1 Arc Welding An Overview

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1 Arc welding - an overview1.I History of weldingMethods for joining metals have been known for thousands of years, but for most of thisperiod the only form of welding was forge welding by a blacksmith.A number of totally new welding principles emerged at the end of 19th century;sufficient electrical current could then be generated for resistance welding and arcwelding. Arc welding was initially carried out using carbon electrodes, developed byBemados, and was shortly followed by the use of steel rods. The Swede Oskar Kjellbergmade an important advance when he developed and patented the coated electrode. Thewelding result was amazing and formed the foundation of the ESAB welding company.Figure 1. I Principle of Manual Metal Arc ( M M ) welding.Another early method of welding which was also developed at that time was gaswelding. The use of acetylene and oxygen made it possible to produce a comparativelyhigh flame temperature, 3100 C, which is higher than that of other hydrocarbon basedgas.The intensity of all these heat sources enables heat to be generated in, or applied to,the workpiece quicker than it is conducted away into the surrounding metal. Consequently it is possible to generate a molten pool, which solidifies to form the unifyingbond between the parts being joined.Figure 1.2 Submerged arc welding. 2003, Woodhead Publishing Ltd

WELDING PROCESSES HANDBOOKLater, in the 1930s, new methods were developed. Up until then, all metal-arcwelding had been carried out manually. Attempts were made to automate the processusing a continuous wire. The most successful process was submerged arc welding(SAW) where the arc is "submerged" in a blanket of granular fusible flux.During the Second World War the aircraft industry required a new method for thewelding of magnesium and aluminium. In 1940 experiments began in the USA with theshielding of the arc by inert gases. By using an electrode of tungsten, the arc could bestruck without melting the electrode, which made it possible to weld with or withoutfiller material. The method is called TIG welding (Tungsten Inert Gas).Filler material(if necessary)electrodeIFigure 1.3 The TIG welding method.Some years later the MIG welding process (Metal Inert Gas) was also developedusing a continuously fed metal wire as the electrode. Initially, the shielding gases wereinert such as helium or argon. Zaruba and Potapevski tried to use C02 as this was mucheasier to obtain and by using the "dip transfer" method they did manage to reduce someof the problems caused by the intense generation of spatter; however when using a relatively reactive gas such as C02 or mixed gases such as argon/C02, the process isgenerally called MAG welding (Metal Active Gas).IFigure 1.4 The MIG/MAG welding method.The power-beam processes electron beam (EB) welding and laser welding have themost intensive of heat sources. The breakthrough of EB-welding came in 1958. Theaircraft and nuclear power industries were the first to utilise the method. The main characteristics of EB-welding are its deep and narrow penetration. Its one limitation is theneed for a vacuum chamber to contain the electron beam gun and the workpiece. 2003, Woodhead Publishing Ltd

ARC WELDING - A N OVERVIEWIn some respects, Laser welding (and cutting) have ideal characteristics. The laserbeam is a concentrated heat source, which permits high speed and very low distortion ofthe workpiece, unfortunately, a high power laser is large and expensive. The beam mustalso be conducted to the joint in some way. The light from a C02 laser must be transmitted by mirrors, while that from a Nd:YAG-laser can be carried by a thin glass fibre,which makes it attractive for use with robotic welding.In the future it should be possible to utilise lightweight diode lasers with sufficientpower for welding. The diode laser has a higher efficiency in converting electricalenergy into the light beam. Although it has not yet been possible to produce diode laserswith the same power output and beam quality as present welding laser sources; these arealready being used for welding metal up to about 1 mrn thick. The low weight and sizemake them an interesting power source for use with robotic welding.TerminologyWelding methodsDefinitions of welding processes are given in IS0 857. Reference numbers for the processes are defined in IS0 4063. These numbers are then used on drawings (IS0 2553) orin welding procedure specifications (EN 288) as references.TABLE 1.1 Reference numbers for some fusion welding methods (IS0 4063).1 Welding- method1I Metal-arc welding with coated electrode/ Reference numberIFlux-cored wire metal-arc welding without gas shield1 Submerged arc welding1141TIC weldingPlasma arc weldingOxy-fuel gas welding12I131MIG weldingMAG weldingMAG welding with flux-cored wire/I1111351361141I1531Basic termsPressure welding. Welding in which sufficient outer force is applied to cause more orless plastic deformation of both the facing surfaces, generally without the addition offiller metal. Usually, but not necessarily, the facing surfaces are heated in order to permitor to facilitate bonding.Fusion welding. Welding without application of outer force in which the facingsurface(s) must be melted. Usually, but not necessarily, molten filler metal is added.Surfacing. Producing a layer of different metal by welding, e.g. with highercorrosion, abrasion or heat resistance than the parent metal.Welding procedure specijication (WPS). A document specifying the details of therequired variables for a specific application in order to assure repeatability (EN 288).Deposition rate. Amount of metal supplied to the joint per unit time during welding. 2003, Woodhead Publishing Ltd

