Steel–Aluminum Hybrid Die Casting: Microstructures related to the applied Al–Si bond coating

Abstract

The microstructure of a successfully applied Al–Si bond coating for the interconnection of cast aluminum alloy with a conventional steel substrate is analyzed in detail. The bond coating formed a 10–20 μm columnar Al layer which is attached to the steel substrate with an intermetallic layer (2–3 μm) during physical vapor deposition at approx. 853 K. Silicon grains precipitated in between the columnar Al layer and the intermetallic phases, giving rise to the ternary compounds on top of the IMC layer, whereas binary intermetallics were found in vicinity to the steel. Peculiarities of the crystallographic structural models of the observed intermetallics are discussed in light of the existing literature and are evaluated integrally using X-ray diffraction and locally by electron backscatter diffraction. The sequence of the phases of the bond coating and a resulting diffusion path is discussed on basis of the isothermal section of the ternary Fe–Al–Si phase diagram. After mechanical shear tests it was found that the applied bond coating did not detach from the steel substrate and instead failure was observed at the interface between columnar Al and cast Al or even within the microstructure the cast Al, proving the successful connection of the bond coating with the steel substrate.