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Novel cavitation fluid jet polishing process based on negative pressure effects
Accepted Manuscript Novel Cavitation Fluid Jet Polishing Process Based on Negative Pressure Effects Fengjun Chen, Hui Wang, Yu Tang, Shaohui Yin, Shuai Huang, Guanghua Zhang PII: DOI: Reference:
S1350-4177(17)30528-X https://doi.org/10.1016/j.ultsonch.2017.11.016 ULTSON 3953
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
17 July 2017 12 October 2017 13 November 2017
Please cite this article as: F. Chen, H. Wang, Y. Tang, S. Yin, S. Huang, G. Zhang, Novel Cavitation Fluid Jet Polishing Process Based on Negative Pressure Effects, Ultrasonics Sonochemistry (2017), doi: https://doi.org/ 10.1016/j.ultsonch.2017.11.016
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Novel Cavitation Fluid Jet Polishing Process Based on Negative Pressure Effects
Fengjun Chen*, Hui Wang, Yu Tang, Shaohui Yin, Shuai Huang, Guanghua Zhang National Engineering Research Center for High Efficiency Grinding, Hunan University, Changsha 410082, Hunan, China *Corresponding author. E-mail: [email protected]. Tel: +86 0731 88829817
Abstract：Traditional abrasive fluid jet polishing (FJP) is limited by its high-pressure equipment, unstable material removal rate, and applicability to ultra-smooth surfaces because of the evident air turbulence, fluid expansion, and a large polishing spot in high-pressure FJP. This paper presents a novel cavitation fluid jet polishing (CFJP) method and process based on FJP technology. It can implement high-efficiency polishing on small-scale surfaces in a low-pressure environment. CFJP uses the purposely designed polishing equipment with a sealed chamber, which can generate a cavitation effect in negative pressure environment. Moreover, the collapse of cavitation bubbles can spray out a high-energy microjet and shock wave to enhance the material removal. Its feasibility is verified through researching the flow behavior and the cavitation results of the negative pressure cavitation machining of pure water in reversing suction flow. The mechanism is analyzed through a computational fluid dynamics simulation. Thus, its cavitation and surface removal mechanisms in the vertical CFJP and inclined CFJP are studied. A series of polishing experiments on different materials and polishing parameters are conducted to validate its polishing performance compared with FJP. The maximum removal depth increases, and surface roughness gradually decreases with increasing negative outlet pressures. The surface becomes smooth with the increase of polishing time. The experimental results confirm that the CFJP process can realize a high material removal rate and smooth surface with low energy consumption in the low-pressure environment, together with compatible surface roughness to FJP.
Key words：Cavitation fluid jet polishing; negative pressure; cavitation bubble; material removal; Computational fluid dynamic; Abrasive jet
1. Introduction The application of micro optics and molds has increased for high precision and miniaturization machining. Moreover, the quality requirement of a machined surface becomes high [1]. For the hard brittle material mold, surface defects and damages must be polished by free abrasives. Furthermore, the conventional automatic process is difficult to polish small-scale surface [2]. One suitable way is to use a jet of abrasive/fluid mixture. The fine surface can be obtained by spraying fine particles onto the microstructure of a workpiece surface. The fine abrasive fluid can be used as a flexible polishing tool to further improve surface quality and reduce surface defects. Tno et al. [3] confirmed the possibility of polishing BK7 glass by abrasive water jet (AWJ). Shiou et al. [4] presented the surface roughness improvement of Zerodur optical glass using a rotary abrasive fluid multi-jet polishing process, and the surface roughness was achieved to Ra 0.006 µm. Yuan et al. [5] presented a spiral-rotating abrasive flow polishing technology to address the issue of precision polishing of a cylinder’s internal surface for 6061 aluminum alloy. Wang et al. [6] presented a multi-jet polishing process and tools based on fluid jet polishing (FJP), which can implement high-efficiency polishing on large-scale or lens array surfaces. The new tool can realize a high material removal rate. A hydrodynamic suspension polishing method [7] was presented, and the computational fluid dynamics (CFD) model has been numerically developed to explore the particle impact velocities and impact angles. Cao et al. [8] predicted the material removal characteristics and surface generation mechanisms in FJP based on CFD modeling. Zhu et al. [9] analyzed the ductile erosion mechanism of hard-brittle materials by AWJ in a small erosion angle. The ductile erosion can achieve micro-material removal. Yuvaraj et al. [10] conducted surface integrity studies on the AWJ, and the cryogenic assisted AWJ cutting of AA5083-H32 aluminum alloy by varying the jet impact angles and abrasive sizes. Mieszala et al. [11] investigated the influence of microstructure and mechanical properties on the erosion mechanisms using AWJ-controlled depth milling. This flexible tool may resolve a challenging problem of polishing steep concave surfaces and cavities [12]. A fluid jet with a small nozzle can be used to generate sufficient shear stress and to remove materials under condition of high-speed impact. Traditional high-pressure FJP is carried out in an atmospheric air environment, the air turbulence of a high-speed fluid jet is evident and violent, and the disturbance of entrainment expansion is also violent to generate the large polishing spot.
Thus, the minimum polishing dimension of the workpiece is limited. After a thorough cavitation study, a cavitation jet technology emerges. Lauterborn et al. [13] investigated the dynamics of cavitation bubbles on water for bubbles produced optically and acoustically. The impact pressure of cavitation water jet is 9–124 times higher than that of the ordinary jet, which can reduce the requirement of pressure pump. Wijngaarden [14] surveyed the dynamical phenomena that accompany the collapse of cavitation bubbles and discussed shock waves, microjets, and the various ways in which collapsing bubbles produce damage. Chahine et al. [15] numerically investigated surface cleaning using cavitation bubble dynamics. Bubble deformation and reentrant jet formation are responsible for generating concentrated pressures, shear, and lift forces on dirt particles, and high impulsive loads on a material layer to remove. Wu et al. [16] corroborated that cavitation structures are stable under unaltered experimental conditions and that the cavitation bubble cloud returns to the original structure and remains stable even in the face of large perturbations. Beaucamp et al. [17] reported on a novel system in which ultrasonic cavitation causes microbubble generation directly upstream of the nozzle outlet. These microbubbles boost the removal rate by up to 380%. Parthasarathy et al. [18] focused on activated carbon adsorption and ultrasound cavitation for polishing the palm oil mill effluent. Sreedhar et al. [19] discussed the theoretical formulation of bubble collapse, the estimate of collapse pressures, and the techniques for the measurement of cavitation damages. Hutli et al. [20] demonstrated a possible application of the cavitation phenomenon as an efficient method to modify surface properties. Aluminum alloy (AlSiMg) specimens were subjected to high-speed submerged cavitating jets under various working conditions. Li et al. [21] investigated the cavitation erosion of Inconel 718 nickel-base alloy using an ultrasonic vibratory apparatus, and the cavitation-induced nanosize precipitates of intermetallic phase Ni3Nb were found in the local zones of eroded surfaces. Verhaagen et al. [22] proposed an “ideal sensor” with high spatial and temporal resolution to investigate bubble jetting, shockwaves, streaming, and chemical effects by correlating cleaning processes with cavitation effects generated by hydrodynamics, lasers, or ultrasound. Water jet cavitation was also applied to degrade an azo dye [23]. Huang et al. [24] conclusively established that a combination of jet cavitation and impacting stream can be effectively used for the complete degradation of chitosan. The fine abrasive fluid is easy to flow into a microstructure as a flexible polishing tool in
traditional FJP. However, this fluid is limited by high-pressure equipment, air turbulence, and unstable material removal rates in high-pressure FJP. A novel cavitation fluid jet polishing (CFJP) method based on the negative pressure cavitation (NPC) effect is presented.
