Welding and Joining Processes

Welding and Joining Processes

Welding and joining processes are necessary for various engineering activities and in the development of virtually every manufactured product. However, these processes frequently appear to consume greater fractions of the product cost and to create more of the production difficulties than normally expected. There are a number of reasons for this.

First reason is that the welding and joining are multifaceted, both in terms of 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 processes. The second reason is that welding or joining difficulties normally occur far into the manufacturing process, where the relative value of scrapped parts is high. The third reason is that a very large percentage of product failures occur at joints because 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 great incentives in manufacturing economy and product reliability.

There are several fusion welding processes. One of the greatest 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.  The different fusion welding processes are described below.

Shielded metal arc welding

Shielded metal arc welding (SMAW) process is normally called stick, or covered electrode, welding. It is a manual welding process whereby an arc is generated between a flux-covered consumable electrode and the work piece. The process uses the decomposition of the flux covering to generate a shielding gas and to provide fluxing elements to protect the molten weld-metal droplets and the weld pool.

In the SMAW process (Fig 1), the arc is initiated by momentarily touching or ‘scratching’ the electrode on the base metal. The resulting arc melts both the base metal and the tip of the welding electrode. The molten electrode metal/flux is transferred across the arc (by arc forces) to the base-metal pool, where it becomes the weld deposit covered by the protective, less-dense slag from the electrode covering. The SMAW process is the most widely used welding process. It is the simplest, in terms of equipment requirements, but it is, perhaps, the most difficult in terms of welder training and skill-level requirements.

Fig 1 Shielded metal arc welding process

SMAW process has the greatest flexibility of all the welding processes, since it can be used in all positions (flat, vertical, horizontal, and overhead), with virtually all base-metal thicknesses (1.6 mm and higher), and in areas of limited accessibility, which is a very important capability. Flat position butt welds and horizontal fillet welds are generally considered the easiest to weld. Out-of-position welding (vertical, overhead) requires greater skill.

The circuit diagram for the SMAW process is shown in Fig 1. The equipment consists of a power source, electrode holder, and welding cables that connect the power source to the electrode holder and the work piece. Alternating current (AC), or direct current electrode negative (DCEN), or direct current electrode positive (DCEP) can be used, depending on the electrode coating characteristics.

The welding machine, or power source, is the crux of the SMAW process. Its primary purpose is to provide electrical power of the proper current and voltage to maintain a controllable and stable welding arc. Its output characteristics are to be of the constant current (CC) type. SMAW electrodes operate within the range from 25 A to 500 A. Operating arc voltage varies between 15 V and 35 V. The electrode holder, which is held by the welder, firmly grips the electrode and transmits the welding current to it.

Gas metal arc welding (GMAW) process

Gas metal arc welding (GMAW) process is an arc welding process which joins metals together by heating them with an electric arc which is established between a consumable electrode (wire) and the work piece. An externally supplied gas or gas mixture acts to shield the arc and molten weld pool. Although the basic GMAW concept was introduced in the 1920s, it was not commercially available until 1948. At first, it was considered to be fundamentally a high-current-density, small-diameter, bare-metal electrode process using an inert gas for arc shielding. Its primary application was aluminum welding. As a result, it became known as metal-inert gas (MIG) welding, which is still common nomenclature.

Subsequent process developments included operation at low current densities and pulsed direct current, application to a broader range of materials, and the use of reactive gases particularly carbon dioxide (CO2) and gas mixtures. The latter development, in which both inert and reactive gases are used, led to the formal acceptance of the term gas-metal arc welding.

The GMAW process can be operated in semi-automatic and automatic modes. All commercially important metals, such as carbon (C) steel, high-strength low-alloy steel, stainless steel, and aluminum, copper, and nickel alloys can be welded in all positions by this process if appropriate shielding gases, electrodes, and welding parameters are chosen. The advantages make the process particularly well suited to high production and automated welding applications. With the advent of robotics, GMAW process has become the predominant process choice.

In the GMAW process (Fig 2), an arc is established between a continuously fed electrode of filler metal and the work piece. After proper settings are made by the operator, the arc length is maintained at the set value, despite the reasonable changes which are to be expected in the gun-to-work distance during normal operation. This automatic arc regulation is achieved in one of the two ways. The most common method is to utilize a constant-speed (but adjustable) electrode feed unit with a variable-current (constant-voltage) power source. As the gun-to-work relationship changes, which instantaneously alters the arc length, the power source delivers either more current (if the arc length is decreased) or less current (if the arc length is increased). This changes in current causes a corresponding change in the electrode melt-off rate, thus maintaining the desired arc length.

Fig 2 Gas metal arc welding process

The second method of arc regulation utilizes a constant-current power source and a variable-speed, voltage-sensing electrode feeder. In this case, as the arc length changes, there is a corresponding change in the voltage across the arc. As this voltage change is detected, the speed of the electrode feed unit changes to provide either more or less electrode per unit of time. This method of regulation is normally limited to larger electrodes with lower feed speeds.

The characteristics of the GMAW process are best described by reviewing the three basic means by which metal is transferred from the electrode to the work namely (i) short-circuiting transfer, (ii) globular transfer, or (iii) spray transfer. The type of transfer is determined by a number of factors.

Short-circuiting transfer encompasses the lowest range of welding currents and electrode diameters associated with the GMAW process. This type of transfer produces a small, fast-freezing weld pool which is normally suited for joining thin sections, for out-of-position welding, and for bridging of large root openings. Metal is transferred from the electrode to the work piece only during a period when the electrode is in contact with the weld pool, and there is no metal transfer across the arc gap.

The electrode contacts the molten weld pool at a steady rate which can range from 20 times to over 200 times per second. As the wire touches the weld metal, the current increases and the liquid metal at the wire tip is pinched off, initiating an arc. The rate of current increase is to be high enough to heat the electrode and promote metal transfer, yet low enough to minimize spatter caused by violent separation of the molten metal drop. The rate of current increase is controlled by adjusting the power source inductance.

The optimum setting depends on the electrical resistance of the welding circuit and the melting temperature of the electrode. When the arc is initiated, the wire melts at the tip as it is fed forward toward the next short circuit. The open-circuit voltage of the power source is to be low enough so that the drop of molten metal cannot transfer until it contacts the weld metal.

Since metal transfer only occurs during short circuiting, the shielding gas has very little effect on the transfer itself. However, the gas does influence the operating characteristics of the arc and the base-metal penetration. The use of CO2 normally produces high spatter levels, when compared with inert gases, but it allows deeper penetration when welding steels. To achieve a good compromise between spatter and penetration, mixtures of CO2 and argon are frequently used.

Flux cored arc welding

In the flux cored arc welding (FCAW) process, the heat for welding is produced by an electric arc between a continuous filler metal electrode and the work piece. A tubular, flux-cored electrode makes this welding process unique. The flux contained within the electrode can make the electrode self-shielding. Alternatively, an external shielding gas is needed. FCAW process has two major variations. The gas-shielded FCAW process (Fig 3) uses an externally supplied gas to assist in shielding the arc from nitrogen (N2) and oxygen (O2) in the atmosphere. Normally, the core ingredients in gas shielded electrodes are slag formers, deoxidizers, arc stabilizers, and alloying elements. In the self-shielded FCAW process, the core ingredients protect the weld metal from the atmosphere without external shielding. Some self-shielded electrodes provide their own shielding gas through the decomposition of core ingredients. Others rely on slag shielding, where the metal drops being transferred across the arc and the molten weld pool are protected from the atmosphere by a slag covering. Many self-shielded electrodes also contain substantial amounts of deoxidizing and denitrifying ingredients to help achieve sound weld metal. Self-shielded electrodes can also contain arc stabilizers and alloying elements.

Fig 3 Flux core arc welding process

Since FCAW process combines the productivity of continuous welding with the benefits of having a flux present, it has several advantages relative to other welding processes. FCAW process enjoys widespread use in many industries. Both the gas-shielded and self-shielded FCAW processes are used to fabricate structures from C steel and low alloy steels. Both process variants are used for shop fabrication, but the self-shielded FCAW process is preferred for field use. Gas-shielded flux-cored electrodes are normally used to weld C steel, low alloy steel, and stainless steels in the construction of pressure vessels and piping. Flux-cored electrodes are also used in the automotive and heavy-equipment industries in the fabrication of frame members, axle housings, wheel rims, suspension components, and other parts.

The FCAW process utilizes semi-automatic, mechanized, and fully automatic welding systems. The basic equipment includes a power supply, wire feed system, and welding gun. The required auxiliary equipment, such as shielding gas, depends on the process variant used and the degree of automation. Fume removal equipment is also to be considered in most applications of the FCAW process. Typical semiautomatic equipment is shown in Fig 3. The recommended power supply for the semi-automatic FCAW process is a constant-voltage direct current (DC) power. Majority of power supplies used for semi-automatic FCAW have output ratings of 600 A or less.

