Picosecond laser trepan drilling of vertical sidewalls micro-holes for 631 stainless steel

doi:10.21203/rs.3.rs-2568305/v1

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Picosecond laser trepan drilling of vertical sidewalls micro-holes for 631 stainless steel

https://doi.org/10.21203/rs.3.rs-2568305/v1

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Ultrashort-pulse laser is widely used in drilling micro-hole (diameter<100μm) because of its higher energy density and less thermal effect. However, ultrashort-pulse laser drilling still has some defects, such as taper, poor entrance edge quality, etc. In this paper, the method of trepanning drilling was used to process micro-holes (the entrance diameter was about 55μm) on the stainless steel sheet (the thickness was 80μm) with a picosecond laser. The taper (7.63º ~ -1.8º) of the micro-hole was successfully adjusted by changing the laser angle of incidence (AOI) and laser energy. The experimental results showed that the entrance roundness increased from 85.2% to 94.1% with the increase of the AOI. In addition, the entrance roundness decreased with the rise of the laser fluence. Observing the hole wall surface showed that the remelts and micro-cracks on the hole wall surface increased with the increase of the AOI. The remelts and micro-cracks on the hole wall surface were obviously reduced when the laser fluence was increased. In summary, AOI and laser energy are essential parameters to adjust the geometric size of the hole. By optimizing the AOI and fluence parameters, the taper of the hole can be controlled (from positive taper to negative taper), and the morphology of the entrance and exit of the hole can be improved.

laser drilling

trepan

taper control

picosecond laser

Inkjet printing technology has become one of the indispensable tools in industrial production[1–3]. Hundreds of microholes (usually tens of microns in diameter) are generally distributed on the nozzle board of the inkjet print head[4]. The mainstream microhole machining methods mainly include mechanical processing[5], electrochemical machining[6, 7], micro-EDM[8], and laser machining[9]. Yun et al.[10] machined straight-wall microholes with a depth-to-diameter ratio of 1:1 on 100µm, 50µm, and 25µm thick copper and stainless steel plates, respectively. Zhang et al.[11] used EDM technology to produce high-quality micro-holes with a diameter of 73µm (± 1.1µm) at a thickness of 200µm, but the processing time of a single hole was as long as 1min 10s. Meng et al.[12] successfully fabricated micro blind holes (entrance diameter was 80µm, depth was 100µm) on the alloy substrate using a 20W fiber laser. There are still many problems in these mainstream micro-hole processing methods, such as high equipment costs, low efficiency, severe pollution and high cost of sewage treatment, poor processing quality, and limited processing materials.

With the development of laser technology, the ultrashort-pulse laser (picosecond and femtosecond laser) drilling technology has gradually become the main processing method in the field of high-precision micro-hole processing because of its non-contact, high precision, material-independent, wide processing size, and low thermal effects[13, 14]. The quality of ultrafast laser machining of micro-holes mainly depends on the process parameters, such as scanning speed, scanning path, laser energy, and angle of the incident (AOI)[15–17]. Among them, the laser incident angle and laser energy significantly affect the taper control of the hole[18]. Zai et al.[19] successfully machined an inverted cone hole with a diameter of 120um on a 500um thick stainless steel sheet by using the method of trepan drilling and changing the AOI. Zhu et al.[20] obtained the straight wall hole and improved the sharpness of the entrance edge by changing the laser incident angle. From the above research, it is evident that the laser incident angle has an essential influence on the geometry of the hole, especially the taper of the hole. The effect of laser energy on drilling is mainly reflected in the intensity of pulse ablation of the material and the discharge of melt (such as material removal mechanism and recoil pressure)[21].

