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ISO 25239-4:2020 – FSW Welding Procedure Specification and Qualification
Developing and qualifying welding procedures for friction stir welding
ISO 25239-4:2020 specifies requirements for welding procedure specification (WPS) and qualification for friction stir welding of aluminium alloys. A properly qualified welding procedure is the foundation of consistent, defect-free FSW production. The standard establishes a systematic methodology for developing, testing, and qualifying FSW procedures across different alloys, thicknesses, and joint configurations.
Unlike fusion welding where pre-qualified WPSs (e.g., according to AWS D1.2) are common, FSW currently requires procedure qualification for each new application due to the strong interdependence between tool design, material characteristics, and process parameters.
Welding Procedure Specification (WPS) Content
The standard specifies the minimum information that must be documented in a FSW-WPS. Essential variables include material group and thickness, joint type and dimensions, tool geometry (shoulder diameter, pin diameter, pin length, feature type), tool material, rotational speed (range), traverse speed (range), axial force (range), tool tilt angle, and plunge depth. Any change to an essential variable requires requalification of the procedure.
Non-essential variables include clamping method, backing anvil material, shielding gas (when used), and preheating — changes to these do not require requalification but must be documented. The standard provides a template for FSW-WPS format, including fields for tool drawing references, parameter monitoring methods, and acceptance criteria for production welds.
| Parameter Category | Examples | Change Requires Requalification? |
|---|---|---|
| Essential | Material group, thickness, joint type, tool geometry, rotation speed, traverse speed, axial force | Yes |
| Non-Essential | Clamping method, backing material, shielding gas, preheating | No |
| Supplementary Essential | Heat treatment, post-weld aging parameters | Yes, when applicable |
Tool geometry is frequently underestimated as an essential variable. Two tools with identical shoulder and pin diameters but different feature geometries (e.g., threaded vs. fluted pin, concave vs. flat shoulder) can produce dramatically different weld qualities. Always include a detailed tool drawing with the WPS and require requalification when any tool geometry dimension changes by more than 0.1 mm.
Procedure Qualification Testing
Qualification requires welding a test piece representative of the production joint, followed by comprehensive testing. Destructive tests include transverse tensile test (per ISO 4136), bend test (per ISO 5173), and macroscopic examination (per ISO 17639). The standard specifies minimum acceptance criteria: tensile strength must reach at least 90% of the parent material strength for structural applications, bend specimens must not show defects exceeding 3 mm on the tension surface, and macro sections must be free of voids, cracks, and lack of fusion.
Non-destructive testing includes visual inspection (100% of production welds), and when specified, radiographic testing (ISO 17636) or ultrasonic testing (ISO 17640) for critical applications. The qualification test piece must be produced under conditions that replicate production constraints, including clamping configuration, backing conditions, and equipment used. The standard defines ranges of qualification for thickness and diameter — for example, qualifying on 6 mm material typically covers 3-12 mm for the same alloy group and joint type.
Use design of experiments (DOE) methodology during procedure development rather than one-factor-at-a-time (OFAT) approaches. FSW parameters interact strongly — for example, rotational speed and traverse speed jointly determine heat input and cooling rate. A DOE approach with 15-20 trials can efficiently map the parameter window and identify the optimal operating point with fewer total tests than OFAT.
Frequently Asked Questions
Q: How many test pieces are required for procedure qualification?
A: The standard typically requires one test piece from which multiple test specimens are extracted: 2 tensile, 4 bend (2 face, 2 root), and 1 macro section. Additional specimens may be required for fracture toughness, fatigue, or corrosion testing depending on the application.
A: The standard typically requires one test piece from which multiple test specimens are extracted: 2 tensile, 4 bend (2 face, 2 root), and 1 macro section. Additional specimens may be required for fracture toughness, fatigue, or corrosion testing depending on the application.
Q: Can a qualified procedure be transferred between different FSW machines?
A: Yes, but only if the machine characteristics (stiffness, force capability, control system, spindle power) are equivalent. Significant differences in machine dynamics can alter the effective parameter window. Verification welding and testing on the new machine is recommended even if requalification is not formally required.
A: Yes, but only if the machine characteristics (stiffness, force capability, control system, spindle power) are equivalent. Significant differences in machine dynamics can alter the effective parameter window. Verification welding and testing on the new machine is recommended even if requalification is not formally required.
Q: What is the validity period of a qualified FSW procedure?
A: ISO 25239-4 does not specify a fixed validity period for the procedure itself. However, the WPS remains valid as long as production conditions do not change. If the procedure is not used for more than 3 years, a review and verification weld is recommended before resuming production.
A: ISO 25239-4 does not specify a fixed validity period for the procedure itself. However, the WPS remains valid as long as production conditions do not change. If the procedure is not used for more than 3 years, a review and verification weld is recommended before resuming production.
Q: How are parameter tolerances defined in the WPS?
A: Typical ranges are ±10% for rotational speed, ±15% for traverse speed, and ±20% for axial force. These ranges must be validated during qualification testing — the test piece should be welded at the extreme conditions (e.g., minimum rotational speed with maximum traverse speed) to confirm the procedure’s robustness at parameter boundaries.
A: Typical ranges are ±10% for rotational speed, ±15% for traverse speed, and ±20% for axial force. These ranges must be validated during qualification testing — the test piece should be welded at the extreme conditions (e.g., minimum rotational speed with maximum traverse speed) to confirm the procedure’s robustness at parameter boundaries.
