Method and Device for Increasing Laser-Induced Shock Wave Pressure

Abstract

A method and a device for increasing a laser induced shock wave pressure. According to the method, plasmas (21) are generated by impinging an aluminium foil (20) using lasers; a high-voltage pulse electrode (22) discharges to the plasmas (21) to induce and form a photoelectric combined energy field and then high-temperature plasmas (21) having the characteristics of an ultra-high density and an ultra-high speed expansion are induced and generated; a surface to be processed is impacted by the high-temperature plasmas (21) in a restrained state; the laser induced shock wave pressure is increased substantially; the surface of a high-strength material is reinforced, and the strength, hardness, abrasion resistance and anti-fatigue performances of the high-strength material are improved. The device comprises a laser, the electrode (22), a high-voltage power supply (4), a discharging medium (12), a moving platform, etc.

Claims

1. A method for increasing laser-induced shock wave pressure, wherein, the method uses a laser to induce aluminum foil to generate plasma, high-voltage pulse electrodes discharging to the plasma to induce and form a photoelectric composite energy field, thereby inducing and generating a high-temperature plasma with ultra-high density and ultra-high speed expansion characteristics, and the high-temperature plasma in a restrained state impacts a surface to be processed, significantly increasing the laser induced shock wave pressure, strengthening the surface of high-strength materials and improving strength, hardness, abrasion resistance and anti-fatigue performances of high-strength materials.

2. The method according to claim 1, wherein, the specific steps are as follows: A) the surface of the workpiece is affixed with aluminum foil, and the nanosecond pulse laser is irradiated on the surface of the aluminum foil; B) the aluminum foil absorbs laser energy to gasify; C) under the action of laser, the gasified substances ionize and generate a plasmoid with conductive characteristics; D) the plasmoid continues to absorb the laser energy and expands, making the outer surface of the plasmoid rapidly expand outward; E) after the outer surface of the plasmoid enters the discharge gap of the symmetrical electrodes, the symmetrical electrodes automatically discharge to automatically induce and form a photoelectric composite energy field and generate the high temperature; F) under the action of high temperature, the plasma density rapidly increases and explodes, thereby realizing the ultra-high speed expansion of the plasma; and G) under the constraint of the discharge medium, the ultra-high speed expansive plasma gas impacts the surface of the workpiece with an ultra-high pressure shock wave and produces a significant strengthening effect.

3. The method according to claim 1, wherein, the laser pulse width is 10 to 100 ns.

4. The method according to claim 2, wherein, the number of symmetrical electrodes is 2 to 6, and the symmetrical electrodes are symmetrically arranged in the horizontal plane around the center of the light spot.

5. The method according to claim 1, wherein, the critical voltage of the high-voltage power supply of the electrode is $\begin{array}{cc}{U}_0=E_r.Math.D=E_r\left(L_1-{\mathrm{Vt}}_0\right)=E_r\left(L_1-{\mathrm{Bt}}_0.Math.\sqrt{\frac{E}{.Math..Math.d^2}}\right),& \end{array}$ where E.sub.r is the critical electric field strength when the discharge medium is broken down, D is the distance between the outer surface of the plasma and the electrode when the discharge medium is broken down, L.sub.1 is the distance between the discharge end of the electrode and the spot center on the surface of the aluminum foil, V is the speed of plasma expansion, t.sub.0 is the time of the discharge medium being broken down, E is the laser energy, r is the pulse width, d is the diameter of the laser spot, and the constant B is obtained from the experimental data.

6. The method according to claim 5, wherein, the electrode spacing L.sub.2 between the symmetrical electrodes is 0.81.0 R, and R is the diameter of the laser spot; the distance L.sub.1 between the discharge end of the electrode and the spot center on the surface of the aluminum foil is maintained at 0.50.8 R.

7. The method according to claim 5, wherein, the time distribution of laser-induced shock wave is adjusted by the time t.sub.0 of the discharge medium being broken down, and the time t.sub.0 of the discharge medium being broken down is adjusted by the distance L.sub.1 between the discharge end of the electrode and the spot center on the surface of the aluminum foil, the time t.sub.0 is detailed as $\begin{array}{cc}{t}_0=\frac{L_1-D}{V},& \end{array}$ and the time distribution function of shock wave pressure is $\begin{array}{cc}P\left(t\right)=\{\begin{array}{c}\stackrel{\_}{P}.Math.A\left(t\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(t<t_0\right)\\ C\left(U\right).Math.\stackrel{\_}{P}.Math.A\left(t\right).Math..Math.\mathrm{.Math.}.Math..Math.\left(t{t}_0\right)\end{array},& \end{array}$ wherein, A(t) is the pressure factor of the laser induced shock wave, the average pressure of shock wave $\begin{array}{cc}\stackrel{\_}{P}=0.01.Math.\sqrt{\frac{}{2.Math..Math.+3}.Math.Z.Math.I_0},& \end{array}$ wherein is the laser absorption coefficient and Z is the relative acoustic impedance; I.sub.0 is the laser power density, C(U)U is the composite field coefficient, U is the voltage of the high voltage power supply.