Gas tungsten arc welding

Gas tungsten arc welding (GTAW) process is also known as HeliArc, tungsten inert gas (TIG), and tungsten arc welding. It was developed in the late 1930s when a need to weld magnesium became apparent. The melting temperature necessary to weld materials in the GTAW process is achieved by maintaining an arc between a tungsten alloy electrode and the work piece (Fig 4). Weld pool temperatures can approach 2500 deg C. An inert gas sustains the arc and protects the molten metal from atmospheric contamination. The inert gas is normally argon, helium, or a mixture of helium and argon.

Fig 4 Gas tungsten arc welding process

GTAW process is used extensively for welding stainless steel, aluminum, magnesium, copper, and reactive materials (for example, titanium and tantalum). The process can also be used to join C and alloy steels. In C steels, it is mainly used for root-pass welding with the application of consumable inserts or open-root techniques on pipe. The materials welded range from 0.05 mm to several millimeters in thickness. The GTAW process is applicable when the highest weld quality is needed.

Power supplies for GTAW are normally the constant-current type with a drooping (negative) volt-ampere (V-A) curve. Transistorized DC power supplies are in common use, and the newer rectifier-inverter supplies are very compact and versatile. The inverter supplies can be switched from constant current to constant voltage for GMAW, resulting in a very versatile piece of equipment. The inverter-controlled power supplies are more stable and have faster response times than conventional silicon-controlled rectifier (SCR) power supplies.

The welding torch (Fig 4) holds the tungsten electrode which conducts the current to the arc, and it provides a means of shielding the arc and molten metal. Welding torches rated at less than 200 A are normally gas-cooled (that is, the shielding gas flows around the conductor cable, providing the necessary cooling). Water-cooled torches are used for continuous operation or at higher welding currents and are common for mechanized or automatic welding.

The non-consumable electrodes used in GTAW are composed of tungsten or alloys of tungsten. The most common electrode is a 2 % ThO2-W alloy. This material has good operating characteristics and good stability. Thoria is radioactive, so care is to be taken when sharpening electrodes not to inhale metal dust. Lanthaniated and yttriated tungsten electrodes have the best starting characteristics in that an arc can be started and maintained at a lower voltage. Ceriated tungsten is only slightly better than the thoriated tungsten with respect to arc starting and melt-off rate. Any of the afore-mentioned electrodes produce acceptable welds. Pure tungsten is used primarily in AC welding and has the highest consumption rate. Alloys of zirconium are also used.

The shape of the electrode tip can affect the resulting weld shape. Electrodes with included angles from 60 degrees to 120 degrees are stable and give good weld penetration depth-to-width ratios. Electrodes with smaller included angles (5 degrees to 30 degrees) are used for grooved weld joints to eliminate arcing to the part side walls.

Wire feed systems are made from a number of components and vary from simple to complex. The basic system consists of a means of gripping the wire sufficiently to pull it from the spool and push it through the guide tube to the point of welding.  The wire is fed into the leading edge for cold wire feeds and into the trailing edge for hot wire feeds. Cables, hoses, and gas regulators are necessary to deliver the process consumable of electricity, water, and inert gas to the welding torch.

Plasma arc welding

Plasma arc welding (PAW) process can be defined as a gas-shielded arc welding process where the coalescence of metals is achieved via the heat transferred by an arc which is created between a tungsten electrode and a work piece. The arc is constricted by a copper alloy nozzle orifice to form a highly collimated arc column (Fig 5). The plasma is formed through the ionization of a portion of the plasma (orifice) gas. The process can be operated with or without a filler wire addition.

Fig 5 Plasma welding process

Once the equipment is set up and the welding sequence is initiated, the plasma and shielding gases are switched on. A pilot arc is then struck between a tungsten alloy electrode and the copper alloy nozzle within the torch (non-transferred arc mode), normally by applying a high-frequency open-circuit voltage. When the torch is brought in close proximity to the work piece or when the selected welding current is initiated, the arc is transferred from the electrode to the work piece through the orifice in the copper alloy nozzle (transferred arc mode), at which point a weld pool is formed (Fig 5).

The PAW process can be used in two distinct operating modes, often described as the melt-in mode and the keyhole mode. The melt-in-mode refers to a weld pool similar to that which typically forms in the GTAW process, where a bowl-shaped portion of the work piece material which is under the arc is melted. In the keyhole mode, the arc fully penetrates the work piece material, forming a nominally concentric hole, or keyhole, through the thickness. The molten weld metal flows around the arc and re-solidifies behind the keyhole, as the torch traverses the work piece.

The PAW process uses three current modes namely (i) micro-plasma (melt-in mode), (ii) medium current plasma (melt-in mode), and (iii) keyhole plasma (keyhole mode). This categorization is primarily based on the level of welding current. The micro-plasma mode is normally defined in the current range from 0.1 A to 15 A. The medium-current plasma mode ranges from 15 A to 100 A. The keyhole plasma mode is above 100 A. There is a certain degree of overlap between these current ranges. Micro-plasma and medium-current melt-in modes are used for material upto 3 mm thick, whereas the keyhole plasma mode is used for greater thicknesses and higher travel speeds.

In addition to operating in a continuous and steady direct current electrode negative (DCEN) mode, the PAW process can be carried out using DCEN pulsed current, as well as in the variable polarity mode, which uses both direct current electrode positive (DCEP) and electrode negative polarity switching. The pulsed current mode (both DCEN and DCEN/DCEP) is most frequently used when current levels (typically, above 100 A) are employed for keyhole plasma welding. Pulsing of the current widens the tolerance region of acceptance welding parameters, primarily by further stabilizing the formation of the keyhole itself.

The PAW process is generally applied when the high penetration of the keyhole welding mode can be exploited to minimize the number of welding passes and, hence, welding time. The time saved can reduce the direct labour element of the welding operation. At the other end of the scale, the micro-plasma operating mode is used to weld small, thin-section components (as low as 0.025 mm thick), where the high arc constriction and low welding current can be beneficial in controlling heat input and distortion.

The advantages of the PAW process are primarily intrinsic to the keyhole mode of operation, since greater thicknesses of metal can be penetrated in a single pass, compared with other processes, such as GTAW. This greater amount of penetration allows a reduced amount of joint preparation.

A basic PAW system consists of a power source, a plasma control console, a water cooler, a welding torch, and a gas supply system for the plasma and shielding gases (Fig 5). The power source, which supplies the main power for the welding system, is normally supplemented with a sequence controller and control console. The sequence controller sequences the timing of gas flow, arc initiation, main welding current control, and any up-slope and down-slope parameters. In its simplest form, the plasma control console controls the gas flow for plasma and shielding gases from separate flow-meters and incorporates the high-frequency pilot arc initiation circuit. The welding torch can be manual or mechanized and is water cooled to avoid torch overheating and to maximize component life.

In most PAW installations, plasma and shielding gases are supplied from separate gas cylinders, although bulk gas can readily be used. The gas supply is normally routed through the plasma control console, where the individual flow rates are set by the operator.

The power source is to be of a constant-current design. Transistorized power sources are most common, although inverter power supplies are also available. It is to have a minimum open-circuit voltage of 80 V to ensure the reliable initiation and transfer of the main arc current. The power source can be adjusted for welding current and it is to have the capability to adjust the up slope and down slope of the current.

Carbon arc welding

Carbon arc welding (CAW) utilizes what is considered to be a non-consumable electrode, made of C or graphite, to establish an arc between itself and either the work piece or another C electrode. However, this electrode erodes fairly quickly and generates carbon monoxide (CO) gas which partially replaces the air around the arc, thereby providing the molten weld with some protection.

The CAW process, which uses either single or twin electrodes (Fig 6), resembles with GTAW process, where the arc is used only as a source of heat. The single-electrode arrangement normally operates with DC electrode negative (straight polarity), using most DC power supplies. The twin-electrode arrangement normally operates with AC, normally with small AC power supplies. Although the CAW process has been almost completely replaced by the newer processes used in the welding industry, it is still used in certain applications.

Fig 6 Carbon arc welding process

CAW electrodes can be either C or graphite, although baked C electrodes are normally used since they last longer and are more readily available. The typical torch is air cooled for the smaller electrodes, but water cooled for the larger ones.

In the single-electrode operation, the C electrode is prepared to a long tapered point which is one half the diameter of the electrode. The arc is established by ‘scratch starting’, that is, by bringing the electrode into contact with the work piece and immediately withdrawing it to the correct length for welding. The generally accepted arc length is maintained between 6.4 mm and 9.5 mm. Too short of holding of the length can result in carburization of the base metal, creating potentially brittle welds. This is particularly true when welding without filler metal.

