Transcription
Applying Metal Inert Gas(MIG)Welding Techniques
Interest Approach Have you heard theterm MIG Welding What are theadvantages of MIGWelding? How is MIG Weldingdone?
Student Learning Objectives 1. Explain the advantages of the metalinert gas (MIG) welding process. 2. Describe the equipment, types ofshielding gases, and electrodes used inthe MIG welding process. 3. Describe the types of metal transferpatterns used in MIG welding and relatetheir applications.
Student Learning Objectives 4. Describe the correct techniquesfor starting, controlling, andstopping an MIG bead. 5. Explain how to adjust andmaintain the MIG welder. 6. Identify safety practices thatshould be observed in MIG welding.
Terms Burnback Ductility Globular transfer Inert gas Short arc transfer Spray arc transfer Stickout Transition current Travel angle Whiskers
What are the advantages ofthe MIG welding process?
MIG Welding Metal inert gas welding (MIG) is aprocess in which a consumablewire electrode is fed into an arc andweld pool at a steady but adjustablerate, while a continuous envelopeof inert gas flows out around thewire and shields the weld fromcontamination by the atmosphere.
MIG Welding The MIG welding process hasseveral advantages which accountfor its popularity and increased usein the agricultural and weldingindustries.
MIG Welding Advantages A. Welding jobs can be performedfaster with the MIG process. The continuous wire feedeliminates the need to changeelectrodes.
MIG Welding Advantages B. Weld cleaning and preparationtime is less for MIG welding than forstick electrode welds. Since the gaseous shield protectsthe molten metal from theatmospheric gases, there is no fluxor slag, and spatter is minimal.
MIG Welding Advantages C. Little time is required to teachindividuals how to MIG weld.
MIG Welding Advantages D. Because of the fast travel speedat which MIG welding can be done,there is a smaller heat-affectedzone than with the shielded metalarc welding process. The smaller heat-affected zoneresults in less grain growth, lessdistortion, and less loss of temper inthe base metal.
MIG Welding Advantages E. Both thick and thin metals can bewelded successfully andeconomically with the MIG process. F. Less time is needed to prepareweld joints since the MIG welds aredeep penetrating. Narrow weld joints can be used withMIG welding and still secure soundweldments.
MIG Welding Advantages G. The MIG welding process canbe used to join both ferrous andnonferrous metals. The development of electrode wireand the use of spool guns has madethe MIG process widely used foraluminum, stainless steel, highcarbon-steel, and alloy-steelfabrication.
MIG Welding Advantages H. The weld visibility is generallygood. There is less smoke and fumes sooperator environment is improved.
What equipment, types ofshielding gases, andelectrodes are used in theMIG welding process?
MIG Welders To understand the MIG weldingprocess, you must understand theequipment needed. It consists of a welder, a wire feedsystem, cable and welding gunassembly, shielding gas supply, andelectrode wire.
MIG Welders A. Most welders used for MIGwelding are direct current machinesof the constant voltage type. B. MIG welding machines must bedesigned to produce a constantvoltage. With a constant voltage MIG machine,the output voltage will change verylittle with large changes in current.
MIG Welders C. Welding voltage has an effect onbead width, spatter, undercutting,and penetration. D. The constant voltage weldingmachines are designed so thatwhen the arc voltage changes, thearc current is automaticallyadjusted or self-corrected.
MIG Welders E. Most MIG welding units havethree adjustments which must be inbalance to achieve a quality weld. These are voltage control, wire feedspeed, and shielding gas flow rate.
Wire Feeder 1. The wire feeder continually drawsa small diameter electrode wirefrom the spool and drives it throughthe cable assembly and gun at aconstant rate of speed.
Wire Feeder 2. The constant rate of wire feed isnecessary to assure a smooth evenarc. This must be adjustable to providefor different welding current settingsthat may be desired.
Wire Feeder 3. Wire speed varies with the metalthickness being welded, type ofjoint, and position of the weld.
Wire FeederF. To move the electrode wire fromthe spool to the MIG welding gun,run the wire through a conduit andsystem of drive wheels. These drive wheels, dependingupon their location in the wire feedunit, are either the push type or thepull type.
Wire Feeder F1. The pull-type drive wheels arelocated relatively close to the MIGgun and exert a pulling action onthe wire. Pull-type drive wheels are usedon most spool guns.
