Friction Stir Welding

 Friction Stir Welding (FSW) is a solid state welding process in which a rotating tool is fed along the joint line between two workpieces. It is used to join two facing surfaces. During welding, heat is generated due to friction and the metal is mechanically stirred to form the weld seam. FSW differs from normal friction welding in such a way by generating friction heat by a separate wear-resistant tool instead of the parts between them.

In friction stir welding process, the rotating tool consists of a cylindrical shoulder and a smaller probe or pin projecting beneath it as shown in Figure 2.33, is used. During welding, the shoulder rubs against the top surfaces of two parts thereby developing friction heat and the pin or probe simultaneously generates additional heat by mechanical mixing of the metal along the butt surfaces. At the same time, the probe has been designed in order to perform the mixing perfectly.

The heat is produced by the combination of friction and mixing. During the process, the metal will not melt but it softens. The softening of metal takes place up a highly plastic condition. When the tool moves forward along the joint, the leading surface of the rotating probe is forcing the metal around it. Then, the developed force forges the metal into a weld seam. So, the shoulder helps to limit the plasticized metal flowing around the probe.

Since workpiece does not melt, problems such as porosity, solidification cracks and thermal distortion are non-existent.

It is extremely important to identify appropriate combinations of tool geometry, tool rotational speed and welding speed to ensure a proper material flow. The harder is the workpiece material, the stronger has to be the tool material because the tool experiences severe atmosphere of stress and temperature. The commercial use of FSW for hard alloys still remains intangible.

1. Advantages, Limitations and Applications of Friction Stir Welding

Advantages:

1. It ensures the good mechanical properties of the weld joint.

2. It avoids toxic fumes, warping, shielding issues and other problems associated with arc welding.

3. It permits less distortion or shrinkage on joints.

4. It provides good weld appearance:

5. There is no use of consumables.

6. It can be easily automated on simple milling machines due to lower setup costs and less training.

7. It can be operated in all positions as there is no weld pool.

8. It can be used for thinner materials with same joint strength.

9. Simplicity of operation and simple equipment are obtained.

10. It requires less time to perform welding.

11. Heat affected zone is small as compared to conventional flash welding.

12. Dissimilar metals can be joined.

13. High quality weld can be obtained.

14. There is no use of shielding gases.

15. Only less surface impurities and oxide films are produced. Hence, there is no surface cleaning required.

Limitations:

1. An exit hole remains the same after the tool is withdrawn from the work.

2. Large down forces are required with heavy-duty clamping necessary to hold the plates together.

3. It is less flexible than manual and arc processes.

4. Process is restricted to flat and angular butt welds.

5. It can be used only for joining small parts.

6. It requires the heavy rigid machine due to high thrust pressure.

7. In case of tube welding process, it becomes complicated.

8. In case of high carbon steels, it is difficult to remove flash.

Applications: 

Friction stir welding processes are mainly used in the following industries:

1. FSW process is used in aerospace, automotive, railway and shipbuilding industries. The main applications are butt joints on large aluminium parts.

2. Automobile: Bimetallic engine valve, universal joint yoke, gear hub etc.

3. Aerospace: Turbine blade joining, seamless joining etc.

4. Industrial machines: Spindles, tapers and tools.

5. Medical: Stainless steel joining of containers.

6. Mining/Drilling: Twist drills.

7. It is used in rolling stock for railways, general fabrication, robotics and computers. 8. Sometimes, steel, copper and titanium as well as polymers and composites are also joined by using FSW.