Laser welding method

Abstract

The invention discloses a laser welding method which comprises the following steps: firstly, fixing a PMMA (Polymethyl Methacrylate) workpiece, adopting laser with the wavelength of 1900-2000nm, forming an elliptical light spot of which the long axis is parallel to a welding seam through a beam shaping module, preheating, and superposing transverse oscillation; then, a light spot is switched into a small-size circle, the power and the scanning speed are remarkably improved, a laser focus is accurately aligned with an upper-layer interface and a lower-layer interface, high-speed spiral scanning motion is superposed, and sufficient fusion and stirring of an interface material are achieved; after welding is completed, the light spot form, the laser power, the scanning speed and the focus position are linearly and gradually changed back to the initial state within the transition time, and a heat source is closed; and finally, the workpiece is subjected to parameter-controllable forced air cooling. By dynamically regulating and controlling the light spot shape, the energy distribution and the scanning track, the problems of thermal stress concentration, material degradation, joint performance degradation and the like in the PMMA welding process are effectively solved, and the mechanical strength, the optical transparency and the overall quality consistency of a welded joint are remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser welding, in particular to a laser welding method. BACKGROUND

[0002] As a thermoplastic engineering plastic with high transparency, excellent weather resistance and good processing performance, polymethyl methacrylate (PMMA) is widely used in high value-added fields such as optical devices, medical equipment, automotive lighting and precision electronic packaging. In these application scenarios, the connection quality of PMMA components is extremely demanding: not only the intrinsic high transmittance and low dispersion characteristics need to be maintained, but also the joint needs to have sufficient mechanical strength and geometric stability.

[0003] Under this background, laser transmission welding gradually develops into the mainstream process path for PMMA material connection due to its non-contact, high precision, low heat input and automation advantages. The existing technology generally uses near-infrared waveband laser source to realize internal heating by using the "bulk absorption" characteristics of the material. The typical process configuration includes a circular Gaussian or quasi-flat-top spot combined with a straight uniform scanning path, and is assisted by a preset combination of laser power and scanning speed parameters to achieve clean connection without slag and splashing.

[0004] However, with the continuous improvement of performance requirements in application scenarios, the above-mentioned traditional laser welding scheme gradually exposes its inherent limitations when dealing with the unique high thermal expansion coefficient and extremely low thermal conductivity of PMMA material. Because PMMA is prone to form significant temperature gradient and internal stress field during laser heating, the energy distribution of the traditional circular spot in the depth direction is difficult to match the thermal response requirements of the material thickness dimension, and the straight scanning method aggravates the uneven accumulation of heat along the weld direction, resulting in local overheating or thermal stress concentration. If the energy input is insufficient or uneven during the preheating stage, it will cause asymmetric thermal expansion of the workpiece, causing macroscopic warping or microscopic cracking; if the power is increased or the speed is reduced during the main welding stage to pursue the penetration depth, it is easy to make the heat excessively diffuse to the material body, triggering the thermal degradation of PMMA molecular chain, which manifests as joint yellowing, bubble generation, mechanical strength reduction and optical performance degradation.

[0005] Therefore, it is necessary to provide a laser welding method to solve the above technical problems. SUMMARY

[0006] The present application overcomes the shortcomings of the prior art and provides a laser welding method.

[0007] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows: a laser welding method, comprising the following steps:

[0008] S1, a polymethyl methacrylate (PMMA) workpiece is fixed, and laser welding preparation is performed using laser with a wavelength of 1900-2000 nm;

[0009] S2, an elliptical light spot is formed by adjusting a beam shaping module, the major axis of the light spot is parallel to the weld direction, and the light spot is used to preheat the workpiece at a first power and a first scanning speed, while a transverse oscillation is superimposed on the scanning path;

[0010] S3, the laser light spot is switched to a circular light spot, the laser power is increased to a second power, the scanning speed is increased to a second scanning speed, the laser focal point is aligned with the upper and lower layer interface, and a spiral scanning motion is superimposed;

[0011] S4, during the transition time, the circular light spot is continuously transitioned back to the elliptical light spot, the laser power is linearly attenuated from the second power to zero, the scanning speed is linearly reduced from the second scanning speed, and the laser focal point is moved away from the interface;

[0012] S5, forced air cooling is performed on the workpiece, the cooling air flow temperature is 20-30 DEG C, the flow rate is 2-4 m / s, and the cooling duration is 3-7 s.