2. Polishing theory 2.1. CFJP process Fig. 1 shows the principle of the CFJP process. A small workpiece is placed in a sealed container, in which a micro nozzle with an inflow pipe is installed above the workpiece. The suction pipe with large caliber is connected with the suction pump. The suction pump starts, the air is sucked out from the suction chamber in the sealed container, and negative pressure is generated. The high-pressure difference is formed between the exterior and the interior of the micro nozzle given the high pressure produced by a jet pump. The suction flow generated by the pressure difference makes the fine abrasive fluid spray at a high speed from the micro nozzle and scatter around in the negative pressure environment. Fine abrasive fluid shears the micro surface. Because of the violent cavitation effect, a large number of microbubbles in the fluid generate the continuous growth, compression, and impact on the workpiece surface. 2.2. Fluid jet polishing mechanism The jet beam of abrasive fluid expands with the increase of polishing distance due to air disturbance. The cross-sectional area of the jet increases, and its velocity decreases. The diffused pyramidal jet has three different functional stages. The initial stage is suitable for jet cutting and drilling. The basic stage is highly favorable for polishing or finishing. The dissipation stage is mainly used in dust removal. An abrasive jet is controlled in the basic stage to erode the workpiece surface. The shearing erosion process between single abrasive and the workpiece surface includes the following three stages: elastic sliding, plowing, and microcutting. At the elastic sliding stage, the abrasives shallowly squeeze into the material surface and show elastic deformation. At the plowing stage, the abrasives further cut into the material and show the ridge on both sides of the erosion groove. When the cutting depth continues to increase, debris accumulation gradually increases, and the shear crack nucleate grow and propagate near the shear sliding surface, which finally leads to material removal. Moreover, these ridges incur crack propagation and are removed. Therefore, the erosion of abrasives should be controlled in the basic stage to ensure the shear deformation, plowing,
and the microcutting of the erosion surface. 2.3 NPC-enhanced polishing 2.3.1 NPC mechanism The inflow pipe with a micro nozzle is in the negative pressure state. The fluid in the jet pipe is in a normal atmospheric or high-pressure state. The fluid will easily generate the cavitation effect. Fig. 2 shows the erosion modes of a cavitation jet within a negative pressure environment. The collapsing bubble near or on the workpiece surface forms microjet impingement and produces instantaneous high temperature and high pressure in a small area. A strong shock wave and a high-speed microjet are formed to rapidly destroy the minimum eroded surface. The cavitation effect enhances the friction and the impact of micro-abrasive fluid on the workpiece surface, which effectively improves the removal efficiency. Compared with the traditional high-pressure abrasive jet, the cavitation effect of the negative pressure environment is enhanced, and the flow entrainment is small. The material is removed in cavitation erosion. The bubble collapse is a major factor in which the erosion ability of a cavitation jet is higher than that of an ordinary jet. In bubble annihilation, transient pressure can be expressed as
= . ( ⁄ )
(1)
Where, pmax is the instantaneous maximum pressure, p∞ is the outlet pressure, R0 is the initial bubble radius, and R is the instantaneous radius. When R0 = 20 R and pmax = 1260 p ∞, the maximum pressure is 1260 times of the ambient pressure. It indicates that the destructive effect of the bubble collapse is extremely strong. The enhancement effect of a cavitation jet is generated due to the high-speed microjet and shock wave induced by the collapses of microbubbles. However, because of the high-speed microjet (130-180m/s), the small size (0.01-1mm) and short impact time (1-10µs) of bubble, the bubble collapses is an extremely complex motions and physical processes. It is hard to observe clearly the change of bubbles on the workpiece wall in turbulent fluid. 2.3.2 NPC verification Fig. 3 shows that the feasibility is verified through researching the cavitation effect of pure water on the aluminum surface in reversing the suction flow through a simplified negative pressure equipment from Fig. 1. Fig. 3(a) shows the schematic diagram of the experimental set-up of the NPC machining under a negative pressure of −60 kPa. The pure water, which is sucked into the inflow pipe from a tank, is supplied to a suction chamber through the micro nozzle with diameter of
2 mm. Owing to the suction flow of a suction pump, pure water is sucked from a suction chamber along the arrow direction flow, as shown in the arrows in Fig.3(a). The cavitation phenomenon occurs under the micro nozzle by the rapid pressure decrease. The cavitation flow of pure water turns around near the aluminum surface. Figs. 3(b)–3(h) show the flow behavior of pure water in the proposed cavitation flow. Fig. 3(b) shows that, when the suction pump runs, the air in the suction chamber and inflow pipe is pumped out to form a negative pressure environment. Given the suction flow, pure water would be sucked into an inflow pipe in Fig. 3(c) and jet into the suction chamber through a micro nozzle in Fig. 3(d). The gas dissolved in pure water becomes supersaturated and breaks down into microbubbles at high speed in the suction chamber. With the continuous injection of water, several microbubbles are continuously generated between the micro nozzle and the workpiece surface because of the rapid pressure decrease (Fig. 3(e)). Such cavitation phenomenon is caused by the rapid expansion of the section area of the fluid channel inside and outside a nozzle. Figs. 3(f) to 3(h) show that the cavitation effect of pure water gradually increases to form the high-speed microjet and high-pressure shock wave for impacting on the workpiece surface when the bubble collapses. After the NPC machining of pure water for 5 h, Fig. 3(i) shows the following three regions: the initial aluminum surface, the transition region, and the cavitation removal region. The initial aluminum surface is smooth in Fig. 3(j). Fig. 3(k) shows the cavitation removal region on the aluminum surface. The cavitation effect produces large cavitation bubbles to form some sheet pits given the pressure difference between the atmospheric pressure and the suction chamber. The pressure difference is a large near nozzle when increasing the inlet pressure to 0.2 MPa in the inflow pipe. The cavitation effect would produce many small-size cavitation bubbles to erode and form many pits and cavernous cavities on the aluminum surface in Fig. 3(m).
3. Simulation modeling 3.1 Modeling flow The polishing simulation of the fluid jet in the NPC condition is achieved using the fluid dynamics software Fluent. The mixing and cavitation models derived from a multiphase flow model are used. The flow of cavitation simulation includes the following steps: a) Determine the geometry model of the abrasive jet based on NPC;
b) Model and discrete using a GAMBIT module; c) Determine the initial and boundary conditions, and set solver parameters; d) Solve the model; e) Check the computational convergence property or revise the initial and boundary conditions again; and f) If convergence is achieved, show the simulation results. 3.2 Geometrical modeling Fig. 4 shows the 3D geometrical modeling of vertical and inclined CFJP. The two-dimensional solver is used to reduce the convergence time. In order to analyze the content and position of cavitation bubbles in turbulent flow with high Reynolds number, a standard k-ɛ equation is used in mixture flow model as below: ()
(%)
+
( )
! = + + + − #$
+
(% )
! = & + ( + +
= #+
!
'
%
!
!
%)* !
− +, #
%
(2)
%
Where, ρ denotes fluid density. k is the turbulence kinetic energy, and ɛ is the turbulent dissipation rate. µ is the kinematic viscosity of the fluid. For incompressible fluid, C1=1.44, C2=1.92, Cµ=0.09, σk=1.0, σε=1.3. Suppose no slip exists between the two phases in this model, the boundary conditions of the Fluent solver are set as follows. a) The pressure inlet is adopted, and the inlet pressure is the positive pressure from the jet pump. b) The pressure outlet is adopted, and the outlet pressure is the negative pressure from the suction pump. c) The workpiece is considered a rigid wall without slip because no brittle fracture on the workpiece is found during low-pressure polishing. d) The particle size is 1 µm, and the density is 2500 kg/m3. The density of the mixed jet is 1500 kg m3. 3.3 Simulation results and analysis 3.3.1 Cavitation in a vertical jet The simulation conditions of a vertical jet are set as follows. The inlet pressure is 0.8 MPa, and
the outlet pressures are −15, −60, −80, and −95 kPa. The polishing distance is 10 mm, and the nozzle diameter is 1 mm. Fig. 5 shows the content of cavitation bubbles in different outlet pressures. The cavitation bubbles are symmetrically distributed on both sides of the axis line of the vertical jet, and the content of the cavitation bubbles increases with the increase of the negative pressure value. Fig. 6 shows the effect of different outlet pressures on the content of cavitation bubbles. The low outlet pressure generates a high content of cavitation bubbles and makes the cavitation effect become profoundly significant. The effect of negative outlet pressure on cavitation intensity can be divided into two aspects. When the outlet pressure is higher than −60 kPa, the average value and the largest value of cavitation bubbles are below 10%. The cavitation intensity is weak, and the effect of outlet pressure on bubble contents is not evident. When the outlet pressure is lower than −70 kPa, the content of cavitation bubbles increases quickly with increasing gradually negative pressure and the cavitation intensity. The average value and the maximum value of cavitation bubbles are 35% and 90%, respectively. The cavitation effect in the vertical jet is higher than the inclined jet in the same outlet pressure. The simulation results confirm that the outlet pressure within −65 to −80 kPa can be controlled to obtain the evident cavitation effect. 3.3.2 Cavitation in an inclined jet Fig. 7 shows the content of cavitation bubbles in the flow field at the outlet pressures of −60 and −80 kPa. Given the inclined nozzle, the distribution of the cavitation bubbles is asymmetrical and offset to the near wall side of the nozzle. The jet velocity and dynamic pressure distribution are asymmetric on both sides of the inclined nozzle. The jet velocity is large, and the entrainment diffusion is apparent near the wall. The content of cavitation bubbles near the wall is smaller than that far from the wall, which leads to the difference in the distribution of cavitation intensity on both sides of the jet fluid. The shape of removal spot is not circular but meniscus in the inclined jet.
