Abstract

The hydrodynamic cavitation abrasive finishing (HCAF) technology, as an innovative, clean, and efficient polishing method, has been proven effective for processing the internal surfaces of additive manufacturing flow channels. However, in-depth mechanistic studies on the key factors affecting the cavitation intensity in the HCAF processing remain limited, even though they play a crucial role in optimizing polishing performance and enhancing process stability. This study aims to apply the HCAF process to the flow channels fabricated by the laser powder bed fusion (LPBF). By adjusting the abrasive inlet pressure and throat diameter, the optimal process parameter combination was obtained, resulting in a 90% reduction in surface roughness near the inlet. fluent simulations and high-speed imaging were conducted to further validate its effect on the cavitation intensity. Furthermore, the channel diameter was found to have a significant impact on the polishing performance. Additionally, predictions of cavitation intensity were used to guide the application of the HCAF polishing for channels of different diameters. The results indicate that although the abrasive inlet pressure has a minor effect on the incipient cavitation number, it significantly alters the pressure distribution in the mixed-flow chamber, thereby influencing cavitation dynamics. The high-pressure region accelerates cavitation bubble contraction and collapse, significantly reducing bubble lifespan and weakening both the intensity and persistence of the cavitation effect. This instability makes sustained cavitation enhancement in the HCAF difficult, affecting material removal efficiency and jet stability. Therefore, the abrasive inlet pressure plays a crucial role in controlling cavitation behavior and enhancing machining performance.

1 Introduction

Internal surface machining is a critical step in manufacturing channel components, ensuring surface quality and cleanliness after complex design and fabrication [1]. Laser powder bed fusion (LPBF) has demonstrated tremendous capability in designing and manufacturing parts containing fluid channels for reducing weight, and improving flow resistance and thermal properties [2,3]. However, LPBF fabricated channels inevitably exhibit issues such as non-uniform material distribution, poor surface quality in the overhanging regions, and slag and powder adhesion on the inner walls [4]. In mechanical and aerospace applications, higher inner surface quality is often required to minimize fluid pressure loss and particle shedding. Currently, inner surface processing methods for channels, including electrochemical polishing [5], chemical polishing [6], and abrasive flow polishing [7,8], face challenges such as uneven inner treatment, excessive material removal, and environmental pollution, particularly in the LPBF channels. In contrast, the hydrodynamic cavitation abrasive finishing (HCAF), a novel non-contact, flexible, and clean fluid polishing technology, effectively mitigates issues such as abrasive agglomeration and material loss [9]. When processing highly uneven inner surfaces in additive manufacturing (AM), it can improve the surface finish by 90% [10].

The HCAF builds upon traditional cavitation jet surface finishing by introducing micron-sized abrasive particles (e.g., SiC, Al2O3, or CeO2). These particles interact synergistically with cavitation bubbles, enhancing the erosion effect to facilitate the material removal, thereby achieving finer and more efficient surface polishing. Nagalingam and Yeo first proposed the use of an HCAF method and developed the first cavitation jet system [9]. In this system, abrasives first mix uniformly with water in a high-pressure pipeline to form an abrasive slurry, which then passes through the cavitation region to generate a jet, known as a pre-mixed hydrodynamic cavitation abrasive finishing. This method ensures good mixing performance but involves a complex setup and causes wear on the high-pressure pump and pipelines. Using this pre-mixed jet machining approach, the inner surface of AlSi10Mg flow channels was polished. Nagalingam et al. revealed the synergistic mechanism between cavitation erosion and abrasives and systematically investigated the effects of pressure, flow velocity, and temperature on the surface roughness [11]. Subsequently, a multi-jet hydrodynamic cavitation abrasive finishing (MJ-HCAF) system was developed and applied to the surface finishing of complex internal channels in additive manufacturing [10]. Compared to the previous pre-mixed system, the new post-mixed HCAF device delivers abrasives and water separately, mixing them at or after the cavitation region. This design effectively prevents abrasive-induced damage to the high-pressure pump and pipelines while enhancing the durability and maintenance convenience of the equipment. A summary of relevant literature on the HCAF of AM parts is presented in Table 1.

