manual pentru mig, lincoln

Gas Metal Arc WeldingCarbon, Low Alloy, and Stainless Steels and Aluminum

GMAWWelding Guide

2 www.lincolnelectric.com

GMAW

The gas metal arc process is dominant today as ajoining process among the worlds welding fabrica-tors. Despite its sixty years of history, research anddevelopment continue to provide improvements tothis process, and the effort has been rewarded withhigh quality results.

This publications purpose is to provide the readerwith the basic concepts of the gas metal arc welding(GMAW) process, and then provide an examination ofmore recent process developments. Additionally, thereader will find technical data and direction, providingthe opportunity to optimize the operation of theGMAW process and all of its variants.

Process DefinitionGas Metal Arc Welding (GMAW), by definition, is anarc welding process which produces the coalescenceof metals by heating them with an arc between a con-tinuously fed filler metal electrode and the work. Theprocess uses shielding from an externally suppliedgas to protect the molten weld pool. The applicationof GMAW generally requires DC+ (reverse) polarity tothe electrode.

In non-standard terminology, GMAW is commonlyknown as MIG (Metal Inert Gas) welding and it is lesscommonly known as MAG (Metal Active Gas) welding.In either case, the GMAW process lends itself to welda wide range of both solid carbon steel and tubularmetal-cored electrodes. The alloy material range forGMAW includes: carbon steel, stainless steel,aluminum, magnesium, copper, nickel, silicon bronze and tubular metal-cored surfacing alloys. The GMAW process lends itself to semiautomatic,robotic automation and hard automation weldingapplications.

Advantages of GMAWThe GMAW process enjoys widespread use becauseof its ability to provide high quality welds, for a widerange of ferrous and non-ferrous alloys, at a low price.GMAW also has the following advantages:

The ability to join a wide range of material types andthicknesses.

Simple equipment components are readily availableand affordable.

GMAW has higher electrode efficiencies, usually between93% and 98%, when compared to other welding processes.

Higher welder efficiencies and operator factor, when comparedto other open arc welding processes.

GMAW is easily adapted for high-speed robotic, hardautomation and semiautomatic welding applications.

All-position welding capability.

Excellent weld bead appearance.

Lower hydrogen weld deposit generally less than5 mL/100 g of weld metal.

Lower heat input when compared to other welding processes.

A minimum of weld spatter and slag makes weld clean up fastand easy.

Less welding fumes when compared to SMAW (ShieldedMetal Arc Welding) and FCAW (Flux-Cored Arc Welding)processes.

Benefits of GMAW Generally, lower cost per length of weld metal deposited when

compared to other open arc welding processes.

Lower cost electrode.

Less distortion with GMAW-P (Pulsed Spray Transfer Mode),GMAW-S (Short-Circuit Transfer Mode) and STT (SurfaceTension Transfer).

Handles poor fit-up with GMAW-S and STT modes.

Reduced welding fume generation.

Minimal post-weld cleanup.

Limitations of GMAW The lower heat input characteristic of the short-circuiting

mode of metal transfer restricts its use to thin materials.

The higher heat input axial spray transfer generally restricts itsuse to thicker base materials.

The higher heat input mode of axial spray is restricted to flator horizontal welding positions.

The use of argon based shielding gas for axial spray andpulsed spray transfer modes is more expensive than 100%carbon dioxide (CO2).

Gas Metal Arc Welding

3GMAW

www.lincolnelectric.com

Editor:Jeff Nadzam, Senior Application Engineer

Contributors:Frank Armao, Senior Application Engineer

Lisa Byall, Marketing GMAW ProductsDamian Kotecki, Ph.D., Consumable Research and Development

Duane Miller, Design and Engineering Services

Gas Metal Arc Welding Guidelines

Important Information on our WebsiteConsumable AWS Certificates:www.lincolnelectric.com/products/certificates/Material Safety Data Sheets (MSDS):www.lincolnelectric.com/products/msds/ANSI Z49.1 Safety in Welding and Cutting and Arc WeldingSafety Checklist:www.lincolnelectric.com/community/safely/Request E205 Safety Booklet:www.lincolnelectric.com/pdfs/products/literature/e205.pdf


GMAW

Contents Page

History of Gas Metal Arc Welding (GMAW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Modes of Metal Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-10Short-Circuit Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7Globular Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8Axial Spray Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9Pulsed Spray Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Components of the Welding Arc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Shielding Gases for GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-15Inert Shielding Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12Reactive Shielding Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-13Binary Shielding Gas Blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13-14Ternary Shielding Gas Blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14GMAW Shielding Gas Selection Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

Effects of Variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16-17Current Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16Electrode Efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16Deposition Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16-17Electrode Extension and CTWD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

Advanced Welding Processes for GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18-19Waveform Control Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18-19

The Adaptive Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20-21Advanced Waveform Control Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

Surface Tension Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20-21

Tandem GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22-23Features of Tandem GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22Modes of Metal Transfer for Tandem GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22-23

Equipment for GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24-31The Power Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25The Wire Drive System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26-27

Special Wire Feeding Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28-29Shielding Gas Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28Bulk Electrode Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

Typical GMAW Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30-31 Semiautomatic GMAW System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30Automatic GMAW System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30Portable Engine Driven GMAW System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

GMAW Torches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31-33For Semiautomatic GMAW Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31-32For Hard and Robotic Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

5GMAW


Contents Page

GMAW of Carbon and Low Alloy Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34-39Selecting Carbon and Low Alloy Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34Types of GMAW Carbon and Low Alloy Steel Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35-36Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36Chemical Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37AWS Specifications for Manufacturing GMAW Wires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38Selecting Carbon and Low Alloy Electrodes for GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38-39

GMAW of Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40-57Types of Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40-42Sensitization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43-44Hot Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44-45Precipitation Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46Duplex Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47Physical and Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47-49Selecting Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49Corrosion Resistance of Stainless Steels in Various Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50Design for Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51Selecting Stainless Steel Electrodes for GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52-54GMAW of Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54-56

GMAW of Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57-64Properties of Aluminum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57Aluminum GMAW Modes of Metal Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57Power Supplies and Wire Drives for Aluminum GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58-59Shielding Gases for Aluminum GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60Filler Alloy for Aluminum GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60Aluminum GMAW Welding Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60-61Filler Metal Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62-63Chemical Composition for Aluminum Wires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63Selecting Aluminum Electrodes for GMAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63Aluminum Filler Metal Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64

General Welding Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65-77Current vs. Wire Feed Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65-66General Welding Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67-77

STT II Welding Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78-81For Carbon Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78-79For Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80For Nickel Alloy and Silicon Bronze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81For Pipe Root Pass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81

Rapid-Arc Welding Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .82-87For Solid Wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .82-84For Metal-Cored Wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84-86Application Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .86-87

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88-89

Safety Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .90-93


GMAW

The history of GMAW, gas metal arc welding, had its industrialintroduction in the late 1940s. The site was the BattelleMemorial Institute, and it was there that Hobart and Devers,sponsored by the Air Reduction Company, researched anddeveloped the first use of a continuously fed aluminum wireelectrode, shielded with 100% argon gas.

Axial spray transfer for aluminum was the earliest metal transfermode for the process. This eventually led to the use of argonplus small additions of oxygen. The oxygen improved arc stabilityand finally permitted the use of axial spray transfer on ferrousmaterials. The process was limited because of the high energylevel of axial spray transfer to plate thickness material.

In the early 1950s, the work of Lyubavshkii and Novoshilovinitiated the development of the GMAW process to include theuse of large diameters of steel electrode shielded with carbondioxide, a reactive gas. The process development at this stagewas high in weld spatter, and the level of heat generated by thearc made the process uninviting to welders.

In the late 1950s improvements in power source technologyand the interface of small diameter electrodes, in the 0.035" -0.062" (0.9 - 1.6 mm) diameter range, permitted the implemen-tation of the discrete mode known as short-circuiting transfer.This development permitted the use of lower heat input weldingon thin sections of base material, and it provided the opportunityfor all-position welding.

In the early 1960s, power source research and development ledto the introduction of pulsed spray in the GMAW mode. Theidea for pulsed spray transfer, GMAW-P, occurred in the 1950sand it conceptually involved the use of a high-speed transitionbetween a high-energy peak current to a low backgroundcurrent. The motivation behind the idea was the need todecrease spatter and eliminate incomplete fusion defects. Thepulsed arc process incorporated the benefits of axial spraytransfer clean, spatter-free welds having excellent fusion,with lower heat input. The lower average current provided byGMAW-P allowed for out-of-position welding capability withimproved weld quality, when compared with short-circuittransfer.

