Properties of electric arc plasma for metal cutting

The properties of plasmas generated for the air plasma cutting process have been investigated in this study. The plasma are cutting process employs a plasma torch with a very narrow bore to produce a transferred are to the workpiece at an average current density of similar to 5 kA cm(-2) within the bore of the torch. The energy and momentum of the high-velocity plasma jet generated by the plasma torch melts, vaporizes and removes the metal from the region of impingement of the jet. Measurements have been made of the total are voltage, nozzle voltage, air flow rate and nozzle pressure over a range of are currents of 40-160 A for a nozzle with a bore of 1.5 mm. Using high-resolution digital photography, the radius of the are at the nozzle exit has been measured over the current range. Photographic observations indicate that an underexpanded supersonic plasma jet emanates from the nozzle. An approximate two-zone are model has been developed to estimate the are radius, voltage and pressure of the are at the nozzle exit as a function of current and the predicted results have been compared with experiments. The study reveals that the nozzle of the plasma torch is heavily clogged because of the presence of an electric are with a very high current density in the nozzle. The nozzle clogging effect increases the pressure in the chamber upstream of the nozzle as the are current is increased for a constant mass flow rate of air. The nozzle clogging phenomenon is crucial to generate a plasma jet with the high momentum required to remove material from the molten workpiece and to maintain plasma stability.