Transcript Gas Metal Arc Welding Practice: Jobs 22

PowerPoint to accompany
Welding
Principles and Practices
Third Edition
Sacks and Bohnart
Gas Metal Arc
Welding Practice:
Jobs 22-J1–J23
(Plate)
Chapter 22
1
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Permission required for reproduction or display.
Objectives
1.
2.
3.
4.
Describe GMAW operating variables
Describe GMAW weld defects
Describe GMAW safe operation
Describe and demonstrate proper care, use, and
troubleshooting of equipment
5. Describe and demonstrate welding techniques
6. Make various groove and fillet welds with the
various modes of metal transfer with both solid and
metal cored electrodes
22 - 2
Operating Variables That Affect
Weld Formation
• Factors that affect operation of arc and weld
deposit
• Sound welding of good appearance results
when variables in balance
• Necessary to become familiar with all variables
22 - 3
Direct Current Electrode
Positive (DCEP)
• Generally used for gas metal arc welding
– Provides maximum heat input into work allowing relatively
deep penetration to take place
– Assists in removal of oxides from plate
– Low current values produce globular transfer of metal from
electrode
• On carbon steel shielding gas must contain minimum
of 80% argon
• Ferrous metals need addition of 2 to 5% oxygen to gas
mixture
22 - 4
Gas Metal Arc DCEP Welding:
Wire Positive, Work Negative
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22 - 5
Direct Current Electrode
Negative (DCEN)
•
•
•
•
•
Limited use in welding of thin gauge materials
Greatest amount of heat occurs at electrode tip
Wire meltoff rate great deal faster than DCEP
Penetration also less than with DCEP
Arc not stable at end of filler wire
– Corrected by use of shielding gas mixture of 5%
oxygen added to argon
– Meltoff rate reduced so benefit cancelled
22 - 6
Gas Metal Arc DCEN Welding:
Wire Negative, Work Positive
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22 - 7
Alternating Current
• Seldom used in gas metal arc welding
• Arc unstable because of current reversal
• Combination of both DCEN and DCEP
polarity, rate of metal transfer and depth of
penetration falls between those polarities
• Found some use for welding of aluminum
22 - 8
Shielding Gas
• Argon and helium first used for gas metal arc
– Continue to be basic gases
• Argon used more than helium on ferrous metals
to keep spatter at minimum
– Also heavier than air so good weld coverage
• Oxygen or carbon dioxide added to pure gases
to improve arc stability, minimize undercut,
reduce porosity, and improve appearance of
weld
22 - 9
Shielding Gas
• Helium added to argon to increase penetration
• Hydrogen and nitrogen used for only limited
number of special applications
• Carbon dioxide has following advantages:
– Low cost
– High density, resulting in low flow rates
– Less burn-back problems because of its shorter arc
characteristics
22 - 10
Specific Metal Recommendations
• Aluminum alloys: argon
• Magnesium and aluminum alloys: 75 percent
helium, 25 percent argon
• Stainless steels: argon plus oxygen
• Magnesium: argon
• Deoxidized copper: 75 percent helium, 25
percent argon preferred
• Low alloy steel: argon, plus 2 percent oxygen
22 - 11
Specific Metal Recommendations
• Mild steel: 15 percent argon, 25 percent carbon
dioxide (dip transfer); 100 percent CO2 may
also be used with deoxidized wire
• Nickel, Monel®, and Inconel®: argon
• Titanium: argon
• Silicon bronze: argon
• Aluminum bronze: argon
22 - 12
Joint Preparation
• Joint design should provide for most
economical use of filler metal
• Correct design for job depends on:
–
–
–
–
–
–
–
Type of material being welded
Thickness of material
Position of welding
Welding process
Final results desired
Type and size of filler wire
Welding technique
22 - 13
Joint Preparation
• Arc in gas metal arc welding more penetrating
and narrower than arc in shielded metal arc
welding therefore, smaller root openings may
be used for groove welds
– Change in joint design increase speed of welding
• 100% penetration may be secured in ¼" plate
in square butt joint welded from both sides
22 - 14
Joint Preparation
• No root face recommended for 60º single- or
double-V butt joints
– Root opening should range from 0 to 3/32"
– Double-V joints may have wider root openings
than single-V
• Plates thicker than 1 inch should have
U-groove preparation
– Require less weld metal; root face thickness should
be less than 3/32" and root spacing 1/32 and 3/32"
22 - 15
V-Groove, Butt Joint
Comparison
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22 - 16
Joint Preparation
• Multipass welding easier since absence of slag
ensures easier cleaning
• For fillet welds deposit smaller weld beads on
surface of material
• Certain types of joints backed up to prevent
weld from projecting through back side
– Blocks, strips and bars of copper, steel or ceramics
22 - 17
Comparison of Penetration in a
Fillet Weld
Carbon dioxide shielded
MAG weld versus coated
electrode weld.
