# A Novel Ultrasonic Cavitation Peening Approach Assisted by Water Jet
## 3 Experiments  
Figure 2 shows the schematic illustration of the equipment during this novel ultrasonic cavitation peening approach. A classic sandwich transducer with different excitation amplitudes and a resonance frequency of approximately 23 kHz was used as vibration source. The transducer was designed to vibrate at its second longitudinal resonance frequency. The mechanical amplitude is amplified by a stepped ultrasonic horn with a diameter at the tip end of 5 mm. The amplitude was controlled by a digital phase control system which keeps vibration amplitude constant, even if a changing load is applied to the transducer [17; 24]. There is a correlation (120 \(\upmu\)m/A) between the vibration amplitude of the ultrasonic horn is proportional to the transducer's driving current [25].  
The water jet is only required to make the gap full of water. Thus, the slowest velocity of the water jet is required. According to pre-tests, the water flow rate of 75 mL/min for the experiments is minimum and used in the experiments. The pipe with an inner diameter of 3 mm stayed at 45 degrees  
Figure 1: Photos of the small gaps in different experimental conditions: (**a**) No vibration loading without water jet; (**b**) Under vibration loading without water jet; (**c**) No vibration loading with water jet; (**d**) Under vibration loading with water jet.  
Figure 2: Experimental setup of the novel ultrasonic cavitation peening and the partial schematic illustration.  
from the horizontal plane. The 45 degrees is arbitrarily chosen, since the angle between the pipe and the horizontal plane is available from 15 degrees to 75 degrees with the water jet velocity of 75  mL/min according to pre-tests. When the ultrasonic horn vibrates up and down, cavitation bubbles are generated on the surface of the specimen to which a piezoelectric force sensor is attached. The output signals by the force sensor during ultrasonic cavitation peening was recorded for 20  s with an oscilloscope. The sensor was made of two piezoelectric disks with the diameter of 12 mm. The square specimen plate (10 mm \(\times\) 10 mm) was fixed over the sensor with two side tape. The specimens were made of aluminium alloy 5005 and the surfaces were polished with the roughness of 0.1 \(\upmu\)m. Aluminium alloy 5005 is erosion-resistant and is prone to plastic deformation. Therefore, it is a better choice to evaluate the effectiveness of ultrasonic cavitation peening. Three specimens were treated under the same process condition. The duration of the process time for each specimen was 60 s.  
After the treatments, the volume loss and microhardness generated with different standoff distances (from 0.1 mm to 1 mm with the interval of 0.1 mm) and different driving currents (0.166 A, 0.208 A, 0.250 A) were measured. The change of the standoff distance was measured by a linear variable differential transformer. The specimens were dried by absorbent paper after treatment. Since the specimen surfaces are polished under the same condition, the topographies of the specimen surfaces were nearly the same. Thus the volume loss of the specimen was only measured after treatment using a Alicona microscope which can also measure the surface roughness. The magnifications of the microscope lenses that were used in the measurements were five times and fifty times, respectively.  
A Vikers tester designed at Institute of Dynamics and Vibration Research was used to measure the surface microhardness. The hardness tester consists of a force sensor (HBM PW4M C3), a Vikers indenter of the load (SIANTEC SV1032M) and an eddy-current displacement sensor (Lion precision ECL101 with probe U 5). The microhardness along the diameter of the treated pattern was measured after treatment. Ten measurement points were distributed evenly along a measurement path.