FRICTION STIR WELDING PROCESS

Aluminum can be welded, but certain alloy families such as 2xxx (Al-Cu) and 7xxx (Al-Zn) are difficult to weld using conventional fusion methods due to their high susceptibility to hot cracking and loss of mechanical strength. In addition, aluminum’s high thermal conductivity and thermal expansion make it prone to distortion during welding. Friction Stir Welding (FSW), a solid-state process, overcomes these limitations and allows joining of all aluminum alloys — including 2xxx and 7xxx series — with excellent mechanical properties and minimal deformation.

Friction Stir Welding produces joints with mechanical strength close to or even exceeding that of the base material, thanks to the absence of melting and solidification defects.

An exit hole is left by the FSW tool during the removal phase. This hole at the end of the bead has no impact on the quality of the weld. It can be prohibitive for certain applications of circular welds in specialized fields such as the aeronautical industry.
To avoid this exit hole, we have designed an FSW welding head with a retractable pin. In this context, the welding phase is carried out over a complete turn in order to close the weld. The retractable pin is pulled up when the weld area is overwritten.

Yes, a wormhole is a FSW defect also called a tunnel in friction stir welding jargon.
Identifiable to the naked eye, this weld defect is characterised by porosity along the weld joint.
The wormhole is a defect that can occur in FSW butt or lap welds.

The quality of FSW welds can be verified by non-destructive testing (NDT) and destructive testing to ensure compliance with the FSW ISO 25239 standard.
The visual inspection permits the observation of visible defects to the naked eye which appear as well on the butt and lap configurations. The flash appears as a burr on the edges of the joint. The wormhole is identifiable by an extended porosity on all or part of the length of the weld.
Other defects related to friction stir welding operations are internal. Only destructive tests can verify these welding defects. Control parts can be examined by different destructive testing techniques: microstructure, bending tests, tensile tests, macrographic examinations, chemical analyses…

In this FSW configuration, the parts to be joined are positioned one on top of the other. This joint geometry allows for the positioning of dissimilar materials or parts with different thicknesses to be avoided.
The FSW process keeps the back side of the assembled parts intact. This avoids the need for grinding.
Lap welding is one of the most common welding configurations in friction stir welding.

Yes, the friction stir welding process can be used to join thick aluminium alloys.
Our FSW welding head allows access to quality welds up to 20 millimetres thick of the base material.

Welding aluminium is a much more complex operation than for other metals. Welding aluminium requires a great deal of skill and the right tools.
The best way to weld aluminium is by cold welding, or more precisely, by welding in the pasty state. Friction stir welding is an innovative cold joining process. FSW technology permits parts to be joined together by the application of pressure and friction without reaching the melting point.
This makes it an excellent way to weld aluminium without using heat. FSW also allows the mechanical properties of the material to be retained.

Wormhole is a defect that can be encountered in friction stir welding (FSW). It can occur on FSW butt or lap welds.
Visible to the naked eye, the wormhole is an internal cavity in the welded joint. This porosity is present along all or part of the length of the weld. Mechanically, this weld defect behaves like a crack.
The wormhole is mainly due to a bad definition of the welding parameters.

FSW technology offers many technical and ecological advantages.
Green welding technology, it does not involve any fusion or addition of material. It does not emit toxic gases or UV radiation. Energy consumption is reduced compared to other welding techniques (TIG, MIG, EBW).
It allows the assembly of materials such as aluminum alloys, which are known to be difficult to weld. Finally, being a mechanical process, FSW is easy to automate and industrialize.
The limitations of FSW are mainly the size and price of the machines. Stirweld’s welding heads adapt to the existing CNC machine tools and strongly limit the investment. This solution becomes a new advantage for the company.
To learn more about the benefits of FSW, check out our article here.

FSW stands for Friction Stir Welding.
Invented in 1991 by The Welding Institute (TWI), the FSW process is based on the assembly of two parts by heating the materials. The pasty state is obtained thanks to the friction of a rotating tool coupled with a kneading that generates the plastic deformation.
The major advantage of friction stir welding is its ability to join two parts without reaching fusion temperature. This is called cold welding. Thus, the material does not undergo cracks, deformations or loss of resistance as during a traditional fusion welding.

Clamping has an important impact on the quality of welds, for example on large panels from machine tools. The two parts to be joined must be clamped tightly on a support in order to withstand the welding forces.
The clamping system must be sized to counteract the forces generated by the rotating tool. We design a clamping jig to clamp the parts during the friction stir welding process. Each clamping jig is unique and corresponds to the specificity of the large panels to be joined.
When welding mass-produced parts, we recommend the development of a specific clamping system for the part.

