Gas Timgstem Arc Welding Ew-470

104 Pages·1995·6.44 MB·English

EW-470
TECHNICAL
GUIDE
Gas Tungsten Arc Welding
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Table of Contents
Chapter Page
1. Introduction to the Process . . . . . . . . . . . . . . . . . . . . . . . . . 1
2. Principles of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Equipment for Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Electrodes, Shielding Gas, and Filler Metals . . . . . . . . . . . . . . . 15
5. Welding Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6. Cost of Gas Tungsten Arc Welding . . . . . . . . . . . . . . . . . . . . 30
7. Welding Metallurgy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
8. Weld and Joint Design . . . . . . . . . . . . . . . . . . . . . . . . . . 46
9. Welding Procedure Variables . . . . . . . . . . . . . . . . . . . . . . . 60
10. Welding Procedure Schedules . . . . . . . . . . . . . . . . . . . . . . 66
11. Preweld Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
12. Welding Discontinuities and Defects . . . . . . . . . . . . . . . . . . . 77
13. Postweld Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
14. Welder Training and Qualification . . . . . . . . . . . . . . . . . . . . . 84
15. Welding Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
Appendix: Sources for Standards . . . . . . . . . . . . . . . . . . . . 102
© 2012, 2002, 1995 Hobart Institute of Welding Technology
ISBN 978-1-936058-15-0
This publication includes information available at the time of production. The Hobart Institute of Welding Technology presents
this information as a guideline. Relevant standards may have been updated and should be reviewed together with this book
for accuracy. Federal or other laws and standards may govern different operations and facilities.

Hobart Institute of Welding Technology disclaims liability for any injury to persons or to property, or other damages of any na-
ture whatsoever, whether special, indirect, consequential or compensatory, directly or indirectly resulting from the publication,
use of or reliance on this book. Hobart Institute of Welding Technology makes no guarantee or warranty as to the accuracy
or completeness of any information published herein.
CHAPTER 1
INTRODUCTION TO THE PROCESS
Gas tungsten arc welding (GTAW) is an arc welding pro- Methods of Application
cess which produces coalescence of metals by heating
them with an arc between a tungsten (nonconsumable) The gas tungsten arc welding process can be applied
electrode and the work. Shielding is obtained from a gas by the manual, semiautomatic, machine, or automatic
or gas mixture. Both pressure and filler metal may or may methods. The manual method of applying is used for the
not be used. This process is sometimes called TIG weld- greatest majority of work. It is done when the torch is op-
ing, which indicates “tungsten inert gas welding.” In Eu- erated by hand and filler metal, if used, is added by hand.
rope, it is called WIG welding, using Wolfgram, the Ger- A foot pedal that controls the amount of welding current
man word for tungsten. and switches the current on and off, is a refinement that
can be added to manual welding. The process allows the
The gas tungsten arc welding process is very versatile. welder extreme control for precision work. Heat can be
This process may be used to weld ferrous and a wide controlled very closely and the arc can be accurately di-
variety of nonferrous metals. It is an all-position weld- rected.
ing process. Welding in other than flat positions depends
on the base metal, the welding current and the skill of This process can also be operated semiautomatically,
the welder. The process was originally developed for the where the torch is operated by hand and the filler metal
“hard-to-weld” metals and can be used to weld more dif- is added automatically by a wire feeder. Semiautomatic
ferent kinds of metals than any other arc welding pro- gas tungsten arc welding is rarely used, however, and the
cess. machine and automatic methods are becoming increas-
ingly popular for many applications.
Gas tungsten arc welding has an arc and a weld pool
that are clearly visible to the welder. No slag is produced Machine welding occurs when the equipment performs
which might otherwise be entrapped in the weld. There the welding under the control and observation of the
is no filler metal carried across the arc, so there is little or welding operator.
no spatter. Because the electrode is nonconsumable, a
weld can be made by fusing the base metal without the Automatic welding occurs when the equipment performs
addition of a filler wire. the welding without adjustment or control by a welding
operator. The amount of automation or mechanization
Commercially developed for the welding of magnesium applied to the process depends on the accessibility of
and aluminum in the 1930’s for the aircraft industry, gas the joint, quality control requirements, number of identi-
tungsten arc welding made use of helium shielding gas cal welds to be made, and the availability of capital.
and a tungsten electrode. At first, only direct current with
a positive electrode was used. However, the electrode Advantages of Gas Tungsten Arc Welding
tended to overheat and deposit particles of the tungsten
electrode in the weld. It was then discovered that the Gas tungsten arc welding has many advantages over
overheating problem would be avoided by making the most other types of welding processes. The outstanding
electrode negative. This made it satisfactory for welding features are:
stainless steel.
