Why invest in a friction stir welding machine?

Because of its wide range of applications, the friction stir welding machine offers many advantages for manufacturers. Designed to adapt to the most specific requirements, the FSW machine has now been adopted in all cutting-edge sectors…

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Friction stir welding (FSW) is a solid-state welding process that combines heat generated by friction with mechanical pressure to join metals.

This heat softens the materials, which are then mixed together to form a solid, homogeneous bond when they cool. It is therefore an innovative welding method that has a number of advantages over other processes.

But what is exactly an FSW machine? How does it work? What types of machines exist today? And what are its advantages in the welding sector? TRA-C industrie takes a look at this revolutionary tool in this article…

What components make up a friction stir welding machine?

A friction stir welding machine is designed to perform a mechanical welding process using a combination of friction, pressure and controlled movement. Here are the main components of a conventional FSW machine:

• The pin: this is the central element of the machine. It is a threaded tip that penetrates the parts to be welded. It rotates at high speeds during the welding process;
• The shoulder: cylindrical or conical in shape, this is the part precedes the pin. It exerts constant pressure on the materials to generate heat through friction;
• The anvil: this supports the parts to be welded during the welding operation. It can be adjusted to suit different workpiece shapes and sizes;
• The clamping system: it holds the parts to be welded firmly against the clamping plate during welding to ensure close contact between the surfaces to be welded;
• The movement system: this system moves the spindle and/or the parts to be welded according to specific parameters, such as speed and direction, in order to weld along the joint line;
• The in-situ control system: this is the ‘brain’ of the machine. It controls all aspects of the welding process, such as the speed of rotation and the force applied by the pin, as well as the movement of the workpieces. It therefore guarantees continuous quality.

How does a friction stir welding machine work?

A friction stir welding machine operates in 5 key stages:

1. Positioning of the parts. The parts to be welded are first placed on the tool, then held firmly in place with the clamping system;
2. Penetration. The tool is then rotating with the machine spindle and inserted into the components to be welded;
3. Pre-heating. A waiting phase allows the material to heat locally and bring the tool up to temperature;
4. Welding. The welding head is then moved along the interface to be welded, forming a mechanical bond. The combined effect of strain, rotation and movement generates friction which produces heat. The materials to be welded then enter a pasty state, allowing them to be mixed;
5. Removing the tool. At the end of the welding process, the tool is finally removed, leaving an exit hole characteristic of the process.

The usual FSW parameters to be considered when welding are as follows:

• The speed of rotation expressed in revolutions per minute
• Feed speed expressed in millimetres per minute
• The normal force expressed in kN
• The tilt angle is expressed in degrees. It allows better forging of the material while improving the surface finish of the welded joint.

What friction stir welding machines are there?

There are several types of friction stir welding machine available today, each adapted to specific applications according to industrial needs. The most commonly used machines include:

• The table-top machine: used to assemble small parts in the X and Y axes. It is ideal for use in research laboratories and training centres;
• The gantry machine: it is composed of a table that moves along the X axis and a welding head that moves along the Y and Z axes. With its higher rotation speed, it is particularly suitable for mass production;

• The rail machine: mounted on rails, this machine is able to weld long components. It is therefore perfect for assembling structural panels in the rail and aerospace industries;
• The robotised FSW machine: the FSW head is installed on an industrial robot to weld various thin-walled shapes. It can also be programmed for automated operations on production lines;
• The twin-spindle FSW machine: finally, this machine is equipped with rotating spindles operating in tandem. It is ideal for welding applications requiring bidirectional access or for welding complex parts.

What are the advantages of investing in an FSW machine?

Economic benefits

First of all, investing in a friction stir welding machine saves on material costs compared with conventional welding machines. As the process takes place in a solid state, there is no material input. The absence of electrodes or shielding gas also makes the process easier to use!

The FSW machine also welds faster than other methods, resulting in shorter production cycle times. Companies can therefore increase their productivity and meet market demand more quickly.

Finally, FSW machines are designed to be durable. They require less maintenance than welding machines. Maintenance costs are therefore lower…

Welding quality

Welds produced with a friction stir welding machine are homogeneous and have a high level of structural integrity. By mixing the materials, the metal components are evenly distributed along the weld zone.

Solid state welding limits the problems of solidification or rapid cooling associated with fusion welding methods. Once qualified, the FSW process therefore produces welded joints without major defects such as porosities, cracks or inclusions. The parameters associated with each application can be used repeatedly.

