I. Research Background: Waste materials are difficult to utilize, and conventional approaches have significant limitations.

After sand‑mold casting, large marine aluminum‑bronze propellers require dry grinding to refine their external geometry. This process generates substantial quantities of irregularly shaped chips. The as‑cast chips exhibit sharp edges, highly irregular morphologies, and extremely poor flowability; if directly fed into a laser‑directed energy deposition process, they readily give rise to welding defects, thereby failing to meet production requirements.

Currently, research has sought to convert various metal chips into additive manufacturing powders, with processes such as ball milling and plasma melting being employed; however, these approaches commonly suffer from insufficient powder sphericity and difficulties in completely removing impurities. Building on this foundation, the present study introduces… Induction remelting + ultrasonic atomization A combined process is employed to explore pathways for the resource‑based recycling of aluminum bronze chips.

II. Core Process: Ultrasonic Atomization Powdering, Reshaping Chip Morphology

This study employs the material generated during the grinding of marine propellers. CuAl10Ni5Fe5 aluminum bronze chips Using it as the raw material, with commercially available gas‑atomized aluminum bronze powder as a reference, the core employs… Ultrasonic Atomization (UA) Technology Complete powder preparation.

Process flow The entire process is carried out under an argon‑inert atmosphere at grade 5.0. First, aluminum bronze chips are heated to 1300°C to achieve complete remelting; the molten metal is then discharged through a 1 mm‑diameter nozzle and dripped onto… 40 kHz high-frequency vibrating carbon-fiber ultrasonic horn Under ultrasonic vibration, high-temperature molten metal breaks up into tiny droplets, which naturally coalesce into spherical shapes due to surface tension. Upon cooling, these droplets solidify into metal powders, with an overall powder yield of 77% for the entire process.

Technological Advantages During the chip remelting process, alumina abrasives and organic impurities introduced by grinding float to form slag, which is then separated, thereby purifying the powder at the source. The entire process is conducted under an inert gas atmosphere, effectively suppressing metal oxidation. However, this process has a minor drawback: the carbon‑fiber sonotrode may shed trace amounts of fibers, leading to slight contamination of the powder; this can be mitigated by replacing the sonotrode with one made from a more compatible material.

III. Powder Performance Testing: Ultrasonic Atomization Yields Superior Overall Performance

The study employed multiple analytical techniques, including dynamic image analysis, SEM scanning electron microscopy, flowability testing, and oxygen content measurement, to compare the original chips, Ultrasonic atomization powder The three major samples of commercially produced gas‑atomized powders are summarized in the table below:

Based on the characterization results, three key highlights can be summarized:

1. Exceptional sphericity : Ultrasonic atomized powder exhibits a particle aspect ratio of ≥0.9 for 82% of particles, approaching perfect sphericity—significantly higher than the 43% observed in commercially available gas-atomized powders. SEM images further confirm that the ultrasonic‑atomized particles are smooth and uniformly shaped, with very few satellite or agglomerated particles.
2. Excellent liquidity In the Carney test, 150 g of ultrasonic‑atomized powder flows out in just 8.6 seconds, with a Hausner ratio of 1.05—placing it firmly within the range of excellent flowability. In the disc‑type powder‑feeding system of laser deposition equipment, its powder feed rate is approximately 30% higher than that of commercial powders, making it well suited to industrial production requirements.
3. Higher purity : The oxygen content of the ultrasonic atomization powder is only 0.0125%, lower than that of commercial powders, indicating excellent inert‑atmosphere protection; ceramic and organic impurities generated during grinding have been completely removed, with only trace amounts of carbon‑fiber contaminants remaining, which have a negligible impact on overall performance.

In terms of particle size distribution, the ultrasonic atomization powder exhibits a broader particle-size range (45–179 μm) and a higher proportion of both coarse and fine particles; this characteristic does not interfere with the normal operation of additive manufacturing.

IV. Properties of Additively Manufactured Specimens: Strength Increases, Mechanical Characteristics Undergo Changes

Research use Laser Directed Energy Deposition (DED-LB) The process employs the original process parameters used for commercial powders, utilizing ultrasonic atomization to recover powder and fabricate formed specimens, followed by microstructural and mechanical property characterization.

