Formation mechanisms of meso-scale element segregation in multi-material metal additive manufacturing

Meso-scale element segregation (MES) critically affects the property and reliability of multi-material metal additive manufacturing (AM) parts, yet its formation principles remain insufficiently understood. In this work, we reveal the formation process of MES through high-fidelity multi-physics simulations combined with in-situ and ex-situ experiments. We find that the fundamental modes of MES can be classified into stratified segregation and vortex segregation, according to the meso-scale fluid transport behaviors near the solidification front. In stratified segregation, melts with different compositions tend to separate and solidify at different positions of solidification front. In vortex segregation, however, composition-distinct melts can solidify at the same position with comparable probability. We further show that the formation process of MES can exhibit strong instability, which results in spatially inconsistent MES in the printed part. The instability mainly originates from the intrinsic chemical inhomogeneity in the feedstocks and is further amplified by the alloying reaction in the melting process. Guided by the mechanistic insights, we propose several strategies to regulate MES in AM process. This work establishes a physical foundation for the precise composition engineering in multi-material AM.