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

Abstract

The effect of high-speed direct-chill (DC) casting on the microstructure and mechanical properties of Al–Mg2Si in situ composites and AA6061 alloy was investigated. The microstructural evolution of the Al–Mg2Si 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 Mg2Si particles (from 28 to 12 μm), the spacing of eutectic Mg2Si (from 3 to 0.5 μm), and the grains of AA6061 alloy (from 102 to 22 μm). The morphology of the eutectic Mg2Si 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 Mg2Si phase and the grains in the Al–Mg2Si composites and the AA6061 alloy. High-speed DC casting is demonstrated to be an effective method to improve the mechanical properties of Al–Mg2Si composites and AA6061 alloy billets.