Welding Processes Complete

12/22/13 Lesson 2 - Common Electric Arc Welding Processes

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BASICWELDING FILLER METAL

TECHNOLOGY

A Correspondence Course

LESSON IICOMMON ELECTRIC ARC

WELDING PROCESSES

ESAB ESAB Welding &

Cutting Products

©COPYRIGHT 2000 THE ESAB GROUP, INC.

Lesson 1The Basics of Arc

Welding

Current

Chapter Table of

Contents

Lesson 2Common Electric

Arc Welding

Processes

Lesson 3

Covered Electrodes

for WeldingMild Steels

Lesson 4

Covered Electrodes

for Welding Low AlloySteels

Go To Test

Lesson 5

Welding Filler Metalsfor Stainless Steels

Print

Lesson 6

Carbon & Low AlloySteel Filler Metals -GMAW,GTAW,SAW

Glossary

Lesson 7

Flux Cored ArcElectrodes CarbonLow Alloy Steels

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Electrodes

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Lesson 9

Estimating &Comparing Weld

Metal Costs

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Lesson 10

Reliability of Welding

Filler Metals

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© COPYRIGHT 2000 THE ESAB GROUP, INC.

TABLE OF CONTENTSLESSON II

COMMON ELECTRIC ARC WELDINGPROCESSES

2.1 INTRODUCTION .............................................................................. 1

2.2 SHIELDED METAL ARC WELDING ............................................... 1

2.2.1 Equipment & Operation ..................................................................... 2

2.2.2 Welding Pow er Sources .................................................................... 2

2.2.3 Electrode Holder................................................................................ 4

2.2.4 Ground Clamp ................................................................................... 4

2.2.5 Welding Cables ................................................................................. 4

2.2.6 Coated Electrodes ............................................................................ 4

2.3 GAS-TUNGSTEN ARC WELDING .................................................. 5

2.3.1 Equipment & Operation ..................................................................... 6

2.3.2 Pow er Sources .................................................................................. 7

2.3.3 Torches.............................................................................................. 10

2.3.4 Shielding Gases ................................................................................ 11

2.3.5 Electrodes ......................................................................................... 12

2.3.6 Summary ........................................................................................... 13

2.4 GAS METAL ARC WELDING .......................................................... 13

2.4.1 Current Density .................................................................................. 14

2.4.2 Metal Transfer Modes ........................................................................ 15

2.4.3 Equipment and Operation .................................................................. 17

2.4.4 Pow er Source.................................................................................... 18

2.4.5 Wire Feeder ...................................................................................... 19

2.4.6 Welding Gun ...................................................................................... 20

2.4.7 Shielding Gases ................................................................................ 21

2.4.7.1 Short Circuiting Transfer .................................................... 22

2.4.7.2 Spray Arc Transfer ............................................................ 23

Section Nr. Section Title Page


Welding

Current

Chapter Table of

Contents


Arc Welding

Processes

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Covered Electrodes


Lesson 4

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Lesson 5


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2.4.7.3 Pulse Spray Transfer ......................................................... 23

2.4.8 Electrodes ......................................................................................... 23

2.5 FLUX CORED ARC WELDING ....................................................... 24

2.5.1 Self-Shielded Process ....................................................................... 24

2.5.2 Gas Shielded Process....................................................................... 25

2.5.3 Current Density .................................................................................. 26

2.5.4 Equipment ......................................................................................... 26

2.5.5 Pow er Source.................................................................................... 26

2.5.6 Wire Feeder ...................................................................................... 26

2.5.7 Welding Guns .................................................................................... 26

2.5.8 Shielding Gases ................................................................................ 27

2.6 SUBMERGED ARC WELDING ....................................................... 27

2.6.1 Submerged Arc Flux .......................................................................... 28

2.6.2 The Welding Gun ............................................................................... 28

2.6.3 Pow er Sources .................................................................................. 28

2.6.4 Equipment ......................................................................................... 28

2.6.5 Electrodes ......................................................................................... 29

2.6.6 Summary ........................................................................................... 29

2.7 ELECTROSLAG AND ELECTROGAS WELDING .......................... 30

2.7.1 Electroslag Welding........................................................................... 30

2.7.2 Flux ................................................................................................... 30

2.7.3 Process ............................................................................................. 30

2.7.4 Equipment......................................................................................... 31

2.7.5 Summary .......................................................................................... 31

Appendix A - GLOSSARY OF TERMS ................................................................. 32

TABLE OF CONTENTSLESSON II - Con't.

Section Nr. Section Title Page


Welding

Current

Chapter Table of

Contents


Arc Welding

Processes

Lesson 3

Covered Electrodes


Lesson 4

Covered Electrodes


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Lesson 6


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Lesson 7


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LESSON II

COMMON ELECTRIC ARC WELDING PROCESSES

2.1 INTRODUCTION

After much experimentation by others in the early 1800's, an Englishman named Wilde

obtained the f irst electric w elding patent in 1865. He successfully joined tw o small pieces of

iron by passing an electric current through both pieces producing a fusion w eld. Approximately

tw enty years later, Bernado, a Russian, w as granted a patent for an electric arc w elding

process in w hich he maintained an arc betw een a carbon electrode and the pieces to be

joined, fusing the metals together as the arc w as manually passed over the joint to be w elded.

2.1.0.1 During the 1890's, arc w elding w as accomplished w ith bare metal electrodes that

w ere consumed in the molten puddle and became part of the w eld metal. The w elds w ere of

poor quality due to the nitrogen and oxygen in the atmosphere forming harmful oxides and

nitrides in the w eld metal. Early in the Tw entieth Century, the importance of shielding the arc

from the atmosphere w as realized. Covering the electrode w ith a material that decomposed in

the heat of the arc to form a gaseous shield appeared to be the best method to accomplish

this end. As a result, various methods of covering electrodes, such as w rapping and dipping,

w ere tried. These efforts culminated in the extruded coated electrode in the mid-1920's,

greatly improving the quality of the w eld metal and providing w hat many consider the most

signif icant advance in electric arc w elding.

2.1.0.2 Since w elding w ith coated electrodes is a rather slow procedure, more rapid

w elding processes w ere developed. This lesson w ill cover the more commonly used electric

arc w elding processes in use today.

2.2 SHIELDED METAL ARC WELDING

Shielded Metal Arc Welding*, also know n as manual metal arc w elding, stick w elding, or

electric arc w elding, is the most w idely used of the various arc w elding processes. Welding is

performed w ith the heat of an electric arc that is maintained betw een the end of a coated metal

electrode and the w ork piece (See Figure 1). The heat produced by the arc melts the base

metal, the electrode core rod, and the coating. As the molten metal droplets are transferred

across the arc and into the molten w eld puddle, they are shielded from the atmosphere by the

gases produced from the decomposition of the f lux coating. The molten slag f loats to the top

of the w eld puddle w here it protects the w eld metal from the atmosphere during solidif ication.


Welding

Current

Chapter Table of

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Arc Welding

Processes

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Covered Electrodes


Lesson 4

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Lesson 5


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LESSON II

Other functions of the coating are to provide

arc stability and control bead shape. More

information on coating functions w ill be

covered in subsequent lessons.

* Shielded Metal Arc Welding (SMAW) is the

terminology approved by the American

Welding Society.

2.2.1 Equipment & Operation - One

reason for the w ide acceptance of the SMAW

process is the simplicity of the necessary equipment.

The equipment consists of the follow ing items. (See

Figure 2)

1. Welding pow er source

2. Electrode holder

3. Ground clamp

4. Welding cables and connectors

5. Accessory equipment (chipping

hammer, w ire brush)

6. Protective equipment (helmet, gloves, etc.)

2.2.2 Welding Power Sources - Shielded metal arc w elding may utilize either

alternating current (AC) or direct current (DC), but in either case, the pow er source selected

must be of the constant current type. This type of pow er source w ill deliver a relatively constant

amperage or w elding current regardless of arc length variations by the operator (See Lesson I,

Section 1.9). The amperage determines the amount of heat at the arc and since it w ill remain

relatively constant, the w eld beads produced w ill be uniform in size and shape.

2.2.2.1 Whether to use an AC, DC, or AC/DC pow er source depends on the type of w elding

to be done and the electrodes used. The follow ing factors should be considered:

1. Electrode Selection - Using a DC pow er source allow s the use of a greater range

of electrode types. While most of the electrodes are designed to be used on AC or

DC, some w ill w ork properly only on DC.

2. Metal Thickness - DC pow er sources may be used for w elding both heavy

sections and light gauge w ork. Sheet metal is more easily w elded w ith DC

because it is easier to strike and maintain the DC arc at low currents.

FIGURE 1

CORE ROD

SHIELDINGGASES

SOLIDIFIEDSLAG

WELD METAL

WORK PIECE

MOLTENPOOL

SHIELDED METAL ARC WELDING

AC OR DCPOWERSOURCE

ELECTRODE

CABLE

ELECTRODEHOLDER

ELECTRODE

GROUND

CABLEWORK

SHIELDED METAL ARC WELDING CIRCUIT

FIGURE 2


Welding

Current

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COATING


















LESSON II

3. Distance from Work - If the distance from the w ork to the pow er source is great,

AC is the best choice since the voltage drop through the cables is low er than w ith

DC. Even though w elding cables are made of copper or aluminum (both good

conductors), the resistance in the cables becomes greater as the cable length

increases. In other w ords, a voltage reading taken betw een the electrode and the

w ork w ill be somew hat low er than a reading taken at the output terminals of the

pow er source. This is know n as voltage drop.