During World War ll, welding machines
producing alternating current and high
frequency stabilization were developed.
Alternating current with a superimposed Gas Cup
high frequency, high voltage current over Shielding Gas
the basic welding current achieved good Tungsten Electrode
quality welding of aluminum and magne- Molten Weld Metal
Arc
sium. Helium, which was used in the early
Solidified
development stages, was largely replaced
Weld Metal
by argon, due to the greater availability of Filler Rod
argon. The gas tungsten arc welding pro-
cess became more widely accepted by the Base Metal
early 1950’s. The process is classified by
the American Welding Society as the gas
tungsten arc welding process.
Illustration 1-1 – Gas Tungsten Arc Welding
– 1 –
1) It will make high-quality welds in almost all metals
and alloys
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
Limitations of Gas Tungsten Arc Welding
The limitations of the gas tungsten arc welding process
include:
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 which
can blow the stream of shielding gas away from
the arc
– 2 –
CHAPTER 2
PRINCIPLES OF OPERATION
The gas tungsten arc welding process uses the heat
produced by an arc between a nonconsumable tung-
True Constant
Open Circuit Voltage
sten electrode and the base metal. The molten weld Current
metal, heated weld zone, and nonconsumable electrode
are shielded from the atmosphere by an inert shielding
gas that is supplied through the torch. The shielding gas Steep Slope – no
current change from
displaces the air, so that the oxygen and the nitrogen of S) short to long arc
the air do not come in contact with the molten metal or T length
L
the hot tungsten electrode. An electric arc is produced O
V
by an electric current passing through an ionized gas. E (
G Flatter Slope –
In this process, the inert gas atoms are ionized by los- A considerable current Moderate Slope –
ing electrons and leaving a positive charge. The positive OLT clehnagntghe with arc scohmaneg ceu wrriethn ta rc
gas ions then flow from the positive pole to the negative V length change
D
pole and the electrons flow from the negative pole to L
E
the positive pole of the arc. The intense heat developed W Long Arc Length
by the arc melts the base metal and filler metal (if used)
Normal Arc Length
to make the weld. As the molten metal cools, coales-
cence occurs and the parts are joined. There is little or Short Arc Length
no spatter or smoke. The resulting weld is smooth and Short Circuit
Current
uniform and requires minimum finishing.
Filler metal is not added when thinner materials, edge WELD CURRENT (AMPERES)
joints, or flange joints are welded. This is known at au-
Illustration 2-1 – Typical Volt-Ampere Curves for
togenous welding. For thicker materials an externally
Constant Current Types of Power Sources
fed or “cold” filler rod is generally used. The filler metal
in gas tungsten arc welding is not transferred across
the arc, but is melted by it. is reduced as the current increases. The changing arc
length causes the arc voltage to increase or decrease
The arc is struck in one of three ways. The first method is
slightly, which in turn changes the welding current. With-
by briefly touching the electrode to the work and quickly
in the welding range, the steeper the slope of the volt-
withdrawing it a short distance. The second method is by
ampere curve, the smaller the current change for a given
using an apparatus that will cause the arc to jump from
change in the arc voltage. Illustration 2-1 shows volt-am-
the electrode to the work. The third method is by using an
pere curves for different welding machine performance
apparatus that starts and maintains a small pilot arc. This
characteristics. This shows several slopes, all of which
pilot arc provides an ionized path from the main arc.
can provide the same normal voltage and current.
The torch then progresses along the weld joint manually
The variations in power sources are caused by differenc-
or mechanically. The torch is left in one place until a weld
es in the basic power source design. A machine with a
puddle is formed. Once adequate fusion has been ob-
higher short circuit current will give more positive starting.
tained at one point, the torch is moved along the joint
A steep volt-ampere characteristic is generally the most
so that the adjoining edges are joined. The weld metal
desirable when the welder wants to achieve maximum
solidifies behind the arc along the joint and completes
welding speeds on some welding jobs. The steeper slope
the welding process.
gives less current variation with changing arc length and
gives softer arc. The types of machines that have this
Arc Systems
kind of curve are especially useful on sheet metal.
The gas tungsten arc welding process uses the constant
These types of machines are also typically used for weld-
current power source, which may produce either direct or
ing at high current levels. On some applications, a less
alternating current. A constant current welding machine
steep volt-ampere characteristic is desired, such as for
provides nearly constant current during welding. Both
all position pipe welding where better arc control with
shielded metal arc welding and gas tungsten arc weld-
high penetration capability is desired. Machines with a
ing can be operated with the same power supply, except
less steep volt-ampere curve are also easier to use for
that a high frequency attachment, often added for gas
depositing the root passes on joints that have varying fit-
tungsten arc welding, is not needed for scratch starting
up. This power source characteristic allows the welder to
the arc.
control the welding current in a specific range by chang-
The constant current output is obtained with a drooping ing the arc length. This type of machine also produces a
volt-ampere characteristic, which means that the voltage more driving arc.
