Research on Surface Strengthening and Fatigue Life Extension of High-Performance Centrifugal Impellers

High-performance centrifugal impellers are widely used in aerospace, energy equipment, and precision transmission systems and generally work in high-speed, high-load, and complex thermo-mechanical coupling environments. They are prone to surface fatigue failure and strength degradation. As a significant factor influencing the service life of centrifugal impellers, surface integrity has drawn widespread attention in recent years. To enhance their fatigue property, surface strengthening methods such as shot peening, laser cladding, surface rolling, arc spraying, carburizing, and ultrasonic composite strengthening have been established and enhanced incessantly.

Introduction

In the modern high-performance mechanical system, being the key component for energy transformation and fluid power transmission, centrifugal impeller structural design has evolved toward lightweight and high-speed characteristics. This not only enhances power density but also aggravates the trend of fatigue crack initiation in the surface layer. In real operation, centrifugal impellers are subjected to severe centrifugal loading, periodic flow disturbance, and thermal alternating stresses, so their surface is locally prone to stress concentration, which leads to the fast development of micro-cracks and ultimately generates macroscopic fatigue damage. 

Experiments have shown that more than 80% of fatigue cracks are initiated in the 5–50 μm area close to the surface. Therefore, surface strengthening treatment has become the priority technique for the improvement of impeller fatigue life. By optimizing the stress field, hardness gradient, and microstructure morphology of the surface layer, it effectively slows down crack initiation and propagation and enhances the overall structural durability. On these grounds, I believe that profound understanding of the strengthening mechanism and scientific determination of process parameters hold significant practical meaning for improving the safety and reliability of high-performance impeller systems.

Operational Loads and Fatigue Failure Mechanism of Centrifugal Impellers

Centrifugal impellers are widely used in high-speed rotating equipment such as aero-engines, gas turbines, compressors, and high-speed pumps. Their working environment is complex and changing, with the coupling action of multiple loads. During long-term operation, their structural fatigue can be easily induced by such complex loads, thereby resulting in cracking, deformation, or even catastrophic failure. Deep understanding of their operational load characteristics and fatigue failure mechanism is the theoretical basis for adopting anti-fatigue design and (life extension) strengthening treatment.

Characteristics of Operational Loads

Centrifugal impellers experience the following few load coupling effects at the same time in operation:

• Centrifugal stress: Increasing with the square of the rotational speed, it is the main factor causing stress concentration at the blade root;
• Fluid disturbance impact: Periodical load variation is caused by the complex flow structures, resulting in high-frequency fatigue;
• Thermal stress effect: Especially for hot-end parts such as aviation turbines, there is a huge thermal gradient, causing thermal fatigue;
• Machining residual stress and micro-defects: Poor initial surface quality will lead to preferential crack growth.

Fatigue Failure Characteristics

The centrifugal impeller failure mainly exhibits the characteristics of surface fatigue damage. Cracks generally originate from the blade suction surface, weld transition region, surface scratches, or stress concentration zones and propagate along grain boundaries. Their fatigue life is closely related to material surface integrity. The surface roughness, residual stress state, microhardness, and microstructure have significant effects on the crack initiation and propagation behavior. Therefore, the improvement of impeller surface integrity has been one of the primary anti-fatigue design methods.

Common Surface Strengthening Technologies and Mechanism Analysis

The nature of various strengthening technologies is to control the surface residual stress field for optimization and enhance hardness and microstructural stability. The specific mechanisms are as follows in the table:

Mechanism of Surface Strengthening in Improving Fatigue Life

In the application of high-performance impeller components, surface strengthening technologies are widely used to improve fatigue resistance and extend service life. Since impellers are long-term subjected to high-frequency vibration, thermal cyclic loads, and complex aerodynamic and mechanical stresses during service, traditional material strength design is hard to meet the requirements of harsh service environments. Therefore, strengthening the surface stress state, microstructure, and surface quality of materials through various surface strengthening methods has become an essential way to improve the fatigue life of impellers. This paper systematically addresses the action mechanism of surface strengthening technologies in improving fatigue performance from four aspects: residual stress regulation, hardness and microstructure evolution, morphology optimization, and phase transformation mechanism.

