TY - JOUR

T1 - Dendrite fragmentation mechanism under forced convection condition by rotating magnetic field during unidirectional solidification of AlSi7 alloy

AU - Zhang, Haijie

AU - Wu, Menghuai

AU - Rodrigues, Christian M.G.

AU - Ludwig, Andreas

AU - Kharicha, Abdellah

AU - Rónaföldi, Arnold

AU - Roósz, András

AU - Veres, Zsolt

AU - Svéda, Mária

N1 - Publisher Copyright: © 2022

PY - 2022/9/28

Y1 - 2022/9/28

N2 - Forced convection and its effect on the microstructure evolution of an Al-7wt.%Si alloy during unidirectional solidification were studied experimentally. Under natural convection (gravity), columnar structures develop. However, under forced convection by activating a rotating magnetic field (RMF: 10 mT, 50 Hz), many equiaxed grains form in the half-radius region of the cylindrical sample, and a severe macrosegregation channel forms at the centre of the sample. Crystal fragmentation is regarded as the main source of equiaxed grains, but their formation mechanism and the fragment transport phenomenon are not fully understood. A mixed columnar-equiaxed solidification model with extension to consider two dendrite fragmentation mechanisms (capillary-driven and flow-driven), was used to reproduce the experiment with the objective to investigate the formation process of the microstructure and macrosegregation. Under the effect of the RMF-induced primary/secondary flow, the capillary-driven fragmentation mechanism, which is associated with dendrite coarsening, operates mainly in the peripheral region of the sample at a certain depth of the mushy zone. These fragments are difficult to be transported out of the (columnar dendritic) mushy zone. The flow-driven fragmentation mechanism associated with the interdendritic flow-induced re-melting of dendrites, operates mostly near the front of the mushy zone and/or around the central segregation channel. Some of these fragments can be transported out of the columnar tip region. In this case, a thin undercooled layer exists. Therefore, fragments can grow and become equiaxed grains. Some fragments are transported distally from the mushy zone into the bulk superheated region and are re-melted/destroyed there. The fragments, which continue to grow in the deep mushy zone or in the thin undercooled layer, are easily trapped by columnar dendrites, thereby competing with the growth of columnar dendrites to form a mixed columnar-equiaxed structure or even leading to a columnar-to-equiaxed transition.

AB - Forced convection and its effect on the microstructure evolution of an Al-7wt.%Si alloy during unidirectional solidification were studied experimentally. Under natural convection (gravity), columnar structures develop. However, under forced convection by activating a rotating magnetic field (RMF: 10 mT, 50 Hz), many equiaxed grains form in the half-radius region of the cylindrical sample, and a severe macrosegregation channel forms at the centre of the sample. Crystal fragmentation is regarded as the main source of equiaxed grains, but their formation mechanism and the fragment transport phenomenon are not fully understood. A mixed columnar-equiaxed solidification model with extension to consider two dendrite fragmentation mechanisms (capillary-driven and flow-driven), was used to reproduce the experiment with the objective to investigate the formation process of the microstructure and macrosegregation. Under the effect of the RMF-induced primary/secondary flow, the capillary-driven fragmentation mechanism, which is associated with dendrite coarsening, operates mainly in the peripheral region of the sample at a certain depth of the mushy zone. These fragments are difficult to be transported out of the (columnar dendritic) mushy zone. The flow-driven fragmentation mechanism associated with the interdendritic flow-induced re-melting of dendrites, operates mostly near the front of the mushy zone and/or around the central segregation channel. Some of these fragments can be transported out of the columnar tip region. In this case, a thin undercooled layer exists. Therefore, fragments can grow and become equiaxed grains. Some fragments are transported distally from the mushy zone into the bulk superheated region and are re-melted/destroyed there. The fragments, which continue to grow in the deep mushy zone or in the thin undercooled layer, are easily trapped by columnar dendrites, thereby competing with the growth of columnar dendrites to form a mixed columnar-equiaxed structure or even leading to a columnar-to-equiaxed transition.

KW - Capillary-driven fragmentation

KW - Flow-driven fragmentation

KW - Macrosegregation

KW - Microstructure

KW - Re-melting, grain destruction

UR - http://www.scopus.com/inward/record.url?scp=85139360811&partnerID=8YFLogxK

U2 - 10.1016/j.actamat.2022.118391

DO - 10.1016/j.actamat.2022.118391

M3 - Article

AN - SCOPUS:85139360811

VL - 241.2022

JO - Acta Materialia

JF - Acta Materialia

SN - 1359-6454

IS - December

M1 - 118391

ER -