Submission (draft) copy: Welding in the World, DOI: 10.1007/s40194-014-0196-x Influence of the CO 2 Content on Operational Performance of Short-Circuit GMAW Olga Liskevych, liskevich@i.ua 1 , Américo Scotti, ascotti@mecanica.ufu.br 1 1 Center for Research and Development of Welding Processes of Federal University of Uberlandia (Laprosolda), Brazil Abstract CO2 blended with Ar is the most common shielding gas used for short-circuit GMAW. There has been some technical knowledge devised from the process application over the years (personal opinion and results from practice) on the selection of the gas blend composition. However, there is still lack of more scientific data to explain the performance of the mixtures. This paper presents a systematic study of the influence that CO2 content in mixture with Argon has on the operational performance of the short-circuit GMAW. The objective of this study was to describe, to quantify and to explain the alterations in the metal transfer behavior, spatter generation, weld bead geometry and bead finish due to the different CO2 contents in the shielding gas. Carbon steel plates were welded in adequate parametric conditions for each CO2+Ar shielding gas composition (CO2 ranging from 2% to 100%). These parametric conditions were found by applying a metal transfer regularity index over welds carried out at different voltage settings for each gas blend. A target of 130 A was applied as base for comparison. Laser shadowgraphy with high speed filming and current and voltage oscillograms were used as analysis tools. The results showed (and confirmed) that the increase of the CO2 content deteriorates metal transfer regularity, leading to excessive spatter generation and uneven bead appearance, but increases the penetration and the fusion area of the weld beads and improves bead convexity. In general, the CO2 content should neither be lower than 10% (unless for thin plates) nor higher than 30%. Keywords: GMAW; shielding gas; CO2 content; short-circuit metal transfer 1 – INTRODUCTION Shielding atmosphere plays a fundamental role in the operational behavior of the GMAW process, since the shielding gas composition controls arc stability, metal transfer mode, fume emission, spatter generation and weld bead geometry. The most common shielding used in GMAW for carbon steel applications is a blend of Ar and CO2. The CO2 content in these blends varies from a very low amount of CO2 up to a 100%, yet commercially the applicable blend composition lay on a range from 8% to 25% of CO2, having the pure CO2 as an alternative for the blend. In general, it is considered that the higher the CO2 content, the “hotter” the shielding becomes and the deeper penetration is reached by the arc. However, the limits for the best percentages of CO2 in a blend are questionable. Zielinska et al. [1] demonstrated that the arc stability becomes poor with CO2 content over 9%. Stenbacka and Persson [2] pointed out that adding more than 15% of CO2 in the shielding gas mixture makes the metal transfer unstable and increases fume and spatter generation. It may be correlated with the conclusion of Soderstrom and Mendez [3], for whom higher percentages of CO2 in the mixture with Argon increase the current density and decrease the anode spot on the droplet surface. Plasma pressure is concentrated in a smaller area, resulting in sufficient force to lift the droplet, leading to its displacement on the electrode tip, which, in turn, disturbs the "soft" transfer of the metal droplet to the welding pool and leads to increased spatter generation. Hermans and den Ouden [4], in