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ISO 25239-1:2020 – Friction Stir Welding Vocabulary: Complete Terminology Guide
Essential terminology for friction stir welding of aluminium alloys
ISO 25239-1:2020 defines the vocabulary and terminology used throughout the ISO 25239 series for friction stir welding (FSW) of aluminium alloys. Friction stir welding is a solid-state joining process invented at The Welding Institute (TWI) in 1991, where a non-consumable rotating tool generates frictional heat to plasticize material without reaching the melting point, producing high-quality joints with minimal distortion.
Unlike conventional fusion welding, FSW operates below the solidus temperature of the workpiece material. This means no solidification cracking, no porosity, and significantly reduced residual stresses — making it ideal for aluminium alloys that are notoriously difficult to weld with traditional methods.
Key Terminology and Definitions
The standard establishes precise definitions for all components, parameters, and phenomena specific to FSW. The welding tool consists of a shoulder (which generates the majority of frictional heat and contains plasticized material) and a pin (which stirs material across the joint interface). The advancing side of the weld is where the tool rotation direction aligns with the traverse direction, while the retreating side is where they oppose each other — a distinction that strongly influences microstructure and mechanical properties.
Critical process parameters defined include rotational speed (ω, typically 500-3000 rpm), traverse speed (v, typically 100-2000 mm/min), axial force (the downward force on the tool), and tool tilt angle (typically 1-3 degrees). The standard also defines weld quality attributes such as nugget zone, thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ).
| Term | Definition | Typical Value/Description |
|---|---|---|
| Shoulder | Larger diameter tool surface contacting workpiece top | 10-25 mm diameter, generates 80% of heat |
| Pin (Probe) | Smaller diameter tool feature extending into workpiece | 3-10 mm diameter, stirs material across joint |
| Advancing Side (AS) | Side where tool rotation and travel directions coincide | Higher temperature, finer grain structure |
| Retreating Side (RS) | Side where tool rotation opposes travel direction | Lower temperature, coarser grain structure |
| Nugget Zone (NZ) | Central weld region experiencing intense plastic deformation | Equiaxed fine grains, 2-10 μm typical |
| Flash | Expelled material extruded from under the shoulder | Excessive flash indicates poor parameter selection |
Confusion between advancing side and retreating side is a common source of inspection errors. The advancing side typically has a sharper boundary between the nugget and TMAZ, while the retreating side shows a more gradual transition. Always verify AS/RS orientation against the tool rotation direction before interpreting weld cross-sections.
Process Variants and Nomenclature
ISO 25239-1 covers terminology for key FSW variants. Bobbin tool FSW uses a tool with two shoulders — one on each side of the workpiece — eliminating the need for a backing anvil. Stationary shoulder FSW decouples shoulder rotation from pin rotation, providing a smoother surface finish. Refill friction stir spot welding (RFSSW) creates spot joints without leaving a keyhole, critical for automotive body panel applications.
The standard provides a systematic nomenclature for weld joint configurations including butt joints (the most common FSW configuration), lap joints, T-joints, and corner joints. Each configuration has specific terminology for tool path, joint preparation, and dimensional features.
When specifying FSW welds in engineering drawings, use the joint designation system from ISO 25239-1 to avoid ambiguity. For example, “FSW-B-6-AA5754” specifies a butt joint in 6 mm AA5754 aluminium alloy, ensuring consistent interpretation across design, production, and quality teams.
Frequently Asked Questions
Q: What is the difference between TMAZ and HAZ in FSW?
A: The TMAZ (thermo-mechanically affected zone) experiences both thermal cycling and plastic deformation, resulting in deformed grain structures. The HAZ (heat-affected zone) experiences only thermal effects — no mechanical deformation — leading to grain growth or precipitate coarsening without morphological change.
A: The TMAZ (thermo-mechanically affected zone) experiences both thermal cycling and plastic deformation, resulting in deformed grain structures. The HAZ (heat-affected zone) experiences only thermal effects — no mechanical deformation — leading to grain growth or precipitate coarsening without morphological change.
Q: Why is FSW referred to as a “solid-state” process?
A: The peak temperature during FSW typically reaches 70-90% of the melting temperature of the workpiece material. The material plasticizes but does not melt, avoiding solidification defects common in fusion welding such as porosity, hot cracking, and segregation.
A: The peak temperature during FSW typically reaches 70-90% of the melting temperature of the workpiece material. The material plasticizes but does not melt, avoiding solidification defects common in fusion welding such as porosity, hot cracking, and segregation.
Q: Can FSW be performed in positions other than flat (1G/PA)?
A: Yes, FSW can be performed in all positions. However, the tool must generate sufficient axial force to contain the plasticized material. Horizontal and overhead positions typically require reduced traverse speeds and optimized tool designs to prevent material sagging or dropout.
A: Yes, FSW can be performed in all positions. However, the tool must generate sufficient axial force to contain the plasticized material. Horizontal and overhead positions typically require reduced traverse speeds and optimized tool designs to prevent material sagging or dropout.
Q: What does “pitch” refer to in FSW parameters?
A: Pitch (or weld pitch) is the ratio of traverse speed to rotational speed (mm/rev). It represents the forward distance traveled per tool revolution. A pitch that is too high leads to void defects, while too low a pitch causes excessive heating and flash formation.
A: Pitch (or weld pitch) is the ratio of traverse speed to rotational speed (mm/rev). It represents the forward distance traveled per tool revolution. A pitch that is too high leads to void defects, while too low a pitch causes excessive heating and flash formation.
