TIGBook_Chpt4.pdf

Magnetic Arc Control

This control uses magnetic fields to deflect the arc in advanta-

geous directions. It is useful for high speed automatic welding

to even out the weld pool, prevent undercut, and promote

uniform penetration. The oscillation and positioning effects of

these magnetic fields on the arc improve weld appearance

and weld bead profiles. See Figure 3.36.

represents a Cold Wire Feed System. Improved penetration

and weld profiles can be had by feeding the filler wire into the

back edge of the weld pool versus the front half of the weld

pool, which is typically done with manual welding. Some sys-

tems can be set up where the filler wire is preheated electri-

cally. These systems are referred to as Hot Wire TIG.

Seam Tracking

In order to keep the welding arc on track when following a

constantly varying weld seam, systems like Figure 3.38 have

been developed. This type control allows the equipment to

constantly monitor the weld joint location both horizontally

and vertically over the joint. In order to have consistency at high

travel speeds, devices like this can control the position of the

welding arc within plus and minus 0.005 inch or 0.13 mm.

Cold Wire Feed System

GTAW is generally considered a low-deposition process.

However, by automating it and adding the filler wire in an

automatic fashion its deposition rate can be increased.

Increased weld deposition means higher travel speeds and

more parts out the door at the end of the day. Figure 3.37

IV. Electrodes and

Consumables

Tungsten Electrodes for GTAW

Electrodes made of tungsten and tungsten alloys are secured

within a GTAW torch to carry current to the welding arc.

Tungsten is preferred for this process because it has the

highest melting point of all metals.

Standard Tungsten Sizes

SI Units

U.S. Customary

Tolerance

Diameter

0.010 a

Diameter

Tolerance

in. b, c

mm b, c

0.300

0.50

1.00

1.60

2.00

2.40

2.50

3.00

3.20

4.00

4.80

5.00

6.40

8.00

0.025

0.05

0.08

0.001

0.020

0.002

0.040

0.002

0.060

0.002

0.093

0.003

0.125 (1/8)

0.003

0.156 (5/32)

0.003

0.187 (3/16)

0.003

0.250 (1/4)

0.003

The tungsten electrode establishes and maintains the arc. It

is said to be a “nonconsumable” in that the electrode is not

melted and included in the weld pool. In fact, great care must

be taken so that the tungsten does not contact the weld pool

in any way, thereby causing a contaminated, faulty weld. This

is generally referred to as a “tungsten inclusion”.

Notes:

a. 0.010 in. (0.30 mm) electrodes are also available in coils.

b. Tolerances, other than those listed, may be supplied as agreed upon

between supplier and user.

c. Tolerances shall apply to electrodes in both the clean finish and

ground finish conditions.

Tungsten electrodes for GTAW come in a variety of sizes and

lengths. They may be composed of pure tungsten, or a com-

bination of tungsten and other elements and oxides.

Electrodes are manufactured to specifications and standards

developed by the American Welding Society and the

American Society For Testing And Materials. Electrodes come

in standard diameters from .010" through 1/4", as seen in

Figure 4.1. The diameter of tungsten electrode needed is

often determined by the thickness of base metal being welded

and the required amperage to make the weld.

Figure 4.1 Diameters of standard tungsten electrodes (Courtesy AWS).

3" (76 mm)

12" (305 mm)

6" (152 mm)

18" (457 mm)

7" 178 mm)

24" (610 mm)

Figure 4.2 Standard tungsten lengths.

most commonly used. For special applications some suppliers

provide them in cut lengths to your specifications. For example,

.200" – .500", .501" – 3.000" and 3.001" – 7.000".

Lengths of tungstens needed are often determined by the

type of torch used for a particular application. Standard

lengths are shown in Figure 4.2. Of these, the 7" length is the

Chemical Composition Requirements for Electrodes a

Weight Percent

AWS

UNS

W Min.

Other Oxides or

(difference) c

Classification

Number b

CeO 2

La 2 O 3

ThO 2

ZrO 2

Elements To tal

EWP

R07900

99.5

—

0.5

EWCe-2

R07932

97.3

1.8 – 2.2

—

0.5

EWLa-1

R07941

98.3

—

0.8 – 1.2

—

0.5

EWLa-1.5

R97942

97.8

—

1.3 – 1.7

—

0.5

EWLa-2

R07943

97.3

—

1.8 – 2.2

—

0.5

EWTh-1

R07911

98.3

—

0.8 – 1.2

—

0.5

EWTh-2

R07912

97.3

—

1.7 – 2.2

—

0.5

EWZr-1

R07920

99.1

—

0.15 – 0.40

0.5

EWG d

—

94.5

NOT SPECIFIED

0.5

Notes:

a. The electrode shall be analyzed for the specific oxides for which values are shown in this table. If the presence of other elements or

oxides is indicated, in the course of the work, the amount of those elements or oxides shall be determined to ensure that their total

does not exceed the limit specified for “Other Oxides or Elements, Total” in the last column of the table.

b. SAE/ASTM Unified Numbering System for Metals and Alloys.

c. Tungsten content shall be determined by subtracting the total of all specified oxides and other oxides and elements from 100%.

d. Classification EWG must contain some compound or element additive and the manufacturer must identify the type and minimal

content of the additive on the packaging.

