TIGBook_Chpt2.pdf

No Slag

There is no requirement for flux with this process; therefore,

there is no slag to obscure the welder’s vision of the molten

weld pool. The finished weld will not have slag to remove

between passes. Entrapment of slag in multiple pass welds is

seldom seen. On occasion with materials like Inconel ®

GTAW Disadvantages

The main disadvantage of the GTAW process is the low filler

metal deposition rate. Another disadvantage is that the

hand-eye coordination necessary to accomplish the weld is

difficult to learn, and requires a great deal of practice to

become proficient. The arc rays produced by the process

tend to be brighter than those produced by SMAW and

GMAW. This is primarily due to the absence of visible fumes

and smoke. The increased amounts of ultraviolet rays from

the arc also cause the formation of ozone and nitrous oxides.

Care should be taken to protect skin with the proper clothing

and protect eyes with the correct shade lens in the welding

hood. When welding in confined areas, concentrations of

shielding gas may build up and displace oxygen. Make sure

that these areas are ventilated properly.

this

may present a concern.

No Sparks or Spatter

In the GTAW process there is no transfer of metal across the

arc. There are no molten globules of spatter to contend with

and no sparks produced if the material being welded is free

of contaminants. Also under normal conditions the GTAW arc

is quiet without the usual cracks, pops, and buzzing of

Shielded Metal Arc Welding (SMAW or Stick) and Gas Metal

Arc Welding (GMAW or MIG). Generally, the only time noise

will be a factor is when a pulsed arc, or AC welding mode is

being used.

Process Summary

GTAW is a clean process. It is desirable from an operator

point of view because of the reasons outlined. The welder

must maintain good welding conditions by properly cleaning

material, using clean filler metal and clean welding gloves,

and by keeping oil, dirt and other contaminants away from

the weld area. Cleanliness cannot be overemphasized,

particularly on aluminum and magnesium. These metals are

more susceptible to contaminants than are ferrous metals.

Porosity in aluminum welds has been shown to be caused by

hydrogen. Consequently, it is most important to eliminate all

sources of hydrogen contamination such as moisture and

hydrocarbons in the form of oils and paint.

No Smoke or Fumes

The process itself does not produce smoke or injurious

fumes. If the base metal contains coatings or elements such as

lead, zinc, nickel or copper that produce fumes, these must

be contended with as in any fusion welding process on these

materials. If the base metal contains oil, grease, paint or other

contaminants, smoke and fumes will definitely be produced

as the heat of the arc burns them away. The base material

should be cleaned to make the conditions most desirable.

II. GTAW Fundamentals

If you’ve ever had the experience of hooking up a car battery

backwards, you were no doubt surprised at the amount of

sparks and heat that can be generated by a 12 volt battery. In

actual fact, a GTAW torch could be hooked directly to a battery

and be used for welding.

When welding was first discovered in the early 1880s it was

done with batteries. (Some batteries used in early welding

experiments reached room size proportions.) The first

welding machine, seen in Figure 2.1, was developed by

N. Benardos and S. Olszewski of Great Britain and was issued

a British patent in 1885. It used a carbon electrode and was

dynamo, a machine that produces electric current by

mechanical means.

Figure 2.1 Original carbon electrode welding apparatus — 1885.

A SIMPLE WELDING CIRCUIT

CURRENT FLOW (AMPS)

BATTERY

(VOLTAGE)

Figure 2.3 The original torch and some of the tips used by Pavlecka and

Meredith to produce the first GTAW welds in 1942. Note the torch still

holds one of the original tungstens used in those experiments.

Although the selenium rectifier had been around for some

time, it was the early 1950s when rectifiers capable of handling

current levels found in the welding circuit came about. The

selenium rectifier had a profound effect on the welding industry.

It allowed AC transformer power sources to produce DC. And

it meant that an AC power source could now be used for

GTAW welding as well as Stick welding.

Figure 2.2 A simple welding circuit showing voltage source and current flow.

Figure 2.2 shows what a welding circuit using a battery as a

power source would look like.

