Welding and joining processes are necessary for the development of virtually every manufactured product. However, these processes frequently appear to consume higher fractions of the product cost and to create more of the production difficulties than can be expected. There are a number of reasons, as given below, which explain this situation.
First, welding and joining are multifaceted, both in terms of the process variations (such as fastening, adhesive bonding, soldering, brazing, arc welding, diffusion bonding, and resistance welding etc.) and in the disciplines needed for problem solving (such as mechanics, materials science, physics, chemistry, and electronics etc.). An engineer with unusually broad and deep training is needed to bring these disciplines together and to apply them effectively to a variety of the processes.
Second, welding or joining difficulties normally occur far into the manufacturing process, where the relative value of scrapped components is high.
Third, a very large percentage of product failures occur at joints since they are normally located at the highest stress points of an assembly and are hence the weakest parts of the assembly. Careful attention to the joining processes can produce large rewards in the manufacturing economy and product reliability.
Since there are several fusion welding processes, one of the main difficulties for the manufacturing engineer is to determine which process is going to produce acceptable properties at the lowest cost. There are no simple answers. Any change in the part geometry, material, value of the end product, or size of the production run, as well as the availability of joining equipment, can influence the choice of joining method. For small lots of complex parts, fastening can be preferable to welding, whereas for long production runs, welds can be stronger and less expensive.
The perfect joint is indistinguishable from the material surrounding it. Although some processes, such as diffusion bonding, can achieve results which are very close to this ideal, they are either expensive or restricted to use with just a few materials. There is no universal process which performs adequately on all materials in all geometries. However, virtually any material can be joined in some way, although joint properties equal to those of the bulk material cannot always be achieved.
The economics of joining a material can limit its usefulness, e.g., aluminum (Al) is used extensively in aircraft manufacturing and can be joined by using adhesives or fasteners, or by welding. However, none of these processes has proven economical enough to allow the extensive replacement of steel by Al in the frames of automobiles. An increased use of composites in aircrafts is limited by an inability to achieve adequate joint strength.
It is necessary that the manufacturing engineer work with the designer from the point of product conception for ensuring that compatible materials, processes, and properties are selected for the final assembly. Frequently, the designer leaves the problem of joining the parts to the manufacturing engineer. This can cause an escalation in cost and a decrease in reliability. If the design has been planned carefully and the parts have been produced accurately, the joining process becomes much easier and cheaper, and both the quality and reliability of the product are improved.
Normally, any two solids bond if their surfaces are brought into intimate contact. One factor which normally prevents this contact is surface contamination. Any freshly produced surface exposed to the atmosphere absorbs oxygen (O2), water vapour, carbon dioxide (CO2), and hydro-carbons very rapidly. If it is assumed that each molecule which hits the surface is absorbed, then the time-pressure value to produce a mono-layer of contamination is around 0.001 Pa.s (Pascal.second), e.g., at a pressure of 1 Pa, the contamination time is 0.001 second, whereas at 0.1 MPa (atmospheric pressure), it is only 0.000000001 seconds.
Welding is a fabrication process which joins materials by causing coalescence. It is a method of joining metals permanently. It normally needs 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. It is an ancient method, around 3,000 years old. The method used in ancient days was forge or blacksmith welding. 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, codes, and standards adopted and there is still a continuous search for new and improved methods of welding.
The origin of welding is perhaps traced to the shaping of metals. The term joining is normally used for adhesive bonding, soldering, brazing, and welding, which form a permanent joint between the parts, joint which cannot easily be separated. The term assembly normally refers to mechanical methods of fastening parts together.
Adhesive bonding is a process in which joining between two or more parts is accomplished by the solidification or hardening of a non-metallic adhesive material, placed between the mating edges of the parts.
In soldering, joint is made on thin metals using solder as a joining medium. The melting point of solder is less than the metals to be joined. The joint can be opened by heating up to the solder melting temperature (below 400 deg C).
In brazing, the joint is similar to soldering but has more strength. The joining medium used is brass, which has a higher melting temperature than solder. The joint can also be opened by heating up to the melting point of brass (850 deg C to 950 deg C).
