Welding joins materials by establishing a permanent bond between the components. Depending on the welding process, the joint may be produced by melting the parent material, by pressure and plastic deformation without bulk melting, or by a combination of heat and pressure.
There are many welding processes because no single method is suitable for every material, joint geometry, component thickness or production requirement. The choice of process can influence weld quality, productivity, distortion, equipment cost, accessibility and the metallurgical condition of the finished joint.
This page provides an engineering introduction to the principal welding processes and explains why each type is used. Detailed welding procedures, qualification requirements and production parameters should be established from the applicable welding specification and process documentation.
A useful first distinction is whether the parent material is intentionally melted at the joint.
Fusion welding produces a joint by melting the parent material at the joint. Filler metal may also be added depending on the process and joint design. The molten weld pool subsequently solidifies to form the joint.
Arc welding, gas welding, laser beam welding and electron beam welding are examples of fusion welding processes.
Solid-state welding produces bonding without intentionally creating bulk melting of the parent materials. Pressure, deformation, friction, temperature, time, vibration or high-velocity impact may be used to produce the required intimate contact and bonding.
Examples include friction welding, friction stir welding, diffusion welding, ultrasonic welding, cold pressure welding and explosion welding.
The following table provides a first comparison of commonly encountered welding processes. It is intended as a process-selection guide rather than a substitute for a qualified welding procedure.
| Process | How the joint is produced | Parent metal melted? | Why it is used | Typical applications |
|---|---|---|---|---|
| MMA / SMAW | An electric arc melts the workpiece and a consumable flux-coated electrode. The electrode supplies filler metal while its coating provides shielding and forms slag. | Yes | Simple and versatile equipment, with good portability and usefulness for site and repair work. | Structural steelwork, fabrication, maintenance and repair. |
| MIG / MAG (GMAW) | An arc operates between the workpiece and a continuously fed consumable wire electrode under a shielding gas. | Yes | High productivity, continuous wire feed and excellent suitability for mechanisation and robotic welding. | General fabrication, automotive manufacture, steel, stainless steel and aluminium fabrication depending on process and shielding gas. |
| TIG (GTAW) | An arc is formed between a non-consumable tungsten electrode and the workpiece. Filler material can be added separately when required. | Yes | Excellent control of the weld pool and high-quality welds, particularly on thin material and demanding alloys. | Stainless steel, aluminium, titanium, pipework and precision fabrication. |
| Submerged Arc Welding | A continuously fed electrode produces an arc beneath a layer of granular flux. | Yes | High deposition rates and productivity on suitable long welds and thicker sections. | Pressure vessels, pipes, beams and heavy fabricated structures. |
| Oxy-Fuel Gas Welding | A fuel-gas and oxygen flame heats and melts the joint region. Filler metal can be added separately. | Yes | Portable, versatile equipment and useful manual heat control, although generally slower than modern arc welding processes. | Repair, maintenance, thin-section work and specialist applications. |
| Resistance Spot Welding | Overlapping components are clamped between electrodes. High electrical current produces local resistance heating while electrode force is applied. | Locally | Extremely rapid and repeatable, requires no separate filler metal and is readily automated. | Automotive bodies and high-volume sheet-metal assemblies. |
| Resistance Seam Welding | Rotating electrode wheels produce a succession of resistance welds along the joint. | Locally | Allows continuous or overlapping welded seams to be produced rapidly. | Sheet-metal containers, tanks and sealed seams. |
| Friction Welding | Relative motion generates frictional heating at the joint interface. Pressure subsequently consolidates the softened material. | No bulk melting | Repeatable solid-state joints and useful capability for joining some dissimilar materials. | Shafts, rods, valves, drill components and automotive parts. |
| Friction Stir Welding | A rotating non-consumable tool plasticises and mechanically transports material along the joint below the bulk melting temperature. | No | Low distortion and useful mechanical properties, particularly for many aluminium alloys. | Aerospace, rail, automotive, marine and aluminium structures. |
