Welding is a fabrication process that joins materials by causing coalescence. Welding is normally carried out by melting the work pieces and adding a filler material to form a pool of molten material that cools to become a strong joint, either with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower melting point material between the work pieces to form a bond between them, without melting the work pieces. Welding usually requires a heat source to produce a high temperature zone to melt the material, though it is possible to weld two metal pieces without much increase in temperature.
There are some methods with solid phase joining. In these methods there is no melting of the electrodes, though heat is produced in the process. Also since the work pieces are closely pressed together, air is excluded during the joining process. In normal welding the melted and solidified material is normally weaker than the wrought material of the same composition. In the solid phase joining such melting does not occur and hence the method can produce joints of high quality. Metals which are dissimilar in nature can also be readily welded by these methods. In the normal welding process, joining of dissimilar metals presents problems since brittle intermetallic compounds are formed during melting.
Modern welding technology started just before the end of the 19th century with the development of methods for generating high temperature in localized zones. There are different methods and standards adopted and there is still a continuous search for new and improved methods of welding.
Though the different welding processes have their own advantages and limitations and are required for special and specific applications, manual metal arc (MMA) welding continues to enjoy the dominant position in terms of total weld metal deposited. Welding processes can be classified based on following criteria.
- Welding with or without filler material – Welding can be carried out with or without the application of filler material. When welding is done without filler material it is called ‘autogenous welding’. Earlier only gas welding was the fusion process in which joining could be achieved with or without filler material. However, with the development of many other welding processes (e.g. TIG, electron etc.) such classification created confusion since these processes fall in both the categories.
- Source of energy of welding – A number of sources of energies such as chemical, electrical, light, sound, mechanical energies etc are used. However except chemical energy all other forms of energies are generated from electrical energy. So this criterion is not a good criterion for proper classification.
- Arc and non-arc welding – In this classification all the arc welding processes come under one class and all other processes come under non arc welding class. However it is difficult to assign either of the class to processes like electro slag welding (ESW) and flash butt welding etc. under this classification and hence such classification is also not perfect.
- Fusion and pressure welding – This classification is the most widely used classification as it covers all processes in both the categories irrespective of heat source and welding with or without filler material. Fusion welding includes all those processes where molten metal solidifies freely while in pressure welding molten metal if any is retained in confined space (e.g. resistance spot welding or arc stud welding) solidifies under pressure or semisolid metal cools under pressure. Fusion and pressure welding processes are given in Fig 1 and Fig 2 respectively.
Fig 1 Fusion welding processes
Fig 2 Pressure welding processes
Major welding processes are described in short below.
Gas welding – Oxy acetylene gas welding is the most important process used for joining a variety of metals. Other fuel gases such as LPG, methane, hydrogen etc. can also be used in the place of acetylene in the gas welding. Depending on the ratio of gas oxygen ratio, three types of flame can be obtained namely (i) reducing flame, (ii) neutral flame, and (iii) oxidizing flame. The reducing flame (also called carburizing flame) has unburned carbon which may be added to the weld during welding. Carburizing flame may be fit for welding high carbon steels or for carburizing the surface of low carbon or mild steels. Neutral flame is invariably used for welding of steels and other metals. In oxidizing flame the inner zone becomes very small and a loud noise is induced. Oxidizing flame gives the highest temperature possible. The maximum temperature of oxy-acetylene flame is around 3200 deg C and the center of this heat concentration is just off the extreme tip of the white cone. Oxidizing flame usually introduces oxygen into the weld metal and is not preferred for welding steels.
The welding torch has a mixing chamber in which oxygen and acetylene is mixed and the mixture is ignited at the torch tip. Welding can be carried out in two ways. In the forehand technique the torch moves in the direction of welding with the torch inclined at 65 deg to the weld deposit. In the back hand technique the torch is inclined at 45 deg to the unweld region. Gas welding is more suitable for thin plates and sheets as its flame is not as piercing as that of arc welding. Welding time is comparatively longer and heat affected zone (HAZ) and distortion are larger than in arc welding
MMA welding or shielded metal arc welding (SMAW) – It is also known as stick welding or flux shielded arc welding (FSAW). It is the very flexible and is the widely used arc welding process. It involves striking an arc between a covered metal electrode and a work piece. The heat of the arc melts the parent metal and the electrode which mix together to form, on cooling, a continuous solid mass. An electric current, in the form of either AC or DC from a welding power supply, is used to form the electric arc between the electrode and the metals to be joined. As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag. Both of these protect the weld area from atmospheric contamination. MMA welding can be used to join steels, stainless steels, cast irons and many non ferrous materials. For many mild and high strength carbon steels, it is the preferred joining method.
