Abstract
A 3-phase Eulerian approach is used to model the macrosegregation during solidification in direct chill (DC) casting of binary bronze (Cu-Sn). The three phases are the melt, the solidifying columnar dendrites and the equiaxed grains. The thermodynamic information of Cu-Sn is included based on published thermodynamic data, which are coupled with the 3-phase solidification model. The occurrence of columnar-to-equiaxed transition (CET), phase interactions, feeding flow, equiaxed sedimentation and their influence on macrosegregation are considered in the model. The model is applied to a laboratory DC casting process of bronze as a benchmark to demonstrate the model potentials. The simulation results of mixed columnar and equiaxed solidification as well as the formation of macrosegregation are presented. The focus of this work is to analyze and discuss the macrosegregation mechanisms by different flow including feeding flow and crystal sedimentation.
Highlights
Bronzes (Cu-Sn) are among the oldest engineering materials, and nowadays are still used in many different applications with which high quality materials are required for example electronics
Due to the fact that bronzes have a rather large mushy zone, macrosegregation often appears in direct chill (DC) casting process of bronze
Experimental observations [13] indicate that the columnar front is blocked by a certain amount of equiaxed crystals in front of it. This condition ensures that the columnar cannot overgrow the equiaxed phase when it has already a certain amount of volume fraction. (iii) The equiaxed phase cannot grow if its diameter is bigger than the space between the primary dendrite arm spacing (λ1) of the columnar phase and the diameter of the columnar dendrite trunk [13]. This condition models the growth of the equiaxed phase within the dendrite trunks; it is thought that an equiaxed crystal is not able to grow further if it's sticking within two dendrite trunks. (iv) The equiaxed phase is blocked in its movement when the sum of the volume fraction of the equiaxed phase and the columnar phase reaches the packing limit of fe + fc > = 0.637 [14]. (v) The equiaxed is trapped and forced to move with the columnar when the volume fraction of columnar reach to a certain value, that is, trapping limit of fc > = 0.2 is reached [14]
Summary
Bronzes (Cu-Sn) are among the oldest engineering materials, and nowadays are still used in many different applications with which high quality materials are required for example electronics. Beckermann et al.[4] applied a coupled multicomponent solidification model with melt convection to a large industry-scale ingot. Wu et al [8] developed a mixed columnar-equiaxed solidification model, which considered the competitive growth of columnar and equiaxed phase, melt convection, equiaxed grain sedimentation, together with their influence on the species transport and macrosegregation. This model successfully predicted the conic negative segregation in the lower region of the ingot and the columnar-toequiaxed-transition (CET) [9]. Based on that study this paper presents an insight investigation on the formation mechanisms of macrosegregation by different flow phenomena, especially the effects of equiaxed crystal sedimentation are discussed in details
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