Figure 4.3 Tungsten electrode requirements (Courtesy AWS).

Types of Electrodes

EWP (100% Tungsten, Green)

These electrodes are unalloyed, “pure” tungsten with a 99.5%

tungsten minimum. They provide good arc stability when using

AC current, with either balanced wave or unbalanced wave and

continuous high-frequency stabilization. Pure tungsten electrodes

are preferred for AC sine wave welding of aluminum and

magnesium because they provide good arc stability with both

argon and helium shielding gas. Because of their inability to

carry much heat, the pure tungsten electrode forms a balled end.

Types of tungsten and tungsten alloy electrodes for GTAW are

classified according to the chemical makeup of the particular

electrode types. Figure 4.3 shows the nine types of electrodes

classified by the American Welding Society.

In the first column of Figure 4.3, the AWS identifies the nine

classifications as they would for filler metal specifications.

The letter “E” is the designation for electrode. The “W” is the

designation for the chemical element tungsten.

The next one or two letters designates the alloying element

used in the particular electrode. The “P” designates a pure

tungsten electrode with no intentionally added alloying

elements. The “Ce”, “La”, “Th”, and “Zr” designate tungsten

electrodes alloyed with cerium, lanthanum, thorium, or

zirconium, respectively.

EWCe-2 (2% Cerium, Orange)

Alloyed with about 2% ceria, a non-radioactive material and

the most abundant of the rare earth elements, the addition of

this small percentage of cerium oxide increases the electron

emission qualities of the electrode which gives them a better

starting characteristic and a higher current carrying capacity

than pure tungsten. These are all-purpose electrodes that will

operate successfully with AC or DC electrode negative.

Compared with pure tungsten, the ceriated tungsten electrodes

provide for greater arc stability. They have excellent arc starting

properties at low current for use on orbital tube, pipe, thin sheet

and small delicate part applications. If used on higher current

applications the cerium oxide may be concentrated to the

excessively hot tip of the electrode. This condition and oxide

change will remove the benefits of the cerium. The

nonradioactive cerium oxide has slightly different electrical

properties as compared to the thoriated tungsten electrodes.

For automated (orbital tube, etc.) welding these slight changes

may require welding parameters and procedures to be adjusted.

The cerium electrodes work well with the Advanced Squarewave

power sources and should be ground to a modified point.

The number “1”, “1.5” or “2” behind this alloy element

indicates the approximate percentage of the alloy addition.

The last electrode designation, “EWG”, indicates a “general”

classification for those tungsten electrodes that do not fit

within the other categories. Obviously, two electrodes bearing

the same “G” classification could be quite different, so the

AWS requires that a manufacturer identify on the label the

type and content of any alloy additions.

Electrodes are color coded for ease of identification. Care

should be exercised when working with these electrodes so

that the color-coding can be kept intact.

EWLa-1 (1% Lanthanum, Black), EWLa-1.5 (1.5%

Lanthanum, Gold) and EWLa-2 (2% Lanthanum, Blue)

Alloyed with nonradioactive lanthanum oxide, often referred

to as lanthana, another of the rare earth elements. These

electrodes have excellent arc starting, low-burn-off rate, arc

stability, and excellent re-ignition characteristics. The addition

of 1 – 2% lanthana increases the maximum current carrying

capacity by approximately 50% for a given size electrode

using alternating current compared to pure tungsten. The

higher the percentage of lanthana, the more expensive the

electrode. Since lanthana electrodes can operate at slightly

different arc voltages than thoriated or ceriated tungsten

electrodes these slight changes may require welding parameters

and procedures to be adjusted. The 1.5% content appears to

most closely match the conductivity properties of 2% thori-

ated tungsten. Compared to cerium and thorium the

lanthana electrodes had less tip wear at given current levels.

Lanthanum electrodes generally have longer life and provide

greater resistance to tungsten contamination of the weld.

The lanthana is dispersed evenly throughout the entire length

of the electrode and it maintains a sharpened point well,

which is an advantage for welding steel and stainless steel on

DC or the AC from Advanced Squarewave power sources.

Thus the lanthana electrodes work well on AC or DC electrode

negative with a pointed end or they can be balled for use with

AC sine wave power sources.

applications, the electrode is ground to a taper or pointed.