Submerged arc welding (SAW) – The welding process is so named because the weld and arc zone are submerged beneath a blanket of flux. SAW heats metals using an electric arc between a bare electrode and the base material, beneath a blanket of flux material. The flux material becomes conductive when it is molten, creating a path for the current to pass between the electrode and the work piece. This process uses a continuous, solid wire electrode shielded by the flux. The flux acts to stabilize the arc during welding while shielding the molten pool from the atmosphere. The flux blanket prevents spatter and sparks, while shielding ultraviolet light and fumes that are normally a part of SMAW. It also covers and protects the weld during cooling and can affect weld composition and its properties. SAW is normally automated, but semi-automated systems are also available. The current can be either AC or DC and for automated systems, the electrodes can be a single wire or multiple solid or tubular wires, or strips. Welding can only be done in a flat or horizontal position due to the use of granular flux and the fluidity of the molten weld pool. High deposition rates can be achieved and very thick and thin materials can be welded with this process.
Flux cored arc welding (FCAW) – FCAW was developed in the early 1950s as an alternative to SMAW. The advantage of FCAW over SMAW is that it eliminates the use of the stick electrodes. This helped FCAW to overcome many of the restrictions associated with SMAW. The process is widely used because of its high welding speed and portability. It is a semi-automatic or automatic welding process designed for carbon steel, stainless steel and low-alloy steels. It uses an electric arc to produce coalescence between a continuous tubular filler metal electrode and the base materials, and can be done with or without a shield gas. FCAW requires a continuously fed consumable tubular electrode containing a flux and a constant voltage or, less commonly, a constant current welding power supply. An externally supplied shielding gas is sometimes used, but often the flux itself is relied upon to generate the necessary protection from the atmosphere, producing both gaseous protection and liquid slag protecting the weld. With gas shielded flux-cored wire, shielding agents are provided by a flux contained within the tubular electrode. An externally supplied gas augments the core elements of the electrode to prevent atmospheric contamination of the molten metal. When a shielding gas is used, the process equipment is virtually the same used in gas metal arc welding. With special voltage sensing feeders it is possible to do high quality flux cored welding with a constant current welding power supply. The process is suitable for all position welding with the correct filler metal and parameters selection.
Gas metal arc welding (GMAW) – It is also called metal inert gas (MIG) welding. GMAW is an arc welding process that incorporates the automatic feeding of a continuous, solid consumable electrode normally shielded by an externally supplied gas. The process is used to weld metals such as steel, aluminum, stainless steel and copper and can be used to weld in any position when appropriate welding parameters and equipment are selected. GMAW uses direct current electrode positive (DCEP) polarity, and the equipment offers automatic arc control. The only manual controls required to be done by the welder are gun positioning, guiding and travel speed. In this process a filler metal is stored on a spool and driven by rollers (current is fed into the wire) through a tube into a ‘torch’. The large amount of filler wire on the spool means that the process can be considered to be continuous and long, uninterrupted welds can easily be made. An inert gas is also fed along the tube and into the torch and exists around the wire. An arc is struck between the wire and the work piece and because of the high temperature of the arc a weld pool forms almost instantly. In this process the key issues are selection of the correct gas mixture, its flow rate, welding wire speed, and current. Once these have been set, the skill level required is lower than with the oxy acetylene process. The process can readily be automated. GMAW welding is now commonly carried out by robots. GMAW welding process is widely used on steels and aluminum. Although the inert gas shield keeps the weld clean, depending upon the process settings, there may be spatter of metal globules adjacent to the weld which detracts from its appearance unless they are removed.
Tandem welding – Compared to a conventional GMAW system, tandem welding uses two in line wires, one behind the other. The welding wires are fed simultaneously and melted using independent contact tips mounted in the same torch. The result is excellent weld quality with little spatter and up to three times the deposition rates and travel speeds of conventional systems. The process has ideal characteristics for automated applications.
Tungsten inert gas (TIG) or gas tungsten arc welding (GTAW) – It is a manual welding process that uses a non consumable tungsten electrode, an inert or semi inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds. In this process a non consumable tungsten electrode is used and an arc struck between this and the work piece surface. GTAW can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important.
Plasma arc welding (PAW) – PAW is a variation of GTAW. The process uses a tungsten electrode but uses plasma gas to make the arc. The arc is more concentrated than the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. The constriction process greatly increases arc voltage and the amount of ionization that takes place. In addition to raising arc temperature, the hottest area of the plasma is extended outside of the nozzle down toward the work surface. The overall result is a more concentrated heat source at a higher temperature that greatly increases the heat transfer efficiency allowing faster travel speeds. When used manually, a high level of operator skill is required. Because of its stable current, the method can be used on a wider range of material thicknesses than the GTAW process. It is much faster and can be applied to all the materials as in GTAW except magnesium. Automated welding of stainless steel is one important application of the process.
