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Lesson 7.6JoinChapter 7 · Joining

Friction stir welding

Solid state: a spinning tool stirs softened edges together

Friction stir welding joins metal plates by pushing a spinning tool along the joint: friction softens the metal and the tool stirs both edges into one, without ever melting them. It is like kneading two lumps of clay together. Invented in 1991 at TWI in the UK, it now builds rocket fuel tanks, train bodies and electric-car battery trays.

Friction stir welding: a real photo
Photo: NASA, Public domain, via Wikimedia Commons
A spinning tool rubs the metal soft and stirs the two edges together without meltingSide view along the joint, cut through the middle of the tool1Plates clamped on a backing barWorkpiece5Solid seam, forged by the shoulderDownward forceRotating toolShoulder2Tool spins and plunges in3Friction heats and softens4Pin stirs the softened metal(hot, but never melted)TravelWorkpiece and solid seamTool and backing barHot, softened metal
Friction stir welding · rotating tool, stirred zone and seam, side viewCircled numbers match the steps below; the tool moves to the right, leaving the seam behind it.

How it works

  1. Clamp firmly. The plates are butted together on a backing bar and clamped hard, because the tool pushes them apart with large forces.

  2. Spin and plunge. A hard tool, typically turning a few hundred to about 2,000 rpm, pushes its pin into the joint until the wider shoulder touches the surface.

  3. Heat by friction. The shoulder rubbing on the metal heats it to roughly 70–90% of its melting temperature. The metal goes soft like modeling clay but stays solid.

  4. Travel and stir. The tool moves along the joint. The threaded pin sweeps softened metal from the front around to the back, mixing the two edges together.

  5. Forge and finish. The shoulder presses the stirred metal down as it cools into a solid seam with fine grains. At the end the tool lifts out, leaving a small exit hole, often on a scrap tab.

Why no melting helps. Melting and refreezing cause most weld defects: gas bubbles, cracks and weak cast grains. Friction stir welds avoid them, so aluminum alloys that are hard to fusion weld join well, with little distortion and no fumes, sparks or filler.

Key facts

Materials
Mainly aluminum alloys; also magnesium, copper and dissimilar pairs; steel and titanium with costly special tools
Typical parts
Rocket fuel tanks, rail car bodies, ship deck panels, battery trays, heat sinks
Part size
Sheets of about 1 mm up to aluminum plates several centimeters thick; seams many meters long
Precision
Low distortion; seam position within a fraction of a millimeter on CNC machines
Surface finish
Smooth seam with fine half-moon ripples, slightly below the surface; small flash (burrs) at the edges
Batch size it suits
Medium to large series of long, straight or gently curved seams
Tooling and machine cost
Tens of thousands for a CNC conversion to well over $1 million for large dedicated machines
Speed
Typically 0.1–2 m of seam per minute, in a single pass

Strengths and limits

Strengths

  • No melting, so no porosity or hot cracks
  • Joins aluminum alloys that are hard to fusion weld
  • Low distortion, and strong, fine-grained welds
  • No filler, no gas, no fumes or sparks

Limits

  • Needs heavy, rigid machines and strong clamping
  • Leaves an exit hole at the end of each seam
  • Best on simple straight or flat joints
  • Steel and titanium wear tools quickly

Where it's used

  • SpaceFuel and oxidizer tanks of launch vehicles
  • RailAluminum car bodies of high-speed and commuter trains
  • ShipbuildingLarge aluminum deck and hull panels
  • AutomotiveBattery trays for electric cars, wheels, suspension parts
  • Electronics and heatingHeat sinks, cold plates, boiler heat exchangers

How to recognize it

  • A smooth, flat seam with fine, evenly spaced half-moon marks all pointing the same way.
  • A slight dip where the shoulder pressed, with thin curled burrs (flash) along the edges.
  • A round exit hole at the end of the seam, unless it was cut off.

Compare with

Arc welding7.1melts the metal and adds filler; easier for complex shapes but causes more distortion and defects in aluminum.Ultrasonic welding7.7also solid-state, but uses vibration for thin foils and wires, not thick plates.Explosion welding7.8another solid-state method, bonding whole plates at once with a blast.

Watch it

TheGrenzebachGroup · The principle, explained by a machine maker

RIFTEC GmbH - Videos · Plunge, travel and the solid-state joint, step by step

Manufacturing Technology, Inc. (MTI) · A real weld from plunge to exit hole

ESAB · An industrial machine at work

See real photos

Remember

  • Friction, not a flame or an arc, heats the metal, and it never melts.
  • The pin stirs the two softened edges into one, and the shoulder forges them down.
  • It shines on long aluminum seams: strong, clean and with little distortion.

More in Joining

All 100 processes
7.1ShortArc welding (MIG/MAG, TIG, stick)Fusion: an electric arc melts the edges and usually a fillerOpen the lesson7.2Resistance spot weldingFusion: a pulse of current melts a nugget between clamped sheetsOpen the lesson7.3Brazing and solderingFiller only: a lower-melting filler flows into a tight gapOpen the lesson7.4Laser weldingFusion: a focused laser drills a keyhole for narrow, deep weldsOpen the lesson7.5Electron beam weldingFusion: an electron beam in a vacuum welds very thick metalOpen the lesson7.7Ultrasonic weldingSolid state: fast vibration bonds small joints in under a secondOpen the lesson7.8Explosion welding (cladding)Solid state: a blast slams one plate onto anotherOpen the lesson

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