Helmut Hügel

A 1-kW CW thin disc laser

The thin disc laser is presented as an optimal laser design for the operation of a quasi-three-level laser active medium in the high power regime with high optical efficiency. Numerical calculations of the laser output power show that operation with an output power up to 1 kW with an optical efficiency of 50% and more is possible at room temperature utilizing 16 absorption passes. Scaling of the output power can be realized by scaling the pumped area using one or more discs. The experimental investigations yield a maximum output power of 647 W at 51% optical efficiency for one crystal and of 1070 W with 48% optical efficiency for four crystals at a temperature of the cooling water of 15/spl deg/C.

Time-resolved observation of gas-dynamic discontinuities arising during excimer laser ablation and their interpretation

Ablation of materials (Cu is presented in this report) in air at an ambient gas pressure of 1 bar with a KrF excimer laser (3-47 J cm-2) leads to gasdynamic processes above the target surface which affect the processing result, the efficiency of the treatment and the debris in the environment of the irradiated area. These laser-induced processes have been diagnosed using fast schlieren photography and shadowgraphy. Five discontinuities have been discerned and their propagation mechanisms have been detected. A physical interpretation of the discontinuities is given along the lines of existing theories and plausible reasoning. The locally most advanced discontinuity can be explained by the classical Sedov-Taylor blast wave theory, and conclusions on the energy content in the shock wave, the pressure distribution and the surface pressure evolution will be presented. The results show that, at high energy densities (3-47 J cm-2), about 80% of the available laser pulse energy is deposited in the shock wave. A reduction in the energy density leads to a decrease in the fraction of the energy that is deposited in the shock wave. Close behind the first discontinuity follows a second one that is interpreted as the ionization front. The contact front, which separates shocked air and ablated material vapour, has been observed within the laser pulse duration. The complex structure of the contact front is interpreted in terms of gas flow phenomena inside the two outer discontinuities.

Micromachining with ultrashort laser pulses: from basic understanding to technical applications

Ultrashort pulses appear very promising for material removal with ultra high precision. Initial investigations showed, however, several unexpected quality problems such as formation of recast, ripples and irregular hole shapes even in the femtosecond pulse regime. After describing the problems, this contribution will present some progress in fundamental understanding of the ultrafast ablation process. On the basis of this knowledge technical means have been developed allowing to achieve an unprecedented level of accuracy at acceptable expenses. The latter being strongly influenced by the shortness of the laser pulse, a comparison between pico- and femto-second regime will be presented.

The effect of plasma formation on beam focusing in deep penetration welding with CO2 lasers

This paper deals with the absorption and defocusing of a CO2 laser beam by the laser-induced plasma plume in deep penetration welding. To derive the `effective` intensity distribution in the focal plane theoretically, the laser beam propagation through the plasma plume is calculated by solving the paraxial wave equation with a finite-difference scheme. Corresponding to experimental results, documented in the literature, the properties of the plasma plume (spatial temperature distribution and shielding gas content) are pre-set within the calculation. Parametric studies demonstrate that the intensity at the focus is reduced due to the defocusing effect of the plasma plume, mainly, and only to a minor extent due to absorption within the plume. Because of refraction within the plume, the intensity distribution in the focal plane is dependent on the plasma`s size, position and temperature. On studying the dependency of the optical properties on plasma temperature and shielding gas composition, it is found that, by applying a shielding gas mixture of He and Ar in the ratio 3:1, the variation of the focal diameter with plasma temperature can be significantly reduced. This shielding gas mixture, therefore, is recommended for enhancing process stability when welding with high-power CO2 lasers.

Investigations of extinction coefficients during excimer laser ablation and their interpretation in terms of Rayleigh scattering

KrF excimer laser ablation in air at an ambient pressure of 1 bar leads to an intense evaporation of target material. The ablated material compresses the surrounding gas and leads to the formation of a shock wave. The incident laser radiation interacts with the compressed, ionized ambient gas behind the shock front and the partially ionized material vapour. This interaction is responsible for extinction of the incident laser radiation and exerts effects on the processing result and the efficiency of the treatment. The transmission of the incident laser power through the laser-induced interaction zone was measured using a target foil prepared with a small aperture within the area of irradiation. Extinction coefficients in the range were measured for PET, copper and aluminium. In order to explain the experimental results, a theoretical study of possible extinction mechanisms was performed on the basis of inverse Bremsstrahlung theory and scattering theories. The thermodynamic properties in the interaction zone were calculated by using a shock wave theory. Under the assumption that this theory describes the thermodynamic properties in a physically correct manner, it will be shown that inverse Bremsstrahlung cannot explain the measured small degrees of transmission. Interactions between the incident laser light and material clusters in the laser-induced material vapour, like Rayleigh scattering, however, can lead to values comparable to the experimental findings. In order to classify the scattered fraction from the incident laser power, information on the size of the scattering particles is necessary. Therefore, a simplified model of cluster condensation in the material vapour and the scattering of incident laser power by these clusters was developed. Theoretically obtained results on this basis will be compared with the experimental data.

Propagation analysis of self-convergent beam width and characterization of hard-edge diffracted beams

For a laser beam diffracted by a hard-edge aperture, propagation of the beam width, defined by the second-order moment of its irradiance distribution truncated according to the self-convergent-width criterion, obeys the familiar hyperbolic law. It is demonstrated numerically that, with the self-convergent-width approach, the beam-propagation parameters for three beam types (Gaussian, Hermite-Gaussian, and flattened Gaussian) diffracted by hard-edge apertures can be determined with the second-moment-based procedure that is recommended by the present draft standard only for unapertured laser beams.

