Chapter 9 Gas Tungsten Arc Welding

147 Pages·2011·12.01 MB·English

Chapter 9
Gas Tungsten Arc Welding
Topics
1.0.0 Introduction to the Process
2.0.0 Principles of Operation
3.0.0 Equipment for Welding
4.0.0 Equipment Setup, Adjustment, and Shut Down
5.0.0 Electrodes, Shielding Gas, and Filler Metal
6.0.0 Welding Applications
7.0.0 Welding Metallurgy
8.0.0 Weld Joint Design
9.0.0 Welding Procedure Variables
10.0.0 Welding Procedure Schedules
11.0.0 Preweld Preparations
12.0.0 Welding Discontinuities and Problems
13.0.0 Postweld Procedures
14.0.0 Welder Training and Qualification
15.0.0 Welding Safety

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Overview
The gas tungsten arc welding (GTAW) process, also known as tungsten inert gas (TIG)
welding, uses a non-consumable tungsten electrode to produce the weld. A shielding
gas (usually an inert gas such as argon), protects the weld area from atmospheric
contamination, and the process normally uses a filler metal, though some welds, known
as autogenous (aw-toj-uh-nuhs) welds, do not require a filler metal.
A constant-current welding power supply produces energy that is conducted across the
arc through a column of highly ionized gas and metal vapors known as plasma. Welders
most commonly use TIG to weld thin sections of stainless steel and non-ferrous metals
such as aluminum, magnesium, and copper alloys.
TIG provides the welder with greater control over the weld than competing procedures
such as shielded metal arc welding (SMAW) and gas metal arc welding (GMAW), thus
allowing for stronger, higher quality welds. However, GTAW/TIG is comparatively more
complex and difficult to master (closer tolerance requirements and filler metal usually
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added by other hand), and is significantly slower than most other welding techniques as
well.
This chapter will present a basic understanding of the GTAW/TIG process and
equipment, along with the key variables that affect the quality of welds. It will also cover
core competencies such as setting up equipment, preparing materials, fitting up, starting
an arc, welding pipes and plates, and repairing welds. Lastly, you will get an
understanding of the safety precautions for GTAW/TIG and an awareness of the
importance of safety in welding.
Although this chapter is very comprehensive, always refer to the manufacturer’s
manuals for specific operating and maintenance instructions.
Objectives
When you have completed this chapter, you will be able to do the following:
1. Describe the process of gas tungsten arc welding.
2. Describe the principles of operation used for gas tungsten arc welding.
3. Describe the equipment associated with gas tungsten arc welding.
4. Describe the processes for installation, setup, and maintenance of equipment for
gas tungsten arc welding.
5. State the shielding gas and electrodes for gas tungsten arc welding.
6. Identify the welding applications for gas tungsten arc welding.
7. Describe the welding metallurgy of gas tungsten arc welding.
8. Identify weld and joint designs used for gas tungsten arc welding.
9. Describe the welding procedure variables associated with gas tungsten arc
welding.
10. Identify welding procedure schedules used for gas tungsten arc welding.
11. Describe preweld preparations for gas tungsten arc welding.
12. Identify defects and problems associated with gas tungsten arc welding.
13. Describe postweld procedures for gas tungsten arc welding.
14. State the welder training and qualifications associated with gas tungsten arc
welding.
15. Describe the welding safety associated with gas tungsten arc welding.
Prerequisites
None
This course map shows all of the chapters in Steelworker Basic. The suggested training
order begins at the bottom and proceeds up. Skill levels increase as you advance on
the course map.
NAVEDTRA 14250A 9-2
Introduction to Reinforcing Steel

Introduction to Structural Steel
S
Pre-Engineered Structures:
T
Buildings, K-Spans, Towers and Antennas
E
Rigging
E
Wire rope
L
Fiber Line W
O
Layout and Fabrication of Sheet-Metal and Fiberglass Duct
R
Welding Quality Control
K
Flux Core Arc Welding-FCAW
E
R
Gas-Metal Arc Welding-GMAW

Gas-Tungsten Arc Welding-GTAW
B
Shielded Metal Arc Welding-SMAW
A
Plasma Arc Cutting Operations S
I
Soldering, Brazing, Braze Welding, Wearfacing
C
Gas Welding

