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Gas Tungsten Arc Welding

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Title: Gas Tungsten Arc Welding


1
Gas Tungsten Arc Welding
2
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3
Objectives
  • Describe the gas tungsten arc welding process
  • List other terms used to describe it
  • What makes tungsten a good electrode
  • Eliminate tungsten erosion
  • Shape and clean a tungsten electrode
  • Grind a point on a tungsten electrode

4
Objectives (continued)
  • Remove a contaminated tungsten end
  • Melt the end of the tungsten electrode into the
    desired shape
  • Compare water-cooled GTA welding torches to
    air-cooled torches
  • The purpose of the three hoses connecting a
    water-cooled torch to the welding machine

5
Objectives (continued)
  • Choose an appropriate nozzle
  • How to get an accurate reading on a flowmeter
  • Compare the three types of welding current used
    for GTA welding
  • Shielding gases used in the GTA welding process

6
Objectives (continued)
  • Define preflow and postflow
  • Problems resulting from an incorrect gas flow
    rate
  • Properly set up a GTA welder
  • Establish a GTA welding arc

7
Introduction
  • The Gas Tungsten Arc Welding (GTAW) process is
    sometimes referred to as a TIG or Heliarc
  • TIG is short for tungsten inert gas
  • An arc is established between a non-consumable
    tungsten electrode (heating element) and the base
    metal
  • The inert gas provides the needed arc
    characteristics and protects the molten weld pool
  • When Argon became plentiful, the GTA process
    became more common

8
Power Source Basics
9
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10
Tungsten Electrodes
11
Tungsten
  • Tungsten has the following properties
  • High tensile strength
  • Hardness
  • High melting temperature
  • High boiling temperature
  • Good electrical conductivity

12
Tungsten (continued)
  • Tungsten is the best choice for a non consumable
    electrode
  • High melting temperature
  • Good electrical conductivity
  • As the tungsten electrode becomes hot the arc
    between the electrode and the work stabilizes
  • But a clean and correctly ground tungsten is
    needed
  • Because of the intense heat some erosion of the
    electrode will occur

13
Figure 15-1 Some tungsten will erode and be
transferred across the arc.
14
Tungsten (continued)
  • Ways to limit erosion
  • Good mechanical and electrical contact
  • Use as low a current as possible
  • Use a water-cooled torch
  • Use as large a tungsten electrode as possible
  • Use DCEN current
  • Use as short an electrode extension as possible
  • Use the proper shape electrode
  • Use an alloyed tungsten electrode

15
Torch Build Out
16
Torch Build Out
17
Tungsten (continued)
  • The collet is the cone-shaped sleeve that holds
    the electrode in the torch
  • Large-diameter electrodes conduct more current
  • The current-carrying capacity at DCEN is about
    ten times greater than at DCEP
  • The preferred electrode shape impacts the
    temperature and erosion of the tungsten
  • With alternating current, the tip is subjected to
    more heat than with DCEN

18
Figure 15-3 The smooth surface of a centerless
ground tungsten electrode. Courtesy of Larry
Jeffus.
19
Types of Tungsten Electrodes
  • Pure tungsten is an excellent nonconsumable
    electrode
  • Pure tungsten can be improved by adding
  • Cerium
  • Lanthanum
  • Thorium
  • Zirconium

20
Tungsten Electrodes
21
Table 15-1 Tungsten Electrode Types and
Identification.
22
Shaping the Tungsten
  • To obtain the desired end shape
  • Grinding (for MS and SS)
  • Breaking (not recommended due to cost)
  • Re melting the end (Aluminium welding)
  • Using chemical compound (doesnt work that well)

23
Grinding
  • Often used to clean a contaminated tungsten or to
    point the end
  • Should have a fine, hard stone
  • A coarse grinding stone with result in more
    tungsten breakage
  • Should be used for grinding tungsten only
  • Metal particles will quickly break free when the
    arc is started, causing contamination

24
Figure 15-8 Correct way of holding a tungsten
when grinding. Courtesy of Larry Jeffus.
25
Breaking and Remelting
  • Tungsten is hard but brittle
  • If struck sharply, it will break without bending
  • Try not to do this because of
  • Holding against a sharp corner and hitting
    results in a square break
  • After breaking squarely, melt back the end

26
Chemical Cleaning and Pointing
  • Tungsten can be cleaned and pointed using one of
    several compounds
  • Heated by shorting it against the work
  • Dipped in the compound
  • When the tungsten is removed, cooled, and
    cleaned, the end will be tapered to a fine point
  • The chemical compound will dissolve the tungsten,
    allowing the contamination to fall free

27
Pointing and Remelting
  • Tapered tungsten with a balled end is made by
    first grinding or chemically pointing
  • The ball should be made large enough so that the
    color of the end stays dull red and bright red
  • Increase ball size by applying more current
  • Surface tension pulls the molten tungsten up onto
    the tapered end

