Study On Mechanisms Of Ultrasonic Cavitation Erosion Of Metallic Materials

Operation stability and safety are extremely important for fluid-related industrial setups such as power plants,underwater vehicle.These systems or equipment involve pipes,valves,and hydraulic components,which frequently encounter the cavitation phenomenon.As cavitation bubbles collapse,enormous energy is released.Consequently,relevant components are damaged.This is cavitation erosion.The cavitation erosion leads to the mass loss of the components and the degradation of the operation efficiency.Severe cavitation erosion gives rise to fracture of the components and even the halt of the whole process.To ensure smooth fluid transport and safe operation of fluid-handling devices,the capability of resisting the cavitation erosion must be improved.To achieve this goal,the optimization design of relevant components is necessary.Furthermore,the cavitation erosion of materials deserves investigating.The present dissertation will study the ultrasonic cavitation erosion mechanisms of metallic materials used in the components exposed to cavitation and then systematically evaluate the resistance of these materials to the cavitation erosion.Major works and conclusions in the present dissertation are as follows:?1?An ultrasonic cavitation erosion experimental rig was constructed based on the ASTM G32 standard.The experiments were performed using this ultrasonic cavitation erosion device.Three liquids,the deionized water,Na₂SO₄ and NaHCO₃solutions were selected for studying the effects of the liquid medium on the cavitation erosion.Temperatures ranging from 15 to 60oC were selected and the standoff distance between the horn tip and the specimen surface was varied.The cumulative mass loss,microhardness,and residual stress of the specimens were measured.Furthermore,the eroded surfaces were observed using the scanning electron microscope,the stereo microscope and the laser confocal microscope.?2?To investigate the material response to the cavitation erosion,a comparative experimental work was carried out on three materials,namely aluminum alloy,copper alloy and titanium.The ultrasonic cavitation erosion was produced on the specimens which were submerged in the deionized water.Cavitation damage patterns were compared between the three materials.The microhardness was measured.The results indicate that the cumulative mass loss of aluminum alloy is the highest among the three materials,while the least material removal is associated with titanium.The cavitation erosion pattern on the copper alloy specimen surface is related to the twin-phase crystal structure and the erosion pits are smaller and shallower than those in the surfaces of the aluminum alloy specimens.The comparison of the surface hardening characteristics also prove that titanium owns the highest resistance to the cavitation erosion among the three materials.?3?To elucidate the effects of liquid medium properties on the cavitation erosion,experiments of the ultrasonic cavitation erosion of the copper alloy were conducted in the Na₂SO₄ and NaHCO₃ solutions.The results associated with the deionized water were introduced for comparison.It shows that the cumulative mass loss rates in the 3%Na₂SO₄ and 3%NaHCO₃ solutions are much higher than that in the deionized water.Moreover,the residual stress of the specimen eroded in the Na₂SO₄ solution is higher than that in the deionized water.The?phase is severely eroded in the 3%Na₂SO₄solution.The maximum microhardness of the eroded specimen arises at the target surface and declines with the increase of the penetration depth.?4?To deepen the understanding of the effect of the liquid medium properties on the cavitation erosion,the copper alloy specimens were tested in 0.3%,3%and 6%NaHCO₃ solutions.It indicates that the cumulative mass loss of the copper alloy specimen submerged in the 6%NaHCO₃ solution is the highest among the three cases.The initiation of the stabilization stage of the cavitation erosion for the specimen in the 6%NaHCO₃ solution is earlier than its counterparts.Small erosion pits are well remained for the 0.3%NaHCO₃ solution as the erosion time increases.For the specimen in the 6%NaHCO₃ solution,erosion pits and cracks expand rapidly.At the later stage of the cavitation erosion,the surface of the specimen in the 6%NaHCO₃solution is dominated by large and connected erosion pits.The highest residual stress and the thickest hardened layer are associated with the specimen submerged in the 6%NaHCO₃ solution.?5?The cavitation erosion of the CuZnAl shape memory alloy?SMA?was studied.The deionized water served as the liquid medium.Emphasis was placed on the effects of the water temperature and the standoff distance on the cavitation erosion.During the experiment,four water temperatures,namely 15,30,45 and 60oC,were selected and standoff distances of 0.5,0.7,1.0,and 3.0 mm were adopted.The results show that there is no monotonous relationship between the cumulative mass loss and the water temperature.At the water temperature of 45oC,the highest cumulative mass loss is attained and the cavitation erosion is the severest.At the same temperature,the standoff distance of 0.5 mm is responsible for the highest resistance to the cavitation erosion.The combination of wrinkles and erosion pits is explicitly identified in the eroded surface;erosion pits and cracks are considerably expanded.?6?A further discussion about the behaviors of the CuZnAl shape memory alloy under the cavitation impact was performed.The shape memory effect is enhanced due to the cavitation impact.The grains are refined evidently at high cavitation intensity.The diversity of grain orientations is improved considerably.The transition from the thermoelastic martensite to the austenite phase tends to occur easily as the cavitation impact proceeds.The stability of the thermoelastic martensite phase decays as the cavitation erosion progresses.