Laser processing method

Abstract

A laser processing method is provided which can reduce the deformation generated in a workpiece even when forming a plurality of through holes. The laser processing method includes the following steps: a gas supply step (S3) of making the air pressure on the back side of the workpiece (1) greater than the air pressure on the front side; a deep hole processing step (S5) of irradiating pulsed laser having a first repetition frequency (f1) from the front side of the workpiece (1) to form a through hole (A) in the workpiece (1); and a hole finishing step (S7) of irradiating pulsed laser having a second repetition frequency (f2) smaller than the first repetition frequency (f1) to the inner surface of the through hole (A).

Description

Technical Field

[0001] The present invention relates to a laser processing method. Background Art

[0002] When forming a through hole in a workpiece by irradiating a laser pulse, the peak output, pulse width, repetition frequency, number of emissions, etc. of the laser are appropriately set according to the depth of the hole, etc. During processing, gas is supplied from the laser irradiation side to prevent soot from flying into the laser processing head. In addition, the molten metal is discharged from the through hole to prevent slag (molten matter) from adhering to the inner surface of the hole.

[0003] However, especially when forming a plurality of through holes close to each other by laser processing, recast (re-melting) caused by the heat input of the laser sometimes occurs, and deformation occurs due to the solidification of the molten metal. Moreover, due to this deformation, cracks may be generated on the workpiece. Considering performing laser processing while suppressing heat input, there is a drawback that the processing time is significantly extended.

[0004] Therefore, Patent Document 1 discloses supplying gas also from the laser emission side of the workpiece to make the air pressure on the emission side higher than that on the incident side. Thereby, an air flow is formed from the emission side toward the incident side, and the slag generated during processing is discharged toward the incident side.

[0005] In addition, Patent Document 2 discloses that after performing bottom hole processing by irradiating a nanosecond laser, a picosecond laser is irradiated for finish machining. Thereby, the temperature rise during finish machining is suppressed, and the finish machining accuracy is improved.

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-35973

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2008-55477 Summary of the Invention

[0010] Problems to be Solved by the Invention

[0011] However, in the prior art described in Patent Document 1 or 2, etc., when forming a plurality of through holes, especially when forming through holes with a high aspect ratio close to each other, there is a problem that the deformation becomes large. This problem particularly occurs significantly when the workpiece is annular.

[0012] In view of the above problems, an object of the present invention is to provide a laser processing method that can reduce the deformation generated in a workpiece even when forming a plurality of through holes.

[0013] Means for Solving the Problems

[0014] The laser processing method of the present invention is characterized by including the following steps: a step of making the air pressure on the back side of the workpiece greater than the air pressure on the front side; a step of irradiating pulsed laser with a first repetition frequency from the front side of the workpiece to form adjacent through-holes on the workpiece; and a finishing step of irradiating pulsed laser with a second repetition frequency smaller than the first repetition frequency to the inner surface of the through-holes. Whenever one through-hole is formed, the pulsed laser is irradiated only by changing the repetition frequency, thereby performing the finishing step on the one through-hole, and arranging a plurality of the through-holes adjacent to each other.

[0015] According to the laser processing method of the present invention, pulsed laser with a second repetition frequency is irradiated to the inner surface of the through-hole, and the second repetition frequency is smaller than the first repetition frequency of the pulsed laser irradiated in the step of forming the through-hole. Thereby, it is possible to achieve a balance between heat discharge caused by gas flowing from the inside of the through-hole and heat input caused by laser irradiated from the surface side of the through-hole. Therefore, it is possible to perform the finishing step of the through-hole by irradiating pulsed laser with a second repetition frequency while suppressing heat accumulation inside the through-hole, and it is possible to suppress deformation generated in the workpiece.

[0016] In addition, since heat accumulation in the workpiece can be suppressed, the number of emissions in the finishing step can be increased, and recast can be reduced. In addition, by reducing recast, generation of deformation and cracks in the workpiece caused by re-solidification of recast can be reduced. In addition, since residual deformation generated in the workpiece can be reduced, post-processing correction steps can be reduced.

