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Evaporation of hafnium cathode in a plasma cutting arc torch was investigated numerically and experimentally to assess the effects of cathode diameter and operating parameters such as gas pressure, gas flow rate and arc current. A numerical model was developed for arc plasma with consideration of hafnium cathode evaporation. Using this model, the s...
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... calculated result can be related to experimentally obtained results [25], showing that an increase in the plasma-gas inlet pressure allows the plasma jet constriction, which improves the cutting quality and reduces nozzle wear. Figure 10 presents the radial distribution of the surface temperature of the hafnium cathode for different gas pressures. This calculation shows that the surface cathode temperature between 0.35 mm in radius increases with gas pressure. ...
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... figures indicate that an increase in gas pressure induces shrinkage of the arc root, which allows a high-current density near the surface cathode, resulting in more efficient heating of cathode surface and therefore a higher temperature of the cathode surface. As a result portrayed in figure 13, the maximum in mass fraction of hafnium vapour is predicted at the radial position off-axis because the arc temperature achieves its highest value there. The peak of radial mass fraction increases markedly as the gas pressure rises. ...
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... effect of gas pressure on evaporation of the hafnium cathode was calculated similarly. Figure 14 shows the net evaporation mass flux as a function of the gas pressure. The increase in the gas pressure results in increased net evaporation mass flux from the hafnium cathode. ...
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... cathode diameter was 1.6 mm, and the arc current was 100 A for different gas flow rates. Figure 15 shows the effects of gas flow rate on the temperature distribution of arc plasma for 20 slm and 50 slm, respectively. It is apparent that a higher gas flow rate will tend to constrict the arc plasma and make it narrower. ...
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... radial distribution of the surface temperature of the hafnium cathode for different gas flow rates is shown in figure 16. As the gas flow rate increases, the cooling effect becomes stronger, causing a drop in temperature at the hafnium insert centre. ...
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... the highest arc temperature is increased in the region that is distant from the arc centre. Furthermore, figure 19 presents a comparison of gas flow fields obtained with the gas flow rates of 20 and 50 slm. It is readily apparent that the gas flow in front of the cathode increases along the centre of the axial direction with a higher gas flow rate. ...
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... the same trend as that of the radial temperature distribution, the maximum in the radial mass fraction increases with the increase in gas flow rate. The results presented in figure 21 and table 4 indicate that the mass flux of hafnium vapour ejected from the cathode and the calculated amount of mass loss of the hafnium cathode evaporation increase simultaneously with an increase in the gas flow rate. ...
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Citations
... Ainsi, une électrode creusée impliquera un allongement de l'arc qui sera compensé par la CNC en rapprochant la torche de l'ouvrage, augmentant les risques dus aux projections. [6]. L'érosion augmente, pour un diamètre supérieur, de par un effet Joule accentué car l'insert est moins bien refroidi par le cuivre environnant. ...
Pour deux configurations d’électrode utilisées sur une torche de découpe par plasma d’air, une étude expérimentale est menée. Elle s'intéresse à l'effet de la tension d'arc sur leur usure. Ces deux configurations d'électrodes (A&B) du commerce présentent des compositions chimiques ainsi que des inserts en hafnium de diamètres différents. Des campagnes expérimentales sont menées afin d’étudier la profondeur des cratères à l'aide d'un comparateur à aiguille. Ces mesures sont complétées par des observations au microscope électronique.
... The welds were conducted for plasma currents of DC25 A, DC50 A, DC75 A and DC100 A. For comparison purposes, conventional MIG welding tests were also conducted using the same plasma MIG welding system, with the plasma current disabled. A two-color temperature measurement method was used to measure the weld bead temperatures during welding [13]. Cross-sections of the resultant welds were micrographically prepared by grinding and polishing to a mirror surface and etched using nital 2%. ...
... The finer grain structures in plasma MIG welding is believed due to the low heat input into base metal during welding as a consequence of the lower droplet temperature as a result of the presence of plasma flow in plasma MIG welding process [9]. During the welding process, the weld bead temperature is measured by using a twocolor temperature measurement method [13]. Figure 3 shows the analysis of the weld pool temperatures for different plasma currents. ...
