Microstructural and mechanical characterization of AA7075 surface composites reinforced with reduced graphene oxide via multipass friction stir processing

AA7075 surface composites with improved hardness, tensile and flexural strength are highly preferable in aerospace sector like wing and fuselage sections and in automotive industries for piston and cylinder liner applications. In order to achieve this, Friction Stir Process (FSP) is adopted in the fabrication of Surface Composites (SCs) with Reduced Graphene Oxide (RGO) as reinforcements using tapered cylindrical pin tool with appropriate process parameters. Microstructural examination affirms that equiaxial grains with fine structure is formed in the stir zone region. Addition of RGO particulates subsequent to five passes lead to curtail the grain size to 9 ± 0.7 µm in comparison to base metal (79 ± 5.4 µm). The immaculate interface are formed between the base metal and the RGO, which is evident in TEM. XRD confirms the phase of reinforcement particulate and absence of intermetallics. Herewith, elemental mapping confirms the reinforcement along with base metal. RGO 5 Pass specimen attains superior microhardness (292.9 ± 3VHN) which increases to 60.9% in comparison to the base metal (182 ± 4VHN) owing to the homogenous dispersion strengthening mechanism accompanied with reduced grain size. Orowan strengthening and the RGO particulates surrounded by reduced oxygen content at five number of passes (withstand more load) are responsible for the enhanced tensile strength (758 ± 3MPa). Despite this, reduced ductility is obtained through fragmentation of fine grains and fractography that dictates the tensile failure mechanism as both ductile and brittle. Whilst, inherent toughness of both AA7075 and RGO particulates at 5 passes significantly improves the flexural strength (126.8 ± 1.4N/mm2) with reduced deflection (19 ± 1.1 mm). Furthermore, the variation of microstructure behavior in the bend sample is discussed.