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22 - 18
Electrode Diameter
• Influences size of weld bead, depth of penetration, and
speed of welding
• General rule
– For same current, arc becomes more penetrating as
electrode diameter decreases and deposition rate increases
• To get maximum deposition rate at given current
density, use smallest wire possible consistent with
acceptable weld profile
• Wire 0.045" and larger provide lower deposition rate
and deposit wider beads than small wires
22 - 19
Electrode Diameter
• Filler wires should be same composition as materials
being welded
• Position of welding may affect size of electrode
• Welding thin material
– Wires with diameters: 0.023/0.025, 0.030, 0.035"
• Medium thick materials
– Wires with diameters: 0.045" or 1/16"
• Heavy materials
– Wire with diameter: 1/8"
• Small diameters recommended for vertical and
overhead positions
22 - 20
Electrode Extension
•
•
•
•
Length of filler wire that extends pas contact tube
Area where preheating of filler wire occurs
Also called the stickout
Controls dimensions of weld bead since length of
extension affect burnoff rate
• Exerts influence on penetration through its effect on
welding current
– As extension length increased, preheating of wire increases
and current reduced which in turn decreases amount of
penetration into work
• Stickout distance may vary from 1/8 to 1 1/4"
22 - 21
Electrode Extension
• Short electrode extensions (1/8–1/2 inch) used for
short circuit mode of transfer, generally with smaller
diameter electrodes (0.023–0.045 inches)
• Stainless steel favors shorter electrode extension
because of its higher resistivity (1/8–1/4 inch)
– Longer and larger diameter electrode extensions used for
spray arcs (1/2–11/4 inches)
• Excessive long arcs with active gases reduce the
mechanical properties in weld
– Various alloys being burned out as metal transferred across
longer arc
22 - 22
Electrode Extension
• Tests indicated that when electrode extension
increased from 3/16 to 5/8 inch, welding current then
drops approximately 60 amperes
• Current reduced because of change in amount of
preheating that takes place in wire
– As electrode extension increased, preheating of wire increases
– Thus less welding current required from power source at a
given feed rate
– Because of self-regulating characteristics of constant voltage
power source, welding current decreased
– As welding current decreased, depth of penetration also
decreases
22 - 23
Nomenclature of Area Between
Nozzle and Workpiece
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22 - 24
Position of the Gun
• Expressed by two angles: travel and work
• Bead shape changed by changing direction of wire as
goes into joint in line of travel
• Gun Angle
– Can be compared to angle of electrode in shielded metal arc
welding
– Drag technique results in high narrow bead with deeper
penetration (10º drag angle)
– As drag angle reduced, bead height decreases, width
increases
– Increased travel speeds characteristic of push technique
22 - 25
Travel and Work Gun Angles
Axis of Weld
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22 - 26
Travel and Work Gun Angles
Work Angle
(W.A.)
Travel Angle
(T.A.)
(Push) Travel Direction
(Drag) Travel Direction
Axis of Weld
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22 - 27
Drag and Push Gun angles
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22 - 28
Work Angle
• Position of wire to joint in plane perpendicular
to line of travel
• Filler weld joints: work angle normally half of
included angle between plates forming joint
• Butt welds: work angle normally 90º to surface
of plate being joined
• Utilizes natural arc force to push weld metal
against vertical surface to prevent undercut and
provide good bead contour
22 - 29
Work and Gun Angles
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22 - 30
Arc Length
• Constant voltage welding machine used for gas metal
arc welding provides for self-adjustment of arc length
– Arc length shortened, arc voltage reduced
– Arc length lengthened, arc voltage increased
• No change in wire-feed speed occurs
• Corrected by automatic increase or decrease of
burnoff rate of filler wire
• Welder has complete control of welding current and
arc length by setting wire-feed speed on wire feeder
and voltage on welding machine
22 - 31
Arc Voltage
• Decided effect upon penetration, bead height,
and bead width
• Chief function to stabilize welding arc and
provide smooth, spatter-free weld bead
• Higher or lower causes arc to become unstable
– Higher: produces wider, flatter bead and increases
possibility of porosity and increases spatter and
increases undercut in fillet welds
– Lower: causes bead to be high and narrow
22 - 32
Arc Voltage
• High arc voltages result in globular transfer
– Spatter prone and reduces deposition efficiency
• Has sharp crackling sound when proper arc
voltage for short circuit transfer
– Spray arc have hissing sound
• Not set to control penetration
• Better control of weld profile and arc stability
22 - 33
Relationship of Arc Length to
Weld Bead Width
Electrode
Electrode
Arc Length
Arc Length
Low Voltage
High Voltage
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22 - 34
Penetration Comparisons
Arc voltage too high
for travel speed.
Proper arc voltage
for speed
Arc voltage too slow
for travel speed
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22 - 35
Wire-Feed Speed
• Fixed relationship between rate of filler wire burn off
and welding current
• Electrode wire-feed speed determines welding current
– Current set by wire-feed speed control on wire feeder
• Excessive speed, welding machine cannot put out
enough current to melt wire fast enough
– Stubbing or roping of wire occurs
– Causes convex weld beads and poor appearance
• Decrease in speed results in less electrode being melted
• Generally – high setting of filler wire speed rate results
in short arc, slow speed in long arc
22 - 36
Effect of Wire-Feed Speeds
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22 - 37
Welding Current
• Determines amount of current delivered at arc
• Often related to current density
– Amperage per square inch of cross-sectional area
of electrode
• At given amperage, current density of electrode 0.035"
in diameter higher than of electrode 0.045" in diameter
• Area of current-carrying sheath of metal cored
electrode more complex to calculate
– Current densities much higher with metal cored
electrodes than solid wire
22 - 38
Welding Current
• If going to maintain given amperage and switch from
solid wire to metal core, either jump one wire
diameter size and keep wire-feed speed same or keep
same wire size and increase wire-feed speed
• Each type and size of electrode has minimum and
maximum current density
– Best working range lies between
• Direct relationship between welding current and
penetration
– Welding current increases, penetration increases
22 - 39
Welding Current
• Table 22-3 gives comparative current ranges and other
parameters for welding carbon steel, stainless steel,
and aluminum
• Increases in current will increase bead height and
width (voltage must also be increased)
• Too high
– Possibility of electrode burn-back into tube, arc unstable
and gas shielding disturbed, spatter
• Too low
– Arc unstable, poor fusion, electrode becomes red hot, arc
may be extinguished, less penetration
22 - 40
Travel Speed
• Has decided effect on penetration, bead size, and
appearance
• At given current density, slower travel speeds provide
proportionally larger weld beads and more heat input
in base metal per unit length of weld
– Too slow, unusual weld buildup occurs
• Progressively increased travel speeds have opposite
effects
– Less weld metal deposited with lower heat input per unit
length of weld
– Produces narrower weld bead and lower contour
22 - 41
Travel Speed
• Excessively fast speeds causes undercut
• Influenced by thickness of metal being welded,
joint design, cleanliness, joint fitup, and
welding position
• If increased, necessary to increase wire-feed
speed, which increases current and burnoff rate
• Too low produces overlap of base metal and
even burn-through on this material
22 - 42
Arc Position
Arc must be on
leading edge of
weld pool to
assure penetration
and fusion.