In FSW, the parts are also welded at lower temperatures, which minimises thermal stresses. There is generally less deformation than with other welding methods, particularly those involving high heat.

Diversity of applications

The FSW machine can assemble a wide range of materials that are difficult to weld using conventional methods. These include aluminium, magnesium, titanium and their alloys…

This versatility means that high quality welds can be produced for a variety of industrial applications. Friction stir welding is particularly well suited to the needs of the aerospace, military and naval industries.

The FSW process also offers great flexibility in terms of part design. Assemblies are lighter, stronger and more efficient, which is ideal for the automotive and e-mobility sectors.

Environmental interests

The friction stir welding machine is environmentally friendly. It consumes less energy during the welding process than some traditional techniques as it does not require an intense external heat source…

Some arc welding techniques use a shielding gas to prevent oxidation of the molten metal. This is not the case with FSW. This helps reducing the greenhouse gas emissions associated with its production and use.

Finally, the FSW machine produces less waste than conventional welding methods, since it does not use consumables such as electrodes or filler materials. Remember that the manufacture of the filler metal accounts for 50% of the CO2 emitted. This limits the processes involved in eliminating or recycling them!

How to choose the right friction stir welding machine?

Most friction stir welding machines are tailor-made to suit the specific needs of the user. Several aspects therefore need to be taken into account to guarantee high-quality assemblies:

• Geometric proportions: choose them to satisfy accessibility, quality criteria and weld configurations depending on the application;
• Specific applications: specify your requirements in terms of materials to be welded, part thicknesses, joint geometries and quality specifications to the manufacturer so that the FSW machine can meet these criteria;
• Machine capacity: also stipulate the dimensions and weight of the parts you plan to weld. The FSW machine must be able to assemble long panels or mass-produced parts without compromising the quality of the weld;
• Flexibility and versatility: depending on the configuration of the parts, opt for a machine that can move along different axes, load and unload in hidden time, or even weld in 3 dimensions. This will enable you to cover a wide range of applications and projects;
• Reliability and durability: when designing your machine, use a manufacturer with a reputation for the reliability and durability of its tools. A reliable machine will require less maintenance and last longer;
• Automation and control: features such as digital control, real-time monitoring and automatic programming make welding operations easy. Ask the designer to add these features if you want to mass-produce;
• Technical support and training: make sure the manufacturer also offers maintenance, repair and training services for your technicians. A responsive after-sales service is essential to help you out when you need it;
• Acquisition cost: finally, evaluate the purchase, maintenance, operating and training costs for your FSW machine. Build a machine that will give you the best revenue.

TRA-C industrie, expert in the design, development and installation of FSW machines

With 9 production sites in France, a technical design office and a workshop in Canada, TRA-C industrie is now the European leader in friction stir welding. We support manufacturers at every stage of their welding projects.

We are experts in the development, design, prototyping and on-site installation of your friction stir welding machine. We can also train your teams in its use and offer you a maintenance contract!

TRA-C industrie is also an integrator: we can load and unload your welding robots, deburring stations, integrate cells on your manufacturing execution system (MES), etc.

Finally, you can count on our strong experience in FSW to subcontract your industrial projects. Our FSW machines allow us to meet all your needs: prototyping, testing, mass production of assemblies, welding of thin (0.8 mm) or thick (40 mm) parts, etc.

With competitive price and timely delivery, World Wide Welding sincerely hope to be your supplier and partner.

Mathilda

Background

A very wide range of Friction Stir Welding (FSW) tool probe and shoulder designs have been developed around the world over the last 30 years, many of which have been used successfully and some patented. However, there has never been a standard FSW tool probe shoulder design that has been incorporated into standards and specifications such as BS EN ISO -1: friction stir welding aluminium, or AWS D17.3/D17.3M: specification for friction stir welding of aluminium alloys for aerospace applications. Although there is not a standard FSW tool probe shoulder design, there is now a few companies offering “off the shelf” FSW tools. However, the majority of FSW users consider their FSW tool designs as confidential and, as such, there are very few public domain papers that discuss detailed FSW tool geometries and dimensions or the exact material composition from which they are made.

Since its invention back in , TWI has continuously developed the FSW process for a wide range of applications for our Member companies. This has involved the design of FSW tools, weld parameter development, FSW machinery operating specifications and design, and also prototype FSW machine manufacture. Throughout that period of time it has always been recognised that the FSW tool is a crucial component for the production of high quality welds.

