With the growing demands on reliability and service life of high-end equipment, surface remanufacturing technology of key components such as impellers has emerged as an effective way to achieve equipment life extension and maintenance cost savings. Laser cladding technology, as a high energy density, low heat input, and favorable metallurgical bonding strength process, is being extensively applied in impeller surface repair and performance enhancement.
Introduction
As the important components in aerospace, petrochemical, power, and water conservancy industries, impellers typically work for a long time in high-temperature, high-speed, high-pressure, and corrosive environments. These severe environments readily cause the impeller surface to experience failure problems such as cavitation, erosion, and fatigue cracks. Traditional repair and replacement methods are not just costly but also involve a long cycle, which is difficult to meet the demands of efficient operation and maintenance. Remanufacturing technology, through reuse and performance restoration of retired impellers, develops an innovative approach to the sustainable use of critical components.
Among many remanufacturing technologies, laser cladding is one of the most significant technologies for the repair of impeller surfaces and functional enhancement because it can precisely control the cladding layer thickness and shape and has minimal thermal impact on the substrate. Especially for impellers with complex structures and harsh working conditions, laser cladding technology has seen widespread use due to its advantages.
Principles and Characteristics of Laser Cladding Technology
Laser cladding is a process in which metal powder or wire is melted by a high-energy laser beam to form a metallurgical combination with the substrate surface. The cladding layer and the base material have excellent bonding force in the process, thereby enhancing the wear resistance, corrosion resistance, high-temperature resistance, and other properties of the part surface.
The key characteristics of laser cladding technology include:
- Low heat input and small heat-affected zone: The high energy density and fast heating rate of the laser beam result in minimal thermal deformation of the impeller base material, avoiding thermal effect-related crack and deformation problems. It is highly suitable for repairing impeller components with complicated geometry and precise dimensions.
- High metallurgical bonding strength: There is a strong metallurgical bonding force between the cladding layer and the base material, and a dense transition layer is produced, significantly improving the adhesion and overall strength of the material.
- Excellent strengthening performance: By selecting different alloy powders, such as Ni-based, Co-based, or Fe-based alloys, laser cladding can significantly improve the wear resistance, corrosion resistance, and oxidation resistance of the impeller surface.
- Precise repair: Laser cladding has the ability to accurately control the thickness, form, and surface quality of the repair area and thus is particularly suitable for complex impeller structures and repair work with high dimensional precision requirements.
- High degree of automation: The automation degree of laser cladding technology is high, and it can be matched with robots or five-axis processing platforms to adapt to the repair of surfaces with complex shapes.
Surface Failure Modes and Repair Requirements of Impellers
During long-term service, the common failure modes of the impeller are mainly cavitation corrosion, erosion wear, thermal fatigue cracks, and corrosion cracking, which are characteristically manifested as:
- Cavitation corrosion: Due to the formation and instant collapse of bubbles on the blade surface by high-speed fluid, the material surface is worn away, especially at the blade tip and back.
- Erosion wear: Particles carried by fluid have a serious impact on the surface of the impeller, gradually making the material on the surface lost and having an impact on the working efficiency of the impeller.
- Thermal fatigue cracks: Because of the alternating stress between repetitive high temperature and cooling operation, micro-cracks readily occur on the surface of the impeller.
- Corrosion cracking: Especially in chemical media environments, the surface of the impeller is likely to be affected by corrosion cracking, which reduces the material strength.
Traditional repair methods, i.e., welding repair or casting repair, not only have high thermal effects, leading to deformation of the impeller and laborious restoration of surface quality, but also high operational difficulty and costly repair expense. Yet, laser cladding technology can effectively control heat input, repair the damaged area with high accuracy, and make up for the shortcomings of traditional repair methods.
Process Flow of Laser Cladding in Impeller Remanufacturing
As high-value impeller components are widely used in aviation, energy, chemical industry, and other areas, the issue of local failure due to fatigue, wear, or corrosion has become more and more prominent. The conventional replacement maintenance is expensive and time-consuming. Hence, laser cladding, a remanufacturing technology that has high precision and high bonding strength, is increasingly being applied to local repair and surface functional improvement of impellers. To ensure cladding repair quality stability and service reliability of remanufactured parts, the entire process flow needs to be system-designed and strictly controlled, which mainly includes the following five steps.
