Cavitation erosion mechanism of titanium alloy radiation rods in aluminum melt

Ultrasound radiation rods play a key role in introducing ultrasonic to the grain refinement of large-size cast aluminum ingots (with diameter over 800 mm), but the severe cavitation corrosion of radiation rods limit the wide application of ultrasonic in the metallurgy field. In this paper, the cavitation erosion of Ti alloy radiation rod (TARR) in the semi-continuous direct-chill casting of 7050 Al alloy was investigated using a 20 kHz ultrasonic vibrator. The macro/micro characterization of Ti alloy was performed using an optical digital microscopy and a scanning electron microscopy, respectively. The results indicated that the cavitation erosion and the chemical reaction play different roles throughout different corrosion periods. Meanwhile, the relationship between mass-loss and time during cavitation erosion was measured and analyzed. According to the rate of mass-loss to time, the whole cavitation erosion process was divided into four individual periods and the mechanism in each period was studied accordingly.     

Cavitation erosion mechanisms in Co-based coatings exposed to seawater

The cavitation erosion (CE) of most materials in seawater is more serious than in fresh water due to the onset of corrosion; however, in a previous study we reported results that contradict this widely accepted trend. In this research our objective is to provide fundamental insight into the mechanisms that may be responsible for these earlier results. To accomplish this objective, two types of Co-based coatings, prepared by high velocity oxygen fuel (HVOF) spraying system, were used to further investigate the underlying corrosion-mitigating CE mechanism in seawater. Accordingly, the influence of spraying parameters on microstructure, composition and mechanical properties of the coatings was analyzed on the basis of SEM, XRD, Raman spectroscopy, Vicker’s hardness and nano-indentation results. Electrochemical corrosion tests were used to evaluate the corrosion behavior of the Co-based coatings. Their CE performances in seawater and deionized water were comparatively studied by a vibratory apparatus. Results demonstrated that a higher flame temperature facilitated the oxides formation with associated improvements in compactness, hardness and toughness of the coatings. The presence of alumina in combination with the oxides formed in-situ facilitated the formation of an oxidation film on surfaces, and effectively enhanced the charge transfer resistance of the coating, thereby significantly improving the corrosion resistance in seawater. Metallic Co was not only more easily oxidized but also more readily corroded than the alloyed Co. Compactness was identified as an important factor affecting CE resistance of coatings in deionized water, because defects facilitate the nucleation and eventual collapse of bubbles. Moreover, bubble collapse produced a transient high temperature spike in excess of 600 °C that also caused Co and Cr elements to oxidize. Because the CE tests were carried out in seawater, additional Co₃O₄ and Cr₂O₃ were generated owing to corrosion that more effectively increased the surface compactness and mechanical properties of the coatings. This behavior was particular notable for coatings with metallic Co and Cr, which should be why seawater corrosion could weaken the CE of Co-based coatings.     

激光空泡在刚性壁面附近空蚀特性

采用脉冲激光聚焦于透明液体中产生单个激光空泡,利用高速相机拍摄空泡在透明液体中整个运动过程,并对刚性壁面造成的空蚀现象进行了观测.实验研究发现,空泡溃灭将产生高速冲击波和高速射流,这是造成刚性壁面损伤的两种主要原因.空泡与刚性壁面的无量纲距离在0.4～1.4之同时,刚性壁面首先受到高速冲击波的破坏,由于空泡的趋壁效应,空泡在第二次收缩过程中将在壁面附近对实验靶材产生高速微射流的空蚀破坏.且这两种作用机制在无量纲距离为1.0时,高速微射流对壁面的空蚀效果更加明显.     

