Plasma – arc welding (PAW) developed in the 1960s, is produced a concentrated plasma arc and aimed at the weld area. The temperature of this process is stable and reaches as high as 33,000 oC. A plasma torch is ionized hot gas, composed of nearly equal numbers of electrons and ions. Plasma cutting usually used to cut steel and other metals using a plasma torch. In this process, an inert gas is blown at high speed out of a nozzle or orifice, at the same time an electrical arc is formed through that gas from the nozzle to the surface being cut, turning some of that gas to plasma. This plasma is sufficiently hot to melt the metal and moving sufficiently fast to blow molten metal away from the cut. The result is very much like cutting butter with a hot jet of air.
Working Principle
There are two methods of plasma arc welding. Firstly, transferred – arc, the process happened which the workpiece made the part of the electrical circuit, so the transferred arc from electrode to the workpiece occurred. Meanwhile for second process is non – transferred, the arc occurred between the electrode and the nozzle, and the heat is carried to the workpiece by the plasma gas. The arc used a two cycle approach to producing plasma. First, a high-voltage, low current circuit is used to initialize a very small high intensity spark within the torch body, thereby generating a small pocket of plasma gas. This is referred to as the pilot arc. The now conductive plasma contacts the workpiece, which is the anode. The plasma completes the circuit between the electrode and the workpiece, and the low voltage, high current now conducts. If the plasma cutter uses a high frequency/high voltage starting circuit, the circuit is usually turned off to avoid excessive consumable wear. The plasma, which is maintained between the workpiece and electrode, travels at over 15,000 km/h. For new types plasma welding machines operational capabilities. There are likes,
Ø Manual plasma-arc welding is usually modified to non key hole fusion type welding.
Ø Mechanized plasma-arc welding is required for high current plasma-arc applications such as making key hole-mode welds or high current filler passes. Metals welded by these processes: Weld unalloyed, low alloy and high alloy steels, nickle, copper, titanium, zircon and their alloys and special
materials.
Ø Powder Surfacing (PTA) is used for wear facing and corrosion resistance applications using a wide variety of cobalt, nickel, tungsten carbide, stellite and iron based alloys in powder form.
Figure 1 : Two types of plasma arc welding, left picture transferred arc, right picture non – transferred arc.
Advantages
i. Adding filler metal is accomplished more easily with Paw than LBW or EBW, during process in tolerance to joint gaps and misalignment. It is because although the arc is constricted, the plasma column has a significantly larger diameter than the beam.
ii. Reduced weld time results in less embrittlement by carbide and complrx intermetallic compounds for stainless steel and super alloys.
iii. Equalization of distortion stresses results in less residual stress.
iv. Weld in a single pass up to 6mm plates in squre butt position and 10mm plates in only two passes.
v. Less filler metal required in keyhole mode significantly reduces porosity and less sensitivity in arc length.
Limitations
i. The equipment of plasma welding is more complex and costly, and the need for water cooling of the torch limits how small the torch can be made compare to GTAW (GTAW torches may be gas-cooled and can be made to fit into smaller areas).
ii. The process plasma welding is its greater heat input, which produces wider welds and heat-affected zones than LBW and EBW. This may result in more distortion and loss of mechanical properties.
iii. Plasma torches need to be bulky, making them uncomfortable for precision manual use.
iv. For the plasma welding, a careful balance of current, plasma and filler control is needed.
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