Microscale surface topography and its forming mechanism in abrasive suspension jet processing

doi:10.21203/rs.3.rs-1951931/v1

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Microscale surface topography and its forming mechanism in abrasive suspension jet processing

https://doi.org/10.21203/rs.3.rs-1951931/v1

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Microscale, nanometer to micrometer, is critical to research machining quality of abrasive suspension jet processing (ASJP), in which scale the surface topography of the processing section can be clearly shown and the material removal mechanism can be easily determined. In this paper, an experiment was carried out that three kinds of materials, Ti-6Al-4V (TC4), marble, and glass, cut by abrasive suspension jet (ASJ) and the microscale surface topography captured by an ultra-deep three-dimensional microscope. The experimental results showed that microscale surface topography of the three materials cut by ASJ is different due to their brittleness and toughness. TC4 cutting surface is covered by long and thin scratches with straight down direction, marble cutting surface is covered by pot holes, and glass cutting surface is covered by both of the scratches and pot holes. The scratches are between 40 and 140 microns in length, while the pot holes are between 10 and 40 microns in width. Moreover, forming mechanism of the three materials’ microscale surface topography is also different due to their brittleness and toughness. The scratch on TC4 cutting surface is the plastic damage caused by the abrasive particles, and the pot hole is the brittle fracture caused by the abrasive particles. Intermediate between brittle and plastic materials, glass cut by ASJ with both plastic damage and brittle fracture. The conclusions can provide theory guidance in material precision processing with ASJ.

abrasive suspension jet

microscale surface topography

forming mechanism

plastic and brittle materials

Abrasive suspension jet (ASJ) becomes a most potential non-traditional processing technology with the advantages of no residual stress, no thermal effects, good machining precision, and high applicability of materials [1–4]. As a liquid-solid two-phases medium jet just composed of water and abrasive particles, it is of good convergence to lead a material processing quality. Therefore, ASJ has huge potential in terms of the precision machining [5–8].

Surface roughness is the most significant evaluation standard for the machining quality of the ASJ, which is studied by many scholars for a long time. Shanmugam [9] researched the surface roughness of an AWJ machined surface of 7075 Aluminum metal matrix composite and found that increasing the pressure and jet traverse speed results in increasing the surface roughness and taper angle. Ipar [10] presented a comparative roughness criterion to evaluate the surface quality of agate plates under different AWJ processing conditions. Jagadish [11] used fuzzy logic to establish a surface roughness prediction model of AWJM process on machining of green composites, which can reflect the effects of jet parameters on the surface roughness. Banon [12] studied the surface quality generated by water jet machining in a low melting point thermoplastic matrix composite material and established a mathematical model that relates the surface quality with the cutting parameters by means of a response surface methodology.

The surface machined by ASJ possesses a unique surface topography, which have a significant influence on the surface roughness. In macroscopic view, the cutting surface is divided into four zones to describe the unique surface topography [13–14]. These are initial zone, smooth zone, transition zone and rough zone from top to bottom. In the initial zone, surfaces roughness is larger than the one in the smooth zone [15]. In the rough zone, surfaces roughness is the largest due to numerous grooves and striations locate on the cutting surface[16]. Reason for the unique surface topography is definitely related to deformation wear mode of material removal in the water jet machining. The most common view is that water jet, just like a soft knife, will deform to a tail lag in material machining process [17–18]. In microscopic view, scratches are formed on the cutting surface with a clear consistent direction. Obviously, the micro surface topography is caused by the abrasion of particles [19].

The previous research works aimed at the macro surface topography machined by the ASJ, while the analysis is lack for the micro surface topography. However, the micro surface topography is significant to reveal the forming mechanics of the surface roughness and reduce it. Therefore, this paper will use an ultra-deep three-dimensional microscope to capture the 3D micro surface topography of different materials cut by the ASJ. Moreover, the forming mechanics of the micro surface topography carries out in the ASJ cutting plastic and brittle materials. The conclusions can provide theory guidance in material precision processing with the ASJ.

In the experiment, an ASJ system was used to generate a high-speed coherent-jet to cut the materials along with a straight line. As shown in Fig. 1(a), water in the water tank is pressured by the plunger pump and mixed with abrasive particles in the abrasive tank, forming the high-speed ASJ through the nozzle finally. Pump is driven by a variable frequency motor, which can change the pressure of the system according to the experimental requirements. The cutting table is a CNC machine tool, which can control the moving route and the traverse speed of the nozzle accurately. As shown in Fig. 1(b), an ultra-depth three-dimensional microscope was used to collect the microscale morphological characteristics of the cutting surface. The microscope has two ways to reflect the morphological characteristics, which are intuitive picture and three-dimensional size measurement.

