GMAW chapter22[1]
Transcript of GMAW chapter22[1]
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PowerPoint to accompany
WeldingPrinciples and PracticesThird Edition
Sacks and Bohnart
1
Gas Metal Arc
Welding Practice:
Jobs 22-J1±J23
(Plate)
Chapter 22
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O bjectives
1. Describe GMAW operating variables
2. Describe GMAW weld defects
3. Describe GMAW safe operation4. 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
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O perating Variables ThatAffect Weld Formation
Factors that affect operation of arc and weld
deposit
Sound welding of good appearance resultswhen variables in balance
Necessary to become familiar with all variables
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Direct Current ElectrodePositive (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
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Gas Metal Arc DCEP Welding:Wire Positive, Work Negative
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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
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Gas Metal Arc DCEN Welding:Wire Negative, Work Positive
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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
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Shielding Gas
Argon and helium first used for gas metal arc
± Continue to be basic gases
Argon used more than helium on ferrous metalsto 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
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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
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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
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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
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Joint Preparation
Joint design should provide for mosteconomical 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
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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
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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 openingsthan 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"
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V-Groove, Butt JointComparison
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Joint Preparation
Multipass welding easier since absence of slag
ensures easier cleaning
For fillet welds deposit smaller weld beads onsurface 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
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Comparison of Penetration in aFillet Weld
Carbon dioxide shieldedMAG weld versus coated
electrode weld.
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Electrode Diameter
Influences size of weld bead, depth of penetration, andspeed of welding
General rule
± For same current, arc becomes more penetrating aselectrode diameter decreases and deposition rate increases
To get maximum deposition rate at given currentdensity, 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
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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 andoverhead positions
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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 onwelding 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"
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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
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Electrode Extension
Tests indicated that when electrode extensionincreased from 3/16 to 5/8 inch, welding current thendrops 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 agiven feed rate
± Because of self-regulating characteristics of constant voltage power source, welding current decreased
± As welding current decreased, depth of penetration alsodecreases
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Nomenclature of Area Between Nozzle and Workpiece
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Position of the Gun
Expressed by two angles: travel and work
Bead shape changed by changing direction of wire asgoes 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
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Travel and Work Gun Angles
Axis of Weld
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Travel and Work Gun Angles
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Travel Angle
(T. A.)
Axis of Weld
(Drag) Travel Direction
(Push) Travel Direction
Work Angle
(W. A.)
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Drag and Push Gun angles
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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 surfaceof plate being joined
Utilizes natural arc force to push weld metalagainst vertical surface to prevent undercut and provide good bead contour
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Work and Gun Angles
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Arc Length
Constant voltage welding machine used for gas metalarc 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 andarc length by setting wire-feed speed on wire feeder and voltage on welding machine
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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 andincreases undercut in fillet welds
± Lower: causes bead to be high and narrow
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Arc Voltage
High arc voltages result in globular transfer
± Spatter prone and reduces deposition efficiency
Has sharp crackling sound when proper arcvoltage for short circuit transfer
± Spray arc have hissing sound
Not set to control penetration Better control of weld profile and arc stability
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Relationship of Arc Length toWeld Bead Width
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High VoltageLow Voltage
Arc Length
Arc Length
Electrode
Electrode
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Penetration Comparisons
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Arc voltage too high
for travel speed.
Arc voltage too slow
for travel speed
Proper arc voltage
for speed
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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 outenough 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 resultsin short arc, slow speed in long arc
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Effect of Wire-Feed Speeds
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Welding Current
Determines amount of current delivered at arc
Often related to current density
± Amperage per square inch of cross-sectional areaof 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 coredelectrode more complex to calculate
± Current densities much higher with metal coredelectrodes than solid wire
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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
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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 andwidth (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, arcmay be extinguished, less penetration
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Travel Speed
Has decided effect on penetration, bead size, andappearance
At given current density, slower travel speeds provide
proportionally larger weld beads and more heat inputin 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
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Travel Speed
Excessively fast speeds causes undercut
Influenced by thickness of metal being welded,
joint design, cleanliness, joint fitup, andwelding 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
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Arc Position
Arc must be on
leading edge of
weld pool to
assure penetrationand fusion.