Welding is normally carried out by melting the work-pieces at the mating surfaces, and adding a filler material to form a pool of molten material which 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 mating edges of the work-pieces.
During welding, the joining edges are heated and fused together with or without filler metal to form a permanent (homogeneous) bond. In other words, welding is a process of joining two or more pieces of the same or dissimilar materials to achieve complete coalescence. This is the only method of developing monolithic structures and it is frequently accomplished by the use of heat and / or pressure.
Joining methods like riveting, assembling with bolt, seaming, soldering and brazing all result in temporary joints. Welding is the only method to join metals permanently. The temporary joints can be separated if (i) the head of the rivet is cut, (ii) nut of the bolt is unscrewed, (iii) hook of the seam is opened, and (iv) more heat is given than that needed for soldering and brazing. Welded joints cannot be separated like soldering and brazing since it is made homogeneous by heating and fusing the joining edges together.
Welding is superior to other metal joining processes since it (i) produces a permanent pressure tight joint, (ii) occupies less space, (iii) gives more economy of the material, (iv) has less weight, (v) withstands high temperature and pressure equal to joined material, (vi) can be done quickly, and (vii) gives no colour change to the joints. It is the strongest joint and any type of metal of any thickness can be joined.
Welding involves localized coalescence or joining together of two metallic parts at their mating edges. The mating edges are the parts’ surfaces in contact or close proximity which are to be joined. Welding is normally performed on parts made of the same metal, but some welding operations can be used to join dissimilar metals.
Several welding processes are accomplished by heat alone, with no pressure applied, others by a combination of heat and pressure, and still others by pressure alone with no external heat supplied. In some welding processes, a filler material is added to facilitate coalescence. The assemblage of parts which are joined by welding is called a ‘weldment’.
Though the different welding processes have their own advantages and limitations and are needed for special and specific applications, manual metal arc welding (MMAW) continues to enjoy the dominant position in terms of total weld metal deposited.
Welding is normally associated with metal parts, but the process is also used for joining plastics. Its commercial and technological importance derives from (i) it provides a permanent joint and the welded parts become a single entity, (ii) the welded joint can be stronger than the parent materials if a filler metal is used which has strength properties superior to those of the parents, and if proper welding techniques are used, (iii) it is normally the most economical way to join parts in terms of material usage and fabrication costs, since alternative mechanical methods of assembly need more complex shape alterations (e.g., drilling of holes) and addition of fasteners (e.g., rivets or bolts), hence making the resulting mechanical assembly normally heavier than a corresponding weldment, and (iv) welding is not restricted to the factory environment and can be accomplished ‘in the field’.
Although welding has the advantages indicated above, it also has certain limitations and draw-backs (or potential draw-backs). These are (i) majority of the welding operations are performed manually and are expensive in terms of man-power cost, (ii) several welding operations are considered ‘skilled trades’, and the skilled personnel needed to perform these operations can be scarce, (iii) majority of the welding processes are inherently dangerous since they involve the use of high energy, (iv) since welding accomplishes a permanent bond between the components, it does not allow for convenient disassembly, in case the product is to be occasionally disassembled (e.g., for repair or maintenance), and (v) the welded joint can suffer from certain quality defects which are difficult to detect. These defects can reduce the strength of the joint.
Classification of welding processes – Welding processes can be classified based on several criteria. Important classification criteria are described below.
Welding can be carried out with or without the application of filler material. When welding is carried out without filler material it is called ‘autogenous welding’. Earlier only gas welding was the fusion process in which joining can be achieved with or without filler material. However, with the development of several other welding processes, e.g., tungsten inert gas (TIG) welding, and electron beam (EB) welding etc., such classification created confusion since these processes fall in both the categories.
A number of sources of energies such as chemical, electrical, light, sound, and mechanical energies etc. are used. However, except chemical energy all other forms of energies are generated from electrical energy. Hence, this criterion is not a good criterion for proper classification.
In the classification based on arc and non-arc welding, 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.
The classification based on fusion and pressure welding is the most widely used classification since 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 semi-solid metal cools under pressure.
One of the methods of classifying welded joints is the method used which causes the joint between metal pieces. As per this classification the methods are (i) fusion method without pressure / with pressure, and (ii) non-fusion method.
