Numerical study of the influence of mold filling conditions on the as-cast structure of Al-4 wt.% Cu ingots M Ahmadein 1 , M Wu 1,2 , A Ludwig 1 1 Chair for Simulation and Modelling of Metallurgical Processes, 2 Christian-Doppler Lab for Advanced Process Simulation of Solidification & Melting, Dept. of Metallurgy, Univ. of Leoben A-8700 Leoben, Austria E-mail: mahmoud.ahmadein@unileoben.ac.at Abstract—In the last few decades research efforts were conducted to grasp good understanding about the origin of equiaxed and columnar grains formed during solidification. The morphological evolutions such as globular/cellular to dendritic or columnar-to-equiaxed transition were generally studied. Correspondingly, some empirical models were introduced. Nevertheless, no sufficient attention was paid to incorporation of such models together with macroscopic phenomena. A 5-phase mixed columnar-equiaxed solidification model recently proposed by the current authors was used to predict the macrostructure formation. However previous results showed that the initial melt conditions can influence the predicted structure particularly at low pouring temperature. In the current work, the impact of mold filling conditions on the final solidification structure is numerically verified in two stages: during pouring using 3-phase globular-equiaxed model; and after filling using the 5-phase mixed columnar-equiaxed model. The calculated results are compared to the as-cast structures obtained from experiments. The results demonstrated the significance of the ‘big bang’ nucleation and the ‘premature’ solidification occurred during pouring at low melt superheat on the as-cast structure. Keywords—modeling, as-cast structure, nucleation, CET I. INTRODUCTION The as-cast structure has crucial impact on the mechanical properties of cast products. Researchers and metallurgists studied over decades the factors affecting the as-cast structure [1]-[5]. In recent times, they modeled the micro- and macro-structure formation to predict and improve the properties of the final products. Once the nucleation of equiaxed grains occurs (heterogeneously or under dynamically stimulated conditions), grain grow in either columnar (cellular or dendritic) or in equiaxed (globular or dendritic) form. The final as-cast structure may contain one or more of these morphologies with columnar-to-equiaxed transition (CET). In addition, various interacting multi-phase / multi-scale processes during solidification, e.g. transport of heat, momentum, mass, species, and melt convection increase the complexity of the prediction of the as-cast structure. Progress was also made in the understanding of CET since the pioneer work of Hunt [6] in 1980s. A CET map, the correlation of the columnar primary dendrite tip growth velocity with the local temperature gradient at the moment of CET, was established to analyze the occurrence of CET. This CET map was later confirmed and further improved by many authors [7]-[12]. In the meantime, empirical correlations were proposed as indirect criteria to predict the CET for engineering castings [13]-[17]. In the late work of Wu et al. [20]-[22], a 5-phase mixed columnar-equiaxed model that treats the solidification as a multiphase transport problem which takes into account the impacts of flow and grain transport based on the above mentioned empirical knowledge. Preliminary simulations were compared to Al-Cu alloy ingots poured at various temperatures [23]. The results showed that the model is capable to qualitatively produce similar structures. However, the detailed structural agreement could not be achieved. Further improvement to the model results were obtained after the application of experimentally determined nucleation parameters [24] for the nucleation model instead of the assumed nucleation parameters used in [23]. As a consequence, a good qualitative agreement in addition to satisfactory quantitative agreement between simulation and experiment were obtained for the ingot poured at high temperature [25]. Nevertheless, the results of the ingot poured at lower temperature disagree with the experiment. The authors argued some possible reasons for the disagreement. The initial grain number density, n o , was the most important parameter that influences the profile of CET-line and the proportion of the formed equiaxed crystals. By recalling the previous experimental investigations on the influence of the mold filling conditions on the as-cast structure [4][5][26], it can concluded that the pouring temperature, mold temperature, and pouring technique play very important role. In the present work and based on the above discussion, the influence of various mold filling parameters on the initial state of the melt is numerically investigated using a 3-phase flow and solidification model developed at earlier time by current authors. Afterwards, the mold filling simulation outcomes are used as initial conditions for the simulation of the as-cast structure using the 5-phase mixed columnar/equiaxed model.