The two most basic parameters we deal with in welding are

the amount of current in the circuit, and the amount of voltage

pushing it. Current and voltage are further defined as follows:

The realization is that high frequency added to the weld circuit

would make AC power usable for TIG welding. The addition

of this voltage to the circuit keeps the arc established as

the weld power passes through zero. Thus stabilizing the

GTAW arc, it also aids in arc starting without the risk of

contamination. The later addition of remote current control,

remote contactor control, and gas solenoid control devices

evolved into the modern GTAW power source. Further

advances such as Squarewave, and Advanced Squarewave

power sources have further refined the capabilities of this

already versatile process.

Current — The number of electrons flowing past a given

point in one second. Measured in amperes (amps).

Voltage — The amount of pressure induced in the circuit to

produce current flow. Measured in voltage (volts).

Resistance in the welding circuit is represented mostly by the

welding arc and to a lesser extent by the natural resistance of

the cables, connections, and other internal components.

Chapters could be written on the theory of current flow in an

electrical circuit, but for the sake of simplicity just remember

that current flow is from negative to positive. Early

researchers were surprised at the results obtained when the

battery leads were switched. We’ll examine these differences

in more detail later in the section when we discuss welding

with alternating current.

Alternating Current

Alternating current (AC) is an electrical current that has both

positive and negative half-cycles. These components do not

occur simultaneously, but alternately, thus the term alternating

current. Current flows in one direction during one half of the

cycle and reverses direction for the other half cycle. The half

cycles are called the positive half and the negative half of the

complete AC cycle.

Even after alternating current (AC) became available for welding

with the use of transformer power sources, welds produced

were more difficult to accomplish and of lesser quality than

those produced with direct current (DC). Although these AC

transformer power sources greatly expanded the use of com-

mercial power for SMAW (Stick), they could not be used for

GTAW because as the current approached the zero value, the

arc would go out. (see Figure 2.4). Motor generators followed

quickly. These were machines that consisted of an AC motor, that

turned a generator, that produced DC for welding. The output

of these machines could be used for both SMAW and GTAW.

Frequency

The rate at which alternating current makes a complete cycle

of reversals is termed frequency. Electrical power in the

United States is delivered as 60 cycles per second frequency,

or to use its proper term 60 hertz (Hz). This means there are

120 reversals of current flow directions per second. The

power input to an AC welding machine and other electrical

equipment in the United States today is 60 Hz power. Outside

of North America and the United States, 50 Hz power is more

commonly used. As this frequency goes up, the magnetic

effects accelerate and become more efficient for use in trans-

formers, motors and other electrical devices. This is the

It was with a motor generator power source that GTAW was

first accomplished in 1942 by V.H. Pavlecka and Russ

Meredith while working for the Northrup Aviation Company.

Pavlecka and Meredith were searching for a means to join

magnesium, aluminum and nickel, which were coming into

use in the military aircraft of that era.

fundamental principal on how an “inverter power source

works”. Frequency has major effect on welding arc perform-

ance. As frequencies go up, the arc gets more stable,

narrows, and becomes stiffer and more directional. Figure 2.4

represents some various frequencies.

WELDING

POWER

SUPPLY

3/32" ELECTRODE

WORK

ELECTRODE

–

GAS

IONS

–

ELECTRONS

–

WORK

Figure 2.6 AC welding machine connection.

Squarewave AC

Some GTAW power sources, due to refinements of electronics,

have the ability to rapidly make the transition between the

positive and negative half cycles of alternating current. It is

obvious that when welding with AC, the faster you could

transition between the two polarities (EN and EP), and the

more time you spent at their maximum values, the more

effective the machine could be. Electronic circuitry makes it

possible to make this transition almost instantaneously. Plus

the effective use of the energy stored in magnetic fields

results in waveforms that are relatively square. They are not

truly square due to electrical inefficiencies in the Squarewave

power source. However, the Advanced Squarewave GTAW

power source has improved efficiencies and can produce a

nearly square wave as compared in Figure 2.5.

Figure 2.4 An oscilloscope representation of normal 50 and 60 Hz in

relation to increased frequency rate.