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O ptimum Travel Speed
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Summary of O perating Variables
Height and width of bead depend on adjustment of these variables
± Joint preparation
± Gas flow rate ± Voltage
± Speed of travel
± Arc length
± Polarity Variables adjusted on basis of type of material being
welded, thickness of material, position of welding,deposition rate required, and final weld specifications
± Gun angle
± Size and type of filler wire ± Electrode extension
± Characteristics of the shieldinggas
± Wire-feed speed (current)
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Summary of O perating Variables
Welding current and travel speed have similar
effect on both bead height and width
± Each variable increases or decreases both beadheight 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
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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
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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
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Whiskers
Short lengths of electrode wire sticking through
weld on root side of joint
Caused by pushing electrode wire past leadingedge of weld pool
Can be prevented by
± Reducing wire-feed speed
± Increasing electrode extension distance
± Weaving gun
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Voids
Referred to as wagon tracks because of resemblancein 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
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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
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Porosity
Most common defect in welds
Exists on face of weld readily detected
Below surface must be determined byradiograph ultrasonic or other testing methods
Causes of most porosity are contamination byatmosphere, change in physical qualities of
filler wire, and improper welding technique Also caused by entrapment of gas evolved
during weld metal solidification
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Causes of Porosity
Travel so fast that part or all of shielding gaslost, and atmospheric contamination occurs
Shielding gas flow rate too low so gas does notfully 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
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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
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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
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Other Defects
Weld cracking
± Comes from compositional problems, poor joint design, and poor welding technique
± Prevent by making sure filler metal has compositionsuitable for base metal and providing for expansion andcontraction 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
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Undercutting
Cutting away of base material along toes of weld
May be present in cover pass weld bead or inmultipass welding
Condition usually result of high current, high voltage,excessive travel speed, low wire-feed speed, poor guntechnique, 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 toopposite side
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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
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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
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Clothing Regulations
Standard arc welding helmets with lenses ranging inshade from no. 6 for work using up to 30 amperes tono. 14 for work using more than 400 amperes should
be worn ± Arc should never be viewed with the naked eye whenstanding closer than 20 feet
Skin should be covered completely to prevent burnsand other damage from ultraviolet light
± Back of the head and neck should be protected fromreflected radiation
± Gloves should always be worn
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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
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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
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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
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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 hazardfrom 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
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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 raysfrom arc
± Locate degreasing operations far away from welding
activities
Much of welding smoke and fumes can be engineeredout of GMAW arc by use of higher argon percent and
pulse-spray mode of transfer
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Ventilation
During welding, certain metals emit toxicfumes that may cause respiratory irritation andstomach upset
± Most common toxic metal vapors given off bywelding 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
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Electrical Safety
Hazard less than that with shielded meal arc
process
± O pen circuit voltage considerably less
Electrical maintenance should be done only by
qualified person
± NEVER worked on in electrical HOT condition
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Wire-Feeder Safety
Turn power off when aligning and adjustingdrive rolls
Avoid pinch points when working near drive
rolls
Remember force being applied to wiresufficient to push it through your hand or other
body parts Never let exposed wire come in contact with or
be pointed at your body
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Fire Safety
Welding should not be done near areas whereflammable 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 previouslycontained combustible materials, special precautionsshould be taken
Use ³hot work permit´ as required
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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 penetrationand deposition rate characteristic of the MIG/MAG process
Small fillets and butt welds should be positioned soarc can run slightly downhill
Equipment has to be kept clean, in proper adjustment,and in good mechanical condition
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Drag and Push Methods
Produces large wide beads Produces flatter bead shape
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Care of Nozzles
Keep the gun nozzle, contact tube, and wire-feeding system clean to eliminate wire-feedingstoppages
± Nozzle is natural spatter collector
If spatter builds up thick enough, it can actually bridge gap and electrically connectinsulated nozzle to contact tube
To remove spatter, use soft, blunttool for prying
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Care of Nozzles
Spatter almost falls out by itself if nozzle kept
clean, shiny and smooth
Antispatter compound may be applied to gunnozzle 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
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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
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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 reducesfriction
Clean every time spool or coil changes
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Bird Nesting
Wire coils sideways between wire-feed cable
and drive rolls
Prevent by accurate alignment of wire-feedcable 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
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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
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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
Connecting Work to Minimize
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Connecting Work to MinimizeArc Blow
Suggestions to shorten trial-and-error process
to correct or minimize arc blow
Attach work lead or leads directly onworkpiece if possible
Connect both ends of long, narrow weldments
Use electrical conductors of proper length
Weld away from work connection
Connecting Work to Minimize
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Connecting Work to MinimizeArc Blow
On parts that rotate, use rotating work connection or allow work cable to wind up nomore than one or two turns
In making longitudinal welds on cylinders, usetwo work connections²one on each side of theseam as close as possible to point of starting
If multiple work connections necessary, makesure cables are same size and length and haveidentical terminals
Connecting Work to Minimize
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Connecting Work to MinimizeArc Blow
