On the subsurface erosion, micro-tunneling and deformation induced by high-frequency droplet impingement in gravity-cast aluminum, analyzed through micro-computed tomography

• Microchannels form along grain boundaries. • Subsurface hydrodynamic penetration ignores grain boundaries. • Increased water droplet impacts expand the microchannel network. • Wear rates differ when using μ-CT versus a laser confocal microscope. This research focuses on monitoring the erosion evolution of 99.8% pure, gravity-cast aluminum, utilized in its basic, un-processed state, when subjected to precisely timed, high-frequency droplet impacts. The investigation aims to ascertain if subsurface micro-tunneling occurs with repeated impacts at a single point. Additionally, the study seeks to understand the development of micro-tunneling over time and the mechanics of subsurface penetration. The material, in its raw state devoid of any processing history, was subjected to an ultrasonic pulsating water jet (PWJ) at a fundamental frequency of f = 40 kHz and a supply pressure of p = 20 MPa. The specimen, pre-examined by micro-CT, underwent varying PWJ exposure, ranging from 0.125s to 1.25s. Erosion metrics such as the volume of material extruded above the treated surface, the volume of material removed, maximum depth, and subsurface micro-tunneling were derived from X-ray μ-CT analysis. The measurement results showed that for the increasing time interval from t = 0.125 to 1.25 s, the maximum depth increased from 0.118 mm to 0.75 mm and 0.085 mm to 1.565 mm as measured from confocal and X-ray μ-CT, respectively. Similarly, the eroded volume removed increased from 82.73 mm 3 to 3408.28 mm 3 and 79.86 mm 3 to 10256.8 mm 3 as measured from confocal and X-ray μ-CT, respectively. The results showed a clear differentiation between both measurement techniques. The μ-CT observations showed early signs of micro-tunneling during short exposures to the PWJ. In many cases, exit vents emerged at a considerable distance from the point of contact. The micro-tunnel channels appeared straight and narrowed progressively. Micro-channels that undercut the material and re-emerged on the surface were identified. Further scrutiny was conducted through microhardness testing, particularly detecting potential hardening near the micro-tunnels. SEM analysis showed that entry points of penetration predominantly occurred along the edges of the erosion groove.