How to Optimize Mild Steel GMAW - CONCOAmetal transfer for a gas metal arc welding (GMAW) procedure as classified by cur-rent range need to be understood. Figure 1 illustrates the

DECEMBER 200430

Two divergent forces are hard atwork in today’s business world: oneis the constant updating of the lat-

est and greatest technology, and the sec-ond is the ongoing political rhetoric aboutoutsourcing jobs to lower-wage-payingcountries.

Inverter technology, for example, of-fers better power efficiency and, in some

cases, more stable arc characteristics. In-ternational competition, however, utilizessimpler technology coupled with loweroverhead costs to put pressure on manu-facturing jobs in the United States.

Being pressured into making a capitalinvestment of tens of thousands of dol-lars that may only achieve incrementalcost savings over current optimized prac-

RICHARD GREEN (richard.green@ concoa.com) is Product Manager, CONCOA, Virginia Beach, Va.

How toOptimize MildSteel GMAW

How toOptimize MildSteel GMAW

Make yourself more

competitive globally

through improved shielding

gas selections

BY RICHARD GREEN

The hybrid laser beam welding process combines the traditional GMAW processwith laser beam processing. (Photo courtesyof Craig Bratt, Fraunhofer USA.)

31WELDING JOURNAL

tices is obviously a pitfall to avoid. In-stead, American business and its employees can offer the world the inge-nuity it takes to produce quality weld-ments cost-competitively.

What follows is a strategy for optimiz-ing the cost to produce a mild steel gasmetal arc weldment. This includes evalu-ating the mode of transfer as well as laborand overhead rates, deposition efficiency,electrode cost, and power consumption.It also shows the gas system required toobtain a competitive rate using existingassets.

To begin, the three basic modes ofmetal transfer for a gas metal arc welding(GMAW) procedure as classified by cur-rent range need to be understood. Figure1 illustrates the approximate currentranges for the three modes of transfer forboth 0.035- and 0.045-in.-diameter solidwire. Short-circuiting arc occurs between60 and 175 A for 0.035 wire, and 90 to 220A for 0.045 wire. Short-circuiting arc weld-ing offers low thermal input, which facili-tates welding in all positions and reducespart distortion. Metallurgical propertiesare not adversely affected by the low en-ergy input and subsequent dilution of thebase material.

Figure 2 illustrates the voltage and cur-rent relationship as the metal is trans-ferred from the wire to the workpiece. Asthe wire is fed into the weld pool, the tipof the wire connected to the positive ter-minal of the power supply comes in con-tact with the workpiece that is connectedto the negative terminal, and a short is cre-ated in the circuit. The welding machineoutput current rises to a minimum currentlevel of 320 A for 0.035 wire, and 370 Afor 0.045 wire, to separate it from the weldpool. The short-circuit process will occur50 to 230 times per second depending onprocess design.

Welding machine manufacturers havedeveloped both fixed and variable slopewelding power supplies to control the out-put voltage with increasing amperage.This limits the maximum energy availableto separate the wire from the pool. If thereis too much energy, the result is excessivespatter, which lowers the deposition effi-ciency; with too little energy, the wire pilesup, resulting in incomplete fusion andpoor weld quality.

Secondly, welding equipment manu-facturers have developed both fixed andvariable inductance to control the rate ofthe current rise as illustrated by the cur-rent curve sequence A-B in Fig. 2. As in-ductance is increased, the amount of arc-ing time also increases as illustrated bythe voltage curve sequence E-H in Fig. 2.The additional arc-on time produces amore fluid weld pool, which yields a flat-ter weld bead with better wetting at theedges. In turn, this affects the cosmeticsand load-bearing capacity of the joint.

The proper selection of shielding gaswill drastically affect the energy transferand deposition efficiency of the GMAWshort-circuit transfer mode. Carbon diox-ide was the first shielding gas used becauseof its availability and cost. The arc plasmahas a narrow inner core and a low outerenvelope resulting from its low thermalconductivity that produces narrow anddeep penetration. This presents problemsfor thin materials.

More expensive GMAW wire contain-ing higher amounts of deoxidizing ele-ments is typically needed to balance theoxidizing nature of carbon dioxide. Also,because of centerline crowning and exces-sive spatter that result in 85 to 95% dep-osition efficiencies, manufacturers devel-oped binary mixtures of argon and carbondioxide.

Additions of up to 80% argon (with the

balance being carbon dioxide) will pro-duce less crowning, better edge tie-in, and94 to 98% deposition efficiencies. Argonadditions offer better arc ignition and sta-bility based on argon’s low ionization po-tential. Argon has a low thermal conduc-tivity that yields similar arc constrictionbut a shallower penetration profile thancarbon dioxide. And, argon-carbon diox-ide mixtures yield higher deposition rateswith less spatter, which is ideal for all-position welding and thin materials.

As additional welding current is ap-plied, the end of the welding wire becomesoverheated and balls up 1.5 to 3 times thewire diameter. This establishes a longerarc length as illustrated in sequence F-Hof Fig. 2. Gravity facilitates the metaltransfer, which creates instability and ex-cessive spatter. Deposition efficiencytends to fall between 80 and 90% depend-ing on gas selection and processing pa-rameters. For this reason and welding po-sition limitations, it is wise to stay outsidethe globular transition range of 160 to 185 A for 0.035 wire, and 200 to 220 A for0.045 wire.

