A riveted joint is thousands of discrete parts, each with a drilled hole, a stress concentration, a leak path and a piece of added mass. A friction stir weld replaces that with a continuous joint of forged parent metal: no fasteners to buy, drill for, insert and inspect; no fastener weight carried for the life of the product; no hole-edge fatigue initiation; hermetic by nature; and faster per metre of joint in production. Fasteners keep the advantage for mixed-material stacks, field assembly, and structures whose certification basis is built around inspecting fastened joints.
What fastening really costs
Riveting earned its place in aluminium structures because early fusion welds could not be trusted in the strong 2xxx and 7xxx aerospace alloys — those grades are essentially unweldable by arc processes. The fastened joint was the workaround, and it carries four permanent costs:
- Mass. Every fastener, and every overlap flange sized to accept it, is weight the structure carries forever. In aerospace and space structures, that mass is paid for on every flight.
- Fatigue. Every drilled hole is a stress concentration. Fastened structures live and die by hole quality, fit and inspection intervals.
- Rate. Drill, deburr, seal, insert, set, inspect — multiplied by thousands of fasteners per assembly.
- Sealing. A fastened joint is not inherently tight; sealants and gaskets add process steps, cure times and their own ageing behaviour.
What changes with friction stir welding
Friction stir welding joins the alloys that made riveting necessary. Because it never melts the metal, the 2xxx and 7xxx families weld reliably in the solid state, with typical joint strengths above 90% of parent ultimate tensile strength in thin sections.
| Criterion | Riveted / fastened joint | Friction stir welded joint |
|---|---|---|
| Joint character | Discrete points, drilled holes | Continuous forged parent metal |
| Added mass | Fasteners, overlap flanges, sealant | None — often enables thinner design |
| Fatigue behaviour | Hole-edge initiation; inspection-driven | Fine-grained forged weld; no holes |
| Leak-tightness | Requires sealant | Hermetic by nature |
| Operations per metre of joint | Dozens to hundreds | One pass |
| Part count / logistics | Thousands of fasteners to source and track | Zero consumables |
| 2xxx / 7xxx alloys | Fastening was the workaround | Welds them directly |
| Mixed-material stacks (Al-composite) | Strong — fasteners do not care | Limited |
| Field assembly and repair | Strong | Machine-based |
Proof this substitution works
- The Eclipse 500 business jet was certified with friction stir welding replacing thousands of rivets in cabin and aft fuselage structures. The joints were produced far faster than riveting and the approach passed FAA certification (Assembly Magazine, Friction Stir Welding Replaces Rivets).
- Launch vehicles. NASA programmes friction stir welded aluminium-lithium tankage on the Shuttle Super Lightweight Tank. SpaceX then put an all friction stir welded aluminium-lithium tank into Falcon 9 production and has kept it there since the vehicle entered service in 2010. See the TIG comparison for the cited sources. StirLight has no commercial relationship with SpaceX; this is a public manufacturing case study, not a StirLight programme.
- Consumer hardware: Apple’s 2012 iMac enclosure replaced bonded and fastened joints with friction stir welding to achieve a thin, seamless aluminium body.
When fastening is still the right answer
- Dissimilar and mixed stacks — aluminium to composite, or joints that must be disassembled for service. Fasteners do not care what they clamp.
- Field assembly far from any machine.
- Certification regimes written around fastened joints, where the cost of re-substantiating a welded primary structure outweighs the weight saving — though the Eclipse precedent shows the path exists.
- Structures that cannot react welding forces. Friction stir welding needs rigid fixturing and backing.
What a conversion programme looks like
Moving a fastened assembly to friction stir welding is a staged engineering exercise: a design-for-process review of the joint lines, a feasibility phase producing welded samples and a documented parameter set on your geometry, then qualification testing (tensile, fatigue, corrosion as required) against your certification basis, and finally a costed route to production — in-house equipment or subcontract welding while capacity builds. Each stage gates the next, so commitment tracks evidence. See services.
Frequently asked questions
How does friction stir welding fatigue performance compare with riveted joints?
The weld is a continuous, fine-grained forged band with no holes. Fatigue initiation sites are removed rather than managed. Programme-specific fatigue substantiation is still developed per application.
What happens at the end of the weld?
The tool leaves an exit keyhole. It is placed in a run-off tab or machined region, used as a functional hole, or eliminated with retracting-pin tooling.
Can friction stir welding reach the rates aerospace programmes need?
Weld traverse in aluminium runs at hundreds of millimetres per minute and the process is machine-run, so rate scales with fixturing and machine time rather than certified-welder headcount.
Next step A structured feasibility trial answers the question on your geometry, with measurements rather than argument.