– 3 –
CHAPTER 3
EQUIPMENT FOR WELDING
The equipment used for the gas tungsten arc welding tungsten arc welding have a 60% duty cycle. For the ma-
process consists of a power source, a welding torch, an chine and automatic methods, a welding machine with
electrode, welding cables and a gas shielding system. 100% duty cycle rating would be best, but these are not
The non-consumable electrodes used for this process will normally available.
be discussed in Chapter 4. There are several items of op-
tional equipment that are often used. These include water The formula for determining the duty cycle of a welding
circulators, foot rheostats, programmers, motion devices, machine for a given current load is:
oscillators, automatic voltage controls (AVC), and wire Rated Current2
feeders. Since gas tungsten arc welding can be applied % Duty Cycle = X Rated Duty Cycle
Load Current2
by various methods, a wide variety of equipment configu-
rations are possible. Illustration 3-1 shows a diagram of For example, if a welding machine is rated at a 60% duty
the equipment used for a manual welding setup. cycle at 300 amperes, the duty cycle of the machine
when operated at 250 amperes would be:
POWER SOURCES 3002
% Duty Cycle = X 60 = 86%
2502
The purpose of the power source or welding machine is
to provide the electric power of the proper current and Illustration 3-2 represents the ratio of the square of the
voltage to maintain a welding arc. Several various siz- rated current to the square of the load current, multiplied
es and types of power sources have been designed for by the rated duty cycle. This chart can be used instead
gas tungsten arc welding. Most of these power sources of working out the formula. A line is drawn parallel to the
operate on 230 or 460 volt input electric power. Power sloping lines through the intersection of the subject ma-
sources that operate on 200 or 575 volt input power are chines rated current output and rated duty cycle. For ex-
available as options. ample, 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.
Power Source Duty Cycle
The chart shows that the machine can be safely used at
The duty cycle of a power source is defined as the ratio slightly over 230 amperes at a 100% duty cycle. Con-
of arc time to total time. For rating a welding machine, versely, there may be a need to draw more than the rated
a ten minute time period is used. Thus, for a machine current from a welding machine, but for a shorter period.
rated at a 60% duty cycle, the rated welding current load This illustration can be used to compare various ma-
could be safely applied continuously for six minutes and chines. All machines should be rated to the same duty
be off for four minutes. Most power sources used for gas cycle for comparison.
Rated Current2
% Duty Cycle = X Rated DInuetyr tC Gyaclse
Water Circulator Load Current2 Supply
Power
Source
Filler
Metal
Water Out Water In
Torch Shielding Gas
Electrode Lead
Work Lead
Base Metal
Foot Pedal (Optional)
Illustration 3-1 – Equipment for Gas Tungsten Arc Welding
– 4 –
800
700
600
) Power Source Rating:
S
E 500
R
600 Amps, 60% Duty Cycle
E
P
M 400
500 Amps, 60% Duty Cycle
A
(

T
N 400 Amps, 60% Duty Cycle
300
E
R
R
U
C 300 Amps, 60% Duty Cycle

G
200
N
I
D
L
E 200 Amps, 60% Duty Cycle
W 150
250 Amps, 30% Duty Cycle
L
A 295 Amps, 20% Duty Cycle
U
T
C
230 Amps, 20% Duty Cycle
A 100
90
80
70
20 30 40 50 60 70 80 90 100
% DUTY CYCLE
Illustration 3-2 – Duty Cycle vs Current Load
TYPES OF WELDING CURRENT
DCEN DCEP
The type of current available is determined by the type
(Straight Polarity) (Reverse Polarity)
of power source. The most important factor in selecting
Welding Welding
the type of current is the type of metal to be welded. Power SMALL LARGE Power
tungsten electrode tungsten electrode
The thickness of the metal can also have an influence. Source Source
Cathode Anode
Illustration 4-2 in the next chapter shows the type of cur-
rent used for welding various base metals along with the
types of electrodes and shielding gases used. Alternating
or direct current may be used for gas tungsten arc weld-
ing and high frequency arc ignition current is used with
Electrons Ions Electrons Ions
either alternating or direct current. The welding current ( ) ( ) ( ) ( )
Flow Flow Flow Flow
may also be pulsed.
