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 0.6wt.% 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.
Highlights
The excellent mechanical properties of Al–Si–Cu casting alloys have enabled this alloy family to find usage in many automotive applications, such as engine blocks and cylinder heads [1]
The influence of Fe-rich b-intermetallics on solidification defect formation has been quantitatively investigated using 4-D synchrotron X-ray tomographic microscopy
Two experiments were conducted in this work: an in situ solidification study of porosity formation and an in situ isothermal semisolid deformation experiment simulating hot tearing
Summary
The excellent mechanical properties of Al–Si–Cu casting alloys have enabled this alloy family to find usage in many automotive applications, such as engine blocks and cylinder heads [1]. Metallic compounds, including plate-like b-Al5FeSi intermetallics, which have been shown to act as crack initiators and are detrimental to in-service mechanical properties [2,3,4,5,6] In addition to their negative effects on the mechanical properties of the final product, b-Al5FeSi intermetallics are reported to be deleterious to castability, increasing the as-cast porosity content [7,8,9,10]. Many mechanisms by which b-Al5FeSi intermetallics affect pore formation have been suggested, including blocking of interdendritic flow [10,11], acting as nucleation sites [12] and aiding pore growth [13] They may influence pore growth indirectly, e.g. inducing larger Al–Si eutectic grains that reduce feeding and increase porosity [14].
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