Researchers from the University of Waterloo’s Multi-Scale Additive Manufacturing Laboratory and the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) have developed a single-step reductive sintering process that allows binder jet additive manufacturing (BJAM) of steel using as-water-atomized powders. The method eliminates the conventional annealing stage used to refine powders before printing, achieving in-situ oxide reduction, carbon control, and densification exceeding 99.7 % solid. Published in Journal of Materials Processing Technology, the study outlines a pathway to lower-cost, lower-energy metal production using unrefined feedstock.
Steel additive manufacturing typically relies on gas- or plasma-atomized powders that undergo multiple pre-processing steps to meet strict purity and flowability requirements. Water-atomized powders, produced by high-pressure water jets that fragment molten metal, are far cheaper but contain surface oxides that require annealing in pure hydrogen at 900–1200 °C. According to earlier analyses, this annealing stage can account for more than half of the total energy used in powder production.
The new approach bypasses annealing entirely. Using as-atomized AISI 4340-type steel powder supplied by Rio Tinto Metal Powders, a Canadian manufacturer of ferrous metal powders, researchers printed components on an ExOne M-Flex BJAM system with a 20 % binder saturation. Subsequent reductive sintering was carried out under a 5 % H₂ – 95 % N₂ atmosphere. This atmosphere was selected to suppress hydrocarbon formation and prevent excessive carbon loss while maintaining a chemically reducing environment. At peak temperatures near 1455 °C, the team achieved near-full densification without geometric distortion.
Mechanisms governing chemical refinement
Characterization of the as-atomized powder revealed irregular particle morphology and oxide-covered surfaces containing iron, chromium, manganese, and silicon in various oxidation states. Thermodynamic modeling using modified Ellingham diagrams showed that high-valence oxides such as Fe₂O₃ and MoO₃ can be reduced by hydrogen at relatively low temperatures, whereas more stable oxides including Cr₂O₃ and MnCr₂O₄ require carbon-based reduction.
During heating, binder decomposition below 500 °C produced CO and CO₂ gases, followed by a hydrogen-driven reduction stage up to 600 °C. Beyond this temperature, carbon-based redox reactions became dominant, consistent with thermogravimetric and mass-spectrometric data showing increased CO evolution. Above 735 °C, corresponding to the body-centered cubic to face-centered cubic transformation in steel, CO formation accelerated, driving the reduction of remaining oxides through the Boudouard equilibrium.
At temperatures above 1200 °C, oxygen levels dropped below 0.03 wt %, while carbon stabilized within the AISI 4340 specification range of 0.38–0.43 wt %. This carbon balance proved critical for initiating supersolidus liquid-phase sintering (SLPS), where partial melting near 1445 °C enhanced particle rearrangement and pore elimination. X-ray tomography confirmed porosity below 0.3 %, validating the densification mechanism.
Comparative analysis and process control
Tests under a pure hydrogen atmosphere resulted in severe decarburization without significant oxide reduction, supporting the hypothesis that high hydrogen partial pressure promotes internal oxygen transfer reactions rather than net de-oxidation. In contrast, the diluted hydrogen mixture maintained a balance between carbon retention and chemical reduction.
Simultaneous thermal analysis combined with mass spectrometry identified distinct reaction intervals: binder burnout below 500 °C, hydrogen-mediated oxide reduction from 500 to 600 °C, transition to CO-driven redox reactions above 600 °C, and full de-oxidation near 1200 °C. These results align with thermodynamic predictions showing that carbon monoxide becomes the dominant reductant at higher temperatures, rendering hydrogen concentration less relevant to the final oxide removal.
Tomographic reconstructions of lattice-type BJAM specimens sintered at 1445 °C and 1455 °C revealed progressive pore annihilation and fragmentation of large, interconnected pores into isolated micro-pores. Bulk density measurements reached 99.44 ± 0.24 %, substantially higher than the < 92 % reported for BJAM 4340 steel produced from gas-atomized powders in earlier studies.
Water-atomized powders are widely used in powder metallurgy and metal-injection molding because of their low cost, yet their use in additive manufacturing has been limited by oxide contamination and poor sintering response. Previous research on nominally oxygen-free Cr–Mn steels still found coarse oxide inclusions after vacuum sintering, indicating an insufficient chemical driving force for reduction. Graphite additions in conventional powder metallurgy partially alleviate this but require multiple process stages.
By merging powder refinement and part consolidation into a single operation, the reductive sintering framework removes the need for separate annealing and external carbon additives. The methodology provides both the thermal energy and chemical potential necessary to reduce mixed oxides while precisely controlling carbon content. This integrated route supports the direct use of as-atomized powders for binder jet 3D printing of structural steels, lowering feedstock costs and energy consumption across the supply chain.
Results demonstrate that impurity-bearing powders can be transformed into dense, compositionally balanced alloys suitable for functional parts. The process maintains geometric fidelity in complex lattices, indicating potential for scaling to larger industrial components. Researchers plan to investigate how part volume influences gas diffusion and redox kinetics, as larger cross-sections may require adjusted dwell times to ensure uniform de-oxidation.
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Featured image shows Graphical Abstract. Image via ScienceDirect.
