The influence of different heat treatment processes on the microstructure and mechanical properties of 18Ni350 maraging steel manufactured by selective laser melting and the corrosion resistance of TiB₂ ceramic coatings Electro-Spark-Deposited on its surface when immersed in high-temperature molten aluminum have been investigated in the present study. The microstructures and mechanical properties of the differently heat-treated samples were analyzed using various precision instruments. The results reveal that the as-built sample exhibits a microstructure composed of cellular and columnar dendritic grains. After solution treatment, the microstructure fully transforms into lath-like martensite. After direct aging treatment, the cellular structures diminish, while precipitates at grain boundaries proliferate with increasing aging temperature. SAT- treatment achieves full microstructural homogenization, featuring fine-lath martensite and a small amount of randomly distributed austenite particles. DA- and SAT- significantly improve the strength, hardness and modulus of samples and were found to reduce the toughness and plasticity. After solution treatment at 800 °C for 1 h followed by aging treatment at 520 °C for 6 h (SAT 800-520), the specimen achieved an UTS of 2476 MPa while maintaining an EL of 4.6%. The TiB₂ coating and the Cr interlayer deposited via ESD form a continuous interfacial bond with the substrate, demonstrating favorable adhesion. After 4 h of static immersion in high-temperature molten aluminum, the coating remains intact without complete delamination, delivering effective protection to the substrate.

In this study, ZnO thin-film ultrasonic transducers were directly fabricated by reactive radio-frequency magnetron sputtering for bolt preload detection. The effects of deposition pressure and Ar/O₂ ratio on the morphology, crystal structure, piezoelectric response, and ultrasonic performance of ZnO films were investigated. The ZnO film deposited at 2.0 Pa with an Ar/O₂ ratio of 3:1 exhibited strong c-axis preferred orientation and generated stable high-frequency longitudinal ultrasonic waves with a center frequency above 50 MHz. After being deposited on the bolt surface, the optimized transducer showed a strong linear relationship between ultrasonic time-of-flight and axial load, with an R² value greater than 99.99% and an average measurement error below 1% within the preload range. Field monitoring at a hydropower station further showed that the preload variation of randomly selected bolts was negatively correlated with temperature, mainly due to the thermal deformation mismatch between 35CrMo bolts and Q235-B flanges. The maximum preload variation was approximately 40 kN, accounting for about 6.7% of the specified preload of 600 kN, while no abnormal preload loss was observed. These results demonstrate that ZnO thin-film ultrasonic transducers provide a reliable and accurate approach for hydraulic bolt preload measurement and in-service monitoring, with better applicability than conventional torque-based methods.

The detection of bolt preload force is of vital importance for ensuring the structural reliability of equipment under extreme operating conditions. Traditional ultrasonic transducers based on bulk piezoelectric materials suffer from poor long-term coupling stability and high brittleness of the material, which limits their practical applications. In this work, AlN piezoelectric thin films were fabricated by RF magnetron sputtering, and the effects of RF power and target-to-substrate distance on film morphology, crystal structure, and ultrasonic response were investigated. The results show that increasing RF power increased the film thickness and deposition rate, reduced the detected O content on the film surface, and changed the XRD response. The film deposited at 900 W generated ultrasonic longitudinal wave echoes with a relatively high signal amplitude among the tested RF powers. Among the tested target-to-substrate distances, the film deposited at 60 mm showed a relatively higher deposition rate and generated an ultrasonic longitudinal wave echo with a relatively higher amplitude. The measured d₃₃ value of this film was approximately 4.8 pC/N. The AlN thin-film ultrasonic transducers prepared under the selected deposition conditions were directly deposited on bolts, and the effects of temperature and axial load were calibrated using the ultrasonic TOF measurement method. There was a linear correlation between the TOF and the temperature (R² > 99.99%), as well as between the TOF and the axial load. These results indicate that the deposited AlN thin-film transducer has potential for bolt preload measurement in wind turbine bolts.