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22 - 43
Optimum Travel Speed
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22 - 44
Summary of Operating Variables
• Height and width of bead depend on adjustment of
these variables
–
–
–
–
–
–
Joint preparation
Gas flow rate
Voltage
Speed of travel
Arc length
Polarity
–
–
–
–
Gun angle
Size and type of filler wire
Electrode extension
Characteristics of the shielding
gas
– Wire-feed speed (current)
• Variables adjusted on basis of type of material being
welded, thickness of material, position of welding,
deposition rate required, and final weld specifications
22 - 45
Summary of Operating Variables
• Welding current and travel speed have similar
effect on both bead height and width
– Each variable increases or decreases both bead
height and width at same time
• Arc voltage
– As arc voltage increases, bead height decreases and
bead width increases, flattening bead
– Affects shape and size of bead
22 - 46
Weld Defects
• Defects found in welds made by gas metal arc
process similar to those in other welding
– Causes and corrective action entirely different
• Incomplete penetration
– Result of too little heat input in weld area
– Correct by increasing wire-feed speed and reducing
electrode extension to obtain maximum current for
particular wire-feed setting
– Also causes by improper welding techniques
22 - 47
Excessive Penetration
• Usually causes excessive melt-through
• Result of too much heat in weld area
– Reducing wire-feed speed to obtain lower amperage or
increasing speed of travel
• Another cause is improper joint design
– Root opening too wide or root face too small
– Correct by checking position of welding and root face and
opening
• Remedied during welding by increasing electrode
extension distance and weaving gun
22 - 48
Whiskers
• Short lengths of electrode wire sticking through
weld on root side of joint
• Caused by pushing electrode wire past leading
edge of weld pool
• Can be prevented by
– Reducing wire-feed speed
– Increasing electrode extension distance
– Weaving gun
22 - 49
Voids
• Referred to as wagon tracks because of resemblance
in radiographs to ruts in dirt road
• May be continuous along both sides of weld
• Found in multipass welding
– Underneath pass has bead with large contour or bead with
too much convexity or undercut
– Next bead does not completely fill void between previous
pass and plate
• Prevent by making sure edges of all passes filled in so
undercut cannot take place and arc melts previous
bead and fuses into sides of joint
22 - 50
Incomplete Fusion
• Also referred to as overlap
• Result of improper gun handling, low heat and
improper speed of travel
• To prevent:
–
–
–
–
Direct arc so it covers all areas of joint
Keep electrode at leading edge of pool
Reduce size of pool as necessary by adjusting travel speed
Check current values carefully; keep short electrode
extension
22 - 51
Porosity
• Most common defect in welds
• Exists on face of weld readily detected
• Below surface must be determined by
radiograph ultrasonic or other testing methods
• Causes of most porosity are contamination by
atmosphere, change in physical qualities of
filler wire, and improper welding technique
• Also caused by entrapment of gas evolved
during weld metal solidification
22 - 52
Causes of Porosity
• Travel so fast that part or all of shielding gas
lost, and atmospheric contamination occurs
• Shielding gas flow rate too low so gas does not
fully displace all air in arc area
• Shielding gas flow rate too high drawing air
into arc area and causing turbulence
• Shielding gases must be of right type for metal
being welded
• Shielding gases must be pure and dry
22 - 53
Causes of Porosity
• Gas shield may be blown away by wind or
drafts
• May be defects in gas system
• Excessive voltage for arc required can cause
loss of its deoxidizers and alloying elements
• Foreign material such as oil, dirt, rust, grease,
and paint on wire or material to be welded
• Improper welding techniques are used
22 - 54
Other Defects
• Warpage
– Occurs when forces of expansion and contraction
poorly controlled
• Spatter
– Made up of very fine particles of metal on plate
surface adjoining weld area
– Usually caused by high current, long arc, irregular
and unstable arc, improper shielding gas, improper
gun angle, electrode extension, or clogged nozzle
22 - 55
Other Defects
• Weld cracking
– Comes from compositional problems, poor joint design, and
poor welding technique
– Prevent by making sure filler metal has composition
suitable for base metal and providing for expansion and
contraction forces during welding
• Irregular weld shape
– Include too wide, too narrow, excessively convex or
concave surface and those with coarse, irregular ripples
– Caused by poor gun manipulation, too fast or too slow
speed of gun travel, too high or too low current, improper
arc voltage, improper shielding gas, improper extension
22 - 56
Undercutting
• Cutting away of base material along toes of weld
• May be present in cover pass weld bead or in
multipass welding
• Condition usually result of high current, high voltage,
excessive travel speed, low wire-feed speed, poor gun
technique, improper gas shielding, or wrong filler
wire
• To correct, move welding gun from side to side in
joint, and hesitate at each side before returning to
opposite side
22 - 57
Safe Practices
• Safety most important consideration to both
worker and employer
• Welding no more dangerous than other
industrial operations
• Safety precautions and protective equipment
required for MIG/MAG process essentially
same as for any other electric welding process
22 - 58
Eye, Face, and Body Protection
• Welding helmets and protective clothing
necessary
• Radiant energy produced by gas-shielded
process 5 to 30 times more intense than
produced by shielded metal arc welding
– Lowest intensities produced by gas tungsten arc
– Highest by gas metal arc
– Argon produces greater intensities than helium
22 - 59
Clothing Regulations
• Standard arc welding helmets with lenses ranging in
shade from no. 6 for work using up to 30 amperes to
no. 14 for work using more than 400 amperes should
be worn
– Arc should never be viewed with the naked eye when
standing closer than 20 feet
• Skin should be covered completely to prevent burns
and other damage from ultraviolet light
– Back of the head and neck should be protected from
reflected radiation
– Gloves should always be worn
22 - 60
Clothing Regulations
• Shirts should be dark in color to reduce
reflections
– Have tight collar and long sleeves
– Leather, wool and aluminum-coated cloth can
withstand action of radiant energy reasonably welld
22 - 61
Handling of Gas Cylinders
• Stored cylinders should be in protected area
away from fire, cold, and grease and away
from general shop activity
• Cylinders must be secured to equipment to
prevent their being knocked over
• Proper regulators and flowmeters must be used
with each special type of cylinder
22 - 62
Handling of Gas Cylinders
• Cylinders should not be dropped, used as
rollers, lifted with magnets, connected into
electric circuit, or handled in any other way
that might damage cylinder or regulator
• When cylinders empty, should be stored in
upright position with valve closed
22 - 63
Ventilation
• Ozone generated in small quantities, generally
below allowable limits of concentration
• Nitrogen dioxide also present around area of
arc in quantities below allowable limits
• Carbon dioxide shielding may create hazard
from carbon monoxide and carbon dioxide if
welder’s head in path of the fumes or if
welding done in confined space
– Special ventilation should be provided
22 - 64
Ventilation
• Eye, nose, and throat irritation can be produced when
welding near such degreasers as carbon tetrachloride,
trichlorethylene, and perchloroethylene
