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ABSTRACT. A cascade impactor was em-ployed to separate the fume particles inorder to determine the size distribution ofwelding fume. Clear images of coarsewelding fume particles (microspatter)from scanning electron microscopy arepresented. The particle size distribution ofthe welding fume reveals that gas metalarc welding (GMAW) fume consists pre-dominately of particle agglomeratessmaller than approximately one microme-ter. Less than 10% of the fume by weightis microspatter, which is larger than a mi-crometer. This fraction of microspatterdoes not change greatly as the GMAW pa-rameters are changed. Flux cored arcwelding (FCAW) fume contains more mi-crospatter, approximately 30% by weight.

Introduction

Arc welding poses many hazards, in-cluding heat, noise, vibration, and elec-tricity. Radiation from the arc can causeeye and skin damage. Gases and res-pirable particles in the welding environ-ment contain chemicals that can createadverse side effects after inhalation, if de-livered in the appropriate dose and chem-ical state. A major source of respirableparticles is welding fume.

Aerosol scientists use the term “fume”to describe any airborne metal or metaloxide particles that condense from vapor(Ref. 1), which is indeed the case for mostparticles formed from welding. However,some airborne particles generated bywelding are not formed from vapor con-densation, but from liquid droplet ejec-tion, and thus are not technically fumeparticles. Welding spatter is formed fromliquid droplets and is for the most part toolarge to remain airborne; droplets smallenough to remain airborne have been

termed microspatter. The welding com-munity has traditionally labeled all air-borne particles formed during welding as“welding fume,” including microspatter,despite it not technically being fume. Thisnaming convention is used here.

Particles smaller than 20 µm in diame-ter can remain airborne (Ref. 2), but notall airborne particles are deposited thesame way in the lungs, because airborneparticles of different sizes behave differ-ently aerodynamically. Objects greaterthan a few micrometers in size are trappedon the walls of the human airway beforethey reach the lungs. They are carriedaway in the mucus, which is then trans-ported to the digestive tract. Particlessmaller than 0.1 µm are inhaled and de-posited in the lungs. The path of entry intothe body strongly affects the biologicalfate of the chemicals present in the in-haled particles. Particles or agglomeratesbetween 0.1 and 1 µm can be exhaled,meaning that only about 30% of particlesof this size eventually deposit in the lungs(Ref. 3). Therefore, it is important to mea-sure the size distribution of particles inorder to ascertain the respirable fractionthereof.

Zimmer and Biswas (Ref. 4) reportedthe results of using two different airborneparticle counters with different size rangecapabilities to measure the particle sizeof gas metal arc welding (GMAW) fume.This provided a particle size distributionover all sizes of interest. Three modes ofparticle sizes can clearly be distinguished:

a nucleation mode of individual particlesfrom a few nanometers to ~0.1 µm, anaccumulation mode (~0.1 to ~1 µm) ofagglomerated, aggregated, and coalescedparticles formed from the nucleationmode, and a coarse mode of unagglom-erated particles in the range of ~1 to ~20 µm.

Particles in the nucleation mode formby vapor condensation. Because particlesin the accumulation mode agglomeratefrom nucleation mode particles, vaporchemistry also controls accumulation par-ticle composition. The particles formedfrom nucleation are called primary parti-cles whenever found in fractal-like ag-glomerates or aggregates. Aggregatesrefer to clumps of primary particles thathave fused together. Agglomerates arethose particles made up of primary parti-cles that adhere together because of elec-trostatic or van der Waals forces. The nu-cleation and accumulation modes aretherefore often grouped together as “fineparticles,” which distinguishes them fromthe coarse mode particles created throughliquid ejection (Ref. 5).

Coarse particles, or microspatter, inwelding fume have been reported earlier(Ref. 6). Because the median diameter ofcoarse particles is an order of magnitudelarger than the median diameter of ag-glomerates (Ref. 2), coarse particles maypresumably dominate the bulk chemistryof welding fume, even if these coarse par-ticles are few in number.

