Microstructure and Mechanical Properties of an In Situ Al 356-Mg2Si-TiB2 Hybrid Composite Prepared by Stir and Cooling Slope Casting

In this work, cooling slope (CS) casting process has been used successfully to fabricate in situ Al 356-10%Mg2Si-3%TiB2 hybrid composite. The Mg2Si particles were formed by adding excess Mg into the Al 356 alloy, and potassium fluorotitanate (K2TiF6) and potassium fluoroborate (KBF4) salts were used to produce in situ TiB2 particles. The formation of the dendritic structure of primary Al and Mg2Si phase, with conventional casting, is always a concern due to the deterioration of mechanical properties. The CS casting process favours dendrites fragmentation, leading to the spherical and non-dendritic structure formation. The TiB2 particles used as reinforcement also act as the heterogeneous nuclei for the matrix and Mg2Si phase, also contributing to grain refinement; otherwise, coarse Mg2Si particles are detrimental to ultimate mechanical properties. The effect of CS casting process leads to the refinement of matrix and Mg2Si phase and the uniform distribution of reinforced particles enhances the mechanical behaviour of Al 356-Mg2Si-TiB2 hybrid composite. X-ray diffraction (XRD) was used for phase identification and microstructural features were examined by optical microscope (OM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results demonstrated that refined Mg2Si and TiB2 particles are homogeneously dispersed in the composite using the CS casting technique. This casting route improved the tensile strength, ductility and hardness of hybrid composite by 11%, 13%, and 20%, respectively, as compared to the stir casting. While tensile strength of the hybrid composite increased by 68% and 23% compared with the Al 356 alloy and Al 356-Mg2Si composite, respectively. Mechanism of refinement of Mg2Si phase and alloy matrix has also been discussed.