Enhanced plasticity due to melt pool flow induced uniform dispersion of reinforcing particles in additively manufactured metallic composites

Additive manufacturing has gained increasing interest to fabricate metal matrix composites. Particle agglomeration remains a critical challenge and can severely compromise the mechanical properties, in particular tensile ductility. Here we investigated and utilized laser powder-bed-fusion (L-PBF) to uniformly disperse particles in metal matrices, taking advantages of particle flow dynamics during melting and remelting. We demonstrate our approach in a TiB2 reinforced aluminum system and produce a high dispersion of fine TiB2 particles (with sizes down to tens of nanometers) in a single print. Both micron- (1-5 μm) and nano-sized (40 nm) TiB2 particles are found to be uniformly dispersed in composites, with a volume fraction of up to 25%. High-fidelity computer modeling indicates that although particles flow with Marangoni vortexes, the melt pool overlapping and subsequent re-melting/re-solidification play an important role for the successful dispersion. The propelling effect of vapor depression and particle filling via the Marangoni vortex flow are the two main mechanisms that lead to the uniform dispersion. A geometric criterion for hatch spacing is derived and verified by experiments for the successful uniform dispersion of reinforcing particles. Compared to composites with nonuniform particle dispersion, L-PBF aluminum 2024 composites show ∼30% higher tensile strength and ∼16% higher Young's modulus, together with a marked improvement of tensile ductility (up to three times higher). The uniform dispersion of reinforcing particles is also achieved in copper materials and thus demonstrates the versatility of our approach to other metals and alloys.