Measurement of the inner surface roughness of laser-processed micro-holes

1 Introduction

The processing of micro-holes has always been a difficult problem in mechanical manufacturing, and researchers have conducted a lot of research on this issue. Currently, the methods that can be used to process micro-holes include: mechanical processing, laser processing, electrical discharge machining, ultrasonic processing, electron beam machining and composite processing [1]. There have been many reports on the diameter range of micro-holes that can be processed by various methods, but there are relatively few studies on the roughness of the side walls of the processed micro-holes. With the development of science and technology and the increasing precision, integration and miniaturization of cutting-edge products, micro-holes are increasingly used in the fields of automobiles, electronics, optical fiber communications and fluid control. These applications have also put forward higher requirements for the processing of micro-holes. For example, the nozzle used in the fused deposition rapid prototyping machine is a high-precision micro-hole, which requires not only accurate aperture size, but also smooth hole wall, which is conducive to the accurate control of melt extrusion and fluid resistance of the micro-hole during extrusion. This paper measures the roughness of the side walls of micro-holes that can be used in the nozzle of the rapid prototyping machine, and further studies the influence of the roughness of the micro-holes on the working quality of the nozzle used in the fused deposition rapid prototyping machine. The results of this study can also provide a reference for the study of similar micro-hole surface roughness in other industries such as spinning and inkjet printers.

Rapid prototyping (RP) technology is an advanced manufacturing technology that emerged in the late 1980s[2]. Rapid prototyping technology can be used to quickly evaluate and modify product designs, respond to market demands in a timely manner, and improve the competitiveness of enterprises. Fused deposition modeling (FDm) is a rapid prototyping manufacturing process that uses a hot melt nozzle to extrude semi-fluid materials layer by layer along a path controlled by CAD layered data, and then accumulate and solidify to form the entire prototype or part[3]. The common nozzle diameter used for FDm is about 0.2mm, which belongs to the micro-hole range. At present, such micro-holes can be processed by methods such as electric spark, high-speed drilling and laser. Laser processing technology has developed rapidly in recent years. Now, lasers can be used to process micro-holes with a diameter of 0.3mm and a depth-to-diameter ratio of 50:1 on ruby ​​and sapphire[4]; micro-holes with a diameter of 0.1 to 0.3mm can also be easily drilled using a focused extremely fine laser beam[5]. Considering the advantages of micro-hole laser processing technology and its increasing application trend, this paper focuses on the measurement of the inner surface roughness of micro-holes processed by laser.

2 Measurement experiment

(1) Determination of the micro-hole to be measured

The depth of the micro-hole to be measured is 4mm; the hole diameters are 0.2mm, 0.25mm and 0.3mm respectively; 3 micro-holes of each diameter are processed in the experiment.

(2) Measurement method

For through holes with a hole depth of less than 1mm, the roughness of the inner wall of the hole can be roughly observed with the help of a magnifying glass. This study uses a reflection microscope to directly observe the inner surface of the hole mouth as a control for the actual roughness test. For the inner wall roughness of micro-holes with a hole depth of 4mm, it is obviously impossible to accurately measure it using this method. Since the diameter of the micro-hole to be measured is small, the controllable light source cannot accurately penetrate into the hole, so it is impossible to measure it using the method of light interference principle. If the direct contact measurement method is used, although the probe diameter is smaller than the inner diameter of the micro-hole, the subsequent part connected to it is too large, so the probe cannot penetrate into the micro-hole for direct measurement. Therefore, the author adopts the splitting method for the micro-holes and uses a profilometer with a taper of 60° to directly measure the inner surface of the micro-holes exposed after splitting to obtain accurate data.

