Hardness and Microstructural Characterization of Al/FA Composites Fabricated by Compo Casting and the Equal Channel Angular Extrusion

Fly ash (FA) is a low-cost industrial waste material mostly composed of oxides. These small, hard particles can be used as reinforcements in composite production. In this study, an A356.0 aluminum alloy reinforced with 4 wt.% FA was synthesized by compo casting and subsequently subjected to multiple passes of equal channel angular extrusion (ECAE) to investigate the influence of intense plastic deformation on the composite hardness and microstructure. Microstructure analysis was performed on an optical microscope and by computer tomography (CT). The as-cast alloy contains a relatively homogeneous microstructure with minor FA agglomerations and very low porosity. The severe plastic deformation induced by ECAE results in a directed structure and additional integration of FA into the matrix with the disappearance of pores. Vickers hardness measurement of aluminum/fly ash (Al/FA) composite was carried out with different indentation loads: 0.196 N (HV0.02), 0.490 N (HV0.05), 0.981 N (HV0.1), and 1.960 N (HV0.2). The results showed that hardness increases after each ECAE pass because of microstructure changes. Already after the first pass, a significant increase in hardness is achieved, ranging from 27% (HV0.05) to 62% (HV0.2). A Meyer’s index (n) value greater than 2 indicates that the hardness of single and double extruded composite depends on the indentation load. Extruded samples show a hardness enhancement with increasing applied load, so the examined composite exhibits a reverse indentation size effect (RISE).