The evolution of water droplet erosion on copper with varying grain sizes altered by heat treatment was investigated. An ultrasonically driven pulsating water jet was employed as a droplet generator. The water jet treatment was performed at a frequency of 40 kHz using two levels of supply pressure of 20 and 30 MPa. The measurements of removed volume and achieved depth show a critical interplay between droplet-surface interaction development and material properties, including hardness, grain size, and plasticity. Two distinct erosion phenomena were observed, influenced by the effective interaction governed by the feed rates of the PWJ with the material. At the lowest feed rate (0.5 mm/s), the erosion resistance of the material modestly increased with the intensity of heat treatment for both levels of supply pressure. Suggesting plasticity is the main factor of erosion resistance. The largest mean depth was recorded in the basic state of the material (without heat treatment), namely 90.06 µm at a pressure of 20 MPa, and the minimum value was 42.23 µm at the highest heat treatment of 900°C/1h with a pressure of 30 MPa. This contrasts with the behaviour at the higher feed rate (0.67 mm/s), where more severe erosion with heat treatment intensity was observed. The hardness decreases and grain size increase caused by heat treatment intensity are the driving factors of erosion resistance at this feedrate. In this case, the lowest mean depth of 69.46 µm was observed in the basic heat-untreated state of the material at a pressure of 20 MPa, and the maximum mean depth of 103.17 µm at 30 MPa was observed in the treated sample at 900°C/1h. The results demonstrated different erosion responses of the material after its heat treatment depending on the change in the input feed rates. These findings contribute to a deeper understanding of the mechanisms of PWJ-induced erosion when interacting with materials of different hardness and grain size.
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