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"NPJ" New Study: The Role of Interface Strategies in Multi-Scale Hybrid Directed Energy Deposition

September 24, 2025 Materials 《npj Advanced Manufacturing》 111

The related research has been published in the journal npj Advanced Manufacturing under the paper title "On the role of interface strategy in multi-scale hybrid additive manufacturing".

Engineers at the Georgia Institute of Technology have conducted a systematic study investigating how interface strategies affect part quality in hybrid additive manufacturing, encompassing both wire and powder-based Directed Energy Deposition (DED) techniques.

Paper Link: https://www.nature.com/articles/s44334-025-00034-z

The study evaluated seven sample configurations and found that while machined surfaces can improve density, printed interfaces sometimes perform equally well or even better in terms of toughness, highlighting the trade-offs faced by industrial users.

The experiment paired ER70S-6 low-carbon steel deposited by wire-arc DED (DED-arc) with H13 tool steel deposited by laser-powder DED (DED-LP). Four interface strategies were tested: mapping a laser-isolated plane onto the arc surface, applying a least-squares best-fit plane, selecting the maximum surface height as a reference, and machining the arc surface. Each strategy was combined with either a parallel ("co-directional") or perpendicular ("reverse") direction for the first layer of powder. In these configurations, the self-regulating effect of the DED-LP process reduced surface flatness variation by 55% compared to the underlying DED-arc surface. However, surface contaminants inherent to arc welding, such as silicates and oxides, reduced the density from 99.5% in machined samples to 92.4% in unmachined samples.

Visualization of the asynchronous DED-arc and DED-LP processes. Image credit: Georgia Institute of Technology.

Porosity analysis revealed that when the DED-LP coating was deposited directly onto the contaminated surface, pore sizes ranged between 100 and 425 micrometers, whereas the machined control group showed no large pores. Hardness mapping showed a sharp transition from about 650 HV in the H13 coating to about 200 HV in the steel substrate when the DED-LP coating was deposited in the reverse direction, while a more gradual hardness transition was observed with co-directional deposition, indicating differences in hybrid bonding. Charpy impact tests highlighted the complexity of these effects.

The absorbed energy ranged from 105 J for machined samples to 145 J for unmachined samples, indicating that higher density does not necessarily equate to higher toughness. Statistical analysis confirmed this: density showed only a weak negative correlation with toughness (-0.46), while arc surface flatness exhibited a strong positive correlation (0.93). This suggests that rougher arc surfaces enhance toughness by increasing the proportion of ductile ER70S-6 in the cross-section.

Conditions of the interface strategies. Image credit: Georgia Institute of Technology.

Directed Energy Deposition has become a core technology for large-scale metal additive manufacturing. Arc-based systems offer deposition rates exceeding 200 mm³/s, enabling rapid bulk production but resulting in uneven surfaces with millimeter-scale deviations. Laser-powder systems operate at lower rates, around 20 mm³/s, but achieve finer feature sizes below 0.6 mm and support a wider range of materials, from stainless steels to high-entropy alloys.

A key advantage of DED-LP is its self-regulating effect, which stabilizes layer height by dynamically adjusting powder capture efficiency. Combining these processes—using arc for structural bulk and laser-powder for local details—holds significant industrial value, but the interfacial mechanisms between them remain poorly understood. Previous studies combining powder bed fusion and DED have shown that mismatches in roughness and oxidation at the interface can lead to defects, reducing hardness and crack resistance. Building on this, the Georgia Tech study directly compares interface strategies in macro-scale hybrid DED.

Comparison of surface profiles under different test conditions evaluated using optical profilometry. Image credit: Georgia Institute of Technology.

The findings indicate that machining or cleaning the arc surface provides predictable density and geometric uniformity but is not always necessary for mechanical resilience. In some cases, samples deposited on contaminated as-printed surfaces exhibited Charpy toughness equal to or greater than machined controls, reflecting the influence of the ductile ER70S-6 fraction. For high-value, low-volume components where fatigue life is critical (e.g., tooling), machining or thorough cleaning may be justified. For low-value, high-volume production, strategies such as using a best-fit plane without machining can provide sufficient performance while reducing cost and cycle time.