Direct Current
Anode Cathode
Current Flow Current Flow
Direct current can be connected in one of two ways: Conventional Deep Penetration Shallow Penetration Conventional
(base metal
electrode negative (straight polarity) and electrode posi- Narrow Melted Area Wide Melted Area cleaning)
tive (reverse polarity). The electrically charged particles Approximate 30% Heat Approximate 70% Heat
in Electrode and 70% in Electrode and 30%
flow between the tip of the electrode and the work, as Heat in Base Metal Heat in Base Metal
shown in Illustration 3-3. The electrode negative connec-
tion can be used for welding all metals. Special proce-
dures must be used to weld alloys of magnesium and Illustration 3-3 – The Tungsten Arc.
aluminum which have a refractory surface oxide that Negative and Positive Polarity.
– 5 –
hinders their fusion. Welds can be made on aluminum
and magnesium with a short arc length using electrode
negative and a helium bearing shielding gas. The metals
can be more easily welded using electrode positive be-
cause this connection breaks down the oxide layers on
the surfaces. The main problem with using the electrode
positive connection is that the current carrying capacity
of the electrode is extremely low. The electrode will begin
to melt if the currents used are too high. For this reason,
electrode positive is rarely used, except for the welding
of thin sheet metal.
Pulsed Current
The pulsed current method of the gas tungsten arc weld-
ing process employs two levels of welding current instead
of a steady current. The welding current is switched pe-
riodically between the high and low levels to produce a
pulsating current or arc. A diagram of pulsed direct cur-
rent is shown in Illustration 3-4. This pulsed current pro-
duces a continuously welded seam consisting of over-
lapping arc spot welds. Illustration 3-5 shows a GTAW
weld produced with pulsed current. Each of the spots is
produced by the high level welding current after which
the current is switched to the lower level. This lower level
allows the weld to partially solidify between spots and
maintains the arc to avoid re-ignition problems. Pulsed Illustration 3-5 – Weld Produced by the
current may be used with direct or alternating current, Pulsed Current GTAW Process
but it is most commonly used with direct current.
ing, the high pulse provides high current for complete
The pulsed direct current method of gas tungsten arc penetration, but the low pulse cools the puddle down to
welding has several advantages over steady direct cur- prevent burning through at the root of the joint. Pulsing
rent for welding thin materials. The pulsed method is reduces the heat input to the base metal. This is par-
more tolerant of edge misalignment, normal fixturing can ticularly good for welding thin stainless steel sheet metal,
be used with thinner materials, and it gives better dis- which distorts very easily without pulsed current. Another
tortion control and root penetration. For open root weld- advantage of pulsed current is that it is very good for
)
s
p
m
a
(

t
n
e
r High Low High Low
r
u Pulse Pulse Pulse Pulse
C
Time Time Current Current
Time (sec)
Illustration 3-4 – Pulsed Current Terminology
– 6 –
welding in the vertical and overhead positions, because High-Frequency Current
good penetration can be obtained with less heat input.
Pulsing keeps the weld puddle from getting too large to The high-frequency current is separate, superimposed
control because of the partial solidification that occurs current that is used to maintain a pilot arc and help start
during the low current. the arc. The pilot arc does not do any welding but it is
needed to start the welding arc without touching the
The number of pulses used can vary from about ten per electrode to the work when using either direct or alter-
second down to about one or one-half per second. The nating current. When alternating current is used, the high
length of time the high current is on and the length of time frequency current keeps the arc from going out when the
the low current is on can be varied as well as the percent- alternating current is changing cycles, from either the
age of low current with respect to the high current. positive to the negative or the negative to the positive.
Alternating Current When direct current is used, the high frequency is only
needed to help start the arc and may be turned off after
Alternating current is a combination of both polarities the arc is established. The use of a high frequency cur-
that alternate in regular cycles. In each cycle, the cur- rent is the best starting method because touching the tip
rent starts at zero, builds up to a maximum value in one of the electrode to the work or starting on a piece of car-
direction, decays back to zero, builds up to a maximum bon can cause contamination of the tungsten electrode.
value in the other direction, and decays back to zero. The
arc goes out during the zero portion of the cycle, so a When this superimposed high frequency current is used
high frequency current in the welding circuit is used to with AC gas tungsten arc welding, certain precautions
re-ignite the arc. Using alternating current gives the ad- are required. These are necessary because the high fre-
vantages of direct current electrode positive (reverse po- quency spark gap oscillators in the power sources radiate
larity) without the current limitations and the advantages power at frequencies that can interfere with commercial,
of direct current electrode negative without the oxide police, and aviation radio broadcasts. This can also in-
cleaning problems. For this reason, alternating current is terfere with television transmissions. Because of this, the
generally used for the manual welding of aluminum and operation of high frequency for AC is subject to control
magnesium. by the Federal Communication Commission in the United
States. Most other countries have similar regulations.