激光空泡在刚性壁面附近空蚀特性

采用脉冲激光聚焦于透明液体中产生单个激光空泡,利用高速相机拍摄空泡在透明液体中整个运动过程,并对刚性壁面造成的空蚀现象进行了观测。实验研究发现,空泡溃灭将产生高速冲击波和高速射流,这是造成刚性壁面损伤的两种主要原因。空泡与刚性壁面的无量纲距离在0.4~1.4之间时,刚性壁面首先受到高速冲击波的破坏,由于空泡的趋壁效应,空泡在第二次收缩过程中将在壁面附近对实验靶材产生高速微射流的空蚀破坏。且这两种作用机制在无量纲距离为1.0时,高速微射流对壁面的空蚀效果更加明显。     

Cavitation bubble collapse studied at 20 million frames per second

The collapse of laser-induced bubbles in water is investigated by high speed photography at framing rates as high as 20 million frames per second. The case of a spherical bubble in an unbounded liquid is compared with the Gilmore model. Bubbles collapsing in front of a solid wall show a rich dynamics depending on their normalized distance. Unprecedented details are given of the generic sequence of events leading to multiple shock waves and bubble shape metamorphosis upon collapse.     

Optical investigation of cavitation erosion by laser-induced bubble collapse

By means of a new force sensor based on optical beam deflection (OBD), the mechanical effects of laser-matter interaction underwater at different incident laser energy are investigated in detail. The experimental results show that a target underwater is impacted in turn by laser-plasma ablation force and high-speed liquid-jet impulse induced by bubbles collapse in the vicinity of a solid boundary. Furthermore, the amplitudes of the two forces increase monotonously with laser energy. According to the ablation force detected by the experiment and the theoretical relationship between laser intensity and ablation pressure, the value of liquid-jet impact against a solid boundary can be easily obtained. In addition, based on the model of a collapsing bubble, some characteristic parameters, such as the liquid-jet impact velocity, the maximum bubble radius, the bubble energy can also be obtained at different laser energy, which are valuable in the corresponding research fields.     

Cavitation mapping by sonochemiluminescence with less bubble displacement induced by acoustic radiation force in a 1.2 MHz HIFU

An acoustic radiation force counterbalanced appliance was employed to map the cavitation distribution in water. The appliance was made up of a focused ultrasound transducer and an aluminum alloy reflector with the exactly same shape. They were centrosymmetry around the focus of the source transducer. Spatial–temporal dynamics of cavitation bubble clouds in the 1.2 MHz ultrasonic field within this appliance were observed in water. And they were mapped by sonochemiluminescence (SCL) recordings and high-speed photography. There were significant differences in spatial distribution and temporal evolution between normal group and counterbalanced group. The reflector could avoid bubble directional displacement induced by acoustic radiation force under certain electric power (⩽50 W). As a result, the SCL intensity in the pre-focal region was larger than that of normal group. In event of high electric power (⩾70 W), most of the bubbles were moving in acoustic streaming. When electric power decreased, bubbles kept stable and showed stripe structure in SCL images. Both stationary bubbles and moving bubbles have been captured, and exhibited analytical potential with respect to bubbles in therapeutic ultrasound.     

Experiment investigation on effects of elastic modulus on cavitation erosion of silicone rubber

Research into cavitation phenomena in various fields shows that the elastic modulus of a boundary has a potential impact on cavitation erosion. To obtain the direct relationship between the elastic modulus of the boundary and cavitiation erosion, single-layer samples with different chemical composition and moduli, and double-layer samples with different elastic moduli and the same surface layer material, were prepared with silicone rubber. The results of cavitation experiments on single-layer samples, show that the coating chemical composition and mechanical properties together affect the cavitation morphology of the coating, and dominant factors vary with erosion stage. Through the cavitation test of double-layer samples, it was found that there is a positive correlation between the elastic modulus of the coating and the degree of cavitation. This study helps us to understand the relationship between coating elastic modulus and cavitation more directly, and provides theoretical and technical guidance for the application of anti-cavitation for elastic coating in engineering.     