The 1970s introduced power source technology, which furtherenhanced the development of the GMAW process and GMAW-Pin particular. This period saw the incorporation of the earliestthyristor power sources for pulsed GMAW. The WeldingInstitute of the United Kingdom is largely responsible fordetermining the linear relationship between pulsed frequencyand wire feed speed. The algorithm for this mathematical relation-ship permitted a fundamental base for subsequent synergictransistor controlled power sources. The new high speedelectronic controls improved the interface between weldingsophistication and the welding shop floor. The new descriptorfor this development was the word "Synergic." As it relates,synergy means: one knob control as the welder increases ordecreases wire feed speed, a predetermined pulsed energy isautomatically applied to the arc. Synergic power sources madeit easier to use GMAW-P.

In the 1990s, research and development in welding powersource technology continued to evolve. The Lincoln ElectricCompany took the lead in developing a wide range of powersource platforms designed with the optimized arc in mind.Widely recognized as Waveform Control Technology theLincoln Electric welding systems incorporate an inverter basedtransformer design with a high speed, computerized controlcircuit. Software developed programs provide an expansivearray of synergic and non-synergic optimized arc weldingprograms for the following welding processes:GMAW Gas Metal Arc WeldingFCAW Flux-Cored Arc WeldingGTAW Gas Tungsten Arc WeldingSMAW Shielded Metal Arc WeldingCAC-A Carbon Arc Cutting Process

Among the newer advanced Waveform Control Technologyprocesses is Surface Tension Transfer, or STT. STT is alow heat input mode of weld metal transfer, which incorporatesa high-speed reactive power source to meet the instantaneousneeds of the arc. The power source is a waveform generator,which is therefore neither a constant current nor constantvoltage power source.

Unique to STT, is the application of applying welding currentindependent of the wire feed speed. This feature has the benefitof increasing or decreasing the welding current to increase ordecrease heat input. Fundamentally, STT provides an answerfor controlling the welding conditions, that can produceincomplete fusion. The STT welding mode has the dual benefitof increasing productivity, and improving overall weld quality.See Advanced Welding Processes for GMAW on page 18.

The GMAW process is flexible in its ability to provide soundwelds for a very wide base material type and thickness range.Central to the application of GMAW is a basic understanding ofthe interplay between several essential variables:

The thickness range of the base material to be welded will dictate the electrode diameter, and the useable currentrange.

The shielding gas selection will influence the selection of themode of metal transfer, and will have a definite effect on thefinished weld profile.

History of Gas Metal Arc Welding

7GMAW


Short-Circuit Metal Transfer

Short-circuiting metal transfer, known by the acronym GMAW-S,is a mode of metal transfer, whereby a continuously fed solid ormetal-cored wire electrode is deposited during repeated electricalshort-circuits.

The short-circuiting metal transfer mode is the low heat inputmode of metal transfer for GMAW. All of the metal transferoccurs when the electrode is electrically shorted (in physicalcontact) with the base material or molten puddle. Central to thesuccessful operation of short-circuiting transfer is the diameterof electrode, the shielding gas type and the welding procedureemployed. This mode of metal transfer typically supports the useof 0.025 - 0.045 (0.6 - 1.1 mm) diameter electrodes shieldedwith either 100% CO2 or a mixture of 75-80% argon, plus25-20% CO2. The low heat input attribute makes it ideal forsheet metal thickness materials. The useable base materialthickness range for short-circuiting transfer is typically consideredto be 0.024 0.20 (0.6 5.0 mm) material. Other namescommonly applied to short-circuiting transfer include short arcmicrowire welding, fine wire welding, and dip transfer.

Advantages of Short-Circuiting Transfer All-position capability, including flat, horizontal, vertical-up,

vertical-down and overhead.

Handles poor fit-up extremely well, and is capable of rootpass work on pipe applications.

Lower heat input reduces weldment distortion.

Higher operator appeal and ease of use.

Higher electrode efficiencies, 93% or more.

Limitations of Short-Circuiting Transfer Restricted to sheet metal thickness range and open roots of

groove joints on heavier sections of base material.

Poor welding procedure control can result in incompletefusion. Cold lap and cold shut are additional terms that serveto describe incomplete fusion defects.

Poor procedure control can result in excessive spatter, andwill increase weldment cleanup cost.

To prevent the loss of shielding gas to the wind, welding out-doors may require the use of a windscreen(s).

FIGURE 1: Pinch Effect During Short-Circuiting Transfer

Current (A)

Electrode

P A2 Pinch effect force, P

Description of Short-Circuiting TransferThe transfer of a single molten droplet of electrode occursduring the shorting phase of the transfer cycle (See Figure 2).Physical contact of the electrode occurs with the molten weldpool, and the number of short-circuiting events can occur up to200 times per second. The current delivered by the weldingpower supply rises, and the rise in current accompanies anincrease in the magnetic force applied to the end of theelectrode. The electromagnetic field, which surrounds theelectrode, provides the force, which squeezes (more commonlyknown as pinch) the molten droplet from the end of the electrode.

Because of the low-heat input associated with short-circuitingtransfer, it is more commonly applied to sheet metal thicknessmaterial. However, it has frequently found use for welding theroot pass in thicker sections of material in open groove joints.The short-circuiting mode lends itself to root pass applicationson heavier plate groove welds or pipe.

Solid wire electrodes for short-circuiting transfer range from0.025 - 0.045 (0.6 1.1 mm). The shielding gas selectionincludes 100% CO2, and binary blends of argon + CO2 orargon + O2. Occasionally ternary blends, (three part mixes), ofargon + CO2 + oxygen are sometimes employed to meet theneeds of a particular application.

A B C D E

Short

Volta

geCu

rrent

Extin

ction

Reignition

Arcing Period

Time

Zero

Zero

FIGURE 2: Oscillograms and Sketches of Short Circuiting Transfer

Time

Short

Zero

Zero

Cur

rent

Vol

tage R

eign

ition

Ext

inct

ion

Arcing Period

AA BB CC DD EE

Modes of Metal Transfer

AA The solid or metal-cored electrode makes physical contact with the molten puddle.The arc voltage approaches zero, and the current level increases. The rate of rise to the peak current is affected by the amount of applied inductance.

BB This point demonstrates the effect of electromagnetic forces that are applied uniformly around the electrode. The application of this force necks or pinches the electrode. The voltage very slowly begins to climb through the period before detachment, and the current continues to climb to a peak value.

CC This is the point where the molten droplet is forced from the tip of the electrode. The current reaches its maximum peak at this point. Jet forces are applied to the molten puddle and their action prevents the molten puddle from rebounding and reattaching itself to the electrode.

DD This is the tail-out region of the short-circuit waveform, and it is during this down-ward excursion toward the background current when the molten droplet reforms.

EE The electrode at this point is, once again, making contact with the molten puddle, preparing for the transfer of another droplet. The frequency of this varies between 20 and 200 times per second. The frequency of the short-circuit events is influenced by the amount of inductance and the type of shielding gas. Additions of argon increase the frequency of short-circuits and it reduces the size of the molten droplet.


GMAW

Inductance ControlKeywords:

Rate of Current Rise

Henries

Variable Inductance

Fixed Inductance

The application of an inductance control feature is typical formost GMAW power sources. Inductance has effects only in theshort-circuit transfer mode. Usually, inductance is either fixed orvariable; and this depends upon the design of the power source.A fixed inductance power source indicates that an optimum levelof inductance is built into the power source, and variableinductance indicates that the amount of inductance applied tothe arc is adjustable. Inductance controls the rate of current risefollowing the short-circuit condition. Consequently, its use isbeneficial because its adjustment facilitates adding or decreasingenergy to the short-circuit condition. Inductance plays a role inthe frequency of droplet transfer per unit of time: as theinductance increases, the frequency of short-circuit metaltransfer decreases. Each droplet contains more energy and toewetting improves. As the inductance decreases, the short-circuit events increase, and the size of the molten dropletdecreases. The objective for the variable inductance controlfeature, on any given power source, is to transfer the smallestmolten droplet possible with the least amount of spatter, andwith sufficient energy to ensure good fusion. Additions ofinductance will provide the essential energy to improve toe wetting.

Inductance is measured in Henries, and in a variable inductancepower source it is the resulting arc performance characteristicthat results from the interplay of a combination of electricalcomponents. These components typically include the chokefilter, capacitors, and power resistors.

Globular Transfer

Globular metal transfer is a GMAW mode of metal transfer,whereby a continuously fed solid or metal-cored wire electrodeis deposited in a combination of short-circuits and gravity-assistedlarge drops. The larger droplets are irregularly shaped.

