The influence of Cr content on the microstructure and mechanical properties of AlSi9Cu3(Fe) die-casting alloys

The effect of chromium concentration on the microstructure and mechanical properties of a high-pressure die-cast AlSi9Cu3(Fe) alloy is reported. The Cr content ranges within the composition tolerance limits of the standard EN AB-46000 alloy, from 0.057 up to 0.15 wt.%. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring with Cr addition. The results indicate that the morphology of the primary and proeutectic α-Alx(Fe,Mn,Cr)ySiz intermetallics are mainly polyhedral, star-like and blocky. The area fraction and the size of intermetallic compounds increases as the Cr content increases. At the holding temperature used for the different EN AB-46000 type alloys, sludge forms mainly in the shot sleeve before the melt entered the die cavity and segregates in the central region of the cross-section of the castings. The entrapped sludge, whose value can be estimated from the sludge factor by a linear relationship, affects the ultimate tensile strength and the ductility of the alloy at the highest Cr level.