With the development of efficiency and stability requirements of modern fluid machinery, mirror-class impellers have become key hardware in aviation, energy, chemical, and high-quality pump industries due to their high surface finish quality and anti-friction corrosion. As a priority secondary processing technology, mirror finishing not only has the ability to reduce flow channel resistance effectively and inhibit surface crack growth but also enhance the fatigue life and appearance effect of impellers.
Performance Demands and Surface Challenges of Mirror-Class Impellers
Mirror-class impellers typically refer to impellers after high-precision mirror surface processing, with Ra surface roughness value typically less than 0.05μm, even reaching 0.02μm. In engineering practice, these impellers are widely used in the following industries:
- High-speed rotating equipment: such as aviation compressors, gas turbine impellers, etc.;
- High-cleanliness environments: such as pharmaceutical pumps, food-grade centrifugal equipment, etc.;
- High-corrosion/fatigue resistance conditions: such as seawater pumps, chemical circulation systems, etc.
They impose very stringent specifications on impeller surface quality, in particular quantified as:
- Extremely low surface roughness: reducing boundary layer disturbance and flow resistance;
- No micro-cracks or scratches: avoiding fatigue crack sources;
- Integrity of the anti-corrosion passivation layer: ensuring material stability;
- Appearance quality and reflection performance: with special requirements in medical and testing equipment on high reflectivity and mirror effects.
The complex curved surface shape of impellers, high-hardness materials, and hard-to-reach internal cavity regions by traditional polishing processes, however, make mirror finishing processes face some technical challenges. The selection of appropriate mirror finishing processes for different impeller materials and shape characteristics, therefore, has been the key factor in maximizing impeller performance.
Typical Mirror Finishing Processes and Technical Characteristics
In precision parts production, especially in applications with very high surface quality requirements such as aviation impellers, mold cavities, and high-end medical tools, mirror finishing processes have come to be major post-processing interfaces. Different mirror finishing processes differ in principles, materials that can be processed, surface roughness that can be obtained, and part structures that are best suited for them. The following technically compares five widely used mirror processes and notes their pros and cons.
Ultra-precision Polishing (Manual/Mechanical): High Flexibility for Local Trimming
Ultra-precision polishing makes use of (abrasive paste) or polishing liquid as the medium, in combination with velvet wheels, wool wheels, or rubber soft tools for successive mechanical grinding, ending up with a highly surface roughness reduction. Its main advantage is elastic operation, for example, controlling the trajectory of polishing and force according to the actual surface state of the part, particularly suitable for small-batch production and delicate repair of the critical regions. In mold surfaces or at turbine blade corners, for example, this technique can effectively compensate traces resulting from milling or electrical discharge machining.
But this process is highly operator skill and ability dependent, not only low efficiency but also difficult to ensure consistency in mass production. In addition, poor repeatability and hard edge control restrict its promotion in automated manufacturing processes.
Electrochemical Polishing: Micro-peak Removal, Suitable for Metal Mirrors
Electrochemical polishing is based on the metal’s micro-dissolution reaction when used as an anode in the electrolyte to lead to surface smoothing by preferentially dissolving micro-peaks and preserving micro-valleys. Its optimum technical advantage is the potential for achieving extremely low surface roughness, with some stainless steel materials having Ra < 0.05 μm, and are suitable for high-cleanliness mirror finishing of food-grade and medical-grade stainless steel components.
Yet, the technology has definite geometrical constraints on parts—deep holes, blind holes, and other areas are difficult to process due to non-uniform electric field distribution. On the contrary, the electrolyte is corrosive itself and has strict requirements on processing environment and safety protection. Especially with complicated spatially structured parts, polishing uniformity is difficult to ensure without auxiliary electrodes or flow field regulation.
Chemical Mechanical Polishing (CMP): Nano-level Surface Refinement Technology
CMP involves the two mechanisms of chemical corrosion and mechanical abrasion, widely used in surface repairing. of semiconductor wafers, ceramic substrates, and critical optical components of aero-engines. It uses a special polishing pad and buffer solution to make abrasives particles act in a controllable fashion to remove surface material in the chemical reaction layer to ultimately reach the Ra 0.01 μm level, one of the key processes of production in modern nano-precision mirrors.
