A novel inner surface enhancement method for holes utilizing ultrasonic cavitation.

Holes in housings, shafts and flanges lead to stress concentrations when the components are working under high dynamic loads. Peening methods are commonly used to improve the stress concentration and extend the working life. These methods, however, are difficult to treat the inner surface of the holes in the components because these surfaces are fully shadowed and limit the access of the shot streams, water jets or laser beams. A new developed method using ultrasonic cavitation can be expected to solve these problems by using a sonotrode with a special shape. The working principle is that the fluid enters through a narrow gap between the sonotrode and the inner surface to create a cavitation. In this paper, a new sonotrode was designed and manufactured, then tested at a resonance frequency of 23.8 kHz. The sono-chemiluminescence experiments were carried out to detect the cavitation intensity on the inner surfaces. The stainless-steel tubes were treated, and their surface properties were evaluated as well. The results show that the cavitation intensity is strongest at the working distance of 1 mm. The hardness increased by about 12% without the significant change of surface roughness.

Impacts of ultrasound on oxide removal - An attempt towards acid-free cleaning.

Strong or mid-strong acids are always used to remove oxides from part surfaces in remanufacturing, painting, metallurgical and mineral industries, which is not environmentally benign. In this work, a green cleaning method - ultrasonic (US) cleaning with distilled water is proposed. The impacts of ultrasonic cleaning process parameters including the distance between the sonotrode end surface and the specimen surface, the vibration amplitude, the process time and the concentration of oxalic acid, on the surface oxide removal rate were systematically studied based on a Box-Behnken Design. The results show a significant increase of the oxide removal rate on the specimen surface with the decrease of the distance, the increase of the vibration amplitude, the increase of the process time and the presence of oxalic acid. Based on the experimental results, an empirical model was established to quantitatively describe the effects of these factors on the oxide removal rate. In addition to all the linear factors, the square factors of time and the concentration of oxalic acid as well as the interaction factors among time, driving current and the concentration of oxalic acid are significant on the oxide removal. Compared to the cleaning with acids, a high level removal rate is still achievable with acid-free distilled water even though the process window gets narrower. This study enhances the potential application of US cleaning on oxide removal with a small amount of weak acid or without any acid in the cleaning liquid in industries.

Theoretical and experimental investigations of ultrasonic sound fields in thin bubbly liquid layers for ultrasonic cavitation peening.

Ultrasonic cavitation peening is a potential surface enhancement process. During this process a high input power is necessary to obtain an effective process result. A small gap, usually less than 1 mm, between the sonotrode tip and the treated surface is also required to avoid substantial energy loss. Due to the high vibration of the sonotrode, many cavitation bubbles are generated, forming a thin bubbly liquid layer in the small gap. The cavitation bubbles in the layer seriously disturb the sound wave propagation and interact with each other. The disturbances and interactions change the intensity and the spatial distribution of cavitation bubbles, resulting in the different interactions between cavitation bubbles and workpiece surfaces. The variations of the interactions cause different surface properties of the workpieces after ultrasonic cavitation peening. Therefore, quantifying the ultrasound field in different conditions is of great important to improve the ultrasonic cavitation peening process. A current model of the sound propagation in the bubbly liquid was already developed but did not include the bubble interactions. In this work, the bubble interactions are taken into account to improve the current model. The calculated results of the sound field with the improved model are validated by sonochemiluminescence experiments in various standoff distances and vibration amplitudes. Both of the experimental and the calculated results show that the highest sound pressure is generated when the vibration amplitude is around 25 µm. The strongest cavitation intensity occurs at the gap width of 0.5-0.7 mm.

Impact of time on ultrasonic cavitation peening via detection of surface plastic deformation.

During ultrasonic cavitation peening, bubbles repeatedly form and collapse, which leads to high impact loads on the treated surface. At the initial stage of ultrasonic cavitation peening, the most obvious change is plastic deformation instead of mass loss on the treated specimen surface. Meanwhile the plastic deformation is beneficial for mechanical surface properties. As the cavitation exposure time increases, erosion and damage are inflicted on the metal surface due to the increase in the number of collapse events. In this respect, the treatment time is a key parameter to improve the specimen surface properties during this manufacturing process. However, the influence of treatment time on the surface properties has not yet been thoroughly investigated. In this paper, it is the first time to utilize the plastic deformation to evaluate the optimal treatment time at different input power. The plastic deformation can be deduced by the mass loss and the volume change on the treated specimen surface. Using plastic deformation, the modification of surface hardness and roughness are investigated at different cavitation exposure intervals and vibration amplitudes. It is found that significant improvement of the microhardness on the treated surface occurs at the end of incubation period. Higher vibration amplitudes of the horn tip lead to shorter incubation period and higher microhardness.

Capability evaluation of ultrasonic cavitation peening at different standoff distances.

Ultrasonic cavitation peening is a novel surface treatment technology which utilizes the effect of cavitation bubble collapses to improve the properties of metal surfaces. In order to obtain high impact during ultrasonic cavitation peening, a small standoff distance between a sound radiator and a rigid reflector (the surface of treated specimen) is necessary. However, the effects of different standoff distances on the capability of ultrasonic cavitation peening are not yet clear. In this paper, a simplified model was developed to evaluate the cavitation capability at different standoff distances. Meanwhile, to validate the theoretical model, the plastic deformation or erosion on the peening surface before and after treatment were compared. It was found that at a very small standoff distance the impact pressure generated by cavitation bubbles did not cause much deformation or erosion, as the dynamics of cavitation bubbles was limited. At a large standoff distance, due to much attenuation of sound propagation in the bubbly liquid, little impact pressure was generated by the collapse of cavitation bubbles and reached the treated surface. A fixed vibration amplitude, however, corresponded to a standoff distance which caused the largest deformation or erosion on the treated surface.