Abstract This work investigates the in situ production of the Fe64Ni36 alloy (Invar 36) using a powder-based Directed Energy Deposition process with a plasma arc as energy source (DED-Arc). The alloy composition is achieved during fabrication by melting elemental iron and nickel powders directly in the plasma arc. The study focuses on the resulting microstructure, chemical composition, mechanical and thermophysical properties, with comparisons to conventional material. Results confirm a homogeneous alloy composition throughout the additively manufactured structure, with an average of 63.5 wt.% Fe and 36.5 wt.% Ni, validating the in situ alloying approach. Thermophysical and mechanical testing demonstrated that the characteristic low coefficient of thermal expansion (CTE) up to 200 °C and tensile strength is partly comparable to conventionally manufactured Invar 36. However, cross-sectional analysis revealed occasional unmolten iron particles positioned between layers, which reduced tensile strength in the build direction. To address this, an improved parameter set and an adapted powder-feeding strategy were developed. These modifications resulted in complete melting and mixing of the elemental powders, eliminating particle residues. The resulting mechanical behavior showed further improvement in the critical build up direction, despite this, interlayer bonding remained fully intact. Overall, the results confirm the feasibility of in situ alloying using elemental powders in the DED-Arc process and demonstrate that appropriate process tuning enables fully homogeneous alloy formation. This approach provides a flexible and cost-efficient pathway for on-demand alloy development and can be extended to more complex systems.
Rimpl et al. (Fri,) studied this question.