During the use of all metal-cored or solid wire electrodes forGMAW, there is a transition where short-circuiting transfer endsand globular transfer begins. Globular transfer characteristicallygives the appearance of large irregularly shaped molten dropletsthat are larger than the diameter of the electrode. The irregularlyshaped molten droplets do not follow an axial detachment fromthe electrode, instead they can fall out of the path of the weld or

move towards the contact tip. Cathode jet forces, that moveupwards from the work-piece, are responsible for the irregularshape and the upward spinning motion of the molten droplets.

The process at this current level is difficult to control, and spatteris severe. Gravity is instrumental in the transfer of the largemolten droplets, with occasional short-circuits.

During the 1960s and 1970s, globular transfer was a popularmode of metal transfer for high production sheet metal fabrica-tion. The transfer mode is associated with the use of 100% CO2shielding, but it has also seen heavy use with argon/CO2 blends.For general fabrication on carbon steel, it provides a mode oftransfer, just below the transition to axial spray transfer, whichhas lent itself to higher speed welding.

The use of globular transfer in high production settings is beingreplaced with advanced forms of GMAW. The change is beingmade to GMAW-P, which results in lower fume levels, lower orabsent spatter levels, and elimination of incomplete fusiondefects.

Advantages of Globular Transfer Uses inexpensive CO2 shielding gas, but is frequently used

with argon/CO2 blends.

Is capable of making welds at very high travel speeds.

Inexpensive solid or metal-cored electrodes.

Welding equipment is inexpensive.

Limitations of Globular Transfer: Higher spatter levels result in costly cleanup.

Reduced operator appeal.

Prone to cold lap or cold shut incomplete fusion defects,which results in costly repairs.

Weld bead shape is convex, and welds exhibit poor wetting atthe toes.

High spatter level reduces electrode efficiency to a range of87 93%.

FIGURE 3: Globular Weld Metal Transfer CharacteristicsGlobular Transfer

9GMAW


Axial Spray TransferKeywords:

Globular to Axial Spray Transition Current

Weld Interface

Axial spray metal transfer is the higher energy mode of metaltransfer, whereby a continuously fed solid or metal-cored wireelectrode is deposited at a higher energy level, resulting in astream of small molten droplets. The droplets are propelledaxially across the arc.

Axial spray transfer is the higher energy form of GMAW metaltransfer. To achieve axial spray transfer, binary blends containingargon + 1-5 % oxygen or argon + CO2, where the CO2 levelsare 18% or less. Axial spray transfer is supported by either theuse of solid wire or metal-cored electrodes. Axial spray transfermay be used with all of the common alloys including: aluminum,magnesium, carbon steel, stainless steel, nickel alloys, and copper alloys.

For most of the diameters of filler metal alloys, the change toaxial spray transfer takes place at the globular to spray transitioncurrent. A stream of fine metal droplets that travel axially fromthe end of the electrode characterizes the axial spray mode ofmetal transfer. The high puddle fluidity restricts its use to thehorizontal and flat welding positions.

For carbon steel, axial spray transfer is applied to heavier sectionthickness material for fillets and for use in groove type weldjoints. The use of argon shielding gas compositions of 95%, witha balance of oxygen, creates a deep finger-like penetrationprofile, while shielding gas mixes that contain more than 10%CO2 reduce the finger-like penetration profile and provide amore rounded type of penetration.

The selection of axial spray metal transfer is dependent upon thethickness of base material and the ability to position the weldjoint into the horizontal or flat welding positions. Finished weldbead appearance is excellent, and operator appeal is very high.Axial spray transfer provides its best results when the weld jointis free of oil, dirt, rust, and millscale.

Advantages of Axial Spray Transfer High deposition rates. High electrode efficiency of 98% or more. Employs a wide range of filler metal types in an equally wide

range of electrode diameters. Excellent weld bead appearance.

TABLE 1 Transition Currents for Axial Spray Transfer

Electrode ApproximateFiller Metal Diameter Shielding Current

Type Inches (mm) Gas (Amps)

0.030 (0.8) 90% Argon, 10% CO2 155 - 1650.035 (0.9) 90% Argon, 10% CO2 175 - 1850.045 (1.2) 90% Argon, 10% CO2 215 - 2250.052 (1.3) 90% Argon, 10% CO2 265 - 275

Carbon and 0.062 (1.6) 90% Argon, 10% CO2 280 - 290Low AlloySolid Steel 0.035 (0.9) 98% Argon, 2% O2 130 - 140

0.045 (1.2) 98% Argon, 2% O2 205 - 2150.052 (1.3) 98% Argon, 2% O2 240 - 2500.062 (1.6) 98% Argon, 2% O2 265 - 275

Carbon and 0.040 (1.0) 90% Argon, 10% CO2 140 - 150Low Alloy 0.045 (1.2) 90% Argon, 10% CO2 160 - 170

Composite 0.052 (1.3) 90% Argon, 10% CO2 170 - 180Steel 0.062 (1.6) 90% Argon, 10% CO2 220 - 230

0.030 (0.8) 98% Argon, 2% O2 120 - 1300.035 (0.9) 98% Argon, 2% O2 140 - 1500.045 (1.2) 98% Argon, 2% O2 185 - 195

Stainless 0.062 (1.6) 98% Argon, 2% O2 250 - 260Steel

0.030 (0.8) 98% Argon, 2% CO2 130 - 1400.035 (0.9) 98% Argon, 2% CO2 200 - 2100.045 (1.2) 98% Argon, 2% CO2 145 - 1550.062 (1.6) 98% Argon, 2% CO2 255 - 265

GMAW Axial Spray Transition Currents for Solid and Composite

Carbon Steel Electrodes and Stainless Steel Solid Wire Electrodes

FIGURE 4: Axial Spray Weld Metal Transfer Characteristics

High operator appeal and ease of use. Requires little post weld cleanup. Absence of weld spatter. Excellent weld fusion. Lends itself to semiautomatic, robotic, and hard automation

applications.

Limitations of Axial Spray Transfer Restricted to the flat and horizontal welding positions. Welding fume generation is higher. The higher-radiated heat and the generation of a very bright

arc require extra welder and bystander protection. The use of axial spray transfer outdoors requires the use of a

windscreen(s). The shielding used to support axial spray transfer costs more

than 100% CO2.


GMAW

Pulsed Spray TransferKeywords:

Period

Peak Current

Background Current

Frequency

Pulsed spray metal transfer, known by the acronym GMAW-P,is a highly controlled variant of axial spray transfer, in which thewelding current is cycled between a high peak current level to alow background current level. Metal transfer occurs during thehigh energy peak level in the form of a single molten droplet.

GMAW-P was developed for two demanding reasons: control ofweld spatter and the elimination of incomplete fusion defectscommon to globular and short-circuiting transfer. Its earliestapplication included the welding of high strength low alloy basematerial for out-of-position ship hull fabrication. The advantagesthat it brought to the shipbuilding industry included: higherefficiency electrodes than FCAW, and the ability to deliver lowerhydrogen weld deposits. The mode employs electrode diametersfrom 0.030 1/16 (0.8 1.6 mm) solid wire electrodes andmetal-cored electrodes from 0.045 5/64 (1.1 2.0 mm)diameter. It is used for welding a wide range of material types.Argon based shielding gas selection with a maximum of 18%CO2 supports the use of pulsed spray metal transfer withcarbon steels.

The welding current alternates between a peak current and alower background current, and this controlled dynamic of thecurrent results in a lower average current than is found with axialspray transfer. The time, which includes the peak current andthe background current, is a period, and the period is known asa cycle (Hz). The high current excursion exceeds the globular tospray transition current, and the low current is reduced to avalue lower than is seen with short-circuiting transfer. Ideally,during the peak current, the high point of the period, a singledroplet of molten metal is detached and transferred across thearc. The descent to the lower current, known as the backgroundcurrent, provides arc stability and is largely responsible for theoverall heat input into the weld. The frequency is the number oftimes the period occurs per second, or cycles per second. Thefrequency of the period increases in proportion to the wire feedspeed. Taken together they produce an average current, whichleverages its use in a wide material thickness range.

1

2

4

3

5

89

6

7

CURRENT

TIME (mS)

(1) Front Flank Ramp-up Rate

(2) Overshoot

(3) Peak Current

(4) Peak Time

(5) Tail-out

(6) Tail-out Speed

(7) Step-off Current

(8) Background Current

(9) Period and Frequency

FIGURE 5: A Single Pulsed Event

Time (mS)

Cur

rent

(4)

(2)

(3)

(5)

(6)

(7)

(8)(9)

(1)

Advantages of Pulsed Spray Transfer Absent or very low levels of spatter. More resistant to lack of fusion defects than other modes of

GMAW metal transfer. Excellent weld bead appearance. High operator appeal. Offers an engineered solution for the control of weld fume

generation. Reduced levels of heat induced distortion. Ability to weld out-of-position. Lower hydrogen deposit. Reduces the tendency for arc blow. Handles poor fit-up. When compared to FCAW, SMAW, and GMAW-S, pulsed

spray transfer provides a low cost high-electrode efficiencyof 98%.