There seems to be a correlation be-tween fume formation rate (FFR) andspatter formation rate. Some researchers(Refs. 6, 7) have proposed that the forma-tion of additional microspatter explainswhy processes that create more spatter,such as globular GMAW or flux cored arcwelding, have greater fume formationrates. A comparison of the particle sizedistributions of various welding processeswould yield insight into how welding fumeforms, which would help determine whichprocess controls are most effective in re-ducing fume formation.

Several studies on the size of welding

Particle Size Distribution of Gas Metal andFlux Cored Arc Welding Fumes

Impactor separation of welding fumes shows at least two size modes, the relativemagnitude of which is determined by the welding process

BY N. T. JENKINS, W. M.–G. PIERCE, AND T. W. EAGAR

KEYWORDS

Airborne ParticlesArc WeldingFlux Cored Arc WeldingFumesGas Metal Arc WeldingParticulateWelding Fume

N. T. JENKINS is with the College of Medicine,The Ohio State University, Columbus, Ohio. W.M.–G. PIERCE is with the College of Medicine,Drexel University, Philadelphia, Pa. T. W. EAGARis with the Department of Materials Science andEngineering, Massachusetts Institute of Technol-ogy, Cambridge, Mass.

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fume have previously been reported. Re-searchers have measured the size distrib-ution of welding fume with impactors(Refs. 3, 8–13), and with airborne particlecounters (Refs. 4, 12, 14–16). Because ofthe large size range of particles, no singletechnique can measure the size of everyparticle. Only expensive impactors candistinguish between particles smaller than0.1 µm. Airborne particle counters (e.g.,laser particle counter or electric aerosolanalyzer) are only effective in a limitedsize range; most do not detect particleslarger than 2 µm. In addition, there areconcerns that airborne particle countersmay not measure the true particle diame-ter for welding fume (Ref. 17). Opticalparticle counters detect particle light scat-tering, which is determined by particleshape and refraction index. Particles canbe counted while airborne by measuringelectric mobility, which is strongly depen-dent on the surface area of the particles.Since welding fume particles are primarilyfractal-shaped agglomerates with complexand nonuniform shapes and surfaces, it isdifficult to calibrate such measurementtechniques to welding fume with stan-dards established with spherical particles.

Impactors separate particles by aero-

dynamic diameter(daerodynamic), whichis not necessarilythe true particle di-ameter. However,the daerodynamic isthe particle prop-erty that deter-mines inhalationand lung deposi-tion. Unlike air-borne particle counters, an impactor canbe used to divide welding fume into sizegroups that then can be examined sepa-rately in a scanning electron microscope(SEM). Without such size separation, it isdifficult to resolve useful images of weld-ing fume in a SEM because the fume oth-erwise clumps in indistinguishable masses.The size groups separated by impactioncan also be analyzed chemically in order tocharacterize the compositional depen-dence of particle size (Ref. 18).

In this paper, a cascade impactor wasused to separate welding fume into sizegroups for analysis. Although the im-pactor used could not distinguish betweenparticles in the nucleation mode and ac-cumulation mode, it was able to collectively separate these two groups of

fine particles from the coarse particles.The mass distribution found with cascadeimpaction can be used to determinewhether spatter contributes to weldingfumeformation and help determine howfume is inhaled.

Methods and Results

Fume was created by arc welding in afume chamber described elsewhere (Ref.7). Four different methods were employed:gas metal arc welding with three differentsets of parameters and correspondingly dif-ferent metal transfer modes (globular,spray, and pulsed), and one type of self-shielded flux cored arc welding (Table 1).

A cascade impactor (Thermo Ander-sen Nonviable Eight Stage Cascade Im-

Fig. 1 — Andersen cascade impactor. Only stages 1, 5, 6, and F, and the

bottom filter were used.

Fig. 2 — Scanning electron microscopy of stainless steel GMAW fumeparticles collected with a cascade impactor on stage 1, designed to cap-ture particles with daerodynamic > 5.8 µm. The count median diameter(CMD) of these particles is 5.0 µm with a geometric standard devia-tion (σg) of 1.7.

