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JMEPEG https://doi.org/10.1007/s11665-020-04947-y

Multipass Friction Stir Processing of Steel/SiC Nanocomposite: Assessment of Microstructure and Tribological Properties Mostafa Fotoohi Nezhad Khales, Seyed Abdolkarim Sajjadi, and Ata Kamyabi-Gol (Submitted June 6, 2019; in revised form May 7, 2020) A surface layer of steel/SiC nanocomposite was fabricated by multiple passes of friction stir processing (FSP) on a low-carbon steel. The characterization methods such as optical microscopy, field emission scanning electron microscopy, microhardness, and wear test were performed to obtain the microstructure and mechanical properties of the produced layers. The results revealed that the dispersion of nano-SiC particles is directly related to the FSP passes and that the superior scattering of the nanosized SiC particles is achieved at higher FSP passes. Nano-composite surface layers demonstrated an average microhardness value of  424 HV, which is  3.4 times higher than that of the as-received substrate. Moreover, ultra-fine submicron grains were formed by FSP due to the role of SiC particles as obstacles to grain growth. The produced surface-layer nanocomposite exhibited an outstanding enhancement in wear resistance in comparison with the FSPed sample without reinforcing particles. Keywords

friction stir processing, plain low-carbon steel, silicon carbide, surface nano-composite, wear resistance

1. Introduction Most engineering failures are caused by surface-related processes such as wear (Ref 1, 2), fatigue (Ref 3, 4), corrosion (Ref 5), thermal stress (Ref 6), or a combination of these processes (Ref 7, 8). Having a good understanding and a further improvement of the tribological properties of surfaces can increase the life expectancy of industrial parts (Ref 9, 10). The production of surface nanocomposite layers (SNLs) is an effective method to improve the overall wear resistance and mechanical properties of the materials without any detrimental effects on the bulk properties of materials such as toughness and ductility (Ref 11-15). Friction stir processing (FSP) is a relatively new solid-state processing technique for the fabrication of SNLs. In fact, FSP contributes efficaciously to the dispersion of nanoparticles and the breakdown of the agglomerated clusters (Ref 16). Moreover, FSP can also be used to refine microstructural properties of a material surface (Ref 17). FSP is performed using the same method as friction stir welding (FSW). During FSP, a nonconsumable, which is in most cases, a cylindrical tool with a specific geometry comprising a shoulder and a pin, plunges into the surface of the base material (Ref 18, 19). The rotational spinning motion of the tool generates thermal energy by the friction between the tool and the base material. As the input thermal energy increases, and as the temperature of the stir zone Mostafa Fotoohi Nezhad Khales, Seyed Abdolkarim Sajjadi, and Ata Kamyabi-Gol, Department of Metallurgical and Materials Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad,