Relationship between process, microstructure, design, and properties of cast aluminum foams and lattice structures

RWTH Publications (RWTH Aachen)

Abstract

This study investigates the fabrication, microstructural evolution, and mechanical performance of open-cell aluminium foams and lattice structures produced by investment casting route. Aluminum cellular materials are of significant interest for lightweight structural applications due to their high specific strength and energy absorption capacity. However, the combined effects of casting conditions, alloy composition, and architectural design on the deformation behavior of such structures remain insufficiently understood. By integrating the effect of alloy alteration with structural design, this work establishes new insights into the process–structure–property relationships of cast cellular aluminium materials. In Al–Si alloy foams, the mold temperature was found to exert a dominant influence on solidification microstructure and deformation behavior. Higher mold temperatures promoted dendritic microstructures with coarse eutectic silicon phases localized at the strut surfaces, which resulted in brittle deformation, premature failure, and serrated stress–strain responses. In contrast, lower mold temperatures favored globular aluminium grains and a more homogeneous silicon distribution, enabling ductile deformation and smoother stress–strain curves. These benefits, however, were offset by processing challenges, as insufficient mold filling and thinner struts limited the properties improvement of foams produced under such conditions. Local cooling rate variations during foam casting further influenced finer microstructural features. Although the overall change in cooling rate was moderate, faster cooling enhanced the effectiveness of strontium modification, refined the eutectic phase, and altered the morphology of iron-rich intermetallics. These findings highlight that sensitivity to microstructural change is higher, when solidification happens at slow cooling rate and in confined solidification space, where local diffusion and solidification dynamics strongly affect phase formation. The deformation behavior was also shown to be highly sensitive to the size and distribution of second-phase particles within micrometer-sized struts. Coarse, brittle phases such as eutectic silicon led to strain localization and premature failure, while refinement of these phases or their substitution by fine precipitates, as obtained in heat-treated Al–Cu alloys, promoted ductile and homogeneous deformation. In foams with a uniform and refined microstructure, the plateau stress correlated directly with intrinsic material properties such as strut hardness, reflecting similar trends to those observed in bulk alloys. This demonstrates that when brittle constituents are minimized, the foam-level mechanical response is governed by fundamental material parameters. Lattice structures produced by indirect additive manufacturing enabled systematic variation of cell geometry, which provided further insight into the interaction between architecture and material behavior. The results showed that relative density is less dominant in structures with thicker struts, where deformation is primarily governed by cell design. In bending-dominated lattices, brittle failure without bending could occur when unfavorable alloy chemistry or casting defects were present, indicating that architectural advantages can be neutralized by poor microstructural quality. At thicker strut dimensions, fine-scale microstructural modifications had little influence on deformation. However, as struts became thinner, microstructural sensitivity increased markedly, revealing an interaction between geometry and material behavior. Moreover, tensile load-bearing elements in cast lattice structures were shown to be mechanically ineffective, suggesting that such features can be avoided in design to improve efficiency. Overall, this work clarifies how processing, microstructure, and geometry interact to determine the performance of cast Aluminum foams and lattices. By linking alloy design to structural behavior across multiple length scales, the study provides a comprehensive framework for tailoring lightweight cellular metals through combined microstructural and geometrical optimization. The findings contribute to advancing the understanding of cast cellular Aluminum materials and offer guidelines for designing structures with improved mechanical responses.

Authors 1

1. Mahan Firoozbakht corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen