Evolution of sludge particles in secondary die-cast aluminum alloys as function of Fe, Mn and Cr contents

The evolution of sludge particles in a secondary high-pressure die-cast AlSi9Cu3(Fe) alloy has been investigated over three levels of iron (0.80, 1.00, 1.20 wt.%) and manganese (0.25, 0.40, 0.55 wt.%), and two levels of chromium (0.06, 0.10 wt.%). Metallographic and image analysis techniques have been used in order to quantitatively evaluate the morphological and dimensional variations of sludge with different Fe, Mn and Cr contents. The results indicate that any increase of the Fe, Mn and Cr level promotes the formation of coarser sludge, with polyhedral and star-like morphologies. Fe and Mn produce an increase on the number of primary Fe-rich particles, while Cr promotes the formation of a large number of secondary Fe-rich particles. No relationship is revealed between the morphological variation of sludge and the Fe, Mn or Cr contents, as well as between the sludge factor and the Fe:Mn ratio. In the analyzed range of composition, the design of experiment methodology and the analysis of variance have been used in order to develop statistical models that accurately predict the average size and number of sludge particles in the AlSi9Cu3(Fe) diecasting alloys.