28
Figure 15-14 Melting the tungsten end shape.
29
GTAW Equipment
30
GTA Welding EquipmentCadillac Stick Welder
  • GTA welding torches are water- or air-cooled
  • Water-cooled GTA welding torch is more efficient
  • Water-cooled torch has three hoses connecting it
    to the welding machine
  • Nozzle directs the shielding gas directly on the
    welding zone
  • Flowmeter regulates the rate of gas flow

31
Figure 15-21 Schematic of a GTA welding setup
with a water-cooled torch.
32
Types of Welding Current
  • DCEN concentrates about 2/3 of its welding heat
    on the work
  • Max penetration
  • High Freq. start only
  • DCEP concentrates about 1/3 of its welding heat
    on the work
  • Max cleaning action
  • 2/3 of heat at tungsten primarily used for
    balling tungsten for aluminium welding
  • High Freq. start only

33
Types of Welding Current
  • AC concentrates its heat at 50/50
  • Sign wave provides for DCRP (cleaning action) and
    DCSP (penetration action)
  • Square wave technology allows for adjusting the
    cleaning or penetration cycle.
  • High Freq. is on Continuous so there is equal
    firing of both sides of sign wave.
  • DC Component will take place if there is no High
    Freq.

34
Figure 15-29 Electrons collect under the oxide
layer during the DCEP portion of the cycle.
35
Figure 15-30 Sine wave of alternating current at
60 cycle.
36
Shielding Gas
37
Shielding Gases
  • Shielding gases used for GTA welding process
  • Argon (Ar)
  • Helium (He)
  • Or a mixture of two or more gases

38
Shielding Gases (continued)
  • Argon effectively shields welds in deep grooves
    in flat positions
  • Helium offers the advantage of deeper penetration

39
Shielding Gases (continued)
  • Hot start allows a surge of welding current
  • Preflow is the time gas flows to clear out air in
    the nozzle
  • Some machines do not have preflow
  • Postflow is the time the gas continues flowing
    after the welding current has stopped

40
Shielding Gases (continued)
  • Ionization Potential
  • Amount of voltage needed to kick start the arc
  • The ionization potential, or ionization energy,
    of a gas atom is the energy required to strip it
    of an electron. That is why a shielding gas such
    as helium, with only 2 electrons in its outer
    shell, requires more energy (higher voltage
    parameters) for welding. The ionization potential
    of a shielding gas also establishes how easily an
    arc will initiate and stabilize. A low ionization
    potential means the arc will start relatively
    easy and stabilize quite well. A high ionization
    potential has difficulty initiating and may have
    difficulty keeping the arc stable.
  • Argon
  • 15.7 electron volts
  • Helium
  • 24.4 electron volts
  • More penetration

41
Figure 15-35 Too steep an angle between the torch
and work may draw in air.
42
Remote ControlsFoot or Finger
43
Remote Controls
  • Can be used to
  • Start the weld
  • Increase the current
  • Decrease the current
  • Stop the weld
  • Remote can be foot-operated or hand-operated
    device

44
Welding Techniques
45
Objectives
  • Applications using the gas tungsten arc welding
    process
  • Effects on the weld of varying torch angles
  • Why and how the filler rod is kept inside the
    protective zone of the shielding gas
  • How tungsten contamination occurs and what to do
  • Causes of change in welding amperage
  • Correct settings for the minimum and maximum
    welding current

46
Objectives (continued)
  • Types and sizes of tungsten and metal
  • Factors affecting gas preflow and postflow times
  • Minimum and maximum gas flow settings
  • Nozzle size
  • Tungsten size
  • Amperage setting
  • Characteristics of low carbon and mild steels,
    stainless steel, and aluminum
  • Metal preparation for GTA welding
  • Make GTA welds in all positions

47
Introduction
  • Gas tungsten arc is also called GTA welding
  • GTA welding can be used to for nearly all types
    and thicknesses of metal
  • GTA welding is fluxless, slagless, and smokeless
  • Welders have fine control of the welding process
  • GTA welding is ideal for close-tolerance welds
  • Some GTA welds make the critical root pass
  • GTA used when appearance is important

48
Introduction (continued)
  • Setup of GTA equipment affects weld quality
  • Charts give correct settings
  • Field conditions affect the variables in the
    charts
  • Experiments designed to evaluate the appearance
    of a weld
  • After welding in the lab, troubleshooting field
    welding problems is easier
  • To make a weld is good to solve a welding
    problem is better

49
Torch Angle
  • As close to perpendicular as possible
  • May be angled 0-15 degrees from perpendicular for
    better visibility
  • As the gas flows out it forms a protective zone
    around the weld
  • Too much tilt distorts protective shielding gas
    zone

50
Figure 16-5 Filler being remelted as the weld is
continued. Courtesy of Larry Jeffus.
51
Torch Angle (continued)
  • Velocity of shielding gas affects protective zone
  • Low-pressure area develops behind the cup when
    velocity increases
  • Sharper angle and higher flow rate increases
    contamination

