The influence of operating parameters on number-weighted aerosol size distribution generated from a gas metal arc welding process
Introduction
Census data indicates that over 600,000 workers in the United States are involved in welding or allied processes. Animal and epidemiological studies suggest that welding is associated with a wide range of adverse health effects such as metal fume fever, pneumonitis, chronic bronchitis, and decrements in pulmonary function (NIOSH, 1988). Welding processes are known to generate sub-micrometer sized aerosols, and recent research has indicated sub-micrometer aerosols may cause adverse health effects due to their size (Ferin et al., 1990; Oberdorster et al., 1990; Takenaka, Dornhofer-Takenaka, & Muhle, 1986). This research suggests that a lower moment of the particle size distribution (e.g., particle number or particle surface area) may be more toxicologically relevant.
Welding processes are known to generate high fume formation rates (FFR) with values typically ranging from 1.7 to (Gray & Hewitt, 1982; Heile & Hill, 1975; Hewitt & Hirst, 1993; Hilton & Plumridge, 1991). In light of mass-based occupational safety and health standards, prior welding research has primarily focused upon characterizing how various welding parameters affect the FFR to indicate the relative cleanliness of a welding process. Gas Metal Arc Welding (GMAW) processes, frequently encountered in the welding industry, use filler metals (welding alloys) that require shield gases (e.g., argon/carbon dioxide gases) to protect the molten metals within the arc against oxidation and to provide the desired arc characteristics (Fig. 1). Prior research has demonstrated that several parameters affect the FFR of GMAW processes including the mass transfer mode of molten droplets within the arc, the shield gas composition, and welding spatter (Gray & Hewitt, 1982; Heile & Hill, 1975; Hewitt & Hirst, 1993; Hilton & Plumridge, 1991).
In GMAW processes, welding professionals typically operate using two modes of droplet metal transfer across the arc and these modes are termed globular and spray transfer (Heile & Hill, 1975; Hilton & Plumridge, 1991; Ma & Apps, 1982). Globular droplet transfer is typically associated with low applied currents and have the following characteristics: a diffuse arc with the root covering all of the globule surface, high electrical resistance with a calculated mean droplet temperature of ∼2750°C, and enhanced transfer of the metal vapors resulting in high fume generation rates (Ma and Apps, 1982). Spray droplet transfer is associated with high applied currents and has the following characteristics: a conical arc that surrounds the column of irregularly shaped/sized metal droplets, high electrical resistance with a calculated mean droplet temperature of ∼2750°C, enhanced transfer of metal vapors resulting in high fume generation rates (Ma and Apps, 1982).
The shield gas composition also influences the fume formation rate in GMAW processes. Typically, an oxidizing gas, such as carbon dioxide and oxygen, is used to assist in arc stability. In GMAW operations involving argon/carbon dioxide/oxygen mixtures, an increase in the percentage of oxygen increased the FFR (Heile & Hill, 1975; Hilton & Plumridge, 1991). For example, the FFR for a GMAW operation involving mild steel solid wires welded at increased from using a shield gas composed of 93% Ar, 5% CO2, 2% O2 to using a more oxidizing shield gas composed of 78% Ar, 20% CO2, 2% O2. Additionally, when pure CO2 was used the FFR markedly increased to (Hilton & Plumridge, 1991). Prior research has also shown that an increase in oxidizing gases in the shield gas will increase spatter formation (Gray, Hewitt, & Dare, 1982; Hewitt & Hirst, 1993). High spatter emissions are considered undesirable because of the potential for occupational injury due to skin burns.
The formation of welding spatter can also have a significant effect on the composition of the fume and its rate of formation. During welding, spatter is associated with an unstable arc condition and a turbulent weld-pool (Hewitt & Hirst, 1993). In GMAW processes, the electrical current is delivered through the filler wire. As the filler wire constricts during droplet separation, the current density in the reducing cross section increases until separation occurs explosively (Gray, Hewitt, & Hicks, 1980). This results in the almost instantaneous evaporation of some of the remaining metal filament and the ejection of a spray of very hot metal droplets called spatter. Spatter ejected this way, and by other arc forces, represents a large surface area from which further evaporation can occur and increase the formation of fumes. It was experimentally shown that at least 25% of the fume from solid wire GMAW welding originated from outside of the arc region (Gray, 1980b), and that an increase in oxidizing gases in the shield gas will increase spatter formation (Gray et al., 1982; Hewitt & Hirst, 1993). However, it is thought that the ejected spatter particles are too large to remain airborne and do not contribute directly to the fume (Gray, 1980a; Hewitt & Hirst, 1993).
Although the effect of mass transfer mode, shield gas composition, and welding spatter on the mass-based fume formation rate is known, no information is available on how these factors affect the resultant particle size distribution, especially particle size distributions weighted by particle number or particle surface area. In light of recent research on the potential health problems associated with sub-micrometer aerosols, an apparatus was designed to characterize aerosols generated from GMAW and other welding processes based upon particle number (Zimmer & Biswas, 2001). The results demonstrated that the aerosols generated from these processes were multi-modal and temporally changing. In this paper, the role of mass transfer of molten droplets through the plasma, shield gas composition, and welding spatter on the resultant particle number size distribution is established.
Section snippets
Experimental
An experimental apparatus (Fig. 1) was designed to promote the steady-state generation of welding fumes over a period of several minutes and details are provided in an earlier paper (Zimmer & Biswas, 2001). A high quality, commercial arc welding system (Miller Deltaweld Series 452 power source and Miller S-62 wire feeder) was selected to generate GMAW aerosols. In this setup, wire alloy, fed at a constant speed, provided the filler material for the arc welding process. Within the
The effect of mass transfer on the particle size distribution of sub-micrometer aerosols
To characterize the effect that the mass transfer mode had on the sub-micrometer particle size distribution, SMPS measurements were taken during globular and spray transfer (Table 1). These modes were studied because they represent those typically used by welding professionals. During the globular mass transfer, the extension height was , and the applied arc voltage was (the current during these measurements varied between 194 and ). During the spray mode of metal transfer, the
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