4. Welding Position (See Appendix A - Glossary of Terms) - Because DC may be

operated at low er w elding currents, it is more suitable for overhead and vertical

w elding than AC. AC can successfully be used for out-of-position w ork if proper

electrodes are selected.

5. Arc Blow - When w elding w ith DC, magnetic f ields are set up throughout the

w eldment. In w eldments that have varying thickness and protrusions, this magnetic

f ield can affect the arc by making it stray or f luctuate in direction. This condition is

especially troublesome w hen w elding in corners. AC seldom causes this problem

because of the rapidly reversing magnetic f ield produced.

2.2.2.2 Combination pow er sources that produce both AC and DC are available and

provide the versatility necessary to select the proper w elding current for the application.

2.2.2.3 When using a DC pow er source, the question of w hether to use electrode negative

or positive polarity arises. Some electrodes operate on both DC straight and reverse polarity,

and others on DC negative or DC positive polarity only. Direct current f low s in one direction in

an electrical circuit and the direction of current f low and the composition of the electrode

coating w ill have a definite effect on the w elding arc and w eld bead. Figure 3 show s the

connections and effects of straight and reverse polarity.

2.2.2.4 Electrode negative (-) produces w elds w ith shallow penetration; how ever, the

electrode melt-off rate is high. The w eld bead is rather w ide and shallow as show n at "A" in

Figure 3. Electrode

positive (+)

produces w elds w ith

deep penetration

and a narrow er w eld

bead as show n at

"B" in Figure 3.

FIGURE 3

DCPOWER SOURCE

ELECTRODE

DCPOWER SOURCE

ELECTRODE

A

HIGHER BURN-OFF RATE,LESS PENETRATION

DEEP PENETRATION,LOW BURN-OFF RATE

WORK PIECE

B

STRAIGHT POLARITY REVERSE POLARITY

WORK PIECE

Current

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LESSON II

2.2.2.5 While polarity affects the penetration and burn-off rate, the electrode coating also

has a strong influence on arc characteristics. Performance of individual electrodes w ill be

discussed in succeeding lessons.

2.2.3 Electrode Holder - The electrode holder connects to the w elding cable and con-

ducts the w elding current to the electrode. The insulated handle is used to guide the electrode

over the w eld joint and feed the electrode over the w eld joint and feed the electrode into the

w eld puddle as it is consumed. Electrode holders are available in different sizes and are rated

on their current carrying capacity.

2.2.4 Ground Clamp - The ground clamp is used to connect the ground cable to the w ork

piece. It may be connected directly to the w ork or to the table or f ixture upon w hich the w ork is

positioned. Being a part of the w elding circuit, the ground clamp must be capable of carrying

the w elding current w ithout overheating due to electrical resistance.

2.2.5 Welding Cables - The electrode cable and the ground cable are important parts of

the w elding circuit. They must be very f lexible and have a tough heat-resistant insulation.

Connections at the electrode holder, the ground clamp, and at the pow er source lugs must be

soldered or w ell crimped to assure low electrical resistance. The cross-sectional area of the

cable must be suff icient size to carry the w elding current w ith a minimum of voltage drop.

Increasing the cable length necessitates increasing the cable diameter to lessen resistance

and voltage drop. The table in Figure 4 lists the suggested American Wire Gauge (AWG)

cable size to be used for various w elding currents and cable lengths.

Total Cable Length (Ground Lead Plus Electrode Lead)

Up to 50 f t. Up to 100 f t. Up to 250 f t. Up to 500 f t.

Cable Voltage Cable Voltage Cable Voltage Cable Voltage

Size Drop Size Drop Size Drop Size Drop

20 to 180 #3 1.8 #2 2.9 #1 5.7 #0 9.1 180 Amps30 to 250 #2 1.8 #1 2.5 #0 5.0 #0 9.9 200 Amps

60 to 375 #0 1.7 #0 3.0 #00 5.9 #000 9.3 300 Amps80 to 500 #00 1.8 #000 2.5 #0000 5.0 #0000 9.9 400 Amps

100 to 600 #00 2.0 #0000 2.5 ... ... ... 500 Amps

Voltage drops indicated do not include any drop caused by poor connection, electrode holder, or work metal

WeldingServ ice

Range(Amperes)

Voltage

DropFigured

At

FIGURE 4

2.2.6 Coated Electrodes - Various types of coated electrodes are used in shielded

metal arc w elding. Electrodes used for w elding mild or carbon steels are quite different than

those used for w elding the low alloys and stainless steels. Details on the specif ic types w ill be

covered in subsequent lessons.

Current

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LESSON II

2.3 GAS TUNGSTEN ARC WELDING

Gas Tungsten Arc Welding* is a w elding process performed using the heat of an arc

established betw een a nonconsumable tungsten electrode and the w ork piece. See Figure 5.

The electrode, the arc, and the area surrounding the molten w eld puddle are protected from

the atmosphere by an inert gas shield. The electrode is not consumed in the w eld puddle as in

shielded metal arc w elding. If a f iller metal is

necessary, it is added to the leading

the molten puddle as show n in

2.3.0.1 Gas tungsten arc w elding

produces exceptionally clean w elds

no slag is produced, the chance

inclusions in the w eld metal is

and the f inished w eld requires

virtually no cleaning. Argon

and Helium, the primary

shielding gases employed,

are inert gases. Inert gases

do not chemically combine

w ith other elements and

therefore, are used to exclude

the reactive gases, such as oxygen and

nitrogen, from forming compounds that could

be detrimental to the w eld metal.

2.3.0.2 Gas tungsten arc w elding may be used for w elding almost all metals — mild steel,

low alloys, stainless steel, copper and copper alloys, aluminum and aluminum alloys, nickel

and nickel alloys, magnesium and magnesium alloys, titanium, and others. This process is

most extensively used for w elding aluminum and stainless steel alloys w here w eld integrity is of

the utmost importance. Another use is for the root pass (initial pass) in pipe w elding, w hich

requires a w eld of the highest quality. Full penetration w ithout an excessively high inside bead

is important in the root pass, and due to the ease of current control of this process, it lends

itself to control of back-bead size. For high quality w elds, it is usually necessary to provide an

inert shielding gas inside the pipe to prevent oxidation of the inside w eld bead.

* Gas Tungsten Arc Welding (GTAW) is the current terminology approv ed by the American Welding Society ,

f ormerly known as "TIG" (Tungsten Inert Gas) welding.

FIGURE 5

TRAVELDIRECTION

TORCH

SHIELDING GASNOZZLE

INERT GAS

SHIELD

WORK PIECE

TUNGSTEN

ELECTRODE

ARC

FILLER

METAL

GAS TUNGSTEN ARC WELDING

Current

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LESSON II

2.3.0.3 Gas tungsten arc w elding lends itself to both manual and automatic operation. In

manual operation, the w elder holds the torch in one hand and directs the arc into the w eld joint.

The f iller metal is fed manually into the leading edge of the puddle. In automatic applications,

the torch may be automatically moved over a stationary w ork piece or the torch may be

stationary w ith the w ork moved or rotated in relation to the torch. Filler metal, if required, is

also fed automatically.

2.3.1 EQUIPMENT AND OPERATION - Gas tungsten arc w elding may be accomplished

w ith relatively simple equipment, or it may require some highly sophisticated components.

Choice of equipment depends upon the type of metal being joined, the position of the w eld

being made, and the quality of the w eld metal necessary for the application. The basic equip-

ment consists of the follow ing:

1. The pow er source

2. Electrode holder (torch)

3. Shielding gas

4. Tungsten electrode

5. Water supply w hen necessary

6. Ground cable

7. Protective equipment

Figure 6 show s a basic gas tungsten arc w elding schematic.

FIGURE 6

REGULATORFLOW METER

GAS HOSE (WATER COOLED ONLY)

TORCH

* COMPOSITE CABLE

WATER COOLER

GAS COOLED ONLY

WELDING CABLE

SHIELDINGGAS SUPPLY

POWERSOURCE WATER

FROMTORCH

WATERTO

TORCH

GROUND CABLE

WORK

* COMPOSITE CABLE

GAS COOLED TORCH.CURRENT IN & GAS IN.

WATER COOLED TORCH.CURRENT IN & WATER OUT

GAS TUNGSTEN ARC WELDING CONNECTION SCHEMATIC

Current

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2.3.2 Power Sources - Both AC and DC pow er sources are used in gas tungsten arc

w elding. They are the constant current type w ith a drooping volt-ampere curve. This type of

pow er source produces very slight changes in the arc current w hen the arc length (voltage) is

varied. Refer to Lesson I, Section 1.9.

2.3.2.1 The choice betw een an AC or DC w elder depends on the type and thickness of the

metal to be w elded. Distinct differences exist betw een AC and DC arc characteristics, and if

DC is chosen, the polarity also becomes an important factor. The effects of polarity in GTAW

are directly opposite the effects of polarity in SMAW as described in paragraphs 2.2.2.3

through 2.2.2.5. In SMAW, the distribution of heat betw een the electrode and w ork, w hich

determines the penetration and w eld bead w idth, is controlled mainly by the ingredients in the

flux coating on the electrode. In GTAW w here no f lux coating exists, heat distribution betw een

the electrode and the w ork is controlled solely by the polarity. The choice of the proper w elding

current w ill be better understood by analyzing each type separately. The chart in Figure 7 lists

current recommendations.

