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The effect of sand particle concentrations on the vibratory cavitation erosion
Author’s Accepted Manuscript The effect of sand particle concentrations on the vibratory cavitation erosion H.X. Hu, Y.G. Zheng
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S0043-1648(16)30558-0 http://dx.doi.org/10.1016/j.wear.2017.05.003 WEA102161
To appear in: Wear Received date: 2 November 2016 Revised date: 11 May 2017 Accepted date: 11 May 2017 Cite this article as: H.X. Hu and Y.G. Zheng, The effect of sand particle concentrations on the vibratory cavitation erosion, Wear, http://dx.doi.org/10.1016/j.wear.2017.05.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The effect of sand particle concentrations on the vibratory cavitation erosion H.X. Hu*, Y.G. Zheng CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China
Abstract Comparisons between the cavitation erosion (CE) in pure water and cavitation-silt erosion (CSE) in sand suspensions were carried out experimentally on 304 stainless steel. The damage was evaluated by mass loss, scanning electron microscopy, roughness and surface residual stress. The effects of the particles and the sand concentrations on the CSE were analyzed. The results show that cumulative mass loss decreases with increasing sand concentration until the sand concentration exceeds 3 wt.%. A center region and the periphery of the test surface are dominated by particles erosion wear and CE respectively at lower sand concentrations. While the entire surface is covered by the particles erosion wear at higher sand concentrations. However, the roughness declines with increasing sand concentration in both regions. The damage mechanism is also proposed based on the comprehensive analyses.
Keywords: cavitation erosion; cavitation-silt erosion; sand concentration; impingement erosion.
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1. Introduction Cavitation erosion (CE) is a common problem in flow-handling components especially in the places where flow is suddenly disturbed, such as the reducer and expander pipes, valves and orifice plate structures. It can cause severe damage on the materials in a short time. There are many challenges in researching CE of the metallic materials because it is related to not only materials properties such as hardness [1], work hardening [2], phase’s transformation property [3] and stacking fault energy [4], but also the environmental parameters including corrosion medium [5-8] and temperature [9-13]. Moreover, local load caused by collapsing bubbles is difficult to understand which is also challenging the CE research. Solid particles effect on the cavitation erosion especially the vibratory cavitation erosion is far more complicated and few researchers pay attention to them. Most investigations concentrated on the cavitation-silt erosion (CSE) in fluid system using rotating disk devices with cavitation inducer and Venturi tube type devices. It has been reported that this erosion is more severe than the silt erosion in the fluid system without CE, which indicates a cavitation enhanced erosion [14]. Solid particles are carried by the high velocity fluid and impact on the materials surface resulting in direct erosion wear. Apart from this damage, synergistic action with the CE also affects the CSE processes by changing the fluid viscosity [15], the number of bubbles nucleation and the size of the bubbles [16]. While for the vibratory CE in sand suspensions, particles move in different directions due to the collapse of cavitation bubbles, which is the big difference from the movements of the particles in the CSE in high velocity slurry flow. Therefore, the particles’ effect on the vibratory CSE is more complex than that on the fluid CSE. Liu et. al [17] reported that the mass loss of the CSE was more, but the roughness was less than that of the CE for 20SiMn and 0Cr13Ni5Mo (corresponding to ASTM S-135).Chen et al. [18] revealed that the CSE damage was almost independent of particles shape, but the irregular particles erosion
on the material surface needed to be considered. They also proposed a critical particle size of approximately 500 nm, which had the highest CSE level [19]. Laguna-Camacho et. al [20] compared the vibratory CE of materials in tap water with that in water-solid particles mixture (75 μm in size). They found that the damaged surface caused by the pure CE in tap water was coarser than that caused by the CSE in the mixture of water and solid. Sand concentration is another parameter affecting the vibratory CSE mechanism, but few attentions have been paid to it. Hong [21, 22] investigated the vibratory CSE of HVOF coatings in different sediment concentrations and found that the erosion rate increased as the sediment concentration was increased. However, only two sand concentrations (20 kg/m3 and 40 kg/m3) were selected, which is not enough to fully obtain the relationship between the sand concentration and the CSE damage. Wu and Guo [23] studied vibratory CE of 1045 carbon steel in sand suspensions with three concentrations (25 kg/m3, 50 kg/m3 and 85 kg/m3) and five sand particle sizes (26 μm-531μm), but their focus centered on the critical size effect of the particles on CE. They proposed that the mass loss decreased with increase in sand concentrations on the condition of small sand particles. The synergetic action between the CE and the particles erosion is extremely complex. The CSE damage not only includes the separate CE and particle erosion wear, but also the mutual effect between them. Furthermore, each role played by cavitation and particle may change to different extent and dominate the entire damage on different sand concentrations. Unfortunately, the mechanism is yet far unknown due to the complexities. Commercial stainless steel 304 was selected as the target material in this study. Five silica sand concentrations of 0.5, 1, 3, 5, 10 wt.% were employed to investigate the effect of sand concentration on CSE. Cumulative mass loss, erosion morphologies, surface roughness and the residual stress analyses were examined to clarify the CSE mechanism. Damage models on different sand concentrations were proposed based on the analyses above.
2. Experimental 2.1 Test facilities and material The CE experiments were conducted using a 300 W magnetostrictive-induced cavitation facility with a vibratory frequency of 20 kHz and peak-to-peak vibratory amplitude of 60 μm. The apparatus is shown in Fig. 1 and the details can be seen in our previous work [24]. It is noted that this rig was homemade in China. All the parameters given with the rig were not adjustable. The tested specimens were screwed at the tip of the horn. The experiments were performed in a sand-pure water system contained in a beaker dipped in a water container at the room temperature, 25±1℃. To avoid the fluctuations of the experimental temperature, the beaker was cooled by the flowing tap water outside. A small beaker of 50 ml was used as the container of the solid-liquid solution, where a full dispersion of the sand particles in the suspension could be achieved with the supersonic vibration stir. Another reason for using such a small beaker as the solution container is to favor the collection of the experimental products including the broken sand particles and the residue removed from the specimens. These mixed powders were obtained after precipitation, separation and drying.
Fig. 1 Schematic of a magnetostrictive-induced cavitation facility and a specimen’s dimension
The experimental material is stainless steel 304 (UNS 30400) with chemical compositions of C: 0.06 wt.%, Si: 0.50%, Mn: 1.30%, P: 0.036%, S: 0.020%, Cr: 18.10%, Ni: 8.05% and Fe balanced. Its hardness was approximately 156 HV. Mass loss was determined using precision electronic balance with an accuracy of 0.1 mg after
ultrasonic cleaning of the specimen by ethanol and drying with air. All the data of mass loss were the average results of at least three parallel experiments. The morphologies of the surfaces and cross sections of the specimens were observed by scanning electron microscopy (SEM, FEI Inspect F). 2D and 3D surface roughness were measured by a non-contact surface roughness measuring instrument (MicroXAM-1200). The residual stress of the surface before and after experiments was examined by X-ray diffraction method (X-Pert PRO).
2.2 Particles and suspensions Silica sands of 75-150# were mixed with pure water to form suspension solutions with the sand concentration of 0.5 wt.%, 1 wt.%, 3 wt.%, 5 wt.% and 10 wt.%. The particle shape observed by SEM is shown in Fig. 2a. It is an irregular shape with sharp edges. The normal distribution of sand particles was measured with a wet dispersion method by a laser diffraction particle size distribution analyzer and the result is presented in Fig. 2b. The average size of the particles, D50 is approximately 152 μm. The hardness of the sand particles was approximately 1100 HV.
