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.