WELDING PROCESSES HANDBOOKFigure 1.5 Schematic presentation of the most common welding methods. 2003, Woodhead Publishing Ltd

ARC WELDING - AN OVERVIEWHeat input. The heat input has great importance for the rate of cooling of the weld. Itcan be calculated from the formula:Efficiencv*:Q m.EfficiencyV . 1000MMA: '0.75MIGIMAG: 0.90SAW: 0.90TIG: 0.80where Q heat input (kJ1mrn)U voltage (V)I current (A)V welding speed (mndmin)*) These eflciencies are close to physical measured values. Always check ifother valuesare given in the regulations or standards used by your company.Heat Afected Zone (HAZ). The heat affected zone, (Figure 1.6), is that area of thebase metal not melted during the welding operation but whose physical properties arealtered by the heat induced from the weld joint.l lToe\Heat affected zoneFigure 1.6 Fillet weld showing the location of weld toes, weld face, root and heataffected zone.ROO'Throat thickness. Fillet welds are calculated by reference to the throat size. The sizerequired is specified on drawings in terms of throat thickness, t, or the leg length, 1, seeFigure 1.7.Figure 1.7 Throat thickness (t) and leg length (I) in afillet weld 2003, Woodhead Publishing Ltd

WELDING PROCESSES HANDBOOKJoint typesJoint types are chosen with regard to the welding method and plate thickness. The idealjoint provides the required structural strength and quality without an unnecessarily largejoint volume. The weld cost increases with the size of the joint, and the higher heat inputwill cause problems with impact strength and distortion.Joint preparation can also be expensive; therefore it is preferable to use joint types wherethe joint faces are parts of the workpiece. This means that fillet welds are probably themost commonly used joints.Included angle/-\Root faceRoot gap widthFigure 1.8 Joint terminology.I IUSquare butt preparationSingle V preparationDouble V preparationuuSingle U preparationLap joint assemblyFigure 1.9 Examples ofjoint types. 2003, Woodhead Publishing LtdT-joint with singleJ preparation

ARC WELDING - AN OVERVIEWWelding positionsThere are essentially four different fundamental welding positions, namely flat, horizontal-vertical, overhead and vertical position. Vertical position welding can be carriedout as vertical upward or vertical downward welding. In addition, fillet welds can bemade in the horizontal-vertical position or in the flat position, see Figure 1.10.PA (1G) Flat or downhandPE (4G) OverheadFigure 1.10 Definitions of weldingpositions for butt welds, as given in EN 287-1. AWSdesignation in [email protected],#"PB (2F) HorizontaVVerticalPA (IF) Flat positionPD (4F) OverheadFigure 1.11 Definitions of weldingpositions forfillet welds, as given in EN 287-1. AWSdesignation in parenthesis.I.3 DistortionAll fusion-welding methods produce the weld by moving a molten pool along the joint;when the heated metal cools, the shrinkage introduces residual stresses and distortion inthe welded structure. The stresses produce longitudinal and rotational distortion.Longitudinal distortion. "Shortens" the weld, but may in many cases not be a seriousproblem. An example of this type of distortion is a welded beam that can be bent if theweld is not located symmetrically (in the centre of gravity of the cross section). If morethan one weld is used, they must be symmetrical.Rotational distortion. The rotational distortion (see Figure 1.13) can be minimised bymaking the weld bead symmetrical about the neutral axis or by having a parallel-sidedsingle pass weld, as with electron beam welding. A stiff section can also prevent thistype of distortion from appearing. 2003, Woodhead Publishing Ltd

