CN1695873A - Method and device for laser shot peening forming of medium-thick plate - Google Patents

Abstract

A laser blasting apparatus for shaping on the surface of medium-thick plate is composed of laser device, optical guide system, laser blasting head, fixture system for workpiece, and control system. Its shaping process includes computer simulation of the stress field distribution according to the curved surface shape to be processed, optimizing it to obtain the distribution of shock wave pressure and blasting tracing on the surface of plate, automatically choosing the parameters of laser pulse optical beam converte and optical mask, writing control program, generating strong short laser pulses, covering them to several pulse clusters, passing through optical mask, acting on energy converting body on the surface of workpiece to obtain shock waves, acting on the surface of workpiece for shaping.

Description

Method and device for laser shot peening forming of medium-thick plate

technical field

The invention relates to the field of mechanical manufacturing, in particular to a plate forming method and device based on high-energy pulse laser peening technology, which is suitable for the forming process of medium-thick plate parts with smooth curvature requirements. It is especially suitable for the processing of small-curvature formed parts of medium-thick plates that are difficult to form or cannot be formed at all by conventional methods.

Background technique

Plate forming parts are used in a wide range of applications in the aerospace and defense industries. Large-scale structural components on aircraft, such as wings, skins, integral panels, and turbine engine blades, require precise and complex curvature shapes and maintain the original shape of the material in order to withstand effective structural loads and meet aerodynamic performance. Structural characteristics; the forming process of missiles, rockets in the defense industry, nuclear reaction metal tanks and other parts in nuclear reactors requires high mechanical properties and surface quality in addition to precise shapes. For the processing of these medium-thick plate parts, traditional processing methods, such as mechanical shot peening, heating-assisted bending forming, etc., are difficult to meet the use requirements and performance of the parts.

The traditional mechanical shot peening is to use the qualitative projectile flow to hit the surface of the plate at high speed, which is easy to cause work hardening, and makes the sprayed surface rough and reduces the surface quality. At the same time, the existing shot peening system is very complicated. After each shot peening, the shot must be collected, cleaned, classified and broken to remove. Due to the many parameters of mechanical shot peening and it is difficult to control accurately, the method of test spraying and sample inspection is still used for mechanical shot peening at present, resulting in long production cycle and high cost. And because the residual stress produced by mechanical shot peening is small, the thickness of the plate that can be formed is small (<5mm), and the applicable materials are limited. Heating-assisted bending forming is the main method of forming medium-thick plates at present. The process is to first heat the plate to be processed to a certain temperature to reduce the yield strength of the material, and then use the mold pressure to bend it, because the heating changes the material’s yield strength. The structure reduces the mechanical properties of the parts after forming and produces residual tensile stress on the surface, which affects the service life.

The closest to the present invention is the patent "CONTOUR FORMIMG OF METALS BY LASER PEENING" (Patent No.: WO0105549) applied by Hackel Lloyd and Harris Fritz of the University of California, USA. Residual stress release produces micro-deformation forming with micro-curvature. By accumulating the micro-curvature of each hammering at each point or performing multiple impacts at the same point, larger-scale bending can be obtained. The disadvantages are as follows: (1) Since the bending is induced by the residual compressive stress generated by laser single-point hammering, the deformation is small; (2) Before laser hammering, an energy absorbing layer must be applied on the surface to be impacted, and then coated Water is used as a constraint layer on top of the layer. This separate processing method causes disadvantages such as low laser energy utilization rate, unstable data, and poor reliability, and also requires a post-processing process to remove the residual energy-absorbing layer; (3) an additional water supply system is required when using water as a constrained layer , and make the fixture complicated, and the splash of water in the process of laser hammering is difficult to control, and the optical lens is easy to be damaged.

The existing patent "a laser shock precision forming method and device" (patent number: ZL01134063.0) proposes a high-performance laser shock precision forming technology, which directly uses a strong pulsed laser beam (power density greater than 10 ⁹ W/cm ² , pulse width 8-30ns) impacts the flexible film on the surface of the workpiece, gasifies and ionizes the surface and forms a shock wave, and the peak pressure of the shock wave exceeds the dynamic yield strength of the material, resulting in obvious plastic deformation of the plate. This method adopts the macroscopic plastic deformation of the sheet metal under a single laser impact, that is, the process of directly forming concave and concave overlapping joints, which is suitable for forming parts with drawing ratio requirements. Since the shock wave pressure induced by each laser pulse must produce a macroscopic concave change in the sheet, the energy of the laser pulse must be large enough, which brings higher requirements to the laser. Since the laser shock forming process is accompanied by surface strengthening, the yield strength of the plate is increased, and subsequent deformation is difficult, resulting in poor plastic flow of the material in the overlapping area, the thickness of the plate that can be formed is limited, and the surface roughness cannot be guaranteed. Used for forming medium and thick plates with smooth curvature requirements.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned shortcomings, and provide a method and device for forming medium-thick plates (>6-15mm) that are difficult to be formed by conventional methods, and are especially suitable for precision forming of smooth and small-curvature plate parts.