The major drawback of CMP is its high system complexity and narrow process window. It requires very high stability demands from equipment, cleanliness, slurry composition, and workpiece flatness, typically requiring a specialized clean room operating environment, with high cost investment. Thus, it is more appropriate for high-value-added, high-precision demand environments.
Magnetic Abrasive Finishing (Magnetorheological/Magnetic Needle): High-precision Polishing for Complex Internal Surfaces
Magnetic abrasive finishing controls the flow of magnetic needle clusters or abrasive suspension in a magnetic field, and they grind evenly on the workpiece surface or channel as an ultra-high precision method for cutting hard-to-reach internal cavities and closed curved surfaces. Especially in complex structures such as aviation compressor blades and high-precision injection mold cooling channels, magnetorheological finishing can bypass the limitations of traditional tools that cannot make contact.
But magnetic abrasive finishing is usually characterized by poor processing efficiency and is a traditional “high-precision, low-output” process. At the same time, it requires stringent conditions for distribution of magnetic field strength, workpiece magnetic permeability, and abrasive properties, and the equipment cost and maintenance complexity also limit its extensive use in traditional processing.
Polishing Shot Peening (Micron Beads): A Combination of Rapid Cleaning and Pre-polishing
Polishing shot peening employs micron-sized abrasive beads whose diameter is between 10~100 μm, propelled by compressed air to strike the surface of the workpiece at high speed, and realizes the function of removing stuck particles simultaneously and enhancing 光洁度 (smoothness). Polishing shot peening can be used for pre-polishing treatment of large-area surfaces, which is capable of homogenizing the surface texture efficiently and enhancing the uniformity of following processes.
But since shot peening is an impact process, there can be a possibility of some plastic deformation on the surface, and its final surface roughness is difficult to attain mirror level, usually required to be combined with ultra-precision polishing or chemical mechanical polishing as secondary process. It is suitable for applications with high requirements for processing efficiency but moderate requirements for final precision.
Adaptability of Mirror Finishing for Different Material Impellers
The selection of mirror finishing processes usually depends on the material of the impeller, and materials have different flexibility towards mirror finishing:
- Stainless Steel Impellers (such as SUS304/316L): Suitable to combine electrochemical polishing with magnetic abrasive finishing. Electro-polishing can peel off the oxide layer and machining texture quickly, while magnetic abrasive finishing is suitable for fine polishing of inner walls with complicate structures.
- Titanium Alloy Impellers (such as TC4): The titanium alloy surface oxide film is very stable, and hence electrochemical polishing is inappropriate. Chemical Mechanical Polishing (e.g., hydrofluoric acid system + grinding fluid) and ultrasonic cleaning in combination must be used to achieve mirror effects.
- Nickel-based Superalloy Impellers (such as Inconel 718): The alloy is hard and also electrically conductive, ideal for coarse electrochemical polishing followed by fine polishing of complex curved surfaces with magnetic needle grinding.
Case Analysis: Performance Improvement Effect of Mirror Finishing on Impellers
In one nickel-based impeller sample of an aviation compressor, the following treatment process was used:
- Pretreatment: After five-axis machining, the Ra value was 0.8μm;
- Mirror finishing process: rough electro-polishing + precision magnetic abrasive finishing + ultrasonic cleaning.
Treatment effects:
- Surface roughness Ra decreased to 0.03μm;
- Evenness of the oxidation passivation film thickness augmented by 22%;
- Efficiency in high-speed airflow tests increased by about 3.5%, and rise in temperature decreased by 1.2°C;
- Fatigue crack initiation life was extended by about 25%.
This indicates that mirror finishing not only improves the looks and surface quality of impellers but also has major impacts on hydrodynamic performance and fatigue life, especially in those high-precision applications where irreplaceable benefits exist.
Conclusion
Mirror finishing technology plays an important role in mirror-class impeller manufacturing, not only improving the surface quality of impellers but also the hydrodynamic performance and fatigue life of impellers. With reasonable process selection for different materials and structures and the combination of intelligent control and digital technology, mirror finishing processes in the future will continue to develop towards efficiency, greenness, and intelligence, and be an important supporting force for high-end impeller product manufacturing.
