超声振动对激光熔覆高熵合金涂层组织与耐磨性能的影响OA北大核心CSTPCD

High entropy alloys (HEA) is a type of multi-major element alloys composed of various metal elements. Among many HEA systems, dual-phase HEAs (such as AlCoCuFeNi, AlCoCrCuFeNi, and CrCuFeMnNi, etc.) can combine strength and toughness perfectly under specific conditions. They possess good ductility, oxidation, wear, and corrosion resistance, making them excellent choices for the preparation of protective coating materials for metals. The excellent choices for protective coating materials include the following methods: magnetron sputtering, electrochemical deposition, arc melting, and laser melting. Laser cladding technology offers advantages such as a low dilution rate, strong metallurgical bonding, good densification, and controllable thickness. However, differences in melting points of many elements in high-entropy alloys, and different solidification sequences, can lead to porosity. Moreover, the rapid solidification effect can produce large local residual stresses, potentially resulting in cracks. Therefore, the coating technology of high-entropy alloys prepared by laser melting and cladding needs further optimization. In recent years, researchers have found that ultrasonic vibration, as an external energy field, can decrease the defects in laser cladding technology and regulate the organization and properties of the cladding layer. The cavitation effect, acoustic flow effect, and thermal effect induced by ultrasonic vibration have a positive influence on the solidification process of metal melt, which can alter the flow mode of the melt, reduce the temperature gradient, refine the grain, and inhibit segregation. The work aims to investigate the effect of ultrasonic vibration on the solidification process of metal melt and to explore its impact on mechanical properties, particularly the inhibition of crack formation in high-entropy alloy coatings. A self-designed ultrasonic vibration platform with a vibration frequency of 20.1 kHz was used to conduct the experiment. The vibration of the platform was measured by a laser vibrometer, and the ultrasonic amplitude generated by the platform ranged from 1.5 to 9.5 μm. After preliminary exploration and process optimization, an ultrasonic amplitude of 5 μm was selected. 304 stainless steel was used as the coated substrate, and the particle size range of the AlCoCuFeNi high-entropy alloy powder was 53 μm. The particle size of the AlCoCuFeNi high-entropy alloy ranged from 53 to 150 μm. The laser power used for the coating test was 900 W, with a scanning speed of 7 mm/s. At the conclusion of the test, the cross-sectional morphology of the high-entropy alloy coatings was observed with a laser confocal microscope to compare the number and distribution of cracks before and after the addition of ultrasonic vibration. A scanning electron microscopy, an X-ray diffractometry, and other testing equipment were utilized to characterize the microstructural transformation of the coating before and after ultrasonic treatment, including trends in element distribution and grain size. The microhardness and wear resistance of the coatings were examined with a microhardness tester and a reciprocating friction and wear tester. Under the influence of ultrasonic vibration, the macroscopic morphology of the coating changed. High-frequency vibration reduced the contact angle between the molten pool and the substrate, causing the coating cross section to change from semicircular to elliptical. In the original coating, cracks ran through the coating at the overlap joints; however, after the addition of ultrasonic vibration, the coating exhibited improved continuity and metallurgical bonding at the overlap joints. The acoustic flow effect generated by ultrasonic vibration altered the flow pattern of the melt pool, while the stirring effect reduced the temperature gradient and suppressed the formation of dense dendritic crystals at the root of the coating. The micro-shock waves generated by cavitation and the instantaneous high temperature interrupted the columnar crystals inside the coating, leading to significant grain refinement. The change in the temperature gradient of the molten pool and the grain refinement increased the precipitation of FCC phase, which was distributed in the form of a "mesh" at the overlap joints of the coatings, inhibiting crack extension. Consequently, the mechanical properties of the coatings were improved by ultrasonic vibration. The average microhardness increased from 503HV0.5 to 526HV0.5, and the coefficient of friction decreased, with reduced fluctuation in the curve. The coefficient of friction decreased from 0.669 to 0.586, the catalytic cracking precipitation phase induced by ultrasonic vibration reduced the formation of microcracks in the coating during the friction process, the grain refinement led to improved friction performance, and the wear mechanism was abrasive wear with slight oxidative wear.