Yes, it is possible to weld in a cold or, more precisely, in a pasty state.
This process is based on the joining of two materials in a solid state without exceeding the melting temperature. Cold welding is used in particular on metal and plastic parts that can be deformed by heat. It is therefore a particularly interesting process for aluminium. Cold welding is also suitable for joining dissimilar materials.
Cold welding is generally used to design butt or lap/transparent joints.
Friction stir welding (FSW) is one of the techniques for joining in the paste state.

In welding, flash refers to the burr on the retreating side due to the rotation of FSW tool.
The flash is the main defect that can be encountered during a friction stir welding operation. Flashes occur mainly when too much heat is applied to FSW butt or lap welds. This intense heat is due to a high welding effort or too much penetration. Moreover, flashes could be also due to the gap between the welded parts. Indeed, in presence of gap bridging, the FSW tool plunges more (thanks to the force control) in order to fill the void between the parts and to avoid any hole defect (wormhole).

These flashes are visible to the naked eye on the joint edges. It is more a question of an aesthetic imperfection. Indeed, this loss of plasticised material on the edge of the shoulder has no impact on the strength of the weld.

In order to ensure the solidity of your weld, it must meet two essential conditions: the right penetration of the weld and homogeneity along the whole length.
For maximum strength, it is necessary that the parts are fully penetrated. The greater the penetration of the material, the stronger the FSW weld.
The materials must be mixed along their entire length at the weld. Otherwise, the strength of the joint is reduced.
The strength of the FSW weld is guaranteed by compliance with the FSW ISO 25239 standard. An industrial study ensures the successful implementation of the FSW process in critical sectors such as aeronautics, space or defense.
Our prototyping department is at your disposal for any questions regarding the quality control of your FSW welded parts.

Friction stir welding is a solid state welding process without the need for filler metal. This technique is more commonly known as FSW for Friction Stir Welding. This breakthrough technology was invented and patented in 1991 by The Welding Institute (TWI).
As its name indicates, the FSW process consists in obtaining a weld by combining heating and mixing thanks to the rotation of the tool. The heat necessary for welding is obtained by the constant friction of the tool shoulder on the parts to be welded. The tool pin ensures the mixing of the material brought to a pasty state. It thus plastically deforms the material by penetrating the parts to be welded.
To learn more, download our white paper on FSW principles and trends.

The machining centre is designed to operate in position control. Friction stir welding requires force control, and therefore a force sensor to control the friction of the shoulder on the workpieces.
Integrated directly into the Stirweld head, this sensor measures and maintains a constant pressure. The force control allows the positioning of the tool to be adjusted according to variations in the relief of the parts to be joined. Unlike the position control of the CNC, the force control allows for adaptation to these variations.
Force control is an essential element in ensuring a high quality FSW weld. Perfect FSW welding requires control of the force exerted by the FSW head during the welding operation.
In addition, the force control provides additional safety for your machine tool. The sensor ensures that the maximum force applicable by the traditional milling machine is not reached.

There are 5 main FSW defects:

• Flash or burr on the edges of the weld ;
• Wormhole or tunnel which i.e. a porosity present on all or part of the length of the weld ;
• Lack of penetration which is an internal defect of the weld ;
• Hooking which i.e. a rise in surface of the material;
• Kissing-bond that appears when the adhesion is zero.

Flashes and wormholes are common defects in butt and through weld configurations. Find in our article detailed information on the main defects encountered in friction stir welding.

Friction welding (or inertial friction welding) is a process where the heat required for welding is obtained by rubbing the parts to be joined together under axial pressure.
Friction stir welding (FSW) is a variant of friction welding. FSW is particularly suitable for joining parts that are not rotationally symmetrical. By means of a threaded pin, the material is heated and softened in order to ensure the joining of the materials and the quality of the weld.
Friction welding consists of only one phase: the friction of the parts to be welded against each other. In contrast, friction stir welding combines two phases: friction and stirring thanks to a rotating tool that penetrates the parts to be welded while advancing.

The effectiveness of friction stir welding is particularly well known for joining materials identified as difficult to weld, such as aluminium. Friction stir welding technology can be used to weld all aluminum alloys. FSW welding of the 5000 series retains 100% of the mechanical properties. For the 2,000, 6,000 and 7,000 series, a reduction in properties of 10 to 20% has been observed. Its efficiency is recognized and allows to dimension the structures to be welded according to the alloys
The choice of an appropriate FSW tool also contributes to the efficiency of aluminum welds.

