Effects of cooling rate on microstructure and microhardness of directionally solidified Galvalume alloy.

The influences of cooling rate on the phase constitution, microstructural length scale, and microhardness of directionally solidified Galvalume (Zn-55Al-1.6Si) alloy were investigated by directional solidification experiments at different withdrawal speeds (5, 10, 20, 50, 100, 200, and 400 µm·s-1). The results show that the microstructure of directionally solidified Galvalume alloys is composed of primary Al dendrites, Si-rich phase and (Zn-Al-Si) ternary eutectics at the withdrawal speed ranging from 5 to 400 µm·s-1. As the withdrawal speed increases, the segregation of Si element intensifies, resulting in an increase in the area fraction of the Si-rich phase. In addition, the primary Al dendrites show significant refinement with an increase in the withdrawal speed. The relationship between the primary dendrite arm spacing (λ1) and the thermal parameters of solidification is obtained: λ1=127.3V-0.31. Moreover, as the withdrawal speed increases from 5 to 400 µm·s-1, the microhardness of the alloy increases from 90 HV to 151 HV. This is a combined effect of grain refinement and second-phase strengthening. [ABSTRACT FROM AUTHOR]