Femtosecond-nanosecond double-beam laser cooperative shock peening method

Abstract

The invention belongs to the technical field of laser surface modification processing, and particularly relates to a femtosecond-nanosecond double-beam laser cooperative shock peening method, which comprises the following steps of: taking a nanosecond laser and a femtosecond laser as shock peening light sources at the same time, and combining the two beams of laser through a beam combiner, focusing on the surface of the workpiece through a focusing optical system for impact strengthening; when the workpiece is subjected to shock peening, the time delay between the femtosecond pulse and the nanosecond pulse is controlled through the time delay device, so that the femtosecond laser reaches the surface of the workpiece before the nanosecond laser; the femtosecond laser firstly ionizes a material on the surface of a workpiece to form plasma, and realizes strong absorption of subsequent nanosecond laser, so that the depth of an influence layer of nanosecond impact strengthening can be achieved without an absorption layer and a water restraint layer. The method has the advantages of femtosecond laser shock peening, that is, a restraint layer and an absorption layer are not needed, and the method has the advantage of nanosecond shock peening, so that the depth of a deep influence layer can be realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser surface modification processing, and particularly relates to a femtosecond-nanosecond dual-beam synergistic strengthening method, which is suitable for surface strengthening treatment of metal materials. BACKGROUND

[0002] Laser shock peening (LSP) technology can significantly improve the fatigue life, surface compressive stress and corrosion resistance of a workpiece by introducing residual compressive stress on the surface of the metal through a plasma shock wave induced by a high-energy laser.

[0003] Current mainstream nanosecond shock peening technology usually adopts a Nd:YAG laser (wavelength 1064 nm / 532 nm) with a pulse width of 5-20 ns. However, the plasma shielding effect caused by the long pulse width leads to significant attenuation of the shock wave pressure, and produces an unavoidable thermal effect; and the process must rely on an absorbing layer and a water confinement layer, which greatly increases the operation complexity and limits the applicable scenarios (such as the inability to process inner holes, micro-slots and other limited spaces).

[0004] In order to suppress thermal damage, femtosecond shock peening (pulse width <1 ps) is proposed. Although femtosecond laser can effectively avoid thermal damage, the expansion speed of the plasma induced by the femtosecond laser is too fast, which leads to a very short duration of the shock wave and a significantly insufficient residual stress layer depth.

[0005] In summary, the existing nanosecond laser shock peening technology is still limited by the plasma shielding effect, thermal damage and dependence on the confinement layer; while the femtosecond laser shock peening, although excellent in thermal management, is restricted in engineering practicability due to insufficient residual stress layer depth. Therefore, there is an urgent need to develop a breakthrough technology in this field to balance the shock intensity and deep modification effect. SUMMARY

[0006] The present application aims to provide a femtosecond-nanosecond dual-beam laser synergistic shock peening method, which can significantly improve the plasma generation efficiency and shock wave pressure by precisely controlling the action timing of the femtosecond pulse prior to the nanosecond pulse, and realize high-precision and low-thermal-damage material strengthening. The technical solution is as follows: A femtosecond-nanosecond dual-beam laser synergistic shock peening method, comprising a femtosecond laser, a nanosecond laser, a beam combiner, a delay device, a focusing optical system, a scanning galvanometer and a moving platform; the beams emitted by the femtosecond laser and the nanosecond laser are combined in the beam combiner and then form a spot on the workpiece on the moving platform through the focusing optical system; the delay device is used to control the nanosecond laser; the scanning galvanometer is used to adjust the position of the spot; the delay device controls the time sequence delay Δt of the femtosecond laser pulse and the nanosecond laser pulse, so that the femtosecond laser pulse reaches the workpiece surface prior to the nanosecond laser pulse and interacts with the workpiece to generate plasma.

[0007] Preferably, the light beam combiner is a polarization beam combiner or a dichroic mirror.

[0008] Preferably, the focusing optical system comprises at least one focusing lens, which can be placed before or after the scanning galvanometer as a front focusing system or a rear focusing system.

[0009] Preferably, a beam splitter is arranged behind the light beam combiner to cooperate with a CCD and an oscilloscope to complete the detection and calibration of the timing delay of the femtosecond pulse and the nanosecond pulse and the light beam coincidence degree.

[0010] Preferably, the time when the subsequent nanosecond pulse intervenes in the plasma is changed by adjusting the timing delay Δt, thereby indirectly regulating the evolution of the plasma.

[0011] Preferably, the femtosecond laser spot and the nanosecond laser spot on the workpiece surface are completely or partially overlapped in space.

[0012] Preferably, the polarization state of the femtosecond light beam is adjusted to S polarization and the polarization state of the nanosecond light beam is adjusted to P polarization by a λ / 2 wave plate or a polarization adjuster provided by the laser.