Table 1

Reference using the HCAF in AM parts

Literature and material of polished samplesFocusContributionRemarksPre-mixed HCAF/Post-mixed HCAFInternal surface/External surface
[9]
AlSi10Mg
The HCAF method was proposed for machining additive manufacturing parts.A novel, efficient, and controllable surface processing method was proposed.The study covers a comparative analysis of abrasive concentration and processing time, with comprehensive data support.Pre-mixedInternal surface
[10]
AlSi10Mg
The effects of the synergistic interaction between the cavitation and abrasives on the finishing of AM internal surfaces were investigated.A quantitative model of the synergistic effect was established.The synergistic effect mechanism was analyzed in depth.Pre-mixedInternal surface
[12]
Inconel 625
The MJ-HCAF method was proposed for the surface treatment of complex internal channels in AM.The MJ-HCAF system was developed, resulting in a 60–90% reduction in the surface roughness.The processing time is short, but parameter optimization is required to control dimensional deviations.Post-mixedInternal surface
[13]
Inconel 625
The MJ-HCAF technology was applied to process LPBF fuel nozzles, integrating the X-CT inspection for dimensional integrity.This technology reduced the internal surface roughness to ≤1 μm while keeping the multi-branch outlets structurally intact.The X-CT was integrated for non-destructive testing, ensuring both surface quality and dimensional accuracy.Post-mixedInternal surface
[14]
Inconel 625
The MJ-HCAF technology allows efficient polishing of complex LPBF components while maintaining dimensional integrity.The cavitation abrasive synergistic effect was quantitatively analyzed.Achieves efficient polishing within 10 min, reducing abrasive consumption by 98%.Post-mixedExternal surface
Literature and material of polished samplesFocusContributionRemarksPre-mixed HCAF/Post-mixed HCAFInternal surface/External surface
[9]
AlSi10Mg
The HCAF method was proposed for machining additive manufacturing parts.A novel, efficient, and controllable surface processing method was proposed.The study covers a comparative analysis of abrasive concentration and processing time, with comprehensive data support.Pre-mixedInternal surface
[10]
AlSi10Mg
The effects of the synergistic interaction between the cavitation and abrasives on the finishing of AM internal surfaces were investigated.A quantitative model of the synergistic effect was established.The synergistic effect mechanism was analyzed in depth.Pre-mixedInternal surface
[12]
Inconel 625
The MJ-HCAF method was proposed for the surface treatment of complex internal channels in AM.The MJ-HCAF system was developed, resulting in a 60–90% reduction in the surface roughness.The processing time is short, but parameter optimization is required to control dimensional deviations.Post-mixedInternal surface
[13]
Inconel 625
The MJ-HCAF technology was applied to process LPBF fuel nozzles, integrating the X-CT inspection for dimensional integrity.This technology reduced the internal surface roughness to ≤1 μm while keeping the multi-branch outlets structurally intact.The X-CT was integrated for non-destructive testing, ensuring both surface quality and dimensional accuracy.Post-mixedInternal surface
[14]
Inconel 625
The MJ-HCAF technology allows efficient polishing of complex LPBF components while maintaining dimensional integrity.The cavitation abrasive synergistic effect was quantitatively analyzed.Achieves efficient polishing within 10 min, reducing abrasive consumption by 98%.Post-mixedExternal surface

Therefore, to extend the system lifespan, this study employs a post-mixed MJ-HCAF system. It is well known that in HCAF, which is based on the synergistic interaction between the cavitation bubbles and abrasives, the cavitation intensity significantly influences machining efficiency. Previous studies have shown that the cavitation intensity of water jets is influenced by the concentration of cavitation bubbles, and modifying nozzle structures and hydrodynamic parameters can enhance the cavitation intensity [15,16]. Wu et al. [17] used a high-speed camera to capture the growth and development of cavitation clouds, revealing that the nozzle throat cross-sectional size has the most significant impact on the cavitation effect. Additionally, Soyama [18] found that in non-submerged cavitation water jets, the velocity of the external low-speed water jet also significantly affects the cavitation intensity. However, little research has been conducted on the evolution of cavitation bubbles in the high-speed jet region, including their generation, growth, collapse, and influencing factors within the mixing chamber and processing pipeline.

This study focuses on an in-depth investigation of the cavitation jet mechanism in the HCAF. The study examines the effects of various parameters, including abrasive branch-end pressure, nozzle size, and channel dimensions, on the cavitation intensity during the HCAF processing of channels. Additionally, it explores their influence on the evolution of cavitation bubbles. Furthermore, a high-speed camera was used to record the cavitation cloud intensity under different parameters. Destructive measurement methods were employed to document the surface morphology after internal channel polishing, followed by analysis and interpretation of the results. The findings provide guidance for the optimization of the HCAF mixed-flow chamber design, as well as the selection of polishing channel dimensions and operating pressure.

2 Experimental Apparatus and Methods

2.1 Experimental Apparatus.

Based on the hydrodynamic cavitation abrasive finishing mechanism, a hydrodynamic cavitation abrasive finishing system was constructed. Figure 1 illustrates the overall structure of the hydrodynamic cavitation abrasive finishing system, which consists of a high-pressure cavitation circuit, a low-pressure abrasive circuit, and a mixed-flow chamber that connects both circuits. Figure 2 presents a photograph of the actual HCAF system. The primary function of the high-pressure cavitation circuit is to generate cavitation jets. It comprises a water tank, a pre-filter, a high-pressure plunger pump, an accumulator, a pressure regulating valve, and a mechanical pressure gauge, all integrated within the control cabinet. The high-pressure plunger pump is a three-piston pump with a maximum pressure of 25 MPa and a maximum flowrate of 15 l/min. The primary function of the low-pressure abrasive circuit is to transport abrasives. It consists of a carrier water tank, a ball valve, a filtration water tank, a precision filter, an electric diaphragm pump, and a pressure gauge.

