Computational Modeling of Columnar to Equiaxed Transition in Alloy Solidification

Abstract

The columnar to equiaxed transition (CET) provides a challenging simulation goal for computational models of alloy solidification, in addition to being an important technological feature of many casting processes. CET thus provides an industrially relevant test‐case for those developing numerical models across a range of scales. Whether or not CET occurs depends on numerous experimental parameters such as cooling rate, speed of columnar growth, thermal gradient in the liquid, and level of grain refiner in the alloy. Information on columnar and equiaxed grain structure, and the transition between the two, is very useful for foundry engineers, at the macroscopic scale of the casting. The detailed microstructure within each grain is determined by typically dendritic growth and local transport of solute and heat. This paper presents a review of recent progress on modeling CET at multiple length scales. It is evident that, whilst micro‐models can provide simulations of physical phenomena, such as the evolution of dendrite morphology, at scales 10−3 to 10−5 m, finite computational resources preclude this resolution over the length scale of castings which is in the 10−2–100 m range. Instead, reasonably accurate models of CET formation in castings can be achieved by meso‐scale modeling featuring 10−3–10−2 m phenomena. Such meso‐scale models make use of analytical expressions to simulate dendrite growth in undercooled melts. Recent progress in modeling of CET, at both macro/meso‐ and micro‐scales is reviewed, and computational challenges yet to be met are summarized.

Numerical Simulation of Microstructural Evolution via Phase-Field Model Coupled to the Solutal Interaction Mechanism

“…However, after columnar-to-equiaxed transition (CET), Figure 10(c) and Figure 10(d) show smaller columnar dendrites, as expected, for a higher nuclei density present in the melt, at a shorter distance between mold wall and the equiaxed zone. The simulated results showed that the solidification features are consistent with those observed based on the metallographic examinations of cast microstructures reported in the literature [31] [32] and [34]. During the solidification process the extradendritic liquid (l) decreases, while both solid (s) and interdendritic liquid (d) increase gradually through that period.…”

“…The columnar-to-equiaxed transition (CET) will occur, when the solute rejected from the equiaxed grains is sufficient to dissipate the solutal undercooling at the columnar front, such that C l has increased to * l C . Through numerical examples (Figure 8), one can see that the columnar-to-equiaxed transition (CET) is strongly influenced by nuclei density (n), as suggested by previous studies [31]- [33]. The time of columnar-to-equiaxed transition (CET), in turn, is used as an input data for phasefield model.…”

Overview of In Situ X-Ray Studies of Light Alloy Solidification in Microgravity

“…During their upward motion, the size of the dendrite fragments decreased because they gradually melted, forming a final white cloud, which corresponded to the melting of the aluminum-rich dendritic fragment. It is worth noting that these dendrite fragments could not promote columnar-to-equiaxed transition [17][18][19] or CET, because they were carried up far into the liquid where they were re-melted. In addition, a strong segregation along the sample occurred during solidification because all Al-enriched dendrite fragments were transported by buoyancy forces into the upper part of the sample and mixed in the liquid phase after melting.…”

Controlling Phase Growth During Solidification by Nanoparticles

“…[1][2][3][4][5][6][7] It is well known that the properties of materials are dictated by their microstructures. Thus, controlling the microstructure evolution during solidification is essential.…”