Construction method of bimodal coherent combination laser and additive manufacturing method

Abstract

The invention relates to a construction method of bimodal coherent combined laser and an additive manufacturing method. The method comprises the steps that pulse seed laser and continuous seed laser are generated and synchronized; pulse laser and continuous laser are obtained; s3, enabling the phases of the pulse laser and the continuous laser to tend to be synchronous to reach a phase-locked state, and then carrying out beam combination to form coherent combined laser; s4, performing phase compensation and optical path difference adjustment on the coherent combined laser to obtain high-quality coherent combined laser; and S5, the coherent combined laser and the auxiliary oscillation laser are shaped into a flat-topped light beam, the flat-topped light beam is shaped into flat-topped annular coherent combined laser and flat-topped annular oscillation laser, and the flat-topped annular oscillation laser can be subjected to coaxial and paraxial switching. The method has the advantage of stable continuous laser energy output and the advantages of small pulse laser heat affected zone and difficult reflection, the additive manufacturing process is high in precision and high in efficiency, the mechanical property of parts is excellent macroscopically, and the structure of the parts is uniform microcosmically.

Description

Technical Field

[0001] The present invention relates to the technical field of additive manufacturing, and specifically to a method for constructing a dual-modal coherent composite laser and an additive manufacturing method, which are applicable to the rapid prototyping of high-precision complex metal structures. Background Art

[0002] With the rapid development of high-end manufacturing industries such as aerospace, automotive, and energy, the requirements for the performance and manufacturing precision of components are constantly increasing. Traditional manufacturing processes often face problems such as serious material waste, long processing cycles, and difficulty in realizing the integrated manufacturing of complex structures when manufacturing components with complex shapes and high-performance materials.

[0003] Since the birth of additive manufacturing technology in the 1980s, it has manufactured components in a layer-by-layer stacking manner, fundamentally changing the traditional concept of "subtractive manufacturing". It can directly manufacture components with complex shapes according to three-dimensional models, greatly shortening the product R & D cycle, improving material utilization rate, and reducing production costs. However, early additive manufacturing technologies were difficult to meet the manufacturing requirements for large-scale and high-performance metal components in high-end fields such as aerospace.

[0004] To solve the problems of additive manufacturing of large metal components, coaxial laser additive manufacturing technology has gradually developed. The coaxial powder / wire feeding process is to feed metal powder / wire along the axis of the laser beam into the molten pool through a powder / wire feeding device. The laser beam melts the powder / wire and fuses it with the substrate material to achieve layer-by-layer stacking of materials. This process has the advantages of fast powder / wire feeding speed, high material utilization rate, and the ability to manufacture large and complex components, and can effectively meet the manufacturing requirements for large metal structures in fields such as aerospace, shipbuilding, and energy, becoming a research hotspot and development direction in the field of laser additive manufacturing in recent years. Currently, the coaxial powder / wire laser additive manufacturing technology has the following technical bottlenecks: 1. Coaxial powder / wire feeding technology: Currently, the mainstream coaxial powder / wire feeding systems mostly adopt the external optical powder / wire feeding mode and use Gaussian laser as the energy source. The characteristics of large energy distribution gradient and concentrated energy at the center of the light spot are prone to causing metal spatter. And the existing internal optical powder / wire feeding modes, such as multi-beam convergence or fixed annular light spot design (such as CN112584962A), still have disadvantages such as sensitivity to the optical fiber coupling accuracy (±0.1 mm tolerance).

[0005] 2. Laser heat source mode: Single-modal lasers (continuous or pulsed) are difficult to balance the requirements of the thermal input stability of the molten pool and grain refinement, and the matching relationship between laser energy input and powder / wire feeding speed is complex. When the laser energy is too high and the powder / wire feeding speed is too slow, the temperature of the molten pool is too high, which is easy to cause excessive metal evaporation and pore generation; conversely, if the laser energy is insufficient and the powder / wire feeding speed is too fast, the powder / wire material cannot be fully melted, resulting in unfused defects.

[0006] 3. Material Compatibility Limitations: Although coaxial powder / wire laser additive manufacturing can use a variety of metal powders / wires, there are still limitations in material compatibility. For example, materials with high reflectivity to infrared lasers, such as aluminum alloys (the reflectivity can be as high as 90%), using expensive blue or green lasers will significantly increase costs.

