WO2024087248A1

WO2024087248A1 - Ultrafast laser micro-hole machining method for in-situ hole trimming

Info

Publication number: WO2024087248A1
Application number: PCT/CN2022/130050
Authority: WO
Inventors: 张丰云; 孙树峰; 王茜; 王津; 王萍萍; 庞泳; 王雅迪; 邵晶; 张健
Original assignee: 青岛理工大学
Priority date: 2022-10-27
Filing date: 2022-11-04
Publication date: 2024-05-02
Also published as: CN115446480A

Abstract

An ultrafast laser micro-hole machining method for in-situ hole trimming, which relates to the field of laser micro-hole machining. For the problems of it being difficult to optimize micro-hole geometric shapes and recast layers adhering to hole walls in laser drilling processes, through hole machining is carried out firstly, then reaming modification is carried out, and the geometric shape of a through hole is optimized in two steps, such that a micro-hole with high roundness and small taper and a large-area micro-hole array can be machined. Therefore, the micro-hole size accuracy is ensured, and the machining efficiency is also improved.

Description

An in-situ hole repair ultrafast laser micro-hole processing method

The present invention claims the priority of the Chinese patent application filed with the Chinese Patent Office on October 27, 2022, with application number 202211324552.9 and invention name “A method for in-situ hole repair ultrafast laser micro-hole processing”, the entire contents of which are incorporated by reference in the present invention.

Technical Field

The invention relates to the field of laser micro-hole processing, and in particular to an in-situ hole repair ultrafast laser micro-hole processing method.

Background technique

Laser micro-hole processing has the advantages of fast speed, high efficiency, low cost, high precision and green environmental protection. It is suitable for complex processing of turbine blade materials such as high-temperature alloys and ceramic composites. During the laser processing process, recast layers and oxide layers are prone to appear on the hole wall, and microcracks are caused, which spread directly to the material matrix, thus affecting the performance and life of the turbine blade. The pulse time of ultrafast laser is extremely short, reaching the order of picoseconds and femtoseconds. Under the same single pulse energy, it can obtain a greatly improved peak power, which fundamentally changes its interaction mechanism with the material. The material removal process is no longer a hot melt process. In theory, high-precision, heat-affected zone-free, recast layer-free, and microcrack-free high-quality micro-hole processing can be achieved. However, experiments have found that ultrafast lasers cannot fully achieve true "cold processing". Simply reducing the pulse width to picoseconds or even femtoseconds is still difficult to effectively prevent the generation of heat-affected zones, recast layers, pores and micro-pits, and problems such as micro-hole shape deformation and large taper caused by polarization may occur, and the processing efficiency is low.

At present, the processing size of microholes is constantly decreasing, from micron to submicron and even nanometer. The laser drilling process has the problem of difficulty in optimizing the geometry of microholes and the problem of sticky recast layer on the hole wall. In the actual application of microholes, it is difficult to take into account both processing accuracy and efficiency at different scales.

Summary of the invention

The purpose of the present invention is to provide an in-situ hole repair ultrafast laser micro-hole processing method to address the defects of the prior art. The through-hole processing is first performed and then the hole expansion modification is performed. The through-hole geometry is optimized by the two-step in-situ hole repair method, and micro-holes with high roundness and small taper and large-area micro-hole arrays can be processed, thereby improving the processing efficiency while ensuring the micro-hole size accuracy.

In order to achieve the above objectives, the following scheme is adopted:

An in-situ hole repair ultrafast laser micro-hole processing method, comprising:

Through-hole processing: The laser is focused on the upper surface of the target material, and the through-hole is processed by circular scanning and axial feeding;

Adjust the laser focus to the middle of the target material in the through hole;

Hole expansion modification: The lower part and exit area of the through hole are processed by circular scanning and axial feed to modify the through hole morphology and size so that the through hole exit roundness, through hole wall morphology and through hole taper meet the set requirements.

Furthermore, in the through hole processing stage and the hole expansion modification stage, a nested circular cutting scanning method is adopted to process the through hole.

Furthermore, the inner circle diameter of the laser processing area in the hole expansion and modification stage is larger than the inner circle diameter of the laser processing area in the through hole processing stage, thereby reducing the number of annular scanning circles in the hole expansion and modification stage.