– Break down into phosgene under action of powerful rays
from arc
– Locate degreasing operations far away from welding
activities
• Much of welding smoke and fumes can be engineered
out of GMAW arc by use of higher argon percent and
pulse-spray mode of transfer
22 - 65
Ventilation
• During welding, certain metals emit toxic
fumes that may cause respiratory irritation and
stomach upset
– Most common toxic metal vapors given off by
welding of lead, cadmium, copper, zinc, and
beryllium
– Fumes can be controlled by general ventilation,
local exhaust ventilation, or respiratory protective
equipment
• Welding guns can be purchased with smoke
extractor capability
22 - 66
Electrical Safety
• Hazard less than that with shielded meal arc
process
– Open circuit voltage considerably less
• Electrical maintenance should be done only by
qualified person
– NEVER worked on in electrical HOT condition
22 - 67
Wire-Feeder Safety
• Turn power off when aligning and adjusting
drive rolls
• Avoid pinch points when working near drive
rolls
• Remember force being applied to wire
sufficient to push it through your hand or other
body parts
• Never let exposed wire come in contact with or
be pointed at your body
22 - 68
Fire Safety
• Welding should not be done near areas where
flammable materials or explosive fumes present
• Paint spray or dipping operations should not be
located close to any welding operation
• Combustible material should not be used for floors,
walls, welding tables, or in immediate vicinity of
welding operation
• When welding on containers that have previously
contained combustible materials, special precautions
should be taken
• Use “hot work permit” as required
22 - 69
Care and Use of Equipment
• Do not push gun into arc like an electrode
– Wire feeder pushes wire into arc
• Either push or drag travel angle can be used
• If possible, welding should be done in flat welding
position to take advantage of increased penetration
and deposition rate characteristic of the MIG/MAG
process
• Small fillets and butt welds should be positioned so
arc can run slightly downhill
• Equipment has to be kept clean, in proper adjustment,
and in good mechanical condition
22 - 70
Drag and Push Methods
Produces large wide beads
Produces flatter bead shape
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22 - 71
Care of Nozzles
• Keep the gun nozzle, contact tube, and wirefeeding system clean to eliminate wire-feeding
stoppages
– Nozzle is natural spatter collector
• If spatter builds up thick enough, it can actually
bridge gap and electrically connect
insulated nozzle to contact tube
• To remove spatter, use soft, blunt
tool for prying
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22 - 72
Care of Nozzles
• Spatter almost falls out by itself if nozzle kept
clean, shiny and smooth
• Antispatter compound may be applied to gun
nozzle and contact tube end
• Do not clean by tapping or pounding on solid
object
– Bends gun nozzles, damages threads and high
temperature insulation in nozzle can break
22 - 73
Care of Contact Tubes
• Transfers welding current to electrode wire
• Hole has to be big enough to allow wire with slight
cast to pass through easily
• Wire wears hole to oval shape
– Wire slides more easily, but transfer of current not as good
and arcing in tub results
– Spatter flies up into bore and wire slows down because of
friction
– Must be replace; secure tightly in gun and check
periodically for tightness
22 - 74
Care of Wire-Feed Cables
• Wire-feed conduit flexible steel tube that does
not stretch
• Main source of friction in wire-feed system
• Should be kept clean and straight as possible
– Clean with dry compressed air
• Lubricate with dry powdered graphite reduces
friction
• Clean every time spool or coil changes
22 - 75
Bird Nesting
• Wire coils sideways between wire-feed cable
and drive rolls
• Prevent by accurate alignment of wire-feed
cable inlet guide
– Aligned exactly with rollers so wire does not have
to make reverse bend
– Notch in drive rolls must be in perfect alignment to
provide smooth passage for wire
22 - 76
Cleanliness of Base Metal
• Clean area thoroughly before welding
• Remove all rust, scale, burned edges and
chemical coatings
– Gas producers
– Porosity is result
• Intense heat of arc burns away some of the
contaminants
22 - 77
Arc Blow
• Arc blown to one side or other by condition of pull
and counter-pull as magnetic field is distorted
– Ionized gases carrying arc from end of electrode wire to
work act as flexible conductor with magnetic field around it
– When placed in location such as corner of joint or end of
plate, magnetic field distorted and pulls in another direction
– Magnetic field tries to return to state of equilibrium
• Does not occur with a.c. welding arcs
– Forces exerted by magnetic field reversed 120 times per
second thus keeping magnetic field in equilibrium
22 - 78
Connecting Work to Minimize
Arc Blow
•
•
•
•
Suggestions to shorten trial-and-error process
to correct or minimize arc blow
Attach work lead or leads directly on
workpiece if possible
Connect both ends of long, narrow weldments
Use electrical conductors of proper length
Weld away from work connection
22 - 79
Connecting Work to Minimize
Arc Blow
• On parts that rotate, use rotating work
connection or allow work cable to wind up no
more than one or two turns
• In making longitudinal welds on cylinders, use
two work connections—one on each side of the
seam as close as possible to point of starting
• If multiple work connections necessary, make
sure cables are same size and length and have
identical terminals
22 - 80
Connecting Work to Minimize
Arc Blow
• On multiple-head installations, all heads should
weld in same direction and away from work
connection
• Use individual work circuits on multiple-head
installations
• Do not place two or more arcs close to one
another on weldments that are prone to
magnetic disturbance with one arc such as
tubes or tanks requiring longitudinal seams
22 - 81
Setting Up Equipment
• Constant voltage d.c. power source
• Wire-feeding mechanism with controls and spooled or
reeled filler wire mounted on fixture
• Gas-shielding system consisting of one or more
cylinders of compressed gas, pressure-reducing
cylinder regulator, flowmeter assembly
• Combination gas, water, wire, and cable control
assembly and welding gun of correct type and size
• Connecting hoses and cables, work lead, and clamp
• Face helmet, gloves, sleeves (if necessary), and
assortment of hand tools
22 - 82
Assumed Safety Precautions
• Welding equipment installed properly
• Welding machine in dry location, and no water
on floor of welding booth
• Welding booth lighted and ventilated properly
• All connections tight, and all hoses and leads
arranged so they cannot be burned or damaged
• Gas cylinders securely fastened so they cannot
fall over and not part of electrical circuit
22 - 83
Starting Procedure
1. Check power cable connections; connect gun
cable to proper welding terminal on welding
machine and work cable end connected to
proper terminal on welding machine
2. Start welding machine by pressing on button
or, in case of engine drive, start engine
3. Turn on wire-feed unit
4. Check gas-shielding supply system
5. Check water flow if gun water cooled
22 - 84
Starting Procedure
5. Set wire-feed speed control for type and size of filler
wire and for job
6. Voltage rheostat should be set to conform to type
and thickness of material being welded, diameter of
filler wire, the type of shielding gas, and type of arc
7. Adjust for proper electrode extension beyond
contact tube
8. To start arc, touch end of electrode wire to proper
place on weld joint, usually just ahead of weld bead,
with current shut off; lower helmet and press gun
trigger on torch
22 - 85
Shutting Down the Equipment
1.