Cavitation onset caused by a dynamic pressure wave in liquid pipelines

When liquids flow in the pipelines, the onset of cavitation can be characterized by a variant of the Euler number known as the cavitation number (CN), which is based on the velocity and denoted by C in this paper. Conventionally, cavitation is considered to be induced when C ~ 1. However, experimental observations and several pipe bursts indicate that the CN may incorrectly predict the onset of cavitation. For example, when leakage occurs in the pipeline or a valve in the pipeline is opened, the resultant pressure loss generates a dynamic pressure wave with a small amplitude, which may lead to bubble formation, even though C ~ 1 is not satisfied. Hence, this paper proposes another CN based on the amplitude of the generated dynamic pressure wave, rather than the velocity, for ascertaining the onset of cavitation. The validity of the proposed CN was verified through experiments and a case study. The results indicated that the proposed CN can be effectively used for cavitation prediction induced by pressure fluctuations and for investigating phenomena such as pressure fluctuation, leakage, and corrosion in liquid pipelines, tanks, and pressure vessels, as well as the safety design of liquefied natural gas tanks and tankers.     

Delivery by shock waves of active principle embedded in gelatin-based capsules

PURPOSE: Delivering a drug close to the targeted cells improves its benefit versus risk ratio. A possible method for local drug delivery is to encapsulate the drug into solid microscopic carriers and to release it by ultrasound. The objective of this work was to use shock waves for delivering a molecule loaded in polymeric microcapsules. MATERIAL AND METHODS: Ethyl benzoate (EBZ) was encapsulated in spherical gelatin shells by complex coacervation. A piezocomposite shock wave generator (120 mm in diameter, focused at 97 mm, pulse length 1.4 micros) was used for sonicating the capsules and delivering the molecule. Shock parameters (acoustic pressure, number of shocks and shock repetition frequency) were varied in order to measure their influence on EBZ release. A cavitation-inhibitor liquid (Ablasonic) was then used to evaluate the role of cavitation in the capsule disruption. RESULTS: The measurements showed that the mean quantity of released EBZ was proportional to the acoustic pressure of the shock wave (r2 > 0.99), and increased with the number of applied shocks. Up to 88% of encapsulated EBZ could be released within 4 min only (240 shocks, 1 Hz). However, the quantity of released EBZ dropped at high shock rates (above 2Hz). Ultrasound imaging sequences showed that cavitation clouds might form, at high shock rates, along the acoustic axis making the exposure inefficient. Measurements done in Ablasonic showed that cavitation plays a major role in microcapsules disruption. CONCLUSIONS: In this study, we designed polymeric capsules that can be disrupted by shock waves. This type of microcapsule is theoretically a suitable vehicle for carrying hydrophobic drugs. Following these positive results, encapsulation of drugs is considered for further medical applications.     

In-situ SEM observations of ultrasonic cavitation erosion behavior of HVOF-sprayed coatings

Several typical high-velocity oxy-fuel (HVOF)-sprayed coatings, including WC-10Co4Cr coatings, Co-based coatings, WC-10Co4Cr/Co-based composite coatings, and Fe-based amorphous/nanocrystalline coatings were fabricated, and their cavitation behavior was evaluated in deionized water. Further, in-situ SEM surface observations were used to understand the microstructure of tested coatings. The results show that cavitation erosion initially occurred at pre-existing defects in the coatings. Meanwhile, it was found that cavitation erosion damage of the WC-10Co4Cr/Co-based composite coating, which contained a hard reinforcing phase (WC-10Co4Cr phase) and a soft matrix phase (Co-based phase), preferentially occurred at or around pores and microcracks in the reinforcement, rather than in the defect free matrix. This suggested that defects were a critical contributing factor to cavitation damage of the composite coatings. Furthermore, a mechanism was suggested to explicate the cavitation behavior of composite coatings. The approach of using in-situ SEM surface observations proved to be useful for the analysis of the cavitation mechanism of engineering materials and protective coatings.     