WELDING PROCESSES HANDBOOKOriginal shapeLongitudinal distortion after weldingFigure 1.12 Longitudinal distortion.Distortion is often minimised by offsetting the joints prior to welding, or by placingweld beads in a suitable sequence.Original shape1Shape after weldingtt/ Rotational distortionPresettingIFigure 1.13 Rotational distortion can be prevented by pre-setting to compensate fordistortion.Limiting the heat input can also reduce distortion. A more intense heat source allowshigher speed, lower heat input and less distortion. See Figure 1.14.Electron beamLaserPlasmaTIGFigure 1.14 Penetration profile for some different welding methods.1.4The welding arcA welding arc is an electric discharge between two electrodes. The welding current isconducted from the electrode to the workpiece through a heated and ionised gas, calledplasma. The voltage drop and current in the arc give the amount of electric power that isreleased, the heat of which, melts the electrode and the joint faces.The power must also be high enough to keep the temperature of the arc sufficient forthe continued transport of the current. The temperature maintains ionisation of the gas,i.e. it creates electrically charged particles that carry the current. 2003, Woodhead Publishing Ltd

ARC WELDING - AN OVERVIEWDepending on the choice of shielding gas, different temperatures are needed to keep theplasma ionised. Argon, for example, is easier to ionise than helium. That means thatwelding in helium or helium-mixed gases produces a higher voltage drop and higher heatinput to the weld pool.When welding with a consumable electrode, such as MIG/MAG welding, the arc hastwo main functions. One is the above-mentioned supply of heat for melting the materials; the other is the transport of the molten electrode material down to the weld pool.This droplet transfer is very dependent on the electromagnetic forces and surface tensionin the arc region. These forces have a great influence on the behaviour of the weldingprocess, and enable one to distinguish between different arc types.Spray arcAt high current, the resulting magnetic forces are directed downwards which helps thedroplet to be released from the surface tension at the electrode. The droplet transfer ischaracterised by a stream of small droplets.Short arcAt lower current it has the opposite effect. The magnetic forces are smaller and are alsodirected upwards. The droplet hanging at the tip of the electrode tends to increase in sizeand the process runs the risk of being unstable. A way to overcome this problem is tokeep the arc length so short that the droplets will dip into the pool before they havegrown too much. Surface tension will then start the transfer of the melted material andthe tail of the droplet will be constricted by the magnetic forces, the so-called "pincheffect".No metal is transferred in the form of free droplets across the arc gap. The stability ofthe short circuiting transfer is very sensitive to variations in the shielding gas, thechemical composition of the electrode and the properties of the power source and wirefeed system.Magnetic arc blowThe force or 'arc blow' that arises when the magnetic field around the arc is notcompletely symmetrical, is a well-known problem with arc welding. In critical cases, itcan result in a defective weld.The weld pool, and thus the weld bead, can be deflected towards one side, producinga defective weld.If the arc is deflected along the joint, the width of the bead and the penetration can beaffected.The protection provided by molten slag or gas can be affected, resulting in the formation of pores.The problem becomes worse, and more noticeable, as the welding current increases,as this results in a corresponding rapid increase in all the electromagnetic forces inand around the arc. 2003, Woodhead Publishing Ltd

WELDING PROCESSES HANDBOOKPossible causesThe return current connection is asymmetricWelding close to a return current connection, or with an asymmetrically connectedconnection, is a common cause of this problem.ElectrodeArcWelding current)Workpiece,/Return cableFigure 1.15 Rule of thumb no. I: The magnetic forcespom the welding current attemptto widen the current path.The workpiece is asymmetricThe magnetic arc blow that arises when welding close to an edge or where the metalthickness increases.IFigure 1.16 Rule of thumb no. 2: umagnetic material (iron) in the workpiece isasymmetrically distributed, the arc will move in the direction where there is the mostmetal.Electrodes close to each other when using multi-electrode weldingCommon in connection with, for example, submerged arc welding. Each currentcarrying conductor is surrounded by its own magnetic field. The magnetic field from oneelectrode can interfere with the arc from an adjacent electrode.Figure 1.1 7 EfSect from a nearby electrode. 2003, Woodhead Publishing Ltd