In addition, laser-assisted drilling technology (such as auxiliary air blowing, magnetic field, and water jet.) is also one of the crucial ways to improve machining quality. Wang et al. [22] found that high-pressure air blowing and magnetic field-assisted laser drilling could reduce the shielding and absorption effect of steam and plasma on laser energy and greatly improve the drilling efficiency. Li et al. [23] studied the experiment of laser drilling under static water, the taper of the hole was reduced, and the roundness was improved. Besides, the recasting layer and microcracks were significantly reduced. Wang et al. [24] proposed a novel lens ultrasonic vibration-assisted laser machining device to change the laser energy density on the workpiece surface and reduce the taper of laser drilling. In a word, laser-assisted drilling technology could significantly improve the quality of laser drilling. However, these auxiliary processing technologies had higher equipment, scenes, and materials requirements and increased the drilling cost. The air-blowing auxiliary drilling technology was widely used because of its simple equipment, low cost, and high processing quality.

In this research, the effects of laser incident angle and energy on the geometric morphology and micro-morphology of the hole wall in the process of picosecond laser trepan drilling were studied. In the process of laser drilling, the taper of the hole was successfully changed by changing the laser incident angle and energy, and the straight wall hole and negative taper (7.63º~-2.11º) micro-hole with an entrance diameter of fewer than 60µm were successfully machined on a stainless steel sheet with a thickness of 80µm. In addition, by increasing the laser energy, it was found that the remelting material of the hole wall was significantly reduced, indicating that the laser energy directly affects the material removal. In general, this study shows the potential and advantages of picosecond laser in the field of micro-hole processing. It provides a reference for the engineering application of picosecond laser in precision processing.

2.1. Materials

631 stainless steel is widely used in electronic equipment because of its excellent corrosion resistance, wear resistance and processability. The 631 steel sheet (40mm×20mm×0.08mm) was chosen as sample material. The elementary compositions of 631 stainless steel are shown in Table 1. Before processing, the sample was ultrasonically cleaned for 10 minutes to eliminate the scratches and dirt on the sample surface on processing.

Table 1

Chemical compositions of the 631 stainless steel[25].
Element	C	Si	Mn	Al	N	Ni	Cu	Cr	Ti
Wt.%	0.07	0.4	0.6	1.2	0.04	7.0	1.5	17	0.1

2.2. Drilling strategy and device

In this study, a trepan drilling strategy was proposed. As shown in Fig. 1, picosecond laser pulse machined the micro-hole along the circular scanning path (the circular diameter was set to be 30µm) with a speed of 50Hz (rounds per second) in the X-Y plane. At the same time, the laser beam was given a certain angle (AOI = 0º ~ 7º). The pulse repetition rate was kept constant at 500kHz, while the laser energy was adjusted in a range of 7µJ ~ 9µJ. The laser fluence was adjusted in a range of 2.53J/cm² ~ 2.69J/cm². The whole machining path was repeated 30 times and machined 14 holes per test parameter.

The trepan micro-hole machining system was shown in Fig. 1. Trepan micro-hole machining system included a picosecond laser system and PrecSYS 5-axis Micromachining System (SCANLAB GmbH). The linearly polarized laser beam passed through a quarter-wave plate and was output circularly polarized by adjusting the quarter-wave plate. The PrecSYS 5-axis Micromachining System can modify the AOI in a range of -7.5º to + 7.5º. The main parameters of the picosecond laser are listed in Table 1. The whole laser drilling process was carried out in the air environment. The system sprayed auxiliary processing gas (argon) through the nozzle, and the process gas direction was parallel to the laser beam. The environment temperature of the experiments is maintained at 21°C. After processing, the sample was ultrasonically cleaned for 10 minutes. Then, the morphology of the entrance and exit of the hole were observed under an optical microscope (Leica DM750, Leica Germany) and a scanning electron microscope (S-3400N, Hitachi). The profile of the hole was polished with 800, 1000, 1500 and 2000 mesh sandpaper and observed by the scanning electron microscope.