| Diffusion Welding | Prepared surfaces are held together under pressure at elevated temperature for sufficient time for interfacial bonding and diffusion to occur. | No | High-integrity joints with little macroscopic deformation and capability for specialised or difficult material combinations. | Aerospace components, titanium structures and specialist assemblies. |
| Ultrasonic Welding | High-frequency mechanical vibration is applied while the components are held together under force. | No | Very short joining cycles and limited bulk heating. | Electrical connections, foils, thin sheet and battery manufacture. |
| Explosion Welding | Controlled explosive energy accelerates one metal against another at high velocity, producing a metallurgical bond at the collision interface. | Essentially no bulk melting | Particularly useful for joining large areas of dissimilar metals which can be difficult to fusion weld. | Clad plate, transition joints and corrosion-resistant composite plate. |
| Cold Pressure Welding | Large compressive deformation disrupts surface films and brings clean, ductile metal surfaces into intimate contact. | No | No external heat source or filler metal is required. | Wire, electrical conductors and suitable ductile non-ferrous metals. |
| Forge Welding | Components are heated below melting temperature and mechanically worked together to produce a solid-state bond. | No | Historically important and still useful in specialist forging and traditional manufacturing. | Forged components and specialist manufacture. |
| Laser Beam Welding | A concentrated laser beam supplies highly localised energy to melt and join the material. | Yes | High welding speed, concentrated heat input, precision and excellent automation potential. | Automotive manufacture, electronics, precision and automated production. |
| Electron Beam Welding | A focused high-energy electron beam deposits energy into the workpiece and produces a narrow fusion zone. | Yes | Very high energy density can produce deep, narrow welds with controlled heat input. | Aerospace, power generation and high-integrity precision components. |
Soldering and brazing are joining processes rather than fusion welding processes because the parent materials are not intentionally melted. A lower-melting filler material flows into and bonds with the prepared joint surfaces.
Soldering uses a relatively low-melting filler metal. The parent materials are heated sufficiently to allow the solder to wet the prepared surfaces without melting the components themselves.
Flux may be used to remove or disrupt oxide films, protect the surfaces during heating and promote wetting.
Where joint geometry is suitable, molten solder can be drawn through a small joint clearance by capillary action.
Soldering is widely used for electrical and electronic connections and for light-duty engineering assemblies where the temperatures and mechanical requirements are appropriate.
Brazing uses a filler metal which melts above 450°C while remaining below the melting temperature of the parent materials. The molten filler wets the prepared joint surfaces and is commonly distributed through a close-fitting joint by capillary action.
Brazing filler systems include copper-, silver- and nickel-based materials among others. Heating can be provided by torch, furnace, induction, resistance or other methods.
Brazing is useful where:
Applications include heat exchangers, pipework, tooling, electrical components and many mechanical assemblies.
Gas welding uses combustion of a fuel gas with oxygen to produce the heat required to melt the joint region. Oxy-acetylene is the best-known form because the oxygen-acetylene flame provides sufficiently high temperature for welding many engineering metals.
The torch mixes the gases and allows the welder to control the flame. Depending on the process and material, filler metal may be added separately.
Oxy-Acetylene Welding
Gas welding equipment is relatively portable and can provide excellent manual control of heat input. For substantial production welding, however, arc processes are generally considerably more productive.
Oxy-fuel cutting is an allied thermal-cutting process rather than a welding process.
The material is initially heated to a temperature at which rapid oxidation can occur. A high-purity oxygen jet is then directed at the heated region. The metal reacts with the oxygen and the resulting oxides and molten material are expelled from the cut.
Oxy-Fuel Cutting
The process is particularly associated with suitable carbon and low-alloy steels. It should not be assumed that every metal which can be melted by an oxy-fuel flame can also be efficiently cut by the oxidation process.
Arc welding processes use an electrical arc as the principal heat source. The arc is established between an electrode and the workpiece, generating sufficient local heat to form the weld.
