Gregor Gött

High-speed three-dimensional plasma temperature determination of axially symmetric free-burning arcs

In this paper we introduce an experimental technique that allows for high-speed, three-dimensional determination of electron density and temperature in axially symmetric free-burning arcs. Optical filters with narrow spectral bands of 487.5?488.5?nm and 689?699?nm are utilized to gain two-dimensional spectral information of a free-burning argon tungsten inert gas arc. A setup of mirrors allows one to image identical arc sections of the two spectral bands onto a single camera chip. Two-different Abel inversion algorithms have been developed to reconstruct the original radial distribution of emission coefficients detected with each spectral window and to confirm the results. With the assumption of local thermodynamic equilibrium we calculate emission coefficients as a function of temperature by application of the Saha equation, the ideal gas law, the quasineutral gas condition and the NIST compilation of spectral lines. Ratios of calculated emission coefficients are compared with measured ones yielding local plasma temperatures. In the case of axial symmetry the three-dimensional plasma temperature distributions have been determined at dc currents of 100, 125, 150 and 200?A yielding temperatures up to 20000?K in the hot cathode region. These measurements have been validated by four different techniques utilizing a high-resolution spectrometer at different positions in the plasma. Plasma temperatures show good agreement throughout the different methods. Additionally spatially resolved transient plasma temperatures have been measured of a dc pulsed process employing a high-speed frame rate of 33000 frames per second showing the modulation of the arc isothermals with time and providing information about the sensitivity of the experimental approach.

Study of the welding gas influence on a controlled short-arc GMAW process by optical emission spectroscopy

The controlled short-arc processes, variants of the gas metal arc welding (GMAW) process, which have recently been developed, are used to reduce the heat input into the workpiece. Such a process with a wire feeding speed which varies periodically, using a steel wire and a steel workpiece to produce bead-on-plate welds has been investigated. As welding gases CO2 and a mixture of Ar and O2 have been used. Depending on the gas, the properties of the plasma change, and as a consequence the weldseams themselves also differ distinctly. Optical emission spectroscopy has been applied to analyse the plasma. The radial profiles of the emission coefficients of an iron line and an argon line or an atomic oxygen line, respectively, have been determined. These profiles indicate the establishment of a metal vapour arc core which has a broader profile under CO2 but is more focused in the centre for argon. The measured iron line emission was near to its norm maximum in the case of CO2. From this fact, temperatures around 8000 K and a metal vapour molar fraction above 75% in the arc centre could be roughly estimated for this case. Estimations of the electrical conductivity and the arc field indicate that the current path must include not only the metal vapour arc core but also outer hot regions dominated by welding gas properties in the case of argon.

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.

Temperature and emissivity determination of liquid steel S235

Temperature determination of liquid metals is difficult but a necessary tool for improving materials and processes such as arc welding in the metal-working industry. A method to determine the surface temperature of the weld pool is described. A TIG welding process and absolute calibrated optical emission spectroscopy are used. This method is combined with high-speed photography. 2D temperature profiles are obtained. The emissivity of the radiating surface has an important influence on the temperature determination. A temperature dependent emissivity for liquid steel is given for the spectral region between 650 and 850 nm.

Study of an ablation-dominated arc in a model circuit breaker

A switching arc in a model circuit breaker is studied by means of CFD simulations and optical emission spectroscopy. The arc is initiated between tungsten–copper electrodes in a carbon dioxide atmosphere and is led through PTFE (polytetrafluorethylene) nozzles. Radiation emitted by the arc plasma is absorbed by the surface of the nozzles leading to ablation of the wall material. The CFD simulations are based on a model of the arcing zone including a consistent treatment of the radiation transport and wall ablation. Carbon ion line radiation is analysed in the experiment using an optical path in the heating channel between the nozzles. Temperature profiles obtained from spectroscopy and simulations are compared. The pressure value in the arc is estimated based on the line width-intensity dependence. The obtained values correspond to the measured pressure outside the arc. Coincidence in temperature for the peak current and discrepancy on the falling edge are found and discussed.

Improvement of the control of a gas metal arc welding process

Up to now, the use of the electrical characteristics for process control is state of the art in gas metal arc welding (GMAW). The aim of the work is the improvement of GMAW processes by using additional information from the arc. Therefore, the emitted light of the arc is analysed spectroscopically and compared with high-speed camera images. With this information, a conclusion about the plasma arc and the droplet formation is reasonable. With the correlation of the spectral and local information of the plasma, a specific control of the power supply can be applied. A corresponding spectral control unit (SCU) is introduced.