Nakagami-m parametric imaging for characterization of thermal coagulation and cavitation erosion induced by HIFU

Nowadays, both thermal and mechanical ablation techniques of HIFU associated with cavitation have been developed for noninvasive treatment. A specific challenge for the successful clinical implementation of HIFU is to achieve real-time imaging for the evaluation and determination of therapy outcomes such as necrosis or homogenization. Ultrasound Nakagami-m parametric imaging highlights the degrading shadowing effects of bubbles and can be used for tissue characterization. The aim of this study is to investigate the performance of Nakagami-m parametric imaging for evaluating and differentiating thermal coagulation and cavitation erosion induced by HIFU. Lesions were induced in basic bovine serum albumin (BSA) phantoms and ex vivo porcine livers using a 1.6 MHz single-element transducer. Thermal and mechanical lesions induced by two types of HIFU sequences respectively were evaluated using Nakagami-m parametric imaging and ultrasound B-mode imaging. The lesion sizes estimated using Nakagami-m parametric imaging technique were all closer to the actual sizes than those of B-mode imaging. The p-value obtained from the t-test between the mean m values of thermal coagulation and cavitation erosion was smaller than 0.05, demonstrating that the m values of thermal lesions were significantly different from that of mechanical lesions, which was confirmed by ex vivo experiments and histologic examination showed that different changes result from HIFU exposure, one of tissue dehydration resulting from the thermal effect, and the other of tissue homogenate resulting from mechanical effect. This study demonstrated that Nakagami-m parametric imaging is a potential real-time imaging technique for evaluating and differentiating thermal coagulation and cavitation erosion.     

Research on synergistic erosion by cavitation and sediment: A review

Sediment erosion frequently occurs in areas with high incidences of cavitation. The collaborative impact of abrasion and cavitation presents a host of challenges, threats, and damages to hydraulic engineering. However, little is known about the synergistic wear mechanism, and research conclusions remain inconsistent. In this work, relevant studies on synergistic erosion have been collected, classified, and analyzed. Presently, research on synergistic wear primarily operates at the macro and micro levels. The microscopic level enables the visualization and quantification of the process by which particles gain momentum from bubbles, the trajectory of particle acceleration, and the mechanism that triggers strong interactions between bubble-particle. At the macro level, erosion is understood as the summation of damage effects on the wall that is caused by the interaction between a plethora of bubbles of varying scales and numerous particles. The synergistic bubble-particle effect is reflected in the dual inhibiting or promoting mechanism. Furthermore, while numerical simulations could be realized by coupling cavitation, multiphase flow, and erosion models, their accuracy is not infallible. In the future, the dual role of particles, and particles driven by micro-jets or shock waves should be fully considered when establishing a combined erosion model. In addition, enhancing the influence of flow field and boundary parameters around bubbles and utilizing FSI would improve the predictive accuracy of erosion location and erosion rate. This work helps to elucidate the combined wear mechanism of hydraulic machinery components in sediment-laden flow environments and provides a theoretical basis for the design, manufacture, processing, and maintenance of hydraulic machinery.     

含气量对激光空泡在刚性壁面空蚀的影响

采用强脉冲激光器设计液体环境下刚性壁面空蚀实验平台,改变液体中含气量,利用高速相机观察不同含气量条件下激光空泡在壁面附近的脉动过程,并对刚性壁面造成的空蚀结果进行了观测。实验研究发现,随着液体中相对空气含量的提高,激光空泡脉动的最大尺寸增大,空泡的膨胀运动变剧烈,溃灭运动速度降低,空泡的溃灭强度降低,从而影响到溃灭冲击波和壁面微射流对刚性壁面的冲击速度,减弱了壁面空蚀,而液体中含气量的提高能够降低激光空泡对刚性壁面的空蚀程度。     

Cavitation activity stimulation by low frequency field pulses

The influence of a short-time action of a low-frequency ultrasound on the sonoluminescence generation by a high frequency pulsed field has been studied. This action remarkably lowers the cavitation thresholds and increases the sonoluminescence intensity. The stimulating effect of the low-frequency field action depends on its duration and on the intensities of both fields. Possible mechanisms of this effect are discussed.     