Three kinds of materials, Ti-6Al-4V (TC4), marble, and glass, were chosen as the cutting target to study the different microscale surface topography of plastic and brittle materials cut by ASJ. TC4 is a plastic material commonly used in aviation industry. Marble is a brittle material with large porosity, while glass is the one with small porosity. Parameters keep constant in the cutting experiment. Thickness of the three workpieces are 10mm. Jet pressure used is 30MPa, traverse speed is 60mm/min, and stand-off distance is 1mm. Abrasive used is garnet particle with 80 meshes grade, whose mass flow rate is 330g/min. The diameter of the nozzle is 0.8 mm, while the length is 46.2mm.

3.1 Microscale surface topography

Surface cut by ASJ is divided into four zones in terms of the roughness, which are the initial zone, smooth zone, transition zone, and rough zone from top to bottom[20]. The cutting surface has different microscale topography along with the cutting depth. For studying the microscale surface topography of different areas on the cutting surface, ten lines have been chosen as analysis line. They are evenly located on the cutting surface with 1mm separation distance. In statistical analysis of microscale surface topography size, three zones were adopted at one line to reduce the error. Each zone is a 0.5×0.5mm square areas, which depend on the microscope field size.

The ultra-deep microscope was used to enlarge the cutting surface at different analysis zones, which can demonstrate the characteristics of the microscopic surface topography.

Figure 3 shows the microscopic surface topography of TC4 cut by ASJ. There are ten partial images in figure showing the characteristics of the microscopic surface topography at zones on the different analysis lines. There are a large number of long and thin grooves with straight down direction cover the surface, which is caused by the high-speed abrasive particles obviously. Thus, the groove is called “scratch” in this paper. The enlarged images with measuring scale show that the length of the scratch is approximate 100 microns, and the width of the scratch is approximate 10 microns. Moreover, it also can be seen from the images that the scratch length increases and the scratch width decreases with the increase of the cutting depth.

The length, width, and depth of the scratches are measured by the three-dimensional microscope to quantitatively analyze the microscopic surface topography of TC4. As shown in Fig. 4, 10 feature scratches on one enlarged image were selected as the evaluation criteria for surface morphology. Then, it’s width measurement, height measurement, and length measurement were carried out for these feature scratches in order.

Figure 5 shows the average size measurement results of representative scratches at the ten analysis lines. Length of the scratches on the cutting surface is between 38.32-100.21µm, width is between 5.32–19.07µm, and depth is between 1.28–4.86µm. The scratches length increases with the growth of the cutting depth, while both of the scratches width and depth decrease with the growth of the cutting depth. The measurement results are consistent with the analysis results for Fig. 3. The reason for the results is that abrasive particles near the nozzle outlet have enough energy to remove TC4 materials, leading to a wide and deep scratches on the upper part of the cutting surface. However, energy of the abrasive particles away from the nozzle outlet is insufficient to wear the surface due to jet divergence and material removing energy consumer, leading to a narrow and shallow scratches on the lower part of the cutting surface.

Figure 6 shows the microscopic surface topography of marble cut by ASJ. There are ten partial images which can show the characteristics of the microscopic surface topography at zones on the different analysis lines. No scratches were observed in the figure. Instead, lots of pot holes cover the cutting surface, which were formed by the impact of the high-speed abrasive particles. They are called the “crushing pit” in this paper. The enlarged images with measuring scale show that the width of the crushing pit is approximate 20–30 microns.

Figure 7 shows the average size measurement results of representative crushing pits at the ten analysis lines. Width of the crushing pits on the cutting surface is between 11.58–33.84µm, depth is between 1.49–6.46µm. Both of the crushing pit width and depth decrease at first and then increase with the growth of the cutting depth. It means crushing pit width and depth in the smooth zone is smaller than the one in the smooth zone and the rough zone. The reason is that the smooth area is fully polished by abrasive particles.

Figure 8 shows the microscopic surface topography of glass cut by ASJ. There are ten partial images to show the characteristics of the microscopic surface topography at zones on the different analysis lines. Both scratches and crushing pits are appeared on the cutting surface. The enlarged images with measuring scale show that the length of the scratch is approximate 150 microns, the width of the scratch is approximate 5 microns, and the width of the crushing pit is approximate 20–30 microns. Obviously, the scratches on the glass surface is longer and thinner than those on the TC4 surface, while crushing pits on the glass surface have the similar dimensions with those cover the marble surface. Moreover, the size of the scratches and crushing pits change greatly with the increase of the cutting depth.

Figure 9 shows the average size measurement results of representative scratches and crushing pits at the ten analysis lines. Width of the crushing pits on the cutting surface is between 11.58–33.84µm, depth is between 1.49–6.46µm. Length of the scratches on the cutting surface is between 85.43-137.77µm, width is between 5.09–7.92µm, and depth is between 0.90–1.17µm. Both of the crushing pit width and depth decrease with the growth of the cutting depth. The scratches length increases with the growth of the cutting depth, while the scratches width and scratches depth have no evident changes.

3.2 Forming mechanism of the microscale surface topography

In the ASJ machining, material will be damaged and removed from the workpiece under the coupled force of high-speed abrasive and water jet. Stress caused by water jet is relatively small due to its less density, while stress caused by abrasive is large enough to make material failure. Therefore, abrasive particles play a decisive role on removal process in the ASJ machining. According to analyzing the micro-scale morphology of the cutting surface, the abrasion mechanism of abrasive is different for materials with different physical properties.