Fig. 2 SEM profiles of (a) sand particles and (b) normal distribution of sand particle size
3. Results 3.1 Mass loss Fig. 3 presents the cumulative mass loss against time at different sand concentrations. The error bar means the dispersity of the mass loss for three parallel experiments. It can be observed that the cumulative mass loss almost increases linearly with the test time for all the cases (Fig. 3a). However, the rising slope of 10 wt.% is higher than that of 5 wt.%, which is in turn higher than 0.5, 1 and 3 wt.%. The one for CE in pure water is between the cases of 10 wt.% and 5 wt.% . Moreover, the mass loss for 5 wt.% is higher than that of CE in pure water before the first 4 h, but lower at the end of the test. On the conditions of lower sand concentrations, 0.5, 1 and 3 wt.%, the variation of the mass loss of the CSE are almost the same. That is probably because the changes in the sand concentration are too small to make obvious damage difference.
Fig. 3 Variations of (a) cumulative mass loss and (b) cumulative mass loss rate of UNS30400 as a function of CE time at different sand concentrations, and (c) cumulative mass loss of 10 h test of UNS30400 as a function of sand concentration
The plot of the cumulative mass loss rate with time is shown in Fig. 3b. It can be observed that the cumulative mass loss rate at different sand concentrations increases for CSE sharply in the first 1 h, and then shows a reversible trend and thereafter remains relatively stable as the time is increased. In contrast, the rate for the CE in pure water increases quickly in the first 2 h, and then it reaches a stable period without decline. The quick rising period of the CE is longer than that of the CSE by 1 h. The relationship between the cumulative mass loss for 10 h and the sand concentrations can be seen in Fig. 3c. The mass loss initially decreases after adding sand particles in the pure water, and then it declines slightly until the sand concentration of 3 wt.%. It means that the sand particles inhibit CE. With further
increase in sand concentration, the mass loss increases linearly at relatively higher slope. It finally exceeds the mass loss of the CE in pure water at a sand concentration between 5 wt.% and 10 wt.%. It indicates that the sand particles start promoting the damage at certain sand concentration. Moreover, a critical sand concentration can be observed at approximately 3 wt.%, above which the mass loss begins to rise as the sand concentration is increased.
3.2 Morphology
Fig. 4 Macro morphology comparison of the damaged surfaces between the cases of (a) CE in pure water, (b) CSE in 0.5 wt.%, (c) 1 wt.% and (d) 5 wt.% sand concentrations after 10 h test
Digital images of the macro morphologies of the surfaces after CE in different sand concentrations are presented in Fig. 4. A perimeter encountered slight CE is along the edge of tested surface (Fig. 4a) and this agrees with the report of Wu et. al on the 1Cr18Ni9Ti stainless steel [25]. This is mainly caused by the special distribution of the bubbles under the condition of vibrating cavitation. Besides the original perimeter, a ring-like region and an ellipse center region can be distinguished
according to the damage extent and morphology after adding 0.5 wt.% sand particles (Fig. 4b). The differences in colors between these two distinctive erosion regions indicate different microscopic morphologies and erosion mechanisms. As the sand concentration is increased to 1 wt.%, the center region enlarges and its boundary becomes vague (Fig. 4c). Megascopic pits and craters are located in the ring-like region. When the sand concentration is increased to 3 wt.% and above, the entire surface is covered by uniform rough erosion appearance, which is similar to that in the center region after CSE at lower sand concentrations. The case of 5 wt.% representing those of 3 wt.% and 10 wt.% is shown in Fig. 4d. Details on the further morphology observation are displayed below.
Fig. 5 SEM morphologies of material surfaces after CE for 10 h in (a) pure water, (b and c) 0.5 wt.%, (d and e) 1 wt.%, (f and g) 3 wt.%, (h) 5 wt.% and (i) 10 wt.% sand suspensions, where b, d and f are obtained in the ring-like region, and c, e and g are from the center regions. The inserted images are local magnification for the representative morphologies.
Fig. 5 presents the surface morphologies examined by SEM after CE for 10 h in pure water and sand suspensions with different sand concentrations. It can be observed that the surface after CE in pure water is severely damaged and covered by dense CE craters and pits (Fig. 5a). In contrast, the surfaces after CSE are greatly different from that after CE in pure water. In the ring-like region for the case of 0.5 wt.% (Fig. 5b), most parts of the surface have been removed leaving island structures. Small scratches and pits can be observed on the remaining island surfaces. In contrast, though experienced deformation to certain extent, original surfaces remains mostly in the center region of the sample (Fig. 5c). A lot of pits and deformation ridges and valleys are distributed on the surface. Unlike the damage in the ring-like region, micro cutting and scratching occur on the surface instead of large area of material removal as shown in the magnified picture in the inserted figure (Fig. 5c). The surface is also relatively smooth compared to that in the ring-like region. As the sand concentration increases to 1 wt.%, the area of materials removal is obviously reduced in the ring-like region compared to that in the same region of 0.5 wt.% (Fig. 5d). In the center region (Fig. 5e), both the size and the number of the CSE craters decrease compared to those in the ring-like region (Fig. 5d), and those in the same region for 0.5 wt.% (Fig. 5c). Particles erosion wear scars dominate the parts without CE craters. As the sand concentrations increases, the number of CE craters in the ring-like region further decreases (Fig. 5f) and few CE pits can be observed in the center region of the sample (Fig. 5g). When the sand concentration increases to 5 wt.% and 10 wt.%, only erosion wear is left on the surface and no CE craters and pits can be observed (Fig. 5h and i). Shot scratches and impingement scars with no directions cover the entire surface. The heterogeneous distribution of the scratches indicates that the surface suffered solid particles impacting and ploughing in different directions and angles, which is different from that caused by high flow velocity slurry.
Fig. 6 SEM morphologies of the cross sectional surfaces of the material for (a) CE in pure water, (b) CSE in the center region and (c) ring-like region at 1 wt.% sand concentration, (d) for CSE at 5 wt.% and (e) 10 wt.% sand concentrations after test for 10 h
Fig. 6 shows the CE and CSE damage in the view of the cross sectional surfaces. In Fig. 6a, it can be observed that superficial layer of the sample is loose and the entire surface is covered by the big CE craters after 10 h CE in pure water. Some of them penetrate deep inside the mateiral indicating severe damage. In contrast, the cross sectional surface in the center region is much smoother for the CSE at 1 wt.% sand concentration than that for CE (Fig. 6b), although a few slight craters are located in the field of view (Fig. 6b). Compared with the center region, the crater is relatively deep in the ring-like region (Fig. 6c). However, they are much less than those for the CE in pure water. For the cases at higher sand concentrations of 5 wt.% and 10 wt.% (Fig. 6d and e), the profiles of the cross section is even smoother than that in the
center region of 1 wt.% (Fig. 6b). No obvious big crater can be observed, which is consistent with those observed on the surface morphologies (Fig. 5h and i).
3.3 Roughness
Fig. 7 Variation of the surface roughness as a function of the sand concentration after 10 h CE and CSE
The surface roughness before and after CE and CSE was measured and the results are shown in Fig. 7. Ra is arithmetical mean deviation of the profile. It was utilized as a parameter to evaluate the surface roughness. The data before test was obtained on the samples after being ground with 800# sand paper in turn. It is the lowest of all the cases, which made it easier to understand and so increase the roughness after CE in pure water. The roughness in the center region slightly decreases with increase in sand concentration until it reaches 3 wt.%, and then it remains in a relatively stable period. While for the roughness in the ring-like region, it is much higher than that in the center region and this is consistent with the SEM morphology observation that the damage in the ring-like region is much more severe than that in the center region (Fig. 5). It also decreases with increasing sand concentration but still much higher than that in the center region. The roughness in the ring-like region for the condition of 0.5 wt.% is approximately Ra=3.6
, which is 6 times that of the center region and 4.5 times
that of CE in pure water. As the sand concentration rises up to 5 wt.% and 10 wt.%, the ring-like region disappears and the roughness is uniform in the entire surface.
