Electromagnetic Pinch Force Research Articles

Optimization of the Circular Channel Size and the A.C. Magnetic Field Parameters for Application in a Channel-Type Induction-Heating Tundish

In this paper, the appropriate channel design and operating conditions for the simultaneous operation of the inclusion removal and the induction heating of the molten steel by imposing the A.C. magnetic field around the circular channel have been studied. (1) The effect of the lift force and the turbulence force on the inclusion in the transition zone of the channel has been computed; the results show that when the diameter of the inclusion is not less than 0.1 mm, both forces can be neglected because they are less than the electromagnetic pinch force, especially when the shielding parameter is not less than 5.0. (2) The minimal channel length to remove the inclusions out of the dead zone was computed without the lift force and the turbulence force; the shielding parameter of 10.0~15.9 is optimal to obtain the shortest channel length. Furthermore, the slow velocity of the molten steel is desirable. (3) The molten steel temperature increase per unit channel length by the induction heating can be controlled by the A.C. magnetic field frequency; a high frequency condition is better for the efficient thermal supply in this calculation condition. (4) When the flow velocity of the molten steel in the channel with a length of 1 m and a radius of 0.075 m is 0.1 m/s, the shielding parameter of 15.9 is the optimal parameter to simultaneously remove the inclusions and heat up the molten steel in the circular channel. And when the non-dimensional magnetic field intensity of the A.C. magnetic field is 31.7, the removal rate of the 0.1 mm inclusion in the channel can reach more than 95% and the molten steel temperature can be heated over 10 K.

A new approach to controlling metal transfer by dynamic modification in gas composition of arc atmosphere: studies on pulsed gas MAG welding*

ABSTRACT It has been well known that, in GMA welding, various welding phenomena are largely affected by the type of shielding gas used. However, up to the present time, almost no attempt has been made to utilize ‘gas composition’ for controlling welding phenomena. This study focused on this point and, thus, an investigation was made to understand how dynamic gas compositional modification affected various welding phenomena. In this study, pulsed Ar gas was injected periodically into CO2 shielding gas so that the gas composition of arc atmosphere largely modified in a very short period of time. It was found that this quick gas compositional change from CO2 to Ar led to a large decrease in arc voltage and an arc shape change from a constricted shape to a flare shape and that a molten droplet formed at the electrode tip during the CO2 period was released due to a sudden change in force balance between the molten droplet and the arc. The increase in current stimulated by the voltage drop may have assisted the metal drop detachment due to an increase in electromagnetic pinch force. If the cycle of this periodical Ar gas injection was selected to be shorter than that of repelled metal transfer in CO2 atmosphere, the repelled transfer was avoided and stable drop transfer occurred instead with far less spatter. Appropriate selections of the duration time of pulsed Ar gas injections avoided spray transfer which might cause undesirable finger-like penetration profiles. This new conceptual welding method, namely ‘Pulsed Gas MAG Welding’, required only a small amount of Ar gas to obtain the effects mentioned above.

A comparison of double shielded GMAW-P with conventional GMAW-P in the arc, droplet and bead formation

Abstract Double shielded pulsed gas metal arc welding (D-GMAW-P) was proposed to feed the shielding gas of Argon (Ar) and carbon dioxide (CO2) with two independent coaxial channels. The molten pool was protected from blowholes by improving the flow rates of shielding gas in the inner channel and outer channel. The feeding of Ar in inner channel and CO2 in outer channel made the current density distribution of the double-shielded arc generate a strong electromagnetic pinch force at the necking positon of droplet, which was favorable for droplet transfer, thereby improving welding stability. In the case of avoiding short circuit and spatter, the content of CO2 in shielding gas could be increased to 50 % by the double-channel torch. Compared with the conventional GMAW-P arc, the outer ring of the double-shielded arc had a more significant thermal compression effect on its inner ring, which made the arc heat-pressure distribution more concentrated and the droplet impingement much weaker. It decreased the backward flow velocity of molten metal, thereby weakening its eroding action on the gouging region and weld toe. Both the finger penetration and undercut defect were suppressed. The weld fusion line varied from an S-type to a C-type, which could effectively alleviate stress concentration.