52
Filler Rod Manipulation
  • Filler rod must be kept inside the protective
    zone
  • If filler rod is removed from the gas protection,
    it oxidizes rapidly
  • Oxide is added to the molten weld pool
  • When a weld is temporarily stopped, the shielding
    gas must be kept flowing

53
Filler Rod Manipulation (continued)
  • If the rod tip becomes oxidized, if should be cut
    off before restarting
  • The rod should enter the shielding gas as close
    to the base metal as possible
  • An angle less than 15 degrees prevents air from
    being pulled in the welding zone

54
Figure 16-2 The hot filler rod end is well
within the protective gas envelope. Courtesy of
Larry Jeffus.
55
Figure 16-7 Too much filler rod angle has caused
oxides to be formed on the filler rod end.
Courtesy of Larry Jeffus.
56
Tungsten Contamination
  • Most frequent problem is tungsten contamination
  • Tungsten becomes contaminated if it touches
  • Molten weld pool
  • Filler metal
  • Surface tension pulls the contamination up onto
    the hot tungsten
  • Extreme heat causes some of the metal to vaporize
    and form a large oxide layer

57
Tungsten Contamination (continued)
  • Contamination caused by the tungsten touching the
    molten pool or filler metal forms a weak weld
  • The weld and tungsten must be cleaned before any
    more welding can be done
  • Tiny tungsten particles will show up if the weld
    is x-rayed
  • Contamination can be knocked off quickly by
    flipping the torch head
  • This procedure should never be used with heavy
    contamination or in the field

58
Figure 16-8 Contaminated tungsten. Courtesy of
Larry Jeffus.
59
Current Setting
  • Amperage on the machine's control is the same at
    the arc when
  • Power to the machine is exactly correct
  • Lead length is very short
  • All cable connections are perfect
  • Arc length is exactly right
  • Remote current control is in the full on position

60
Figure 16-10 Melting first occurring. Courtesy
of Larry Jeffus.
61
Figure 16-12 Oxides forming due to inadequate gas
shielding. Courtesy of Larry Jeffus.
62
Gas Flow
  • Gas preflow and postflow times depend upon
  • Wind or draft speed
  • Tungsten size used
  • Amperage
  • Joint design
  • Welding position
  • Type of metal welded
  • Maximum flow rates must never be exceeded
  • Air can be sucked into the weld zone

63
Practice Welds
  • Practice welds are grouped according to the weld
    position and type of joint
  • Mild steel is inexpensive and requires the least
    amount of cleaning
  • With aluminum, cleanliness is a critical factor
  • Try each weld with each metal to determine which
    metal will be easier to master

64
Low Carbon and Mild Steels
  • Low carbon and mild steel are two basic steel
    classifications
  • Small pockets of primary carbon dioxide gas
    become trapped
  • Porosity most likely when not using a filler
    metal
  • Most filler metals have some alloys, called
    deoxidizers

65
Stainless Steel
  • Setup and manipulation are nearly the same as
    for low carbon and mild steels
  • Most welds on stainless steels show effects of
    contamination
  • Most common problem is the bead color after the
    weld
  • Using a low arc current with faster travel speeds
    is important

66
Aluminum
  • Molten aluminum weld pool has high surface
    tension
  • Preheat the base metal in thick sections
  • Preheat temperature is around 300 Fahrenheit
  • Cleaning and keeping the metal clean is time
    consuming
  • Aluminum rapidly oxidizes at welding temperatures

67
Metal Preparation
  • Base and filler metals must be thoroughly cleaned
  • Contamination will be deposited into the weld
  • Oxides, oil, and dirt are the most common
  • Contaminants can be removed mechanically or
    chemically

68
Figure 16-15 Aluminum filler being correctly
added to the molten weld pool. Courtesy of Larry
Jeffus.
69
Figure 16-16 Filler rod being melted before it is
added to the molten pool. Courtesy of Larry
Jeffus.
70
Figure 16-18 Surfacing weld. Courtesy of Larry
Jeffus.
71
Figure 16-20 Establish a molten weld pool and dip
the filler rod into it. Courtesy of Larry Jeffus.
72
Figure 16-21 Note the difference in the weld
produced when different size filler rods are
used. Courtesy of Larry Jeffus.
73
Figure 16-22 Move the electrode back as the
filler rod is added. Courtesy of Larry Jeffus.
74
Figure 16-34 Be sure both the top and bottom
pieces are melted. Courtesy of Larry Jeffus.
75
Figure 16-35 Oxides form during tack welding.
Courtesy of Larry Jeffus.
76
Figure 16-36 A notch indicates the root was not
properly melted and fused. Courtesy of Larry
Jeffus.
77
Figure 16-37 Watch the leading edge of the molten
weld pool. Courtesy of Larry Jeffus.
78
Summary
  • Positioning yourself to control the electrode
    filler metal and to see the joint is critical
  • Experienced welders realize they need to see only
    the leading edge of the weld pool
  • Good idea to gradually reduce your need for
    seeing 100 of the weld pool
  • Increasing this skill is significant advantage in
    the field
  • Welding in the field may have to be done out of
    position
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