2.
3.
4.
Stop welding and release gun trigger
Return feed speed to zero position
Close gas outlet valve in top of gas cylinder
Squeeze welding gun trigger, hold it down,
and bleed gas lines
5. Close gas flowmeter valve until finger-tight
6. Shut off welding machine and wire feeder
7. Hang up welding gun and cable assembly
22 - 86
Starting the Weld
• Running start
–
–
–
–
Arc started at beginning of weld
Electrode end put in contact with base metal
Trigger on torch pressed
Tends to be too cold at beginning of weld
• Scratch start
– Arc struck approximately 1 inch ahead of beginning of weld
– Arc quickly moved back to starting point of weld, direction
of travel reversed, and weld started
– Arc may also be struck outside of weld area on starting tab
22 - 87
Finishing the Weld
• Arc should be manipulated to reduce penetration
depth and weld pool size when completing weld bead
– Decreases final shrinkage area
– Reduction accomplished by rapidly increasing speed of
welding for approximately 1 to 2 inches of weld length
– Trigger released, stopping wire feed and interrupting
welding current
• Gun trigger can be turned on and off several times at
end of weld to fill crater
22 - 88
Gun Angle
• Push angle of 5° to 15° generally employed when
welding in flat position
– Take care push angle not changed as end of weld
approached
• Work angle equal on all sides when welding uniform
thicknesses
• Welding in horizontal position, point gun upward
slightly
• Thick-to-thin joints, direct arc toward heavier section
• Slight drag angle may help when welding thin
sections
22 - 89
Control of Arc
• Arc voltage controls penetration, bead contour,
and such defects as undercutting, porosity and
weld discontinuities
• Arc should be occasionally noisy for most
applications of spray arcs
22 - 90
Process and Equipment Problems
• Study tables 22-6 which lists problems with
MIG/MAG short arc process and their
correction
• Table 22-7 lists problems with MIG/MAG
process and equipment, their causes, and
possible remedies
22 - 91
Practice Jobs
• Practice gas metal arc welding on mild steel,
aluminum, and stainless steel
• Specifications given in Job Outline in order
assigned by instructor
• Beyond these job, practice other forms of joints
in all positions
– Use various types and sizes of filler wire and
different shielding gases
22 - 92
Precautions to Observe When
Doing Practice Jobs
•
•
•
•
Avoid excessive current values
Check your welding speed
Make sure that gas flow adequate
Keep wire centered in gas pattern and in center of
joint; make sure correct electrode angle maintained at
all times
• Select proper filler wire for material being welded and
for such situations as rust, scale, and excessive oxygen
• When welding from both sides of plate, be sure root
pass on first side deeply penetrated by root pass on
second side
22 - 93
MIG/MAG Welding of
Carbon Steel
• Bulk of all welding done on carbon steel
• MIG/MAG welding on increase
– Welders find it relatively easy to master
– Consistently produces sound welds at high rate of
speed
22 - 94
Groove Welds:
Jobs 22-J1 and J2
• Plate up to 1/8" thick may be butt welded with
square edges with root opening of 0 to 1/16"
• Heavier plate, 3/16 and 1/4 inch may be
welded without beveling edges if 1/16 to 3/32"
opening provided
• Bead should be wider than root spacing for
proper fusion
• Two passes, one from each side usually needed
22 - 95
Groove Welds:
Jobs 22-J1 and J2
• For code welding, plate thicknesses from 3/16
to 1" should be beveled
– 60º single- or double-V without root face
recommended
– Root opening of 0 to 1/16" should be maintained
– Wider root openings may be provided for double-V
joints
– Single-V grooves backing pass from reverse side
generally required
• Less distortion when welding from both sides
of joint
22 - 96
Groove Welds:
Jobs 22-J1 and J2
• Open root joint should be run using short
circuit or pulse spray for ferrous metals
• Practice 3G using both uphill and downhill
techniques
• U-grooves used on plate thicker than 1 inch
– Root spacing between 1/32 and 3/32" maintained
– Root face of 3/32" or less to assure penetration
– Requires less filler metal than V groove butt joint
22 - 97
Groove Welds:
Jobs 22-J1 and J2
• Argon-oxygen mixture containing 1-5% oxygen
recommended for spray arc welding
– Oxygen improves flow of weld metal and reduces tendency
to undercut
• Argon with 10% CO2 sometimes used
• Carbon dioxide at 100% used by arc not true spray arc
– Popular for MAG small wire welding
• Short arc welding of carbon steel uses mixture of 75%
argon and 25% carbon dioxide
22 - 98
Fillet Welds: Jobs 22-J3-J10
• Used in T-joints, lap joints, and corner joints
• Deposit rate and rate of travel high with deep
penetration
• Permits smaller fillet welds than with stick electrode
welding
• Position of nozzle and speed of welding important
• Welding may be single pass or multipass
– Multipass may be done with stringer or weave beads
• Each pass must be cleaned carefully
22 - 99
Inspection and Testing
Outside corner joint in steel plate welded with gas metal
arc welding process in the flat position.
