Isabelle Choquet

A review of cathode-arc coupling modeling in GTAW

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.

On the choice of electromagnetic model for short high-intensity arcs, applied to welding

We have considered four different approaches for modelling the electromagnetic fields of high-intensity electric arcs: (i) three-dimensional, (ii) two-dimensional axi-symmetric, (iii) the electric potential formulation and (iv) the magnetic field formulation. The underlying assumptions and the differences between these models are described in detail. Models (i) to (iii) reduce to the same limit for an axi-symmetric configuration with negligible radial current density, contrary to model (iv). Models (i) to (iii) were retained and implemented in the open source CFD software OpenFOAM. The simulation results were first validated against the analytic solution of an infinite electric rod. Perfect agreement was obtained for all the models tested. The electromagnetic models (i) to (iii) were then coupled with thermal fluid mechanics, and applied to axi-symmetric gas tungsten arc welding test cases with short arc (2, 3 and 5 mm) and truncated conical electrode tip. Models (i) and (ii) lead to the same simulation results, but not model (iii). Model (iii) is suited in the specific limit of long axi-symmetric arc with negligible electrode tip effect, i.e. negligible radial current density. For short axi-symmetric arc with significant electrode tip effect, the more general axi-symmetric formulation of model (ii) should instead be used.

Effect of cathode model on arc attachment for short high-intensity arc on a refractory cathode

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.

Main issues for a fully predictive plasma spray torch model and numerical considerations

Plasma spray is one of the most versatile and established techniques for the deposition of thick coatings that provide functional surfaces to protect or improve the performance of the substrate material. However, a greater understanding of plasma spray torch operation will result in improved control of process and coating properties and in the development of novel plasma spray processes and applications. The operation of plasma torches is controlled by coupled dynamic, thermal, chemical, electromagnetic, and acoustic phenomena that take place at different time and space scales. Computational modeling makes it possible to gain important insight into torch characteristics that are not practically accessible to experimental observations, such as the dynamics of the arc inside the plasma torch. This article describes the current main issues in carrying out plasma spray torch numerical simulations at a high level of fidelity. These issues encompass the use of non-chemical and non-thermodynamic equilibrium models, incorporation of electrodes with sheath models in the computational domain, and resolution of rapid transient events, including the so-called arc reattachment process. Practical considerations regarding model implementation are also discussed, particularly the need for the model to naturally reproduce the observed torch operation modes in terms of voltage and pressure fluctuations.

A strong ionization model in plasma physics

We derive a fluid model for partially ionized plasmas in which the dominant collisional process is not only driven by elastic collisions against neutrals, but also by ionization/recombination processes, especially concerning the electrons. In terms of applications, these models correspond to the interaction of a glow discharge with an ion beam.

Incubation during laser ablation with bursts of femtosecond pulses with picosecond delays

We report on an experimental investigation of the incubation effect during irradiation of stainless steel with bursts of ultrashort laser pulses. A series of birefringent crystals was used to split the pristine 650-fs pulses into bursts of up to 32 sub-pulses with time separations of 1.5 ps and 3 ps, respectively. The number of selected bursts was varied between 50 and 1600. The threshold fluence was measured in case of Burst Mode (BM) processing depending on the burst features, i.e. the number of sub-pulses and their separation time, and on the number of bursts. We found as many values of threshold fluence as the combinations of the number of bursts and of sub-pulses constituting the bursts set to give the same total number of impinging sub-pulses. However, existing incubation models developed for Normal Pulse Mode (NPM) return, for a given number of impinging pulses, a constant value of threshold fluence. Therefore, a dependence of the incubation coefficient with the burst features was hypothesized and experimentally investigated. Numerical solutions of the Two Temperature Model (TTM) in case of irradiation with single bursts of up to 4 sub-pulses have been performed to interpret the experimental results.