Weld pool temperatures of steel S235 while applying a controlled short-circuit gas metal arc welding process and various shielding gases

The temperature determination of liquid metals is difficult and depends strongly on the emissivity. However, the surface temperature distribution of the weld pool is an important characteristic of an arc weld process. As an example, short-arc welding of steel with a cold metal transfer (CMT) process is considered. With optical emission spectroscopy in the spectral region between 660 and 840 nm and absolute calibrated high-speed camera images the relation between temperature and emissivity of the weld pool is determined. This method is used to obtain two-dimensional temperature profiles in the pictures. Results are presented for welding materials (wire G3Si1 on base material S235) using different welding CMT processes with CO2 (100%), Corgon 18 (18% CO2 + 82% Ar), VarigonH6 (93.5% Ar + 6.5% H2) and He (100%) as shielding gases. The different gases are used to study their influence on the weld pool temperature.

Spectral diagnostics of a pulsed gas metal arc welding process

IntroductionPlasma properties in the pulsed arc determine the welding process. They will have influence on the consumable electrode and the weld pool. For that reason, the accurate gauging of the plasma properties is of special importance for deeper understanding of the processes.Material and methodsUsing spectroscopic methods and plasma physical diagnostics, the temperature in the arc during the high-current phase of a pulsed gas metal arc welding process is determined. With this knowledge and composition calculation, the electrical conductivity is also derived. A one-drop-per-pulse process with workpiece and wire made of steel and an argon-dominated shielding gas is considered. Boltzmann plots applied to iron lines, broadening of argon lines or the emission coefficient of optically thin lines are used for the determination of plasma parameters.ResultsIntersections of the arc at different distances from the workpiece are analysed for different times during the pulse. It is observed that the brighter central part of such an arc has a minimum in the temperature profile and contains a high amount of iron.ConclusionConsequently, the central part of the arc has lower electrical conductivity than the outer part dominated by the shielding gas argon.

Study of flux-cored arc welding processes for mild steel hardfacing by applying high-speed imaging and a semi-empirical approach

Arc welding processes with flux-cored wire electrodes are often applied for steel hardfacing. The optimal choice of the process parameters is a key issue for the process stability and the surface quality of the weld seam. However, this complex as a relation between the process characteristics, the predominant mechanisms of the arc, the material transfer, the solidification of the molten pool, the metallurgical properties and therefore the wear behaviour of the weld seam surface is not well studied. Synchronous high-speed imaging with spectral filters and different viewing angles is used for a detailed analysis of the arc attachment at the wire electrode, the wire melting and the behaviour of the weld pool. Two different gas metal arc welding processes, a pulsed process without short circuits and a modified short arc process, and different flux-cored wire electrodes have been used. All the combinations have been studied under different shielding gases: mixtures of Ar with CO2, O2 or He. Macrosections have been used to characterize seam width and dilution. Vickers hardness (HV 0.1) was tested to quantify the hardness of the different phases. The modified short arc processes have turned out to be more stable and go along with a reduced energy transfer to the substrate. As a consequence of the lower energy input, the short arc processes cause a lower dilution but a poor weld seam geometry in comparison with the pulsed processes. The choice of the shielding gas has a significant impact on the melting of the wire and the weld pool behaviour in particular in case of the modified short arc processes. A flatter and regular seam but with more coarse-grained surface structure is obtained with larger admixture of a molecular gas in the shielding gas flow. A semi-empirical approach of the correlation of power input and weld seam geometry demonstrates the potential decrease of the dilution and only smaller changes of the seam form factor by decreasing the electric power of a pulsed process.

High-Speed Visualization of Filament Instabilities and Self-Organization Effect in RF Argon Plasma Jet at Atmospheric Pressure

An RF argon plasma jet has been explored using high-speed camera imaging at 10000 frames/s. Small variations of gas flow and/or RF power lead to instabilities of the filament movement. Two types of instabilities have been observed depending on the interrelated azimuthal velocities of filaments. In the case of antiparallel filament velocities, one filament is collapsing and fuses with the other filament, while the collapsing filament exhibits a striated structure. In the case of parallel velocities, both filaments establish a symmetric configuration and rotate with constant velocity in the jet. Spatially and temporally resolved features are visualized with a time-colored stroboscopic image.