ARC WELDING - AN OVERVIEWInduced magnetic fields from the welding currentWhen welding in steel, the workpiece can provide a path for the magnetic field. Anexample of this occurs in connection with internal longitudinal welding of a pipe or tube,where the welding current supply cable induces a magnetic flux in the tube. The jointproduces a break (also known as an air gap) in the magnetic path, so that the magneticflux spreads out and affects the arc.Permanent magnetic fieldsThese are magnetic fields from magnetic clamping bedplates, or remanence (residualmagnetisation) in the workpiece from, for example, lifting magnets, magnetic nondestructive testing or parts of jigs that have become magnetised by the welding current.Even the earth's magnetic field can be concentrated close to the ends of long steel itemslying in a north-south direction, affecting the arc.Figure 1.18 Example: Holding the electrode at an angle (see rule of thumb no. 1) cancompensate for the arc blow on asymmetric workpieces (rule of thumb no. 2).Recommended measuresDo not connect the return current connector close to the position of the weld.Welding towards the return current connection is often preferable. When weldinglong items, the current can be more evenly distributed by attaching equally longreturn current cables to each end of the object.The use of adequately sized starting and finishing discs can reduce problems at thebeginning and the end of a joint.Eddy curre tIIllICI,Magnetic fluxFigure 1.19 Eddy currents in the workpiece limit the magneticflux when welding withA C.AC welding is often better than DC welding: the interference from an externalmagnetic field is symmetrical, due to the constantly changing direction of the current,and there is less risk of interference resulting from induced fields. This is because theconstantly reversing magnetic flux is opposed by eddy currents in the workpiece. 2003, Woodhead Publishing Ltd

WELDING PROCESSES HANDBOOK1.5Shielding gasesThe most important reason to use a shielding gas is to prevent the molten metal from theharmful effect of the air. Even small amounts of oxygen in the air will oxidise thealloying elements and create slag inclusions. Nitrogen is solved in the hot melted material but when it solidifies the solubility decreases and the evaporating gas will formpores. Nitrogen can also be a cause of brittleness. The shielding gas also influences thewelding properties and has great importance for the penetration and weld bead geometry.Argon (Ar)Argon is one of the most popular shielding gases thanks to its suitable properties. As aninert gas it has no chemical interaction with other materials. Therefore it is suitable forsensible materials such as aluminium and stainless steel. At MIG welding of mild steelan addition of C02 or a small amount of oxygen will increase the welding properties,especially for short arc welding. Contents of up to 20 % C02 improves the penetration(limits the risk of lack of fusion) while 5-8 % will give reduced spatter.Helium (He)Helium like argon is an inert gas. It gives more heat input to the joint. Mixed with argonit increases welding speed and is advantageous for the penetration in thick-walledaluminium or copper where it compensates for the high heat conduction.Drawbacks with helium is a high cost and the low density. At TIG welding, highcontents of helium will reduce the ignition properties.Carbon dioxide (C02)Pure carbon dioxide (C02) can be used for short arc welding. It is a cheap gas, it hasgood properties for welding of galvanised steel and gives better safety against lack offusion than argon based gases. Drawbacks are a higher amount of spatter and the factthat the gas cannot be used for spray arc.Hydrogen (H2)Small additions of hydrogen can be used to increase heat input and welding speed in thesame manner as helium, but it is much cheaper. Because of the risk of cracks, hydrogencan only be used for welding of austenitic stainless steel. It actively reduces the oxidesand is therefore also used in root gases.Oxygen is also used as a small addition to stabilise the arc at MIG welding.Nitrogen (N2)Nitrogen can be used as an alloying element in ferritic-austenitic stainless steels. A smalladditive of nitrogen in the shielding gas compensates for the losses when welding. 2003, Woodhead Publishing Ltd

ARC WELDING - AN OVERVIEW1.6Power sourcesThe importance of the power source for the welding processThe main purpose of the power source is to supply the system with suitable electricpower. Furthermore, the power source performance is of vital importance for the weldingprocess; the ignition of the arc, the stability of the transfer of the melted electrode material and for the amount of spatter that will be generated. For this purpose it is importantthat the static and dynamic characteristics of the power source is optimised for the particular welding process.Static characteristicsThe static characteristics of a power unit can be plotted by loading the power unit withan adjustable resistive load. We speak of drooping characteristics, constant-currentcharacteristics and straight characteristics (constant-voltage hort-circuit currentbC/tCurrentFigure 1.20 Examples of a) a drooping characteristic, b) a constant-currentcharacteristic and c) a straight or slightly drooping characteristic.A constant-current characteristic is used when the arc length is controlled by thewelder, e.g. in TIG welding. If the arc length is unintentionally changed, the arc voltagechanges to maintain a constant current.A drooping characteristic is used for MMA welding, where it is an advantage if theshort-circuit cu

Figure 1.14 Penetration profile for some different welding methods. 1.4 The welding arc A welding arc is an electric discharge between two electrodes. The welding current is conducted from the electrode to the workpiece through a heated and ionised gas, called plasma. The voltage drop and current in the arc g