Coupling in situ synchrotron X-ray tomographic microscopy and numerical simulation to quantify the influence of intermetallic formation on permeability in aluminium–silicon–copper alloys

AbstractThe influence of the β-Al5FeSi intermetallic phase on permeability evolution during solidification in an Al–Si–Cu alloy with a columnar dendritic microstructure has been numerically studied at solid fractions between 0.10 and 0.85. The fluid flow simulations were performed on a semisolid microstructure extracted directly from a single solidifying specimen, enabling the first study of permeability variation on an individual microstructure morphology that is evolving in solid fraction. The 3-D geometries were imaged at the TOMCAT beamline using 4-D (3-D+time) in situ synchrotron-based X-ray tomographic microscopy. The results illustrate the major effect of intermetallic particles on flow blockage and permeability. Intermetallics that grow normal to the flow direction were found to have a greater impact on the flow field in comparison to intermetallics in the parallel flow direction. An analytical expression, based on the anisotropic Blake–Kozeny model, was developed with a particle blockage term that takes into account the effects of intermetallic particles on permeability. In the regime of primary-phase solidification, a good fit between the analytical expression and the simulation results is found

Laser-based Additive Manufacturing of Bulk Metallic Glasses: Recent Advances and Future Perspectives for Biomedical Applications

Bulk metallic glasses (BMGs) are non-crystalline class of advanced materials and have found potential applications in the biomedical field. Although there are numerous conventional manufacturing approaches for processing BMGs, the most commonly used like copper-mould casting have some limitations. It is not easy to manage and control the critical cooling rate, especially when the fabrication of complex BMG geometries is involved. Other limitations of these techniques include the size constraints, non-flexibility, and the tooling and accessories are costly. The emergence of additive manufacturing (AM) has opened another promising manufacturing route for processing BMGs. AM processes, particularly laser powder-bed fusion (PBF-LB/M) builds parts layer-by-layer and successively fused the powder-melted feedstocks using prescribed computer-controlled laser scanner system, thereby forming a BMGs part upon sufficiently rapid cooling to ensure the glass forming-ability. PBF-LB/M overcomes the limitations of the pre-existing BMGs processing techniques by not only improving the part size, but also produces exceptionally complex structures and patient-specific implants. This review article aims to summarise and discuss the mechanism of BMGs formation through PBF-LB/M for biomedical applications and to highlight the current scientific and technological challenges as well as the future research perspectives towards overcoming the pore-mediated microcracks, partial crystallisation, brittleness and BMG size constraint

Effects of iron-rich intermetallics and grain structure on semisolid tensile properties of Al-Cu 206 cast alloys near solidus temperature

The effects of iron-rich intermetallics and grain size on the semisolid tensile properties of Al-Cu 206 cast alloys near the solidus were evaluated in relation to the mush microstructure. Analyses of the stress–displacement curves showed that the damage expanded faster in the mush structure dominated by plate-like β-Fe compared to the mush structure dominated by Chinese script-like α-Fe. While there was no evidence of void formation on the β-Fe intermetallics, they blocked the interdendritic liquid channels and thus hindered liquid flow and feeding during semisolid deformation. In contrast, the interdendritic liquid flows more freely within the mush structure containing α-Fe. The tensile properties of the alloy containing α-Fe are generally higher than those containing β-Fe over the crucial liquid fraction range of ~0.6 to 2.8 pct, indicating that the latter alloy may be more susceptible to stress-related casting defects such as hot tearing. A comparison of the semisolid tensile properties of the alloy containing α-Fe with different grain sizes showed that the maximum stress and elongation of the alloy with finer grains were moderately higher for the liquid fractions of ~2.2 to 3.6 pct. The application of semisolid tensile properties for the evaluation of the hot tearing susceptibility of experimental alloys is discussed

Combined deformation and solidification-driven porosity formation in aluminum alloys

In die-casting processes, the high cooling rates and pressures affect the alloy solidification and deformation behavior, and thereby impact the final mechanical properties of cast components. In this study, isothermal semi-solid compression and subsequent cooling of aluminum die-cast alloy specimens were characterized using fast synchrotron tomography. This enabled the investigation and quantification of gas and shrinkage porosity evolution during deformation and solidification. The analysis of the 4D images (3D plus time) revealed two distinct mechanisms by which porosity formed; (i) deformation-induced growth due to the enrichment of local hydrogen content by the advective hydrogen transport, as well as a pressure drop in the dilatant shear bands, and (ii) diffusion-controlled growth during the solidification. The rates of pore growth were quantified throughout the process, and a Gaussian distribution function was found to represent the variation in the pore growth rate in both regimes. Using a one-dimensional diffusion model for hydrogen pore growth, the hydrogen flux required for driving pore growth during these regimes was estimated, providing a new insight into the role of advective transport associated with the deformation in the mushy region

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

AbstractTo 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

In situ quantification of the nucleation and growth of Fe-rich intermetallics during Al alloy solidification

AbstractReal-time in situ synchrotron X-ray tomographic microscopy was used to gain new insights into and quantify the nucleation mechanisms and growth kinetics of β-Al5FeSi intermetallics during solidification of an aluminium Al–7.5Si–3.5Cu–0.6Fe (wt.%) alloy. Three new insights were obtained. First, the plate-like β-intermetallics appeared to nucleate mainly on or near the primary aluminium dendrites and to a lesser extent off the oxide skin on the surface of the specimen. Second, for this alloy composition, β-intermetallic formation was largely complete before the formation of Al–Si eutectic. Third, the β-intermetallics formed via fast lateral growth, wrapping around and in between the primary dendrite arms. Further, the nucleation and growth dynamics of β-intermetallics were quantified as a function of undercooling in a functional form that could be easily used in microstructural simulations. The frequency of intermetallic interaction mechanisms, such as plate nucleation vs. impingement and branching, were also quantified