a disturbance of some kind as may occasionally
occur when use is made of the known method. Moreover,
it has been found that when use is made of the method in F`
accordance with the invention, it is no longer necessary
to adjust the consumable electrode and plasma electrode
potentials exactly to each ~ther in order to prevent
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PHN ~342
4.3.1977
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disturbances or to obtain a satisfactory welding process.
The range within which satisfactory transfer of material
takes place is much larger. This is probably due to the
shorter extension of the consumable electrode, so that a
voltage gradient across the consumable electrode occurs
to a iesser degree.
- In the case of positive polarity of plasma
electrode and consumable electrode, the short extension of
the consumable electrode may be of importance, because
this inter alia enables further penetration of the workpiece
~ than in the case of a large extension of the consumable
: electrode. The rotation of the MIG-arc which occurs when
use is made of the known method then takes place at a
higher transition current intensity of the welding current
~, 15 , so enabling larger welding currents below this transition
current intensity for the same deposited quantity of
electrode material, as a result of the reduced resistance
heating of the consumable electrode and reduced heating in
the gas plasma. Such a stabilization of the plasma arc as
achieved with negative polarity is not achieved with positive
polarity, but it is often possible to select a more favourable
position of the plasma electrode relative to the plasma
aperture, because a smaller distance between the plasma
electrode and the consumable electrode does not give rise
to disturbances.
, According to a preferred method in accordance
with the invention, the welding current is fed to the
consumable electrode via a contact area which is situated
in a thermal]y ionized gas atmosphere. Such an "open"
PHN 8342
4.3.1977
o
( ~aq~s
contact area which is flushed by a gas plasma has more
favourable current feeding properties than a closed contact
area as in the case of a contact tube or a contact ring.
According to a preferred method, where the
gas flow is guided through the plasma aperture of a nozzle,
the consumable electrode is brought into contact with
the nozzle, the welding current being supplied to the
consumable electrode via the nozzle.
The invention also relates to a welding torch
for performing the method, this welding torch comprises a
housing with a non-consumable plasma electrode, a wire guide
and a connection for the supply of a plasma gas; characteriz~
by a contact member having a contact face which is arranged
; downstream of the plasma electrode, at least part ~f the
contact face being substantially in alignment with the centre
line of the wire guide.
For the contact member use can be made of any
convenient element, provided it is suitably cooled so that
it can withstand the combination of heat development by the
current transition and by heat transfer from the gas plasma.
A preferred embodiment of the welding torch
in accordance with the invention is characterized by a
nozzle with a plasma aperture, said nozzle acting as the
contact member, the wire guide being eccentrically arranged
relative to the plasma aperture, and the centre line of the
wire guide being substantially in alignment with a point
on the circumference of the plasma aperture. The nozzle is
very suitable for use as contact member, because it is
usually cooled very well. The plasma aperture of this
PllN ~342
4.3.1~7
949
embodiment may have a circular or oval shape.
It has been found that the asymmetrical
- arrangement of the wire guide relative to the nozzle does
not affect the welding process. Evidently, the electro- ¦
magnetic field of the current-carrying portion of the
welding wire outside the nozzle symmetrically distributes
the gas plasma around this extending portion.
A further advantage of the use of the nozzle
as the contact member consists in that the supply (f the
welding current to the contact area on the nozzle is
effected via the wall of the housing of the torch. As a
result, the supply of current to the welding wire will cause
no, or on]y a weak, magnetic field to prevail in the
interior of the housing where the plasma electrode is situated,
so that the effect of the current through the welding wire
on the part of the plasma arc which is present in the torch
is minimized.
A further embodiment of the welding torch in
accordance with the invention is characterized by a nozzle
with a plasma aperture, said nozzle acting as the plasma
electrode, the contact member being arranged downstream of
the nozzle. As a result, the contact member will be situated
comparatively near to the workpiece, so that the current-
carrying part of the welding wire, and hence its extension,
is extrernely short.
