Introduction
The friction stir welding technique is regarded as the most suitable
method of joining aluminium alloys and other soft metals [1]. This
includes for both similar and dissimilar metals and alloys. There are
numerous investigations that are being in progress which are looking
into the compatibility of FSW on the dissimilar aluminium alloys
[1-4]. However, it has been noted that there are many factors which
compromise the quality and strength of the dissimilar joint. Some of the
factors include welding parameters, material positioning and welding
surrounding conditions [5-6]. Additionally, the joining of
dissimilar materials was found to result in the formation of
intermetallic compounds, which is also a major contributing factor
towards the weakening of the joint strength [6-7]. This, therefore,
opened a gap for a post welding technique to strengthen the joint
strength. The friction stir processing technique was found to be a
suitable method in modifying the joint/surface properties. The friction
stir processing (FSP) works the same way as the from friction stir
welding, but it does not join material together but modifies the
microstructure of the material [8-9]. FSP has been used successfully
to modify both single surfaces and welded joints [10-13].
The FSP technique still has many areas that still need to be
investigated. Some of those areas include the impact of FSP on the
fatigue life of the processed joint. There is very limited literature
available in this area. However, it has been shown by various authors
that the occurrence of most structural failures originates from fatigue
[14-15]. This then makes it a need that special attention be given
to this area as there are high possibility of utilizing dissimilar
aluminium alloys in the aerospace and automotive industries. These
industries require very high safety, which makes fatigue resistance of
joints very pivotal for ensuring the integrity of such structures
[16-17].
Hussein and Shammari [18] comparatively studied the FSWed and FSPed
AA5083-H111 joint to determine the respective fatigue and fracture
behaviour of the joints. The fatigue test were performed under constant
stress amplitude cantilever. The analysis of the fatigue properties
revealed that the FSPed samples yielded better fatigue limits compared
to the FSWed samples. Uematsu and Tokaji [19] employed FSP on the
cast aluminium alloy A356-T6 with the purpose of studying its fatigue
strength. The fatigue strength of the FSPed A356-T6 was found to be
lower than that of the base material, nonetheless, the fatigue limit was
significantly improved by FSP. The increase in fatigue limit was said to
be due to the elimination of the casting defects during FSP, while the
decrease in fatigue strength was due to matrix softening by the
dissolution of precipitates caused by heat input during FSP leading to a
faster crack growth rate in the FSPed region.
Park et al. [20] discovered that the application of FSP increased
the fatigue life of the processed joint by 42% compared to that of the
welded one. Additionally, the fatigue strength was found to increase
with a decrease in the microstructural grain size. Similar results were
in consistent with those reported in the literature [21]. FSP was
employed on the TIG welded AA6061 joint with the aim of increasing the
fatigue life of the joint [22]. The constant amplitude loading and a
stress ratio of zero were used. The application of FSP resulted in the
tensile strength being improved by 5-13% while the hardness was
increased by 5-10%. Subsequently the fatigue strength improvement of
about 30-60% was observed and these improvements were due to the
microstructural grain refinement in the stir zone as well as the
modification in the geometry occurred at the weld toe.
There is a very limited to no work available that involves the
processing of the friction stir welded dissimilar joint. This work is
focussing on analysing the impact of the FSP on the friction stir welded
AA8011/AA6082 dissimilar joint. The analysis was being performed in
relation to the material positioning during FSP.