Additive manufacturing assisted investment casting: A low-cost method to fabricate periodic metallic cellular lattices

Metallic cellular solids are a class of materials known for their high specific mechanical properties, being desirable in applications where a combination of high strength or stiffness and low density are important. These lightweight materials are often stochastic and manufactured by foaming or casting. If regular (periodic) lattice structures are desired, they may be manufactured by metallic additive manufacturing techniques. However, these have characteristic issues, such as un-melted powders, porosity and heterogeneous microstructures. This study reports a novel low-cost route for producing regular lattice structures by an additive manufacturing assisted investment casting technique. Fused filament fabrication is used to produce the lattice structure pattern which is infiltrated with plaster. The pattern is then burnt off and the aluminum is cast in vacuum. In this way we can manufacture non-stochastic metallic lattices having fine struts/ribs (0.6 mm cross-section using a 0.4 mm nozzle) and relative densities down to 0.036. X-ray micro computed tomography (mu CT) showed that as-cast A356 Aluminium alloy frameworks have high dimensional tolerances and fine detail control. Frameworks based on units of six connected struts ranging from intruding (auxetic) to protruding (hexagonal) strut angles are studied. Vertical struts are finer than expected, reducing their moment of area which could impact their compressive strength. This new, low cost, route for producing high precision metallic cellular lattices offers an attractive alternative to other additive manufacturing techniques (e.g. selective laser and electron beam melting).