Tiny Titanium Doses Transform 3D-Printed Marine Bronze Into Stronger, Corrosion-Proof Alloy

Ship propellers, valve systems, and underwater pipelines endure some of the harshest conditions on Earth: relentless saltwater, mechanical stress, and constant cyclic loading. The material of choice for these components has long been nickel-aluminium bronze, a copper-based alloy prized for combining strength with outstanding corrosion resistance. But as manufacturers increasingly turn to wire arc additive manufacturing (WAAM), a 3D-printing technique that builds metal parts layer by layer with an electric arc, they have run into a stubborn problem. The printing process tends to produce coarse, columnar grains that grow preferentially along the build direction, leaving the finished part mechanically anisotropic and structurally uneven. A new study from researchers at Harbin Institute of Technology and De Montfort University, published in Advanced Materials Joining, shows that a dash of titanium nanoparticles, added in astonishingly small quantities, can rewrite that story, delivering simultaneously stronger and more corrosion-resistant printed bronze.

The team, led by Lamei Zhang and corresponding author Xiaoyu Cai, used a cold metal transfer WAAM system, a variant of the technique that keeps heat input low and reduces spatter and residual stress. Their feedstock was a 1.2-millimetre nickel-aluminium bronze wire deposited onto a 5-millimetre substrate, building single-pass multilayer walls measuring 200 by 6 by 65 millimetres. The innovation lay in how the titanium was introduced. Rather than pre-alloying the wire, the researchers prepared ethanol solutions containing 50-nanometre titanium powder at concentrations of 0.03, 0.05, and 0.1 weight percent, ultrasonically dispersed them, and brushed the solution onto each layer between deposition passes. Once the ethanol evaporated, the next arc pass remelted the surface, dissolving the nanoparticles directly into the molten pool. This in-situ strategy merges manufacturing and modification into a single integrated process, sidestepping the need for separate post-print heat treatments that have historically been the main tool for homogenising WAAM bronzes.

The results revealed a strikingly non-monotonic relationship between titanium content and grain structure, one that defies the simple intuition that more refinement agent always means finer grains. At 0.03 weight percent titanium, the opposite of refinement occurred: average grain size ballooned to 75.2 micrometres, nearly double the 39.1 micrometres of the unmodified printed alloy, and columnar grains proliferated. The explanation lies in a nucleation threshold. Using classical solidification theory, the team calculated that at this low loading, roughly 65 percent of the titanium particles dissolve in the ferociously hot arc molten pool, dropping the effective nucleation site density below the critical value of about 2 times 10 to the 12 per cubic centimetre. With too few active nucleants and a solute drag force far too weak to impede grain boundary migration, the surviving particles could not arrest growth, and the microstructure coarsened.

At 0.05 and 0.1 weight percent, however, the physics flipped decisively. Enough titanium particles survived the thermal assault to push nucleation density past the threshold, triggering prolific heterogeneous nucleation that shattered the coarse columnar regime. Average grain sizes plummeted to 17.3 and 21.6 micrometres respectively. The team also invoked Zener pinning theory, showing that the calculated limiting grain size of about 20.6 micrometres at 0.05 weight percent, after applying a particle distribution correction, matches the measured 17.3 micrometres remarkably well. Electron backscatter diffraction added another dimension: the strong (100) crystallographic texture of the low-titanium sample, with a pole density peak of 7.13, weakened dramatically to 3.13 and then 2.91 as titanium content rose, signalling grains oriented far more randomly and uniformly through the build.

Crucially, transmission electron microscopy confirmed that the titanium did not create any new phases or disrupt the alloy’s delicate multiphase constitution. The printed bronze retained its characteristic alpha matrix, martensitic beta-prime phase, spherical kappa-II precipitates, lamellar kappa-III precipitates, and coffee-bean-shaped kappa-IV iron-rich particles. Energy-dispersive spectroscopy showed the titanium dissolving predominantly into the alpha matrix at concentrations matching the added amounts, while the lamellar kappa-III phase shrank in axial size as titanium content climbed. This means the modification works through grain refinement, solid-solution strengthening, and dislocation pinning rather than by interfering with the thermodynamics of precipitation, preserving the corrosion advantages inherent to conventional nickel-aluminium bronze chemistry.