[0013] Compared with the prior art, the application has the following beneficial effects: Compared with the single nanosecond laser shock peening method, the femtosecond pulse is used to excite the plasma, and the nanosecond pulse is used to provide energy, thereby reducing the plasma shielding effect and thermal damage. Meanwhile, the technology provided by the application does not need an absorbing layer and a constraining layer, thereby reducing the processing steps and being more suitable for the shock peening of a precision structure.

[0014] 2. Compared with the single femtosecond laser shock peening method, the femtosecond-nanosecond double-beam technology deposits energy through the nanosecond pulse, thereby generating a shock wave with higher pressure and longer duration, and greatly improving the shock peening influence layer depth and the compressive stress. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 It is a structure diagram of a double-beam shock peening device, in which 1 is a femtosecond laser, 2 is a light beam combiner, 3 is a delay device, 4 is a focusing lens, 5 is a scanning galvanometer, 6 is a moving platform, 7 is a workpiece, and 8 is a nanosecond laser.

[0016] Figure 2 It is a schematic diagram of a double-beam coupling spot.

[0017] Figure 3 It is a schematic diagram of the principles of femtosecond shock peening, nanosecond shock peening and double-beam shock peening.

[0018] Figure 4 It is a timing diagram of the plasma density excited by a single nanosecond laser pulse.

[0019] Figure 5 The density time sequence chart of the femtosecond laser pulse excited plasma.

[0020] Figure 6 The density time sequence chart of the femtosecond-nanosecond dual-beam excited plasma. DETAILED DESCRIPTION

[0021] The technical scheme of the present application will be described in detail below with specific examples and drawings. It should be understood that the specific examples and specific features in the examples are detailed descriptions of the technical scheme of the present application, but not limitations of the technical scheme of the present application, and the specific technical features can be combined with each other.

[0022] Figure 1 As shown in the figure, a femtosecond-nanosecond dual-beam laser synergistic strengthening device includes a femtosecond laser 1, a nanosecond laser 8, a beam combiner 2, a delay device 3, a focusing optical system (focusing lens 4), a scanning galvanometer 5, and a moving platform 6 (a three-dimensional moving platform is adopted), and a λ / 4 wave plate is arranged between the beam combiner 2 and the delay device 3 (to ensure that the light beam reflected by 90 degrees when returning to the delay device 3); the light beams emitted by the femtosecond laser and the nanosecond laser are combined by the beam combiner, then pass through the focusing optical system to form a light spot, and are focused on a workpiece 7 on the moving platform; the delay device is used to control the nanosecond laser; and the scanning galvanometer is used to adjust the light spot position.

[0023] After the light beam passes through the beam combiner 2, it can change the direction of the light beam through a 45-degree mirror, and then enter the focusing optical system (focusing lens 4), so as to optimize the equipment space. The focusing optical system includes at least one focusing lens, which can be placed in front of or behind the scanning galvanometer as a front focusing system or a rear focusing system. In this embodiment, one focusing lens is placed in front of the scanning galvanometer as a front focusing system. Similarly, it can also be placed behind the scanning galvanometer as a rear focusing system.

[0024] The femtosecond laser pulse width is <1 ps, and the emission wavelength is 1030 nm; the nanosecond laser pulse width is 5 ns to 20 ns, and the emission wavelength is 1064 nm.

[0025] The femtosecond laser and the nanosecond laser are synergistically strengthened, the two light beams are coaxially combined through the beam combiner, and are focused on the workpiece surface through the focusing optical system. At the same time, the delay device is used to ensure that the femtosecond pulse reaches the workpiece surface first. This composite impact strengthening method can achieve the same impact strengthening depth as single nanosecond pulse impact strengthening without adding the absorption layer and the water constraint layer in the nanosecond impact strengthening process.

[0026] The working frequencies of the femtosecond laser and the nanosecond laser are the same.

[0027] A femtosecond-nanosecond dual-beam laser synergistic strengthening method, a nanosecond laser beam emitted by a nanosecond laser and a femtosecond laser beam emitted by the femtosecond laser are combined in a beam combiner; the combined beam is focused on the surface of a workpiece to be strengthened through a focusing optical system (including a focusing lens 4); the time sequence delay Δt of the femtosecond laser pulse and the nanosecond laser pulse is controlled through a delay device, so that the femtosecond laser pulse reaches the workpiece surface in advance of the nanosecond laser pulse and interacts with the workpiece to generate plasma. By adjusting the time sequence delay Δt, the time of the subsequent nanosecond pulse intervention into the plasma is changed, and thus the evolution of the plasma is indirectly regulated.

[0028] The femtosecond laser spot and the nanosecond laser spot on the surface of the workpiece are completely overlapped or partially overlapped in space, Figure 2 The dual-beam coupling spot is obtained.

[0029] The beam combiner is a polarization combiner or a dichroic mirror combiner.

[0030] The focusing optical system selects a focusing lens group, and the focusing lens group includes at least one focusing lens 4, which cooperates with a high-speed scanning galvanometer and a three-dimensional moving platform to complete focusing and scanning.