8. A device implementing the method for improving laser induced shock wave pressure according to claim 1, comprising a laser shock system, a pulse discharge system, and a motion platform system; the laser shock system comprising a laser, a control system and a 45 total reflection mirror; the laser is connected with the control system through the data lines; the pulse discharge system includes symmetrical electrodes, a hanging bracket, a horizontal sliding rail, a fluid pump, a storage tank and a high voltage power supply; the symmetrical electrodes are mounted on the horizontal sliding rail which is connected with the hanging bracket; the electrode spacing L.sub.2 between the symmetrical electrodes is adjustable along the horizontal sliding rail; the symmetrical electrodes are connected to the high-voltage power supply through a high-power wire; the discharge medium is pumped from the storage tank by the fluid pump, which then flows through the entrance of discharge tank into the discharge tank; the discharge medium in the discharge tank flows back to the storage tank through outlet port of the discharge tank; a filter is arranged between the fluid pump and storage tank; the motion platform system comprising X-direction table, Y-direction table and Z-direction table; and they are provided with an X-direction manual adjustment knob, a Y-direction manual adjustment knob and a Z-direction manual adjustment knob, respectively.

9. The device according to claim 9, wherein, the discharge medium are mineral oil or deionized water, electrodes adopt electroforming Cu or Cu-based composite material.

Description

DESCRIPTION OF DRAWINGS

[0023] FIG. 1 is a schematic diagram of the method for greatly increasing the pressure of laser-induced shock waves by using an auto-induced composite energy field.

[0024] FIG. 2 is diagram of a device for improving the pressure of laser-induced shock wave. Wherein, 1. laser control system, 2. nanosecond laser, 3. 45 total reflection mirror, 4. high voltage power supply, 5. control system of motion platform control system, 6. discharge tank, 7. Z-direction motion platform, 8. X-direction platform manual knob, 9. X-direction motion platform, 10. Y-direction platform manual knob, 11. Y-direction motion platform, 12. discharge medium, 13. the entrance of the discharge tank, 14. fluid pump, 15. the outlet port of the discharge tank, 16. storage tank, 17. filter.

[0025] FIG. 3 is a local device diagram that improves the laser-induced shock wave pressure. Wherein, 18. Z-direction platform manual knob, 19. workpiece, 20. aluminum foil, 21. plasma, 22. symmetrical electrodes, 23. hanging bracket, 24. horizontal sliding rail.

[0026] FIG. 4 is a schematic diagram of critical distance parameters (L.sub.1, L.sub.2, D, d) in the method of improving laser-induced shock wave pressure.

EMBODIMENTS

[0027] Hereunder the present invention will be further described combined with the drawings and examples.

Example 1

[0028] Taking TC4 aerospace titanium alloy as an example, a method that greatly increases the laser induced shock wave pressure by utilizing the auto-induced composite energy field is employed to strengthen this material. Wherein, the laser is a French Thales nanosecond laser, and the laser pulse width is 20 ns. An aluminum foil with a thickness of 120 m is selected as the absorbing layer. Deionized water is selected as the discharge medium. Symmetrical electrodes are electroforming Cu. The number of electrodes is 2 and the voltage of the high voltage power supply is set to 480 V. The methods and steps are as follows:

[0029] A. Adjusting the electrode spacing L.sub.2 between the symmetrical electrodes 22 and the distance L.sub.1 between the discharge end of electrode and the spot center on the surface of aluminum foil 20 to 0.50.8 R; adjusting the output voltage of the high voltage power supply 4 to the critical voltage to ensure that the symmetrical electrodes 22 can be automatically triggered to discharge by the laser-induced plasma cloud.

[0030] B. Turning on the fluid pump 14 to deliver the deionized water 12 to the discharge tank and ensure that the fluid level of the deionized water 12 in the discharge tank is higher than the discharge end of the symmetrical electrodes 22, and then, adjusting the flow rate of the fluid pump 14 to keep the fluid level of the deionized water 12 constant.

[0031] C. Turning on the laser shock system, pulse discharge system and the motion platform system in turn and preheating for 5 to 15 minutes.

[0032] D. The collaborative control of laser control system 1 and the motion platform control system 5 is realized by an external computer so that the laser 2 and the motion platforms 9, 11 and 7 can complete the laser shock strengthening under the photoelectric composite energy field.

