| Heat transfer behavior plays a key role in solidification process and determines cooling conditions within the casting,especially in foundry systems with high thermal diffusivity such as castings with metallic mold.High heat transfer can induce fine microstructure and therefore good mechanical properties and high productivity of castings.The behavior between the casting and mold or die interface could be characterized by interfacial heat transfer coefficient(IHTC)or interfacial heat flux(IHF)quantitatively.The accurate data of interfacial boundary conditions are helpful in improving the accuracy of casting numerical simulation.Almost all studies have been devoted to determining IHTC or IHF assuming a one-dimensional problem at the casting/mold interface for simplicity.However,the interfacial heat transfer is multi-dimensional in actual conditions.In the present work,inverse heat-conduction solutions with spatial interfaces(two interfaces)were performed for quantifying transient values of IHTC or IHF.The study includes two parts:(1)The inverse method based on nonlinear estimation method(NEM)was applied for determining the spatial IHTC or IHF during the solidification of A356 and Al-3wt.%Cu alloy.The casting surface characteristics of A356 casting on different interfaces were inspected and compared.Furthermore,the relations between the characteristics and evolution of IHTCs were investigated.(2)Inversely-calculated IHF during initial solidification of Al-3wt.%Cu alloy was adopted as metal/mold heat transfer boundary for dendritic growth simulations with cellular automaton(CA)method.Then,the formation of surface microstructure affected by heat flux,was simulated.The electron backscattered diffraction(EBSD)observation of different domains was conducted for validation.Thus,the effect of heat flux on dendritic morphology and tip growth velocity was investigated.Firstly,temperature history of several thermocouples was obtained from solidification experiments.Based on NEM,a C/C++program was designed for solving the inverse problem of spatial interface heat transfer using ANSYS parametric design language(APDL).The feasibility and accuracy of the developed algorithm were verified by a virtual test.Applying the experimental temperature profiles as inputs,the metal/mold interfacial heat transfer conditions during solidification of A356 were identified by the designed program.The calculated results showed that IHTCs of bottom interface reached the stable value since the casting surface from interface solidified,while those of lateral interface became stable at the end of volumetric solidification shrinkage of casting.The stable value at the lateral interface was 250 W/(m~2·°C),which was only one-third of that at the bottom interface.Further analysis of the interplay between spatial IHTCs and observed surface morphology reveals that spatial heat transfer across casting/mold interfaces is the direct result of different interface evolution during solidification process.To investigate the effect of spatial interfacial heat transfer on microstructure formation,a mathematical model was established.Discrete models of physical fields during the solidification process were established by finite difference method(FDM).A correspongding Python program was designed.The solidification process of Al-3wt.%Cu alloy was selected for simulations.Dendritic growth simulations,which containing the growth kinetics model and nucleation model,were performed based on the CA method.The inversely-calculated thermal boundaries were applied in the dendritic growth simulations.The dendritic formation and morphology calculated near different casting/mold interfaces were compared and discussed based on local heat flux.Dendritic tip velocities resulting from different heat flux were analyzed.The relationship between crystallographic orientations and heat flux directions were verified by EBSD observation.Thus,the anisotropy of solidification texture was investigated numerically and experimentally. |
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