Poor tribological properties restrict structural applications of aluminum alloys, surface composites of aluminum alloys have gained more attention in material processing. Addition of solid lubricant reinforcement particles along with abrasive ceramics contributes to the enhancement of tribological performance of surface composites. In the present study, solid-state technique, friction stir processing (FSP) was used to develop mono (B₄C) and hybrid (B₄C+MoS₂) surface composites in AA6061-T651 aluminum alloy. The hybrid surface composites were produced by varying an amount of MoS₂. Multipass FSP with different direction strategy was adopted for achieving uniform distribution of reinforcement powders in the aluminum matrix. Microstructure analysis showed a uniform dispersal of reinforcement particles without any clustering or agglomeration in the processing zone. Microhardness and wear performance of mono and hybrid composites improved in comparison to the base metal. Mono surface composite exhibited the highest hardness while hybrid surface composite (75%B₄C+25%MoS₂) achieved the highest wear resistance. This was attributed to the solid lubricant nature of MoS₂. Furthermore, dissolution of strengthening precipitate condition during multipass FSP without reinforcement particles resulted in the reduction of hardness and wear resistance.

The metal matrix composites combine the metallic properties of a tough and ductile matrix with properties of reinforcement particles, simultaneously develop the functional properties by proper selection of reinforcements for projected applications. However, hard ceramics reinforcements decrease toughness and ductility of soft matrix and restrict their wide applications. The surface metal matrix composites (SMMCs) preserve the matrix properties with added advanced surface properties by reinforcing particles only in the surface layer. The hybrid surface metal matrix composites (HSMMCs) with more than one reinforcement gained attention in material processing due to their noble tribological behavior and surface properties, which cannot be attained in mono composites. Conventional liquid-phase processing techniques to fabricate hybrid surface composites result in the formation of undesirable brittle compounds, detrimental to desirable properties of composites. Friction stir processing (FSP), a solid-state processing technique, has been used by many investigators using different reinforcements to fabricate mono as well as hybrid surface composites. Friction stir processed (FSPed) hybrid surface composites have not been extensively reviewed. The current review provides a comprehensive understanding of the latest developments of FSP in hybrid surface composites manufacturing. This paper review different reinforcement strategies in the fabrication of FSPed hybrid surface composites and also the effects of single-pass, multipass, and change in pass direction on microstructure and resultant properties. Finally, future directions and challenges to FSPed hybrid surface composites are summarized. This review article containing important information on hybrid surface composites fabrication by FSP will be useful to academicians and investigators in the field.

Aluminum surface composites have gained huge importance in material processing due to their noble tribological characteristics. The reinforcement of solid lubricant particles with hard ceramics further enriches the tribological characteristics of surface composites. In the current study, friction stir processing was chosen to synthesize hybrid surface composites of aluminum containing B₄C and MoS₂ particles with anticipated improved tribological behavior. B₄C and MoS₂ powder particles in 87.5: 12.5 ratio were reinforced into the AA6061 by hole and groove method. Microstructural observations indicated that reinforcement particles are well distributed in the matrix. The hardness and wear resistance of hybrid surface composites improved as compared to the base material, due to well distributed abrasive B₄C and solid lubricant MoS₂ particles in AA6061. The hybrid surface composites achieved ∼32 % increased average hardness as compared to the base material. Hole method revealed ∼13 % better wear resistance compared to the groove method for friction stir processed hybrid surface composite, attributing to an improved homogeneity of particle distribution shown by zigzag hole pattern. Moreover, friction stir processed AA6061 without reinforcement particles exhibited reduced hardness and wear resistance due to loss of strengthening precipitates during multi‐pass friction stir processing.

Aluminum matrix composites (AMCs) demonstrating a good combination of properties that are hard to acquire by a monolithic aluminum material. Since the last few decades, investigators have shown their keen interest to advance these materials for complex applications. Homogeneous reinforcement distribution, defect-free microstructure, and improved resultant properties depends on the fabrication method along with matrix and reinforcement materials and size. Two-step melt stirring technique and K2TiF6 flux enhanced the wettability and improve the particle distribution of boron carbide (B4C) in AMCs. The mechanical properties of the AMCs were enriched by either extrusion process or thermal treatment. Hybrid composites exhibited better characteristics than mono composites. Surface composites manufactured by incorporation of reinforcement in the surface layer; offer good surface properties without losing toughness and ductility. The B4C-Al interfacial reactions produce different precipitates in AMCs and damaged the composite's age-hardening ability. B4C reinforced friction stir processed surface composites exhibits refined structure and better properties compared to the aluminum matrix. The strength, hardness, and wear resistance of AMCs increased with rising fraction and reducing the size of B4C up to a certain level. Wear rate increases with rising applied load, sliding time and speed. A review of effect of B4C reinforcement on different properties of mono and hybrid AMCs with summarized results attained and concluded by different investigators is presented in this paper to help researchers in the field. At the end of this paper a position given to conclusions and future directions.