Objective As the core part of energy conversion of industrial steam turbine, the blade plays an important role in the safe operation of a steam turbine. However, the last stage blade usually suffers from cavitation, leading to severe vibration, blade fracture, and other malignant events. Since cavitation usually starts from the blade surface, it is an economic and effective method to prepare anti-cavitation coating on the blade surface by coating technology, which has attracted significant attention. Cobalt-based alloy Stellite-6 is widely considered as one of the most ideal materials for cavitation-resistant coating of steam turbine blades due to its good corrosion resistance, wear resistance, and high-temperature resistance. Traditional coating technologies, such as laser cladding (LC) and thermal spraying, have adverse thermally-induced effects, such as phase transformation, dilution, and decomposition. Supersonic laser deposition (SLD) technology is a material deposition technology combining laser and cold spraying. It can realize the deposition of high-strength materials (e.g., Stellite-6) while avoiding the adverse effects caused by massive heat input. In this study, SLD and LC are employed to prepare Stellite-6 coating. The cavitation-resistant properties of the two kinds of Stellite-6 coatings are evaluated. The underlying mechanisms are clarified based on microstructure, dilution ratio, elastic modulus, and hardness. This study is expected to provide process support and theoretical guidance for the fabrication and performance optimization of cavitation-resistant coating for steam turbine blades.

Methods Stellite-6 coating is prepared on 17-4 PH stainless steel through SLD and LC processes. The cavitation-resistant properties of the two kinds of coatings are tested using an ultrasonic cavitation method according to ASTM G32. The cavitation sample is assembled with the bottom of the ultrasonic horn through a thread connection. The test medium is NaCl (the mass fraction is 3.5%) solution and the constant temperature is 25 ℃. During the test, the coating side of the cavitation sample is immersed in the medium solution for 20 mm, the ultrasonic vibration frequency is 20 kHz, and the peak-peak amplitude is 50 μm. The duration of the cavitation test is 14 h. After every 1 h of cavitation, the sample is taken out, cleaned with alcohol, and dried. Then, the sample is weighed with an electronic scale (accuracy of 0.001 mg) three times to take the average value. Mass loss is recorded before continuing the experiment. The cavitation-resistance is characterized by cavitation mass loss and cavitation rate.

Results and Discussions As shown in Table 1, in the first 2 h, Stellite-6 coatings prepared by LC and SLD processes have similar cavitation mass loss and cavitation rate, which corresponds to the incubation stage of the cavitation process, and the cavitation rate is slow (less than 1 mg/h). In the following stage, the cavitation mass loss of the LC sample increased rapidly, and the cavitation rate increased rapidly and remained above 2 mg/h. However, the cavitation mass loss of SLD sample increased slowly, and the cavitation rate remained at about 0.7 mg/h during the whole cavitation process and increased to more than 1 mg/h only when the cavitation time is 14 h.

The LC coating has a typical coarse cladding dendrite structure (Fig. 7 (b)), while the SLD coating retained the fine dendrite structure inside the deposited powder particles (Fig. 7 (d)), which is related to the laser energy input during the two processes. The laser energy density is calculated to be 72.79 J/mm² and 35.03 J/mm² for LC and SLD processes, respectively. The fine dendrite structure of the original powder particles remained in the SLD coating due to lower heat input. It is reported that grain refinement is essential for improving the cavitation-resistance of materials. Thus, the finer dendrite structure in SLD coating is responsible for its better cavitation-resistance than LC coating.

As shown in Fig. 8 (a), the LC coating had severe element dilution of Fe from the substrate while Fe element is almost not detected in SLD coating (Fig. 8 (b)). The Fe element from the substrate changes the original chemical composition of the Stellite-6 alloy and affects its cavitation-resistance. The higher dilution degree of the LC coating is responsible for its inferior cavitation-resistance compared to that of the SLD coating. SLD is a material deposition process based on plastic deformation of powder and substrate. During the coating preparation process, the material will undergo work-hardening; thus, its hardness is higher than that of LC coating (Fig.9), which is essential for cavitation-resistance.

To investigate the cavitation mechanism of the Stellite-6 coating prepared through LC and SLD, the surface morphology of the coating after different cavitation time is analyzed. The phase/grain boundary is the preferred position of cavitation in the LC coating, indicating a uniform surface morphology (Fig.10). The pores between particles are the initial position of cavitation in the SLD coating, indicating a non-uniform cavitation process (Fig.11).

Conclusions In this study, the cavitation-resistant properties of Stellite-6 coatings prepared by SLD and LC processes are compared. The reasons for the advantages and disadvantages of the two coatings are clarified from the perspective of micro characteristics. Through the analysis of cavitation surface morphology, the differences in cavitation mechanism between the two coatings are elucidated.

Due to the lower laser input energy density in the SLD process, the SLD coating has a finer dendrite structure and a lower element dilution ratio than the LC coating. Besides, SLD is a powder deposition process based on material plastic deformation, which induces a work-hardening effect. Thus, SLD coating has a higher hardness/elastic modulus ratio than LC coating. These factors lead to better cavitation-resistance of SLD coating than LC coating.

LC coating is formed through the material melting/re-solidification process, resulting in a typical dendrite structure. The phase/grain boundary is the preferred position of cavitation in the LC coating, which shows a uniform surface morphology. Since SLD relies on mechanical bonding instead of metallurgical bonding to fabricate coatings, there will be pores between particles due to poor bonding. These pores are the initial position of cavitation in SLD coating, showing a non-uniform cavitation process.