4. Methods and materials 4.1. Experimental setup Fig. 8 shows the polishing experiment device, which mainly includes the jet pump, booster pump, suction pump, circulation mechanism, and sealing mechanism. The polishing procedure includes the following steps: a) Install a small workpiece on the worktable and adjust the polishing distance;
b) Start the stirrer, jet pump, and suction pump; c) Form a negative pressure state in the sealed container and spray the polishing fluid around at a high speed; and d) Control the jet pump and suction pump to adjust the inlet and the outlet pressure. Both the abrasive jet and the cavitation erosion in the negative pressure environment are conducted simultaneously to accomplish the polishing of small-scale elements. 4.2. Experimental conditions 4.2.1 Comparison between FJP and CFJP of Cu materials A single-point polishing experiment of the CFJP of Cu materials are carried out by comparing with FJP. The nozzle diameter is 1 mm, and the inclined angle is 45°. Abrasives are alumina oxide with a grit size of 1 µm. The inlet pressure is 0.7 Mpa, the outlet pressure is −70 kPa, and the polishing time is 90 min. Meanwhile, the polishing distance is 12 mm, and the abrasive concentration (in volume) is 10%. The maximum removal depths, removal diameters, and removal areas of the polishing spot are compared after FJP and CFJP. 4.2.2 Comparison between FJP and CFJP of BK7 glass material 4.2.2.1 Removal depth in different polishing conditions The CFJP experiment on BK7 glass are carried out to study the effect of polishing conditions on the removal depth of CFJP. The nozzle diameter is 1 mm, and the inclined angle is 45°. Abrasives is alumina oxide with a grit size of 1 µm. Meanwhile, the inlet pressure is 0.7 Mpa, the polishing time is 90 min, the polishing distance is 15 mm, and the abrasive concentration is 10%. The outlet pressures are the atmospheric pressure, −50, −60, −70, and −80 kPa. The maximum removal depth of the polished material is measured by using a Keyence VHX-1000 3D digital microscope. 4.2.2.2 Surface roughness in different outlet pressures For the demonstration of the surface roughness obtained by CFJP, the BK7 glass is ground before polishing, and the initial surface roughness is approximately Ra 136 nm. The nozzle diameter is 1 mm, and the inclined angle is 45°. Abrasives are cerium oxide with a grit size of 1 µm. Meanwhile, the inlet pressure is 0.55 Mpa, the polishing time is 45 min, the polishing distance is 15 mm, and the abrasive concentration is 6%. The outlet pressures are the atmospheric pressures, −45, −50, −60, −65, and −73 kPa.
4.2.2.3 Surface roughness in different CFJP times The CFJP experiments of the BK7 glass are conducted before polishing and after 10, 60, and 150 mins at the outlet pressure of −75 kPa. The other conditions are the same with that described in Section 4.2.2.2. The surface roughness values aremeasured by using a white light interferometer (Zygo Newview 7100).
5. Results and discussions 5.1 Comparison between FJP and CFJP Fig. 9 shows the removal characteristics of the polishing spot after the FJP and CFJP of Cu materials. The maximum removal depths in two polishing methods are 9.5 and 16.2 µm, respectively. The diameters of the polishing spot are 1.6 and 3.0 mm. The spot areas are 0.24 and 0.73 mm2. The removal depth after CFJP is larger than the one after FJP because the cavitation effect induced by the negative pressure can enhance the removal of materials and reduce the air turbulence of the jet beam. Therefore, the abrasive jet plays a dominant role in material removal, and cavitation effect plays an auxiliary enhancement role. The material removal on the workpiece surface is evident after polishing for 90 min. The removal shape of the polishing spots in two polishing methods has a similar meniscus shape. The center is deep, whereas the sides are shallow. 5.2 Removal depth in different polishing conditions Fig. 10 shows the maximum removal depth variation against polishing conditions after the CFJP BK7 glass. The effect of the negative pressure on the material removal rate is small when the negative pressure is approximately 0 to −43 kPa. The cavitation effect is not evident in this pressure range. Thus, the content of bubbles is less, and the microjet interaction is extremely weak. The enhanced removal effect of cavitation is weak. The removal depth of the workpiece has a linear growth trend when the outlet pressure is higher than −55 kPa. The cavitation has reached a certain strength to effect material removal. The workpiece surface would be damaged when the outlet pressure is high because of violent cavitation erosion. Moreover, the removal depth increases with the increase of inlet pressure and abrasive concentration but decreases with the increase of polishing distance. The machining quality and polishing efficiency are ensured by controlling the negative pressure in CFJP.
5.3 Surface roughness in different outlet pressures Fig. 11 shows the surface roughness of the BK7 material after CFJP in different outlet pressures. The surface roughness decreases with the increase of the negative outlet pressure. The surface roughness can be up to Ra 10.2 nm after 45 min of polishing at the outlet pressure of −73 kPa. The enhancement effect of negative pressure cavitation on the material removal is remarkable compared with the result of Ra 28.7 nm under atmospheric pressure condition. Several surface micro-topographies are measured by using the white light interferometer after FJP and CFJP in Fig. 11. The surface roughness is Ra 33.6 nm after atmospheric pressure FJP. The surface roughness of Ra 10.2 nm is improved after CFJP at the outlet pressure of −73 kPa. CFJP can obtain smoother surface than FJP in atmospheric pressure, except small pits caused by the cavitation. Cavitation bubbles enhance the removal capacity and accelerate the material removal. However, the enhancement effect has limited influence on the overall shape of the polishing spots. 5.4 Surface roughness in different CFJP times Fig. 12 shows the surface topography of the BK7 glass before polishing and after 10, 60, and 150 mins at the outlet pressure of −75 kPa. In Fig 12(a), some grinding scratch before polishing is visible, and the surface ground by the #2000 diamond grinding wheel is covered with spikes and valleys. After polishing for 10 min, the abrasive jet leaves the erosion mark on the ground surface, and the grinding mark is removed. The surface quality has improved but still with many micro-pits and erosive texture. The surface roughness value dropped from 136 nm to 78 nm. After 30 min of polishing, the surface quality is further improved, and the surface roughness decreases to 22 nm. After polishing for 150 min, the surface roughness is 2.5 nm. The grinding marks and the erosive texture have almost been removed, and the surface became smoother. When the outlet pressure is −75 kPa, the cavitation effect has a positive enhancement effect on the smooth surface quality as a polishing result. No evident cavitation pits and other defects caused by excessive cavitation strength are observed.
6. Conclusions 1) The cavitation effect and the enhancement removal of the CFJP are analyzed by comparing the traditional high-pressure FJP. The enhancement effect of a cavitation jet is generated due to the high-speed microjet and shock wave induced by the collapses of microbubbles. Its feasibility is verified by researching the flow behavior and the cavitation results of the NPC
machining of pure water in the reversing suction flow. 2) The flow fields of the vertical and inclined jets are simulated in different negative outlet pressures by Fluent software. The cavitation bubbles and removal shape are symmetrically distributed on both sides of the axis line of the vertical jet. The content of the cavitation bubbles increases with the increase of the negative pressure. The shape of polishing pot is not circular but meniscus in the inclined jet. The outlet pressure value within −65 to −80 kPa can be controlled to obtain the enhanced cavitation effect and polishing surface. 3) After the FJP and CFJP of Cu materials, the maximum removal depths are 9.5 and 16.2 µm, and the diameters of the polishing spot are 1.6 and 3.0 mm. The spot areas are 0.24 and 0.73 mm2. CFJP can obtain a smoother polishing surface than FJP. The cavitation effect caused by negative pressure can enhance the material removal and reduce the air turbulence of the jet beam. 4) The maximum removal depth and surface roughness have been obtained after the CFJP of BK7 glass under different polishing conditions. The maximum removal depth increases, and surface roughness gradually decreases with increasing negative outlet pressure. The surface becomes extremely smooth with the increase of polishing time. The surface roughness of BK7 glass is 2.5 nm after polishing for 150 min. 5) CFJP process can realize a high material removal rate in the low-pressure environment, together with compatible surface roughness to FJP. In addition, the CFJP has also possible applications as an efficient method to remove materials, modify surface properties, promote catalytic reactions, etc.
Acknowledgements The authors gratefully acknowledge the financial support for this work by the National Natural Science Foundation of China (No. 51205120)
References [1] F.J. Chen, S.H. Yin, H. Huang, H. Ohmori, Profile error compensation in ultra-precision grinding of aspheric surfaces with on-machine measurement, Int. J. Mach. Tool. Manu. 50 (2010) 480-486.
