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During the solidification process, microstructures are affected by the experimental conditions, the thermophysical characteristics of the alloy, and the type of grain-refining particles. Unidirectional solidification experiments were performed in a vertical Bridgman-type furnace to investigate the effect of the solidification front velocity on the solidified microstructure of a non-refined and refined Al-20%Cu alloy. The samples were solidified by rapidly increasing the sample velocity (v) range from 0.02 mm/s to 0.2 mm/s while maintaining an almost constant temperature gradient (~5 K/mm). As a result, despite changes in the solid/liquid front velocity along the sample, the microstructure of the non-refined alloys remained columnar. In the refined alloy, the columnar structure changed into an equiaxed structure at two different front velocities. [ABSTRACT FROM AUTHOR]

During the solidification process, microstructures are affected by the experimental conditions, the thermophysical characteristics of the alloy, and the type of grain-refining particles. Unidirectional solidification experiments were performed in a vertical Bridgman-type furnace to investigate the effect of the solidification front velocity on the solidified microstructure of a non-refined and refined Al-20%Cu alloy. The samples were solidified by rapidly increasing the sample velocity (v) range from 0.02 mm/s to 0.2 mm/s while maintaining an almost constant temperature gradient (~5 K/mm). As a result, despite changes in the solid/liquid front velocity along the sample, the microstructure of the non-refined alloys remained columnar. In the refined alloy, the columnar structure changed into an equiaxed structure at two different front velocities.

This paper provides insights into the effect of high thermal gradients and cooling rates on equiaxed grain nucleation and growth in conditions similar to those experienced during Additive Manufacturing (AM) processes. Bridgman type solidification is numerically simulated with columnar grains growing at a fixed pull rate under a user-imposed thermal gradient. Controlled inoculants of known nucleation undercooling were placed ahead of the growing columnar grains to allow quantitative analysis of nucleation events. At low thermal gradient and cooling rate only the inoculants with low nucleation undercooling were activated due to low melt undercooling driven by constitutional supercooling (CS). As the cooling rate is increased, for a given thermal gradient, a larger number of inoculants with higher nucleation undercoolings were activated. At higher cooling rates, thermal undercooling was generated by a lag in the growth rate of the solid-liquid (S–L) interface compared to the theoretical pull rate. Thus, thermal undercooling becomes dominant leading to the facilitation of nucleation on less potent substrates requiring higher undercooling. The results show a transition from solute-driven undercooling to cooling rate driven thermal undercooling which contributes to the undercooling that activates the nucleation events. Invoking the Interdependence model, it is also shown that the high cooling rate induced thermal undercooling reduces the size of the nucleation free zone substantially. Image, graphical abstract [ABSTRACT FROM AUTHOR]

A solidification benchmark experiment based on a Sn-10 wt% Pb alloy was performed at the SIMAP Laboratory in Grenoble, France, to investigate the formation of as-cast structure and macro- and mesosegregation (Wang et al., 2009; Hachani et al., 2015). The current study simulated this benchmark using a mixed columnar-equiaxed solidification model. This model was developed by the current authors (Wu et al., 2006), and recently extended to consider the crystal fragmentation of columnar dendrites as the source of equiaxed grains. A satisfactory experiment-simulation agreement in the temperature evolution, the as-cast structure, and the distribution of macro- and mesosegregation, was obtained. By analysis of the modelling results, new knowledge regarding the formation of the as-cast structure (mixed columnar-equiaxed) was obtained: The fragmentation-induced equiaxed crystals are transported away from the front region of the columnar zone by the melt flow; those crystals continue to grow or re-melt depending on the condition of local constitutional undercooling or supersaturation; the density of the melt varies with the Pb enrichment in the melt, and the equiaxed crystals can settle down or float up through the balance of the flow-induced drag force and the gravity-driven sedimentation; it is the motion of equiaxed crystals that determines the final distribution of the equiaxed zone in this benchmark experiment; the interdendritic flow and its interaction with the solidification in the front region of the columnar zone lead to the formation of segregation channels. [ABSTRACT FROM AUTHOR]

