Michael Schnick

Metal vapour causes a central minimum in arc temperature in gas-metal arc welding through increased radiative emission

A computational model of the argon arc plasma in gas‐metal arc welding (GMAW) that includes the influence of metal vapour from the electrode is presented. The occurrence of a central minimum in the radial distributions of temperature and current density is demonstrated. This is in agreement with some recent measurements of arc temperatures in GMAW, but contradicts other measurements and also the predictions of previous models, which do not take metal vapour into account. It is shown that the central minimum is a consequence of the strong radiative emission from the metal vapour. Other effects of the metal vapour, such as the flux of relatively cold vapour from the electrode and the increased electrical conductivity, are found to be less significant. The different effects of metal vapour in gas‐tungsten arc welding and GMAW are explained.

Modelling of gas–metal arc welding taking into account metal vapour

The most advanced numerical models of gas–metal arc welding (GMAW) neglect vaporization of metal, and assume an argon atmosphere for the arc region, as is also common practice for models of gas–tungsten arc welding (GTAW). These models predict temperatures above 20 000 K and a temperature distribution similar to GTAW arcs. However, spectroscopic temperature measurements in GMAW arcs demonstrate much lower arc temperatures. In contrast to measurements of GTAW arcs, they have shown the presence of a central local minimum of the radial temperature distribution.This paper presents a GMAW model that takes into account metal vapour and that is able to predict the local central minimum in the radial distributions of temperature and electric current density. The influence of different values for the net radiative emission coefficient of iron vapour, which vary by up to a factor of hundred, is examined. It is shown that these net emission coefficients cause differences in the magnitudes, but not in the overall trends, of the radial distribution of temperature and current density. Further, the influence of the metal vaporization rate is investigated. We present evidence that, for higher vaporization rates, the central flow velocity inside the arc is decreased and can even change direction so that it is directed from the workpiece towards the wire, although the outer plasma flow is still directed towards the workpiece. In support of this thesis, we have attempted to reproduce the measurements of Zielinska et al for spray-transfer mode GMAW numerically, and have obtained reasonable agreement.

Numerical simulation of droplet detachment in pulsed gas–metal arc welding including the influence of metal vapour

A numerical model of the droplet detachment of a gas–metal arc welding process is presented. The model is based on the volume of fluid method and focuses on the detailed description of the interaction between the arc and the anodic wire electrode. The influence of metal vapour on the arc plasma and the arc attachment at the wire is taken into account. The formation of metal vapour at the wire is described self-consistently as a function of the wire temperature by the help of the Hertz–Knudsen–Langmuir equation. Results are presented for a pulsed gas–metal arc welding process with a wire of mild steel and argon as the shielding gas.

Three-dimensional modelling of arc behaviour and gas shield quality in tandem gas–metal arc welding using anti-phase pulse synchronization

The paper presents a transient three-dimensional model of an anti-phase-synchronized pulsed tandem gas–metal arc welding process, which is used to analyse arc interactions and their influence on the gas shield flow. The shielding gases considered are pure argon and a mixture of argon with 18% CO2. Comparison of the temperature fields predicted by the model with high-speed images indicates that the essential features of the interactions between the arcs are captured. The paper demonstrates strong arc deflection and kinking, especially during the low-current phase of the pulse, in agreement with experimental observations. These effects are more distinct for the argon mixture with 18% CO2. The second part of the paper demonstrates the effects of arc deflection and instabilities on the shielding gas flow and the occurrence of air contamination in the process region. The results allow an improved understanding of the causes of periodic instabilities and weld seam imperfections such as porosity, spatter, heat-tint oxidation and fume deposits.

Visualization and Optimization of Shielding Gas Flows in Arc Welding

GMA welding is one of the most frequently applied welding techniques in industry. Particularly the joining of aluminium, high alloyed steels or titanium requires a cover of shielding gas in order to provide a low PPM concentration of oxygen. The result of the welding process depends essentially on the chemical and thermophysical properties of the process gas used. Consequently, it is necessary to be able to describe and to analyse its flow with respect to various influencing variables. However, it is very difficult to realize this during arc welding processes; a poor access is predominant due to the covered areas inside the welding torch and temperatures of up to 20 000 K cause the strong radiation of the arc and electromagnetic fields. This paper deals with experimental and numerical methods for visualization and quantification of process gas flows in arc welding and gives examples for their technical applications. Unlike previous work, the described methods consider the arc as a dynamic element which determinates the gas flow. Advanced Particle Image Velocimetry (PIV) and Schlieren measurement were used for characterization of the flow field in the direct vicinity of the arc in GTA and GMA welding. Furthermore, a numerical model including magneto-hydrodynamics and turbulence models was used for a detailed visualization of the flow in the free jet and in the hidden interior of the torch. It is based on a commercial CFD code which allows to model complex 3-D geometries of torch and workpiece design. Mixing effects and turbulence model were validated by oxygen measurements in the gas shield.

Thermal plasma properties for Ar–Cu, Ar–Fe and Ar–Al mixtures used in welding plasmas processes: II. Transport coefficients at atmospheric pressure

This article is devoted to the calculation of thermodynamic properties (mass density, enthalpy and specific heat at constant pressure) and transport coefficients (viscosity, electrical conductivity, thermal conductivity and combined diffusion coefficients) of mixtures of argon and aluminum, iron and copper vapour at atmospheric pressure. Data are presented for the temperature range 300 to 30 000 K, for different concentrations of the metal vapours. The dependence of the properties on metal vapour type and concentration are discussed. Mixtures of argon and metal vapour occur in the arc welding and in other plasma applications. Tabulations of the data are presented, and will be of use in computational modelling of such applications.

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.

Numerical Investigations of the Influence of Design Parameters, Gas Composition and Electric Current in Plasma Arc Welding (PAW)

Plasma Tungsten Arc Welding (PTAW) compared with TIG Welding enables an increased welding speed, a reduced energy input per unit length and butt joint welding of plates without preparation of the welded seam due to keyhole effect. However, because of missed profound understanding of effects in plasma arcs, the indisputable advantages of this process increase demands on education and especially on the experience of developers and operators. In this paper, process parameters, properties of the plasma jet and the molten pool are derived from the numerical modelling of arc and sheath layer under consideration of the process gas properties and the effects of demixing. Basics of the model and the testing site are introduced in this paper. Additionally, influences of current intensity, plasma gas quantity, process gases and their specific properties, torch geometry on plasma jet and energy input into work piece is shown by an exemplary torch. Model and numerical results have been validated by impact pressure measurements at the surface of the work piece and penetration profiles (cross section).

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.