Microtomographic characterization of columnar Al–Cu dendrites for fluid flow and flow stress determination

During the twin roll casting of Al alloys, the interdendritic liquid may flow as the two solidification fronts are compressed together between the rolls. This can lead to defects such as centerline segregation. To understand the flow properties of the interdendritic liquid, samples of Al–12 wt.% Cu were solidified directionally in a Bridgman furnace and quenched to capture the growing columnar dendritic structures. The quenched samples were scanned using a laboratory X-ray microtomography (XMT) unit to obtain the 3D structure with a voxel resolution of 7.2 μm. Image analysis was used to separate the Al dendrite from the interdendritic Al–Al₂Cu eutectic. Flow between the dendrites was simulated by solving the Stokes equation to calculate the permeability tensor as a function of the fraction solid. The results were compared to prior experimental measurements and calculations using synchrotron tomography observations of equiaxed structures. Elasto–plastic finite element (FE) simulations were performed on the dendritic structures to determine flow stress behavior as a function of fraction solid. It was found that the standard approximations for the reduction in flow stress in the semi-solid have a variation in excess of 100% from that calculated using the true structure. Therefore, it is critical to simulate the actual dendrite for effective flow stress determination.     

An X-ray microtomographic and finite element modeling approach for the prediction of semi-solid deformation behaviour in Al–Cu alloys

There is a dearth of published experimental measurements of flow stress behaviour of semi-solids, yet it is critical for simulating phenomena ranging from the processing of metals to the flow of magma. In this paper, a method for calculating flow stress behaviour of semi-solids was developed using a combination of high-temperature compression testing, X-ray microtomography (XMT) imaging and direct finite element modeling (DFEM). This novel methodology was applied to columnar dendritic structures in semi-solid Al–Cu alloys via first quantifying the complex geometry of the semi-solid using XMT. Then these three-dimensional datasets were meshed and their behaviour was simulated using DFEM to derive the stress–strain relationship with a fraction solid (f_S) dependency term. The mechanical behaviour of the solid dendrites near the liquidus temperature was not available in the literature; therefore, samples were fabricated and compression tested using a Gleeble 3500 thermomechanical simulator. The resulting XMT–DFEM-derived constitutive equation predicts the flow stress behaviour of semi-solid in the range of f_S equal to 0.1–0.9, showing good correlation to prior experimental data for both other aluminium and ferrous alloys.     

Solution Annealing of Super Duplex Stainless Steel: Correlating Corrosion Performance with Grain Size and Phase-Specific Chemistry

Metallurgical and Materials Transactions A - A super duplex stainless steel was subjected to solution annealing at 1323 K. This led to grain coarsening, ‘limited’ (~ 7 pct by...     

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