Table 1 — Description of Welding Processes Used to Create Fume Studied

Welding Process Shield Gas Electrode Current Voltage Wire PulseAWS (amp) (volt) Speed WidthDesignation (ipm) (ms)

(mm/s)Globular GMAW 2%O2-Ar ER308L ˜130 30 180 NA

0.045 in. (76)Spray GMAW 2%O2-Ar ER308L ˜185 30 300 NA

0.045 in. (127)Pulsed GMAW 2%O2-Ar ER308L peak:-325 average: ˜30 180 2.8

0.045 in. ave:-100 background:19 (76)FCAW none ER308FC-0 ˜170 30 530 NA

0.045 in. (224)

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pactor Series 20-800 Mark II, Fig. 1) wasconnected to the chimney of the fumechamber, with approximately 0.5 m be-tween the welding arc and the impactor.The vacuum supplied with the impactordrew 28.3 L/min of air from the chamberinto the impactor where airborne particleswere collected at various stages onto im-paction plates during welding. The platesfrom each stage were weighed before andafter fume collection using a Mettler AE163 microbalance with ± 50 µg precision.Before collection, a larger vacuum pulledair and fume out of the chamber to pre-vent buildup. It was turned off when theimpactor vacuum was turned on. Collec-tion times ranged from 10 s for FCAW to4 min for pulsed GMAW and were suffi-

cient to obtain adequate material toweigh, but care was also taken to collect nolonger than necessary. Overloading on theimpaction plates can cause some impactedparticles to become reentrained in the air-flow and to reimpact on the incorrectstage (Ref. 19).

The relative fume formation rate wasdetermined by dividing the total mass col-lected in the cascade impactor by the col-lection time. The fume formation rate ofglobular GMAW was about 2.7 timesgreater than that of spray GMAW, whichwas in turn 1.4 times greater than pulsedGMAW; and FCAW formed fume at rates10 times that of spray GMAW.

The airflow through an impactor iscontrolled by a jet plate in each stage so

that particles impact on the correspondingcollection plate according to their inertia.Therefore only particles with a smallenough daerodynamic can pass a given stage.This cutoff aerodynamic diameter issmaller for each successive stage. There-fore, the largest particles impact on thefirst plate and are removed from the airstream, then slightly smaller particles im-pact on the next plate, and so on. An oil orgrease on the stages can be used to preventparticle bounce, if that is a concern. Thiswas not done, because welding fume isknown to adhere well to clean metal sur-faces. At the final stage, a filter collects allparticles with daerodynamic smaller than acertain size.

Not all of the eight stages of the im-

Fig. 3 — Scanning electron microscopy of stainless steel GMAWfume particles collected with a cascade impactor onto stage 5, de-signed to capture particles with 1.1 µm < daerodynamic < 5.8 µm.The count median diameter (CMD) of these particles is 3.7 µmwith a geometric standard deviation (σg) of 1.5.

Fig. 4 — Scanning electron microscopy of stainless steel GMAWfume particles collected with a cascade impactor onto stage 6, de-signed to capture particles with 0.7µm < daerodynamic < 1.1 µm.The count median diameter (CMD) of these particles is 0.7 µmwith a geometric standard deviation (σg) of 1.9.

Table 2 — Mass Distribution (in milligrams) of Various Welding Fumes Measured with Cascade Impactor

FCAW globular GMAW spray GMAW pulsed GMAW

Run 1 2 3 4 1 2 3 1 2 3 1 2 3

Sampling Time (s) 10 12 13 15 60 65 75 120 180 180 120 150 240

Stage d aerodynamic

(µm) Mass (mg)

1 >5.8 1.18 1.23 1.64 2.37 0.20 0.27 0.39 0.09 0.14 0.20 0.40 0.38 0.73

5 1.1 – 5.8 0.64 0.67 0.82 1.05 0.20 0.22 0.25 0.09 0.12 0.12 0.12 0.17 0.13

6 0.7 – 1.1 3.43 4.61 4.82 8.77 1.93 3.17 4.21 1.16 1.70 1.69 1.42 0.82 1.42

F 0.4 – 0.7 4.79 5.38 5.87 8.45 4.60 5.24 7.00 2.10 2.89 2.96 0.89 1.43 2.59

Filter <0.4 9.21 10.95 13.4 16.64 24.71 26.18 31.29 20.68 31.04 31.24 15.06 19.33 31.17

Sum 19.25 22.84 26.55 37.28 31.64 35.08 43.14 24.12 35.89 36.21 17.89 22.13 36.04

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pactor were used in this study; accordingto the manufacturer, this does not changethe aerodynamics of the remaining stages,which simply collect the particles thatwould have impacted on the missingstages.