[2] H.C. Kim, I.H. Lee, T.J. Ko, 3D tool path generation for micro-abrasive jet machining on 3D curved surface, Int. J. Precis. Eng. Manuf. 14 (2013) 1519-1525. [3] O.W. Faehnle, H.V. Brug, H.J. Frankena, Fluid jet polishing of optical surfaces, Appl. Optics 37 (1998) 6771-6773 [4] F.J. Shiou, A. Asmare, Parameters optimization on surface roughness improvement of Zerodur optical glass using an innovative rotary abrasive fluid multi-jet polishing process, Precis. Eng. 42 (2015) 93-100 [5] Q.L. Yuan, H. Qi, D.H. Wen, Numerical and experimental study on the spiral-rotating abrasive flow in polishing of the internal surface of 6061 aluminum alloy cylinder, Powder Technol. 302 (2016) 153-159 [6] C.J. Wang, C.F. Cheung, L.T. Ho, M.Y. Liu, W.B. Lee. A novel multi-jet polishing process and tool for high-efficiency polishing, Int. J. Mach. Tool. Manu. 115 (2017), 115(4):60-73. [7] H. Qi, Z. Xie, T. Hong, Y.Y. Wang, F.Z. Kong, D.H. Wen. CFD modelling of a novel hydrodynamic suspension polishing process for ultra-smooth surface with low residual stress, Powder Technol. 2317 (2017) 320-328 [8] Z.C. Cao, C.F. Cheung, Theoretical modelling and analysis of the material removal characteristics in fluid jet polishing, Int. J. Mech. Sci. 89 (2014) 158-166. [9] H.T. Zhu, C.Z. Huang, J. Wang, Q.L. Li, C.L. Che, Experimental study on abrasive waterjet polishing for hard–brittle materials, Int. J. Mach. Tool. Manu. 49 (2009) 569-578 [10] N. Yuvaraj, M.P. Kumar, Cutting of aluminium alloy with abrasive water jet and cryogenic assisted abrasive water jet: A comparative study of the surface integrity approach, Wear 362–363 (2016) 18-32. [11] M. Mieszala, P.L. Torrubia, D.A. Axinte, J.J. Schwiedrzik, Y. Guo, S. Mischler, J. Michler, L. Philippe, Erosion mechanisms during abrasive waterjet machining: model microstructures and single particle experiments, Mater. Process. Technol. 247 (2017) 92-102. [12] F.H. Zhang, X.Z. Song, Y. Zhang, D.R. Luan, Figuring of an ultra-smooth surface in nanoparticle colloid jet machining, J. Micromech. Microeng. 19 (2009) 1-6 [13] W. Lauterborn, C.D. Ohl, Cavitation bubble dynamics, Ultrason. Sonochem. 4 (1997) 65-75 [14] L. Wijngaarden, Mechanics of collapsing cavitation bubbles, Ultrason. Sonochem. 29(2016)524-527
[15] G.L. Chahine, A. Kapahi, J.K. Choi, C.T. Hsiao, Modeling of surface cleaning by cavitation bubble dynamics and collapse, Ultrason. Sonochem. 29 (2016) 528-549 [16] P.F. Wu, L.X. Bai, W.J. Lin, J.C. Yan, Stability of cavitation structures in a thin liquid layer, Ultrason. Sonochem. 38 (2017) 75-83 [17] A. Beaucamp, T. Katsuura, Z. Kawara, A novel ultrasonic cavitation assisted fluid jet polishing system, CIRP ANN-Manuf. Techn. In Press, Available online 5 May 2017 [18] S. Parthasarathy, R.R. Mohammed, M.F. Chong, R.L. Gomes, S. Manickam. A novel hybrid approach of activated carbon and ultrasound cavitation for the intensification of palm oil mill effluent (POME) polishing, J. Clean. Prod. 112 (2016) 1218-1226 [19] B.K. Sreedhar, S.K. Albert, A.B. Pandit. Cavitation damage: Theory and measurements – A review, Wear 372–373 (2017) 177-196 [20] E. Hutli, A. Bonyár, D. Oszetzky, M.S. Nedeljkovic, Plastic deformation and modification of surface characteristics in nano-and micro-levels and enhancement of electric field of FCC materials using cavitation phenomenon, Mech. Mater. 92 (2016): 289-298 [21] L. Zhen, J.S. Zhou, J.S. Han, J.M. Chen. Formation of cavitation-induced nanosize precipitates on the eroded surface for Inconel 718 alloy. Matter. Lett. 164 (2016) 267-269 [22] B. Verhaagen, D. F. Rivas, Measuring cavitation and its cleaning effect, Ultrason. Sonochem. 29 (2016) 619-628 [23] X.N. Wang, J. P. Jia, Y. L. Wang, Enhanced photocatalytic-electrolytic degradation of Reactive Brilliant Red X-3B in the presence of water jet cavitation, Ultrason. Sonochem. 23 (2015) 93-99 [24] Y.C. Huang, P.F. Wang，Y. Yuan. Synergistic degradation of chitosan by impinging stream and jet cavitation, Ultrason. Sonochem. 27 (2015) 592-601.
Figure captions
Fig. 1 CFJP principle and polishing process Fig. 2 Erosion modes of cavitation jet: (a) suspending bubble and (b) adhering bubble Fig. 3 Schematic diagram, flow behavior, and cavitation results of NPC machining in reversing suction flow using a high-speed camera and Zygo 7100: (a) schematic diagram, (b) flow behavior of pure water when the suction pump runs at 0 ms, (c) pure water sucked from a micro nozzle at 415 ms, (d) pure water with bubbles impacting on an aluminum surface at 439 ms, (e) microbubbles are continuously generated at 452 ms and (f) at 455 ms, (g) pure water with bubbles are sucked from suction chamber at 457 ms and (h) at 465 ms, (i) three regions after NPC machining on the (j) initial aluminum surface, (k) certain sheet pits induced by cavitation erosion at the negative pressure of −60 kPa, and (m) bubbles induced by cavitation erosion at the inlet pressure of 0.2 MPa and the negative pressure of −60 kPa Fig. 4 3D modeling of vertical jet and inclined jet polishing Fig. 5 Effect of outlet pressure on the content of cavitation bubbles: (a) −15 kPa, (b) −60 kPa, (c) −80 kPa, and (d) −95 kPa Fig. 6 Effect of outlet pressure on the content of cavitation bubbles in (a) vertical jet and (b) inclined jet Fig.7 Simulation of cavitation flow field in different outlet pressures: (a) -60kPa, and (b) -80kPa Fig.8 Polishing device of CFJP Fig.9 Removal characteristics of polishing spot after FJP and CFJP Fig.10 Maximum removal depth variation against various parameters Fig.11 Surface roughness variation against outlet pressures
Fig.12 Surface topography (a) before polishing and after (b) 10 min, (c) 60min, and (d) 150 min at the outlet pressure of -75kPa
Fig. 1 CFJP principle and polishing process
Fig. 2 Erosion modes of cavitation jet: (a) suspending bubble and (b) adhering bubble
Fig. 3 Schematic diagram, flow behavior, and cavitation results of NPC machining in reversing suction flow using a high-speed camera and Zygo 7100: (a) schematic diagram, (b) flow behavior of pure water when the suction pump runs at 0 ms, (c) pure water sucked from a micro nozzle at 415 ms, (d) pure water with bubbles impacting on an aluminum surface at 439 ms, (e) microbubbles are continuously generated at 452 ms and (f) at 455 ms, (g) pure water with bubbles are sucked from suction chamber at 457 ms and (h) at 465 ms, (i) three regions after NPC machining on the (j) initial aluminum surface, (k) certain sheet pits induced by cavitation erosion at the negative pressure of −0.60 kPa, and (m) bubbles induced by cavitation erosion at the inlet pressure of 0.2 MPa and the negative pressure of −0.60 kPa
Fig. 4 3D modeling of vertical jet and inclined jet polishing
Fig. 5 Effect of outlet pressure on the content of cavitation bubbles: (a) −15 kPa, (b) −60 kPa, (c) −80 kPa, and (d) −95 kPa
Fig. 6 Effect of outlet pressure on the content of cavitation bubbles in (a) vertical jet and (b) inclined jet
Fig.7 Simulation of cavitation flow field in different outlet pressures: (a) -60kPa, and (b) -80kPa
Fig.8 Polishing device of CFJP
Fig.9 Removal characteristics of polishing spot after FJP and CFJP
Fig.10 Maximum removal depth variation against various parameters
Fig.11 Surface roughness variation against outlet pressures
Fig.12 Surface topography (a) before polishing and after (b) 10 min, (c) 60min, and (d) 150 min at the outlet pressure of -75kPa
Highlights • A cavitation fluid jet polishing method under a negative pressure condition was proposed. • The feasibility is verified through researching the cavitation results of NPC machining using pure water. • A CFD model of CFJP for simulating the cavitation effect was described. • The material removal characteristics of CFJP were analyzed by comparing with FJP. • The experimental results agreed well with the results of simulated model.