Laser beam welding (LBW) – LBW is an automated process that utilizes the heat from a concentrated beam of coherent light to join two materials. The process is used to weld all metals including steel, stainless steel, aluminum, titanium, nickel and copper, and delivers high mechanical properties and travel speeds, with low distortion and no slag or spatter. Welds may be made with or without filler metal and in many applications a shielding gas is used to protect the molten pool. The equipment used requires a significant capital investment and high level of operator skill due to the very high welding speeds and small area affected by the laser beam,
Electric resistance welding – It is a non fusion welding process. Heat is generated when high electric current is passed through a small area of the two contacting metal surfaces. The heat H generated is given by H = I² × R × t where I is current, R is resistance of the interface and t is the time of application of current. When the rise in temperature is sufficient, a large pressure is applied at the heated interface to form a weld joint. The process variables are current, time of application of current, pressure, duration of pressure applications, materials to be welded and their thickness. There are five main types of resistance welding. These are (i) spot welding, (ii) seam welding, (iii) projection welding, (iv) upset butt welding, and (v) flash butt welding.
- Spot welding – In spot welding the plates to be welded are kept one over the other, after cleaning the two surfaces in contact. Two stick electrodes are kept on both sides of the plate. A pressure is applied to the electrodes and maintained for a particular interval known as squeeze time before starting further operation. Then the current is passed through the electrodes. The time of application of current known as weld time is measured in terms of the number of cycles. The pressure is maintained during this time also. After the current is cut off, the pressure is maintained for a brief time known as hold time, so that the heated metal solidifies and forms a weld nugget. After hold time, the pressure will be released and an off-time is given before starting another spot welding operation. Too high a current causes weld expulsion, cavitation and weld cracking, reduced mechanical properties and electrode embedment in the surface. On the other hand, less current results in unfused surface and poor weld. High pressure increases the contact and decrease the contact resistance and so less heat is generated. It may lead to distortion and reduced electrode life. More time of application of current may lead to boiling, porosity, growth of nugget upto electrode face. The conductivity of the materials plays an important role in deciding the thickness of the plates that could be easily welded by spot welding. Spot welding of high carbon steels requires post weld heat treatment (PWHT). The advantages of spot welding are its adaptability to mass production, high speed of operation, cleanliness, no welding rods and less operational skill. Materials having high thermal and electrical conductivities are difficult to weld by spot welding and require special procedure.
- Seam welding – In seam welding roller type of electrodes are used. The rollers are rotated over the job as the welding proceeds. By controlling the power supply, it is possible to obtain a good heat control. The seam cools under pressure at definite intervals. The weld has less surface disturbances. As the welding proceeds the applied current tries to pass through the already welded portion, thus reducing the heating in the portion to be welded. One way of overcoming this difficulty is to increase the current as the welding progresses. Sometimes external heating like high frequency heating is adopted to offset the effect of reduced current due to shunting. The heat generated during welding is high and the rollers must be cooled by using water cooling arrangements to avoid distortion of rollers. Current interruption can also be employed so that the current flows for a specific time to supply the requisite heat to the weld and then ceases for another predetermined length of time before the next spot weld is begun. This way also controls the heating of the rollers. Seam welding can be carried out on steels, aluminum, magnesium and nickel alloys and not recommended for copper and its alloys. High frequency seam welding is suitable for finned tubes and other tubings.
- Projection welding – It is similar to spot welding except that welding is carried out at places in the materials where there are projections made for this purpose. The projections are created by pressing at the selected places in the sheet. Resistance to heat being confined to the projections welding between the parts takes place by the application of adequate pressure at the appropriate time at these points of contact. Projection welding is particularly applicable to mass production work, and is quite suitable where many spot welds are required in a restricted area. This welding method is used in welding brackets, heavy steel stampings, in the encapsulation of thyristors etc.
- Upset butt weld – Upset butt weld is obtained by bringing two pieces of metals to end to end contact under pressure and then allowing current to flow from one piece to the other. The contact surfaces should be as smooth as possible. In upset welding (as also in flash butt welding) a forge structure results as against the cast structure obtained in spot and projection welding. Welding of tools to the shank is carried out by upset welding. Resistance butt welding is employed for joining tubes
- Flash butt welding – In this welding method, the two pieces to be welded are pressed against each other by applying a pressure so that contact will be at points due to surface roughness. A high welding current is passed. The surfaces are heated upto molten condition, and as one piece is slowly advanced towards the other the molten metal is flashed out. After the faces attain plastic stage upsetting pressure is applied, leading to bonding of the two faces. Flash butt welding is different from resistance pressure welding in the sense that in this weld contacts between the two surfaces are made at some point only due to the roughness of the surface. In resistance butt weld a smooth full contact surface is preferred.