Introducing Residual Compressive Stress to Counteract Tensile Stress Concentration

Most of the surface strengthening processes (such as shot peening, rolling, laser shock, etc.) can generate a residual compressive stress field in the surface layer of workpieces. Their main role is to neutralize the inevitable periodic tensile service stress, thereby effectively inhibiting the initiation and growth of cracks. Especially at the critical load-carrying areas of impellers such as the blade root, disk connection, and weld heat-affected zone, these are usually the origin of fatigue cracks. By introducing a deep and stable compressive stress, the strengthening layer acts as a “mechanical buffer zone,” which can effectively reduce the actual stress peak carried by the surface and improve the reliability of the impeller under high-load fluctuation. Studies have shown that if the residual compressive stress value is over −300MPa, the fatigue life can be extended by 30% to 70%.

Synergistic Enhancement of Fatigue Strength through Surface Hardness Improvement and Microstructure Refinement

A number of strengthening methods will induce plastic deformation and microstructure evolution of the metal surface layer during the action process, such as grain refinement, dislocation densification, and precipitation hardening of the hardening phase, ultimately expressed as a clear increase in surface microhardness. Hardness strengthening can significantly improve the resistance of the material to fretting fatigue, erosion, and micro-crack propagation. For example, the rolling strengthening process can increase the surface hardness of aluminum alloy impellers by more than 50% without changing the geometric size and form a high-strength refined microstructure layer with a thickness of up to 200μm. For aviation compression impellers subjected to high-frequency vibration and multi-axial stress cycles, the hardness improvement is directly related to their stability and cracking resistance in long-term operation.

Surface Morphology Optimization Reduces Stress Concentration Sources

The aluminum alloy or titanium alloy impellers after processing typically have stress concentration locations in the form of microscopic pits, scratches, or process textures, which are (most likely to) become the starting location of crack formation. Surface strengthening technologies can make the surface more (smoother) through local extrusion, striking, or remelting, significantly reducing the surface roughness (Ra value) and number of micro-defects. Especially ultrasonic rolling, laser polishing, and shot peening with secondary rolling composite treatment not only can remove roughness but also can superimpose residual compressive stress and strengthening phase precipitation, forming a multiple strengthening mechanism superposition effect. According to my observation and actual measurement, by the composite strengthening process, the Ra value of the general aviation turbine impellers can be reduced from the original 0.8μm to 0.15μm, and the surface quality and resistance to crack initiation are significantly improved.

Metal Phase Transformation and Microstructure Reconstruction Improve Thermal Stability

Apart from residual compressive stress inducing, certain thermally induced strengthening processes (for example, laser cladding, laser shock strengthening, surface rapid annealing, etc.) are also accompanied by structure reconstruction processes such as metastable phase precipitation, martensite transformation, or carbon/nitride precipitation. These metal phase transformations can form a very stable strengthening microstructure zone in the surface layer, which improves its resistance to high-temperature creep, thermal fatigue, and corrosion. For example, nickel superalloys can form a fine MC-type carbide precipitation zone after laser shock, which can effectively seal the crack propagation path and improve thermal fatigue life. Similarly, the ultrafine lamellar martensite microstructure formed by laser surface rapid solidification of titanium alloys has superior strength-toughness matching capability and thermal stability, and is an optimal structural form for impellers operating under severe thermal loads.

Analysis of Test Data and Engineering Cases

Following composite strengthening (shot peening + rolling) of a 7075 aluminum alloy impeller of an aviation turbocharger, its fatigue performance was significantly improved. As shown in the following table:

Microstructure analysis shows that the grains of surface layer are significantly refined after strengthening, and the direction of crack is deflected and extended, effectively delaying the crack penetration time. The above example intuitively verifies the apparent effect of multi-strengthening method in engineering application.

Conclusion

From the systematic study in this paper, it can be observed that surface strengthening technology, being one of the important ways to enhance the fatigue performance of centrifugal impellers, has great potential in service life extension. Surface strengthening is not just a processing technology but an significant interdisciplinary subject linking materials science, mechanics, and manufacturing engineering. Continuing to develop its theoretical research and process innovation has invaluable significance in promoting the localization and reliability of high-performance power devices.