[0031] The femtosecond-nanosecond dual-beam strengthening device adopts a femtosecond laser with a central wavelength of 1030 nm and a nanosecond laser with a central wavelength of 1064 nm. The two lasers are set to have the same repetition frequency. The polarization state of the femtosecond beam is adjusted to be S-polarized by a λ / 2 wave plate or a polarization adjuster provided in the laser, and the polarization state of the nanosecond beam is adjusted to be P-polarized. After the time sequence delay (0-20 ns) of the femtosecond pulse and the nanosecond pulse is adjusted by a delay device, coaxial combination is realized by using a polarization combiner. Meanwhile, a beam splitter is arranged behind the beam combiner, and a CCD, a photoelectric detector and an oscilloscope are used to detect and calibrate the time sequence delay of the femtosecond pulse and the nanosecond pulse and the degree of beam overlap. The combined beam is focused by a focusing lens and transmitted to the surface of a workpiece by a high-speed scanning galvanometer, and a three-dimensional moving platform is used to scan and strengthen the workpiece according to a predetermined track. In the face of different workpieces to be strengthened, the pulse energy, the pulse width, the time sequence delay and the repetition frequency can be adjusted to achieve the optimal strengthening modification effect.

[0032] Figure 3 It is a schematic diagram of the principles of femtosecond strengthening, nanosecond strengthening and dual-beam strengthening. The nanosecond strengthening requires an absorption layer and a constraint layer.

[0033] Figure 4 It is a time sequence diagram of the plasma density excited by a single nanosecond laser pulse. It can be seen that the duration of the plasma excited by the nanosecond pulse is of the order of nanoseconds, and the plasma has hysteresis due to the existence of the absorption layer and the constraint layer.

[0034] Figure 5 The density time sequence diagram of the plasma excited by the single femtosecond laser pulse shows that the femtosecond pulse excites the plasma without hysteresis, but the duration is only femtosecond.

[0035] Figure 6 The density time sequence diagram of the plasma excited by the femtosecond-nanosecond double-beam shows that the cooperation of the femtosecond pulse and the nanosecond pulse can excite the plasma without the absorption layer and the confinement layer, and the duration of the plasma can reach nanosecond.

[0036] The above only describes the preferred embodiments of the present application, and it should be noted that the ordinary skilled in the art can make several improvements and modifications without departing from the technical principles of the present application, and these improvements and modifications should also be considered as the protection scope of the present application.

Claims

1. A femto-nano dual-beam laser synergistic shock reinforcement method, characterized in that, The application relates to a femtosecond laser and nanosecond laser, a beam combiner, a time delay device, a focusing optical system, a scanning galvanometer and a moving platform; the light beams emitted by the femtosecond laser and the nanosecond laser are combined in the beam combiner, then pass through the focusing optical system to form a light spot, and are focused on a workpiece of the moving platform; the time delay device is used for controlling the nanosecond laser; the scanning galvanometer is used for adjusting the light spot position; the time delay device controls the time sequence delay Delta t of the femtosecond laser pulse and the nanosecond laser pulse, so that the femtosecond laser pulse reaches the workpiece surface prior to the nanosecond laser pulse, and interacts with the workpiece to generate plasma.

2. The femtosecond-nanosecond dual-beam laser synergistic strengthening method according to claim 1, characterized in that, The beam combiner is a polarization combiner or a dichroic mirror.

3. The femtosecond-nanosecond dual-beam laser synergistic strengthening method according to claim 1, characterized in that, The focusing optical system comprises at least one focusing lens which can be arranged in front of or behind the scanning galvanometer as a front focusing system or a rear focusing system.

4. The femtosecond-nanosecond dual-beam laser synergistic strengthening method according to claim 1, characterized in that, A light splitter is arranged behind the beam combiner to cooperate with a CCD and an oscilloscope to detect and calibrate the time sequence delay of the femtosecond pulse and the nanosecond pulse and the beam coincidence degree.

5. The femtosecond-nanosecond dual-beam laser synergistic strengthening method according to claim 1, characterized in that, The femtosecond laser and the nanosecond laser have the same working frequency.

6. The femtosecond-nanosecond dual-beam laser synergistic strengthening method according to claim 1, characterized in that, The time when the subsequent nanosecond pulse intervenes in the plasma is changed by adjusting the time sequence delay Delta t, so as to indirectly regulate the evolution of the plasma.

7. The femtosecond-nanosecond dual-beam laser synergistic strengthening method according to claim 1, characterized in that, The femtosecond laser spot and the nanosecond laser spot on the workpiece surface are completely or partially overlapped in space.

8. The femtosecond-nanosecond dual-beam laser synergistic strengthening method according to claim 1, characterized in that, The polarization state of the femtosecond light beam is adjusted to S polarization by a lambda / 2 wave plate or a polarization adjuster of the laser, and the polarization state of the nanosecond light beam is adjusted to P polarization.

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