Wire Feeder 2. With the push-type drive wheels,the wire goes through the wheelsand is pushed through theelectrode lead and out through theMIG gun.
Wire Feeder G. Correct tension on the wire feeddrive wheels is very important. 1. Too little tension results in drivewheel slippage which causes thewire to be fed into the puddle at anuneven rate, giving a poor-qualityweld.
Wire Feeder 2. Too much tension on the wirefeed wheels results in deformationof the wire shape. This altered wire shape can make itdifficult to thread the electrodethrough the conduit and the contacttip in the MIG gun.
Wire Feeder H. When a blockage or burnbackoccurs, the MIG gun should beturned off immediately to prevententanglement. A burnback occurs when theelectrode wire is fused to the contacttip.
Wire Feeder I. The wire feeders have differentsized drive rolls so they canaccommodate different sizes andtypes of wire.
MIG GunJ. The electrode holder is commonlyreferred to as the MIG gun. The MIG gun has a trigger switchfor activating the welding operation,a gas nozzle for directing the flowof the shielding gas, and a contacttip.
MIG Gun J1. The nozzle on the MIG gundirects the shielding gas over thepuddle during welding. A nozzle that is too large or too smallmay result in air from the atmospherereaching the puddle andcontaminating the weld.
MIG Gun 2. The nozzle is made of copperalloy to help remove the heat fromthe welding zone.
MIG Gun K. When welding outside, wherethe weld zone is subjected to draftsand wind currents, the flow ofshielding gas needs to be strongenough so that drafts do not blowthe shielding gas from the weldzone.
Contact Tip L. The contact tip helps to guide thewire electrode into the puddle aswell as transmit the weld current tothe electrode wire.The electrode wire actually touchesthe contact tip as it is fed through theMIG gun. During this contact, the weld currentis transmitted to the electrode.
Shielding GasM. Shielding Gas - the shielding gasdisplaces the atmospheric air with acover of protective gas. The welding arc is then struckunder the shielding gas cover andthe molten puddle is notcontaminated by the elements inthe atmosphere
Shielding Gas Inert and non-inert gases are usedfor shielding in MIG welding. An inert gas is one whose atomsare very stable and will not reacteasily with atoms of other elements.
1. Argon Has a low ionization potential andtherefore creates a very stable arcwhen used as a shielding gas. The arc is quiet and smoothsounding and has very little spatter.
Argon Argon is a good shielding gas forwelding sheet metal and thin metalsections. Pure argon is also used for weldingaluminum, copper, magnesium, andnickel. Pure argon is not recommended foruse on carbon steels.
2. Helium gas Conducts heat well and is preferred forwelding thick metal stock. It is good for welding metals thatconduct heat well, such as aluminum,copper, and magnesium. Helium requires higher arc voltagesthan argon. Helium-shielded welds are wider, haveless penetration and more spatter thanargon-shielded welds.
3. Carbon Dioxide The most often used gas in MIGwelding because it gives good beadpenetration, wide beads, noundercutting and good bead contourand it costs much less than argon orhelium.
Carbon Dioxide The main application of carbondioxide shielding gas is welding lowand medium carbon steels. When using carbon dioxideshielding gas, the arc is unstable,which causes a lot of spatter.
3. Carbon dioxide Carbon dioxide gas has a tendencyto disassociate. At high temperatures encountered inthe arc zone, carbon dioxide willpartially break up into oxygen andcarbon monoxide. Good ventilation is essential toremove this deadly gas
4. Gas Mixtures When used in a mixture with argon,oxygen helps to stabilize the arc,reduce spatter, eliminateundercutting, and improve weldcontour. The mixture is primarily used forwelding stainless steel, carbonsteels, and low alloy steels.
Gas Mixtures An argon-helium mixture is used forwelding thick non-ferrous metals. This mixture gives the same arcstability as pure argon with verylittle spatter, and produces a deeppenetrating bead.
Gas Mixtures The argon-carbon dioxide mixtureis used mainly for carbon steels,low alloy steels, and some stainlesssteel. The gas mixture helps to stabilizethe arc, reduce spatter, eliminateundercutting and improve metaltransfer straight through the arc.
Gas Mixtures The fabrication of austenitic stainlesssteel by the MIG process requires ahelium, argon, carbon dioxideshielding gas mixture. The mixture allows a weld with verylittle bead height to be formed.
