Economy of FSW

As the matter of fact, FSW is very efficient welding technology. The main reason is that it does not reach the melting point of the material during the process of welding, thus heat and energy delivery into weld is much lower in comparison with any other welding technology, where melting point of the material is reached due to absence of melting heat.

Other aspect of FSW economy is high speed of welding reachable on modern machines assuring high productivity and low production time. Easy gripping and modern CNC machinery keeps staff skill requirements low as well.

4.1      Speed

Speed of FSW can be generally higher than the ones in conventional welding. Modern machines allow welding speed up to 2 m.min-1 and some studies showed [72] that higher welding speeds produce welds with better mechanical properties. Although optimal welding speed has to be determined with regard to the shape of the tool as well as rotational speed with regard to material used.

4.2      Energy consumption

Energy consumed by FSW process has three main components [30].

1.      Stirring the material (deformation energy) of the work pieces being joined

2.      Production of heat due to friction between the welding tool and the work pieces

3.      Energy consumed by machine/work piece movement

Points 1 and 2 are closely connected and take the major part of the FSW energy requirements. As the energy in point 3 can be considered as roughly equal with the energy used for the same purpose in conventional welding the energy saving can be determined by investigating and comparing energy in points 1 and 2.

Studies conducted on material 7050 showed that energy consumptions vary from 800 to 2800 [30] depending on used welding properties, mainly axial force applied to the tool. It is apparent that the most of the heat is delivered through the tool wear, as also other studies have shown [9].

There was a statistically significant correlation between the ration maximum weld temperature to solidus temperature and energy needed for one millimetre of the weld as shown in Figure12. The correlation is valid for different materials.

Figure 12. Temperature ratio as a function of effective energy per weld length with linear regression curve added. [30]

4.3      Machinery cost

Machinery used for FSW process does not require the use of any new or very advanced technology. The technology level is comparable to standard CNC milling machines. There is very little information available about the costs of the machinery. Due to technological compatibility with milling machines, it can be presumed that the prices are also comparable.

There is a wide range of different types of FSW machines. The main differences lie in the gripping equipment and the maximum size of the work pieces and number of welding heads and their power output. Most of the current machines are armed with CNC unit allowing automated welding.

A study by Minton showed a possibility of utilizing usual milling machine as a FSW machine [67] with good results. Many companies are equipped with universal welding machines that can be easily transformed into FSW machines at very low costs. The main issue is to figure out FSW tool gripping. Bearing load capacity, machine stiffness and power unit performance are usually above standard FSW requirements. Nevertheless using adapted machines lowers machine productivity in comparison with its original purpose and does not allow high productivity levels of FSW to be reached either. Adapting of milling machines is preferably recommended for technology testing purpose only.

4.4      Staff requirements

FSW is a process with high possible level of automation and does not require special staff training or certification for running the welding machines. The level of skills and knowledge of worker is comparable to the one needed for running CNC milling machines. Specific required knowledge varies with complexity of the machinery i.e. degrees of freedom of welding heads and gripping equipment.

Complexity of programming of CNC programs is also on the similar level of difficulty as for milling machines.

4.5      Work piece preparations costs

As mentioned before, one of the advantages of FSW lies in the fact that there is no need for special preparation treatment of work piece surfaces that are being connected. The only requirement is making sure the surfaces are grease free [69] which usually does not require any special treatment or only a simple degreasing procedure with alcohol or acetone being required.

Low sensibility of FSW technology to corrosion spots and other particles other than the base material significantly lower the costs of work piece preparation, and thus reduce overall costs of application of FSW technology overall.