Penetration through back side of corner joint welded
in the flat position.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
22 - 100
Inspection and Testing
Fillet weld on lap joint in steel plate
welded with gas metal arc welding
process in 2F position.
Fillet weld on lap joint in steel plate welded
with gas metal arc welding process in 3F
position, downhill. Note porosity caused
by poor gas shielding.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
22 - 101
Inspection and Testing
Fillet weld on T-joint welded
in the 2F position with the
gas metal arc welding
process in steel plate.
Penetration through back side of a
V-groove butt joint welded
in the 1G position.
The first (root) pass of a V-groove
butt joint welded in the 1G position
with the gas metal arc welding
process in steel plate.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
22 - 102
Fillet and Groove Welding Combination Project:
Job Qualification Test 1
• Purpose
–
–
–
–
–
Ability to read print
Develop bill of materials
Thermally cut
Fit components together
Tack and weld carbon steel project
• Follow instructions found in Fig. 22-26
22 - 103
Fillet and Groove Welding Combination Project:
Job Qualification Test 1
• Inspection and testing (visual inspection only)
– Shall be no cracks or incomplete fusion
– Shall be no incomplete joint penetration in groove welds
except as permitted for partial joint penetration groove
welds
– Undercut shall not exceed lesser of 10% of base metal
thickness or 1/32 inch
– Frequency of porosity shall not exceed one in each 4 inches
of weld length, and maximum diameter shall not exceed
3/32 inch
– Welds shall be free from overlap
– Only minimal weld spatter shall be accepted
22 - 104
Fillet and Groove Welding Combination
Project: Job Qualification Test2
• Purpose
–
–
–
–
–
–
–
Ability to read print
Develop bill of materials
Thermally cut
Fit components together
Tack and weld carbon steel project
Use spray arc mode of metal transfer
Note on Fig. 22-27
22 - 105
Fillet and Groove Welding Combination
Project: Job Qualification Test2
• Inspection and testing (visual inspection only)
– Shall be no cracks or incomplete fusion
– Shall be no incomplete joint penetration in groove welds
except as permitted for partial joint penetration groove
welds
– Undercut shall not exceed lesser of 10% of base metal
thickness or 1/32 inch
– Frequency of porosity shall not exceed one in each 4 inches
of weld length, and the maximum diameter shall not exceed
3/32 inch
– Welds shall be free from overlap
– Only minimal weld spatter shall be accepted
22 - 106
Groove Weld Project: Job
Qualification Test 3
• Project
– Ability to read print
– Fit components together
– Tack and weld carbon steel unlimited thickness test
plate
– Using spray arc mode of metal transfer
– Instructions in notes in Fig. 22-28
22 - 107
Inspection and Testing
• After tacking, have it inspected
• After complete welding, use visual inspection
and cut specimens for bend testing
• Use side bend test procedures and check:
• Testing criteria:
– No cracks or incomplete fusion
– No incomplete joint penetration in groove welds
except as permitted for partial joint penetration
groove welds
22 - 108
Inspection and Testing
• Testing criteria (cont.):
– Undercut shall not exceed lesser of 10 percent of
base metal thickness or 1/32 inch
– Frequency of porosity shall not exceed one in each
4 inches of weld length and maximum diameter
shall not exceed 3/32 inch
– Welds shall be free from overlap
– Only minimal weld spatter shall be accepted
22 - 109
Side Bend Acceptance Criteria as Measured on
Convex Surface of Bend Specimen
• No single indication shall exceed 1/8 inch measured in
any direction on surface
• Sum of greatest dimensions of all indications on
surface, which exceed 1/32 inch, but are less than or
equal to 1/8 inch, shall not exceed 3/8 inch
• Cracks occurring at corner of specimens shall not be
considered unless there definite evidence that they
result from slag inclusions or other internal
discontinuities
22 - 110
MIG Welding of Aluminum
• Readily joined by welding, brazing, soldering,
adhesive bonding, and mechanical fastening
• Lightweight
• Alloyed readily with many other metals
• Highly ductile and retains ductility at subzero
temperatures
• High resistance to corrosion, no colored salts, not toxic
• Good electrical and thermal conductivity
• High reflectivity to both heat and light
• Nonsparking and nonmagnetic
22 - 111
MIG Welding of Aluminum
• Easy to fabricate
• May be given wide variety of mechanical,
electrochemical, chemical and paint finishes
• Needs high heat input for fusion welding
• Aluminum and its alloys rapidly develop oxide
film when exposed to air (melting point 3600ºF)
– Must be removed during welding
• Removed by fluxes, action of arc in inert gas
atmosphere or mechanical and chemical means
22 - 112
MIG Welding of Aluminum
• MIG and TIG replaced stick electrode
welding for aluminum and its alloys
– Small percentage still using stick electrodes
• Type of joint and position of welding
determines process to used on thicknesses
1/8 inch and under
22 - 113
Factors that Make Gas Metal Arc Welding
Desirable Joining Process for Aluminum
• Cleaning time reduced because there no flux on
weld
• Absence of slag in weld pool eliminates
possibility of entrapment
• Weld pool highly visible due to absence of
smoke and fumes
• Welding can be done in all positions
22 - 114
Joint Preparation
• Designed like those for steel
• Narrower joint spacing and lower welding currents
used
• Foreign substances must be removed
– Wiped off or removed by vapor degreasing
– Oxide film removed by chemical and mechanical cleaning
methods
• Weld as soon as possible before oxide film has chance
to form again
• Sheared edges can also cause poor quality welds
22 - 115
Shielding Gas
• Argon preferred for welding aluminum plate
thicknesses up to 1 inch
• Plate thicknesses 1-2 inches may use:
– Pure argon, mixture of 50% argon and 50% helium, or
mixture of 75% argon and 25% helium
– Helium provides high heat and argon excellent cleaning
action
• Plate thicknesses from 2-3 inches
– Mixture of 50% argon and 50% helium or 25% argon and
75% helium
• Plate thicknesses greater than 3 inches
– Mixture of 25% argon and 75% helium
22 - 116
Spray Arc Welding
• Weld metal deposited continuously
• More arc energy and greater heat provided for melting
filler wire and base material
• Helium, helium-argon mixtures and argon used as
shielding gases
– Choice dependent upon type of material, thickness and
welding position
• Welding can be done in all positions
• GMAW-P very effective when welding aluminum
22 - 117
Out-of-Position Welding
• Horizontal position
– Care must be taken to penetrate to root of joint
when welding butt joints and T-joints
– Overheating in any one area causes sagging,
undercutting or melt-through to back of joint
– Weld metal should be directed against upper plate
– In multipass welding, be sure fusion between
passes
22 - 118
Horizontal Position
Welding T-joint in aluminum
plate in 2F position
Welding V-groove butt joint
in aluminum plate in 2G position.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
22 - 119
Out-Of-Position Welding
• Vertical position
– Travel-up technique on fillet and groove welds
– Do not use too high welding current nor deposit too large
weld bead
– Slight side-to-side motion helpful
• Overhead position
–
–
–
–
No problem with fillet and groove welds
Welding current and travel speed lower than flat position
Gas flow rate higher because gas has tendency to leave area
Somewhat awkward – assume relaxed position as possible
22 - 120
Out-Of-Position Welding
Welding T-joint in aluminum
plate in 3F position, uphill.