Table 2

Parameters of the picosecond laser
Parameters	value
Average power (W)	≤ 50
Pulse duration (ps)	9
A pulse energy (µJ)	≤ 200
Repetition rate (kHz)	≤ 1000
Beam quality	M² ≤ 1.5
Wavelength (nm)	1064
Beam diameter (µm)	20

2.3. Quality characterizations method

In this work, the quality of the hole is evaluated by observing the entrance and exit morphology and roundness of the hole, the taper of the hole, and the recasting layer of the hole wall. Among them, the section of the hole wall was taken by scanning electron microscope to observe the recast layer of the hole wall. The roundness of the entrance and exit are calculated by Eq. (1) and Eq .(2), respectively[17].

$${\text{HC}}_{\text{entrance}}\text{=}{\text{D}}_{\text{min}}/{\text{D}}_{\text{max}}$$

$${\text{HC}}_{\text{exit}}\text{=}{\text{d}}_{\text{min}}/{\text{d}}_{\text{max}}$$

where ${HC}_{entrance}$ and ${HC}_{exit}$ denotes the roundness of entrance and exit, respectively; ${D}_{min}$ and ${D}_{max}$ are the minimum and maximum of the entrance diameters, respectively; ${d}_{min}$ and ${d}_{max}$ are the minimum and maximum of the exit diameters, respectively.

The taper of the hole is calculated by Eq .(3–5)[26].

$$\text{TA=arctan[}\left(\text{D-d}\right)/\text{2T}\text{]}$$

$$\text{D=}\text{(}{\text{D}}_{\text{max}}\text{-}{\text{D}}_{\text{min}}\text{)}/\text{2}$$

$$\text{d=}\text{(}{\text{d}}_{\text{max}}\text{-}{\text{d}}_{\text{min}}\text{)}/\text{2}$$

where $TA$ denotes the taper of hole, $D$ is the average diameter of entrance, $d$ is the average diameter of exit.

3.1. Evolution of geometric morphology

3.1.1. Ablation morphology under the different angles of incidence

The AOI is an essential parameter for adjusting the geometry of the hole. In this experiment, the AOI of 0º, 1º, 2º, 3º, 4º, 5º, 6º, and 7º were selected. Other parameters were kept constant (the laser fluence was set to 2.58J/cm², the scanning speed was 50Hz, and the repetition time was 30). The experiment result of the geometric morphology of the holes was shown in Fig. 2. As seen from Fig. 2(a), with the increase of the AOI (0º~ 7º), the entrance diameter of the hole was not changed obviously, but the exit diameter increased significantly. Meanwhile, the taper of the hole decreased from 7.63º to -1.8º. Especially when the AOI was 6º and 7º, the taper of the hole was reduced to 0.039º and − 1.8º. It showed that the taper of the hole could be adjusted by changing the laser incident angle, and the machining of straight wall holes and even inverted cone hole could be realized. In addition, the exit roundness and the entrance roundness of the holes were above 85%, and the exit roundness was better than the entrance roundness, as shown in Fig. 2(b). In the process of changing the AOI from 0º to 7º, the entrance roundness increased from 85.2–94.1%. That was, the entrance roundness increased obviously with the increase of the AOI. The laser incident angle has an apparent relationship between the taper of the microhole and the entrance roundness. A high-quality microhole with zero taper and higher entrance roundness can be obtained by selecting the appropriate incident angle.

Figure 3 shows the two-dimensional morphology of the exit and entrance of the hole. It was obvious that the entrance roundness of the hole increased with the increase of the AOI. The entrance edge of the hole was rough, and there was more denudation, while the exit edge was smooth, and there was no obvious erosion pit defect. It was consistent with the experimental phenomenon reported by Mincuzzi al.[27]. In the interaction between the laser pulse and the material, many molten droplets and a large amount of steam are discharged from the entrance[28–30], eroding the edge of the entrance, shielding and absorbing the beam, and resulting in a rough edge. On the other hand, less material is excluded near the hole exit, and the edge quality is much better than that of the entrance, thus forming the found experimental phenomenon. Figure 4 shows the micrograph of holes profiles with different AOI. It could be observed that the taper of the hole wall decreased gradually with the increase of the AOI and even formed a negative taper hole.

