One of the typical methods to manufacture 3D lattice metals is the direct-metal additive manufacturing (AM) process such as Selective Laser Melting (SLM) and Electron Beam Melting (EBM). In spite of its potential processing capability, the direct AM method has several disadvantages such as high cost, poor surface finish of final products, limitation in material selection, high thermal stress, and anisotropic properties of parts. We propose a cost-effective method to manufacture 3D lattice metals. The objective of this study is to provide a detailed protocol on fabrication of 3D lattice metals having a complex shape and a thin wall thickness; e.g., octet truss made of Al and Cu alloys having a unit cell length of 5 mm and a cell wall thickness of 0.5 mm. An overall experimental procedure is divided into eight sections: (a) 3D printing of sacrificial patterns (b) melt-out of support materials (c) removal of residue of support materials (d) pattern assembly (e) investment (f) burn-out of sacrificial patterns (g) centrifugal casting (h) post-processing for final products. The suggested indirect AM technique provides the potential to manufacture ultra-lightweight lattice metals; e.g., lattice structures with Al alloys. It appears that the process parameters should be properly controlled depending on materials and lattice geometry, observing the final products of octet truss metals by the indirect AM technique.

The centrifugal casting is a classical manufacturing method and it has been widely studied. However, when it comes to manufacture thin walled lattice materials with complex three-dimensional meso-structures, a multiscale flow-fill analysis may be needed for macro-filling at the sprue system and micro-filling at lattice structures. On the micro-filing analysis for a thin walled lattice structure, the surface tension of molten metal appears to be an important factor. On the other hand, flow inertia may affect the flow-filling process more than the surface tension of molten metal does. Our hypothesis is that there exist a range of ratios of cell wall thickness to length that are primarily affected by surface tension or density. From comparison with two different molten metals — aluminum and copper alloys, we can estimate the characteristic of flow, which will be of benefit when designing lattice structures and selecting materials for the manufacturing process. The objective of this study is to test the hypothesis by constructing an analytical model on flow filling of molten metals (aluminum alloy and copper alloy) associated with manufacturing lattice structures. The Naiver-Stokes equation with surface tension is considered for modeling of the flow of molten metal along the micro-channel of lattice structures and is numerically implemented with MATLAB. Temperature dependent properties of the liquid metals; e.g., density, viscosity, and conductivity, are considered for building the analytical model. Numerical simulations with a commercial code, ANSYS are conducted using a user defined function. Experimental validation is followed to manufacture a cubic truss lattice structure with a varying wall thickness; 0.5–1mm. Two molten metals — aluminum alloy and copper alloy are used for filling the mold at the centrifugal casting system. The mold is prepared by removing sacrificial lattice patterns made by a polyjet 3D printer. The preliminary result shows that the final lattice structures with an aluminum alloy through the 3D printing of sacrificial pattern followed by centrifugal casting have relatively good flow filling property at thin wall thickness (∼0.5mm) due to low surface tension of aluminum alloy. On the other hand, the high surface tension of a copper alloy prevents flow-fill to micro-channel mold cavity, resulting in early solidification. The indirect additive manufacturing based casting shows an excellent surface quality, which can be used for manufacturing cellular structures. A coupled flow and heat transfer of molten metal successfully simulate flow-fill and solidification and is compared with the experiment. Faster filling-time and faster solidification for the temperature-dependent material properties were shown.

