Modification of Al–Si alloy melts containing graphite particle dispersions

Abstract

The variation in the size of silicon in commercial eutectic and hyper-eutectic Al–Si alloys owing to the presence of graphite particles of average size 40 11m, dispersed by the UPAL process, has been studied. The presence of dispersions equivalent to 3 wt-% of graphite particles (in gravity die cast Al–Si–graphite castings) results in about 37% reduction in the size of eutectic silicon in LM 13 alloy and 44% reduction in the size of primary silicon in LM 28 alloy, indicating the occurrence of partial modification and refinement of silicon in Al–Si alloys. When the concentration of graphite particles was as high as 15 wt-%, complete modification of silicon was observed. Procedures for achieving complete modification in Al–Si alloys containing 3 wt-% graphite using conventional modifiers have also been established. Silicon in LM 13–3graphite composite melts can be completely modified using sodium in elemental form or in commercially available salt form (Coveral-29A). With the help of microscopic and thermal studies, an attempt has been made to understand the mechanisms of modification and refinement occurring in Al–Si because of the presence of dispersed graphite particles. Important mechanical and tribological properties of partially and completely modified cast Al–Si–graphite-particle composites have been studied in comparison with the unmodified base alloy.