The mechanical consequences were substantial. Hardness rose by 6 to 8 percent across all titanium-modified samples, reaching peaks near 195.6 HV0.3 compared with 181.2 HV0.3 for the unmodified deposit, and remained remarkably uniform along the build height because each Vickers indentation samples many grains and phases at once. More importantly, tensile testing in both longitudinal and transverse directions showed that the strength-orienting effects of columnar growth had been tamed. The 0.1 weight percent titanium alloy delivered the best overall package: a longitudinal ultimate tensile strength of 718.2 megapascals, up from 664.2 megapascals in the as-deposited alloy, with a yield strength of 372.5 megapascals and 28 percent elongation. The gap between testing directions narrowed to within experimental error, evidence that mechanical anisotropy, the Achilles heel of printed metals, had been effectively suppressed.

Fractography told a coherent mechanistic story. The 0.03 weight percent sample, cursed with coarse columnar grains and a preferred crack path, failed in a quasi-cleavage brittle mode in the transverse orientation, posting the lowest elongation at 20 percent and a yield strength scatter of plus or minus 33.4 megapascals that betrayed deep microstructural heterogeneity. In contrast, the higher-titanium alloys fractured in a ductile, dimpled fashion even in transverse specimens. Pores and cracks in all samples originated at hard precipitate phases and then extended into the alpha matrix, but the refined and spheroidised precipitate morphology in the 0.1 weight percent alloy improved resistance to crack propagation without altering the fundamental initiation mechanism.

Corrosion performance, arguably the decisive metric for marine service, tracked the microstructural improvements with impressive fidelity. After 21 days of immersion in 3.5 weight percent sodium chloride solution, the 0.1 weight percent titanium alloy showed the lowest corrosion rate at 0.167 grams per square metre per day, with mass loss stabilising near 3 milligrams. Its corrosion product layer thinned to 1.83 micrometres, compared with 2.43 and 2.15 micrometres for the lower-titanium variants, and its corrosion interface was straight and uniform, indicating stable passivation. Electrochemical testing confirmed the trend: corrosion current density fell 15.6 percent to 8.49 microamperes per square centimetre, corrosion potential shifted positively by 4.34 millivolts, and the open-circuit potential stabilised in just 18 seconds, the fastest of any sample, reflecting rapid formation of a dense passive film. X-ray diffraction identified the protective products as aluminium oxide and cuprous oxide, the same chemistry that makes bronze a marine workhorse, but now deployed across a finer, more homogeneous microstructure that lengthens the path corrosive ions must travel.

What makes this study resonate beyond one alloy system is the demonstration that microalloying can break the traditional trade-off in which improving one property degrades another. Previous composition tweaks to nickel-aluminium bronze, whether raising nickel content, adjusting the aluminium-to-nickel ratio, or adding tin or chromium, typically boosted strength at the expense of ductility or corrosion behaviour. Here, the coordinated action of grain refinement, precipitate morphology optimisation, and texture weakening allowed strength, ductility, and corrosion resistance to rise together, a synergy the authors connect to the broader concept of escaping performance trade-offs through multi-microstructural feature cooperation. For naval architects and offshore engineers, the practical message is that a titanium addition of 0.05 to 0.1 weight percent, applied during printing rather than after, offers a low-cost route to printed bronze components that could finally match or exceed their cast and forged ancestors, opening the door to large, complex, corrosion-proof marine hardware built directly at the arc.

Subject of Research: Titanium nanoparticle microalloying of wire-arc additive manufactured nickel-aluminium bronze

Article Title: Effect of low-level Ti microalloying on the microstructure, mechanical properties, and corrosion behaviour of WAAM-processed nickel-aluminium bronze

Article References: Zhang, L., Cai, X., Song, B., Dong, B., Lin, S., & Khoshnaw, F. (2026). Effect of low-level Ti microalloying on the microstructure, mechanical properties, and corrosion behaviour of WAAM-processed nickel-aluminium bronze. Advanced Materials Joining, 1(1), Article 17. https://doi.org/10.1007/s44500-026-00016-x

Image Credits: AI Generated

DOI: 10.1007/s44500-026-00016-x

Keywords: nickel-aluminium bronze, wire arc additive manufacturing, titanium nanoparticles, microalloying, grain refinement, corrosion resistance, mechanical properties, cold metal transfer, heterogeneous nucleation, marine engineering, texture weakening, passivation