[0007] 4. Application of Auxiliary Light Sources: Existing oscillating auxiliary lasers mostly adopt a single path (such as linear or circular), lacking dynamic regulation ability, and generally not having the ability to switch between paraxial / coaxial, with relatively single functions.

[0008] To solve the technical bottlenecks of coaxial laser additive manufacturing technology and achieve the rapid prototyping of high-precision complex metal components, an additive manufacturing method combining continuous / pulse dual-mode laser coherent synthesis, annular flat-top laser shaping, and multi-degree-of-freedom oscillating auxiliary light sources is required. Summary of the Invention

[0009] The purpose of the present invention is to provide a method for constructing a dual-mode coherent synthesis laser and an additive manufacturing method to solve the bottleneck problems of traditional laser additive manufacturing technology that uses a single-mode laser as the energy source, such as large heat-affected zones, low processing efficiency, low processing accuracy, and easy generation of composition segregation.

[0010] To achieve the above purpose, the present invention provides the following technical solutions: A method for constructing a dual-mode coherent synthesis laser, the method sequentially includes the following steps: S1. Generation and Synchronization of Seed Sources: Generate a pulsed seed source and a continuous seed source. The pulsed seed source is processed to generate a pulsed seed laser, and the continuous seed source is processed to generate a continuous seed laser; S2. Beam Splitting and Amplification of Lasers: The pulsed seed laser undergoes beam splitting and amplification to become a pulsed laser, and the continuous seed laser undergoes beam splitting and amplification to become a continuous laser; S3. Phase Locking and Synthesis of Lasers: After the phases of the pulsed laser and the continuous laser tend to be synchronized to reach the phase-locked state, they are combined to form a coherent synthesis laser; S4. Output Optimization and Monitoring of the Coherent Synthesis Laser: Perform phase compensation and adjustment of the optical path difference on the coherent synthesis laser to obtain a high-quality coherent synthesis laser; S5. Shaping and Coupling of Beams: The coherent synthesis laser and the auxiliary oscillating laser obtained in step 4 undergo spatial reconstruction of energy through a flat-top beam shaping unit to form a flat-top beam. The flat-top beam then passes through an annular beam shaping unit to form a flat-top annular coherent synthesis laser and a flat-top annular oscillating laser. The flat-top annular oscillating laser can be switched to a coaxial oscillating laser or a paraxial oscillating laser, and real-time switching of the path can be achieved.

[0011] Preferably, the pulsed seed source in step 1 forms a pulsed seed laser after mode-locking technology and modulation processing, and the continuous seed source in step 1 forms a continuous seed laser after frequency stabilization technology processing.

[0012] Preferably, the pulsed seed laser and the continuous seed laser in step 2 are split by a polarization beam splitter and amplified by an amplifier chain. The pulsed seed laser needs to be additionally chirped pulse amplified.

[0013] Preferably, the pulsed laser and the continuous laser in step 3 reach a phase-locked state through active phase-locking technology or passive phase-locking technology, and are combined after phase-locking through a polarization beam combiner.

[0014] Preferably, the coherently combined laser in step 4 is phase-compensated and the optical path difference is adjusted through a wavefront sensor and an M 2 factor meter to achieve output optimization and monitoring of the coherently combined laser.

[0015] Preferably, the wavelength of the continuous laser in step 3 is 1060 - 1080 nm, the power is 1 - 6 kW, the wavelength of the pulsed laser is 1030 - 1070 nm, the peak power is 10 7 -10 8 W / cm 2 , the pulse width is 10 - 100 ns, the power fluctuation of the coherently combined laser is ≤±1.5%, and the ratio of the continuous laser and the pulsed laser in the coherently combined laser is adjustable to match different materials. For example, materials with high reflectivity such as aluminum alloy require an increased ratio of the pulsed laser, and materials with low reflectivity such as steel can reduce the ratio of the pulsed laser.

[0016] Preferably, the spot diameter of the flat-top annular coherently combined laser in step 5 is adjustable from 2 to 8 mm, and the energy uniformity is ≥95%. The spot distortion is corrected in real time by Zernike polynomials, which can adapt to wire / powder feeding at an inclination of ±90°, and the material utilization rate can reach nearly 100%.