[0013] In one preferred embodiment of the present application, in the S2, the ratio of the major and minor axes of the elliptical light spot is 1.5-2.5, and the equivalent diameter is 1.5-2 mm; the first power is 80-120 W, and the first scanning speed is 100-200 mm / s; the transverse oscillation is a sinusoidal oscillation, the frequency is 8-12 Hz, and the amplitude is 0.2-0.4 mm.

[0014] In one preferred embodiment of the present application, in the S3, the diameter of the circular light spot is 0.5-0.8 mm; the second power is 230-270 W, and the second scanning speed is 1000-1500 mm / s.

[0015] In one preferred embodiment of the present application, in the S4, the transition time is 80-120 ms; during the transition, the ratio of the major and minor axes of the light spot is linearly increased from 1 to 2, the equivalent diameter is linearly expanded, and the focal point position is moved away from the interface by 40-60 mu m.

[0016] In one preferred embodiment of the present application, in the S2, the beam shaping module is used to realize the switching and transformation of the light spot shape; the beam shaping module is one of a deformable mirror, a diffractive optical element, or a spatial light modulator.

[0017] In one preferred embodiment of the present application, when a deformable mirror is used, the surface deformation precision is lambda / 8 to lambda / 12, and the response time is 0.5-1 ms.

[0018] In a preferred embodiment of the present application, when a diffractive optical element is used, the diffraction efficiency is 90-95%, and the switching time is 40-60 ms.

[0019] In a preferred embodiment of the present application, when a spatial light modulator is used, the number of pixels is 1900x1000-2000x1100, and the refresh rate is 100-150 Hz.

[0020] In a preferred embodiment of the present application, in the S2, the workpiece temperature is monitored in real time by an infrared thermal imaging system, and when the average temperature of the central region reaches 80-90℃ and the standard deviation of the temperature difference along the weld direction is less than 5℃, the S3 step is entered.

[0021] In a preferred embodiment of the present application, in the S3, the spiral radius of the spiral scanning is 10-20μm, the pitch is 0.4-0.6mm, and the rotation frequency is 500-700Hz.

[0022] The present application solves the defects in the background art and has the following beneficial effects:

[0023] (1) The present application provides a laser welding method, which uses an elliptical spot to preheat in combination with transverse oscillation, effectively broadens the heat-affected zone and realizes uniform energy distribution, can significantly reduce the local temperature gradient caused by the low thermal conductivity of PMMA material, can avoid the asymmetric thermal expansion and micro-cracks of the workpiece caused by uneven heating in the preheating stage, and compared with the prior art using a circular spot for linear preheating, the present application establishes a stable and uniform initial temperature field for subsequent welding; further, this gentle preheating process provides sufficient conditions for the relaxation and rearrangement of molecular chains, thereby reducing the residual stress level of the joint from the root.

[0024] (2) The present application provides a laser welding method, which switches the spot to a small size circle and superimposes a spiral scanning motion in step S3, so that the energy is highly concentrated on the interface region and produces strong molten pool stirring effect, promotes the mutual penetration and sufficient entanglement between the molten PMMA molecular chains, realizes firm interface metallurgical bonding, and at the same time inhibits the degradation of the material caused by overheating; compared with the single linear scanning mode in the prior art, this dynamic scanning trajectory can effectively break the temperature boundary layer, making the heat distribution more uniform, further, this process ensures that the joint has high mechanical strength while also maximally maintaining the original high optical transmittance of PMMA.

[0025] (3) The application provides a laser welding method, through linear gradual transition of multiple parameters such as spot shape, power and scanning speed in step S4, smooth closing of the heat source is realized, the molten material at the end of the weld is allowed to have more sufficient time for solidification and shrinkage, and the risk of stress concentration at the end of the weld is reduced; compared with the simple way of laser direct shutdown in the prior art, the progressive ending strategy of the application effectively avoids the generation of defects such as end cracks and warping, further ensures the quality consistency in the length direction of the whole weld, and improves the overall reliability and geometric stability of the welded structure. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments described in the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings;

[0027] Figure 1 is a flowchart of the preferred embodiment of the present application. DETAILED DESCRIPTION

[0028] The technical solutions in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0029] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application, but the present application can also be implemented in other ways different from those described herein, therefore, the scope of protection of the present application is not limited by the specific embodiments disclosed below.

[0030] In the description of this application, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the scope of protection of this application. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0031] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art will understand the specific meaning of the above terms in this application based on the specific circumstances.

[0032] like Figure 1 As shown, the present invention provides a laser welding method, comprising the following steps:

[0033] S1. Fix the polymethyl methacrylate (PMMA) workpiece and prepare for welding using a laser with a wavelength of 1900-2000nm;

[0034] S2. Adjust the beam shaping module to form an elliptical beam spot, making its major axis parallel to the weld direction, and use the beam spot to preheat the workpiece with the first power and the first scanning speed, while superimposing transverse oscillations on the scanning path.

