Utilização da oscilação magnética do arco elétrico seguindo figuras de Lissajous na soldagem GMAW para passe de raiz

Abstract

The purpose of this work is to study the use of controlled magnetic oscillation of the electric arc according to complex geometric patterns applied to the GMAW process for root pass welding. These patterns were represented here by what is known as Figures of Lissajous. These one are parametric curves mathematically produced that can offer versatility for the creation of wetting movements similar to those practiced by experienced welders. These curves are basically produced by applying and controlling sine equations for the orthogonal axes "X" and "Y". An electrical voltage generator/controller system was developed for this work and its software combines and controls the variables, frequency (Hz), amplitude (V), phase (rad) and offset (%) of the sine equation of each axis. This system was built to feed two pairs of electromagnets magnetic field generators, each pair corresponding to its respective axis. The electromagnets, in turn, receive from the system the appropriate electrical voltage to produce magnetic fields with specific magnetic density (mT) values. Once the electromagnets are positioned on their respective orthogonal axes with reference to the vertical center line of the electric arc, it becomes possible to make weave movements through the magnetic flux that was generated and directed to the arc. With this equipment, the welding was performed in order to deflect the electric arc following the forms of Figures of Lissajous. With this resource/equipment in the process, in addition to promoting controlled deflection of the electric arc (difficult to be achieved manually or mechanically), it was possible to deposit addition metal in specific locations, modify the characteristics of the thermal, distribution located in the welding region, contribute to the formation of a fusion pool less concentrated in the weld region and produce beads with potential to overcome obstacles such as misalignments and uneven clearances typical of the stage known as root pass. Prior to the groove joints welding using the pulsed GMAW process, the magnetic oscillator system that was built was put to the test in a sequence of initial trials performed with the autogenous GTAW and GMAW processes with short circuit transfer. Imposing weave movements to the arc following two different Lissajous Figure in the autogenous GTAW welds was clearly evidenced transverse and longitudinal movements of the molten pool as a visual and dimensional characteristics of the beads that were produced. In short-circuited GMAW welds such movement was not clearly visualized. However, through thermographic records obtained on the opposite side of weld beads made on plate, it was possible to record thermal differences with higher or lower heat concentration between beads produced without and with weaving patterns. This fact suggests that the magnetic oscillation imposed on the process interfered in the thermal distribution of the weld. The welds in the GMAW process pulsed in groove joints parts were produced by applying weave following four different Lissajous Figures. The results were analyzed according to dimensional characteristics of the respective beads and electrical signals recorded in oscillograms. These results showed that the application of the controlled magnetic oscillation imposed to the arc was able to produce root pass welds in flat positioned groove joints parts and these with an average clearance of the 3.7 mm between their faces, and without the application of the oscillation the average root opening was 2.4 mm. Additionally, this Thesis established a basic concept called "Rate of variation of electrical voltage" which proved to be a consequence of the magnetic field applied in the arc.