Fusion welding without pressure is a method of welding in which similar and dissimilar metals are joined together by melting and fusion their joining edges with or without the addition of filler metal but without the application of any kind of pressure. In fusion welding, intimate interfacial contact is achieved by interposing a liquid of substantially similar composition as the base metal. If the surface contamination is soluble, then it is dissolved in the liquid. If it is insoluble, then it floats away from the liquid-solid interface. Fig 1 shows fusion welding processes.
Fig 1 Fusion welding processes
During the fusion welding (2a), the joint made is permanent. The normal heating sources are (i) electric arc, (ii) fuel gas, and (iii) chemical reaction (thermit welding). One distinguishing feature of all fusion welding processes is the intensity of the heat source used to melt the liquid. Almost every concentrated heat source has been applied to the welding process. However, several of the characteristics of each type of heat source are determined by its intensity, e.g., when considering a planar heat source diffusing into a very thick slab, the surface temperature is a function of both the surface power density and the time.
Fig 2 Fusion and pressure welding and butt and bead weld
Pressure welding (Fig 2b) is a method of welding in which similar metals are joined together by heating their mating edges to plastic or partially molten state and then joined by pressing or hammering without the use of filler metal. This is fusion method of joining with pressure. Heat source can be blacksmith forge (forge welding) or electric resistance (resistance welding), or friction (friction welding). Fig 3 shows the pressure welding processes.
Fig 3 Pressure welding processes
Non-fusion welding is a method in which similar or dissimilar metals are joined together without melting the edges of the base metal by using a low melting point filler rod but without the application of pressure.
According to the sources of heat, welding processes can be broadly classified as (i) electric welding processes (heat source is electricity), (ii) gas welding processes (heat source is gas flame), and (iii) other welding processes (heat source is neither electricity nor gas flame).
Electric welding processes can be classified as (i) electric arc welding, (ii) electric resistance welding, (iii) laser welding, (iv) electron beam welding, and (v) induction welding. Electric arc welding can be further classified as (i) metallic arc welding, (ii) carbon arc welding, (iii) atomic hydrogen arc welding, (iv) inert gas arc welding / tungsten inert gas (TIG) welding, (v) CO2 gas arc welding, (vi) flux cored arc welding, (vii) submerged arc welding, (viii) electro-slag welding, and (ix) plasma arc welding. Electric resistance welding can be further classified as (i) spot welding, (ii) seam welding, (iii) butt welding, (iv) flash butt welding, and (v) projection welding.
Gas welding processes can be classified as (i) oxy-acetylene gas welding, (ii) oxy-hydrogen gas welding, (iii) oxy-coal gas welding, (iv) oxy-liquefied petroleum gas (LPG) / natural gas (NG) welding, and (v) air acetylene gas welding.
The other welding processes are (i) thermit welding, (ii) forge welding, (iii) friction welding, (iv) ultrasonic welding, (v) explosive welding, (vi) cold pressure welding, and (vii) plastic welding.
Arc welding is a group of welding processes in which the arc generated by electric power is used to melt the wire and weld pool to allow the joining of the parts. However, the process can face difficulties in welding some materials. The need to widen the range of weldable materials and to increase productivity has contributed to new arc welding processes modifications. Although the modifications techniques have been introduced at the end of the nineteenth century, widespread implementation of the arc welding process has not been possible in the past because of the poor capability of power sources to control and provide the needed dynamic and static characteristics. Fig 4 shows arc types and their working ranges.
Fig 4 Arc types and their working ranges
There is another method of classifying welding and allied processes. Different positive processes involving addition or deposition of metal are first broadly grouped as welding process and allied welding processes. Welding processes are (i) cast welding processes, (ii) fusion welding processes, (iii) resistance welding processes, and (iv) solid state welding processes. Allied welding processes are (i) metal depositing processes, (ii) soldering, (iii) brazing, (iv) adhesive bonding, (v) weld surfacing, and (vi) metal spraying.