The invention also relates to apparatus for
performing the method in accordance with the invention, ~hich
comprises a welding torch, having a non-consumable plasma
electrode, a wire guide, and means for the supply of a
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4.3.1~77
~q~g49
plasma gas; and a power supply source whereto the plasma
electrode is connected; characterized by a contact member
having a contact surface arranged downstream of the plasma
electrode and connected to a second power supply source,
at least part of said contact surface being substantially in
alignment with the centre line of the wire guide.
In a preferred embodiment of the apparatus in
accordance with the invention, in which the torch has a
nozzle with a plasma aperture, the nozzle is connected to
the second power supply source, the centre line of the wire
guide being substantially in alignment with a point on the
periphery of the plasma aperture. As a result of the use of
the nozzle as the contact member, separate contact means
can be dispensed with. This apparatus is suitable for welding
with a transferred as well as with a non-transferred plasma
arc. The transferred plasma arc is maintained between the
plasma electrode and a workpiece which acts as a second
electrode. The non-transferred plasma arc is maintained
between the plasma electrode and a second electrode other
than the workpiece; for example the nozzle, an annular
electrode arranged upstream or downstream of the nozzle, or
a separate rod electrode.
In a further preferred embodiment of the
apparatus in accordance with the invention, the two power
supply sources are connected in series, the nozzle also
being connected to -the first power supply source whilst a
switch is connected to the second power supply source. As
a result of this step, the welding process can be started in
a simple manner by first igniting a non-transferred plasma arc,~
_ g _
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p~ \T ~ 3ll2
4.3.1977
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i9~
.
followed by welding, after the striking of the MIG-arc, with
a transferred plasma arc.
A further preferred embodiment provides a
simpler and cheaper apparatus in that the first power supply
source is formed by a transformer, the nozzle being provided
with an auxiliary electrode.
Yet a further preferred embodiment of the
apparatus in accordance with the invention, in which the
~ torch has a nozzle with a plasma aperture, is characterized
m 10 in that the contact member is arranged downstream of the
nozzle. Welding with an extremely short extension of the
welding wire is possible when the contact member is arranged
`~ between the nozzle and the workpiece.
A further preferred embodirnent provides a very
compact and simple device, in that the nozzle is connected
to the first power supply source and thus acts as non-
consumable plasma electrode. A separate plasma electrode can
thus be dispensed with.
Embodiments of the invention will now be
described in detail with reference to the accompanying
drawings, of which
Figure 1 shows a practical embodiment of
apparatus in accordance with the invention;
Figures 2 to 5 diagrammatically show further
embodiments of apparatus for performing the method in
accordance with the invention. Corresponding elements are
denoted by corresponding references in the Figures.
Figure 1 shows welding apparatus 1 comprising
a welding torch 3 having a non-consuma~le plasma electrode
.5 and a wire guide 15 arranged one on each side of the
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PHN 8342
i~.3.1977
., .
lQ9~ 9
centre line T of a housing 21 of the welding torch. The
housing 21 comprises a nozzle 11 with a plasma aper-ture 13
and connections 23 for the supply of an inert gas flow A.
A shielding gas S can be supplied via connections 27 in
a shield 25. The plasma elec~rode 5, being made of tungsten
, in the present example, is mounted on a copper holder 29
which is cooled via cooling water connections 31 and 33
and cooling ducts (not shown). The nozz~e 11 and the housing
21 are provided with cooling ducts (not shown) in a
conventional manner. The electrode holder 29 is connected,
, by means of a connection termina] 35, via an H~-generator 7
to one of the two poles of a first power supply source'9,
the other pole of which is connected to a workpiece W.
By means of a connection terminal 3l, the nozzle 11 is
connected to one of thepoles of a second power supply source
191 the other pole of which is connected to the workpiece W.
