Effects of fluid condition and material on surface damage in ultrasonic wet peening

Cavitation in solid-water mixtures generated by ultrasonic waves can be used to modify the topography of metallic surfaces. During ultrasonic irradiation two kinds of processes occur in the liquid: (a) material removal by bubble formation plus collapse and the resulting micro jets; (b) material removal by colliding or sliding of accelerated particles due to bubble formation plus collapse. Both processes allow the removal of burrs after micro milling. However high impact forces by bubble collapse often cause erosion pits on the surface which are detrimental to further processing. The aim of this research was to avoid the creation of erosion pits on metal surfaces which often lead to increased surface roughness. Therefore these investigations focus on reducing this effect by the variation of material properties of the workpiece and fluid conditions. Distilled water or a solution of sodium chloride both mixed with alumina grains of a mean diameter of 25 μm were used. Additionally in case of distilled water the gas content was varied to change the intensity of the bubble collapse. Two different workpiece materials, a tempered low-alloyed tool steel and a precipitation-hardened steel were investigated. Flat samples as well as micro milled grooves have been exposed to ultrasonic waves in the alumina grain-water mixtures within a time period of 20 minutes. The results show that the resulting surface topographies strongly depend on the hardness of the workpiece material used. With increasing material hardness the influence of cavitation on the surface roughness decreases. Furthermore it is demonstrated, that a reduction of amount, size and depth of erosion pits can be achieved by optimizing the fluid conditions namely gas content and density while deburring without shape errors or edge rounding is enabled effectively.