Sascha Rose

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.

Spatial structure of the arc in a pulsed GMAW process

A pulsed gas metal arc welding (GMAW) process of steel under argon shielding gas in the globular mode is investigated by measurements and simulation. The analysis is focussed on the spatial structure of the arc during the current pulse. Therefore, the radial profiles of the temperature, the metal vapour species and the electric conductivity are determined at different heights above the workpiece by optical emission spectroscopy (OES). It is shown that under the presence of metal vapour the temperature minimum occurs at the centre of the arc. This minimum is preserved at different axial positions up to 1 mm above the workpiece. In addition, estimations of the electric field in the arc from the measurements are given. All these results are compared with magneto-hydrodynamic simulations which include the evaporation of the wire material and the change of the plasma properties due to the metal vapour admixture in particular. The experimental method and the simulation model are validated by means of the satisfactory correspondence between the results. Possible reasons for the remaining deviations and improvements of the methods which should be aspired are discussed.

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.

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.

Numerical simulation of arc and droplet transfer in pulsed GMAW of mild steel in argon

The process capability of gas metal arc welding (GMAW) processes is mainly determined by the arc properties and the material transfer. In recent years, numerical methods are being used increasingly in order to understand the complex interactions between the arc and material transfer in gas metal arc welding. In this paper, we summarize a procedure to describe the interaction between an arc and a melting and vaporizing electrode. Thereafter, the presented numerical model is used to investigate the arc properties and the metal transfer for a pulsed GMAW process of mild steel in argon. The results of the numerical simulation are compared with OES measurements as well as high-speed images at different times in the pulse cycle and show excellent agreement. The results illustrate the high influence of the changing vaporization rate on the arc attachment at the wire electrode in the high current phase. It could be shown that a substantial part of the current does not participate in the constriction of the wire electrode via the resulting lorentz force which explains the nearly spatter-free droplet detachment in pulsed GMAW processes of mild steel in argon shielding gas.

Methods and results concerning the shielding gas flow in GMAW

Gas metal arc welding (GMAW) of aluminium, high alloyed steel or titanium requires a shielding gas cover in order to provide preferably low parts per million concentration of oxygen at the joint. Consequently, it is necessary to be able to describe and to analyse the flow of shielding gas. The paper presents numerical and diagnostic investigations concerning the shielding gas flow in gas metal arc welding. Therefore, a numerical model and several diagnostic methods have been developed. The model used is based on ANSYS CFX and includes the effects of magneto hydrodynamic, turbulence and diffusion depending on temperature. The model is verified by Particle Image Velocimetry, Schlieren-technique, and gauging the oxygen concentration. Advantages and disadvantages of these particular methods and the potential of their combined application to analyse welding processes and torch design are shown. The methods introduced were used for the precise analysis of the shielding gas flow and the construction of torches in GMAW. The formation of turbulence by actual concepts of gas distributors and the advantages of optimised and innovative torch constructions are demonstrated. Furthermore the interaction between the process and the shielding gas flow is described and the explicit dependency of the gas cover based on the current profile employed (pulse welding) is visualised. Based on these results, the way in which different gas nozzles influence the shielding gas flow and what happens if the position of the torch or the type of joint changes is explained. In summary, the paper details the profound physical correlations between the construction of the torch and the shielding gas cover as well giving concrete advice for users of the GMAW processes.

TIG narrow gap welding — new approaches to evaluate and improve the shielding gas coverage and the energy input

TIG narrow gap welding is one of the most important methods for joining thick components. Using narrow gap welding, steel plates with a gap, being only a few millimeters wide but several centimeters deep can be joined. Today’s developments are aimed at increasing the process reliability as well as the filling speed. Basic requirements are a sophisticated process understanding — especially concerning the arc attachment and the resulting energy input in the workpiece. Furthermore, the quality of the shielding gas coverage is one of the main important aspects for arc stabilization and the final welding result. In this article, new experimental and numerical methods for the determination of the energy input into the workpiece and the shielding gas coverage are presented. Measurements of electrical current and heat flow distribution as well as oxygen concentration at the workpiece surface are used to validate the numerical model. With the help of the validated model, physical effects can be considered separately, which allows a better investigation of narrow gap welding. The numerical model is also capable of visualizing high-speed processes, such as the transient gas flow and shielding gas coverage.

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.

Numerical Investigations of the Influence of Metal Vapour in GMA Welding

Current numerical models of gas metal arc welding (GMAW) attempt to combine a magnetohydrodynamic (MHD) model of the arc and a volume-of-fluid (VoF) model of metal transfer. But in these models vaporization of metal is neglected and the arc region is assumed to be composed of pure argon, as it is common practice for models of gas tungsten arc welding (GTAW). These models predict temperatures over 20 000 K and a temperature distribution similar to GTAW arcs. However, recent spectroscopic temperature measurements in GMAW arcs have demonstrated much lower arc temperatures. In contrast to GTAW arcs, they found a central local minimum of the radial temperature distribution. The paper presents a GMAW arc model that considers metal vapour and which is in very good agreement with experimentally observed temperatures. Furthermore, the model is able to predict the local central minimum in the radial temperature and the radial electric current density distributions for the first time. The axially symmetric model of the welding torch, the workpiece, the wire and the arc (fluid domain) implements MHD as well as turbulent mixing and thermal demixing of metal vapour in argon. The mass fraction of iron vapour obtained from the simulation shows an accumulation in the arc core and another accumulation on the fringes of the arc at 2 000 to 5 000 K. The demixing effects lead to very low concentrations of iron between these two regions. Sensitive analyses demonstrate the influence of the transport and radiation properties of metal vapour, the welding current and the evaporation rate.