As a key component in rotary machinery, impellers are usually subjected to severe working conditions such as high temperature, high speed, corrosion, and erosion, which impose extremely high requirements on surface performance. Surface treatment technology plays an indispensable role in both the manufacturing and remanufacturing of impellers in order to improve their fatigue life, corrosion resistance, and wear resistance. This article systematically reviews a number of surface treatment processes of high-performance impellers, including heat treatment, laser shock peening, thermal spraying, nano-surface modification, and traditional electroplating, phosphating, passivation, and Dacromet treatment. The principles, characteristics, and specific influence of these processes on the performance of impellers are compared, and, combined with typical application examples, the development trend of surface treatment technology under the background of green intelligent manufacturing is predicted.
Introduction
With increasingly demanding performance requirements for impellers in high-end manufacturing fields such as aerospace, energy equipment, and marine engineering, how to enhance the overall performance of impellers under harsh service conditions has been a research focus for engineering applications. Defects on the surface of the impeller have a tendency to become origins of cracking or sites of corrosion failure, and traditional processing methods have not met the demands of their long-term and high-reliability performance. Therefore, the redesign of the surface structure and performance of impellers through advanced surface treatment technologies is one of the effective ways for improving their service reliability and extending their service life. Especially under the “double carbon” policy and the requirements for clean production, green, efficient, and controllable surface treatment processes of impellers are increasingly replacing traditional polluting processes, becoming an important development trend for impeller manufacturing.
Overview of Impeller Surface Treatment Processes
As key components under high-speed rotation and harsh service conditions, the surface performance of impellers directly affects their fatigue strength, corrosion resistance, and operation stability. To improve the service life and operation safety of impellers, a variety of surface treatment technologies have been widely applied to different materials and application conditions. The following is a systematic review of current mainstream and advanced impeller surface treatment technologies.
Heat Treatment Technology
Heat treatment is a basic way to improve the comprehensive surface performance of impellers. Common techniques include surface quenching, nitriding, and nitrocarburizing:
- Surface quenching uses induction heating and rapid cooling technology to form a high-hardness hardened layer on the impeller surface, significantly improving its wear resistance and fatigue strength, suitable for low-alloy steel and medium-carbon steel materials;
- Nitriding treatment introduces nitrogen elements in the environment of low-temperature ammonia or ion to create a dense nitride layer with high hardness on the surface of the impeller for the purpose of improving corrosion resistance and wear resistance, which is suitable for titanium alloy and stainless steel components;
- Nitrocarburizing utilizes the synergistic diffusion effect of nitrogen and carbon to obtain a deeper infiltration layer and more superior performance gradient distribution, which is suitable for impellers with complex working conditions requiring strength and toughness.
Laser Shock Peening (LSP)
Laser shock peening is a non-thermal and non-contact process, using high-energy laser pulses to generate shock waves to excite a deep residual compressive stress layer on the surface of the impeller. The stress field inhibits the initiation and propagation of cracks, and fatigue resistance is significantly improved. LSP is widely used in aero-engine impellers, especially for titanium alloy and nickel-based superalloy components, and the fatigue life improvement is 30% to 70%.
Thermal Spraying Technology
Thermal spraying technologies form dense functional protective coatings on the impeller surface through high-speed spraying of metal or ceramic powders in the high-temperature molten state. The commonly used materials include NiCrAlY metal coatings and Al₂O₃-TiO₂ ceramic composite coatings, with the ability to resist high-temperature oxidation, corrosion, and wear effectively, and have extensive applications in ship propulsion systems, aviation gas turbines, and other severe environments.
Nano-Surface Modification Technology
With the cross-disciplinary development of materials engineering and nano-technology, nano-structured coating building or nano-particle doping on the impeller surface can accurately dominate the mechanical and physical-chemical behavior of the impeller surface at the micro level. For example:
- Nano-coatings can significantly improve hardness and wear resistance;
- Nano-doped layers have excellent oxidation resistance, self-cleaning, and corrosion resistance.
- This kind of technology has great application potential in high-performance turbine blades and chemical corrosion-resistant applications and is one of the main directions for future high-end impeller surface engineering.
Traditional Surface Treatment Processes
In spite of the fact that new treatment technologies are being developed continuously during these years, traditional processes still hold an important position in a broad area of industrial applications due to their mature technology and low cost:
- Electroplating treatment: Deposits metal coatings on the impeller surface through electrolysis to improve corrosion resistance, aesthetics, and electrical conductivity. Electrogalvanizing is widely used in general industrial impellers due to its low cost;
- Phosphating and passivation: Phosphating films can improve the anti-friction and corrosion resistance of steel impellers; passivation forms a stable oxide film on the surface of stainless steel, suitable for long-term service in high-humidity and high-salt environments;
- Dacromet treatment: Low-pollution and high-corrosion-resistance coating method, which has extensive uses in high-strength steel impellers with particular adaptability in marine and high-temperature and high-humidity environments;
- Zinc infiltration and electrostatic spraying: Zinc infiltration can achieve metallurgical bonding between the substrate and the zinc layer, which can improve the corrosion resistance life of impellers; electrostatic spraying can be used for rapid protection of impellers with common contours, but the evenness of adhesion on complex geometries is yet to be enhanced;
- Polishing and electrophoretic coating: Polishing improves smoothness and reduces flow resistance; electrophoretic coatings are dense and uniform, suitable for metal impellers with high corrosion resistance requirements in automotive, water pump, and other fields.
Influence of Surface Treatment Processes on Impeller Performance
Surface treatment not only changes the physical and chemical properties of the impeller surface but also significantly affects its overall service performance:
- Fatigue strength improvement: LSP and nitriding treatment can directly inhibit the initiation and propagation of micro-cracks by introducing a compressive stress layer, which is the key to high-cycle fatigue life improvement;
- Enhanced corrosion resistance: Thermal spray coatings, nano-anticorrosive films, and passivation treatment form barrier layers to effectively prevent the penetration of moisture and ions, especially suitable for salt spray and acid-base environments;
- Improved wear resistance: Surface hardening, ceramic coating, electroplated alloy layer, etc., can efficiently enhance the impeller’s resistance to particle erosion and abrasion, and are critical ways to extend the high-load impeller’s service cycle.
Typical Application Cases
- A type of aero-engine titanium alloy impeller is treated with laser shock peening and NiCrAlY thermal spraying. Tests have shown that its fatigue life is increased by 40%, high-temperature stability is enhanced, and maintenance costs are reduced effectively.
- Another bimetallic composite impeller for seawater circulation pumps, through the combined surface treatment of nitriding and ceramic spraying, showed no obvious corrosion appearances in the 1200-hour salt spray test, and the cycle was extended by more than 50%.
Conclusion
High-performance impeller surface treatment technology is the key to improving its reliability and service life. Heat treatment, laser shock, thermal spraying, nano-modification, and traditional electrochemical treatment each has its own characteristics and needs to be reasonably matched according to application conditions and material properties. In the future, it needs to be green manufacturing, intelligent optimization, and multi-functional integration to power the technological upgrading of high-end equipment impellers.
