Microstructural observations on Fe-intermetallics in unmodified and Sr-modified Al-Si-Cu (A380.1) die casting alloy

Abstract

The present work was performed on an Al-9%Si-3.2 % Cu-1 % Fe-0.17 % Mn-0.04 % Mg (A380.1) alloy, in the unmodified and Sr-modified (350 ppm Sr) conditions. Three pouring temperatures, 750 °C, 720 °C and 620 °C, were used in each case. The liquid metal was poured into a hot graphite mould, cold metallic mould and cold water which produced castings whose microstructures exhibited average dendrite arm spacings (DASs) of ∼45 μm, ∼15 μm, and ∼5 μm, respectively. The microstructures were analysed using optical microscopy, and electron microprobe analysis (EMPA) coupled with energy dispersive X-ray (EDX) analysis. Tensile test bars were also prepared using a Stahl permanent mould (type ASTM 108-B), each Stahl mould casting providing two test bars. The test bars were tensile tested in the T6 condition (6–8 bars per composition). The results obtained show that the presence of Sr leads to precipitation of a large part of the iron intermetallics in the form of coarse pre-dendritic particles situated within the α-Al dendrites, instead of in the interdendritic regions. This is expected to homogenise the stress distribution across the alloy matrix, and hence lead to better tensile properties. In the alloy containing a higher Fe-content (1.4% Fe), but the same amount of Mn, addition of 0.08% Be together with 0.02% Sr is equivalent to the addition of 1% Mn (for an Mn/Fe ratio of 0.7) in terms of the tensile properties. The results are discussed in detail in the paper.