TIGBook_Chpt1.pdf

I. The GTAW (TIG) Process

The necessary heat for Gas Tungsten Arc Welding (TIG)

is produced by an electric arc maintained between a

nonconsumable tungsten electrode and the part to be welded.

The heat-affected zone, the molten metal, and the tungsten

electrode are all shielded from the atmosphere by a blanket of

inert gas fed through the GTAW torch. Inert gas is that which

is inactive, or deficient in active chemical properties. The

shielding gas serves to blanket the weld and exclude the

active properties in the surrounding air. It does not burn, and

adds nothing to or takes anything from the metal. Inert gases

such as argon and helium do not chemically react or combine

with other gases. They possess no odor and are transparent,

permitting the welder maximum visibility of the arc. In some

instances a small amount of reactive gas such as hydrogen

can be added to enhance travel speeds.

Advantages of the GTAW Process

The greatest advantage of the GTAW process is that it will

weld more kinds of metals and metal alloys than any other arc

welding process. TIG can be used to weld most steels including

stainless steel, nickel alloys such as Monel ® and Inconel ® ,

titanium, aluminum, magnesium, copper, brass, bronze, and

even gold. GTAW can also weld dissimilar metals to one

another such as copper to brass and stainless to mild steel.

Concentrated Arc

The concentrated nature of the GTAW arc permits pin point

control of heat input to the workpiece resulting in a narrow

heat-affected zone. A high concentration of heat is an advantage

when welding metals with high heat conductivity such as

aluminum and copper. A narrow heat-affected zone is an

advantage because this is where the base metal has undergone

a change due to the superheating of the arc and fast cooling

rate. The heat-affected zone is where the welded joint is

weakest and is the area along the edge of a properly made

weld that would be expected to break under a destructive test.

The GTAW process can produce temperatures of up to

35,000˚ F / 19,426˚ C. The torch contributes only heat to the

workpiece. If filler metal is required to make the weld, it may

be added manually in the same manner as it is added in the

oxyacetylene welding process. There are also a number of

filler metal feeding systems available to accomplish the task

automatically. Figure 1.1 shows the essentials of the manual

GTAW process.

Gas

Valve

Regulator/

Flowmeter

Power Source

Coolant System

Power Cord

Shielding

Gas

Remote Control

Gas

Out

Work Cable

Work

Clamp

Coolant In

Adapter

Block

Coolant Out

To r c h

Coolant System

Work

Figure 1.1 Essentials of the GTAW process (water cooled).

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.

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