Influence of Fe-rich intermetallics on solidification defects in Al–Si–Cu alloys

TLDR

The study investigates how Fe‑rich intermetallics influence solidification defects in Al–Si–Cu alloys by performing fast in situ synchrotron X‑ray tomographic microscopy on A319 alloy containing 0.2–0.6 wt % Fe. Fast in situ synchrotron X‑ray tomographic microscopy was used to observe solidification and semi‑solid deformation in the alloy, enabling real‑time tracking of defect evolution. β‑intermetallics promote porosity and hot tearing by blocking interdendritic channels, increasing pore tortuosity, and providing low‑energy surfaces for pore growth, and their abundance shifts semi‑solid failure from ductile to brittle with damage oriented perpendicular to loading.

Abstract

To better understand the influence of Fe-rich intermetallics on solidification defect formation, fast in situ synchrotron X-ray tomographic microscopy experiments were performed on a commercial A319 alloy (Al–7.5Si–3.5Cu, wt.%) with 0.2 and 0.6 wt.% Fe. Real-time observations during solidification and semi-solid deformation experiments reveal that β-intermetallics contribute via several different mechanisms to porosity formation and hot tearing susceptibility. While β-intermetallics were not observed to nucleate porosity directly, they do block interdendritic channels, thereby reducing the shrinkage feeding, and increasing pore tortuosity. Pores also grow preferentially along the surface of the β-intermetallics, suggesting that the β-phase has a lower gas–solid interfacial energy than α-Al, thus assisting in increasing pore volume. During uniaxial tension experiments, the ductile failure of the semi-solid, intermetallic-poor, base alloy transitions to a brittle-like failure when a large amount of β-intermetallics are present. In all post-failure microstructures, internal damage was preferentially orientated perpendicular to the loading direction, agreeing with prior experimental and numerical studies.