The evolution of microstructure and mechanical properties during high-speed direct-chill casting in different Al–Mg2Si in situ composites

The effect of high-speed direct-chill (DC) casting on the microstructure and mechanical properties of Al–Mg 2 Si in situ composites and AA6061 alloy was investigated. The microstructural evolution of the Al–Mg 2 Si composites and AA6061 alloy was examined by optical microscopy, field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The results revealed that an increase of the casting speed substantially refined the primary Mg 2 Si particles (from 28 to 12 μm), the spacing of eutectic Mg 2 Si (from 3 to 0.5 μm), and the grains of AA6061 alloy (from 102 to 22 μm). The morphology of the eutectic Mg 2 Si transformed from lamellar to rod-like and fibrous with increasing casting speed. The tensile tests showed that the yield strength, tensile strength, and elongation improved at higher casting speeds because of refinement of the Mg 2 Si phase and the grains in the Al–Mg 2 Si composites and the AA6061 alloy. High-speed DC casting is demonstrated to be an effective method to improve the mechanical properties of Al–Mg 2 Si composites and AA6061 alloy billets.