A magnetic field externally applied to the welding arc deflects the arc by the electromagnetic force (Lorentz force) in the plane normal to the field lines. The magnetic field exerts the force on the electrons and ions within the arc, which causes the arc to be deflected away from the normal arc path. The welding arc can be deflected forward, backward or sideways with respect to electrode and welding direction, depending upon the direction of an external magnetic field. A transverse magnetic field deflects the arc in the welding direction, whereas a longitudinal magnetic field deflects the arc perpendicular to the bead. If a unidirectional magnetic field is applied to an AC arc, or an alternating field is applied to a DC arc, then the arc can be oscillated in the position normal to the direction of welding, and this has been used to improve the welding with both TIG and MIG arcs. Oscillating the arc in sideways with respect to welding direction could be used for strip cladding and welding of a material which is sensitive to hot cracking, because this gives the wide bead and the uniform and shallow penetration. Subjecting the welding arc to transverse magnetic fields increases the welding speed several times at which under-cut free and no-porosity welds can be made. Also, magnetic arc oscillation could be applied to high-speed seam tracking because the magnetically oscillating arc possesses virtually no inertia.
Narrow-gap welding is an important technique for increasing the productivity in the manufacture of thick-walled components. Narrow-gap welding has many advantages such as high productivity and quality, minimal distortion and all-position capability. But lack-of-fusion into sidewalls is the most frequent defect in narrow-gap GMAW due to the low heat input and small molten weld pool. In order to improve the weld quality, an arc weaving technique has to be used. Arc length control and weld seam tracking are also needed because the weld quality is sensitive to any disturbance of the arc. Therefore, the nature of the process demands an automated approach and precise control to ensure consistently high weld quality. The most important objective of narrow gap welding processes is to maintain uniform and sufficient penetration at both sidewalls.
Characteristics of electromagnetic arc oscillation were examined by experiments and numerical analysis. Electromagnetic arc oscillation could be applied to narrow gap welding by thorough understanding of its characteristics obtained by experimental and theoretical investigation.
First, fundamental phenomena such as arc shape, arc oscillation width, temperature distribution in arc column, arc inertia and arc constriction during electromagnetic arc oscillation were analyzed. Operation method of electromagnet and control system of electromagnet was also studied. Shape of arc column image captured by high speed camera was changed according to various optical filters. Temperature at boundary of arc column images was predicted by numerical analysis of arc. Current on the coil of electromagnet for maintaining constant magnetic flux at high oscillation frequency was obtained by electromagnetic analysis. It is known that the extent of an arc deflection is dependent upon the flux density of the applied magnetic field, the current in arc, and the length of arc and, so on. In order to apply the magnetic arc oscillation to welding automation such as weld quality control and seam tracking, therefore, quantitative information have to be obtained about the effect of welding conditions on the deflection of arc. A mathematical model of the magnetic arc deflection was introduced for theoretical prediction and the model was confirmed by corresponding experiments. And the effects of magnetic field intensity, initial arc length and arc current on the deflection of the gas tungsten welding arc in a magnetic field were investigated.
In order to analyze welding bead characteristics of electromagnetic arc oscillation, the calculation of the molten drop flight trajectory and the thermal analysis for the temperature distribution of base metal was carried out. The temperature and plasma velocity distribution was considered in calculation of the flight trajectory. The analysis results showed a good agreement with experimental ones. The beta distribution heat source was introduced to analyze temperature distribution of base metal during electromagnetic arc oscillation. The simulation results by using beta distribution heat source were more similar to experimental ones than using gaussian heat source.
The arc sensor using the magnetic arc oscillation was also modeled mathematically by considering the model of magnetic arc deflection and electromagnet as well as welding arc and welding power source. Experiments and simulations were carried out in order to clarify how welding conditions such as initial arc length and arc current affect the sensitivity characteristics of the arc sensor. The simulated results base on the model showed a good agreement with the experimental ones.
Finally, a welding system using electromagnetic arc oscillation was developed for narrow-gap welding. The welding arc is periodically oscillated by the electromagnetic force in the plane normal to the magnetic field lines when an alternating magnetic field is applied. Magnetic arc oscillation method is easy to control the weaving width and frequency by the control of magnitude and frequency of current applied to the electromagnet. The frequency of arc oscillation is the same as that of the controlling magnetic field. The electromagnetic analysis was performed for the design of electromagnet used in narrow gap GMA welding. The narrow gap GMA welding torch was newly developed. The electromagnet was built in the torch. This paper shows the arc and bead characteristics in narrow gap GMA welding using electromagnetic arc oscillation. Arc signal characteristics for automatic seam tracking were also investigated. The periodical change of welding current in electromagnetic arc oscillation was examined by experiments and numerical analysis and adopted for developing an automatic seam tracking system in narrow gap GMA welding.
