Welding allows parts to coalesce along their contacting surfaces by applying heat, pressure, or both, and often adding a filler material. The most common industrial welding methods are fusion processes in which workpieces are melted at their common surfaces.

Solid-state welding joins parts by applying heat and pressure. The temperature is usually below the melting point of the materials joined. Hot-press, ultrasonic, diffusion, and explosive welding are examples of solid-state welding.

Fusion welding methods, chiefly gas, arc, and resistance, are the most widely used and are less restrictive as to the materials that can be joined. Diffusion welding is employed primarily to join high-strength materials.

Arc welding: In arc welding, an arc between an electrode and the workpiece generates heat. Shielding the molten weld metal from the atmosphere with gases fed in or generated by the weld reaction is often critical. Unwanted gases react with the molten metal to cause strength-reducing oxides and inclusions within the weld. Arc-welding processes vary mainly in the way the weld is shielded and the methods of applying filler material.

Shielded-metal arc welding (SMAW): This form of welding, also called stick welding, is usually done manually, with the welder feeding a consumable, coated electrode in the work area. The flux coating provides arc stabilizers; gases to displace air; metal; and slag to protect, support, and insulate the weld metal. Many electrode and flux coatings are available and are matched to type, size, and position of the material welded.

SMAW is suitable for portable applications. Labor and material costs are high, but its simplicity and versatility make it the most commonly used welding process.

Gas-metal arc welding (GMAW): This process generates an arc between a consumable electrode wire and the workpiece. Shielding is provided by gas that flows over the weld area from the welding gun. Combinations of shielding gas, power source, and electrode significantly affect metal transfer across the arc. A variety of gases are used, depending on the metals reactivity and the joint design. For example, argon is used most often, though carbon dioxide is added when welding ferrous metals. Carbon dioxide is sometimes used alone to shield steels. GMAW processes are fast, can be automated, and work in all positions.

Flux-core arc welding (FCAW): This process is very similar to gas-metal arc welding. Weld heat is produced from an arc between the work and a continuously fed filler-metal electrode. The electrode is hollow with flux in the core. The core may provide shielding gases, deoxidizers, and slag-forming materials. In some cases, materials may be added to promote arc stability, enhance weld-metal properties, and improve weld contour. FCAW is used only on ferrous metals, chiefly mild and low-alloy steels. Some FCAW electrodes are self-shielding, but others require an external shielding gas, usually carbon dioxide, supplied through a nozzle. This process works in all positions, producing a fast, clean weld. Both GMAW and FCAW are sometimes referred to as MIG welding. They require less skill than SMAW.

Gas-tungsten arc welding (GTAW): In GTAW, an arc is generated between a nonconsumable tungsten electrode and the work areas. Wire filler metal is fed in separately. Work is shielded by helium or argon gas. The process, also called TIG (tungsten inert-gas welding), is more expensive than GMAW but is used on thinner metals such as aluminum, magnesium, titanium, and high-alloy steels.

GTAW may use dc or ac. When ac is used with argon shielding, arc cleaning action is produced at aluminum and magnesium joint surfaces. This removes oxides and is useful in removing porosity in aluminum. Helium used with dc provides deeper penetration but requires stringent cleaning of aluminum and magnesium. Ac is preferred for aluminum and magnesium.

GTAW costs more than SMAW and is much slower than GMAW, but it provides a high-quality weld in a very wide range of thicknesses, positions, and geometries. The process can be fully automated.

Submerged arc welding (SAW): Heat is produced from an arc between the work and a continuously fed filler-metal electrode. A blanket of granular fluxing material preplaced on the work protects the molten weld puddle from the surrounding atmosphere. The process is limited to flat, or nearly flat, workpieces. Both flux and filler wire can be fed automatically, and the process is most commonly used in mechanized operations.

Plasma arc welding (PAW):Hot ionized gases shield the work area in this process. The plasma is sometimes supplemented with a separate shielding gas. The welding gun, like that used with GTAW, has a nonconsumable tungsten electrode. Filler material, if used, is fed in separately. The weld produced is also similar to GTAW, but the process is much faster and arc control is superior. Plasma arc welding produces a deep, narrow, uniform beam and is suitable for refractory metals, low-alloy steels, stainless steels, aluminum, and titanium. It is most frequently used for high-quality welds on high-strength, thin-section material. The process tolerates great variations in joint alignment and does not generate a high-frequency arc.

Resistance welding: In resistance welding, coalescence is produced by heat generated by resistance to the flow of electric current through the parts being joined. The assembly heats up, and pressure is applied by the welding machine through the electrodes. No fluxes or filler metals are needed.

The process is commonly used as a mass-production technique requiring special fixtures and automatic handling equipment. Resistance welding can be applied to almost all steels and aluminum alloys and some dissimilar metal bonds. Heating is localized. Common stock thickness range is 0.004 to 0.75 in.

The process may be continuous, as when welding pipe seams, or finite, as in spot welding. There are no limitations on welding position. This process works well with robotics.

Percussive arc welding: This technique can be considered a special case of dc resistance welding. In percussive arc welding, power is usually supplied by a capacitor bank that is directly short-circuited across the parts to be welded. The charge is concentrated at the point where the electrode nib contacts with the part. The nib vaporizes, establishing an ionized area. This provides a localized arc for the remaining capacitor charge that puddles an area on the surface of both parts.

The weld head moves forward quickly before the materials solidify. This forces the interface to alloy, producing an excellent low-penetration, low heat-affected-zone weld, usually having excellent character and grain structure. Percussive arc welding is most commonly used for stud-welding precision parts.