Basic Principles

In order to discuss how a FSW tool is designed, we first must understand its various roles...

To generate a solid state weld between pieces of metal, the FSW tool probe and shoulder combination are rotated and plunged into the interface between two plates/sheets under an applied axial force, which keeps the FSW tool in the correct location during the weld cycle, as shown in Figure 1. It is very important that the plates/sheets are supported in a clamping fixture, on the underside by (usually) a steel backing bar. This bar has the purpose of reacting to the axial force. In addition, side clamping is required to prevent the plates/sheets from separating as the FSW tool is traversed along the weld interface. Rotation of the tool generates frictional heating and softens the weld interface region and when the aluminium alloy is sufficiently softened the tool is traversed along the weld interface.

As it rotates and is traversed the thread form on the probe body disrupts the softened weld zone material and also crushes and disperses any oxide film at the joint interfaces. Complex forging and extrusion occurs and softened material is transferred through 180° from the leading edge to the trailing edge of the probe, generating a solid-state weld as a result of time, temperature and pressure. As the rotating shoulder (shown in Figure 1) is traversed along the weld interface it applies a compressive force onto the surface of the plates/sheets, both heating and containing the softened material beneath. The plates/sheets can be joined using lap welding or butt welding approaches.

Early Developments

The starting point FSW tool probe design consisted of a simple parallel-sided (cylindrical) threaded probe body, which rapidly became the first industrially successful probe in . Since then, TWI has progressively developed a family of FSW tool probes, as shown in Figures 2a-e, with the original parallel, threaded pin shown in Figure 2a. The thread form was cut onto the probe body in the left hand direction.

Figure 2 FSW probe designs: a) Threaded pin; b) MX-Triflute™; c) Parallel flute; d) MX-Triflat™; e) Thru-flow tip feature applied to a parallel flute tool.

Tool Design Evolution

As the development of FSW has progressed, commercial users of the process demanded faster welding speeds in much higher strength aluminium alloys and that is the point at which the MX-Triflute™ probe was developed. A version of an MX-Triflute™ probe is shown in Figure 2b which, although not obvious, has a slightly tapered body. The tapered probe body and the three equally spaced helical flutes, identified in Figure 2b, displaced very much less material during the weld cycle than the original cylindrical body probe and thus much faster welding speeds could be achieved, whilst maintaining high quality. The three flutes and the MX thread form also created a more active disruption of weld zone materials and more rapid generation of frictional heating, which improved the FSW process efficiency. MX-Triflute™ FSW tool probes tend to be used for welding thinner <15mm workpieces and the MX-Triflat™ FSW probe >15mm.

FSW tool shoulders are generally less complex in their design than probes. The tool shoulder does not necessarily run parallel to the workpiece surface, in simple linear welds the tool is often tilted such that the trailing edge of the shoulder penetrates the workpiece and applies additional forging pressure. Dawes et al () developed a concave shoulder design which worked reliably at an operating (tool tilt) angle of 2-3˚ (Figure 3a). The desire to increase welding speeds in 5xxx series aluminium alloys led to the development of a scroll shoulder (Figure 3a-b), in which a scroll feature is machined into the face of the shoulder which pulls in material from the outer edge of the shoulder to the root of the probe (Dawes and Thomas, ). This idea was developed to promote the vertical flow of material, but initial trials showed that such a design modification also allowed the use of a vertical (zero-tilt) tool. This shoulder design is now widely used for applications requiring 2 and 3-dimensional weld paths.

The tool shoulder profile significantly influences frictional heat generation during FSW. Tool shoulder profiles that restrict material flow, such as the scroll, give the greatest heat input, due to increased surface area. Thus, reduced scroll shoulder diameters may be used. This has proved particularly beneficial as joint designs and weld paths are becoming increasingly complex, since tool design can often be driven by joint geometry constraints.

Figure 3 Tool shoulder designs (shown with a plain probe): a) Concave; b) Scroll.

Friction Stir Welding Tool Materials (for Al alloys)

Materials such as intermetallic alloys, silicides, Laves phase alloys (two phase Nb-Ti-Cr alloys), platinum alloys, iridium alloys and ceramics have all been identified as having potential (in terms of high temperature strength) to be used as FSW tool probes for welding aluminium alloys. However, previous research at TWI has shown that most of these materials have very poor fracture toughness and fail rapidly by brittle fracture when used as friction stir tools. In addition, this group of materials are difficult to source, and challenging to machine to the FSW probe geometries currently considered necessary to generate good quality welds.