Gas Cutting

Introduction to Welding
Basic Heat Treatment
Introduction to Types and Identification of Metal
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1.0.0 INTRODUCTION to the PROCESS
Gas tungsten arc welding (GTAW) is an arc welding process that produces coalescence
of metals by heating them with an arc between a tungsten (non-consumable) electrode
and the work. Shielding comes from a gas or gas mixture (Figure 9-1). Both pressure
and filler metal may or may not be used. This process is also known as TIG welding,
which stands for tungsten inert gas welding, unless you are on deployment in Europe,
where you may hear it called WIG welding, using Wolfgram, the German word for
tungsten. Throughout this chapter, the process will be referred to as TIG.
Figure 9-1 — Gas tungsten arc welding.
The gas tungsten arc welding process is very versatile. This process may be used to
weld f e r r o u s and a wide variety of n o n - f e r r o u s metals. It is an all-position welding
process. Welding in other than flat positions depends on the base metal, the welding
current, and the skill of the welder. The process was developed for the "hard-to-weld"
metals and can be used to weld more different kinds of metals than any other arc
welding process.
Gas tungsten arc welding has an arc and a weld pool clearly visible to the welder. It
produces no slag for entrapment in the weld, and no filler metal carries across the arc,
so there is little or no spatter. Because the electrode is non-consumable, you can make
a TIG weld by fusing the base metal without a filler wire.
The TIG welding process was invented by Russell Meredith of Northrop Aircraft’s
welding group in 1941. Mr. Jack Northrop's dream was to build a magnesium airframe
for lighter, faster warplanes. This new process was called "Heliarc," as it used an
electric arc to melt the base material and helium (He) to shield the molten puddle. The
Linde Division of Union Carbide bought the patents, developed a number of torches for
different applications, and sold them under the brand name Heliarc. Linde also
developed procedures for using argon (Ar) gas, a more readily available and less
expensive gas than helium.
At first, only direct current with a positive electrode was used. However, the electrode
tended to overheat and deposit particles of the tungsten electrode in the weld. This
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problem was overcome by making the electrode negative, which then also made it
satisfactory for welding stainless steel.
During World War II, welding machines producing alternating current and high
frequency stabilization were developed. Alternating current with a superimposed high
frequency, high voltage current over the basic welding current achieved good quality
welding of aluminum and magnesium. With helium largely replaced by argon due to its
greater availability, the gas tungsten arc welding process became more widely accepted
by the early 1950s, and today is classified by the American Welding Society by that
term.
1.1.0 Methods of Application
Welders can apply the gas tungsten arc welding process by the manual, semiautomatic,
machine, or automatic methods, although the manual method produces the greatest
majority of work; the torch is operated by hand, and filler metal, if used, is added with
the other hand. A foot pedal is an additional refinement that controls the amount of
welding current and switches the current on and off. TIG allows the welder extreme
control for precision work by very closely controlling the heat and accurately directing
the arc.
Operators can also use TIG semi automatically, that is by operating the torch by hand
with a wire feeder adding the filler metal automatically. Semiautomatic gas tungsten arc
welding is rarely used; however, machine and automatic methods are becoming
increasingly popular for many applications.
TIG machine welding occurs when equipment performs the welding only under the
control and observation of the welding operator.
Automatic welding occurs when the equipment performs the welding without adjustment
or control by a welding operator. The amount of automation or mechanization applied to
the process depends on the accessibility of the joint, quality control requirements,
number of identical welds to be made, and the availability of capital.
1.2.0 Advantages and Limitations
TIG welding generally produces welds far superior to those produced by metallic arc
welding electrodes. Especially useful for welding aluminum, it is quite useful for welding
many other types of metals as well. The TIG process is most effective for joining metals
up to 1/8 inch thick, although you can use it to weld thicker material with appropriate
preheating.
Gas tungsten arc welding has many advantages over most other types of welding
processes. The outstanding features are the following:
1. It makes high quality welds in almost all metals and a lloys.
2. There is no slag, so very little, if any, postweld cleaning is required.
3. There is no filler metal carried across the arc, so there is little or no spatter.
4. Welding can be performed in all positions.
5. Filler metal is not always required.
6. Pulsing may be used to reduce the heat input.
7. The arc and weld pool are clearly visible to the welder.
8. Because the filler metal does not cross the arc, the amount added is not
dependent on the weld current level.
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The limitations of the gas tungsten arc welding process include the following:
1. The welding speed is relatively slow.
2. The electrode is easily contaminated.
3. It is not very efficient for welding thick sections because deposition rates are low.
4. The arc requires protection from wind drafts that can blow the stream of shielding
gas away from the arc.
2.0.0 PRINCIPLES of OPERATION
TIG uses the heat produced by the arc between the non-consumable tungsten electrode
and the base metal. An inert shielding gas supplied through the torch shields the molten
weld metal, heated weld zone, and non-consumable electrode from the atmosphere.
The gas protects the electrode and molten material from oxidation, and provides a
conducting path for the arc current.
An electric current passing through an ionized gas produces an electric arc. In this
process, the inert gas atoms are ionized by losing electrons and leaving a positive
charge. Then the positive gas ions flow to the negative pole and the negative electrons
flow to the positive pole of the arc. The
intense heat developed by the arc melts the
base metal and filler metal (if used) to make
the weld. As the molten metal cools,
coalescence occurs and the parts join.
There is little or no spatter or smoke. The
resulting weld is smooth and uniform, and
requires minimum finishing (Figure 9-2).
You do not need to add filler metal when
welding thinner materials, edge joints, or
flange joints. This is known as autogenous
welding. For thicker materials, an externally
fed or "cold" filler rod is generally used. The
filler metal in gas tungsten arc welding does
not transfer across the arc, but is melted by
it.
You strike the arc in one of three ways:
Figure 9-2 — TIG process.
1. By briefly touching the electrode to
the work and quickly withdrawing it a short distance.
2. By using an apparatus that will cause the arc to jump from the electrode to the
work.
3. By using an apparatus that starts and maintains a small pilot arc. This pilot arc
provides an ionized path from the main arc.
The torch then progresses along the weld joint manually or mechanically after remaining
in one place until a weld puddle forms. Once the welder obtains adequate fusion, the
torch moves along the joint so the adjacent edges join and the weld metal solidifies
along the joint behind the arc, thus completing the welding process.