Furthermore, the outer diameters of the laser processing areas corresponding to the hole expansion and modification stage and the through hole processing stage are equal, and are both equal to the set diameter of the through hole.

Furthermore, in both the through-hole processing stage and the hole expansion and modification stage, the morphology and size of the through-hole entrance and exit are detected, and subsequent steps are carried out after the morphology and size meet the set requirements.

Furthermore, the axial feeding is performed step by step, and after completing one axial circular scanning step, the next circular scanning step is performed.

Furthermore, in the hole expansion and modification stage, the number of single-layer annular cutting scans and the spot overlap rate are increased to modify the through-hole exit roundness, the through-hole wall morphology and reduce the through-hole taper.

Furthermore, pre-processing preparation is performed before the through-hole processing stage:

Fix the target on the working platform and adjust the position so that the upper surface of the target is located at the laser focus;

Adjust the laser ring cutting scanning process parameters and use the scanning galvanometer to prepare for through-hole processing.

Furthermore, the laser annular cutting scanning process parameters include scanning inner circle diameter, scanning outer circle diameter, scanning spacing, laser power, scanning speed, scanning times, and axial feed spacing.

Furthermore, after the hole expansion and modification of a through hole is completed, the target material is moved to the next through hole processing position, the laser is adjusted to the initial processing position, and the through hole processing and hole expansion and modification are repeated.

Compared with the prior art, the present invention has the following advantages and positive effects:

(1) In view of the difficulty in optimizing the microhole geometry and the problem of sticky recast layer on the hole wall in the laser drilling process, through-hole processing is first performed and then hole expansion modification is performed. The through-hole geometry is optimized by a two-step in-situ hole repair method, and microholes with high roundness and small taper and large-area microhole arrays can be processed, which improves the processing efficiency while ensuring the microhole size accuracy.

(2) During the through-hole processing stage, it is necessary to ensure that the through-hole is processed and the entrance roundness reaches the preset size to obtain the fastest processing efficiency, so as to avoid the impact of the processing effect of the hole repairing stage due to the small hole opening hindering the laser incidence.

(3) In the hole expansion and modification stage, the lower half of the hole is mainly modified. The hole is initially formed in the through-hole processing stage. The scanning diameter of the inner circle of the annular nested circle is reasonably selected according to the size of the outlet to obtain the maximum laser utilization rate and accelerate the processing efficiency.

(4) The problem of micro-hole shape deformation and large taper caused by low spot roundness and galvanometer scanning of ultrafast laser is solved. By reducing the number of circular cutting circles in the hole expansion and modification stage, the repeated empty scanning and excessive ablation of the laser beam on the processed area are avoided, which effectively improves the laser utilization rate and ultrafast laser micro-hole processing efficiency, and provides a technical reference for the application of ultrafast laser in micro-nano processing.

(5) There is no need to replace the processing platform and laser source during the through-hole processing stage and the hole expansion modification stage. This can be achieved by optimizing the process flow and parameters, avoiding secondary positioning and processing errors. It is simple and feasible and improves processing efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

FIG. 1 is a schematic flow chart of the in-situ hole repair ultrafast laser micro-hole processing method in Examples 1-4 of the present invention.

FIG. 2 is a schematic diagram of a device for implementing laser micro-hole processing in Examples 1-4 of the present invention.

FIG. 3 is a schematic diagram of the processing principle of the through-hole processing stage in Examples 1-4 of the present invention.

FIG. 4 is a schematic diagram of the processing principle of the hole expansion and modification stage in Examples 1-4 of the present invention.

FIG. 5 is a schematic diagram of a laser single-layer scanning path during the through-hole processing stage in Examples 1-4 of the present invention.

FIG6 is a schematic diagram of the laser single layer scanning path during the hole expansion and modification stage in Examples 1-4 of the present invention.

FIG. 7 is a schematic diagram of the axial feed of the laser circular cutting process in Examples 1-4 of the present invention.

Among them, 1-ultrafast laser, 2-optical gate, 3-first reflector, 4-beam expander, 5-second reflector, 6-scanning galvanometer, 7-focusing mirror, 8-target material, 9-processing platform, 10-CCD camera, 11-processing control system.

