Friction Stir Welding: Principle, Benefits, and Applications

Friction Stir Welding or FSW is a contact welding process that uses the heat generated by friction to fuse two different materials. This joining technique does not use any consumables in the process.

One of the key advantages of using friction stir welding is the improved aesthetics that the finished parts have when compared with other welding methods.

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Friction stir welding was developed and patented by TWI (The Welding Institute) in 1991. Since then, countless industries use this welding technique due to its unique characteristics.

This comes as a result of how friction stir welding is applied over two workpieces.

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 weld together. As the tool rotates over the metal, heat generates 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 shoulder that rotates on the seam, and a profiled pin that 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 for fusing them.

TWI has detailed how the welding process takes place as the tool moves over two materials.

As the contact surfaces heat up, three types of regains form within the materials – A weld nugget, TMAZ or Thermo-Mechanically Affected Zone, and HAZ or Heat Affected Zone.

In friction stir welding, the weld nugget undergoes dynamic recrystallization. In simple terms, the microstructural grains of the metal grow at the same time when the metal undergoes deformation.

This leads to a much better bond, even more so than what is achievable with arc welding.

Friction stir 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 generates 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 unweldable 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
• Limitations of friction stir welding

Unlike other welding tools, the tool used in friction welding has to provide forging action to the workpieces besides heating them. So, the tool must withstand significant loads and should not produce excessive wear.

This is one of the reasons why frictions stir welding is not used commonly on hard steel or metals. Also, a hole will be left behind by the profiled pin when it leaves the workpiece.

Stir welding is mainly used to weld aluminum and it can weld nearly all types of aluminum, even the modern Al-Li alloys. The reason why it is restricted to aluminum is because aluminum welds can happen at relatively low peak temperatures.

Applications of friction stir welding

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 utilizes FSW. The joining technique welds the domes to the cylindrical structure that makes up these fuel tanks.

Boeing used FSW in Interstage Module of a Delta II rocket which had a successful liftoff on 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 in FSW technology.

Conventional welding methods cannot reproduce high tolerance parts like that of FSW. The quick weld times of FSW also makes it more appealing than other forms of welding for aluminum.

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FSW or Friction Stir Welding is one of the most unique types of welding methods available today. It produces very strong welds without compromising on weight or aesthetics.

Within the welding ecosystem, friction stir welding stands out as a unique candidate.