Friction stir welding is a high-performance method of joining, suitable for the most critical applications. It is finding increased use within aerospace structures as a lower cost and higher-strength alternative to the use of solid rivets.
Welding is normally thought of as a process in which material is melted, causing it to fuse together as it solidifies. Although welding is an extremely useful way of creating strong and permanent joints, the melting and resolidification of material can cause a few issues. While in the liquid phase, porosity can develop and alloying components can vaporize. As the material solidifies, cracking can occur and inferior grain structures can develop. These issues can reduce the strength of the weld affected area, particularly its fatigue and damage tolerance. This means that conventional welding is rarely considered reliable enough for certain critical applications, such as joining aerospace structures.
However, welding does not have to involve fully melting material. Friction stir welding (FSW) uses friction to heat material to just below its melting temperature while applying pressure that causes solid state fusion. This avoids the above issues, resulting in grain structures similar to forged components with very little risk of porosity or cracking.
The Friction Stir Welding Process
FSW uses a rotating tool that generates friction when it is brought into contact with the workpiece. This heats a region of material, causing it to soften. The tool is then plunged into the joint and moved along the join line. Material flows plastically across the joint, without fully melting, in a similar way to the flow of solid material in a forging process. FSW is primarily used to join high-strength alloys within aerospace structures, although it can also be used to join polymers. It was originally patented by The Welding Institute (TWI) in the United Kingdom. The original patents have expired, putting this technology into the public domain.
FSW uses a tool with a shoulder and probe. After an initial dwell period to soften the material, the probe is pushed into the joint until the shoulder rests on the surface. As the tool continues to rotate, friction at the shoulder softens material in the region around the joint while the probe stirs the material at the joint interface, causing it to mix. Since the material has not been melted, this mixing may be considered as a forging process.
Standard machine tools can be programmed to perform FSW. They only need to be fitted with a relatively simple tool with a shoulder and an extended probe. For welding aluminum, the tool can be produced from ordinary hardened tool steel. Machine tool manufacturers are, therefore, also leading in the production of specialized machines intended for FSW, although these often retain a multi-functional capability as a machine tool. For example, Mazak produces VTC-300C FSW machines that are essentially machine tools fully tested for FSW.
For welding within large structures and assemblies, articulated robots are preferred. Airbus has supported the development of the DeltaN FS tool, which allows a spindle and FSW tool to be mounted on a robot, such as the Kuka KR500-3MT heavy-duty robot.
Advantages of Friction Stir Welding
Friction stir welding has many advantages. It is lighter than conventional welds, with no filler materials added and no raised weld bead. This flat weld can also be more aesthetically desirable than a raised weld and more easily hidden under paint. Another advantage of FSW is that it is considerably faster than drilling and riveting. However, it may be slower than conventional welding.
FSW is also stronger than both conventional welds and riveted joints. Because material is not melted, alloying components are not vaporized or displaced while the solid-state mixing creates a wrought grain structure. This means that the weld affected zone maintains the bulk material properties of the alloy. Fatigue resistance and damage tolerance are further aided by the fact that there is very little risk of defects forming.
Despite these structural advantages, a number of defects may still occur in FSW. If the critical parameters of rotational speed and traverse speed are set incorrectly, there may be insufficient heat to enable forging a material. In some cases, this can result in a visible groove where the weld should be. However, it can also result in a void below the surface or a light unfused contact between the parts. These defects can be more difficult to detect. Increasing rotational speed or reducing traverse speed will increase heat in the weld. The temperature must be high enough to enable plastic flow without excessive tool reaction forces while not fully melting the material.
From a health and safety perspective, because material is not fully melted, there are no fumes or spatter. There are also no arcs, which often cause an eye hazard within the vicinity of welding operations.
Applications of Friction Stir Welding
Several manufacturers have significant experience with friction stir welding. Boeing has been using FSW extensively in spacecraft structures for many years, and Airbus used it to attach stringers to fuselage panels for the A380. FSW was seen as a key technology for the production of low-cost very light jets (VLJ), a new class of small business jet that Eclipse Aviation launched in 2006. Although the company failed, it was at least partially successful in demonstrating that the VLJ was a viable class of aircraft. It also demonstrated the way FSW can significantly reduce the cost of producing aircraft structures when fully adopted throughout assemblies.
Within ship building, FSW has been used extensively since the 1990s. It has been used to join the hull and deck panels of both commercial and naval ships, as well as other components within the superstructure. Another growing application for FSW is within the automotive industry. This is largely driven by increased interest in the use of aluminum alloys within car bodies. It has also been widely used within railway rolling stock.
If you are interested in using friction stir welding, perhaps the best place to start would be a training course from The Welding Institute (TWI). TWI invented FSW back in the early 1990s and provides training worldwide in its application.