Influence of Metal Transfer Stability and Shielding Gas Composition on CO and CO2 Emissions during Short-circuiting MIG/MAG Welding

Soldagem & Inspeção. 2016;21(3):253-268 Technical Papers http://dx.doi.org/10.1590/0104-9224/SI2103.02 Influence of Metal Transfer Stability and Shielding Gas Composition on CO and CO2 Emissions during Short-circuiting MIG/MAG Welding Valter Alves de Meneses1, Valdemar Silva Leal1, Américo Scotti2 1 2 Instituto Federal do Maranhão – IFMA, São Luis, MA, Brazil. Universidade Federal de Uberlândia – UFU, Center for Research and Development of Welding Processes, Uberlândia, MG, Brazil. Received: 15 Mar., 2016 Accepted: 05 May, 2016 E-mails: (VAM), (VSL), (AS) Abstract: Several studies have demonstrated the influence of parameters and shielding gas on metal transfer stability or on the generation of fumes in MIG/MAG welding, but little or nothing has been discussed regarding the emission of toxic and asphyxiating gases, particularly as it pertains to parameterization of the process. The purpose of this study was to analyze and evaluate the effect of manufacturing aspects of welding processes (short-circuit metal transfer stability and shielding gas composition) on the gas emission levels during MIG/MAG welding (occupational health and environmental aspects). Using mixtures of Argon with CO2 and O2 and maintaining the same average current and the same weld bead volume, short-circuit welding was performed with carbon steel welding wire in open (welder’s breathing zone) and confined environments. The welding voltage was adjusted to gradually vary the transfer stability. It was found that the richer the composition of the shielding gas is in CO2, the more CO and CO2 are generated by the arc. However, unlike fume emission, voltage and transfer stability had no effect on the generation of these gases. It was also found that despite the large quantity of CO and CO2 emitted by the arc, especially when using pure CO2 shielding gas, there was no high level residual concentration of CO and CO2 in or near the worker’s breathing zone, even in confined work cells. Key-words: MIG/MAG welding; Metal Transfer Stability; Occupational health; Toxic and asphyxiating gases. Influência da Estabilidade da Transferência Metálica e Composição do Gás de Proteção sobre Emissão de CO e CO2 durante Soldagem MIG/MAG Resumo: Vários estudos têm demonstrado a influência dos parâmetros e proteção gasosa sobre a estabilidade de transferência metálica ou sobre a geração de fumos em soldagem MIG/MAG, mas pouco ou nada tem sido discutido sobre a emissão de gases tóxicos e asfixiantes, particularmente no que tange a parametrização do processo. O objetivo deste estudo foi analisar e avaliar o efeito de aspectos operacionais dos processos de soldagem (estabilidade da transferência metálica e composição do gás de proteção) sobre os níveis de emissão gasosa de durante a soldagem MIG/MAG (aspectos ambientais e de saúde ocupacional). Utilizando-se misturas de argônio com CO 2 e O2 e mantendo-se a mesma corrente média e o mesmo volume de cordão de solda, soldagens com transferência por curto-circuito foram realizadas com arame de aço carbono em ambientes aberto (zona de respiração do soldador) e confinados. A tensão de soldagem foi ajustada gradualmente para modificar a estabilidade da transferência. Verificou-se que quanto mais rica a composição do gás em CO2, mais CO2 e O2 são emitidos pelo arco. No entanto, ao contrário de emissão de fumos, tensão e estabilidade da transferência metálica não têm efeito sobre a geração de gases. Verificou-se também que, apesar da grande quantidade de CO e CO2 emitida pelo arco, especialmente quando se utiliza gás de proteção CO2 puro, não se forma concentração residual de alto nível de CO e CO2 perto de zona de respiração do trabalhador, mesmo em células de trabalho confinadas. Palavras-chave: Soldagem MIG/MAG; Estabilidade da Transferência Metálica; Saúde ocupacional; Gases tóxicos e asfixiantes. Symbols: U = Welding power supply voltage setting; This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License which permits unrestricted non-commercial use, distribution, and reproduction in any medium provided the original work is properly cited. WFS = electrode-wire feed speed setting; TS = welding travel speed; Meneses et al. CTWD = contact tip to workpiece distance; Umean = monitored mean voltage; Imean = monitored mean current; RIsc = short-circuit metal transfer regularity index. 1. Introduction MIG/MAG is unquestionably the welding process most firmly established in the industry and one of the operating modes most widely applied in this process is the short-circuit mode. Despite its significant operational advantages, the main problem of short-circuit transfer is spatter generation, which decreases production capacity, due to loss of filler material or to the need to spend resources on its removal. Spatter is caused by unstable metal transfer from the tip of the wire to the weld pool. However, Meneses et al. [1] recently demonstrated that, although greater transfer stability mitigates spattering, this condition does not necessarily reduce welding fumes or affect the composition and morphology of fume particles. The parameters that influence short-circuit transfer stability include welding voltage, inductance and shielding gas. The influence of the welding voltage on transfer regularity lies in its proportionality with the arc length. An example of the influence of the arc length on this regularity is given by Liskevych and Scotti [2], who show that there is an acceptable range of droplet size to ensure good short-circuit metal transfer (the volume of the droplet should be neither too large nor too small before it detaches from the wire). Following the reasoning of these authors, in very short arc lengths, the arc time (time during which the electrode melts) is very short and the molten tip of the electrode accumulates a small volume before the droplet that is being formed comes into contact with the weld pool. At the moment of this contact, short circuits occur abruptly (without the droplet being sucked into the molten pool aided by surface tension) and the metal transfer is erratic, with abundant spatter. On the other hand, a very long arc increases the wire melting time, favoring the formation of droplets with larger volumes prior to detachment. Larger droplets should be more easily transferred by sucking into the molten pool due to surface tension (stable). However, larger droplets may be repelled as they approach the pool. As Scotti et al. [3] explain, this repulsion occurs by constriction of the arc attachment points, generating vapor and pinch effect on the weld pool towards the droplet. As a result, incipient short circuits are generated, but which do not lead to actual short-circuit transfer (by surface tension). Because the transfer will occur only after a few incipient short circuits, when the weight of the droplet overcomes the repulsive forces, the transfer is also unstable. Inductance influences (...truncated)