[0017] The present invention also includes an additive manufacturing method based on the dual-mode coherently combined laser, and the method is as follows: the metal wire / powder is transported into the laser head through a wire / powder feeding mechanism and is coaxial with the flat-top annular coherently combined laser. The metal wire / powder is melted under the action of the flat-top annular coherently combined laser to form a molten pool, and the molten pool solidifies to form a weld bead. The additive manufacturing of the part is finally completed by continuously running along a predetermined trajectory; during the additive process, the auxiliary oscillating laser is adjusted to a paraxial oscillating laser or a coaxial oscillating laser in real time based on the molten pool monitoring data, and at the same time, the path of the auxiliary oscillating laser is dynamically adjusted. The coaxial mode is mainly used for stirring the center of the molten pool, and the paraxial mode is mainly used for balancing the edge temperature field and precisely shaping complex contours.

[0018] Preferably, the wire / powder feeding mechanism supports wire feeding / powder feeding switching or the cooperative feeding of wire and powder. The wire diameter is 0.8 - 1.6 mm, the powder diameter is 50 - 200 μm, and the powder is spherical powder.

[0019] Preferably, the path of the auxiliary oscillating laser can be dynamically adjusted by a galvanometer. The path includes linear, circular, figure-eight, or ∞-shaped oscillation, with a frequency of 10 - 500 Hz and an amplitude of 0.3 - 3 mm.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention uses continuous laser and pulsed laser for coherent synthesis, and uses the coherent synthesis laser as an energy source for additive manufacturing. The coherent synthesis laser can combine the advantages of continuous laser (continuous and stable heat input, high beam quality, etc.) and pulsed laser (small heat affected zone, easy to refine grains, can process high reflectivity materials, etc.), and compared with independent continuous laser and pulsed laser, it has the advantages of high peak power density, good beam quality, and stable transmission.

[0021] (2) The present invention adopts an annular flat-top laser shaping technology (integrating a free-form lens group and a diffractive optical element), which can make the energy distribution of the laser present a uniform ring shape. Compared with the energy distribution characteristics of Gaussian laser with high center and low edge, it can significantly reduce the temperature gradient of the molten pool, optimize the macroscopic morphology of the weld bead, and reduce the microscopic segregation and residual stress after solidification of the molten pool. Further, the spatial reorganization of energy can also avoid metal gasification and molten pool splash caused by too high center energy.

[0022] (3) The present invention uses oscillating laser as an auxiliary laser to assist the additive process. The oscillating laser can promote the overflow of bubbles through high-frequency agitation to reduce porosity, and at the same time promote the transformation from columnar crystals to equiaxed crystals and weaken the texture, ultimately improving the surface quality and mechanical properties of the part.

[0023] (4) The present invention realizes linear / circular / figure-eight / ∞-shaped laser oscillation through a galvanometer system, and based on the molten pool monitoring data, it can automatically switch the laser oscillation mode (coaxial mode or off-axis mode), combining the advantages of coaxial laser with omnidirectional unrestriction and low forming porosity, and the advantages of off-axis laser that can precisely adjust the incident angle, power, and scanning path to complete the precise shaping of complex contours without affecting the already formed part.

[0024] (5) The present invention has strong material adaptability. The present invention can freely adjust the proportion of continuous laser, pulsed laser, and oscillating laser, so it can be applied to the processing of different types of materials (such as high reflectivity materials like aluminum and copper, and low reflectivity materials like steel and titanium).

[0025] (6) The process of the present invention has strong scalability. The present invention does not limit the physical state of the material and is equally applicable to powder additive manufacturing and wire-powder collaborative additive manufacturing. The control quality of the present invention depends on the accuracy of the monitoring system and the control algorithm, and artificial intelligence (AI) can be used to assist in real-time adjustment of the laser process parameters. Description of the Drawings