TWI development over the last 25 years had led to a small group of materials, which can be reasonably easily procured and machined. Previous Al alloy research has included the following probe materials:

• Hot work tool steels (AISI H13 HWTS has been used extensively)
• High speed steels
• Superalloys (Ni- and Co- based)
• Cemented carbides (WC-Co – have limited use)

A comparative summary of these materials is shown in Table 1.

Tool materialSuitability Relative machinability Relative cost Relative availability  WC Good strength, poor toughness at low temperature Poor Low Reasonable

Densimet D176

Reasonable strength and toughness Good Medium Good TZM Reasonable strength and toughness but produces a hazardous oxide fume at elevated temperatures Good Medium Good Nimonic alloys Good strength but relatively low toughness Reasonable  High Poor MP159 Good strength and toughness Reasonable High Reasonable

Table 1 Relative properties of various possible FSW tool materials welding high strength Al alloys

For Al alloys, the cobalt-based high strength MP159 alloy has proven itself the best choice available. MP159 was first used at TWI as a FSW probe material in . This alloy was developed by SPS Technologies Inc. to be a fastener alloy capable of operating up to 590°C. At the time of writing, the alloy is manufactured by the Latrobe Speciality Steel Company in the USA. The nominal chemical composition of MP159 alloy is shown in Table 2.

Element Co Ni CrFe MoTiNi Al  Wt. % 35.7  25.5  19 9 7 3  0.6  0.2

The attractive properties that have led to the use of MP159 for FSW probes are as follows:

• High strength ( N/mm2 at 540°C) combined with good ductility and toughness
• High operating temperature (up to 590°C)
• High creep strength up to 590°C
• Can be forged to complex shapes
• Good fatigue resistance
• Can be purchased at a commercially viable price
• Can be machined to complex shapes

Weld Quality and Acceptability

Although often requested, it is often difficult to provide an FSW probe geometry and dimensions for a particular application, because the size of the component to be welded and the heat sink effect of the clamping fixtures required need to be accommodated. Therefore, a particular FSW tool probe type is initially chosen from prior experience, and the design, geometry and dimensions for a starting point probe. A short weld parameter test matrix study is most often carried out to establish if this probe can produce good weld quality, or where the boundary of the weld parameter tolerance envelope is located. In many cases, the original FSW probe design selected produces good quality welds but occasionally a redesign of the tool is required to accommodate any drawbacks that were identified or inconsistencies in the material being welded – e.g. variations in extrusion thickness, edge quality or straightness.

In the absence of any truly reliable modelling information that can accurately identify the exact FSW probe shape for a particular application, alloy type and plate/sheet thickness, TWI find that an empirical and iterative approach still tends to be the best way of developing the FSW technology for our TWI Member companies (see Figure 4). Our approach has so far proved to be successful and has provided confidence for the industrial end user. One aspect which is often over-looked when exploring elaborate ‘optimised’ tool designs with computer modelling is the final ‘manufacturability’ of the tool.

Figure 4 Example of various design iterations explored during a large FSW tool development programme for the welding of thick section Cu-alloy. Machinability is an important factor to consider when designing new FSW tools.

FSW Tools in TWI Research Projects

A FSW Single Client Project at TWI will most often include the delivery an FSW tool design (which then becomes the property of the client), accompanied by tool manufacturing drawings and the most effective and robust production weld parameters, derived from an agreed weld parameter development programme against client KPIs.

Quality is assessed in terms of metallurgical and mechanical joint properties, with the acceptance criteria most often in line with the standard ISO , or dictated by client specification. If a criteria is not available, TWI are able to advise on suitable assessment testing plans to help the Member company establish the tolerable quality and performance acceptance boundaries. This can short circuit an FSW programme that might well have taken a long time to develop successful results in a company with an application but little internal FSW process experience.

FSW Support Offering at TWI

Since inventing FSW in , TWI has been working hard to maintain our world-leading industrial support and innovation for the technology. Below is a non-exhaustive selection of our FSW technical support offerings and a number of the FSW process variants.

Typical FSW Project Cycle at TWI

Over the years, TWI has devised a risk-managed, staged approach to FSW process development and technology transfer. The diagram below provides an overview of a typical FSW project cycle, which is suggested to be a risk-limiting, confidence-building road map when exploring and adopting the technology.

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