Arc welding includes several distinct processes which differ in electrode type, shielding method, filler-metal supply and degree of automation.
Manual Metal Arc welding, also known as Shielded Metal Arc Welding (SMAW), uses a consumable flux-coated electrode.
An arc forms between the electrode and workpiece, melting both the joint region and the end of the electrode. The electrode core provides filler metal while the coating decomposes and melts to provide shielding and fluxing action.
After solidification, slag formed from the electrode coating normally covers the weld and is removed before inspection or subsequent weld passes.
MMA is valued for:
Gas Metal Arc Welding (GMAW) uses a continuously fed consumable wire electrode. An arc between the wire and workpiece melts the wire and parent material while shielding gas protects the weld region.
The terminology depends on the shielding gas.
Continuous wire feed gives the process high productivity and makes GMAW particularly suitable for mechanised and robotic production.
Tungsten Inert Gas welding, also known as Gas Tungsten Arc Welding (GTAW), uses a non-consumable tungsten electrode to establish the arc.
An inert shielding gas protects the electrode and weld region. Filler metal is supplied separately where required, allowing the process to be used either autogenously or with added filler.
TIG provides excellent control over heat input and the weld pool and is widely used for stainless steels, aluminium alloys, titanium and other materials where weld quality and control are particularly important.
Its principal disadvantage for many production applications is a lower deposition rate than continuously fed consumable-electrode processes.
Submerged Arc Welding (SAW) uses an arc beneath a layer of granular flux. A consumable electrode is continuously fed into the weld region.
The flux shields the arc and molten weld pool from the surrounding atmosphere. Because the arc is submerged, visible arc radiation and spatter around the process are greatly reduced.
Side View - Submerged Arc Welding
SAW is particularly useful for long welds and heavy fabrication where high deposition rates and mechanised production are advantageous.
Applications include pressure vessels, large pipes, structural fabrication and other heavy plate assemblies.
Resistance welding generates heat from electrical resistance at and around the joint while mechanical force is applied to the components.
The process is particularly important in high-volume sheet-metal production because individual welds can be made very rapidly and the process can be readily automated.
In resistance spot welding, overlapping sheets are clamped between electrodes. A high electrical current passes through the joint for a controlled time.
Electrical resistance generates concentrated heat at the interface, producing a local weld nugget while electrode force maintains contact and consolidates the joint.
| Stage | Purpose |
|---|---|
| Squeeze | Electrode force is applied and the components are brought into controlled contact before welding current is applied. |
| Weld | Current passes through the components and generates the heat required to form the weld. |
| Hold | Electrode force is maintained while the weld nugget solidifies. |
| Off / Release | Current has stopped and the electrodes are released so that the component or electrodes can move to the next position. |
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Projection welding is related to resistance spot welding, but the current and force are concentrated at deliberately formed projections or local features in one or both components.
The projection concentrates electrical resistance and heating at the required weld position. As the joint forms, the projection deforms or collapses.
The process is useful for components such as nuts, studs, brackets and other parts where one or more accurately positioned resistance welds are required.
Resistance seam welding commonly uses rotating wheel electrodes to pass current through overlapping sheet material as the joint travels between the wheels.
A series of individual or overlapping weld nuggets can produce a substantially continuous seam. The process is useful where rapid production of sealed or near-continuous sheet-metal joints is required.
Solid-state welding joins materials without intentionally producing bulk melting of the parent materials. Bonding is achieved using combinations of pressure, plastic deformation, temperature, time, friction, vibration or high-velocity impact.
Avoiding a conventional molten weld pool can provide important engineering advantages. Solid-state processes can reduce solidification-related defects, limit some thermal effects and make certain dissimilar-material combinations easier to join than by fusion welding.
Friction welding generates heat through relative movement between the surfaces being joined while force is applied.
The interface becomes hot and plastic without intentionally producing bulk melting. Relative motion is then stopped and forging force consolidates the joint.
