Effects of individual and combined additions of Be, Mn, Ca and Sr on the solidification behaviour, structure and mechanical properties of Al-7Si-0.3Mg-0.8Fe alloy

The effects of individual and combined additions of Be, Mn, Ca and Sr on the solidification, structure and mechanical properties of Al-7Si-0.3Mg-0.8Fe alloy has been investigated. Thermal analysis study shows that all additions except Be show a peak corresponding to β-Fe intermetallic phase formation. Ca and Sr additions decrease the eutectic temperature (by 6.6 and 8.7 K, respectively) compared to the untreated alloy and lead to modification of eutectic Si from platelet to fibrous form. The peak corresponding to Be-Fe phase has been identified by DTA. Mn (0.4%) and Be (0.2%) additions to Al-7Si-0.3Mg-0.8Fe alloy change the morphology of platelet β-phase to script form leading to significant improvement in tensile and both tensile and impact properties, respectively. Addition of Be, Ca and Sr to Mn added Al-7Si-0.3Mg-0.8Fe alloy improves the impact strength significantly due to the modification of the eutectic Si from acicular to fibrous form and refinement of platelet β-phase in addition to its morphological change from platelet to Chinese script form. Combined additions of Be + Mn, Ca + Mn and Sr + Mn result in a reduction in the impact strength compared to individual additions of Be, Ca and Sr. The best combinations of both tensile and impact properties have been obtained with the combined additions of Be + Mn, Ca + Mn and Sr + Mn to Al-7Si-0.3Mg-0.8Fe alloy. A trace amount of Be (0.005%) addition to Ca and Sr leads to superior tensile properties compared to individual additions of Ca and Sr.