FIGURE 7

Material &Thickness DCEN DCEP

ACHigh Freq. Argon Helium Ar/He

AluminumUnder 1/8"Ov er 1/8"

22 & 3

11

11

23 2

MagnesiumUnder 1/16"Ov er 1/16"

2 11

11

2

Carbon SteelUnder 1/8"Ov er 1/8"

11

11 2 3

Stainless SteelUnder 1/8"Ov er 1/8"

11

11

22

CopperUnder 1/8"Ov er 1/8"

11 1

1

Nickel Alloy sUnder 1/8"Ov er 1/8"

11

1 32

21

TitaniumUnder 1/8"Ov er 1/8"

1 12

21

WELDING CURRENT SHIELDING GAS

1. Pref erred Choice - Manual Welding2. Pref erred Choice - Automatic Welding3. Second Choice - Automatic Welding

CURRENT/SHIELDING GAS SELECTION, TUNGSTEN GAS ARC WELDING


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2.3.2.2 Direct current electrode negative (DCEN) is produced w hen the electrode is

connected to the negative terminal of the pow er source. Since the electrons f low from the

electrode to the plate, approximately 70% of the heat of the arc is concentrated at the w ork,

and approximately 30% at the electrode end. This allow s the use of smaller tungsten elec-

trodes that produce a relatively narrow concentrated arc. The w eld shape has deep penetra-

tion and is quite narrow . See Figure 8. Direct current electrode negative is suitable for w eld-

ing most metals. Magnesium and aluminum have a refractory oxide coating on the surface that

must be physically removed immediately prior to w elding if DCSP is to be used.

2.3.2.3 Direct current electrode positive (DCEP) is produced w hen the electrode is

connected to the positive terminal of the w elding pow er source. In this condition, the electrons

flow from the w ork to the electrode tip, concentrating approximately 70% of the heat of the arc

at the electrode and 30% at the w ork. This higher heat at the electrode necessitates using

larger diameter tungsten to prevent it from melting and contaminating the w eld metal. Since

the electrode diameter is larger and the heat is less concentrated at the w ork, the resultant

w eld bead is relatively w ide and shallow . See Figure 8.

2.3.2.4 Aluminum and magnesium are tw o metals that have a heavy oxide coating that acts

as an insulator and must be removed before successful w elding can take place. Welding w ith

electrode positive provides a good oxide cleaning action in the arc. If w e w ere to study the

physics of the w elding arc, w e f ind that the electric current causes the shielding gas atoms to

lose some of their electrons. Since electrons are negatively charged, these gas atoms now

are unbalanced and have an excessive positive charge. As w e learned in Lesson I, unlike

charges attract. These positively charged atoms (or positive ions as they are know n in

FIGURE 8

Electrode Oxide HeatPolarity Penetration Cleaning Concentration

Direct Current

Alternating Current

Medium Penetration

Medium WidthBead

Good

Cleans Oxideon Each Half

Cy cleAlternates BetweenElectrode and Work

Straight PolarityElectrode Negativ e

DeepPenetration

NarrowBead

Direct Current

Rev erse PolarityElectrode Positiv e

Shallow Penetration

Wide BeadMaximum

None AtWork

AtElectrode

GAS IONS

+

_

ELECTRONFLOW

_

_

+

+

EFFECTS OF CURRENT TYPE - GAS TUNGSTEN ARC WELDING


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LESSON II

chemical terminology) are attracted to the negative pole, in this case the w ork, at high velocity.

Upon striking the w ork surface, they dislodge the oxide coating permitting good electrical

conductivity for the maintenance of the arc, and eliminate the impurities in the w eld metal that

could be caused by these oxides.

2.3.2.5 Direct current electrode positive is rarely used in gas-tungsten arc w elding. Despite

the excellent oxide cleaning action, the low er heat input in the w eld area makes it a slow

process, and in metals having higher thermal conductivity, the heat is rapidly conducted aw ay

from the w eld zone. When used, DCEP is restricted to w elding thin sections (under 1/8") of

magnesium and aluminum.

2.3.2.6 Alternating current is actually a combination of DCEN and DCEP and is w idely

used for w elding aluminum. In a sense, the advantages of both DC processes are combined,

and the w eld bead produced is a compromise of the tw o. Remember that w hen w elding w ith

60 Hz current, the electron f low from the electrode tip to the w ork reverses direction 120 times

every second. Thereby, the intense heat alternates from electrode to w ork piece, allow ing the

use of an intermediate size electrode. The w eld bead is a compromise having medium

penetration and bead w idth. The gas ions blast the oxides from the surface of aluminum and

magnesium during the positive half cycle. Figure 8 illustrates the effects of the different types

of current used in gas-tungsten arc w elding.

2.3.2.7 DC constant current power sources - Constant current pow er sources, used for

shielded metal arc w elding, may also be used for gas-tungsten arc w elding. In applications

w here w eld integrity is not of utmost importance, these pow er sources w ill suff ice. With

machines of this type, the arc must be initiated by touching the tungsten electrode to the w ork

and quickly w ithdraw ing it to maintain the proper arc length. This starting method

contaminates the electrode and blunts the point w hich has been grounded on the electrode

end. These conditions can cause w eld metal inclusions and poor arc direction. Using a

pow er source designed for gas tungsten arc w elding w ith a high frequency stabilizer w ill

eliminate this problem. The electrode need not be touched to the w ork for arc initiation.

Instead, the high frequency voltage, at very low current, is superimposed onto the w elding

current. When the electrode is brought to w ithin approximately 1/8 inch of the base metal, the

high frequency ionizes the gas path, making it conductive and a w elding arc is established.

The high frequency is automatically turned off immediately after arc initiation w hen using direct

current.

2.3.2.8 AC Constant Current Power Source - Designed for gas tungsten arc w elding,

alw ays incorporates high frequency, and it is turned on throughout the w eld cycle to maintain a

stable arc. When w elding w ith AC, the current passes through 0 tw ice in every cycle and the


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arc must be reestablished each time it does so. The oxide coating on metals, such as

aluminum and magnesium, can act much like a rectif ier as discussed in Lesson I. The positive

half-cycle w ill be eliminated if the arc does not reignite, causing an unstable condition.

Continuous high frequency maintains an ionized path for the w elding arc, and assures arc re-

ignition each time the current changes direction. AC is extensively used for w elding aluminum

and magnesium.

2.3.2.9 AC/DC Constant Current Power Sources - Designed for gas tungsten arc

w elding, are available, and can be used for w elding practically all metals. The gas tungsten

arc w elding process is usually chosen because of the high quality w elds it can produce. The

metals that are commonly w elded w ith this process, such as stainless steel, aluminum and

some of the more exotic metals, cost many times the price of mild steel; and therefore, the

pow er sources designed for this process have many desirable features to insure high quality

w elds. Among these are:

1. Remote current control, w hich allow s the operator to control w elding amperage

w ith a hand control on the torch, or a foot control at the w elding station.

2. Automatic soft-start, w hich prevents a high current surge w hen the arc is

initiated.

3. Shielding gas and cooling water solenoid valves, w hich automatically control

f low before, during and for an adjustable length of time after the w eld is completed.

4. Spot-weld timers, w hich automatically control all elements during each

spot-w eld cycle.

Other options and accessories are also available.

2.3.2.10 Pow er sources for automatic w elding w ith complete programmable output are also

available. Such units are used extensively for the automatic w elding of pipe in position. The

w elding current is automatically varied as the torch travels around the pipe. Some units

provide a pulsed w elding current w here the amperage is automatically varied betw een a low

and high several times per second. This produces w elds w ith good penetration and improved

w eld bead shape.

2.3.3 Torches - The torch is actually an electrode holder that supplies w elding current to

the tungsten electrode, and an inert gas shield to the arc zone. The electrode is held in a

collet-like clamping device that allow s adjustment so that the proper length of electrode pro-

trudes beyond the shielding gas cup. Manual torches are designed to accept electrodes of 3

Current

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inch or 7 inch lengths. Torches may be either air or w ater-cooled. The air-cooled types actu-

ally are cooled to a degree by the shielding gas that is fed to the torch head through a compos-

ite cable. The gas actually surrounds the copper w elding cable, affording some degree of

cooling. Water-cooled torches are usually used for applications w here the w elding current

exceeds 200 amperes. The w ater inlet hose is connected to the torch head. Circulating

around the torch head, the w ater leaves the torch via the current-in hose and cable assembly.

Cooling the w elding cable in this manner allow s the use of a smaller diameter cable that is

more f lexible and lighter in w eight.

2.3.3.1 The gas nozzles are made of ceramic materials and are available in various sizes

and shapes. In some heavy duty, high current applications, metal w ater-cooled nozzles are

used.

2.3.3.2 A sw itch on the torch is used to energize the electrode w ith w elding current and start

the shielding gas f low . High frequency current and w ater f low are also initiated by this sw itch if

the pow er source is so equipped. In many installations, these functions are initiated by a foot

control that also is capable of controlling the w elding current. This method gives the operator

full control of the arc. The usual w elding method is to start the arc at a low current, gradually

increase the current until a molten pool is achieved, and w elding begins. At the end of the

w eld, current is slow ly decreases and the arc extinguished, preventing the crater that forms at

the end of the w eld w hen the arc is broken abruptly.

2.3.4 Shielding Gases - Argon and helium are the major shielding gases used in gas

tungsten arc w elding. In some applications, mixtures of the tw o gases prove advantageous.

