Directional Solidification of AlSi7Fe1 Alloy Under Forced Flow Conditions: Effect of Intermetallic Phase Precipitation and Dendrite Coarsening

A forced flow was experimentally shown to influence the solidification microstructure of metal alloys by modifying the coarsening/ripening law. In some technical alloys (AlSi7Fe1), this flow effect can also be significantly suppressed due to the formation of intermetallic precipitates (β-Al5FeSi) that can block the flow in the mushy region. The forced flow was induced by a rotating magnetic field (RMF). Herein, a three-phase volume-average-based solidification model is introduced to reproduce the above experiment. The three phases are the melt, the primary solid phase of columnar dendrites, and the second solid phase of intermetallic precipitates. The dynamic precipitation of the intermetallic phase is modelled, and its blocking effect on the flow is considered by a modified permeability. Dendrite coarsening, which influences the permeability, is also considered. The RMF induces a strong azimuthal flow and a relatively weak meridional flow (Ekman effect) at the front of the mushy zone during unidirectional solidification. This forced flow reduces the mushy zone thickness, induces the central segregation channel, affects the distribution of the intermetallic precipitates, and influences dendrite coarsening, which in turn modifies the interdendritic flow. Both interdendritic flow and the microstructure formation are strongly coupled. The modelling results support the explanation of Steinbach and Ratke—the formed intermetallic precipitates (β-Al5FeSi) can block the interdendritic flow, and hence influence the coarsening law. The distribution of β-Al5FeSi is dominantly influenced by the flow-induced macrosegregation. The simulation results of the Si and Fe distribution across the sample section are compared with the experimental results, showing good simulation–experiment agreement. Graphic Abstract During alloy solidifications the flow can influence the mushy zone by inducing macrosegregation, modifying the solidification microstructure, and influencing the formation of intermetallic precipitates. The resulting microstructural features can in turn affect the melt flow by changing the flow intensity and flow pattern. A three-phase volume-average-based solidification model is introduced to study the flow-solidification interaction, and hence to improve the knowledge on the formation mechanism of intermetallics and their effect on solidification. (a) Schematic for the flow pattern and formation of different phases; (b) experiment–simulation comparison of macrosegregation (Fe) across the diameter of as-solidified sample.

Characteristics, Mechanism and Criterion of Channel Segregation in NbTi Alloy via Numerical Simulations and Experimental Characterizations

Channel segregation (CS) is the most typical defect during solidification of NbTi alloy. Based on numerical simulation and experimental characterizations, we deeply elucidated its characteristics, formation mechanism, effecting factor and prediction criterion. According to acid etching, industrial X-ray transmission imaging, 3D X-ray microtomography and chemical analysis, it was found that in a casing ingot, by He cooling, finer grain size, weaker segregation and slighter CS can be obtained compared with air-cooled ingot. The simulation results of macrosegregation show that CS is caused by the strong natural convection in the mushy zone triggered by the thermo-solutal gradient. Its formation can be divided into two stages including channel initiation and growth. In addition, due to the stronger cooling effect of the He treatment, the interdendritic flow velocity becomes smaller, consequently lowering the positive segregation and CS and improving the global homogenization of the final ingot. Finally, to predict the formation of CS, the Rayleigh number model was proposed and its critical value was found to be 15 in NbTi alloy for the first time. When it is lower than the threshold, CS disappears. It provides an effective tool to evaluate and optimize the solidification parameters to fabricate the homogenized NbTi ingot in engineering practice.

Porosity Development and Modification in Al-Si Alloys: Effect of P and Sr

The benefits of Sr additions to Al–Si alloys to modify the eutectic are often impaired by the development of porosity, sometimes to the degree that benefits are negated. Experimental reports are reviewed in this paper, suggesting an explanation in terms of the oxide population in the melt. The unmodified silicon particles are nucleated by AlP, which has in turn nucleated on oxide bifilms. The oxide bifilms, which are essentially cracks, are straightened by the crystalline growth of Si particles, leading to increased crack size and consequently reduced mechanical properties. The addition of Sr improves properties by suppressing the formation of Si on bifilms and thereby preventing the straightening of the pre-existing cracks. Si is now forced to precipitate at a lower temperature as a coral-like eutectic. Unfortunately, the bifilms are now freed (the primary Si particles no longer exist to grow around and sequester the bifilms), remaining in suspension in the liquid metal, allowing them to act to block interdendritic flow and aid the initiation of the formation of pores, countering the benefits of the improved structure.

Influence of Crystal Morphological Parameters on the Solidification of ESR Ingot

Electroslag remelting (ESR) is an advanced process to produce high quality steel. During the ESR process, the steel electrode is melted and then solidified directionally in a water-cooled mold. The quality of the ingot is strongly dependent on the shape of melt pool, i.e. the depth and thickness of mushy zone, which is in turn influenced by the bulk and interdendritic flow. Here, we perform a numerical study to investigate the effect of crystal morphological parameter such as primary dendrite arm spacing on the solidification of the ESR ingot ( 750 mm). The crystal morphology is dominantly columnar and dendritic, thus a mixture enthalpy-based solidification model is used. Accordingly the mushy zone is considered as a porous media where the interdendritic flow is calculated based on the permeability. The permeability is determined as function of the liquid fraction and primary dendrite arm spacing according to Heinrich and Poirier [Comptes Rendus Mecanique, 2004, pp. 429-44]. The modeling results were verified against experimental results.

Numerical Simulation of Macrosegregation Formation in Binary Alloy Solidification Processing Based on Modified Projection Method

A simplified mathematical model for describing the solidification processing of fluid flow, heat and solute transfer in binary alloys is developed. The conservation equations are numerically solved on a staggered grid by using the control volume-based finite difference method. As the interdendritic flow in the mushy zone is governed by Darcy’s law, the Modified Projection Method is presented for solving the momentum and continuity equations. The solidification processing of Fe-C alloy in rectangular domains is simulated and discussed. The numerical result shows that, by using the Modified Projection Method technique, the presented model is able to predict the macrosegregation formation effectively.