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1、Unit A Mechanical Systems and Technology Lesson 7Applying Metal Inert Gas (MIG)Welding TechniquesNext Generation Science/Common Core Standards Addressed!CCSS.ELALiteracy.RST.910.1 Cite specific textual evidence to support analysis of science and technical texts, attending to the precise details of e
2、xplanations or descriptions.CCSS.ELALiteracy.RST.910.3 Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks, attending to special cases or exceptions defined in the text.CCSS.Math.Content.HSGCO.D.12 Make formal geometric con
3、structions with a variety of tools and methods (compass and straightedge,string, reflective devices, paper folding, dynamic geometric software, etc.). Copying a segment; copying anangle; bisecting a segment; bisecting an angle; constructing perpendicular lines, including theperpendicular bisector of
4、 a line segment; and constructing a line parallel to a given line through a point not on the line.Bell Work!1. Explain the advantages of the metal inert gas (MIG) welding process.2. Describe the equipment, types of shielding gases, and electrodes used in the MIG welding process.3. Describe the types
5、 of metal transfer patterns used in MIG welding and relate their applications.4. What are the advantages of MIG welding?TermsBurnbackDuctilityGlobular transferInert gasShort arc transferSpray arc transferStickoutTransition currentTravel angleWhiskersHave you heard the term MIG WeldingWhat are the ad
6、vantages of MIG Welding?How is MIG Welding done?Interest ApproachWhat are the advantages of the MIG welding process?Metal inert gas welding (MIG) is a process in which a consumable wire electrode is fed into an arc and weld pool at a steady but adjustable rate, while a continuous envelope of inert g
7、as flows out around the wire and shields the weld from contamination by the atmosphere.The MIG welding process has several advantages which account for its popularity and increased use in the welding industries.MIG Welding AdvantagesA. Welding jobs can be performed faster with the MIG process. The c
8、ontinuous wire feed eliminates the need to change electrodes.MIG Welding AdvantagesB. Weld cleaning and preparation time is less for MIG welding than for stick electrode welds. Since the gaseous shield protects the molten metal from the atmospheric gases, there is no flux or slag, and spatter is min
9、imal.C. Less time is required to teach individuals how to MIG weld. If an individual has learned how to stick weld they can pick up on MIG welding quickly.MIG Welding AdvantagesD. Because of the fast travel speed at which MIG welding can be done, there is a smaller heat-affected zone than with the s
10、hielded metal arc welding process. The smaller heat-affected zone results in less grain growth, less distortion, and less loss of temper in the base metal.MIG Welding AdvantagesE. Both thick and thin metals can be welded successfully and economically with the MIG process.F. Less time is needed to pr
11、epare weld joints since the MIG welds are deep penetrating. Narrow weld joints can be used with MIG welding and still secure sound weldments.MIG Welding AdvantagesG. The MIG welding process can be used to join both ferrous and nonferrous metals. The development of electrode wire and the use of spool
12、 guns has made the MIG process widely used for aluminum, stainless steel, high-carbon-steel, and alloy-steel fabrication. Simply change the wire and shielding gas to adapt to different types of metal.MIG Welding AdvantagesH. The weld visibility is generally good. There is less smoke and fumes so ope
13、rator environment is improved and it is easier to see the weld formation.What equipment, types of shielding gases, and electrodes are used in the MIG welding process?To understand the MIG welding process, you must understand the equipment needed. It consists of a welder, a wire feed system, cable an
14、d welding gun assembly, shielding gas supply, and electrode wire.MIG WeldersA. Most welders used for MIG welding are direct current machines of the constant voltage type.B. MIG welding machines must be designed to produce a constant voltage. With a constant voltage MIG machine, the output voltage wi
15、ll change very little with large changes in current.MIG WeldersC. Welding voltage has an effect on bead width, spatter, undercutting, and penetration.D. The constant voltage welding machines are designed so that when the arc voltage changes, the arc current is automatically adjusted or self-correcte
