How Does Friction Stir Welding Work?

Considered one of the most significant innovations in welding over the past decade, Friction Stir Welding (FSW) is a solid-state joining process that is environmentally-friendly, versatile, and efficient in terms of energy use.

Sow does friction stir welding work?

To put it simply, it uses the energy generated by friction to weld two materials together. No consumables are used in the process.

Friction stir welding was invented in 1991, by Wayne Thomas, and patents were filed in Europe, the USA, Japan, and Australia. The process was further studied in 1992 at The Welding Institute and industrial production using friction stir welding was in progress by the mid-1990s, making it one of the shortest time periods that any welding process took to go from invention to being so used so widely.

The Process of Friction Stir Welding

The concept is simplistic – friction stir welding involves rotation to create friction and then heat generated from that friction to weld.

A specially designed tool is required for friction stir welding. This tool rotates at high speeds over the seams that are to be welded together. The heat is generated as the tool rotates at high speeds over the metal.

Friction stir welding is capable of welding two types of joints:

• Lap joints
• Butt joints

The tool that is used for friction stir welding has two parts. There is a cylindrical part called the shoulder that is what acts upon the seam, and a pin that extends out of the shoulder.

First, the pin is inserted into the seam to hold it in place, and the shoulder rotates on top of the workpiece for enough time to create the optimum temperature and let it absorb into the material. Once this is done, the tool moves across the seam, creating a continuous weld. This is possible only due to the heating produced by the tool and the metal particles that are mixed by the profiled pin.

The softened particles here are being ‘stirred’ by the pin to fuse them, and this is where the process gets the name ‘friction stir welding.’

As the contact surface temperature rises, three types of regains are formed within the material. A weld nugget, thermo-mechanically affected zone, and heat-affected zone. A weld nugget is a pool of molten metal that cools and solidifies into a round joint.

The thermo-mechanically affected zone is an exclusive zone formed in friction welded joints as a transition zone between the heat-affected zone and the stir zone. The heat-affected zone is the area of the base material which has not melted but the microstructure and properties have been affected because of the heat-intensive cutting operations.

Friction Stir Welding vs. Friction Welding

There are a number of welding techniques that use friction to generate heat, and the most common one of these is friction welding. In this method, heat is typically generated by moving one workpiece in relation to another at the seams. This friction causes the surfaces to melt and fuse.

However, this puts a limitation on friction welding when it comes to set up because to move the workpiece at a high speed, there should be a linear reciprocating movement, which means that the pieces can only be placed a certain way.

Friction stir welding overcomes this limitation because the workpieces are fixed into place and the tool is moved along the seam to create the weld.

Applications of Friction Stir Welding

Friction stir welding is used primarily in industries to join aluminum alloys from all series, regardless of whether they are cast or rolled, or even extruded. The process is shown to be able to weld aluminum alloy butt joints with thicknesses between 0.3mm and 75mm in a single attempt, depending on factors such as the material being used, the machine power used and structural stiffness.

Other materials joined by using friction stir welding include metals such as magnesium, titanium, copper, and alloys of these metals as well as steel. But plastics and composite materials with metal constituents have also been tested out. Friction stir welding has shown to be able to join combinations of these materials that are not similar to each other.

The friction stir welding process can be applied in a number of ways across various industries including aerospace, shipbuilding, rail and electronic, including electric vehicle battery tray friction stir welding.

Advantages of Friction Stir Welding

There are a number of advantages friction stir welding holds over other types of conventional arc welding processes.

• Friction stir welding is a solid-state welding process, thus it is a largely defect-free joining method and does not have any problems with hot or solidification cracking, or porosity
• The lower temperature compared to other processes means that shrinking and distorting after the material is joined can be reduced
• No filler materials or consumables such as flux or shielding gas for aluminum alloys are required in friction stir welding.
• Friction stir welding is an environmentally friendly process that does not produce any fume, spatter or radiation. There are no solvents required for degreasing or any waste produced, and no surface cleaning needed.
• Less energy is needed than in laser welding – about 2.5%.
• Decreased fuel consumption is needed in automobiles, aircraft, and ships where friction stir welding is used.
• The process uses machine tool technology which makes automation easier and highly repeatable with a reduced need for welders of very high skill.
• Friction stir welding can work at any angle and does not have to be linear
• Mechanical and microstructural properties of friction stir welded aluminum alloys have been proven to be better than those with other welding processes.
• This process also has the capability to join many alloys of aluminum that are generally considered non-weldable, such as those from the 2xxx or 7xxx grades.
• The process usually does not require special edge preparation in most cases.

Disadvantages of Friction Stir Welding

Although there are many advantages to friction stir welding, there are also some limitations that come with the process.

• The exit hole is left behind from the pin after the tool is withdrawn from the materials that were to be welded.
• Significant levels of downforce are needed, as well as traversing forces, which means that clamping becomes more likely and will affect the material more.
• There is a lack of flexibility compared to manual and arc processes, for example, friction stir welding cannot be used where deposition of the metal is necessary.

Still, despite its disadvantages, friction stir welding is a genius concept for the versatility, sustainability, and functionality it offers. Joining metals of different compositions, and alloys, while maintaining excellent tensile strength is now possible because of friction stir welding. Because of the energy efficiency, compatibility for automation, and use by robots, this is a reliable welding process for many industries.

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