Fe-Cu multi-materials manufactured via laser directed energy deposition by in-situ alloying

Copper-steel multi-materials and functionally graded materials combine the excellent electrical and thermal conductivity of copper with the high mechanical strength of steel. Although Fe and Cu are substantially insoluble at the solid stage, their combination can create a sort of composite material at the microstructural scale which can potentially provide special features to properly designed multi-material systems. This study examines the microstructural, mechanical, and thermophysical properties of constitutive layers of commercially-pure Cu - 4130 steel structures fabricated via laser directed energy deposition using an in-situ alloying strategy. Four intermediate compositions were investigated: 100% 4130, 75% 4130 – 25% Cu, 50% 4130 – 50% Cu, and 25% 4130 – 75% Cu. The composition with lowest amount of Cu experienced solidification cracking due to large thermal stresses and insufficient liquid replenishment, while more abundant Cu-rich terminal liquid in other compositions could successfully backfill microcracks formed during early solidification stages. In general, Fe-Cu liquid phase separation and Marangoni convection generated a recursive macrostructure in samples, with copper and steel alternating as matrix and dispersed phase. Measurements of the thermal properties showed that interconnected Cu-rich regions in Fe50Cu50 and Fe25Cu75 could increase the diffusivity by ~50% and ~200%, respectively, compared to full steel samples. In situ tensile tests carried out inside a FEG-SEM allowed investigating the crack growth mechanisms of the different samples. These findings provide insights into the critical factors governing laser directed energy deposition of copper-steel multi-materials, contributing to process optimization and highlighting new design opportunities for multi-materials produced by additive manufacturing.