Lends itself to robotic and hard automation applications. Is combined for use with Tandem GMAW Twinarc or other

multiple arc scenarios. Capable of arc travel speeds greater than 50 inches per

minute (1.2 M/min.).

Limitations of Pulsed Spray Transfer Equipment to support the process is more expensive than

traditional systems.

Blends of argon based shielding gas are more expensive thancarbon dioxide.

Higher arc energy requires the use of additional safetyprotection for welders and bystanders.

Adds complexity to welding.

Requires the use of windscreens outdoors.

Pulsed Spray Transfer

Surface Tension Transfer

Axial Spray Transfer

Short-Circuit Transfer

GMAW Mode of Metal Transfer Selector

Material Thickness RangeUT(1) 19.0mm 12.5mm 6.4mm 3.2mm 1.6mm 0.9mm

3/4 1/2 1/4 1/8 1/16 0.035(1) UT = Unlimited Base Material Thickness.

11

GMAW


Keywords:

Anode Region

Cathode Region

Arc Plasma Region

Electromagnetic Forces

Gravity Droplet Weight

Surface Tension Forces

Jet Forces

The area of the welding arc is a region of high complexity that iscomprised of physical forces and chemical reactions. Theinteraction of the components of the arc affects metal transferand the quality of the finished weld. The behavior of the arc isinfluenced by:

The type and diameter of the filler metal.

The base metal conditions clean or millscale.

The shielding gas.

The welding parameters voltage and current.

The interaction of physical forces gravity, surface tension,jet forces, and electromagnetic force.

The character of the mode of metal transfer, the penetrationprofile, and the bead shape are influenced by the forces appliedto the metal as it moves from the electrode end to the work-piece.

Anode (+)

Cathode ()

Plasma Ionized GasMetal Vapor

Electromagnetic ForcesWhen current flows through a conductor, a magnetic field buildsand surrounds the conductor. In GMAW the electro-magneticforces, which are mathematically proportional to the square ofthe applied current, affect the mode of metal transfer. The mostcommon term applied to the electromagnetic force is the pincheffect. As the molten drop forms, it is uniformly squeezed fromthe electrode anode end by the electromagnetic force. The sizeof the droplet transferred depends upon this force, the appliedwelding current, and the shielding gas.

Surface Tension ForcesSurface tension forces are those forces, which are normal to thesurface of a molten droplet. They act on both the interior andthe exterior surface of the droplet. Together they serve tosupport the form of a molten droplet. There is always an inwardpull of the forces applied to the surface.

Jet ForcesIn the short-circuiting mode of metal transfer, during the shortingportion of the metal transfer cycle, higher currents cause theelectrode to heat to the point of melting. The high current drivesan increase in the electromagnetic force, which causes themolten metal to detach from the electrode. As the dropletmeets the weld pool, the surface tension forces supporting themolten droplet release and the molten droplet then adds itself tothe molten weld pool.

In the globular transfer mode, a large molten droplet develops.Surface tension forces support the formation of the moltendroplet, and jet forces push against the large droplet. The jetforces are responsible for supporting, spinning, and pushing thelarge droplet in an irregular fashion within the arc. The transferoccurs by the occasional shorting of the large droplet to theweld pool and the force of gravity. Once the droplet contactsthe molten pool or work-piece, the surface tension forces in thedroplet collapse, and the volume of weld metal is absorbed bythe puddle.

The shielding gas employed in a welding application has aneffect on the surface tension forces. If the energy level within thearc is high, as is the case with a 100% argon gas employed witha carbon steel electrode, then the bead shape will be extremelyconvex. If the surface tension value is low, because of the addi-tion of carbon dioxide or oxygen, then the bead shape will beless convex, and more acceptable. So the addition of active gascomponents will result in improved weld bead and overall arcperformance with carbon steel electrodes.

FIGURE 6: Cross Section of a GMAW Arc

Anode (+)

Cathode (-)

Ionized GasPlasma Metal Vapor{

Components of the Welding Arc


GMAW

The selection of the correct shielding gas for a given applicationis critical to the quality of the finished weld. The criteria used tomake the selection includes, but is not restricted to, the following:

Alloy of wire electrode. Desired mechanical properties of the deposited weld metal. Material thickness and joint design. Material condition the presence of millscale, corrosion,

resistant coatings, or oil. The mode of GMAW metal transfer. The welding position. Fit-up conditions. Desired penetration profile. Desired final weld bead appearance. Cost.

Under the heat of the arc, shielding gases respond in differentways. The flow of current in the arc, and its magnitude, has aprofound effect on the behavior of the molten droplet. In somecases, a given shielding gas will optimally lend itself to onetransfer mode, but will be incapable of meeting the needs ofanother. Three basic criteria are useful in understanding theproperties of shielding gas:

Ionization potential of the gas components Thermal conductivity of the shielding gas components The chemical reactivity of the shielding gas with the molten

weld puddle

The following discussion details the arc physics associated withspecific shielding gases, and permits the selection of the bestshielding gas for the application.

Shielding GasesArgon and helium are the two inert shielding gases used forprotecting the molten weld pool. The inert classification indicatesthat neither argon nor helium will react chemically with themolten weld pool. However, in order to become a conductivegas, that is, a plasma, the gas must be ionized. Different gasesrequire different amounts of energy to ionize, and this ismeasured in terms of the ionization energy. For argon, theionization energy is 15.7 eV. Helium, on the other hand, has anionization energy of 24.5 eV. Thus, it is easier to ionize argonthan helium. For this reason argon facilitates better arc startingthan helium.

The thermal conductivity, or the ability of the gas to transferthermal energy, is the most important consideration for selectinga shielding gas. High thermal conductivity levels result in moreconduction of the thermal energy into the workpiece. Thethermal conductivity also affects the shape of the arc and thetemperature distribution within the region. Argon has a lowerthermal conductivity rate about 10% of the level for bothhelium and hydrogen. The high thermal conductivity of heliumwill provide a broader penetration pattern and will reduce thedepth of penetration. Gas mixtures with high percentages of

argon will result in a penetration profile with a finger-likeprojection into the base material, and this is due to the lowerthermal conductivity of argon.

Inert Shielding GasesArgon is the most commonly used inert gas. Compared tohelium its thermal conductivity is low. Its energy required togive up an electron, ionization energy, is low, and this results inthe finger-like penetration profile associated with its use. Argonsupports axial spray transfer. Nickel, copper, aluminum, titanium,and magnesium alloyed base materials use 100% argonshielding. Argon, because of its lower ionization energy, assistswith arc starting. It is the main component gas used in binary(two-part) or ternary (three-part) mixes for GMAW welding. Italso increases the molten droplet transfer rate.

Helium is commonly added to the gas mix for stainless andaluminum applications. Its thermal conductivity is very high,resulting in the broad but less deep penetration profile.When in use, arc stability will require additions of arc voltage.Helium additions to argon are effective in reducing the dilution ofbase material in corrosion resistant applications. Helium/argonblends are commonly used for welding aluminum greater than 1(25 mm) thick.

Reactive Shielding GasesOxygen, hydrogen, nitrogen, and carbon dioxide (CO2) arereactive gases. Reactive gases combine chemically with theweld pool to produce a desirable effect.

Carbon Dioxide (CO2) is inert at room temperature. In thepresence of the arc plasma and the molten weld puddle it isreactive. In the high energy of the arc plasma the CO2 moleculebreaks apart in a process known as dissociation. In thisprocess, free carbon, carbon monoxide, and oxygen releasefrom the CO2 molecule. This occurs at the DC+ anode regionof the arc. At the DC- cathode region, which is invariably thework piece for GMAW, the released elements of the CO2molecule undergo the process of recombination. During recom-bination higher energy levels exist and are responsible for thedeep and broad penetration profile that characterizes the use ofcarbon dioxide.

Dissociation and RecombinationDuring the process of dissociation, the free elements of the CO2molecule (carbon, carbon monoxide, and oxygen) mix with themolten weld pool or recombine at the colder cathode region ofthe arc to form, once again, carbon dioxide. The free oxygencombines chemically with the silicon, manganese, and iron toform oxides of silicon, manganese and iron. Formed oxides,commonly referred to as silica islands, float to the surface of theweld pool, then solidify into islands on the surface of the finishedweld or collect at the toes of a weld. Higher levels of carbondioxide (higher oxidation potential) increases the amount of slagformed on the surface of the weld. Lower levels of carbondioxide (lower oxidation potential) increase the amount of alloy,

Shielding Gases for GMAW

13

GMAW


Argon Oxygen

Argon Argon Helium Helium

Argon CO2

CO2

CO2

silicon and manganese retained in the weld. As a result, lowercarbon dioxide levels, in a binary or ternary shielding gas blend,increase the yield and ultimate tensile strength of a finished weld(see Shielding Gas section on page 12).