Detailed ways

Example 1

In a typical embodiment of the present invention, as shown in FIGS. 1 to 7 , an in-situ hole repair ultrafast laser micro-hole processing method is provided.

Laser micro-hole processing has been widely studied as a new processing method and is also one of the important application areas of laser processing technology. The pulse time of ultrafast laser is extremely short, reaching the order of picoseconds and femtoseconds. Under the same single pulse energy, it can obtain greatly improved peak power, which fundamentally changes the interaction mechanism between it and the material. The material removal process is no longer a hot melt process. In theory, it can achieve high-precision, high-quality micro-hole processing without heat-affected zone, recast layer, and micro-cracks. However, experiments have found that ultrafast lasers cannot fully achieve true "cold processing". Simply reducing the pulse width to picoseconds or even femtoseconds is still difficult to effectively prevent the generation of heat-affected zones, recast layers, pores and micro-pits, and problems such as micro-hole shape deformation and large taper caused by polarization may occur, resulting in low processing efficiency.

At the same time, with the continuous improvement of the manufacturing level, the structural design requirements are constantly increasing, and the processing size of microholes is constantly decreasing, from micron level to submicron level, and even nanometer level. There are still many unresolved problems in laser drilling technology, two of which are the optimization of microhole geometry and the sticky recast layer on the hole wall. In addition, in the actual application of microholes, it is difficult to take into account both processing accuracy and efficiency at different scales, which restricts the development of ultrafast laser in the direction of microhole processing.

Based on this, this embodiment provides an in-situ hole repair ultrafast laser micro-hole processing method. Through the two-step in-situ hole repair strategy, micro-holes with high roundness and small taper and large-area micro-hole arrays can be processed, which improves the processing efficiency while ensuring the micro-hole size accuracy.

First, through-hole processing is performed, the laser is focused on the upper surface of the target, and the through-hole is processed by circular scanning and axial feeding;

Then perform laser adjustment to adjust the laser focus to the middle of the target material in the through hole;

Finally, the hole expansion modification is carried out, and the lower part and the exit area of the through hole are processed by circular cutting scanning and axial feeding to modify the through hole morphology and size so that the through hole exit roundness, through hole wall morphology and through hole taper meet the set requirements.

The in-situ hole repair ultrafast laser micro-hole processing method in this embodiment is described in detail below with reference to the accompanying drawings.

See Figure 2, which is a schematic diagram of a device capable of executing the in-situ hole repair ultrafast laser micro-hole processing method of this embodiment, using an existing laser micro-hole processing device.

The system includes an ultrafast laser 1, a shutter 2, a first reflector 3, a beam expander 4, a second reflector 5, a scanning galvanometer 6 and a focusing mirror 7 which are sequentially arranged along the optical path. The scanning galvanometer 6 can output an annular scanning laser, and after passing through the focusing mirror 7, it acts on a target material 8 on a processing platform 9. A CCD camera 10 is arranged at the laser processing position to obtain an image of the processing position. The CCD camera 10 and the processing platform 9 are respectively connected to a processing control system 11, and the action of the processing platform 9 is controlled by the processing control system 11.

Referring to FIG. 1 , in order to improve the shape and size accuracy of micro-hole processing and the processing efficiency of ultrafast laser, a two-step in-situ hole repair laser ring cutting micro-hole processing strategy is adopted in this embodiment, which mainly includes a through-hole processing stage and a hole expansion modification stage. The specific steps are as follows:

S1: Fix the workpiece to be processed on the working platform, and move the working platform after positioning so that the workpiece is located in the focal plane of the laser beam;

S2: According to the material properties and sample thickness, set the optimal laser cutting process parameters and use the scanning galvanometer for laser processing;

S3: Through-hole processing stage: A nested circular cutting scanning method is adopted, as shown in FIG5 , and the laser is focused on the surface of the target material to process a preliminary through-hole by axial feeding, as shown in FIG3 ;

S4: Use a real-time monitoring device to detect the morphology of the micropore entrance and exit, and proceed to the next step when the requirements are met;

S5: The laser beam descends along the Z axis to the center of the target;

S6: Hole expansion and modification stage: By increasing the number of single-layer scanning times and the spot overlap rate, the lower part of the microhole and the exit morphology and size are modified, as shown in Figures 4 and 6, to achieve the purpose of modifying the exit roundness, hole wall morphology and reducing the microhole taper. When the exit morphology meets the requirements, proceed to the next step;

S7: Move the target to the next processing position, adjust the laser beam to the initial processing setting, and repeat the micro-hole processing.