Fig. 8 Three dimensional roughness of the surfaces (a) before CE, (b) after CE and after CSE in (c and d) 0.5 wt.%, (e and f) 1 wt.%, (g and h) 3 wt.%, (i) 5 wt.% and (j) 10 wt.% for 10 h
Surface roughness is visualized by the three-dimensional roughness images displayed in Fig. 8. Before the test (Fig. 8a), the original surface has traces of grinding characteristics in the same direction during grinding. After CE in pure water for 10 h (Fig. 8b), the original surface is completely destroyed leaving an uneven surface with partial indentations and bumps which can be distinguished by the color differences. While for the CSE in 0.5 wt.% suspension (Fig. 8c), the surface is dominated by small peaks of protrusion-like stone forest and this is similar to the CSE pits in the center region (Fig. 5c and e). In contrast, hill-like morphologies with no sharp peak but
relatively big rugged appearances cover the surface of the ring-like region (Fig. 8d). As the sand concentration increases, the characteristics of the surface are similar to each other in the center region (Fig. 8e and g), while in the ring-like region, the surface fluctuation is alleviated and becomes relatively smooth with increasing sand concentration, which is consistent with the variation of the roughness shown in Fig. 7. However, compared with the morphologies in the center region, large area of protrusions and indentations can be observed in the ring-like region suggesting severe materials peeling. At higher sand concentration conditions (Fig. 8i and j), micro protrusions and indentations cover the entire surface, which are more like those in the center regions of lower sand concentrations (0.5-3 wt.%). They are greatly different from those in the ring-like region. Different roughness characteristics suggest different damage mechanisms, which will be analyzed in the discussion section.
Fig. 9 Two dimensional roughness distribution on the surfaces before and after CE and CSE at different sand concentrations for 10 h
Fig. 9 displays the roughness characteristics with two-dimensional curves at different conditions and it presents the roughness in another view. The scales of the roughness in the center region and ring-like region both show uniform properties. Compared with the roughness in the ring-like region, the roughness curves for the center regions are characteristic of many micro peaks, which is similar to those on the conditions of 0 wt.%, 5 wt.% and 10 wt.%. It indicates that relatively uniform surfaces resulted from sand erosion wear combined with less CE. However, in the ring-like region, local big and deep craters are more obvious. Moreover, the space
between big craters is very smooth suggesting that ring-like region suffers not only CE but also particles erosion wear. The characteristics of quantized roughness lines are well consistent with the SEM morphologies (Fig. 5) and the three dimensional roughness (Fig. 8).
3.4 Residual stress
Fig. 10 Variation of the surface residual stress as a function of sand concentration after 10 h CE and CSE
Fig. 10 displays the residual stress variations with the sand concentrations after CE and CSE for 10 h. The error bars indicate the measuring error. The negative value of residual stress on the surface before the test must have been caused during the processes of sample preparation, such as materials machining and grinding. Moreover, both the residual stress in the center region and ring-like region after CE and CSE are compressive stress and higher than the one before test, which indicates that the compressive stress is resulted from the CE and CSE and increases with the sand concentration. It can be deduced that particles erosion enhances the residual stress and the more the sand particles, the higher the residual stress. In the center region, as the sand concentration increases up to 5 wt.%, the residual stress has access to a relatively stable period. It is -434.18±4.75 MPa at 10 wt.% sand concentration, which is approximately 1.76 times that for CE in pure water and approximately 2.86 times that
before test. Compared with the residual stress in the center region, it is relatively high in the ring-like region but it generally maintains an increasing trend. This indicates that the ring-like region encountered more bubble attacks and suffered more compressive stress than the center region. The stress in the ring-like region is approximately 1.3, 1.1 and 1.2 times as high as that in the center region at 0.5 wt.%, 1 wt.% and 3 wt.% sand concentration respectively. Different extents of stress between the two regions probably suggest different damage mechanisms.
4. Discussion 4.1 Particles effect on damage Multiple synergetic roles are played by the sand particles including grind impact on the material surface [21, 22, 26], change of the liquid viscosity [15, 27], shielding effect to the collapsed bubbles attack [28, 29] and effect on the nucleation and growth of the bubbles. It is extremely difficult to evaluate each action quantitatively and independently. However, two complex competitive actions affect CSE processes all the time. One is the positive action promoting the CE. The other is the negative action inhibiting the CE. For the former, the particles in turbulence are captured and accelerated by the collapsing bubbles in the formation of shock wave or micro jet [26]. They impact the material surface and cause extra materials removal in the formation of micro cutting and fatigue. The micro cutting and shot scratches on the surface are caused by the particle erosion instead of bubbles (Fig. 5b-i), which can also be observed from the differences between the particle size and shape apart from surface morphologies. There are sharp edges and points on the particles before test (Fig. 11a), which are worn away after 10 h CSE (Fig. 11b). Therefore, the particles with high hardness must cause severe damage on the material surface. Moreover, the material is removed in the formation of small particles as shown in Fig. 11c and d. Its size is approximately 1/50 of that of the solid particles in the field of view.
Fig. 11 SEM of sand particles (a) before test, (b) after 10 h test and (c and d) particles dropping from the sample after 10 h test
As for the negative effect, the erosion wear of the particles can also smoothen the surface and reduce the number of nuclei of bubbles, which will reduce the CE. For the CE in pure water, small and shallow craters initially caused by CE can guide the shock wave of collapsed bubbles and concentrate them inside the craters, which will enlarge and dig the craters gradually to be the big and deep craters [30]. While for the CSE, it is inevitable for the sample to suffer CE. However, particles will grind the edges of the craters and reflect the shock wave inhibiting the further growth of the CE craters. This is why the surface roughness after CE is much higher than that after CSE (Fig. 7). Besides that, the collision between the sand particles and bubbles can break the bubbles before they grow up. Therefore, the damage caused by the small bubbles is smaller than that caused by the big ones. In addition to the particles erosion and breaking to the free bubbles, the changes of the liquid viscosity due to the addition of sand particles can also affect the CSE. The viscosity rises as the sand particles are added, which will inhibit the growth of the bubbles and consequently alleviate the CSE [19, 31]. All of the effects result in the
fact that the cumulative mass loss of CSE at lower sand concentrations is less than that of CE in pure water. As the sand concentration increases to a certain value, prior to the negative effect, the positive effect of the sand particles causes more CSE than the pure CE (Fig. 3). It is difficult to quantitatively evaluate the pure particles impingement erosion and pure CE independently according to the analyses above, because the synergetic actions between the particles and bubbles are quite complex.