[0017] In the laser processing method of the present invention, in the step of irradiating pulsed laser with the second repetition frequency, the state where the air pressure on the back side of the workpiece is greater than the air pressure on the front side is continuously maintained.

[0018] Thereby, in the step of irradiating pulsed laser with a second repetition frequency, gas continuously flows from the back side of the workpiece toward the front side. Thereby, generation of deformation and cracks in the workpiece can be further reduced. Description of the Drawings

[0019] Figure 1 is a cross-sectional view schematically showing an example of a laser processing apparatus used in the laser processing method according to an embodiment of the present invention.

[0020] Figure 2 is a flowchart showing the laser processing method according to an embodiment of the present invention.

[0021] Figure 3 is a graph showing the relationship between the number of laser emissions and cracks.

[0022] Figure 4 It is a graph showing the relationship between the second repetition frequency and the deformation of the workpiece.

[0023] Figure 5 It is a graph showing the temperature change of the test piece caused by the difference in the second repetition frequency. Detailed implementation mode

[0024] Hereinafter, with reference to Figure 1 An example of the laser processing apparatus 100 used when performing the laser processing method of the embodiment of the present invention will be described. The laser processing apparatus 100 forms a plurality of through holes A on a workpiece (workpiece to be processed) 1 having a substantially annular shape by laser processing.

[0025] The workpiece 1 has a wall portion 2 as a whole. The wall portion 2 has a substantially circular ring shape in a plan view and has a substantially semi-circular arched longitudinal section that bulges upward. Moreover, on the workpiece 1, a plurality of port holes 3 are formed at equal intervals on the same circle in a plan view. Here, eight port holes 3 are formed. The plurality of port holes 3 have the same diameter and they penetrate the top of the wall portion 2. However, in order to perform phase determination, the diameter or the tolerance of the diameter of a specific one of the port holes 3 may be different from that of the other port holes 3. The workpiece 1 is made of, for example, an alloy such as a nickel alloy added with molybdenum, chromium, etc., which has excellent heat resistance and corrosion resistance.

[0026] And, on substantially the entire surface of the substantially upper half portion of the wall portion 2 of the workpiece 1, a plurality of fine through holes A inclined at the same angle are formed from above along the clockwise rotation direction using the laser processing apparatus 100. Thus, since the through holes A are inclined and have a small hole diameter, the aspect ratio is large and they are formed close to each other. In addition, the hole diameters and the inclination angles of the through holes A may be the same or different.

[0027] The laser processing apparatus 100 includes: a laser processing head 10, a workpiece support portion 20, jigs (30, 40, 50), a gas supply mechanism 60, and a control portion 70.

[0028] The processing head 10 is composed of, for example, a laser source 11 and a laser irradiation portion 13. The laser source 11 is a laser source capable of generating laser pulses such as a YAG laser, a semiconductor laser, or a fiber laser. The laser irradiation portion 13 has an optical system such as a mirror and a lens and is connected to the laser source 11 through a transmission optical fiber 12. The laser generated by the laser source 11 according to the conditions set by the control portion 70 is guided to the laser irradiation portion 13 through the transmission optical fiber 12. The laser irradiation portion 13 converges the laser transmitted through the transmission optical fiber 12 to the portion set by the control portion 70.

[0029] The workpiece support portion 20 supports the workpiece 1 from the lower surface (back surface) side. This lower surface side serves as the laser emission side. The workpiece support portion 20 supports the workpiece 1 before machining the through-hole A, thereby forming a sealed space S between the lower surface of the workpiece 1 and the workpiece support portion 20. Here, the workpiece support portion 20 places and supports the workpiece 1 at three locations: the peripheral portion of the portion of the workpiece 1 where the port hole 3 is formed, the lower surface of the inner peripheral side wall portion 2, and the lower surface of the outer peripheral side wall portion.