The present invention adopts the following methods to achieve the above object: according to the curved surface shape of the processed parts, the required stress field distribution shape is simulated by the computer control system, and the shock wave pressure and shot blasting track distribution acting on the surface of the sheet are optimized; according to the optimized shock wave Pressure and shot blasting track distribution parameters, the computer control system automatically selects the laser pulse parameters, the parameters of the beam conversion device and the optical mask, and compiles the NC processing program for the computer control system to control the light guide system and the trajectory of the workbench; A high-energy short-pulse intense laser can be converted into several group pulses through the beam conversion device, and then act on the energy conversion body on the surface of the workpiece through the optical mask. The energy conversion body converts the absorbed group pulse laser energy into shock wave pressure It acts on the surface of the workpiece to realize forming.

The spot mode of the laser beam emitted by the laser can be one of fundamental mode and multi-mode, the pulse energy is 15-60J, and the pulse width is 20-30ns.

The computer control system controls the contour scanning measurement system to measure the deformation of the sheet during the shot peening of the sheet, and then the computer control system performs error analysis, stress field and dynamic simulation of the deformation of the sheet, and the laser controller and laser shot peening head Adjust the laser pulse energy, shot peening intensity and shot peening trajectory to realize the precise forming of the sheet.

The device for implementing the method includes a laser generator, a light guide system, a laser shot peening head, a workpiece fixture system and a control system.

The laser generator consists of three stages: a first-stage Q-switched neodymium glass (φ16mm×280mm) laser oscillator, a resonant cavity length of 800mm, a first-stage pre-positioned neodymium glass (φ16mm×200mm) laser amplifier and a first-stage neodymium glass (φ20mm×500mm ) laser main amplifier, the output laser wavelength is 1.06μm, the pulse energy is 10-60J, the pulse width is 5-30ns, the repetition frequency is 2Hz, and the peak power density is 5×10 ⁹ W/cm ² .

The light guide system is an optical path transmission system connecting the laser generator to the workpiece, which is composed of a light guide tube, a turning mirror, and a beam conversion device;

The laser shot peening head is equipped with a focusing mirror and an optical mask. The focusing lens group includes a focusing mirror, a rotatable mirror, and a prism. The optical mask can screen the laser pulses according to the requirements of the shot peening, so as to obtain the required pulses. distribution method.

The workpiece fixture system includes a five-axis linkage CNC worktable and a workpiece fixture. The workpiece sheet with an energy conversion body attached to the surface is clamped in the workpiece fixture, and the workpiece fixture is installed on the CNC workbench. Axis movement and two-axis rotation meet the motion requirements required for the shot peening process;

The control system consists of a contour scanning measurement system, a machine tool controller, a laser controller, and a computer control system. The computer control system controls the laser controller, the machine tool controller, and the contour scanning measurement system respectively.

The laser emitted by the laser generator is connected to the laser shot peening head through the light guide system, and the shot peening action is performed on the workpiece sheet under the control of the laser controller. The required shape; the laser controller, machine tool controller and contour scanning measurement system, laser shot peening head are connected with the computer control system, so as to realize the effective control of the working process of the whole system.

The implementation process of the present invention is as follows:

According to the curved surface shape of the processed parts, the required stress field distribution shape is simulated by a special computer control system, and the shock wave pressure and track distribution acting on the surface of the sheet are optimized;

(1) According to the parameters such as optimized shock wave pressure and trajectory distribution, the computer control system automatically selects the parameters of the laser pulse, the optical conversion device and the optical mask, and compiles the NC processing program for the control system to control the light guide system and the workbench. track;

(2) Cover the surface of the workpiece with an energy conversion body to excite a high-energy pulse laser, and the group pulse formed by the optical conversion device impacts the energy conversion body through an optical mask, and converts it into a high-amplitude shock wave. Forming deformability. By changing the laser pulse energy, pulse width, pulse shape and laser beam mode, different plastic deformation forces can be obtained;

(3) According to the selected pulse laser process parameters, laser peening trajectory and the number of pulses, the stress pulse is applied in an orderly manner through the five-axis linkage numerical control system to realize the predetermined stress field distribution in the sheet and realize the required shape. Sheet metal forming;

(4) Through the contour scanning measurement system, the deformation of the sheet metal in the shot peening of the sheet metal is measured, the error analysis is carried out by the computer control system, the stress field and the deformation of the sheet metal are dynamically simulated, and the laser pulse energy and shot peening are adjusted by the control system. Strength and shot blasting trajectory, to achieve precision forming of sheet metal.