4.2 Effect of sand concentration on the damage It is generally reported that the mass loss increases with increasing sand concentration in the CSE [21, 22, 32]. Meanwhile Wu et. al [23] observed that it did not decrease with the sand concentration until the particle size exceeded a critical value. Apart from the particle size, the sand concentration plays an important role in the damage process. Hong et. al [21, 22] selected only two sand concentrations (20 kg/m3 and 40 kg/m3) in their CSE experiments and found a monotonic increasing curve of mass loss with the sand concentrations. However, it is difficult to completely identify the relationship between the effect of the sand concentrations and the CE. Larger region (0.5-10 wt.%) of sand concentrations was employed in the present study. It is observed that the mass loss initially decreases and then increases with the increasing sand concentration with a certain particle size of approximately D50=152 μm. Based on the two distinctive morphologies at different sand concentrations (Fig. 4 and Fig. 5), it can be deduced that they are caused by different mechanisms. At lower sand concentrations, the CE still dominates the removal of the materials. Therefore, the equation below can be employed to describe the relationship between the bubbles and particles combined with the observed results. (1) Where Na is the total number of collapsed bubbles, Nm is the number of the collapsed bubbles from homogenous nucleation in the body of flow caused by ultrasonic vibration, Nt is the number of the collapsed bubbles from heterogeneous
nucleation on the particle surfaces, and Nc is the number of the bubbles coalesced with particles through collisions. For the CE in pure water, the CE intensity can be described by Na=Nm. After adding sand particles with low sand concentrations, the number of homogeneous bubbles, Nm in the solution will be reduced due to the collision of the particles. The reduction number depends on the size of the particles [16]. The crevices and concaves on the particle surface can also create bubbles nucleation, which is called heterogeneous nucleation [33]. Therefore, Nt will increase with the increasing sand concentration. As for the Nc，according to Sutherland’s theory [16], the particles with the size of approximately 0.5 μm have the lowest probability to collide with the bubbles. The size of the particles used in the present study (approximately 152 μm) is much larger than the critical size, which probably indicates more collision chances. Moreover, the collision will also increase with the particle concentration [32]. The bubble-particle coalescence is more difficult to collapse than the pure bubbles once the coalescence forms. As a result, the coalescence will occur as the addition of the particles and increase with increasing sand concentration in a certain range. Based on the damage extent (Fig. 3),
is probably less in the
sand/water solution than that in the pure water. For CSE at high sand concentrations, the entire surface of the sample is covered by micro cutting and ploughing and there is no obvious CE crater (Fig. 4d and Fig. 5i). It indicates that the particles impingement erosion has dominated damage instead of the CE. Moreover, the stress caused by particles impingements is higher than that caused by either collapsed bubbles or its combination effect with particles (Fig. 10). As the sand concentration increases, the stress caused by the particles impingement is also relatively high compared to that with combination effect (Fig. 10). It continues to increase independently on the variation of cumulative mass loss, which suggests that surface residual stress is a superficial phenomenon and can not be used to explain the mass loss. At least, it is not the only factor affecting the damage mechanism. Moreover, the damage mechanism changes from the combination effect of the bubbles and particles at low sand concentrations to the one dominated by particles
impingement at high sand concentrations. This can be proved by the morphology evolutions that it changes from two different erosion regions (center region and ring-like region) to one uniform erosion region as the sand concentration increases (Fig. 4d). On the other hand, the mass loss roughly rises with the increase of sand concentration by and large (Fig. 3) despite the initial slight decrease from 0.5-3 wt.% sand concentrations. This trend is consistent with that discovered by Kang et. al [21, 32]. Moreover, the mass loss for CSE does not exceed the one for CE in pure water until the sand concentration rises up to a value between 5 wt.% and 10 wt.%. It indicates that the sand particles can alleviate the CE at low sand concentrations. However, higher concentration of sand particles greatly enhances the erosion of the material. The cumulative mass loss at 10 wt.% is approximately 1.6 times that for CE in pure water, which is mainly due to the particles impingement erosion.
4.3 Morphology evolution The difference in erosion morphologies of the center region from that of the ring-like region is mainly caused by the competitive actions of sand particle erosion wear and collapse bubble attack. It is also dependent on the turbulence distribution near the sample surface. In general, there is a CE boundary on the sample surface even in pure water CE (Fig. 4), which is similar to that observed by Hong et. al. [21] and Hattori et al. [34]. Under the micro vibration at a high frequency, the bubbles are created and concentrate in the center part resulting in an obvious CE region in the center region and a much less damaged region in the perimeter (Fig. 4) [20]. After adding sand particles with low sand concentration ≤3 wt.%, the sand particles are stirred by the ultrasonic vibration and gain high velocity from the bombardment of the collapsed bubbles. They slide on the surface at different angles leaving chaotic scratches [20]. Moreover, it is reported that the bubbles are concentrated below the center region [20], which will drive more sand particles to impact on the samples. Therefore, particle impingement erosion in the center region is more than that in the
ring-like region (Fig. 5). However, the damage caused by CE is more severe than that caused by particles impingement erosion. It can be deduced that both the roughness and the residual stress in the ring-like region are higher than those in the center region. At higher sand concentrations ≥5 wt.% (Fig. 5h, i and Fig. 6d, e), the numbers of the sand particles are high enough to shield and take over the damaged energy of the collapsing bubbles, which is transferred to impact and scratch the surface. The erosion morphology for 10 wt. % (Fig. 5i) is similar to that in the center region for 0.5 wt.% (Fig. 5c). They are both characteristic of the particles impingement erosion. It can be deduced that the particles erosion wear area enlarges with the increasing sand concentration. Although the roughness decreases (Fig. 7), the cumulative mass loss and mass loss rate increase (Fig. 3). Therefore, it is bias to evaluate the CSE extent only by the roughness and morphologies. In addition to the relationship between the roughness and the mass loss, the variation of the roughness with sand concentration (Fig. 7) is opposite to that of the residual stress (Fig. 10). It is roughly applied to both the center region and the ring-like region. After adding sand particles, the surface absorbs compressive energy from not only the bubbles collapsing but also the particles impingement. This is why the residual stress due to CSE is higher than that due to pure CE, which is more obvious in the ring-like region because of stronger synergistic effect. Under this condition, plastic deformation and crack growth are difficult to develop, which results in the decreasing surface roughness. When the compressive stress exceeds material’s yield limit, the mass loss will increase due to fatigue mechanism.
4.4 Damage mechanism
Fig. 12 CE and CSE mechanism at different sand concentrations
Based on the analyses above, the damage mechanism can be deduced as shown in Fig. 12. For the CE in pure water (Fig. 12a), the material loss is all attributed to the CE in the mechanisms of micro jet flow or shock wave. The damaged surface is characteristic of big and dense craters due to the guided wave effect [30]. After adding sand particles with low concentration (Fig. 12b), the center region is dominated by
particles impingement erosion and the number of big CE craters decreases due to the polishing of the sand particles (Fig. 5c, e and g). While in the ring-like region, less enhanced particle impingement results in the dominant CE (Fig. 5b, d and f), owing to the cone-like bubble structure below the samples [20]. Therefore, the roughness of the center region is much lower than that of the ring-like region because center region suffers more polishing of sand particles (Fig. 7). The phenomenon is well consistent with that obtained by Laguna-Camacho et. al [20]. As the sand concentration exceeds a critical value (Fig. 12c), sand particles are more enough to shield the attacks of the collapsing bubbles and take over the attacking energy. The damage turns from the combined attack of the direct CE and particles impingement erosion to the single particles impingement erosion. It results in no presence of big craters on the surfaces (Fig. 5h and i, Fig. 6d and e) and small roughness. However, the mass loss (Fig. 3) and the residual stress (Fig. 10) are much higher than those for low sand concentrations. In other words, the damage deteriorates, although it seems alleviated in morphologies. Therefore, comprehensive analyses including morphologies, surface roughness, residual stress and so on are needed to evaluate the real CSE.
5. Conclusions (1) A critical sand concentration was observed at approximately 3 wt.%, beyond which the cumulative mass loss increased with the increasing sand concentration. Moreover, the cumulative mass loss of CSE at sand concentration regions of 0.5 wt.%- 5 wt.% were less than that of CE in pure water. (2) A center region dominated by particles impingement erosion and a ring-like region dominated by direct CE were observed on the surfaces after CSE at 0.5 wt.% - 3 wt.% sand concentrations, while uniform particles impingement erosion covered the entire surfaces after CSE at higher sand concentrations. (3) The roughness after CSE decreased with sand concentration until ≥5 wt.%
sand concentration, where the roughness experienced small changes. (4) The compressive residual stress on the surface after CSE was higher than that after CE in pure water. It initially increased with increasing sand concentration and then ended with a stable period between 5 wt.% and 10 wt.%. (5) A schematic of CSE was proposed in this paper. CE was responsible for the big craters and higher roughness. Particles impingement erosion competed with CE for dominating the damage of CSE, which changed with the sand concentrations.
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant No: 51501206). The authors appreciate Ekerenam, Okpo Okpo and Wilfred Emori for their help in modifying the manuscript language.
