What is Friction Stir Welding?
Friction Stir Welding or FSW is an innovative joining process. This approach uses a non-consumable tool to join two pieces of material together without melting material of the tools, making it an efficient solution.
In this process, friction generates heat between the tool and the material; this softens the material. Whilst the tool moves, it allows the two materials to mix somewhat similarly to joining clay or dough. This method is mainly used on wrought or extruded aluminium and for structures that require a higher weld strength. It can also be used to join a variety of alloys and steels.
Recent advancements have allowed this method to weld polymers together and dissimilar metals. As a result, FSW application can be found in a variety of industries. The practice was invented in 1991, with further work going into the development of the process the following year. By the mid-1990s, it had been adopted for widespread use by many different manufacturers and industries.
Mechanical Properties & Microstructure
FSW has proven to have better mechanical properties when welding alloys than other more traditional methods of welding, such as arc welding. The mechanical properties of FSW usually consist of three primary microstructures – a weld nugget, a heat-affected zone and a thermo-mechanically affected zone or TMAZ.
Although technically, both the weld nugget and the TMAZ are thermomechanically affected zones, they are considered to be separate microstructures. This is because the weld nugget undergoes a dynamic recrystallisation, but the TMAZ doesn’t. That being said, the exact compositions and the extent of the microstructural composition in the zones mentioned above depends entirely on the material and the pressing conditions.
These alter under the influence of a variety of different factors such as welding parameters or the design of the FSW tool used.
Advantages and Limitations
FSW has several advantages over other methods of welding due to its solid-state nature. The problems associated with the cooling process from other forms of welding such as cracking, porosity or redistribution are avoided. It also has a better record for tolerating variations in the process and materials and is found to produce fewer defects. That isn’t to say that there aren’t defects, although these tend to occur if the process isn’t carried out correctly.
The advantages that FSW has been found to have over other more traditional forms of welding can be extremely beneficial for its industrial applications. In its as-welded condition is has strong mechanical properties. It is also cheaper as there is no need for consumables as with other forms of welding. In addition, it can be carried out from many different positions as there is no weld pool. There are other performance and cost benefits, too, such as the fact that it can be automated easily on simple milling machines, which have lower setup costs and require less training to operate.
It also doesn’t require any special preparation of the area before welding, which helps speed the process along. In addition, the impact on the environment is lower as it does not produce any fumes, spatter, or UV radiation. Finally, it also provides an aesthetically pleasing outcome minimising the need for expensive machining after welding. It can also be used to weld thinner materials as long as they have the same joint strength.
As with any technology, there are also disadvantages. For example, this method can leave an exit hole when the tool is withdrawn. It can also require more downforce and heavy-duty clamping to hold the plates together. There is also a question around whether or not it takes longer, the traverse rate may be slower than other techniques, but this time could be made back up if it doesn’t need as many passes.
The process requires more care between the edges that are being joined as there is no extra material to stabilise the materials and fill the gaps as they are welded together. Finally, it can be less flexible than other processes such as manual and arc.
What Industries Can Benefit from FSW?
There are numerous areas of business that could benefit from Industrial welding solutions. Firstly, shipbuilding and offshore manufacturing. Commercially it has been used to manufacture various parts of ships, from building fish freezer panels to deck panels and even helicopter landing platforms. FSW has also been used as the primary method in the construction of entire ships from stern to bow. Many companies reap the benefits of using FSW in the construction of ships, but it is perhaps most notable used in armour plating for amphibious assault ships. It is used as it is one of the simplest ways to increase efficiency as it reduces weight. The ships don’t have the unnecessary extra weight of rivets, clinch nuts or traditional welding. FSW does not add any weight to the structure.
The friction stir welding applications in industry do not stop there. It has also seen applications in the aerospace industry. It has been used on many expendable launch vehicles, the first of which had an interstage module that was friction stir welded. It has also been used for external tanks, rockets, ramps and even a test article at Nasa. FSW is a popular choice for the construction of many aspects of aerospace travel. For this industry, FSW simply makes the welding process more manageable. Aerospace engineering requires some difficult to weld aluminium alloys which can frustrate attempts at traditional welding methods. It also makes it easier to join dissimilar alloys, which has long since been a challenge due to the different properties of the metal.
FSW has also been used in the manufacture of automotive parts such as engine cradles and suspension struts. Many companies successfully exploit this method of production for the production of car parts such as engine tunnels or flatbed trailers. They can be used to join aluminium sheets to brackets for the boot or the bonnet and even doors of many vehicles. The potential applications are vast. Vehicle manufacturers choose FSW for the stability it offers, which in turn results in less wear and tear and better fuel mileage.
FSW has also had applications for many railways. It has been used to make roof and side panels for many different trains in several countries. It has been used to secure floor panels in double-decker train cars. It is an ideal choice as heat sinks for cooling high-powered electronic locomotives, and FSW provides excellent heat transfer. More so than any other, this industry has seemingly honed in on the advantages FSW offers in crash safety; it is one of the best welding practices for creating safe designs.
It is also used for fabrication purposes. Façade panels and cathode sheets are made using this method by many different manufacturers. It is also used in the production of meat slicers, HVAC units and X-Ray vacuum vessels. FSW is also used in the containment of nuclear waste in thick copper canisters. It is also used to produce heat exchangers, hunting knives and ship propellers.
The use of FSW can also be found in the robotics industry, although this is a relatively new development that hinges on the advancement of other technologies. Nevertheless, the application of FSW has a massive potential in this industry that is largely untapped due in part to the gradual emergence of the industry as a whole.
FSW has many applications for a myriad of different industries all of which could benefit from the adoption of this technique. Whether because it is a less heavy form of welding, or that it offers more stability and safety aspects. Or maybe simply because it has performance and cost benefits or that it is quicker and is better for the environment.