{"id":184,"date":"2022-04-16T12:27:00","date_gmt":"2022-04-16T12:27:00","guid":{"rendered":"https:\/\/mewelding.com\/?p=184"},"modified":"2022-04-17T11:22:08","modified_gmt":"2022-04-17T11:22:08","slug":"plasma-arc-welding-paw","status":"publish","type":"post","link":"https:\/\/mewelding.com\/plasma-arc-welding-paw\/","title":{"rendered":"Plasma Arc Welding (PAW)"},"content":{"rendered":"\n
PAW, short for Plasma Arc Welding is a type of metal joining process in which coalescence is obtained by heating the work-piece with a constricted arc. The constricted arc can be struck in two ways: one, between the tungsten electrode and the work-piece. This is called a transfer arc. Two, the arc is struck between the electrode and the constricting nozzle. This is called a non-transfer arc.<\/p>\n\n\n\n
A schematic of the process is shown in figure 1 below. The shielding is provided by a stream of hot ionized gas that flows from the annular gap between the tungsten electrode and the nozzle. This ionized gas may be supplemented by another stream of gas issuing from the annular gap between the nozzle and the outer casing of the welding gun.<\/p>\n\n\n\n
Pressure may or may not be applied between the parts being joined. A filler rod may or may not be used in the process. For welding metals of low thickness, welding can be done without a filler. For higher section thicknesses, use of a filler is imperative.<\/p>\n\n\n\n Following paragraphs discuss a little about power source, welding torch, control console and wire feeder used in plasma arc welding.<\/p>\n\n\n\n A power source of constant current type is generally used. The C-V characteristics are of drooping type. Direct current is generally used, although AC can be used as well. The circuit has an OCV (open circuit voltage) of 80 volts. The equipment comes with a rating of 60 % duty cycle.<\/p>\n\n\n\n The amperage range on most PAW equipment comes between 2 amperes to 300 amperes. The equipment comes with an inbuilt welding contactor, and a remote control for adjusting the parameters.<\/p>\n\n\n\n The PAW welding torch appears similar in construction to a GTAW torch. However, the two are quite different. The PAW carries more sophistication in its\u2019 construction.<\/p>\n\n\n\n The first notable difference is that all PAW torches are water cooled. This is necessary because, unlike GTAW, the plasma in PAW gets generated inside the body of the nozzle itself. The temperature of the plasma is substantially high, enough to melt most metals. The nozzles therefore must be cooled continuously by a constant flow of cooling water circulating through the body of the nozzle.<\/p>\n\n\n\n If a constant circulation of cooling water is not maintained, the plasma heat is enough to melt the nozzle.<\/p>\n\n\n\n A cut-section of a PAW nozzle is shown in figure 2 below. PAW torches come in various ratings, from 100 amps to 300 amps. The torches for both manual operation and automatic operation are available.<\/p>\n\n\n\n The tungsten electrode used in PAW is 2% thoriated tungsten, similar to the one used in GTAW process. In GTAW process, the tungsten electrode pokes out from the nozzle, while in PAW \u2013 it is housed completely inside the nozzle. <\/p>\n\n\n\n Due to this reason, tungsten inclusion is not a problem in case of plasma arc welding, unlike GTAW. <\/p>\n\n\n\n In GTAW process, a piece of tungsten electrode can dissociate and get mixed with the molten metal, thus contaminating it. A tungsten inclusion is a hard element inside the weld, and is not desirable.<\/p>\n\n\n\n The PAW torch is generally connected to the control console, instead of directly connecting to the power source. The control console contains a high frequency generator for initiating the pilot arc. It also contains separate flowmeters for measuring the flow rates of shielding gas and plasma gas.<\/p>\n\n\n\n The control console is also equipped with delay timing systems for transferring the pilot non-transferred arc to the main transferred arc, besides water valves and gas valves. Two pressure switches that control the pressure of plasma gas and cooling-water are also fitted on the console. These two switches act as protective devices for the welding torch.<\/p>\n\n\n\n In manual PAW welding, the filler rod is manually fed. In machine welding or automatic welding, a wire-feed mechanism is provided on the PAW equipment. This mechanism feeds filler wire at a constant rate. Controls are provided on the equipment to adjust this speed from 10 in\/min to 125 in\/min (250 mm\/min to 3180 mm\/min).<\/p>\n\n\n\n The plasma in PAW has significantly higher concentration of heat than the GTAW arc. The temperature of the plasma is significantly higher. The stream of plasma is delivered through a constricted cross section, and at a higher speed than the GTAW arc.<\/p>\n\n\n\n The plasma stream is a powerful thin column of concentrated heat, in comparison to the conical arc emanating from the tungsten tip.