Shuya Zhang

Magnetic field-assisted casting technologies have received extensive research attention. Applying a transverse magnetic field during directional solidification offers an effective, non-contact approach to control the microstructure of Cu-Ag alloys by modulating melt convection. However, a comprehensive quantitative understanding of its mesoscopic influence on solute-driven dendritic growth and the columnar-to-equiaxed transition (CET) is still lacking. In this study, a coupled phase-field and lattice Boltzmann (PF-LBM) model is employed to systematically investigate the effect of a transverse magnetic field on columnar dendrite evolution and CET kinetics during the directional solidification of Cu-Ag alloys. Results show that the field-induced Lorentz force disrupts the symmetry of the solute field ahead of the dendrite tip, driving tilted dendritic growth that intensifies with decreasing initial primary spacing. During the CET process, the magnetic field overrides the randomness of grain nucleation by selecting specific crystallographic orientations. More importantly, it effectively mitigates the thermodynamic suppression of equiaxed nucleation typically caused by high temperature gradients. Furthermore, an increased magnetic field intensity not only accelerates the occurrence of CET but also significantly diminishes the inhibitory effect of high nucleation barriers on the transition. Ultimately, this work provides fundamental insights into external field-induced orientation selection mechanisms of as-cast grains and establishes a theoretical framework for quantitatively controlling texture evolution during the subsequent processing of high-flexure Cu-Ag alloys.

Directional solidification technology is the core process for manufacturing single-crystal blades in aero-engines, but transverse grain boundaries caused by the competitive growth of polycrystals severely degrade blade performance. To gain a deeper understanding of polycrystalline competitive growth behavior, this study investigates the competitive growth of polycrystals during directional solidification under natural convection based on the phase field and lattice Boltzmann coupling model. By adjusting the solutal expansion coefficient, grain configuration, and pulling velocity, the influence of the flow field on polycrystalline competitive growth is analyzed. The results indicate that changes in the solutal expansion coefficient affect the dendritic competition process and outcome, particularly for dendrites with larger favorably oriented (FO) angles, which are more likely to be eliminated at higher solutal expansion coefficients. Additionally, grain configurations with greater orientation differences between adjacent dendrites are more sensitive to changes in the solutal expansion coefficient, whereas configurations with smaller orientation differences are less affected. It was also found that as the pulling velocity increases, the primary dendrite arm spacing decreases and the growth direction of the dendrites deflects towards the temperature gradient direction. This leads to a reduction in vortices at the dendrite tips and grain boundaries, thereby decreasing the overall flow field intensity. During dendrite growth, solute is rejected from the solid phase, creating a concentration gradient between the dendrite tips and the liquid region. This induces convection in the liquid phase. The interaction between the flow field and the solute concentration in the liquid phase causes the flow field strength and solute concentration to exhibit periodic fluctuations.