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PII: DOI: Reference:
S0043-1648(16)30558-0 http://dx.doi.org/10.1016/j.wear.2017.05.003 WEA102161
To appear in: Wear Received date: 2 November 2016 Revised date: 11 May 2017 Accepted date: 11 May 2017 Cite this article as: H.X. Hu and Y.G. Zheng, The effect of sand particle concentrations on the vibratory cavitation erosion, Wear, http://dx.doi.org/10.1016/j.wear.2017.05.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The effect of sand particle concentrations on the vibratory cavitation erosion H.X. Hu*, Y.G. Zheng CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China
Abstract Comparisons between the cavitation erosion (CE) in pure water and cavitation-silt erosion (CSE) in sand suspensions were carried out experimentally on 304 stainless steel. The damage was evaluated by mass loss, scanning electron microscopy, roughness and surface residual stress. The effects of the particles and the sand concentrations on the CSE were analyzed. The results show that cumulative mass loss decreases with increasing sand concentration until the sand concentration exceeds 3 wt.%. A center region and the periphery of the test surface are dominated by particles erosion wear and CE respectively at lower sand concentrations. While the entire surface is covered by the particles erosion wear at higher sand concentrations. However, the roughness declines with increasing sand concentration in both regions. The damage mechanism is also proposed based on the comprehensive analyses.
Keywords: cavitation erosion; cavitation-silt erosion; sand concentration; impingement erosion.
*
Corresponding author Tel: +86-24-23915904;
E-mail addresses: [email protected]
1. Introduction Cavitation erosion (CE) is a common problem in flow-handling components especially in the places where flow is suddenly disturbed, such as the reducer and expander pipes, valves and orifice plate structures. It can cause severe damage on the materials in a short time. There are many challenges in researching CE of the metallic materials because it is related to not only materials properties such as hardness [1], work hardening [2], phase’s transformation property [3] and stacking fault energy [4], but also the environmental parameters including corrosion medium [5-8] and temperature [9-13]. Moreover, local load caused by collapsing bubbles is difficult to understand which is also challenging the CE research. Solid particles effect on the cavitation erosion especially the vibratory cavitation erosion is far more complicated and few researchers pay attention to them. Most investigations concentrated on the cavitation-silt erosion (CSE) in fluid system using rotating disk devices with cavitation inducer and Venturi tube type devices. It has been reported that this erosion is more severe than the silt erosion in the fluid system without CE, which indicates a cavitation enhanced erosion [14]. Solid particles are carried by the high velocity fluid and impact on the materials surface resulting in direct erosion wear. Apart from this damage, synergistic action with the CE also affects the CSE processes by changing the fluid viscosity [15], the number of bubbles nucleation and the size of the bubbles [16]. While for the vibratory CE in sand suspensions, particles move in different directions due to the collapse of cavitation bubbles, which is the big difference from the movements of the particles in the CSE in high velocity slurry flow. Therefore, the particles’ effect on the vibratory CSE is more complex than that on the fluid CSE. Liu et. al [17] reported that the mass loss of the CSE was more, but the roughness was less than that of the CE for 20SiMn and 0Cr13Ni5Mo (corresponding to ASTM S-135).Chen et al. [18] revealed that the CSE damage was almost independent of particles shape, but the irregular particles erosion
on the material surface needed to be considered. They also proposed a critical particle size of approximately 500 nm, which had the highest CSE level [19]. Laguna-Camacho et. al [20] compared the vibratory CE of materials in tap water with that in water-solid particles mixture (75 μm in size). They found that the damaged surface caused by the pure CE in tap water was coarser than that caused by the CSE in the mixture of water and solid. Sand concentration is another parameter affecting the vibratory CSE mechanism, but few attentions have been paid to it. Hong [21, 22] investigated the vibratory CSE of HVOF coatings in different sediment concentrations and found that the erosion rate increased as the sediment concentration was increased. However, only two sand concentrations (20 kg/m3 and 40 kg/m3) were selected, which is not enough to fully obtain the relationship between the sand concentration and the CSE damage. Wu and Guo [23] studied vibratory CE of 1045 carbon steel in sand suspensions with three concentrations (25 kg/m3, 50 kg/m3 and 85 kg/m3) and five sand particle sizes (26 μm-531μm), but their focus centered on the critical size effect of the particles on CE. They proposed that the mass loss decreased with increase in sand concentrations on the condition of small sand particles. The synergetic action between the CE and the particles erosion is extremely complex. The CSE damage not only includes the separate CE and particle erosion wear, but also the mutual effect between them. Furthermore, each role played by cavitation and particle may change to different extent and dominate the entire damage on different sand concentrations. Unfortunately, the mechanism is yet far unknown due to the complexities. Commercial stainless steel 304 was selected as the target material in this study. Five silica sand concentrations of 0.5, 1, 3, 5, 10 wt.% were employed to investigate the effect of sand concentration on CSE. Cumulative mass loss, erosion morphologies, surface roughness and the residual stress analyses were examined to clarify the CSE mechanism. Damage models on different sand concentrations were proposed based on the analyses above.
2. Experimental 2.1 Test facilities and material The CE experiments were conducted using a 300 W magnetostrictive-induced cavitation facility with a vibratory frequency of 20 kHz and peak-to-peak vibratory amplitude of 60 μm. The apparatus is shown in Fig. 1 and the details can be seen in our previous work [24]. It is noted that this rig was homemade in China. All the parameters given with the rig were not adjustable. The tested specimens were screwed at the tip of the horn. The experiments were performed in a sand-pure water system contained in a beaker dipped in a water container at the room temperature, 25±1℃. To avoid the fluctuations of the experimental temperature, the beaker was cooled by the flowing tap water outside. A small beaker of 50 ml was used as the container of the solid-liquid solution, where a full dispersion of the sand particles in the suspension could be achieved with the supersonic vibration stir. Another reason for using such a small beaker as the solution container is to favor the collection of the experimental products including the broken sand particles and the residue removed from the specimens. These mixed powders were obtained after precipitation, separation and drying.
Fig. 1 Schematic of a magnetostrictive-induced cavitation facility and a specimen’s dimension
The experimental material is stainless steel 304 (UNS 30400) with chemical compositions of C: 0.06 wt.%, Si: 0.50%, Mn: 1.30%, P: 0.036%, S: 0.020%, Cr: 18.10%, Ni: 8.05% and Fe balanced. Its hardness was approximately 156 HV. Mass loss was determined using precision electronic balance with an accuracy of 0.1 mg after
ultrasonic cleaning of the specimen by ethanol and drying with air. All the data of mass loss were the average results of at least three parallel experiments. The morphologies of the surfaces and cross sections of the specimens were observed by scanning electron microscopy (SEM, FEI Inspect F). 2D and 3D surface roughness were measured by a non-contact surface roughness measuring instrument (MicroXAM-1200). The residual stress of the surface before and after experiments was examined by X-ray diffraction method (X-Pert PRO).
2.2 Particles and suspensions Silica sands of 75-150# were mixed with pure water to form suspension solutions with the sand concentration of 0.5 wt.%, 1 wt.%, 3 wt.%, 5 wt.% and 10 wt.%. The particle shape observed by SEM is shown in Fig. 2a. It is an irregular shape with sharp edges. The normal distribution of sand particles was measured with a wet dispersion method by a laser diffraction particle size distribution analyzer and the result is presented in Fig. 2b. The average size of the particles, D50 is approximately 152 μm. The hardness of the sand particles was approximately 1100 HV.