The process is highly repeatable and is particularly useful for rotational components such as shafts, rods and tubes. It can also join some dissimilar metal combinations which are difficult to fusion weld.
Friction Stir Welding (FSW) uses a rotating non-consumable tool which is plunged into the joint and traversed along it.
Frictional heating and severe plastic deformation soften the surrounding material without intentionally melting it. The rotating tool transports and mechanically mixes the plasticised material across the joint, which consolidates behind the tool.
The process became particularly important for aluminium alloys, including applications where conventional fusion welding can be difficult.
Diffusion welding uses accurately prepared mating surfaces held together under pressure at elevated temperature for sufficient time to develop bonding across the interface.
The temperature remains below that required to produce conventional bulk fusion. Intimate surface contact, deformation at microscopic asperities and atomic diffusion progressively eliminate the original interface.
Surface condition, atmosphere, temperature, pressure and time are therefore important to the process.
Ultrasonic welding applies high-frequency mechanical vibration while the components are held together under force.
Relative microscopic movement at the interface disrupts surface films and produces local plastic deformation, allowing sufficiently intimate contact for a solid-state bond to develop.
Explosion welding uses a controlled explosive charge to accelerate one metal component against another at high velocity and at a controlled collision angle.
The high-energy impact disrupts surface contamination and causes intense local deformation, producing a metallurgical bond without conventional bulk fusion of the components.
Cold pressure welding joins suitable ductile materials through substantial compressive deformation of clean mating surfaces.
The deformation disrupts oxide films and contamination and brings fresh metal surfaces into sufficiently intimate contact for bonding to occur. No external heat source or filler metal is required.
Forge welding is one of the oldest solid-state joining processes. The components are heated until sufficiently plastic and the prepared surfaces are mechanically worked together by hammering or pressing.
The process has largely been replaced by more controllable modern welding methods for general fabrication but remains important historically and in specialist forging and traditional manufacture.
Roll welding bonds prepared materials by passing them between rolls under sufficient compressive force to produce substantial deformation and intimate contact at the interface.
The process is particularly applicable to sheet and plate and can be used to manufacture clad or laminated metallic products.
Hot pressure welding combines elevated temperature with mechanical pressure to produce a joint without intentionally melting the parent materials. Heating reduces the force required to deform the mating surfaces and assists the development of intimate interfacial contact.
The term encompasses several pressure-assisted joining techniques, and the actual process conditions depend strongly on the materials and component geometry.
Electron Beam Welding (EBW) uses a focused beam of high-energy electrons. When the electrons strike the workpiece their kinetic energy is converted into heat, producing highly concentrated local melting.
The high energy density allows narrow, deeply penetrating welds to be produced with a relatively small heat-affected region compared with many conventional fusion processes.
Electron beam systems commonly operate under vacuum conditions, although equipment and process arrangements vary. Vacuum operation helps control the electron beam and provides a clean welding environment.
The process is used where weld quality, penetration, precision or control of heat input justify the specialised equipment. Aerospace, power-generation and other high-integrity components are important applications.
Laser Beam Welding (LBW) uses a concentrated beam of electromagnetic radiation to deliver energy to the joint. The focused beam can produce a small and highly controlled fusion zone.
Modern industrial laser welding systems can operate continuously or in pulsed modes depending on the laser source and application. The process is highly compatible with automation and precision motion systems.
Laser welding can provide high travel speeds, narrow welds and controlled heat input. Depending on equipment and application it can be used on thin precision components as well as substantially thicker engineering sections.
In a fusion weld the material immediately adjacent to the molten weld pool does not necessarily melt, but it experiences a thermal cycle capable of changing its microstructure and properties. This region is known as the heat-affected zone (HAZ).
The significance of the HAZ depends on the parent material and welding procedure. Potential effects include:
Welding procedure, preheat, heat input, interpass temperature and post-weld treatment can therefore be important engineering controls.
