Influence Of The Refining Element On The Solidification Characteristics Of Hypereutectic Al-Si Alloy

Hypereutectic Al-Si alloy is an important casting alloy and widely used in aerospace, automobile and electronic packaging industry. The mechanical properties of hypereutectic Al-Si alloy mainly depend on the morphology and the size of primary silicon, which is influenced by the solidification behavior. In this paper, Al-70%Si alloy was highly undercooled by an electromagnetic levitation combined with a laser heating unit. Employing a high-speed camera(HSC), solidification behaviors of Al-70%Si alloy with different undercoolings during the solidification process were investigated. The scanning electron microscope(SEM) was used to examine the surface morphologies of the as-solidified alloys. The effects of undercooling, refining elements of mischmetal and phosphorus on the growth mode of the primary silicon were studied. The main results are as follows:(1) The solidification behavior and microstructure of Al-70%Si alloy depend on the undercooling. When the undercooling is less than 122 K, the primary silicon grows in coarse stripe morphology, which shows the facet growth characteristic. As the undercooling is in the range of 122 K and 230 K. the primary silicons are partly coarse strip and lump with edges, sharp corners and re-entrants, and partly dendrite and spherulite with smooth surface. The growth mode presents both characteristic of facet and characteristic of no-facet. When the undercooling increases to 230 K. the primary silicon grows in non-facet manner with smooth rod-like or spherical bulge in shape. So it can be determined that the primary silicon has three growth mode, facet growth mode, intermediate growth mode and no-facet growth mode in the Al-70%Si alloy. The critical undercoolings of growth mode transition from facet mode to intermediate mode ΔT1 and from intermediate mode to no-facet mode ΔT2 for primary silicon are 122 K and 230 K, respectively.(2) The refining element RE has a remarkable effect on solidification process and micro structure of Al-70%Si alloy. The critical undercoolings ΔT1 and ΔT2 are 104 K and 212 K respectively for the Al-70%Si-0.5%RE alloy. The critical undercoolings ΔT1 and ΔT2 are 80 K and 200 K respectively for the Al-70%Si-1.0%RE alloy, while they are 65 K and 190 K for the Al-70%Si-1.5%RE alloy. The Energy dispersive spectrometer (EDS) results indicates that RE element mainly distributed along the grain boundaries. The refining element RE reduces critical undercoolings ΔT1 and ΔT2, because the enrichment of RE element in front of the solid-liquid interface restricts the growth of primary silicon.(3) The refining element P can accumulate the nucleation of the primary silicon, thus decreases the the critical undercoolings ΔT1 and ΔT2. The critical undercoolings ΔT1 and ΔT2 are 92 K and 206 K respectively for the Al-70%Si-0.5%P alloy, while they are 56 K and 179 K respectively for the Al-70%Si-1.0%P alloy.