To a lesser extent, hydrogen is mixed w ith argon or helium for special applications.

2.3.4.1 Argon and helium are colorless, odorless, tasteless and nontoxic gases. Both are

inert gases, w hich means that they do not readily combine w ith other elements. They w ill not

burn nor support combustion. Commercial grades used for w elding are 99.99% pure. Argon

is .38% heavier than air and about 10 times heavier than helium. Both gases ionize w hen

present in an electric arc. This means that the gas atoms lose some of their electrons that

have a negative charge. These unbalanced gas atoms, properly called positive ions, now

have a positive charge and are attracted to the negative pole in the arc. When the arc is

positive and the w ork is negative, these positive ions impinge upon the w ork and remove

surface oxides or scale in the w eld area.

2.3.4.2 Argon is most commonly used of the shielding gases. Excellent arc starting and

ease of use make it most desirable for manual w elding. Argon produces a better cleaning

action w hen w elding aluminum and magnesium w ith alternating current. The arc produced is

Current

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relatively narrow . Argon is more suitable for w elding thinner material. At equal amperage,

helium produces a higher arc voltage than argon. Since w elding heat is the product of volts

times amperes, helium produces more available heat at the arc. This makes it more suitable

for w elding heavy sections of metal that have high heat conductivity, or for automatic w elding

operations w here higher w elding speeds are required.

2.3.4.3 Argon-helium gas mixtures are used in applications w here higher heat input and the

desirable characteristics of argon are required. Argon, being a relatively heavy gas, blankets

the w eld area at low er f low rates. Argon is preferred for many applications because it costs

less than helium.

2.3.4.4 Helium, being approximately 10 times lighter than argon, requires f low rates of 2 to

3 times that of argon to satisfactorily shield the arc.

2.3.5 Electrodes - Electrodes for gas tungsten arc w elding are available in diameters

from .010" to 1/4" in diameter and standard lengths range from 3" to 24". The most commonly

used sizes, how ever, are the .040", 1/16", 3/32", and 1/8" diameters.

2.3.5.1 The shape of the tip of the electrode is an important factor in gas tungsten arc

w elding. When w elding w ith DCEN, the tip must be ground to a point. The included angle at

w hich the tip is ground varies w ith the application, the electrode diameter, and the w elding

current. Narrow joints require a relatively small included angle. When w elding very thin

material at low currents, a needlelike point ground onto the smallest available electrode may

be necessary to stabilize the arc. Properly ground electrodes w ill assure easy arc starting,

good arc stability, and proper bead w idth.

2.3.5.2 When w elding w ith AC, grinding the electrode tip is not necessary. When proper

w elding current is used, the electrode w ill form a hemispherical end. If the proper w elding

current is exceeded, the end w ill become bulbous in shape and possibly melt off to

contaminate the w eld metal.

2.3.5.3 The American Welding Society has published Specif ication AWS A5.12-80 for

tungsten arc w elding electrodes that classif ies the electrodes on the basis of their chemical

composition, size and f inish. Briefly, the types specif ied are listed below :

1) Pure Tungsten (AWS EWP) Color Code: Green

Used for less critical applications. The cost is low and they give good results at

relatively low currents on a variety of metals. Most stable arc w hen used on AC, either

balanced w ave or continuous high frequency.


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2) 1% Thoriated Tungsten (AWS EWTh-1) Color Code: Yellow

Good current carrying capacity, easy arc starting and provide a stable arc. Less

susceptible to contamination. Designed for DC applications of nonferrous materials.

3) 2% Thoriated Tungsten (AWS EWTh-2) Color Code: Red

Longer life than 1% Thoriated electrodes. Maintain the pointed end longer, used for

light gauge critical w elds in aircraft w ork. Like 1%, designed for DC applications for

nonferrous materials.

4) .5% Thoriated Tungsten (AWS EWTh-3) Color Code: Blue

Sometimes called "striped" electrode because it has 1.0-2.0% Thoria inserted in a

w edge-shaped groove throughout its length. Combines the good properties of pure

and thoriated electrodes. Can be used on either AC or DC applications.

5) Zirconia Tungsten (AWS EWZr) Color Code: Brown

Longer life than pure tungsten. Better performance w hen w elding w ith AC. Melts more

easily than thoriam-tungsten w hen forming rounded or tapered tungsten end. Ideal for

applications w here tungsten contamination must be minimized.

2.3.6 Summary - Gas Tungsten Arc Welding is one of the major w elding processes

today. The quality of the w elds produced and the ability to w eld very thin metals are the major

features. The w eld metal quality is high since no f lux is used, eliminating the problem of slag

inclusions in the w eld metal. It is used extensively in the aircraft and aerospace industry, w here

high quality w elds are necessary and also for w elding the more expensive metals w here the

w eld defects become very costly. Metals as thin as .005" can be w elded due to the ease of

controlling the current.

2.3.6.1 The major disadvantages of the process are that it is slow er than w elding w ith

consumable electrodes and is little used on thicknesses over 1/4" for this reason. Shielding

gas and tungsten electrode costs make the process relatively expensive.

2.4 GAS METAL ARC WELDING

Gas Metal Arc Welding* is an arc w elding process that uses the heat of an electric arc

established betw een a consumable metal electrode and the w ork to be w elded. The electrode

is a bare metal w ire that is transferred across the arc and into the molten w eld puddle. The

* Gas Metal Arc Welding (GMAW) is the current technology approved by the American Welding Society.

Formerly known as "MIG" (Metal Inert Gas) Welding.


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w ire, the w eld puddle, and the area in the arc zone are protected from the atmosphere by a

gaseous shield. Inert gases, reactive gases, and gas mixtures are used for shielding. The

metal transfer mode is dependent on shielding gas choice and w elding current level. Figure 9

is a sketch of the process show ing the basic features.

FIGURE 9

WELDING WIRE

WELDING CABLE

SHIELDING GAS

GAS NOZZLE

CONTACT TIP

WORK PIECE

MOLTEN POOL

WELD METAL

ARC

GAS SHIELD

SOLID WIREELECTRODE

TRAVELDIRECTION

GAS METAL ARC WELDING

2.4.0.1 Gas metal arc w elding is a versatile process that may be used to w eld a w ide

variety of metals including carbon steels, low alloy steels, stainless steels, aluminum alloys,

magnesium, copper and copper alloys, and nickel alloys. It can be used to w eld sheet metal or

relatively heavy sections. Welds may be made in all positions, and the process may be used

for semiautomatic w elding or automatic w elding. In semiautomatic w elding, the w ire feed

speed, voltage, amperage, and gas f low are all preset on the control equipment. The operator

needs merely to guide the w elding gun along the joint at a uniform speed and hold a relatively

constant arc length. In automatic w elding, the gun is mounted on a travel carriage that moves

along the joint, or the gun may be stationary w ith the w ork moving or revolving beneath it.

2.4.0.2 Practically all GMAW is done using DCEP (Electrode positive). This polarity

provides deep penetration, a stable arc and low spatter levels. A small amount of GMAW

w elding is done w ith DCEN and although the melting rate of the electrode is high, the arc is

erratic. Alternating current is not used for gas metal arc w elding.

2.4.1 Current Density - To understand w hy gas metal arc w elding can deposit w eld

metal at a rapid rate, it is necessary that the term "current density" be understood. Figure 10

show s a 1/4" coated electrode and a 1/16" solid w ire draw n to scale. Both are capable of

carrying 400 amperes. Notice that the area of the 1/16" w ire is only 1/16 that of the core w ire

of the coated electrode. We can say that the current density of the 1/16" w ire is 16 times

Current

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greater than the current density

of the 1/4" w ire at equal w elding

currents. The resultant melt-off

rate of the solid w ire is very high.

If w e w ere to increase the current

through the 1/4" coated

electrode to increase the current

density, the resistance heating

through the 14" electrode length w ould be

excessive, and the rod w ould become so

hot that the coating w ould crack, rendering

it useless. The 1/16" w ire carries the high

current a distance of less than 3/4", the

approximate distance from the end of the contact tip to the arc.

2.4.2 Metal Transfer Modes

2.4.2.1 Spray transfer is a high current density process that rapidly deposits w eld metal in

droplets smaller than the electrode diameter. They are propelled in a straight line from the

center of the electrode. A shielding gas mixture of Argon w ith 1% to 2% Oxygen is used for

w elding mild and low alloy steel, and pure Argon or Argon-Helium mixtures are used for w eld-

ing aluminum, magnesium, copper, and nickel alloys. Welding current at w hich spray transfer

FIGURE 10

AREA = .049 SQ. IN.

AREA = .0031 SQ. IN.CORE WIRE

FLUXCOATING

COATED ELECTRODE

RELATIVE SIZE OF ELECTRODES FOR WELDING AT 400 AMPS

SOLID WIRE

1/4"

1/16"

.049 ÷ .0031 = 16

AA × 16

FIGURE 11

SPRAYTRANSFER

GLOBULARTRANSFER

PULSETRANSFER

MODES OF METAL TRANSFER

1 2 3SHORT CIRCUITING ARC METAL TRANSFER

takes place is relatively high and w ill vary w ith the metal being w elded, electrode diameter, and

the shielding gas being used. Deposition rates are high and w elding is usually limited to the

flat or horizontal f illet position. See Figure 11.