The AC Sine Wave

In some of the following sections we will be seeing alternating

current waveforms which represent the current flow in a

circuit. The drawing in the first part of Figure 2.5 is what

would be seen on an oscilloscope connected to a wall recep-

tacle and shows the AC waveform known as a sine wave. The

other two types of waveforms that will be discussed are

Squarewave and Advanced Squarewave. Figure 2.5 shows a

comparison of these three waveforms. These waveforms

represent the current flow as it builds in amount and time in

the positive direction and then decreases in value and finally

reaches zero. Then current changes direction and polarity

reaching a maximum negative value before rising to the zero

value. This “hill” (positive half) and “valley” (negative half)

together represent one cycle of alternating current. This is

true no matter what the waveform is. Note however, the

amount of time at each half cycle is not adjustable on the sine

wave power sources. Also notice the reduced current high

points with either of Squarewave type power sources.

Advanced Squarewave

–

Figure 2.7 Advanced Squarewave superimposed over a sine wave.

Advanced Squarewave allows additional control over the

alternating current waveforms. Figure 2.7 shows an AC sine

wave and an Advanced Squarewave superimposed over it.

Squarewave machines allow us to change the amount of time

within each cycle that the machine is outputting electrode

positive or electrode negative current flow. This is known

as balance control. They also reduce arc rectification and

resultant tungsten spitting. With Advanced Squarewave

technology, AC power sources incorporate fast switching

electronics capable of switching current up to 50,000 times

per second, thus allowing the inverter type power source to

be much more responsive to the needs of the welding arc.

These electronic switches allow for the switching of the

direction the output welding current will be traveling. The

output frequency of Squarewave or sine wave power sources

is limited to 60 cycles per second, the same as the input

power from the power company. With this technology and

200

Sine

Wave

Square

Wave

Advanced

Square

Wave

Figure 2.5 Comparison of the three different AC waveforms all

representing a time balanced condition and operating at 200 amperes.

Direct Current Electrode Negative

(Nonstandard Term is Straight Polarity)

advancements in design, the positive and negative amplitude

of the waveform can be controlled independently as well as

the ability to change the number of cycles per second.

Alternating current is made up of direct current electrode

negative (DCEN) and direct current electrode positive

(DCEP). To better understand all the implications this has on

AC TIG welding, let’s take a closer look at DCEN and DCEP.

WELDING

POWER

SUPPLY

1/16" ELECTRODE

Direct Current

Direct current (DC) is an electrical current that flows in one

direction only. Direct current can be compared to water flowing

through a pipe in one direction. Most welding power sources

are capable of welding with direct current output. They

accomplish this with internal circuitry that changes or rectifies

the AC into DC.

–

WORK

Figure 2.9 Direct current electrode negative.

Direct current electrode negative is used for TIG welding of

practically all metals. The torch is connected to the negative

terminal of the power source and the work lead is connected

to the positive terminal. Power sources with polarity switches

will have the output terminals marked electrode and work.

Internally, when the polarity switch is set for DCEN, this will

be the connection. When the arc is established, electron flow

is from the negative electrode to the positive workpiece. In a

DCEN arc, approximately 70% of the heat will be concentrated

at the positive side of the arc and the greatest amount of heat

is distributed into the workpiece. This accounts for the deep

penetration obtained when using DCEN for GTAW. The elec-

trode receives a smaller portion of the heat energy and will

operate at a lower temperature than when using alternating

current or direct current electrode positive polarity. This

accounts for the higher current carrying capacity of a given

size tungsten electrode with DCEN than with DCEP or AC. At the

same time the electrons are striking the work, the positively

charged gas ions are attracted toward the negative electrode.

Figure 2.8 shows what one cycle of AC sine wave power

would look like and what it would look like after it has been

rectified into DC power.

180˚

360˚

0˚

Alternating Current

Single Phase Direct Current

(Rectified AC)

Figure 2.8 Single-phase AC — single-phase direct current (rectified AC).

Polarity

Earlier in this section it was stated how the earliest welders

used batteries for their welding power sources. These early

welders found there were profound differences in the welding

arc and the resulting weld beads when they changed the battery

connections. This polarity is best described by what electrical

charge the electrode is connected for, such as direct current

electrode negative (DCEN) or direct current electrode positive

(DCEP). The workpiece would obviously be connected to the

opposite electrical charge in order to complete the circuit.

Review Figure 2.2.

When GTAW welding, the welder has three choices of welding

current type and polarity. They are: direct current electrode

negative, direct current electrode positive and alternating

current. Alternating current, as we are beginning to under-

stand, is actually a combination of both electrode negative

and electrode positive polarity. Each of these current types

has its applications, its advantages, and its disadvantages.

