Microstructural evolution and hardness of rapidly solidified hypereutectic Al-Si surface layers by laser remelting

Selective laser melting technique is used to manufacture Al-25 wt.%Si and Al-30 wt.%Si surface layers with a thickness of 0.5–0.6 mm followed by a laser rapid solidification process to refine the silicon to a nano-size and increase the hardness. The as-produced alloyed layers were heterogeneous and composed of clustered aggregates of primary Si crystals of size 5–7 µm, decorated with α-Al cells/dendrites, and embedded in a matrix consisting of α-Al dendritic crystals and Al-Si eutectic. However, after several remelting scanning treatments at a fast speed and low power, significant refinement of all phases was achieved, and a dramatic increase in the proportion of the interwoven nano-sized Al-Si fibrous eutectic. The primary Si sizes were reduced to ≤1 µm, while the eutectic spacing decreased and approached as low as 10–15 nm at the highest cooling rate. The high cooling rate significantly reduced the size and the amount of the α-Al cells/dendrites especially those surrounding the primary Si, thereby stimulating ultrafine Al-Si fibrous eutectic to grow adjacent to the primary Si crystals and spread massively, which in turn contributed to the increase in hardness. Transmission electron microscopy revealed block-like primary Si, worm-like Si, and nanoscale fibrous eutectic Si, which was internally nano-twinned.