The holder 29 is insulated relative to the housing 21 by
means of an insert 39 of a synthetic material. The transport
of a welding wire 17 to be deposited is effected by means of
rollers 41 which are driven at a controllable speed by a
motor 43. The wire guide 15 is arranged relative to the
nozzle 11 so that the centre line Y of the wire guide is
substantially in alignment with a point on the inner
circumference 45 of the nozzle 11 which bounds the plasma
aperture 13, so that the welding wire 17 contacts the inner
circumference 45 of the nozzle,which serves as a contact
member for feeding the welding current to the welding wire.
Welding can be effected with either positive or negative
polarity of the welding wire, the nozzle, and the p]asma
electrode, as long as their polarity is the same.
A transferred plasma arc P between the plasma
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PHN 83~2
4.3.1977
electrode 5 and the workpiece W is ignited by means of a
high frequency discharge and is maintained by the power
supply source 9. Subsequently, the welding wire 17 can be
supplied, after the striking of a MIG-arc M between the
welding wire and the workpiece; the welding wire being
maintained at a suitable potential by the power supply
source 19.
Positive polarlty has an advantage over
negative polaFity in that the welding process is smoother
in the high current range, for example, above 225A. An
advantage of negative polarity over positive polarity
consists in that the stability of the plasma is ensured
~j as long as the electrical discharge from welding wire to
workpiece takes place. Another advantage in certain cases
consists in that the material transfer disturbs the molten
pool less and in that the root of the combined arc on the
molten pool is more regularly distributed and hence has
a less disturbing effect on the surface of the weld.
~or both polarities, welding with a non-
transferred plasma arc betwecn the plasma electrode 5 and
the nozzle 11 is alternatively possible. For this it is
sufficient to connect the power supply source 9 for the
plasma arc, by means of a connection terminal 47, to the
nozzle 11 instead of to the connection on the workpiece W,
so that the two power supply sources 9 and 19 are then
connected in series. It may be advantageous for the plasma
arc to be completely independent of the workpiece. In the
latter embodiment with a non-transferred plasma arc
between the plasma electrode and the nozzle, the simple
PH~ 834-2
4.3.1977
.
l~ g
introduction of a switch 49 between the workpiece W and the
second power supply source 19 for the MIG-arc M offers
substantial advantages. When the switch 4~ is open, a plasma
arc which is not transferred to the workpiece W is struck
between the plasma electrode 5 and the nozzle 11. When the
feeding of the welding wire 17 star-ts, the switch 49 is
closed in order to establish the conneetrn workpiece-current
source-nozzle-welding wire, and to bring the welding wire
at the suitable potential relative to the workpiece. As a
result, the plasma electrode is then also at a higher
potential relative to the workpiece. Consequently, the
. plasma arc starting from the plasma electrode 5 jumps from
the nozzle 11 to the workpiece W, and the disc~rging from
the welding wire 17 to the workpiece W commences immediately.
When the wire feed stops and the switch 49 is interrupted,
the plasma arc automatically returns to the non-transferred
form between the plasma elec~r~de 5 and the nozzle 11, The
device is then ready again for renewed starting of the
welding process, so that ignition of the plasma by means of
a high frequency discharge is no longer required.
As is shown in Figure 2, the nozzle of the
described embodiment with a non-transferred plasma arc between
the plasma electrode 5 and the nozzle 11, via the connection
terminal ~7, can be replaced in a manner kno~n ~ se by
a second plasma electrode 51 in the housing 21, the non-
transferred plasma arc then being maintained between the
two plasma electrodes 5 and 51. If both plasma electrodes 5
and 51 are made of tungsten, the power supply source 9 may
consist of a transformer. If a third power supply source 53 is
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PH~T 83l-l2
4.3.1~77
lQ~ 9
connected to one of the two plaslna electrodes and to the
workpiece W via a switch 55, the stability of the plasma
arc between the plasma electrode 5 and the workpiece W is
ensured by the simultaneous presence of the plasma arc
between the two plasma electrodes 5 and 51 in the housing
21 which is not transferred to the workpiece. This is
because, should the plasma arc to the workpiece extinguish
. due to a disturbance during welding, it would immediately
be ignited again by the flow of thermally ionized gas
produced by the plasma arc between the two plasma electrodes,
whlch is independent of the workpiece.