Oxygen (O2) is an oxidizer that reacts with components in themolten puddle to form oxides. In small additions (1-5%), with abalance of argon, it provides good arc stability and excellentweld bead appearance. The use of deoxidizers within thechemistry of filler alloys compensates for the oxidizing effect ofoxygen. Silicon and manganese combine with oxygen to formoxides. The oxides float to the surface of the weld bead to formsmall islands, and are more abundant under CO2 shielding thanwith blends of argon and oxygen gas.

Hydrogen (H2) in small percentages (1-5%), is added to argonfor shielding stainless steel and nickel alloys. Its higher thermalconductivity produces a fluid puddle, which promotes improvedtoe wetting and permits the use of faster travel speeds.

Binary Shielding Gas BlendsTwo-part shielding gas blends are the most common and theyare typically made up of either argon + helium, argon + CO2, orargon + oxygen.

Argon + HeliumArgon/helium binary blends are useful for welding nickel basedalloys and aluminum. The mode of metal transfer used is eitheraxial spray transfer or pulsed spray transfer. The addition ofhelium provides more puddle fluidity and flatter bead shape.Helium promotes higher travel speeds. For aluminum GMAW,helium reduces the finger-like projection found with pure argon.Helium is also linked to reducing the appearance of hydrogenpores in welds that are made using aluminum magnesium fillerswith 5XXX series base alloys. The argon component providesexcellent arc starting and promotes cleaning action onaluminum.

FIGURE 7: Bead contour and penetration patterns for various shielding gases

FIGURE 8: Relative effect of Oxygen versus CO2 additions to the argon shield

Common Argon + Helium Blends75% Argon + 25% Helium this binary blend is frequentlyapplied to improve the penetration profile for aluminum, copper,and nickel applications. The puddle is more fluid than with 100%argon.

75% Helium + 25% Argon the higher helium content increasesthe thermal conductivity and puddle fluidity. The penetrationprofile is broad, and it exhibits excellent sidewall penetration.

Argon + CO2The most commonly found binary gas blends are those used forcarbon steel GMAW welding. All four traditional modes ofGMAW metal transfer are used with argon/CO2 binary blends.They have also enjoyed success in pulsed GMAW applicationson stainless steel where the CO2 does not exceed 4%.

Axial spray transfer requires CO2 contents less than 18%.Argon/CO2 combinations are preferred where millscale is anunavoidable welding condition. As the CO2 percentage increases,so does the tendency to increase heat input and risk burn-through. Argon/CO2 blends up to 18% CO2 support pulsedspray transfer.

Short-circuiting transfer is a low heat input mode of metaltransfer that can use argon/CO2 combinations. Optimally, thesemodes benefit from CO2 levels greater than or equal to 20%.Use caution with higher levels of argon with short-circuit metaltransfer.

Argon Argon - Helium Helium CO2

Argon - Oxygen Argon - CO2 CO2


GMAW

Common Short-Circuiting Transfer Shielding Gas Blends75% Argon + 25% CO2 reduces spatter and improves weldbead appearance on carbon steel applications. 80% Argon + 20% CO2 another popular blend, which furtherreduces spatter and enhances weld bead appearance on carbonsteel applications.

Common Axial Spray Transfer shielding gas blends98% Argon + 2% CO2 for axial or pulsed spray with stainlesssteel electrodes and carbon steel electrodes. This blend hasseen repeated success on high-speed sheet metal applications.There is excellent puddle fluidity and fast travel speeds associatedwith this shielding gas blend.95% Argon + 5% CO2 for pulsed spray with carbon steelelectrodes. The addition of 5% CO2 provides for additionalpuddle fluidity, and it lends itself to heavier fabrication thanblends with 2% CO2.92% Argon + 8% CO2 for both axial and pulsed sprayapplications on carbon steel. Higher energy in axial spraytransfer increases puddle fluidity.90% Argon + 10% CO2 for either axial spray or GMAW-Papplications on carbon steel. The penetration is broader and itreduces the depth of the finger-like penetration exhibited byargon + oxygen mixes.85% Argon + 15% CO2 the higher CO2 level in axial orpulsed spray transfer increases sidewall fusion on sheet metal orplate thickness material. Generally produces improved toewetting on carbon steel with low levels of millscale. In GMAW-S,short circuiting transfer, the lower CO2 level translates to lessheat for welding parts with less risk of burnthrough. 82% Argon + 18% CO2 the effective limit for axial spray withCO2. Popular European blend used for a wide range of weldingthicknesses. Broad arc enhances penetration profile along theweld interface. Also lends itself well for use in short-circuitingtransfer or STT applications.

Argon + OxygenArgon/oxygen blends attain axial spray transfer at lower currentsthan argon/CO2 blends. The droplet sizes are smaller, and theweld pool is more fluid. The use of argon + oxygen hashistorically been associated with high travel speed welding onthin materials. Both stainless steel and carbon steel benefit fromthe use of argon/oxygen blends.99% Argon + 1% Oxygen used for stainless steel applications.The use of oxygen as an arc stabilizer enhances the fine droplettransfer and maintains the puddle fluidity for this gas blend.Stainless steel welds will appear gray because of the oxidizingeffect on the weld pool.98% Argon + 2% Oxygen used as a shielding gas for eithercarbon or stainless steel applications. The earliest use ofargon/oxygen blends for axial spray transfer on carbon steelemployed 2% oxygen level. It is typically applied to applicationsthat require high travel speed on sheet metal. Applied witheither axial spray or pulsed spray transfer modes. Stainlessdeposits are dull gray in appearance. This blend is often usedwhen superior mechanical properties are required from low alloycarbon steel electrodes.

95% Argon + 5% Oxygen general purpose axial spray orpulsed spray transfer shielding gas applied to heavier sections ofcarbon steel. The base material is usually required to be free ofcontaminants with a low level of millscale.

Ternary Gas Shielding BlendsThree-part shielding gas blends continue to be popular forcarbon steel, stainless steel, and, in restricted cases, nickelalloys. For short-circuiting transfer on carbon steel the additionof 40% helium, to argon and CO2, as a third component to theshielding gas blend, provides a broader penetration profile.Helium provides greater thermal conductivity for short-circuitingtransfer applications on carbon steel and stainless steel basematerials. The broader penetration profile and increasedsidewall fusion reduces the tendency for incomplete fusion.

For stainless steel applications, three-part mixes are quitecommon. Helium additions of 55% to 90% are added to argonand 2.5% CO2 for short-circuiting transfer. They are favored forreducing spatter, improving puddle fluidity, and for providing aflatter weld bead shape.

Common Ternary Gas Shielding Blends90% Helium + 7.5% Argon + 2.5% CO2 is the most popularof the short-circuiting blends for stainless steel applications. Thehigh thermal conductivity of helium provides a flat bead shapeand excellent fusion. This blend has also been adapted for usein pulsed spray transfer applications, but it is limited to stainlessor nickel base materials greater than .062" (1.6 mm) thick. It isassociated with high travel speeds on stainless steel applications.55% Helium + 42.5% Argon + 2.5% CO2 although lesspopular than the 90% helium mix discussed above, this blendfeatures a cooler arc for pulsed spray transfer. It also lends itselfvery well to the short-circuiting mode of metal transfer forstainless and nickel alloy applications. The lower heliumconcentration permits its use with axial spray transfer.38% Helium + 65% Argon + 7% CO2 this ternary blend is foruse with short-circuiting transfer on mild and low alloy steelapplications. It can also be used on pipe for open root welding.The high thermal conductivity broadens the penetration profileand reduces the tendency to cold lap. 90% Argon + 8% CO2 + 2% Oxygen this ternary mix isapplied to short-circuiting, pulsed spray, and axial spray modesof metal transfer on carbon steel applications. The high inertgas component reduces spatter.

