Structure refinement and improved mechanical properties of Al-20wt.% Si by rapid solidification in conjunction with strontium modification

Abstract

The effect of the addition of strontium on the morphology, microstructure and hardness of melt-spun AlSi ribbons as well as on the morphology, hardness and tensile properties (in both the longitudinal and the transverse directions) of the ribbon consolidates (hot extrusion) was investigated. Longitudinal sections of AlSi ribbons both without strontium and with strontium showed three zones: a chill zone at the wheel side with primary aluminium dendrites grown more or less perpendicular to the wheel surface, a central zone consisting of a submicroscopic eutectic with randomly distributed primary aluminium dendrites and a top zone of a submicroscopic eutectic with primary silicon crystallites. For the ribbons with strontium, the primary aluminium dendrites were coarser while, in the chill zone of the ribbons without strontium, silicon particles were identified. Optical microscopy and X-ray diffraction analyses indicate that the addition of strontium led to a much finer and more homogeneous structure with respect to grain size, preferred orientation, lattice defects and solid solubility. The mechanical properties of the consolidates were examined by tensile testing and hardness measurements. Chopping of the ribbons and subsequent extrusion eliminated the anisotropy of mechanical properties; that is, the transverse properties were substantially improved. Ultimately, structure refinement due to liquid quenching in conjunction with strontium modification led to both an increase of about 85% in tensile strength and an increase of about 300% in ductility compared with extruded conventionally cast material with a corresponding silicon content. However, it is probable that, because of the process of consolidation, about half of the innate strength improvement reached as a result of rapid solidification and strontium modification has not been utilized yet.