Synchronous ultrasonic impact assisted laser cladding CoCrFeNiMo high entropy alloy coating: Microstructure and wear property

Synchronous ultrasonic impact treatment (UIT) is proposed to enhance the microstructure and wear property of laser cladded CoCrFeNiMo high entropy alloy (HEA) coatings on 45 steel. FCC phase with minor tetragonal sigma phase and rhombic eta phase are form in both conventional and UIT treated coatings. Due to the plastic deformation and high-frequency vibrations induced by UIT, dislocation interactions lead to recrystallization process at the top region coating. With the application of UIT, the columnar grains at top region of the coating transform into fine equiaxed grains owing to recrystallization. The fragmentation of dendrites occurs in the middle region of the coating, resulting in reduced dendrite arm spacing. The results also show the defects, such as pores and microcracks, are effectively eliminated within the coating. Microhardness and wear property of the coating are remarkably enhanced owing to fine-grain strengthening and second-phase strengthening. The average friction coefficient decreases from 0.56 for the conventional sample to 0.49 for the UIT treated sample, and the wear mechanism evolved from a combination of abrasive, fatigue, and oxidized wear to predominantly fatigue and oxidized wear. It indicates that synchronous ultrasonic impact assisted laser cladding is a potential method to fabricate high-performance HEA coatings.