Bulk and Surface AMMC for Airframe Applications

Aluminum alloy as base metal (BM) or matrix has attracted material researchers for airframe applications due to their better corrosion resistance, high strength, and good formability. Al-based Metal Matrix Composites (AMMC) possess high specific strength, high thermal conductivity, and superior damage tolerance; which make them promising structural materials for many industries. The utilization of AMMC in the aircraft and aerospace industries increased dramatically due to their superior structural performance over un-reinforced aluminum alloys. The significant increase in the availability of reinforcement infusion processes has ensured the fabrication of bulk and SCs economically. Various liquid- and solid-state practices are investigated by researchers for the fabrication of bulk and surface AMMCs. One of the major conventional practices for fabrication of bulk composites through the liquid processing method is the stir casting process [17]. It involves the incorporation of reinforcement particles into molten aluminum during stirring by rotating impeller and allowing the mixture to solidify. The major advantages of stir casting are its simplicity, low cost, large range of shape, and large size up to 500 kg can be economically produced. Squeeze casting has also been investigated due to rapid solidification advantages but it can fabricate limited shape and size of the components [17,18]. It is well known that casting/liquid metallurgy practices, if not controlled precisely, are associated with inhomogeneous particle distribution, segregation, clustering of reinforcement, deleterious interfacial reactions, and unavoidable casting defects which lead to poor mechanical properties.

Solid-state processing method such as powder metallurgy (PM) is widely investigated for AMMC fabrication to overcome the casting discontinuities [19]. It involves blending of aluminum alloy powder with reinforcement particle, which is generally followed by cold compaction and sintering. Vacuum heating, hot pressing, and forging have also been utilized after blending. PM practices can produce a wide range of composites; however, the size of products restricts their application. Moreover, the fabricated composites often possess porosity that affects mechanical properties significantly. Bulk composites, due to the incorporation of hard ceramic reinforcements, show low toughness and ductility. In many circumstances where useful life of components depends on surface properties, a combination of special properties at the surface and high toughness in the interior bulk material is required; then the surface layer of components composited (called SCs) becomes more utile.

There is an increasing interest in the fabrication of SCs on aluminum substrate due to aforementioned advantages. In SCs fabricated through fusion- based techniques, the deposition by spray-based additive manufacturing methods including high-energy laser melt treatment, high-energy electron beam irradiation, plasma spraying, cast sintering, cold spraying, etc., are reported [20, 21]. The main shortcomings of these techniques are interfacial reactions between reinforcement particle and matrix material due to high-processing temperature, formation of undesirable phases, poor bonding between composited surface layer and substrate, and agglomeration of particles. Moreover, these techniques require a lot of capital investment, are very sophisticated in nature, and have higher processing cost. FSP is one of the most recent solid-state thermo-mechanical processing techniques that have overcome the shortcomings of all the fusion-based methods for the fabrication of SCs [22]. This technique is regarded as emerging surfaceengineering technology. Although, FSP is more popular to fabricate SCs but it has also been successfully tried for bulk composite fabrication [23]. FSP utilizes the heat (caused by friction between tool and workpiece material) and extreme plastic deformation (due to stirring of rotating tool) and produces ultrafine-grained (UFG) microstructure. There has been a huge interest in FSP due to its potential and numerous advantages such as the formation of UFG and equiaxed grained structure, low processing temperature, and simplicity. Furthermore, if controlled carefully, it can be used efficiently for age-hardened aluminum alloys as well with a tolerable loss (or even without the loss) of the precipitates. Recently, FSP has proven its great success by further enhancement in strength through SC fabrication on an age-hardened AA7050-T7451 aluminum alloy that is widely employed for airframe applications such as in fuselage frames, bulkheads, and wing skins [24].

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