The daerodynamic is the equivalent diam-eter of a spherical particle with 1 g/cm3

density with the same inertial propertiesas the particles in question. For sphericalparticles, the actual diameter can be foundby dividing the aerodynamic diameter bythe square root of the density of the parti-cles (Ref. 2). If the density or particleshape is unknown, it is useful to directlymeasure the particle size collected at eachimpactor stage for each type of particulatematerial that is collected. This was donefor GMAW fume created in spray modeand for FCAW fume.

Fume particles were transferred to ad-hesive SEM stubs by pressing the stubsagainst impaction plates after collection.Scanning electron micrographs of thestubs (Figs. 2–11) were analyzed on a Mac-intosh Powerbook G4 computer using the

public domain NIH Image program (de-veloped at the U.S. National Institutes ofHealth and available on the Internet athttp://rsb.info.nih.gov/nih-image/) to deter-mine the particle size collected on eachplate. Because of the design of the im-pactor, many particles impacted in pilesthat made measurement of a single parti-cle difficult. Care was taken to create mi-crographs only of regions where particlepileup did not occur. In addition, smallerwelding fume particles often aggregate inthe air before collection to form largerparticle agglomerates that behave and im-pact like larger spherical particles. Theseagglomerates appear like foam or fine tan-gled hair in the micrographs (Figs. 5, 6, 10,11). The effective particle diameter of anaggregate was calculated from the area ofthe two-dimensional image. However, themajority of the analyzed particles in thetop three stages were individual spheres(or circles in the two-dimensional micro-graphs), so this approximation did not in-troduce substantial error into the mea-surement of those stages. At least 100

particles were measured per stage and acount median diameter (CMD) deter-mined for each of the top three stages,along with the respective geometric stan-dard deviation (σg). Individual fume par-ticles smaller than approximately 0.5µmwere difficult to resolve in the standardSEM used in the study. Agglomeration ofthese ultrafine particles also made resolu-tion difficult. Therefore, only the approxi-mate maximum particle size found is re-ported for the bottom two stages (stage Fand the filter).

These data compared favorably tothose obtained from measuring particlescollected in the cascade impactor fromiron powder with a known diameterdusted into the air. This iron powder wasused to test the measurement techniquebecause it was commercially preparedthrough liquid atomization and did notcontain ultrafine particles to obscure themeasurements.

As stated previously, the mass of eachimpaction plate was measured before andafter collection. Fume collection and mea-

Fig. 5— Scanning electron microscopy of stainless steel GMAWfume particles collected with a cascade impactor onto stage F, de-signed to capture particles with 0.4 µm < daerodynamic < 0.7 µm. Noindividual particles > 1 µm detected.

Fig. 6 — Scanning electron microscopy of stainless steel GMAWfume particles collected with a cascade impactor onto the filter, de-signed to capture particles with daerodynamic < 0.4 µm. No individualparticles > 0.5 µm detected.

Table 3 — Average Particle Size Distribution, by Percentage (%) of Total Fume Mass Collected

Impactor Stage Aerodynamic Diameter FCAW Globular GMAW Spray GMAW Pulsed GMAW

1 >5.8 6.01 0.77 0.44 1.99

5 1.1 –5.8 3.04 0.61 0.35 0.60

6 0.7 – 1.1 19.9 8.30 4.74 5.19

F 0.4 – 0.7 23.3 15.2 8.31 6.21

Filter <0.4 47.7 75.1 86.2 86.0

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surement were performed successfullythree or four times for each fume type andthen averaged. The resulting data are re-ported in Table 2.

The mass distribution data must betransformed for useful comparison be-tween various welding fumes. One way todo this is by reporting the average distrib-utions for each process as percentages ofthe total mass collected, as shown in Table 3.

