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The new technology of intermetallic phases treatment by fluid flow in Al-Si casting alloys

Final Report Summary - IPHASEFLOW (The new technology of intermetallic phases treatment by fluid flow in Al-Si casting alloys)

Al-Si base alloys are widely used in the automotive and aerospace industry, where they have been steadily replacing many conventional ferrous alloys due to their excellent combination of properties like high strength-to-weight ratio, good corrosion resistance, weldability and minimum energy requirement for recycling.

Application of recycled aluminum alloys with even small amounts of iron causes formation of a rich variety of intermediate Fe-rich phases having morphologies described as polyhedral, Chinese script, needles, cubes, platelets. One important phase is the so-called ß-Al5FeSi phase. Its needles are hard and brittle and have a detrimental effect on castings: it causes porosity, promotes crack initiation, lowers fatigue life, but also reduces soldering to molds in especially high-pressure die casting.

The project "iPhaseFlow" aimed to understand the effect of fluid flow on the microstructure and intermetallics in Al-alloys with 5, 7 and 9 wt. % Si. These base materials are alloyed with Fe and Mn in amounts of 0. 2 to 1. 0 wt. %. Experimentally directional solidification was employed under well defined thermal and fluid flow conditions (rotating magnetic field, RMF). The microstructure with the iron intermetallic phases were studied using light microscopy and SEM with EDX, X-Ray tomography and numerical simulation.

The results showed that forced flow causes shortening of the platelets in dendritic and lengthening in eutectic regions with an increasing number density. The application of RMF to AlSi alloys could better shorten ß-Al5FeSi needles compared to variations of the Fe content. It can be deduced for directional solidification, that the melt loaded with ß particles transports these to the overheated bulk liquid where the intermetallics can be fragmented or are dissolved. This leads to microstructures with shorter ß-Al5FeSi and a higher number density.

X-Ray nano-tomography gave a 3D visualisation of the intermetallics and revealed complicated ß-Al5FeSi being structures formed within the mushy zone. These results resolve an open problem in the literature. It was claimed, that small amounts of intermetallics precipitating during solidification restore diffusive mass transport in the transition zone between the fully liquid and the fully solid state (the mush). This observation was not understandable, since the amount of intermetallics even in alloys with 1 wt. % Fe is that small, that the spongy structure of the Al-dendritic network is hardly being affected by them. The nano-tomography clearly shows for the first time, that the intermetallics grow between the dendrites a few millimeters behind the their tips almost transverse to the solidification direction and block fluid flow from the bulk into the mush. Thus even small amount of "dirt" can clog the mushy zone filter.

The Project results are relevant for secondary aluminum alloys, since these are always enriched with Fe and Mn. The project allowed to realise and visualise interactions between fluid flow, microstructure and intermetallics in AlSi alloys. It was clearly shown how dedicated fluid flow can change the morphology of intermetallics and how suitable electromagnetic stirring could improve the microstructure and thus the properties.

The results were presented at several international conferences and several scientific publications are currently being prepared to be presented in peer reviewed journals.

The project also included a list of training activities for the fellow which all were executed. Special emphasis was taken on networking the fellow to researchers in the area of solidification. He therefore also attended the internationally well recommended Solidification Course, organised by EPFL Lausanne, Calcom and ESI Group in 2010 and a course on Successful Presentations in English in 2011.

Therefore the project enhanced the European Union scientific excellence, since it shows European foundries how to manage fluid flow to yield better castings.