Presentation on theme: "Arc Welding Processes The first series of welding processes that we will investigate are the arc welding processes."— Presentation transcript:
1 Arc Welding ProcessesThe first series of welding processes that we will investigate are the arc welding processes.
2 Arc Welding Processes Lesson Objectives When you finish this lesson you will understand:The similarities and difference between some of the various arc welding processesFlux and gas shielding methodsAdvantages and disadvantages of the arc welding processesNeed to select between the processesLearning ActivitiesRead Handbook Pp 1-16,Look up KeywordsView Slides;Read Notes,Listen tolectureDo on-line workbookDo homeworkKeywordsWelding Flux, Inert Shielding Gas, Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), Metal Transfer Mode, Flux Cored Arc Welding FCAW), Submerged Arc Welding (SAW),
3 As seen from this slide, the arc welding processes fall under a larger category labeled fusion welding, with an electrical energy source. We will be paying particular attention to processes represented by the top most lines where shielding is provided by either a flux or a shielding gas.Linnert, Welding Metallurgy,AWS, 1994
4 Arc Welding ProcessesWelding processes that employ an electric arc are the most prevalent in industryShielded Metal Arc WeldingGas Metal Arc WeldingFlux Cored Arc WeldingSubmerged Arc WeldingGas Tungsten Arc WeldingThese processes are associated with molten metalElectric ArcArc welding processes are used primarily on metals. The processes we will review are shielded metal arc welding or SMAW and sometimes called “stick” welding; gas metal arc welding GMAW, sometimes call Mig welding; flux cored arc welding FCAW; submerged arc welding SAW; and gas tungsten arc welding GTAW, sometimes called Tig welding.In addition, we will briefly look at plasma arc welding, Electrogas and electroslag welding and Arc Stud Welding processes.
5 Arc welding processes use an electric arc as a heat source to melt metal. The arc is struck between an electrode and the workpiece to be joined. The electrode can consist of consumable wire or rod, or may be a non-consumable tungsten electrode.The process can be manual, mechanized, or automated. The electrode can move along the work or remain stationary while the workpiece itself is moved. A flux or shielding gas is employed to protect the molten metal from atmosphere.If no filler metal is added, the melted weld is referred to as autogenous. If the filler metal matches the base metal, it is referred to as homogenous. If the filler metal is different from the base metal, it is referred to as heterogeneous.Linnert, Welding Metallurgy,AWS, 1994
6 Protection of the Molten Weld Pool Molten metal reacts with the atmosphereOxides and nitrides are formedDiscontinuities such as porosityPoor weld metal propertiesAll arc welding processes employ some means of shielding the molten weld pool from the airContamination of the weld pool, by the atmosphere, can cause weld defects. These defects can have an adverse effect on the joint efficiency, which may lead to failure. Therefore, the weld pool should be protected from the atmosphere until it has completely solidified.A variety of fluxes and shielding gases are employed by the arc welding process to provide atmospheric shielding.
7 Welding Flux Three forms GranularElectrode wire coatingElectrode coreFluxes melt to form a protective slag over the weld poolOther purposesContain scavenger elements to purify weld metalContain metal powder added to increase deposition rateAdd alloy elements to weld metalDecompose to form a shielding gasWelding fluxes have three forms. Flux can be poured over the weld pool in a granular form as in submerged arc welding, it can be coated on the exterior of the electrode as in shielded arc welding, or it can be placed in the interior core of the electrode as in flux-cored arc welding.During the process, the flux melts or vaporizes. The vaporized flux forms a protective atmosphere around the molten pool. The melted flux flows over the surface of the weld pool, further protecting it from atmosphere contamination. This molten flux is referred to as slag, and solidifies upon cooling as a hard covering over the weld pool.Slag must be removed after each pass of a multi-pass weld. Any trapped slag is one of many welding defects, and can degrade joint properties. Flux compositions can be designed so that the slag peels away from the weld during cooling. This occurs because of the thermal contraction mismatch between the weld metal and slag.
