same solidifies and forms the weld bead. Key-holing aids deep penetration at faster speeds and produces high-quality bead. While welding thicker pieces, in laying others than root run, and using filler metal, the force of plasma jet is reduced by suitably controlling the amount of orifice gas. Plasma arc welding is an advancement over the GTAW process. This process uses a non-consumable tungsten electrode and an arc constricted through a fine-bore copper nozzle. PAW can be used to join all metals that are weldable with GTAW (i.e., most commercial metals and alloys). Difficult-to-weld in metals by PAW include bronze, cast iron, lead and magnesium. Several basic PAW process variations are possible by varying the current, plasma gas-flow rate, and the orifice diameter, including: Micro-plasma (< 15 Amperes) Melt-in mode (15–100 Amperes) Keyhole mode (>100 Amperes) Plasma arc welding has a greater energy concentration as compared to GTAW. A deep, narrow penetration is achievable, with a maximum depth of depending on the material. Greater arc stability allows a much longer arc length (stand-off), and much greater tolerance to arc-length changes. PAW requires relatively expensive and complex equipment as compared to GTAW; proper torch maintenance is critical. Welding procedures tend to be more complex and less tolerant to variations in fit-up, etc. Operator skill required is slightly greater than for GTAW. Orifice replacement is necessary. Process variables Gases At least two separate (and possibly three) flows of gas are used in PAW: Plasma gas – flows through the orifice and becomes ionized. Shielding gas – flows through the outer nozzle and shields the molten weld from the atmosphere. Back-purge and trailing gas – required for certain materials and applications. These gases can all be same, or of differing composition. Key process variables Current Type and Polarity DCEN from a CC source is standard AC square-wave is common on aluminum and magnesium Welding current and pulsing - Current can vary from 0.5 A to 1200 A; the current can be constant or pulsed at frequencies up to 20 kHz Gas-flow rate (This critical variable must be carefully controlled based upon the current, orifice diameter and shape, gas mixture, and the base material and thickness.) Other plasma arc processes Depending upon the design of the torch (e.g., orifice diameter), electrode design, gas type and velocities, and the current levels, several variations of the plasma process are achievable, including: Plasma arc cutting (PAC) Plasma arc