Segregation by convection
Solidification processes are affected by natural convection as soon as instable density stratifications arise in the melt from local variations of the temperature and/or the concentration field. In particular, the rejection of solute at the solidification front being lighter as the initial melt composition results in the formation of rising plumes in the liquid driven by the buoyancy force. Such convective plumes have been identified as the main reason for the development of solute-rich channels in solidifying metal alloys. These channels may lead to freckle formation in fully solidified castings which is considered as a serious material defect deteriorating the mechanical properties of the final product.
The development of such kind of segregation channels was studied by means of in situ X-ray radiography. The Ga–25wt%In alloy was solidified directionally in vertical direction from the bottom of the solidification cell.
Figure 1: Snapshot of the growing dendritic structure with a segregation channel in a Ga-25wt%In alloy. The colour scale represents the Ga concentration (red – Ga-rich melt, blue/green – the Indium dendrites).
Different scenarios have been observed concerning the formation of segregation channels in the mushy zone which can be related to variations of the vertical temperature gradient along the solidification cell. Variations of the temperature gradients applied over the solidification cell induce modifications of the melt flow pattern, which lead to different segregation structures.
Video 1: Channel formation triggered by instabilities of the growth process (low temperature gradient 1K/mm)
Video 2: Channel formation triggered by mesoscopic convection (high temperature gradient 2K/mm)
Perturbations of the dendritic structure are the origin of development of segregation freckles in case of low temperature gradients. The situation at higher temperature gradients is characterized by two dominating convection rolls in the liquid phase which are driven by a lateral temperature gradient and the convex shape of the solidification front. The penetration of this flow pattern into the mushy zone results in continuous accumulation of solute in the central part of the mushy zone followed by a remelting of the solid fraction and the occurrence of a stable chimney.
Shevchenko, N., Neumann-Heyme, H., Pickmann, C., Schaberger-Zimmermann, E., Zimmermann, G., Eckert, K., Eckert, S.
Investigations of fluid flow effects on dendritic solidification: Consequences on fragmentation, macrosegregation and the influence of electromagnetic stirring
IOP Conference Series: Materials Science and Engineering 228(2017), 012005
Shevchenko, N.; Roshchupkina, O.; Sokolova, O.; Eckert, S.
The effect of natural and forced melt convection on dendritic solidification in Ga-In alloys
Journal of Crystal Growth 417(2015), 1-8
Saad, A.; Gandin, C.-A.; Bellet, M.; Shevchenko, N.; Eckert, S.
Simulation of Channel Segregation During Directional Solidification of In—75 wt pct Ga. Qualitative Comparison with In Situ Observations
Metallurgical and Materials Transactions A 46(2015)11, 4886-4897
Karagadde, S., Yuan, L., Shevchenko, N., Eckert, S., Lee, P.D.
3-D microstructural model of freckle formation validated using in situ experiments
Acta Materialia, 79 (2014), 168-180
Shevchenko, N., Boden, S., Gerbeth, G., Eckert, S.
Chimney formation in solidifying Ga-25wt pct in alloys under the influence of thermosolutal melt convection
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 44 (8) (2013), 3797-3808
Shevchenko, N., Boden, S., Eckert, S., Borin, D., Heinze, M., Odenbach, S.
Application of X-ray radioscopic methods for characterization of two-phase phenomena and solidification processes in metallic melts
European Physical Journal: Special Topics, 220 (1) (2013), 63-77
Shevchenko, N., Eckert, S., Boden, S., Gerbeth, G.
In situ X-ray monitoring of convection effects on segregation freckle formation
IOP Conference Series: Materials Science and Engineering, 33 (1) (2012), 012035
Shevchenko, N., Boden, S., Eckert, S., Gerbeth, G.
Observation of segregation freckle formation under the influence of melt convection
IOP Conference Series: Materials Science and Engineering, 27 (1) (2011), 012085