Dieter Schuöcker

Dynamic phenomena in laser cutting and cut quality

The quality obtained with laser cutting, one of the most important processes of laser machining, is characterized by a nearly periodic pattern of striations that cause a certain roughness of the cut surfaces. Improvements of the cut quality, that would be of major technical importance, can only be obtained if the mechanism that leads to the formation of these striations is entirely understood.The present author has argued already in 1984 that temporal fluctuations of laser power can induce oscillations of the liquid layer formed at the momentary end of the cut and its temperature, thus causing distortions of the cut surfaces due to the movement of that layer in cutting direction.The numerical evaluation of the theoretical model yields satisfactory agreement of the “wavelength” of the striations and their dependance on the thickness of the workpiece with experimental values. So far, no attempt has been made to calculate not only the wavelength but also the depth of these striations.It is the purpose of this paper to determine the depth of the striations and to compare the numerical values with the experimental situation. Moreover, the paper is devoted to the explanation of the fact that the laser cut surfaces are showing not only one but usually two different striation patterns, one with a finer structure adjacent to the upper surface and one with a coarser pattern adjacent to the lower surface of the workpiece.

On the reliability of amorphous chalcogenide switching devices

Abstract Switching devices consisting of a thin amorphous AsTeGe film sandwiched between two molybdenum electrodes were prepared by electron-beam evaporation and investigated by subsequent switching. The number of switching events from the beginning of the test until a prefixed change in switching voltage appeared, was defined as the “lifetime”. Samples which were subjected to an ageing process showed relatively stable operation, and longer lifetime than unaged samples. In addition, the lifetime was found to depend strongly on device geometry and to be limited by the formation of crystalline regions in the amorphous film. This was explained to be due to heating effects during the preswitching region, during the transition from off-state to on-state, and during the on-state, although the underlying switching model is assumed to be non-thermal.

Dynamic Effects In Laser Cutting And Formation Of Periodic Striations

Quality of laser cuts is mainly determined by the roughness of the cut edges. That roughness is caused by nearly periodic striations that are typical for laser cutting and appear during laser cutting without reactive gas flow as well as for reactive gas assisted laser cutting and also for other thermal cutting processes /1/.

Heat Conduction And Mass Transfer In Laser Cutting

The most important phenomena contributing to laser cutting are heatiu of the workpiece by absorbed laser radiation and exothermic reaction, heat loss due to t rmal conduction and evaporation and melting of the workpiece in the vicinity of the focus of the laser beam. Material removal takes place since liquid material is blown away by the oxygen flow and due to evaporation. To obtain a deeper insight into these main phenomena of laser cutting and to understand their relative importance, heating of the workpiece and material removal will ne closely regarded and analyzed.

Dual-mode single-crystal photoelastic modulator and possible applications

A new type of acousto-optic device based on a LiTaO(3) crystal is presented. A harmonic voltage with a proper frequency applied to the piezoelectric LiTaO(3) crystal generates mechanical oscillations in the material. Due to photoelasticity, an artificial modulated birefringence is induced by this oscillation. By using a properly adjusted polarizer and analyzer, the transmission of trough-going polarized light can be modulated. By simultaneous excitation of two modes, an advanced optical response can be achieved. For the applications presented here, the first shear eigenmode must have exactly three times the frequency of the first longitudinal eigenmode.

Theoretical Model Of Reactive Gas Assisted Laser Cutting

Laser cutting has reached a high degree of industrial maturity.Nevertheless to extend the present range of industrial applications, the performance and quality of laser cutting must be improved.Such improvements can only be achieved if the physical mechanism of laser cutting is deeply understood.Special attention has to be payed to dynamic effects, since they are most important for cut quality.Examples for dynamic effects are the formation of periodic striations, that cause a certain roughness or pulsed operation, that allows cutting of parts with changing curvature without overheating/18/. In the following bulk of the paper, a mathematical analysis of reactive gas assisted laser cutting including dynamic effects is presented. Although that work has partly been published before, the present paper contains an important task, that has never been published before, namely the calculation of the reactive energy gain.Laser cutting has reached a high degree of industrial maturity.Nevertheless to extend the present range of industrial applications, the performance and quality of laser cutting must be improved.Such improvements can only be achieved if the physical mechanism of laser cutting is deeply understood.Special attention has to be payed to dynamic effects, since they are most important for cut quality.Examples for dynamic effects are the formation of periodic striations, that cause a certain roughness or pulsed operation, that allows cutting of parts with changing curvature without overheating/18/. In the following bulk of the paper, a mathematical analysis of reactive gas assisted laser cutting including dynamic effects is presented. Although that work has partly been published before, the present paper contains an important task, that has never been published before, namely the calculation of the reactive energy gain.

Dynamic Model Of Laser Cutting Including; Pulsed Operation

A dynamic description of laser cutting based on a previously developed steady state model has been elaborated. With that dynamic model the periodic striation pattern that is formed on the cut surfaces can be satisfactory explained. i2loreover, the above model can be used to analyse cutting with pulsed radiation and elucidates some important technical aspects of pulsed cutting.A dynamic description of laser cutting based on a previously developed steady state model has been elaborated. With that dynamic model the periodic striation pattern that is formed on the cut surfaces can be satisfactory explained. i2loreover, the above model can be used to analyse cutting with pulsed radiation and elucidates some important technical aspects of pulsed cutting.

Rf-Excited-CO[sub]2[/sub] Laser With Improved Electrode Geometry

A new laser gas flow design for transversely rf-excited CO2-lasers has been presented at the GCL VII Conference. Due to the imperfect electrode shape the electrical input power was limited by the occurrance of filaments in the discharge. The prensent paper deals with improvements of the rf-electrodes. Metals with high electrical conductivity and low permeability i.e. diamagnetic materials as copper, aluminum, and brass are proper electrode materials. Furthermore, the shape of the electrodes has main influence on the distribution of the current of the glow discharge. The use of small contoured electrodes enhances ignition of the discharge, however, the maximum input power for arc free operation is low Electrodes with a larger outer diameter enable a streamer free discharge at higher input powers. Consequently the electrodes were optimized concerning length, diameter, and alignment to ensure the use of the maximum electrical power available from the generator. The electrode temperature is determined by heat conduction from the discharge tube. Electrodes which are separated from the tube need no cooling. Air cooling with a blower improves the performance of the system.

System for monitoring the focal position in laser material processing

A novel system for on-line measurement of the distance from the melt pool to the lens in laser material processing is described. A spectrometer analyzes the broadband radiation that is produced at the laser-workpiece interaction zone and then focused by the effector optics in the direction of the laser source. From the influence of chromatic aberration on the detected radiation spectrum, the distance from the workpiece to the lens can be derived. An analytical calculation of the shift of the melt pool spectrum that depends on the distance between the molten zone and the focusing lens shows good agreement with the measured results.

Active length control of two phase locked CO2 lasers with a digital signal processor

Two CO2 lasers are phase locked together by injection locking, where radiation between the two lasers is exchanged using a copper prism as a beam folding device in the resonators. Extraction of the output radiation is achieved by a common output coupler. As this method is only utilizing reflective optics (except for the output coupler), it is also practical as a technique for phase-locking lasers with powers in the multikilowatt range. Single frequency oscillation is only achieved if the lengths of both resonators are within a so-called locking range. This can only be fulfilled if the length of one resonator is actively controlled. The method of detection of the phase difference between the lasers and actively controlling the length of one resonator is presented as well as its implementation into a digital signal processor. Stable phase locked operation is achieved and proved through detection of twice the intensity in the far field in the phase-locked regime.