FAQs – Friction Stir Welding

1. A pin tool is mounted in a drive spindle. Before the two parts can be joined, they are clamped together to keep them stationary.
2. The pin tool begins to rotate and plunges into the two pieces of metal, maintaining a downward (Z-axis) force. This is referred to as the plunge or entry hole at the start of the weld path.
3. The rotating pin tool shoulder creates friction. The resulting friction then preheats the materials, creating a plasticized state at a temperature range below the melting point, as described above.
4. The pin forms a bond as it travels along the joint and consolidates the material along the weld profile/path while maintaining a downward force and Z-axis position.
5. At the end of the weld path, the tool is withdrawn in the Z axis. This extraction point is sometimes referred to as the “exit hole” because the pin probe will leave an impression in the material at the point of extraction.

2. What is Friction Stir Welding used for?

Because Friction Stir Welding creates extremely high-quality, high-strength joints with low distortion, the solid-state joining process is the preferred technology for welding aluminum sheets, extrusions, panels, and other products.  

At MTI, Friction Stir Welding is also commonly used to join dissimilar lightweight metals and hybrid electric vehicle applications.

3. What is a pin tool in Friction Stir Welding?

The pin tool is key to Friction Stir Welding.  In rotary and linear friction welding, one part is rotated or oscillated while the other part is held stationary. 

However, in Friction Stir Welding, both parts are held stationary, and a non-consumable spinning pin tool creates the frictional heat. This also makes Friction Stir Welding ideal for joining very large, long, or thin parts, such as sheet metal or an extrusion.  

The key features of pin tools are the shoulder and the cone-shaped pin.  This FSW head rotates and penetrates the material along the seam of the two parts while the shoulder rides along the surface of the parts and typically inputs most of the heat and force.

 It’s important to remember that the pin tool’s features and geometry will differ depending on your application, joint, and the materials you’re joining. 

 When designing the shoulder, MTI considers several factors, including the profile geometry and the diameter of the shoulder:

• The profile geometry of the shoulder can either be flat, convex, or concave. Some shoulder designs also incorporate a pattern of groves that are used to generate and channel the desired amount of heat to the joint.
• The diameter of the shoulder depends on your part material, type of joint, and depth of weld penetration required for the weld joint and part application scope/heat requirement.

 When designing the pin, MTI’s engineers consider material flow and material displacement.

• For proper material flow, the tapered probe pin can contain a series of flutes, faces, or a combination of the two.
• The pin depth or thickness and taper angle design contribute to properly displacing and consolidating the plasticized material(s) being joined.

4. What are the advantages of using Friction Stir Welding?

 There are several advantages to using friction stir welding, especially over fusion welding processes. Here’s just a few: 

• Virtually Defect-Free Bonding: Because Friction Stir Welding is a solid-state joining process, many of the limitations associated with conventional fusion welding techniques do not apply to the Friction Stir Welding process—including shrinkage, solidification, cracking, and porosity.
• Superior Mechanical Characteristics: Friction Stir Welding produces a weld with high strength, toughness, and a fine grain structure that resists fatigue stress. Due to the low heat and small heat-affected zone, the joined parts are minimally distorted, reducing the costs associated with preparing them for subsequent use. FSW is frequently used for aluminum welding where strong but lightweight material is needed.
• Machine-Controlled Process: Friction Stir Welding occurs via a machine-controlled process and program with fine-tuned technical positions and values that can be saved and repeated with the exact same results each time. Part programs and their associated welded part data can be viewed live, recorded, and stored for future use. The weld path profile and pin tool position can utilize proprietary closed-loop system software—such as MTI’s IntelliStir temperature control—which can monitor, adjust, and maintain critical weld features to ensure consistent mechanical properties and a solid, repeatable, successful weld. Examples of critical weld features include Z Axis force, position, or tool temperature.  The resulting part, production, and weld quality are, therefore, very operator-independent.
• Environmentally Friendly Process: Friction Stir Welding is a cleaner, greener process with low energy input and cost that requires no consumables, flux, filler material, or shielding gases to run. Friction Stir Welding also does not emit smoke, fumes, or gases that need to be exhausted from the process or require the operator to use a traditional bulky welding helmet and spark resistant clothing.
• Join Dissimilar Alloys: Friction Stir Welding may be used to weld dissimilar alloys – including combinations that aren’t compatible with conventional welding methods. That’s because fusion methods rely on melting to join the two materials, and differences in melting temperatures could make it impossible to join certain combinations with fusion welding.  Fusion processes also change the material properties of one or both materials due to melting.  On the other hand, the Friction Stir Welding process happens below the melting temperature and works only the parent material(s).  This means that in Friction Stir Welding, no additional filler materials, metals, or flux are used that can cause additional changes to the parent material properties.  The result is a stronger weld.

5. What are the Applications of FSW?