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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.

You can find more information on our web, so please take a look.

Our detailed guide to FSW provides a deeper understanding of its unique capabilities and diverse applications. 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 70-90 percent of the solidus temperature. Solidus is the highest temperature at which an alloy is completely solid. 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 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 At the end of the weld path, the tool is withdrawn in the Z axis.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?

Friction stir welding is especially beneficial for industries where efficient high-strength welding is key. Here are some of the examples:

• Aerospace Industry: FSW is used to create stiffened skins and panels for high-speed engineering solutions.
• Electronics and Machines: This procedure plays a significant role in small parts fabrication. It seals cold boxes, hard drive cases, and welds panels in heat exchangers and electronic enclosures.
• Automotive Industry: FSW’s high-strength welding of lightweight aluminum and aluminum alloys is vital for producing safer, lighter, and more affordable vehicles.
• Nuclear Storage: FSW contributed to the development of long-term storage units for nuclear waste management. 
• Electrical Vehicle (EV): EVs benefit from FSW’s ability to create a watertight seal to protect battery enclosures and electrical components from moisture and environmental contaminants.
• Rail Industry: Rail cars are made from welding aluminum extrusions with FSW for superior joint strength and durability.

6. What are the Mechanical Properties and Microstructure of Friction Stir Welded Materials?

When friction stir welding is used to bond materials, it will have a nugget zone, a thermo-mechanically affected zone (TMAZ), and a heat-affected zone (HAZ). While the nugget zone is the welded area where the two metals joined, the TMAZ does not experience dynamic recrystallization. However, the extent of the microstructural composition of these zones will depend on the material and processing.

7. When Was FSW Invented?

The Welding Institute invested in friction stir welding in . Through years of development, FSW technology has been used to prepare light metal materials into components with a high strength-to-weight ratio in products, construction, and vehicles.

8. What Does Friction Stir Welding Look Like?

To answer this question, view our video on friction stir welding to learn how friction stir welding bonds materials together.

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.

At MTI Welding, we are committed to providing robust support to our customers throughout the entire process.. While we hope our FAQs have enlightened you. You may have more questions. To learn if Friction Stir Welding is right for your project, contact us today.

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How to choose my FSW tool for my application?

Since the emergence of friction stir welding, many innovations have been made to improve the quality of FSW tools. These improvements result in better quality, more stable welds, and a certified lifetime. Today, many people are asking how to choose their FSW tool and how this choice will influence the quality of their weld. We propose to answer this legitimate question throughout this article. To start with, it is important to detail the composition of an FSW tool.

World Wide Welding Product Page

The different components of a FSW tool: what is a shoulder and what is a pin?

A FSW tool consists of three parts: the body, the shoulder and the pin.

The shoulder is the part that heats the material by friction in order to soften the workpieces and thus enable the welding operation. This is its primary function. The larger the diameter of the shoulder, the larger the contact area, which increases the frictional capacity of the tool. However, too large a shoulder will cause the workpiece to heat up too much and reduce the strength of the weld. The size of the shoulder will therefore depend on the thickness to be welded.

In addition, the shoulder with its counter-cyclical spiral shape allows the material flow to be pushed back towards the centre to prevent it from leaving the welding area. This second feature will improve the surface finish of the weld and the quality of the weld. With this feature, there is no need to tilt the tool during the welding operation. This is because the spirals pull the material towards the axis, which prevents the material from slipping (see infographic below: “No more tilt angle required on the FSW tool”).

Tool attachment – Whistle notch systemShoulderPin

In addition, the spiral shape improves tolerance to weld seam offset, reduces burrs and makes variable thickness welds possible.

Second part: the pin – the extremity of the tool.

The pin is the part of the tool that will penetrate the material by mixing it through a shearing effect: this is the stirring. Indeed, due to its conical shape and its grooves, the pin heats the material and softens it in order to ensure the mixing of the materials and the quality of the weld. The flat tip of the pin allows better penetration of the tool and therefore reduces the risk of lack of penetration. The pin and its flutes allow stirring in the direction of tool rotation as well as vertical mixing of the material.

Heat is generated by the friction between the workpiece and the tool, which produces plastic deformation of the workpiece and welds the two parts together.

How does the quality of my FSW tool contribute to the quality of my weld?

The quality of the weld depends directly on the quality of the tool and the choice of the tool. We will see later how to choose the right tool for your specific application.
But first, let’s see how the quality of the tool affects the quality of your FSW weld. To understand this, it is important to keep in mind how a FSW welding operation works.