8 Shielding GasShielding gas forms a protective atmosphere over the molten weld pool to prevent contaminationInert shielding gases, argon or helium, keep out oxygen, nitrogen, and other gasesActive gases, such as oxygen and carbon dioxide, are sometimes added to improve variables such as arc stability and spatter reductionShielding gas can be a single pure gas or a mixture of two or more gases.Inert gases, as the name implies, do not react with the weld metal. Argon is often used in the flat and horizontal position, since it is heavier than air. Helium can be used in the overhead position, since it is lighter than air. Helium has a characteristic of producing a “hotter” arc than argon.Active gases, such as oxygen and carbon dioxide, are often added to inert gases in order to improve arc properties. These properties include arc stability and spatter reduction.Shielding gases should be free of moisture, which decompose to hydrogen and oxygen in the arc. Moisture in the gas can result in porosity, and in steels, hydrogen can lead to cracking..The gas is regulated and measured as a flow rate in cubic feet per hour or liters per minute as it passes over the weld pool.ArgonHeliumOxygenCarbon Dioxide
9 Questions?Turn to the person sitting next to you and discuss (1 min.):What would happen if there was no flux on the wire to decompose into gas or no inert shielding gas was provided?What would the weld metal look like?
10 Shielded Metal Arc Welding SMAW Now let’s consider the Shielded Metal Arc Welding Process in more detail. This is typically a manual welding process where the heat source is an electric arc which is formed between a consumable electrode and the base material. The electrode is covered by a coating, which is extruded on the surface of the electrode. During welding, the electrode coating decomposes and melts, providing the protective atmosphere around the weld area and forming a protective slag over the weld pool.
11 Shielded Metal Arc Welding (SMAW) The arc is struck between the tip of the electrode and the workpiece. The arc is moved over the work at the appropriate arc length and travel speed, melting and fusing a portion of the base metal and continuously adding filler metal. Electrode types and sizes, (with predetermined coating compositions) are selected to correspond to the required strength levels of base metal, the types of welding power supplies utilized and depth of penetration and amount of weld metal fill required.
12 SMAW Electrode Classification Example E indicates electrode70 indicates 70,000 psi tensile strength1 indicates use for welding in all positions8 indicates low hydrogenThis slide illustrates the American Welding Society Electrode Classification System found in Code A The E7018 electrode is probably one of the most commonly used. The E indicates that this is an electrocde, the 70 indicates that the weld metal deposited has at least 70,000 pounds per square inch tnesile strength, the 1 indicates that the electorde can be used in all positions, and the 8 indicates that it is a low hydrogen electrode.Additional information May be given by a series of suffixes separated by dashes. In the second example, the “A1” indicates Chemical composition for undiluted weld metal. “H8” indicates conformity to the diffusable hydrogen test. (8 ml of H2 per 100g of deposited metal). Finally, the “R” indicates conformity to an absorbed moisture test (less than 0.4% Moisture Content).E7018-A1-H8R
13 AWS Website: http://www.aws.org ANSI/AWS : Specification for Covered Carbon SteelANSI/AWS : Specification for Low Alloy SteelANSI/AWS : Specification for Corrosion Resistant SteelCommon electrode specification can become confusing to the user and manufacturer, depending upon the number of special requirements desired for an application. Some common American Welding Society SMAW specifications for electrode classifications are presented here. You are urged to examine these. They can be obtained from AWS at the website listed.AWS Website:
14 Coating Materials -Partial List Slipping Agents to Aid ExtrusionClayTalcGlycerinBinding AgentsSodium SilicateAsbestosStarchSugarAlloying and Deoxidizing ElementsSi, Al, Ti, Mn, Ni, CrArc StabilizersTitania TiO2Gas-Forming MaterialsWood PulpLimestone CaCO3Slag-Forming MaterialsAlumina Al2O3TiO2SiO2Fe3O4
17 Shielded Metal Arc Welding SMAW AdvantagesEasily implementedInexpensiveFlexibleNot as sensitive to part fit-up variancesAn additional advantage is the familiarity most welders have with the process. This process is usually the first one taught to welders. From a cost standpoint, initial investment in the process is low in comparison to other welding processes such as gas metal arc welding. SMAW’s flexibility is unprecedented in narrow access applications and, as the above photograph shows, even in underwater welding.