[0026] Figure 1 is the general flow chart of the additive manufacturing method based on the present invention; Figure 2 is the flow chart of the dual-mode laser coherent synthesis of the present invention; Figure 3 is the schematic diagram of the dual-mode laser coherent synthesis in the embodiment of the present invention; Figure 4 is the schematic diagram of the dual-mode laser coherent synthesis and dynamic oscillation-assisted additive manufacturing in the embodiment of the present invention; Reference numerals in the drawings: 1. Additive substrate; 2. Coherent synthesis laser pulse laser component; 3. Coherent synthesis laser continuous laser component; 4. Coaxial flat-top ring oscillation laser; 5. Paraxial flat-top ring oscillation laser; 6. Controller; 7. Metal wire. Detailed Embodiments

[0027] In order to clearly and completely describe the objectives, technical solutions of the present invention and make the advantages more clearly understood, the following further details the embodiments of the present invention with reference to the drawings. It should be understood that the specific embodiments described herein are part of the embodiments of the present invention, rather than all of the embodiments, and are only used to explain the embodiments of the present invention, not to limit the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. Embodiment

[0028] Please refer to Figures 1 to 4 , the additive manufacturing method of this embodiment includes the following steps: 1. Pre-treatment before additive manufacturing: According to the structural characteristics of the part to be additively manufactured, its typical structure is divided, and the additive manufacturing path is accurately planned. Subsequently, the program is input into the controller 6 to ensure the smooth and efficient progress of subsequent processing operations. The surface of the additive substrate 1 carrying the additively manufactured part is treated. The specific treatment method is as follows: First, its surface is polished to ensure it is flat and smooth, and then the shot peening process is used to increase its surface roughness to improve the absorption rate of the additive substrate 1 for the laser. After shot peening, it is cleaned with acetone to remove surface dirt. Subsequently, the additive substrate 1 is placed in a preheating device and preheated to 400 °C and maintained at this temperature throughout the subsequent processing.

[0029] 2. Constructing a dual-mode coherent synthesis laser AsFigure 1 , 2 As shown in 2 , the construction method of the dual - mode coherent combined laser successively includes the following steps: S2.1 Generation and synchronization of seed sources: Turn on the first laser generator and the second laser generator simultaneously. A pulsed seed source is formed inside the first laser generator, and a continuous seed source is formed inside the second laser generator. The pulsed seed source is formed into pulsed seed laser through mode - locking technology and modulation processing, and the continuous seed source is formed into continuous seed laser through frequency - stabilization technology processing.

[0030] S2.2 Beam splitting and amplification of lasers: After the pulsed seed laser passes through the polarization beam splitter, it is separated according to the polarization state. Subsequently, with the help of chirped pulse amplification technology, the initial low - energy ultrashort pulsed laser is broadened, increasing its pulse width and dispersing the energy in time. Immediately afterwards, the broadened pulsed laser is amplified to a high - energy level through an amplifier chain. Finally, the amplified pulse is compressed by a compressor to transform it back into pulsed laser, and at this time the peak power of this laser is greatly increased.

[0031] The continuous seed laser also passes through the polarization beam splitter and is separated according to the polarization state, then the continuous seed laser is amplified to a high - energy level using an amplifier chain, and finally a high - energy continuous laser is formed.

[0032] S2.3 Phase locking and combination of lasers: The first method is the active phase - locking method. Set a high - precision phase modulator. Based on the feedback control signal, the phase modulator precisely regulates the phase of the pulsed laser to make it match the phase of the continuous laser. The second method is the passive phase - locking method. The pulsed laser and the continuous laser are jointly introduced into a shared resonant cavity. Special components such as nonlinear optical crystals in the cavity cause unique optical effects. Affected by the nonlinear effect, there will be an interaction between the two lights, which can change the phase characteristics of the laser and prompt the phases of the pulsed laser and the continuous laser to gradually and spontaneously tend to be synchronized.

[0033] After reaching the phase - locked state, use a polarization beam combiner to combine the phase - adjusted pulsed laser and continuous laser into the same optical path, forming the coherent combined laser pulsed laser component 2 and the coherent combined laser continuous laser component 3, and finally realizing coherent combination.

[0034] The wavelength of the continuous laser described in step 2.3 is 1060 - 1080 nm, the power is 1 - 6 kW, the wavelength of the pulsed laser is 1030 - 1070 nm, and the peak power is 10 7 -10 8 W / cm 2, the pulse width is 10 - 100 ns, the power fluctuation of the coherently combined laser is ≤ ±1.5%, and the ratio of the continuous laser to the pulsed laser in the coherently combined laser is adjustable to match different materials. For example, materials with high reflectivity such as aluminum alloy require an increase in the ratio of the pulsed laser, while materials with low reflectivity such as steel can reduce the ratio of the pulsed laser.