Base Electrode Lincoln GMAW Mode ofMaterial Type Product Name Metal Transfer Shielding Gas Blends

ER70S-3GMAW-S or 100% CO2

ER70S-4SuperArc STT 75-90% Argon + 10-25% CO2

Carbon SteelER70S-6 SuperGlide

orAxial Spray 82-98% Argon + 2-18% CO2

E70C-6MMetalshield or 95-98% Argon + 2-5% Oxygen

GMAW-P 90% Argon + 7.5% CO2 + 2.5% Oxygen

ER80S-Ni1 GMAW-S or 100% CO2ER80S-D2 SuperArc STT 75-80% Argon + 20-25% CO2

Low Alloy ER100S-G andSteel ER110S-G Metalshield

95% Argon + 5% CO2E90C-G Axial Spray or95-98% Argon + 2-5% OxygenE110C-G GMAW-P

ER1100 Axial Spray 100% ArgonER4043, ER4047 SuperGlaze or 75% Helium + 25% Argon

Aluminum ER5183, ER5356 GMAW-P 75% Argon + 25% HeliumER5554, ER5556 (No GMAW-S) 100% Helium

GMAW-S 98-99% Argon + 1-2% Oxygenor 90% Helium + 7.5% Argon + 2.5% CO2

ER308LSiSTT 55% Helium + 42.5% Argon + 2.5 CO2

AusteniticER309LSi Blue MaxStainless SteelER316LSi Axial Spray

98-99% Argon + 1-2% Oxygen

or98% Argon + 2% CO2

GMAW-P97-99% Argon + 1-3% Hydrogen

55% Helium + 42.5% Argon + 2.5% CO2

90% Helium + 7.5% Argon + 2.5% CO2GMAW-S 89% Argon + 10.5% Helium + .5% CO2or 66.1% Argon + 33% Helium + .9% CO2ERNiCr-3 Blue Max STT 75% Argon + 25% HeliumERNiCrMo-4 75% Helium + 25% Argon

Nickel ERNiCrMo-3Alloys ERNiCrMo-10 100% Argon

ERNiCrMo-14 89% Argon + 10.5% helium + .5% CO2ERNiCrMo-17 Axial Spray 66.1% Argon + 33% Helium + .9% CO2

or 75% Helium + 25% ArgonGMAW-P 75% Argon + 25% Helium

97-99% Argon + 1-3% Hydrogen

66.1% Argon + 33% Helium + .9% CO2GMAW-S 90% Helium + 7.5% Argon + 2.5% CO2

or 98-99% Argon + 1-2% OxygenDuplex STT 98% Argon + 2% CO2

Stainless Steel 2209 Blue Max(Second Generation) 2304 75% Argon + 25% Helium

Axial Spray 75% Helium + 25% Argonor 100% Argon

GMAW-P 100% Helium66.1% Argon + 33% helium + .9% CO2

90/10 Copper ERCuNiAxial Spray

100% ArgonNickel Alloys Type 70/30

or75% Argon + 25% HeliumGMAW-P75% Helium + 25% Argon(No GMAW-S)

Copper Alloys ERCu Axial Spray 100% Argon(Deoxidized) or 75% Argon + 25% Helium

GMAW-P 75% Helium + 25% Argon

GMAW-S,Silicon Bronze STT,

and ERCuSi Axial Spray 100% ArgonBrasses or

GMAW-P

ERCuAl-A1Axial Spray

AluminumERCuAl-A2

or 100% ArgonBronze

ERCuAl-A3GMAW-P

Limited GMAW-S

15

GMAW


GMAW SHIELDING GAS SELECTION GUIDE


GMAW

Current Density

Keywords:

Current Density

Cross-Sectional Area

Saturated

Current density is defined as the current employed with aparticular electrode diameter divided by its current carryingcross-sectional area. If the wire feed speed is low, then the cur-rent density will be low, and vice versa. From this you can deter-mine that:

Lower current density applied to a given electrode is associatedwith the short-circuit mode of metal transfer.

Higher current density is associated with the higher energymodes of metal transfer: globular, axial spray transfer or themore advanced pulsed spray metal transfer.

The current for a given GMAW solid or metal-cored electrodewill reach a maximum density level. Once this level of currentdensity is attained, no additional current can be carried by theelectrode. In other words, the electrode has reached its maxi-mum current density. In particular, Figure 9 demonstrates thisphenomenon for 0.035 (0.9 mm) diameter solid wire. It can beseen that the current is relatively linear to approximately 200ampere, but as the current reaches just beyond 210 ampere,the rise in current becomes exponential. At approximately 280ampere [720 ipm (18.3 M/min.) wire feed speed], the electrodereaches its maximum current density. The electrode at thispoint becomes saturated with current and no more current canbe added to the electrode. Therefore, the maximum currentdensity for a given electrode diameter is synonymous with theconcept of current saturation. So it can be speculated that thisphenomenon occurs for all diameters and material types ofelectrodes used for GMAW.

It is important to note that once the electrode reaches itsmaximum current density, the saturation point, any added wirefeed speed will provide a higher deposition rate with no increasein current.

Electrode EfficienciesElectrode efficiency is a term that is applied to the percentage ofelectrode that actually ends up in the weld deposit. Spatterlevels, smoke, and slag formers affect the electrode efficiency inGMAW. The electrode efficiency is a numeric value that isassigned to the particular mode of metal transfer:

GMAW-S, short-circuit transfer, shielded with an argon + CO2gas blend, will typically operate with an electrode efficiencyequal to or greater than 93%. Shielded by 100% CO2, theelectrode efficiency will range from 90 to 93%. Typically, CO2increases spatter levels to some extent, and argon blends aretypically useful in reducing, but not completely eliminating,spatter.

STT, a dynamically controlled form of GMAW-S, will attainelectrode efficiencies of 98% .

Globular transfer is associated with higher spatter levels thatprofoundly impact electrode efficiency. The efficiency ofglobular transfer can vary from 85 to 88%, when shielded with100% CO2. Under argon blends the efficiency may vary from88 to 90%.

Axial spray has a higher electrode efficiency. This higher ener-gy mode of metal transfer is associated with electrode effi-ciencies of 98%.

The electrode efficiency for GMAW-P varies depending uponthe welding application and the sophistication of the powersource. Generally, the efficiency factor applied for GMAW-P is98%, like that for axial spray, but there may be the need for ahigher travel speed application that requires shorter arclengths. High speed pulsed spray transfer types of applica-tions generally introduce higher spatter levels. This necessarilyreduces the electrode efficiency to some lower value.

All of this is related to the amount of electrode that actually endsup in the weld. If 100 lbs. (45 kg) of 0.035 (0.9 mm) diameterelectrode is purchased for use on a particular project, and theproject calls for the use of GMAW-S, then the effective amountof electrode that will be expected to end up in the welds will be:

EE x (lbs. Electrode)= 0.93 x 100 lbs.

= 93 lbs.NOTE: The calculation assumes no loss of material due to wire clipping.

Deposition Rate

Keywords:

Deposition Rate

Melt-off Rate

The melt-off rate for a particular electrode does not include con-sideration for the efficiency of the mode of metal transfer or theprocess. Its interest is in how much electrode is being melted.

Deposition rate is applied to the amount of electrode, measuredin wire feed speed per unit of time, that is fed into the moltenpuddle. Importantly, its value reflects the use of the factor forelectrode efficiency.

800

700

600

500

400

300

200

100

0 0

5

20

15

10

0 50 100 150 200 250 300 350 400Welding current, A (DCEP)

Wire

fe

ed

speed,

inch

es pe

r m

inute

Wire

fe

ed

speed,

m

ete

rs pe

r m

inute

0.03

0 in.

(0.8 mm)

0.03

5 in.

(0.9 mm)

0.045

in. (1.2

mm)

0.052

in. (1.3

mm)

0.062 in.

(1.6 mm)

450

FIGURE 9: Typical Welding Currents vs. Wire Feed Speeds

Effects of Variables

17

GMAW


Electrode Extension and Contact Tip to Work DistanceKeywords:

Electrode Extension

Electrical Stickout (ESO)

Contact Tip to Work Distance (CTWD)

The electrode extended from the end of the contact tip to thearc is properly known as electrode extension. The popularnon-standard term is electrical stickout (ESO). In GMAW, this isthe amount of electrode that is visible to the welder. Theelectrode extension includes only the length of the electrode, notthe extension plus the length of the arc. The use of the termelectrode extension is more commonly applied for semiautomaticwelding than it is for robotic or mechanized welding operations.Contact tip to work distance (CTWD) is the standard term usedin the latter.

Contact tip to work distance (CTWD) is a term that lends itselfwell to the electrode extension for mechanized or roboticwelding applications. It is measured from the end of the contacttip to the work piece.

In a non-adaptive constant voltage (CV) system the electrodeextension or the CTWD acts as a resistor. Varying the length ofthe electrode affects the current applied to the arc:

Increasing electrode extension increases the resistance to theflow of current in the electrode, and the current in the arc isdecreased.

Decreasing the electrode extension decreases the resistanceto the flow of current in the electrode, and the current in thearc increases.

Because the current can vary with an increase or decrease inextension, the consistency of the extension is important to theconsistency of weld penetration. It is important to maintain avery steady hand during semiautomatic welding. It is equally asimportant to establish and maintain the correct CTWD formechanized or robotic welding.

