Inverse segregation
- PDF / 703,439 Bytes
- 10 Pages / 614.28 x 794.28 pts Page_size
- 85 Downloads / 436 Views
Io
INTRODUCTION
INVERSE segregation results from the back flow of solute enriched interdendritic liquid due to volume shrinkage during solidification. Analytical expressions which quantitatively describe the segregation have been developed at the chill face by Scheil, 1 and remote from the chill face by Kirkaldy and Youdelis. 2 The analysis has been further extended for general macrosegregation by Flemings and Nereo. 3 Measurements of inverse segregation at the chill face, as a function of alloy composition in AICu alloys, -~ agree well with the calculated values. Reasonable agreement is also obtained for the A1Zn~ and SbBi 5 alloy systems, In the theoretical development and calculations of inverse segregation, a number of assumptions are made. The most important assumptions relate to the specific volume of the liquid (vt) and solid (vs), since the back flow of solute enriched liquid is entirely due to the volume changes during solidification. Both vs and vt, in general, are a function of the temperature and composition of the material. In the case of the A1Cu alloys in the Scheil analysis, v~ is assumed constant with temperature. Values of vt are taken from those reported by Sauerwald 6 in which v~ at a given temperature is a function of the liquid composition as defined by the equilibrium phase diagram. This neglects the change in vt with temperature and applies equilibrium conditions to nonequilibrium dendritic solidification. In careful measurements of inverse segregation in A1Cu alloys Prabhakar and Weinberg 7 found that inverse segregation was confined to a region very close to the chill face, contrary to the results reported previously, 2 and was dependent on factors not considered in the theoretical development. Specifically, the chill face segregation was found to decrease with increasing pouring temperature of the melt and to decrease as the thermal conductivity of the chill material decreased. In the present investigation the theoretical model and calculations are reevaluated and extended using finite difference computer modeling. Chill face segregation is S. MINAKAWA Ls Engineer with Riken Corporation, Tokyo, Japan. 1. V SAMARASEKERA, Assistant Professor, and E WEINBERG, Prolessor and Head. are with the Department of Metallurgical Engineering. University of British Columbia, Vancouver, BC V6T 1W5, Canada Manuscript submitted November 26, 1984.
METALLURGICAL TRANSACTIONS B
determined for AICu, AIZn, and SbBi alloys in which the temperature and composition dependence of v, and vt are determined from density data and nonequilibrium segregation. The distance from the chill face in which inverse segregation is present is considered, as well as the effect of pouring temperature and chill material conductivity on the chill face segregation.
II.
PROCEDURE
A. Model Formulation
The basic model geometry and formulation follows that of Scheil. ~Solidification is dendritic and the shape of the dendrite is taken as pyramidal as shown in Figure 1. The following assumptions are made: (1) no surface exudations o
Data Loading...