The size distributions can also be plot-ted in a relative mass histogram with log-normal axes — Fig. 12. Two importantpoints should be made about histogramsof particle size distributions. First, thewidth of a histogram bin affects the total

mass in each bin. To account for this, themass should be divided by the width of thebin, which here is the logarithmic range inparticle size collected by each impactorstage. This range is related to the secondpoint: the lowest cutoff and highest cutofffor cascade impactors must sometimes beestimated in order to incorporate the datafrom the top and bottom stages into thehistogram. For example, the top stage inthe impactor used in this study collectedall particles larger than 5.8 µm. To dividethe mass from this stage by the range inparticle size collected, one must estimatethe effective upper cutoff to the particlesize collected on the top stage. Only parti-cles smaller than 20 µm remain airborne,

so this was used for the upper cutoff. Thelargest particle found on the top stage withSEM was 18 µm, so this estimate is reasonable. The bottom stage in the im-pactor contains a filter that collects all par-ticles smaller than 0.4 µm. The approxi-mate size limit for the accumulationmode, 0.1 µm, was used as the lower cut-off estimate for the bottom stage. Thereare likely smaller welding fume particlesin the air, but they would have accumu-lated into agglomerates upon depositionon the filter.

One can see that the accuracy of an im-pactor histogram can be dependent oncertain estimates. A less error-pronemethod of reporting the data is with a cu-

Fig. 7 — Scanning electron microscopy of stainless steel FCAW fume parti-cles collected with a cascade impactor on stage 1, designed to capture parti-cles with daerodynamic > 5.8 µm. The count median diameter (CMD) of theseparticles is 6.3 µm with a geometric standard deviation (σg) of 1.2.

Fig. 9 — Scanning electron microscopy of stainless steel FCAW fume parti-cles collected with a cascade impactor onto stage 6, designed to capture par-ticles with 0.7 µm < daerodynamic < 1.1 µm. The count median diameter(CMD) of these particles is 0.9 µm with a geometric standard deviation (σg)of 2.0.

Fig. 10 — Scanning electron microscopy of stainless steel FCAW fume par-ticles collected with a cascade impactor onto stage F, designed to captureparticles with 0.4 µm < daerodynamic < 0.7 µm. No individual particles > 1µm detected.

Fig. 8 — Scanning electron microscopy of stainless steel FCAW fume parti-cles collected with a cascade impactor onto stage 5, designed to capture par-ticles with 1.1 µm < daerodynamic < 5.8 µm. The count median diameter(CMD) of these particles is 3.3 µm with a geometric standard deviation (σg)of 1.4.

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mulative probability plot using probit ver-sus linear axes — Fig. 13. Here one reportsthe percentage of total mass that is smallerthan the upper cutoff size of each stage.The need for finding the lower cutoff forthe bottom stage can thus be avoided. Theonly drawback to a probability plot is thatit can be more difficult to see multiple sizemodes if present. However, it is easy tofind the mean mass aerodynamic diameter(MMAD) on a cumulative probabilityplot. It is simply the intersection or ex-trapolation of the data through the 50th

percentile. The MMAD along with thecorresponding geometric standard devia-tion can be used as the descriptive valuesof the entire size distribution. See Table 4for the MMADs of the various weldingfumes.

Discussion

Berner and Berner (Ref. 10) noted thatthe welding fume particle size distributionthat they measured could be explainedwith the trimodal model of atmosphericaerosols, because it was close to the loga-rithmic normal except for high concentra-tions at the tails.

The size distributions shown in Fig. 12also seem to fit the trimodal model, al-though because of size limitations with thecascade impactor, no direct evidence ofthe nucleation mode is available. How-ever, the coarse fraction is clearly distinctfrom the accumulation mode.

This can also be seen in the cumulativeprobability plot, in that the data do not fol-low a straight line, as they would if they

were in a single lognormal mode. The geo-metric standard deviations in Table 4 foreach process are also too large for singlemodes.

