Formation of Heterostructure with Appearance of α-Al Phase in Hyper- and Eutectic Al-Si Alloys by Solidification at High Rate

Abstract

Normally, the microstructure of eutectic and hypereutectic Al-Si alloys consists of Al-Si eutectic or Al-Si eutectic + SiI, that is why they have good wear-resistance properties, but their ductility decreases somewhat, which can be improved by the appearance of non-equilibrium α-aluminum grains. In present work, a recrystallization and partial melting (RAP) technique was applied in the preliminary research to investigate the solidification behavior of the eutectics at high cooling rate. The results show that numerous α-Al dendrites appeared instead of the eutectic structure. In the followed researches, the combination of some technological procedures, based on the competitive growth of dendrites and eutectics, such as heterogeneous nucleation and high cooling rate, were applied: A413.0 and A390.0 alloys were melted in resistant furnace and poured into a mold, made of copper at one side and of steel at another, which is connected to the temperature measuring device via four K-class thermocouples. The various pouring regimes: gravity, via 450 tilt cooling slope and injection with Argon gas for 2 min. before pouring, were applied at different temperatures of 680, 650, 6300 C. The results show that the hetero-structure of eutectic and hyper-eutectic Al-Si alloys, consists of non-equilibrium α-Al grains, primary silicon and eutectics was obtained. Grain size varies with cooling rate, with minimum value of 10 μm when the specimen thickness is 5 mm. Non-dendritic grains were achieved with semi-solid treatment (pouring via cooling slope). The elongation of alloy is expected to be enhanced due to the appearance of α-Al grains.