[0030] Specifically, the workpiece support portion 20 has a cylindrical pin 22, and the pin 22 is fixed to a support portion main body 21 that constitutes the main body of the workpiece support portion 20. Moreover, a short cylindrical protrusion portion 23 having a diameter smaller than that of the pin 22 is integrally provided at the top of the pin 22. And, in a state where the protrusion portion 23 is located within the port hole 3, the workpiece 1 is supported in a state where the lower surface of the outer peripheral portion of the port hole 3 of the workpiece 1 is placed on the upper surface of the pin 22.

[0031] Moreover, a bolt hole 24 is formed at the top of the protrusion portion 23, and the port fixture 30 is fixed to the upper surface of the protrusion portion 23 by a bolt 25 that is screwed into the bolt hole 24. Thereby, the workpiece 1 is fixed to a specified position of the workpiece support portion 20.

[0032] And, the inner fixture 40 is fixed to the upper surface of the inner side of the workpiece 1 supported by the workpiece support portion 20 in a state where it abuts against the inner peripheral side wall portion 2 of the workpiece 1 from above using a bolt 41. In addition, the outer fixture 50 is fixed to the upper surface of the outer side of the workpiece 1 supported by the workpiece support portion 20 in a state where it abuts against the outer peripheral side wall portion 2 of the workpiece 1 from above using a bolt 51.

[0033] The gas supply mechanism 60 supplies gases such as air, argon, and nitrogen into the sealed space S via a gas supply passage 61 formed in the support portion main body 21, thereby making the air pressure in the sealed space S greater than the air pressure outside, particularly greater than the air pressure in the space on the front side of the workpiece 1. Specifically, by supplying gas into the sealed space S by the gas supply mechanism 60, a pressure difference of 0.1 MPa or more, more preferably 0.5 MPa or more, is generated with respect to the external air pressure. In addition, although not shown, it is preferable to supply gas to the front side of the workpiece 1 as well.

[0034] The control unit 70 is connected to the laser processing head 10 and the gas supply mechanism 60 and controls their operations. In addition, the control unit 70 can be separately provided in a manner of separately controlling the laser processing head 10 and the gas supply mechanism 60, or can be integrally provided in a manner of controlling the whole.

[0035] Hereinafter, a laser processing method according to an embodiment of the present invention using the above-described laser processing apparatus 100 will be described.

[0036] First, a workpiece placement process (S1) is performed to place the workpiece 1 at a specified position on the workpiece support portion 20. Specifically, the jigs 30, 40, and 50 are pre-removed from the workpiece support portion 20. Then, the operator places the workpiece 1 at the specified position on the workpiece support portion 20 with each port hole 3 of the workpiece 1 located within the protrusion 23.

[0037] Then, a workpiece fixing process (S2) is performed to fix the workpiece 1 placed at the specified position on the workpiece support portion 20 using the jigs 30, 40, and 50. Specifically, the operator fixes the workpiece 1 to the workpiece support portion 20 using each port jig 30. Also, the workpiece 1 is fixed to the workpiece support portion 20 using the inner jig 40 and the outer jig 50.

[0038] Next, a gas supply process (S3) is performed to supply gas into the sealed space S through the gas supply mechanism 60 to make the pressure inside the sealed space S a specified pressure.

[0039] After making the pressure inside the sealed space S a pressure above the specified value, a hole entrance machining process (S4) is performed to form a hole near the entrance of the through hole A on the workpiece 1 using the laser processing head 10. After that, a deep hole machining process (S5) is performed to make the through hole A a deep hole. In these processes, pulsed laser with a first repetition frequency f1 is irradiated from the laser processing head 10. The first repetition frequency f1 can be the same as the previous frequency suitable for forming the through hole A, for example, 50 Hz to 100 Hz. In addition, the set conditions such as the peak output, pulse width, and number of emissions of the pulsed laser irradiated in the hole entrance machining process (S4) and the deep hole machining process (S5) can be the same as before.

[0040] Next, a hole through process (S6) is performed to make the through hole A penetrate. After that, a hole finishing process (S7) is performed to irradiate pulsed laser on the inner surface of the penetrated through hole A. The hole finishing process (S7) is performed every time one through hole A is penetrated.