In order to guarantee the quality of a FSW weld, it is necessary to carry out a study of prototypes in accordance with the ISO 25239 standard. These prototypes are controlled by destructive and non-destructive tests.
Non-destructive testing (NDT) includes both visual surface inspection and volumetric inspection. Some defects are visible to the naked eye, such as flash and wormholes. They can therefore be easily detected. Other NDT tests can be performed using ultrasonic (UT) devices or radiography.
Destructive testing will involve an action on the weld such as a bend test, a tensile test or a macrographic examination.
Thus, once these controls are carried out and validated, the FSW welding can be repeated in an industrialized way, ensuring its quality at 100%.

The WPS (Welding Procedure Standard) certification is valid for a specific application. It depends on the material, its thickness and the welding configuration. For each application, it is therefore necessary to qualify the weld according to these parameters.
The WPS certification thus describes all the parameters of a quality FSW weld for a specific application. It is obtained after successful qualification tests on prototypes.

Yes, it is no longer necessary to tilt the tool during the FSW process with our technology.
Other friction welding processes will require a tool tilt angle to be set to improve the surface finish of the joints. The purpose of the tilt is to keep the material being welded below the shoulder and thus limit burrs.
By choosing Stirweld’s FSW tool for your welds, you can do away with this tilt parameter. The shoulder of our FSW tool is designed as a counter-cyclical spiral. This feature allows it to push the material flow towards the centre to keep it in the joint area.

Hooking is a defect in the surface of the material that can occur in FSW welding operations. The material rises on the edges of the feed zone and it thins on the receding side. This internal defect can be detected by macrography.
The hooking is mainly due to a lack of stirring creating a reduction of the welded section.
This defect of non-welded zones leads to a decrease in the mechanical resistance of the weld to tensile stress. To avoid hooking, it is important to use the appropriate FSW tools and make the appropriate settings.

In butt and through welds configurations, there are several ways to position the parts to be joined. The T-joint is one of them.
FSW T-joint welding is achieved by intersecting the two parts at right angles with one part in the centre of the second. It is called a T-joint because of the positioning of the parts to be welded looks like the letter “T”.

Stirweld’s FSW solutions can be equipped with temperature measuring tools.
Stirweld’s intelligent tool holder is simply installed on your welding heads and spindles. It allows to check that the heat is constant during the friction stir welding process. For quality welding and safe operation, the monitoring control interface tracks the tool temperature.

The qualification of the welding operator depends on the technology chosen to weld aluminium.
It is possible to weld certain types of aluminium alloys with traditional welding processes such as TIG or MIG. These techniques require the welder to have a welder’s qualification (QS) issued by an official certification body.
Friction stir welding is used on an existing machine tool. The machinist or operator trained on your machine can easily learn to weld aluminium with FSW. Friction stir welding does not require a welder’s qualification.
Striweld supports you throughout the entire FSW implementation cycle. We train your technicians in the use of our FSW tools. Several webinars FSW courses are already available to train you in friction stir welding of aluminium.

Friction stir welding (FSW) guarantees a higher quality and strength of the welds than those obtained by conventional techniques (TIG, MIG, EBW). There are 2 types of FSW welding: butt or lap welding. The choice between these configurations depends on the application.
Our studies prove that butt welding offers a better fatigue life. Moreover, this type of welding does not present any hooking defect.

To learn more, check out our blog post on the comparison between butt and through welds.

Yes, in FSW welding, the shoulder must be in permanent contact with the surface of the workpiece. It is the friction of the shoulder against the workpiece that provides the heating necessary for the friction stir welding operation.
The shoulder is one of the components of the FSW tool. Its main function is to heat the material by friction in order to soften the parts to be welded. To do this, the rotating FSW tool is immersed in the material until the shoulder is in direct contact with the top surface of the workpiece.

Friction stir welding (FSW) consists in assembling two parts by bringing them to a pasty state thanks to a rotating pin, in particular thanks to a robot or a FSW head for CNC.
Specifically, the rotating tool is pressed into the material until the shoulder touches the surface of the material being welded. The friction allows the material to be heated locally to a temperature sufficient to make it easily malleable without melting. The tool can then advance over the parts to be welded. The material flows from the front to the back of the tool to form the welded joint.
To learn more, see our videos on friction stir welding technology.

