Characterisation of innovative friction stir welding (FSW) joints for aeronautical structures
Date
2013-07-12
Authors
Dada, Oluwaseun John
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Abstract
Friction stir welding (FSW) is the fastest evolving joining technology and the principal
prospective technique for implementing integral fuselage structure in aircraft manufacturing.
The viability of FS welds is dependent upon mechanical properties and production turnovers,
which are dependent on welding rates and tool design. This dissertation is about the multiobjective
characterisation of aeronautical alloys (AA5083-H111, AA6056-T4, AA6082-
H111) friction stir welded using the standard FSW tools, scrolled shoulder Triflat tool, and
the Bobbin tool respectively.
The effects of defects and microstructure on the mechanical behaviour of AA5083-H111
welds made from the standard tool were examined. SEM analysis showed that tunnel defects
resulted from partial consolidation at the root of the weld, and the microstructure of the weld
zones determined the failure mechanisms along the weld.
The effect of welding and rotational speeds on the integrity of AA6056-T4 FS welds
produced with scrolled shoulder Triflat tool was studied. Increased welding speeds and
advance per revolution led to improvements in mechanical properties and reduction in
residual stresses maxima. The retreating side TMAZ, which was the microhardness minimum
and tensile residual stress maximum, was also the failure location in most of the welds, hence
the microstructure and properties in this region were critical to the integrity of the weld. Bend
properties of the welds from the Triflat tool were good, because of the absence of root flaws.
The influence of two different tool designs (standard tool and standard Bobbin tool) on
mechanical behaviour and microstructure of AA6082-T6 was also studied. The standard tool
produced welds with better static properties than the bobbin tool, but most of the specimens
failed prematurely, and root bending resulted in cracks, because of alumina rich layers in the
weld nugget and root flaws. Bobbin tool welds had lower mechanical properties (70% weld
efficiency), the bend strength was better, with no flaws.
In the welds, the microstructure and microhardness which were influenced by the welding
rates which determined static properties, failure locations and residual stress minima and
maxima. The study also supported previous findings that the nature and distribution of
precipitates in the 6XXX series aluminium FS welds was important for determining
mechanical properties, unlike the 5083-H111 FS welds where the Hall-Petch effect was more influential.