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Modelling of cathode-plasma interaction in short high-intensity electric arc: Application to Gas Tungsten Arc Welding
University West, Department of Engineering Science, Avdelningen för svetsteknologi (SV). (PTW)ORCID iD: 0000-0002-7897-621X
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 [en]
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: urn:nbn:se:hv:diva-9358ISBN: 9789175973814OAI: oai:DiVA.org:hv-9358DiVA: diva2:932049
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
List of papers
1. A review of cathode-arc coupling modeling in GTAW
Open this publication in new window or tab >>A review of cathode-arc coupling modeling in GTAW
2016 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 60, no 4, 821-835 p.Article in journal (Refereed) Published
Abstract [en]

Material properties of welds are strongly influenced by the thermal history, including the thermo-fluid and electromagnetic phenomena in the weld pool and the arc heat source. A necessary condition for arc heat source models to be predictive is to include the plasma column, the cathode, and the cathode layer providing their thermal and electric coupling. Different cathode layer models based on significantly different physical assumptions are being used. This paper summarizes today’s state of the art of cathode layer modeling of refractory cathodes used in GTAW at atmospheric pressure. The fundamentals of the cathode layer and its physics are addressed. The main modeling approaches, namely (i) the diffusion approach, (ii) the partial LTE approach, and (iii) the hydrodynamic approach are discussed and compared. The most relevant publications are systematically categorized with regard to the respective physical phenomena addressed. Results and process understanding gained with these models are summarized. Finally, some open questions are underlined.

Keyword
GTA Welding, Cathodes, Mathematical models, Tungsten electrodes, Reviews, Simulating, Plasma, Heat flow
National Category
Applied Mechanics Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-9268 (URN)10.1007/s40194-016-0319-7 (DOI)000385025900020 ()2-s2.0-84975709499 (ScopusID)
Note

First online: 21 March 2016

Available from: 2016-04-13 Created: 2016-03-30 Last updated: 2016-12-19Bibliographically approved
2. Modelling of electrode-arc coupling in electric arc welding
Open this publication in new window or tab >>Modelling of electrode-arc coupling in electric arc welding
2014 (English)In: Proceedings of The 6th International Swedish Production Symposium 201416-18 September 2014 / [ed] Johan Stahre, Björn Johansson,Mats Björkman, 2014, 1-8 p.Conference paper (Refereed)
Abstract [en]

Modelling of the arc in electric arc welding is significant to achieve a better pro-cess understanding, thus gain better weld quality and a more efficient production process.It requires knowing the conditions at the surfaces of the anode and cathode. These condi-tions are very difficult to set from measurements and should be calculated. This requiresmodelling the complex physics of the electrode layer coupling electrode and arc. Thispaper presents a self-consistent electrode layer model that 1) is suited to welding applica-tions, 2) accounts for the known physics taking place, and 3) satisfies the basic conservationrequirements. The model is tested for different conditions. Its potentiality for welding ap-plications is shown through calculations coupling plasma arc, electrode and cathode layermodels. The calculations are done for both tungsten and thoriated tungsten electrode.

Keyword
thermal plasma, arc welding, electrode layer, sheath, electrode surface temperature, numerical simulation, OpenFOAM.
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-6622 (URN)978-91-980974-1-2 (ISBN)
Conference
The 6th International Swedish Production Symposium 201416-18 September 2014
Projects
SUMMAN
Funder
Knowledge Foundation
Available from: 2014-09-11 Created: 2014-09-11 Last updated: 2016-05-31Bibliographically approved
3. Coupling boundary condition for high-intensity electricarc attached on a non-homogeneous refractory cathode
Open this publication in new window or tab >>Coupling boundary condition for high-intensity electricarc attached on a non-homogeneous refractory cathode
2016 (English)Article in journal (Other academic) Submitted
Keyword
Coupling boundary
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-9356 (URN)
Note

Ingår i dktorsavhandling

Available from: 2016-05-31 Created: 2016-05-31 Last updated: 2016-05-31Bibliographically approved
4. Effect of cathode model on arc attachment for short high-intensityarc on a refractory cathode
Open this publication in new window or tab >>Effect of cathode model on arc attachment for short high-intensityarc on a refractory cathode
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
Keyword
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:nbn:se:hv:diva-9357 (URN)10.1088/0022-3727/49/48/485201 (DOI)2-s2.0-84997269626 (ScopusID)
Note

Ingår i doktorsavhandling

Available from: 2016-05-31 Created: 2016-05-31 Last updated: 2016-12-27Bibliographically approved

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