Current

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2.4.2.2 Globular transfer takes place at low er w elding currents than spray transfer. There

is a transition current w here the transfer changes to globular even w hen shielding gases using

a high percentage of argon are used. When carbon dioxide (CO2) is used as a shielding gas,

the transfer is alw ays globular. In globular transfer, a molten drop larger than the electrode

diameter forms on the end of the electrode, moves to the outer edge of the electrode and falls

into the molten puddle. Occasionally, a large drop w ill "short circuit" across the arc, causing

the arc to extinguish momentarily, and then instantaneously reignite. As a result, the arc is

somew hat erratic, spatter level is high, and penetration shallow . Globular transfer is not

suitable for out-of-position w elding. See Figure 11.

2.4.2.3 Short circuiting transfer is a much used method in gas metal arc w elding. It is

produced by using the low est current-voltage settings and the smaller w ires, usually .030",

.035", and .045" diameters. The low heat input makes this process ideal for sheet metal, out-

of-position w ork, and poor f it-up applications. Often called "short arc w elding" because metal

transfer is achieved each time the w ire actually short circuits (makes contact) w ith the w eld

puddle. This happens very rapidly. It is feasible for the short circuit frequency to be 20-200

times a second, but in practice, it occurs from 90-100 times a second. Each time the

electrode touches the puddle, the arc is extinguished. It happens so rapidly that it is visible

only on high speed f ilms.

2.4.2.4 Pulse transfer is a mode of metal transfer somew hat betw een spray and short

circuiting. The specif ic pow er source has built into it tw o output levels: a steady background

level, and a high output (peak) level. The later permits the transfer of metal across the arc.

This peak output is controllable betw een high and low values up to several hundred cycles per

second. The result of such a peak output produces a spray arc below the typical transition

current.

2.4.2.4.1 Figure 11 show s the transfer method. The arc is initiated by touching the w ire to the

w ork. Upon initial contact, a bit of the w ire melts off to form a molten puddle. The w ire feeds

forw ard until it actually contacts the w ork again, as at 1 in Figure 11, and the arc is

extinguished. The short circuiting current causes the w ire to neck dow n, as show n in 1, until it

melts off, as show n at 2. As soon as the w ire is free of the puddle, the arc is reignited and a

molten ball forms at the end of the electrode, as at 3. The w ire continues to move forw ard until

it makes contact w ith the puddle, and the cycle is repeated.

2.4.2.5 Gas metal arc spot welding is a variation of the process that allow s spot w elding

of thinner gauge metals, or of a thin gauge metal to a heavier section. The gun is placed

directly against the w ork and is equipped w ith a special nozzle to allow escape of the shielding

gas. When the trigger sw itch is actuated, the follow ing sequence takes place. The shielding

Current

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gas f low s for a short interval before w ire feeding starts; w ire feeding starts; the arc is initiated

and continues for a preset time (usually a few seconds). The w elding current and w ire feeding

stops, and the shielding gas f low s for a short interval before it automatically stops. The

process is also useful for tacking w elding pieces in position prior to running the f inal w eld

bead.

2.4.3 EQUIPMENT AND OPERATION - The equipment used for gas metal arc w elding

is more complicated than that required for shielded metal arc w elding. Initial cost is relatively

high, but the cost is rapidly amortized due to the savings in labor and overhead achieved by

the rapid w eld metal deposition.

2.4.3.1 The equipment necessary for gas metal arc w elding is listed below :

1) Pow er source

2) Wire feeder

3) Welding gun

4) Shielding gas supply

5) Solid electrode w ire

6) Protective equipment

2.4.3.2 The basic equipment necessary for semiautomatic gas metal arc w elding is show n

in Figure 12.

FIGURE 12

FLOWMETERREGULATOR

SHIELDINGGAS

POWERSOURCE

GROUND CABLEWORK

WELDING GUN

WELD CABLE

115V CONTACTOR

MAGNETIC

VALVE

TRIGGERCONTROL LEAD

FEED ROLLS

GAS HOSE

WIRE FEEDER

WIRE SPOOL

+ _

SCHEMATIC DIAGRAM SEMI-AUTOMATIC GMAW EQUIPMENT


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2.4.4 Power Source - A direct current, constant v oltage power source is recommended

for gas metal arc w elding. It may be a transformer-rectif ier or a rotary type unit. The low er

open circuit voltage and self-correcting arc length feature, as described in Lesson I, makes it

most suitable. Constant voltage pow er sources used for spray transfer w elding and for f lux

cored electrode w elding (to be covered later) are the same. How ever, if the unit is to be used

for short-circuiting arc

w elding, it must have

"slope" or slope control.

Slope control is a

means of limiting the

high short-circuit current

that is characteristic of

this type w elder. Figure

13 show s the effect of

slope on the short-

circuiting current.

2.4.4.1 If w e w ere

short-arc w elding at

approximately 150 amperes

and 18 volts, as show n in Figure 13,

and had no slope components in the pow er source, the current at short-circuit or w hen the w ire

touches the w ork, w ould be over 1400 amperes. At this high current, a good length of the w ire

w ould literally explode off the end, cause much spatter, and the arc w ould be erratic. With the

slope components in the circuit, the short-circuiting current is in the neighborhood of 400

amperes, and the molten ball is sort of pinched off the end of the w ire more gently. For those

w ith an electrical background, it might be added that in some machines, slope is achieved by

adding a reactor in the AC secondary of the pow er source. In others, a slope resistor is added

in the DC output portion of the circuit. Slope may be adjustable for varying w ire diameters or it

may be f ixed, giving a good average value for .035" and .045" diameter w ires, the tw o most

popular sizes.

2.4.4.2 Another factor inf luencing the arc in short-circuiting w elding is the rate that the

amperage reaches the short-circuiting current level. Using the example in Figure 13, w e know

that the current goes from 150 amperes to 400 amperes during each shorting period. If w e

w ere to plot the current rise on a graph, as in Figure 14, w e w ould see that the current rise if

very rapid, as show n by the broken line.

FIGURE 13

25

20

15

10

5

200 400 600 800 1000 1200 1400

OPERATING POINT

CONSTANT VOLTAGE V/A CURVE

SHORT CIRCUITINGCURRENT NO SLOPE

SHORT CIRCUITING CURRENT

WITH SLOPE

EFFECT OF SLOPE ON SHORT CIRCUITING CURRENT

VO

LT

S

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This rapid current rise can be

by using a device called an

(sometimes called a stabilizer)

output circuit of the w elder. An

merely an iron core w ound

turns of heavy w ire. It does

current f low , but it acts

somew hat like a f ly w heel or

damper by retarding the rate of

rise as show n by the solid

line. By preventing the

rapid current rise, the arc

becomes smoother,

spatter is reduced, and

bead shape and

appearance are

improved. Because the inductor inf luences the time function, its design determines arc on-off

time, and short-circuit frequency. Some pow er sources have a selector that can sw itch in

several different inductance values to f inely tune the arc.

2.4.4.4 Welding pow er sources designed for gas metal arc w elding have a 115 volt outlet to

provide pow er to operate the w ire feeder. They also have a receptacle to receive the electrical

pow er required to close the main contactor in the pow er source, w hich turns on the w elding

pow er to the w elding gun w hen the gun trigger is actuated.

2.4.4.5 Additional advancements in equipment technology have introduced many new

models. Inverters, as w ell as microprocessor controls, have created the greatest attention. In

addition, multipurpose machines have provided the user w ith greater f lexibility w ith a variety of

capabilities.

2.4.4.6 Global competition w ill continue to have a profound influence on future

advancements in arc w elding equipment. As energy prices rise, greater demands for more

eff icient equipment w ill follow .

2.4.5 Wire Feeder - When w elding w ith a constant voltage pow er source, as is the case

in most gas metal arc w elding applications, the prime function of the w ire feeder is to deliver

the w elding w ire to the arc at a very constant speed. Since the w ire feed speed determines

the amperage, and the amperage determines the amount of heat at the arc, inconsistent w ire

feed speed w ill produce w elds of varying penetration and bead w idth. Advanced electronics

FIGURE 14

TIME - MILLISECONDS

EFFECT OF INDUCTANCE ON CURRENT RISE

400 AMPSWITHOUT INDUCTANCE

WITH INDUCTANCE

150 AMPS

Current

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technology makes it possible to design motor speed controls that w ill produce the same

speed, even though the load on the motor varies or the input voltage to the motor may f luctuate.

2.4.5.1 A limited amount of gas metal arc w elding is performed w ith constant current type

pow er sources. In this case, the motor speed automatically varies to increase or decrease the

w ire feed speed as the arc length varies to maintain a constant voltage.

2.4.5.2 The w ire feeder also controls the main contactor in the pow er source for safety

reasons. This assures that the w elding w ire w ill only be energized w hen the sw itch on the

w elding gun is depressed.

2.4.5.3 The f low of shielding gas is controlled by a solenoid valve (magnetic valve) in the

w ire feeder to turn the shielding gas on and off w hen the gun sw itch is actuated. Most feeders

utilize a dynamic breaking circuit to quickly stop the motor at the end of a w eld to prevent a

long length of w ire protruding from the gun w hen the w eld is terminated. Most feeders have a

burn-back circuit that allow s the w elding current to stay on for a short period of time after w ire

feeding has stopped, to allow the w ire to burn back exactly the right amount for the next arc

initiation.

