Hi

The most important thing is the capability of a joining process in adjusting with every process conditions. for example, electric arc welding can be accomplished in a wide range of conditions, e.g. size and geometry of the joining parts, environmental conditions (temperature, humidity, ...), materials used in the process and also simplicity of the whole process. About FSW, the situation will change. you always need high strength materials (the process in not applicable on relatively soft metals and materials). Geometric conditions are so important in the mentioned process (it is mainly used to join circular cross sections to other surfaces). Also several other factors can limit the application of FSW, includes the availability of friction machines (in addition to electricity), etc. Therefore, although it is a promising process, but it still evokes more research and developments.

Hi,

The single most important reason is availability of a Friction stir welding Machine or in other words initial investment in the system. 

There are secondary limitations like geometrical limitation of  the work piece and the cost of the welding tool.

The other reason in addition to initial machine investment and geometry limitation is the fixture to hold the specimens. In some cases, due to the difference in specimen dimensions, new fixture design is required. 

The biggest drawback of FSW relative to other industrial joining processes for large volume production is that it is slow. Add to that the already mentioned limitation of part geometry/accessibility then you have a process that is less desirable for mass production and more amenable to low volume and/or specialty/niche products. That said, as I recall Apple Computer is making their IMac computer cases using FSW to obtain a thinner profile. 

all of the replies are good... other than these problems.. can we think.. to make these process fast, robust and also cost efficient process...

if we think basically about the process it needs three most important factors to be controlled. power motor for rotation of tool and movement of the tool or workpiece minimum 10HP, a fixture which can hold two workpieces firmly and a universal tool holding chuck with a smooth backing plate ....

many times my freaky mind thought about vacuum pressure which plays an important role in automobile industry.... there we need forces of different intensity e. g. nut bolt tightening, large car hood holding movement without any scratch. we can explore more things....

till now there are many special purpose FSW machines.. but we need a universal setup... only then it will be popular and adopted for mass production.

I don't think so that FSW process is not popular. Some well reputed tech companies and organisations are using this technique to meet their requirements. For e.g: NASA is leveraging key technologies like friction stir welding from the Space Shuttle Program to design and manufacture the Space Launch System. For other applications, please go through the following link:

https://ed.ted.com/on/Wtwkde5M

All the applications in video, with the exception of Apple, are very low volume.

To the best of my knowledge Friction stir welding is the only specific process which can produce the 90 - 95 % joint efficiencies for the high thermal conductive material e.g copper (Al/ Mg as well). While other processes are not able to produce the same result (Conventional process, TIG, MIG etc). This is where the FSW is the most suited process. The second thing is that when the dissimilar joint has to made e.g SS to Cu, Cu to Al etc. But for this the fixture has to be rigid enough, while in industries the big size plates need to be weld So for them the practically it is not possible, secondly at the end of the process the keyhole generated. So according to me the advantages of the process became the limitation. And the process became limited to the specific but critical application.

In my opinion, the Friction Stir Welding process can be intended to connect materials that are difficult to weld both similar and non-similar materials. But there are some difficulties, namely the more complex shape, position, special tools are needed as a strangler and the result of the end of the process is remaining pinholes.