Friction Stir Welding in 4 steps

1

Clamping of the parts to be welded: side by side in a butt welding configuration, one on top of the other in a lap welding configuration. The tool does not depend on the welding configuration. The same tool is used for butt and lap welding.

2

The tool penetrates the material: the shoulder heats the material by friction and the pin seals the parts together.

3

Welding operation: the tool advances along the weld seam and mixes the parts together.

4

End of welding: the tool moves up vertically, leaving a hole.

As you can see, the tool geometry has an impact on the weld quality. The tool will directly contribute to the mechanical strength of the weld. How? By the strong mixing of the material by the pin and by the heat generated by the shoulder. These two phenomena will cause a recrystallisation of the aluminium which will result in the precipitation of fine grains and thus contribute to the very good mechanical strength of FSW welds.

However, at the end of the weld, the tool will leave an exit hole as you can see in step 4 of the infographic above. This is not a quality issue, but for some applications it is better to remove it. The solution: the retractable pin tool. Where a conventional FSW tool leaves an exit hole at the end of the weld, a retractable pin tool seals the hole directly at the end of the weld, without any further intervention.

Finally, a last improvement has been made to the FSW tools: temperature measurement of the weld in real time. The temperature measurement tool allows you to control your FSW weld in real time: it is a quality tool.

Smart tool holder – the temperature measuring tool: precision for successful welding

In addition to the functionality of a standard tool, the temperature measurement tool allows you to precisely measure the temperature of the weld in order to adjust your welding parameters finely and check 100% of the parts.

How does the temperature measurement tool work?

The FSW temperature measurement has two holders, one fixed and one rotating, each containing an electronic card. These electronic boards receive and send the signal from the probe in the tool to the software. Thanks to this system, it is possible to control the temperature of the weld in order to optimise all the welding parameters (rotation speed, feed speed, Z force).

The benefits of the Stirweld temperature measurement tool are:

• Real time temperature measurement for demanding FSW parts (space, aerospace and automotive applications),
• 100% quality control of FSW parts – compatible with EN ,
• Optimisation of welding parameters: increase of welding speed (from 20 to 100%).

As an example, the temperature measurement tool is very useful for the assembly of aerospace components according to EN .

How to choose my friction stir welding tool according to my application?

To choose your FSW tool, you need to take into account 4 parameters:

• The distance to be welded,
• The depth to be welded, which will determine the size of the pin and the shoulder,
• The tool geometry: for this point, Stirweld experts are present to help you define the tool geometry that will best suit your welding configuration – optimal heat and material stirring.
• The material of the parts to be welded: standard aluminium alloys, high strength aluminium alloys, aluminium casting, copper, aluminium-steel or aluminium-stainless steel dissimilar.

Stirweld has developed a range of standard tools for different markets and materials to be welded. To give you an idea of which FSW tool is best suited to your application, here are some examples of FSW tool choices depending on the application sector, the material used and the thickness to be welded.

FSW tool for your application

For example, for the aerospace sector, the FSW tool will be made of a high-strength alloy to suit the more demanding aluminium parts (2xxx and 7xxx series).

Another example is the automotive sector, where most parts are produced in aluminium casting. These alloys contain silicon, which is inherently abrasive. For this type of aluminium, we use tools made of ceramic, a material that is highly resistant to abrasion.

Stirweld supports you with a complete range of high quality FSW tools. One of our main references, the Ariane company, leader in the aerospace field.

What is the lifetime of an FSW tool?

There are two factors that influence the lifetime of an FSW tool:

• Cyclic fatigue: Fatigue exerted at the base of the pin causing the pin to pull out prematurely. It is due to the drag force, i.e. the force exerted as the tool moves through the workpiece.

• Abrasive wear: Fatigue exerted on the whole tool by friction in the welded material. This will cause wear on the shoulder spiral which affects the quality of the weld.

The life of the tool will also depend on the materials being welded: aluminium/aluminium, aluminium/copper, copper/copper, aluminium/steel.

FSW Stirweld tools have an average life of  metres of weld in a “classic” Al/Al welding configuration with a thickness of less than 8mm. In order to guarantee the quality of its tools, Stirweld uses a test bench.

We will assist you in the choice of your tool. Contact us without commitment.

If you are looking for more details, kindly visit friction stir spot welding.