Study of the Formation Mechanism of A-Segregation
Based on Microstructural Morphology
ZHAO ZHANG, YUCHONG BAO, LIN LIU, SONG PIAN, and RI LI
A model that combines a cellular automaton (CA) and lattice Boltzmann method (LBM) is
presented. The mechanism of A-segregation in an Fe-0.34 wt pct C alloy ingot is analyzed on
the basis of microstructural morphology calculations. The CA is used to capture the solid/liquid
interface, while the LBM is used to calculate the transport phenomena. (1) The solidiﬁcation of
global columnar dendrites was simulated, and two obvious A-segregation bands appeared in the
middle-radius region between the ingot wall surface and the centerline. In addition, the angle of
deﬂection to the centerline increased with the increasing heat dissipation rate of the wall surface.
When natural convection was ignored, the A-segregation disappeared, and only positive
segregation was present in the center and bottom corner of the ingot. (2) Mixed colum-
nar–equiaxed solidiﬁcation was simulated. Many A-segregation bands appeared in the ingot. (3)
Global equiaxed solidiﬁcation was simulated, and no A-segregation bands were found. The
results show that the upward movement of the high-concentration melt is the key to the
formation of A-segregation bands, and remelting and the emergence of equiaxed grains are not
necessary conditions to develop these bands. However, the appearance of equiaxed grains
accelerates the formation of vortexes; thus, many A-segregation bands appear during
Ó The Minerals, Metals & Materials Society and ASM International 2018
refers to variations in
composition that arise in alloy castings or ingots and
range in scale from several millimeters to centimeters or
even meters. These compositional variations detrimen-
tally aﬀect the subsequent processing behavior and
properties of cast materials and can lead to rejection of
cast components or processed products.
is important to study macrosegregation and its forma-
tion mechanism experimentally
or via simulation
such studies are of great signiﬁcance for
applying appropriate technological means to eliminate
these defects. The typical segregation pattern in the
ingot is a positive segregation in the upper region, a
negative segregation in the lower region, V-segregation
along the centerline, and A-segregations in the mid-
dle-radius region between the outer surface of the
casting and the centerline.
A-segregation is a kind of channel segregation caused
by the enrichment of solute in a local area during ingot
solidiﬁcation. Researchers have been investigating the
formation mechanism of A-segregation for decades, and
many mechanisms have been proposed,
ﬂow due to solidiﬁcation shrinkage, thermal and solutal
natural convection of the liquid, motion of free equiaxed
grains, and deformation of the solid skeleton in the
mushy zone. According to the most accepted theory, the
main factor is the ﬂow instability of the solidiﬁcation
front, which is controlled by the solute and temperature
conditions during solidiﬁcation.
Bennon and Incropera
were among the ﬁrst to
predict A-segregation in solidiﬁcation from the side of
O-30 wt pct NH
Cl solution in a rectangular
cavity. Mehrabian et al.
studied the combined eﬀect
of shrinkage and natural convection in the liquid phase.
They showed that the macrosegregation pattern strongly
depends on the orientation of the variation in the
density of the interdendritic liquid and the solid fraction.
They also proposed the hypothesis that A-segregations
develop due to ﬂow instability, which occurs at a critical
ﬂow condition. Combeau et al.
used the volume-av-
eraging method, which takes into account the motion
and the morphology of equiaxed grains, to predict
A-segregation in a large, 3.3-ton ingot. They showed
that many A-segregations formed at the top of the ingot.
They also investigated the role of inertia of ﬂow through
ZHAO ZHANG, YUCHONG BAO, LIN LIU, SONG PIAN, and
RI LI are with the School of Materials Science and Engineering, Hebei
University of Technology, Tianjin, 300130, China. Contact e-mail:
Manuscript submitted November 2, 2017.
Article published online April 9, 2018
2750—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A