However, the process is sensitive to humidity. Also, large welds tend to be very noisy, with a loud percussive repeat which gives the process its name. The process also is relatively slow because of the time required to recharge the capacitor bank.

Oxyfuel gas welding (OFW): This group of welding processes uses the heat produced by a gas flame to melt the base metal (and the filler metal, when used). Various fuel-gas combinations are burned in oxygen to produce the necessary heat. Oxygen and acetylene is the most commonly used combination. The mixture produces a temperature of around 5,600°F. Oxy-acetylene torches can be used with or without a filler metal.

Propane, butane, natural gas, and hydrogen, in combination with air or oxygen, are used to weld nonferrous, low-melting-temperature materials in special applications. However, such hydrocarbon fuel gases are not generally suited to welding ferrous materials, because the flame atmosphere is oxidizing or the heat output of the flame is too low.

Because OFW requires minimal equipment, it is inexpensive and suitable for manual methods. However, it is slow.

Specialty systems include:

Electron-beam welding (EBW): EBW uses energy from a fast-moving beam of electrons focused on the base material. The electrons strike the metal surface, giving up their kinetic energy almost completely in the form of heat. Welds are made in a vacuum, which eliminates contamination of the weld material by gases. The high vacuum produces a stable beam.

With high beam energy, a hole can be melted through the material. This hole is moved along the joint by moving either the electron gun or the workpiece and is maintained as the metal at the front melts and flows around to the rear, where it solidifies. Welds can be made without a hole, where melting takes place by conduction of heat from the surface, but such welds are slower.

EBW can produce deep, narrow, almost parallel-sided welds with low total heat input and relatively narrow heat-affected zones. The depth-to-width ratio can be as high as 30:1 in some cases. The low energy input allows welding close to heat-sensitive components. Also, projecting the electron beam makes it possible to weld in otherwise inaccessible locations.

Though equipment costs are high, indirect savings can result from reduced joint-preparation costs, ability to weld in a single pass, high welding speed, and low distortion. However, the process can only be used to produce a tight butt or lap joint.

Laser-beam welding (LBW): Fusion is obtained by directing a concentrated beam of coherent light to a very small spot. Laser beams combine low heat input with an intensity greater than the electron beam. Since the heat is provided by a beam of light, there is no physical contact between the workpiece and welding equipment. Welds can be made through transparent materials.

LBW is flexible because the laser beam can be moved under digital control to seam weld any shape. Laser welding plays an important role in microelectronics and light-gauge metal-welding applications that require precise welding control.

LBW can be used with a variety of metals, including low-alloy and stainless steels, aluminum alloys, lead, titanium, refractory metals, and high-temperature alloys. It vaporizes the metal at the laser's point of focus, producing a deep penetration column of vapor. The vapor column is surrounded by a liquid pool, which is moved along the joint to produce welds with depth-to-width ratios greater than 8:1. However, speeds may be low in seam welding because the welds actually consist of a series of overlapping spot welds.

Certain characteristics of lasers can affect the costs of applying a system. Joint preparation and fixturing costs may be higher because lasers require close joint fit. However, filler material and edge machining are not used, so joint finishing costs are saved. The time it takes to load and unload parts is reduced because laser welding is done in an open atmosphere. Also, initial equipment costs normally are higher than those for conventional welders, but operating costs are comparable or lower.

Electroslag welding (ESW): Heat for ESW is provided by an electrically conductive molten slag that is resistance heated by the welding current. Electrodes are fed continuously into this molten pool of slag at temperatures over 3,200°F.

Electroslag welding is suitable for welding joints in thick metal plates because the weld pool is confined and molded by copper dams. However, electroslag welds must be completed in one continuous process. Restarting a weld after a stop can result in a large discontinuity. ESW tends to produce a large grain size because of the large mass of metal that is molten at a given time and the slow cooling of the metal.

Solid-state welding techniques include:

Inertia welding:This is one of the few solid-state welding processes that have been generally accepted by industry. Kinetic energy of a flywheel is converted to heat by friction between the workpieces. One part to be joined is fixed; the other is clamped in a spindle chuck. The flywheel (to which the movable part is attached) is accelerated, and at a predetermined speed driving power is cut, and the parts are forced together.

The major limitation of the process is that one of the two parts to be joined must be axially symmetric. But inertia welding has few of the material and part cleanliness limitations of other solid-state welding techniques.

Inertia-welding advantages include fast, uniform production welds, clean operation, low energy costs, and minimum labor skill required. The amount of upset of parts can be controlled to close tolerances, and a complete-interface weld can be obtained.

Ultrasonic welding (USW): This process joins metals by inducing high-frequency vibrations in overlapping metals in the area to be joined. Fluxes and filler metals are not required. Electrical current does not pass through the weld metal, and only localized heating is generated. The temperature produced is below the melting point of the materials, so no melting occurs during the welding.

Workpieces are clamped together between two jaws or sonotrode tips, and vibrations are transmitted through one or both of the tips oscillating in a plane parallel to the weld interface. The oscillating shear stress results in elastic hysteresis, localized slip, and plastic deformation at the contacting surfaces. This disrupts surface films and leads to metal-to-metal contact.

Ultrasonic welding has many applications in the assembly of electrical products. It is typically used to attach oxide-resistant contact buttons to switches; leads to coils of aluminum foil, sheet, or wire; and fine wire leads and elements to other components. For plastics, USW is used for both spot and continuous-seam fabrication and for closures on foil or plastic envelopes and pouches.