Welding of aluminum: A challenging opportunity for laser technology

The increasing demand for lightweight structures has led to a greater industrial use of aluminum alloys. The application of these materials is handicapped by a lack of productivity and reliability in the joining techniques currently available. The laser welding of aluminum is markedly more difficult than for steel. The difficulties to be solved include the threshold for deep penetration welding, the occurrence of material related defects and seam deficiencies such as cavities and blowholes that discourage production engineers from making greater use of lasers. A theoretical analysis reveals the requirements that have to be fulfilled by laser systems in order to obtain process stability. In comparison to steel greater focusability and shorter wavelengths are needed. An intensity distribution leading to a V‐shaped keyhole is also beneficial. The results of this analysis are confirmed by welding results obtained with CO2 and Nd:YAG lasers. An increase in process stability is observed when a dual beam techniq...

Moving humps at the capillary front in laser welding

Welding is a widespread application in laser materials processing and the current development of laser systems with high beam quality helps to enhance the number of applications furthermore. However, the weld-pool dynamics is still a limiting factor leading to weld defects and it seems to be more important for lasers with high focusability and for radiation around 1 µm compared to the 10 µm radiation.In the time past a lot of investigations were performed to analyse the reasons for those instabilities: high speed imaging, X-ray analysis and also modelling. In addition to experiments with real workpieces (iron and aluminium alloys) a set of experiments with transparent materials such as water and ice were performed at IFSW in the last years.As a result, flow components of the melt parallel to the laser beam were identified to be a major reason of the unwanted behaviour of the melt, driven by humps at the capillary front. Although such humps were discovered in X-ray analyses already in the 1980s and described in some models, they were discussed relatively seldom.For this contribution, welding with tracer material in the specimen was used to identify typical flow patterns, whereas the “welding” in transparent material shows directly typical flow structures. Finally, calculations of the beam propagation within the capillary clarify interesting differences for different wavelengths and beam quality. In this context, also the role of the polarisation will be discussed.Welding is a widespread application in laser materials processing and the current development of laser systems with high beam quality helps to enhance the number of applications furthermore. However, the weld-pool dynamics is still a limiting factor leading to weld defects and it seems to be more important for lasers with high focusability and for radiation around 1 µm compared to the 10 µm radiation.In the time past a lot of investigations were performed to analyse the reasons for those instabilities: high speed imaging, X-ray analysis and also modelling. In addition to experiments with real workpieces (iron and aluminium alloys) a set of experiments with transparent materials such as water and ice were performed at IFSW in the last years.As a result, flow components of the melt parallel to the laser beam were identified to be a major reason of the unwanted behaviour of the melt, driven by humps at the capillary front. Although such humps were discovered in X-ray analyses already in the 1980s and describ...

Spatially resolved on-line monitoring during laser beam welding of steel and aluminum

In recent years, laser welding has found increasing use in various industries. One reason is the enhancement of economic efficiency compared to competing techniques. The realization of this aspect necessitates automated process control strategies. One method of on-line process monitoring during laser beam welding is the use of spatially resolved detectors. Normally CMOS cameras are preferred to CCD cameras because, compared to CCD cameras, they have a higher dynamic and direct pixel access, which results in high frame rates when working with subframes.The main difficulty of camera based process monitoring during welding of steel and aluminum is the optical resolution of the large temperature range from the melting point to the evaporation point. The temperature range that can be observed with a camera, on the one hand, depends on the dynamic and the spectral response of the camera and, on the other hand, on the detection wavelength. However because of disturbances by emissions from the excited metal vapor above the work piece not each wavelength range is useful for the process monitoring.Experiments for process monitoring during laser beam welding with a CMOS camera in different spectral ranges show the influence of the selected wavelength on the detection result and, therefore, also on the significance of the different indicators for the process control. Extensive spectroscopic investigations of the emissions of the welding process give information about the feasible spectral ranges. The result is a kind of measurement instruction for the camera based process monitoring.In recent years, laser welding has found increasing use in various industries. One reason is the enhancement of economic efficiency compared to competing techniques. The realization of this aspect necessitates automated process control strategies. One method of on-line process monitoring during laser beam welding is the use of spatially resolved detectors. Normally CMOS cameras are preferred to CCD cameras because, compared to CCD cameras, they have a higher dynamic and direct pixel access, which results in high frame rates when working with subframes.The main difficulty of camera based process monitoring during welding of steel and aluminum is the optical resolution of the large temperature range from the melting point to the evaporation point. The temperature range that can be observed with a camera, on the one hand, depends on the dynamic and the spectral response of the camera and, on the other hand, on the detection wavelength. However because of disturbances by emissions from the excited metal vapor...

Analytical expressions for the threshold of deep-penetration laser welding

Explicit analytical expressions are derived based on a simplified model as a convenient estimation of the requirements to reach the threshold of deep-penetration laser welding. For materials with high heat conductivity and low surface tension the simple formulas allow determining the material-dependent minimum power required for deep-penetration laser welding as a function of the diameter and the travel speed of the beam on the work piece surface. Within this area of application of the model the derived formulas agree well with experimental results.