[0035] S2.4. Output Optimization and Monitoring of the Coherently Combined Laser: The wavefront sensor monitors the wavefront of the combined laser in real time to obtain distortion information, and uses adaptive optical elements such as deformable mirrors to correct it in real time to flatten the wavefront and improve the beam quality. The M2 factor meter measures the divergence characteristics of the coherently combined laser. By adjusting the positions of the elements in the resonant cavity, optimizing the characteristics of the optical medium, changing the laser injection parameters, etc., the laser divergence angle can be reduced, the energy can be concentrated, the long-distance transmission stability and focusing ability can be enhanced, and the optimization of the coherently combined laser can be achieved to output high-quality coherently combined laser.

[0036] S2.5. Beam Shaping and Coupling The coherently combined laser and the auxiliary oscillating laser (the galvanometer required to realize the oscillating laser is installed inside the laser head, which is actually a continuous laser here. To distinguish it from the continuous laser in the coherently combined laser, it is directly described as an oscillating laser here) undergo spatial reconstruction of energy through a flat-top beam shaping unit to form a flat-top beam. The shaped flat-top beam then passes through an annular beam shaping unit to form a flat-top annular coherently combined laser (main light source) and a flat-top annular oscillating laser (auxiliary light source). The flat-top annular oscillating laser can be switched to a coaxial flat-top annular oscillating laser 4 or a paraxial flat-top annular oscillating laser 5 under the control of the controller 6, and real-time switching of the path (linear / circular / 8-shaped / ∞-shaped oscillation) can be achieved. The spot diameter of the flat-top annular coherently combined laser is adjustable from 2 to 8 mm, and the energy uniformity is ≥ 95%. The spot distortion is corrected in real time by the Zernike polynomial, which can adapt to wire / powder feeding with a ±90° tilt, and the material utilization rate can reach nearly 100%.

[0037] 3. Additive Process Such as Figure 4As shown, this embodiment takes coaxial wire feeding additive manufacturing as an example. The wire feeder feeds the metal wire 7 into the collimation mechanism for straightening, and then into the laser head, where it forms a coaxial relationship with the flat-top annular coherent composite laser. The metal wire 7 melts under the action of the flat-top annular coherent composite laser and forms a molten pool on the surface of the additive substrate 1. With the assistance of the coaxial flat-top annular oscillating laser, the molten pool undergoes intense agitation, the gas inside the molten pool will accelerate to escape, reducing the formation of pores, and to a certain extent, the agitation of the molten pool will break the dendritic crystal arms at the solidification front to increase the nucleation rate and refine the grain size. When it is necessary to perform fine processing on a specific area of the part, such as secondary remelting of the local surface to improve the surface quality, or adding additional materials at specific positions to enhance the structural strength, the controller 6 switches the auxiliary oscillating laser to the off-axis mode. By precisely adjusting the incident angle, power, and scanning path of the off-axis flat-top annular oscillating laser 5, the precise shaping of complex contours can be completed without affecting the already formed part. Finally, the above process is cycled according to a predetermined trajectory to complete the additive process of the entire part.

[0038] Furthermore, the wire / powder feeding mechanism supports wire feeding / powder feeding switching or the coordinated feeding of wire and powder. The wire diameter is 0.8 - 1.6 mm, the powder diameter is 50 - 200 μm, and the powder is spherical powder. The path of the auxiliary oscillating laser can be dynamically adjusted through a galvanometer, and the path includes linear, circular, figure-eight, or ∞-shaped oscillations, with a frequency of 10 - 500 Hz and an amplitude of 0.3 - 3 mm. Real-time dynamic monitoring can dynamically adjust the laser power (response time < 10 ms) and wire feeding speed (accuracy ≥ 0.1 m / min) through the PID algorithm.