[0041] In the hole through process (S6) and the hole finishing process (S7), pulsed laser with a second repetition frequency f2 is irradiated. It is preferable to continuously perform the hole entrance machining process (S4), the deep hole machining process (S5), the hole through process (S6), and the hole finishing process (S7). It is particularly preferable to change the repetition frequency from the first repetition frequency f1 to the second repetition frequency f2 immediately after the deep hole machining process (S5) to perform the hole through process (S6).

[0042] The second repetition frequency f2 is less than the first repetition frequency f1, for example, 2 / 3 or less, or half or less of the first repetition frequency f1, or 40 Hz or less, or 30 Hz or less. The setting conditions such as the peak output and pulse width of the pulsed laser irradiated in the hole penetration process (S6) and the hole finishing process (S7) can be the same as those in the deep hole machining process (S5). In addition, the number of firings in the hole penetration process (S6) and the hole finishing process (S7) can be appropriately set according to the hole diameter, depth, etc. of the through hole A, for example, 10 firings or more and 200 firings or less.

[0043] In addition, the setting conditions such as the first repetition frequency f1, peak output, and pulse width of the pulsed laser irradiated in the hole entrance machining process (S4) and the deep hole machining process (S5) can be the same or different. Also, the setting conditions such as the second repetition frequency f2, peak output, and pulse width of the pulsed laser irradiated in the hole penetration process (S6) and the hole finishing process (S7) can be the same or different.

[0044] In addition, if the through hole A is formed, gas will leak out from the through hole A to the outside. Therefore, it is preferable to continuously or intermittently supply gas using the gas supply mechanism 60 after the hole penetration process (S6).

[0045] After all the fine through holes A are formed as described above, an operator performs a workpiece removal process (S8) of releasing the jigs 30, 40, and 50 to remove the workpiece 1. Thus, all the processes are completed.

[0046] As described above, according to the present embodiment, the second repetition frequency f2 of the pulsed laser irradiated in the hole penetration process (S6) and the hole finishing process (S7) is less than the first repetition frequency f1 of the pulsed laser irradiated in the hole entrance machining process (S4) and the deep hole machining process (S5). Thereby, it is possible to achieve a balance between heat discharge caused by gas flowing from the inside of the through hole A and heat input caused by laser irradiation from the surface side of the through hole A. Therefore, it is possible to perform the hole finishing process (S7) of irradiating the pulsed laser with the second repetition frequency f2 while suppressing heat accumulation inside the through hole A, and it is possible to suppress deformation generated in the workpiece 1.

[0047] In addition, since heat accumulation in the workpiece 1 can be suppressed, the number of firings in the hole finishing process (S7) can be increased, and recast can be reduced. In addition, by reducing recast, it is possible to reduce the generation of deformation and cracks in the workpiece 1 due to re-solidification of recast. In addition, since the residual deformation generated in the workpiece 1 can be reduced, the post-processing correction process can be reduced.

[0048] In addition, the inventors measured the depth of cracks generated due to differences in the number of laser emissions of the pulsed laser with the second repetition frequency f2 during the hole finishing process (S7) using test pieces. The measurement results are as Figure 3 shown. In Figure 3 , the circular markers represent the average crack depth, and the triangular markers represent the maximum crack depth. In addition, the number of emissions at the first repetition frequency f1 is approximately 100 times, and all the setting conditions other than the number of laser emissions of the pulsed laser in the hole finishing process (S7) are the same.

[0049] According to Figure 3 , compared with the case where the number of laser emissions is 0 times, that is, when the hole finishing process (S7) is not performed, by irradiating the pulsed laser with the second repetition frequency f2 during the hole finishing process (S7), cracks can be suppressed. This is considered to be because the recast is reduced. In addition, compared with the case where the number of laser emissions is 20 times, when the number of laser emissions is 100 times, the cracks are further suppressed, but the difference between the two is not as large as that on the left.