Fig. 2 SEM profiles of (a) sand particles and (b) normal distribution of sand particle size
3. Results 3.1 Mass loss Fig. 3 presents the cumulative mass loss against time at different sand concentrations. The error bar means the dispersity of the mass loss for three parallel experiments. It can be observed that the cumulative mass loss almost increases linearly with the test time for all the cases (Fig. 3a). However, the rising slope of 10 wt.% is higher than that of 5 wt.%, which is in turn higher than 0.5, 1 and 3 wt.%. The one for CE in pure water is between the cases of 10 wt.% and 5 wt.% . Moreover, the mass loss for 5 wt.% is higher than that of CE in pure water before the first 4 h, but lower at the end of the test. On the conditions of lower sand concentrations, 0.5, 1 and 3 wt.%, the variation of the mass loss of the CSE are almost the same. That is probably because the changes in the sand concentration are too small to make obvious damage difference.
Fig. 3 Variations of (a) cumulative mass loss and (b) cumulative mass loss rate of UNS30400 as a function of CE time at different sand concentrations, and (c) cumulative mass loss of 10 h test of UNS30400 as a function of sand concentration
The plot of the cumulative mass loss rate with time is shown in Fig. 3b. It can be observed that the cumulative mass loss rate at different sand concentrations increases for CSE sharply in the first 1 h, and then shows a reversible trend and thereafter remains relatively stable as the time is increased. In contrast, the rate for the CE in pure water increases quickly in the first 2 h, and then it reaches a stable period without decline. The quick rising period of the CE is longer than that of the CSE by 1 h. The relationship between the cumulative mass loss for 10 h and the sand concentrations can be seen in Fig. 3c. The mass loss initially decreases after adding sand particles in the pure water, and then it declines slightly until the sand concentration of 3 wt.%. It means that the sand particles inhibit CE. With further
increase in sand concentration, the mass loss increases linearly at relatively higher slope. It finally exceeds the mass loss of the CE in pure water at a sand concentration between 5 wt.% and 10 wt.%. It indicates that the sand particles start promoting the damage at certain sand concentration. Moreover, a critical sand concentration can be observed at approximately 3 wt.%, above which the mass loss begins to rise as the sand concentration is increased.
3.2 Morphology
Fig. 4 Macro morphology comparison of the damaged surfaces between the cases of (a) CE in pure water, (b) CSE in 0.5 wt.%, (c) 1 wt.% and (d) 5 wt.% sand concentrations after 10 h test
Digital images of the macro morphologies of the surfaces after CE in different sand concentrations are presented in Fig. 4. A perimeter encountered slight CE is along the edge of tested surface (Fig. 4a) and this agrees with the report of Wu et. al on the 1Cr18Ni9Ti stainless steel [25]. This is mainly caused by the special distribution of the bubbles under the condition of vibrating cavitation. Besides the original perimeter, a ring-like region and an ellipse center region can be distinguished
according to the damage extent and morphology after adding 0.5 wt.% sand particles (Fig. 4b). The differences in colors between these two distinctive erosion regions indicate different microscopic morphologies and erosion mechanisms. As the sand concentration is increased to 1 wt.%, the center region enlarges and its boundary becomes vague (Fig. 4c). Megascopic pits and craters are located in the ring-like region. When the sand concentration is increased to 3 wt.% and above, the entire surface is covered by uniform rough erosion appearance, which is similar to that in the center region after CSE at lower sand concentrations. The case of 5 wt.% representing those of 3 wt.% and 10 wt.% is shown in Fig. 4d. Details on the further morphology observation are displayed below.
Fig. 5 SEM morphologies of material surfaces after CE for 10 h in (a) pure water, (b and c) 0.5 wt.%, (d and e) 1 wt.%, (f and g) 3 wt.%, (h) 5 wt.% and (i) 10 wt.% sand suspensions, where b, d and f are obtained in the ring-like region, and c, e and g are from the center regions. The inserted images are local magnification for the representative morphologies.
Fig. 5 presents the surface morphologies examined by SEM after CE for 10 h in pure water and sand suspensions with different sand concentrations. It can be observed that the surface after CE in pure water is severely damaged and covered by dense CE craters and pits (Fig. 5a). In contrast, the surfaces after CSE are greatly different from that after CE in pure water. In the ring-like region for the case of 0.5 wt.% (Fig. 5b), most parts of the surface have been removed leaving island structures. Small scratches and pits can be observed on the remaining island surfaces. In contrast, though experienced deformation to certain extent, original surfaces remains mostly in the center region of the sample (Fig. 5c). A lot of pits and deformation ridges and valleys are distributed on the surface. Unlike the damage in the ring-like region, micro cutting and scratching occur on the surface instead of large area of material removal as shown in the magnified picture in the inserted figure (Fig. 5c). The surface is also relatively smooth compared to that in the ring-like region. As the sand concentration increases to 1 wt.%, the area of materials removal is obviously reduced in the ring-like region compared to that in the same region of 0.5 wt.% (Fig. 5d). In the center region (Fig. 5e), both the size and the number of the CSE craters decrease compared to those in the ring-like region (Fig. 5d), and those in the same region for 0.5 wt.% (Fig. 5c). Particles erosion wear scars dominate the parts without CE craters. As the sand concentrations increases, the number of CE craters in the ring-like region further decreases (Fig. 5f) and few CE pits can be observed in the center region of the sample (Fig. 5g). When the sand concentration increases to 5 wt.% and 10 wt.%, only erosion wear is left on the surface and no CE craters and pits can be observed (Fig. 5h and i). Shot scratches and impingement scars with no directions cover the entire surface. The heterogeneous distribution of the scratches indicates that the surface suffered solid particles impacting and ploughing in different directions and angles, which is different from that caused by high flow velocity slurry.
Fig. 6 SEM morphologies of the cross sectional surfaces of the material for (a) CE in pure water, (b) CSE in the center region and (c) ring-like region at 1 wt.% sand concentration, (d) for CSE at 5 wt.% and (e) 10 wt.% sand concentrations after test for 10 h
Fig. 6 shows the CE and CSE damage in the view of the cross sectional surfaces. In Fig. 6a, it can be observed that superficial layer of the sample is loose and the entire surface is covered by the big CE craters after 10 h CE in pure water. Some of them penetrate deep inside the mateiral indicating severe damage. In contrast, the cross sectional surface in the center region is much smoother for the CSE at 1 wt.% sand concentration than that for CE (Fig. 6b), although a few slight craters are located in the field of view (Fig. 6b). Compared with the center region, the crater is relatively deep in the ring-like region (Fig. 6c). However, they are much less than those for the CE in pure water. For the cases at higher sand concentrations of 5 wt.% and 10 wt.% (Fig. 6d and e), the profiles of the cross section is even smoother than that in the
center region of 1 wt.% (Fig. 6b). No obvious big crater can be observed, which is consistent with those observed on the surface morphologies (Fig. 5h and i).
3.3 Roughness
Fig. 7 Variation of the surface roughness as a function of the sand concentration after 10 h CE and CSE
The surface roughness before and after CE and CSE was measured and the results are shown in Fig. 7. Ra is arithmetical mean deviation of the profile. It was utilized as a parameter to evaluate the surface roughness. The data before test was obtained on the samples after being ground with 800# sand paper in turn. It is the lowest of all the cases, which made it easier to understand and so increase the roughness after CE in pure water. The roughness in the center region slightly decreases with increase in sand concentration until it reaches 3 wt.%, and then it remains in a relatively stable period. While for the roughness in the ring-like region, it is much higher than that in the center region and this is consistent with the SEM morphology observation that the damage in the ring-like region is much more severe than that in the center region (Fig. 5). It also decreases with increasing sand concentration but still much higher than that in the center region. The roughness in the ring-like region for the condition of 0.5 wt.% is approximately Ra=3.6
, which is 6 times that of the center region and 4.5 times
that of CE in pure water. As the sand concentration rises up to 5 wt.% and 10 wt.%, the ring-like region disappears and the roughness is uniform in the entire surface.
