Concurrent effects of various B additions on grain refinement, Fe intermetallics morphologies, and ductility evolution of Al-7.5Si-0.55 Mg (A357) cast alloy

Abstract

Simultaneous effects of boron (B) additions on grain refinement, morphological changes in Fe-intermetallic compounds, and consequent variations in mechanical responses of the cast Al-7.50Si-0.55 Mg (A357) alloys are evaluated in this research work. The contents B are varied from 0.002 to 0.40 wt. % and subsequent changes in the grain size and Fe-intermetallics morphologies are assessed through polarized light images, optical micrography, and scanning electron microscopy. A small addition of B (0.04 wt. %) yields identical refinement result (620 µm) as noted for commercial grain refiner Al-5Ti-1B (642 µm); whereas, the higher amount (0.40 wt. % of B) reduces the grain size up to 44% (360 µm). In as-cast state, the structure of the π-Fe phase is transmuted from plate/needle-type to Chinese-script particles for increased quantities (0.10 ~ 0.40 wt. %) of B. Contrarily, the heat-treatment (T6) causes the depletion of Mg and B from the π-Fe phase Chinese-script morphologies and disintegrates them by creating segregated regions of Fe particles. These agglomerated areas of Fe particles act as stress concentration points during tensile loading and considerably decrease the ductility values from 6.7 to 1.8%. Therefore, it implies that great care must be taken while developing B containing grain refiners.