From Heat Treatment to Ion Implantation: A Review of Surface Treatment Technologies for Improving Cavitation Erosion Resistance of Titanium Alloys
Titanium alloys have relatively low hardness and poor wear resistance. When used in components such as marine propeller blades, hydro-turbine blades, turbines, draft tubes, pumps, and valves, they are exposed to flowing fluids with pressure variation zones, making them highly susceptible to cavitation erosion. When cavitation bubbles collapse near a solid boundary, material surface spalling occurs, leading to performance degradation, shortened service life, and even component failure. In recent years, significant progress has been made worldwide in protecting titanium alloys from cavitation damage. The following discussion summarizes developments in heat treatment, surface engineering, and corrosion inhibitor addition.
1. Heat Treatment
Heat treatment improves cavitation resistance by modifying the microstructure of titanium alloys. Researchers treated Ti-6Al-4V alloy at different temperatures (1020°C, 950°C, and 850°C), obtaining Widmanstätten structure, duplex structure, and equiaxed structure, respectively. After an 8-hour cavitation test, all heat-treated samples showed lower cumulative mass loss than the as-received material.
Among them, the sample water-quenched from 1020°C with a Widmanstätten structure exhibited the best cavitation resistance, with mass loss only 77.9% of the original sample and a cavitation resistance coefficient increased to 1.83 times.
The strengthening mechanisms vary among different microstructures. The martensitic α′ phase and lamellar α phase in the Widmanstätten structure improve fracture toughness and tensile strength, enhancing the material’s ability to absorb cavitation bubble collapse energy. In the duplex structure, the primary α phase provides strong work-hardening capability, while the secondary α phase in the transformed β matrix improves strength and hardness. Although the equiaxed structure also contains secondary α phase, its content is relatively low, resulting in limited improvement in cavitation resistance. Overall, the Widmanstätten structure shows the best performance, followed by the duplex structure, with the equiaxed structure performing the worst.
2. Surface Treatment
Since cavitation erosion initiates at the material surface, surface engineering has become an effective approach to improving resistance. Current techniques mainly include laser surface texturing, laser gas nitriding, thermochemical treatment, ion implantation, and hot-dip coating.
Laser surface texturing:
This technique selectively melts the workpiece surface using a laser to create specific surface morphologies. It offers short processing time, high precision, a small heat-affected zone, and no environmental pollution. The resulting micro-dimples can trap wear debris and improve wear resistance, while also increasing surface hardness. It has recently been applied to enhance cavitation resistance in titanium alloys.
Laser gas nitriding:
In a nitrogen atmosphere, a high-energy laser beam heats the titanium alloy surface to a molten state. Nitrogen dissolves into the melt pool and reacts to form titanium nitride hard phases, creating a nitrided layer. This method is low-cost, fast, allows localized treatment, and produces a relatively thick metallurgically bonded layer, making it effective for improving cavitation resistance.
Thermochemical treatment:
This method is simple and cost-effective. It forms a dense ceramic layer and diffusion zone on the titanium alloy surface, increasing surface hardness to resist cavitation bubble collapse energy and preventing crack initiation and propagation, thereby extending the incubation period of cavitation damage. However, it requires long processing times, high energy consumption, and may cause environmental pollution. Improving its economic efficiency remains a future goal.
Ion implantation:
High-energy ions are used to bombard and implant elements into the surface. The modified layer has strong adhesion to the substrate with no abrupt interface. Any element can be implanted with controllable concentration and depth. Since it does not change the sample dimensions or surface roughness, it can serve as a final surface treatment step for components.
Other surface treatments:
A combination of hot-dip coating and heat treatment can produce a uniform and dense Al₃Ti coating on Ti-6Al-4V alloy. After 20 hours of cavitation testing, the cumulative mass loss is only 53.7% of that of the untreated alloy. The coating is dense and uniform with significantly higher microhardness than the substrate, enhancing resistance to cavitation bubble collapse impact.
3. Addition of Corrosion Inhibitors
Cavitation damage in titanium alloys in corrosive environments often involves synergistic effects of corrosion and mechanical erosion. Adding corrosion inhibitors is an effective approach.
Studies show that in a 55% LiBr solution with 1% NaNO₂ inhibitor, Ti-6Al-4V exhibits significantly reduced cavitation damage after 1 hour, with fewer surface pits observed. After 8 hours, cumulative mass loss is only 90.5% of that without inhibitor.
NO₂⁻ ions in the inhibitor are adsorbed onto active sites of the passive film, promoting repassivation and suppressing electrochemical corrosion, thereby reducing cavitation damage caused by the synergistic effect of mechanical and electrochemical processes. However, corrosion is not completely eliminated, and further optimization of this technique is still required.
