Gas Tungsten Arc Welding

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Gas Tungsten Arc Welding
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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

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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

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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

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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

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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

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Power Source Basics
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Tungsten Electrodes
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Tungsten

• Tungsten has the following properties
• High tensile strength
• Hardness
• High melting temperature
• High boiling temperature
• Good electrical conductivity

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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

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Figure 15-1 Some tungsten will erode and be
transferred across the arc.
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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

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Torch Build Out
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Torch Build Out
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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

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Figure 15-3 The smooth surface of a centerless
ground tungsten electrode. Courtesy of Larry
Jeffus.
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Types of Tungsten Electrodes

• Pure tungsten is an excellent nonconsumable
  electrode
• Pure tungsten can be improved by adding
• Cerium
• Lanthanum
• Thorium
• Zirconium

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Tungsten Electrodes
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Table 15-1 Tungsten Electrode Types and
Identification.
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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)

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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

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Figure 15-8 Correct way of holding a tungsten
when grinding. Courtesy of Larry Jeffus.
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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

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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

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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

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Figure 15-14 Melting the tungsten end shape.
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GTAW Equipment
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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

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Figure 15-21 Schematic of a GTA welding setup
with a water-cooled torch.
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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

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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.

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Figure 15-29 Electrons collect under the oxide
layer during the DCEP portion of the cycle.
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Figure 15-30 Sine wave of alternating current at
60 cycle.
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Shielding Gas
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Shielding Gases

• Shielding gases used for GTA welding process
• Argon (Ar)
• Helium (He)
• Or a mixture of two or more gases

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Shielding Gases (continued)

• Argon effectively shields welds in deep grooves
  in flat positions
• Helium offers the advantage of deeper penetration

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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

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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

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Figure 15-35 Too steep an angle between the torch
and work may draw in air.
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Remote ControlsFoot or Finger
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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

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Welding Techniques
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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

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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

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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

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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

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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

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Figure 16-5 Filler being remelted as the weld is
continued. Courtesy of Larry Jeffus.
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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

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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

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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

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Figure 16-2 The hot filler rod end is well
within the protective gas envelope. Courtesy of
Larry Jeffus.
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Figure 16-7 Too much filler rod angle has caused
oxides to be formed on the filler rod end.
Courtesy of Larry Jeffus.
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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

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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

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Figure 16-8 Contaminated tungsten. Courtesy of
Larry Jeffus.
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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

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Figure 16-10 Melting first occurring. Courtesy
of Larry Jeffus.
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Figure 16-12 Oxides forming due to inadequate gas
shielding. Courtesy of Larry Jeffus.
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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

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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

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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

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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

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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

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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

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Figure 16-15 Aluminum filler being correctly
added to the molten weld pool. Courtesy of Larry
Jeffus.
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Figure 16-16 Filler rod being melted before it is
added to the molten pool. Courtesy of Larry
Jeffus.
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Figure 16-18 Surfacing weld. Courtesy of Larry
Jeffus.
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Figure 16-20 Establish a molten weld pool and dip
the filler rod into it. Courtesy of Larry Jeffus.
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Figure 16-21 Note the difference in the weld
produced when different size filler rods are
used. Courtesy of Larry Jeffus.
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Figure 16-22 Move the electrode back as the
filler rod is added. Courtesy of Larry Jeffus.
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Figure 16-34 Be sure both the top and bottom
pieces are melted. Courtesy of Larry Jeffus.
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Figure 16-35 Oxides form during tack welding.
Courtesy of Larry Jeffus.
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Figure 16-36 A notch indicates the root was not
properly melted and fused. Courtesy of Larry
Jeffus.
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Figure 16-37 Watch the leading edge of the molten
weld pool. Courtesy of Larry Jeffus.
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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