The thorium electrode will retain the desired shape in those

applications where the pure tungsten would melt back and

form the ball end. The thoria content in the electrode is

responsible for increasing the life of this type over the pure

tungsten, EWP.

EWZr-1 (1% Zirconium, Brown)

A zirconium oxide (zirconia) alloyed tungsten electrode is

preferred for AC welding when the highest quality work is

necessary and where even the smallest amounts of weld pool

contamination cannot be tolerated. This is accomplished

because the zirconium alloyed tungsten produces an

extremely stable arc which resists tungsten spitting in the arc.

The current carrying capability is equal to or slightly greater

than an equal sized cerium, lanthana or thorium alloyed

electrode. Zirconium electrodes are typically used only for

AC welding with a balled end.

EWG (unspecified alloy, Gray)

This classification covers tungsten electrodes containing

unspecified additions of rare earth oxides or combinations of

oxides. As specified by the manufacturer, the purpose of the

additions is to affect the nature or characteristics of the arc.

The manufacturer must identify the specific addition or

additions and the quantity or quantities added.

Some “rare earth” electrodes are in this category and they

contain various percentages of the 17 rare earth metals. One

mixture is 98% tungsten, 1.5% lanthanum oxide, and a .5%

special mixture of other rare earth oxides. Some of these

electrodes work on AC and DC, last longer than thoriated

tungsten, can use a smaller size diameter tungsten for the

same job, can use a higher current than similar sized thoriated

tungstens, reduce tungsten spitting, and are not radioactive.

EWTh-2 (2% Thorium, Red) and EWTh-1

(1% Thorium, Yellow)

Commonly referred to as 1 or 2% thoriated tungstens, these

are very commonly used electrodes since they were the first

to show better arc performance over pure tungsten for DC

welding. However, thoria is a low-level radioactive material,

thus vapors, grinding dust and disposal of thorium raises

health, safety and environmental concerns. The relatively

small amount present has not been found to represent a

health hazard. But if welding will be done in confined spaces

for prolonged periods of time, or if electrode grinding dust

might be ingested, special precautions should be taken

concerning proper ventilation. The welder should consult

informed safety personnel and take the appropriate steps to

avoid the thoria.

Tungsten electrodes for GTAW can easily be recognized by

their color code. See Figure 4.4.

Electrode Identification Requirements a,b

AWS Classification

Color

EWP

Green

EWCe-2

Orange

EWLa-1

Black

EWLa-1.5

Gold

The thoriated electrode does not ball as does the pure tungsten,

cerium or lanthana electrodes. Instead, it forms several small

projections across the face of the electrode when used on

alternating current. When used on AC sine wave machines,

the arc wanders between the multiple projections and is often

undesirable for proper welding. Should it be absolutely necessary

to weld with these type machines, the higher content lanthana

or thoria electrodes should be used. The thoriated electrodes

work well with the Advanced Squarewave power sources and

should be ground to a modified point. These electrodes are

usually preferred for direct current applications. In many DC

EWLa-2

Blue

EWTh-1

Yellow

EWTh-2

Red

EWZr-1

Brown

EWG

Gray

Notes:

a. The actual color may be applied in the form of bands, dots,

etc., at any point on the surface of the electrode.

b. The method of color coding used shall not change the

diameter of the electrode beyond the tolerances permitted.

Figure 4.4 Color codes for tungsten electrodes (Courtesy AWS).

Use of Tungsten Electrodes

Electrodes used for GTAW welding differ greatly in many

respects from electrodes used in consumable metal arc welding.

The tungsten electrode is not melted or used as filler metal as

is the case with SMAW or GMAW electrodes. At least it is not

intended to be melted and become part of the weld deposit.

However, in cases where the wrong electrode type, the wrong

size of electrode, the wrong current, the wrong polarity or

technique is used, tungsten particles can be transferred

across the arc. The power source used may affect the amount

of tungsten which may be transferred across the arc. A

machine designed specifically for GTAW welding will usually

have characteristics advantageous for the process. Excessive

current surges or “spikes” will cause “spitting” of tungsten.

Excessive arc rectification on aluminum or magnesium will

cause a half-wave effect, and cause particles of tungsten to be

transferred across the arc. An understanding of the electrode

materials and types of electrodes and their recommended

uses will enable the user to make the proper electrode selection.

With the choice of several alloy types and a variety of sizes,

many factors must be considered when selecting the electrode.

One of the main considerations is welding current. The welding

current will be determined by several factors including base

metal type and thickness, joint design, fit-up, position, shielding

gas, type of torch, and other job quality specifications.

An electrode of a given diameter will have its greatest current

carrying capacity with direct current electrode negative

(DCEN), less with alternating current and the least with direct

current electrode positive (DCEP). Figure 4.5 lists some typical

current values for electrodes with argon shielding.