In this welding method, surface contaminations are removed in the spatter during flashing and molten metal is expelled in the final upset of forging operation. A small fin is created at the weld joint consisting of the remaining molten metal and oxides. This fin can be trimmed off by grinding. The advantage of this process lies in the fact that the molten metal and the arc afford an efficient protection to the plastic metal which ultimately forms the weld, so that the danger of oxidation can be avoided.
The applied pressure in the cold (not preheated) condition varies on the type of material. With preheating, the applied pressure can be reduced to approximately half the normal values. Flash butt welding is easily applied to highly alloyed steels which cannot be satisfactorily welded by other processes.
Electro-slag welding (ESW) – ESW offers good productivity and quality in heavy structural and pressure vessel fabrications. The weld metal in ESW process is obtained by fusion of electrode wire under the blanket of flux layers. The heat for melting is obtained as resistance heat by passage of current through slag pool covering the complete surface of the weld metal. A pool of molten slag is formed between the edges of the parts to be welded and the travelling moulding shoes. The metal electrode is dipped into the molten slag. The current passing through the electrode and the molten slag heats up the slag pool. The slag melting point is higher than those of the wire and the parent metal. Hence the electrode wire melts and the molten metal settles at the bottom of the slag pool and solidifies to form the weld metal. To keep welding stable, it is necessary for the slag pool to maintain its temperature.
In ESW the slag pool is 40-50 mm deep and it offers a conductive path between the electrode and base metal. Thus the current flow is maintained after the arc is extinguished. In contrast, in the case of SAW which appears to be similar to ESW, the arc remains stable under the molten slag, as the arc voltage is around 25-30 V, and the slag layer is rather shallow. Both non-consumable and consumable guides are used in ESW. The non consumable guide method has a contact tube which directs the wire electrode into the slag bath. The welding head moves upwards steadily along with the shoes as the weld is deposited. In the consumable guide arrangement, a consumable tube is used. The welding head remains fixed at the top of the joint. The axis of the weld is vertical. The welding machine moves upwards consistent with the deposition rate. The amount of slag remains constant. A small amount of flux has to be added to the slag. When the weld is complete the welding machine can be withdrawn. The welding wire chosen must match with the base material.
The ESW process is completely continuous and hence the productivity is higher. No edge preparation of the parts to be joined is necessary. There will be saving in the quantity of filler metal and the flux. After the welding process, the welded parts need heat treatment. The process is to be continuous and should not be interrupted. In case of interruption the molten metal will shrink forming a cavity at the centre. Normal welding defects such as slag inclusion, porosity, undercut, and notch etc., are not encountered in ESW process.
Induction pressure welding – This is a solid phase welding, obtained by the use of high frequency induction heating and by simultaneous application of pressure. Oxidation is avoided by purging with hydrogen gas. The surfaces to be joined are heated by induction current produced by an inductor in series with two capacitors, powered by a transformer with two high frequency alternators. The induced current flows in a longitudinal loop along the edges to be welded, heating them uniformly through their thickness over a certain length. Forging rolls, then weld together the fused lips, leaving a slight external flash, which is removed afterwards. The normal speed of welding depends on the power supplied. Induction pressure welding is extensively used in joining boiler grade Cr-Mo steel tubes.
Electron beam welding (EBW) – Electron beam welding is a process in which the heat required to produce fusion is obtained from the impact of a high velocity high density stream of electrons on the work piece. Upon impact the kinetic energy of the electrons is converted to thermal energy causing both vaporization and melting. The vaporization of the material beneath the beam enables the beam to penetrate into the material to be welded, with the beam and the vapour forming a hole. As the beam moves along the joint, the molten metal flows round the hole leaving the welded joint in the wake of the beam. The EBW has depth to width ratio of more than 10:1 due to the extremely high heat concentration. The beam is very narrow and the welding speed is high. The net heat input is very low.
The electron emitter is a cathode – anode system in a very high vacuum chamber. The cathode is made of tantalum or tungsten and heated to about 2560 deg C. Electron cloud is thus created near its surface. A metallic shield is fixed near the cathode to make the electric field sharper and regulate the electron flow. The electric field between cathode and anode accelerates the electrons and sets them free with considerable energy. Thus an electron beam is created which is made to impinge on the parts to be welded. Magnetic lenses are used to focus the beam on the work piece. Magnetic coils are also used for beam deflection and manipulation of the beam spot on the work piece.
