Correlation of laser-induced single bubbles with cavitation damage via in-situ imaging

Abstract

Experiments with series of laser-induced single cavitation bubbles were performed on Cu-, Fe-, and Al- base alloys, namely 316LVM, a nickel aluminum bronze (NAB), and pure aluminum. The bubble dynamics were recorded by two high-speed cameras, while the resulting surface damage was investigated by a microscope in situ, i.e. between two successive bubbles with the sample remaining in the water. Bubbles with stand-off distances γ (the ratio of distance of the surface from the bubble and the bubble maximum radius) of 1.35 ± 0.5 and bubble radii of about 1.2 mm were directly correlated with the damage they produced. We found a correlation between locally enlarged (“strong”) gas-filled collapse areas (SCAs) during the second bubble collapse and the resulting pits on the sample surface. While all pits could be correlated to a preceding SCA, not all SCAs resulted in pits. This is true on both NAB and 316L. On aluminum, despite the softness of the material, some bubbles did not produce pits, and these where the ones with the most symmetric second-collapse torus. The pits that constitute, in sum, the damage of the material where analyzed in their number and geometry. The pitting rate and the maximum pit depth were roughly inversely depended on the material hardness. Also, beginning wear mechanisms were identified, like emerged grain boundaries on aluminum and light slip lines on steel. Finally, the collapse of a very small, secondary, dissolved-gas bubble close to the surface is documented to cause a pit. This may be one mechanism by which the pits are formed.