16、d.WFS + wire feed speedE. Most MIG welding units have three adjustments which must be in balance to achieve a quality weld. These are voltage control, wire feed speed, and shielding gas flow rate.Wire Feeder1. The wire feeder continually draws a small diameter electrode wire from the spool and drive
17、s it through the cable assembly and gun at a constant rate of speed.2. The constant rate of wire feed is necessary to assure a smooth even arc. This must be adjustable to provide for different welding current settings that may be desired.Wire Feeder3. Wire speed varies with the metal thickness being
18、 welded, type of joint, and position of the weld.Wire FeederF. To move the electrode wire from the spool to the MIG welding gun, run the wire through a conduit and system of drive wheels. These drive wheels, depending upon their location in the wire feed unit, are either the push type or the pull ty
19、pe.Wire FeederF1. The pull-type drive wheels are located relatively close to the MIG gun and exert a pulling action on the wire. Pull-type drive wheels are used on most spool guns.2. With the push-type drive wheels, the wire goes through the wheels and is pushed through the electrode lead and out th
20、rough the MIG gun.G. Correct tension on the wire feed drive wheels is very important.1. Too little tension results in drive wheel slippage which causes the wire to be fed into the puddle at an uneven rate, giving a poor-quality weld.G. Correct tension on the wire feed drive wheels is very important.
21、2. Too much tension on the wire feed wheels results in deformation of the wire shape. This altered wire shape can make it difficult to thread the electrode through the conduit and the contact tip in the MIG gun.H. When a blockage or burnback occurs, the MIG gun should be turned off immediately to pr
22、event entanglement. A burnback occurs when the electrode wire is fused to the contact tip.I. The wire feeders have different sized drive rolls so they can accommodate different sizes and types of wire.MIG GunJ. The electrode holder is commonly referred to as the MIG gun. The MIG gun has a trigger sw
23、itch for activating the welding operation, a gas nozzle for directing the flow of the shielding gas, and a contact tip.MIG GunJ1. The nozzle on the MIG gun directs the shielding gas over the puddle during welding. A nozzle that is too large or too small may result in air from the atmosphere reaching
24、 the puddle and contaminating the weld.2. The nozzle is made of copper alloy to help remove the heat from the welding zone.K. When welding outside, where the weld zone is subjected to drafts and wind currents, the flow of shielding gas needs to be strong enough so that drafts do not blow the shieldi
25、ng gas from the weld zone. Removal of shielding gases by the wind will lead to contaminated, weakened welds.L. The contact tip helps to guide the wire electrode into the puddle as well as transmit the weld current to the electrode wire. The electrode wire actually touches the contact tip as it is fe
26、d through the MIG gun. During this contact, the weld current is transmitted to the electrode. The tip must match the wire diameter being used in the welder.M. Shielding GasThe shielding gas displaces the atmospheric air with a cover of protective gas. The welding arc is then struck under the shieldi
27、ng gas cover and the molten puddle is not contaminated by the elements in the atmosphereM. Shielding GasInert and non-inert gases are used for shielding in MIG welding. An inert gas is one whose atoms are very stable and will not react easily with atoms of other elements.1. ArgonHas a low ionization
28、 potential and therefore creates a very stable arc when used as a shielding gas. The arc is quiet and smooth sounding and has very little spatter.a. Argon is a good shielding gas for welding sheet metal and thin metal sections.Pure argon is also used for welding aluminum, copper, magnesium, and nick
29、el.b. Pure argon is not recommended for use on carbon steels.2. Helium gasConducts heat well and is preferred for welding thick metal stock. It is good for welding metals that conduct heat well, such as aluminum, copper, and magnesium.a. Helium requires higher arc voltages than argon.b. Helium-shiel
