Microstructural characterisation of Al foam/steel interface formed during foaming process in argon flow

Bonding between a metal foam core and a metallic skin is a pre requisite for the technological application of aluminium foam as filling reinforcement material to improve energy absorption and vibration damping of hollow components. In many cases a metallurgical bond provides the best compromise between bond strength and cost effectiveness. Aim of this work was to characterise the composition and microstructure of the interface layer between a commercial PM precursor and a steel plate during foaming in argon flow. Inert atmosphere improves the formation of the intermetallic layer when molten aluminium come in contact with solid iron. Precursors tablets of two different compositions (AlMg1Si0.6 and AlSi10) were foamed on low carbon steel substrates. The microstructure of the intermetallic layer were characterised by electron microscopy, electron probe microanalysis and microhardness measurements on the cross section. X ray diffraction measurements performed on the foam/substrate surface after step wise removing of material allow the identification of the intermetallic phases. Two intermetallic layers, identified as Fe2Al5 and FeAl3, characterise the low Si foam/substrate while the AlSi10 foam/substrate interface evidences the presence of three Fe(Si, Al) intermetallic layer with different composition. Two and three different phases of increasing hardness could be distinguished going from the foam to the steel substrate for AlMg1Si0.6 and AlSi10 precursors respectively. The results suggest the importance of elemental diffusion from the steel substrate in the molten aluminium matrix (foam). The possibility to control and tailoring the microstructural properties of the interface between foam and mould is of fundamental importance in the technological process of foam filled structures manufacturing.