Fibre lasers offer distinct advantages over established laser systems with respect to power efficiency, beam guidance and beam quality. Consequently, the potential of these new laser beam sources will be increasingly exploited for laser cutting applications that are conventionally carried out with CO2 lasers. However, theoretical estimates of the effective absorptivity at the cut front suggest that the shorter wavelength of the fibre laser in combination with its high focusability seems to be primarily advantageous for thin sheet metal cutting whereas the CO2 laser is probably still capable of cutting thicker materials more efficiently. This surprising result is a consequence of the absorptivity behaviour of metals that shows essential quantitative differences for the corresponding wavelengths of both laser sources as a function of the angle of incidence between the laser beam and the material to be cut. In evaluation of the revealed dependences, solution strategies for an improvement of the efficiency of fibre laser cutting of thicker metal sheets are suggested.

Hybrid laser welding technologies can be defined as the combination of a high quality laser heat source with an additional secondary heat source. Based on such a definition, known hybrid techniques are classified by the type of the heat sources used as well as their arrangement relative to each other. The classification is followed by a discussion of different theoretical aspects occuring during a hybrid laser beam welding process concerning the plasma characteristics and the resultant temperature distribution of the superposed heat sources. Finally, some practical examples demonstrate how the potential of hybrid technologies can be used to extend the application spectrum of the laser beam welding process.

Experimental and theoretical investigations on fibre-laser-assisted plasma arc welding (LAPW) were performed. Welding experiments were carried out on aluminium and steel sheets. In the case of a highly focused laser beam and a separate arrangement of plasma torch and laser beam, high-speed video recordings of the plasma arc and corresponding measurements of the time-dependent arc voltage revealed differences in the process behaviour for both materials. In the case of aluminium welding, a sharp decline in arc voltage and stabilization and guiding of the anodic arc root was observed whereas in steel welding the arc voltage was slightly increased after the laser beam was switched on. However, significant improvement of the melting efficiency with the combined action of plasma arc and laser beam was achieved for both types of material. Theoretical results of additional numerical simulations of the arc behaviour suggest that the properties of the arc plasma are mainly influenced not by a direct interaction with the laser radiation but by the laser-induced evaporation of metal. Arc stabilization with increased current densities is predicted for moderate rates of evaporated metal only whereas metal vapour rates above a certain threshold causes a destabilization of the arc and reduced current densities along the arc axis.

Laser cutting is still the most common industrial application of CO ₂ laser systems but currently available high-power fibre lasers seem to be an attractive alternative to the established CO ₂ laser sources for several cutting tasks. Practical experience has shown that fibre lasers enable significantly increased travel rates in the case of inert-gas fusion cutting. This advantage in achieving higher cutting speeds in comparison to CO ₂ laser cutting is however a clear function of the sheet thickness to be cut. In the first part of this article, possible reasons for this experimental fact are derived from a thermodynamic analysis of the process with consideration of the specific beam absorption characteristics under cutting conditions. After that, in the second part, a quite new laser cutting variant, namely the gas-free remote cutting process that considerably benefits from the high beam quality of fibre laser systems, is presented.

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