N. Gas Cylinder and Gauges The tank supplying the shieldinggas will have a gauge and a gasflowmeter. The volume of gas directed overthe weld zone is regulated by theflowmeter.
O. Electrode Wire The selection of the correctelectrode wire is an importantdecision and the success of thewelding operation depends on thecorrect selection.
Electrode Wire There are factors to consider whenselecting the correct electrode. 1. Consider the type of metal to bewelded and choose a filler wire tomatch the base metal in analysisand mechanical properties.
Electrode Wire2. Consider the joint design. Thicker metals and complicatedjoint designs usually require fillerwires that provide high ductility. Ductility is the ability to be fashionedinto a new form without breaking.
Electrode Wire3. Examine the surface condition ofthe metal to be welded. If it is rusty or scaly, it will have aneffect on the type of wire selected.4. Consider the service requirementsthat the welded product willencounter.
P. Electrode Wire Classification MIG electrode wire is classified bythe American Welding Society(AWS). An example is ER70S6. For carbon-steel wire, the “E”identifies it as an electrode “R” notes that it is a rod
P. Electrode Wire Classification The first two digits relate the tensilestrength in 1,000 lbs. psi The “S” signifies the electrode is asolid bare wire Any remaining number andsymbols relate the chemicalcomposition variations ofelectrodes.
What are the types of metaltransfer patterns used in MIGwelding and when are theyused?
Metal Transfer Patterns In MIG welding, the metal from thewire electrode is transferred acrossthe arc plasma to the puddle byglobular, short arc, or spray transferpatterns. The type of transfer used for any givenweld depends upon the arc voltage,current, kind of shielding gas used,and diameter of the wire electrode.
A. Globular Transfer Patterns When the molten metal from thewire electrode travels across thearc in large droplets, it is in theglobular transfer pattern. 1. Globular transfer pattern occursat low wire feed rates, low current,and low arc voltage settings.
Globular Transfer Patterns 2. The current for globular transferis below transition current. Transition current is the minimumcurrent value at which spraytransfer will occur.
Globular Transfer Patterns 3. The molten globules are two tothree times larger than thediameter of the electrode. Surface tension holds the globuleson the end of the wire electrode.
Globular Transfer Patterns When the globules become tooheavy to remain on the electrode,they drop off and move across thearc. The globules do not move acrossthe arc in an even pattern.
Globular Transfer Patterns 4. Welds made with globulartransfer have poor penetration andexcessive spatter and are used littlein MIG welding.
B. Short Arc Transfer Pattern Is actually a series of periodic shortcircuits that occur as the molten tipof the advancing wire electrodecontacts the workpiece andmomentarily extinguishes the arc.
Short Arc Transfer Pattern 1. The droplet forms on the end ofthe electrode and begins to sagwhile the arc is ignited. The droplet sags further andtouches the molten puddle. When the droplet touches thepuddle, the arc is short-circuitedand extinguished.
Short Arc Transfer Pattern The droplet continues to melt andbreaks off the end of the wireelectrode. At this instant, the arc reignites anda new droplet begins to form. 2. New droplet formation and arcshorting may occur from 20 to 200times per second.
Short Arc Transfer Pattern 3. Short arc transfer is also knownas short circuiting transfer and diptransfer. Short arc transfer is especially goodfor welding in the horizontal,vertical, and overhead positionswhere puddle control is usuallyhard to maintain.
Short Arc Transfer Pattern Short arc welding is most feasibleat current levels below 200 ampsand with small-diameter electrodewire.
C. Spray Arc Transfer Pattern Is a spray of very fine droplets. 1. Spray arc transfer is a high-heatmethod of welding with a rapiddeposition of metal. It is used for welding all commonmetals from 3 /32 inch to over 1inch in thickness.
C. Spray Arc Transfer Pattern 2. This transfer occurs only withargon or argon-oxygen mixture ofshielding gas.
What is the correct techniquefor starting, controlling, andstopping an MIG weld?
Follow proper procedureswhen starting, controlling, andstopping an MIG weld.
MIG Welding Procedures A. Preparing to start welding withthe MIG welder requires you tomake adjustments to the machine. 1. Be sure the gun and groundcables are properly connected. If possible, attach the ground directlyto the workpiece and weld away fromthe ground.
