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Did you know how friction stir welding creates strong joints without melting?
Friction stir welding (FSW) is a solid-state joining process that uses a non-consumable tool to join two facing workpieces together without melting the workpiece materials. During this process, the heat generated by friction softens the area close to the tool. After this, the tool is advanced along the joint line, mechanically mixing the metals of both and forging these heat-softened metals through the mechanical pressure applied by the tool, similar to mixing clay or dough together. This has led to the increasing use of friction stir welding for high-strength welded structures, particularly in wrought or extruded aluminum alloys.
How Friction Stir Welding Works
During the friction stir welding process, the tool used is a rotating cylindrical tool with a smaller diameter shaped pin attached to it. As welding proceeds, the tool is applied between two clamped workpieces until the needle penetrates into the workpieces and the shoulder of the tool contacts the surfaces of the workpieces. After a short dwell time, the tool moves forward along the joint line at a preset welding speed.
The generation of frictional heat makes the material soft without melting, thus achieving effective plastic deformation.
Microstructural characteristics
The solid-state nature of friction stir welding, combined with the special tool geometry and asymmetric speed profile, produces a unique microstructure. The microstructure can be divided into several regions:
- Stir Zone: This is the area of heavily deformed material, roughly coinciding with the needle position during welding.
- Flow Arm Zone: Located on the upper surface of the weld, consisting of material dragged by the shoulder.
- Thermo-Mechanically Affected Zone (TMAZ): This is the area on either side of the stir zone where the weld has less effect on the microstructure.
- Heat Affected Zone (HAZ): The area present in all welding processes that undergoes the thermal cycles of the weld without undergoing deformation.
The stir zone of friction stir welding has a unique "onion ring" structure, a phenomenon whose origin has not yet been fully determined.
Advantages and limitations of friction stir welding
Friction stir welding offers several advantages over conventional fusion welding methods. First, the defect concentration during welding is lower because problems caused by cooling from the liquid phase, such as porosity, solute redistribution, and solidification cracking, are avoided. Secondly, friction stir welding has the following advantages:
- The mechanical properties are good after welding.
- Improve safety by avoiding toxic fumes or splashing of molten material.
- Over 1 km of aluminum welding can be achieved with no consumables and only threaded pins made of standard tool steel.
- Operable in all positions without welding pool.
Although friction stir welding has many advantages as mentioned above, it still has unique defects such as "kissing joint" if it is not operated properly.
Importance of welding parameters
The effectiveness of friction stir welding depends on several welding parameters, including tool design, tool rotation and movement speed. The key to tool design is that the material must be strong enough and resistant to high temperatures to ensure the effectiveness of the welding process. Reasonable rotation and movement speed can achieve appropriate heat input. Too high or too low a speed will affect the welding quality and may even cause tool breakage or obvious defects.
Effective welding parameter selection can not only provide ideal welding results, but also improve production efficiency.
Wide Application of Friction Stir Welding
Friction stir welding has a wide range of applications, covering modern shipbuilding, railway transportation, aerospace and other fields. In recent years, this process has even been successfully applied to fields such as welding polymers and joining different metal materials. The economic benefits of this technology and its application potential in various materials will further drive its popularization and development in the industry.
As technology continues to advance, what role will friction stir welding play in the future of manufacturing?
