Ultra-refined primary and eutectic silicon in rapidly solidified laser produced hypereutectic Al–Si alloys

Abstract

ABSTRACT A series of laser-produced Al–Si surface layers of composition 35, 50, and 60 wt%Si was prepared and analysed for microstructure and hardness. Also, laser remelting at different scanning speeds was performed to obtain nano-sized silicon crystals. The results indicated that all the laser fabricated Al–Si layers consisted of fine primary silicon (PSi) crystals of average size 10 µm, surrounded by α-Al cells/dendrites and distributed in dendritic structure of α-Al and Al–Si eutectic. The results of laser remelting at speeds of 10 and 50 mm/s showed slight microstructural refinement. However, a significant reduction of the amount and size of the PSi to <1 µm as well as a favourable change in the morphology of the eutectic have resulted in the sample processed at a speed of 120 mm/s. The high cooling rate led to a dramatic reduction in the amount of the α-Al dendrites and triggers the eutectic silicon fibres to grew from the existing PSi crystals and ramify around its perimeter. The nanoindentation hardness increased as the interlamellar eutectic spacing decreased. The refinement of the PSi crystals and the modification of the eutectic silicon were attributed to the high density of twins which facilitate rapid and directional growth.