Depending on the gas selection, theminimum transition current for spraytransfer occurs between 155 and 195 A for0.035 wire, and 220 and 250 A for 0.045wire. Above this transfer range, the endof the wire electrode develops a taper thatemits fine droplets of metal across the arcwith virtually no spatter, yielding 97 to99% deposition efficiencies. The spraytransfer yields higher travel speeds anddeposition rates because of the superiorarc stability and high droplet rate. How-ever, the high heat input limits the weld-ment to the flat position.

Choosing the optimal shielding gas forspray transfer takes some forethought tounderstand the application and effectseach gas component will contribute to the

Fig. 1 — The approximate current ranges include the threemodes of transfer for both 0.035- and 0.045-in.-diametersolid wire.

Fig. 2 — This graph illustrates voltage and current relationship through a short-circuiting arc sequence transfer. (Reprinted from AWS C5.6-89R, Recom-mended Practices for Gas Metal Arc Welding, p. 6.)

.045” spray

.045” globular.045” short-

circuiting arc.035” spray

.035” globular.035” short-

circuiting arc

Transfer Mode Current Range

deposition efficiency and cost, environ-mental, and mechanical properties.

Pure argon produces higher arc volt-age and subsequent longer arc lengths,which create arc instability and excessiveundercut at the edge of the welds. For thisreason, 5 to 20% carbon dioxide is addedto create an argon mixture that stabilizesthe spray transfer. It is well documentedthat the lower the amount of carbon diox-ide concentration, the lower the minimumspray transfer current and subsequentfume generation rates.

It should also be noted that 8 to 15%carbon dioxide mixtures are flexibleenough to facilitate both spray and short-circuit transfer modes. In some cases, 1 to5% oxygen may be added to argon toachieve superior arc stability and bettertie-in (wetting) at the weld edge. Oxygentends to provide a wider but shallowerpenetration profile, as compared to car-bon dioxide mixtures, because of its lowerionization and higher thermal conductiv-ity properties. Oxygen additions tend toyield better toughness and strengths be-cause of the absence of carbon retentionassociated with carbon dioxide mixtures.Shielding gas development has led manu-facturers to design three-component gasblends that offer the benefits of both car-bon dioxide and oxygen additions toargon-based mild steel gas metal arc applications.

As mentioned previously, each com-pany must evaluate the incremental ben-efits of three-component mixtures as com-pared to two. In most cases, attention toquality and continually training person-nel to meet the basic processing parame-

ters will yield the greatest return with min-imal investment.

For example, assume that the weld-ment is a 1⁄4-in. mild steel, 12-in. fillet weldrequiring 0.106 lb/ft of welding wire. Cur-rent practice calls for a 0.045-in.-diame-ter wire using 75% argon balance carbondioxide. It is assumed that the wire costs$0.80 per pound on a 33-lb spool, and thetypical labor and overhead rate is $40/h.There is a total of ten weld stations eachusing a single “T-size” (330 ft3) high-pres-sure bottle. The company uses eight bot-tles per week at a cost of $18 each. Themanual welding is performed utilizingconventional short-circuit parameters setat 20 V/200 A, yielding a deposition rateof 5.5 lb/h at 96% efficiency.

In today’s market, it is also safe to as-sume that the company is receiving pric-ing pressure from international competi-tors. Utilizing existing equipment and pro-viding the required training, the proce-dure is changed to a spray transfer withthe following parameters listed below.

The shielding gas is changed to 92%argon, balance carbon dioxide. The weld-ing machine parameters are 29 V/300 A,which provide a deposition rate of 9.7 lb/hwith a 98% efficiency. The economic re-sults displayed in Table 1 show that a 38%cost reduction per foot of weld is achiev-able because of the higher deposition rateand efficiency of a spray transfer. As well,Table 1 illustrates that an additional 6%in cost savings can be realized by mixingthe argon-carbon dioxide shielding gason-site.

Simple blending systems as illustratedin Fig. 3 enable the company to realize ad-

ditional duty cycle or productivity savingsby eliminating daily cylinder handling.

Finally, the on-site blending system en-ables the company to adjust the ratio ofcarbon dioxide in the shielding gas, whichwill have a positive effect on the weld-ment’s mechanical properties and workenvironment.

With American business facing somuch competitive pressure today, it is nec-essary to look for the “lowest hangingfruit” to reduce production costs and en-hance the quality of products. Gettingback to the basics will further enhance theincremental cost savings of future invest-ment in technology. To achieve such re-sults, solutions as simple as evaluating themode of transfer for a gas metal arc weldand the gas delivery system are importantin the highly competitive global market-place.◆

DECEMBER 200432

Table 1 — Economic Comparison

Short-Circuiting Arc Spray Transfer On-Site MixingGas System

Labor and Overhead ($/h) 40 40 40Deposition Rate (lb/h) 5.5 9.7 9.7Duty Cycle (%) 0.4 0.4 0.45Electrode Cost ($/lb) 0.8 0.8 0.8Deposition Efficiency (%) 0.96 0.98 0.98Gas Flow Rate (ft3/h) 35 40 40Gas Cost (dollars per hundred cubic feet) 5.45 6.06 5.28Electrical Cost (kWh) 0.06 0.06 0.06Machine Volts 20 29 29Machine Amps 200 300 300Travel Speed (in./min) 9 15 15

Cost per Foot of Weld

Labor and Overhead 2.22 1.33 1.19Wire Cost 0.09 0.09 0.09Shielding Gas Cost 0.04 0.03 0.03Power Cost 0.01 0.01 0.01Total Cost per Foot of Weld 2.36 1.46 1.31

Percent Cost Reduction 38.13% 44.59%

Fig. 3 — Simple blending systems allow foradditional duty cycle or productivity savingsby eliminating daily cylinder handling.

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