On multiple-head installations, all heads shouldweld in same direction and away from work connection
Use individual work circuits on multiple-headinstallations
Do not place two or more arcs close to one
another on weldments that are prone tomagnetic disturbance with one arc such astubes or tanks requiring longitudinal seams
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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 morecylinders of compressed gas, pressure-reducingcylinder regulator, flowmeter assembly
Combination gas, water, wire, and cable controlassembly 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
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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
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Starting Procedure
1. Check power cable connections; connect guncable to proper welding terminal on weldingmachine and work cable end connected to
proper terminal on welding machine2. Start welding machine by pressing on button
or, in case of engine drive, start engine
3. Turn on wire-feed unit4. Check gas-shielding supply system
5. Check water flow if gun water cooled
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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 beyondcontact 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 guntrigger on torch
h i h i
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Shutting Down the Equipment
1. Stop welding and release gun trigger
2. Return feed speed to zero position
3. Close gas outlet valve in top of gas cylinder
4. 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
S i h ld
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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
Fi i hi h W ld
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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 atend of weld to fill crater
G A l
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Gun Angle
Push angle of 5° to 15° generally employed whenwelding in flat position
± Take care push angle not changed as end of weldapproached
Work angle equal on all sides when welding uniformthicknesses
Welding in horizontal position, point gun upwardslightly
Thick-to-thin joints, direct arc toward heavier section
Slight drag angle may help when welding thinsections
C l f A
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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
P d E i P bl
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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
P ti J b
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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
Precautions to O bserve When
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ecaut o s to Obse ve W eDoing 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 atall times
Select proper filler wire for material being welded andfor 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 onsecond side
MIG/MAG Welding of
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gCarbon 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
Groove Welds:
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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
Groove Welds:
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Jobs 22-J1 and J2
For code welding, plate thicknesses from 3/16to 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
Groove Welds:
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Jobs 22-J1 and J2
O pen 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
Groove Welds:
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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
Fill t W ld J b 22 J3 J10
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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 electrodewelding
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
Inspection and Testing
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Inspection and Testing
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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.
Inspection and Testing
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Inspection and Testing
Fillet weld on lap joint in steel plate
welded with gas metal arc welding
process in 2F position.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fillet weld on lap joint in steel plate weldedwith gas metal arc welding process in 3F
position, downhill. Note porosity caused
by poor gas shielding.
Inspection and Testing
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Inspection and Testing
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fillet weld on T- joint welded
in the 2F position with thegas metal arc welding
process in steel plate.
Penetration through back side of a
V-groove butt joint weldedin the 1G position.
The f irst (root) pass of a V-groove
butt joint welded in the 1G positionwith the gas metal arc welding
process in steel plate.
Fillet and Groove Welding Combination Project:
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g j
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
Fillet and Groove Welding Combination Project:
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g j
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 weldsexcept as permitted for partial joint penetration groove
welds
± Undercut shall not exceed lesser of 10% of base metalthickness or 1/32 inch
± Frequency of porosity shall not exceed one in each 4 inches
of weld length, and maximum diameter shall not exceed3/32 inch
± Welds shall be free from overlap
± Only minimal weld spatter shall be accepted
Fillet and Groove Welding Combination
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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
Fillet and Groove Welding Combination
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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 weldsexcept as permitted for partial joint penetration groove
welds
± Undercut shall not exceed lesser of 10% of base metalthickness or 1/32 inch
± Frequency of porosity shall not exceed one in each 4 inches
of weld length, and the maximum diameter shall not exceed3/32 inch
± Welds shall be free from overlap
± Only minimal weld spatter shall be accepted
Groove Weld Project: Joblifi i
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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
Inspection and Testing
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Inspection and Testing
After tacking, have it inspected
After complete welding, use visual inspectionand 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 weldsexcept as permitted for partial joint penetrationgroove welds
Inspection and Testing
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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
Side Bend Acceptance Criteria as Measured
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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 internaldiscontinuities
MIG Welding of Aluminum
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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
MIG Welding of Aluminum
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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 oxidefilm 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
MIG Welding of Aluminum
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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
Factors that Make Gas Metal Arc Welding
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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
Joint Preparation
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Joint Preparation
Designed like those for steel
Narrower joint spacing and lower welding currentsused
Foreign substances must be removed ± Wiped off or removed by vapor degreasing
± Oxide film removed by chemical and mechanical cleaningmethods
Weld as soon as possible before oxide film has chanceto form again
Sheared edges can also cause poor quality welds
Shielding Gas
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Shielding Gas
Argon preferred for welding aluminum platethicknesses 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
Spray Arc Welding
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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 asshielding 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
Out-of-Position Welding
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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
Horizontal Position
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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.
