Thesis (MTech (Mechanical Engineering))--Peninsula Technikon, 1999. / Word processed copy. Summary in English. Includes bibliographical references (leaves 66-68). Also available online.

Thesis (MTech (Mechanical Engineering))--Peninsula Technikon, 1999 / The simulation of Plasma Arc Cutting is presented 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. A technique for modelling plasma arc cutting has been developed by applying the thermo-metallurgical model to the process and integrating a model of material removal to this model. The model is solved using the finite element method using the FE package SYSWORLD, more specifically SYSWELD. The objective is to determine the minimum energy required to cut a plate of some thickness using this virtual model. The characteristics of the cut need to exhibit the characteristics of a "high quality cut". The model presented can predict the kerf size given certain process variable settings. The numerical results obtained are assessed by conducting experiments. By maintaining Ill1rumum energy input cost savings can be made through energy savings, limiting additional finishing processes and reducing expense of shortening the electrode and nozzle lifetimes. The modelling of the PAC process using virtual design techniques provides a cost-effective solution to the manufacturing industries with respect to process specification development. This plays an important role in South Africa's transition into a competitive global market. It is envisaged that the model will provide an alternative more efficient, non-destructive means of determining the optimum process variable settings for the plasma arc cutting process.

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A specially-designed plasma chamber was constructed to study the operating characteristics of a dc plasma transferred arc struck between a fluid convective cathode and a water-cooled anode. The plasmagen gas consisted of pure argon and arc lengths as high as 7 cm were studied. / The arc voltage increased markedly with arc length and with an increase in the inlet velocity of the argon flow past the cathode tip, and much less with an increase in current. The effect of the gas velocity was minor at low velocities (1 to 3 m/s) but became important at higher velocities (20 to 60 m/s). / Arc length and gas velocity were again the major factors in the distribution of the arc energy. For long arcs and at high gas velocities, the energy emitted by the arc to the wall of the chamber was dominant, while for short arcs the energy absorbed by the water-cooled anode was greater. In all cases, the fraction of the input energy absorbed by the water-cooled cathode, or leaving the system as sensible heat in the exit gas was small. At the anode, the major contribution was from electron recombination, with convection heat transfer from the plasma somewhat less. Direct radiation from the plasma to the anode surface was very small. The recombination of the electron stream occurred on a very small area of the anode ((TURN) 5 mm in diameter). In spite of the energetic water cooling, appreciable vaporization took place on that area. / Appropriate instrumentation permitted the measurement of the stagnation-pressure and pressure distribution on the anode, and that of total heat flux and radiative heat transfer. From the latter, it was estimated that the plasma temperature just above the anode was in the range 10,000 - 12,000 K. / Preliminary experiments with an anode consisting of molten copper showed that the arc root was no longer fixed, but moved around continuously. The arc was otherwise quite stable, and its operating characteristics differed little from those reported for solid anodes, in spite of the greater extent of metal vaporization.

The basic characteristics of a transferred arc argon plasma were determined. A cathode assembly suitable for transferring the electric arc to a molten metal bath or to a cooled anode was designed and constructed to produce the plasma in open air. It included provision for the injection of secondary gas into the arc column. / The measured electrical characteristics indicated that the sustained voltage depended strongly on the arc length and much less on current. The inlet gas velocity past the cathode tip was determined to be an important operating parameter, rather than the volumetric gas flowrate. / The measurement of the axial and radial profiles of temperature was effected with (+OR-) 8% accuracy by a novel diagnostic technique based on the determination of the variations in the outputs of a two-wavelength pyrometer that views a small section of the plasma, as the latter is swept rapidly by a blocking element. Temperatures up to 18,500 K were observed on the axis of the plasma column. / A sweeping microprobe was used to measure the axial and radial profiles of velocity. Velocities up to 190 m/s were recorded. The presence of a relatively colder flow surrounding the plasma column was detected. Mass and energy balances performed by taking this flow into account agreed with the measured input rates.

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