On the Supplementation of Magnesium and Usage of Ultrasound Stirring for Fabricating In Situ TiB₂/A356 Composites with Improved Mechanical Properties

Adding mixed K₂TiF₆-KBF₄ salts into mechanical stirred molten A356 alloy (Al-7Si-0.3Mg) provides a simple method to fabricate in situ TiB₂/A356 composites via salts-metal reaction. However, some issues, such as the loss of Mg element in A356 alloy and the clustering of TiB₂ particles in the matrix, usually occur during the fabricating process. These issues easily result in the degradation of mechanical properties of composites. In this study, a quantitative analysis revealed that the Mg element was almost consumed after the reaction, and a thermodynamic analysis inferred that the loss of Mg resulted from the reaction between Mg and mixed salts. In order to fabricate in situ TiB₂/A356 composites with high strength and good ductility, we proposed a developed approach that consisted of supplementing Mg and using ultrasound stirring during the fabricating process. The introduction of ultrasound into the melt could disperse TiB₂ particles uniformly in the matrix, leading to more effective refinement of α-Al grains and modification of Si phase. Compared with the untreated A356, both the strength and ductility of the developed sample were improved significantly, and the coefficient of thermal expansion mismatch strengthening was found as the major contributor to the improvement in the yield strength.