Reaction effect of fly ash with Al–3Mg melt on the microstructure and hardness of aluminum matrix composites

Highlights: • In situ synthesis of Al matrix composites formed by reaction of Al–3Mg melt with fly ash. • A quasi-dendritic structure of fly ash particles formed by the microstructure evolution observed in decomposition reaction. • XPS used to analyze the change of bonds between Al and Si with O-atom on the composite surface. • The hardness and density of Al matrix composites increased with increasing the reaction time of Al–3Mg melt with fly ash. During the reactions between fly ash and Al–3Mg melt, fly ash particles progressively disintegrate in the in-situ-generated reinforcement of composites. In this study, Al–3Mg/5 wt.% fly ash composites were prepared through stir casting with reactions at 850 °C for various durations (0, 10, 20, 30, and 40 h). The effects of the reactions on the microstructure and hardness of the composites were investigated. The experiment indicated that after a 30-h reaction most of the fly ash particles were decomposed because of a reduction reaction, in situ-generated θ-Al₂O₃, and MgAl₂O₄. In reduction reactions, the fly ash released the Si in the melt. A sufficient quantity of Si formed an Mg₂Si phase with the Mg of the melt for precipitation strengthening. Microstructural observations revealed that after the wall of fly ash particles cracked, molten metal infiltrated the pores and cracks in the nonsolid fly ash particles. Increasing the reaction time reduced the porosity in the Al matrix composites, whereas the density and BHN were increased. After a 40-h reaction, the composite exhibited a maximal hardness of BHN 70.74, which was an improvement of 46.7% over that of unreinforced alloys.