The upper, middle and exit stages in this embodiment are all relative to the feed direction during ultrafast laser micro-hole processing as shown in Figure 2. The upper part is the surface position of the target material close to the scanning galvanometer side, and this position forms the entrance of the through hole after processing; the lower part is the surface position of the target material away from the scanning galvanometer side, and this position forms the exit of the through hole after processing; the area between the entrance and exit of the through hole is the middle part of the target material.

During the processing, after the target material is penetrated in the through-hole processing stage, the hole expansion and modification stage is entered. The hole expansion and modification stage can be divided into single step or multi-step, which is determined by the thickness of the target material. For thicker targets, multiple hole expansion and modification are required to make the through-hole exit roundness, through-hole wall morphology and through-hole taper meet the requirements. For thinner targets that can meet the requirements through a single hole expansion and modification, the hole expansion and modification stage can be completed in one step to improve processing efficiency.

Among them, when setting the optimal laser circumferential cutting process parameters according to the material properties and sample thickness, the laser processing parameters are set according to the following principles: in the through-hole processing stage, it is necessary to ensure that the through-hole is processed and the entrance roundness reaches the preset size to obtain the fastest processing efficiency, and avoid hindering the laser incidence due to the small hole opening, which affects the processing effect in the hole repairing stage.

In addition, excessive repeated processing and feeding cannot effectively modify the outlet morphology, but will cause over-ablation of the inlet surface and hole wall to generate a recast layer and expand the outlet size. Therefore, in this embodiment, the hole expansion modification is performed from the middle of the through hole and the outlet position, as shown in FIG4 .

Since the scanning galvanometer processing causes the micro-hole shape to be deformed, especially the outlet is prone to be elliptical, therefore, the lower half of the hole is mainly modified in the hole expansion modification stage, as shown in Figure 4 and Figure 6. Through the through-hole processing stage, the through-hole has been initially formed. According to the size of the outlet, the scanning diameter of the inner circle of the circumferential nesting circle is reasonably selected to omit the scanning path of the formed hole part, obtain the maximum laser utilization and speed up the processing efficiency.

In the process of maintaining axial feeding scanning, increasing the number of single-layer scanning is an effective means of modifying the hole type. Changing the number of single-layer scanning means adjusting the number of circumferential scanning in the step after one axial feeding step, as shown in Figure 7; at least one circumferential scanning is performed after one axial feeding step. In the hole expansion modification stage, the number of circumferential scanning after one axial feeding step can be appropriately increased. In addition, the laser scanning speed can be adjusted to adjust the spot overlap rate, and the laser power can be adjusted for different materials to improve the hole repair effect.

Adjust the laser beam to the initial processing position, adjust the laser beam focus position back to the target surface, laser scanning speed, single layer scanning time and other process parameters, and select automatic focus or manual focus according to the flatness of the target surface to meet the new micro-hole processing process.

Using a scanning galvanometer for micro-hole processing, two layers can be established during the drawing process to achieve in-situ secondary processing of the first step of through-hole processing and the second step of modification and expansion of the hole, which can reduce the secondary operation time and speed up the processing efficiency. The purpose of in-situ hole repair can be achieved by using two-step circular cutting laser processing, without changing the processing platform and laser source. It can be achieved by optimizing the process flow and parameters, avoiding secondary positioning and processing errors, and is simple and feasible.

The problems of microhole shape deformation and large taper caused by low spot roundness and galvanometer scanning of ultrafast laser are solved. By reducing the number of circular cutting circles in the second step, repeated empty scanning and excessive ablation of the processed area by the laser beam are avoided, which effectively improves the laser utilization rate and ultrafast laser microhole processing efficiency, and provides a technical reference for the application of ultrafast laser in micro-nano processing.

When setting the processing parameters, the number of feeds, single-layer scanning time and the number of nested circles of circular scanning are the key to ensuring the micro-hole size processing accuracy and processing efficiency. They need to be repeatedly verified and ensured to obtain the optimal values.