Study on factors affecting the droplet temperature in plasma MIG welding process

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.

Numerical simulation of the transition of metal transfer from globular to spray mode in gas metal arc welding using phase field method

A numerical model is constructed based on the solution of the magnetohydrodynamic equations within the framework of phase field algorithm to simulate the metal transfer process and investigate the mechanism of the transition of metal transfer from globular to spray mode. Surface tension is the strongest driving force acting on the pendent droplet in globular transfer, while the governing force shifts to the electromagnetic pinch force in spray transfer. Driving force balance in the axial direction could be the indicator of detachment in globular transfer, while that force balance doesn’t exist in spray transfer. The condition for the transition from globular to spray transfer is that the local pressure at the root of the droplet caused by the electromagnetic pinch force exceeds the surface tension pressure at the droplet tip corresponding to a droplet radius equals to the wire radius. Compared with volume of fluid method, phase field method shows a more physically realistic estimation of the current path from the drop to the arc plasma and leads to a better agreement with experimental data.

Effect of rare earth metal on plasma properties in GMAW using CO2 shielding gas

Gas metal arc welding (GMAW) process, the effect of welding polarity and rare earth metal (REM) elements on physical properties was investigated through the observation of the plasma characteristics using the optical emission spectroscopy method which is suitable respectively to the O plasma region and the Fe plasma region. At the present experimental conditions, the MG-50 wire electrode and KC-500 wire electrode (REM-added electrode) were used for welding with 100% CO2 shielding gas. The results of the KC-500 wire electrode with DCEN polarity showed that the maximum temperature of the O plasma region was around 13,000–13,500 K. The maximum occurs at the region apart from the arc axis. The effect REM elements can cause high current density and electromagnetic pinch force at the tip of electrode. Meanwhile, the metal droplet diameter became smaller like spray transfer during arc with DCEN polarity.

Characteristics of welding and arc pressure in TIG narrow gap welding using novel magnetic arc oscillation

Tungsten inert gas (TIG) narrow gap welding using magnetic arc oscillation is one of the most important methods for joining thick components. Today’s developments are aimed at increasing a sophisticated process understanding, especially concerning the physical effect of arc behavior, which can provide many strategies to enhance weld prediction and joint quality. In this article, the arc behaviors studied by its arc pressure, static characteristic, and arc profile in terms of arc force are presented. With the help of the forces acting on the welding arc, arc profile can be considered separately to allow an improved investigation of narrow gap welding involved. Measurements of arc pressure distribution are used to validate the weld formation and the final welding result. Under the same conditions, the uniformity of arc pressure distribution in both the bottom and sidewall improved with the increase in magnetic flux density. Furthermore, a comparative study of arc static characteristic with magnetic field and non-magnetic field has also been carried out. The reduction in the difference of arc static characteristic is attributed to the inverse effect between electromagnetic pinch force and Lorentz force with the increasing welding current.

In situ study on solidification controlled by electric current pulse in Al-Bi immiscible alloy via synchrotron radiation imaging technology

The Bi-rich droplets motion in a solidifying Al-Bi immiscible alloy was in situ investigated using a synchrotron microradiography technique. The electric current pulse (ECP) was applied to control the droplets motion and distribution in the Al-matrix during solidification. Without ECP treatment, it was found that the second phase presented spherical droplets and distributed in the Al-matrix without obvious aggregation and segregation. With ECP treatment, the second phase seriously aggregated and formed lots of big clusters in the matrix. The electromagnetic pinch force induced by ECP was used to explain this phenomenon.