[0035] S3. Switch the laser spot to a circular spot, increase the laser power to the second power, increase the scanning speed to the second scanning speed, and simultaneously align the laser focus with the upper and lower interface and superimpose the spiral scanning motion.

[0036] S4. During the transition time, the circular spot is continuously transitioned back to the elliptical spot, while the laser power is linearly reduced from the second power to zero, the scanning speed is linearly reduced from the second scanning speed, and the laser focus is moved up from the interface.

[0037] S5, forcibly air-cooling the workpiece, the cooling airflow temperature being 20-30℃, the flow rate being 2-4m / s, and the cooling duration being 3-7s.

[0038] Next, each step will be described in detail.

[0039] In step S1, the polymethyl methacrylate (PMMA) workpiece is fixed on a fixture to ensure that the workpiece will not displace or vibrate due to thermal stress when it undergoes subsequent thermal cycles, thereby providing a basic guarantee for forming a weld with high geometric precision and good consistency.

[0040] The fixture usually has vacuum adsorption and mechanical limiting functions to cope with the slight deformation of the PMMA material during heating;

[0041] Step S1 selects a laser source with a wavelength in the range of 1900-2000nm. Based on the optical properties of PMMA material, the wavelength range is exactly located in the bulk absorption window of PMMA, which means that the laser energy can effectively penetrate the upper PMMA material, rather than being absorbed by the surface. The energy is mainly deposited at the lower interface or a specific depth inside the material, which is a prerequisite for non-contact and additive-free laser transmission welding.

[0042] Step S2 is to establish an initial temperature field in the workpiece area to be welded, which is achieved by dynamically adjusting the energy distribution pattern and scanning motion mode of the laser;

[0043] In step S2, the laser spot is adjusted from a basic circular shape to an elliptical shape by a beam shaping module, and the long axis direction is kept parallel to the predetermined weld trace. By adjusting, the laser energy is dispersed in the direction perpendicular to the weld, forming a wide and shallow heating band, which effectively avoids the problem of local overheating that may be caused by using a circular spot with high energy density;

[0044] Then, the elliptical spot is driven to move along the weld path at a lower first power of 80-120W and a slower first scanning speed of 100-200mm / s.

[0045] More importantly, the scanning path is generated by a path synthesis algorithm. The mathematical expression of the algorithm is:

[0046] ;

[0047] wherein is the target position of the galvanometer at time , and is the scanning speed component in the weld direction, is the transverse oscillation amplitude 0.2-0.4mm, is the oscillation frequency 8-12Hz;

[0048] This algorithm combines linear scanning and transverse sinusoidal oscillation into a complex scanning trajectory, further widening the heat-affected zone and smoothing out the temperature gradient.

[0049] Step S2 effectively overcomes the challenge posed by the low thermal conductivity of PMMA material. The termination of the preheating process is not based on a fixed time, but is determined by a real-time temperature feedback control algorithm. The infrared thermal imaging system continuously monitors the temperature field on the workpiece surface. When the system detects that the average temperature of the preset area reaches 80-90℃ and the standard deviation of the temperature distribution is less than 5℃, the algorithm triggers a state switching command, causing it to automatically switch to step S3.

[0050] In step S3, this step is the core of the welding process, namely the main welding stage. Its goal is to achieve rapid and precise melting and high-quality bonding of materials at the interface. First, a switching command is executed to immediately change the laser spot from an ellipse used for preheating to a small-diameter circular spot.

[0051] The change in the shape of the light spot means that the energy instantly changes from a dispersed mode to a highly concentrated mode, and the power density increases sharply, providing the necessary conditions for instantaneous melting of materials;

[0052] Subsequently, the laser power was significantly increased to a second power of 230-270W, while the scanning speed was also greatly increased to a second scanning speed of 1000-1500mm / s;

[0053] At this stage, the core algorithm is the galvanometer control algorithm used to generate the helical scanning trajectory. This algorithm is implemented by applying sinusoidal signals with a phase difference of 90d° to the two orthogonal axes of the scanning galvanometer, the X-axis and the Y-axis. The formula is as follows: ;

[0054] in, The helix radius is 10-20 μm. The rotation frequency is 500-700Hz, which makes the laser focus move at high speed and micro-amplitude spiral motion at the interface, generating a strong stirring effect on the molten pool.