Gas tungsten arc models including the physics of the cathode layer : remaining issues

A recent review pointed out that the existing models for gas tungsten arc coupling the electrode (a cathode) and the plasma are not yet complete enough. Their strength is to predict with good accuracy either the electric potential or the temperature field in the region delimited by the electrode and the workpiece. Their weakness is their poor ability to predict with good accuracy these two fields at once. However, both of these fields are important since they govern the heat flux to the workpiece through current density and temperature gradient. New developments have been made since then. They mainly concern the approaches addressing the electrode sheath (or space charge layer) that suffered from an underestimation of the arc temperature. These new developments are summarized and discussed, the modelling assumptions are examined, and important modelling issues that remain unexplored are underlined.

Incubation effect in burst mode fs-laser ablation of stainless steel samples

We report on an experimental study of the incubation effect during irradiation of stainless steel targets with bursts of femtosecond laser pulses at 1030 nm wavelength and 100 kHz repetition rate. The bursts were generated by splitting the pristine 650-fs laser pulses using an array of birefringent crystals which provided time separations between sub-pulses in the range from 1.5 ps to 24 ps. We measured the threshold fluence in Burst Mode, finding that it strongly depends on the bursts features. The comparison with Normal Pulse Mode revealed that the existing models introduced to explain the incubation effect during irradiation with trains of undivided pulses has to be adapted to describe incubation during Burst Mode processing. In fact, those models assume that the threshold fluence has a unique value for each number of impinging pulses in NPM, while in case of BM we observed different values of threshold fluence for fixed amount of sub-pulses but different pulse splitting. Therefore, the incubation factor coefficient depends on the burst features. It was found that incubation effect is higher in BM than NPM and that it increases with the number of sub-pulses and for shorter time delays within the burst. Two-Temperature-Model simulations in case of single pulses and bursts of up to 4 sub-pulses were performed to understand the experimental results.

Coupling boundary condition for high-intensity electric arc attached on a non-homogeneous refractory cathode

The boundary coupling high-intensity electric arc and refractory cathode is characterized by three sub-layers: the cathode sheath, the Knudsen layer and the pre-sheath. A self-consistent coupling boundary condition accounting for these three sub-layers is presented; its novel property is to take into account a non-uniform distribution of electron emitters on the surface of the refractory cathode. This non-uniformity is due to cathode non-homogeneity induced by arcing. The computational model is applied to a one-dimensional test case to evaluate the validity of different modeling assumptions. It is also applied coupling a thoriated tungsten cathode with an argon plasma (assumed to be in local thermal equilibrium) to compare the calculation results with uniform and non-uniform distribution of the electron emitters to experimental measurements. The results show that the non-uniformity of the electron emitters’ distribution has a significant effect on the calculated properties. It leads to good agreement with the cathode surface temperature, and with the plasma temperature in the hottest region. Some differences are observed in colder plasma regions, where deviation from local thermal equilibrium is known to occur.

A physically based model for thermal plasma arc attachment on a W-ThO2 cathode

Summary form only given. Arc attachment radius imposed a priori when modelling the coupling between cathode, cathode layer and thermal plasma still hinders models from being predictive, as underlined in a recent review1. The aim of this work was to find a physical element, still lacking in the models, which could contribute in governing the arc attachment. In this study the cathode layer is modeled within the frame of the partial local thermal equilibrium approach1 including the space charge layer, the Knudsen layer and the ionization layer, while the plasma column is assumed to be in local thermal equilibrium. Several modeling assumptions were questioned based on e.g. contradictory assumptions in the literature, or oversimplified physics compared to experimental observations. For testing model and assumptions, 5 mm argon arc test cases with a sharp cathode geometry that have been investigated experimentally in the literature were calculated. Within this framework, the following conclusions were drawn. The space charge emitted electrons is negligible. The Richardson-Dushman emission law supplemented with Schottky correction is used within its domain of validity when applied to thorium doped tungsten cathodes, which are mainly characterized by a field enhanced thermionic emission regime. The radiative heat absorption from the plasma at the cathode surface is not negligible compared to the radiative emission. Ignoring the non-homogeneous structure and composition of a doped tungsten cathode operated in these conditions leads to a large over-estimation of the extent of the arc attachment, and results in an under-estimation of the arc temperature. A cathode model based on physical criteria for taking into account a first level of the cathode inhomogeneity has a significant effect on the arc attachment and on arc properties such as temperature and pressure. The cathode physics is thus an important element to include for obtaining a comprehensive and predictive arc model.