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.