Formation mechanism of axial macrosegregation of primary phases induced by a static magnetic field during directional solidification

Abstract

Understanding the macrosegregation formed by applying magnetic fields is of high commercial importance. This work investigates how static magnetic fields control the solute and primary phase distributions in four directionally solidified alloys (i.e., Al-Cu, Al-Si, Al-Ni and Zn-Cu alloys). Experimental results demonstrate that significant axial macrosegregation of the solute and primary phases (i.e., Al2Cu, Si, Al3Ni and Zn5Cu phases) occurs at the initial solidification stage of the samples. This finding is accompanied by two interface transitions in the mushy zone: quasi planar → sloping → quasi planar. The amplitude of the macrosegregation of the primary phases under the magnetic field is related to the magnetic field intensity, temperature gradient and growth speed. The corresponding numerical simulations present a unidirectional thermoelectric (TE) magnetic convection pattern in the mushy zone as a consequence of the interaction between the magnetic field and TE current. Furthermore, a model is proposed to explain the peculiar macrosegregation phenomenon by considering the effect of the forced TE magnetic convection on the solute distribution. The present study not only offers a new approach to control the solute distribution by applying a static magnetic field but also facilitates the understanding of crystal growth in the solute that is controlled by the static magnetic field during directional solidification.

Improvement in creep life of a nickel-based single-crystal superalloy via composition homogeneity on the multiscales by magnetic-field-assisted directional solidification

“…The appropriate magnitude of TEMC results in a stability in the liquid-solid interface 27 . However, the strong TEMC could modify the liquid-solid interface shape and the dendrite morphology, or induce the macrosegregation 22 – 24 , 45 , 46 . The two effects compete.…”

Magnetic Effects on Microstructure and Solute Plume Dynamics of Directionally Solidifying Ga-In Alloy

“…These currents interact with an externally applied magnetic field, generating a thermoelectric Lorentz force (TEF) that drives flow through a phenomenon known as thermoelectric magnetohydrodynamics (TEMHD). The TEMHD effect, also known as thermoelectric magnetic convection (TEMC), has been shown to introduce many changes to the microstructure, including macrosegregation [5][6][7] that leads to a tilted interface, 8 the formation of solute pockets, 6,9 banded structures, 10 modification to the primary arm spacing, 11 and the formation of stray grains in single-crystal castings. 12 Most of the cited studies were carried out on alloys where the solute density was higher than in the bulk liquid, hence solute plumes do not spring from the interface; however, similar changes are also observed in systems where the solute is the lighter element.…”

Influence of a transverse static magnetic field on the orientation and peritectic reaction of Cu-10.5 at.% Sn peritectic alloy

“… 13 modeled the mechanism of magnetic field induced macrosegregation and dendritic refinement that demonstrated that static magnetic fields lead to a large scale flow circulation in the liquid phase. Experimental results have shown that the application of a static magnetic field during directional solidification could induce the formation of thermoelectric magnetic convection (TEMC) 14 – 17 . The initial driver for TEMC is indentified as Lorentz force in the liquid phase caused by the interaction of thermoelectric current and the magnetic field 18 .…”