FAQs – Friction Stir Welding

Friction Stir Welding (FSW) is a solid-state joining process that’s especially popular in the aerospace, transportation, and electronics industries.

Below, we’ll explore some commonly asked questions about Friction Stir Welding.

1. What is Friction Stir Welding (FSW)?  

The Friction Stir Welding process uses a non-consumable pin tool to create frictional heat between two materials.  As the pin tool spins, it “stirs” the two materials together, creating plastic deformation between 70 to 90 percent of the solidus temperature. Solidus is the highest temperature at which an alloy is completely solid. And, plastic deformation changes the shape of the solid body without weakening the material.

Because Friction Stir Welding is a solid-state joining process, the two materials being friction welded never melt and joining occurs below the solidus of the equilibrium phase for the materials. As a result, the metals better retain their original mechanical properties.   

Here’s a closer look at how it’s done: 

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 material melting point, as described above.
4. The pin 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 At the end of the weld path, the tool is withdrawn in the Z axis.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 primarily used for joining aluminum sheets, extrusions, and panels.  

At MTI, Friction Stir Welding is also commonly used for joining dissimilar lightweight metals and for 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. 

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

The key features of the pin tool are the shoulder and the cone-shaped pin.  The pin 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 keep in mind that the features and geometry of the pin tool will differ based 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 take into account 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 weld strength and toughness, plus a fine grain structure that resists fatigue stress. Due to the low heat and small heat-affected zone, there is minimal distortion of the joined parts, reducing the costs associated with preparing the part for subsequent use.
• Machine Controlled Process: Friction Stir Welding occurs via a machine-controlled process and program that can be precisely adjusted, fine-tuned, 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 is therefore very operator independent. 

• Environmentally Friendly Process: Friction Stir Welding is a cleaner, greener process that features low energy input and 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 of the materials due to melting.  The Friction Stir Welding process, on the other hand, 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 does Friction Stir Welding Look Like?

 For more Friction Stir Welding videos, check out MTI’s video center.  

About MTI

MTI is an expert in Friction Stir Welding for mass production and industrial applications.  As the only company in the world specializing in all three friction welding technologies—linear, rotary, and friction stir—our on-staff metallurgists, design engineers, and applications engineers  can find the right technology to solve your joining challenges.

To learn if Friction Stir Welding is right for your project, contact us today.