Abstract: Achieving a micro-closed-cell structure with sub-100 µm pore sizes is key to enhancing the strength, energy absorption, and design flexibility of aluminum foams, but remains a significant challenge. This study introduces a novel two-step foaming (TSF) process to fabricate aluminum foams with ∼50 % porosity and an average pore size of ∼49 µm. The process begins with laser powder bed fusion (L-PBF) additive manufacturing to produce a foamable precursor containing uniformly dispersed ZrH₂. A subsequent semi-solid heat treatment triggers gas release through an in-situ reaction between ZrH₂ and the aluminum melt—a mechanism we term reactive foaming, which is a controllable foaming method. High-resolution nano-computed tomography (nano-CT) and scanning electron microscopy reveal that rapid solidification during L-PBF promotes dense pore nucleation, while the high viscosity of the semi-solid melt limits pore growth, resulting in a highly refined, close-pored microcellular structure. This architecture significantly improves the load-bearing capacity of the pore walls, giving the foam higher yield strength and energy absorption than comparable foams. The TSF method offers a scalable, additive manufacturing-based strategy for producing lightweight aluminum foams with superior mechanical performance and structural versatility, opening new possibilities for a myriad of structural and functional applications.

Key words: Aluminum foam, Microcellular structure, Laser powder bed fusion, Additive manufacturing, Reactive foaming