18 Advantages Equipment relatively easy to use, inexpensive, portable Filler metal and means for protecting the weld puddle are provided by the covered electrodeLess sensitive to drafts, dirty parts, poor fit-upCan be used on carbon steels, low alloy steels, stainless steels, cast irons, copper, nickel, aluminumShielded Metal Arc Welding (SMAW) is by far the most widely used arc welding process. It is very popular because of it’s many advantages. The equipment is relatively easy to use, inexpensive, and portable. The filler metal and means for protecting the weld puddle are provided by the covered electrode. It is a versatile process in that it can be used on carbon steels, low alloy steels, stainless steels, cast irons, copper, nickel, and aluminum.
19 Shielded Metal Arc Welding Quality IssuesDiscontinuities associated with manual welding process that utilize flux for pool shieldingSlag inclusionsLack of fusionOther possible effects on quality are porosity, and hydrogen crackingAspects of the SMAW process present disadvantages from a quality standpoint; these include a dependence on operator technique, as well as the starting and stopping of the arc to change electrodes. Slag entrapment and lack of fusion to the basemetal or previous passes can occur during welding as a result of improper torch manipulation by the welder. Improper cleaning can also cause slag inclusion defects.In addition, at each start and stop there is a possibility of porosity being formed since it takes some time for the slag to melt and form a protective gas over the molten weld pool.
20 Shileded Metal Arc Welding LimitationsLow Deposition RatesLow ProductivityOperator DependentSMAW has a low weld metal deposition rate compared to other processes. This is because each welding rod contains a finite amount of metal. As each electrode is used, welding must be stopped and a new rod inserted into the holder. A 12-inch electrode may be able to deposit a bead 6-8 inches long.The overall productivity of the process is impeded by:Frequent changing of electrodes,Interpass cleaning (grinding, brushing, etc.),Grinding of arc initiation points and stopping points,Slag inclusions which require removal of the defect and rewelding of the defective area.
21 Other LimitationsHeat of welding too high for lead, tin, zinc, and their alloysInadequate weld pool shielding for reactive metals such as titanium, zirconium, tantalum, columbiumThere are other limitations of the SMAW process as well. The heat of the welding arc is too high for some lower melting metals. And the shielding of metals that react aggressively with the atmosphere is inadequate.
22 Questions?Turn to the person sitting next to you and discuss (1 min.):Wood (cellulose) and limestone are added to the coating on SMAW Electrodes for gas shielding. What gases might be formed?How do these gases shield?
23 Now let’s examine the gas metal arc welding process or GMAW Now let’s examine the gas metal arc welding process or GMAW. In this process, the arc is struck between a continuous wire electrode and the weld pool. The wire electrode is automatically fed from a spool into the weld pool by a wire feed system. The wire feed draws the electrode through the welding torch.Shielding is supplied by an inert gas which flows down around the wire through a gas cap attached to the torch. In some cases, supplemental trailing shielding gas and back side shielding may be provided.
24 Gas Metal Arc Welding Gas Metal Arc Welding This process utilizes a direct current electrical power supply with the electrode positive (DCEP). The positive electrode attracts electrons flowing in the circuit; these electrons act to melt the electrode wire.The welding current is varied by changing the wire feed speed. Higher wire feed speeds produce higher welding currents. The arc length can be varied by changing the voltage setting. Higher voltages produce longer arc lengths.There are four basic methods in which the wire is transferred to the molten weld pool, short circuiting, globular, pulsed spray and spray transfer. These transfer modes have been used in shops to describe the GMAW process itself. Terms such as “short arc” , “dip transfer MIG”, and “spray” are all common non-standard terms to describe the GMAW process and the mode of operation.Drawing from Welding Handbook, 8th Edition, Volume 2, American Welding Society.
25 GMAW Modes of Metal Transfer Gas Metal Arc WeldingGMAW Modes of Metal TransferSprayGlobularShort circuiting transfer characteristics:At low current and voltages, short circuit transfer occurs. The weld is a shallow penetrating weld with low heat input. Using GMAW in this mode allows welding in all positions since the weld puddle is so small. In comparison to the other three modes of transfer, this method is slowest (low productivity). Used primarily for sheet metal applications. This mode produces large amounts of spatter if welding variables are not optimized. This mode is also known as short arc or dip transfer.Globular transfer characteristics:This mode of transfer is obtained at intermediate current and voltage levels or at high current and voltage levels with 100% CO2 shielding gas. Has higher heat input and penetration than short circuit transfer. Larger weld pool makes it more difficult to weld in over-head position. It produces significant amounts of spatter.Spray and pulsed spray transfer characteristics:Spray is achieved at higher welding currents and voltages with argon or Helium based shielding gas (over 80% Ar). This high-heat-input, deep-penetrating weld limits the application to the flat position. This mode produces little or no spatter and is known for the high deposition rate (higher productivity). Pulsing the current where spray transfer occurs allows for better control for out of position welding.Pulsed SprayShort Circuiting
26 GMAW Filler Metal Designations Gas Metal Arc WeldingGMAW Filler Metal DesignationsER - 70S - 6Composition6 = high siliconElectrodeSolid ElectrodeThe classification of solid wires is shown above. The most important aspects from the classification are:The electrode is a solid wire as designated by the ER and the S,The UTS of the weld metal (70,000 psi in the above example), andThe chemical composition of the weld wire and the shielding gas used to achieve the stated ultimate tensile strength requirements as designated by the number 6.The chemical composition of the welding wire effects the usability on different surface finishes. For example, the S-6 designation shown above is used for welding over rust and oily plates since it contains a higher amount of the deoxidizer silicon.Rod (can be usedwith GMAW)Minimum ultimate tensilestrength of the weld metal
27 AWS Specifications for GMAW Wire AWS A Carbon Steel ElectrodesAWS A Low Alloy Steel ElectrodesThe American Welding Society specifications used for solid wires are presented here. The specific designations for the chemistry and shielding gas requirements can be obtained in these specifications.
28 Shielding Gas Shielding gas can affect He CO2 Ar-He Weld bead shape Gas Metal Arc WeldingShielding GasShielding gas can affectWeld bead shapeArc heat, stability, and startingSurface tensionDrop sizePuddle flowSpatterArHeCO2Ar-HeThe shielding gas, used for atmospheric shielding, affects the type of metal transfer in the process, the penetration depth, and the bead shape. The ionization potential of the gas is the ability of the gas to give up electrons and is the characteristic which determines the plasma characteristics of the arc. The ionization potential (IP) of the gas can have an effect on welding characteristics such as Arc heat, stability, & starting.Helium with high Ionization potential inhibits spray transfer in steelsCO2 with moderate Ionization potential also has limited spray transferArgon with low IP promotes the Spray mode particularly at higher currents.Surface tension of the weld pool and metal droplets are also effected by the type of shielding gas. Surface tension affects:the Drop sizePuddle flow, andSpatter.Argon results in high surface tension with shallower penetration. CO2 results in low surface tension with deeper penetration.
29 GMAW Advantages Deposition rates higher than SMAW Gas Metal Arc WeldingGMAW AdvantagesDeposition rates higher than SMAWProductivity higher than SMAW with no slag removal and continuous weldingEasily automatedThe major advantage of GMAW is the high deposition rates due to the continuously fed wire electrode. No time is lost in order to change electrodes (as in SMAW) or to remove slag (as in SMAW, FCAW, and SAW) .Unlike SMAW, which is a manual process, GMAW is easily mechanized or automated and is very often used in conjunction with robots.Picture courtesy of Panasonic Robots.
30 Quality Spatter Porosity Gas Metal Arc WeldingQualitySpatterDroplets of electrode material that land outside the weld fusion area and may or may not fuse to the base materialPorositySmall volumes of entrapped gas in solidifying weld metalThere are several quality issues.As metal drops transfer from the electrode to the weld pool, some are blown clear of the weld and form drops of spatter and the base plate. All open arc consumable electrode processes produce some spatter.Common causes of spatter are:Magnetic arc blowArc length too longToo much energy inputInsufficient inductance in the GMAW power supplyPreventative and clean up measures include:the Reduction of energy input and/or voltagethe Repositioning the ground clamp or switching to AC current (if possible)the use of Anti-spatter compounds, andScraping or light grinding after weldingThe weld pool must be protected by a shielding gas. Drafts and outdoor use can cause the shielding gas to be blown away, which can lead to porosity and lack of fusion discontinuities.