Small AC machines are normally used in the twin-electrode process because the electrodes wear at an equal rate. DC can be used as long as the positive-side electrode is one size larger than the negative electrode. The larger electrode compensates for the faster consumption rate on the positive side of the direct current.

The single-electrode C arc process is primarily used on steel, cast iron, and copper, although it can be used on most ferrous and non-ferrous materials. With steels, it is principally applied to make outside corner welds on thinner-gauge materials, where no filler metal is used. A good fit-up is needed, and a fluxing agent is frequently used to promote better welds. The resulting welds arc is smoother and frequently, more economical than similar welds created by the SMAW process.

Submerged arc welding

Submerged arc welding (SAW) is an arc welding process in which the arc is concealed by a blanket of granular and fusible flux. Heat for SAW is generated by an arc between a bare, solid-metal (or cored) consumable wire or strip electrode and the work piece. The arc is maintained in a cavity of molten flux or slag, which refines the weld metal and protects it from atmospheric contamination. Alloy ingredients in the flux can be present to enhance the mechanical properties and crack resistance of the weld deposit

Fig 7 shows a typical setup for automatic SAW process. A continuous electrode is being fed into the joint by mechanically powered drive rolls. A layer of granular flux, just deep enough to prevent flash through, is being deposited in front of the arc. Electrical current, which produces the arc, is supplied to the electrode through the contact tube. The current can be DC with electrode positive (reverse polarity), with electrode negative (straight polarity), or AC. Fig 7 also shows the melting and solidification sequence of SAW process. After welding is completed and the weld metal has solidified, the unfused flux and slag are removed. The unfused flux can be screened and reused. The solidified slag can be collected, crushed, resized, and blended back into new flux. Re-crushed slag and blends of re-crushed slag with unused (virgin) flux are chemically different from new flux. Blends of re-crushed slag can be classified as a welding flux, but cannot be considered the same as the original virgin flux.

Fig 7 Submerged arc welding process

SAW process is adaptable to both semi-automatic and fully automatic operation, although the latter, because of its inherent advantages, is more popular. In semi-automatic welding, the welder controls the travel speed, direction, and placement of the weld. A semi-automatic welding gun is designed to transport the flux and wire to the operator, who welds by dragging the gun along the weld joint. In automatic SAW, travel speed and direction are controlled mechanically. Flux can be automatically deposited in front of the arc, while the unfused flux can be picked by a vacuum recovery system behind the arc.

To increase deposition rate or welding speed, more than one wire can be fed simultaneously into the same weld pool. In tandem arc SAW, multiple electrodes are arranged with one in front of the other. Each electrode has an independent power supply and contact tip. The spacing, configuration, and electrical nature of the electrodes can be arranged to optimize welding speed and bead.

If some steel is suitable for welding with GMAW, FCAW, SMAW, or GTAW processes, procedures can be developed to weld the steel with SAW process. The main limitations of SAW are plate thickness and position. Since SAW is a high heat input and high deposition rate process, it is normally used to weld thicker-section steels. Although the welding of 1.6 mm thick steel is possible, most SAW is done on plate over 6.4 mm  in thickness.

SAW process is normally used to join plain C steels. Alloy steels can be readily welded with SAW process if care is taken to limit the heat input as needed to prevent damage to the heat-affected zone (HAZ). Low-heat-input procedures are available for welding alloy steels and heat-treated steels to prevent grain coarsening and cracking in the HAZ. Maintaining proper preheat and inter-pass temperature is also important when welding alloy steels to prevent weld metal and HAZ cracking and to develop the required mechanical properties in the weld deposit. SAW process can be used to join stainless steels and non-ferrous alloys. It is also normally used to produce a stainless or non-ferrous overlay on top of a base metal.

The most common weld deposits made with SAW process are groove, fillet, and plug welds, and surfacing deposits. For groove welds, the characteristic deep penetration of SAW process plays a role in joint selection.

Since SAW process is used to join thick steel sections, it is mainly used for shipbuilding, pipe fabrication, pressure vessels, and structural components for bridges and buildings. Other than joining, SAW process is used to build up parts and overlay with stainless or wear-resistant steel (for example, rolls for continuous casting steel, pressure vessels, rail car wheels, and equipment for mining, mineral processing, construction, and agriculture).

The power supplied to the contact tip can be DC or AC. Currents over 1,000 A DC tend to create arc blow problems. AC is normally used for high-current applications, for applications where arc blow can be a problem, and in multi-wire applications. DC power supplies can be either constant voltage or constant-current power. AC power sources are normally transformers.

To produce a submerged arc weld, both flux and electrode are consumed. Each flux and electrode combination, along with the variation of base material and process parameters, produces a unique weld deposit. Because the integrity of the weld deposit depends on these parameters, specific fluxes and electrodes are to be used in combination to optimize the weld metal properties.

Stud arc welding

Stud arc welding (SW), also known as arc stud welding, is a normally used method for joining a metal stud, or a fastener, to a metal work piece. The process has been used as an alternative metal-fastening method since the 1940s. A large number of specially designed and manufactured metal studs are welded by this process regularly in such diverse industries as construction, shipbuilding, automotive, and hard goods, as well as in miscellaneous industrial applications. The SW process represents an alternative to other welding processes, and is also a substitute for other fastening procedures, such as drilling and tapping, bolting, and self-tapping screws.

Stud arc welding is similar to many other welding processes, including arc and percussion welding, in that the base (weld end) of a specifically designed stud is joined to a base material by heating both parts with an arc which is drawn between the two. Equipment which is unique to this process regulates the arc length and arc dwell time. After an arc is struck, the stud weld end and the work piece surface are brought to the proper temperature for joining and, after a controlled period of time, the two heated surfaces are brought together under pressure, creating a metallurgical bond capable of developing the full strength of the stud.

There are two basic types of stud arc welding, which are differentiated by the source of welding power. One type uses DC power provided by a transformer / rectifier. The second type uses power discharged from a capacitor storage bank. The process based on a DC power source is known as stud arc welding, whereas the process which utilizes capacitors is known as capacitor discharge stud welding (CDSW).

Both the SW and CDSW processes overlap in some areas of application. Normally, the SW process is used in applications which need similar stud and work piece metals, the work piece thickness is greater in relation to the stud diameter, and an accommodation is to be made for the stud flash (fillet). In contrast, the CDSW process is used extensively when welding to thin sheet metal, and is used frequently with dissimilar work piece and stud alloys. It is also used in cases where marks on the opposite side of the work piece are to be avoided or minimized. With this process, the stud diameter is limited to smaller sizes. The factors on which process selection is to be based are fastener size, base-metal thickness, base-metal composition, and reverse-side marking requirements.

As shown in Fig 8, the basic equipment used for stud arc welding consists of (i) a control system, which regulates the arc time and controls gun movement, (ii) a fixed or portable stud-welding gun, which holds the stud in position during the welding process to create the proper arc length and joining pressure, and (iii) connecting cables, which are to be connected to a separate source of DC power. The other items which are needed to weld the work piece are the studs themselves and ceramic arc shields, or ferrules.

Fig 8 Stud arc welding process

A typical stud-welding gun (Fig 8) is normally available in at least two sizes namely (i) standard duty, for use with studs upto 16 mm in diameter, and heavy duty, for use on larger-diameter studs. The stud arc welding process utilizes the same principles as any other arc welding process.  With the exception of special processes, the stud arc welding of steel, stainless steel, and aluminum studs needs the use of a specifically designed weld stud and ceramic ferrule.

Obtaining a full-quality stud arc weld needs a sufficient amount of total energy input to the weld joint in order to produce melting and complete metallurgical bonding of the stud and work piece. The energy input or weld current which is necessary depends on the stud diameter. Other parameters in the stud arc welding process are arc voltage, arc time, and plunge. Plunge is the length of stud which extends past the end of the ceramic ferrule and is available for melt off during the weld cycle. Studs are reduced in length during the weld cycle.

Capacitor discharge stud welding

Capacitor discharge stud welding is a stud arc welding process in which the tip of the stud melts almost instantly when energy stored in capacitors is discharged through it. The three basic modes of capacitor discharge (CD) stud welding are initial-gap welding, initial-contact welding, and drawn-arc welding.

The initial-gap mode (Fig 9) is begun with the stud held away from the work surface by the welding head. At the beginning of the weld cycle, the stud is forced against the work surface, melting the tip, the face of the stud, and the adjoining work surface upon contact with the work surface. The weld is completed using the gun forces (i.e., spring pressure or air pressure) to plunge the stud into the molten materials, forming a strong welded bond between the stud and the work surface. The weld cycle time for this process is from 4 milli seconds (ms) to 6 ms, and the penetration of the weld zone into the work surface is normally from 0.1 mm to 0.15 mm.