S1350-4177(17)30528-X https://doi.org/10.1016/j.ultsonch.2017.11.016 ULTSON 3953
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
17 July 2017 12 October 2017 13 November 2017
Please cite this article as: F. Chen, H. Wang, Y. Tang, S. Yin, S. Huang, G. Zhang, Novel Cavitation Fluid Jet Polishing Process Based on Negative Pressure Effects, Ultrasonics Sonochemistry (2017), doi: https://doi.org/ 10.1016/j.ultsonch.2017.11.016
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Novel Cavitation Fluid Jet Polishing Process Based on Negative Pressure Effects
Fengjun Chen*, Hui Wang, Yu Tang, Shaohui Yin, Shuai Huang, Guanghua Zhang National Engineering Research Center for High Efficiency Grinding, Hunan University, Changsha 410082, Hunan, China *Corresponding author. E-mail: [email protected]. Tel: +86 0731 88829817
Abstract：Traditional abrasive fluid jet polishing (FJP) is limited by its high-pressure equipment, unstable material removal rate, and applicability to ultra-smooth surfaces because of the evident air turbulence, fluid expansion, and a large polishing spot in high-pressure FJP. This paper presents a novel cavitation fluid jet polishing (CFJP) method and process based on FJP technology. It can implement high-efficiency polishing on small-scale surfaces in a low-pressure environment. CFJP uses the purposely designed polishing equipment with a sealed chamber, which can generate a cavitation effect in negative pressure environment. Moreover, the collapse of cavitation bubbles can spray out a high-energy microjet and shock wave to enhance the material removal. Its feasibility is verified through researching the flow behavior and the cavitation results of the negative pressure cavitation machining of pure water in reversing suction flow. The mechanism is analyzed through a computational fluid dynamics simulation. Thus, its cavitation and surface removal mechanisms in the vertical CFJP and inclined CFJP are studied. A series of polishing experiments on different materials and polishing parameters are conducted to validate its polishing performance compared with FJP. The maximum removal depth increases, and surface roughness gradually decreases with increasing negative outlet pressures. The surface becomes smooth with the increase of polishing time. The experimental results confirm that the CFJP process can realize a high material removal rate and smooth surface with low energy consumption in the low-pressure environment, together with compatible surface roughness to FJP.
Key words：Cavitation fluid jet polishing; negative pressure; cavitation bubble; material removal; Computational fluid dynamic; Abrasive jet
1. Introduction The application of micro optics and molds has increased for high precision and miniaturization machining. Moreover, the quality requirement of a machined surface becomes high [1]. For the hard brittle material mold, surface defects and damages must be polished by free abrasives. Furthermore, the conventional automatic process is difficult to polish small-scale surface [2]. One suitable way is to use a jet of abrasive/fluid mixture. The fine surface can be obtained by spraying fine particles onto the microstructure of a workpiece surface. The fine abrasive fluid can be used as a flexible polishing tool to further improve surface quality and reduce surface defects. Tno et al. [3] confirmed the possibility of polishing BK7 glass by abrasive water jet (AWJ). Shiou et al. [4] presented the surface roughness improvement of Zerodur optical glass using a rotary abrasive fluid multi-jet polishing process, and the surface roughness was achieved to Ra 0.006 µm. Yuan et al. [5] presented a spiral-rotating abrasive flow polishing technology to address the issue of precision polishing of a cylinder’s internal surface for 6061 aluminum alloy. Wang et al. [6] presented a multi-jet polishing process and tools based on fluid jet polishing (FJP), which can implement high-efficiency polishing on large-scale or lens array surfaces. The new tool can realize a high material removal rate. A hydrodynamic suspension polishing method [7] was presented, and the computational fluid dynamics (CFD) model has been numerically developed to explore the particle impact velocities and impact angles. Cao et al. [8] predicted the material removal characteristics and surface generation mechanisms in FJP based on CFD modeling. Zhu et al. [9] analyzed the ductile erosion mechanism of hard-brittle materials by AWJ in a small erosion angle. The ductile erosion can achieve micro-material removal. Yuvaraj et al. [10] conducted surface integrity studies on the AWJ, and the cryogenic assisted AWJ cutting of AA5083-H32 aluminum alloy by varying the jet impact angles and abrasive sizes. Mieszala et al. [11] investigated the influence of microstructure and mechanical properties on the erosion mechanisms using AWJ-controlled depth milling. This flexible tool may resolve a challenging problem of polishing steep concave surfaces and cavities [12]. A fluid jet with a small nozzle can be used to generate sufficient shear stress and to remove materials under condition of high-speed impact. Traditional high-pressure FJP is carried out in an atmospheric air environment, the air turbulence of a high-speed fluid jet is evident and violent, and the disturbance of entrainment expansion is also violent to generate the large polishing spot.
Thus, the minimum polishing dimension of the workpiece is limited. After a thorough cavitation study, a cavitation jet technology emerges. Lauterborn et al. [13] investigated the dynamics of cavitation bubbles on water for bubbles produced optically and acoustically. The impact pressure of cavitation water jet is 9–124 times higher than that of the ordinary jet, which can reduce the requirement of pressure pump. Wijngaarden [14] surveyed the dynamical phenomena that accompany the collapse of cavitation bubbles and discussed shock waves, microjets, and the various ways in which collapsing bubbles produce damage. Chahine et al. [15] numerically investigated surface cleaning using cavitation bubble dynamics. Bubble deformation and reentrant jet formation are responsible for generating concentrated pressures, shear, and lift forces on dirt particles, and high impulsive loads on a material layer to remove. Wu et al. [16] corroborated that cavitation structures are stable under unaltered experimental conditions and that the cavitation bubble cloud returns to the original structure and remains stable even in the face of large perturbations. Beaucamp et al. [17] reported on a novel system in which ultrasonic cavitation causes microbubble generation directly upstream of the nozzle outlet. These microbubbles boost the removal rate by up to 380%. Parthasarathy et al. [18] focused on activated carbon adsorption and ultrasound cavitation for polishing the palm oil mill effluent. Sreedhar et al. [19] discussed the theoretical formulation of bubble collapse, the estimate of collapse pressures, and the techniques for the measurement of cavitation damages. Hutli et al. [20] demonstrated a possible application of the cavitation phenomenon as an efficient method to modify surface properties. Aluminum alloy (AlSiMg) specimens were subjected to high-speed submerged cavitating jets under various working conditions. Li et al. [21] investigated the cavitation erosion of Inconel 718 nickel-base alloy using an ultrasonic vibratory apparatus, and the cavitation-induced nanosize precipitates of intermetallic phase Ni3Nb were found in the local zones of eroded surfaces. Verhaagen et al. [22] proposed an “ideal sensor” with high spatial and temporal resolution to investigate bubble jetting, shockwaves, streaming, and chemical effects by correlating cleaning processes with cavitation effects generated by hydrodynamics, lasers, or ultrasound. Water jet cavitation was also applied to degrade an azo dye [23]. Huang et al. [24] conclusively established that a combination of jet cavitation and impacting stream can be effectively used for the complete degradation of chitosan. The fine abrasive fluid is easy to flow into a microstructure as a flexible polishing tool in
traditional FJP. However, this fluid is limited by high-pressure equipment, air turbulence, and unstable material removal rates in high-pressure FJP. A novel cavitation fluid jet polishing (CFJP) method based on the negative pressure cavitation (NPC) effect is presented.
