Friction stir welding

In a typical linear weld, the workpieces are held together while a tool with a shoulder and pin rotates and is plunged into the joint. As the tool travels along the seam, friction and deformation heat the adjacent metal, softening it while it remains solid. The rotating pin moves and mixes the plasticised material across the joint; pressure from the tool shoulder forges material behind the pin, where it cools into a continuous weld. The tool itself is not consumed as filler.

Tool geometry, rotation and travel speeds, applied forces, workpiece material and thickness influence heat generation, material flow and the resulting joint. FSW equipment must provide sufficient force and accurate control of the tool path. The process can make butt and other joint configurations, although the required tooling and constraints depend on the component.

The Welding Institute (TWI) developed friction stir welding in 1991. The technique was first adopted particularly for aluminium alloys, including grades that can be difficult to join by conventional fusion welding. It has also been applied to magnesium, copper, titanium and steel alloys, as well as selected dissimilar-material combinations; suitability depends on the materials, tool and process conditions.

Reported industrial uses span ship and marine structures, railway panels, automotive components, electronics enclosures and aerospace structures. In aerospace manufacturing, the process has been used to join large aluminium structures, including rocket propellant tanks. Researchers and manufacturers have developed variants—such as spot-welding, stationary-shoulder and bobbin-tool approaches—to accommodate different joint shapes and access requirements.

Because the joined metal does not undergo bulk melting and solidification, FSW avoids some defects associated with fusion welding, including certain forms of porosity and solidification cracking. Its lower peak temperatures can also reduce distortion and shrinkage compared with some fusion processes. The weld does not ordinarily require filler metal, flux or shielding gas, and the process can be automated for repeatable production.

A friction stir weld is not microstructurally uniform. It commonly contains a central weld nugget, a thermo-mechanically affected zone and a heat-affected zone. In aluminium alloys, dynamic recrystallisation may occur in the nugget. The properties and dimensions of these zones depend on alloy, tool design and process parameters; claims about weld strength must therefore be judged for the specific material and application.

FSW needs a machine, fixture and tool capable of handling substantial process forces while maintaining the designed joint path. Component shape, backing support, access to the seam, start and end conditions, production throughput and inspection requirements can affect whether it is practical. Tool materials can also limit use with metals that require higher processing temperatures.

The capital cost of dedicated equipment may be greater than for some conventional welding setups, so the full comparison includes not just the machine but also consumables, production rate, defect risk and any post-weld processing. For these reasons, FSW is an engineering option rather than a universal replacement for fusion welding.