Friction stir welding (FSW) is a solid-state joining technique that has advanced the field of welding by dispensing with material melting. Developed in the 1990s, this method distinguishes itself from conventional techniques by joining materials below their melting point, which mitigates common defects associated with solidification and allows combining energy efficiency with high mechanical integrity.
This analysis focuses on the technical fundamentals, equipment components, material compatibility and its profile as a low environmental impact alternative to fusion techniques.
Friction stir welding
FSW is a solid-state joining process that avoids melting of the metal during welding. It uses a non-consumable rotary tool that generates frictional heat and plasticizes the material without melting it. This technique is used to join materials that are difficult to weld by conventional methods, achieving joints with excellent mechanical and metallurgical properties and resistance to corrosion, thanks to the homogeneous microstructure generated during the process.
The term “friction stir welding” has been standardized by the international standard ISO 25239:2020 – Friction Stir Welding of Aluminium; which defines the technical requirements for this process, including qualification procedures, operating parameters and defect classification. This standard ensures technical uniformity and identification of the origin of industrial processes using FSW.
The name of the process reflects its two fundamental principles:
- Friction, which generates the necessary heat without reaching the melting point;
- Agitation, by which the material is plasticized and mechanically mixed by the action of the rotating probe.
Additionally, other relevant standards include AWS D17.3/D17.3M:2021, applicable to aerospace components, and the technical certification programs offered by the American Welding Society (AWS), which reinforce the standardization of this technology in advanced industrial environments.
Development and historical summary
Stir welding is relatively recent, having been developed in 1991 by Wayne Thomas at The Welding Institute (TWI), UK, with the aim of solving the limitations of aluminum fusion welding especially on 2000 and 7000 series alloys, known for their susceptibility to hot cracking.
The development of FSW responded to the critical need to join advanced materials without compromising their integrity. The evolution of this technology has been marked by intense research aimed at innovative tool design and process parameter improvement, broadening its industrial applicability.
Fundamental principles and process mechanics
FSW works by means of a rotating tool consisting of a shoulder and a pin. The shoulder generates frictional heat on the surface, while the pin penetrates the joint and agitates the plasticized material. This action is performed at 70% to 90% of the melting temperature of the material, without actually melting it.
The progressive displacement of the tool forms a homogeneous bond, free of discontinuities. The resulting microstructure is composed of three distinct regions: a nugget zone, with fine grains formed by dynamic recrystallization; a thermomechanically affected zone (TMAZ) subject to heat and shear; and a heat-affected zone (HAZ), where thermal cycling occurs without significant deformation.
This thermomechanical control makes it possible to join materials and also to modify their properties locally, something that is not achieved with fusion welding, where rapid solidification can generate fragile structures.
Process parameters
Control of critical parameters include rotational speed (rpm), traverse speed (mm/min), axial penetration force, tool inclination and tool displacement in dissimilar joints. Correct parameterization ensures proper thermal generation, homogeneous plasticization and intermetallic reduction in heterogeneous configurations. For example, too much rotation can overheat the material causing defects, while too little rotation does not adequately plasticize the joint.
Stages of friction stir welding process
The complete FSW cycle is carried out in well-defined sequential phases, which guarantee the thermal and mechanical continuity of the process:
- Penetration: The FSW tool rotates at high speed and contacts the material surface, generating frictional heat without actually melting the material.
- Plastic agitation: The pin penetrates the bond line, while the shoulder confines the softened material. The rotary and axial motion plasticizes and mixes the material.
- Translation: The tool advances along the joint, forming a continuous bond while maintaining solid-state agitation.
- Exit and cooling: The tool is carefully withdrawn and the material is cooled under pressure, generating a homogeneous bead with superior mechanical properties.
As a complement to the article, the following video presents a research developed at West University on stir welding. The study reflects an integration between metrological characterization and process control, aiming to reinforce the industrial applicability of FSW.
Friction stir welding equipment parts
The stir welding system is composed of a number of fundamental components that act in an integrated manner to ensure process stability, bead quality and operational efficiency. Its main elements are described below:
FSW tool (probe and shoulder)
It is the active component of the welding process. It consists of:
- Probe or pin: Present in the weld line, it plasticizes the material by friction and mechanically agitates it to mix it without melting it.
- Shoulder: Located on the surface, it generates additional surface friction, helps to confine the plasticized material and contributes to bead closure.
The geometry of both (diameter, length, threads, splines, helical channels or conical profile) directly influences material flow, thermal generation, bond integrity and final microstructure.
Drive system and drive head
It includes the motorized spindle and the linear feed system, responsible for the following parameters:
- Rotation speed (rpm).
- Feed or translation speed (mm/min).
- Constant axial force during penetration.
The drive is integrated with CNC machines or robotic systems, allowing high repeatability, optimized cycles and precise control over thermal and mechanical conditions.
Clamping system
The function of this system is to keep the parts firmly aligned during operation by counteracting axial and torsional forces. There are three variants depending on the production environment:
- Manual: with mechanical presses for specific tasks.
- Pneumatic: suitable for low volume intermittent jobs.
- Hydraulic: for automated production lines requiring high rigidity.
Poor clamping can lead to defects such as lack of penetration, burrs, cracks, voids or flash due to misalignment or deformation in the heat-affected zone.
Control and monitoring system
It is the core of process management. It employs CNC or PLC controllers combined with sensors that record and adjust in real time critical variables such as:
- Temperature in the agitation zone.
- Torque applied.
- Axial force.
- Position and depth of penetration.