2.4.5.4 The feed rolls, sometimes called drive rolls, pull the w ire off the spool or reel, and

push it through a feed cable or conduit to the w elding gun. These rolls must usually be

changed to accommodate each different w ire diameter, although some rolls are designed to

feed a combination of sizes.

2.4.6 Welding Gun - The function of the w elding gun, sometimes referred to as a torch, is

to deliver the w elding w ire, w elding current, and shielding gas to the w elding arc. Guns are

available for semi-automatic operation and for automatic operation, w here they are f ixed in the

automatic w elding head.

2.4.6.1 Guns for GMAW have several characteristics in common. All have a copper alloy

shielding gas nozzle, that delivers the gas to the arc area in a nonturbulent, angular pattern to

prevent aspiration of air. The nozzle may be w ater cooled for semiautomatic w elding at high

amperage and for automatic w elding w here the arc time is of long duration. Welding current is

transferred to the w elding w ire as the w ire travels through the contact tip or contact tube

located inside the gas nozzle (Refer to Figure 9). The hole in the contact tip through w hich the

w ire passes is only a few thousandths of an inch larger than the w ire diameter. A w orn contact

tip w ill result in an erratic arc due to poor current transfer. Figure 15 show s a few different

semiautomatic gun configurations that are commonly used for GMAW.

Current

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2.4.6.2 The curved neck or "goose neck" type is probably the most commonly used. It

allow s the best access to a variety of w eld joints. The w ire is pushed to this type of gun by the

feed rolls in the w ire feeder through a feed cable or conduit that usually is 10 or 12 feet in

length. The shielding gas hose, w elding current cable, and trigger sw itch leads are supplied

w ith the w elding gun.

2.4.6.3 The pistol type gun is similar to the curved neck type, but is less adaptable for

diff icult to reach joints. The pistol type is also a "push" type gun and is more suitable for gas

metal arc spot w elding applications.

2.4.6.4 The self contained type has an electric motor in the handle and feed rolls that pull the

w ire from a 1 or 2 pound spool mounted on the gun. The need for a long w ire feed cable is

eliminated, and w ire feed speed may be controlled by the gun. Guns of this type are often

used for aluminum w ire up to .045" diameter, although they may also be used for feeding steel

or other hard w ires.

2.4.6.5 The pull type gun has either an electric motor or an air motor mounted in the handle

that is coupled to a feeding mechanism in the gun. The spool of w ire is located in the control

cabinet that may be located as far as f if ty feet from the gun. When feeding such long

distances, a set of "push" rolls located in the control cabinet assist in feeding the w ire. This

then becomes know n as a push-pull feed system and is especially useful in feeding the softer

w ires such as aluminum.

2.4.7 SHIELDING GASES - In gas metal arc w elding, there are a variety of shielding

gases that can be used, either alone or in combinations of varying degrees. The choice is

dependent on the type of metal transfer employed, the type and thickness of metal, the bead

CURVED NECK PISTOL TYPE

SELF CONTAINEDPULL TYPE

SEMI-AUTOMATIC GMAW GUN TYPES

FIGURE 15

Current

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profile (See Figure 16), penetration, and speed of w elding. In our discussion, w e w ill deal w ith

the more common choices used for the various transfer processes.

2.4.7.1 Short Circuiting Transfer - Straight carbon dioxide (CO2) is often used for short

circuiting arc w elding because of its low cost. The deep penetration usually associated w ith

CO2 is minimized because of the low amperage and voltage settings used w ith this process.

Compared to other gas mixes, CO2 w ill produce a harsher arc and therefore, greater spatter

levels. Usually, this is minimized by maintaining a short arc length and by careful adjustment of

the pow er supply inductance. The temperatures reached in w elding w ill cause carbon dioxide

to decompose into carbon monoxide and oxygen. To reduce the possibility of porosity caused

by entrapped oxygen in the w eld metal, it is w ise to use electrodes that contain deoxidizing

elements, such as silicon and manganese. If the current is increased above the short circuiting

range, the use of carbon dioxide tends to produce a globular transfer.

2.4.7.1.1 Mixing argon in proportions of 50-75% w ith carbon dioxide w ill produce a smoother

arc and reduce spatter levels. It w ill also w iden the bead profile, reduce penetration, and

encourage "w etting". Wetting, i.e., a uniform fusion, along w ith joining edges of the base metal

and the w eld metal, minimizes the w eld imperfection know n as undercutting (See Figure 17).

FERROUS METALS NON-FERROUS METALS

CO2 ARGON + CO2 ARGON + O2 ARGON HELIUM

BEAD PROFILE

FIGURE 16

FIGURE 17

UNDERCUT WETTING

2.4.7.1.2 The 75% Argon/25 CO2 mixture is often chosen for short circuit w elding of thin

sections, w hereas the 50-50 combination w orks w ell on thicker sections.

2.4.7.1.3 It should be noted that shielding gases can affect the metallurgy of the w eld metal.

As an example, a combination of argon and carbon dioxide may be used for w elding stainless

steel, but as the carbon dioxide breaks dow n, excessive carbon may be transferred into the

Current

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w eld metal. Corrosion resistance in stainless steel is reduced as the carbon content

increases. To counteract this possibility, a less reactive mixture of 90% helium - 7-1/2% argon

- 2-1/2% CO2 is sometimes chosen. This combination, know n as a trimix, provides good arc

stability and w etting.

2.4.7.2 Spray Arc Transfer - Pure argon produces a deep constricted penetration at the

center of the bead w ith much shallow er penetration at the edges (Figure 16). Argon performs

w ell on nonferrous metals, but w hen used on ferrous metals, the transfer is somew hat erratic

w ith the tendency for the w eld metal to move aw ay from the center line. To make argon suit-

able for spray transfer on ferrous metals, small additions of 1 to 5% oxygen have proven to

provide remarkable improvements. The arc stabilizes, becomes less spattery, and the w eld

metal w ets out nicely. If the percentage of argon falls below 80%, it is impossible to achieve

true spray transfer.

2.4.7.2.1 Pure helium or combinations of helium and argon are used for w elding nonferrous

metals. The bead profile w ill broaden as the concentration of helium increases.

2.4.7.3 Pulse Spray Transfer - The selection of shielding gas must be adequate enough to

support a spray transfer. Material type, thickness, and w elding position are essential variables

in selecting a particular shielding gas. The follow ing is a list of recommended gases:

Carbon Steel Argon/CO2/O2/He (He less than 50%)

Alloy Steel Argon/CO2/O2/He (He less than 50%)

Stainless Argon/O2/CO2 (CO2 max. 2%)

Copper, Nickel, & Cu-Ni Alloys Argon/Helium

Aluminum Argon/Helium

2.4.8 Electrodes - The solid electrodes used in GMAW are of high purity w hen they come

from the mill. Their chemistry must be closely controlled and some types purposely contain

high levels of deoxidizers for use w ith CO2 shielding.

2.4.8.1 The electrode manufacturer draw s dow n the electrode to a f inished diameter that,

w ith GMAW, is usually quite small. Diameters from .030" thru 1/16" are common.

2.4.8.2 Most steel GMAW electrodes are copper plated as a means of protecting the

surface. The copper inhibits rusting, provides smooth feeding, and helps electrical

conductivity.

2.4.8.3 Information on types and classif ications w ill be covered in a future lesson.

Current

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2.5 FLUX CORED ARC WELDING

Flux Cored Arc Welding (FCAW) is quite similar to GMAW as far as operation and

equipment are concerned. The major difference is that FCAW utilizes an electrode that is very

different from the solid electrode used in GMAW. The f lux cored electrode is a fabricated

electrode and as the name implies, f lux material is deposited into its core. The f lux cored

electrode begins as a f lat metal strip that is formed first into a "U" shape. Flux and alloying

elements are deposited into the "U" and then the shape is closed into a tubular configuration

by a series of forming rolls.

2.5.0.1 The f lux cored electrode is a continuous electrode that is fed into the arc w here it is

melted and transferred into the molten puddle. As in GMAW, the f lux cored process depends

on a gas shield to protect the w eld zone from detrimental atmospheric contamination. With

FCAW, there are tw o primary w ays this is accomplished (See Figure 18). The gas is either

applied externally, in w hich case the electrode is referred to as a gas shielded f lux cored

electrode, or it is generated from the decomposition of gas forming ingredients contained in

the electrode's core. In this instance, the electrode is know n as a self-shielding f lux cored

electrode. In addition to the gas shield, the f lux cored electrode produces a slag covering for

further protection of the w eld metal as it cools. The slag is manually removed w ith a w ire brush

or chipping hammer.

2.5.1 Self Shielded Process - The main advantage of the self shielding method is that

its operation is somew hat simplif ied because of the absence of external shielding equipment.

FIGURE 18

GAS CUP

GAS SHIELD

FLUX CORE

GAS SHIELDED

CONTACT TIP

INSULATEDGUIDE TUBE

SELF SHIELDED

CONTACT TIP

FLUX CORE

FLUX-CORED ARC WELDING

Current

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Although self shielding electrodes have been developed for w elding low alloy and stainless

steels, they are most w idely used on mild steels. The self shielding method generally uses a

long electrical stick-out (distance betw een the contact tube and the end of the unmelted elec-

trode) commonly from one to four inches. Electrical resistance is increased w ith the long

extension, preheating the electrode before it is fed into the arc. This enables the electrode to

burn off at a faster rate and increases deposition. The preheating also decreases the heat

available for melting the base metal, resulting in a more shallow penetration than the gas

shielded process.