Because the present invention uses high-energy pulsed laser beams instead of quality projectiles to implement shot peening of sheet metal, it is non-contact forming, with no obvious mechanical damage on the forming surface, low surface roughness, and good finish; due to the use of energy conversion technology , realize the transformation of laser energy to shock wave pressure, and protect the workpiece surface from the thermal damage of laser; due to the use of optical conversion device, the group pulse pressure action mode is used, which greatly improves the efficiency of laser shot peening; due to the use of control pulse Laser parameters, in the process of each laser action, the sheet does not produce macroscopic plastic deformation, but forms a moderate residual compressive stress on the surface of the sheet, so small energy can be used to achieve large deformation; because by controlling the laser parameters and the action trajectory, Control the stress field on the surface of the workpiece to realize sheet metal forming, so the process and control are easier to realize; because the laser parameters are accurate and controllable, the process is repeatable and easy to realize automatic production; because the laser peening process is on the surface of the material The formation of a hardened layer and high residual compressive stress improves the stability of sheet metal forming parts, and there is almost no springback, so the forming of small curvature parts can be realized. At the same time, the improvement of the surface quality of the formed parts and the existence of high-amplitude residual stress can greatly improve the fatigue resistance and stress corrosion resistance of the workpiece, eliminating the need for subsequent processing procedures and surface treatment procedures. Since the laser peening process leads to deeper residual compressive stress (greater than 1 mm), and the magnitude of the residual stress can be precisely controlled by laser parameters, it has potential advantages in flexible precision forming of medium-thick plates.

Description of drawings

Fig. 1 is a schematic diagram of laser peening forming method and device.

Fig. 2 Schematic diagram of the principle of laser shot peening for medium and thick plates.

Figure 3 The relationship between the number of laser peening times and the bending deformation of the sheet metal under the straight shot peening trajectory

Figure 4 Schematic diagram of laser peening process for cylindrical parts

Fig.5 Schematic diagram of saddle-shaped parts formed by laser shot peening

1-Laser generator 2-Light guide wall 3-Returning mirror 4-Beam conversion device 5-Optical mask 6-Shot blasting head 7-Contour scanning measuring instrument 8-Workpiece fixture 9-Multi-axis linkage workbench 10-Machine control Device 11-Laser controller 12-Computer control system 13-Pulse laser beam 14-Energy conversion body 15-Workpiece 16-Formed parts

Detailed ways

The details and working conditions of the specific device proposed by the present invention will be described in detail below in conjunction with FIG. 1 .

The device includes a laser generator (1), a light guide system, a shot blasting head (6), a work clamp system and a control system. The crystal used in the laser is Nd:YAG, which generates laser pulses with an energy of 10-60 joules and a duration of 5-30 nanoseconds. The spot mode of the laser beam (13) can be a fundamental mode, a multi-mode, etc. It is regulated and controlled by the laser controller (11). The light guide system includes a light guide wall (2), a return mirror (3), and a light beam conversion device (4). The shot blasting head (6) is equipped with multiple focusing mirrors and optical masks (5), which can move vertically , the spot size can be changed by adjusting the size of the focusing mirror and the optical mask (5). The workpiece fixture system includes a workpiece fixture (8) and a multi-axis linkage worktable (9). The device has three coordinates of movement and two rotations, and can realize five-axis linkage, so that the workpiece (15) can be easily laser sprayed. The pill is formed. The control system is composed of a contour scanning measurement system (7), a machine tool controller (10), a laser controller (11), and a computer control system (12). The laser beam generated by the laser (1) passes through the light guide wall (2), the turning mirror (3), the beam conversion device (4), the optical mask (5) and hits the sheet to be processed ( 15), the contour scanning measurement system (7), the machine tool controller (10), and the laser controller (11) are connected with the computer control system, so as to realize the effective control of the working process of the whole system.

Fig. 2 is a schematic diagram of the principle of laser shot peening, the group pulse laser beam (13) output by the light guiding system in Fig. The energy conversion body instantly vaporizes and ionizes the absorbed high-energy laser light into shock wave mechanical energy, which acts on the surface of the workpiece (15) to be processed. The workpiece (15) is fixed on the fixture (8) in Fig. 1, and the workpiece fixture (8) is fixed on the multi-axis linkage workbench (9). The machine tool controller (10) issues numerical control commands to the multi-axis linkage worktable (9) to perform multi-axis movement. Under the action of the group pulse laser, the required high-amplitude stress field distribution is generated inside the sheet metal, and the deformation of the sheet metal is achieved. Forming part (16) of desired shape.