Friction stir welding is a solid state welding technique that does not reach the melting point. Known for its ability to weld difficult to weld materials, FSW is mainly used for the high quality joining of aluminium and copper parts. The process is also suitable for joining dissimilar parts made of aluminium and steel, for example.
FSW replaces the traditional fusion welding techniques due to the excellent repeatability of the process and the low deformation of the material. This technique has many applications in the industry. FSW is used, for example, for the production of cold plates, aluminium tanks in various fields such as railways, aeronautics and space.

Friction Stir Welding (FSW) was invented by Wayne Thomas of the Welding Institute in 1991. The TWI is a British research and technology organization specializing in welding.
In 1995, a second patent was filed to extend the application to the tool level.
In early 2015, the Welding Institute patent fell into the public domain. This allowed FSW to be more widely exploited. Today, this technology finds high-quality applications all over the world.
As an industrial member of TWI, Stirweld participates, with their teams, in several collaborative projects involving FSW expertise.

Compared to a conventional FSW head, the FSW head with retractable pin has an additional function: the progressive removal of the pin in the shoulder in order to fill the exit hole of the FSW weld.
As the only FSW tool technology that does not leave a hole at the end of the weld, it is very popular for certain applications, such as space and aerospace applications (tanks, rivet replacement etc.). The exit hole on a cylindrical part becomes a major leakage defect. The retractable pin is particularly used for circular welds (e.g. tanks) in the aerospace industry in order to ensure the tightness of the weld.
The FSW head with retractable pin also offers the possibility of joining parts with a variable penetration depth.

Yes, it is essential to get the welding force data during the operation.
This information allows real-time control of the quality of the weld.
Our FSW Stirweld heads and spindles are designed to provide this intelligent data through its powerful HMI.

The depth of tool penetration is one of the main parameters of the FSW process to ensure the quality of the weld. For this purpose, it is necessary to carry out prototype tests in order to define the welding parameters as well as possible. One of these indicators is the force exerted (in kN) on which the penetration depth of the tool depends.
The penetration depth will depend on the force exerted during the welding operation but also on the thickness of the parts to be welded.
It is essential that the pin of the FSW tool penetrates the entire length of the workpiece. This ensures that the shoulder rubs against the workpiece and generates the heat required for mixing.
The penetration depth is the lowest point of the shoulder below the surface of the welded part. The depth must be correctly set to ensure the necessary downward pressure for the FSW tool to fully penetrate the welded joint.
A precise study will define the penetration depth of the FSW tool according to its application to ensure an optimal weld.

Lack of Penetration (LoP) occurs when the metal does not completely fill the thickness of the joint. This internal defect can occur during butt welding operations.
This lack of root penetration is mainly due to the fact that the pin does not reach the bottom of the material. Thus, a pin that is too short will not allow the entire thickness to be welded. If the length of the pin is adapted, a lack of penetration indicates an insufficient forging effort.
To avoid a lack of penetration, it is important to choose the right FSW tools and the right settings.

The most common types of friction stir welding are butt and lap welding.
The most usual FSW configuration is butt welding. The parts to be joined, of equal thickness, are placed against each other. In a lap weld, the two parts are placed on top of each other.
The choice of the type of FSW configuration depends on the materials to be welded and the geometry of the part. For example, only lap welding will allow the assembly of heterogeneous materials (aluminium/copper, aluminium/steel).
More details on these types of welding can be found in our article on the comparison between butt and through welding.

No, external preheating is not necessary for a welding operation with FSW technology.
We have integrated a delay phase into our welding process that avoids this phase. The preheating action is thus carried out by the tool itself while continuing to rotate in the same place. It is also possible to increase the heating of the material by reducing the lowering speed or increasing the rotation speed.

During an FSW welding operation, the shoulder brings heat to the top of the workpieces to be welded. This can cause distortion of the material which requires post-weld machining to restore the correct flatness.
To control this distortion during friction stir welding, less heat should be applied to the workpiece. To do this, it is recommended to weld faster. It is also advisable to use the smallest possible FSW tool. Our team will help you to define these welding parameters, to choose the right FSW tool and to design your clamping system.

A friction stir weld may have wormhole or kissing bond defects. In order to repair these defects, it is necessary to pass the FSW tool over the weld again.
In the case of a wormhole defect, the welding operation must be repeated while maintaining the same position. To repair a kissing bond or hooking defect, the FSW tool should be slightly offset from the initial weld. Our FSW experts will assist you in defining your welding parameters. They will also advise you throughout the process of integrating the technology, particularly for the production of prototypes.