The speed of welding which depends on the width and depth of the weld must be properly controlled. Otherwise it leads to either incomplete penetration or overheating. As the fusion zone in the weld joint is very narrow, there are very small disturbances in the base material. Design shrinkage allowance needed is small compared to other arc welding methods and the residual stresses produced in the component are also small. As the focal length of the EB system is quite high, the EB gun can be placed at a distance, as farther as one meter from the work pieces, unlike in electric arc or plasma jet welding. Thus welding narrow and restricted area is possible with EBW. Welding can be done over a wide range of thicknesses (0.1 mm to 100 mm) and dissimilar metals can be easily welded by the process due to precise heat control. Welding speed in EB is much higher than electric arc methods, thus reducing the welding time. Also the repeatability of EB welds is high compared to other processes.
Explosive welding (EW) – EW is a process based on the controlled application of enormous power generated by detonating explosives. The surfaces of the parts to be joined must be clean without contamination of oxides etc. These clean surfaces are pressed at pressure of the order of million kg/sq cm generated by the explosive. Combination of dissimilar metals-aluminum to steel or titanium to steel – can be readily obtained by this process. Metals which are too brittle to withstand the impact of explosion cannot be welded by this process. EW is a well suited process for cladding application. There is no upper limit for the thickness of the backer plate.
Friction welding (FW) – Friction between the two welding faces is used to create heat to the extent that the material at the two surfaces become plastic. Welding is effected by applying axial force. The friction welding process is divided into two distinct modes namely (i) conventional drive friction welding, and (ii) inertia welding.
In the conventional drive friction welding, the pieces are axially aligned. One component is rotated at a constant speed by a direct drive while the other is moved into contact with the former under axial pressure. Sufficient time is allowed for heat generation, so that the interfacial temperature makes the material plastic and permits the components to be forged together. At this stage the rotation is rapidly stopped while the pressure is still maintained to consolidate the joint.
Though the basic principle is the same, in the inertia welding kinetic energy from a rotating flywheel system is used to heat the faces of components to be welded. One component is attached to the flywheel rotating at high speed. The other component is brought to face the first, so that friction between the two generates heat. The flywheel energy is utilized to obtain a good bond between the components.
The principal variables in this process are the relative velocity, heating and forging pressure and the duration of heating. The secondary factors are pressure build up during heating and forging, deceleration during braking and the properties of the material being welded. Peripheral speeds in the range of 75 to 105 meters per minute appear to give satisfactory welds. Too high a speed may result in a wide HAZ. Lower speeds will not be able to generate sufficient heat and raise the temperature upto the required level. The forging pressure depends on the hot strength of the alloy being welded. The pressure chosen must be sufficient to weld the surfaces. Duration of the heating time depends on rotational speed, friction and the pressure. Heating time determines the heat input. Steels of all varieties, stainless steels, copper and its alloy can be easily welded by friction welding process.
Diffusion bonding (DB) – DB is a joining process which requires high temperature to enhance diffusion, but involves very little macroscopic deformation. The joint is formed without any filler metal and the microstructure and composition at the interface are the same as those of the base metals. Pressure is applied which causes local plastic and creep deformation at the temperature of operation. Bonding takes place due to diffusion and depends on temperature, time and the pressure applied. An interlayer foil or coating may be used to improve the bonding characteristics. Recrystallization and grain boundary migration at the interface occur at the final stages of bonding and these processes are essential for obtaining high strength joints and to eliminate the planar boundary interface. The pressure can be applied on the surfaces to be joined through a platen. Gas pressure can also be used to achieve the same. To preserve the clean surface the bonding can be carried out in vacuum. If gas pressure is employed for the application of load, then an argon gas chamber can be used. The important variables that will affect the bond quality are (i) surface roughness, and (ii) surface oxide films. The major advantages of DB are as below.
- Joint strength can approach that of the base metal.
- Sintered products, dissimilar materials including metal and ceramics can be joined by this process.
- Bonding involves minimum deformation and distortion and hence close dimensional control is possible.
- In metal ceramics joints residual stresses can be reduced by multiple interpayers.
- Thin sheets of fine grained superplastic materials can be easily joined and formed to any desired shape and contour by diffusion bonding.
- Large area bonding is possible and thick and thin sections can be easily joined. Process time is independent of area or number of components.
- Machining cost is reduced and no flux or electrode is necessary.