2.5.1.1 A major draw back of the self shielded process is the metallurgical quality of the

deposited w eld metal. In addition to gaining its shielding ability from gas forming ingredients

in the core, the self shielded electrode contains a high level of deoxidizing and denitrifying

alloys, primarily aluminum, in its core. Although the aluminum performs w ell in neutralizing the

affects of oxygen and nitrogen in the arc zone, its presence in the w eld metal w ill reduce

ductility and impact strength at low temperatures. For this reason, the self shielding method is

usually restricted to less critical applications.

2.5.1.2 The self shielding electrodes are more suitable for w elding in drafty locations than

the gas shielded types. Since the molten f iller metal is on the outside of the f lux, the gases

formed by the decomposing f lux are not totally relied upon to shield the arc from the

atmosphere. The deoxidizing and denitrifying elements in the f lux further help to neutralize the

affects of nitrogen and oxygen present in the w eld zone.

2.5.2 The Gas Shielded Process - A major advantage w ith the shielded f lux cored

electrode is the protective envelope formed by the auxiliary gas shield around the molten

puddle. This envelope effectively excludes the natural gases in the atmosphere w ithout the

need for core ingredients such as aluminum. Because of this more thorough shielding, the

w eld metallurgy is cleaner w hich makes this process suitable for w elding not only mild steels,

but also low alloy steels in a w ide range of strength and impact levels.

2.5.2.1 The gas shielded method uses a shorter electrical stickout than the self shielded

process. Extensions from 1/2" to 3/4" are common on all diameters, and 3/4" to 1-1/2" on

larger diameters. Higher w elding currents are also used w ith this process, enabling high

deposition rates to be reached. The auxiliary shielding helps to reduce the arc energy into a

columnar pattern. The combination of high currents and the action of the shielding gas

contributes to the deep penetration inherent w ith this process. Both spray and globular

transfer are utilized w ith the gas shielded process.

Current

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2.5.3 Current Density - Flux cored arc w elding utilizes the same principles of current

density, as explained in section 2.4.1, but there is one signif icant difference betw een the f lux

cored electrode and the solid electrode. With the f lux cored electrode, the granular core

ingredients are poor electrical conductors and therefore, the current is carried primarily

through the outer metal sheathing. When an equal diam-

eter cross section of the tw o are compared (See Figure

19), it is seen that the f lux cored electrode has

a smaller current carrying area than the solid

electrode. This greater concentration of

current in a smaller area increases the

burnoff rate.

2.5.3.1 When all other factors are equal,

the deposition rate of the f lux cored

electrode is somew hat higher than the

solid electrode.

2.5.4 EQUIPMENT - The equipment used for f lux cored arc w elding is the same as

show n previously in Section 2.3.2.2, Figure 12, w ith the exception that the self shielded

method does not need the external gas apparatus.

2.5.4.1 Flux cored arc w elding is done w ith direct current. All of the gas shielded electrodes

are designed for DCEP operation. The self shielded electrodes are either designed

specif ically for DCEN or DCEP.

2.5.5 Power Source - The recommended pow er source is the direct current constant

voltage type. The constant current type can be used but w ith less satisfactory results.

2.5.6 Wire Feeder - The function of the w ire feeder in FCAW is the same as discussed in

the section on GMAW. Since the f lux cored electrode is tubular in construction, precautions

must be taken not to f latten the electrode. To facilitate feeding by means other than pressure

alone, specially designed feed rolls w ith knurled or grooved surfaces are used. Some feeders

use four feed rolls rather than tw o to minimize unit pressure on the electrode.

2.5.7 The Welding Gun - As compared to GMAW, the main difference in FCAW w elding

guns is in those used w ith the self shielding process. The gun is somew hat more compact due

to the absence of an external gas shielding nozzle. Since the self shielding process normally

requires a longer electrode extension, the self shielding gun may have an insulated guide tube

(Refer back to Figure 18) to give stability to the electrode. Water cooled guns are available for

high duty semi-automatic w elding and for automatic w elding.

FIGURE 19

CURRENT PATH

1/16" FLUX-COREDELECTRODE 1/16" SOLID

ELECTRODE

Current

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2.5.7.1 Flux cored w elding generates fumes, that for environmental reasons, must be

removed from the w elding area. This is usually done w ith an external exhaust system, but

w elding guns w ith internal fume extractors have been developed. They are heavier than the

regular gun and must be properly maintained so that the extracting mechanism does not

disturb the shielding gas.

2.5.8 SHIELDING GASES - Carbon dioxide is the most w idely used gas for auxiliary

shielding of the f lux cored electrode. The other commonly used gas is a mixture of 75% Argon

and 25% CO2.

2.5.8.1 A carbon dioxide shield produces deep penetration and the transfer is globular. As

previously discussed, CO2 w ill dissociate in the heat of the arc. To counteract this

characteristic, deoxidizing elements are added to the core ingredients of the electrode. The

deoxidizers react to form solid oxide compounds that f loat to the surface as part of the slag

covering.

2.5.8.2 The addition of Argon to CO2 w ill increase the w etting action, produce a smooth arc

arc, and reduce spatter. The transfer is spray-like, and the penetration is somew hat less than

w ith the straight carbon dioxide.

2.5.8.3 While some flux cored electrodes are designed to operate w ell on both the 100%

CO2 or the 75/25 mixture, others are formulated specif ically for the CO2 shield or the Argon/

CO2 mixture. If the recommended gas is not used w ith these electrodes, the w eld chemistry

may be affected. The reason for this is that inert gas, such as Argon, does not react w ith the

other elements; therefore, allow ing them to be transferred across the arc into the w eld metal.

An electrode designed for CO2 shielding contains deoxidizing elements, such as silicon and

manganese. If a high percentage of Argon is used in the shielding medium, a large portion of

these elements may pass into the w eld metal causing the w eld metallurgy to be less ductile

than intended.

2.5.8.3 The opposite happens w ith electrodes formulated for a 75/25 mixture. These

electrodes are usually designed for high yield and tensile strength. If a high percentage of CO2

is used w ith them, the CO2 may react w ith the elements needed to attain these strength levels,

thereby preventing them from passing into the w eld metal.

2.6 SUBMERGED ARC WELDING

Submerged Arc Welding (SAW) is different from the previously explained arc w elding

processes in that the arc is not visible. The arc is submerged beneath loose granular f lux. A

continuous electrode is fed by automatic drive rolls through an electrode holder w here current

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is picked up at the contact tube. The electrode moves into the loose f lux and the arc is

initiated. The f lux is deposited from a separate container that moves at the same pace as the

electrode assuring complete coverage (See Figure 20).

2.6.1 Submerged Arc Flux - The f lux helps form the molten puddle, slow s the cooling

rate, and acts as a protective shield. The f lux, w hich is in close contact w ith the arc, is fused

into a slag cover and that w hich is not fused is collected for reuse. The f lux can contain alloying

elements that, w hen molten, w ill pass into the w eld metal affecting the metallurgy. Some fluxes

are specif ically prepared for their alloy altering capabilities w hile others, know n as neutral

f luxes, are chosen w hen a minimal alloy change is desired. Although these latter f luxes are

called "neutral", they still have the ability to slightly alter the w eld chemistry.

FIGURE 20

FLUX HOPPER

LOOSE GRANULAR FLUX

MOLTEN PUDDLE

FUSED SLAG COVER

SOLIDIFIED WELD METAL

BASEMETAL

ELECTRODE

SUBMERGED ARC WELDING

2.6.2 The Welding Gun - Although there are hand-held w elding guns for the submerged

arc process, the majority of SAW is done w ith fully automatic equipment. The basic compo-

nents include a w ire feeder, a pow er source, a f lux delivery system, and in some instances, an

automatic f lux recovery system.

2.6.3 Power Sources - The pow er source can be a constant current AC transformer, or it

may be a DC rectif ier or generator of either the constant current or constant voltage variety.

The pow er source must be rated for high current output. When current requirements exceed

the value of a single machine, tw o or more of the same type may be connected in parallel.

2.6.4 Equipment - Most submerged arc w elding is done w ith DCEP because it provides

easy arc starting, deep penetration and excellent bead shape. DCEN provides the highest

Current

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deposition rates but minimum penetration. Alternating current is often used as a trailing arc in

tandem arc applications. In this type of application, the leading DCEP arc provides deep

penetration, and the closely trailing AC arc provides high deposition w ith a minimum of arc

blow .

2.6.5 Electrodes - A variety of ferrous and nonferrous electrodes are used in submerged

arc w elding. They are usually solid electrodes refined w ith the appropriate alloys at the steel

mill, and then shipped to electrode manufacturers w here they are draw n dow n to a specif ic

diameter and packaged. There is another type of sub arc electrode know n as a composite

electrode, that is fabricated in the same manner as a f lux cored electrode. A chief advantage

of this type is that the alloying elements can be added to the core of the electrode more

cheaply than a steel mill can produce those same alloys in a solid form. The electrodes for

SAW vary in diameter from 1/16 inch to 1/4 inch w ith the larger diameters being the most

w idely used.

2.6.6 Summary - Submerged arc w elding has some advantages over other w elding

processes. Since the radiance of the arc is blanketed by the loose f lux, there is no need for a

protective w elding hood (although safety glasses are recommended), there is no spatter and

only a very minimal amount of fumes escape from under the blanket. High w elding currents,

quite commonly in the 300 to 1600 ampere range, are used. These high currents, combined

w ith fast travel speeds, make SAW a high deposition process that is especially suitable for

applications that require a series of repetitious w elds. Some setups allow tw o or more elec-

trodes to be fed simultaneously into the joint, further increasing the deposition rate and speed.

