Understanding defect structures in metal additive manufacturing via molecular dynamics

TL;DR

This study uses molecular dynamics simulations to analyze how defect structures form in metal additive manufacturing, revealing the influence of solidification fronts, cooling rates, temperature, and inclusions on defect formation and reduction.

Contribution

It introduces a detailed atomic-scale analysis of defect evolution during metal additive manufacturing, highlighting the effects of solidification dynamics and inclusions on defect control.

Findings

Faster solidification fronts lead to defect-free crystals up to a critical layer thickness.

Slower cooling rates reduce defects, but benefits plateau below a certain rate.

Higher powder bed temperatures can significantly lower defect content.

Abstract

Additive manufacturing of a single crystalline metallic column is studied using molecular dynamics simulations. In the model, a melt pool is incrementally added and cooled to a target temperature under isobaric conditions to build a metallic column from bottom up. Common neighbor analysis (CNA) is used to observe the evolution of atomic scale defects during this process. The solidification is seen to proceed along two directions for an added molten layer. The molten layer in contact with the cooler lattice has a fast solidification front that competes with the slower solidification front starting from the top layer. Defect structure formed strongly depends on the speeds of the two competing solidification fronts. Up to a critical layer thickness, defect free single crystals are obtained as the faster solidification front reaches the top of the melt pool before the initiation of the slower front from the top. Slower cooling rates lead to reduction in defects, however, the benefits diminish below a critical rate. Defect content can be significantly reduced by raising the temperature of the powder bed to a critical temperature. This temperature is determined by two competing mechanisms: slower cooling rates at higher temperatures competing against increase in amorphousness as one gets closer to the melting point. Finally, effect of added soft inclusion (SiS2) and a hard inclusion (SiC) on the defect structure is studied. Hard inclusions lead to retained defect structure while soft inclusions reduce defective content compared to a pure metal.