Plasma MIG welding is a hybrid welding process that combines two welding methods of conventional metal inert gas (MIG) welding with plasma arc welding. This study investigates the effect of plasma and plasma current values on the microstructure and microhardness properties of welded carbon steel plates. It was found that utilization of the plasma has resulted in a refined microstructure in the heat affected zones (HAZ), and a decrease in microhardness values as compared to conventional MIG welds. This potentially increases the ductility of the plasma MIG weldments. Furthermore, decreasing the plasma currents would result in the decrease of microhardness and grain sizes, thus further increasing the ductility of the welds.
... Many studies have been carried out for regulating the work function through surface engineering, [2][3][4] applying electric field, 5 and dimension reduction. 6 Hafnium (Hf ), usually used as a refractory electrode in plasma cutting due to its high melt temperature and the ability to emit electrons into air, [7][8][9][10] has been widely studied both experimentally and theoretically, [11][12][13][14] and special attention was paid to polycrystalline 11,[15][16][17][18] because, in practice, it is difficult to prepare perfect single crystals due to radiation, stress, and other factors, and the resulting grain boundaries (GBs) have great impacts on the properties of materials. For example, Huang et al. found that the presence of grain boundaries reduces the fracture load of graphene from 1.7 μN to 100 nN 19 and Poudel et al. found that the dimensionless thermoelectric figure of merit (ZT) can be enhanced from 1 to 1.4 at 100°C in a p-type nanocrystalline BiSbTe bulk alloy 20 with grain boundaries and the bandgap of ZnO is reduced from 3.22 to 3.15 eV with the average grain size changing from 76 ± 1 to 25 ± 0.5 nm. ...
Hafnium (Hf) has been used as a cathode material for thermionic emission in high temperature environments for a long time. However, the effect of grain boundaries (GBs) on its work function has not been reported. In this work, by using first-principles calculations, we find that the introduction of GBs would reduce the work function of Hf surface as compared with that of the perfect crystal, and by increasing the distance between two grain boundaries, the work function converges gradually to the value of monocrystalline Hf. By analyzing the surface atomic structure and charge density distribution, we find that the reduced work function of GB-containing structures originates from the increase of atomic distance and the changes of atomic coordination environments at the GB region, which results in redistribution of electrons and enhances the electronic density of states at the Fermi level.
... Table 1 shows the welding parameter for both conventional MIG and plasma MIG welding. Fig. 2 shows the peak temperature of weld pool in conventional MIG welding and plasma MIG welding that was measured by using two-color temperature measurement method [6] [7]. The analysis results clearly showed that the weld pool peak temperature of conventional MIG welding was 2287 K and the one in plasma MIG welding was 1885 K. ...
The use of plasma MIG welding which consists of plasma flow in surrounding of the MIG wire causes the divergence of MIG current from MIG wire to outside. The divergence then disperses the heat and force to the upper side of MIG wire, thus reduces the droplet temperature and affects the droplet size as well as metal transfer frequency. In this study, the effects of plasma flow in plasma MIG welding to the refinement of microstructure at heat affected zone is investigated. The microstructure evaluation is carried out by means of electron backscattered diffraction (EBSD). As a result, we found that the microstructure refined at HAZ area especially at coarse grain HAZ. The difference in grain size at coarse grain HAZ area and fine grain HAZ area was almost eliminated. This result was compared with the one with conventional MIG welding, which shows the clear difference in grain size at coarse grain HAZ and fine grain HAZ area. It is suggested that, the presence of plasma flow in plasma MIG welding process refines the microstructure at HAZ area.
... Temperature distribution on weld pool surfaces (top surface and bottom surface) are captured by a high temperature measurement system [26]. A schematic illustration of temperature distribution measurement on the weld pool surfaces is denoted in Fig. 1. ...
... Due to strong radiation and intensity of the arc, the image is captured immediately after switching off the main arc (within 0.001 s after cutting arc). It is confirmed in a recent paper that the decreased temperature during the cutting arc is enough small to be ignorable [26]. Then, the temperature distribution is computed from the ratio of R sensor signal to G sensor signal in the image by Thermera-HS software. ...