Fig 9 Capacitor discharge stud welding process

The initial-contact mode begins with the weld stud in contact with the work surface. The weld cycle is initiated with a surge of current which disintegrates the weld tip, thus melting the stud face area and the work surface which it immediately contacts. The stud is forced into the molten material, forming a strong homogeneous weld. This process has a weld cycle time of around 6 ms, much like the gap process.

The drawn-arc mode begins with the stud in contact with the work surface. When the weld cycle is initiated, a current surge is applied to the weld tip and the stud is retracted from the work surface, drawing an arc which melts the face of the stud and the work surface directly beneath it. The stud is then plunged into the molten pool of material, forming a welded connection. The weld cycle time for this process is longer than for the other two processes, and the heat-affected zone (HAZ) is thicker than it is in the preceding two processes.

A major reason for using the CD stud welding process is that it provides a strong welded fastener with either a minimum or no reverse side marking of the work material. Another reason is that it is cost effective, especially for small-diameter fasteners. This process also allows fasteners to be welded to very thin sections of work material, as well as to thick sections (as thick as necessary), with reliable results. Furthermore, the process allows the welding of many dissimilar material combinations. The disadvantages of using the CD process are the limited stud diameters available and the fact that the work surface is to be clean of mill scale, dirt, oxidation products, and oil.

The stud and work piece materials can be common low C steel, stainless steel, or aluminum. Also used is medium C steel, lead-free brass and copper, Inconel, titanium etc. CD stud welding is used in many industries in a large variety of applications because it is one of the most versatile and reliable processes available.

The equipment used for CD stud welding can be either portable or stationary production-type units. The portable units consist of the basic controller which utilizes standard 110 V AC power input to charge the capacitors and a lightweight gun-shaped tool used for placing and welding the fasteners. The production-type units typically need 240 V / 480 V AC three-phase incoming power and a compressed air supply. The production units are normally used for higher rates of productivity, close-tolerance work, critical reverse side marking requirements, automatic stud feeding, automatic feeding of the part to be welded, automatic location of the stud on the part, and exotic materials.

Plasma-MIG welding

Plasma-MIG (metal inert gas) welding can be defined as a combination of plasma arc welding (PAW) and gas-metal arc welding (GMAW) within a single torch, where a filler wire is fed through the plasma nozzle orifice. The process can be used for both welding and surfacing.

The principles of operation, in terms of equipment, are shown in Fig 10. Separate power supplies are used for the PAW and the GMAW elements of the equipment. An arc is struck between the tungsten electrode and the work piece in a similar fashion to that of a PAW system. The filler wire can be fed to the plasma arc, either with or without the GMAW arc established. Without power supplied to the filler wire, the system can be operated as a PAW system with concentric feed of filler wire. Later versions of the system incorporated an annular electrode to replace the offset tungsten electrode in the welding torch.

Fig 10 Plasma MIG welding process

The equipment can be operated either with a single power source, effectively as a PAW system with concentric filler wire feed, or with two power sources, for the plasma-MIG operation. The polarity of the tungsten electrode is DC, electrode negative (DCEN), as is that of the GMAW part of the system. The heat of the plasma arc is sufficient to achieve good metal transfer stability for the GMAW element, despite the fact that when this process is used separately, it is almost always used in a DC, electrode positive (DCEP) mode. The filler wire is heated by the constricted plasma arc, as well as by the cathode heating of its own arc, and by resistance heating along the wire extension. Hence, the melting and deposition rates of the wire are higher than the rates achieved by heating with either arc alone.

Metal transfer is governed not only by plasma streaming, but also by arc forces between the wire tip and the work piece. Because the metal droplets are totally enclosed by the plasma stream, spray transfer takes place even though the GMAW element operates on negative polarity.

The advantages of the plasma-MIG process include deposition rates and joint completion rates which are higher than those of the conventional GMAW process. The independent control of the plasma arc and current to the filler wire leads to more control of metal deposition. This capability can yield improved productivity and good flexibility for controlling heat input and arc characteristics in both welding and surfacing operations. Good control of dilution is achieved by running the system without any power applied to the filler wire. Metal transfer stability is increased, compared to that of the conventional GMAW process, and results in lower spatter levels.

The basic equipment includes a power source for the plasma arc and a power source for the GMAW part of the system. A special torch incorporating both a contact tip for the GMAW element and a cathode for the PAW element is needed. The initial design incorporated an offset tungsten electrode, as well as a concentric conduit and contact tip for the delivery of the consumable wire. A later design incorporated a concentric cathode for the plasma arc.

A constant-current power source with a high-frequency circuit to initiate the pilot arc is used for the plasma arc component of the system. The power source for the GMAW component can be used as a constant-voltage or a constant-current rectifier. Power sources have welding currents which typically range from 40 A to 200 A for the plasma arc and from 60 A to 300 A for the GMAW element at 100 % duty cycle.

A special torch with a concentric cathode for the plasma arc and a concentric conduit and contact tip for the delivery of the consumable wire is needed. A water-cooled copper alloy nozzle is used to constrict the arc and to form a collimated plasma jet which exits the nozzle orifice. A plasma orifice gas and a focusing gas from the same supply are used; the latter is delivered via channels between the plasma welding electrode and the constricting nozzle.

Three shielding gases are used: one for the plasma (orifice) gas, one to provide additional arc constriction and arc stability, and one for supplementary shielding. The plasma gas and the focusing gas are normally argon, since an inert gas is needed to prevent oxidation of the PAW electrode. The supplementary shielding gas can be argon, argon-oxygen, argon-carbon dioxide, or argon-hydrogen, depending on the nature of the work piece being welded or, in the case of a surfacing operation, on the material being deposited.

Resistance spot welding

Resistance spot welding (RSW) is a process (Fig 11) in which faying surfaces are joined in one or more spots by the heat generated by resistance to the flow of electric current through work pieces which are held together under force by electrodes. The contacting surfaces in the region of current concentration are heated by a short-time pulse of low-voltage, high amperage current to form a fused nugget of weld metal. When the flow of current ceases, the electrode force is maintained while the weld metal rapidly cools and solidifies. The electrodes are retracted after each weld, which normally is completed in a fraction of a second.

Fig 11 Resistance spot welding process

Spot welding is the most widely used joining technique for the assembly of sheet metal products such as automotive body-in-white assemblies, domestic appliances, furniture, building products, enclosures, and to a limited extent, aircraft components.

Major advantages of spot welding include high operating speeds and suitability for automation or robotization and inclusion in high-production assembly lines together with other fabricating operations. With automatic control of current, timing, and electrode force, sound spot welds can be produced consistently. Majority of metals can be resistance spot welded if the appropriate equipment is used coupled with suitable welding conditions. This is particularly true for thin sheet or strip steel products, whether uncoated or coated.

The equipment needed for RSW can be simple and inexpensive or complex and costly, depending on the degree of automation. The transformer used in a direct energy resistance delivers the power to the work piece by changing the input high voltage, low-amperage, AC in the primary winding to a low-voltage, high-amperage current in the secondary winding. This system forms the basis of pedestal, or gun-type, welding machines where the output of the secondary transformer is applied directly to the welding electrodes. More complex secondary circuits are used in multi welders.

Welding operations in highly automated production lines are based primarily on multiple spot welders and robotic cells. In addition, manual welding operations can be used to manufacture either sub- assemblies, which are fed into the main production / assembly lines, or, in many instances, finished products. These different end uses require welding machines of varying designs and characteristics.

Materials for RSW electrodes are to have sufficiently high thermal and electrical conductivities and sufficiently low contact resistance to prevent burning of the work piece surface or alloying at the electrode face. In addition, the electrodes are to have adequate strength to resist deformation at operating pressures and temperatures. Because the part of the electrode which contacts the work piece becomes heated to high temperatures during welding, hardness and annealing temperatures are also to be considered.

Projection welding

Projection welding (PW) process (Fig 12) is a variation of resistance welding in which current flow is concentrated at the point of contact with a local geometric extension of one (or both) of the parts being welded. These extensions, or projections, are used to concentrate heat generation at the point of contact. The process typically uses lower currents, lower forces, and shorter welding times than does a similar application without the projections. Projection welding is frequently used in the most difficult resistance-welding applications since a number of welds can be made at one time, which speeds up the manufacturing process.