2. Polishing theory 2.1. CFJP process Fig. 1 shows the principle of the CFJP process. A small workpiece is placed in a sealed container, in which a micro nozzle with an inflow pipe is installed above the workpiece. The suction pipe with large caliber is connected with the suction pump. The suction pump starts, the air is sucked out from the suction chamber in the sealed container, and negative pressure is generated. The high-pressure difference is formed between the exterior and the interior of the micro nozzle given the high pressure produced by a jet pump. The suction flow generated by the pressure difference makes the fine abrasive fluid spray at a high speed from the micro nozzle and scatter around in the negative pressure environment. Fine abrasive fluid shears the micro surface. Because of the violent cavitation effect, a large number of microbubbles in the fluid generate the continuous growth, compression, and impact on the workpiece surface. 2.2. Fluid jet polishing mechanism The jet beam of abrasive fluid expands with the increase of polishing distance due to air disturbance. The cross-sectional area of the jet increases, and its velocity decreases. The diffused pyramidal jet has three different functional stages. The initial stage is suitable for jet cutting and drilling. The basic stage is highly favorable for polishing or finishing. The dissipation stage is mainly used in dust removal. An abrasive jet is controlled in the basic stage to erode the workpiece surface. The shearing erosion process between single abrasive and the workpiece surface includes the following three stages: elastic sliding, plowing, and microcutting. At the elastic sliding stage, the abrasives shallowly squeeze into the material surface and show elastic deformation. At the plowing stage, the abrasives further cut into the material and show the ridge on both sides of the erosion groove. When the cutting depth continues to increase, debris accumulation gradually increases, and the shear crack nucleate grow and propagate near the shear sliding surface, which finally leads to material removal. Moreover, these ridges incur crack propagation and are removed. Therefore, the erosion of abrasives should be controlled in the basic stage to ensure the shear deformation, plowing,
and the microcutting of the erosion surface. 2.3 NPC-enhanced polishing 2.3.1 NPC mechanism The inflow pipe with a micro nozzle is in the negative pressure state. The fluid in the jet pipe is in a normal atmospheric or high-pressure state. The fluid will easily generate the cavitation effect. Fig. 2 shows the erosion modes of a cavitation jet within a negative pressure environment. The collapsing bubble near or on the workpiece surface forms microjet impingement and produces instantaneous high temperature and high pressure in a small area. A strong shock wave and a high-speed microjet are formed to rapidly destroy the minimum eroded surface. The cavitation effect enhances the friction and the impact of micro-abrasive fluid on the workpiece surface, which effectively improves the removal efficiency. Compared with the traditional high-pressure abrasive jet, the cavitation effect of the negative pressure environment is enhanced, and the flow entrainment is small. The material is removed in cavitation erosion. The bubble collapse is a major factor in which the erosion ability of a cavitation jet is higher than that of an ordinary jet. In bubble annihilation, transient pressure can be expressed as
= . ( ⁄ )
(1)
Where, pmax is the instantaneous maximum pressure, p∞ is the outlet pressure, R0 is the initial bubble radius, and R is the instantaneous radius. When R0 = 20 R and pmax = 1260 p ∞, the maximum pressure is 1260 times of the ambient pressure. It indicates that the destructive effect of the bubble collapse is extremely strong. The enhancement effect of a cavitation jet is generated due to the high-speed microjet and shock wave induced by the collapses of microbubbles. However, because of the high-speed microjet (130-180m/s), the small size (0.01-1mm) and short impact time (1-10µs) of bubble, the bubble collapses is an extremely complex motions and physical processes. It is hard to observe clearly the change of bubbles on the workpiece wall in turbulent fluid. 2.3.2 NPC verification Fig. 3 shows that the feasibility is verified through researching the cavitation effect of pure water on the aluminum surface in reversing the suction flow through a simplified negative pressure equipment from Fig. 1. Fig. 3(a) shows the schematic diagram of the experimental set-up of the NPC machining under a negative pressure of −60 kPa. The pure water, which is sucked into the inflow pipe from a tank, is supplied to a suction chamber through the micro nozzle with diameter of
2 mm. Owing to the suction flow of a suction pump, pure water is sucked from a suction chamber along the arrow direction flow, as shown in the arrows in Fig.3(a). The cavitation phenomenon occurs under the micro nozzle by the rapid pressure decrease. The cavitation flow of pure water turns around near the aluminum surface. Figs. 3(b)–3(h) show the flow behavior of pure water in the proposed cavitation flow. Fig. 3(b) shows that, when the suction pump runs, the air in the suction chamber and inflow pipe is pumped out to form a negative pressure environment. Given the suction flow, pure water would be sucked into an inflow pipe in Fig. 3(c) and jet into the suction chamber through a micro nozzle in Fig. 3(d). The gas dissolved in pure water becomes supersaturated and breaks down into microbubbles at high speed in the suction chamber. With the continuous injection of water, several microbubbles are continuously generated between the micro nozzle and the workpiece surface because of the rapid pressure decrease (Fig. 3(e)). Such cavitation phenomenon is caused by the rapid expansion of the section area of the fluid channel inside and outside a nozzle. Figs. 3(f) to 3(h) show that the cavitation effect of pure water gradually increases to form the high-speed microjet and high-pressure shock wave for impacting on the workpiece surface when the bubble collapses. After the NPC machining of pure water for 5 h, Fig. 3(i) shows the following three regions: the initial aluminum surface, the transition region, and the cavitation removal region. The initial aluminum surface is smooth in Fig. 3(j). Fig. 3(k) shows the cavitation removal region on the aluminum surface. The cavitation effect produces large cavitation bubbles to form some sheet pits given the pressure difference between the atmospheric pressure and the suction chamber. The pressure difference is a large near nozzle when increasing the inlet pressure to 0.2 MPa in the inflow pipe. The cavitation effect would produce many small-size cavitation bubbles to erode and form many pits and cavernous cavities on the aluminum surface in Fig. 3(m).
3. Simulation modeling 3.1 Modeling flow The polishing simulation of the fluid jet in the NPC condition is achieved using the fluid dynamics software Fluent. The mixing and cavitation models derived from a multiphase flow model are used. The flow of cavitation simulation includes the following steps: a) Determine the geometry model of the abrasive jet based on NPC;
b) Model and discrete using a GAMBIT module; c) Determine the initial and boundary conditions, and set solver parameters; d) Solve the model; e) Check the computational convergence property or revise the initial and boundary conditions again; and f) If convergence is achieved, show the simulation results. 3.2 Geometrical modeling Fig. 4 shows the 3D geometrical modeling of vertical and inclined CFJP. The two-dimensional solver is used to reduce the convergence time. In order to analyze the content and position of cavitation bubbles in turbulent flow with high Reynolds number, a standard k-ɛ equation is used in mixture flow model as below: ()
(%)
+
( )
! = + + + − #$
+
(% )
! = & + ( + +
= #+
!
'
%
!
!
%)* !
− +, #
%
(2)
%
Where, ρ denotes fluid density. k is the turbulence kinetic energy, and ɛ is the turbulent dissipation rate. µ is the kinematic viscosity of the fluid. For incompressible fluid, C1=1.44, C2=1.92, Cµ=0.09, σk=1.0, σε=1.3. Suppose no slip exists between the two phases in this model, the boundary conditions of the Fluent solver are set as follows. a) The pressure inlet is adopted, and the inlet pressure is the positive pressure from the jet pump. b) The pressure outlet is adopted, and the outlet pressure is the negative pressure from the suction pump. c) The workpiece is considered a rigid wall without slip because no brittle fracture on the workpiece is found during low-pressure polishing. d) The particle size is 1 µm, and the density is 2500 kg/m3. The density of the mixed jet is 1500 kg m3. 3.3 Simulation results and analysis 3.3.1 Cavitation in a vertical jet The simulation conditions of a vertical jet are set as follows. The inlet pressure is 0.8 MPa, and
the outlet pressures are −15, −60, −80, and −95 kPa. The polishing distance is 10 mm, and the nozzle diameter is 1 mm. Fig. 5 shows the content of cavitation bubbles in different outlet pressures. The cavitation bubbles are symmetrically distributed on both sides of the axis line of the vertical jet, and the content of the cavitation bubbles increases with the increase of the negative pressure value. Fig. 6 shows the effect of different outlet pressures on the content of cavitation bubbles. The low outlet pressure generates a high content of cavitation bubbles and makes the cavitation effect become profoundly significant. The effect of negative outlet pressure on cavitation intensity can be divided into two aspects. When the outlet pressure is higher than −60 kPa, the average value and the largest value of cavitation bubbles are below 10%. The cavitation intensity is weak, and the effect of outlet pressure on bubble contents is not evident. When the outlet pressure is lower than −70 kPa, the content of cavitation bubbles increases quickly with increasing gradually negative pressure and the cavitation intensity. The average value and the maximum value of cavitation bubbles are 35% and 90%, respectively. The cavitation effect in the vertical jet is higher than the inclined jet in the same outlet pressure. The simulation results confirm that the outlet pressure within −65 to −80 kPa can be controlled to obtain the evident cavitation effect. 3.3.2 Cavitation in an inclined jet Fig. 7 shows the content of cavitation bubbles in the flow field at the outlet pressures of −60 and −80 kPa. Given the inclined nozzle, the distribution of the cavitation bubbles is asymmetrical and offset to the near wall side of the nozzle. The jet velocity and dynamic pressure distribution are asymmetric on both sides of the inclined nozzle. The jet velocity is large, and the entrainment diffusion is apparent near the wall. The content of cavitation bubbles near the wall is smaller than that far from the wall, which leads to the difference in the distribution of cavitation intensity on both sides of the jet fluid. The shape of removal spot is not circular but meniscus in the inclined jet.