30、ded welds are wider, have less penetration and more spatter than argon-shielded welds.C. Helium is quite often used along with flux cored MIG welding wire.3. Carbon dioxideThe most often used gas in MIG welding because it gives good bead penetration, wide beads, no undercutting and good bead contour
31、 and it costs much less than argon or helium.a. The main application of carbon dioxide shielding gas is welding low and medium carbon steels.b. When using carbon dioxide shielding gas, the arc is unstable, which causes a lot of spatter.3. Carbon dioxidec. Carbon dioxide gas has a tendency to disasso
32、ciate. At high temperatures encountered in the arc zone, carbon dioxide will partially break up into oxygen and carbon monoxide.d. Good ventilation is essential to remove this deadly gas4. Gas MixturesArgon / Oxygen is used quite often when welding stainless steel. The oxygen gas helps to stabilize
33、the arc and eliminate much of the weld splatter.4. Gas Mixturesb. An argon-helium mixture is used for welding thick non-ferrous metals. This mixture gives the same arc stability as pure argon with very little spatter, and produces a deep penetrating bead.4. Gas Mixturesc. The argon-carbon dioxide mi
34、xture is used mainly for carbon steels, low alloy steels, and some stainless steel. The gas mixture helps to stabilize the arc, reduce spatter, eliminate undercutting and improve metal transfer straight through the arc.N. Gas Cylinder and GaugesThe tank supplying the shielding gas will have a gauge
35、and a gas flowmeter. The volume of gas directed over the weld zone is regulated by the flowmeter.O. Electrode WireThe selection of the correct electrode wire is an important decision and the success of the welding operation depends on the correct selection. O. Electrode WireThere are factors to cons
36、ider when selecting the correct electrode.1. Consider the type of metal to be welded and choose a filler wire to match the base metal in analysis and mechanical properties.O. Electrode Wire2. Consider the joint design. Thicker metals and complicated joint designs usually require filler wires that pr
37、ovide high ductility. Ductility is the ability to be fashioned into a new form without breaking.O. Electrode Wire3. Examine the surface condition of the metal to be welded. If it is rusty or scaly, it will have an effect on the type of wire selected.4. Consider the service requirements that the weld
38、ed product will encounter.5. MIG welding is not effective on rusty or painted surfaces as the lower voltage is not effective a scouring the surface in the same manner as a stick electrode.P. Electrode Wire ClassificationMIG electrode wire is classified by the American Welding Society (AWS). An examp
39、le is ER70S6. For carbon-steel wire, the “E” identifies it as an electrode “R” notes that it is a rod Note MIG wire is available in different diameters. The diameter of the wire must match the drive wheels and the gun tip.P. Electrode Wire ClassificationThe first two digits relate the tensile streng
40、th in 1,000 lbs. psi The “S” signifies the electrode is a solid bare wireAny remaining number and symbols relate the chemical composition variations of electrodes.What are the types of metal transfer patterns used in MIG welding and when are they used?Metal Transfer PatternsIn MIG welding, the metal
41、 from the wire electrode is transferred across the arc plasma to the puddle by globular, short arc, or spray transfer patterns. The type of transfer used for any given weld depends upon the arc voltage, current, kind of shielding gas used, and diameter of the wire electrode.A. Globular Transfer Patt
42、ernsWhen the molten metal from the wire electrode travels across the arc in large droplets, it is in the globular transfer pattern.1. Globular transfer pattern occurs at low wire feed rates, low current, and low arc volt-age settings.A. Globular Transfer Patterns3. The molten globules are two to thr
43、ee times larger than the diameter of the electrode. Surface tension holds the globules on the end of the wire electrode. When the globules become too heavy to remain on the electrode, they drop off and move across the arc. The globules do not move across the arc in an even pattern.A. Globular Transf
44、er Patterns4. Welds made with globular transfer have poor penetration and excessive spatter and are used little in MIG welding.B. Short Arc Transfer Patternis actually a series of periodic short circuits that occur as the molten tip of the advancing wire electrode contacts the workpiece and momentar