Fig. 1
Fig. 2
Photograph of the HCAF system
Fig. 2
Photograph of the HCAF system
Close modal

The mixed-flow chamber is a key component of the system, designed to merge and homogenize the high-pressure cavitation jet and the low-pressure abrasive flow, forming an abrasive cavitation jet for cavitation-based polishing of the workpiece connected downstream, as shown in Fig. 3(a). Additionally, the diagram of the mechanism of cavitation and abrasive synergistic action is shown in Fig. 4.

Fig. 3
(a) Mixed-flow chamber structure, (b) dimension of the mixed-flow region, and (c) workpiece size
Fig. 3
(a) Mixed-flow chamber structure, (b) dimension of the mixed-flow region, and (c) workpiece size
Close modal
Fig. 4
Schematic diagram of the hydrodynamic cavitation abrasive finishing
Fig. 4
Schematic diagram of the hydrodynamic cavitation abrasive finishing
Close modal

The mixed-flow plate is the core component of the mixed-flow chamber, responsible for blending the high-pressure cavitation jet and the low-pressure abrasive flow. In this study, it was optimized and improved. The mixed-flow plate was designed with a “T”-shaped mixing structure, modified from the “V”-shaped structure originally developed by Nagalingam et al. Compared to the V-shaped mixing structure, which is prone to backflow, the improved T-shaped design features a self-priming negative pressure function. The specific dimensions are shown in Fig. 3(b).

As shown in Fig. 1, the low-pressure abrasive flow loop uses a 25-watt low-flow electric diaphragm pump to deliver the abrasive flow to the mixing chamber. The pump is driven by a stepper motor and allows step-less speed adjustment to achieve variable flowrates and pressures. Specifically, the abrasive flow pressure of –0.04 MPa (Pu2) is generated by the negative pressure structure of the mixing plate during the operation of the 15 MPa cavitation jet. The pressure range of –0.04 to 0.06 MPa is achieved through step-less speed control of the abrasive flow pump. The abrasive particles used in the experiment had an average size of approximately 60 μm. The scanning electron microscope (SEM) image of the abrasives is shown in Fig. 5.

Fig. 5
SEM photographs of the abrasives
Fig. 5
SEM photographs of the abrasives
Close modal

The mixed-flow chamber is connected to the polishing specimen at the bottom. In this study, the workpiece used was a 316L channel with a diameter of 1.8 mm, a length of 100 mm, and a wall thickness of 2.1 mm, as shown in Fig. 3(c). The LPBF machine used in this study was the Zhongrui iSLM160. The feedstock was 316L spherical powder with a loose packing density of 3.9 g/cm3 and a particle size range of 15–53 μm.

2.2 Measurement.

A high-speed camera system was used to capture the distribution of cavitation bubbles inside the flow channel. The high-speed camera used was the FASTCAM Mini AX200, capable of recording 20,000 frames per second at a resolution of 640 × 480. Shooting parameters and image recording were adjusted via dedicated software on a computer. The flow channel was fabricated using photopolymerization-based 3D printing with a transparent resin material (ZR820) that has 85% light transmittance. Dimensional fine-tuning ensured that the printed flow channel closely matched the diameter and length of the metal additive-manufactured counterpart, maintaining similar cavitation bubble distribution characteristics.

A confocal microscope was used to capture the surface morphology of the channel and measure its surface roughness. The confocal microscope used was the KEYENCE VK-X150, which employs a 658 nm red semiconductor laser for imaging and measurement, providing a height range of 7 mm and a maximum resolution of 0.005 μm. The vk analysis software was used to process the collected data, providing three-dimensional morphology and roughness values for the measured area.

2.3 Numerical Methods

2.3.1 Computational Domain.

The workpiece in the HCAF is fully submerged in water, operating under a submerged water jet environment. Simulations were conducted using ansys fluent. Based on the structure of the mixed-flow chamber and flow channel workpiece, as well as the system's operating mechanism, a simplified two-dimensional geometric model was created, as shown in Fig. 6. The computational fluid domain parameters are listed in Table 2.

Fig. 6
Two-dimensional model computational domain: (a) division of model boundary conditions and (b) grid division
Fig. 6
Two-dimensional model computational domain: (a) division of model boundary conditions and (b) grid division
Close modal
Table 2

The size of the fluid domain

ParametersValue
Channel diameter1.8 mm
Channel length100 mm
Cavitation jet nozzle diameterΦ0.4 mm and Φ1 mm
ParametersValue
Channel diameter1.8 mm
Channel length100 mm
Cavitation jet nozzle diameterΦ0.4 mm and Φ1 mm

2.3.2 Mesh Selection.

A mesh independence verification was performed using the maximum velocity in the flow field as a reference value. The mesh independence verification results are presented in Table 3. As the number of mesh elements increases, the maximum velocity in the flow field stabilizes. Using a global mesh size of 0.2 mm as the baseline, the relative error of the maximum velocity in the flow field remained within 0.5% for global mesh sizes of 0.3 mm and 0.15 mm. The accuracy of the numerical simulation no longer varied with mesh refinement. The final selected mesh size was 0.2 mm globally and 0.008 mm locally, resulting in a total of 733,921 mesh elements.

