Solidification Rate and the Partial Modification of 319 Aluminum Alloy with Strontium

Abstract

Trace strontium (Sr) additions to Al–Si alloys can transform coarse, acicular eutectic Si particles into a fine, fibrous morphology. Much research has focused on the production of well-modified structures, given their superior material properties. However, the partial modification of Al–Si alloys has received minimal attention, with little data available on the role of solidification rate. In this study, the partial modification of industrial 319 Al alloy was investigated by producing castings with Sr contents up to 300 ppm and total solidification rates between 0.14 and 5.89 °C s⁻¹. Eutectic Si growth temperatures ($T_{\text{G}}$) and median particle morphologies were characterized using thermal analysis and quantitative image analysis. This revealed synergistic modification effects between Sr and solidification rate on either feature. At intermediate Sr contents, a gradual Si transformation occurred only at slow solidification rates. Yet, at 3.03 °C s⁻¹ and higher, the transformation was drastic with no appreciable partial modification range. Furthermore, an unusual distribution of fine Al₂Cu particles was observed in the eutectic regions of the Sr-containing alloys. Their presence was also influenced by the combination of Sr and solidification rate. The knowledge and alloy-specific $T_{\text{G}}$ and modification data presented will enable effective alloy processing and property control of 319 Al components.