Effect of Ultrasonic Vibration on the Grain Characteristics and Defects in Laser Repaired Nickel-Based Superalloy
Owing to the severe service conditions, components made from nickel-based superalloys are susceptible to cracking and wear. Laser repair technology can restore the mechanical properties of damaged structural components, thereby prolonging their service life. To enhance the quality of repaired components, ultrasonic vibration has been integrated into the laser repair process to suppress defects and improve performance. The work aims to investigate the impact of ultrasonic vibration on the microstructure and mechanical properties of the trapezoidal groove repair zone in nickel-based superalloy components repaired by laser technology.Ultrasonic vibration was applied to the laser additive repair process by bottom ultrasonic device in this study. A laser beam with power of 1000 W and a spot diameter of 1.2 mm was adopted. The wire was fed with a speed of 20 mm/s at a 45° angle. The scanning speed was 10 mm/s. Following the experiments, the macro morphology and microstructure of the trapezoidal groove repair zone were observed by a Zeiss optical microscope (Axio Imager 2). The grain orientation and distribution in the trapezoidal groove repair zone were analyzed by electron backscatter diffraction (EBSD). Scanning electron microscopy (SEM, Zeiss Evo 18) and energy dispersive spectroscopy (EDS, Brukerxflash 6130) were used to analyze the distribution, morphology, and energy spectrum of the precipitated phase in the trapezoidal groove repair zone. Microhardness was measured from the top of the trapezoidal groove repair zone to the underlying matrix by a Vickers microhardness tester (HMV-2TADWXY).When ultrasonic vibration was applied, poor fusion defects at the bottom zone of the trapezoidal groove repair zone were reduced. The epitaxial growth of columnar dendrites was inhibited and the growth angle of dendrites were altered. A transition from columnar grains to equiaxed grains was observed at the top of the repair zone. The maximum multiples of uniform distribution value of the grains at the top of the repair zone reduced from 18.39 to 4.3, and the direction of grain growth shifted from the <100> to the <110> or <111> direction. Notably, the average primary dendrite spacing in the trapezoidal groove repair zone decreased from 6.74 μm to 3.38 μm. The average grain size reduced from 58.4 μm to 50.2 μm, and the proportion of equiaxed grain increased from 46.6％ to 63.4％. Moreover, the precipitation of the long-strip shape Laves phase was inhibited and the Laves phase was diffusely distributed. The average microhardness of the trapezoidal groove repair zone under an ultrasonic power of 4000 W was 256.2HV0.2, indicating an increase of 17.7HV0.2 compared to the specimen under no ultrasonic vibration. The effects of ultrasonic vibration on the grain characteristics and defects of the repaired zone were discussed.The findings of this study confirm the benefit of ultrasonic vibration to the trapezoidal groove repair zone. With application of ultrasonic vibration, the poor fusion defects of the repair zone are suppressed, and the primary dendrite spacing is noticeably reduced. Furthermore, the shape of the Laves phase is transformed from a long-strip shape into a granular shape, and the average microhardness increases with an increase of ultrasonic power.
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