45、ily extinguishes the arc.B. Short Arc Transfer Pattern1. The droplet forms on the end of the electrode and begins to sag while the arc is ignited. The droplet sags further and touches the molten puddle. When the droplet touches the puddle, the arc is short-circuited and extinguished. MIG Welding Pro
46、cedures b. Long, coiled cables act as reactors and set up stray magnetic fields that affect arc action.2. Check that the wire type, wire size, and shielding gas are correct for the metal to be welded.3. Set the shielding gas flow rate, proper amperage, and wire speed for the metal being welded.B. Sh
47、ort Arc Transfer PatternThe droplet continues to melt and breaks off the end of the wire electrode. At this instant, the arc reignites and a new droplet begins to form.2. New droplet formation and arc shorting may occur from 20 to 200 times per second.B. Short Arc Transfer Pattern3. Short arc transf
48、er is also known as short circuiting transfer and dip transfer.a. Short arc transfer is especially good for welding in the horizontal, vertical, and overhead positions where puddle control is usually hard to maintain.b. Short arc welding is most feasible at current levels below 200 amps and with sma
49、ll-diameter electrode wire.C. Spray Arc Transfer PatternIs a spray of very fine droplets.1. Spray arc transfer is a high-heat method of welding with a rapid deposition of metal. It is used for welding all common metals from 3 /32 inch to over 1 inch in thickness.2. This transfer occurs only with arg
50、on or argon-oxygen mixture of shielding gas.What is the correct technique for starting, controlling, and stopping an MIG weld?Follow proper procedures when starting, controlling, and stopping an MIG weld.MIG Welding ProceduresA. Preparing to start welding with the MIG welder requires you to make adj
51、ustments to the machine.1. Be sure the gun and ground cables are properly connected. a. If possible, attach the ground directly to the work piece and weld away from the ground. b. Long, coiled cables act as reactors and set up stray magnetic fields that affect arc action.2. Check that the wire type,
52、 wire size, and shielding gas are correct for the metal to be welded.3. Set the shielding gas flow rate, proper amperage, and wire speed for the metal being welded.B. When ready to start the welding process, travel speed, stickout, and gun angle are important considerations.MIG Welding Procedures1.
53、The speed at which the arc is moved across the base metal affects the puddle. Proper control of the puddle provides for good penetration, with correct bead width and bead height, and prevents undercutting.a. Travel speed may also affect arc stability and the metal transfer pattern.MIG Welding Proced
54、uresb. Travel speeds vary with the size of the electrode wire, current density, metal thickness, weld position, and kind of metal being fabricated.2. The tip-to-work distance can affect weld penetration and weld shape, and is known as stickout.a. Short stickout distances ( 3 /8 inch or less) are des
55、irable on small-wire, low-amperage applications.MIG Welding Procedures3. Holding the MIG gun at the correct angle is very important since it controls shielding gas distribution, puddle control, and bead formation. Two angles which must be correct to make a quality weld are the travel angle and the w
56、ork angle.a. Travel AngleThe angle at which the MIG gun leans toward or away from the direction of movement.i. A travel angle of 10 degrees to 20 degrees is used for most welding.ii. Travel angle is sometimes referred to as drag angle.b. The Work AngleIs perpendicular to the line of travel and varie
57、s considerably, depending upon the type of weld being made and the welding position. The work angle for a flat position surfacing weld should be 15 degrees to 25 degrees.4. The MIG gun may be held three different ways.a. Perpendicular to the base metal.4. The MIG gun may be held three different ways
58、.b. Leaning in the direction of travel, also known as the backhand or pull position.4. The MIG gun may be held three different ways.c. Leaning opposite the direction of travel, also known as the forehand or push position.C. If the weld current is stopped instantly, the weld puddle freezes, gases bec