Tungsten has a high resistance to current flow and therefore,

heats up during welding. In some applications the extreme tip

forms a molten hemisphere. The "ball" tip is characteristic of

pure tungsten and is most desirable for AC welding with sine

wave power sources. The extreme tip is the only part of

the electrode which should be this hot. The remainder of the

electrode should be kept cool. Excessive electrode stickout

beyond the collet will cause heat build-up in the electrode. In

a water-cooled torch, the heat is more rapidly dissipated from

the collet assembly and helps cool the electrode. Excessive

current on a given size electrode will cause the tip to become

excessively hot.

Tungsten is a very hard steel gray metal. It is a highly refractory

metal and does not melt or vaporize in the heat of the arc. It

has a melting point of 6170˚ F (3410˚ C), and a boiling point

of 10,220˚ F (5600˚ C). Tungsten retains its hardness even

when red hot.

Typical Current Range (Amps)

Direct Current,

Alternating Current,

DCEN

70% Penetration

(50/50) Balanced

Wave A

Gas Cup

Ceriated

Tungsten

Inside

Thoriated

Diameter

Lanthanated

Pure

Lanthanated

Pure

Lanthanated

.040

#5 (3/8 in)

15 – 80

20 – 60

15 – 80

10 – 30

20 – 60

.060 (1/16 in)

#5 (3/8 in)

70 – 150

50 – 100

70 – 150

30 – 80

60 – 120

.093 (3/32 in)

#8 (1/2 in)

150 – 250

100 – 160

140 – 235

0 – 130

100 – 180

.125 (1/8 in)

#8 (1/2 in)

250 – 400

150 – 200

225 – 325

100 – 180

160 – 250

All values are based on the use of Argon as a shielding gas. Other current values may be employed depending on the

shielding gas, type of equipment, and application.

DCEN = Direct Current Electrode Negative (Straight Polarity)

Figure 4.5 Typical current ranges for electrodes with argon shielding.

After the proper size and type of electrode has been selected,

how the electrode is used and maintained will determine its

performance and life. There are many misconceptions about

tungsten electrodes and their correct use. The following

information is intended to serve as a guideline to common

sense decisions about tungsten electrodes.

Pointing of electrodes is a subject which has received much

discussion. There are many theories and opinions on the

degree of the point. Again, the application has a bearing on

the configuration of the point. Along with application experience,

the following should serve as a guide to pointing of electrodes.

A common practice in pointing electrodes is to grind the taper

for a distance of 2 to 2-1/2 electrode diameters in length for

use on DC and usually to a sharp needle point (see top of

Figure 4.7). Using this rule for a 1/8" electrode, the ground

surface would be 1/4 to 5/16" long.

Electrode Preparation

For AC Sine Wave and Conventional Squarewave

These electrodes should have a hemispheric or balled end

formed. The diameter of the end should not exceed the diameter

of the electrode by more than 1.5 times. As an example, a

1/8" electrode should only form a 3/16" diameter end. If it

becomes larger than this because of excessive current, there

is the possibility of it dropping off to contaminate the weld. If the

end is excessively large, and the current is decreased before

the molten tip drops off, the arc tends to wander around on the

large surface of the electrode tip. The arc becomes very hard to

control as it wanders from side to side. If welding conditions

are correct, a visual observation of the electrode should

reveal a ball end of uniform shape and proper size.

2-1/2 Times

Electrode Diameter

1. Tungsten Electrode

2. Tapered End

Grind end of tungsten on fine grit, hard abrasive wheel before welding.

Do not use wheel for other jobs or tungsten can become

contaminated causing lower weld quality.

For improved arc focus set the balance control to maximum

penetration and try a ceriated, lanthanated or thoriated tung-

sten with a modified point.

For Advanced Squarewave Use (Pointed)

With the expanded balance control of up to 90% electrode

negative, the electrode shape is very nearly the same as for

DC electrode negative welding. This improves the ability to

focus the arc along with an even greater localization of the

heat into the work. Do not use with pure tungsten.

Ideal Tungsten Preparation – Stable Arc

1. Stable Arc

2. Flat

3. Grinding Wheel

4. Straight Ground

For DC Electrode Negative Use (Pointed)

Since all of the weld energy is provided by electrode negative,

there is very little heating affect on the tungsten and a sharp

pointed tungsten is generally preferred. Figure 4.6 shows the

preferred shapes for balled and the various types of points

used with the DC and AC wave shaped power sources.

Wrong Tungsten Preparation – Wandering Arc

1. Arc Wander

2. Point

3. Grinding Wheel

4. Radial Ground

Figure 4.6 The ball diameter should never exceed 1.5 times the electrode

diameter. Pointed tungstens are as noted.

Figure 4.7 Preparing tungsten for DC electrode negative welding and AC

with wave shaping power sources.

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