Normally, welding is effected with direct
current for the welding wire as well as for the plasma arc.
. The embodiment which is denoted in Figure 1 by broken.lines
and which comprises the two series-connected power supply
- sources 9 and 19 and the switch 49, can be made simpler
and cheaper by replacement of the rectifier of the first
power supply source 9 by a ch~per transformer 57 (Figure 3);
an auxiliary electrode 59 of tungsten then being arranged
on the inner side of the nozzle 11. By mea.ns of a brief high-
frequency discharge, a non-transferred alternating current
arc is ignited between the plasma electrode 5 and the
auxili.ary electrode 59. Because two tungsten electrodes
. are involved, the alternating current arc can be maintained
by the transformer 57 without high-frequency discharge.
The argon gas flow introduced into the housing 21 blows
the arc plasma through the nozzle 11 to the workpiece W.
The device is then ready for the starting of the plasma-MIG
process without further HF~discharge. To this end, the
feeding of the welding wire 17 is started, the said wire
touching the nozzle, and the switch 49 is closed at the
same time. The welding wire 17 is then connected, via the
P~N 83li2
4.3.1g77
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nozzle 11, to the positive pole of the direct current
source 19, the negative pole of which is connected to the
workpiece W. It has been found -that upon closing of the
switch 49 a transferred plasma arc is formed between the
e1ectrode 5 and the workpiece W and that the discharging
of the welding wire 17 therein is promptly ignited and main-
tained. Once the direct current discharging of the welding
wire 17 has been ignited, a cathode is permanently present
on the workpiece W, so that the discharging from the plasma
electrode 5 to the workpiece W is always readily ignited
during the positive period.
During the negative period of the plasma
electrode 5, the plasma arc is constantly present between
the plasma electrode and the nozzle. It is surprising that
the initial ignition of the transferred plasrna arc is so
smoothly achieved, because a cathode spot must be formed on
the workpiece. The presence of the thermally ionized gas of
the non-transferred plasma arc and the increase of the
potential of the plasma electrode during the positive period
with the voltage of the direct current source 19 evidently
suffices for this purpose, In spite of that fact that only
half the plasma flow flows to the workpi~e, so that the
heating of the weld is less than in the case of a direct
current plasma arc, the difference in the welding result is
only small. When the wire feed terminates and the switch l~g
is opened, a non-transferred alternaiing current plasma arc
remains in which starting can be readily effected again.
All these circuits have been successfully used
for performing welding tests with wire currents of between
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PIIN ~3~12
4.3.1977
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~; 50 A and 500 A and plasma currents of between 50 A and 300 A.
- Welding wires of steel, stainless steel, copper and aluminium,
with diameters of from 1.6 to 0.9 mm were deposited.
Figure 4 shows an embodiment in which a separate
contact member 61, connected to the second power supply
source 19, is arranged between the nozzle 11 and the
workpiece W. The contact member comprises a contact face 63
which is parallel to the centre line Y of the wire guide 15;
the contact member is arranged so that the contact face 63 is
substantially in alignment with the centre line ~'. The
extension L or the welding wire 17, i.e. the current-carrying
portion between the contact member and the free end of the
welding wire is extremely short in this embodiment. The
contact member 61 should be properly cooled and can be
provided with cooling ducts (not shown) for this purpose.
The embodiment shown in Figure 5 comprises
a contact member 71 consisting of a water-cooled copper
holder 75 and a tungsten portion 77 with a contact face 73.
The nozzle 11 is connected to the first power supply source
9 and ser~es as a plasma electrode; a separate plasma
electrode in the housing 21 is thus eliminated. ~ high-
frequency generator is no longer required either in this
embodiment, because the plasma arc is spontaneously ignited
by the MIG-arc which can be ignited by bringing the welding
wire into contact with the workpiece.
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