Frictional energy is generated when the stirring tool comes in contact with the workpiece. This results in the generation of heat that is used to create a weld between two workpieces, hence the process is known as friction stir welding. “Friction” is the mode of energy and “stir” resembles the rotation of the tool. Friction stir welding is a solid-state welding process, This means no melting of the workpiece takes place during the process.
Friction stir welding is an eco-friendly process of welding metals together using a non-consumable tool. No filler metals are used in the process of friction stir welding. Aluminum materials used in industries like automobiles and aviation are welded using friction stir welding. Let us understand the process in more detail.
What is Friction Stir Welding?
Friction stir welding (FSW) is a solid-state joining process that uses frictional heat and pressure to weld two metals together. This technique has been increasingly adopted in the aerospace, automotive, and railway industries due to its many advantages over traditional welding methods. It is a highly efficient and environmentally friendly process that does not require any filler materials or shielding gases, making it a cost-effective and low-risk option for many applications.
In this technique, a rotating tool is inserted between two metal plates, and the friction generated by the tool’s rotation causes the metal to soften and become plastic. The tool then traverses along the joint line, forcing the plasticized material to mix and bond together. The pressure applied by the tool ensures a strong and durable joint, with no defects or distortions. It is suitable for a wide range of metals, including aluminum, copper, steel, and titanium, and can be used to weld metals of different thicknesses and compositions.
This technique has several benefits over conventional welding techniques, including reduced distortion, improved mechanical properties, and increased productivity. It produces a high-quality weld with no porosity or solidification defects, resulting in a higher strength-to-weight ratio and improved fatigue resistance. Additionally, it requires less post-weld processing, such as grinding or polishing, which saves time and reduces costs. Overall, Friction stir welding is an innovative and practical joining technique that offers significant advantages over traditional welding methods.
History of Friction stir welding
The Welding Institute (TWI) was the first organization to file a patent for friction stir welding in the year 1991. The places where TWI patented FSW were the US, Japan, Australia, and Europe. In 1992 TWI launched a project with the name “Development of new friction stir welding technique for welding aluminum”. The project was divided into three phases each had different objectives to be fulfilled.
- Phase1 : This phase focused on proving the technique realistic. It followed welding of 6000 series aluminum alloys.
- Phase2 : Focused on practical use of FSW on ships, aircraft, and automobiles.
- Phase3 : The last phase focused on developing data for further industrialization of the process.
Elements and construction of Friction stir welding
The Construction of friction stir welding is very simple and effective. Motor and guide mechanisms are connected to a power supply. The workpiece is kept on the worktable and the worktable is attached to the base. Workpieces are clamped using fixtures to restrict their degrees of freedom. The position of the tool is adjusted using the guide mechanism.
Following are the important elements used in the basic friction stir welding setup:
- Tool Pin
- Guide Mechanism
- Table and Base
The tool pin is the most critical component of the FSW process as it is responsible for generating the heat required to plasticize the metal. The pin is made of a hard material such as tungsten carbide and is designed to rotate at high speeds, typically between 1000 and 2000 revolutions per minute. As the pin rotates, it generates frictional heat that softens the metal and allows it to be joined together.
The tool pins used in friction stir welding can be of different types. Some of the commonly used pins are:
- Straight cylindrical
- Tapered cylindrical
- Threaded cylindrical
- Square cylindrical
Straight Cylindrical Tools
Straight cylindrical tools are the most common FSW tool design. They are typically used for welding materials with uniform thicknesses. The tool has a cylindrical shape with a flat shoulder that contacts the workpiece, and a profiled pin that rotates and plunges into the material. The tool’s shoulder provides the necessary downward force needed to plasticize the material and create a weld. The profiled pin generates the necessary heat and mixes the material, creating a homogenous weld.
Advantages of Straight cylindrical tools
- Simplicity: They are simple and easy to manufacture, as they do not require complex machining or tooling.
- Versatility: They can be used in a wide range of applications, from pipes and tubes to shafts and rods.
Disadvantages of Straight cylindrical tools
- Limited functionality: They lack the features and contours of more complex shapes, such as tapered or threaded cylinders making them limited in terms of applications.
