This paper presents a comprehensive numerical investigation of the influence of cooling conditions on base separation, void formation and thermally induced stresses during the solidification of a high
INTRODUCTIONCasting is an established and widely used process for manufacturing complex shapes without the necessity of applying large forces. It is especially useful in processing energetic materials which cannot tolerate high forces. The ambient cooling conditions play an important role in determining the quality of the final cast product. One measure of quality is the nearness of the product to the final shape desired with little or no residual stresses. An example of deviation from the desired shape is base separation [1]. Well-designed cooling conditions could help avoid such deleterious effects.In casting processes, the heat transfer, fluid flow, and phase change processes and the induced stresses are strongly coupled. The effects of natural convection in the melt induced by solidification are relatively well understood. Extensive analytical, experimental and numerical efforts have been reported on this topic [1][2][3][4][5][6].Thermal stresses induced during the casting process are, in general, less well understood. Analytical solutions have been suggested under simpler conditions and geometries such as high-pressure die casting [7]. It is also challenging to experimentally visualize thermal stresses. Interesting approaches such as the use of X-ray diffraction [8] and neutron diffraction [9] have been suggested. However, these approaches require high-intensity emitters and can be prohibitively expensive. Moreover, transient development of the three-dimensional stress field is very difficult to map. Numerical simulation of thermal stresses also poses several challenges. The stress analysis must consider dynamic coupling between solidification heat transfer and the resulting stresses induced in order to predict the developing gap between mold and cast product. The governing equations for thermal transport and stress should be coupled to model the transient development of thermal stresses.
3Finite element methods (FEM) have been widely used to numerically simulate casting problems. Thomas [10] reviewed in detail the various FEM thermal stress models and discussed difficulties in practical implementation. Contact conductance between the casting and the mold was modeled by Fackeldey et al. [11] as a simple function of pressure. But the separation of the casting from the mold, which has a detrimental effect on heat transfer, was neglected. Several models have been proposed to address this separation. Seetharamu et al. [12] considered loss of contact between the mold and the cast, and modeled the heat transfer as being across an air gap. Isaac et al. [13] predicted the size and distribution of the separation and modeled it as gap conductance.Accurate materials characterization is also important for modeling thermal stresses as, in general, materials change from viscoelastic/viscoplastic in the liquid phase to el...