This investigation aims to discuss the formation process of eddies and the heat transportation in plasma keyhole arc welding. In order to clarify this issue, the measurement of the convection inside the weld pool, the convection on the weld pool surface, also the temperature distribution on the weld pool surface were carried out. The results showed that two eddies were found in the weld pool, which is controlled mainly through the shear force by the plasma flow acting on the weld pool surface. The magnitude, extent and direction of the shear force are thought to be determined primarily by the variation of keyhole profile. The relative shape and strength of each eddy is largely changed depending on the change of the keyhole profile when nozzle diameter changed. These relative strengths of each eddy are considered to decisively govern the heat transport in the weld pool coinciding with the direction of eddies. A larger eddy near the lower part of the keyhole inside the weld pool was found out in the case of 1.6 mm, meanwhile a upward larger eddy was found out near the upper part of the keyhole inside the weld pool in the case of 2.4 mm.
... Droplet temperature measurement is conducted using a two-color temperature measurement method [35,36]. The color HSVC (Miro eX, Phantom) camera with microlens (Sigma DG 28-300mm, Nikon) is used. ...
The metal transfer behavior is one of the most pivotal links of gas metal arc welding (GMAW) for improving the welding quality. Arc impacts the current path and heat input on the droplet surface, in reverse, the metal droplet detachment makes the arc to flicker. It is hard to make the metal transfer process clear with only experimental methods since the limited measuring space and high temperature in the welding region. In this work, with the help of computational fluid dynamics (CFD) software ANSYS FLUENT, a magnetohydrodynamic (MHD) model including argon arc plasma and steel wire and the droplet is constructed by using two independent governing equations of gas and metal phase for the mass, momentum, and energy balance. Comparing the magnetic field controlled GMAW with conventional GMAW, the temperature field, current density distribution, electric potential field, static pressure field and also metal vapor mass fraction distribution are highlighted. It is shown that the metal transfer frequency decreases and droplet diameter increases with 180A welding current when applying an external constant axial magnetic field 0.002T. So, the arc length decreases before the detaching moment, the current density expands in the arc region and the voltage decreases. The centrifugal movement impedes the downward flow and transfers heat from the droplet center to the outside region.
... The two-color temperature measurement method has been used in past studies to measure molten steel temperatures [18,19]. In this method, three images in wavelength ranges corre sponding to red (R), green (G) and blue (B) are captured simultaneously by different color sensors equipped in the camera. ...
In the present study, the mechanism to control droplet temperature in the plasma MIG welding was discussed based on the measurements of the droplet temperature for a wide range of MIG currents with different plasma electrode diameters. The measurements of the droplet temperatures were conducted using a two color temperature measurement method. The droplet temperatures in the plasma MIG welding were then compared with those in the conventional MIG welding. As a result, the droplet temperature in the plasma MIG welding was found to be reduced in comparison with the conventional MIG welding under the same MIG current. Especially when the small plasma electrode diameter was used, the decrease in the droplet temperature reached maximally 500 K. Also, for a particular WFS, the droplet temperatures in the plasma MIG welding were lower than those in the conventional MIG welding. It is suggested that the use of plasma contributes to reducing the local heat input into the base metal by the droplet. The presence of the plasma surrounding the wire is considered to increase the electron density in its vicinity, resulting in the arc attachment expanding upwards along the wire surface to disperse the MIG current. This dispersion of MIG current causes a decrease in current density on the droplet surface, lowering the droplet temperature. Furthermore, dispersed MIG current also weakens the electromagnetic pinch force acting on the neck of the wire above the droplet. This leads to a larger droplet diameter with increased surface area through lower frequency of droplet detachment to decrease the MIG current density on the droplet surface, as compared to the conventional MIG welding at the same MIG current. Thus, the lower droplet temperature is caused by the reduction of heat flux into the droplet. Consequently, the mechanism to control droplet temperature in the plasma MIG welding was clarified.
