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In this work, the impact of laser cutting and a subsequent pre-deformation on the fatigue behavior of AISI 304 was investigated. This represents a typical process chain in steel sheet manufacturing. Laser cutting trials with variating process parameters were evaluated with regard to resulting dross formation and surface roughness. A subsequent deformation can influence the appearance of the laser cut edge. Fatigue tests up to 108 load cycles were carried out on specimens which were processed with selected parameter sets. It can be shown that dross attachment is the failure relevant feature which sets up a shape dependent, crack inducing notch effect. Furthermore, pre-deformation can detach melt droplets and therefore increase the fatigue strength.

Factorial Design-of-Experiment analyses were applied for conventional and beam oscillation fiber laser cutting of 10 mm thick AISI 304 stainless steel. Considered factors in case of the conventional process with a static beam involve both laser and cutting gas parameters, in particular the laser power, the focal plane position, the cutting gas pressure, the nozzle stand-off distance as well as the nozzle diameter. The conducted trials were evaluated with respect to the achievable cutting speed, the cut kerf geometry and the cut edge roughness. Noticeable correlations between cut edge roughness and cut kerf geometry stimulated the development of a corresponding Computational Fluid Dynamics (CFD) model of the cutting gas flow through the kerf. A specific approach of data synchronization revealed that the experimentally determined roughness values do well correlate with numerically computed values of the backward directed component of the gas-induced shear stress and that the cut kerf geometry as internal process-inherent boundary condition influences relevant cutting characteristics more than controllable external cutting gas parameters. Finally, effects of circular beam oscillation were investigated by an additional factorial analysis considering the laser power, the focal plane position, the oscillation frequency and the oscillation amplitude as factors. The results demonstrate the potential of beam oscillation techniques for quality improvements in laser cutting.

The industrial application of laser materials processing methods is still far ahead of research into the physical phenomena occurring during these processes. In particular, the effect of high temperatures on the energy coupling of laser irradiation of metals is poorly understood. However, most processes in laser materials treatment involve temperatures above the melting point or even cause evaporation. This study therefore evaluates the effect of high temperatures on the energy coupling efficiency of stainless steel experimentally for three typical laser wavelengths (515 nm, 1.07 µm, 10.6 µm). As a result, it is shown that the effect of temperature on the energy coupling efficiency depends on the wavelength. In this context the relevance of the X-point phenomenon known from the emissivity theory could be demonstrated for laser material processing. Further, the effect of a process-induced surface oxidation is analyzed. At temperatures above 650 °C the energy coupling efficiency dramatically increases to around 65% at melting point and stays at this high level even in the liquid phase.

Fiber laser fusion cutting trials on AISI 304 stainless steel sheets of 10 mm thickness were performed with the aim to gain further insights into the striation formation process on cut edges. The study is based on a factorial Design-of-Experiment (DoE) approach with consideration of laser power, focal plane position, gas pressure, nozzle stand-off and nozzle diameter as control factors. Analyzed responses include the achievable cutting speed, the cut kerf geometry and the cut edge roughness. In addition, numerical simulations of the cutting gas flow were carried out for adapted kerf geometries and gas parameters. The evaluation of these results reveals strong correlations between experimental roughness values and the numerically computed rear-directed shear stress component within the transition zone between cutting front and cut edge surface. This finding supports the hypothesis that the particular characteristics of the boundary layer zone of the cutting gas flow affects the structure and roughness of cut edges.

Achievable benefits in hybrid laser-arc welding are closely related to suitable parameter settings. Basic optimizations consequently need a profound understanding of the relevance of involved interaction mechanisms. These are however differently evaluated and discussed in literature. This paper gives an over-view on the most popular hypotheses in the field of laser-arc processing. The importance of direct interactions between laser radiation and arc plasma as well as the role of metal evaporation are particularly considered by own experimental and numerical investigations. The results of this analysis suggest that these phenomena cannot be regarded as main driving mechanisms for synergistic achievements in hybrid laser-arc welding with respect to process stability and performance.Achievable benefits in hybrid laser-arc welding are closely related to suitable parameter settings. Basic optimizations consequently need a profound understanding of the relevance of involved interaction mechanisms. These are however differently evaluated and discussed in literature. This paper gives an over-view on the most popular hypotheses in the field of laser-arc processing. The importance of direct interactions between laser radiation and arc plasma as well as the role of metal evaporation are particularly considered by own experimental and numerical investigations. The results of this analysis suggest that these phenomena cannot be regarded as main driving mechanisms for synergistic achievements in hybrid laser-arc welding with respect to process stability and performance.

The high power laser market has been remarkably influenced by the introduction of high-brightness lasers, i.e., laser sources offering a high optical output power in combination with a high beam quality or a low beam parameter product, respectively. These features are primarily exhibited by fiber and disk laser systems, but diode lasers are also increasingly available with improved beam quality and higher output levels. The development of new processes that make use of the advantages of high- brightness lasers was intensively pursued in recent years and some examples of innovative solutions were given in this paper.

Results of an experimental study are presented in which the impact of a low power laser beam on the stability behaviour of plasma arcs was investigated. Both heat sources were arranged in a recently developed coaxial set-up. Bead on plate welding trials on AISI 304 stainless steel sheets demonstrated that the additional laser beam is capable of extending existing limitations of low current plasma arc welding processes with respect to realisable welding speeds. In particular, an effective stabilisation of the anodic arc attachment under conditions of direct current electrode negative plasma arc welding was achieved by use of a laser beam with only 100 W output power. As a result of a performed stability analysis, the particular conditions for arc stabilisation are specified and co nclusions about the most probable physical reasons for the stabilising action of the laser beam are drawn.

A plasma arc and a low-power/high beam quality laser beam were coaxially combined into one process in order to obtain a more efficient and stable arc for thin sheet welding applications. Theoretical discussion of interaction mechanisms between the laser beam and the plasma arc and results of bead-on-plate welding trials carried out on AISI 304 stainless steel will be presented. Measurements were made of electrical and geometrical arc properties with and without assistance from the laser beam. Additionally, the impact of the laser beam on the weld seam geometry was evaluated. The results showed that the use of the additional low-power laser beam is capable of producing significant process improvements in comparison to the individual plasma arc process alone with respect to arc stability and welding performance.

Results of experimental investigations on disk and CO 2 laser beam fusion cutting cold-work tool steel 90MnCrV8 are presented. The study was performed with the aim to detect features of the cut front geometry and differences in the corresponding absorptivity behavior as a function of the laser wavelength. Longitudinal sections of the cut front were prepared for different sheet thicknesses, focal plane positions, and cutting speeds. The digitalization of the geometrical cut front data enabled the determination of local inclination angles and the calculation of corresponding Fresnel absorptivity values. The analysis revealed that particular areas of the cut front geometry are preferably inclined to values close to the Brewster angle which offer the theoretical maximum absorptivity for both l aser types.