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Effect of cathode model on arc attachment for short high-intensity arc on a refractory cathode
University West, Department of Engineering Science, Division of Welding Technology. (PTW)ORCID iD: 0000-0002-7897-621X
University West, Department of Engineering Science, Division of Manufacturing Processes. (PTW)ORCID iD: 0000-0003-2535-8132
Chalmers University of Technology.
2016 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 49, no 3 November 2016, 1-17 p., 485201Article in journal (Other academic) Published
Abstract [en]

Various models coupling the refractory cathode, the cathode sheath and the arc at atmospheric pressure exist. They assume a homogeneous cathode with a uniform physical state, and differ by the cathode layer and the plasma arc model. However even the most advanced of these models still fail in predicting the extent of the arc attachment when applied to short high-intensity arcs such as gas tungsten arcs. Cathodes operating in these conditions present a non-uniform physical state. A model taking into account the first level of this non-homogeneity is proposed based on physical criteria. Calculations are done for 5 mm argon arcs with a thoriated tungsten cathode. The results obtained show that radiative heating and cooling of the cathode surface are of the same order. They also show that cathode inhomogeneity has a significant effect on the arc attachment, the arc temperature and pressure. When changing the arc current (100 A, 200 A) the proposed model allows predicting trends observed experimentally that cannot be captured by the homogeneous cathode model unless restricting a priori the size of the arc attachment. The cathode physics is thus an important element to include to obtain a comprehensive and predictive arc model

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2016. Vol. 49, no 3 November 2016, 1-17 p., 485201
Keyword [en]
activated tungsten cathode, arc attachment, electron emission, gas tungsten arc, modelling, thermal plasma
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
URN: urn:nbn:se:hv:diva-9357DOI: 10.1088/0022-3727/49/48/485201ISI: 000404349900001Scopus ID: 2-s2.0-84997269626OAI: oai:DiVA.org:hv-9357DiVA: diva2:932039
Note

Ingår i doktorsavhandling

Available from: 2016-05-31 Created: 2016-05-31 Last updated: 2017-09-19Bibliographically approved
In thesis
1. Modelling of cathode-plasma interaction in short high-intensity electric arc: Application to Gas Tungsten Arc Welding
Open this publication in new window or tab >>Modelling of cathode-plasma interaction in short high-intensity electric arc: Application to Gas Tungsten Arc Welding
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In arc welding the quality of the weld is strongly influenced by the thermal history of the workpiece which is itself governed by the electric arc heat source. The models for predicting weld properties thus need a good evaluation of the distribution of the heat input from thearc to the workpiece. To have a predictive model of arc heat source it is necessary to take into account the cathode and its coupling with the plasma. The coupling allows to calculate the temperature and current density distributions along the cathode surface rather than prescribing them. This thesis focuses on the arc-cathode coupling for a plasma assumed to be in local thermal equilibrium. A self-consistent coupling boundary model for high-intensity electric arc on a refractory cathode (thoriated tungsten) was developed accounting for the physics of the sub-layers of the cathode layer and the non-uniformity of the cathode surface physical state. The cathode layer model accounts for the non-equilibria in the cathode layer. It was tested in one-dimensional calculations and then extended to a cathode-plasma coupling boundary condition for gas tungsten arc implemented in OpenFOAM. Different modelling assumptions commonly used for developing the model were questioned and investigated. It was checked that the secondary electron emission is negligible compared to the effect of emitted electrons and ions. It was verified that it is justified to neglect the space charge of emitted electron when calculating the cathode surface electric field. It was verified that Richardson-Dushman electron emission law supplemented with Schottky correction is used within its domain of validity in GTA applications even for low work function emitters. It was shown that the radiative absorption of the cathode surface is not negligible compared to the radiative emission. The cathode layer model was also further developed to take into account the in homogeneity of the cathode material. It was shown that the cathode in homogeneityhas a significant effect on the size of the arc attachment and consequently on the cathode surface and the plasma temperature. Good agreement was obtained with the measured cathode surface and plasma temperatures without imposing any adjustable parameters. The results showed that the proposed model, which is only based on physical principles, is ableto predict the trends observed experimentally.

Place, publisher, year, edition, pages
Göteborg: Chalmers University of Technology, 2016. 78 p.
Series
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie, ISSN 0346-718X ; 4062
Keyword
Electric arc discharge, sheath, pre-sheath, Knudsen layer, doped refractory cathode, arc-cathode coupling, Gas Tungsten Arc simulation, OpenFOAM.
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-9358 (URN)9789175973814 (ISBN)
Public defence
2016-06-10, VDL, Chalmers Tvärgata 4C, Chalmers, Göteborg, 10:00 (English)
Opponent
Supervisors
Available from: 2016-05-31 Created: 2016-05-31 Last updated: 2016-05-31Bibliographically approved

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