Plasma arc welding (PAW)
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 work piece (transfer arc) or the electrode and the constricting nozzle (non-transfer 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.
Plasma Arc Welding Equipment
Plasma arc welding : 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.
Plasma arc welding torch The welding torch for plasma arc welding is similar in appearance to a gas tungsten arc torch, but more complex.
All plasma arc welding 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 non-transferred 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.
The plasma arc welding 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.
Plasma arc welding : 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 non-transferred 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.
Plasma arc welding : 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.
Plasma Arc Welding Advantages and Major Uses
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:
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.
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.
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.
Plasma Arc Welding : Uses
Some of the major uses of plasma arc welding 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.
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.
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.
This process is also used to do work similar to electron beam welding, but with a much lower equipment cost.
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).
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.
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.
Limitations of the Plasma arc welding 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.
Plasma Arc Welding : Principles of Operation.
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.
Two modes of operation are the non-transferred arc and the transferred arc.
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 nonmetals.
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.
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.
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.
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.
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.
Plasma Arc Welding : Joint design.
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.
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.
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.
Plasma Arc Welding : 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.
Tips for Using the Plasma Arc Welding Process
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.
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.
Heat input is important. Plasma gas flow also has an important effect. These factors are shown by figure 10-40.
Plasma Arc Welding : Filler Metal and Other Equipment
Filler metal is normally used when plasma arc welding, 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.
Plasma Arc Welding : Shielding gas
An inert gas, either argon, helium, or a mixture, is used for shielding the plasma 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.
Plasma Arc Welding : Quality, Deposition Rates, and Variables
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 welding process are shown by the data in table 10-5.
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.
Plasma Arc Welding : Variations of the Process
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.
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.
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.