The Influence of controlled convection on the growth and the morphology of intermetallic phases in Al-cast alloys

Kurzfassung

The recycling of processed aluminum scrap is difficult. The elements iron and manganese are enriched to such an extent that during casting intermetallic phases such as Fe- Mn-aluminides appear leading to casting failures like porously and shrinkage holes. These intermetallic phases interfere with mechanical properties of Al-cast alloys. The aim of this work is to show how controlled convections can influence the growth and morphology of Fe- and Mn- rich intermetallic phases during solidification. The samples were directional solidified in an aerogel based furnace facility, which allows controlling and analyzing the solidification process with optical methods. A ternary Al-7wt.%Si-1wt.%Fe alloy and an Al-7wt.%Si-1wt.%Mn alloy were solidified over a wide range of solidification velocities (0.015- 0.18mm/s) at a constant temperature gradient G of 3K/mm. A coil system around the sample induces a rotating magnetic field with a magnetic induction of 3mT at a constant frequency of 50Hz. After processing the samples were metallographically prepared and evaluated in terms of the primary dendrite spacing and the secondary dendrite arm spacing. The microstructure of the samples and the spatial arrangement of the intermetallic phases are influenced by the rotating magnetic field in a unique manner. The microstructural parameters were also compared to parameters measured on pure binary Al-7wt.%Si alloys solidified under the same solidification conditions.