What is friction stir welding (FSW), and where is it used?

How FSW works

A rotating tool with a shoulder and pin plunges into the joint line. Friction and deformation heat the material until it becomes plastic. As the tool travels along the joint, metal flows around the pin and consolidates behind it, forming a solid-state joint.

Quality depends on more than one parameter. Rotational speed, travel speed, axial load, tool geometry, plunge depth and workpiece clamping rigidity all matter. The process parameters must therefore be considered as an integrated system.

Why the technology is popular for aluminium

The absence of molten metal reduces the risk of solidification defects and usually limits distortion in long components. The process is readily automated and suits series production where repeatability is important.

Many aluminium applications require neither filler material nor shielding gas. However, FSW is not a heat-free process: the material structure changes in the joint zone, so its properties must be confirmed through process testing.

Practical advantages of FSW over argon arc welding of aluminium workpieces include productivity, cost efficiency, lower energy consumption, environmental benefits and reduced residual distortion. These benefits relate to specific production tasks and must be confirmed for the chosen component and process parameters.

• the joint forms in the solid state;
• high repeatability of the automated process;
• less spatter and no welding arc;
• suitable for certain aluminium alloys that are difficult to fusion-weld.

How quality is controlled

Typical problems arise from insufficient or excessive heat input, incorrect tool plunge depth, unstable force and inadequate clamping. Possible defects include root discontinuities, tunnel defects, local thinning, surface imperfections and deviations in joint geometry.

GOST ISO 25239-5 treats inspection as a system that starts before the process begins. Before welding, the applicability of the qualified procedure and the readiness of equipment and fixtures are checked. During welding, compliance with specified parameters is monitored. After welding, visual, non-destructive and, where required, destructive testing is performed, together with checks of shape and dimensions.

The non-destructive testing method is selected according to the joint type and expected defects. For critical components, appearance alone is insufficient: quality is confirmed by combining recorded process parameters with inspection results for the finished joint.

Where the technology is used

FSW is particularly effective for long aluminium panels, profiles, tanks and transport structures where low distortion and dimensional stability are important.

Suitability depends not only on the material, but also on tool access, the ability to provide backing support, weld length, required productivity and fixture economics.

Key equipment requirements

The machine must provide sufficient axial force, frame rigidity, spindle stability and accurate path control. For large components, the working envelope, locating method and clamping system are especially important.

Equipment selection begins with the process task: material, thickness, joint type, weld geometry and quality requirements. The machine configuration is then selected and parameters are developed using test specimens.

Archival specifications for the STP-4PL machine list a welding thickness of up to 50 mm, a weld length of 3,890 mm and an axial force of 4,000 kgf. These figures describe a specific machine and its condition in 2017, rather than universal FSW parameters.

Sespel’s industrial experience

The company progressively developed aluminium alloy welding capabilities, using STP-14 and STP-4PL machines alongside Fronius manual argon arc welding and Fanuc robotic welding. Technology development included a partnership with the Institute of Strength Physics and Materials Science of the Siberian Branch of the Russian Academy of Sciences.

FSW development for 25–35 mm AD0 aluminium plates covered different welding arrangements and tools, including single-sided, double-sided and bobbin-tool welding. Selected trials used rotational speeds of 400–550 rpm and travel speeds of 70–150 mm/min. This illustrates that parameters are selected for the thickness, tool and joint arrangement, rather than transferred automatically between products.

Demonstrated applications include welding a 9,200 × 8,300 mm panel from 28 mm aluminium plates to form a tank wagon shell, as well as manufacturing and testing aluminium tank wagons. This also demonstrates the use of aluminium technologies in large transport and bridge structures.

• FSW was used as part of an integrated manufacturing process for aluminium structures;
• for series production, parameters are validated through testing and weld inspection;
• scaling the technology requires rigid fixtures, stable equipment and repeatable parameter control.

Which parameters must be repeatable?

The main engineering benefit of a formalised procedure is repeatability. Even a sound weld on a test specimen is insufficient justification for series production unless the process parameters and limits have been defined.

For position-controlled equipment, plunge depth and tool position relative to the surface are particularly important; for force-controlled systems, axial load stability is critical. In both cases, results depend on rotational speed, travel speed, tool tilt, pin and shoulder geometry, temperature and clamping conditions.

How the GOST ISO 25239 series is applied

GOST ISO 25239-2-2026 covers weld joint design and helps define joint geometry for FSW. GOST ISO 25239-4-2026 establishes an approach to specifying and qualifying welding procedures, while GOST ISO 25239-5-2026 sets requirements for quality and inspection.

Process documentation records more than welding speed and rotational speed. Repeatability requires the parent material grade and condition, component thickness, equipment and tool details, process control method, edge preparation, clamping and controlled parameter ranges.

Procedure qualification confirms that the selected combination of equipment, material, joint and parameters can achieve the required result. Successful testing establishes a working procedure whose scope is limited to the conditions validated during qualification.

• pWPS — a preliminary specification of the proposed welding procedure;
• WPQR — a record of procedure qualification test results;
• WPS — the approved working procedure used for production welding;
• exceeding the qualified range requires engineering assessment and, where specified, requalification.