Influence of Mg and stirring on the strength of an aluminium matrix composite obtained by compocasting

A critical step in the processing of particle reinforced aluminium matrix composites is the insertion of the ceramic particles into the molten metal. This greatly influences the strength of the composite since it is controlled by the matrix-particle interfacial bond strength. It is known that Mg has a great influence on the confinement of the reinforcement within the matrix, possibly due to the formation of a reactive layer of MgAl₂O₃ at the particle-matrix interface. The propose of this work was to study the influence of both Mg additions and stirring speed on the mechanical properties of an Al-2%Si-xMg-15% vol. Al₂O₃ composite obtained by compocasting, as well as, to gain a better understanding about the way the reactive layer influences the strength; by means of metallografic and fractographic observations. It was found that the strength is maximum at 4.5 wt.% Mg content decreasing for higher contents. This effect is attributed to an enhanced wettability of the particles with Mg. At Mg contents of more than 4.5 wt.%, the Mg tends to combine with Si to form the intermetallic Mg₂Si, loosing it's effect on the wettability. In addition, the strength was higher at higher stirring speeds due to a better liquid-solid contact and microstructure refinement; nevertheless, the beneficial effect of Mg is lost at very high stirring speeds because it is oxidized.