MIG Welding Procedures Long, coiled cables act as reactorsand set up stray magnetic fields thataffect arc action. 2. Check that the wire type, wiresize, and shielding gas are correctfor the metal to be welded. 3. Set the shielding gas flow rate,proper amperage, and wire speedfor the metal being welded.
MIG Welding Procedures4. In MIG welding there are two typesof starts that may be employed to getthe bead going. In the fuse start technique, the end ofthe wire electrode acts like a fuse. Thewelding current flows through the wireuntil it becomes hot and begins to melt. When the welding gun trigger is “on”, thewire is moving out of the wire contact tip.
MIG Welding ProceduresThe object of a fuse start is to melt thewire fed out of the gun before it touchesthe base metal. When the arc first occurs, it should takeplace between the tip of the wire and thebase metal. If the arc starts at some other point along thewire, other than the tip, then an unmelted sectionwill reach the base metal. Unmelted electrode wires, stuck in the bead, arecalled whiskers.
MIG Welding Procedures The scratch start requires theelectrode wire to touch and movealong the base metal as the arcignites. The contact point between theelectrode tip and the base metal actslike a fuse.
MIG Welding Procedures Dragging the wire over the basemetal is the preferred method ofscratching.The lighter the drag pressure, thesmaller the amount of currentneeded and the better the start.
B. When ready to start thewelding process, travel speed,stickout, and gun angle areimportant considerations.
MIG Welding Procedures 1. The speed at which the arc ismoved across the base metalaffects the puddle. Proper control of the puddleprovides for good penetration, withcorrect bead width and bead height,and prevents undercutting.
MIG Welding Procedures Travel speed may also affect arcstability and the metal transferpattern. Travel speeds vary with the size ofthe electrode wire, current density,metal thickness, weld position, andkind of metal being fabricated.
MIG Welding Procedures 2. The tip-to-work distance canaffect weld penetration and weldshape, and is known as stickout. Short stickout distances (3/8 inch orless) are desirable on small-wire, lowamperage applications.
MIG Welding Procedures It is desirable to keep this distanceas short as possible to get precisionwire alignment over the joint andproper placement in the puddle.
MIG Welding Procedures 3. Holding the MIG gun at thecorrect angle is very importantsince it controls shielding gasdistribution, puddle control, andbead formation. Two angles which must be correctto make a quality weld are thetravel angle and the work angle.
Travel Angle The angle at which the MIG gunleans toward or away from thedirection of movement. A travel angle of 10 degrees to 20degrees is used for most welding. Travel angle is sometimesreferred to as drag angle.
The Work Angle Is perpendicular to the line of traveland varies considerably, dependingupon the type of weld being madeand the welding position. The work angle for a flat positionsurfacing weld should be 15degrees to 25 degrees.
4. The MIG gun may be heldthree different ways. Perpendicularto the basemetal.
4. The MIG gun may be heldthree different ways. Leaning in thedirection oftravel, alsoknown as thebackhand orpull position.
4. The MIG gun may be heldthree different ways. Leaning oppositethe direction oftravel, alsoknown as theforehand or pushposition.
C. If the weld current is stoppedinstantly, the weld puddle freezes,gases become entrapped in thebead, and porosity results.
Stopping the Weld 1. The best stop is achieved byallowing the weld current to taperdown. 2. Stopping the wire feed as quicklyas possible after the MIG guntrigger is off is desirable.
Stopping the Weld 3. Stopping the flow of shieldinggas is the last thing to be donewhen stopping a weld. The shielding gas needs to flowover the puddle until it is fullysolidified
How is the MIG welderadjusted and maintained?
The MIG welder must be setcorrectly in order to do thebest job.Machine adjustment andmaintenance are important.
Most MIG machines have a voltageadjustment in addition to the wire feedcontrol. 1. Determine what the voltage shouldbe for the kind and thickness of metaland the shielding gas being used. 2. Fine adjustments may then need tobe made so welding occurs with theright sound, bead penetration, shape,and contour.
Check specifications to see whatthe correct gas volume should befor the weld. 1. Stand to one side of the regulator,open the tank valve completely. 2. Adjust the flowmeter to thepredetermined gas volume. 3. Hold the MIG gun “on” to set to thecorrect operating volume.
Some machines have a selfcontained coolant system,while others must beconnected to a water source.If it is water cooled, be surethe water is turned on.
