Microstructural features of primary and secondary ductile high pressure die casting alloys for the automotive industry

Primary AlSi10MnMg alloy is the most widely used alloy for manufacturing of vacuum assisted high pressure die castings (VPDC) with high ductility requirements. In this alloy, die soldering is avoided by a high Mn level (0.5 – 0.6 wt. %) while Fe is kept low (< 0.2 wt. %). Such combination guarantees that the Al-Fe-Mn-Si

intermetallic phases are of the α-Al15(FeMn)3Si2 polyhedral or Chinese script type which is least harmful to the ductility. But the use of a secondary alloy allows to significantly reduction of the cost of the castings as the raw material is cheaper and also requires less energy for their manufacturing.

On the other hand, secondary alloys contain a higher amount of Fe, a common impurity, which also reduces die soldering but is detrimental to the ductility due to the formation of needle/platelet shaped β-Al5FeSi phase. Microadditions based on Mn have been found to be effective in transforming the needle/platelet shaped β phases into Chinese script α- iron phases with a less harmful morphology even in relatively high Fe alloys.

In this work a secondary alloy with 0.60 % Fe and different Mn microadditions, has been cast in test parts with different wall thicknesses using VPDC technology. The effect of microadditions and wall thickness on the morphology, amount and size of intermetallic iron phases has been investigated using image analysis and compared to the corresponding AlSi10MnMg primary alloy. The results show that the cooling rate is an important factor to control the size and area fraction of the intermetallic iron compounds. The additions of Mn results in modification of the β-Al5FeSi compounds into a less harmful Chinese script and/or fine a-iron compounds. However, an increase in the area fraction of the intermetallic compounds is observed.