Achim Mahrle

Process characteristics of fibre-laser-assisted plasma arc welding

Experimental and theoretical investigations on fibre -laser assisted plasma arc welding (LAPW) have been per formed . Welding experim ents were carried out on alumin ium and steel sheets. In case of a highly focused laser beam and a separate arrangement of plasma torch and laser beam, h igh -speed video recordings of the plasma arc and corresponding mea surements of th e time -dependent arc vol tage revealed dif ferences in the process behavio ur for bo th materials. In case of alumin ium welding, a sharp decline in arc voltage and stabiliz ation and guiding of the anodic arc root was observed whereas in steel welding the arc v ol tage was slightly in creased after the laser beam was switched on. However, significant improv ement 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 behavio ur suggest that the properties of the arc plasma are mainly influenced not by a d irect interaction with the laser radiation but by the laser induced evaporation of metal. Arc stabil ization with increased current dens ities is predicted for moderate rates of evaporated metal only whereas metal vapo ur rates above a certain threshold causes a destabilization of the arc and reduced current densities along the arc axis.

A comparative study of cut front profiles and absorptivity behavior for disk and CO2 laser beam inert gas fusion cutting

Results of experimental investigations on disk and CO2 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 laser types.

Experimental and Numerical Investigations of the Interaction between a Plasma Arc And a Laser

Plasma arc welding (PAW) is a modern welding technique for challenging joining tasks in a wide range of materials and plate thicknesses, A further improvement of the welding characteristics involving achievable welding speed, process stability and penetration depth is expected by an additional low energy laser beam with a maximum output power of 600 W, The paper presents an experimental and numerical analysis of the interaction between a plasma arc and a superimposed laser beam, The experiments are carried out with a non-concentric set-up of the plasma arc column and the laser beam, As results of bead-on-plate welding trials the cross-sectional weld areas were presented in order to demonstrate benefits of the combined process in comparison to separately conducted arc and laser welding, Furthermore, high speed video images (1 kHz frame rate) with synchronized current and voltage recording (1 MHz frame rate) were used, The experimental results demonstrate a different behaviour for welding steel and aluminium, In case of welding aluminium, an arc guidance was observed whereas destabilization effects occur for welding ferrous alloys, A numerical magneto hydro dynamical (MHD) arc model with a concentric set-up of arc column and laser beam set-up was aimed to improve our understanding of relevant interaction phenomena between the plasma arc and the laser beam.

A comparative study on fusion cutting with disk and CO2 lasers

In recent years, several studies on laser beam fusion cutting demonstrated significant differences in the characteristics of the well-established CO2 laser cutting and cutting with solid-state disk and fiber lasers. The reasons for the observed differences in cutting efficiency and cut edge quality are still the subject of the current research and not finally clarified. In order to further the understanding of the involved phenomena, a series of cutting experiments with CO2 and disk lasers was carried out using a Design of Experiment (DoE) approach. The particular feature of the applied experimental setup was the similar geometry of both the CO2 and the disk laser beam with comparable values of the focus diameter and the Rayleigh length. Cutting trials on cold work steel test specimens with different sheet thicknesses were performed. The extensions of the generated cut kerf and of the heat-affected zone as well as the recast layer were analyzed in order to reach a better understanding of the physical mechanisms which take part in the cutting process. The experimental evaluation of transmitted and reflected energy losses throughout the kerf was realized by means of PMMA (Polymethylmethacrylate) blocks placed under the sheet during the cutting process.In recent years, several studies on laser beam fusion cutting demonstrated significant differences in the characteristics of the well-established CO2 laser cutting and cutting with solid-state disk and fiber lasers. The reasons for the observed differences in cutting efficiency and cut edge quality are still the subject of the current research and not finally clarified. In order to further the understanding of the involved phenomena, a series of cutting experiments with CO2 and disk lasers was carried out using a Design of Experiment (DoE) approach. The particular feature of the applied experimental setup was the similar geometry of both the CO2 and the disk laser beam with comparable values of the focus diameter and the Rayleigh length. Cutting trials on cold work steel test specimens with different sheet thicknesses were performed. The extensions of the generated cut kerf and of the heat-affected zone as well as the recast layer were analyzed in order to reach a better understanding of the physical mech...

Stabilisation of plasma welding arcs by low power laser beams

Abstract 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 conclusions about the most probable physical reasons for the stabilising action of the laser beam are drawn.

Laser-assisted plasma arc welding of stainless steel

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.

Experimental investigation on the cut front geometry in the inert gas laser fusion cutting with disk and CO2 lasers