Table 3

Mesh independence analysis

Mesh sizeMesh numberThe maximum velocity (m/s)Relative error of maximum velocity
Global—0.4 mm, local—0.016 mm180,639177.91182.58%
Global—0.3 mm, local—0.012 mm313,155173.65070.12%
Global—0.2 mm, local—0.008 mm733,921173.43750%
Global 0.15 mm, local—0.006 mm1,238,952173.79000.20%
Mesh sizeMesh numberThe maximum velocity (m/s)Relative error of maximum velocity
Global—0.4 mm, local—0.016 mm180,639177.91182.58%
Global—0.3 mm, local—0.012 mm313,155173.65070.12%
Global—0.2 mm, local—0.008 mm733,921173.43750%
Global 0.15 mm, local—0.006 mm1,238,952173.79000.20%

2.3.3 Multiphase Model.

The SST kω turbulence model was selected for the numerical simulation. The mixture multiphase flow model was chosen, with slip velocity ignored. The Schnerr–Sauer cavitation model was applied, with bubble size set to 1 × 10−5 m and bubble nucleus density of 1 × 1013 nuclei/m3.

The ambient pressure for the numerical simulation was set to standard atmospheric pressure. Pressure inlet 1 was designated as the cavitation jet pressure (Pu1) and set to 15 MPa. Pressure inlet 2 (Pu2) was varied between −0.04 MPa and 0.06 MPa across different cases. The pressure outlet was set to 0 MPa. The computational domain walls were defined as no-slip stationary walls, with scalable wall functions applied for near-wall treatment.

The numerical simulation employed a pressure–velocity coupling approach, with the coupled solver selected. The pseudo transient was applied to enhance the convergence. First-order upwind schemes were used for the momentum, swirl velocity, volume fraction, turbulence dissipation, and turbulent kinetic energy equations. The residual convergence criterion was set to 1 × 10−5, with a maximum iteration count of 2000. The boundary condition parameters are listed in Table 4.

Table 4

Simulation boundary condition parameters for different abrasive flow pressures

ParametersValue
Pressure inlet115 MPa
Pressure inlet2−0.04 MPa, 0 MPa, 0.02 MPa, 0.04 MPa, 0.06 MPa
Pressure outlet0 MPa
Cavitation jet nozzle diameterΦ0.4 mm
Channel sizeΦ1.8 mm; length—100 mm
ParametersValue
Pressure inlet115 MPa
Pressure inlet2−0.04 MPa, 0 MPa, 0.02 MPa, 0.04 MPa, 0.06 MPa
Pressure outlet0 MPa
Cavitation jet nozzle diameterΦ0.4 mm
Channel sizeΦ1.8 mm; length—100 mm

3 Result

3.1 The Impact of the Abrasive Inlet Pressure on the Polishing.

Figure 7 presents the surface roughness measurements of the top and bottom surfaces of the channel after polishing. Analysis of the surface roughness (Ra) curve indicates that the best polishing effect was achieved at a pressure of Pu2 = −0.04 MPa. In the 5–55 mm section of the flow channel, the surface roughness (Ra) was reduced from an average of 15 μm to approximately 1–2 μm, achieving an improvement of about 90%. In the 65–75 mm section, the roughness was reduced to approximately 7 μm, with an improvement of about 50%. In the 85–95 mm section, which is closer to the outlet, only a slight improvement in the surface roughness was observed. Furthermore, comparing the polishing effects under different abrasive pressures reveals that the HCAF process is highly sensitive to variations in abrasive pressure, with Pu2 = −0.04 MPa identified as the optimal process parameter.

Fig. 7
Surface roughness after treatment with different abrasive flow pressures: (a) the bottom surface, (b) the top surface, and (c) the diagram of measurement points
Fig. 7
Surface roughness after treatment with different abrasive flow pressures: (a) the bottom surface, (b) the top surface, and (c) the diagram of measurement points
Close modal

Since the data at an abrasive flow pressure of Pu2 = −0.04 MPa are representative of this study, optical images and 3D profile images of the internal surface of the channel were extracted for the surface morphology analysis.

Figure 8 illustrates the longitudinal surface morphology of the bottom surface of the flow channel under an abrasive flow pressure of Pu2 = −0.04 MPa. As the distance increases, the polishing performance of the HCAF gradually decreases. In the 5–35 mm section of the flow channel, the original surface morphology was completely removed, revealing a smooth and uniform surface. From the 45 mm region onward, the original surface morphology begins to reappear. The 3D profile images reveal that the region near the inlet has a larger arc radius due to greater material removal, resulting in improved surface quality. Conversely, the region near the outlet experiences less material removal, retaining a significant portion of the original morphology.