59、ome entrapped in the bead, and porosity results.Stopping the Weld1. The best stop is achieved by allowing the weld current to taper down.2. Stopping the wire feed as quickly as possible after the MIG gun trigger is off is desirable.3. Stopping the flow of shielding gas is the last thing to be done w
60、hen stopping a weld. The shielding gas needs to flow over the puddle until it is fully solidifiedHow is the MIG welder adjusted and maintained?A. Most MIG machines have a voltage adjustment in addition to the wire feed control.1. Determine what the voltage should be set for the kind and thickness of
61、 metal and the shielding gas being used.2. Fine adjustments may then need to be made so welding occurs with the right sound, bead penetration, shape, and contour.B. Check specifications to see what the correct gas volume should be for the weld.1. Stand to one side of the regulator, open the tank val
62、ve completely.2. Adjust the flowmeter to the predetermined gas volume.3. Hold the MIG gun “on” to set to the correct operating volume.D. The nozzle should be kept clean and free of spatter in order to properly direct the flow of shielding gases over the puddle.1. If filled with spatter, the nozzle m
63、ay be cleaned with a nozzle reamer or a round file. Be careful not to deform the tip while cleaning.2. Anti-spatter dip or spray may be put on the nozzle to help prevent spatter build-up and to make cleaning easier.E. Contact tips need to be sized to fit the diameter of electrode wire being used.1.
64、The current is transmitted to the wire electrode in the contact tip.2. Tips are usually threaded into the MIG gun so that good electrical contact is made.What are the safety practices that are observed in MIG welding?Safety Practices and ProceduresA. Make sure that all welding cables and their conne
65、ctions are in good repair. Do not use cables that are cracked or cut or have damaged insulation. Electrical connections on each cable should be tight and not have frayed ends or bare wires exposed.Safety Practices and ProceduresB. Wear welding gloves, helmet, leather apron, welding chaps, leather sh
66、oes, and other personal protective equipment to help prevent weld burns.Safety Practices and ProceduresC. When operating a MIG welder, never touch an electrical connection, a bare wire, or a machine part which may cause electrical shock. Never weld in damp locations because of the shock hazard.Safet
67、y Practices and ProceduresD. Never weld with flammables (matches, butane lighters, fuel stick, etc.) in your pockets.Safety Practices and ProceduresE. Use pliers or tongs to handle hot metal from the MIG welding process. Never leave hot metal where others may touch it and be burned.F. Select the cor
68、rect shaded lens for the electrode size being used. Shades 10 and 12 are recommended.Safety Practices and ProceduresG. Perform all welds in a well-ventilated area. Welding fumes should be ventilated away from the welder, not across the welders face. Remember that shielding gases are asphyxiants, and
69、 welding fumes are harmful. Work in well-ventilated areas to prevent suffocation or fume sickness.Safety Practices and ProceduresH. Store inert gas cylinders in a cool, dry storage area. Do not drop or abuse gas cylinders in any way. Do not move cylinders unless the valve protection cap is in place
70、and tight. Check all connections with soapy water to detect leaks.Safety Practices and ProceduresI.Hang the welding gun on a hook when 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 or worktable.Safety Practices and ProceduresJ. Prote
71、ct other workers by using a welding screen to enclose your area. Warn persons standing nearby, by saying “flash” or“cover”, to cover their eyes when your are ready to strike an arc.Safety Practices and ProceduresK. Before starting to weld, clear the surrounding area of possible fire hazards. Remove
72、straw, shavings, rags, paper, and other combustible materials.Safety Practices and ProceduresL. Be alert for fires at all times.Because the operators helmet is lowered, clothing may catch fire without being noticed. Depend on your senses of touch, smell, and hearing to indicate that something is wro
73、ng. In case of a clothing fire, strip off the article if possible.Safety Practices and ProceduresL. Be alert for fires at all times. (Cont.)Do not run, as running fans the flames. Wrap yourself in a fire blanket, or improvise with a coat or piece of canvas. If there is nothing at hand to wrap in, drop to the floor and roll slowly.The End!