- Weakness under torsion: They are weaker under torsion, which can limit their use for high torque applications.
Tapered Cylindrical Tools
Tapered cylindrical tools are similar to straight cylindrical tools, but they have a tapered profile instead of a flat shoulder. The taper helps to reduce the amount of plasticized material and heat generated during the welding process. This is particularly useful when welding materials with varying thicknesses. The taper also helps to reduce the amount of force required to forge the material together, making it easier to use the tool.
Advantages of Tapered cylindrical tools
- Strength and stability: Since the taper helps them distribute stress and load evenly they are much more strong and stable.
- Increased functionality: The tapered shape can be used to create a variety of features such as seals or mating surfaces.
Disadvantages of Tapered cylindrical tools
- More complex to manufacture: They require more complex machining which can increase their manufacturing cost.
- Limited versatility: They are best suited for specific applications where their unique features, such as a seal or mating surface, are required.
Threaded Cylindrical Tools
Threaded cylindrical tools are designed with threads on the pin, which help to mix the material and generate heat. The threads also help to reduce the amount of force required to forge the material together. Threaded cylindrical tools are particularly useful when welding materials with high melting points, such as titanium and aluminum alloys. The threads help to mix the material and generate heat without causing excessive wear on the tool.
Advantages of Threaded cylindrical tools
- Secure fastening: They can be used to create secure, threaded connections that are resistant to loosening or vibration.
- Easy to disassemble: They can be easily disassembled for maintenance or repair.
Disadvantages of Threaded cylindrical tools
- Complex manufacturing: They require complex machining to create the threads increasing their manufacturing cost.
- Limited functionality: They may not be as versatile as other cylindrical shapes, as they are primarily used for threaded connections.
Square Cylindrical Tools
Square cylindrical tools are used for welding materials with square or rectangular cross-sections. The tool has a square or rectangular shape with a flat shoulder that contacts the workpiece, and a profiled pin that rotates and plunges into the material. The tool’s shoulder provides the necessary downward force needed to plasticize the material and create a weld. The profiled pin generates the necessary heat and mixes the material, creating a homogenous weld.
Advantages of Square cylindrical tools
- Increased strength: They are stronger than round cylinders of the same size, as they have more material at the corners.
- Easy to stack or pack: They are easier to stack or pack than round cylinders, as they have flat sides that can be aligned more easily.
Disadvantages of Square cylindrical tools
- More complex to manufacture: Square cylindrical shapes require more complex machining or tooling than round cylinders.
- Limited functionality: They may not be as versatile as round cylinders, as they are limited to applications.
Fixtures are another essential element of Friction stir welding, as they hold the workpieces in place during the welding process. Fixtures are typically made of a heat-resistant material such as steel or aluminum and are designed to withstand the high temperatures generated during the welding process. They are also designed to provide support and stability to the workpieces, ensuring that they remain in the correct position during the welding process.
Motors are used to rotate the tool pin and provide the necessary torque to generate the heat required for the welding process. The motors used in Friction stir welding are typically high-powered and designed to operate at high speeds without overheating or burning out.
Table and base
The table and base is used to support the workpieces during the welding process. It is typically made of a heat-resistant material and is designed to provide a stable and level surface for the workpieces to rest on during the welding process.
The guide mechanism is used to control the movement of the tool pin during the welding process. The guide mechanism ensures that the tool pin remains in the correct position and moves along the correct path during the welding process, ensuring that the weld is of high quality and free from defects.
Few important parameters should be taken under consideration before moving on to the working of friction stir welding. Following is the list of parameters:
- Tool material
- Tool speed
- Tool tilt
Tool material is an important parameter while working on a friction stir welding machine. The tool must be strong enough to withstand high temperatures and pressure. Damage to the tool should be nil or minimal. It should retain its properties at high temperatures. Generally, tools are made of high carbon steel to fulfill the above requirements.
There are 2 types of tool speeds involved in friction stir welding:
- Rotational speed of the tool expressed in RPM
- Linear speed of the tool expressed in mm/min.
RPM of the tool must vary from 200 to 2000. Lower and upper limits for linear speed are 10mm/min and 500mm/min respectively.
