Secondary Al-Si alloys Al7.1Si(1.5-x Mn )Fe(x Mn )Mn with x Mn = 0, 0.3, 0.375, 0.6, 0.75 at.% have been solidified with different cooling rates: 0.05 K/s, 1.4 K/s, 11.4 K/s and 200 K/s. In the ternary alloy with x Mn = 0 at.%, formation of the primary α h phase is suppressed upon higher cooling rates at the cost of formation of plate-shaped β and δ phase particles. In the quaternary alloys, with increasing Mn content, α c -phase particles with Chinese-script morphology form and replace the plate-shaped intermetallic particles. While the α c phase forms at intermediate cooling rates only, plate-shaped particles additionally form at low and high cooling rates. The β phase dominates after solidification with lower cooling rates and the δ phase dominates upon higher cooling rates in the plate-shaped particles. The kinetic effect in terms of solidification rate and the chemical composition effect on the phase selection of Fe-containing intermetallic particles in the alloys along the solidification path have been discussed.

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Crystal structure characteristics of coexisting  and δ phase were investigated in a purely intermetallic Al-Fe-Si alloy and in plate-shaped intermetallic particles formed in a model secondary Al7.1Si1.2Fe0.3Mn alloy. The βand the δ-Al-Fe-Si intermetallic phase are thermodynamic and crystallographic distinct phases despite some structural similarities. Nevertheless, both phases can appear severely intergrown within single plate-shaped particles after solidification of Fe-containing secondary Al-Si alloys under non-equilibrium conditions which is attributed to the good fit along the (001) planes of both phases. Thereby, few layers of the δ-like stacking sequences within the β phase can be considered as a typical planar defect of the β phase. The β phase exhibits polytype structures for which structural models have been formulated based on ordering of the constituting double layers being shifted by a/2 or b/2 displacements leading to 4 different double layer positions A, B, C, and D. An AB polytype with idealized Acam symmetry actually shows a monoclinic distortion towards A12/a1 symmetry. An ABCD polytype with idealized I4 1 /acd symmetry has been derived, too. Next to the ordered polytypes, stackings with non-periodic sequences of A, B, C and D double layers exist.

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In aluminium‐steel welding bonding occurs via intermetallic compound (IMC) formation. Therefore, the reaction layer thickness X is a key parameter controlling the bond strength. To enable prediction of the layer thickness in a real welding situation, a simple diffusion model has been developed. The model, which is isokinetic in nature, allows X to be calculated via the Scheil integral from knowledge of the weld thermal cycle. Its relevance to solid state butt welding of aluminium to steel is illustrated in two different case studies, representing best practice for friction stir welding (FSW) and hybrid metal extrusion and bonding (HYB), respectively.