Example 2

In another typical embodiment of the present invention, as shown in FIGS. 1 to 7 , an in-situ hole repairing ultrafast laser micro-hole processing method is provided.

A solid femtosecond laser with a wavelength of 1035nm and a power of 40W is used as the laser processing source 1 to process nickel-based high-temperature alloy micropores with a thickness of 1.30mm and an aperture of 0.60mm. After ultrasonic cleaning and drying, the nickel-based high-temperature alloy target 8 is fixed on the working platform 9, and the processing parameters are set to perform through-hole processing and in-situ hole repairing work in sequence. The first step of through-hole processing: the focus is set on the material surface, as shown in Figure 3; the inner circle diameter of the nested circle scanning is 0.10mm and the outer circle diameter is 0.60mm (as shown in Figure 5); the axial feed is 4 times, and the axial feed distance is set to 0.15mm (as shown in Figure 7); the scanning spacing is 0.02mm, the laser power is 24W, the laser scanning speed is 300mm/S, and the number of single-layer scans is 80 times. After the first step of processing is completed, a preliminary test hole with a taper of about 3° and an entrance roundness greater than 99.8% can be obtained, and the exit roundness is about 85%. The second step is the in-situ hole expansion stage: focus on processing the lower half of the microhole, improve the exit roundness and reduce the microhole taper. First, the processing focus is lowered to the middle of the target, as shown in Figure 4. At this time, the through hole has been formed. To improve the processing efficiency, the inner diameter of the nested circle scanning is 0.40mm (as shown in Figure 6), and the number of single-layer scanning is increased to 120 times. Other parameters remain unchanged. After the processing is completed, the exit roundness can reach more than 99.5%, and the taper is 0.8~1.1°. High-quality nickel-based high-temperature alloy microholes have no defects such as heat-affected zones, recast layers and microcracks. Compared with the processing results of the one-step unrepaired process, the exit roundness of the microhole processed by the modified invention method can be increased by 10%, the taper can be reduced by 4°, and the efficiency can be increased by 30%.

Example 3

A solid femtosecond laser with a wavelength of 1035nm and a power of 40W is used as the laser processing source 1 to process nickel-based high-temperature alloy micropores with a thickness of 2.20mm and an aperture of 0.60mm. After ultrasonic cleaning and drying, the nickel-based high-temperature alloy target 8 is fixed on the working platform 9, and the processing parameters are set to perform through-hole processing and in-situ hole repairing work in sequence. The first step of through-hole processing: the focus is set on the material surface, as shown in Figure 3; the inner diameter of the nested circle scanning is 0.06mm and the outer diameter is 0.50mm (as shown in Figure 5); the axial feed is 3 times, and the axial feed distance is set to 0.1mm (as shown in Figure 7); the scanning spacing is 0.02mm, the laser power is 20W, the laser scanning speed is 350mm/s, and the number of single-layer scans is 80 times. After the first step of processing is completed, a preliminary test hole with a taper of about 6° and an entrance roundness greater than 99.5% can be obtained, and the exit roundness is about 80%. The second step is the in-situ hole expansion stage: the processing focus is lowered to the middle of the target, as shown in Figure 4, the inner diameter of the nested circle scanning is 0.35mm (as shown in Figure 6), the number of single-layer scanning times is increased to 100 times, and the scanning speed is 250mm/s. After the processing is completed, the outlet roundness can reach more than 99.0%, and the taper is 1.0~1.5°. High-quality nickel-based high-temperature alloy micropores have no defects such as heat-affected zone, recast layer and microcracks.

