Effect Of Macrosegregation Research Articles

Formation and segregation of primary silicon in Bridgman solidified Al-18.3 wt% Si alloy

The effect of withdrawal velocity V and temperature gradient G on the formation and segregation characteristics of primary silicon on Bridgman-solidified Al-18.3 wt% Si is reported. The morphology of primary silicon changes from plate-like to polyhedral with increase of GV over the range 1.0–3.4K/s. The polyhedral primary silicon forms as linked clusters by successive branching on edges following attainment of a critical size in each polyhedral element of the cluster. Primary silicon macrosegregates at lower velocities towards the bottom of samples and around the embedded thermocouple attributable to heterogenous nucleation on refractoryware followed by localized growth as found previously in isothermal holding studies. After correction of local silicon alloy concentration for such effects of macrosegregation, local undercooling for the first formation of primary silicon shows no significant dependence on V, G or GV. Corresponding measurements of number density of polyhedral clusters indicate an operative contact angle for heterogeneous nucleation of primary silicon of 30 ± 4° for the range of conditions studied.

A study of the effects of macrosegregation and buoyancy-driven flow in binary mixture solidification

A generalized anisotropic porous medium approach is developed for modelling the flow, heat and mass transport processes during binary mixture solidification. Transient predictions are obtained using FEM, coupled with an implicit time-marching scheme, for solidification inside a two-dimensional rectangular enclosure. A parametric study focusing attention on the effects of solutal buoyancy and thermal buoyancy is presented. It is observed that three parameters, namely the thermal Rayleigh number ( Ra v ), the solutal Rayleigh number ( Ra F ) and the relative density change parameter (σ 2), significantly alter the flow fields in the liquid and the mushy regions. Depending upon the nature of these flow fields, the solute enrichment caused by macrosegregation may occur on the top or the bottom region of the enclosure.