Numerical simulation of a solidifying Pb-Sn alloy: The effects of cooling rate on thermosolutal convection and macrosegr
- PDF / 2,269,908 Bytes
- 12 Pages / 603.28 x 783.28 pts Page_size
- 96 Downloads / 176 Views
I.
INTRODUCTION
THERMOSOLUTAL convection often accompanies the solidification of multicomponent substances, such as metal alloys. That is, buoyancy-driven flow due to both temperature and liquid composition gradients occurs during solidification, and depending on the relative densities of the alloy constituents, solutal buoyancy forces may either augment or oppose thermal buoyancy forces. Furthermore, fluid motion is not restricted to the all-liquid melt but may also occur in the two-phase (mushy) region. Macroscopically, thermosolutal convection can cause large-scale redistribution of the constituents, which resuits in a nonhomogeneous solid product. I~'2j This socalled macrosegregation is an undesirable feature of final castings, and methods by which it may be reduced are sought. However, prescription of such methods requires a better understanding of convection transport phenomena occurring during the solidification process. Since binary alloys are the simplest form of multicomponent systems to exhibit thermosolutal convection, they are well suited for studying convective transport phenomena during alloy solidification. Because of the high temperatures and inherent opacity of liquid metal systems, convection is difficult to discern experimentally, especially for buoyancy-driven flows. [3] Accordingly, researchers have often resorted to studying low-temperature, semitransparent salt solutions, which are analogous to metallurgical systems in that a dendritic mushy zone forms during solidification, t4-8~ Although experiments have been performed using relatively lowtemperature alloys of lead and tin or aluminum and copper, [6,9-141 flow conditions could not be determined with certainty. Stewart and Weinberg were able to infer flow patterns in tin L15~and tin-lead c~61 melts by introducing radioactive tracers into the melt and by taking autoradiographs of sectioned specimens after solidification. Ricou and Vives have developed and tested a magnetic
P.J. PRESCOTT, Doctoral Student, and F.P. INCROPERA, Professor, are with the School of Mechanical Engineering, Purdue University, West Lafayette, 1N 47907. Manuscript submitted June 11, 1990. METALLURGICAL TRANSACTIONS B
velocity probe t~71which works well in forced, liquid metal flows up to 700 ~ but is not sensitive enough to resolve fluid velocities associated with natural convection. To circumvent limitations associated with experimental methods, mathematical models have been developed to establish the relationship between convection and macrosegregation in binary systems, tS'lS-22j However, while useful insights were provided by the models, limiting assumptions related to coupling between the mushy and all-liquid zones precluded prediction of important phenemena, such as double-diffusive layering, remelting, and irregular liquidus morphology, which strongly influence the solidification process. More recently, a set of continuum conservation equations for total mass, momentum, energy, and species has been developed by Bennon and Incropera. [231 A single fixed grid
Data Loading...