Fig. 8
The bottom surface after HCAF treatment under abrasive flow pressure of −0.04 MPa, the optical images of the surface at (a) 5 mm, (b) 15 mm, (c) 25 mm, (d) 35 mm, (e) 45 mm, (f) 55 mm, (g) 65 mm, (h) 75 mm, (i) 85 mm, and (j) 95 mm; and 3D profile images at (a') 5 mm, (b') 15 mm, (c') 25 mm, (d') 35 mm, (e') 45 mm, (f') 55 mm, (g') 65 mm, (h') 75 mm, (i') 85 mm, and (j') 95 mm
Fig. 8
The bottom surface after HCAF treatment under abrasive flow pressure of −0.04 MPa, the optical images of the surface at (a) 5 mm, (b) 15 mm, (c) 25 mm, (d) 35 mm, (e) 45 mm, (f) 55 mm, (g) 65 mm, (h) 75 mm, (i) 85 mm, and (j) 95 mm; and 3D profile images at (a') 5 mm, (b') 15 mm, (c') 25 mm, (d') 35 mm, (e') 45 mm, (f') 55 mm, (g') 65 mm, (h') 75 mm, (i') 85 mm, and (j') 95 mm
Close modal

Compared to the bottom surface of the flow channel, the top surface is more challenging to polish due to its lower initial surface quality, the top surface retains more of the original morphology after polishing due to its lower initial surface quality. Additionally, the reduction of surface profile peaks is more limited, as shown in Fig. 9.

Fig. 9
The top surface after HCAF treatment under abrasive flow pressure of −0.04 MPa, the optical images of the surface at (a) 5 mm, (b) 15 mm, (c) 25 mm, (d) 35 mm, (e) 45 mm, (f) 55 mm, (g) 65 mm, (h) 75 mm, (i) 85 mm, and (j) 95 mm; and 3D profile images at (a') 5 mm, (b') 15 mm, (c') 25 mm, (d') 35 mm, (e') 45 mm, (f') 55 mm, (g') 65 mm, (h') 75 mm, (i') 85 mm, and (j') 95 mm
Fig. 9
The top surface after HCAF treatment under abrasive flow pressure of −0.04 MPa, the optical images of the surface at (a) 5 mm, (b) 15 mm, (c) 25 mm, (d) 35 mm, (e) 45 mm, (f) 55 mm, (g) 65 mm, (h) 75 mm, (i) 85 mm, and (j) 95 mm; and 3D profile images at (a') 5 mm, (b') 15 mm, (c') 25 mm, (d') 35 mm, (e') 45 mm, (f') 55 mm, (g') 65 mm, (h') 75 mm, (i') 85 mm, and (j') 95 mm
Close modal

The simulation results in Fig. 10 illustrate the pressure distribution inside the flow channel under different abrasive flow pressures. By limiting the maximum displayed pressure in the flow field to a relative pressure of 0 MPa, emphasis is placed on regions where the pressure is below 0 MPa. The minimum pressure in the flow field is −97,785 Pa (relative pressure), corresponding to an absolute pressure of 3540 Pa, which is the saturated vapor pressure of water under the current operating conditions. The dark blue regions in the figure indicate areas near the minimum pressure in the flow field, where cavitation bubbles can form and remain stable. Additionally, a pressure transition point exists in the flow channel under all abrasive flow pressure conditions. The pressure in the flow channel rises sharply from a lower pressure to atmospheric pressure, and as the abrasive flow pressure (Pu2) increases, this transition point gradually shifts closer to the flow channel inlet.

Fig. 10
Pressure contour maps under different abrasive flow pressures: (a) Pu2 = −0.04 MPa, (b) Pu2 = 0 MPa, (c) Pu2 = 0.02 MPa, (d) Pu2 = 0.04 MPa, and (e) Pu2 = 0.06 MPa
Fig. 10
Pressure contour maps under different abrasive flow pressures: (a) Pu2 = −0.04 MPa, (b) Pu2 = 0 MPa, (c) Pu2 = 0.02 MPa, (d) Pu2 = 0.04 MPa, and (e) Pu2 = 0.06 MPa
Close modal

Figure 11 presents the simulated cavitation bubble volume fraction along the near-wall region of the flow channel under different pressures. Under all abrasive flow pressures (Pu2), the cavitation bubble volume fraction on the channel wall rapidly increases from approximately 0.1, reaching a peak of around 0.75 at approximately 3 mm. It then gradually decreases along the flow path before abruptly dropping to zero at different positions. In addition to reducing the cavitation bubble distribution area within the flow channel, an increase in the abrasive flow pressure also accelerates the cavitation bubble collapse rate. At an abrasive flow pressure of Pu2 = −0.04 MPa, the cavitation bubble volume fraction exhibits the slowest decline rate, which increases progressively with rising pressure.

