Refinement of primary silicon crystals in a hypereutectic Al-20%Si alloy by addition of metallic selenium

Abstract

Addition of Se was reported to have a stronger effect on refinement of primary silicon in hypereutectic Al-Si alloys than that of P. The present study was conducted to clarify effects of Se addition, holding time and superheating temperature on refinement of primary silicon in an Al-20%Si alloy. Grain size of primary silicon was determined. Changes of microstructures were observed with an optical microscope and an X-ray microanalyzer. When a melt was heated at 900°C for 15min, addition of 0.15% Se resulted in refinement of primary silicon. However, increase of Se up to 0.5% did not show a significant improvement of refining effect. Holding time at 900°C did not affect refining effect of Se. The finest silicon was obtained when 0.5%Se was added to a melt at 1, 200°C for 15min. Grey colored particles were present in primary silicon, at the interface of primary silicon with eutectic structures and in eutectic structures. These grey particles were identified to have Al and Se through a microanalysis by the X-ray microanalyzer. A definite break was observed in a cooling curve of a Se treated specimen when primary silicon started to crystallize. These results were interpreted similarly as the AlP nucleation theory in primary silicon refinement.