[0039] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A construction method of a dual-mode coherent combined laser, characterized in that: The method sequentially includes the following steps: S1. Generation and synchronization of seed sources: Generate a pulsed seed source and a continuous seed source. The pulsed seed source is processed to generate a pulsed seed laser, and the continuous seed source is processed to generate a continuous seed laser. S2. Beam splitting and amplification of the laser: The pulsed seed laser is split and amplified to become a pulsed laser, and the continuous seed laser is split and amplified to become a continuous laser. S3. Phase locking and synthesis of the laser: After the phases of the pulsed laser and the continuous laser tend to be synchronized to reach the phase-locked state, they are combined to form a coherently synthesized laser. S4. Output optimization and monitoring of the coherently synthesized laser: Perform phase compensation and adjustment of the optical path difference on the coherently synthesized laser to obtain a high-quality coherently synthesized laser. S5. Beam shaping and coupling: The coherently synthesized laser obtained in step 4 and the auxiliary oscillating laser undergo spatial reconstruction of energy through a flat-top beam shaping unit to form a flat-top beam. The flat-top beam then passes through an annular beam shaping unit to form a flat-top annular coherently synthesized laser and a flat-top annular oscillating laser. The flat-top annular oscillating laser can be switched to a coaxial oscillating laser or a paraxial oscillating laser, and real-time switching of the path can be achieved.

2. The construction method according to claim 1, wherein: The pulsed seed source described in step 1 forms a pulsed seed laser after mode-locking technology and modulation processing, and the continuous seed source described in step 1 forms a continuous seed laser after frequency stabilization technology processing.

3. The construction method according to claim 1, wherein: The pulsed seed laser and the continuous seed laser described in step 2 are split by a polarization beam splitter and amplified by an amplifier chain. The pulsed seed laser needs to be additionally subjected to chirped pulse amplification.

4. The construction method according to claim 1, characterized in that: The pulsed laser and the continuous laser described in step 3 reach the phase-locked state through active phase-locking technology or passive phase-locking technology, and are combined after phase-locking through a polarization beam combiner.

5. The construction method according to claim 1, characterized in that: The coherent combined laser described in step 4 is subjected to phase compensation and adjustment of the optical path difference through a wavefront sensor and an M 2 factor instrument to achieve output optimization and monitoring of the coherent combined laser.

6. The construction method according to claim 1, characterized in that: The wavelength of the continuous laser described in step 3 is 1060 - 1080 nm, the power is 1 - 6 kW, the wavelength of the pulsed laser is 1030 - 1070 nm, and the peak power is 10 7 - 10 8 W / cm 2 , the pulse width is 10 - 100 ns, the power fluctuation of the coherently combined laser is ≤ ±1.5%, and the ratio of the continuous laser to the pulsed laser in the coherently combined laser is adjustable to match different materials.

7. The construction method according to claim 1, wherein: The spot diameter of the flat-top annular coherently synthesized laser described in step 5 is adjustable from 2 to 8 mm, and the energy uniformity is ≥95%.

8. An additive manufacturing method based on the dual-mode coherent combined laser according to claim 1, characterized in that The method is as follows: The wire / powder is transported into the laser head through a wire / powder feeding mechanism and is coaxial with the flat-top annular coherently synthesized laser. The wire / powder is melted under the action of the flat-top annular coherently synthesized laser to form a molten pool, and the molten pool solidifies to form a weld bead. It continuously runs along a predetermined trajectory to finally complete the additive manufacturing of the part. During the additive process, the auxiliary oscillating laser is adjusted to be a paraxial oscillating laser or a coaxial oscillating laser in real time based on the molten pool monitoring data, and at the same time, the path of the auxiliary oscillating laser is dynamically adjusted. The coaxial mode is mainly used for stirring the center of the molten pool, and the paraxial mode is mainly used for balancing the edge temperature field and precisely shaping complex contours.

9. The additive manufacturing method according to claim 8, wherein: The wire / powder feeding mechanism supports wire feeding / powder feeding switching or coordinated feeding of wire and powder. The wire diameter is 0.8 - 1.6 mm, the powder diameter is 50 - 200 μm, and the powder is spherical powder.

10. The additive manufacturing method according to claim 8, characterized in that: The path of the auxiliary oscillating laser can be dynamically adjusted through a galvanometer. The path includes linear, circular, figure-eight, or ∞-shaped oscillation, with a frequency of 10 - 500 Hz and an amplitude of 0.3 - 3 mm.

Images