Mechanical Analysis On Arc Plasma And Droplet During Short-circuit CO₂Welding Controlled By External Magnetic Fields

In this paper, different longitudinal magnetic fields with a wide range of excitingcurrent or magnetic frequency were introduced to the traditional CO2welding. To revealthe mechanism of external longitudinal magnetic fields in reducing spatter or improvingthe performance of weld metal and building a constructive theoretical foundation for thedevelopment of digital synchronously pulsed magnetic field generator used for CO2welding, the impacts of external longitudinal magnetic fields on electromagnetic force, thegravity, the plasma stream force and surface tension, acting on the liquid metal werestudied by three physical models which were used for imitating the arc plasma and liquidmetal in the arc burning phase, initial stage or final stage of short circuit phase,respectively.To study the impacts of external longitudinal magnetic fields on performance of weldmetal and mass of spatter, bead-on-plate CO2welding was performed on a mild steel sheetQ235with or without the control of external longitudinal magnetic fields. The photos ofarc plasma and droplet and electric signals covering welding current and arc voltage wereacquired synchronously by an analysis and evaluation system based on LabView forGMAWIt was shown that, in terms of spatter and weld metal, the spatter generation ratedecreased with the increase of magnetic frequency in conditions of some exciting current.With a certain magnetic frequency, the bigger the exciting current was, the better itspositive impacts in reducing spatter and improving weld metal were. For example, thespatter generation rate were maximally reduced by3.20%and4.98%under low-frequencymagnetic field(5~25Hz) and high-frequency magnetic field(500~2000Hz) with an excitingcurrent of6A and the reinforcements of weld metals were diminished from2.15mmwithout magnetic fields to1.66mm or1.84mm with a magnetic frequency of5Hz or1000Hz, respectively. As for the impacts on metal transfer, in arc burning phase, the arcplasma and the droplet were both expanded or compressed by the low-frequency magneticfield or high-frequency magnetic field. In short circuit phase, however, the liquid bridgewas compressed by both the two kinds of magnetic fields. According the mechanical analysis, first, in arc burning phase, the resultant force thatcould promote the short circuit action in the axial direction and diminish the scale of arcburning phase was increased or decreased because of the impacts of the low-frequencymagnetic field or high-frequency magnetic field in expanding or compressing the arcplasma and droplet. Second, in the initial stage of short circuit phase, the radius of thesmall contact area between droplet and liquid metal in the molten pool along with anotherresultant force that could drive the movement of liquid metal in the horizontal directionwere increased or decreased by the changes of resultant force in arc burning phase underthe control of the low-frequency magnetic field or high-frequency magnetic field. And it isthese different changes that could promote or hinder the movement of liquid metal in thehorizontal direction and diminish or cause some spatter, also. Third, in the final stage ofshort circuit phase, the resultant force that was responsible to the metal transfer wasincreased by both the low-frequency magnetic field and high-frequency magnetic field.That is to say, the scale of short circuit phase could be diminished and the frequency ofmetal transfer could be improved by the external longitudinal magnetic fields and so wasthe weld metal. In addition, it is just the positive decrease in arc burning time that couldprevent the liquid bridge from being energized too much. So the spatter resulted from thedischarge-induced explosion of liquid bridge was minimized.