Fig 12 Projection welding process

PW applications are normally categorized as being either embossed-projection welding or solid-projection welding. These variations are shown in Fig 12. Embossed-projection welding is normally a sheet-to-sheet joining process, in which a projection is stamped onto one of the sheets to be joined. Then, resistance welding is conducted on a stack of sheets. Heat initially concentrates at the contact point and in the walls of the projection during resistance welding. Early in the process, the projection itself collapses back into the original sheet. However, the initial heating raises the local resistivity of the joint area, allowing continued resistance heating at this location. Weld development then proceeds in the conventional manner, by forming a fused weld nugget.

Solid-projection welding needs that the projection be forged onto one of the two components. Then, during resistance welding, the contact point and the projection itself experience preferential heating. In this case, the projection cannot simply collapse, as it does in embossed-projection welding. Rather, the projection collapses by penetrating the opposing material and by extrusion to the periphery. When compared with embossed-projection welding, the resulting joints are solid-state, rather than fusion, welds. The actual joints are caused by a combination of material forging and diffusion bonding, much like they are in resistance butt and flash butt welding.

Examples of projection welding are shown in Fig 11. These applications, which range from sheet-to-sheet joints, to crosswire welds, to annular attachments, to nut welds, to weld screws, include both embossed- and solid-projection types of welding.

The embossed-projection technique is primarily used for sheet-to-sheet applications. It does involve the additional expense of stamping the projections and ensuring that the electrodes are properly located over the projections. However, the ability of the projection to create highly localized joints allows this technique to extend the applications of resistance welding. It is highly adaptable to the joining of multiple-sheet stacks. A considerably wider range of solid-projection welding processes is normally used in production applications. Annular projection welding, like embossed-projection welding, is normally used to provide a highly localized joint and to minimize thermal damage to other parts of the structure. There are some common variations of solid-projection welding.

Annular projection welding which is one of the most common applications of solid-projection welding is to attach either tubular components or members with circular bases to flat or curved substrates. In this, the projection is machined onto the circular base or tubular section. Then, resistance welding is conducted to set the projection into the substrate.  Annular projection welding generally provides high-integrity joints and can be used for leak-tight applications.

Cross-wire welding is a variation of solid-projection welding in which the projection is formed by the contact point of two crossing wires. Upon resistance welding, heat is maximized at the location of the wire-wire contact, and the parts are subsequently forged together. Depending on the application, either highly localized joints (minimal forging) or heavily forged joints can be made.

Weld nuts represent an application of solid-projection welding in which the projections themselves occur as actual extensions of material from the nut. Edge-to-sheet welds are typically a cross between embossed- and solid-projection welding. They are used to attach the end of a sheet to the flat face of an opposing sheet. The projections for welding are nomally stamped into the face of the attaching sheet. During welding, the projection does collapse locally back into the base sheet. However, bonding is strictly solid state, similar to that of other solid-projection welding processes.

Solid-projection welds are essentially strain-assisted diffusion bonds. Because the projections typically collapse at very high temperatures (normally, within several hundred deg C of the melting point), diffusion bonding can occur within very limited available time (usually, less than 1 second). Not surprisingly, some of the material-related factors which affect diffusion bonding also affect solid-projection welding. The strength-temperature relationship also affects projection weldability. Materials which maintain their strengths at relatively high temperatures permit substantial heating to occur before projection collapse. This heat then becomes available to promote diffusion after projection collapse. Premature collapse results in lower temperatures in which diffusion can occur and in reduced current density, which prohibits further resistance heating. Bulk resistivity also plays a role in projection welding, but to a lesser degree. Increased bulk resistivity can reduce the effectiveness of the projection as a current concentrator. With increasing bulk resistivity, the tendency is for delocalized heating and in general, rather than local, collapse of the projection. As a result, high-resistivity materials are more difficult to projection weld.

Mild steels and low-alloy, nickel-base alloys are ideal materials for projection welding, because they readily dissolve their own oxides and have adequate strength-temperature and resistivity properties. Stainless steels and higher-alloy nickel base materials become slightly more difficult to weld, because of the formation of more-stable chromium and aluminum oxides, increased high-temperature properties, and higher resistivities.

Resistance projection welding generally utilizes slightly modified resistance-welding equipment. Because the nature of projection welding is to direct current flow to the desired weld locations, there is considerably more flexibility in tooling. Projection welding is typically done with large, flat electrodes. In most applications, tooling is simply shaped to match the contour of the part in the contacting location.

Resistance seam welding

Resistance seam welding (RSEW) is a process in which heat generated by resistance to the flow of electric current in the work metal is combined with pressure to produce a welded seam. The resulting seam consists of a series of spot welds. The specific seam weld process can be classified as (i) roll spot welding, (ii) reinforced roll spot welding, and (iii) leak tight seam welding. Two rotating circular electrode wheels are frequently used to apply current, force, and cooling to the work metal. A variety of work piece / wheel configurations are possible (Fig 13). When two electrode wheels are used, one or both wheels are driven. For some applications, the electrode wheels idle while the work piece is driven. Other electrode systems for seam welding can use one wheel and one flat bar-type electrode, or a wire-feed system in which a copper wire is fed into a groove on the wheel.

In seam welding, a series of spot welds is made without retracting the electrode wheels or releasing the electrode force between spots. The electrode wheels can rotate either continuously or intermittently. Weld speed, current magnitude, current waveform, cooling system, and the electrode parameters of force, shape, and diameter are all related and are to be carefully selected to optimize the process and produce the highest quality weld.

A lap seam weld joint is one in which the work pieces are overlapped sufficiently to prevent the sheet edges from becoming part of the weld. Lap seams are popular in automotive applications as well as in non-automotive applications. Lap seam welding of multiple stack ups and dissimilar thicknesses is also possible.

Mash seam weld is a joint in which two sheets are overlapped by only one to two times the sheet thickness. The weld area is forged or mashed down during welding to an over thickness 5 % to 25 % higher than a single sheet thickness. It is necessary to have some amount of over thickness so the current path can be controlled, since the electrode wheels in mash seam welding are wide and flat-edged. Surface appearance and thickness can be improved through the use of hard steel planishing wheels to cold work the joint before or after welding. Because of the excessive hardening induced by cold work, post weld heat treatment can be necessary. Typical applications include drums, buckets, vacuum-jacketed bottles, aerosol cans, water tanks, and steel mill coil joining. Sheets of dissimilar thicknesses and/or coatings have been successfully mash seam welded. This development has created a whole new set of applications in tailored blank manufacturing, primarily for automotive use.

Butt seam weld is a joint in which two abutting edges are welded. The thickness of the weld is to be around the same, or slightly less than, the sheet thickness. Butt seam welding is typically reserved for applications in which other butt welding processes cannot be used (for example, for pipe welding and for sheet metal in railway wagons).

Fig 13 Resistance seam welding process

Empirical seam welding parameters have been developed over the years and have proven useful under many conditions. However, certain concepts from resistance spot welding begin to explain resistance seam welding, since seam welding normally of a series of overlapping spot welds. For seam welding, the same principle of optimum weld time applies, but weld time refers to how long a given point on the weld is to be in contact with the wheel as it passes through the welder. In practice, travel speed and electrode footprint length is to be controlled.

It is important to note that the current range of seam welding is widened by the judicious use of impulse current to form distinct nuggets. Impulse profiles of 25 % to 33 % off-times are normally used. Impulses contain off-time for the electrodes to draw heat away from the weld surface. Hence, the occurrence of surface discontinuities is lessened. In addition, off time allows heat to redistribute itself within the weld, away from local hot spots. Hot spots are places within the weld where thermal spikes can be occurring due to either a relatively high local resistance, a non uniform current density, a multiple stack up, or a stack up of unequal sheets.

Current range is greater in impulse mode than in non impulse (continuous current) mode. The extra range seems to provide more robust penetration under difficult welding conditions, such as when intersecting other welds or turning corners with the seam. One of the disadvantages of seam welding relative to spot welding is that solidification of the weld nugget occurs after the weld metal leaves the pressure field of the electrode wheels. Thus, if there is a large separating force on the weld before it freezes, it can come apart as it leaves the wheels.

Most low C, high C, low alloy, and stainless steels, and many coated steels above around 0.15 % C tend to form areas of hard martensite upon cooling. In critical applications, the welds can need post weld tempering to reduce the hardness and brittleness. In some cases, this can be done in the welding machine. Tempering can also be done in a furnace or by induction heating.

A seam welding machine is similar in construction to a spot welding machine, except that one or two electrode wheels are substituted for the spot welding electrodes. In general, seam welding is done in a press-type resistance welding machine. Most seam welding machines are powered by AC, either three-phase or single phase; some are designed for use with DC.

The upper electrode wheel is mounted to, and insulated from, the operating head. The head, which is actuated by a direct-acting air or hydraulic cylinder, applies the electrode force. The lower electrode is either a wheel, or a platen, or a mandrel and is mounted on a supporting arm, table, or knee. This lower support can be made adjustable for applications in which the work metals are to be maintained at a constant level above the floor. Work pieces are moved by rotating the electrodes with knurl or friction drive, by direct drive of the wheel shaft, by a driven work piece and idling wheels, or by clamping the work pieces to a bar electrode and moving the bar electrode.

