Indirect additive manufacturing based casting of a periodic 3D cellular metal – Flow simulation of molten aluminum alloy

Abstract

Direct-metal additive manufacturing (AM) processes such as Selective Laser Melting (SLM) and Electron Beam Melting (EBM) methods are being used to fabricate three dimensional (3D) metallic mesostructures with a laser or electron beam over metal powder beds. In spite of their good manufacturability on 3D network structures, the direct AM processes still appear to have disadvantages – limited selection of materials, high thermal stress traced to the high local energy source, poor surface finish, anisotropic properties, and high cost on powder materials and manufacturing with high power beams. As an alternative method to manufacture 3D network cellular metals, we suggest and implement an indirect AM method combining an inkjet 3D printing of wax and metal casting – Indirect AM based Casting (I·AM Casting). Due to the high surface area of the cellular structural mold exposed to an ambient temperature during casting, flow and solidification of a molten metal appear to be a strong function of temperature. Therefore, viscosity, density, and thermal conductivity of a molten metal and mold may need to be provided as a function of temperature for characterizing flow and solidification. The objective of this study is to test the hypothesis that casting of a molten metal into a cellular structural mold is highly sensitive to temperature that temperature-dependent viscosity, density, and thermal conductivity should be implemented for the simulations on flow and solidification of a molten metal. A transient flow and heat-transfer analysis of a molten aluminum alloy, AC4C, is conducted through a 3D cellular network mold made of zircon. Solidification of AC4C through the cellular structural mold during casting is simulated with temperature-dependent properties of the molten metal and mold over a range of running temperature using a user defined function (UDF) of ANSYS/FLUENT. We found that solidification is sensitive to viscosity and thermal conductivity of AC4C and the zircon mold, which are a strong function of temperature. The simulation with constant thermal and physical properties of AC4C and the zircon mold overestimates the solidification time with an error of 20% compared to the one with the temperature-dependent properties.