Solidification castings can exhibit a columnar or an equiaxed morphology or a combination of both. Since the relative proportions of these two components strongly influence the internal quality of cast product, the study of morphological transition from columnar to equiaxed structure (CET) becomes important. The transition also affects quality parameters like inclusion distribution in castings which has a significant bearing on the properties of cast products. In this work, a combined model for CET and inclusion distribution in continuously cast steel billets is presented. A conduction based transient thermal solidification model is employed in conjunction with Hunt's criterion for CET to predict the evolution of melt temperature, the location of transition and area-fractions of columnar and equiaxed zones across the billet cross-section. A correlation between melt temperature and equiaxed nuclei density is proposed and incorporated in the model to account for a more realistic variation of CET with melt superheat. The model is compared with available experimental data and is used to explore the effect of process parameters on CET and determine the spatial distribution of non-metallic inclusions in the solidified billet. [ABSTRACT FROM AUTHOR]

Highlights: [•] The magnetic field has caused the occurrence of the columnar-to-equiaxed transition (CET). [•] The value of the TEMF increases as the magnetic field intensity increases. [•] The maximal value of the force is of the order of 105 N/m3 under a 10 T magnetic field. [•] We report a new method to prepare the “aligned equiaxed grains” via an applied high magnetic field. [ABSTRACT FROM AUTHOR]

Abstract: The effects of strong magnetic fields on the columnar-to-equiaxed transition (CET) have been investigated experimentally. Six alloys have been directionally solidified at low growth speeds (1–10μms−1) under magnetic fields up to 10T. Experimental results show that the application of a strong magnetic field causes a dendrite fragmentation and then the CET. The thermoelectric magnetic force acting on cells/dendrites and equiaxed grains in the mushy zone has been studied numerically. Numerical results reveal that the value of the thermoelectric magnetic force increases as the magnetic field intensity and the temperature gradient increase. A torque is created on cells/dendrites and equiaxed grains. This torque breaks cells/dendrites and drives the rotation of equiaxed grains. The rotation of equiaxed grains in the mushy zone will further destroy cells/dendrites. Thus, with the increase of the magnetic field intensity and the temperature gradient, the volume fraction of equiaxed grains in front of columnar dendrites increases. When the magnetic field intensity and the temperature gradient reach a critical value, the growth of columnar dendrites is blocked and the CET then occurs. The present work may initiate a new method of inducing the CET via an applied strong magnetic field during directional solidification. [Copyright &y& Elsevier]

Abstract: The influence of a high magnetic field on the growth of MnBi, α-Al and Al3Ni dendrites in directionally solidified Bi–Mn, Al–Cu and Al–Ni alloys have been investigated. Results indicate that the magnetic field changes the dendrite growth significantly. Indeed, the magnetic field aligns the primary dendrite arm and the effect is different for different dendrites. For the MnBi dendrite, an axial high magnetic field enhanced the growth of the primary dendrite arm along the solidification direction; however, for the α-Al and Al3Ni dendrites, the magnetic field caused the primary dendrite arm to deviate from the solidification direction. At a lower growth speed, a high magnetic field is capable of causing the occurrence of the columnar-to-equiaxed transition (CET). Moreover, it has also been observed that a high magnetic field affects the growth of the high-order (i.e., secondary and tertiary) dendrite arms of the α-Al dendrite at a higher growth speed; as a consequence, the field enhances the branching of the dendrite and the formation of the (111)-twin planes. The above results may be attributed to the alignment of the primary dendrite arm under a high magnetic field and the effect of a high magnetic field on crystalline anisotropy during directional solidification. [Copyright &y& Elsevier]

Abstract: Cylindrical samples of a Sn-15 wt.% Pb alloy were exposed to a rotating magnetic field (RMF) during directional solidification. Results of the soldification experiments were compared with numerical predictions considering the temperature and the velocity field in the liquid phase. The direct impact of the local flow structure on the formation of the mushy zone is demonstrated. Consequences on the occurrence of the CET are discussed. [Copyright &y& Elsevier]

Abstract: An experimental study with respect to the influence of a rotating magnetic field (RMF) on the unidirectional solidification of Pb–Sn alloys is reported. The magnitude of the bulk flow in the melt generated by the RMF varies with the magnetic Taylor number Ta = σωB 2 R 4/2ρν 2. The forced convection causes distinct modifications of the temperature and concentration field such as a reduction of the temperature gradient ahead of the solidification front. Without electromagnetic stirring, the alloy solidifies solely in form of dendrites aligned parallel to the heat flow direction. In contrast, a transition from columnar-to-equiaxed growth (CET) is observed if the solidifying ingot is exposed to an RMF. The position of the CET is shifted towards the bottom of the casting by increasing the Ta number. The CET was found to occur for a cooling rate of about 0.4K/s and temperature gradients in the range between 0.6 and 1.0K/mm. [Copyright &y& Elsevier]