Flash welding

Flash welding (FW) is a resistance welding process (Fig 14) in which a butt joint weld is produced by a flashing action and by the application of pressure. In basic terms, it is a melting and a forging process. The process is capable of producing welded joints with strengths equal to those of the parent materials.It is used to join metallic parts which have similar cross sections, in terms of size and shape. The process lends itself to joining nearly all grades of steel, aluminum, brass, and copper parts, in addition to selected dissimilar materials.

Materials with cross sections ranging from 65 sq mm to 13,000 sq mm are normally flash welded. The process is also used to produce components of solid and tubular shape, in addition to strips and complex configurations. Flash welding is normally used to weld rings which range in diameter from a few mm to as much as 4.5 m. The aircraft engine industry uses flash welded rings made from heat-resistant materials in accordance with military specifications for jet aircraft.

A typical flash-welding machine consists of seven major components consisting of a (i) machine bed, (ii) a movable platen, (iii) two clamping assemblies, (iv) motion control equipment for movable platen, (v) a welding transformer, (vi) control equipment, and(vii)  ventilator / flash catcher. The movable platen of a flash welder can be driven by a number of different methods. The simplest type frequently uses a cam which is motor driven through a gear box. Flashing time is then controlled by the speed of the motor. The cam is machined to produce the desired flashing cam curve profile.

Electrical controls on flash-welding equipment are designed to sequence the machine, control the weld current, and precisely control the platen position during flashing and upsetting. Preheat and post heat cycles are frequently needed procedures in flash welding operations. Timers with phase-shift heat control are used to accomplish these important functions. The most common source of power for flash welders is single-phase AC power.

Fig 14 Flash and upset welding processes

Upset welding

Upset welding (UW) is a resistance welding process (Fig 14) utilizing both heat and deformation to form a weld. The heat is produced by resistance to the flow of electrical current at the interface of the abutting surfaces to be joined. The deformation results from force on the joint in combination with softening from the electrical resistance heat. Upset welding typically results in solid-state welds (no melting at the joint). The deformation at the weld joint provides intimate contact between clean adjoining surfaces, allowing formation of strong metallurgical bonds. If any melting does occur during upset welding, the molten metal is typically extruded out of the weld joint area.

A wide variety of shapes and materials can be joined using upset welding in either a single-pulse or continuous mode. Wire, bar, strip, and piping can be joined end to end with a single pulse of welding current. Seams on pipe or tubing can be joined using continuous upset welding by feeding a coiled strip into a set of forming rolls, resistance heating the edges with wheel electrodes, and applying a force to upset the edges together.

Equipment for single-pulse upset welding is relatively simple. It consists of a pneumatic or hydraulic system for force application, transformers or a bank of diodes as a source of electrical current, and a standard resistance-welding controller. A data acquisition system is normally employed to record the force, current, voltage, and motion of the weld head during welding. Equivalent welds have been made using both AC and DC.

Upset welds have similar characteristics to inertia friction welds, which are also solid-state welds. The amount of deformation is nomally less for upset welds, and the deformation can be more precisely controlled using upset welding. Solid-state upset welding has advantages compared to typical fusion welding processes. These advantages are due to the simplicity of the welding process and the resulting solid-state weld micro-structure.

The effect of welding conditions, other than the basic parameters of force, current, and time, is normally minimal. Surface roughness normally has little effect, since it is overshadowed by the deformation which takes place during welding. Surface cleanliness and welding atmosphere may or may not be important depending on the amount of deformation which occurs to break up any oxide present before, or formed during, welding.

A perceived limitation of the UW process is the lack of a good non-destructive testing technique for determining the quality of solid-state bonds. Normally, control of process variables is sufficient to ensure quality. For upset welding of large parts, the peak power drawn from the electrical supply can be a disadvantage. Fixturing of the parts to be joined is normally necessary to achieve alignment of the parts, carry the electrical current to the joint area, and confine deformation to the joint area. Plug welds are a type of upset weld which needs little fixturing.

A homopolar generator is an alternative method for supplying the electrical current for upset welding. Homopolar generators store energy in a large flywheel and then quickly convert the stored energy to electrical energy. The demand on utility power lines is thus spread over a considerable time period, yet the high peak power needed for upset welding large parts can be met over a period of around 1 second.

High-frequency welding

High-frequency welding is a welding process in which the heat source used to melt the joining surfaces is obtained from high-frequency (HF) AC resistance heating. High-frequency current has certain characteristics which make it useful for welding. Unlike DC or low-frequency AC, HF current tends to flow at high densities along surfaces (skin effect) and seeks adjacent parallel surfaces for its return path (proximity effect). This means that the heating and subsequent melting can be efficiently concentrated and focused to the surfaces where it is needed.

In nearly all HF welding processes, the joining surfaces are mechanically squeezed together (upset) after they are melted. This procedure helps produce a high-quality weld by squeezing out residual oxides and molten metal, which are detrimental to weld integrity. The removal of molten metal from the weld zone is beneficial since it eliminates the potentially harmful effects associated with cast structures, such as lower fracture toughness and poorer corrosion resistance.

The pipe making incorporates the simplest method of controlling HF currents, which is, the generation of an edge ‘V’. High frequency currents can be supplied to the welding process or work piece by using either an induction coil (known as high frequency induction welding, HFIW) or electrical contacts (known as high-frequency resistance welding, HFRW). The edge ‘V’ is a series component of the circuit (Fig 16). Because of proximity and skin effects, current flow is concentrated across the full face width of the strip edges, resulting in controlled surface melting and subsequent welding.

In this process, a coil of strip material is first formed into a longitudinal hollow on a forming mill. The two edges of the strip are then HF welded and squeezed together to form a continuous welded seam. Materials which can be successfully HF welded into tube and pipe include C steels, stainless steels, aluminum, copper, brass, and titanium. Exceptions are materials which are unstable at welding temperatures, have a negligible hot-work capability, or experience property deterioration which cannot be subsequently recovered. Where necessary, a gas shield can be provided for reactive metals. Tube or pipe sizes fabricated by HF welding range in size from 13 mm to 1,220 mm in diameter.

The advantages of HF welding are that it is well suited for high-speed welding and can weld a large range of product sizes and materials. Weld quality is not particularly sensitive to the presence of air, and special atmospheres are normally not needed. Weld quality is relatively (but not completely) tolerant of surface oxides and contamination. The disadvantages of HF welding are that it is not well suited for low welding speeds and small-scale operations where welding is done by hand. HF welding is to be done continuously; continuous welds cannot be made in stop / start operations since a discontinuity in the weld normally occurs.

HF welding is most suitable in applications which involve the continuous edge, or butt, joining of metals. The largest single use of HF welding is in the manufacture of tube and pipe. HF welding is useful in the manufacture of certain types of heat-exchanger tube, where the edge of a rectangular strip is continuously welded onto the outside diameter of a tube to form a cooling fin. The fin can be welded around the tube in a spiral configuration or it can be straight and parallel to the pipe axis. The greatest volume of heat exchanger tube is made with low C steel for both the pipe and fins. However, other alloy combinations are growing in use, such as low C steel fins on both stainless steel and chromium / molybdenum alloy steel tubes, stainless steel fins on low C steel and stainless steel tubes, and aluminum fins on cupro-nickel tube. HF welding can also be used for the production of structural shapes, such as T-sections and I- beams and H-beams.

Units for producing power for HF welding include vacuum tube oscillators and solid-state inverters. Depending on the application, frequency requirements can vary from 100 kHz to 700 kHz, whereas power requirements vary from 30 kW to 1,000 kW. Solid-state inverters produce frequencies ranging from 100 kHz to 400 kHz. Vacuum-tube oscillators are available in frequencies upto 700 kHz. Because of their improved efficiency, solid-state units are replacing vacuum-tube units when frequency requirements permit.

In the production of tube and pipe, induction coils are usually used to make the smaller diameters (less than 300 mm) because their advantages outweigh the loss of efficiency. Contacts are normally used for larger diameters because of their greater efficiency. Contacts are also used in structural shapes and fin heat-exchanger tubes, as well as for end welding, because induction coils cannot induce the current densities necessary for welding. Impeders are used to improve welding efficiency in tube and pipe production. Impeders which consist of one or more water cooled cores of ferrite material are placed inside the tube parallel to and under the weld area.

