Plasma Arc. Plasma arc welding (PAW) is a process in which
coalescence, or the joining of metals, is produced by heating with a constricted
arc between an electrode and the workpiece (transfer arc) or the electrode and
the constricting nozzle (nontransfer arc). Shielding is obtained from the hot
ionized gas issuing from the orifice, which may be supplemented by an auxiliary
source of shielding gas. Shielding gas may be an inert gas or a mixture of
gases. Pressure may or may not be used, and filler metal may or may not be
supplied. The PAW process is shown in figure 10-35.
b. Equipment.
(1) Power source. A constant current drooping characteristic power
source supplying the dc welding current is recommended; however, ac/dc type
power source can be used. It should have an open circuit voltage of 80 volts and
have a duty cycle of 60 percent. It is desirable for the power source to have a
built-in contactor and provisions for remote control current adjustment. For
welding very thin metals, it should have a minimum amperage of 2 amps. A maximum
of 300 is adequate for most plasma welding applications.
(2) Welding torch. The welding torch for plasma arc welding is similar
in appearance to a gas tungsten arc torch, but more complex.
(a) All plasma torches are water cooled, even the lowest-current range torch.
This is because the arc is contained inside a chamber in the torch where it
generates considerable heat. If water flow is interrupted briefly, the nozzle
may melt. A cross section of a plasma arc torch head is shown by figure 10-36.
During the nontransferred period, the arc will be struck between the nozzle or
tip with the orifice and the tungsten electrode. Manual plasma arc torches are
made in various sizes starting with 100 amps through 300 amperes. Automatic
torches for machine operation are also available.
(b) The torch utilizes the 2 percent thoriated tungsten electrode similar to
that used for gas tungsten welding. Since the tungsten electrode is located
inside the torch, it is almost impossible to contaminate it with base metal.
(3) Control console. A control console is required for plasma arc
welding. The plasma arc torches are designed to connect to the control console
rather than the power source. The console includes a power source for the pilot
arc, delay timing systems for transferring from the pilot arc to the transferred
arc, and water and gas valves and separate flow meters for the plasma gas and
the shielding gas. The console is usually connected to the power source and may
operate the contactor. It will also contain a high-frequency arc starting unit,
a nontransferred pilot arc power supply, torch protection circuit, and an
ammeter. The high-frequency generator is used to initiate the pilot arc. Torch
protective devices include water and plasma gas pressure switches which
interlock with the contactor.
(4) Wire feeder. A wire feeder may be used for machine or automatic
welding and must be the constant speed type. The wire feeder must have a speed
adjustment covering the range of from 10 in. per minute (254 mm per minute) to
125 in. per minute (3.18 m per minute) feed speed.
c. Advantages and Major Uses.
(1) Advantages of plasma arc welding when compared to gas tungsten arc
welding stem from the fact that PAW has a higher energy concentration. Its
higher temperature, constricted cross-sectional area, and the velocity of the
plasma jet create a higher heat content. The other advantage is based on the
stiff columnar type of arc or form of the plasma, which doesn’t flare like the
gas tungsten arc. These two factors provide the following advantages:
(a) The torch-to-work distance from the plasma arc is less critical than for
gas tungsten arc welding. This is important for manual operation, since it gives
the welder more freedom to observe and control the weld.
(b) High temperature and high heat concentration of the plasma allow for the
keyhole effect, which provides complete penetration single pass welding of many
joints. In this operation, the heat affected zone and the form of the weld are
more desirable. The heat-affected zone is smaller than with the gas tungsten
arc, and the weld tends to have more parallel sides, which reduces angular
distortion.
(c) The higher heat concentration and the plasma jet allow for higher travel
speeds. The plasma arc is more stable and is not as easily deflected to the
closest point of base metal. Greater variation in joint alignment is possible
with plasma arc welding. This is important when making root pass welds on pipe
and other one-side weld joints. Plasma welding has deeper penetration
capabilities and produces a narrower weld. This means that the depth-to-width
ratio is more advantageous.
(2) Uses.
(a) Some of the major uses of plasma arc are its application for the
manufacture of tubing. Higher production rates based on faster travel speeds
result from plasma over gas tungsten arc welding. Tubing made of stainless
steel, titanium, and other metals is being produced with the plasma process at
higher production rates than previously with gas tungsten arc welding.
(b) Most applications of plasma arc welding are in the low-current range,
from 100 amperes or less. The plasma can be operated at extremely low currents
to allow the welding of foil thickness material.
(c) Plasma arc welding is also used for making small welds on weldments for
instrument manufacturing and other small components made of thin metal. It is
used for making butt joints of wall tubing.