Fig. 11
The influence of different abrasive flow inlet pressures on the volume fraction of cavitation bubbles
Fig. 11
The influence of different abrasive flow inlet pressures on the volume fraction of cavitation bubbles
Close modal

3.2 The Impact of the Nozzle Size on the Polishing.

Figure 12 presents the surface roughness measurements of the bottom and top surfaces of the flow channel after polishing. Regardless of whether surface roughness Ra or Rz is considered, the roughness curves generated using the Φ0.4 mm cavitation jet nozzle and the Φ1.0 mm nozzle do not intersect. Furthermore, the Φ1.0 mm curve is consistently lower than the Φ0.4 mm curve, indicating superior surface quality. A comprehensive analysis of the surface roughness measurements and morphology of the bottom surface of the flow channel indicates that the Φ1.0 mm cavitation jet nozzle exhibits significant advantages over the Φ0.4 mm nozzle in terms of the polishing performance. Under identical polishing conditions, the Φ1.0 mm nozzle achieved greater surface quality improvements and significantly enhanced polishing efficiency.

Fig. 12
Surface roughness after treatment with different nozzle diameters: (a) the bottom surface and (b) the top surface
Fig. 12
Surface roughness after treatment with different nozzle diameters: (a) the bottom surface and (b) the top surface
Close modal

Similarly, by varying the nozzle diameter, the simulation results in Fig. 13 illustrate the pressure distribution inside the flow channel under different cavitation jet nozzle diameters. The pressure in the flow channel regions corresponding to both cavitation jet nozzle diameters is below the saturated vapor pressure, allowing cavitation bubbles to form and remain stable.

Fig. 13
Pressure contour maps under different nozzle diameters: (a) 0.4 mm and (b) 1 mm
Fig. 13
Pressure contour maps under different nozzle diameters: (a) 0.4 mm and (b) 1 mm
Close modal
Table 5

The number of incipient cavitation under different abrasive flow inlet pressures

Experiment no. UnitsNozzle diameter mmCavitation jet pressure
(Pu1) MPa
Abrasive flow pressure
(Pu2) MPa
Abrasive particle concentration %Cavitation number
10.415−0.0410.0010109681
20.415010.0010113854
30.4150.0210.0010120436
40.4150.0410.0010119870
50.4150.0610.0010122722
Experiment no. UnitsNozzle diameter mmCavitation jet pressure
(Pu1) MPa
Abrasive flow pressure
(Pu2) MPa
Abrasive particle concentration %Cavitation number
10.415−0.0410.0010109681
20.415010.0010113854
30.4150.0210.0010120436
40.4150.0410.0010119870
50.4150.0610.0010122722

3.3 Surface Differences at Various Positions Inside the Channel Before and After HCAF Polishing.

A comparison of the inner surface morphology of the top and bottom regions before and after HCAF, as shown in Fig. 14, reveals that the initial overhanging top surface exhibits significantly poorer quality than the bottom region, with numerous partially unmelted particles distributed across the surface. After optimization of the processing parameters, these unmelted particles were completely removed, and dense abrasive micro cuts were observed on the polished surface. However, porosity defects remained on the upper surface region, which are difficult to eliminate during the HCAF process. These results indicate that the HCAF is highly effective at removing protrusion-type defects, such as partially unmelted particles and slag, from the inner surfaces of additively manufactured channels. However, its removal efficiency is relatively low for fine cracks and similar defects.

Fig. 14
Comparison between the initial and post-HCAF surface morphology
Fig. 14
Comparison between the initial and post-HCAF surface morphology
Close modal

Additionally, detailed surface morphology results under different processing parameters (abrasive pressure and nozzle diameter) are shown in Fig. 15.

Fig. 15
Surface morphology differences at the channel inlet under different HCAF processes
Fig. 15
Surface morphology differences at the channel inlet under different HCAF processes
Close modal

4 Discussion

4.1 The Influence of the Cavitation Intensity on the Polishing Effect.

Cavitation occurs when the static pressure of a fluid drops below its vapor pressure at a given temperature [19]. During the HCAF process, cavitation collapse near the machining surface generates localized high-speed microjets with velocities ranging from 200 m/s to 700 m/s. These high-speed jets repeatedly implode on the workpiece surface, eroding the material. Additionally, rigid abrasives near the cavitation implosion point are accelerated by the microjets, impacting the solid surface at higher velocities and enhancing erosion efficiency. Notably, in a static field, microjets and shock waves generated by the cavitation implosion can accelerate abrasives up to 40 m/s [20,21]. Therefore, cavitation intensity has a significant impact on the machining strength of the abrasive cavitation jet.