Inert gases are not needed for HF welding unless certain reactive metals are used. Argon is sometimes used for the welding of stainless steel, and it is normally necessary for the welding of titanium. Although cleanliness of weld surfaces is not particularly necessary for easily welded materials, a lack of cleanliness is frequently the cause of problems in more sensitive metals.

Fig 15 High frequency and electron beam welding processes

Electron beam welding

Electron beam welding (EBW) is a high-energy density fusion process (Fig 15) which is accomplished by bombarding the joint to be welded with an intense (strongly focused) beam of electrons which have been accelerated up to velocities 0.3 times to 0.7 times the speed of light at 25 kV to 200 kV, respectively. The instantaneous conversion of the kinetic energy of these electrons into thermal energy as they impact and penetrate into the work piece on which they are impinging causes the weld-seam interface surfaces to melt and produces the weld-joint coalescence desired. Electron-beam welding is used to weld any metal which can be arc welded. Weld quality in most metals is equal to or superior to that produced by gas tungsten arc welding (GTAW).

Since the total kinetic energy of the electrons can be concentrated onto a small area on the work piece, power densities as high as 108 W/sq cm can be achieved, which is higher than is possible with any other known continuous beam, including laser beams. The high-power density plus the extremely small intrinsic penetration of electrons in a solid work piece results in almost instantaneous local melting and vapourization of the work piece material. This characteristic distinguishes EBW from other welding methods in which the rate of melting is limited by thermal conduction.

Basically, the electron beam is formed (under high-vacuum conditions) by employing a triode style electron gun consisting of a cathode, a heated source (emitter) of electrons that is maintained at some high negative potential; a grid cup, a specially shaped electrode which can be negatively biased with respect to the hot cathode emitter (filament); and an anode, a ground potential electrode through which the electron flow passes in the form of a collimated beam. The hot cathode emitter (filament) is made from a high-emission material, such as tungsten or tantalum. This emitter material, normally available in wire, ribbon, or sheet form, is fabricated into the desired shape for being either directly or indirectly heated to the required emitting temperature of around 2500 deg C.

Electrons emitted from the surface of the filament are accelerated to a high velocity and shaped into a collimated beam by the electrostatic field geometry generated from the cathode / grid /anode configuration employed, thus producing a steady stream of electrons which flows through an aperture in the ground plane anode. By varying the negative potential difference between the grid and cathode, this flow of electrons can be altered easily in a precisely controlled manner.

Diode-style electron guns are also employed, but not to the extent that triode-style electron guns are. In a diode gun, the specially shaped electrode (grid cup) is maintained at the same voltage as the emitter, thus making the diode gun a two element (cathode and anode) device. With this design, the flow of electrons from a diode gun cannot be adjusted by simply varying a grid voltage, as is done with triode guns, and beam current adjustments are normally accomplished by varying the operating temperature of the cathode emitter instead.

Once the electrons exit the anode, they receive the maximum energy input allowable from the operating voltage being applied to the gun. Electrons then pass down through the electron beam column assembly and into the field of an electromagnetic focusing coil (a magnetic lens). This focusing lens reduces the diameter of the electron beam, as it continues in its passage, and focuses the stream of electrons down to a much smaller beam cross section in the plane of the work piece. This reduction in beam diameter increases the energy density, producing a very small, high-intensity beam spot at the work piece. In addition, an electro-magnetic deflection coil (positioned below the magnetic lens) can be employed to ‘bend’ the beam, thus providing the flexibility to move the focused beam spot.

One of the prime advantages of EBW is the ability to make welds which are deeper and narrower than arc welds, with a total heat input which is much lower than that needed in arc welding. This ability to achieve a high weld depth-to-width ratio eliminates the need for multiple-pass welds, as is required in arc welding. The lower heat input results in a narrow work piece heat-affected zone (HAZ) and noticeably fewer thermal effects on the work piece.

In EBW, a high-purity vacuum environment can be used for welding, which results in freedom from impurities such as oxides and nitrides. The ability to employ higher weld speeds, due to the high melting rates associated with the concentrated heat source, reduces the time needed to accomplish welding, thereby resulting in an increased productivity and higher energy efficiency for the process.

Laser beam welding

Laser Beam Welding (LBW) is a welding process (Fig 16), in which heat is generated by a high energy laser beam targeted on the work piece. The laser beam heats and melts the edges of the work piece, forming a joint. Laser beam welding uses a moving high-density coherent optical energy source called a laser as the source of heat. ‘Laser’ is an acronym for ‘light amplification by stimulated emission of radiation’. The coherent nature of the laser beam allows it to be focused to a small spot, leading to high energy densities. The energy of a narrow laser beam is highly concentrated, so a weak weld pool is formed very rapidly.

Fig 16 Laser beam welding

The solidification of the weld pool surrounded by cold metal occurs as rapidly as the melt. Since the time the molten metal is in contact with the atmosphere is low, there is no contamination and hence no gradient (neutral gas, flow) is needed.

In laser beam welding the joint is made either as a sequence of overlapped spot welds or as a continuous weld. Laser welding is used in the electronics, communications and aerospace industries, for the manufacture of medical and scientific equipment, joining small components.

The laser beam welding works on the principle that when the electrons of an atom are excited by receiving some energy, then after some time when it returns to its ground state, it emits a photon of light. The concentration of this emitted photon is increased by the excited emission of radiation and the result is a high energy focused laser beam. The light amplification by stimulated emission of radiation is named as a laser.

Initially, the welding machine is set up (between the two metal pieces to join) at the desired location. Later set up, a high voltage power supply is applied to the laser machine to perform an operation. The lens is used to focus the laser into the area where welding is needed. CAM is used to control the speed of the laser and work piece table during the welding process.

It starts the machine’s flash lamp and it emits light photons. The energy of light photons is absorbed by the atoms of ruby ​​crystals and electrons are excited to their higher energy levels. When they return to their low energy state or ground state they emit a photon of light. This light photon again stimulates the electrons of the atom and produces two photons. This process continues and the result is a focused laser beam which is used on the desired location for welding multiple pieces together.

Laser beam welding needs laser machine, shielding gas and power source.  The laser beam machine generates lasers for welding. Shielding gas can be used during the welding process to prevent work piece from oxidizing.  A high voltage power source is used to the laser machine to produce a laser beam. The main types of laser used in the laser beam welding are gas laser, solid-state laser, and fibre laser.

The gas laser uses a mixture of gases for the production of lasers. It contains gases such as nitrogen, helium, and CO2 are used as the lasing medium. The solid-state laser use many solids in synthetic ruby ​​crystals (chromium in aluminum oxide), neodymium in glass (Nd: glass) and neodymium in yttrium aluminum garnet (Nd-YAG, the most commonly used). The fibre Laser is the optical fibre. LBW process is prominent in the automotive industry. It is employed for high precision welds. As it does not use any electrode, the final weld is light but strong.

Electroslag and electrogas welding

Electroslag and electrogas welding are two related processes which are used to weld thick-section materials in the vertical or near-vertical position between retaining shoes. Primarily applied for joining steels of thicknesses over 50 mm, electroslag welding (ESW) (Fig 17) involves high energy input relative to other welding processes, resulting in normally inferior mechanical properties, specifically lower toughness of the heat-affected zone. However, the high deposition rate and relatively low cost of the process make it attractive for heavy structural fabrication. The as-welded properties of electrogas welding (EGW), normally applied to steels under 50 mm, are normally superior to those of electroslag welds, and the process is normally applied to the field erection of storage vessels and other less critical structures.

Electroslag welding is a vertical welding process producing coalescence with molten slag which melts the filler metal and the surface of the work to be welded. Confined by cooling shoes, the molten weld pool is shielded by the molten slag, which moves along the full cross section of the joint as welding progresses. The conductive slag is maintained in a molten condition by its resistance to electric current passing between the electrode and the work.

Fig 17 Electroslag welding process

ESW can be considered a progressive melting and casting process in which the heat of a bath of molten flux is used to melt the filler metal and the edges of the plates to be welded. Electric arc occurs only at the beginning of the process, and once a molten bath is achieved, the arc is extinguished. During the process, flux is added periodically or continuously to maintain an adequate slag covering over the pool of molten metal. Two or more retaining shoes hold the molten metal in place until it has solidified. In normal operation with a constant potential power source, the electrode melts off while dipping only partly through the flux bath and gathers in the molten metal puddle. In the case of low C steel, the temperature of the bath is reported to be in the vicinity of 1,925  deg C, while the surface temperature is approximately 1,650 deg C.

The major process variables are welding current and voltage. Welding current is directly responsible for the electrode melt rate, while voltage influences the base metal penetration and weld bead width. Both variables are sensitive to the physical properties of the welding flux, such as electrical resistivity and fluidity. Electrogas welding is a method of gas-metal arc welding (if a solid wire is used) or flux-cored arc welding (if a tubular wire is used), wherein an external gas is supplied to shield the arc and moulding shoes are used to confine the molten weld metal for vertical position welding. Electrogas welding may or may not use an added flux.