... The measured temperature is considered to be slightly lower than that at the full current. However, Yamazaki et al reported that the decrease in the surface temperature is negligible within 2 ms after the cutting arc [26]. After the welding, the temperature distribution is calculated from the ratio of the R sensor signal to the G sensor signal in the image through the twocolor pyrometry method by Thermera-HS software. ...
This paper seeks to clarify the weld pool formation process in plasma keyhole arc welding (PKAW). We adopted, for the first time, the measurement of the 3D convection inside the weld pool in PKAW by stereo synchronous imaging of tungsten tracer particles using two sets of x-ray transmission systems. The 2D convection on the weld pool surface was also measured using zirconia tracer particles. Through these measurements, the convection in a wide range of weld pools from the vicinity of the keyhole to the rear region was successfully visualized. In order to discuss the heat transport process in a weld pool, the 2D temperature distribution on the weld pool surface was also measured by two-color pyrometry. The results of the comprehensive experimental measurement indicate that the shear force due to plasma flow is found to be the dominant driving force in the weld pool formation process in PKAW. Thus, heat transport in a weld pool is considered to be governed by two large convective patterns near the keyhole: (1) eddy pairs on the surface (perpendicular to the torch axis), and (2) eddy pairs on the bulk of the weld pool (on the plane of the torch). They are formed with an equal velocity of approximately 0.35 m s⁻¹ and are mainly driven by shear force. Furthermore, the flow velocity of the weld pool convection becomes considerably higher than that of other welding processes, such as TIG welding and GMA welding, due to larger plasma flow velocity.
... Further development of diagnostic methods applicable to non-equilibrium regions of the plasma is also required, for example, the 2D timedependent laser-scattering techniques that have been applied to combustion and circuit breaker arcs. Development of methods that allow access to regions that are hidden from view, such as within plasma spraying and cutting torches [128,132], within the kerf in plasma cutting [128,131], and beneath the flux in submerged arc welding [127], would greatly increase our understanding of these approaches. Figure 25 shows two innovative approaches. ...
Journal of Physics D: Applied Physics published the first Plasma Roadmap in 2012 consisting of the individual perspectives of 16 leading experts in the various sub-fields of low temperature plasma science and technology. The 2017 Plasma Roadmap is the first update of a planned series of periodic updates of the Plasma Roadmap. The continuously growing interdisciplinary nature of the low temperature plasma field and its equally broad range of applications are making it increasingly difficult to identify major challenges that encompass all of the many sub-fields and applications. This intellectual diversity is ultimately a strength of the field. The current state of the art for the 19 sub-fields addressed in this roadmap demonstrates the enviable track record of the low temperature plasma field in the development of plasmas as an enabling technology for a vast range of technologies that underpin our modern society. At the same time, the many important scientific and technological challenges shared in this roadmap show that the path forward is not only scientifically rich but has the potential to make wide and far reaching contributions to many societal challenges.
... During welding process, because too high bright radiation of the arc can impair the camera, images were captured immediately after Plasma arc and MIG arc were switched off from 180 A Plasma welding current and from 160 A MIG welding current, simultaneously. The temperature distribution on the weld pool surfaces was measured optically 6) from the perpendicular direction to the weld seam immediately after the arc vanished via the processing of Thermera-HS software using intensity ratio of green and blue signals for which the calibration was done. ...
This investigation aims to develop a new Plasma-MIG hybrid welding process for butting welding joints of large thickness. In this welding process, Plasma torch and MIG torch are connected in electrode negative (EN) and in electrode positive (EP), respectively. Plasma torch is set up in the leading position, meanwhile the MIG torch is set up in the trailing position. The plasma welding is utilized in keyhole model. The results show that the successful single-sided welding in one pass is fully penetrated. The wettability of welding joints is improved and the penetration is increased in comparison with only conventional MIG welding process and only conventional Plasma welding process. In addition, in order to discuss the research results, the temperature field on the surface of weld pool is also measured. As a result, the temperature on the weld pool surface in Plasma-MIG hybrid welding case is higher in comparison with conventional MIG welding case, especially near the leading edge of weld pool.