Real-Time Observation on Evolution of Droplets Morphology Affected by Electric Current Pulse in Al-Bi Immiscible Alloy

The evolution of Bi-rich droplets morphology in a solidifying Al-Bi immiscible alloy was directly observed using a synchrotron microradiography technique. The electric current pulse (ECP) was applied to control the solidification process of Al-Bi immiscible alloy. It was found that the electromagnetic pinch force and Marangoni force induced by ECP and temperature gradient, respectively, can significantly affect the distribution of Bi-rich droplets. The electromagnetic pinch force drove the droplets from the center to side; meanwhile, the Marangoni force lifted the droplets from the bottom to the top. As a result, the droplets finally distributed with a manner of “inverted triangle.”

Numerical Model of Metal Transfer Using an Electrically Conductive Liquid

In order to understand the mechanism of the metal transfer process, the numerical model, which enables to calculate the time-change of liquid transfer process, was constructed. By using the model, we numerically investigated the influence of liquid properties and electro-magnetic force on the modes of liquid transfer. It is shown that liquid transfer modes depend on the pressure balance at the tip of drop. When the dynamic pressure induced by liquid flow is lower than capillary pressure due to surface tension at the tip of liquid drop, globular transfer occurs. In contrast, when the dynamic pressure of liquid flow is higher, spray transfer occurs. In the case that the electrical current flows in the liquid, electro-magnetic force significantly influences on the transfer mode and the drop detachment. Calculated results using physical properties of molten mild steel show that electro-magnetic pinch force induced by higher current than 250 A changes the transfer mode from globular to spray, for liquid flow-out from the nozzle of 1.2 mm in diameter. Whereas in real GMAW with pure argon gas shielding the transfer mode changes at around 230 A. The behaviour of drop detachment is varied by both spatial distribution and time-change of the electro-magnetic force.

Formation of uniformly sized metal droplets from a capillary jet by electromagnetic force

A new method for producing uniformly sized metal droplets is proposed. In this method, an intermittent electromagnetic pinch force is applied to a capillary jet of liquid metal to generate fluctuations of equal interval on the surface of the jet. As the fluctuations grow, the liquid metal jet breaks into small droplets whose size depends on the frequency of the intermittent electromagnetic pinch force. The breakup of the capillary jet is numerically simulated by performing multiphase fluid flow analysis with surface tracking (volume of fluid method) and electromagnetic force analysis. The simulation results agree well with the results of model experiments. The jet breaks up into uniformly sized droplets when the frequency of the intermittent force equals the frequency that corresponds to the natural disturbance wavelength of the capillary jet.

Metal transfer measurements in gas metal arc welding

The present work reports results obtained using combined lasershadowing and cross correlation techniques to investigate the metal transferprocess. Experimental results of the droplet size and velocity of 0.9 mm and1.2 mm wires from the globular region to the spray transfer region arepresented. The results indicate that the droplet size is not continuous in thenarrow region of mode transition, and also preferred bands of droplet sizeexist. Droplet velocity measurements show good agreement with a simple modelbased only on the electromagnetic pinch force.

Reduction of spatter in high-speed pulsed MAG welding

Summary Undercut and/or humping are generated when the arc length in high-speed pulsed MAG welding is too high, and spatter is generated if it is too low. Refinement of the droplets from the wire and simultaneous maintenance of one droplet transfer per pulse avoid short-circuiting in a short arc and enable spatter and highspeed/high-efficiency welding to be rendered compatible with each other. The requirements are to maintain a stable droplet shape during one droplet transfer per pulse, to maximise the welding speed limit without spatter generation, and to develop a technology for arc length shortening at the limit of short-circuiting by wire droplet refinement as objectives basically centred on pulsed MAG welding. This paper describes an investigation of the factors controlling droplet transfer through a division being made into the driving force imparted to the droplets and the deformability of the weld metal receiving this force as well as a method of droplet refinement based on resolution of these aspects. Through the simple rectangular wave pulse of a high peak current being set for a short time using an inverter-type pulsed power source at a constant wire diameter to increase the electromagnetic pinch force, one droplet transfer per pulse is possible on condition that the droplet volume is reduced by around 30% as compared with that obtained using a conventional power source with a chopping transistor on the secondary side. To increase the droplet deformability, the wire composition can be changed to achieve one droplet transfer per pulse on condition that the droplet volume is reduced by 20%. These droplets also show the same surface tension as conventional droplets as well as a lower viscosity coefficient. Through a combination of a pulsed current waveform and improved wire properties, the droplet volume can be reduced to around one half that of conventional droplets. The critical speed of spatter- and defect-free welding is then increased as an improvement immediately applicable on an actual automotive mass production line.

