A femtosecond-nanosecond dual-beam laser synergistic shock strengthening method

Abstract

The application belongs to the technical field of laser surface modification processing, and particularly relates to a femtosecond-nanosecond dual-beam laser synergistic shock strengthening method, wherein a nanosecond laser and a femtosecond laser are simultaneously used as a shock strengthening light source, the two lasers are combined through a combiner, and then focused on the surface of a workpiece through a focusing optical system for shock strengthening; when the workpiece is subjected to shock strengthening, a time delay between the femtosecond pulse and the nanosecond pulse is controlled through a time delay device, so that the femtosecond laser reaches the surface of the workpiece first; the femtosecond laser first ionizes the material on the surface of the workpiece to form a plasma, and strongly absorbs the subsequent nanosecond laser, so that the influence layer depth of nanosecond shock strengthening can be achieved without an absorbing layer and a water restraining layer. The application has the advantages of femtosecond laser shock strengthening, i.e. without a restraining layer and an absorbing layer, and the advantages of nanosecond shock strengthening, i.e. a relatively deep influence layer depth can be achieved.

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 introduces residual compressive stress in the surface layer of a metal by a plasma shock wave induced by high-energy laser, thereby significantly improving the fatigue life, surface compressive stress and corrosion resistance of the workpiece.

[0003] Current mainstream nanosecond shock peening technology usually uses 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 application scenario limitations (such as the inability to process inner holes, micro-slots and other restricted spaces).

[0004] In order to suppress thermal damage, femtosecond shock peening (pulse width <1 ps) is proposed. Although femtosecond laser can effectively avoid thermal effects, the expansion speed of the plasma induced by the femtosecond laser is too fast, resulting in 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 femtosecond laser shock peening, although excellent in thermal management, is restricted in engineering practicability due to insufficient residual stress layer depth. There is an urgent need in the current field to develop a breakthrough technology 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 significantly improves the plasma generation efficiency and shock wave pressure by precisely controlling the action timing of femtosecond pulses before nanosecond pulses, and realizes high-precision and low-thermal-damage material strengthening. The technical solution is as follows:

[0007] The application discloses a femtosecond-nanosecond dual-beam laser synergistic strengthening method, which comprises a femtosecond laser, a 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 and then form a light spot on a workpiece on the moving platform through the focusing optical system; the time delay device is used for controlling the nanosecond laser; the scanning galvanometer is used for adjusting the position of the light spot; the time 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 surface of the workpiece in advance of the nanosecond laser pulse and interacts with the workpiece to generate plasma.

[0008] Preferably, the beam combiner is a polarization combiner or a dichroic mirror.

[0009] Preferably, 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.

[0010] Preferably, a beam 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 degree of coincidence of the light beams.

[0011] Preferably, the time sequence delay Δt is adjusted to change the time when the subsequent nanosecond pulse intervenes in the plasma, so as to indirectly control the evolution of the plasma.

[0012] Preferably, the femtosecond laser spot and the nanosecond laser spot on the surface of the workpiece are completely coincident or partially coincident in space.

[0013] Preferably, the polarization state of the femtosecond light beam is adjusted to be S polarization and the polarization state of the nanosecond light beam is adjusted to be P polarization through a λ / 2 wave plate or a polarization adjuster of the laser.

[0014] Compared with the prior art, the application has the following beneficial effects:

[0015] Compared with a single nanosecond laser strengthening method, the femtosecond pulse is used to excite plasma, and the nanosecond pulse is used to provide energy, so that the plasma shielding effect and thermal damage are reduced, the technology provided by the application does not need an absorbing layer and a restraining layer, the processing steps are reduced, and the technology is more suitable for the strengthening of precise structures.

[0016] 2. Compared with a single femtosecond laser strengthening method, the femtosecond-nanosecond dual-beam technology deposits energy through the nanosecond pulse, the generated shock wave pressure is stronger, the duration is longer, and the impact strengthening influence layer depth and the compressive stress can be greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1It is a double-beam impact strengthening device structure diagram, wherein: 1-femtosecond laser, 2-beam combiner, 3-delay device, 4-focusing lens, 5-scanning galvanometer, 6-moving platform, 7-workpiece, 8-nanosecond laser.

[0018] Figure 2 It is a double-beam coupling spot diagram.

[0019] Figure 3 It is a femtosecond impact strengthening, nanosecond impact strengthening, double-beam impact strengthening principle diagram.

[0020] Figure 4 It is a single nanosecond laser pulse excitation plasma density timing diagram.

[0021] Figure 5 It is a single femtosecond laser pulse excitation plasma density timing diagram.