(d) This process is also used to do work similar to electron beam welding,
but with a much lower equipment cost.
(3) Plasma arc welding is normally applied as a manual welding process, but
is also used in automatic and machine applications. Manual application is the
most popular. Semiautomatic methods of application are not useful. The normal
methods of applying plasma arc welding are manual (MA), machine (ME), and
automatic (AU).
(4) The plasma arc welding process is an all-position welding process. Table 10-2
shows the welding position capabilities.
(5) The plasma arc welding process is able to join practically all
commercially available metals. It may not be the best selection or the most
economical process for welding some metals. The plasma arc welding process will
join all metals that the gas tungsten arc process will weld. This is illustrated
in table
10-3.
(6) Regarding thickness ranges welded by the plasma process, the keyhole mode
of operation can be used only where the plasma jet can penetrate the joint. In
this mode, it can be used for welding material from 1/16 in. (1.6 mm) through
1/4 in. (12.0 mm). Thickness ranges vary with different metals. The melt-in mode
is used to weld material as thin as 0.002 in. (0.050 mm) up through 1/8 in. (3.2
mm). Using multipass techniques, unlimited thicknesses of metal can be welded.
Note that filler rod is used for making welds in thicker material. Refer to table 10-4 for
base metal thickness ranges.
d. Limitations of the Process. The major limitations of the process
have to do more with the equipment and apparatus. The torch is more delicate and
complex than a gas tungsten arc torch. Even the lowest rated torches must be
water cooled. The tip of the tungsten and the alignment of the orifice in the
nozzle is extremely important and must be maintained within very close limits.
The current level of the torch cannot be exceeded without damaging the tip. The
water-cooling passages in the torch are relatively small and for this reason
water filters and deionized water are recommended for the lower current or
smaller torches. The control console adds another piece of equipment to the
system. This extra equipment makes the system more expensive and may require a
higher level of maintenance.
e. Principles of Operation.
(1) The plasma arc welding process is normally compared to the gas tungsten
arc process. If an electric arc between a tungsten electrode and the work is
constricted in a cross-sectional area, its temperature increases because it
carries the same amount of current. This constricted arc is called a plasma, or
the fourth state of matter.
(2) Two modes of operation are the non-transferred arc and the transferred
arc.
(a) In the non-transferred mode, the current flow is from the electrode
inside the torch to the nozzle containing the orifice and back to the power
supply. It is used for plasma spraying or generating heat in non metals.
(b) In transferred arc mode, the current is transferred from the tungsten
electrode inside the welding torch through the orifice to the workpiece and back
to the power supply.
(c) The difference between these two modes of operation is shown by figure 10-37.
The transferred arc mode is used for welding metals. The gas tungsten arc
process is shown for comparison.
(3) The plasma is generated by constricting the electric arc passing through
the orifice of the nozzle. Hot ionized gases are also forced through this
opening. The plasma has a stiff columnar form and is parallel sided so that it
does not flare out in the same manner as the gas tungsten arc. This high
temperature arc, when directed toward the work, will melt the base metal surface
and the filler metal that is added to make the weld. In this way, the plasma
acts as an extremely high temperature heat source to form a molten weld puddle.
This is similar to the gas tungsten arc. The higher-temperature plasma, however,
causes this to happen faster, and is known as the melt-in mode of operation. Figure 10-36
shows a cross-sectional view of the plasma arc torch head.
(4) The high temperature of the plasma or constricted arc and the high
velocity plasma jet provide an increased heat transfer rate over gas tungsten
arc welding when using the same current. This results in faster welding speeds
and deeper weld penetration. This method of operation is used for welding
extremely thin material. and for welding multipass groove and welds and fillet
welds.
(5) Another method of welding with plasma is the keyhole method of welding.
The plasma jet penetrates through the workpiece and forms a hole, or keyhole.
Surface tension forces the molten base metal to flow around the keyhole to form
the weld. The keyhole method can be used only for joints where the plasma can
pass through the joint. It is used for base metals 1/16 to 1/2 in. (1.6 to 12.0
mm) in thickness. It is affected by the base metal composition and the welding
gases. The keyhole method provides for full penetration single pass welding
which may be applied either manually or automatically in all positions.
(6) Joint design.
(a) Joint design is based on the metal thicknesses and determined by the two
methods of operation. For the keyhole method, the joint design is restricted to
full-penetration types. The preferred joint design is the square groove, with no
minimum root opening. For root pass work, particularly on heavy wall pipe, the U
groove design is used. The root face should be 1/8 in. (3.2 mm) to allow for
full keyhole penetration.
(b) For the melt-in method of operation for welding thin gauge, 0.020 in.