Fig. 8 Three dimensional roughness of the surfaces (a) before CE, (b) after CE and after CSE in (c and d) 0.5 wt.%, (e and f) 1 wt.%, (g and h) 3 wt.%, (i) 5 wt.% and (j) 10 wt.% for 10 h
Surface roughness is visualized by the three-dimensional roughness images displayed in Fig. 8. Before the test (Fig. 8a), the original surface has traces of grinding characteristics in the same direction during grinding. After CE in pure water for 10 h (Fig. 8b), the original surface is completely destroyed leaving an uneven surface with partial indentations and bumps which can be distinguished by the color differences. While for the CSE in 0.5 wt.% suspension (Fig. 8c), the surface is dominated by small peaks of protrusion-like stone forest and this is similar to the CSE pits in the center region (Fig. 5c and e). In contrast, hill-like morphologies with no sharp peak but
relatively big rugged appearances cover the surface of the ring-like region (Fig. 8d). As the sand concentration increases, the characteristics of the surface are similar to each other in the center region (Fig. 8e and g), while in the ring-like region, the surface fluctuation is alleviated and becomes relatively smooth with increasing sand concentration, which is consistent with the variation of the roughness shown in Fig. 7. However, compared with the morphologies in the center region, large area of protrusions and indentations can be observed in the ring-like region suggesting severe materials peeling. At higher sand concentration conditions (Fig. 8i and j), micro protrusions and indentations cover the entire surface, which are more like those in the center regions of lower sand concentrations (0.5-3 wt.%). They are greatly different from those in the ring-like region. Different roughness characteristics suggest different damage mechanisms, which will be analyzed in the discussion section.
Fig. 9 Two dimensional roughness distribution on the surfaces before and after CE and CSE at different sand concentrations for 10 h
Fig. 9 displays the roughness characteristics with two-dimensional curves at different conditions and it presents the roughness in another view. The scales of the roughness in the center region and ring-like region both show uniform properties. Compared with the roughness in the ring-like region, the roughness curves for the center regions are characteristic of many micro peaks, which is similar to those on the conditions of 0 wt.%, 5 wt.% and 10 wt.%. It indicates that relatively uniform surfaces resulted from sand erosion wear combined with less CE. However, in the ring-like region, local big and deep craters are more obvious. Moreover, the space
between big craters is very smooth suggesting that ring-like region suffers not only CE but also particles erosion wear. The characteristics of quantized roughness lines are well consistent with the SEM morphologies (Fig. 5) and the three dimensional roughness (Fig. 8).
3.4 Residual stress
Fig. 10 Variation of the surface residual stress as a function of sand concentration after 10 h CE and CSE
Fig. 10 displays the residual stress variations with the sand concentrations after CE and CSE for 10 h. The error bars indicate the measuring error. The negative value of residual stress on the surface before the test must have been caused during the processes of sample preparation, such as materials machining and grinding. Moreover, both the residual stress in the center region and ring-like region after CE and CSE are compressive stress and higher than the one before test, which indicates that the compressive stress is resulted from the CE and CSE and increases with the sand concentration. It can be deduced that particles erosion enhances the residual stress and the more the sand particles, the higher the residual stress. In the center region, as the sand concentration increases up to 5 wt.%, the residual stress has access to a relatively stable period. It is -434.18±4.75 MPa at 10 wt.% sand concentration, which is approximately 1.76 times that for CE in pure water and approximately 2.86 times that
before test. Compared with the residual stress in the center region, it is relatively high in the ring-like region but it generally maintains an increasing trend. This indicates that the ring-like region encountered more bubble attacks and suffered more compressive stress than the center region. The stress in the ring-like region is approximately 1.3, 1.1 and 1.2 times as high as that in the center region at 0.5 wt.%, 1 wt.% and 3 wt.% sand concentration respectively. Different extents of stress between the two regions probably suggest different damage mechanisms.
4. Discussion 4.1 Particles effect on damage Multiple synergetic roles are played by the sand particles including grind impact on the material surface [21, 22, 26], change of the liquid viscosity [15, 27], shielding effect to the collapsed bubbles attack [28, 29] and effect on the nucleation and growth of the bubbles. It is extremely difficult to evaluate each action quantitatively and independently. However, two complex competitive actions affect CSE processes all the time. One is the positive action promoting the CE. The other is the negative action inhibiting the CE. For the former, the particles in turbulence are captured and accelerated by the collapsing bubbles in the formation of shock wave or micro jet [26]. They impact the material surface and cause extra materials removal in the formation of micro cutting and fatigue. The micro cutting and shot scratches on the surface are caused by the particle erosion instead of bubbles (Fig. 5b-i), which can also be observed from the differences between the particle size and shape apart from surface morphologies. There are sharp edges and points on the particles before test (Fig. 11a), which are worn away after 10 h CSE (Fig. 11b). Therefore, the particles with high hardness must cause severe damage on the material surface. Moreover, the material is removed in the formation of small particles as shown in Fig. 11c and d. Its size is approximately 1/50 of that of the solid particles in the field of view.
Fig. 11 SEM of sand particles (a) before test, (b) after 10 h test and (c and d) particles dropping from the sample after 10 h test
As for the negative effect, the erosion wear of the particles can also smoothen the surface and reduce the number of nuclei of bubbles, which will reduce the CE. For the CE in pure water, small and shallow craters initially caused by CE can guide the shock wave of collapsed bubbles and concentrate them inside the craters, which will enlarge and dig the craters gradually to be the big and deep craters [30]. While for the CSE, it is inevitable for the sample to suffer CE. However, particles will grind the edges of the craters and reflect the shock wave inhibiting the further growth of the CE craters. This is why the surface roughness after CE is much higher than that after CSE (Fig. 7). Besides that, the collision between the sand particles and bubbles can break the bubbles before they grow up. Therefore, the damage caused by the small bubbles is smaller than that caused by the big ones. In addition to the particles erosion and breaking to the free bubbles, the changes of the liquid viscosity due to the addition of sand particles can also affect the CSE. The viscosity rises as the sand particles are added, which will inhibit the growth of the bubbles and consequently alleviate the CSE [19, 31]. All of the effects result in the
fact that the cumulative mass loss of CSE at lower sand concentrations is less than that of CE in pure water. As the sand concentration increases to a certain value, prior to the negative effect, the positive effect of the sand particles causes more CSE than the pure CE (Fig. 3). It is difficult to quantitatively evaluate the pure particles impingement erosion and pure CE independently according to the analyses above, because the synergetic actions between the particles and bubbles are quite complex.
