Effect of casting temperature on halo in the solidification microstructure of direct-chill casting Al-16.5Si-4.5Cu-0.6Mg-0.1Zr alloys

The effect of casting temperature on the halo structure of direct-chill (DC) casting Al-16.5Si-4.5Cu-0.6Mg-0.1Zr alloys was investigated. The results indicated that the microstructure consists of fine primary Si, alpha-Al dendrite halo, and fine fibrous coupled eutectic structure, Al2Cu, and Al8FeMg3Si6 phase. The formation of halos was systematically analyzed. The temperature gradient at the solid-liquid interface front played an essential role in the development and growth of the halo. As the casting temperature elevated, the temperature gradient of the solid-liquid interface front increased, the solid-liquid interface became stable, the alpha-Al dendrite halo was less developed, the number of large-sized Si atomic clusters in the melt decreased and the growth of primary Si was inhibited.