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University ofTechnology, Inst. for High Power Beam Technology 2University Erlangen, Inst. for Manufacturing Technology AB STRACTIn the laser cutting process the gas flow is of main importance to remove molten material from the cut kerf. Changes in nozzle geometry, gas supply pressure and nozzle workpiece stand-.offdistance have a strong influence on the cut quality.The aim of the investigation is the numerical simulation of supersonic gas flow.Supersonic nozzles for laser applications are designed Which can be manufactured more easily and cost effective than conventionally used Laval nozzles. For the simulation wall friction is regarded and the flow field is calculated two dimensional and assumed turbulent. Normal jet impingement for the designed nozzles and underexpanded conic-cylindrical nozzles is compared at various nozzle stand-off distances and shows a more constant pressure on the workpiece for the designed nozzles.Furthermore supersonic effects in the cut kerf are investigated and compared with results from Schlieren photographs. The nonlinear pressure distribution inside the kerf indicates both the shock formation and detachment ofthe gas flow.In the presence of shock waves the existing equations in finite volume form have been modified, as they lead to wrong predictions both in the shock location and strength due to an incorrect inertia term in the momentum equations.

Ill the laser cuffing process the gas flow is of main importance for exhausting molten and evaporised material from the cut kerf. The present paper investigates the gas flow of conventionally used conic-cylindrical nozzles and newly designed supersonic nozzles in a cut kerf A quantitative description of pressure gradients and velocity profiles of the flow field is given as a result of numerical simulation. The calculations are compared with experimental Schlieren photographs. For both nozzle types the dependence ofthe flow field on the variation ofthe nozzle adjustment and the nozzle stand-off distance is investigated. In case of the conic-cylindrical nozzle the non-linear pressure and velocity distribution inside the kerf indicates both the shock formation caused by expansion to ambient pressure and the detaching of the flow field. The important result for the supersonic nozzle is the independence of the flow field from the nozzle stand-off distance which leads to an unchanged cut quality. Therefore the supersonic nozzle is of special interest for 3D laser cutting and cutting ofthick materials.

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In designing classical controllers it is necessary to assume that the parameters of the system are reasonably well known. However the ablation process involves inherent nonlinearities and significant uncertainties in the model parameters, which are one of the main problems for designing an appropriate controller. Nonlinearities occur e.g. due to the cheinical process or due to changing the absorption or the gas flow. The concentration of added oxygen to the assisting gas stream influences the system behaviour significantly. It will be shown that the nonlinearities depend on the process parameters. A simplified nonlinear process model based on an analytical investigation is presented and discussed. To get a glimpse of the time constants, nonstationary investigations are carried out. The best way to control such a complex nonlinear system is to use an input-output map. This caii he settled with an experimental off-loop identification or by fuzzy inference. A fuzzy controller itself has a nonlinear behaviour and allows designing an appropriate controller without any mathematical model of the process.