3.1.2. Ablation morphology under the different Laser fluence

The laser power significantly influences the profile size of the hole[21]. This section explores the influence of laser power on the entrance and exit morphology and taper of the hole in trepan drilling, and the laser fluence of 2.53J/cm², 2.55J/cm², 2.61J/cm², 2.64J/cm², 2.66J/cm² and 2.69J/cm² were selected. As seen from Section 3.2.1, when the AOI was 6º, the taper of the hole was 0.039º, which was close to the straight wall hole, so the AOI set to 6º in this experiment. Figure 5 shows the data of the experimental results. It could be observed from Fig. 5(a) that the diameter of the holes exit increased from 41.8µm to 55µm with the increase of laser fluence. At the same time, there was no noticeable change trend in the entrance diameter of the holes. There was no obvious change in the inlet diameter, which meant that the ablation boundary at the entrance basically reached saturation when the laser fluence was 2.55J/cm². Combined with Fig. 5(b) and Fig. 6, the taper of the hole decreases (1.55º to -2.11º) with the increase of laser fluence. This is because the directional ablation effect caused by increasing the AOI is enhanced by increasing the laser energy. However, there is also a specific limit to this enhancement effect. It could be seen from Fig. 5(b) that when the laser fluence reached 2.66J/cm², the taper of the hole was − 0.68º, but when the laser fluence was further increased, the taper of the hole was not further significantly reduced.

As shown in Fig. 5(b), the entrance roundness decreased with the increase of laser fluence. On the contrary, the increase of laser fluence does not affect the exit roundness. It could also be observed from Fig. 7 that the entrance edge became rougher with the increase of laser fluence, while the exit edge had no apparent change. One of the important reasons for this phenomenon is the two mechanisms of laser material removal[21, 31]: (1) vaporization removal and (2) phase explosion removal. When the laser energy is low, the laser vaporization removal material is dominant, and a large amount of molten splash will not be produced. However, when the laser energy is increased, the laser pulse strongly interacts with the material, mainly through phase explosion. This process will produce many unsteady melts, which will be discharged from the entrance of the hole under a high-pressure environment. as a result, the morphology near the entrance is eroded and even accumulated near the entrance[32], thus reducing the roundness of the entrance and so on.

3.2. Hole sidewall ablation morphology evolution

Observing the morphology of the hole wall is an important way to show the laser ablation process and explore the laser ablation mechanism. Figure 8 shows the SEM image of holes profiles with different AOI. When the AOI was 0º, the ball-like profile deposited particles could be clearly observed on the hole wall, as shown in Fig. 8(A). The ball-like profile of the particles was due to the circularly polarized laser used in this experiment[33]. These particles were composed of molten materials and spherical particles, with sizes ranging from a few microns to hundreds of nanometers. In addition, the exfoliated recast layer could be seen obviously on the hole wall, and the size of the deposited particles on the surface of the recast layer was much larger than that of the exposed particles on the substrate surface. Similarly, many nano-clusters and residual remelting layers were found on the pore wall surface (AOI were 2º and 5º), as shown in Fig. 8(B) and Fig. 8 (D). These deposited particles were due to the rapid cooling of metal particles and agglomeration on the hole wall [32].

When the AOI were 6º and 7º, the remelting material attached to the hole wall increased obviously (as shown in Fig. 8(E) and Fig. 8(F)) because the hole taper decreased or even reached the negative taper. So, in the same processing time, compared with the positive taper hole, the material removal increased, and the molten material was easier to adhere to the hole wall. Nano-scale pores and micron-scale pits could be observed on the remelting material of the hole wall (as shown in Fig. 8(C), Fig. 8(c), and Fig. 8(F)). There are two forms of ultrashort pulse laser ablation, vaporization and phase explosion, corresponding to gentle ablation and strong ablation, respectively[18, 21, 31, 34]. When the laser intensity is much higher than the ablation threshold of the material, the phase explosion dominates the removal of the material, and the ablation boiling occurs violently at this time. At the same time, due to coaxial blowing and other reasons, the material cools rapidly, thus forming many pores and pits. In addition, many micro-cracks (as shown in Fig. 8(e) and Fig. 8(F)) were found on the remelting material on the hole wall. In the interaction between the laser pulse and the material, the remelting layer bore larger thermal stress, and the inclined laser beam increased the energy distribution of the pulse on the hole wall surface, which made the hole wall surface temperature higher, resulting in greater thermal stress.