2.6.6.1 Although SAW has these advantages, it does have some limitations. The f lux must

be deposited and collected for every w elding pass. This requires additional equipment and

handling. Also because of the loose f lux, the process is limited to the f lat and horizontal

positions. The equipment for SAW is commonly quite bulky w hich limits its mobility, and

although the process w orks w ell on thick materials, it usually is not satisfactory for thin gauge

material. The process requires care in the operation. The amperages commonly used may

cause excessive heat buildup in the base metal, that may result in distortion or brittleness.

Current

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LESSON II

2.7 ELECTROSLAG AND ELECTROGAS WELDING

Electroslag Welding (ESW) and Electrogas Welding (EGW) comprise only a minor portion

of all w elding done in the country, but they are uniquely adapted to certain applications,

primarily the joining of very thick materials. The joining of a 12 inch material along a 40 foot

line is not an uncommon application for the Electroslag process.

2.7.1 Electroslag Welding (See Figure 21) is technically not an arc w elding process,

although it utilizes a current carrying consumable electrode. The only time there is an arc

betw een the electrode and the w ork piece is w hen current is initially charged through the

electrode. This initial charge heats a layer of loose f lux that becomes molten and extinguishes

the arc.

2.7.2 Flux - The f lux used in ESW is high in electrical resistance. As current is applied,

enough heat is generated from this resistance to keep the f lux, base metal, and electrode in a

molten state. This axis of the w eld joint is on a vertical plane. The tw o pieces of metal, usually

of the same thickness, are positioned so that there is an opening betw een them. One or more

electrodes are fed into the opening through a w elding bead that travels vertically as the joint is

f illed. To contain the molten puddle, w ater cooled copper shoes or dams are placed on the

sides of the vertical cavity. As the w eld joint solidif ies, the dams move vertically so as to

alw ays remain in contact w ith the molten puddle.

2.7.3 Process - A variation of ESW is the consumable guide method. The process is the

same w ith this method except that the guide tube that feeds the electrode to the molten pool is

WATER INLET/OUTLET

COPPER SHOE

BASE METAL

SOLIDIFIED METALWELD POOL

MOLTEN FLUX

GUIDE TUBE(CONSUMABLE GUIDE METHOD)

ELECTRODE

ELECTROSLAG WELDING

FIGURE 21

Current

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LESSON II

also consumed. The chief advantage w ith this method is the elimination of the electrode

holder w hich must move vertically w ith the w eld pool. Also since the guide tube is consumed,

the deposition rate is slightly increased w ith this method.

2.7.4 Equipment - The equipment used in ESW is all automatic and of special design.

The pow er source may use either AC or DC current. The electrode may be either solid or f lux

cored, although if the f lux cored is used, it must be specially formulated so as not to contain its

normal amount of slag forming ingredients.

2.7.5 Summary - Electrogas Welding is similar to ESW as far as the mechanical as-

pects are concerned. The equipment is automatic, the w elding head travels vertically, and the

molten puddle is retained by shoes on the sides of the joint. The difference is that Electrogas

Welding utilizes an arc and it is externally gas shielded. The pow er source is also limited to

DC operation. The electrodes used in EGW can be either solid or f lux cored.

Current

Chapter Table of

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Welding


Arc Welding

Processes

Lesson 3

Covered Electrodes


Go To Test

Lesson 4

Covered Electrodes


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LESSON II

APPENDIX A

LESSON II - GLOSSARY OF TERMS

Arc Blow - Deviation of the direction of the w elding arc caused by magnetic f ields in the

w ork piece w hen w elding w ith direct current.

Straight

Polarity- Welding condition w hen the electrode is connected to the negative terminal

and the w ork is connected to the positive terminal of the w elding pow er source.

Reverse

Polarity- Welding condition w hen the electrode is connected to the positive terminal

and the w ork is connected to the negative terminal of the w elding pow er

source.

Slag - The brittle mass that forms over the w eld bead on w elds made w ith coated

electrodes, f lux cored electrodes, submerged arc w elding and other slag

producing w elding processes. Welds made w ith the gas metal arc and the

gas tungsten arc w elding processes are slag free.

Manual Arc

Welding- Welding w ith a coated electrode w here the operator's hand controls travel

speed and the rate the electrode is fed into the arc.

Semi-Automatic

Welding- Welding w ith a continuous solid w ire or f lux cored electrode w here the w ire

feed speed, shielding gas f low rate, and voltage are preset on the equipment,

and the operator guides the hand held w elding gun along the joint to be

w elded.

Slag

Inclusion- A w eld defect w here slag is entrapped in the w eld metal before it can f loat to

the surface.

Root Pass - The initial pass in a multi-pass w eld, usually requiring 100% penetration.

Current

Chapter Table of

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Welding


Arc Welding

Processes

Lesson 3

Covered Electrodes


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Lesson 4

Covered Electrodes


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LESSON II

Gas Ions - Shielding gas atoms that, in the presence of an electrical current, lose one or

more electrons and therefore, carry a positive electrical charge. The provide

a more electrically conductive path for the arc betw een the electrode and the

w ork piece.

High

Frequency - (as applied to gas-tungsten arc w elding)

An alternating current consisting of over 50,000 cycles per second at high

voltage, low amperage that is superimposed on the w elding circuit in GTAW

pow er sources. It ionizes a path for non-touch arc starting and stabilizes the

arc w hen w elding w ith alternating current.

Inert Gases - Gases that are chemically inactive. They do not readily combine w ith other

elements.

Flux - In arc w elding, f luxes are formulations that, w hen subjected to the arc, act as

a cleaning agent by dissolving oxides, releasing trapped gases and slag and

generally cleaning the w eld metal by f loating the impurities to the surface

w here they solidify in the slag covering. The f lux also serves to reduce spatter

and contributes to w eld bead shape. The f lux may be the coating on the

electrode, inside the electrode as in f lux cored types, or in a granular form as

used in submerged arc w elding.

Current

Density - The amperes per square inch of cross-sectional area of an electrode. High

current density results in high electrode melt-off rate and a concentrated, deep

penetrating arc.

Slope or Slope

Control - A necessary feature in w elding pow er sources used for short circuiting arc

w elding. Slope Control reduces the short circuiting current each time the

electrode touches the w eld puddle (See Section 2.5.3).

Inductance - (as applies to short circuiting arc w elding)

A feature in w elding pow er sources designed for short circuiting arc w elding

to retard the rate of current rise each time the electrode touches the w eld

puddle.

Current

Chapter Table of

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Welding


Arc Welding

Processes

Lesson 3

Covered Electrodes


Go To Test

Lesson 4

Covered Electrodes


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Glossary

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LESSON II

Contact Tip - That part of a gas metal arc w elding gun or f lux cored arc w elding gun that

transfers the w elding current to the w elding w ire immediately before the w ire

enters the arc.

Spray

Transfer

- Mode of metal transfer across the arc w here the molten metal droplets are

smaller than the electrode diameter and are axially directed to the w eld puddle.

Requires high voltage and amperage settings and a shielding gas of at least

80% argon.

Globular

Transfer

- Mode of metal transfer across the arc w here a molten ball larger than the

electrode diameter forms at the tip of the electrode. On detachment, it takes

on an irregular shape and tumbles tow ards the w eld puddle sometimes

shorting betw een the electrode and w ork at irregular intervals. Occurs w hen

using shielding gases other than those consisting of at least 80% argon and

at medium current settings.

Pulse

Transfer

- Mode of metal transfer somew hat betw een spray and short circuiting. The

specif ic pow er source has built into it tw o output levels: a steady background

level, and a high output (peak) level. The later permits the transfer of metal

across the arc. This peak output is controllable betw een high and low values

up to several hundred cycles per second. The result of such a peak output

produces a spray arc below the typical transition current.

Short-circuiting

Transfer

- Mode of metal transfer in gas metal arc w elding at low voltage and amperage.

Transfer takes place each time the electrode touches or short-circuits to the

w eld puddle, extinguishing the arc. The short-circuiting current causes the

electrode to neck dow n, melt off, and then repeats the cycle.

Trimix or

Triple Mix

- A shielding gas consisting of approximately 90% helium, 7-1/2% argon, and

2-1/2% carbon dioxide used primarily for short-circuiting arc w elding of

stainless steels. Maintains corrosion resistance of the stainless steel and

produces good w etting and excellent w eld bead shape.

Current

Chapter Table of

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Welding


Arc Welding

Processes

Lesson 3

Covered Electrodes


Go To Test

Lesson 4

Covered Electrodes


Print

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Glossary

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LESSON II

Electrical

Stick-Out

- In any w elding process using a solid or f lux cored w ire, the electrical stick-out

is the distance from the contact tip to the unmelted electrode end. Sometimes

called the "amount of w ire in resistance". This distance influences melt-off

rate, penetration, and w eld bead shape.

Out-of-Position

Welds

- Welds made in positions other than f lat or horizontal f illets.

Weld

Positions

-

FLAT HORIZONTAL FILLET

VERTICAL OVERHEAD

HORIZONTALBUTT

POSITIONED FILLET(FLAT)

Current

Chapter Table of

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Welding


Arc Welding

Processes

Lesson 3

Covered Electrodes


Go To Test

Lesson 4

Covered Electrodes


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