Masao Ushio

Numerical study of a free-burning argon arc with anode melting

Numerical modeling of free burning arcs and their electrodes is useful for clarifying the heat transfer phenomena in the welding process and to elucidate those effects which determine the weld penetration. This paper presents predictions for a stationary welding process by the free-burning argon arc. The whole region of the welding process, namely, tungsten cathode, arc plasma and stainless steel anode is treated in a unified numerical model to take into account the close interaction between the arc plasma and the molten anode. The time dependent development of two-dimensional distributions of temperature and velocity, in the whole region of the welding process, are predicted at a current of 150 A. The weld penetration geometry as a function of time is thus predicted. It is shown also that different surface tension properties can change the direction of re-circulatory flow in the molten anode and dramatically vary the weld penetration geometry.

Analysis of the TIG welding arc behavior

Abstract Achieving an effective utilization and exploitation of TIG welding arcs requires a thorough understanding of the plasma properties and its physical processes. Through simultaneous solutions of the set of conservation equations for mass, momentum, energy, and current, a mathematical model has been developed to predict the velocity, temperature, and current density distributions in argon welding arcs. The predicted temperature fields in arc regions, and distribution of current density and heat flux at the anode agree well with measurements reported in literature. This work could lay the foundation for developing a comprehensive model of the TIG welding process, where a dynamic two-way coupling between the welding arc and the weld pool surface is properly represented.

Effect of rare earth metal oxide additions to tungsten electrodes

A comparative study has been made on the operating characteristics of gas-tungsten arc (GTA) welding for several types of electrodes. The work was carried out with a pure tungsten electrode and tungsten electrodes activated with a small quantity of the rare earth metal oxides, La2O3, Y2O3, CeO2, and with ZrO2, ThO2, and MgO. Their behaviors during arcing were analyzed and compared from the points of view of arc starting characteristics, electrode consumption, change in shape due to long-term operation, and incompleteness of insert gas shielding and electrode temperature. The results indicated that W-La2O3 electrodes have superior characteristics among those tested. Metallographic studies of the electrodes indicate that the superiority of operating characteristics strongly depends on the behavior of the rare earth metal oxides during arc burning. It is observed that the rare earth metal oxides form tungstate or oxytungstate during arc burning. These newly formed compounds have low melting points and migrate from the low temperature zones to the high temperature zones throughout the electrode tip, while ThO2 reacts with tungsten, forming pure Th. Also, the investigation demonstrates good stability of La2O3 during arc burning compared with the other oxides. Particular attention was also paid to the electrode temperature measurement and the important phenomena concerning the emissivity of a particular surface as one of the thermal properties. The investigation reveals the effects of temperature and oxide distribution on the spectral emissivity of the electrode in addition to the main different effect of oxides added to tungsten. Observations of the cathode tip microstructure during and after arc burning were made, and important phenomena concerning the formation of a tungsten “rim” at the periphery of the cathode area, which governs the durability of the electrode and the stability of the arc, are discussed theoretically and experimentally based on the temperature measurement of the tip and the oxidation of tungsten.

Anode melting from free-burning argon arcs

Predictions have been made of anode melting for free-burning arcs by making two-dimensional calculations of temperature profiles of the arc and the electrodes to make predictions of weld depth and weld shape in arc welding. Predicted properties at 150 A, for various arc lengths, are compared with experimental results of: 1) weld shape; 2) heat intensity as a function of radius; and 3) current density as a function of radius at the anode. The whole region of the arc system is modeled, including the tungsten cathode, the arc plasma and the solid and molten anode, including convection within the molten weld-pool. Theoretical and experimental results are also obtained for highand low-sulfur steel, which have markedly different surface tension properties of the molten liquid, resulting in weld depths that differ by a factor of three because of changed convective circulation properties in the weld pool. Although total power to the anode increases with increasing electrode separation, the current density becomes less, with the result that for electrode distances above 2 mm, for argon, the width and depth of the molten region become less, in agreement with experimental results that we have obtained.

