Arc-based ultra-short laser pulse assisted workpiece machining

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The processing of metallic workpieces by joining, cutting, structuring or forming has had a long tradition but is, nonwithstanding, still being intensively investigated in present-day research. Nevertheless, for certain applications only few distinct processing technologies have emerged. For instance remote laser processing where the laser beam is deflected by a fast galvanometric scanner and focused by imaging optics on the workpiece offers a high processing speed and allows to process workpieces that have spatial extents. The disadvantages are mainly of system technological nature such as the effort to generate the laser of sufficient power and to guide the beam towards the work piece. In opposition to remote processing various electric arc techniques are present that offer high power but have to be guided closely to the workpieces surface. Consequently, any automation must utilize robot or portal systems and requires workpieces with flat surface geometry in order to prevent a deflection of the electric arc due to an inappropriate electric field gradient originating from a curved surface. The proposed project investigates a technology that combines the advantages of speed and ease-of-access of laser remote processing with those of high power availability in electric arc processing. The proposed technology is based on the generation of electrically conductive plasma filaments in air or other gases using ultrashort laser pulses with high pulse energy. The plasma filaments can act as long channels to deliver high power electrical discharges precisely onto an arbitrary spot of the workpiece, since they significantly reduce the breakdown voltage necessary to bridge the gap between an electrode and the workpiece. Moreover, while the filaments will act as conductive channels between electrodes with different polarity, they can also act as guiding channels for electrical discharges initiated by field ionization due to a high potential on an electrode, without the need to have a second electrode with opposite polarity. This makes new processing methods possible, even for isolating materials. These properties can be utilized for a fast, remote and high-power processing of the workpiece. For this purpose an ultrashort pulsed laser beam will be deflected using a galvanometric scanner. For focusing a modified imaging system is used that provides a fixed convergence point for the laser beam, independently on its actual deflection, on the workpieces side. Thus, an electrode can be placed near to the convergence point ensuring the channeling of electrical discharges is set off and directed by a nearby laser generated plasma filament. Furthermore, the laser can be prefocused in such a way that the filament is generated at the convergence point. The aim of this project is to investigate the proposed technology in order to assess the suitability of filament-induced electrical discharges for material processing such as welding or cutting.