31 Limitations Equipment is more expensive and complex than SMAW Gas Metal Arc WeldingLimitationsEquipment is more expensive and complex than SMAWProcess variants/metal transfer mechanisms make the process more complex and the process window more difficult to controlRestricted accessGMAW gun is larger than SMAW holderGMAW equipment is more complex and expensive than SMAW equipment. The equipment consists of a power supply, wire feeder and a welding gun. Each of these items require more skill and knowledge to operate and maintain as compared to SMAW.The GMAW torch (or gun) is bulky compared to the SMAW electrode. This restricts the conventional use of the process to applications where there is adequate access to the weld joint by the torch. Methods have been developed for narrow gap welding using the GMAW process, however, these methods require extensive equipment and procedure development to implement.
32 Questions?Turn to the person sitting next to you and discuss (1 min.):When comparing processes that have spray and globular metal transfer, which type of transfer mode do you thnk results in more spatter? Why?
33 Flux Cored Arc Welding (FCAW) Flux Cored Arc Welding (FCAW) uses a tubular wire that is filled with a flux. The arc is initiated between the continuous wire electrode and the workpiece. The flux, which is contained within the core of the tubular electrode, melts during welding and shields the weld pool from the atmosphere. Direct current, electrode positive (DCEP) is commonly employed as in the FCAW process.
34 There are two basic process variants; self shielded FCAW (without shielding gas) and gas shielded FCAW (with shielding gas). The difference in the two is due to different fluxing agents in the consumables, which provide different benefits to the user. Usually, self-shielded FCAW is used in outdoor conditions where wind would blow away a shielding gas. The fluxing agents in self shielded FCAW are designed to not only deoxidize the weld pool but also to allow for shielding of the weld pool and metal droplets from the atmosphere.The flux in gas-shielded FCAW provides for deoxidation of the weld pool and, to a smaller degree than in self-shielded FCAW, provides secondary shielding from the atmosphere. The flux is designed to support the weld pool for out-of position welds. This variation of the process is used for increasing productivity of out-of-position welds and for deeper penetration.Linnert, Welding Metallurgy,AWS, 1994
35 FCAW Electrode Classification Flux-Cored Arc WeldingFCAW Electrode ClassificationE70 T - 1ElectrodeType Gas, Usabilityand PerformanceMinimum UTS70,000 psiClassification for FCAW wire is designed to tell the user the ultimate tensile strength of the as welded weld metal, the position(s) it can be used in, and its usability characteristics.In the example above, The ultimate tensile strength of the weld metal is specified as 70 ksi. Positions the electrode can be used in are specified by the third item in the specification, 0- for flat and 1 for all positions. The “T” designates that this is a flux cored wire. The usability and performance of the consumable is specified after the dash. In the example above the 1 stands for a general purpose electrode using DCEP and for multi-pass welding.Flux Cored /TubularElectrodePositionAmerican Welding Society SpecificationAWS A5.20 and AWS A5.29.
37 Flux-Cored Arc Welding AdvantagesHigh deposition ratesDeeper penetration than SMAWHigh-qualityLess pre-cleaning than GMAWSlag covering helps with larger out-of-position weldsSelf-shielded FCAW is draft tolerant.The FCAW process combines the best characteristics of SMAW and GMAW. It uses a flux to shield the weld pool, although a supplemental shielding gas can be used. A continuous wire electrode provides high deposition rates.The flux for FCAW consumables can be designed to support larger weld pools out of position and provide higher penetration compared to using a solid wire (GMAW). Larger welds can be made in a single pass with larger diameter electrodes where GMAW and SMAW would need multiple passes for equivalent weld sizes. This improves productivity and reduces distortion of a weldment.