Figure 16 presents the results captured by the high-speed camera system. To better observe the visual behavior of cavitation bubbles and avoid interference from abrasive particles, a zero-abrasive concentration condition was used. As the abrasive flow pressure (Pu2) increases, the collapse rate of cavitation bubbles accelerates significantly, reducing their proportion in the flow channel. Compared to the flow channel being almost filled with cavitation bubbles at Pu2 = −0.04 MPa, at Pu2 = 0.06 MPa, cavitation bubbles are present only in a small region near the inlet. Based on the working mechanism of the HCAF, cavitation bubble collapse accelerates abrasives, enhancing the cutting force. Therefore, the distribution of cavitation bubbles directly influences the polishing results. The distribution of cavitation bubbles suggests that the polishing effectiveness of the HCAF decreases progressively with increasing abrasive flow pressure (Pu2) and declines along the flow direction.

Fig. 16
The distribution of cavitation bubbles within the flow channel was captured using a high-speed camera under varying abrasive flow pressures (Pu2): (a) Pu2 = −0.04 MPa, (b) Pu2 = 0 MPa, (c) Pu2 = 0.02 MPa, (d) Pu2 = 0.04 MPa, and (e) Pu2 = 0.06 MPa
Fig. 16
The distribution of cavitation bubbles within the flow channel was captured using a high-speed camera under varying abrasive flow pressures (Pu2): (a) Pu2 = −0.04 MPa, (b) Pu2 = 0 MPa, (c) Pu2 = 0.02 MPa, (d) Pu2 = 0.04 MPa, and (e) Pu2 = 0.06 MPa
Close modal
To investigate the effect of abrasive inlet pressure on the nozzle cavitation intensity, the cavitation number at the throat inlet was calculated. The cavitation number Cv is a dimensionless parameter that describes the intensity of cavitation, defined as
(1)

In the equation, Pout represents the outlet pressure, which is set to standard atmospheric pressure in this study, Pv represents the liquid's saturated vapor pressure, and Vthroat is the velocity at the throat, obtained as the average velocity across the throat cross section in the calculations.

The different initial cavitation numbers under different abrasive inlet pressures were obtained through calculation,as shown in Table 5.

Calculations reveal that when the water jet pressure remains constant, an increase in the abrasive flow pressure from −0.04 MPa to −0.06 MPa results in a slight increase in the cavitation number, with a growth rate of no more than 0.13%. Therefore, it can be concluded that the abrasive inlet pressure has a negligible effect on the incipient cavitation number.

Similarly, by varying the nozzle diameter, the calculated incipient cavitation number at the nozzle throat is presented in Table 6.

Table 6

The number of incipient cavitation under different throat sizes

Experiment no. UnitsNozzle diameter mmCavitation jet pressure
(Pu1) MPa
Abrasive flow pressure
(Pu2) MPa
Abrasive particle concentration %Cavitation number
10.415−0.0410.0010109681
21.015−0.0410.000990484
Experiment no. UnitsNozzle diameter mmCavitation jet pressure
(Pu1) MPa
Abrasive flow pressure
(Pu2) MPa
Abrasive particle concentration %Cavitation number
10.415−0.0410.0010109681
21.015−0.0410.000990484

As the nozzle diameter increases, the cavitation number decreases significantly, with a reduction of 2.07%, following the trend described in the relevant literature. This indicates that variations in the nozzle size significantly affect the incipient cavitation number.

Additionally, during the experiment, the size of the flow channel significantly influences the polishing performance. A smaller flow channel diameter causes a rapid pressure rise under an adverse pressure gradient, leading to quicker cavitation bubble collapse, which prevents the bubbles from reaching a sufficient concentration for effective polishing. A larger flow channel diameter results in a rapid decrease in the flow velocity, thereby reducing the cutting force of abrasives and the synergistic polishing effect between the cavitation bubbles and abrasives, ultimately decreasing the polishing efficiency. The appropriate cavitation jet pressure for different flow channel diameters was analyzed through simulation, with the results presented in Fig. 17. The simulation results were compiled to extract the minimum cavitation jet pressure under the conditions where the near-wall flow velocity exceeded 30 m/s and the cavitation bubble volume fraction along the near-wall region remained above 0.4. A fitted curve was plotted in Fig. 18, revealing that variations in the cavitation jet pressure can, to some extent, be correlated with changes in the flow channel diameter.