Figure 9 shows the SEM image and EDS analysis of holes profiles with laser power of 2.53J/cm², 2.55J/cm² and 2.69J/cm², respectively. It could be observed that when the laser fluence was low (power was 2.53J/cm²), there were a large number of remelting materials on the hole wall, including a large number of deposited metal particles, clusters, and micro-cracks (as shown in Fig. 9(A) and Fig. 9(A1)). When the laser fluence increased (power was 2.55J/cm² and 2.69J/cm²), the remelting material attached to the hole wall decreased significantly (as shown in Fig. 9(B1) and Fig. 9(C1)), and the size of deposited metal particles was smaller than that on the low-power (power was 2.53J/cm²) surface. At the same time, with the decrease of remelting material, the microcracks on the hole wall were significantly reduced. This phenomenon shows that the increased laser fluence makes the melt more easily discharged from the hole. Among them, the steam recoil pressure is one of the main driving forces to push the melt out of the hole, which is related to the surface temperature of the melt[35]. The relationship between the vapor pressure applied to the melt and the surface temperature of the melt is expressed in the following equation[36]

$${\text{p}}_{\text{vapor}}\text{=}{\text{p}}_{\text{0}}\text{∙}\text{exp}\left[\frac{{\text{∆}}_{\text{vap}}\text{H}}{\text{R}}\left(\frac{\text{1}}{{\text{T}}_{\text{vap}}}\text{-}\frac{\text{1}}{{\text{T}}_{\text{s}}}\right)\right] \text{ (6)}$$

where ${\text{∆}}_{\text{vap}}H$ is the enthalpy of vaporization, R is a gas constant, ${\text{T}}_{\text{vap}}$ is the boiling point of the liquid melt at atmospheric ${\text{p}}_{\text{0}}$, and ${\text{T}}_{\text{s}}$ is the melt surface temperature. The vapor pressure on the melt surface increases rapidly when the melt surface temperature increases. Therefore, when the laser fluence is increased, the vapor pressure of the melt in the hole increases rapidly to discharge the melt from the hole. On the contrary, when the laser fluence is lower, the vapor pressure on the surface of the melt is also lower, so that part of the melt can not be discharged from the hole in time before cooling. According to the EDS analysis at point a, b, c and d, the remelting materials were mainly oxides (C, O). Among them, the oxygen content at point b was much higher than that at point a, which indicated that these nanoparticles at point b might have been formed by rapid cooling after the completion of the laser pulse when the coaxial blowing was turned off and had complete contact with the air. On the other hand, the oxygen content of the remelting material at point a was lower, which may be due to the coaxial blowing of argon during the processing process, which blocked most of the air and leads to low oxygen content. As can be seen from Fig. 9 (C1), the remelting layer where point d was located has an obvious fracture, indicating that spalling has occurred. In addition, there were no nanoparticles in the region of point b in Fig. 9 (B1), so the remelting layer of point d may have covered this area, and the oxygen content of point d was significantly higher than that of the material at point c.

