An investigation of cathode erosion has been conducted for conditions similar to those encountered in a plasma cutting process. A hafnium insert in a water-cooled copper sleeve serves as the cathode. Modifications to the torch allowed the observation of the cathode surface during operation and measurement of material loss from the cathode during different phases of an operating cycle. Erosion has been found to be predominantly due to ejection of molten material droplets. Most ejection events are associated with changes in the conditions of the plasma, e.g. during start-up, change of gas flow and shutdown. The ejections can be explained by imbalances of the forces acting on the molten surface, those associated with the arc current, those due to surface tension, and those associated with the fluid dynamics within the torch.

Plasma properties in a highly constricted arc were measured using spectroscopic techniques. The arc configuration studied is typical of those produced by plasma cutting equipment. Temperatures and electron densities in the arc were measured using several different emission lines and several different methods for property calculation. Factors such as fluctuations in the arc intensity, arc asymmetry and the validity of the assumptions used for property calculation were shown to affect the accuracy of the measurements. Selection of the temperature measurement method can influence the measurement results. For this arc configuration, methods which use oxygen ion emission lines provide the most accurate results. The measurement methods developed in this study were used to investigate differences in arc properties related to arc current and nozzle diameter. Large changes in the temperature and electron density due to shocks were measured in the under-expanded region near the nozzle exit.

The relationship between plasma properties and cutting performance for a plasma arc cutting system was investigated. Plasma properties such as temperature and composition were measured using spectroscopic techniques in a 200 amp oxygen plasma cutting system. In addition to the plasma properties, the symmetry of the cylindrical cutting arc was also quantified. Cutting performance was measured by analyzing the edge quality of sample cuts. The most important measure of edge quality for this study was the angle of the cut edge. Operating parameters investigated included the effect of shield gas flow and geometry changes caused by cathode erosion. The measured plasma properties are used to account for the observed increase in recommended cutting speed for different consumable designs which operated at the same current level. A strong correlation was also shown between the measured arc symmetry and the cutting performance.

Plasma surfacing is'an important enabling technology in highperformance coating applications. Recently, it is being applied to rapid prototyping/tooling to reduce development time and manufacturing cost for the development of new product. In this technology, a plasma arc beam is used as thermal energy source, metal powders are preheated in plasma arc and deposited in melt pool on the hase plate or deposited layer synchonously, with the move ment of plasma gun and/or worktable controlled by CNC according to CAD slice model in computer, deposition layer grows gradually along z direction until the total part is fabricated. However, this technology is in its infancy, it is essential to understand clearly how process variablff relate to deposit microstructure and properties for plasma deposition manufacturing (PDM) process control. In this article, the microstructure, mechanical properties and coating appearance for single surfacing and multiple surfacing under ditferent parameters, such as plasma power, powder feedrate and scanning speed, etc., were studied in detail, and then a hybrid BP artificial neural network (ANN) and genetic algorithms (GAS) methodwhich can overcome the shortcoming of BP neural network and obtain a global convergence result, was presented to optimize processing parameters. Using the optimized paramet,ers, metal parts with complex shape and excellent mechanical properties and microstructure were fabricated successfully.Erosion of a hafnium cathode imbedded in a water-cooled c a p per sleeve was investigated during arc ignition, steady state operation and arc shutdown using a high speed imaging system. The cathode was operated in a commercially available 200 A oxygen plasma cutting torch. Spectroscopic measurements have shown that the erosion by evaporation is very small for relatively short cutting cycles. Observation of the cathode during arc operation shows erosion of the hafnium is dominated by ejection of small metal droplets from the molten cathode surface. These droplet ejections occur predominantly during arc ignition and shutdown, and a special device allowed determination of the cathode erosion mte during the cutting cycle. The variation in erosion rate, droplet size and trajectory for the different times during the arc operation points towards different erosion mechanism. Occurrence of the droplet ejection was correlated to the changes in the operating parameters of the cutting torch, such as current, voltage, gas pressure in the nozzle, plasma gas flow-rate and plasma gas type. A pressure imbalance on the molten cathode pool has been hypothesized as the cause of the ejectioas during arc ignition, while aerodynamic drag forces appear to be dominant during arc shutdown.I .

Plasma cutting is commonly used to cut metals in a variety of applications. This research is motivated by the lack of fundamental understanding of the dynamics of the plasma flow interacting with cold gas environment. It is focused on the characterization of the arc instabilities that affect the quality and consistency of a cut. The characterization of instabilities has been performed with high-speed Schlieren imaging technology to visualize the arc width and the light intensity fluctuations in the arc boundary layer. Measurements of arc symmetry using Fourier and wavelet transforms of the light intensity fluctuations in individual pixels in the Schlieren images are able to provide a quantitative measure for the degree of instability. Ambient gas entrainment into the plasma jet is measured using a CCD camera with a narrow bandpass filter centred on the nitrogen atomic line to detect nitrogen concentrations throughout the arc, thus showing the penetration of shield and ambient gas into the pure oxygen plasma. The effects of torch design features on these characteristics are investigated by comparing two different torch designs. The results show that the developed diagnostics are useful for evaluating new torch designs.