Welding V-groove butt joint in
aluminum plate in 3G position, uphill.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
22 - 121
Butt Joints: Jobs 22-J11 and J12
•
•
•
•
•
Easy to design
Require minimum of base material
Perform better under fatigue loading
Require accurate alignment and edge preparation
Usually necessary to bevel edge on thicknesses of ¼"
or more to permit root pass penetration
– On heavier plate, chipping back side and welding back side
with one pass
– Sections with different thicknesses should be beveled before
welding
22 - 122
Lap Joints: Job 22-J13
• More widely used on aluminum alloys than on
other materials
• Use double-welded, single-lap joints in
thicknesses of aluminum up to ½"
• Require no edge preparation
• Easy to fit
• Require less jigging than butt joints
22 - 123
T-Joints: Jobs 22-J14-J16
• Seldom require edge preparation on material ¼" or
less in thickness
• Fully penetrated if weld fused into root of joint
• Easily fitted and normally require no back chipping
• Jigging usually quite simple
• Better to put small continuous fillet weld on each side
of joint rather than one large weld on one side
• Continuous fillet welding recommended over
intermittent welding for longer fatigue life
22 - 124
Edge and Corner Joints
• Economical from standpoint of preparation,
base metal used, and welding requirements
• Harder to fit up
• Prone to fatigue failure
• Edges do not require preparation
22 - 125
Inspection and Testing
• Inspect carefully for defects
• Use same inspection and testing procedures
used previously
• Look for surface defects
• High quality welds in aluminum can be
produced only if proper welding conditions and
good cleaning procedures been established and
maintained
22 - 126
Effect of Current on
Aluminum Welds
Kaiser Aluminum & Chemical Corporation
Aluminum weld bead
made with current
too high
Kaiser Aluminum & Chemical Corporation
Aluminum weld bead
made with current
too low
Kaiser Aluminum & Chemical Corporation
Aluminum weld bead
made with
correct current
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
22 - 127
Main Causes of Cracking in
Aluminum Welds
• Generally in crater or longitudinal form
• Crater cracks
– Cause: arc broken sharply and leaves crater
– Cure: manipulate gun properly
• Longitudinal cracks caused by
– Incorrect weld metal composition
– Improper welding procedure
– High stresses imposed during welding by poor joint
design or poor jigging
22 - 128
Main Causes of Porosity in
Aluminum Welds
•
•
•
•
•
Hydrogen in the weld area
Moisture, oil, grease, or heavy oxides in the weld area
Improper voltage or arc length
Improper or erratic wire feed
Contaminated filler wire (Use as large a diameter as
possible and GMAW-P if lower heat is needed.)
