Metal vapour is formed in arc welding processes by the evaporation of molten metal in the weld pool, and in the case of gas–metal arc welding, in the wire electrode and droplets. The presence of metal vapour can have a major influence on the properties of the arc and the size and shape of the weld pool. Previous experimental and computational works on the production and transport of metal vapour in welding arcs, in particular those relevant to gas–metal arc welding and gas–tungsten arc welding, are reviewed. The influence of metal vapour on the thermodynamic, transport and radiative properties of plasmas is discussed. The effect of metal vapour on the distributions of temperature, current density and heat flux in arcs is examined in terms of these thermophysical properties. Different approaches to treating diffusion of metal vapour in plasmas, and the production of vapour from molten metal, are compared. The production of welding fume by the nucleation and subsequent condensation of metal vapour is considered. Recommendations are presented about subjects requiring further investigation, and the requirements for accurate computational modelling of welding arcs.

https://iopscience.iop.org/article/10.1088/0022-3727/43/43/434001/pdf

The effects of metal vapour in arc welding

A two–dimensional dynamic theory for predictions of arc and electrode properties in arc welding has been used to investigate heat transfer phenomena in the welding wire in gas metal arc welding (GMAW). The theory is a unified treatment of the welding wire, the plasma and the workpiece and includes a free surface treatment for the welding drops, accounting for the effects of inertia, gravity, surface tension, arc pressure, magnetic forces, and viscous drag by the gas flow around the drop. Also, the theory accounts for the variation of the surface tension coefficient with temperature and includes thermal and dynamic phenomena within the solid and liquid phases of the wire, together with a detailed treatment for the electrode sheath regions. Calculations are made for arcs in argon with wires of mild steel at currents between 150 and 325 A. Results of calculations for heat fluxes within the wire suggest that evaporation from the surface of the droplet during droplet growth has an important influence on the he...

An analysis of heat transfer and fume production in gas metal arc welding. III

Airborne particles fall into one of three size ranges. The nucleation range consists of nanoparticles created from vapor atom collisions. The decisive parameter for particle size and composition is the supercooling of the vapor. The accumulation range, which comprises particles less than 2 micrometers, consists of particles formed from the collision of smaller primary particles from the nucleation range. The composition of agglomerates and coalesced particles is the same as the bulk vapor composition. Coarse particles, the composition of which is determined by a liquid precursor, are greater than 1 micrometer and solidify from droplets whose sizes are controlled by surface, viscous, and inertial forces. The relationship between size and composition of airborne particles could be seen in welding fume, a typical metallurgical aerosol. This analysis was performed with a cascade impactor and energy dispersive spectrometry with both scanning electron microscopy (SEM-EDS) and scanning transmission electron microscopy (STEM-EDS). Other methods for properly characterizing particles were discussed. In the analysis, less than 10% of the mass of fume particles for various types of gas metal arc welding (GMAW) were coarse, while one-third of flux cored arc welding (FCAW) fume particles were coarse. Coarse particles had a composition closer to that of the welding electrode than did fine particles. Primary particles were not homogeneous. Particles larger than the mean free path of the carrier gas had the same composition as that of the vapor, but for particles 20 to 60 nanometers, smaller particles were more enriched in volatile metals than larger particles were. This was explained by the cooling path along the bubble point line of a binary phase diagram. Particles were not necessarily homogenous internally. Because nanoparticles homogenize quickly, they may form in a metastable state, but will not remain in that state. In this analysis, the presence of multiple stable immiscible phases explains this internal heterogeneity. The knowledge contained herein is important for industries that depend on the properties of nanoparticles, and for manufacturing, where industrial hygiene is important because of respirable particle by-products, such as high-energy-density metallurgical processing. Thesis Supervisor: Dr. Thomas W. Eagar, ScD. P.E. Title: Thomas Lord Professor of Materials Engineering & Materials Systems

Chemistry of airborne particles from metallurgical processing

The present paper compares thirteen shielding gases and their impact on FFR and fume particle while welding in the spray transfer regime. There was no obvious influence from the shielding gas on particle composition and fume particles were identified as (Fe,Mn)3O4. There was a slight peak shift that indicated that small levels of Mn, as detected by TEM-EDS, substituted for Fe in the Fe3O4 phase.Shielding gas composition is an important parameter for successful gas metal arc welding (GMAW) and has been shown to affect the fume formation rate (FFR). In Ar-based mixtures, increasing CO2 had a greater impact than raising O2 on FFR. When O2 was increased in ternary mixtures, the FFR increased for Ar–5%CO2 but no discernable increase was observed for the Ar–12%CO2 mixtures. Results indicate that CO2 additions in Ar-based shielding gases are the controlling factor in determining FFR due to the effect of CO2 on welding arc characteristics. Ar–He–CO2 mixtures had the most stable FFR's.

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Analysis of Fume Formation Rate and Fume Particle Composition for Gas Metal Arc Welding (GMAW) of Plain Carbon Steel Using Different Shielding Gas Compositions

Fume formation in a pulsed gas metal arc welding (GMAW) process is investigated by coupling a time-dependent axi-symmetric two-dimensional model, which takes into account both droplet detachment and production of metal vapour, with a model for fume formation and transport based on the method of moments for the solution of the aerosol general dynamic equation. We report simulative results of a pulsed process (peak current = 350 A, background current 30 A, period = 9 ms) for a 1 mm diameter iron wire, with Ar shielding gas. Results showed that metal vapour production occurs mainly at the wire tip, whereas fume formation is concentrated in the fringes of the arc in the spatial region close to the workpiece, where metal vapours are transported by convection. The proposed modelling approach allows time-dependent tracking of fumes also in plasma processes where temperature–time variations occur faster than nanoparticle transport from the nucleation region to the surrounding atmosphere, as is the case for most pulsed GMAW processes.

http://iopscience.iop.org/article/10.1088/0022-3727/46/22/224006/pdf

Two-dimensional time-dependent modelling of fume formation in a pulsed gas metal arc welding process

The decrease in the fume formation rate at the transition from globular to spray mode during welding has been investigated. A physical model describing this phenomenon has been developed. The essence of the consideration is that the model takes into account the different heating of droplets of the liquid metal by the are plasma in the two welding modes. New experimental results have been obtained for welding of stainless steel in a shielding gas containing oxygen. The decrease of the fume formation rate estimated on the basis of the model is in agreement with experimental data obtained and in the literature.

https://iopscience.iop.org/article/10.1088/0022-3727/28/12/013/pdf

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Fume formation rate at globular to spray mode transition during welding