Improved Prevention of Die Soldering for Aluminum HPDC

Die soldering is a crucial part of aluminum high pressure die casting. It describes a casting defect, occurring when the aluminum component sticks to the steel surface of the die, thus hindering demolding. It is one of the most common reasons for poor casting quality and damages to the casting mold, decreasing its lifespan decisively. A widely used method for prevention is to add manganese to the alloy. However, manganese also contributes to the formation of intermetallic phases with aluminum, silicon and iron. Those phases are very brittle, which is why increasing its content in the material leads to a significant reduction in ductility. For structural casting parts, like used in the automotive industry, a sufficiently high ductility is necessary to ensure processability. Especially when using high strength alloys to achieve thin wall strengths and reduced component weights, this problem turns into focus. Therefore, an alternative for manganese to prevent die soldering could improve the ductility of the aluminum alloys and by that increase the lightweight potential.

The Mechanism Behind the Formation of Die Soldering

When the molten aluminum contacts the steel die strong interdiffusion takes place, rooting in the high affinity of aluminum and iron. This leads to the formation of different intermetallic layers at the interface. Responsible for the joint between the aluminum component and the steel die, in the case of Al-Si alloys, is the irregular shape of the β-Al5FeSi phase. This monocline phase grows in the shape of needles from the die surface into the aluminum alloy, creating a vast interface, consequently interlinking the two components. However, when adding manganese to the alloy, the shape of the interface changes. Manganese prevents the formation of the β-phase by occupying lattice sites of iron, promoting the creation of the α-Al15(Fe,Mn)3Si2 phase. In contrast to the β-phase, the α-phase does not grow in the shape of needles, but develops a Chinese script like morphology. This results in a smooth interface between the aluminum alloy and the die steel, which facilitates demolding.

Alternative Elements for Manganese

Based on the effect mechanism of manganese, other chemical elements noted in literature were found, exhibiting a similar behavior. Dipping experiments showed, that molybdenum, cobalt and chromium create the same kind of intermetallic interface and should therefore prevent die soldering. Especially chromium shows promising results. While molybdenum and cobalt, compared to manganese, achieve the same level of effect by nearly half of the addition, for chromium even less was sufficient. Furthermore, it does not only create a smooth interface between the aluminum alloy and the die steel, but also decreases the size of the intermetallic layers. With this unique behavior, chromium is favorable alternative for manganese to prevent die soldering and improve ductility likewise. First validations on various structural components from an automotive body of the AUDI AG confirmed the results.