• Leaky gun
• Contaminated or insufficient shielding gas
22 - 129
Major Causes of Incomplete Fusion of
Weld Metal with Base Metal
• Incomplete removal of oxide film before
welding
• Unsatisfactory cleaning between passes
• Insufficient bevel or back chipping
• Improper amperage (WFS) or voltage
22 - 130
Causes of Inadequate Penetration at Root
of Weld and Into Side Walls of Joint
•
•
•
•
Low welding current (WFS)
Improper filler metal size
Improper joint preparation
Too fast travel speeds for the selected wire-feed
speed
22 - 131
Causes of Metallic and Nonmetallic
Inclusions in Aluminum Welds
• Copper inclusions caused by burn-back of
electrode to contact tube
• Metallic inclusions from cleaning weld with
wire brush which leaves bristles in weld
• Nonmetallic inclusions from poor cleaning of
base metal
• Always use push gun travel angle when
welding aluminum
22 - 132
Groove Weld Project:
Job Qualification Test 4
• Purpose
–
–
–
–
–
Ability to read print
Fit components together
Tack
Weld aluminum test plates
Using spray arc mode of metal transfer
• Inspection and testing
– Visual inspection
– Perform side bend tests
22 - 133
AWS SENSE
Performance Qualification
Test GMAW Spray Transfer, Aluminum
3G and 4G Positions
Shown only to illustrate what a qualification test would
look like. Follow it and inspect and test as listed in text.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
22 - 134
MAG Welding of Stainless
Steel
• Heat and corrosion resistant alloy
– Always contains high percentage of chromium in
addition to nickel and manganese
• Excellent strength-to-weight ratios
• Many alloys possess high degree of ductility
• Widely used in products such as tubing, piping,
kitchen equipment, ball bearings
• Supplied in sheets, strip, plate, shapes, tubing,
pipe and wire extrusions
22 - 135
MAG Welding of Stainless
Steel
• Lower rate of thermal conductivity than carbon steel
– Heat retained in weld zone longer
• Thermal expansion greater than carbon steel
– Causes greater shrinkage stresses and warpage
• Has tendency to undercut
• All standard forms of joints used in fabrications
• Copper backing bars necessary for welding sections
up to 1/16" thick
• No air must be permitted to reach underside of weld
while weld pool solidifying (air weakens it)
– If no backing bar, argon should be used as purge gas shield
22 - 136
Advantages of MAG Welding
Stainless Steel
• Absence of slag-forming flux reduces cleaning
time and makes it possible to observe weld
pool
• Continuous wire feed permits uninterrupted
welding
• MAG lends itself to automation
• Welding may be performed with shortcircuiting, spray, or pulsed spray modes of
transfer
22 - 137
Spray Arc Welding
• Electrode diameters as large as 3/32" can be used for
stainless steel
– 1/16" wire used with high current to create spray arc
transfer of metal
• DCEP used for most stainless-steel welding
• Most common gas: mixture of Ar and 1 to 2% O
– Recommended for single-pass welding
• Push travel angle should be employed on plate ¼"
thick or more
• Gun should be moved back and forth in direction of
travel and slightly from side to side
22 - 138
Short Arc Welding (GMAW-S)
• Requires low current ranging form 20 to 175 amperes;
low voltage of 12 to 20 volts, small diameter wires
• Metal transfer occurs when filler wire short circuits
with base metal
• Ideally suited for most stainless-steel welding on
thicknesses from 16 gauge to 1/16"
– Also for first pass in which fitup is poor or copper
backing unsuitable
– Very desirable in vertical and overhead positions for
first pass
22 - 139
Short Arc Welding (GMAW-S)
• For stainless steel in light gauges, triple
mixture of gas gives good arc stability and
excellent coalescence
– 90% helium, 7 ½% argon and 2 ½% carbon dioxide
– Produces small heat-affected zone that eliminates
undercutting and reduces distortion
– Does not lower corrosion resistance
– Flow rates must be increased because of lower
density of helium
22 - 140
Pulse Spray Arc (GMAW-P)
• Can be done with lower current levels and
higher wire-feed speeds
• Can be used on all thickness ranges
• Spray-type gas: 1 and 2% oxygen with
remainder being argon most common
• Weld more fluid and flows well because arc on
all the time
• Spatter reduced on thin base metals as
compared to short-circuiting mode of transfer
22 - 141
Hot Cracking
• Tendency of some stainless steels
– More welding passes needed
– Stringer beads recommended instead of weave
• Reduce contraction stresses and cooling more rapid
• Can reduce when welding sections 1 inch or
thicker by preheating to 500ºF
– Also reduce by GMAW-S or P welding
22 - 142
Stainless-Steel Sensitization
• Carbide precipitation
– Sensitizing chromium out of individual grains of austenitic
types of stainless steel
– Occurs most readily in 1,200ºF heat range
• To reduce situation
– Use GMAW process with its rapid speed and high
deposition rate
– Use stabilized and low carbon grades of stainless steel
– Using proper filler metals such as ER308L which is low in
carbon
22 - 143
Inspection and Testing:
Jobs 22-J17-J23
• Inspect each weld carefully for defects
Fillet weld on lap joint in 3/8" stainless-steel plate welded
in the 1F position with the gas metal arc welding process.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
22 - 144
Inspection and Testing:
Jobs 22-J17-J23
Fillet weld on T-joint in 3/8" stainless-steel plate
welded in 1F position with gas metal arc welding process.
Fillet weld on T-joint in 3/8" stainless-steel plate
welded in 2F position with gas metal arc welding process.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
22 - 145
Copper and Its Alloys
• May be welded successfully by gas metal arc process
• Electrolytic copper can be joined by using special
techniques, but weldability not good
• Various grades of deoxidized copper readily weldable
with MIG process
– Deoxidized filler wires necessary
• Filler wires of approximately matching chemistry
used
• Argon preferred shielding gas for material 1" and
thinner
– Flow of 50 cubic feet per hour sufficient
– Heavier material uses 65% and 35% argon
22 - 146
Copper and Its Alloys
• Joint design like any other metal
– Steel backup necessary for sheets 1/8" or thinner
• Welding currents on high side required
– Preheat not required when welding ¼" or less
• Always provide good ventilation when welding
copper and its alloys
– Beryllium-copper alloy dangerous
22 - 147
Copper and Its Alloys
• GMAW-B
– Variation of GMAW process where B indicates
brazing or just MIG brazing
– Uses silicon-bronze type electrode with inert
shielding with Argon 100% most common
– Main application for coated carbon steel sheet
metal (light gauge)
– Zinc coating applied for corrosion resistance
– Base metal not melted (hence brazing operation)
22 - 148
Nickel and Nickel-Copper
Alloys
• Can be welded using gas metal arc process
• Remove all foreign material in vicinity since
susceptible to severe embrittlement and
cracking when come in contact with foreign
materials
• Argon generally preferable for welding up to
about 3/8 inch in thickness
– Above that thickness, argon-helium mixtures
usually more desirable
• Joint preparation like other metals
22 - 149
Magnesium
• Silvery white metal, two-thirds weight of aluminum
and one-quarter weight of steel
• Melting point of 1,204ºF
• Strength-to-weight ratio high when compared to steel
• Welding techniques like aluminum
– Rate of expansion greater
– Care taken that surface clean before welding
• Arc characteristics of helium and argon with
magnesium different than with other metals
– Argon recommended in most cases
22 - 150
Titanium
•
•
•
•
Bright white metal that burns in air
Only element that burns in nitrogen
Melting point of about 3,500ºF
Most important compound titanium dioxide
– Used extensively in welding electrode coatings
• Used as stabilizer in stainless steel
22 - 151
Zirconium
• Bright gray metal
• Melting point above 4,500ºF
• Very hard and brittle and readily scratches
glass
• Used in hard-facing materials
• Often alloyed with iron and aluminum
• Argon or helium-argon mixtures used for gas
shielding
22 - 152