Study of the Formation Mechanism of A-Segregation Based on Microstructural Morphology
- PDF / 5,313,748 Bytes
- 16 Pages / 593.972 x 792 pts Page_size
- 59 Downloads / 148 Views
ION
MACROSEGREGATION 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 detrimentally affect the subsequent processing behavior and properties of cast materials and can lead to rejection of cast components or processed products.[1] Therefore, it is important to study macrosegregation and its formation mechanism experimentally[2,3] or via simulation methods,[4-7] 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 middle-radius region between the outer surface of the casting and the centerline.[8,9]
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: [email protected] Manuscript submitted November 2, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
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,[10] including 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.[11] Bennon and Incropera[12] were among the first to predict A-segregation in solidification from the side of an H2O-30 wt pct NH4Cl solution in a rectangular cavity. Mehrabian et al.[13] 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.[14] used the volume-averaging 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
the dendritic mushy zone in channel segregations.[15] Wu and co-workers[11] used the three-phase columnar–equiaxed mixed model to simulate macrosegregation in a 2.45-ton ingot. The results showed that equiaxed grains are not necessary to producing A-segregation. However, both the appearance of equiaxed grains and their interaction with the growing columnar dendrite tips strengthen
Data Loading...