4.2 Effect of sand concentration on the damage It is generally reported that the mass loss increases with increasing sand concentration in the CSE [21, 22, 32]. Meanwhile Wu et. al [23] observed that it did not decrease with the sand concentration until the particle size exceeded a critical value. Apart from the particle size, the sand concentration plays an important role in the damage process. Hong et. al [21, 22] selected only two sand concentrations (20 kg/m3 and 40 kg/m3) in their CSE experiments and found a monotonic increasing curve of mass loss with the sand concentrations. However, it is difficult to completely identify the relationship between the effect of the sand concentrations and the CE. Larger region (0.5-10 wt.%) of sand concentrations was employed in the present study. It is observed that the mass loss initially decreases and then increases with the increasing sand concentration with a certain particle size of approximately D50=152 μm. Based on the two distinctive morphologies at different sand concentrations (Fig. 4 and Fig. 5), it can be deduced that they are caused by different mechanisms. At lower sand concentrations, the CE still dominates the removal of the materials. Therefore, the equation below can be employed to describe the relationship between the bubbles and particles combined with the observed results. (1) Where Na is the total number of collapsed bubbles, Nm is the number of the collapsed bubbles from homogenous nucleation in the body of flow caused by ultrasonic vibration, Nt is the number of the collapsed bubbles from heterogeneous
nucleation on the particle surfaces, and Nc is the number of the bubbles coalesced with particles through collisions. For the CE in pure water, the CE intensity can be described by Na=Nm. After adding sand particles with low sand concentrations, the number of homogeneous bubbles, Nm in the solution will be reduced due to the collision of the particles. The reduction number depends on the size of the particles [16]. The crevices and concaves on the particle surface can also create bubbles nucleation, which is called heterogeneous nucleation [33]. Therefore, Nt will increase with the increasing sand concentration. As for the Nc，according to Sutherland’s theory [16], the particles with the size of approximately 0.5 μm have the lowest probability to collide with the bubbles. The size of the particles used in the present study (approximately 152 μm) is much larger than the critical size, which probably indicates more collision chances. Moreover, the collision will also increase with the particle concentration [32]. The bubble-particle coalescence is more difficult to collapse than the pure bubbles once the coalescence forms. As a result, the coalescence will occur as the addition of the particles and increase with increasing sand concentration in a certain range. Based on the damage extent (Fig. 3),
is probably less in the
sand/water solution than that in the pure water. For CSE at high sand concentrations, the entire surface of the sample is covered by micro cutting and ploughing and there is no obvious CE crater (Fig. 4d and Fig. 5i). It indicates that the particles impingement erosion has dominated damage instead of the CE. Moreover, the stress caused by particles impingements is higher than that caused by either collapsed bubbles or its combination effect with particles (Fig. 10). As the sand concentration increases, the stress caused by the particles impingement is also relatively high compared to that with combination effect (Fig. 10). It continues to increase independently on the variation of cumulative mass loss, which suggests that surface residual stress is a superficial phenomenon and can not be used to explain the mass loss. At least, it is not the only factor affecting the damage mechanism. Moreover, the damage mechanism changes from the combination effect of the bubbles and particles at low sand concentrations to the one dominated by particles
impingement at high sand concentrations. This can be proved by the morphology evolutions that it changes from two different erosion regions (center region and ring-like region) to one uniform erosion region as the sand concentration increases (Fig. 4d). On the other hand, the mass loss roughly rises with the increase of sand concentration by and large (Fig. 3) despite the initial slight decrease from 0.5-3 wt.% sand concentrations. This trend is consistent with that discovered by Kang et. al [21, 32]. Moreover, the mass loss for CSE does not exceed the one for CE in pure water until the sand concentration rises up to a value between 5 wt.% and 10 wt.%. It indicates that the sand particles can alleviate the CE at low sand concentrations. However, higher concentration of sand particles greatly enhances the erosion of the material. The cumulative mass loss at 10 wt.% is approximately 1.6 times that for CE in pure water, which is mainly due to the particles impingement erosion.
4.3 Morphology evolution The difference in erosion morphologies of the center region from that of the ring-like region is mainly caused by the competitive actions of sand particle erosion wear and collapse bubble attack. It is also dependent on the turbulence distribution near the sample surface. In general, there is a CE boundary on the sample surface even in pure water CE (Fig. 4), which is similar to that observed by Hong et. al. [21] and Hattori et al. [34]. Under the micro vibration at a high frequency, the bubbles are created and concentrate in the center part resulting in an obvious CE region in the center region and a much less damaged region in the perimeter (Fig. 4) [20]. After adding sand particles with low sand concentration ≤3 wt.%, the sand particles are stirred by the ultrasonic vibration and gain high velocity from the bombardment of the collapsed bubbles. They slide on the surface at different angles leaving chaotic scratches [20]. Moreover, it is reported that the bubbles are concentrated below the center region [20], which will drive more sand particles to impact on the samples. Therefore, particle impingement erosion in the center region is more than that in the
ring-like region (Fig. 5). However, the damage caused by CE is more severe than that caused by particles impingement erosion. It can be deduced that both the roughness and the residual stress in the ring-like region are higher than those in the center region. At higher sand concentrations ≥5 wt.% (Fig. 5h, i and Fig. 6d, e), the numbers of the sand particles are high enough to shield and take over the damaged energy of the collapsing bubbles, which is transferred to impact and scratch the surface. The erosion morphology for 10 wt. % (Fig. 5i) is similar to that in the center region for 0.5 wt.% (Fig. 5c). They are both characteristic of the particles impingement erosion. It can be deduced that the particles erosion wear area enlarges with the increasing sand concentration. Although the roughness decreases (Fig. 7), the cumulative mass loss and mass loss rate increase (Fig. 3). Therefore, it is bias to evaluate the CSE extent only by the roughness and morphologies. In addition to the relationship between the roughness and the mass loss, the variation of the roughness with sand concentration (Fig. 7) is opposite to that of the residual stress (Fig. 10). It is roughly applied to both the center region and the ring-like region. After adding sand particles, the surface absorbs compressive energy from not only the bubbles collapsing but also the particles impingement. This is why the residual stress due to CSE is higher than that due to pure CE, which is more obvious in the ring-like region because of stronger synergistic effect. Under this condition, plastic deformation and crack growth are difficult to develop, which results in the decreasing surface roughness. When the compressive stress exceeds material’s yield limit, the mass loss will increase due to fatigue mechanism.
4.4 Damage mechanism
Fig. 12 CE and CSE mechanism at different sand concentrations
Based on the analyses above, the damage mechanism can be deduced as shown in Fig. 12. For the CE in pure water (Fig. 12a), the material loss is all attributed to the CE in the mechanisms of micro jet flow or shock wave. The damaged surface is characteristic of big and dense craters due to the guided wave effect [30]. After adding sand particles with low concentration (Fig. 12b), the center region is dominated by
particles impingement erosion and the number of big CE craters decreases due to the polishing of the sand particles (Fig. 5c, e and g). While in the ring-like region, less enhanced particle impingement results in the dominant CE (Fig. 5b, d and f), owing to the cone-like bubble structure below the samples [20]. Therefore, the roughness of the center region is much lower than that of the ring-like region because center region suffers more polishing of sand particles (Fig. 7). The phenomenon is well consistent with that obtained by Laguna-Camacho et. al [20]. As the sand concentration exceeds a critical value (Fig. 12c), sand particles are more enough to shield the attacks of the collapsing bubbles and take over the attacking energy. The damage turns from the combined attack of the direct CE and particles impingement erosion to the single particles impingement erosion. It results in no presence of big craters on the surfaces (Fig. 5h and i, Fig. 6d and e) and small roughness. However, the mass loss (Fig. 3) and the residual stress (Fig. 10) are much higher than those for low sand concentrations. In other words, the damage deteriorates, although it seems alleviated in morphologies. Therefore, comprehensive analyses including morphologies, surface roughness, residual stress and so on are needed to evaluate the real CSE.
5. Conclusions (1) A critical sand concentration was observed at approximately 3 wt.%, beyond which the cumulative mass loss increased with the increasing sand concentration. Moreover, the cumulative mass loss of CSE at sand concentration regions of 0.5 wt.%- 5 wt.% were less than that of CE in pure water. (2) A center region dominated by particles impingement erosion and a ring-like region dominated by direct CE were observed on the surfaces after CSE at 0.5 wt.% - 3 wt.% sand concentrations, while uniform particles impingement erosion covered the entire surfaces after CSE at higher sand concentrations. (3) The roughness after CSE decreased with sand concentration until ≥5 wt.%
sand concentration, where the roughness experienced small changes. (4) The compressive residual stress on the surface after CSE was higher than that after CE in pure water. It initially increased with increasing sand concentration and then ended with a stable period between 5 wt.% and 10 wt.%. (5) A schematic of CSE was proposed in this paper. CE was responsible for the big craters and higher roughness. Particles impingement erosion competed with CE for dominating the damage of CSE, which changed with the sand concentrations.
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant No: 51501206). The authors appreciate Ekerenam, Okpo Okpo and Wilfred Emori for their help in modifying the manuscript language.
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