In recent years, several studies on laser beam fusion cutting demonstrated significant differences in the characteristics of the well-established CO2 laser cutting and cutting with solid-state disk and fibre lasers. The absorptivity behaviour of metals shows essential quantitative differences for the corresponding wavelengths of both laser sources as a function of the inclination angle between the laser beam and the material to be irradiated. In order to further the understanding of the involved phenomena, a series of cutting experiments with CO2 and disk lasers was carried out. Similar geometry of CO2 and disk laser beam in terms of focus diameter and Rayleigh length was used. In this work, the two dimensional stationary geometric shape of the cutting front is acquired by the longitudinal sections of the cut specimen. The cutting front geometry and the corresponding local inclination angle are related with the absorptivity calculated in each point of the cutting front. The effects of various laser processing parameters, as focal position and cutting speed, on the geometric shape of the cutting front have been analyzed systematically on steel sheet 5 and 8 mm thick.In recent years, several studies on laser beam fusion cutting demonstrated significant differences in the characteristics of the well-established CO2 laser cutting and cutting with solid-state disk and fibre lasers. The absorptivity behaviour of metals shows essential quantitative differences for the corresponding wavelengths of both laser sources as a function of the inclination angle between the laser beam and the material to be irradiated. In order to further the understanding of the involved phenomena, a series of cutting experiments with CO2 and disk lasers was carried out. Similar geometry of CO2 and disk laser beam in terms of focus diameter and Rayleigh length was used. In this work, the two dimensional stationary geometric shape of the cutting front is acquired by the longitudinal sections of the cut specimen. The cutting front geometry and the corresponding local inclination angle are related with the absorptivity calculated in each point of the cutting front. The effects of various laser proces...

Improvements of the welding performance of plasma arcs by a superimposed fibre laser beam

Details and results of experimental investigations of a laser-supported plasma arc welding process are presented. The particular feature of the realized experimental set-up is the coaxial arrangement of a single-mode fibre laser beam through a hollow tungsten electrode in combination with a modified plasma welding torch. The analysis of the welding capabilities of the combined laser-arc source comprises high-speed video recordings of the arc shape and size, corresponding simultaneous measurements of the arc voltage as well as an evaluation of the resultant weld seam geometries. Results of welding trials on different types of steel and aluminum alloys are discussed. The corresponding investigations reveal that a fibre laser beam with a wavelength of 1.07 microns can have a crucial impact on the arc and welding characteristics for both categories of materials even at very low laser power output levels. Beneficial effects are especially observed with high welding speeds. In that particular case the arc root and therefore arc column can be substantially stabilized and guided by the laser-induced hot spot.

Thermodynamic evaluation of inert-gas laser beam fusion cutting with CO2, disk and fiber lasers

Inert-gas laser beam fusion cutting is an established thermal cutting technology for stainless and other high-alloyed steels. At the present time, CO2 lasers are particularly applied for corresponding cutting applications but the nowadays available solid-state disk and fiber lasers seem to be an attractive alternative as efficient cutting heat sources due to their advantages in power efficiency, beam guidance and beam quality. It is consequently anticipated that disk and fiber lasers possess a great potential to change the limits of the cutting performance.However, comparative experimental studies have also shown some distinct differences in the process behavior of CO2, disk and fiber lasers cutting which are not easy to understand at the first sight. Consequently, the available cutting data were thermodynamically analyzed. The theoretical investigations revealed some possible reasons for the detected quantitative and qualitative differences between the cutting results with different laser sources and the achieved deeper understanding of the relevant interactions between the beam properties and the cutting efficiency can be useful for the further establishment of disk and fiber laser cutting.Inert-gas laser beam fusion cutting is an established thermal cutting technology for stainless and other high-alloyed steels. At the present time, CO2 lasers are particularly applied for corresponding cutting applications but the nowadays available solid-state disk and fiber lasers seem to be an attractive alternative as efficient cutting heat sources due to their advantages in power efficiency, beam guidance and beam quality. It is consequently anticipated that disk and fiber lasers possess a great potential to change the limits of the cutting performance.However, comparative experimental studies have also shown some distinct differences in the process behavior of CO2, disk and fiber lasers cutting which are not easy to understand at the first sight. Consequently, the available cutting data were thermodynamically analyzed. The theoretical investigations revealed some possible reasons for the detected quantitative and qualitative differences between the cutting results with different laser sources and the...

Development and experimental analysis of laser-assisted plasma ARC welding

A new welding heat source consisting of a plasma arc and a coaxially superimposed low-power laser beam was developed. For this purpose, a single-mode fiber laser beam was guided through the hollow tungsten electrode of a commercial plasma welding torch. The capabilities of the corresponding welding process were experimentally analyzed for bead-on-plate welding trials on steels and aluminum alloys. Besides the evaluation of the resultant seam geometries also the behavior of the arc under the action of the additional laser beam was studied by means of arc voltage measurements and high-speed camera observations. The results of this study demonstrate so far beneficial improvements of the combined process in comparison to the single welding techniques. First, the melting efficiency can be significantly increased for particular parameter constellations. Secondly, the additional laser beam is very capable of stabilizing the arc attachment under conditions of low arc currents and/or high welding speeds. The experimental results also provided some new insights into possible physical reasons of the observed laser-arc interactions.A new welding heat source consisting of a plasma arc and a coaxially superimposed low-power laser beam was developed. For this purpose, a single-mode fiber laser beam was guided through the hollow tungsten electrode of a commercial plasma welding torch. The capabilities of the corresponding welding process were experimentally analyzed for bead-on-plate welding trials on steels and aluminum alloys. Besides the evaluation of the resultant seam geometries also the behavior of the arc under the action of the additional laser beam was studied by means of arc voltage measurements and high-speed camera observations. The results of this study demonstrate so far beneficial improvements of the combined process in comparison to the single welding techniques. First, the melting efficiency can be significantly increased for particular parameter constellations. Secondly, the additional laser beam is very capable of stabilizing the arc attachment under conditions of low arc currents and/or high welding speeds. The exper...