Out-Of-Position Welding
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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
Out-Of-Position Welding
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Out Of Position Welding
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Welding V-groove butt joint in
aluminum plate in 3G position, uphill.
Welding T- joint in aluminum
plate in 3F position, uphill.
Butt Joints: Jobs 22-J11 and J12
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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
Lap Joints: Job 22-J13
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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
T-Joints: Jobs 22-J14-J16
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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
Edge and Corner Joints
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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
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Main Causes of Cracking inAluminum Welds
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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
Main Causes of Porosity inAluminum Welds
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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
Major Causes of Incomplete Fusion of
Weld Metal ith Base Metal
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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
Causes of Inadequate Penetration at Root
of Weld and Into Side Walls of Joint
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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
Causes of Metallic and Nonmetallic
Inclusions in Aluminum Welds
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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
Groove Weld Project:Job Qualification Test 4
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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
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22 - 134Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Performance Qualif ication
Test GM AW Spray Transfer, Aluminum
3G and 4G Positions
AWS SENSE
Shown only to illustrate what a qualif ication test would
look like. Follow it and inspect and test as listed in text.
MAG Welding of StainlessSteel
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Steel
Heat and corrosion resistant alloy
± Always contains high percentage of chromium inaddition 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
MAG Welding of StainlessSteel
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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 sectionsup to 1/16" thick
No air must be permitted to reach underside of weldwhile weld pool solidifying (air weakens it) ± If no backing bar, argon should be used as purge gas shield
Advantages of MAG WeldingStainless Steel
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Stainless Steel
Absence of slag-forming flux reduces cleaningtime and makes it possible to observe weld pool
Continuous wire feed permits uninterruptedwelding
MAG lends itself to automation
Welding may be performed with short-circuiting, spray, or pulsed spray modes of transfer
Spray Arc Welding
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p y g
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
Short Arc Welding (GMAW-S)
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g ( )
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
Short Arc Welding (GMAW-S)
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g ( )
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
Pulse Spray Arc (GMAW-P)
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Can be done with lower current levels andhigher wire-feed speeds
Can be used on all thickness ranges
Spray-type gas: 1 and 2% oxygen withremainder being argon most common
Weld more fluid and flows well because arc on
all the time Spatter reduced on thin base metals ascompared to short-circuiting mode of transfer
Hot Cracking
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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
Stainless-Steel Sensitization
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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
Inspection and Testing:Jobs 22-J17-J23
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Jobs 22 J17 J23
Inspect each weld carefully for defects
Fillet weld on lap joint in 3/8" stainless-steel plate weldedin the 1F position with the gas metal arc welding process.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Inspection and Testing:Jobs 22-J17-J23
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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.
Copper and Its Alloys
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May be welded successfully by gas metal arc process
Electrolytic copper can be joined by using specialtechniques, but weldability not good
Various grades of deoxidized copper readily weldable
with MIG process ± Deoxidized filler wires necessary
Filler wires of approximately matching chemistryused
Argon preferred shielding gas for material 1" andthinner ± Flow of 50 cubic feet per hour sufficient
± Heavier material uses 65% and 35% argon
Copper and Its Alloys
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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 weldingcopper and its alloys
± Beryllium-copper alloy dangerous
Copper and Its Alloys
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GMAW-B
± Variation of GMAW process where B indicates
brazing or just MIG brazing
± Uses silicon-bronze type electrode with inertshielding 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)
Nickel and Nickel-Copper Alloys
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oys
Can be welded using gas metal arc process
Remove all foreign material in vicinity sincesusceptible to severe embrittlement andcracking when come in contact with foreignmaterials
Argon generally preferable for welding up toabout 3/8 inch in thickness ± Above that thickness, argon-helium mixtures
usually more desirable
Joint preparation like other metals
Magnesium
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Silvery white metal, two-thirds weight of aluminumand 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 withmagnesium different than with other metals
± Argon recommended in most cases
Titanium
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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
Zirconium
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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