Fig. 17
Simulation results of the volume fraction of bubbles in the HCAF with different flow channel diameters: (a) Φ1.8 mm, (b) Φ2.8 mm, (c) Φ4 mm, (d) Φ5.6 mm, and (e) Φ8 mm
Fig. 17
Simulation results of the volume fraction of bubbles in the HCAF with different flow channel diameters: (a) Φ1.8 mm, (b) Φ2.8 mm, (c) Φ4 mm, (d) Φ5.6 mm, and (e) Φ8 mm
Close modal
Fig. 18
The curve of the applicable jet pressure (Pu1) to the channel of various diameters
Fig. 18
The curve of the applicable jet pressure (Pu1) to the channel of various diameters
Close modal

4.2 Analysis of the Cavitation Cloud Evolution Mechanism in the Mixed-Flow Chamber.

Based on the previous polishing and simulation results, the optimal abrasive flow pressure parameter (Pu2) is −0.04 MPa. A comprehensive analysis of the polishing results indicates that the abrasive cavitation jet polishing process is highly sensitive to variations in abrasive flow pressure, with minor pressure fluctuations leading to a rapid decline in polishing efficiency. Combined with the previous analysis, the impact of abrasive flow inlet pressure on the incipient cavitation at the nozzle is negligible. Additionally, as the abrasive flow inlet pressure increases, the abrasive flow inlet flowrate also increases, exhibiting an approximately linear relationship. The relationship between the abrasive flow pressure and abrasive flowrate is presented in Fig. 19.

Fig. 19
The relationship between the abrasive flow inlet pressure and flowrate
Fig. 19
The relationship between the abrasive flow inlet pressure and flowrate
Close modal
However, both high-speed camera observations and polishing results indicate that lower abrasive flow inlet pressure results in higher jet cavitation intensity. Therefore, the abrasive flow pressure significantly influences the evolution and collapse of cavitation clouds within the mixed-flow chamber. This study explains the phenomenon based on the mechanism of abrasive cavitation evolution and presents the corresponding schematic diagram, as shown in Fig. 20. Cavitation occurs through several stages: bubble formation, growth under sub-saturated vapor pressure, further expansion due to inertia, contraction as the ambient pressure increases, and ultimately, violent collapse [22,23]. Additionally, by mapping the pressure distribution in the mixed-flow chamber, it is observed that there is a localized pressure recovery region at the junction with the abrasive inlet, though its coverage is limited. Meanwhile, bubbles exhibit a certain velocity as they travel through the pipeline. Cavitation bubbles move rapidly through the throat region and into the high-pressure zone (above the saturated vapor pressure), where they continue expanding due to inertia for a short duration. Subsequently, they undergo rapid contraction but, after a certain period, re-enter the low-pressure zone (below the saturated vapor pressure). Throughout this process, if the cavitation bubble size is not particularly large, it may persist through this region without reaching its collapse time. The collapse time can be estimated using the following equation:
(2)
Fig. 20
Mechanism diagram of the cavitation bubble evolution inside the channel
Fig. 20
Mechanism diagram of the cavitation bubble evolution inside the channel
Close modal

R0 represents the maximum bubble radius at the onset of collapse, and ρ represents the liquid density [23]. Previously, it was observed that the abrasive inlet pressure has minimal impact on the overall flowrate. Therefore, assuming that the flow velocity remains constant across different abrasive flow pressures (−0.04 MPa to 0.06 MPa), the effect of abrasive flow pressure on the cavitation intensity is primarily reflected in the duration of high-pressure fluctuations (ΔT) at the branch junction. The pressure contour plots indicate that at 0.06 MPa, the duration of high-pressure fluctuations is significantly longer than at −0.04 MPa.

Additionally, an increase in abrasive flow pressure allows excessive fluid to enter the mixed-flow chamber, disrupting the flow field.

5 Conclusions

The effects of abrasive inlet pressure and nozzle size on the cavitation intensity in the mixed-flow chamber region were analyzed. Based on the simulation results, the underlying mechanisms were explained, and the optimal cavitation inlet pressure for different polishing channel diameters was determined. The above results were validated through observations of post-polishing surface roughness and high-speed camera imaging. The conclusions are as follows:

  • The surface roughness results after polishing indicate that the best polishing performance is achieved at an abrasive inlet pressure of Pu2 = −0.04 MPa, while other pressure conditions result in less effective polishing. A nozzle throat diameter of 1 mm results in better polishing quality than 0.4 mm. Additionally, the closer the inner surface of the flow channel specimen is to the inlet, the better the polishing effect, achieving up to a 90% improvement in the surface quality.

  • The simulation results indicate that flow channel diameter significantly affects the polishing performance. Both larger and smaller diameters require higher cavitation jet intensity, suggesting an optimal process window for the HCAF.

  • Analysis of the simulation results reveals the impact of abrasive inlet pressure on the cavitation intensity within the mixed-flow chamber. The abrasive inlet pressure has only a minor effect on incipient cavitation at the nozzle throat, with an impact of approximately 0.13%. However, it significantly influences the distribution of high-pressure regions within the mixed-flow chamber. The high-pressure regions accelerate cavitation bubble contraction and collapse, reducing bubble lifespan and weakening the intensity and persistence of the cavitation effect.

Funding Data

  • The National Key Research and Development Program of China (Grant No. 2022YFB4602502).

  • The National Natural Science Foundation of China (Grant Nos. 52222503 and 52005437).

  • Zhejiang Pioneer Plan Project (Grant No. 2025C01050).

Conflict of Interest

There are no conflicts of interest.

Data Availability Statement

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.

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