Multilevel Metal Flow-Fill Analysis of Centrifugal Casting for Indirect Additive Manufacturing of Lattice Structures

Abstract

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.