## 1 Introduction  
Friction stir processing (FSP) is a relatively new solid-state process adapted from friction stir welding (FSW) and has been explored to homogenise the microstructure and refine the grain size [1; 2; 3; 4]. The efficient grain refinement during the FSP results from severe plastic deformation and dynamic recrystallisation [5]. During the last decade, the FSP has been considered to modify the microstructure and increase the mechanical strength of components made via various manufacturing processes, such as castings [6; 7]. A critical advantage of utilising the FSP is introducing reinforcements particles in the metal matrix, resulting in refined microstructure and excellent mechanical properties [8; 9; 10]. Additionally, considering the simplicity of the process, FSP has the potential to be used for homogenisation, fabrication of fine-grained alloys, surface treatment, and fabricating the hybrid metal matrix composites [11; 12; 13]. All mentioned benefits, alongside the possibility of automation of the process, recently made the FSP a candidate to be used in remanufacturing [14]. Recently, considerable research has been done on strengthening the metal matrix with more than one reinforcing material in order to improve the properties of the matrix metal/alloy [15; 16]. These types of composites are called hybrid composites by various research scientists. Hybrid composites have shown better properties than composites reinforced with one kind of particle, as the advantages of different particles are combined [15; 16; 17; 18]. FSP is one of the new methods to process these types of composites. Hybrid composites can be fabricated additively and in situ using the reactive FSP [15; 16]. Chemical reactions between aluminium and suitable metal oxides have been utilised to produce in-situ hybrid composites by combining the FSP and the thermite reaction. The mechanical properties increase due to the formation of the fine microstructure and nanosized products after the reaction. Various aluminium-based hybrid nanocomposites have been fabricated by FSP using Al-Fe\({}_{2}\)O\({}_{3}\)[19], Al-Fe\({}_{3}\)O\({}_{4}\)[20], Al-TiO\({}_{2}\)[21; 22], Al-CeO\({}_{2}\)[23], Al-SiC [24] and Al-CuO [25] systems, in which particles (e.g., Al\({}_{2}\)O\({}_{3}\)) are formed in situ by reactive mechanisms. Various researchers have investigated mechanisms that lead to grain refinement during FSP. They concluded that severe deformation and high temperature during FSP lead to microstructure refinement [8]. Based on the literature review and our reported findings in this study, during the FSP, materials have been subjected to hot deformation, and as a result, dynamic recovery (DRV) and dynamic recrystallisation (DRX) of the highly distorted grains take place and cause grain refinements. It is worth noting that in materials with high stacking fault energy (SFE), such as aluminium, continuous dynamic recrystallisation (CDRX) is the mechanism of new grain formation [26; 27]. The CDRX mechanism has been detected in some metals with high SFE, such as ferritic steels, \(\beta\) titanium alloys, and aluminium alloys [27]. New grains develop due to a gradual increase in misorientations between subgrains formed by plastic deformation [28]. Studies show that misorientation angles at subgrain boundaries developed near the primary grain boundary are larger than those within the grain and can quickly transfer to high-angle boundaries. This observation indicates that the grain boundaries are preferred as the nucleation sites for new grains. Therefore, metals with smaller primary grains must display greater kinetics for grain refinement during severe strain deformation. It should be noted that in addition to the initial microstructure, the presence of particles also affects the grain refinement rate [29]. Both inside and outside of the grains, these second-phase particles are dispersed throughout the matrix.