In the solid wire process, CO2 shielding gas is normally used and no flux is added. With the flux-cored process, the core ingredients provide a small amount of flux to form a thin deposit of slag between the weld and the shoes. Self-shielding electrodes eliminate the need for external shielding gas. A major difference between ESW and EGW is that the former relies on slag conduction to carry the welding current and the latter uses arc conduction. Despite the differences, similarities between ESW and EGW in terms of equipment, joint preparation, and welding procedures are such that they can be grouped into one category and described as allied processes.

Electroslag welding is quite similar to in situ casting, with large volumes of molten metal and high heat content. When compared with other arc welding processes, electroslag welds have a long thermal cycle with very slow cooling rate. The droplet transfer rate and the length of time each droplet is in contact with the slag layer profoundly affect the chemical composition and the metallurgical properties of the weld pool.

In an electroslag weldment, solidification begins at the fusion line, surfaces adjacent to the retaining shoes, and progresses toward the centre of the weld. Since the process is continuous in a vertical or near vertical-up position, solidification also progresses from the bottom toward the top part of the joint. The angles at which the columnar grains meet at the centre of the weld depend on the shape of the weld pool, which can be described by the weld pool form factor. Form factor is defined as the ratio between the width and the maximum depth of the pool. Welds having a high form factor (higher than 2) have grains meeting at an acute angle at the centre line, while welds with a low form factor (less than 1) solidify with grains meeting at an obtuse angle. Low form factor is highly undesirable because of the potential accumulation of residual elements at the centre of the weld joint. High welding current normally results in a low form factor, while low welding current normally results in a high form factor and shallow metal pool. High voltage promotes shallow pools and low voltage.

Oxyfuel gas welding

Oxyfuel gas welding is a manual process in which the metal surfaces to be joined are melted progressively by heat from a gas flame, with or without filler metal, and are caused to flow together and solidify without the application of pressure to the parts being joined. The most important source of heat for OFW is the oxyacetylene welding (OAW) torch (Fig 18).

The simplest and most frequently used OFW system consists of compressed gas cylinders, gas pressure regulators, hoses, and a welding torch. Oxygen and fuel are stored in separate cylinders. The gas regulator attached to each cylinder, whether fuel gas or oxygen, controls the pressure at which the gas flows to the welding torch. At the torch, the gas passes through an inlet control valve and into the torch body, through a tube or tubes within the handle, through the torch head, and into the mixing chamber of the welding nozzle or other device attached to the welding torch. The mixed gases then pass through the welding tip and produce the flame at the exit end of the tip. This equipment can be mounted on and operated from a cylinder cart, or it can be a stationary installation. Filler metal, when needed, is provided by a welding rod which is melted progressively along with the surfaces to be joined.

Fig 18 Oxyfuel gas welding process

In OFW, the welder has considerable control over the temperature of the metal in the weld zone. When the rate of heat input from the flame is properly coordinated with the speed of welding, the size, viscosity, and surface tension of the welding pool can be controlled, permitting the pressure of the flame to be used to aid in positioning and shaping the weld.

The welder has control over filler-metal deposition rates since the sources of heat and filler metal are separate. Heat can be applied preferentially to the base metal or the filler metal without removing either from the flame envelope. With these capabilities, OFW can be used for joining thin sheet metal, thin-walled tube, small pipe, and assemblies with poor fit-up, as well as for smoothing or repairing rough arc welds.

The equipment is versatile, low-cost, self-sufficient, and usually portable. It can be used for pre-heating, post-heating, welding, braze welding, and torch brazing, and it is readily converted into oxygen cutting. The process can be adapted to short production runs, field work, repairs, and alterations. Most ferrous and non-ferrous metals can be oxyfuel gas welded.

Oxyacetylene supplies the heat intensity and flame atmosphere necessary for welding C steel, cast iron, and other ferrous, copper, and nickel alloys. Aluminum and zinc alloys can also be welded by the oxyacetylene process. Oxyfuel gas welding of steel is done almost exclusively with an oxyacetylene flame. Hydrogen, natural gas, propane, and several proprietary gases are used as fuel gases in welding metals with lower melting temperatures, such as aluminum, magnesium, zinc, lead, and some precious metals. Metals unsuited to OFW are the refractory metals, such as niobium, tantalum, molybdenum, and tungsten, as well as the reactive metals, such as titanium and zirconium.

Except for lead, zinc, and some precious metals, OFW of non-ferrous metals, cast irons, and stainless steels normally needs a flux. In welding C steel, the gas flame shields the weld adequately, and no flux is needed. Adjustment for correct flame atmosphere is important, but the absence of flux results in one less variable to control.

Oxyfuel gas welding can be used to join thin C steel sheet and C steel tube and pipe. The advantages of OFW include the ability to control heat input, bridge large gaps, avoid melt-through, and clearly view the weld pool. C steel sheet, formed in a variety of shapes, can frequently be welded more economically by OFW than by other processes. Oxyfuel gas welding is capable of joining small-diameter C steel pipe (upto around 75 mm in diameter) with resulting weld quality equal to competitive processes and often with greater economy. Pipe with wall thickness upto 4.8 mm can be welded in a single pass.

Oxyfuel gas welding needs skill in manipulating the welding rod and the torch flame. In depositing a weld, the welder uses both hands to melt base metal and filler metal, control the weld pool, and obtain progressive solidification of weld metal in the correct bead shape.

The principal function of OFW equipment is to supply the oxyfuel gas mixture to the welding tip at the correct rate of flow, exit velocity, and mixture ratio. The rate of gas flow affects the quantity of metal melted; the pressure and velocity affect the manipulation of the weld pool and the rate of heating; and the ratio of oxygen to fuel gas determines the flame temperature and the atmosphere, which is to be chemically suited to the metal being welded. Important elements in an OFW system include gas storage facilities, pressure regulators, hoses, torches, related safety devices, and accessories.

Thermite welding

Thermite 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, thermite welding is widely used in the field welding of track, where its portability and versatility are strong assets. It is to be noted that the standard term for this process is frequently used as ‘thermit’ welding. However, Thermit is a registered trademark of Th. Goldschmidt AG of Essen, Germany.

The alumino-thermic reaction which occurs in thermite welding of rails follows the equation Fe2O3 + 2Al = 2Fe + AL2O3 + 850 KJ heat and the typical reaction which occurs in the welding of copper conductors to steel rails is 3Cu2O + 2Al = 6Cu + AL2O3 +1,060 KJ heat. This exothermic reaction is extremely violent if only the metal oxide and aluminum reducing agent are used. Pellets of ferroalloy are added to cool this reaction from a typical temperature of 3,090 deg C to 2,480 deg C. These additions also are used to produce the desired chemistry. The amount of added alloy is very critical, because larger amounts cool the reaction to temperatures below 2,040 deg C, at which point the slag / metal separation can be incomplete

Metallurgical structures which are present in thermite welds depend on the chemical composition of the weld metal and on the cooling rate of the joint after pouring is completed. Fig 19 shows a typical macrostructure of a C steel rail thermite 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 due to the slow cooling rate which completes transformation before reaching the martensite start temperature and the formation of untempered martensite.

Rail welding is the most widely used application of the thermite welding process. The use of that process, as well as the electric flash butt welding process, has eliminated joint bars (mechanical fasteners) and greatly lessened track maintenance. Long sections of continuous welded rail (CWR) (typically, 440 m) that are welded by the flash butt welding process are transported to the track site and set into place. Then, the CWR sections are joined by thermite welding. A short preheating process is typically used in the thermite welding of rail.

Fig 19 Thermite welding of rails and microstructure

The thermite process is also utilized to replace rail defects, to weld insulated joint assemblies into track, and to make electrical bonding connections using copper conductors between rails which are not to be welded together.

The thermite welding process also is used to weld electrical conducting joints, particularly to provide electrical continuity for railroad signal systems. A copper oxide powder is reduced by aluminum to form a metallurgical bond between the steel rail and the copper conductor. The resulting joint has an excellent current-carrying capacity and the ability to withstand corrosion, when compared with a mechanical bond.

Large-diameter rolls, shafts, ingot moulds, and heavy mill housings can be successfully repaired by using the thermite welding process. The metallurgical aspects of the process, when used to make repair welds, are similar to those of rail welding, except for the large volume of the weld. The primary difference is that a customized mould, which is normally manufactured on-site, is used. A wax pattern is built around the weld cavity, and a collar is shaped in wax to overlap the base metal. Sand is hand rammed around the pattern within a mould box. Then, sufficient gates and risers are incorporated into the system.

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