What is Friction Stir Welding?
Friction stir welding (FSW) is a solid-state joining process that uses frictional heat generated by a rotating tool to join two facing workpieces without melting the workpiece material. Heat is generated by friction between the rotating tool and the workpiece material, which leads to a softened region near the FSW tool.
While the tool is traversed along the joint line, it mechanically intermixes the two pieces of metal and forges the hot and softened metal by the mechanical pressure, which is applied by the tool, much like joining clay, or dough.
It is primarily used on wrought or extruded aluminum and particularly for structures that need very high weld strength. FSW is capable of joining aluminum alloys, copper alloys, titanium alloys, mild steel, stainless steel, and magnesium alloys.
More recently, it was successfully used in welding polymers. In addition, the joining of dissimilar metals, such as aluminum to magnesium alloys, has been recently achieved by FSW. The application of FSW can be found in modern shipbuilding, trains, and aerospace applications.
Principle of operation
The progress of the tool through the joint, also shows the weld zone and the region affected by the tool’s shoulder
The FSW is performed with a rotating cylindrical tool that has a profiled pin having a diameter smaller than the diameter of the shoulder. During welding, the tool is fed into a butt joint between two clamped workpieces until the probe pierces into the workpiece and the shoulder touches the surface of the workpieces.
The probe is slightly shorter than the weld depth required, with the tool shoulder riding atop the work surface. After a short dwell time, the tool is moved forward along the joint line at the pre-set welding speed.
Frictional heat is generated between the wear-resistant tool and the workpieces. This heat, along with that generated by the mechanical mixing process and the adiabatic heat within the material, causes the stirred materials to soften without melting.
As the tool is moved forward, a special profile on the probe forces plasticized material from the leading face to the rear, where the high forces assist in a forged consolidation of the weld.
This process of the tool traversing along the weld line in a plasticized tubular shaft of metal results in severe solid-state deformation involving dynamic recrystallization of the base material.
The process of friction stir welding
Friction stir welding uses a specially designed tool that rotates at high speeds over the seams that need to be welded together. As the tool rotates over the metal, heat is generated between them. This heat causes the metals to become plastic and fuse into one another. Friction stir welding is capable of welding two types of joints:
- Lap Joints
- Butt joints
The tool used for friction stir welding has two parts. A cylindrical part called a shoulder rotates on the seam, and a profiled pin extends from the shoulder.
The pin is first drilled into the seam. Then, the shoulder rotates on top of the workpiece for a certain amount of time until an optimum temperature is reached and absorbed into the materials.
Then, the tool moves across the seam, which creates a continuous weld. This is possible due to the volumetric heating produced by the tool and the mixing of the metal particles by the profiled pin. It is from the profiled pin that goes into the workspace where it gets the name “Stir” as the pin is literally stirring the softened particles to fuse them.
Applications of friction stir welding
The FSW process has initially been patented by TWI in most industrialized countries and licensed for over 183 users. Friction stir welding and its variants friction stir spot welding and friction stir processing are used for the following industrial applications: shipbuilding and offshore, aerospace, automotive, rolling stock for railways, general fabrication, robotics, and computers.
Friction stir welding finds application in a wide variety of industries that use aluminum.
Shipbuilding: FSW was first used to weld hollow aluminum panels for fishing boats. Today, this welding technique is common in welding aluminum freezer panels used in the body and hull of ships. Since FSW creates minimal distortion, aluminum panels will hold their shape even with lengthy welds.
Aerospace: Aluminum fuel tanks used in spacecraft to store cryogenic oxygen utilize FSW. The joining technique welds the domes to the cylindrical structure that makes up these fuel tanks. Boeing used FSW in the Interstage Module of a Delta II rocket which had a successful liftoff in August 1999.
FSW is also utilized to join lightweight aluminum frames seen in the aircraft fuselage. This is because the technique offers a much lighter alternative to bolting or riveting.
Railroad: Friction stir welding finds its use on hollow profiles and T-stiffener extrusions for manufacturing high-speed trains.
Automotive industry: The automotive industry has turned to aluminum as the optimal material for preparing car chassis. Hence, it is one of the major adopters of FSW technology. Conventional welding methods cannot reproduce high-tolerance parts like that of FSW. The quick weld times of FSW also make it more appealing than other forms of welding for aluminum.
Friction stirs welding vs. friction welding – the difference
Several welding techniques use friction to generate heat, and among them, the most common is friction welding. In a typical friction welding method, the heat is generated between the two workpieces by moving one workpiece in relation to another at the seams.
The friction between the two surfaces causes them to melt and fuse.
However, this puts a limitation on friction welding when it comes to the workpiece setup, as it must be able to move a workpiece at high speeds through linear reciprocating movement.
Friction stir welding overcomes this limitation by fixing the workpieces in place and then moving the tool along the seam, creating the weld in the process.
Advantages of friction stir welding
The advantages of friction stir welding arise from its unique weld where there is no use of consumables or shielding element techniques. This gives the following characteristics to the weld:
- The finished weld is seamless and aesthetical
- It can weld otherwise unwieldable alloys like the Aluminum 2xxx and 7xxx range
- Fully automated process
- No form of flux or shielding agent required
- Low peak temperatures prevent shrinkage and porosity of the cracks
Disadvantages of friction stir welding
- Complicated or special fixture arrangement required.
- It creates a visible hole in welding plates.
- High initial or setup cost.
- It is less flexible compared to the arc welding process.
- FSW cannot make filler joints.
- Non-forgeable material cannot be welded.