<\/p>\n\n\n\n The above two factors result in two advantages for PAW over GTAW:<\/p>\n\n\n\n The nozzle to work distance is not a critical factor for PAW, like it is for GTAW. This allows the welder to observe the weld more clearly and control it.<\/p>\n\n\n\n The high concentration of heat in the plasma stream produces a keyhole effect. This effect results in a satisfactory fusion between two mating parts in a single pass. The joint can be made with minimal edge preparation, and weld can be completed quite fast.<\/p>\n\n\n\n The HAZ created in the base metal in keyhole welding has a narrower width in comparison to GTAW weld. Since the weld concludes in a single pass, the amount of total heat input to the weld is less. This results in less distortion and warpage<\/a>. <\/p>\n\n\n\n The rate of linear travel along the weld is much higher in plasma arc welding than in gas tungsten arc welding. The depth of penetration is higher in PAW. The depth-to-width ratio is better in PAW than in GTAW.<\/p>\n\n\n\n A major application of plasma arc welding is in tubing weld joints. Owing to the higher travel speed of plasma than the GTAW arc, these joints in materials such as SS, titanium, etc. can be produced at a faster rate than GTAW process.<\/p>\n\n\n\n Another interesting feature of PAW is that it can operate at really low amperages as well. In GTA and GMA welds, welding with amperages lower than 50 amps is not practical. However, very low currents in the range of 10-15 amps can be used in PAW for welding foil thickness materials. In fact, most applications of PAW utilize an amperage of less than 100 A.<\/p>\n\n\n\n In small instruments in which welding of high precision is required, involving base metals of low thickness, PAW yields high quality welds.<\/p>\n\n\n\n PAW does similar function as EBW (electron beam welding), at a far lesser cost of equipment. The operating cost of PAW is also lower than that of electron beam welding.<\/p>\n\n\n\n Plasma arc welding is most commonly used in manual operation. However, less commonly – it is used for automatic welding as well as machine welding too.<\/p>\n\n\n\n PAW can be used to perform welding in almost all common welding positions, viz. flat, horizontal, vertical, and overhead.<\/p>\n\n\n\n Almost all commercially used metals can be welded with plasma arc welding, although it is not the best choice of welding process for some grades of base metals. The following table indicates the weldability of commonly used grades of base metals found in the industry.<\/p>\n\n\n\n The keyhole mode of operation is useful only for limited thicknesses. Because, despite its\u2019 high energy \u2013 the plasma stream cannot achieve complete penetration in thicknesses higher than \u00bd inch (12 mm). So, keyhole penetration technique is generally used in base metal thickness range of 1\/16 inch to \u00bd inch (1.6 mm to 12 mm).<\/p>\n\n\n\n The penetration depends on the hardness of the metal being welded. For hard metals, keyhole mode can be operated only up to a limited thickness.<\/p>\n\n\n\n The ordinary single pass melt-in mode of plasma arc welding can be used to weld thicknesses as low as 0.002 in (0.050 mm) to up to 1\/8 in (3.2 mm). With multipass welding<\/a>, base metal of any higher thickness can be welded. <\/p>\n\n\n\n Of course, high thickness metals would require use of a filler rod. The table below lists thickness ranges of base metals that can be welded with the above-discussed three modes of operation in PAW.<\/p>\n\n\n\nPlasma Arc Welding Equipment<\/strong><\/h2>\n\n\n\n
Power Source<\/strong><\/h3>\n\n\n\n
Welding Torch<\/strong><\/h3>\n\n\n\n
Control Console<\/strong><\/h3>\n\n\n\n
Wire Feeder<\/strong><\/h3>\n\n\n\n
Plasma Arc Welding Advantages<\/strong><\/h2>\n\n\n\n
Applications Of Plasma Arc Welding<\/strong><\/h2>\n\n\n\n
What Base Metals Can Be Welded With PAW?<\/strong><\/h2>\n\n\n\n
Base Metal<\/strong><\/td> Can be welded with Plasma Arc Welding?<\/strong><\/td><\/tr> Aluminum<\/td> Weldable<\/td><\/tr> Bronze<\/td> Possible but not popular<\/td><\/tr> Copper<\/td> Weldable<\/td><\/tr> Wrought iron<\/td> Possible but not popular<\/td><\/tr> Lead<\/td> Possible but not popular<\/td><\/tr> Magnesium<\/td> Possible but not popular<\/td><\/tr> Inconel<\/td> Weldable<\/td><\/tr> Nickel<\/td> Weldable<\/td><\/tr> Monel<\/td> Weldable<\/td><\/tr> Precious Metals<\/td> Weldable<\/td><\/tr> Low carbon steel<\/td> Weldable<\/td><\/tr> Low alloy steel<\/td> Weldable<\/td><\/tr> Medium carbon steel<\/td> Weldable<\/td><\/tr> High carbon steel<\/td> Weldable<\/td><\/tr> Alloy Steel<\/td> Weldable<\/td><\/tr> Stainless Steel<\/td> Weldable<\/td><\/tr> Tool steel<\/td> Weldable<\/td><\/tr> Titanium<\/td> Weldable<\/td><\/tr> Tungsten<\/td> Weldable<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n What Thickness Of Base Metals Can Be Welded With PAW?<\/h2>\n\n\n\n