In the alternating current circuit, there is a tendency for
the current to become unbalanced. The arc current flows Welding machines that use high frequency stabilizers
more easily in one direction because it takes greater en- must be installed with special attention to provide earth
ergy to obtain electrons from the base metal than from grounding and special shielding. Manufacturers provide
the tungsten electrode. The tungsten electrode emits special installation instructions which require that all
electrons more easily because it becomes much hotter metal conductors in the area of the machine must also
during welding than the base metal does. The amplitude be earth grounded. These requirements help to limit high
of the current in the cycle, when the electrode is nega- frequency radiation. If these instructions are followed
tive, is normally higher than it is during the cycle when carefully, the user can post a certificate stating the high
the electrode is positive. This tends to produce an un- frequency stabilizer may be reasonably expected to meet
balanced current. Either series connected capacitators FCC regulations.
or a direct current voltage inserted in the welding circuit
can be used to balance the current. Balanced current is
TYPES OF POWER SOURCES
desirable for some applications like high-speed mecha-
nized welding, but it is not necessary for most manual
Constant current (CC) machines can produce AC or DC
welding applications. Balanced current flow has three
welding power and can be rotating (generators) or static
main advantages:
(transformer/rectifier) or three phase inverter machines.
1) A better oxide-cleaning action
Generator and Alternator Welding Machines
2) Better and smoother welding action
The generator welding machine can be powered by an
3) There is no reduction in the output rating of a conven- electric motor, for shop use, or by an internal combustion
tional welding transformer engine (gasoline, gas or diesel) for field use. Generator
welding machines used for shielded metal arc welding
Disadvantages of a balanced current flow are: can be adjusted for gas tungsten arc welding if an inert
gas and high frequency attachment is added.
1) Larger electrodes are needed
Engine driven welding machines can also be adapted.
2) Wave balancing systems are more expensive These can be either water or air cooled engines. Many
– 7 –
DC current for the application. The transformer-rectifier
welding machines are available in different sizes. These
welding machines have several advantages over rotating
power sources which are:
1) Lower operating costs
2) Lower maintenance costs
3) Quieter operation
4) Lower power consumption while idling
5) No rotating parts
Inverter Welding Machines
These widely used welding machines use the inverter
and different levels of programming. They operate on
three phase input power. The three phase input helps
overcome the line unbalance that occurs with the single
phase transformer-rectifier machines. Inverters provide
power down to .5 ampere with a very fast response time
of one millisecond and less than 1% ripple, Different pro-
gramming is available, depending on the complexity of
the job. The high frequency inverters are very quiet and
Illustration 3-6 – 350 Ampere Transformer Rectifier
provide outstanding arc stability.
Welding Machine
Photo courtesy of Miller Electric Manufacturing Co.
Transformer Welding Machines
of them also provide auxiliary power for emergency light-
ing, power tools, etc. Generator welding machines can Transformer welding machines are not often used for
provide DC power, and in some cases, both AC and DC gas tungsten arc welding except for home shop use or
power, to the arc, depending on the machine design. a small job shop where gas tungsten arc welding is only
occasionally used. Transformer welding machines pro-
Alternator welding machines can also be adapted for duce AC power only and operate on single phase input
gas tungsten arc welding. These machines consist of power. Transformer welding machines used for shielded
an electric generator that is made to produce AC power. metal arc welding can be adapted for gas tungsten arc
These machines are also called rotating or revolving field welding by adding an inert gas and a high frequency at-
machines. tachment.
Transformer-Rectifier Welding Machines
The transformer-rectifier welding machines are much
more widely used for gas tungsten arc welding than mo-
tor generator welding machines. Transformer-rectifier
machines provide both AC and DC welding current to
the arc. A single phase transformer producing alternating
current is connected to the rectifier, which then produces
DC current for the arc. The rectifier is an electrical device
which changes alternating current into direct current.
Transformer-rectifier welding machines operate on single
phase input power.
Because of this, an unbalance may be created in the
power supply lines, which is objectionable to most pow-
er companies. This type of welding machine is the most
versatile for gas tungsten arc welding because it can be
used for welding of a variety of base metals. A program-
mable type of transformer-rectifier power source is of-
ten used for welding this process. By means of a switch Illustration 3-7 – 180 Ampere Inverter,
which can change the output terminals to the transform- Direct Current Arc Welding Package
er or to the rectifier, the welder can select either AC or Photo courtesy of Miller Electric Manufacturing Co.
– 8 –