Gas flows

Gases are often applied in laser material processing for different tasks such as shielding the process zone from the ambient atmosphere, blowing out the molten material in case of laser cutting or air quality control in processing stations. The corresponding gas flows generally need particular attention with respect to process stability, quality control and last but not least operational costs.

The numerical analysis and computation of gas flow usually provides very reliable results. This is caused by the fact that the state behavior of gas flows is well known and can be described by the equation of state of ideal gases or gas mixtures. In addition, appropriate turbulence models for description of high-velocity gas flows are also available. As an example, Figure 1 shows the supersonic gas flow through a cutting gas nozzle in function of experimentally visualized flow fields by means of a Schlieren analysis. The results show a very good agreement regarding the positions of the characteristic shock waves.

Corresponding simulation models can be applied as helpful tools for the design and the optimization of fluidic processes and components (nozzles, processing heads, blowers, suction removals, etc.) by means of parameter studies and sensitivity analyses. Current studies involve investigations of the gas flow characteristics in laser beam fusion cutting and the air-flow optimization in laser material processing stations.

Achievable cutting speeds and cut edge qualities in thick-section laser beam cutting with sheet thicknesses of > 6 mm are strongly influenced by the efficiency of the blow out of molten material from the cut kerf in interaction with the cutting gas flow. The profound knowledge of gas flow characteristics might be a good base for improvements of the process. Adequate simulation models are efficient tools to describe the gas flow in a reliable manner. This is demonstrated in Figure 1 that shows a comparison of calculated and experimental flow fields of the cutting gas in interaction with a cut kerf model. The location, the dimensions and also the positions of typical flow features (supersonic and subsonic regions, straight and oblique shock fronts, boundary layer separation point) are well described by the simulation results.

The use of corresponding simulation models allows for an evaluation of promising approaches for process optimization. New nozzle concepts can be virtually tested or designed with respect to geometrical degrees of freedom (size, shape). In addition, the total gas consumption can be calculated in relation to the part of gas that is actually coupled into the cut kerf. Calculated values according to Figure 2 demonstrate that there is a great potential to improve the utilization ratio of the cutting gas particularly under conditions of high gas pressures.

The interest of the laser community is not only limited to primary effects of process gases such as cutting gas flows in laser beam cutting or shielding gas flows in laser beam welding. Furthermore gases are also applied with secondary functions for example to shield optical components from process emmisions (plumes, spatters) or to remove these emissions out of the direct processing zone. One example is the remote welding technique with modern and high-brilliant solid-state lasers (fiber, disc). For a number of applications the interaction between laser radiation and process emissions gives rise to an increased process sucseptibilty to instabilities and decreased weld penetration depths. In this case blowers and exhaust ventilations might be very beneficial to avoid these effects.

Corresponding simulation models allow for a design and optimization of the forced air flow in remote laser material processing stations. Figure 1 shows the numerical model (including the adapted computational mesh) of a station for remote laser welding. Calculated flow fields with consideration of different flow configurations are shown in Figure 2. Local inflows near the processing zone are highly efficient in removing local process emissions and ensure reliable processing results. Global flows serve to keep clean the air in the station. The simulation model can be applied to improve the flow design with respect to position components and to determine the required flow rates. As a result, optimal configurations can be identified ensuring best processing results at lowest gas consumption rates.