4. Methods and materials 4.1. Experimental setup Fig. 8 shows the polishing experiment device, which mainly includes the jet pump, booster pump, suction pump, circulation mechanism, and sealing mechanism. The polishing procedure includes the following steps: a) Install a small workpiece on the worktable and adjust the polishing distance;
b) Start the stirrer, jet pump, and suction pump; c) Form a negative pressure state in the sealed container and spray the polishing fluid around at a high speed; and d) Control the jet pump and suction pump to adjust the inlet and the outlet pressure. Both the abrasive jet and the cavitation erosion in the negative pressure environment are conducted simultaneously to accomplish the polishing of small-scale elements. 4.2. Experimental conditions 4.2.1 Comparison between FJP and CFJP of Cu materials A single-point polishing experiment of the CFJP of Cu materials are carried out by comparing with FJP. The nozzle diameter is 1 mm, and the inclined angle is 45°. Abrasives are alumina oxide with a grit size of 1 µm. The inlet pressure is 0.7 Mpa, the outlet pressure is −70 kPa, and the polishing time is 90 min. Meanwhile, the polishing distance is 12 mm, and the abrasive concentration (in volume) is 10%. The maximum removal depths, removal diameters, and removal areas of the polishing spot are compared after FJP and CFJP. 4.2.2 Comparison between FJP and CFJP of BK7 glass material 4.2.2.1 Removal depth in different polishing conditions The CFJP experiment on BK7 glass are carried out to study the effect of polishing conditions on the removal depth of CFJP. The nozzle diameter is 1 mm, and the inclined angle is 45°. Abrasives is alumina oxide with a grit size of 1 µm. Meanwhile, the inlet pressure is 0.7 Mpa, the polishing time is 90 min, the polishing distance is 15 mm, and the abrasive concentration is 10%. The outlet pressures are the atmospheric pressure, −50, −60, −70, and −80 kPa. The maximum removal depth of the polished material is measured by using a Keyence VHX-1000 3D digital microscope. 4.2.2.2 Surface roughness in different outlet pressures For the demonstration of the surface roughness obtained by CFJP, the BK7 glass is ground before polishing, and the initial surface roughness is approximately Ra 136 nm. The nozzle diameter is 1 mm, and the inclined angle is 45°. Abrasives are cerium oxide with a grit size of 1 µm. Meanwhile, the inlet pressure is 0.55 Mpa, the polishing time is 45 min, the polishing distance is 15 mm, and the abrasive concentration is 6%. The outlet pressures are the atmospheric pressures, −45, −50, −60, −65, and −73 kPa.
4.2.2.3 Surface roughness in different CFJP times The CFJP experiments of the BK7 glass are conducted before polishing and after 10, 60, and 150 mins at the outlet pressure of −75 kPa. The other conditions are the same with that described in Section 4.2.2.2. The surface roughness values aremeasured by using a white light interferometer (Zygo Newview 7100).
5. Results and discussions 5.1 Comparison between FJP and CFJP Fig. 9 shows the removal characteristics of the polishing spot after the FJP and CFJP of Cu materials. The maximum removal depths in two polishing methods are 9.5 and 16.2 µm, respectively. The diameters of the polishing spot are 1.6 and 3.0 mm. The spot areas are 0.24 and 0.73 mm2. The removal depth after CFJP is larger than the one after FJP because the cavitation effect induced by the negative pressure can enhance the removal of materials and reduce the air turbulence of the jet beam. Therefore, the abrasive jet plays a dominant role in material removal, and cavitation effect plays an auxiliary enhancement role. The material removal on the workpiece surface is evident after polishing for 90 min. The removal shape of the polishing spots in two polishing methods has a similar meniscus shape. The center is deep, whereas the sides are shallow. 5.2 Removal depth in different polishing conditions Fig. 10 shows the maximum removal depth variation against polishing conditions after the CFJP BK7 glass. The effect of the negative pressure on the material removal rate is small when the negative pressure is approximately 0 to −43 kPa. The cavitation effect is not evident in this pressure range. Thus, the content of bubbles is less, and the microjet interaction is extremely weak. The enhanced removal effect of cavitation is weak. The removal depth of the workpiece has a linear growth trend when the outlet pressure is higher than −55 kPa. The cavitation has reached a certain strength to effect material removal. The workpiece surface would be damaged when the outlet pressure is high because of violent cavitation erosion. Moreover, the removal depth increases with the increase of inlet pressure and abrasive concentration but decreases with the increase of polishing distance. The machining quality and polishing efficiency are ensured by controlling the negative pressure in CFJP.
5.3 Surface roughness in different outlet pressures Fig. 11 shows the surface roughness of the BK7 material after CFJP in different outlet pressures. The surface roughness decreases with the increase of the negative outlet pressure. The surface roughness can be up to Ra 10.2 nm after 45 min of polishing at the outlet pressure of −73 kPa. The enhancement effect of negative pressure cavitation on the material removal is remarkable compared with the result of Ra 28.7 nm under atmospheric pressure condition. Several surface micro-topographies are measured by using the white light interferometer after FJP and CFJP in Fig. 11. The surface roughness is Ra 33.6 nm after atmospheric pressure FJP. The surface roughness of Ra 10.2 nm is improved after CFJP at the outlet pressure of −73 kPa. CFJP can obtain smoother surface than FJP in atmospheric pressure, except small pits caused by the cavitation. Cavitation bubbles enhance the removal capacity and accelerate the material removal. However, the enhancement effect has limited influence on the overall shape of the polishing spots. 5.4 Surface roughness in different CFJP times Fig. 12 shows the surface topography of the BK7 glass before polishing and after 10, 60, and 150 mins at the outlet pressure of −75 kPa. In Fig 12(a), some grinding scratch before polishing is visible, and the surface ground by the #2000 diamond grinding wheel is covered with spikes and valleys. After polishing for 10 min, the abrasive jet leaves the erosion mark on the ground surface, and the grinding mark is removed. The surface quality has improved but still with many micro-pits and erosive texture. The surface roughness value dropped from 136 nm to 78 nm. After 30 min of polishing, the surface quality is further improved, and the surface roughness decreases to 22 nm. After polishing for 150 min, the surface roughness is 2.5 nm. The grinding marks and the erosive texture have almost been removed, and the surface became smoother. When the outlet pressure is −75 kPa, the cavitation effect has a positive enhancement effect on the smooth surface quality as a polishing result. No evident cavitation pits and other defects caused by excessive cavitation strength are observed.
6. Conclusions 1) The cavitation effect and the enhancement removal of the CFJP are analyzed by comparing the traditional high-pressure FJP. The enhancement effect of a cavitation jet is generated due to the high-speed microjet and shock wave induced by the collapses of microbubbles. Its feasibility is verified by researching the flow behavior and the cavitation results of the NPC
machining of pure water in the reversing suction flow. 2) The flow fields of the vertical and inclined jets are simulated in different negative outlet pressures by Fluent software. The cavitation bubbles and removal shape are symmetrically distributed on both sides of the axis line of the vertical jet. The content of the cavitation bubbles increases with the increase of the negative pressure. The shape of polishing pot is not circular but meniscus in the inclined jet. The outlet pressure value within −65 to −80 kPa can be controlled to obtain the enhanced cavitation effect and polishing surface. 3) After the FJP and CFJP of Cu materials, the maximum removal depths are 9.5 and 16.2 µm, and the diameters of the polishing spot are 1.6 and 3.0 mm. The spot areas are 0.24 and 0.73 mm2. CFJP can obtain a smoother polishing surface than FJP. The cavitation effect caused by negative pressure can enhance the material removal and reduce the air turbulence of the jet beam. 4) The maximum removal depth and surface roughness have been obtained after the CFJP of BK7 glass under different polishing conditions. The maximum removal depth increases, and surface roughness gradually decreases with increasing negative outlet pressure. The surface becomes extremely smooth with the increase of polishing time. The surface roughness of BK7 glass is 2.5 nm after polishing for 150 min. 5) CFJP process can realize a high material removal rate in the low-pressure environment, together with compatible surface roughness to FJP. In addition, the CFJP has also possible applications as an efficient method to remove materials, modify surface properties, promote catalytic reactions, etc.
Acknowledgements The authors gratefully acknowledge the financial support for this work by the National Natural Science Foundation of China (No. 51205120)
References [1] F.J. Chen, S.H. Yin, H. Huang, H. Ohmori, Profile error compensation in ultra-precision grinding of aspheric surfaces with on-machine measurement, Int. J. Mach. Tool. Manu. 50 (2010) 480-486.