The nozzle should be kept clean andfree of spatter in order to properly directthe flow of shielding gases over thepuddle. 1. If filled with spatter, the nozzle may becleaned with a nozzle reamer or a roundfile. Be careful not to deform the tip whilecleaning. 2. Anti-spatter dip or spray may be put onthe nozzle to help prevent spatter buildup and to make cleaning easier.
Contact tips need to be sizedto fit the diameter of electrodewire being used. 1. The current is transmitted to thewire electrode in the contact tip. 2. Tips are usually threaded into theMIG gun so that good electricalcontact is made.
What are the safety practicesthat are observed in MIGwelding?
The following are suggestedpractices and tips that will helpto eliminate shop accidentswhen MIG welding.
Safety Practices and ProceduresA. Make sure that all welding cablesand their connections are in goodrepair. Do not use cables that are crackedor cut or have damaged insulation. Electrical connections on eachcable should be tight and not havefrayed ends or bare wires exposed.
Safety Practices and Procedures B. Wear welding gloves, helmet,leather apron, welding chaps,leather shoes, and other personalprotective equipment to helpprevent weld burns.
Safety Practices and Procedures C. When operating a MIG welder,never touch an electricalconnection, a bare wire, or amachine part which may causeelectrical shock. Never weld in damp locationsbecause of the shock hazard.
Safety Practices and Procedures D. Never weld with flammables(matches, butane lighters, fuelstick, etc.) in your pockets.
Safety Practices and Procedures E. Use pliers or tongs to handle hotmetal from the MIG weldingprocess. Never leave hot metal where othersmay touch it and be burned. F. Select the correct shaded lensfor the electrode size being used.Shades 10 and 12 arerecommended.
Safety Practices and ProceduresG. Perform all welds in a well-ventilatedarea. Welding fumes should be ventilated awayfrom the welder, not across the welder'sface. Remember that shielding gases areasphyxiants, and welding fumes areharmful. Work in well-ventilated areas to preventsuffocation or fume sickness.
Safety Practices and ProceduresH. Store inert gas cylinders in a cool,dry storage area. Do not drop or abuse gas cylinders inany way. Do not move cylinders unless the valveprotection cap is in place and tight. Check all connections with soapy waterto detect leaks.
Safety Practices and ProceduresI. Hang the welding gun on a hookwhen it is not in use. Do not hang it on the flow meter,regulator, or cylinder valve. Do not lay the gun on the work orworktable.
Safety Practices and Procedures J. Protect other workers by using awelding screen to enclose yourarea. Warn persons standing nearby, bysaying “cover”, to cover their eyeswhen your are ready to strike anarc.
Safety Practices and ProceduresK. Before starting to weld, clear thesurrounding area of possible firehazards. Remove straw, shavings, rags,paper, and other combustiblematerials.
Safety Practices and ProceduresL. Be alert for fires at all times. Because the operator’s helmet islowered, clothing may catch fire withoutbeing noticed. Depend on your senses of touch, smell,and hearing to indicate that somethingis wrong. In case of a clothing fire, strip off thearticle if possible.
Safety Practices and ProceduresL. Be alert for fires at all times. Do not run, as running fans theflames. Wrap yourself in a fire blanket, orimprovise with a coat or piece ofcanvas. If there is nothing at hand to wrapin, drop to the floor and roll slowly.
Safety Practices and ProceduresM. Protect hoses and welding cablesfrom being stepped on or run over byvehicles. Do not allow them to become tangled orkinked. Position them so they are not a trippinghazard. Protect them from flying sparks, hot metal, oropen flame, and from oil and grease that willcause rubber to deteriorate.
Safety Practices and Procedures N. Always unplug the welder andput all equipment away when youhave finished welding for the day.
Review/Summary. 1. Explain the advantages of the metalinert gas (MIG) welding process. 2. Describe the equipment, types ofshielding gases, and electrodes used inthe MIG welding process. 3. Describe the types of metal transferpatterns used in MIG welding and relatetheir applications.
Review/Summary. 4. Describe the correct techniquesfor starting, controlling, andstopping an MIG bead. 5. Explain how to adjust andmaintain the MIG welder. 6. Identify safety practices thatshould be observed in MIG welding.
Describe the correct techniques for starting, controlling, and stopping an MIG bead. 5. Explain how to adjust and maintain the MIG welder. 6. Identify safety practices that . current, kind of shielding gas used, and diameter of the wire electrode. A. Globular Transfer Patterns When the molten metal from the wire electrode travels across the .