Figure 10 shows the formation principle of the TC4 microscale surface topography. With the cutting surface worn by a high-speed particle, a scratch and plastic deforming area is formed on the cutting surface, and a metal swarf is scattered in the jet stream. The microscopic surface topography is composed of scratches and plastic deforming areas caused by a large number of the abrasive particles. The scratch we observed is a kind of plastic damage of abrasive particles to TC4 material. In addition to scratches of different sizes found on the cutting surface, there is certain metal accumulation at the end of the scratches. Metal accumulation also mainly occurs in the rough area, related to the water jet cutting process closely. The length of the scratches gets longer as the cutting depth increases probably because of the subsequent abrasive particle effect. The rear abrasive particles continue to grind in the scratch area formed by the front abrasive particles, increasing the length of the scratch. At the same time, the metal accumulation increases the grinding resistance of the later abrasive particles, and the depth and width of the scratches are difficult to maintain the previous size.

Scratches were not found on the marble cutting surface but crushing pits because of the obvious differences in the materials. The different properties of solid and liquid contact erosion have important influence on the morphology of the cut surface. Marble is easy to fracture, the material itself is easy to carry cracks. As shown in Fig. 11, when a large number of high-speed abrasive particles and water cut the marble, the brittle fracturing of the marble causes abrasive particles and water to take away part of the material particles and form a pit morphology called initial morphology. Once a bumpy shape begins to form, abrasive particles and water will carry away a large amount of marble material when they hit the bump or enter the pit interior. Therefore, there is the smallest crushing pit in the smooth zone of the cutting surface because the initial morphology size of the surface is too small. The formation of crushing pits in the later period is mainly due to the impact of abrasive particles rather than the erosion and grinding.

From the above analysis, it is easy to find that different microscopic surface morphology will be appeared with the ASJ cutting plastic and brittle materials. Glass, a kind of complex nonmetallic material composed of many kinds of materials, belong the materials between plastic and brittle. As Fig. 8 and Fig. 12 shown, both obvious scratches and crushing pits can be seen on the cutting surface of the glass. So the process of cutting glass is very complex, combining the plastic damage and the brittle fracturing caused by abrasive erosion. The former causes the slender scratches, while the latter make the rugged crushing pits on the glass cutting surface.

To study the micro surface topography of different materials machined by ASJ and its forming mechanics, this paper carried out a cutting experiment with an ASJ system, and capture the 3D micro surface topography and measured the characteristic size with an ultra-deep three-dimensional microscope. Moreover, the forming mechanics of the micro surface topography carries out in the ASJ cutting plastic and brittle materials. The conclusions can provide theory guidance in material precision processing with ASJ. The details of the conclusions were shown as follow.

(1) Microscale surface topography of the three materials cut by ASJ is different due to their brittleness and toughness. TC4 cutting surface is covered by long and thin scratches with straight down direction, marble cutting surface is covered by pot holes, and glass cutting surface is covered by both of the scratches and pot holes.

(2) The scratches are between 40 and 140 microns in length, 5 and 20 microns in width, and 1 and 5 microns in depth.

(3) The pot holes are between 10 and 40 microns in width, and 1 and 8 microns in depth.

(4) Forming mechanism of the microscale surface topography for the three materials cut by ASJ is different due to their brittleness and toughness. The scratch on TC4 cutting surface is the plastic damage caused by the abrasive particles, and the pot hole is the brittle fracture caused by the abrasive particles. Intermediate between brittle and plastic materials, glass cut by ASJ with both plastic damage and brittle fracture.

Availability of data and materials: The data and materials with experiments and tests in the paper can be provided if needed.

Competing interests: The authors declare no conflict of interest. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by the Opening Project of Shanxi Key Laboratory of Advanced Manufacturing Technology (Grant No. XJZZ201906), Hebei Natural Science Foundation Ecological Wisdom Mine Joint Fund Project (Grant No: E2020402075) and the Natural Science Foundation of Jiangsu Province (Grant No. BK20200657).

Authors' contributions: Conceptualization, Wang F.; methodology, Li X.; soft-ware, Xie H., Wang F.; formal analysis, Wang F.; investigation, Wang F.; resources, Zhao W.; data curation, Li X.; writing-original draft preparation, Wang F.; writing-review and editing, Li X.; visualization, Wang F.; supervision, Zhao W.; project administration, Li X.; funding acquisition, Wang F.

Acknowledgements: Thanks for the Honghe Technologies Co Ltd for the measurement of the microscale surface topography.

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Microscale surface topography and its forming mechanism in abrasive suspension jet processing

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Abstract

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1 Introduction

2 Experiment

3 Results And Discussion

3.1 Microscale surface topography

3.2 Forming mechanism of the microscale surface topography

4 Conclusions

Declarations

References

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