This approach of classifying the welding process is mainly based on the way metallic pieces are united together during welding such as (i) availability and solidification of molten weld metal between parts being joined are similar to that of casting, e.g., cast welding process, (ii) fusion of mating edges for developing a weld, e.g., fusion welding process, (iii) heating of metal only to plasticize then applying pressure to forge them together, e.g., resistance welding process, and (iv) use pressure only to produce a weld joint in solid state, e.g., solid state welding process.
Cast welding processes are the welding processes in which either molten weld metal is supplied from external source or melted and solidified at very low rate during solidification like castings. The two common welding processes which are grouped under cast welding processes are (i) thermit welding, and (ii) electroslag welding. In case of thermit welding, weld metal is melted externally using exothermic heat generated by chemical reactions and the melt is supplied between the parts to be joined while in electroslag welding weld metal is melted by electrical resistance heating and then it is allowed to cool very slowly for solidification similar to that of casting.
The classification for the cast welding process is true for thermit welding where casting melt is supplied from external source but in case of electroslag welding, weld metal is obtained by melting of both electrode and base metal and is not supplied from the external source.
Fusion welding processes are those welding processes in which mating edges of plates to be welded are brought to the molten state by applying heat and cooling rates experienced by weld metal in these processes are much higher than that of casting. The heat needed for melting can be produced using electric arc, plasma, laser and electron beam, and combustion of fuel gases. Probably this is the most popular way of classifying few welding processes. Common fusion weld processes are (i) carbon arc welding, (ii) shielded metal arc welding, (iii) submerged arc welding, (iv) gas metal arc welding, (v) gas tungsten arc welding, (vi) plasma arc welding, (vi) electro-gas welding, (vii) laser beam welding, (viii) electron beam welding, and (ix) oxy-fuel gas welding.
Resistance welding processes are those welding processes in which heat needed for softening or partial melting of base metal is generated by electrical resistance heating followed by application of pressure for developing a weld joint. However, flash butt welding begins with sparks between the parts during welding instead of heat generation by resistance heating. Common resistance welding processes are (i) spot welding, (ii) projection welding, (iii) seam welding, (iv) high frequency resistance welding, (v) high frequency induction welding, (vi) resistance butt welding, (vii) flash butt welding, and (viii) stud welding.
Solid state welding processes are those welding processes in which weld joint is developed mainly by application of pressure and heat through different mechanisms such as mechanical interacting, large scale interfacial plastic deformation and diffusion etc. During solid state joining, 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 the normal welding the melted and solidified material is normally weaker than the wrought material of the same composition. In the solid state 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 processes. In the normal welding process, joining of dissimilar metals can present problems since brittle inter-metallic compounds are formed during melting.
Depending upon the quantity of heat generated during welding these are further categorized as (i) low heat input processes such as (a) ultrasonic welding, (b) cold pressure welding, and (c) explosion welding etc., and (ii) high heat input processes such as (a) friction welding, (b) forge welding, and (c) diffusion welding etc.
Types of welds – For getting different welding joints, the types of welds which are normally used are (i) bead weld (Fig 2d), (ii) groove or butt weld (Fig 2c), (iii) fillet weld (Fig 5), and (iv) plug or slot weld (Fig 5).
Bead weld is a type of weld composed of one or more stringer or weave beads deposited on an unbroken surface to get the desired properties and dimensions. Butt weld or groove weld is a weld made in the groove between two members to be joined as butt joint. Groove welds are also done on T fillet joints if the plate thickness is more than 12 millimetres (mm). Fillet weld is a weld, having a triangular cross-section, joining two surfaces at right angle to each other such as (i) butt joint (Fig 5), (ii) lap joint (Fig 5), (iii) tee joint (Fig 5), and (iv) corner joint (Fig 5). Plug or slot welds are welds used to join two over-lapping pieces of metal by welding through circular holes or slots. These welds are frequently used in the place of rivets.
Fig 5 Types of joints and welds
Some of the major welding processes are described in short below.
Oxy-fuel gas welding – Oxy-acetylene gas welding is the most popular process used for joining a variety of metals. Other fuel gases such as LPG, NG, methane, hydrogen etc. can also be used in the place of acetylene gas in gas welding. Depending on the 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 can be added to the weld during welding. Carburizing flame is suitable for welding high carbon steels or for carburizing the surface of low carbon steel 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 the oxy-acetylene flame is around 3,200 deg C and the centre of this heat concentration is just off the extreme tip of the white cone. Oxidizing flame normally introduces oxygen into the weld metal and is not preferred for the welding of steels.