There are two methods for splitting micro-holes: one is to split the micro-holes after processing, and the other is to drill holes along the joint seam on two smooth plates that are tightly connected. Due to the small hole diameter, the splitting after processing should be thin plate cutting. In this case, laser cutting should be used to obtain higher cutting accuracy. However, due to the large cutting spot diameter (for example, when the thin plate thickness is 5mm and the required cutting speed is 1.5m/min, the spot diameter is 0.2mm[6]), which is close to the diameter of the processed micro-hole, the inner surface of the micro-hole remaining after cutting is too small to measure the roughness. At the same time, in order to protect the inner wall of the micro-hole from the impact of splashes during splitting, wax or other substances are usually injected into the micro-hole before splitting to protect the inner wall of the hole. However, the influence of the protective material on the roughness measurement result of the inner wall of the micro-hole cannot be evaluated at this time. Therefore, it is necessary to be very cautious when using this splitting process to avoid measurement difficulties. In view of the above reasons, this test adopts the second micro-hole processing method: after processing two flat plates, close them tightly and punch the saddle hole along the contact surface of the two plates, then separate the two plates and directly measure the inner surface of the micro-hole exposed to the outside. The roughness of the inner wall of the micro-hole measured by this method can accurately reflect the actual processing situation of the inner surface of the micro-hole.

When drilling, the two plates are clamped with flat-nose pliers along their entire length to avoid bending or uneven force on the plates during laser drilling. A microscope magnification device with a magnification of 57 times is provided on the laser drilling device, which can clearly observe the contact surface of the two plates, so the relative position of the laser beam and the contact surface of the plate can be better guaranteed and the saddle hole can be punched along the contact surface. The verticality of the contact surface of the plate and the processing workbench can be guaranteed by adjustment.

(3) Experimental specimens and equipment

The laser drilling machine model is JD-50, with a laser voltage of 1000V, a laser pulse width of 300μs, and a laser wavelength of 1.06μm; the test plate material is 45# steel, and the surface roughness of its grinding contact surface is 3.2μm. The 1# test plate after laser drilling is shown in Figure 1. Two test plates were processed with three micro holes of 0.2mm, 0.25mm and 0.3mm in diameter respectively. The measuring instrument is a British Talysurf6 roughness measuring instrument, with a stylus radius of 2μm, a stylus pressure of 1mN, and a straightness of 0.5μm within 150mm from the left end of the bracket to the right. The micro-hole mouth condition and the inner surface of the hole were observed using a reflection microscope with a magnification of 450 times.

Figure 1 Schematic diagram of the 1# test plate

3 Experimental results and analysis

3.1 Measurement results

The surface roughness of each hole on the two test plates after laser drilling was measured, and the measurement results are listed in Table 1 and Table 2 respectively (the value in brackets after the hole diameter in the table is the hole number of the laser drilling):

3.2 Result Analysis

(1) The surface roughness detection methods usually include: comparison method, impression method, light section method, interference method and pin trace method. The applicable ranges of each method are different. The applicable ranges of the above methods are: comparison method: Ra 50 μm ~ 0.2 μm; impression method: Ra 50 μm ~ 3.2 μm; light section method (using light section microscope): Ra 50 μm ~ 3.2 μm; interference method (using interference
microscope): Ra 0.1 μm ~ 0.032 μm; pin trace method (using profilometer): Ra 3.2 μm ~ 0.025 μm [7]. This study uses the pin trace method, and the profilometer used has a measurement range of Ra 0.01 μm ~ 20 μm. According to the final measurement results (see Table 1 and Table 2), all the measured Ra data fall within the measurement range of the profilometer selected for this experiment, and 83% of the data fall within the range of Ra3.2μm to 0.025μm. This shows that the needle tracing method used in this experiment and the range of the selected profilometer are appropriate.

(2) According to the national standard GB10610-1998, it can be seen that the reliability of determining whether the inspected surface meets the technical requirements and the accuracy of the average value of the surface roughness parameters obtained from the same surface depend on the number of sampling lengths and the number of evaluation scales within the evaluation length. The minimum evaluation length is equal to the sampling length. The evaluation length

taken in this article is 0.25mm, and the evaluation direction is along the axis of the microhole. The average value of the surface roughness parameter can be calculated according to the following formula:

In the formula, k is the number of evaluation lengths,
Rj is the surface roughness parameter value determined within each sampling length,
and N is the number of sampling lengths within one evaluation length.