Fusion welding introduces highly localised heating followed by cooling. The resulting non-uniform thermal expansion and contraction can produce residual stress and distortion.
Distortion can take several forms, including:
Joint design, welding sequence, restraint, heat input, balanced welding and fabrication planning can all influence the resulting distortion.
A filler metal is not required by every welding process. Where filler is used, it must be compatible with the parent materials and the required mechanical, metallurgical and service properties of the joint.
Protection from the surrounding atmosphere can be provided by several methods depending on the process:
The shielding method is an integral part of the welding process rather than an optional addition.
Weldability describes how readily a material can be welded to produce a joint which satisfies the required performance under the intended manufacturing conditions.
It depends on more than whether two pieces of material can physically be joined. Important considerations include:
Joining dissimilar metals can introduce additional problems because the materials may have different melting temperatures, thermal expansion, thermal conductivity and metallurgical behaviour.
A fusion process may create brittle phases or undesirable dilution between some material combinations. This is one reason solid-state processes such as friction welding, diffusion welding and explosion welding can be valuable.
Brazing can also provide a useful solution because the parent materials do not have to be melted.
Process selection should begin with the engineering requirements of the joint.
| Question | Why it matters |
|---|---|
| What materials are being joined? | Material composition and metallurgical compatibility influence weldability, filler selection and the processes which can be used. |
| What thickness is being welded? | Some processes are particularly suited to thin sheet while others provide high deposition rate or deep penetration for heavy sections. |
| What is the joint geometry? | Access, welding position and joint preparation can eliminate otherwise attractive processes. |
| What production quantity is required? | A portable manual process may be ideal for repair but uneconomic for hundreds of thousands of identical production welds. |
| Can the parent material be melted? | Metallurgical or distortion requirements may favour brazing or a solid-state process. |
| How important is distortion? | Heat input, joint restraint and process concentration strongly affect dimensional stability. |
| Does the process need to be automated? | GMAW, resistance welding, laser welding and several solid-state processes are particularly suited to automated production. |
| What weld quality is required? | Inspection, defect tolerance, fatigue duty and service environment influence process and procedure selection. |
| What equipment is practical? | Vacuum chambers, large welding machines and specialised tooling may be justified in production but inappropriate for a one-off repair. |
The quality and cost of a welded fabrication are influenced strongly by component design.
Design considerations include:
Weld quality should be assessed against the requirements of the component and the applicable welding specification.
Inspection methods can include:
The appropriate method depends on material, joint geometry, process, possible discontinuities and the required acceptance criteria.
For controlled engineering applications, welding is carried out to a defined procedure rather than relying only on the skill and judgement of the individual welder.
A welding procedure can define matters such as:
Where required by the governing standard, code or contract, the procedure and the personnel carrying out the welding may require formal qualification.
Standardised numerical process designations are widely used on engineering documentation, welding procedures and production records. RoyMech's legacy page contained a process list based on BS EN ISO 4063.
Commonly encountered examples include:
| Process number | Process |
|---|---|
| 111 | Manual metal arc welding with covered electrode |
| 121 | Submerged arc welding with one wire electrode |
| 131 | MIG welding with solid wire electrode |
| 135 | MAG welding with solid wire electrode |
| 136 | MAG welding with flux-cored electrode |
| 141 | TIG welding with solid filler material |
| 21 | Resistance spot welding |
| 22 | Resistance seam welding |
| 23 | Projection welding |
| 311 | Oxy-acetylene welding |
| 42 | Friction welding |
| 43 | Forge welding |
| 441 | Explosion welding |
| 45 | Diffusion welding |
| 48 | Cold pressure welding |
| 52 | Laser welding |
| 51 | Electron beam welding |
The welding process is only one part of welded-joint design. The engineer must also establish the joint geometry, weld size and loading requirements.
RoyMech provides separate information on:
The following external resources were included on the original RoyMech welding page and are retained for further information and subsequent link checking.