To produce a good quality weld the tool must be tilted to an angle of 2 to 5 degrees. This prevents the joints from being affected due to the force applied in the downward direction.
Working Principle of Friction stir welding
The weld formed in the case of friction stir welding is due to the intermolecular bonding of metals at the time of heat diffusion. This is the basic working principle of friction stir welding. The non-consumable tool creates shear stress and friction on the surface of the workpiece. Due to this heat generation takes place at the interface of the tool and the workpiece. The heat generated is sufficient enough to create an intermolecular bond between the two workpieces without Melting the respective metal.
Given below is the step-wise working of friction stir welding:
- The workpiece to be welded is clamped using fixtures so that the degrees of freedom of the workpiece should be restricted. The operator must manually check if the workpiece is clamped tightly.
- The workpiece should be kept in an abutting position (there must be a small gap between them).
- The tool must be kept in the correct position, that is the pin must be inserted in the gap between the workpieces and the shoulder must touch the joint.
- After the tool is placed in the correct position, the rotation of the tool begins.
- Friction is created due to which heat is generated. Because of heat and force applied in a downward direction weld is created between the two workpieces.
- The weld is moved forward with the linear motion of the tool w.r.t to the workpiece.
- When the required area is welded the tool is moved upwards and the welding stops.
- The workpiece is then removed by loosening the fixtures.
Materials that can be manufactured using friction stir welding are aluminum and its alloys, copper and its alloys, titanium, nickels, and plastics.
Advantages of Friction stir welding
- High accuracy: Accuracy is important while working on different industrial products. Dimensional stability in the case of friction stir welding is high and weld formation is extremely accurate.
- No cracking: Cracks are developed on the workpiece due to traditional welding methods. But in the case of friction stir welding, there is no generation of cracks because no Melting of the workpiece takes place and the weld is created due to plastic Melting of the workpiece.
- No loss of metal: There is a loss of metal in different welding processes. But in friction stir welding there is no generation of Arc hence the loss of metal is negligible. Hence no parent metal is lost during friction stir welding.
- Clean process: There are no waste materials formed in friction stir welding. The process is clean and hence it is said to be an eco-friendly process of welding metals.
- Non-consumable tool: The use of a non-consumable tool also gives environmental benefits to the process of friction stir welding. As there are no harmful fumes produced during the welding operation.
- Surface finish: Excellent surface finish is obtained by friction stir welding. This results in a smooth welded surface and eliminates the requirement of grinding.
- Automation: The process of friction stir welding has an advantage over traditional welding methods as it can be fully automated. The operator just needs to clamp the workpiece, further the process can be automated.
- Safe process: Once the workpiece is clamped there is no need to touch the workpiece with hands. This ensures the safety of the operator performing the welding operation.
Disadvantages of Friction stir welding
- Clamping: Clamping of the workpiece in case of friction stir welding is a crucial operation. As the forces applied during the welding may dislocate the workpiece from its desired position. For this heavy fixtures are used to hold the workpiece. Clamping should be rechecked before starting the process.
- Slow process: One of the drawbacks of friction stir welding is that the process is sometimes slower than other welding processes. This results in a decreased production rate of the plant.
- High tool cost: The non-consumable tool used in FSW is made of high carbon steel and hence is expensive. The tool is specially designed for a specific purpose which makes it costlier.
- Keyhole is left: When the process of welding is stopped the tool leaves an unfilled keyhole. This is not desired in some of the products, hence is considered as a drawback.
- Large downward force: A large downward force is required at the initial stage to insert the pin in the abutting workpiece.
Applications of Friction stir welding
The following are the applications of friction stir welding:
- Many shipbuilding companies have adopted the friction stir welding process for joining bigger parts of ships.
- FSW is used in aviation industries to weld huge parts like the wings of an aircraft.
- Friction stir welding is also used in the railway industry for the welding of large aluminum panels.
- In automotive industries for welding chassis made of aluminum friction stir welding is used over other welding processes due to its high tolerance.
- Fabrication of metals and their alloys can also be done using friction stir welding.
Video of Friction Stir Welding