A look at each type and its uses will help the welder select the

best current type for the job. Figures 2.9 and 2.11 illustrate

power supply connections for each current type in a typical

100 amp circuit.

Figure 2.10 GTAW with DCEN produces deep penetration because it

concentrates the heat in the joint area. No cleaning action occurs with this polarity.

The heat generated by the arc using this polarity occurs in the workpiece,

thus a smaller electrode can be used as well as a smaller gas cup and reduced

gas flow. The more concentrated arc allows for faster travel speeds.

Direct Current Electrode Positive

(Nonstandard Term is Reverse Polarity)

used. The positively charged gas ions which were flowing

from the workpiece to the tungsten when welding with DCEN

are now flowing from the tungsten to the negative workpiece

with DCEP. They strike the workpiece with sufficient force to

break up and chip away the brittle aluminum oxide, and

provide what is called a cleaning action. Because of this

beneficial oxide removal, this polarity would seem to be

excellent for welding aluminum and magnesium. There are,

however, some disadvantages.

WELDING

POWER

SUPPLY

1/4" ELECTRODE

–

GAS

IONS

–

ELECTRONS

For example, to weld at 100 amperes it would take a tungsten

1/4" in diameter. This large electrode would naturally produce

a wide pool resulting in the heat being widely spread over the

joint area. Because most of the heat is now being generated

at the electrode rather than the workpiece, the resulting

penetration would probably prove to be insufficient. If DCEN

were being used at 100 amperes, a tungsten electrode of

1/16" would be sufficient. This smaller electrode would

also concentrate the heat into a smaller area resulting in

satisfactory penetration.

–

WORK

Figure 2.11 Direct current electrode positive.

When welding with direct current electrode positive (DCEP),

the torch is connected to the positive terminal on the welding

power source and the ground or work lead is connected to

the negative terminal. Power sources with polarity switches

will have the output terminals marked electrode and work.

Internally, when the polarity switch is set for DCEP, this will

be the connection. When using this polarity, the electron flow

is still from negative to positive, however the electrode is now

the positive side of the arc and the work is the negative side.

The electrons are now leaving the work. Approximately 70%

of the heat will be concentrated at the positive side of the arc;

therefore, the greatest amount of heat is distributed into the

electrode. Since the electrode receives the greatest amount of

heat and becomes very hot, the electrode must be very large

even when low amperages are used, to prevent overheating

and possible melting. The workpiece receives a smaller

amount of the total heat resulting in shallow penetration.

Another disadvantage of this polarity is that due to magnetic

forces the arc will sometimes wander from side to side when

making a fillet weld when two pieces of metal are at a close

angle to one another. This phenomena is similar to what is

known as arc blow and can occur in DCEN, but DCEP polarity

is more susceptible.

The good penetration of electrode negative plus the cleaning

action of electrode positive would seem to be the best

combination for welding aluminum. To obtain the advantages

of both polarities, alternating current can be used.

Figure 2.12 GTAW with DCEP produces good cleaning action as the argon

gas ions flowing toward the work strike with sufficient force to break up

oxides on the surface. Since the electrons flowing toward the electrode

cause a heating effect at the electrode, weld penetration is shallow.

Because of the lack of penetration and the required use of very large

tungsten, continuous use of this polarity is rarely used for GTAW.

At this point, one might wonder how this polarity could be of

any use in GTAW. The answer lies in the fact that some non-

ferrous metals, such as aluminum and magnesium, quickly

form an oxide coating when exposed to the atmosphere. This

material is formed in the same way rust accumulates on iron.

It’s a result of the interaction of the material with oxygen. The

oxide that forms on aluminum, however, is one of the hardest

materials known to man. Before aluminum can be welded,

this oxide, because it has a much higher melting point than

the base metal, must be removed. The oxide can be removed

by mechanical means like wire brushing or with a chemical

cleaner, but as soon as the cleaning is stopped the oxides

begin forming again. It is advantageous to have cleaning

done continuously while the welding is being done.

Figure 2.13 GTAW with AC combines the good weld penetration of DCEN

with the desired cleaning action of DCEP. With certain types of AC waveforms

high frequency helps re-establish the arc, which breaks each half cycle.

Medium size tungstens are generally used with this process.

The oxide can be removed by the welding arc during the

welding process when direct current electrode positive is

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