3.3. Machining mechanism of trepan drilling with different beam angles of incidence

Based on above comparison between different AOI of laser beam, it can be found that the taper of holes can be controlled by the AOI and the laser power. Figure 10 presents underlying ablation mechanisim at the different laser beam angles of incidence. When the laser beam was incident vertically on the sample surface, the side of the ablation pit showed a certain taper because of the Gaussian laser beam (the middle energy of the laser spot was the highest and gradually decreased around) and the defocusing effect. At the same time, due to the vapor pressure and blowing effect on the melt surface, the melt was ejected from the entrance along the hole wall. There were two mechanisms of laser ablation: evaporation and phase explosion. When the laser energy was much larger than the ablation threshold of the material, the phase explosion removal was dominant, and a large amount of molten metal was produced. Due to the short action time of the laser pulse, the melting would cool off quickly, and the undischarged part adhered to the surface of the hole wall to form a recasting layer and deposited metal particles. When the laser beam was given a certain incident angle, the inclined beam increased the amount of radial ablation of the material. The laser beam was still Gaussian light, and when the tilt angle of the beam was appropriate, the taper of the hole wall was greatly reduced. As shown in Fig. 10, the taper of the outer wall of the pit after ablation by an inclined beam was greatly reduced. After increasing the laser energy, the amount of laser pulse ablation increased, which strengthened the ablation direction of the beam (mainly enhanced the radial ablation amount), further reduced the taper of the hole wall, and even realized the processing of the inverted cone hole.

In addition, the increase of laser energy also increased the temperature of the melt during ablation. The higher surface temperature of the melt increased the vapour pressure above the surface of the melt. In the same processing time, compared with the ablation with lower energy, the melt was discharged more easily from the outlet, dramatically reducing the residual melt material on the hole wall and thus reducing the thickness of the recast layer.

In this paper, straight-wall holes and negative taper holes with a diameter of 50 to 60µm were fabricated on a stainless steel sheet with a thickness of 80µm by picosecond trepan drilling. The relevant conclusions are as follows:

By adjusting the incident angle (0º ~ 7º), the taper of the hole decreased gradually and reached a negative taper (7.63º ~ -1.8º). The AOI changed the ablation direction of the laser and increased the radial ablation of the laser pulse, compensated the taper of the Gaussian beam, and then adjusted the taper of the hole. In addition, with the increase of the incident angle, the melt attached to the hole wall increases obviously, and many spherical particles, remelting materials and microcracks appear. With the decrease of the taper of the hole, the amount of material removed increased in the same processing time, thus increasing the amount of remelting material attached to the surface of the hole wall.
With the increased laser power, the taper of the hole decreased obviously (1.55º ~ -2.11º). At the same time, the entrance roundness of the hole also reduced with the increase of laser fluence (94.2 ~ 87.3), but the hole entrance diameter and exit roundness changed little. The increase in laser fluence strengthened the direction of laser beam ablation, further increased the laser radial ablation, expanded the radial ablation range, increased the outlet diameter, and then reduced the taper of the hole. In addition, as the taper decreased and the removed material increased, a large amount of spatter eroded the edge of the entrance, shielding and absorbing the beam, resulting in a rough edge. With the increase of laser fluence, the temperature on the surface of the melt increased, which in turn increased the vapor pressure on the surface of the melt, which made it easier for the melt to discharge out of the hole, and the remelts and micro-cracks on the surface of the hole was reduced.

CRediT authorship contribution statement

Zehui Gu: Conceptualization, Formal analysis, Investigation, Writing - review and editing; Yuyang He: Formal analysis, Investigation, Data curation; Jinghu Ji: Writing - review and editing, Resources; Yonghong Fu: Conceptualization, Funding acquisition, Resources, Supervision, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (Grant numbers 52075225), Science and Technology Planning Social Development Project of Zhenjiang City (CN) (Grant numbers 2020004).

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Editorial decision: Major Revisions Needed
15 Jul, 2023
Reviewers invited by journal
11 Jun, 2023
Reviewers agreed at journal
21 Feb, 2023
Editor assigned by journal
09 Feb, 2023
First submitted to journal
09 Feb, 2023

You are reading this latest preprint version

Picosecond laser trepan drilling of vertical sidewalls micro-holes for 631 stainless steel

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experiment Procedure

2.1. Materials

2.2. Drilling strategy and device

2.3. Quality characterizations method

3. Results And Discussion

3.1. Evolution of geometric morphology

3.1.1. Ablation morphology under the different angles of incidence

3.1.2. Ablation morphology under the different Laser fluence

3.2. Hole sidewall ablation morphology evolution

3.3. Machining mechanism of trepan drilling with different beam angles of incidence

4. Conclusions

Declarations

References

Status:

Version 1