In advanced environments, SCADA systems, closed-loop feedback control and algorithms for automatic defect detection are integrated, facilitating traceability and quality assurance.
Anvil or support plate
Rigid surface on which the parts are positioned. Its main functions are:
- Absorb the vertical pressure of the tool and avoid displacements during welding.
- Dissipate residual heat, avoiding distortions or burrs on the underside of the material.
In applications with materials of high thermal conductivity or resistance, such as steels or titanium, liquid cooling can be incorporated into the anvil or drive head to stabilize the temperature and prevent thermal failure.
Types of materials and metals that can be bonded with FSW
Although predominantly applied to aluminum alloys, stir welding is also used for the assembly of lightweight materials with specific structural applications, such as magnesium, copper, titanium and certain steels. It is also effective in heterometallic configurations that require structural continuity without resorting to fusion.
- Aluminum and its alloys: FSW is highly effective for joining aluminum alloys in all their forms (cast, rolled or extruded), even those high-strength alloys considered “unweldable” by melting, such as the 2000 and 7000 series. Due to its good plasticity in the solid state and low thermal conductivity, it allows confining heat in the agitation zone and avoiding defects such as hot cracking. In addition, it favors microstructural refinement through dynamic recrystallization. Its application is predominant in demanding industrial sectors such as aerospace, rail and shipbuilding.
- Magnesium and its alloys: FSW on magnesium alloys such as AZ31 and AZ91 is feasible due to their low density and moderate melting point. The process avoids cracks, improves ductility and refines the microstructure by plastic stirring; this increases mechanical strength, being useful in sectors such as light automotive and electronics with thin structures.
- Copper and Al-Cu systems: FSW allows soldering copper and Al-Cu systems avoiding excessive temperatures, minimizing oxidation and brittle compounds. In dissimilar joints with aluminum, it achieves intermetallic layers of less than 2 µm, preserving ductility and electrical conductivity. It is ideal for busbars, heat exchangers and high demand electrical connections.
- Titanium and Ti-6Al-4V alloys: It requires high operating conditions due to the high mechanical resistance of these elements: reinforced tools (such as PCBN or tungsten carbide) and higher axial pressures. However, the process allows achieving integral joints without the need of inert atmospheres or preheating; maintaining metallurgical stability and reducing the possibility of subsurface defects.
- Carbon and stainless steels (304, 316L, DQSK): The use in steels is more limited due to higher tool wear, its application in thin sheets (less than 6 mm) has shown good mechanical and thermal results, one of the challenges is the cooling system of the spindle or anvil. Currently, it is used in railway components, light marine structures and pressure vessels with specific sealing requirements.
- Dissimilar metals: FSW: Used to join metals with very different physical and chemical properties, such as aluminum with copper, magnesium, steel or titanium. The control of parameters such as tool position relative to the joint axis, feed rate and heat balance limits the formation of brittle intermetallic phases.
Unusual material combinations
- Mild steel and stainless steel welding.
- Aluminum and titanium welding.
- Aluminum and copper welding.
- Copper and stainless steel welding.
- Carbon steel and stainless steel welding.
- Aluminum and stainless steel welding.
Friction stir welding as a green technology
As a low environmental impact alternative to fusion welding techniques, friction stir welding is one of the most widely applied, due to its high energy efficiency and safe operating conditions. Operating in a solid state, fusion-free welding significantly reduces emissions, energy consumption and waste generation.
In addition, by minimizing the use of consumables, process traceability is optimized, the logistic supply chain is simplified and waste production is reduced, consolidating its profile as a cleaner, safer option aligned with current sustainable production models.
Resource efficiency and circular economy
- Optimized energy consumption: CNC systems have an energy consumption range of 7 to 11 kW, compared to the typical 20-40 kW for arc welding, which represents an estimated energy saving of 40% to 70%.
- Elimination of consumables: This welding process does not require filler material, flux, shielding gases, solvents or post-treatment.
- Defect reduction: By avoiding solidification, porosity, cracking and rejection rates are minimized, also reducing the need for non-destructive testing (NDT).
- Reduced distortion: Low heat input reduces the need for post-weld machining or straightening.
Real-time optimization case study
A recent article in Scientific Reports (Nature, 2024) entitled “Real-time analysis of tool-workpiece interaction and optimization in friction stir welding using sensor data fusion and AI” addresses a very relevant improvement for FSW. The study describes a system that integrates sensor data fusion (such as torque, temperature and axial force) together with artificial intelligence algorithms (neural networks) to analyze and optimize in real time the welding parameters.
As a result, clear progress was achieved in:
- Dynamic control of operating conditions, improving precision during execution.
- Reduction of microstructural defects by maintaining ideal plasticizing conditions.
- Improved joining efficiency, visualizing and correcting deviations during the process.
Conclusions
The friction stir welding technique has redefined joining standards in metallic materials, overcoming common limitations of fusion welding methods, such as solidification defects, thermal distortion or weakened zones. Its application in high-strength alloys (especially aluminum) allows homogeneous, stable and reliable joints to be obtained.
In addition, its compliance with ISO 25239 guarantees the traceability and quality of the process, enabling its implementation in sectors that demand high precision and regulatory compliance. From an energy and environmental point of view, FSW stands out as a clean technique, with no emission of toxic gases or use of consumables, compatible with principles of sustainability and responsible production.
References
- https://en.wikipedia.org/wiki/Friction_stir_welding
- https://www.iso.org/obp/ui/en/#iso:std:iso:25239:-1:ed-2:v1: enTWI (The Welding Institute) – Job Knowledge: Friction Stir Welding.