The fragmentation of wires carrying electric current

Exploding wires are widely used in pulsed power systems. However, many aspects of the wire explosion remain unclear. If the electric current density is not too high, the wires may disintegrate in the solid state without signs of significant melting. The experiments show that the wires break in tension due to some longitudinal force, the nature of this force being unknown. Simple estimates made in the past by other authors showed that neither the pinching effect nor thermal expansion was responsible for the disintegration of the wire, since the tension produced was too low to extrude the wires. A search for a longitudinal force had even led to a certain controversy in electrodynamics. In this investigation we employ the equations of magneto-thermo-elasticity to study stress waves induced in metal wires by a high pulsed current, when the current increases from zero to a constant value in a step-function manner. Two main aspects are studied, namely (i) the possible amplification of stress waves induced by the electromagnetic pinch force and (ii) the dynamic stress induced by the thermal expansion. It is shown that, for wires with free ends, the magnitude of tensile stress produced by the thermal expansion may well exceed the ultimate strength of the material.

懸垂液滴の橋絡移行現象と電磁ピンチ力の効果 橋絡移行におけるピンチ力の効果 （第３報）

The dynamics of bridging transfer are theoretically and experimentally investigated. Timedependent deformation of the liquid metal bridge, which.is formed between the electrode and the pool by a contact of a pendent drop with the pool surface, is observed by high speed photography and simulated with aids of the numerical solution technique for the dynamics of a viscous incompressible fluid with a free surface (MAC method) based on Finite-Difference Method. It is shown that the liquid of the pendent drop is driven to flow into the pool by the capillary force and the electromagnetic pinch force acting on the conducting bridge.

橋絡移行におけるピンチ力の効果 第２報 橋絡液柱の破断に要する電流値と通電時間

Electric current and time required to break up the conducting liquid bridge between the electrode and the pool are theoretically and experimentally investigated. Time-dependent deformation and breakup of the mercury bridge is observed by high speed photography. In the instability of the bridge due to electromagnetic pinch force, it is possible to simulate the initial stage in the necking process and evalutea the time to breakup, using the numerical solution technique for the dynamics of a viscous incompressible fluid with a free surface (MAC method) based on Finite-Difference Method. It is shown that there exists the critical current level responsible for occurring the instability of the bridging liquid and the time to breakup varies inversely as increasing current.

Quasisteady MPD arcjet with hollow cathode

Hollow cathodes with various diameters and interelectrode lengths were tested to evaluate their effects on the performance of a magnetoplasmadynamic (MPD) thruster. The thruster was operated in a quasisteady mode in the specific impulse (Isp) range of 1000-2000 s. The arc chamber was designed so that its wall formed a divergent nozzle and the hollow cathode constituted a part of the flow tube. The entire propellant was injected through the hollow cathode and forced to be heated in the arc column so as to enhance the aerodynamic acceleration. An appropriate combination of cathode interior diameter and interelectrode length resulted in the maximum thrust power ratio of 25 mN/kW for hydrogen and a lower cathode erosion rate than occurs with a solid cathode. However, a similar configuration was not effective for ammonia propellant because the mass flow was interrupted by excessively high pressure due to induced electromagnetic pinch force in the hollow cathode interior. Consequently, the hollow cathode exhibited inferior thrust performance but superior erosion characteristics compared to the solid cathode.