Friction Stir Welding (FSW) is comparatively an advanced process, where to get the best weld quality several parameters are very crucial to control. Knowledge of these parameters and settings are critical in controlling FSW and achieving the appropriate value of welds in terms of the required strength and performance. In this article the author briefly describes critical aspects of FSW and how specific configurations of the process can affect it.
Key parameters in FSW
There are several factors that control FSW process and all are very much essential to get a proper weld. Making the above parameters work can be a challenge especially for this reason; the more so because uniform quality is only possible if the factors of production are held constant.
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Tool geometry
A critical component to the FSW process is the design of the tool which consists of the shoulder and the pin. The shoulder has frictional heat whereas the pin causes intermixing of the material. The geometry of the tool that is the dimensions including diameter, length and geometry influences the heat distribution and the stirring action of the tool. This is because a good tool delivers an equal heat source across the required material and good mix of materials to obtain strong and unblemished weld. The selection of tools depends with the type of material and the joints that are in the course of being welded.
The rotation speed and such feed rate
Rotation per minute also referred as RPM and feeding rate in millimeter per minute are two controlling factors, which affect heat generation as well as material flow in FSW. The rotation speed has an influence on the frictional heat generation and therefore, the material’s flow and plasticity. The feed rate refers to the rate of advancement of the tool from the joint forward and in doing so, it affects how long the material, which has to be heated and stirred, takes to go through the process. Both of these parameters require fine tuning depending on the type of material, its thickness and the characteristics of the weld that is required.
Vertical force
The other important parameter is the vertical force which is used by the FSW tool. This force impact on the interaction of tool with the material and on weld quality. Less than adequate force may produce under stirring and poorly developed joints leaving the tools wanting while more than adequate force can lead to tool deterioration coupled with warping of the workpieces. Fluctuations of the vertical force should be maintained within the correct level to achieve the necessary homogeneity of the material and the strength of the weld.
Welding configurations
Within FSW, various welding arrangements can be employed; in fact there are various configurations of FSW that can be applied depending on the material type and their combinations. The configuration that is chosen will affect the appearance of the weld, its strength and even its performance.
Butt welding
One of the most typical connectors that can be distinguished are butt welding where two parts are placed side by side. This method offers an advantage in applications requiring a normal continuous contact joint. The true alignment of the edges results to a polished, high quality weld bead that is pleasing to the eyes and safe as well. This method is used in the situation where uniform material properties and low amounts of joint distortion is desirable.
Lap welding
This is another welding technique which entails the forming of one piece of metal over another. This configuration is appropriate to join materials of various thickness and kind and give a strong joint in the overlapping area. There are several reasons why lap welding can be useful; firstly if stress distribution plays an important factor then the presence of the lap helps to distribute the load. This type of configuration is widely employed in such applications as the adjoining of materials that do not have compatible coefficients of thermal expansion or in cases where it is impossible to access one face of the joint.
Keyhole welding
Keyhole welding is another process which is similar to FSW, it is used for joining thicker materials or where more penetration is desired. In this position the tool produces a narrow and elongated keyhole shaped weld pool to improve the penetration of thicker sections. Keyhole welding is especially valuable in applications requiring strength, and the process can handle a greater stock thickness than the standard configurations.
