Cathode erosion mechanisms in an oxygen plasma cutting arc

Abstract

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 .

Non-equilibrium modelling of an oxygen-plasma cutting torch

“…For a given torch design, arc current, plasma gas and gas flow rate, the final distribution of the potential inside the torch is obtained self-consistently with the realized plasma resistivity and the corresponding current distribution. In the plot of voltage drop along the central axis (figure 17), the total voltage drop across the segment of the arc is found to be 64 V, a value, close to the 67 ± 4 V, realized experimentally [39].…”

Cyclic erosion of a cathode in high-pressure arcs

“…This type of erosion dominates when cutting small parts. In spite of the obvious importance of this phenomenon in practice, only a little attention was paid to this problem in the scientific literature [2,3]. One of the most important questions regarding CE is when this erosion occurs: does it occur during the arc start (on-erosion) or during the arc termination (off-erosion)?…”

“…One of the most important questions regarding CE is when this erosion occurs: does it occur during the arc start (on-erosion) or during the arc termination (off-erosion)? In the report [2], it has been shown that either type of cycling erosion may dominate depending on how fast the arc current goes down: off-erosion prevails when the arc current is shut down fast and vice versa. Also, it has been shown in this report that the cathode erodes in the form of particles (agglomerates); small agglomerates during the on-erosion and relatively large agglomerates during the off-erosion.…”

Methods to evaluate arc stability in plasma arc cutting torches

“…LGF construction. In order to analyse the light intensity signal in terms of symmetry, a LGF, one of the wavelet transform functions, is applied to the light intensity signal [26]. Figure 13 shows two examples of LGFs in the frequency domain according to the different centre frequencies, w 0 , which will be discussed later.…”

“…By changing the centre frequency (w 0 ), the filters cover different local frequency regions where the filter gives high weighting factors (figure 13). Each value of w 0 is designed to pick out a particular band of frequencies from the signal being analysed [26]. By performing the inverse FFT (IFFT) on the above filters even and odd responses of the log-Gabor wavelets in the physical domain (not in the frequency domain) are obtained as shown in figure 14, at the different centre frequencies, respectively.…”