[2] H.C. Kim, I.H. Lee, T.J. Ko, 3D tool path generation for micro-abrasive jet machining on 3D curved surface, Int. J. Precis. Eng. Manuf. 14 (2013) 1519-1525. [3] O.W. Faehnle, H.V. Brug, H.J. Frankena, Fluid jet polishing of optical surfaces, Appl. Optics 37 (1998) 6771-6773 [4] F.J. Shiou, A. Asmare, Parameters optimization on surface roughness improvement of Zerodur optical glass using an innovative rotary abrasive fluid multi-jet polishing process, Precis. Eng. 42 (2015) 93-100 [5] Q.L. Yuan, H. Qi, D.H. Wen, Numerical and experimental study on the spiral-rotating abrasive flow in polishing of the internal surface of 6061 aluminum alloy cylinder, Powder Technol. 302 (2016) 153-159 [6] C.J. Wang, C.F. Cheung, L.T. Ho, M.Y. Liu, W.B. Lee. A novel multi-jet polishing process and tool for high-efficiency polishing, Int. J. Mach. Tool. Manu. 115 (2017), 115(4):60-73. [7] H. Qi, Z. Xie, T. Hong, Y.Y. Wang, F.Z. Kong, D.H. Wen. CFD modelling of a novel hydrodynamic suspension polishing process for ultra-smooth surface with low residual stress, Powder Technol. 2317 (2017) 320-328 [8] Z.C. Cao, C.F. Cheung, Theoretical modelling and analysis of the material removal characteristics in fluid jet polishing, Int. J. Mech. Sci. 89 (2014) 158-166. [9] H.T. Zhu, C.Z. Huang, J. Wang, Q.L. Li, C.L. Che, Experimental study on abrasive waterjet polishing for hard–brittle materials, Int. J. Mach. Tool. Manu. 49 (2009) 569-578 [10] N. Yuvaraj, M.P. Kumar, Cutting of aluminium alloy with abrasive water jet and cryogenic assisted abrasive water jet: A comparative study of the surface integrity approach, Wear 362–363 (2016) 18-32. [11] M. Mieszala, P.L. Torrubia, D.A. Axinte, J.J. Schwiedrzik, Y. Guo, S. Mischler, J. Michler, L. Philippe, Erosion mechanisms during abrasive waterjet machining: model microstructures and single particle experiments, Mater. Process. Technol. 247 (2017) 92-102. [12] F.H. Zhang, X.Z. Song, Y. Zhang, D.R. Luan, Figuring of an ultra-smooth surface in nanoparticle colloid jet machining, J. Micromech. Microeng. 19 (2009) 1-6 [13] W. Lauterborn, C.D. Ohl, Cavitation bubble dynamics, Ultrason. Sonochem. 4 (1997) 65-75 [14] L. Wijngaarden, Mechanics of collapsing cavitation bubbles, Ultrason. Sonochem. 29(2016)524-527
[15] G.L. Chahine, A. Kapahi, J.K. Choi, C.T. Hsiao, Modeling of surface cleaning by cavitation bubble dynamics and collapse, Ultrason. Sonochem. 29 (2016) 528-549 [16] P.F. Wu, L.X. Bai, W.J. Lin, J.C. Yan, Stability of cavitation structures in a thin liquid layer, Ultrason. Sonochem. 38 (2017) 75-83 [17] A. Beaucamp, T. Katsuura, Z. Kawara, A novel ultrasonic cavitation assisted fluid jet polishing system, CIRP ANN-Manuf. Techn. In Press, Available online 5 May 2017 [18] S. Parthasarathy, R.R. Mohammed, M.F. Chong, R.L. Gomes, S. Manickam. A novel hybrid approach of activated carbon and ultrasound cavitation for the intensification of palm oil mill effluent (POME) polishing, J. Clean. Prod. 112 (2016) 1218-1226 [19] B.K. Sreedhar, S.K. Albert, A.B. Pandit. Cavitation damage: Theory and measurements – A review, Wear 372–373 (2017) 177-196 [20] E. Hutli, A. Bonyár, D. Oszetzky, M.S. Nedeljkovic, Plastic deformation and modification of surface characteristics in nano-and micro-levels and enhancement of electric field of FCC materials using cavitation phenomenon, Mech. Mater. 92 (2016): 289-298 [21] L. Zhen, J.S. Zhou, J.S. Han, J.M. Chen. Formation of cavitation-induced nanosize precipitates on the eroded surface for Inconel 718 alloy. Matter. Lett. 164 (2016) 267-269 [22] B. Verhaagen, D. F. Rivas, Measuring cavitation and its cleaning effect, Ultrason. Sonochem. 29 (2016) 619-628 [23] X.N. Wang, J. P. Jia, Y. L. Wang, Enhanced photocatalytic-electrolytic degradation of Reactive Brilliant Red X-3B in the presence of water jet cavitation, Ultrason. Sonochem. 23 (2015) 93-99 [24] Y.C. Huang, P.F. Wang，Y. Yuan. Synergistic degradation of chitosan by impinging stream and jet cavitation, Ultrason. Sonochem. 27 (2015) 592-601.
Figure captions
Fig. 1 CFJP principle and polishing process Fig. 2 Erosion modes of cavitation jet: (a) suspending bubble and (b) adhering bubble Fig. 3 Schematic diagram, flow behavior, and cavitation results of NPC machining in reversing suction flow using a high-speed camera and Zygo 7100: (a) schematic diagram, (b) flow behavior of pure water when the suction pump runs at 0 ms, (c) pure water sucked from a micro nozzle at 415 ms, (d) pure water with bubbles impacting on an aluminum surface at 439 ms, (e) microbubbles are continuously generated at 452 ms and (f) at 455 ms, (g) pure water with bubbles are sucked from suction chamber at 457 ms and (h) at 465 ms, (i) three regions after NPC machining on the (j) initial aluminum surface, (k) certain sheet pits induced by cavitation erosion at the negative pressure of −60 kPa, and (m) bubbles induced by cavitation erosion at the inlet pressure of 0.2 MPa and the negative pressure of −60 kPa Fig. 4 3D modeling of vertical jet and inclined jet polishing Fig. 5 Effect of outlet pressure on the content of cavitation bubbles: (a) −15 kPa, (b) −60 kPa, (c) −80 kPa, and (d) −95 kPa Fig. 6 Effect of outlet pressure on the content of cavitation bubbles in (a) vertical jet and (b) inclined jet Fig.7 Simulation of cavitation flow field in different outlet pressures: (a) -60kPa, and (b) -80kPa Fig.8 Polishing device of CFJP Fig.9 Removal characteristics of polishing spot after FJP and CFJP Fig.10 Maximum removal depth variation against various parameters Fig.11 Surface roughness variation against outlet pressures
Fig.12 Surface topography (a) before polishing and after (b) 10 min, (c) 60min, and (d) 150 min at the outlet pressure of -75kPa
Fig. 1 CFJP principle and polishing process
Fig. 2 Erosion modes of cavitation jet: (a) suspending bubble and (b) adhering bubble
Fig. 3 Schematic diagram, flow behavior, and cavitation results of NPC machining in reversing suction flow using a high-speed camera and Zygo 7100: (a) schematic diagram, (b) flow behavior of pure water when the suction pump runs at 0 ms, (c) pure water sucked from a micro nozzle at 415 ms, (d) pure water with bubbles impacting on an aluminum surface at 439 ms, (e) microbubbles are continuously generated at 452 ms and (f) at 455 ms, (g) pure water with bubbles are sucked from suction chamber at 457 ms and (h) at 465 ms, (i) three regions after NPC machining on the (j) initial aluminum surface, (k) certain sheet pits induced by cavitation erosion at the negative pressure of −0.60 kPa, and (m) bubbles induced by cavitation erosion at the inlet pressure of 0.2 MPa and the negative pressure of −0.60 kPa
Fig. 4 3D modeling of vertical jet and inclined jet polishing
Fig. 5 Effect of outlet pressure on the content of cavitation bubbles: (a) −15 kPa, (b) −60 kPa, (c) −80 kPa, and (d) −95 kPa
Fig. 6 Effect of outlet pressure on the content of cavitation bubbles in (a) vertical jet and (b) inclined jet
Fig.7 Simulation of cavitation flow field in different outlet pressures: (a) -60kPa, and (b) -80kPa
Fig.8 Polishing device of CFJP
Fig.9 Removal characteristics of polishing spot after FJP and CFJP
Fig.10 Maximum removal depth variation against various parameters
Fig.11 Surface roughness variation against outlet pressures
Fig.12 Surface topography (a) before polishing and after (b) 10 min, (c) 60min, and (d) 150 min at the outlet pressure of -75kPa
Highlights • A cavitation fluid jet polishing method under a negative pressure condition was proposed. • The feasibility is verified through researching the cavitation results of NPC machining using pure water. • A CFD model of CFJP for simulating the cavitation effect was described. • The material removal characteristics of CFJP were analyzed by comparing with FJP. • The experimental results agreed well with the results of simulated model.