• Microstructure : The specimen exhibits a relative density greater than 99.8%, with only a very small number of minute pores, resulting in an overall dense microstructure free of significant defects. It was fabricated by laser deposition. Rapid cooling As a result, the material develops a fine needle‑like martensitic microstructure, with grains exhibiting preferential growth along the build direction—characteristic features of additively manufactured materials.
• Hardness performance The average Vickers hardness of the specimen reaches 266 HV, significantly exceeding that of conventionally sand-cast aluminum bronze (150–180 HV), demonstrating a pronounced hardening effect.
• Tensile properties The specimen exhibits a yield strength of 646 MPa and an ultimate tensile strength of 911 MPa, with strength significantly superior to that of conventional castings; however, the fracture elongation is only 9%, indicating a reduction in both ductility and plasticity, resulting in an overall tendency toward brittleness.

The underlying reason is that the casting process features a slow cooling rate, leading to the precipitation of ductile phases in the alloy. In contrast, laser deposition involves an extremely rapid cooling rate, which suppresses the formation of such ductile phases and ultimately results in a microstructure characterized by high strength but low ductility. Although this type of material is not suitable for the main body of marine propellers, it can be employed in applications such as wear‑resistant hard coatings, and its microstructure can be further tailored through subsequent heat treatment to enhance ductility.

V. Research Summary and Future Prospects

• Core Outcomes: With Ultrasonic atomization technology The core recycling process can convert aluminum‑bronze machining chips into high‑quality additive‑manufacturing powders. The resulting powders exhibit superior sphericity, flowability, and purity compared with commercially available gas‑atomized powders. Laser‑deposition specimens fabricated from these powders demonstrate high densification and exceptional mechanical strength, successfully enabling the circular reuse of metal waste.
• Existing problems : Ultrasonic atomization requires complete remelting of the metal, resulting in relatively high overall energy consumption that conflicts with low‑carbon, sustainable development principles; meanwhile, the carbon‑fiber amplitude‑modulating horn introduces trace carbon impurities, necessitating further optimization of component materials.
• Future Direction On the one hand, we optimize the components of ultrasonic atomization to eliminate impurity contamination; on the other hand, we explore low‑energy mechanical recycling processes and integrate them with atomization technology to reduce overall energy consumption. Furthermore, by coupling these approaches with heat‑treatment processes, we can tailor the mechanical properties of deposited parts, thereby expanding the range of applications for recycled powders.

This study has validated Ultrasonic atomization powder-making technology Its immense potential in the field of metal chip recycling not only establishes a closed-loop manufacturing chain—“waste–powder–finished product”—for the shipbuilding industry, but also offers a replicable technological paradigm for the resource recovery and high-value reuse of various metal‑processing waste streams.

Image and Text Guide

Figure 1. Scanning electron microscope (SEM) images of grinding debris prior to ultrasonic atomization. These debris consist of elongated particles with sharp edges, exhibiting highly non-uniform particle size distribution (PSD) and morphology.

Table 1. DED-LB welding process parameters used to fabricate the specimens in this study

Table 2. Summary of material characterization test results for sieved fragments, ultrasonic atomized powders, and gas‑atomized reference powders.

Figure 2. Particle size distributions of recycled ultrasonic atomized powders and commercially available gas-atomized powders.

Figure 3. a) Scanning electron microscope image of the reference powder produced by gas atomization; b) Ultrasonic‑atomized milled particles; c) Intact milled particles and carbon fibers recovered from the powder; d) Detailed structure of adherent carbon fiber particles.

Figure 4. Analysis results of the flowability of recycled powder and commercially available powder in the disc-type powder delivery system used in the DED-LB system employed in this study.

Figure 5. a) CAD drawing showing the nominal dimensions, micro‑section locations, and the flat tensile specimen; b) The base material and the fabricated specimen block after sawing the micro‑section specimen.

Figure 6. a) Overall image of the etched microsection; b) Close-up micrograph showing the microstructure, with grain structures exhibiting a fine needle-like morphology.

Figure 7. As the height of the additive-manufactured sample (produced from ultrasonic atomized powder) increases, the curves depicting the three hardness measurements at different locations exhibit a uniform distribution.

Figure 8. Tensile test results for DED-LB specimens fabricated using recycled powder (blue curve). The figure also shows the tensile curves obtained for the same alloy via sand casting (gray curve, with a very low cooling rate) and permanent‑mold casting (orange curve, with a relatively rapid cooling rate). Tensile test data for these casting processes were all derived from material testing conducted by Mecklenburger Metallguss GmbH.