Dipl.-Ing. Michael Schnick (Technische Universität Dresden)
Current simulation as a development tool for arc and plasma torches
The arc is the most important technical tool for thermally joining metal materials. But to ensure safe applications in extreme physical situations, the arc processes must be further developed and adapted in order for the ever increasing demands of products with complex constructions, materials and material combinations to be met. The conventional methods of procedure and torch development, based on parameter studies and cost-intensive prototype construction, are not suitable for this process as the physical
active principles in the arc, the electrode and the oven as a black box are observed simply, which means that the laboriously gained results are few and cannot be generalised.
Within the framework of the research work, a simulation tool for plasma arc processes has been developed which describes both timely and with high local resolution, the complex physical processes in the process gas feed, the inert gas coverage, the arc as well as on and in the electrode based on the base numeric model in the basic physics equation. The simulation has been used specifically as a development tool in order to visualise the weak points of existing torch concepts and to develop methods for improvement. For this purpose, effects are visualised which are hidden in the torch and, as a result of the high radiation exposure and temperature loads, are very difficult or even impossible to access under experimental
conditions. Firstly, numerical sensitivity analysis of the understanding of the physical cause-effect relationships between the arc, the work piece, the torch and the process parameters have been entered into in detail and conclusions drawn with regard to the potential and restrictions of the processes.
So, for example, the inert gas flow can be optimised for a plasma pulse welding torch by preventing unsteady current displacement at the inert gas nozzle by inserting a defined break-away edge.
On plasma cutting torches, new results have been gained on the influence of the torch design on cathode wear and pinch instabilities of the plasma column between the cathode and the nozzle hole.
In plasma welding, alongside the geometry of the plasma nozzle cathode system, the influence of process gases has also been examined. The sensitivity analysis of individual gas properties enable us to see that the current of the plasma jet is particularly influenced by the process gases containing helium, whereas the heat introduction into the work piece is, above all, increased by hydrogen admixtures. It was shown that the enrichment of the process gas in the arc (helium) and at the work piece surface (hydrogen) can be caused by complex diffusion processes. These strengthen the extent of the effects even with very low admixtures. Based on this, strategies and process parameters could be derived which allow the effects of the individual process gases to be provoked and used in a targeted manner. The results benefit the economic use of process gases.
Physically founded simulation results are perfectly suited to represent the development results visually and plausibly. They can, at the same time, also be used to pass on knowledge, for example, the transfer to applications or for training welding engineers and welders.