For short-circuiting metal transfer or GMAW-S, semiautomaticwelding, the electrode extension should be held between3/8-1/2 (10 12 mm). For either axial spray or GMAW-P,pulsed spray metal transfer, the electrode extension should beheld between 3/4 1 (19 25 mm). Maintaining the correctelectrode extension is important to the uniformity of thepenetration profile along the length of a weld, and it is consideredto be an important variable for any GMAW procedure.

Electrode Extension

Arc Length

FIGURE 10: Electrical Stickout (ESO)

CTWDContact Tip toWork Distance

FIGURE 11: Contact Tip to Work Distance (CTWD)

Depending upon the mode of metal transfer, as indicated in theElectrode Efficiency section on page 16, the factor for theparticular mode of metal transfer employed is applied to themelt-off rate.

To determine the deposition rate for a given diameter of solidcarbon or low alloy steel wire electrode the following mathemati-cal formula will be useful:

13.1 (D2)(WFS)(EE)where: D = electrode diameter

WFS = wire feed speed (inches per minute)EE = electrode efficiency

13.1 = is a constant that is based upon the density of steel and its cross-sectional area.

If the melt off rate is all that is required, then use the sameformula and remove the factor for EE.

Aluminum is approximately 33% the density of carbon steel, andits constant will be 13.1 x .33, or 4.32. Stainless steel, typically,is only slightly greater in density than carbon steel, 0.284 lbs/in3

versus 0.283 lbs/in.3, and therefore the 13.1 constant is sufficient.

Electrode Extension

CTWD (Contact Tipto Work Distance)

Arc Length

Keywords:Waveform Control Technology

Output Modulation

Waveform Control

Adaptive Control

Synergic Control

Real-Time

Waveform Control TechnologyThe inverter power source in the early 1980s introduced a newera in the development of arc welding power sources. Theyaffected the development of the full range of welding processes,but in the specific areas of GMAW the results from intenseresearch and development are staggering.

The unique concept of Waveform Control Technology featuresan inverter transformer power supply and a central processingunit. The welding power output is produced by a high speedamplifier. The software developed to drive the output isenhanced to provide superior optimized welding output for avariety of GMAW modes of metal transfer. The most notable ofthese developments is the Surface Tension Transfer, (STT),Constant Power, and a variety of special pulsed spray transfermodes of metal transfer.

The newer power sources feature the ability of the power sourceto interact with the end-user and permit the worker to createtheir own GMAW-P welding software program. Wave Designer2000 software is a commercially available Windows softwareprogram that provides real-time output control of the powersource. RS232 connectivity to the power source establishes acommunication link with the computer. For pulsed spray transfer,short-circuiting transfer and STT, the output is modulated inresponse to changes made to the components of the waveform.

The use of waveform control software allows further optimizationfor a given mode of metal transfer. Templates for pulsed spraytransfer, short-circuiting transfer and STT are available foradjustment to meet critical weld requirements. The objective forthe development may be to improve toe wetting action, reducedilution levels or to improve high travel speed performance of apulsed waveform. In any case, the interaction between the arcperformance and the adaptable output are central to thesuccess of Waveform Control Technology.

Data acquisition tools that are an important part of the softwareallow the further ability to monitor the waveform during itsdevelopment. The information collected permits alteration andor final documentation of the suitability of the waveform for theapplication.

Synergic control is designed to support all GMAW modes ofmetal transfer. One knob control permits the welder to select thewire feed speed, and then the voltage/trim value automaticallyfollows. For all of the synergic modes of metal transfer the con-cept of synergy eases the use of higher technology on the shopfloor.

The adaptive arc is an arc that quickly adjusts to changes in theelectrode extension to maintain the same arc length. Theobjective for adaptive control is to improve arc performance andmaintain finished weld quality.

Components of GMAW-P WaveformKeywords:

Front Flank Ramp-up Rate

Percent Overshoot

Peak Current

Peak Time

Tail-out

Tail-out Speed

Step-off Current

Background Current

Pulse Frequency

Nine essential components are useful for manipulating the outputcharacter of the GMAW-P waveform. The interaction of thecomponents determines the specific outcome character of thewaveform. Important in this basic understanding is the effectthat shielding gas (see shielding gases for GMAW on page 12),electrode diameter, and electrode type have on the finishedweld.

The Wave Designer 2000 graphical user interface provides avisual image for the theoretical waveform. It is plotted on aCurrent vs. Time grid which reproduces changes made to thewaveform. The changes made to a given pulsed waveformeither add to, or subtract from, the area under the waveform. Asthe area under the waveform increases, there will be an increasein energy to the arc. The reverse is also true, when the areaunder the waveform decreases, the energy to the arc decreases.

Input Power

Inverter

WaveformGenerator

High SpeedAmplifier

Arc

CPU - CentralProcessingUnit

FIGURE 12: Wave Designer 2000 Pulse Editor


GMAW

Advanced Welding Processes for GMAW

Input Power

Arc

WaveformGenerator

High SpeedAmplifier

CPU - CentralProcessing Unit

Inverter

19

GMAW


GMAW-P Waveform Components

Front Flank Ramp-up Rate (1)The ramp-up rate determines how rapidly the current willincrease from the background current to the peak current. Theramp-up rate assists in the formation of the molten droplet atthe end of the electrode. The rate is measured in terms ofamps/millisecond. The rate of rise can reach 1000amps/millisecond. As the slope of the ramp-up rate increases,the stiffness of the arc also increases. A fast ramp-up rate isassociated with arc stiffness and louder arc noise. Decreasingthe rate of rise contributes to a softer sounding arc.

Overshoot (2)Overshoot describes the condition where the front flank increasesto a predetermined level beyond the level of the peak current. Itis expressed in units of percent. Increasing overshoot isassociated with a more rigid arc that is less prone to deflection.Overshoot adds to the pinch current and it increases the elec-tromagnetic pinch force applied to the molten droplet.

Peak Current (3)Peak current is the nominal current for the high energy pulse. Itis adjusted to a level that is set consistently above the globularto spray transition current. Peak current is expressed in units ofampere. During the time when the peak current is delivered, themolten droplet detaches from the electrode. An increase inpeak current increases the average welding current and theweld penetration.

FIGURE 13: Waveform Development Editor

Peak Current Time (4)Peak current time describes the length of time that the current isat its peak. It is associated with droplet size. Peak time isexpressed in terms of milliseconds. As the peak time increases,the droplets decrease in size. As the peak time decreases, thedroplet size increases. The traditional expectation is that a singlemolten droplet is transferred with each pulse peak. The effectivetime at peak can range from less than 1 millisecond to 3 or moremilliseconds. An increase in peak time increases average current,and it also increases weld penetration.

Tail-out (5)Tail-out is associated with current decay from the peak to thebackground current. It generally follows an exponential path tothe background current. The increase in tail-out time increasesthe average current and marginally increases penetration.Tail-out time is increased to provide an increase in droplet fluidity.This results in improved toe wetting, a softer arc sound, andincreased puddle fluidity.

Tail-out Speed (6)Tail-out speed defines the rate at which the waveform movesfrom the peak current to either the step-off current or thebackground current. Manipulation of this portion of the waveformincreases or decreases the exponential fall to the backgroundcurrent.

Step-off Current (7)Step-off current defines the current level at the portion of thewaveform where tail-out ends. It can add to, or take away from,the area under the waveform. It is associated with stabilizing thearc with stainless or nickel alloy filler metals.

Background Current (8)Background current refers to the lower nominal current of theoutput. The unit of measure for the background current isampere. Increases in background current will increase penetra-tion.

Pulse Frequency (9)Pulse frequency is responsible for how often the pulse cycleoccurs in one second. As the frequency increases, the arc nar-rows, the average current increases, and the molten dropletsbecome smaller. As the frequency decreases, the weld beadand the arc become wider. Frequency is generally proportionalto the wire feed speed.

1

2

4

3

5

89

6

7

CURRENT

TIME (mS)

(1) Front Flank Ramp-up Rate

(2) Overshoot

(3) Peak Current

(4) Peak Time

(5) Tail-Out

(6) Tail-Out Speed

(7) Step-off Current

(8) Background Current

(9) Period and Frequency

Time (mS)

Cur

rent

(4)

(2)

(3)

(5)(6)

(7)

(8)(9)

(1)


GMAW

Keywords:

Scale Factor

Adaptive Loop

Arc Length Regulation

Constant Current

In a constant current scenario, as the CTWD is increased, thearc length also increases. As the CTWD decreases, the arclength also decreases. To control the length of the arc despitechanges in CTWD, an adaptive control is necessary. Theadaptive control will add energy to the arc as the CTWDdecreases, and it will take energy out of the waveform as theCTWD is increased. This provides stability to the arc length, andincreases the overall usability of the waveform.