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2.1.0 Arc Systems
The TIG process uses a constant current power source, either direct or alternating
current. A constant current welding machine provides nearly constant current during
welding, so both stick (SMAW) and TIG (GTAW) can operate from the same power
supply. The exception is that you do not need a high frequency attachment, often added
for gas tungsten arc welding, to scratch start the arc.
The constant current output is obtained with
a drooping volt-ampere characteristic, which
means that the voltage is reduced as the
current increases. The changing arc length
causes the arc voltage to increase or
decrease slightly, which in turn changes the
welding current. Within the welding range,
the steeper the slope of the volt-ampere
curve, the smaller the current change for a
given change in the arc voltage. Figure 9-3
shows volt-ampere curves for different
welding machine performance
characteristics. This shows several slopes,
all of which can provide the same normal
voltage and current.
Differences in the basic power source
design cause the variations in power
sources. A machine with a higher short Figure 9-3 — Volt-ampere curves.
circuit current will give more positive
starting. A steep volt-ampere characteristic is generally the most desirable when the
welder wants to achieve maximum welding speeds on some welding jobs. The steeper
slope gives less current variation with changing arc length, and gives a softer arc.
The types of machines that have this kind of curve are especially useful on sheet metal.
These types of machines are also typically used for welding at high current levels. On
some applications, such as all-position pipe welding, a welder may want a less steep
volt-ampere characteristic for better arc control with high penetration capability.
Machines with a less steep volt-ampere curve are also easier to use for depositing the
root passes on joints that have varying fitup. This power source characteristic allows the
welder to control the welding current in a specific range by changing the arc length. This
type of machine also produces a more driving arc.
Test your Knowledge (Select the Correct Response)
1. The predominant shielding gas used for TIG is _____.

A. O
2
B. NO
2
C. Ar
D. He
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2. How is the arc struck using the manual TIG process?

A. Holding the electrode to the work until a puddle is formed.
B. Briefly tapping the electrode on the work.
C. Depressing the torch trigger and the arc will start.
D. Clipping the grounding strap on the workpiece.
3.0.0 EQUIPMENT for WELDING
A typical TIG welding system usually consists of the following elements:
1. Welding power supply
2. Welding torch
3. Tungsten electrode
4. Welding cables
5. Gas shielding system
Since welders can apply TIG by various methods with a wide variety of equipment
configurations, often they will include several available items of optional equipment such
as water circulators, foot rheostats, programmers, motion devices, oscillators, automatic
voltage controls (AVC), and wire feeders. Figure 9-4 shows a diagram of the equipment
used for a manual welding setup.
Figure 9-4 — Equipment for gas tungsten arc welding.
3.1.0 Power Sources
The purpose of the power source or welding machine is to provide the electric power of
the proper current and voltage to maintain a welding arc. Manufacturers offer several
various sizes and types of power sources for gas tungsten arc welding. Most of these
power sources operate on 230 or 460 volt input electric power. Power sources that
operate on 200 or 575 volt input power are available as options.
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3.1.1 Power Source Duty Cycle
The duty cycle of a power source is defined as the ratio of arc time to total time. For
rating a welding machine, a ten minute time period is used. Thus, for a machine rated at
a 60% duty cycle, the rated welding current load could be safely applied continuously
for six minutes and be off for four minutes. Most power sources used for gas tungsten
arc welding have a 60% duty cycle. For the machine and automatic methods, a welding
machine with 100% duty cycle rating would be best, but these are not normally
available.
The formula for determining the duty cycle of a welding machine for a given current load
is:

For example, if a welding machine is rated at a 60% duty cycle at 300 amperes, the
duty cycle of the machine when operated at 250 amperes would be:

Figure 9-5 represents the ratio of the square of the rated current to the square of the
load current, multiplied by the rated duty cycle. This chart can be used instead of
working out the formula. A line is drawn parallel to the sloping lines through the
intersection of the subject machine’s rated current output and rated duty cycle. For
example, a question might arise whether a 300 amp 60% duty cycle machine could be
used for a fully automatic requirement of 225 amps for a 10-minute welding job. The
chart shows that the machine can be safely used at slightly over 230 amperes at a
100% duty cycle. Conversely, there may be a need to draw more than the rated current
from a welding machine, but for a shorter period. This graph can be used to compare
various machines. All machines should be rated to the same duty cycle for comparison.
Figure 9-5 — Duty cycle vs. current load.
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