Plasma Transferred Arc Welding

The Plasma Transferred Arc (PTA) process employs the plasma principle hence it may be considered an evolution of the Gas Tungsten Arc Welding (GTAW) process, where the high-energy concentration is due to the use of a constrictor nose, which restrains the column diameter of an electric arc established between a tungsten electrode and the workpiece in an inert gas atmosphere, usually argon.

The feeding material is carried to the plasma jet by a gas stream, which might be inert, active or a mixture of active and inert gases. A third gas flow is employed to protect the metal pool from atmospheric contamination. Even though there is the possibility of using mixtures of active and inert gases, argon is typically employed for all three-gas systems.

The PTA process can be considered a derivation of the PAW process. The similarities between the two processes can be observed in Figure 1. Both welding processes employ a non-consumable tungsten electrode located inside the torch, a water-cooled constrictor nozzle, shield gas for the protection of the molten pool, and the plasma gas. The difference between the two welding processes lies in the nature of the filler material, powder instead of wire, which requires a gas for its transport to the arc region. The diagram in Figure 1 shows the two processes with their differences and similarities.

Figure 1: Comparison of Plasma Transferred Arc processes and PAW

As mentioned above, the PTAW welding process and equipment are a modification of Gas Tungsten Arc Welding (GTAW). The significant difference involves a redesign of the GTAW torch to provide a concentration and collimation of the arc plasma. This is achieved by generating the arc and its plasma within the confines of the torch followed by discharging the high temperature ionized plasma through an orifice.

The diameter, configuration and throat length of the orifice are carefully designed to maximize the desirable properties of the arc, i.e. culmination, high temperature and stability. Orifice details are interchangeable so that variations in these dimensions are available to satisfy a variety of weld requirements. The orifice concentrates the plasma arc and collimates the flow so that it assumes a beam configuration as opposed to the "umbrella" shaped arc of the GTAW process.

Figure 2: Schematic view of Plasma Transferred Arc (PTA) Welding

Applications:

• Hard Surfacing: PTAW is well suited to apply hard alloys for wear resistance. Stellite, Colmonoy, Hastelloy, and Tungsten Carbide can all be successfully applied with PTAW.
• Corrosion Resistant Overlays: The localize heat input characteristics of PTAW allow corrosion resistant alloys to be applied with very little dilution into the base material. PTAW can achieve subsea chemistry requirements of <5% Fe in as little as 0.040” of overlay thickness.
• Industries / Components:
  - Non-Mag Down Hole Tools: Drill Collars, Wear Bands for MWDs, Flex Ponies etc.
  - Down Hole Components: Mud Motor Bearings, Stabilizers, Topsubs, Piston SubsFlow Restrictors, Drill Bits etc.
  - Flow Control: Valve Bores, Gates, Seats, Seat Pockets, Ring Grooves, Valve Stems etc...
  - Power Generation: Turbine Blades, Shafts, Bearing Surfaces, etc.
  - Riser Equipment o Pins, Boxes, etc.