(0.500 mm) to 0.100 in. (2.500 mm) metals, the square groove weld should be
utilized. For welding foil thickness, 0.005 in. (0.130 mm) to 0.020 in. (0.0500
mm), the edge flange joint should be used. The flanges are melted to provide
filler metal for making the weld.
(c) When using the melt-in mode of operation for thick materials, the same
general joint detail as used for shielded metal arc welding and gas tungsten arc
welding can be employed. It can be used for fillets, flange welds, all types of
groove welds, etc., and for lap joints using arc spot welds and arc seam welds.
Figure
10-38 shows various joint designs that can be welded by the plasma arc
process.
(7) Welding circuit and current. The welding circuit for plasma arc
welding is more complex than for gas tungsten arc welding. An extra component is
required as the control circuit to aid in starting and stopping the plasma arc.
The same power source is used. There are two gas systems, one to supply the
plasma gas and the second for the shielding gas. The welding circuit for plasma
arc welding is shown by figure 10-39.
Direct current of a constant current (CC) type is used. Alternating current is
used for only a few applications.
(8) Tips for Using the Process.
(a) The tungsten electrode must be precisely centered and located with
respect to the orifice in the nozzle. The pilot arc current must be kept
sufficiently low, just high enough to maintain a stable pilot arc. When welding
extremely thin materials in the foil range, the pilot arc may be all that is
necessary.
(b) When filler metal is used, it is added in the same manner as gas tungsten
arc welding. However, with the torch-to-work distance a little greater there is
more freedom for adding filler metal. Equipment must be properly adjusting so
that the shielding gas and plasma gas are in the right proportions. Proper gases
must also be used.
(c) Heat input is important. Plasma gas flow also has an important effect.
These factors are shown by figure
10-40.
e. Filler Metal and Other Equipment.
(1) Filler metal is normally used except when welding the thinnest metals.
Composition of the filler metal should match the base metal. The filler metal
rod size depends on the base metal thickness and welding current. The filler
metal is usually added to the puddle manually, but can be added automatically.
(2) Plasma and shielding gas. An inert gas, either argon, helium, or a
mixture, is used for shielding the arc area from the atmosphere. Argon is more
common because it is heavier and provides better shielding at lower flow rates.
For flat and vertical welding, a shielding gas flow of 15 to 30 cu ft per hour
(7 to 14 liters per minute) is sufficient. Overhead position welding requires a
slightly higher flow rate. Argon is used for plasma gas at the flew rate of 1 cu
ft per hour (0.5 liters per minute) up to 5 cu ft per hour (2.4 liters per
minute) for welding, depending on torch size and application. Active gases are
not recommended for plasma gas. In addition, cooling water is required.
f. Quality, Deposition Rates, and Variables.
(1) The quality of the plasma arc welds is extremely high and usually higher
than gas tungsten arc welds because there is little or no possibility of
tungsten inclusions in the weld. Deposition rates for plasma arc welding are
somewhat higher than for gas tungsten arc welding and are shown by the curve in
figure
10-41. Weld schedules for the plasma arc process are shown by the data in table
10-5.
(2) The process variables for plasma arc welding are shown by figure 10-41.
Most of the variables shown for plasma arc are similar to the other arc welding
processes. There are two exceptions: the plasma gas flow and the orifice
diameter in the nozzle. The major variables exert considerable control in the
process. The minor variables are generally fixed at optimum conditions for the
given application. All variables should appear in the welding procedure.
Variables such as the angle and setback of the electrode and electrode type are
considered fixed for the application. The plasma arc process does respond
differently to these variables than does the gas tungsten arc process. The
standoff, or torch-to-work distance, is less sensitive with plasma but the torch
angle when welding parts of unequal thicknesses is more important than with gas
tungsten arc.
g. Variations of the Process.
(1) The welding current may be pulsed to gain the same advantages pulsing
provides for gas tungsten arc welding. A high current pulse is used for maximum
penetration but is not on full time to allow for metal solidification. This
gives a more easily controlled puddle for out-of-position work. Pulsing can be
accomplished by the same apparatus as is used for gas tungsten arc welding.
(2) Programmed welding can also be employed for plasma arc welding in the
same manner as it is used for gas tungsten arc welding. The same power source
with programming abilities is used and offers advantages for certain types of
work. The complexity of the programming depends on the needs of the specific
application. In addition to programming the welding current, it is often
necessary to program the plasma gas flow. This is particularly important when
closing a keyhole which is required to make the root pass of a weld joining two
pieces of pipe.
(3) The method of feeding the filler wire with plasma is essentially the same
as for gas tungsten arc welding. The "hot wire" concept can be used. This means
that low-voltage current is applied to the filler wire to preheat it prior to
going into the weld puddle.