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, the method comprising: using a laser to induce aluminum foil to generate plasma; pulse electrodes discharging to the plasma to induce and form a photoelectric composite energy field, thereby inducing and generating the plasma, wherein the plasma is a high-temperature plasma; and the plasma in a restrained state impacting a surface to be processed, 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, wherein a voltage of the pulse electrodes is at least 400 Volts.

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

3. The method according to claim 2, wherein the pulse electrodes comprise 2 to 6 symmetrical electrodes, and wherein the symmetrical electrodes are symmetrically arranged in a horizontal plane around a center of a light spot on the aluminum foil.

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

5. The method according to claim 4, wherein a spacing L.sub.2 between the pulse electrodes is 0.81.0R, and R is a diameter of the laser spot; and wherein the distance L.sub.1 between the discharge end of the pulse electrodes and the spot center on the surface of the aluminum foil is maintained in a range of 0.5R to 0.8R.

6. The method according to claim 4, wherein a time distribution of the 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 pulse electrodes and the spot center on the surface of the aluminum foil, the time t.sub.0 is calculated as $\begin{array}{cc}{t}_0=\frac{L_1-D}{V},& \end{array}$ and a time distribution function of a pressure of the shock wave is $\begin{array}{cc}P\left(t\right)=\{\begin{array}{c}\stackrel{\_}{P}.Math.A\left(t\right)\mathrm{.Math.}\left(t<t_0\right)\\ C\left(U\right).Math.\stackrel{\_}{P}.Math.A\left(t\right)\mathrm{.Math.}\left(t{t}_0\right)\end{array},& \end{array}$ where A(t) is a pressure factor of the shock wave, an average pressure of the shock wave, is $\begin{array}{cc}\stackrel{\_}{P}=0.01\sqrt{\frac{}{2+3}.Math.Z.Math.I_0},& \end{array}$ is a, laser absorption coefficient and Z is a relative acoustic impedance; I.sub.0 is a power density of the laser, C (U)U is a composite field coefficient, and U is a voltage of the power supply.

7. The method according to claim 1, wherein, a pulse width of the laser is 10 nanoseconds (ns) to 100 ns.

8. A device implementing the method for improving laser induced shock wave pressure according to claim 1, the device comprising: a laser shock system; a pulse discharge system; and a motion platform system, wherein the laser shock system comprises the laser, a control system, data lines, and a 45 total reflection mirror, wherein the laser is connected with the control system through the data lines, wherein the pulse discharge system includes the pulse electrodes, a hanging bracket, a horizontal sliding rail, a fluid pump, a storage tank, and a power supply supplying the voltage of at least 400 Volts, wherein the pulse electrodes are mounted on the horizontal sliding rail that is connected with the hanging bracket, wherein an electrode spacing L.sub.2 between the pulse electrodes is adjustable along the horizontal sliding rail wherein the pulse electrodes are connected to the power supply through a high-power wire, wherein a discharge medium is pumped from the storage tank by the fluid pump, and then flows through the entrance of a discharge tank into the discharge tank wherein the discharge medium in the discharge tank flows back to the storage tank through an outlet port of the discharge tank, wherein a filter is arranged between the fluid pump and the storage tank, wherein the motion platform system comprises an X-direction table, a Y-direction table, and a Z-direction table which 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 8, wherein the discharge medium comprises mineral oil or deionized water, and wherein the pulse electrodes comprise electroforming Cu or a, Cu-based composite material.

Description

DESCRIPTION OF DRAWINGS

(1) 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.

(2) 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.

(3) 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.

(4) 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

(5) Hereunder the present invention will be further described combined with the drawings and examples.

Example 1

(6) 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:

(7) 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.

(8) 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.

(9) C. Turning on the laser shock system, pulse discharge system and the motion platform system in turn and preheating for 5 to 15 minutes.

(10) 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.

(11) 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.

(12) 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.

(13) 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.

(14) 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

(15) 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.

(16) 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.

(17) 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.

(18) 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.