| dc.description.abstract | The demand for new and reliable welding processes comes from the need to improve productivity and quality in welding of new materials used in infrastructure projects. Among the areas where this development is crucial, is pipeline welding, which is performed in large infrastructure projects. The development of new grades of high strength steels for line pipes has also driven the need for newer welding process which offer reduced heat input into the work piece, thus avoiding deterioration of the material properties. Since these steels are heat sensitive, the general goal is to increase deposition while maintaining similar nominal heat input into the steel. Cold wire gas metal arc welding (CW-GMAW) is a process that could offer such performance, however a comprehensive understanding of the process features is still lacking in the available literature. To address this lack of understanding, the present work will present a comprehensive analysis of the CW-GMAW process while employing a constant voltage welding power source. First, this work reports a study on the metal transfer dynamics of the CW-GMAW process using high speed imaging synchronized with current and voltage. The droplet diameter and its frequency are estimated and correlated to different transfer mode regimes. In addition, estimates of melting efficiency of the welds and geometry measurements of the beads are reported. Subsequently, a study on the application of CW-GMAW to fill 4 mm wide narrow gaps is reported, it was found that CW-GMAW provides better consistency during welding and avoids sidewall penetration which hamper the mechanical properties of the joints. An overview on current pulsation, on CW-GMAW, is also reported. It was found that cold wire feed rates affect welding with low peak to background current ratio causing differences on weld bead cooling rate, leading to differences in hardness. Globular to spray transition in CW-GMAW was also studied, and it was found that CW-GMAW transits faster to spray regime than standard GMAW. In order to understand the effect of the cold wire feed rate on the energy transferred to the workpiece during welding, an uncertainty study on a water-flow calorimeter was performed, followed by calorimetry of CW-GMAW for the three natural transfer regimes (short-circuit, globular, and spray). Results show that the efficiency on CW-GMAW depends on cold wire feed rate and metal transfer. Lastly, the mechanical properties of welding joints are tested and the CW-GMAW provided better performance for the welding parameters tested. In summary, the CW-GMAW process offers higher productivity than standard GMAW and can be employed successfully with or without pulsation control of current. The thermal efficiency of the process for the same metal transfer decreases with an increase in cold wire feed rate along with reduced dilution of the filler metal. This interesting process feature also qualifies the CW-GMAW for hardfacing applications. | en |