The MMADs in Table 4 are rathersmall and do not match the peaks of themass histogram in Fig. 12 well. This is be-cause the data from both modes were usedto fit the straight line extrapolationthrough the 50th percentile. The coarsefraction flattens the slope of the line, caus-ing it to intersect the 50th percentile at asmaller particle size than expected. If thedata from the impactor are split into twomodes (Fig. 14), more reasonableMMADs can be calculated from each setof data. See Table 4 and compare it toTable 5, a list of MMADs from other fumeresearchers. The values for σg for the ac-

Fig. 11 — Scanning electron microscopy of stainless steel GMAW fumeparticles collected with a cascade impactor onto the filter, designed tocapture particles with daerodynamic < 0.4 µm. No individual particles >0.5 µm detected.

Fig. 12 — Relative mass histogram of welding fume particles capturedwith cascade impactor.

Table 4 — Overall Mass Median Aerodynamic Diameter (MMAD) and Geometric StandardDeviation (σg) of Welding Fume Particles Collected with Cascade Impactor

globularFCAW GMAW spray GMAW pulsed GMAW

Mass Median Diameter (mm) MMAD σg MMAD σg MMAD σg MMAD σg

All Stages 0.33 4.0 0.07 5.8 0.02 7.9 0.04 8.2

(SD = 0.03) (SD = 0.001) (SD = 0.02) (SD = 0.001)

Bottom Three Stages Only 0.43 1.4 0.33 1.5 0.25 1.6 0.24 1.7

(Accumulation Mode) (SD = 0.003) (SD=0.003) (SD = 0.02) (SD = 0.02)

Top Two Stages Only 6.7 n.a. 6.1 n.a. 6.1 n.a 7.3 n.a

(Coarse Mode) (SD = 0.06) (SD = 0.3) (SD = 0.3) (SD = 0.5)

Note: that the standard deviation (SD) is from comparison of MMADs of various runs, whereas σg is a measure of how the averaged data of a group of runs varies fromthe average MMAD of that group of runs.

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cumulation mode are now more typical ofaerosols, including welding fume (Ref. 4).The MMADs from each run of the variousprocesses were compared to one anotherwith Student’s t-test, assuming equal vari-ance. The MMADs of pulsed GMAW andspray GMAW were not significantly dif-ferent from one another (t(4) = 1.57,p<0.05 if the MMADs from all stageswere compared, t(4) = 0.98, p<0.05 ifonly the MMADs from the bottom threestages were compared). It has been spec-ulated (Ref. 20) that pulsed GMAW cre-ated differently sized particles than con-

stant current GMAW, but this appears notto be the case. The MMADs of the otherprocesses were significantly different fromone another.

The accumulation MMAD of globularGMAW fume may be larger than those ofspray or pulsed GMAW fumes becausethe initial particle concentration isgreater, as evidenced by the greater fumeformation rate of globular GMAW. Parti-cle concentration speeds agglomeration(Refs. 4, 5, 21), therefore, in the same time(as determined by the distance from weld-ing arc to collection site) larger agglomer-

ates would accumulate from the nucle-ation mode if the fume formation ratewere greater. Jin (Ref. 14) also found thatwhen larger currents were used in GMAW,fume formation rates were greater andwelding fume particles were larger.FCAW has a greater fume formation ratethan globular GMAW, which explains whyFCAW has the largest accumulationMMAD.

The MMAD values for the coarse frac-tion are not as reliable as those for the ac-cumulation mode, both because there areonly two data points, but also because ofthe 20 µm estimate of the upper cutoff.However, if an upper cutoff of 60 µm wereused, the MMADs of the coarse fractionwould increase by less than 1 µm, so thismay not be an issue.

The distance from weld to collectionpoint naturally affects how many coarsemicrospatter particles are collected beforethey fall out of the air (Ref. 16). The dis-tance used in this study was a typical dis-tance from a weld to the welder helmet,but this distance can vary, both in practiceand in experiment, which would changethe coarse mode MMAD. The same dis-tance was used for each of the weldingprocesses studied, so this may explain thesimilarities in coarse fraction MMADs forthe various processes in this study. SeeRef. 5 for more discussion on the size sim-ilarities of coarse particles from metallur-gical processing.

Since no coarse particles were foundon stage F or on the filter (Figs. 5, 6, 10,11), one can easily say that, by mass, theFCAW fume contains no more than 30%coarse particles (or microspatter), sprayGMAW fume has less than 6%, globularGMAW contains less than 10%, andpulsed GMAW fume has less than 8% mi-

Fig. 13 — Lognormal probability plot of welding fume particles capturedwith cascade impactor.