[0022] Figure 6 It is a femtosecond-nanosecond double-beam excitation plasma density timing diagram. DETAILED DESCRIPTION

[0023] The technical scheme of the present application will be described in detail below through specific embodiments and drawings. It should be understood that the embodiments and specific features in the embodiments are detailed descriptions of the technical scheme of the present application, rather than limitations of the technical scheme of the present application, and the specific technical features can be combined with each other.

[0024] Figure 1 As shown, a femtosecond-nanosecond double-beam laser collaborative impact 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). 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 and then form a spot through the focusing optical system, and the spot is focused on the workpiece 7 of the moving platform; the delay device is used to control the nanosecond laser; and the scanning galvanometer is used to adjust the spot position.

[0025] 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 before or after the scanning galvanometer as a front focusing system or a rear focusing system. In this embodiment, one focusing lens is placed before the scanning galvanometer as a front focusing system. Similarly, it can also be placed behind the scanning galvanometer as a rear focusing system.

[0026] The femtosecond laser pulse width is less than 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.

[0027] The femtosecond laser and the nanosecond laser are used for synergistic strengthening, the two lasers are coaxially combined through a beam combiner, and are focused on the workpiece surface through a focusing optical system. At the same time, the femtosecond pulse is ensured to arrive at the workpiece surface in advance of the nanosecond pulse through a time delay device. The method of composite impact strengthening can achieve the same impact strengthening depth as single nanosecond pulse impact strengthening without adding an absorbing layer and a water confinement layer in the nanosecond impact strengthening process.

[0028] The femtosecond laser and the nanosecond laser have the same working frequency.

[0029] 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 time delay device, so that the femtosecond laser pulse arrives at 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 intervening in the plasma is changed, and then the evolution of the plasma is indirectly regulated.

[0030] 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 It is a dual-beam coupling spot.

[0031] The beam combiner is a polarization combiner or a dichroic mirror combiner.

[0032] The focusing optical system selects a focusing lens group, the focusing lens group includes at least one focusing lens 4, and a high-speed scanning galvanometer and a three-dimensional moving platform are matched to complete focusing and scanning.

[0033] The femtosecond-nanosecond dual-beam impact strengthening device adopts a femtosecond laser with a center wavelength of 1030 nm and a nanosecond laser with a center wavelength of 1064 nm. The two lasers are set with the same repetition frequency. The polarization state of the femtosecond beam is adjusted to S polarization by a λ / 2 wave plate or a polarization adjuster provided by the laser, and the polarization state of the nanosecond beam is adjusted to P polarization. After adjusting the time delay (0-20 ns) of the femtosecond pulse and the nanosecond pulse by a time delay device, coaxial beam combination is realized by using a polarization beam combiner. At the same time, a beam splitter is arranged after the beam combiner to complete the detection and calibration of the time delay and the beam coincidence degree of the femtosecond pulse and the nanosecond pulse by cooperating with a CCD, a photodetector and an oscilloscope. The combined beam is focused by a focusing lens and transmitted to the workpiece surface by a high-speed scanning galvanometer. By cooperating with a three-dimensional moving platform, the workpiece is scanned and impact strengthened according to a predetermined track. In the face of different workpieces to be strengthened, the pulse energy, the pulse width, the time delay and the repetition frequency can be adjusted to achieve the optimal impact strengthening modification effect.

[0034] Figure 3 It is a schematic diagram of the principles of femtosecond impact strengthening, nanosecond impact strengthening and dual-beam impact strengthening. The nanosecond impact strengthening requires an absorption layer and a confinement layer.

[0035] 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 nanosecond order, and the plasma has hysteresis due to the presence of the absorption layer and the confinement layer.

[0036] Figure 5 It is a time sequence diagram of the plasma density excited by a single femtosecond laser pulse. It can be seen that the plasma excited by the femtosecond pulse has no hysteresis, but the duration is only of femtosecond order.

[0037] Figure 6 It is a time sequence diagram of the plasma density excited by a femtosecond-nanosecond dual-beam. By cooperating the femtosecond pulse with the nanosecond pulse, the plasma can be excited without the absorption layer and the confinement layer, and the duration of the plasma can reach nanosecond order.

[0038] The above only describes the preferred embodiments of the present application. It should be noted that those skilled in the art can make some improvements and modifications without departing from the technical principles of the present application. 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, Delta t is set as 0-20 ns, the femtosecond laser pulse reaches the workpiece surface in advance of the nanosecond laser pulse, and plasma is generated by the interaction between the femtosecond laser pulse and the workpiece.

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, and the time sequence delay of the femtosecond pulse and the nanosecond pulse and the beam coincidence degree are detected and calibrated by cooperating with a CCD and an oscilloscope.

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

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 that the evolution of the plasma is indirectly regulated.

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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