Defect Detection and Damage Assessment
The primary link of the process is to carry out general flaw detection and damage examination on the impeller to be repaired for accurate definition of the scope of repair and steady matching of repair proposals. There can be non-destructive testing methods such as ultrasonic flaw detection, X-ray examination, and eddy current testing, supplemented with three-dimensional optical scanning to obtain surface wear level, crack depth, and contour deviation information. In the most critical application scenarios, 3D modeling comparison technology will be capable of providing more direct deformation recognition results. Based on these data, technicians can accurately determine which regions can be utilized for cladding repair, which need to be replaced or strengthened, and formulate personalized process plans.
Surface Pretreatment and Contamination Cleaning
Pre-treatment before cladding directly affects the wettability of the molten pool, the bonding strength of the cladding layer, and the forming quality. First, the impeller should be released from dust and oil by alkaline cleaning agents or ultrasonic cleaning, and then the surface topography with a definite roughness should be established by sandblast or mechanical roughening to improve the metallurgical bonding effect. In the case of severe high-temperature oxidation, local pickling or laser removal of the oxide layer is also required. The entire pretreatment process needs to strictly regulate the cleanliness and roughness index so that inclusions do not enter the molten pool and cause cladding defects such as pores, lack of fusion, or inclusion layers.
Selection and Preplacement Process of Metal Powder
Depending on the type of raw material (i.e., stainless steel, nickel-based superalloy, titanium alloy, etc.) and service environment (high temperature, harsh corrosion, high-speed impact, etc.) of the impeller, compatible alloy powders with corrosion resistance, wear resistance, and thermal fatigue resistance are selected. For example, CoCrWC alloys for steam turbine impeller repair, Ni60, Inconel625 and other superalloy powders for aviation impellers. The powder needs to possess the characteristics of suitable particle size distribution, high sphericity, and low oxygen content, and the particle size is usually between 45-105μm. For single deep pits and (defect) areas, 3D printing-type preplaced powder filling technology can also be used to assist forming, improving repair efficiency and material utilization.
Setting of Laser Cladding Parameters and Process Control
The key to laser cladding is the molten pool stability and interlayer metallurgical bonding quality control. Technicians ought to appropriately set the laser power (generally 800-2000W), scanning speed (5-15 mm/s), overlap rate (30-50%), powder feeding rate, and protective gas (for example, argon) flow rate according to the material and workpiece geometry. In order to avoid the change of base metal microstructure caused by too many heat-affected zones, the heat input should be controlled by the multi-pass small amount and staggered cladding method. In complex curved surface areas, such as the root or trailing edge of the blade, the five-axis linkage head can be used to follow the curved surface trajectory to ensure that the cladding layer is uniform and dense, without delamination, cracks, or ablation defects.
Post-Treatment and Quality Inspection
The laser cladding repaired area typically needs mechanical post-treatment in order to restore the original geometric dimension and surface roughness of the impeller. Common post-treatments include CNC grinding, polishing, shot peening, or hot isostatic pressing (HIP). Grinding and precision polishing among them can effectively keep the cladding layer thickness error within ±0.05mm, and shot peening can generate residual compressive stress for fatigue life improvement. Finally, systematical check of the repaired area is made by metallographic microscopic inspection, microhardness testing, surface roughness testing, bonding strength testing, and corrosion resistance testing to ensure its structural integrity and service performance to satisfy the requirements of use.
Analysis of Typical Application Cases
In practical application, laser cladding technology has been utilized in repairing a number of impellers. For example, the stainless steel centrifugal pump impeller in a power plant formed (large-area) erosion and blade edge wear after 6 years’ running. By repairing with laser cladding of Ni60A powder, the performance of the repaired impeller was greatly improved:
- Cladding layer thickness: within 0.8mm control.
- Hardness improvement: increased from the original 190 HV to 620 HV.
- Surface roughness: controlled below Ra1.6 μm.
The repaired impeller has not experienced secondary spalling or cracking after 3 years of operation, and regular inspection has not detected any abnormalities. The repair cost is only 35% of the purchase price of the new impeller, actually saving a large amount of cost and reducing the replacement cycle of spare parts by about 60%.
Conclusion
Laser cladding technology has shown great potential in the remanufacturing and repairing of high-value complicated components such as impellers. Through appropriate process design and materials selection, laser cladding can significantly improve the surface performance of impellers, prolong their service life, and substantially reduce maintenance costs. With the further maturity of the technology, laser cladding will find use in more industrial sectors, and the prospect for future development is broad.