Frequency, background current, peak time, and peak currentare the typical components of the waveform used to regulate thearc length. Scale factor is the term attached to arc lengthregulation, and percentage is the term applied for its relativemagnitude. If the background current is set to a value of 100amps and the corresponding scale factor is expressed as 10%,then as the CTWD decreases, 10% more background currentwill be added to the present level for background current. If theCTWD increases, then up to 10% background current willdecrease from the original 100 amps. This is how the arc lengthregulation operates, and it is coordinated to include the valuesfor the other scale factor components detailed above. Theregulation of the arc length occurs automatically, and it isfunctional within limits of the CTWD. The effective CTWD rangefor the adaptive loop is 0.50 1.25 (12 30 mm).

The adjustment of trim relates directly to the scale factorsemployed in the adaptive loop. As the trim decreases from anominal value of 1.00, then the scale factors apply themselvestogether to decrease the arc length. As the trim is increased to avalue greater than 1.00, then the scale factors work together toincrease the arc length. Additionally, the "arc control" feature inthe GMAW-P mode is directly tied to the adaptive loop. As thearc control is moved to +1 through +10, then frequencyincreases while background current decreases. The result is thatthe arc column narrows. If the "arc control" feature is moved to1 through 10, then the result is a wider arc column and awider finished weld.

The absence of the use of scale factors assumes that the arc isstable for a given wire feed speed or for a wide range of wirefeed speeds. Arc stability means that the arc will not vary inlength with a consistent CTWD. In this scenario, the weldingprogram is non-adaptive, and only by adjusting the length of theCTWD, will there be a variance in arc length. When using a truenon-adaptive program, trim and arc control will produce nochanges in arc performance or level of arc energy.

Advanced Waveform Control Technology Surface Tension Transfer (STT)Keywords:

Peak Current

Background Current

Tail-Out Current

Reactive Power Source

Sensing Lead

The Surface Tension Transfer (STT) welding mode of metaltransfer is a low heat input welding mode. It specializes in itsability to provide smooth even rippled weld beads, free of weldspatter, and with consistently good fusion. It is ideal for sheetmetal applications requiring excellent weld bead appearanceand it is successfully applied for root pass welding of open rootpipe joints.

The STT welding mode is reactive. The power source monitorsthe arc and responds instantaneously to the changes in the arcdynamics. A sensing lead attaches to the work piece to providefeedback information to the power source. Uniquely, the STTpower source provides current to the electrode independent ofthe wire feed speed. This feature permits the ability to add orreduce current to meet application requirements.

The power source that supports STT is neither constant currentnor constant voltage. It provides controls for the essentialcomponents of the STT waveform. Among these are controlsfor peak current, background current, and tail-out current.See Figure 14 on page 21.

The Adaptive Loop

21

GMAW


BackgroundCurrent

PinchStart

Tail-out

CURRENT

TIME (mS)

PeakTime Peak

CurrentSh

ort

Exit

Pre

dict

ion

1

2

3

4

FIGURE 14: Typical Waveform for STT

A B C D E F G

A. The molten tip of the electrode makes physical contact with the molten poolat the background current level.

B. The background current is reduced to a lower level to prevent the occurrenceof a premature molten droplet detachment.

C. The current then ramps up quickly to a point where the pinch force associatedwith the rise in current (electromagnetic force) starts to neck down themolten column of the electrode. The power source at this point begins tomonitor the changes in voltage over time as it relates to the necking of themolten droplet. The molten metal is still in contact with the molten weldpool. Via the sensing lead, the power source references the observedvoltage, and continuously compares the new voltage value to the previousvoltage value.

D. At the point where the molten metal is about to disconnect from the end ofthe electrode, the power source reduces the current to a lower than back-ground current level. At this point in the waveform, surface tension forcescollapse and the molten droplet transfers to the weld pool. This controlleddetachment of the molten droplet is free of spatter.

E. The power source then rises to the peak current level where a new dropletbegins to form. Anode jet forces depress the molten weld puddle to preventit from reattaching to the electrode. On its descent to the backgroundcurrent, the tail-out current provides the molten droplet with additionalenergy. The added energy increases puddle fluidity, and the result isimproved wetting at the toes of the weld.

F. A plasma boost is applied which provides the energy to re-establish the arclength, provide a new molten droplet, and force the molten puddle awayfrom the molten droplet. The length of time is nominally 1 mS for carbonsteel electrodes and 2 mS for both stainless and nickel alloyed filler metals.

G. The tail-out region is employed in applications where the energy added tothe molten droplet provides faster travel speeds and improved finished weldwetting action at the toes. In most pipe root applications, this value is keptto a minimum.

Time (mS)

PeakCurrent

BackgroundCurrent

Tailout

Sho

rt E

xit

Pre

dic

tion

PeakTime

Pinch Start

Current

A B C D E F G

(1)

(2)

(3)

(4)

STT Arc Controls

The peak current control is responsible for establishing the arclength, and it provides sufficient energy to preheat the workpiece to insure good fusion. If it is set too high, the moltendroplets will become too large. The molten droplet formedshould be equal to 1-1/2 of the electrode diameter.

Background current is the essential component responsible forproviding weld penetration into the base material, and it is largelyresponsible for the overall heat input into the weld. Manipulationof this component controls the level of weld penetration, and iteffects the size of the molten droplet.

Tail-out current is responsible for adding energy to the moltendroplet to provide increased droplet fluidity. It applies addedenergy without effecting droplet size. Increasing the tail-outcurrent permits faster travel speeds and improves weld toewetting action. The use of tail-out has proven to be a greatvalue in increasing puddle fluidity, and this translates into higherarc travel speeds.

Tailout Control

Hot Start Control

Peak Current

Peak Time 1mS or 2mS Switch

.035/.045 Electrode Switch

Power Switch

Background Current Control

TailoutControl

Hot StartControl

.035/.045 Electrode Switch

Peak Time 1mS or2mS Switch

BackgroundCurrent Control

PowerSwitch

PeakCurrent

Surface Tension Transfer (STT)


GMAW

Lower hydrogen weld deposit. Lower spatter levels when compared to other processes and

modes of metal transfer. Capable of high deposition welding for heavy plate fabrication. May be used for out-of-position welding.

Modes of Metal Transfer for Tandem GMAWThe modes of metal transfer used for the tandem GMAWvariant are axial spray metal transfer or pulsed spray metaltransfer. The combinations of the modes that are popularlyemployed include:

Spray + Pulse Axial spray transfer on the lead arc followedby pulsed spray transfer on the trail arc.

Pulse + Pulse Pulsed spray transfer on both the lead andthe trail arc.

Spray + Spray Axial spray transfer on both the lead andthe trail arc.

The software programs designed to support the Pulse + Pulseconfiguration require that the wire feed speed setting for eachthe lead and the trail are the same. Trim values can be adjustedto account for arc length requirements.

The higher energy spray + spray configuration is used forspecial heavy plate welding where deeper penetration isrequired. Pulse + pulse allows for heavy welding or high speedsheet metal welding.

FIGURE 15: Tandem GMAW System

Tandem GMAW

PF 10RWire Drive

PLCMotion Control

Dual Torch(or)

Integrated Torch

Power Wave455R/655R

PLCController

Filed Bus Interface

Keywords:

High Deposition

Higher Travel Speed

The Tandem GMAW system was developed to take advantageof the potential for higher travel speeds and higher depositionrates when using two electrodes in the same molten puddle.The system employs two power sources, two wire drives, and asystem control. It is adapted for either repetitive side-beam typeapplications or it is employed with a welding robot. This variantof the gas metal arc welding process is capable of higher travelspeeds, 1-1/2 to 2 times the speed of a single electrode. Sometravel speeds may exceed 150/min. (3.81 m/min). Depositionrates to 42 lbs/hr (19.1 kg/hr) are achievable for heavier platewelding.

The arc components are broken into two parts: the lead arc andthe trail arc. Generally, two electrodes of the same diameter arefed into the same puddle. Typical applications include the use of0.035 1/16 (0.9 1.6 mm) diameter electrodes. The arcsare employed in a single barrel torch, and each electrode is fedthrough its own conductor tube. Similarly, there are two contacttips and two diffusers, see page 23.

Features of Tandem GMAW Capable of higher travel speeds on sheet metal than

conventional single electrode GMAW.

23

GMAW


Tandem Torch Alignment and Contact Tip to WorkDistanceSheet Metal ApplicationsCen

manual pentru mig, lincoln

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