[0033] E. Turning off the laser 2, the motion platforms 9, 11 and 7 and the pulse power supply 4 in turn, removing the TC4 aerospace titanium alloy workpiece 19, cleaning and maintaining the whole system.

[0034] The result shows that the shock wave pressure induced by traditional laser shock process strengthening technology on the surface of TC4 aerospace titanium alloy is about 2.57 GPa with the laser energy of 6 J and the spot diameter of 3 mm. However, when the electrode spacing L.sub.2 between symmetry electrodes is 2.7 mm (0.9 R) and the distance L.sub.1 between the discharge end of the electrode and the spot center on the surface of the aluminum foil 20 is 1.8 mm (0.6 R), the shock wave pressure induced by the method of the present invention on the surface of TC4 aerospace titanium alloy reaches up to about 8.93 GPa, which is 3.5 times as much as that of the traditional laser shock strengthening technology, and realizes exponential growth of the shock wave pressure.

[0035] In addition, the result of the research reveals that when the electrode spacing L.sub.2 of between the symmetry electrodes is 2.7 mm (0.9 R) and the distance L.sub.1 between the discharge end of the electrode and the spot center on the surface of the aluminum foil 20 is 3.6 mm (1.2 R), the shock wave pressure induced by the method of the present invention on the surface of TC4 aerospace titanium alloy is about 2.52 GPa. It means that the symmetrical electrodes did not be completely discharged and proves that the distance L.sub.1 between the discharge end of the electrode and the spot center on the surface of the aluminum foil is one of the key technical parameters to achieve the present invention.

[0036] The result of the research also indicates that when the electrode spacing L.sub.2 of between the symmetrical electrodes is 4.2 mm (1.4 R) and the distance L.sub.1 between the discharge end of the electrode and the spot center on the surface of the aluminum foil 20 is 1.8 mm (0.6 R), the shock wave pressure induced by the method of the present invention on the surface of TC4 aerospace titanium alloy is about 2.43 GPa which also means that the symmetrical electrodes did not be completely discharged. It proves that the electrode spacing L.sub.2 between the symmetrical electrodes is also one of the key technical parameters to achieve the present invention.

Example 2

[0037] A device for improving the laser induced shock wave pressure includes a laser shock system, a pulse discharge system and a motion platform system. Wherein, the laser shock system comprises a laser 2, a control system 1 and a 45 total reflection mirror 3. The laser 2 is connected with the control system 1 through the data lines. In the pulse discharge system, the symmetrical electrodes are connected with the horizontal sliding rail 24 by the hanging bracket 23 to ensure the electrode spacing L.sub.2 is adjustable. The symmetrical electrodes 22 are connected to the high-voltage power supply 4 through high-power wire. The discharge medium 12 is pumped into the discharge tank via the entrance 13 of the discharge tank by the fluid pump 14. A filter is arranged between the fluid pump 14 and storage tank 16. The discharge medium 12 in the discharge tank flows back into the storage tank 16 through outlet port 15 of the discharge tank. The motion platform system comprises an X direction table 9, a Y direction table 11 and a Z direction table 7, and they are provided with manual adjustment knobs 8, 10, and 18, respectively, wherein Z-direction table 9 is used to adjust the distance L.sub.1 between the discharge end of the electrode and the spot center on the surface of the aluminum foil 20.

[0038] Taking nickel-based alloy IN718 as an example, a device for increasing laser induced shock wave pressure is used to strengthen it. Wherein, the laser is a French Thales nanosecond laser system and the laser pulse width is 20 ns. An aluminum foil with the thickness of 120 m is used as the absorbing layer. The mineral oil is selected as discharge medium 12. The symmetrical electrodes 22 are made of Cu-based composite material. The number of electrodes is 2 and the voltage of high voltage power supply is 400 V.

[0039] The shock wave pressure induced by traditional laser shock strengthening technology on the surface of nickel-based alloy IN718 is about 2.89 GPa with the laser energy of 10 J and the spot diameter of 4 mm. However, when the electrode spacing L.sub.2 between the symmetrical electrodes is 3.2 mm (0.8 R) and the distance L.sub.1 between the discharge end of the electrode and the spot center on the surface of the aluminum foil 20 is 2 mm (0.5 R), the shock wave pressure induced by the method of this invention on the surface of nickel-based alloy IN718 reaches up to about 8.59 GPa, which is 2.97 times as much as that of the traditional laser shock strengthening technology. It realizes the exponential growth of the shock wave pressure.

[0040] The parts not covered by the present invention are the same as those in the prior art or can be implemented by the prior art.