Discussion


7.1      Assessment of knowledge available in open literature

FSW, as a relatively new technology, has not yet gained standardised procedures for setting basic welding parameters and tool selection for given types of joints and materials. Open literature provides a wide range of information about microstructure of FS welds, mechanical, fatigue and corrosion properties of welds manufactured under a low range of specific welding parameters for a low selection of materials. Availability of specific information about welding tool design is very limited. In addition, paper presenting results of testing of mechanical properties present used tool detailed characteristics in an incomplete way. All these facts together cause that the value of the information in open literature for setting welding parameters and deciding about effectiveness of FSW implementation into the manufacturing processes of aircraft producers is low. It makes adoption of FSW technology more expensive, because optimisation of welding parameters requires further detailed investigation by the adopting manufacturers.

The lack of standardisation in the area of FSW can bring additional obstacles into the certification process of the aircraft, which uses FS welds in its construction, because no standardised calculation of fatigue life which can be used as a proof of the lifespan of the welded part. In addition, the effect of corrosion and residuals stresses in the welded area and their influence on the fatigue life have not yet been fully understood.

Generally, there are significant gaps in the literature leaving the field open for larger scale research allowing easier setting of welding parameters and making assessment of benefits of FSW technology implementation less problematic and more accurate.



7.2      Substitutability of riveting by FSW and its application

Riveting is a technology substitutable by FSW if the specifics of the technology are respected and preferably design changes are executed. However, there are some areas where riveting technology could be used more conveniently.

The design changes in the transition from the riveted construction to FS welded construction are the vital part of adoption of FSW technology ensuring its effectiveness. Changes in design utilizing the FSW technology benefits make the resulting lighter and possibly stiffer than the original construction.

The main benefit of FSW in comparison with riveting lies in the fatigue life field. Riveting introduces many stress concentrators into the construction and thus, raises risk of crack initiation. In contrary, FSW technology does not create additional stress concentrators. The only risk factors towards the fatigue life introduced by FSW are residual stresses and crack in the non-optimised welds.

The bending test performed on the FEM modelled samples of riveted and FS welded panels showed that, for the construction of this type, FSW variant could bear the same load as the riveted construction with 6 to 9 % less deformation, depending on loading direction, and weight 7 % less than riveted construction. Switch from riveted to FSW technology saved 3 kg per panel of this size, again, comparing with riveted construction.

Economical benefits of introducing the FSW technology as riveting substitute lay mainly in the possibility of higher automation of manufacturing process and requirement of less skilled labour. For the example of FS welded panel the time consumption of the welding process is nearly 140 min, which is comparable with time consumed by production of riveted construction design. However, the FSW technology is fully automated, not requiring labour during the welding process. Comparison of energy requirements would need detailed investigation of riveting process requirements, which is not in the scope of this thesis.

The area of substitution of riveting by FSW requires further investigation. We suggest to perform examination of real samples of riveted and FS welded construction with accent on the static mechanical properties and mainly fatigue life of FS welded structures.


 

7.3      Evaluation of the use of FSW as rivet substitution on the stringer-reinforced panel

The substitution of riveting on the stringer-reinforced panel can be considered as successful. The FS welded panel demonstrates higher stiffness and weights approximately 3 kg less than its riveted version, which gives promise of further significant weight savings on other parts of the fuselage if FSW technology is used.

Fatigue and corrosion resistance properties of the used FS welds require further investigation and high cycle fatigue mechanical testing in order to allow aircraft certification.

From the technological point of view, the set welding parameters require further optimisation and experimental confirmation to acquire the ideal welding and rotational speed of the tool. Another set of experiments would be required to determine ideal shape of the welding tool, alternatively paid consultation with some of FSW tools and machines manufacturers can fasten and ease the process of determining the ideal welding parameters.

Used technology provides possibility of full automation of the welding process, thus, improving production efficiency.



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