What Is Friction Stir Welding?- Process, And Application

What is Friction Stir Welding?

Friction stir welding (FSW) is a joining process that simply applies frictional heat generated by the rotating tool, used to join two facing workpieces, without melting the workpiece material. Friction between the rotating tool and workpiece material generates the heat, whereupon a softened region is created close to the FSW tool.

As the tool is traversed along the joint line, the two felt pieces of metal are mechanically intermixed and then forged the hot – soft metal, much like one would join clay or dough, by applying mechanical pressure to the joint by the tool.

FSW is primarily used on wrought or extruded aluminum, and in particular for structural applications where very high weld strength is needed. FSW is capable of being used to join aluminum alloys, copper alloys, titanium alloys, mild steel, stainless steel, and magnesium alloys.

Mostly recently it has been used in welding polymers. In addition, FSW has been recently used to join dissimilar metals, such as aluminum to magnesium alloys. Application of FSW can be found in modern shipbuilding, trains and aerospace.

Principle of operation

As the tool advances through the joint, it is evident that you now have and can see the weld zone and the tool shoulder affected space.

The FSW process is performed by means of rotating cylindrical tool that has a profiled pin, with a pin diameter smaller than the shoulder diameter.

The tool is fed into a butt joint between two workpieces that are held in place until the probe protrudes into the workpiece and the shoulder touches the surface of the workpieces.

The probe is slightly less than the depth of weld required, with the tool shoulder riding on top of the work surface. After a brief dwell time, the tool is moved in the forward direction in the joint line at the predetermined welding speed.

The sheer action of the wear resistant tool and workpieces creates frictional heat. The heat generated from mechanical mixing, along with the adiabatic heat in the material generates sufficient heat to cause degeneration to basin materials and softening of the stirred materials without melting.

Where the tool is moved in the forward direction the special profile on the probe allows for plasticized material to be transported from the leading face of the tool to the rear, where the high forces allow for a forge type consolidation of the weld.

This action of the tool traversing along the weld line in a plasticized tubular shaft of metal, promotes extreme solid state deformation accompanied by dynamic recrystallization of the base material.

The process of friction stir welding

Friction stir welding involves a specially designed tool that rotates at high speeds over the seams to be welded together.

During the tooling process, heat is generated as the tool rotates over the metal, causing the metals to become plastic and fuse into one another. Friction stir welding can weld two types of joints:

• Lap joints and
• Butt joints.

The tool used in friction stir welding has two parts: a cylindrical part termed a shoulder that rotates on the seam, and a profiled pin that extends from the shoulder.

The pic is first drilled into the seam and then the shoulder rotates on top of the workpiece for a period of time until optimum temperature is reached and absorbed into the materials.

Finally, the tool traverses across the seam to generate continuous weld due to the volumetric heating of the tool, and the metal particles mixing by the profiled pin. It is from the profiled pin where it gets the name ‘stir’, as the pin is literally stirring the softened particles to fuse.

Applications of friction stir welding

TWI initially patented the FSW process in most industrialized countries and extensively licensed it to over 183 users.

Friction stir welding and its variants, friction stir spot welding and friction stir processing, have been used for the following industrial applications: shipbuilding and offshore, aerospace, automotive, rolling stock for railways, general fabrication, robotics, and computers.

Friction stir welding is used in a large variety of industries that utilize aluminum.

Shipbuilding: FSW was first used to weld hollow aluminum panels on fishing boats. Today, FSW is commonplace in welding aluminum freezer panels in ships’ tubs and hulls. Because FSW causes very little distortion, the aluminum panels maintain the same shape after welding even after long seams are completed.

Aerospace: Aluminum fuel tanks in space vehicles that store cryogenic oxygen also employ FSW. The joining technique welds the curved domes to the cylindrical structure of the fuel tank. Boeing employed FSW in the Interstage Module of a Delta II rocket that launched successfully in August of 1999. Friction stir welding also joins various thin-walled lightweight aluminum frames seen on airplane fuselages, which provides a lighter option from traditional bolting or riveting.

Railroad: Friction stir welding is used on hollow profiles and T-stiffener extrusions for building high-speed trains.

Automotive Industry: The automotive industry has embraced aluminum as the best overall pedaling material for car chassis, becoming one of the major industries to adopt FSW technology. Traditional welding methods cannot produce high-tolerance parts in the same way FSW can. In addition, FSW is more attractive to aluminum due to the time effects of longer welds compared to traditional welding.

Friction stirs welding vs. friction welding – the difference

Many welding processes use friction to create heat, the most recognized of which is friction welding. In a standard friction welding process, heat is created between the two workpieces by moving one workpiece in relation to the other at the seams.

The friction between the two surfaces causes them to melt and be fused together.

This does limit friction welding in that the workpiece must allow for movement of the workpiece at high speed through linear reciprocating motion.

Friction stir welding eliminates this limitation in that both workpieces are fixed in place and only the tool is moved along the seam to create the weld.

Advantages of friction stir welding

Friction stir welding benefits from its unique weld since it does not use consumables, or shielding element techniques which provide the following benefits to the weld:

• The finished weld is seamless and pleasing to the eye.
• It can weld alloys, such as Aluminum 2xxx series and 7xxx series, that would be unweldable otherwise.
• Fully automated process.
• No flux or shielding agent needed.
• Low peak temperature reduces shrink-age and porosity of cracks.

Disadvantages of friction stir welding

• Complicated or special fixture arrangement needed
• Creates a hole visible too the welding plates
• High initial or setup cost.
• Less flexible than arc process
• FSW cannot make filler joint
• Cannot weld non-forgeable material.

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