Taking hole No. 4 as an example, the surface roughness values ​​Ra within each sampling length of hole No. 4 of plate No. 1 were measured to be 1.78μm, 1.58μm, 1.59μm, 1.38μm and 1.63μm, respectively. Substitute each value into formula (1), and the calculated result (1.59μm) is listed in Table 3. Similarly, the other holes of plates No. 1 and No. 2 were measured and calculated, and the results are listed in Table 3. [page]

According to research data, the surface roughness Ra obtained by laser processing is 1.6 to 0.4 μm [8]. As shown in Table 3, the average value of the roughness parameter tested in this experiment is about 3.2, and the surface feature corresponding to this value is micro-visible processing marks [7], which is basically consistent with the results observed by reflection microscope. Since the

diameter of the micro-hole processed by laser is greater than 0.5 mm, the trepanning method should be used to consider the processing efficiency. Therefore, the diameter of the micro-hole in this experiment is close to the larger value of the hole diameter formed by laser one-time processing.

Considering the requirements for the number of samples when measuring the roughness, the depth-to-diameter ratio of the micro-hole in this experiment is up to 20. In actual applications, since the hole depth has a great influence on the flow resistance of the fluid flowing through the micro-hole, the depth-to-diameter ratio of micro-holes such as the nozzle used in the fused deposition rapid prototyping machine rarely reaches the value of this experiment. When processing micro-holes with the same diameter but smaller depth-to-diameter ratio, due to the smaller penetration force required, a smaller diameter spot can be used for processing, so the processing accuracy will be higher. Therefore, under normal circumstances, when laser processing micro holes of similar diameters, the results obtained in this experiment can be used as the range of surface roughness that can be achieved for the side wall of the hole.

(3) Laser processing can achieve good roughness accuracy locally. As can be seen from Table 1, the Ra measured for hole No. 6 on plate No. 1 shows that the hole has good roughness accuracy. From the local continuous roughness measurement results of hole No. 4 on plate No. 1 shown in Figure 2, it can be seen that the peak value of Ry is 13.6μm within the selected 1.9mm range and the fluctuation range of the roughness curve is not large, which is consistent with the measured Ry results of 8.1, 10.9, 10.8, 10.7, and 8.8 for the hole;

Figure 2 Roughness curve

A reflection microscope with a magnification of 450 times was used to observe the two plates at the laser drilling entrance and the micro-hole splitting plane, and a direct image of the inner surface of the micro-hole was obtained on the computer display (Figure 3 is the observation image of the No. 7 hole of the 2# plate). The results show that when the micro-holes are directly processed by laser, the roughness of most surfaces is uniform and the values ​​are close. For the local abnormalities that occur in some places, the causes of their formation need to be further studied, and solutions and avoidance methods should be found.

Figure 3 Reflection microscope observation results

(4) Since the principle of laser processing to remove materials is to focus the laser into a very small spot through an optical system, thereby obtaining extremely high energy density and extremely high temperature, the material is instantly melted and vaporized, forming pits on the surface of the workpiece. At the same time, the melt is ejected at a very high speed under the pressure of the metal vapor generated by the vaporization. This processing mechanism makes it difficult to determine where the maximum roughness will appear in the micro-hole. The experimental results also confirmed this point. The experimental results also showed that the minimum roughness did not appear at the entrance of the laser drilling. The author believes that the hole mouth during processing becomes the passage for the melted and vaporized metal to discharge the micro-hole during subsequent processing. These etched materials will obviously affect the roughness of the hole mouth, so the roughness value at this place cannot be the smallest.

(5) This experiment also measured Ry. The results showed that the maximum and minimum values ​​of Ry basically appeared at the corresponding Ra.

4 Conclusion

(1) The surface roughness of the side wall of the laser-processed micro-hole can be directly measured using the dissection method. This roughness should be measured using a profilometer.
(2) The roughness value of the side wall of the laser-processed micro-hole obtained in this experiment is within 3.2, which can be used as the roughness range that can be guaranteed by laser processing the side wall of micro-holes of similar diameter under normal circumstances.
(3) When laser processing micro-holes, the roughness of most of the inner surfaces of the holes is uniform, and the causes and locations of local anomalies in some places need further study.
(4) The location of the maximum value of the surface roughness of the hole is difficult to determine, and the minimum value does not appear at the entrance of the laser drilling.
(5) The extreme value of Ry basically appears at the corresponding Ra.

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