The role of Cr additions and Fe-rich compounds on microstructural features and impact toughness of AlSi9Cu3(Fe) diecasting alloys

The effect of chromium content on the microstructural evolution of primary and secondary Fe-rich compounds in a high-pressure die-cast AlSi9Cu3(Fe) alloy is reported and then addressed to the impact toughness of the material. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring at different Cr levels. The results reveal that the salient microstructural features of diecastings, such as segregation bands and externally solidified grains, are not affected by Cr content, which contrary promotes the formation of primary sludge particles and proeutectic α-Alx(Fe,Mn,Cr)ySiz intermetallic compounds. Any increase in the amount of these Fe-rich particles is coupled with an increase in their size. Upon increasing the Cr level, the modification of sludge morphology from more or less well-formed hexagon to branched or star-like shapes occurs. The polyhedral sludge is associated to a regular rhombic dodecahedron, while the star-like and branched morphologies originate from hollowed rhombic dodecahedron. On the other hand, the morphology of secondary Fe-rich particles seems to not be influenced by Cr content and mainly determined by high cooling rate of diecasting. The impact toughness and the maximum load of the diecast AlSi9Cu3(Fe) alloy decrease as the Cr level exceeds 0.12 wt.%.