Fig. 3 shows the pulsed laser beam (13) output through the shot peening head, along a certain straight track, the experimental result of the 45 steel with a thickness of 8mm being shot peened, the shape of the plate deformation after the shot peening is with V-shape with rounded corners. Increase the number of shot peening along the same path, respectively 1 to 3 times, and the bending deformation of the sheet metal will increase.

Fig. 4 is the schematic diagram of laser shot-peening forming cylindrical surface, laser beam (13) carries out shot-peening to workpiece along three straight lines parallel to each other after outputting from shot-peening head (6), if the distance between shot-peening narrow strips is large enough, When there is a plane between the two shot-peened narrow strips, a prism surface can be formed. When the distance between the centerlines of adjacent shot-peened narrow strips is relatively close, the deformation zones are connected to each other and a smooth cylindrical surface can be formed.

Fig. 5 shows that according to the trajectory shot peening optimized by the computer control system, with 6 * 6 Φ2 group pulse laser beams (13) to the workpiece surface (15) continuous shot peening, the saddle-shaped curved surface shape schematic diagram that can be obtained. During the shot peening process, according to the curvature of the formed part, the laser parameters can be continuously changed to achieve precise forming. When shot peening along one or several curves, special-shaped parts with compound curvature can be obtained, such as the upper and lower aerodynamic bending areas of the double curvature of the aircraft wing.

More forming examples are too numerous to enumerate. The difference of shot peening trajectory and shot peening intensity can form various complicated shapes, and here only illustrate its technical scheme and some embodiments.

Claims (5)

1. A method for laser shot peening of medium-thick plates, characterized in that: according to the curved surface shape of the processed parts, the required stress field distribution shape is simulated by a computer system, and the shock wave pressure and spraying force acting on the surface of the plate are optimized. Shot trajectory distribution; according to the optimized shock wave pressure and shot peening trajectory distribution parameters, the computer system automatically selects the parameters of the laser pulse, beam conversion device and optical mask, and compiles the NC processing program for the computer control system to control the light guide system and work. The trajectory of the stage; a high-energy short-pulse intense laser emitted by the laser can be converted into several group pulses through the beam conversion device, and then act on the energy conversion body on the surface of the workpiece through the optical mask, and the energy conversion body converts the absorbed group pulses The pulsed laser energy is converted into shock wave pressure and acts on the surface of the workpiece to realize forming. 2. A method for laser shot peening of medium-thick plates according to claim 1, characterized in that: the spot mode of the laser beam emitted by the laser can be one of fundamental mode and multi-mode, and the pulse energy is 15- 60J, pulse width 20-30ns. 3. A method for laser shot peening forming of medium-thick plates according to claim 1, characterized in that: at the same time as the laser impact forming, the computer system controls the contour scanning measurement system to measure the thickness of the plate during the shot peening of the plate. The amount of deformation is analyzed by the computer system, and the dynamic simulation of the stress field and the deformation of the sheet metal is carried out. The laser pulse energy, shot peening intensity and shot peening trajectory are adjusted by the laser controller and the laser shot peening head to realize the precise forming of the sheet metal. 4. A laser shot peening device for medium-thick plates, characterized in that the device includes a laser generator, a light guide system, a laser shot peening head, a workpiece fixture system, and a control system; the laser generator is composed of three stages: the first stage Q-switched neodymium glass (φ16mm×280mm) laser oscillator, resonant cavity length 800mm, first-stage front-stage neodymium glass (φ16mm×200mm) laser amplifier and first-stage neodymium glass (φ20mm×500mm) laser main amplifier, the output laser wavelength 1.06μm, pulse energy 10~60J, pulse width 5~30ns, repetition frequency 2Hz, maximum peak power density 5×10 ⁹ W/cm ² ; the light guide system is the optical path transmission between the laser generator and the workpiece The system consists of a light pipe, a turning mirror, and a beam conversion device; a focusing mirror and an optical mask are arranged in the laser peening head, and the focusing lens group includes a focusing mirror, a rotatable mirror, and a prism; the workpiece fixture system includes a five-axis The CNC workbench and the workpiece fixture are linked together, the workpiece sheet with the energy conversion body attached to the surface is clamped in the workpiece fixture, and the workpiece fixture is installed on the CNC workbench; the control system is composed of a machine tool controller, a laser controller, and a computer system. The computer system controls the laser controller and the machine tool controller respectively; the laser emitted by the laser generator is connected to the laser shot peening head through the light guide system, the workpiece fixture system is located under the shot peening head and connected to the machine tool controller, the laser generator and the laser control The laser shot peening head, the machine tool controller and the laser controller are connected to the computer system respectively. 5. A laser shot-peening forming device for medium-thick plates according to claim 4, characterized in that: a contour scanning measurement system is provided, which is connected with a computer control system.

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