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The properties of plasmas generated for the air plasma cutting process have been investigated in this study. The plasma arc cutting process employs a plasma torch with a very narrow bore to produce a transferred arc to the workpiece at an average current density of ∼ 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 arc voltage, nozzle voltage, air flow rate and nozzle pressure over a range of arc currents of 40-160 A for a nozzle with a bore of 1.5 mm. Using high-resolution digital photography, the radius of the arc 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 arc model has been developed to estimate the arc radius, voltage and pressure of the arc 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 arc 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 arc 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.

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A single-dimensional model, based on an integral formulation of the conservation equations, has been developed which makes it possible to predict the behaviour of these arcs. The radial distribution of axial plasma velocity v2(r,z) is represented by a function vz(0,z)(1-(r/ra)2)n where ra is the arc radius and r and z are the radial and axial coordinates. The solution for n by by the use of radial integral of the axial momentum equation makes it possible to develop a simple but detailed picture of the axial velocity distribution in the arc column. A comparison of the axial distribution of plasma velocities with published experimental results shows that turbulent exchange processes are dominant for currents greater than about 500A.

We report the results of a study on the influence of oxygen in the plasma gas used in the plasma arc cutting process on cuts obtained in mild steel plates. Experimental results of shapes of kerfs and the leading edges of the cut front formed while cutting a 6 mm mild steel plate at 100 A with nitrogen, air and oxygen as plasma gases are presented. These results are discussed in the light of the overall energy balance of the process. It is found that the exothermic reaction of oxygen in the plasma gas with the iron in mild steel enables the cutting of mild steel at higher speeds with both air and oxygen than the maximum cutting speed attainable with nitrogen. A comparison of the melting rates for oxygen with those of air reveals that although oxygen can produce more exothermal energy by oxidation, oxygen is not superior to air in melting metal near the bottom of the kerf formed at high cutting speeds.The study shows that the dross formed at the bottom of the cut is determined by the shape of the cut-front surface over which the molten metal from the cut front flows to be ejected at the plate bottom. Any improvement of metal ejection to be gained with oxygen as the plasma gas may be the result of enhanced superheating of the metal melted from the cut-front surface.