[0050] In addition, the inventors measured the deformation generated due to differences in the second repetition frequency f2 of the pulsed laser irradiated during the hole penetration process (S6) and the hole finishing process (S7) using test pieces. Referring to Figure 4 showing the measurement results, when the second repetition frequency f2 is as low as 10 Hz, 20 Hz, and 30 Hz, the amount of deformation is suppressed to a small extent. On the other hand, when the second repetition frequency f2 is the same as the first repetition frequency f1 at 75 Hz, the amount of deformation is large. In addition, all the setting conditions other than the frequency of the repetition frequency f2 of the pulsed laser in the hole penetration process (S6) and the hole finishing process (S7) are the same.

[0051] Furthermore, the inventors measured the temperature change of the test piece caused by differences in the second repetition frequency f2 of the pulsed laser irradiated during the hole finishing process (S7) using test pieces. The measurement results are as Figure 5 shown. In Figure 5 , the horizontal line represents the maximum temperature reached at each second repetition frequency f2.

[0052] According to Figure 5 , when the second repetition frequency f2 is 10 Hz, 20 Hz, and 30 Hz, the heat storage in the test piece does not increase as much as on the left, and the reached temperature does not rise as much as on the left. It can be considered from this that if the second repetition frequency f2 is 30 Hz or less, even if the number of emissions is increased and the hole finishing process (S7) is performed for a long time, the workpiece 1 will not become as hot as on the left, and the recast is suppressed.

[0053] On the other hand, when the second repetition frequency f2 is 75 Hz, the heat storage in the test piece increases with the passage of time, and the temperature reached becomes higher. Therefore, it is considered that if the emission number increases and the hole finishing process (S7) is performed for a long time, the workpiece 1 becomes hot and recasting increases. Therefore, it is preferable to suppress the emission number.

[0054] In addition, in the hole finishing process (S7), when the second repetition frequency f2 is 10 Hz but the gas supply process (S3) of supplying gas into the closed space S by the gas supply mechanism 60 is omitted, the heat storage in the test piece increases with the passage of time, and the temperature reached becomes very high. Therefore, the workpiece 1 becomes hot, recasting increases, and cracks become larger, so this case is not preferable.

[0055] As described above, the embodiments of the present invention have been described, but the present invention is not limited thereto, and the structures and forms of the workpiece 1 and the laser processing apparatus 100 used in the present invention can be appropriately changed. In addition, the case where the through hole A is formed obliquely has been described, but the through hole A may also be formed vertically.

[0056] In addition, the case where the pulsed laser having the second repetition frequency f2 is irradiated in the hole penetration process (S6) has been described. However, the present invention is not limited thereto, and the pulsed laser similar to that in the deep hole processing process (S5), that is, the pulsed laser having the first repetition frequency f1, may also be irradiated in the hole penetration process (S6). In this case, the deep hole processing process (S5) and the hole penetration process (S6) are integrated.

[0057] Description of reference numerals

[0058] 1 Workpiece

[0059] 2 Wall portion

[0060] 3 Port hole

[0061] 10 Laser processing head

[0062] 11 Laser source

[0063] 12 Transmission optical fiber

[0064] 13 Laser irradiation unit

[0065] 20 Workpiece support portion

[0066] 21 Support portion main body

[0067] 22 Pin

[0068] 23 Protrusion

[0069] 24 Bolt hole

[0070] 25 Bolt

[0071] 30-port fixture

[0072] 40-inner fixture

[0073] 41-bolt

[0074] 50-outer fixture

[0075] 51-bolt

[0076] 60-gas supply mechanism

[0077] 61-gas supply passage

[0078] 70-control unit

[0079] 100-laser processing device.

Claims

1. A laser processing method, characterized in that, including the following steps: a step of making the air pressure on the back side of the workpiece greater than the air pressure on the front side; a step of irradiating pulsed laser with a first repetition frequency from the front side of the workpiece to form a through hole in the workpiece; and a finishing step of irradiating pulsed laser with a second repetition frequency smaller than the first repetition frequency to the inner surface of the through hole, each time one through hole is formed, irradiating the pulsed laser by only changing the repetition frequency, thereby performing the finishing step on the one through hole, and arranging a plurality of the through holes closely, in the finishing step of irradiating pulsed laser with the second repetition frequency, continuously maintaining the state where the air pressure on the back side of the workpiece is greater than the air pressure on the front side.

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