Investment casting processes are influenced by a variety of parameters. Many researches considering viscosity as a constant have been conducted up to this point. In particular, however, viscosity with temperature change has not been much accounted for solidification and heat transfer simulation of molten metal in the investment casting process. In addition, analysis of behavior of metal flow as well as air gap problems for complex network structures have not been investigated much. The aim of this study is to build transient metal flow and velocity profile models considering temperature dependent viscosity in investment casting processes of cellular structures. In this study, a Computational Fluid Dynamics (CFD) modeling tool was used for metal flow and velocity profile in investment casting processing using User Defined Function (UDF) for temperature dependent viscosity. The results of the metal flow and velocity profile inside of the simple cylindrical geometry are represented. It is shown that for the validation of the numerical simulation, the velocity profile between analytical and numerical approaches showed very good agreement. Analytical approaches showed that velocity was reduced with the increase in viscosity, which is applied as a function of temperature. In particular, rapid decreasing in velocity was shown from under the melting temperature of the molten metal. There was no movement on metal flow at the room temperature. Numerical approaches showed that the liquid metal began to be solidified from the wall surface inside of the mold. For the same simulation time, it was shown that the metal flow in a cylinder that has 1mm diameter showed better fluidity rather than that of the cylinder that has 2mm diameter due to the increase in adhesion between liquid metal and the surface of the mold and surface tension between molten metal and air. The effective diameter by solidification is decreased with the time change.

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.

Direct-metal additive manufacturing (AM) processes such as Selective Laser Melting (SLM) and Electron Beam Melting (EBM) methods are being used to fabricate complex metallic cellular structures with a laser or electron beam over a metal powder bed. Even though these processes have excellent capabilities to fabricate parts with cellular mesostructures, there exist several constraints in the processes and applications: limited selection of materials, high thermal stress by the high local energy source, poor surface finish, and anisotropic properties of parts caused by combined effects of one-dimensional (1D) energy based patterning mechanism, the deposition layer thickness, powder size, power and travel speed of laser or electron beam. In addition, manufacturing cost is still high with the Direct-metal AM processes. As an alternative for manufacturing metallic 3D cellular structures, which can overcome the disadvantages of direct-metal AM techniques, polymer AM methods may be combined with metal casting. We may call this "Indirect AM based Casting (I AM casting)". The objective of this study is to explore the potential of I AM Casting associated with development of a novel manufacturing process - Indirect 3D Printing based centrifugal casting which is capable of producing multifunctional metallic cellular structures with internal cooling channels having a 2mm inner diameter and 0.5mm wall thickness. We characterize polymers by making expendable patterns with a polyj et type 3D printer; e.g., modulus, strength, melting and glass transition temperatures and thermal expansion coefficients. A transient flow and heat transfer analysis of molten metal through 3D cellular network mold will be conducted. Solidification of molten metal through cellular mold during casting will be simulated with temperature dependent properties of molten metal and mold over a range of running temperatures. The volume of fluid (VOF) method will be implemented to simulate the solidification of molten metal together with a user defined function (UDF) of ANSYS/FLUENT. Finally, experimental validation will be conducted.

Direct-metal additive manufacturing (AM) processes such as Selective Laser Melting (SLM) and Electron Beam Melting (EBM) methods are being used to fabricate complex metallic cellular structures with a laser or electron beam over a metal powder bed. Even though their excellent capabilities to fabricate parts with cellular mesostructure, there exist several constraints in the processes and applications; limited selection of materials, high thermal stress by the high local energy source, poor surface finish and anisotropic properties of parts caused by combined effects of one-dimensional (1D) energy based patterning mechanism, the deposition layer thickness, powder size, power and travel speed of laser or electron beam. In addition, manufacturing cost is still high with the Direct-metal AM processes. As an alternative way to manufacture metallic 3D cellular structures, which can overcome the disadvantages of direct-metal AM techniques, polymer AM methods can be combined with metal casting. We may call this “Indirect AM based Casting (I AM casting)”. The objective of the study is to explore the potential of I AM Casting associated with development of a novel manufacturing process Indirect 3D Printing based casting which is capable of producing metallic cellular structures within a cell size of 3mm and cell thickness of 0.5mm. We will characterize polymers making sacrificial patterns by PolyJet typed 3D printers; e.g., melting and glass transition temperatures and thermal expansion coefficients. A transient flow and heat-transfer analysis of molten metal through 3D cellular network mold will be conducted. Solidification of molten metal through cellular mold during casting will be simulated with temperature dependent properties of molten metal and mold over a range of running temperature. The volume of fluid (VOF) method will be implemented to simulate the solidification of molten metal together with a user defined function (UDF) of ANSYS/FLUENT. Finally, experimental validation will be followed.