Macro-, meso- and microstructural characterization of metallic lattice structures manufactured by additive manufacturing assisted investment casting

V H Carneiro,P J Withers,H Puga,S D Rawson

doi:10.1038/s41598-021-84524-y

Abstract

Cellular materials are recognized for their high specific mechanical properties, making them desirable in ultra-lightweight applications. Periodic lattices have tunable properties and may be manufactured by metallic additive manufacturing (AM) techniques. However, AM can lead to issues with un-melted powder, macro/micro porosity, dimensional control and heterogeneous microstructures. This study overcomes these problems through a novel technique, combining additive manufacturing and investment casting to produce detailed investment cast lattice structures. Fused filament fabrication is used to fabricate a pattern used as the mold for the investment casting of aluminium A356 alloy into high-conformity thin-ribbed (~ 0.6 mm thickness) scaffolds. X-ray micro-computed tomography (CT) is used to characterize macro- and meso-scale defects. Optical and scanning electron (SEM) microscopies are used to characterize the microstructure of the cast structures. Slight dimensional (macroscale) variations originate from the 3D printing of the pattern. At the mesoscale, the casting process introduces very fine (~ 3 µm) porosity, along with small numbers of (~ 25 µm) gas entrapment defects in the horizontal struts. At a microstructural level, both the (~ 70 μm) globular/dendritic grains and secondary phases show no significant variations across the lattices. This method is a promising alternative means for producing highly detailed non-stochastic metallic cellular lattices and offers scope for further improvement through refinement of filament fabrication.

Highlights

Cellular materials are recognized for their high specific mechanical properties, making them desirable in ultra-lightweight applications
As must be the case, given the castings derive from the molds made from the 3D printed patterns, the highest deviations are observed in the final as-cast A356 samples, it is clear that most of the deviation is transferred over from those introduced during fabrication of the pattern
Regarding the deviations introduced during the production of the Polylactic Acid (PLA) pattern (Step I), these can be primarily attributed to the precision of the 3D-printer stepper motor

Summary

Introduction

Cellular materials are recognized for their high specific mechanical properties, making them desirable in ultra-lightweight applications. At a finer scale metal-based additive manufacturing techniques have been reported to promote meso- and micro-scale defects, such as: porosity, delamination, and un-melted powders, as well as highly anisotropic grain morphologies as a function of build d irection[35,36,37,38,39,40,41,42,43,44] Some of these issues may be addressed by metal casting techniques, where there have been recent advances on the filling of thin-walled molds[45] allowing good dimensional control[46,47,48,49,50] coupled with good microstructural control[51,52,53]. In as-cast samples, previous to solution treatment, M g2Si are precipitated in the grain b oundaries[70,71] and, due to the presence of Mg, Chinese script-shaped π-Al8FeMg3Si6 may be observed[48,49]

Methods

Results

Conclusion