Dieter Schuoecker

Nozzle design and simulation of gas flow for the laser cutting process

In 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-off distance 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 of the 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.

6-kW coaxial high-power CO2-laser resonator

The application of laser welding has been introduced to industry during the last decade. Most of the work done has concentrated on the reliability of the process itself. Now, the next step is to increase the reliability of the laser source to build lasers more simple and to make them cheaper. This paper presents the results of the theoretical calculation of the coaxial laser resonator. The data of the raw and the focused beam have been investigated and first welding samples are demonstrated.

INFLUENCE OF THE FOCUSING ELEMENT FOR CO2 LASER BEAMS ON MONITORED PROCESS SIGNALS

In typical optical sensor setups for CO2 laser material processing scraper mirrors or beam selectors (ZnSe mirrors) are used to allow sensor positions which have a coaxial path for both the laser beam and the monitored visible or near- infrared signal. Calculations and experiments have shown a vital influence of the focusing element for the laser beam on the measured signal. Critical topics are the distance between focusing element and monitored process, as well as the dependence of focal length on wavelength.

Welding of aluminium, copper, and brass

The new concept of a coaxial high power CO2 laser offers the possibility of welding at high power with an excellent beam quality. The first applications show great progress in welding quality compared to competitive systems. In further experiments it is provided that the beam of this coaxial high power CO2 laser allows welding of different materials, especially aluminum, copper and brass with good quality and properties. The results are presented with pictures of the cross section and the properties of the weldseam.

Concept of technological high-power multichannel waveguide diffusion-cooled CO2 lasers with electrodeless sound-frequency AC-discharge

The paper presents the results on investigations and development of multichannel CO2 lasers with diffusion cooling of active medium excited by discharge of audio-frequency alternating current. Various methods of improving the laser beam quality and energy efficiency have been considered and experimentally tested.

Theoretical and experimental investigations on the role of oxidation and gas flow in the machining of steel molds due to melt removal with high-power CO2 lasers

For the machining of metal cavities and reliefs with high power lasers, both the gas mixture and the actual formation of the flow is essential. High ablation rates with an appropriate surface quality can be reached only with an optimized gas flow and a sound process understanding. The impinging gas flow results in a shear force and a pressure gradient over the melt pool. These values also depend on the actual geometry of the erosion front and can not be predicted in advance. To investigate the gas flow with different nozzle arrangements numeric simulations are carried out. Calculations and experiments show that working with an inclined gas flow -- the so called laser planing process -- improves the material removal rate significantly. An optimum angle for the gas- nozzle can be shown. By using a mixture of oxygen and nitrogen as working gas, additional heat can be supplied to the process. Knowing the fraction of material which can be oxidized per time unit is important to calculate the additional heat from the chemical oxidation. Furthermore, the thickness of the formed oxide layer influences the absorption of the laser power effectively. In this paper it will be shown that the change of the absorptivity is the key parameter for a high ablation rate. The chemical reaction also changes the properties of the melt pool. These effects influence the process significantly and they are of fundamental importance to achieve a high material removal rate. This will be shown in various experiments. A simplified process model for melt removal will be presented and compared with experimental results. The model includes two important oxidation effects, which are improved absorptivity and the additional heat transfer. Furthermore the shear force and the pressure gradient over the melt pool will be taken into account to calculate the thickness of the melt film.

High-speed thin metal sheet welding with high-power CO2-lasers

Welding of thin metal sheet is an application, which is more and more often applied in the field of material processing with laser. In many of these applications it is necessary to weld the thin metal with high speed. A new coaxial high power CO2-laser concept offers the possibility to weld these metals. The presented welding has been realized with sheet steels, which are coated with a zinc layer. The results of the experiments are presented with pictures of the application station and pictures of the weldseam. What is furthermore demonstrated, are the quality and the cross section of the welded materials.

Machining and modeling of high-quality reliefs in steel with pulsed-CO2-laser radiation

Machining of reliefs is a fairly new laser application and on the verge of industrial usage. The main problem is that the surface quality degrades with increasing ablation rate. Experiments and simulations are carried out to improve the process. It will be shown that fine structures and excellent surface qualities can be achieved with CO2 lasers. For rapid tooling a complex 3D relief for embossing and stamping applications will be presented. The machining process of reliefs in steel is described by a mathematical model. The ablation geometry is calculated analytically by the temperature field due to pulsed laser radiation. As working gas a mixture of oxygen and nitrogen is used. The oxidation process plays a vital role in the laser ablation process and will be discussed in more detail. The molten film of oxidized and non-oxidized material at the ablation front is described as a stationary boundary layer flow. The thickness of the oxidized layer determined the ablation process significantly, e.g. absorption increases with an oxidized layer rapidly. To determine the absorption of the laser radiation, the interferences between the oxidized and the non-oxidized melt films are investigated. A comparison between the calculated and the machined ablation geometry shows a good correspondence for mild steel.

Process monitoring for laser welding using a quality sensor for cutting

In typical optical sensor setups for CO2 laser material processing there are scraper mirrors or beam selectors used to allow sensor positions, which have an coaxial path for both the laser beam and the monitored visible or near- infrared signal. For the monitoring and controlling of the laser welding process it is of interest to get information on the plasma stability. A two color photodiode sensor which has been successfully tested for quality monitoring of both cutting and ablation has been used for welding in order to find correlations between the sensor signal and the quality of the weld seam. Results and comparisons between cutting and welding will be presented.

Mathematical means of the process data acquisition for sensor engineering

Economical considerations in the industrial manufacturing of technical components force for improvement of established material machining processes. In the field of laser cutting and welding as a young technology, higher decollating and welding rates do always require quality inspection with new sensor combinations. Higher mathematical means are used to find relations between quality and measured signals with its aid the process is to be controlled. Combination of derivatives, integrals or statistical data give information of the quality. Even knowledge about the material weighted with rules of fuzzy-logic helps to control cutting speed and laserpower. In addition to different algorithms, those lead to evaluation offline the process, means of the online check are developed and presented.