Microstructural characterization of PM-Al and PM-Al/Si composites reinforced with lignite fly ash

In the present paper, two Class C, lignite fly ashes (FAs) were utilized for the fabrication of Al/fly ash metal matrix composites (MMCs) using the powder metallurgy (PM) technique. Aluminum-fly ash and aluminum/silicon alloy (Al-12% Si)-fly ash mixtures, containing 5, 10 and 15 wt.% of - both pozzolanic and hydraulic - FAs were prepared and compacted. The green products were tested for their thermal behavior in order for the sintering temperature to be determined. Then, they were sintered for 2 and 6 h. The density of composites was decreasing with the increase of the percentage participation of fly ash particles into them. Apart from that, although FA clusters had been developing in the metal matrix, there was a significant enhancement in the hardness of MMCs when increasing FA content, which is mainly attributed to the intense calcareous nature of the reinforcement material. Volume and mass changes throughout calcination were also estimated as a function of increasing FA weight percent and it seems that CaO (%) of FAs plays a crucial role regarding the post-sintering raise in the mass and volume of composites. Microscopic studies of green and sintered compacts clearly indicated the effectiveness of the sintering process. Additionally, the composites were characterized in terms of their chemical composition and mineralogical structure. The rise of Ca-Si- and Si-mineral phases is intense, particularly for the composites with the highest percentage presence of CaO in FAs (15 wt.%) and does not differentiate as a function of the sintering time.