The welding torch has a mixing chamber in which oxygen and fuel gas 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-degree to the weld deposit. In the back hand technique, the torch is inclined at 45-degree to the not welded 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. Fig 6 shows oxy-acetylene gas welding process.
Fig 6 Oxy-acetylene gas welding process.
Manual metal arc welding (MMAW) or shielded metal arc welding (SMAW) – It is also known as stick welding or flux shielded arc welding (FSAW). It is a 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 vapours which serve as a shielding gas and providing a layer of slag. Both of these protect the weld area from atmospheric contamination. MMAW can be used to join steels, stainless steels, cast irons, and several non-ferrous materials. For several mild and high strength carbon steels, it is the preferred joining method. Fig 7 shows shielded metal arc welding process.
Fig 7 Shielded metal arc welding process
Submerged arc welding (SAW) – The welding process is so named since 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 ultra-violet light and fumes which are normally a part of SMAW. It also covers and protects the weld during cooling and can affect weld composition and its properties.
SAW process is normally automated, but semi-automated systems are also available. The current can be either AC or DC and for the 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 because of 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. Fig 8 shows submerged arc welding process.
Fig 8 Submerged arc welding 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 several 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 steels, 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 needs 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 frequently 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 (GMAW). 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. Fig 9 shows flux cord arc welding process.
Fig 9 Flux cored arc welding process
Gas metal arc welding (GMAW) – It is also called metal inert gas (MIG) welding. GMAW is an arc welding process which 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 needed 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 quantity 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 needed 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 can be spatter of metal globules adjacent to the weld which detracts from its appearance unless they are removed. Fig 10 shows gas metal arc welding process.
Fig 10 Gas metal arc welding process
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 which 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 needs high operator skill and can only be accomplished at relatively low speeds. In this process, a non-consumable tungsten electrode is used and an arc is struck between this electrode and the work-piece surface. GTAW can be used on nearly all weldable metals, though it is more frequently applied to stainless steel and light metals. It is frequently used when quality welds are extremely important. Fig 11 shows gas tungsten arc welding process.
Fig 11 Gas tungsten arc welding process
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 hence normally restricting the technique to a mechanized process. The constriction of the process greatly increases arc voltage and the quantity of ionization which 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 which greatly increases the heat transfer efficiency allowing faster travel speeds.
When used manually, a high level of operator skill is needed. 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. Fig 12 shows plasma arc welding process.
Fig 12 Plasma arc welding process
Laser beam welding (LBW) – LBW is an automated process which 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 can be made with or without filler metal and in several applications a shielding gas is used to protect the molten pool. The equipment used needs a substantial capital investment and high level of operator skill because of the very high welding speeds and small area affected by the laser beam. Fig 13 shows laser beam welding.
Fig 13 Laser beam welding
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 the equation H = I square × 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 processes. These are (i) spot welding, (ii) seam welding, (iii) projection welding, (iv) upset butt welding, and (v) flash butt 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 is to be released and an off-time is given before starting another spot-welding operation.
Very 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 decreases the contact resistance and so less heat is generated. It can lead to distortion and reduced electrode life. More time of application of current can lead to boiling, porosity, and of nugget up to 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 needs 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 need special procedure. Fig 14 shows resistance spot welding process.
Fig 14 Resistance spot welding process
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 get 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, hence 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 because of the shunting.
The heat generated during welding is high and the rollers are to be cooled by using water cooling arrangements to avoid distortion of rollers. Current interruption can also be used so that the current flows for a specific time to supply the requisite heat to the weld and then ceases for another pre-determined 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 types of tubes. Fig 15 shows resistance seam welding process.
Fig 15 Resistance seam welding process
Projection welding 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 several spot welds are needed in a restricted area. This welding method is used in welding brackets, heavy steel stampings, and in the encapsulation of thyristors etc. Fig 16 shows projection welding process.
Fig 16 Projection welding process.
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 are to 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 used for joining tubes. Fig 17 shows the flash and upset welding processes.
Fig 17 Flash and upset welding processes
In flash butt welding method (fig 17), 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 process, weld contacts between the two surfaces are made at some point only because of the roughness of the surface. In resistance butt weld a smooth full contact surface is preferred.
In flash butt 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 around 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) – It 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 mm to 50 mm deep and it offers a conductive path between the electrode and base metal. Hence, 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 V (volts) to 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 quantity of slag remains constant. A small quantity of flux is to be added to the slag. When the weld is complete the welding machine can be withdrawn. The welding wire chosen is to 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 is 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 is not to be interrupted. In case of interruption, the molten metal shrinks, forming a cavity at the centre. Normal welding defects such as slag inclusion, porosity, undercut, and notch etc., are not encountered in ESW process. Fig 18 shows electro-slag welding process.
Fig 18 Electro-slag welding process
Induction pressure welding – It is a solid -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) – It is a process in which the heat needed 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 vapourization and melting. The vapourization 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 because of 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 around 2,560 deg C. Electron cloud is hence 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. Hence, 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 is to 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 processes 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 metre from the work-piece, unlike in electric arc or plasma jet welding. Hence, welding in 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 because of precise heat control. Welding speed in EB is much higher than electric arc methods, accordingly there is reduction of the welding time. Also, the repeatability of EB welds is high compared to other processes. Fig 19 shows electron beam welding process.
Fig 19 Electron beam welding process
Thermit welding – Thermit welding (TW) is a fusion welding process in which two metals become bonded after being heated by superheated metal which has experienced an alumino-thermic reaction. The liquid metal which results from the reaction between a metal oxide and aluminum acts as the filler metal. This exothermic process was discovered in 1898, in Germany. Presently, thermit welding is widely used in the field welding of track, where its portability and versatility are strong assets.
Metallurgical structures which are present in thermit welds depend on the chemical composition of the weld metal and on the cooling rate of the joint after pouring is completed. Fig 20 shows a typical macrostructure of a C steel rail thermit weld and the microstructure of the fusion zone, the fusion line, the end of the heat-affected zone (HAZ), and the unaffected rail. Typically, the weld is 100 % pearlite with varying degrees of coarseness. This is because of the slow cooling rate which completes transformation before reaching the martensite start (Ms) temperature and the formation of untempered martensite. Fig 20 shows thermit welding of rails and micro-structures.
Fig 20 Thermit welding of rails and micro-structures
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 are to be clean without contamination of oxides etc. These clean surfaces are pressed at pressure of the order of million kilograms per square centimeter generated by the explosive. Combination of dissimilar metals, e.g., aluminum to steel or titanium to steel, can be readily obtained by this process. Metals which are very 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 produced 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 fly-wheel 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 fly-wheel energy is utilized to get 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 metres per minute to 105 metres per minute appear to give satisfactory welds. Very high speed can result in a wide HAZ. Lower speeds are not able to generate sufficient heat and raise the temperature up to the needed level. The forging pressure depends on the hot strength of the alloy being welded. The pressure is to 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, and copper and its alloys can be easily welded by friction welding process.
Diffusion bonding (DB) – DB is a joining process which needs high temperature to improve diffusion, but involves very little macroscopic deformation. The joint is formed without any filler metal and the micro-structure 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 because of the diffusion and depends on temperature, time, and the pressure applied. An inter-layer foil or coating can be used for improving the bonding characteristics.
Recrystallization and grain boundary migration at the inter-face occur at the final stages of bonding and these processes are necessary for getting 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. For preserving 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 which affect the bond quality are (i) surface roughness, and (ii) surface oxide films.
The major advantages of DB are (i) joint strength can approach that of the base metal, (ii) sintered products, dissimilar materials including metal and ceramics can be joined by this process, (iii) bonding involves minimum deformation and distortion and hence close dimensional control is possible, (iv) in metal ceramics joints residual stresses can be reduced by multiple inter-layers, (v) thin sheets of fine grained super-plastic materials can be easily joined and formed to any desired shape and contour by diffusion bonding, (vi) large area bonding is possible and thick and thin sections can be easily joined, (vii) process time is independent of area or number of components, and (viii) machining cost is reduced and no flux or electrode is necessary.