38 Flux-Cored Arc Welding LimitationsSlag must be removedMore smoke and fumes than GMAW and SAWSpatterFCAW wire is more expensiveEquipment is more expensive and complex than for SMAWAs with SMAW, the slag must be removed between passes on multipass welds. This can slow down the productivity of the application and result in possible slag inclusion discontinuities. For gas shielded FCAW, porosity can occur as a result of insufficient gas coverage.Large amounts of fume are produced by the FCAW process due to the high currents, voltages, and the flux inherent with the process. Increased costs could be incurred through the need for ventilation equipment for proper health and safety.FCAW is more complex and more expensive than SMAW because it requires a wire feeder and welding gun. The complexity of the equipment also makes the process less portable than SMAW.
39 Questions?Turn to the person sitting next to you and discuss (1 min.):What do you suppose would happen if the powder inside the core did not get compacted good?
40 Submerged arc welding (SAW) employs a granular flux which is fed into the joint around the tip of the welding torch by a hose from a flux hopper. The arc is struck between the wire and the workpiece beneath the flux cover. Both the arc and the molten weld pool are shielded by the resulting envelope of molten flux and a layer of unfused granular flux particles. A vacuum follows behind the torch to collect the unfused flux for future use.
41 Submerged Arc Welding Submerged Arc Welding The filler metal is a continuously-fed wire electrode like GMAW and FCAW. However, higher deposition rates can be achieved using SAW by using larger diameter electrodes (up to 1/4”) and higher currents ( Amperes). Since the process is almost fully mechanized, several variants of the process can be utilized such as multiple torches and narrow gap welding.
42 SAW Flux / Filler Metal Compositions Submerged Arc WeldingSAW Flux / Filler Metal CompositionsF7A2-EM12KF indicates flux70-95 ksi UTS, 58 ksi minimum yield strength, 22% elongationA - as welded; P - postweld heat treated2 - minimum impact properties of 20 20°FE indicates electrode (EC - composite electrode)M - medium manganese per AWS Specifications% nominal carbon content in electrodeK - produced from a heat of aluminum killed steelFor SAW, The American Welding Society specifies both the flux and the filler metal in a single specification. The “F” part of the specification relates to the flux, and the “E” part to the electrode. Note that an “A” or a “P” is specified to indicate whether the strength is obtained as welded or after a post weld heat treatment.A typical specification for a flux-filler metal combination is described above. Manganese and carbon content levels are important to specify for hardness and strengths that are required and for resistance of the material to cold cracking
43 Advantages High deposition rates No arc flash or glare Submerged Arc WeldingAdvantagesHigh deposition ratesNo arc flash or glareMinimal smoke and fumesFlux and wire added separately - extra dimension of controlEasily automatedJoints can be prepared with narrow groovesCan be used to weld carbon steels, low alloy steels, stainless steels, chromium-molybdenum steels, nickel base alloysSAW has the highest deposition rate of all the deep penetrating arc welding processes making it ideal for thick section and multi-pass welding. Variations of the process can utilize dual arc welding, twin arc welding, multiple torch, and narrow groove welding to increase productivity.Since the arc is completely submerged in the flux, there is no arc radiation. Screens or light filtering lenses are not needed. Additionally, the smoke and fumes are trapped within the flux and thus minimizing smoke and fumes .Since the process is simple to mechanize and easily automated, it is extremely consistent once a procedure is qualified. And it can be used on a wide variety of materials.
44 Limitations Flux obstructs view of joint during welding Submerged Arc WeldingLimitationsFlux obstructs view of joint during weldingFlux is subject to contamination Þ porosityNormally not suitable for thin materialRestricted to the flat position for grooves - flat and horizontal for filletsSlag removal requiredFlux handling equipmentThere are some limitations with the process, however. The flux which shields the arc and weld pool in SAW also obstructs the operator’s view of the joint and molten weld pool. This makes observation of the pool and joint impossible during welding; thus, correction of problems during welding can be very difficult.Because of the high current levels common to this process, it is normally not suited for thinner materials.Due to the presence of a granulated flux, submerged arc welding is limited to the flat and horizontal positions. As with SMAW and FCAW, SAW produces a slag which must be completely removed after each pass.Finally, additional flux handling equipment is required.
45 HomeworkDo Homework Assignment 2, “Arc Welding Processes” from the Assignment Page of the WE300 Website. Turn in next Class Period.
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