Estimation of chemical and physical effects of cavitation by analysis of cavitating single bubble dynamics

Cavitation events create extreme conditions in a localized ‘bubble collapse’ region, leading to the formation of hydroxyl radicals, shockwaves and microscopic high-speed jets, which are useful for many chemical and physical transformation processes. Single bubble dynamics equations have been used previously to investigate the chemical and physical effects of cavitation. In the present study, the state of the art of the single bubble dynamics equations was reviewed and certain noteworthy modifications were implemented. Simulations reaffirmed previously reported collapse temperatures of the order ~5,000 K and collapse pressures well over ~1,000 bar under varying operating conditions. The chemical effects were assessed in terms of the hydroxyl radical generation rate (OHG), calculated by applying the minimization of the Gibb’s Free Energy method using simulated collapse conditions. OHG values as high as 1x10¹² OH molecules per collapse event were found under certain operating conditions. A new equation was proposed to assess the physical effects, in terms of the impact pressure of the water jet - termed as the jet hammer pressure (JHP), formed due to the asymmetrical collapse of a bubble near a wall. The predicted JHP were found to be within a range of ~100 to 1000 bar under varying operating conditions. Important issues such as the onset of cavitation and chaotic solutions, for a cavitating single bubble dynamics were discussed. The Blake threshold pressure was found to be a sufficient criterion to capture the onset of cavitation. The impact of key operating parameters on the chemical and physical effects of cavitation were investigated exhaustively through simulations, over the parameter ranges relevant to acoustic and hydrodynamic cavitation processes. Presented methodology and results will be useful for optimisation and further investigations of a broad range of acoustic and hydrodynamic cavitation-based applications.     

Theoretical model of ice nucleation induced by inertial acoustic cavitation. Part 2: Number of ice nuclei generated by a single bubble

In the preceding paper (part 1), the pressure and temperature fields close to a bubble undergoing inertial acoustic cavitation were presented. It was shown that extremely high liquid water pressures but quite moderate temperatures were attained near the bubble wall just after the collapse providing the necessary conditions for ice nucleation. In this paper (part 2), the nucleation rate and the nuclei number generated by a single collapsing bubble were determined. The calculations were performed for different driving acoustic pressures, liquid ambient temperatures and bubble initial radius. An optimal acoustic pressure range and a nucleation temperature threshold as function of bubble radius were determined. The capability of moderate power ultrasound to trigger ice nucleation at low undercooling level and for a wide distribution of bubble sizes has thus been assessed on the theoretical ground.     

Evolution of surface roughness of some metallic materials in cavitation erosion

The evolution of surface roughness of three common metallic materials (316L stainless steel, CP titanium, and brass) in ultrasonic vibratory cavitation tests was monitored using profilometric measurements. Three stages of roughness change, based on the rate of change of the mean surface roughness d(Ra)/dt, may be identified. In stage I (initial stage), Ra increases almost linearly with the test time; in stage II (transition stage), the rate decreases until stage III (steady-state stage) is reached, in which Ra remains unchanged. Concurrent measurements of mass loss in the ultrasonic cavitation test indicated that stage I approximately coincides with the incubation stage, stage II approximately coincides with the acceleration stage, and stage III approximately coincides with the maximum erosion rate stage as defined by ASTM Standard G 32. Compared with conventional mass loss measurements in assessing material degradation in cavitation erosion, surface roughness measurements provide an alternative and convenient method which possesses several advantages. In the first place, change in surface roughness provides information of material response before mass loss is detected. Secondly, there is no restriction of the size of the component, while weighing is suitable for small samples only. Thirdly, mass loss reflects erosion of the whole surface under cavitation attack, and the mean depth of penetration or erosion only gives an average loss, while in roughness measurement, damage in specific locations may be studied. The present study indicates that roughness measurement may constitute a practical method for monitoring damage in industrial ultrasonic cleaners.