| GMAW?Gas Metal Arc Welding?is one of the most widely generalized welding methods in manufacturing industry.It is an effective way to increase the welding current for greatly improving its efficiency,however unstable droplet rotating spray transfer process hinders the application of high-current GMAW in industrial production.In order to improve the high-current GMAW metal transfer behavior,an external alternating magnetic field is applied to control the liquid column rotating process and improve the weld appearance.The current density and electromagnetic force in the arc and molten metal liquid column,which are the keys to revealing the high-current GMAW droplet rotating spray transfer mechanism and arc-droplet coupling behavior,are not feasible to be measured since the experimentally measuring methods in GMAW are severely restricted.In this work,a numerical simulation method based on magnetohydrodynamics?MHD?and high-speed photograph method are combined to obtain the temperature field,flow field,electric potential field,and pressure field by calculating the mass,momentum,energy and component transport balance equations.According to the proposed mathematical-physical model,an external magnetic field is added to study the impact on high-current GMAW arc and metal transfer behaviors,which will finally contribute to the theoretical basis of the development and application of magnetic field control high-efficiency GMAW technology.The maximal wire feeding rate and welding current of the improved GMAW power source are respectively 50m/min?almost more than 2 times of conventional wire feeding rate?and 600A.A simply constructed and assembled magnetic control device is designed.The distributions and strengths of excited magnetic fields are experimentally measured and numerically calculated,and the impact of exciting current magnitude and frequency on magnetic field strength is revealed.It is a reasonable way to increase the magnetic flux density of the external magnetic field in the arc region using a 120 turns magnet coil with an iron core.With the same exciting current,the magnetic flux density decreases when increasing the magnetic field frequency because of the eddy current effect in the iron core.The magnetic flux density reaches nearly 0.01T when the exciting current is 10A and the magnetic field frequency is 500Hz.The impact laws of welding current,wire stick out length and shielding gas composition on wire melting rate of GMAW are studied,and empirical formula of high-current GMAW wire melting rate is concluded.The different metal transfer processes between MIG?Metal Inert Gas?and MAG?Metal Active Gas?welding processes are compared with the help of a high-speed photography system and welding electrical signal acquisition system when the welding voltage is 50V and wire stick out length is30mm.It is found to be only rotating spray transfer in high-current MIG welding process with pure argon shielding gas,and the deflected angle increases to about 55°while the rotating frequency decreases from about 700Hz to about 500Hz.The metal liquid column can rotate and swing in high-current MAG welding process with shielding gas of 80%Ar and 20%CO2 mixture,the swing angle is nearly 90°,and the rotating/swing frequency increases from about 400Hz to about 500Hz when the arc is buried in the weld pool with increasing of wire feeding rate to 40m/min.Based on the research of high-current GMAW metal transfer behavior,the external constant and alternating axial magnetic fields are considered.It is found that the welding arc plasma constricts when applying the external axial magnetic field,and the metal liquid column changes its motion modes from the different magnetic field frequencies:the external constant axial magnetic field makes the metal liquid column rotating angle increase;the external100Hz axial magnetic field makes the metal liquid column rotating direction change as the same frequency,and the rotating angle decreases when the rotating direction changes;increasing the magnetic frequency to 200Hz,the rotating motion is controlled and the rotating angle decreases in the first half period while the rotating motion is promoted and the rotating angle increases in the other half period;with external 500Hz axial magnetic field,the metal liquid column neither changes its rotating direction nor includes promoted and controlled periods,nonetheless the arc is still constricted by the axial magnetic field.A simplified numerical model based on VOF?Volume of Fluid?method for GMAW globular transfer,axial spray transfer and rotating spray transfer is established by using the VOF method and considering the main forces in the molten metal.The force and flow of liquid metal under different welding currents are compared,and the conductive behavior of high-current MIG/MAG welding arc is approximately characterized by changing the electric conductivity in the arc region.It is shown that,as a result of the imbalance of electromagnetic pinch forces,the metal liquid column can rotate or swing.When the unbalanced electromagnetic pinch force is weak,the metal liquid column rotates,and the spiral structure of the welding current can generate a self-induced axial magnetic field to maintain rotating motion.when the unbalanced electromagnetic pinch force is strong,the metal liquid column swings,the bending structure of current generates a self-induced transverse magnetic field to maintain the swing motion.The welding arc behaviors of magnetic field control GTAW?Gas Tungsten Arc Welding?and GMAW without considering the metal transfer process are studied by coupling the arc and weld pool in the numerical model.The magnetic field control arc constricting effect and metal vapor behaviors are revealed.The external axial magnetic field creates the angular electromagnetic force by cross product with radial current,hence the welding arc rotates and constricts since the pressure is larger outside.For GMAW,there is a low-temperature center in arc whatever considering external constant axial magnetic field or metal vapor behavior.A unified GMAW arc-droplet coupled model is proposed by combining the simplified metal transfer model based on the VOF method and GMAW arc model.The arc-droplet coupling behavior and metal vapor behavior of globular transfer,axial spray transfer and rotating spray transfer are researched with an external magnetic field.It is demonstrated that the droplet temperature and arc plasma velocity decreases in the rotating spray transfer process although the welding current is 50A larger than the axial spray transfer process.The calculated results agree well with experimental results about the magnetic field control high-current GMAW metal transfer.The application foundation of magnetic field control high-efficiency GMAW is the arc constricting and controlled rotating motion of the metal liquid column using an external alternating axial magnetic field. |
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