Example 4

A picosecond laser with a wavelength of 1064nm and a power of 30W is used as the laser processing source 1 to process CMC-SiC ceramic matrix composite micropores with a thickness of 2.3mm and an aperture of 0.60mm. After the target material 8 is ultrasonically cleaned and dried, it is fixed on the working platform 9, and the processing parameters are set to perform through-hole processing and in-situ hole repairing work in sequence. The first step of through-hole processing: the focus is set on the material surface, as shown in Figure 3; the inner circle diameter of the nested circle scanning is 0.06mm and the outer circle diameter is 0.60mm (as shown in Figure 5); the axial feed is 10 times, and the axial feed distance is set to 0.10mm (as shown in Figure 7); the scanning spacing is 0.02mm, the laser power is 30W, the laser scanning speed is 400mm/s, and the number of single-layer scans is 120 times. After the first step of processing is completed, a preliminary test hole with a taper of about 4 to 5° and an entrance roundness greater than 98.0% can be obtained, and the exit roundness is about 80%. The second step is the in-situ hole expansion stage: the processing focus is lowered to the middle of the target material, the feed times are 10 times, as shown in Figure 4, the inner diameter of the nested circle scan is 0.20mm (as shown in Figure 6), the single-layer scan times are increased to 120 times, the scanning speed is 350mm/s, and other parameters remain unchanged. After the processing is completed, the entrance roundness can reach 99.7%, the exit roundness can reach more than 94.7%, and the taper is about 3.0° for the ceramic-based composite micropore. The ceramic-based composite micropore processed by this invention can significantly improve the roundness and taper of the micropore, the entrance roundness can be increased by 2%, the exit roundness can be increased by 14%, and the taper is reduced by 2°. For difficult-to-process materials such as ceramic-based composite materials, according to the depth-to-diameter ratio requirements of the hole to be processed, the hole repairing step can be increased, and the multi-step segmented hole repairing can be achieved to achieve the purpose of improving the processing accuracy of the micropore.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

An in-situ hole repair ultrafast laser micro-hole processing method, characterized by comprising:

Through-hole processing: The laser is focused on the upper surface of the target material, and the through-hole is processed by circular scanning and axial feeding;

Adjust the laser focus to the middle of the target material in the through hole;

Hole expansion modification: The lower part and exit area of the through hole are processed by circular scanning and axial feed to modify the through hole morphology and size so that the through hole exit roundness, through hole wall morphology and through hole taper meet the set requirements.
The in-situ hole repair ultrafast laser micro-hole processing method as described in claim 1 is characterized in that in the through hole processing stage and the hole expansion and modification stage, the through hole is processed by a nested circular cutting scanning method.
The in-situ hole repair ultrafast laser micro-hole processing method as described in claim 1 or 2 is characterized in that the inner circle diameter of the laser processing area in the hole expansion and modification stage is larger than the inner circle diameter of the laser processing area in the through hole processing stage, thereby reducing the number of annular cutting scan circles in the hole expansion and modification stage.
The in-situ hole repair ultrafast laser micro-hole processing method as described in claim 3 is characterized in that the outer circle diameters of the laser processing areas corresponding to the hole expansion and modification stage and the through-hole processing stage are equal and equal to the set diameter of the through-hole.
The in-situ hole repair ultrafast laser micro-hole processing method as described in claim 1 is characterized in that the morphology and size of the through-hole entrance and exit are detected in both the through-hole processing stage and the hole expansion modification stage, and the subsequent steps are performed after the morphology and size meet the set requirements.
The in-situ hole repair ultrafast laser micro-hole processing method as described in claim 1 is characterized in that the axial feeding is carried out step by step, and the next step of the axial circular cutting scan is carried out after completing the first step of the axial circular cutting scan.
The in-situ hole repair ultrafast laser micro-hole processing method as described in claim 1 or 6 is characterized in that, in the hole expansion modification stage, the number of single-layer circular cutting scans and the spot overlap rate are increased to modify the through-hole exit roundness, the through-hole wall morphology and reduce the through-hole taper.
The in-situ hole repair ultrafast laser micro-hole processing method according to claim 1 is characterized in that the pre-processing preparation is performed before the through-hole processing stage:

Fix the target on the working platform and adjust the position so that the upper surface of the target is located at the laser focus;

Adjust the laser ring cutting scanning process parameters and use the scanning galvanometer to prepare for through-hole processing.
The in-situ hole repair ultrafast laser micro-hole processing method as described in claim 8 is characterized in that the laser annular cutting scanning process parameters include scanning inner ring diameter, scanning outer ring diameter, scanning spacing, laser power, scanning speed, number of scans, and axial feed spacing.
The in-situ hole repair ultrafast laser micro-hole processing method as described in claim 1 or 9 is characterized in that after the hole expansion modification of a through hole is completed, the target material is moved to the next through hole processing position, the laser is adjusted to the initial processing position, and the through hole processing and hole expansion modification are repeated.