Fig. 14 — Lognormal probability plot of welding fume particles capturedwith cascade impactor, split into two modes.

Table 5 — Mass Median Aerodynamic Diameter (MMAD) of Welding Fume ParticlesCalculated from Data Obtained with Cascade Impactors by Other Researchers

Welding Process MMAD (µm) Reference

SMAW 0.45 – 0.59 (a) 3

SMAW 0.5 – 0.8 8

SMAW 0.35 (a) 10

SMAW 0.2 11

SMAW 0.3 12

FCAW 0.3 8

FCAW 0.4 9

GMAW 0.25 (a) 3

GMAW 0.2 – 0.4 8

GMAW 0.3 9

GMAW 0.2 11

(a) MMAD reported by the researcher

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crospatter. These data confirm the find-ings of Zimmer et al. (Ref. 16) who mea-sured the concentration of particles in thecoarse size range and concluded that mi-crospatter does not significantly con-tribute to GMAW fume.

Regardless of the welding parametersthat cause different metal transfer modes,less than 10% of the mass of GMAW fumeis microspatter (coarse particles). Thismeans that the higher fume formationrates found during globular GMAW,which can be more than double that ofspray GMAW, are not caused by an in-creased fraction of coarse microspatterparticles, but from an increase of vapor-condensed particles.

This suggests that there is more evapo-ration of the electrode in globular GMAWthan in spray GMAW. Increased evapora-tion creates more vapor as well as morespatter, because bursting vapor bubblescreate coarse droplets/particles (Ref. 22).Because higher fume formation rates arelinked with greater vaporization rates dueto increased surface temperatures (Ref.23), it is proposed that the reason for theobserved correlation between fume andspatter rates is not because one causes theother, but rather that both are controlledby the same variables (e.g., surface tem-perature and electrical current).

Microspatter from FCAW is substan-tial. This may be because powder from thecore of the wire is ejected or because theslag-forming flux is more easily atomizedthan liquid metal (Ref. 5). So although theFFR of FCAW is much greater than thatof GMAW, a smaller percentage of FCAWfume may deposit in the lower lungs.

Conclusion

Impactor separation has shown at leasttwo size modes in the mass distribution ofvarious welding fumes, which is consistentwith aerosol theory.

Flux cored arc welding fume containsapproximately 30% microspatter by mass.This contributes to the greater fume for-mation rate of FCAW, but may mean thata smaller fraction of FCAW fume is res-pirable when compared to fume fromGMAW.

Gas metal arc welding fume consistspredominately of particle agglomeratessmaller than 1 µm. This means that mostof the fume is respirable. The size distrib-utions of pulsed GMAW and sprayGMAW are not significantly different.Less than 10% of all types of GMAWfume is microspatter. The microspatterfraction does not greatly change with theGMAW parameters, therefore it can beconcluded that GMAW FFR does not in-crease with welding parameters becauseof greater rates of microspatter formation,

but because of increased evaporation ofthe electrode. Increased FFR may in-crease the median diameter of the fumeparticle agglomerates because of in-creased agglomeration rates.

Knowing how welding parameters af-fect the size distribution of fume particleshelps fume researchers in three ways.First, it adds information to inhalationmodels of welding fume. Second, it yieldsinsight into what process variables can ef-fectively be changed to reduce the forma-tion of respirable welding particles. Third,since it sheds light on the formation pathsof the particles, more rigorous modelsabout fume chemistry can be developed.

Acknowledgment

The funding for this project was pro-vided by a grant from the U.S. Navy, Of-fice of Naval Research.

References

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2. Willeke, K., and Baron, P. A. (eds.) 1993.Aerosol Measurement: Principles, Techniques,and Applications. New York, N.Y.: Van Nos-trand Reinhold.

3. Hewett, P. 1995. The particle size distrib-ution, density and specific surface are of weld-ing fumes from SMAW and GMAW mild andstainless steel consumables. American Indus-trial Hygiene Association Journal 56(2): 128–135.

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