Observations of the anode boundary layer in free-burning argon arcs

In order to make clear the physical grounds of the potential drop in front of the anode, namely, the anode fall in atmospheric free-burning argon arcs, the results of experimental measurements of the laser-scattering method and Langmuir-probe method are presented. The experimental results show that the anode boundary layer at low arc currents such as 50 A remarkably deviates from local thermodynamic equilibrium (LTE), whereas the boundary layer at higher arc currents such as 150 A preserves a similar state to the LTE. The Langmuir-probe measurements also show that the anode fall for 50 A is positive, whereas that for 150 A is negative. From these results, an assumption regarding the physical state of the anode boundary layer in the free-burning argon arcs is presented synthetically and it is also concluded that the sign and magnitude of the anode fall in the arcs relate vary closely to the thermal state of the anode boundary layer and that the thermal state should be influenced strongly by the arc current density, namely, the electron number density.

In situ measurements of electrode work functions in free-burning arcs during operation at atmospheric pressure

A method of in situ measurement of electrode work functions in free‑burning argon arcs during operation is presented. This technique is based on the photoelectric effect ocurring at the surface of a tungsten cathode with the use of a pulse laser system consisting of a NdーYAG laser and a dye laser. Three types of tungsten electrode namely pure W Wー2% ThO2 and Wー2% La 2O3 are used in this work. Free‑burning arcs are operated in argon at atmospheric pressure at cur‑ rents of 100 A and 200 A.The effective work functions of pure W Wー2% ThO2 and Wー2% La2O3 electrodes during ope‑ ration at a current of 200 A are found to be respectively 4.6 2.8 and 3.0 eV from in situ measurements. These results are very close to the work functions of pure W ThO2 and La2O3 obtained from the literature. The in situ measurements how‑ ever show that the effective work functions of pure W Wー2% ThO2 and Wー2% La 2O3 electrodes for an 100 A arc are respectively 2.9 eV 2.6 eV and 2.0 eV. It is shown that each effective work function for an 100 A arc clearly becomes low‑ er than that for a 200 A arc for all types of tungsten electrodes. (Some figures in this article are in colour only in the electronic version

Plasma state in free-burning argon arc and its effect on anode heat transfer

In order to make clear the physical grounds of deviations from local thermodynamic equilibrium (LTE) in atmospheric free-burning argon arcs, the heavy particle temperature, electron temperature and LTE temperature obtained from electron number density were measured by use of line-profile analysis of the laser scattering method without an assumption of LTE. The experimental results showed that the core region of the arc significantly deviated from LTE under both conditions of 50 and 150 A in arc current. As a result, it is suggested that the deviations from LTE in the arc core should be affected strongly by the cathode jet and that aspects of the anode heat transfer were greatly changed by the plasma state in the arc core.

Corrosion and wear behaviors of ferrous powder thermal spray coatings on aluminum alloy

Abstract Atmospheric plasma spray coating was performed on a cast AA383 Aluminum alloy plate, and this process formed a 170-μm thick spray coating layer by using tentative Fe–C powders with nickel added (up to 14 mass%) or without supplementary nickel. Hardness test, microstructure observation, corrosion and wear tests in engine oil with or without sulfuric acid water solutions added were performed as well. The corrosion performance was dependent upon the nickel content. On the contrary, the wear resistance testing under two engine oil lubrication conditions (with or without sulfuric acid water solution) showed the different tendency instead.

One-dimensional analysis of the anode boundary layer in free-burning argon arcs

Numerical analysis of anode boundary layer was carried out to make clear the physical basis of the anode fall in the atmospheric free-burning argon arc. The governing equations of the dominating processes took account of a physical state of non-local thermodynamic equilibrium, and they were solved by applying the finite difference method. The numerical results showed each anode fall, namely a positive anode fall and a negative one, and also showed the distributions of the temperature and electron number density in the case of each anode fall. We conclude that the gradient of electron number density played the most important role in driving an electron flux toward the anode and also determined the sign and magnitude of the anode fall. Furthermore, the electron density gradient was strongly affected by the relation between arc current density and convective flow in the anode boundary layer.

Effect of Fe content on wear resistance of thermal-sprayed Al–17Si–XFe alloy coating on A6063 Al alloy substrate

Abstract To improve the wear resistance of Al alloy, Al–17 mass% Si– X mass% Fe Al alloy powders with different Fe content from 5 to 30 mass% and Al–50 mass% Fe alloy powders were thermal-sprayed on Al alloy A6063 substrate by a low pressure plasma spraying. Microstructure and wear property of the coatings were evaluated. The most beneficial coating can be obtained with Al–17 mass% Si–10 to 15 mass% Fe alloy powders, showing good wear resistance, four times of the substrate and low friction coefficient, approximately 0.4 without cracking and peeling in the coating and the interface between the substrate.