Metal matrix composites (MMCs) are extensively used in various engineering applications due to ease in manufacturing and significant enhancement in properties [1, 2]. MMCs are manufactured by different routes and the resulting characteristics of MMCs are dependent on the production route [3, 4].The stir casting method is commonly used for the production of MMCs due to ease of operation, economic aspect and mass production .
There are some issues and challenges in the manufacturing of MMCs that significantly impair the properties to great extent [6–9]. The poor interfacial bonding and agglomeration of reinforcement particles is an important issue to achieve the full potential of MMCs . A good interfacial bonding between the reinforcement and the matrix is necessarily required in MMCs because the load transfer between the matrix and the reinforcement relationship is controlled by the bonding characteristics. Therefore, the bonding characteristics of a composite dictate its properties and performance [10, 11]. The interfacial bonding mainly depends on the degree or extent of wettability of reinforcement particles with the melt. Reinforcement particles wettability with the melt mainly depends on the surface energy of the matrix and the reinforcement, and also on the surface condition of the particles such as the amount of oxidation or contamination [7, 12]. Most ceramics are not wetted or are poorly wetted by molten metals. Porosity is a major issue in MMCs manufacturing that deteriorates mechanical properties as it reduces the load-bearing area and acts as a crack nucleation site [13, 14]. The presence of porosity at the particle/matrix interface causes debonding of particles from the matrix under very low stresses which reduces the possibility of load transfer to the particle and subsequently decreases in strength [13–15]. Agglomeration of reinforced particles is an unavoidable issue in MMCs. The agglomeration of particles also induces microstructural inhomogeneity that creates stress gradients in MMCs and thus deteriorates mechanical properties [16, 17].
Microstructural phages and intermetallic compounds that are formed during the production of MMC can significantly affect mechanical properties . In Al-Si alloys, the eutectic silicon is present as a coarser phase in the matrix of MMCs which is a deleterious morphology and causes failure emanated from this phase. Dendritic growth typically occurs in aluminum-based MMCs that affect the mechanical properties . Secondary processing or post-processing of MMCs is required to distribute the reinforcement particles homogeneously in the matrix and to improve the mechanical properties [20, 21]. Traditionally various post-processing techniques such as extrusion, rolling, uniform channel angular pressure, selective laser melting, high pressure torsion, accumulation roll bonding, etc. have been used to process MMCs [20–22]. Nonetheless, friction stir processing (FSP) of MMCs emerged as an effective strategy in post-processing of MMCs [23–25]. The FSP is a prominent severe plastic deformation technique in which a simultaneously rotating and traversing tool processes the material with a high strain rate. The grain size refinement in the processed material occurred due to dynamic recrystallisation [25, 26]. It provides a synergistic effect of extrusion, forging, stirring, and severe plastic deformation, which can easily circumvent issues in MMCs manufactured by traditional techniques.
Several studies have been reported on the post-processing of MMCs by the FSP. Kurtyka et al.  demonstrated that after FSP the microstructural heterogeneity of SiC/A339 MMC was transformed into homogeneity. The FSPed MMC exhibited an increase in compressive strength by ~ 40% and hardness by ~ 30% as compared to cast MMC. Huang et al.  reported an increase in yield strength, ultimate tensile strength and elongation of FSPed CNT/Mg-6Zn cast composite. The improvement in mechanical properties was attributed to combined strengthening contributions of grain refinement, load transfer and Orowan mechanism. They also found that grain size reduction in composite is more as compared to FSPed base alloy due to restriction imposed by CNTs in the grain boundaries migration. Yang et al.  investigated the effect of the FSP on Al2O3/AA2024 MMC cold sprayed coating and found that corrosion resistance was increased after two passes of FSP. However, after four passes of FSP the corrosion resistance decreased due to deteriorated interfaces of the inside coatings.
The FSP of MMCs resulted in microstructural refinement and homogeneous distribution of second phase particles. It can be concluded that FSP reinforcement of MMCS is a better strategy for uniform distribution of particles, reduction in the size of reinforcement, grain refinement, break-up of dendritic microstructure and elimination of porosity. In the present work, stir casted Al-Si (LM13) alloy based hybrid MMCs are post-processed by FSP to study the effect of microstructural modification on mechanical properties. The effect of multiple passes of FSP on microstructural modification and its correlation with mechanical properties is also investigated.