[0055] Therefore, step S3 achieves interfacial metallurgical bonding through the synergistic effects of spot switching, power and speed enhancement, focus alignment, and spiral scanning.

[0056] Extremely high energy density and extremely short action time can effectively inhibit the thermal degradation reaction of PMMA materials caused by overheating;

[0057] Simultaneously, a real-time molten pool monitoring and closed-loop control algorithm is running, calculating the molten pool width in real time using molten pool images captured by a coaxial CCD. If it is consistent with the set value If the deviation exceeds 10%, then the inequality is satisfied:

[0058]

[0059] The algorithm then adjusts the laser power automatically according to the following formula: ;

[0060] wherein, is a proportional coefficient, and this closed-loop control mechanism ensures the extreme stability of the welding process, thereby obtaining a high-quality welded joint with high mechanical strength and minimal loss of optical performance.

[0061] In step S4, which is the final process of the main welding phase to the transition to the cooling phase, the purpose is to achieve a smooth closing of the welding heat source and to perform a slow cooling process on the end of the weld to release stress. Step S4 is completed within a transition time of 80-120 ms;

[0062] This phase is a linear interpolation decay algorithm for multiple parameters. The system takes the transition time as a reference to calculate the instantaneous value of laser power, scanning speed, and spot shape parameters such as the ratio of long and short axes and the focal position, which change from 0 to over time; ;

[0063] Specifically, for the laser power , its decay follows the algorithm: ;

[0064] wherein, is the second power of the S3 phase.

[0065] Similarly, the decay of the scanning speed is: ;

[0066] wherein, is the second scanning speed, and is the decay end speed.

[0067] ; The upward movement of the focal position is controlled by the algorithm: ;

[0068] wherein, is the interface position, and is the maximum upward movement of 40-60 μm. The simultaneous operation of the algorithm ensures the gradual change of all key parameters.

[0069] ;

[0070] The gradual ending mode guaranteed by the algorithm, rather than a simple manual or electrical shutdown, effectively reduces the residual stress level to a minimum, significantly reducing the risk of the weld end becoming a stress concentration point or a crack source.

[0071] In step S5, which is the final link of the welding thermal process, i.e., the forced cooling stage, after the laser energy input ends, the workpiece is still at a high temperature and needs to be actively cooled to room temperature as soon as possible and uniformly;

[0072] This step uses forced air cooling to cool the completed workpiece. The temperature of the cooling air flow is controlled in the room temperature range of 20-30°C, and the flow rate is set to 2-4 m / s, forming a stable and controllable cooling environment.

[0073] Example 1:

[0074] Two intrinsic transparent PMMA plates with a thickness of 2.0 mm are welded in a lap joint form, and the weld length is 50 mm, including the following steps:

[0075] S1, fix the PMMA workpiece on the clamp with vacuum adsorption and mechanical limiting function, use the laser with wavelength of 1950 nm for welding preparation, adjust the beam shaping module to form an elliptical spot with its long axis parallel to the weld direction, and the ratio of the major and minor axes of the elliptical spot is 2, and the equivalent diameter is 1.8 mm; use the spot to preheat the workpiece at a laser power of 100 W and a scanning speed of 150 mm / s, while superimposing transverse oscillation on the scanning path; the transverse oscillation is sinusoidal oscillation with a frequency of 10 Hz and an amplitude of 0.3 mm; monitor the workpiece temperature in real time by an infrared thermal imaging system, and when the average temperature in the central region reaches 85°C and the temperature difference standard deviation along the weld direction is less than 5°C, proceed to the next step;

[0076] S3, switch the laser spot to a circular spot with a diameter of 0.65 mm, increase the laser power to 250 W, and increase the scanning speed to 1200 mm / s, while aligning the laser focal point with the upper and lower layer interface and superimposing a spiral scanning motion. The spiral radius of the spiral scanning is 15 μm, the pitch is 0.5 mm, and the rotation frequency is 600 Hz;

[0077] S3, within a transition time of 100 ms, continuously transition the circular spot back to the elliptical spot, while linearly decaying the laser power from 250 W to zero and linearly reducing the scanning speed from 1200 mm / s, and moving the laser focal point 50 μm away from the interface;

[0078] S4, perform forced air cooling on the workpiece, with a cooling air temperature of 25°C and a flow rate of 3 m / s, and a cooling duration of 5 s.

[0079] Example 2:

[0080] This embodiment is basically the same as embodiment 1, the difference is that in the step of S2, the preheating is not performed using the elliptical light spot, but directly using the circular light spot with a diameter of 0.5-0.8mm in the step S3 of embodiment 1 to preheat at a first power of 80-120W and a first scanning speed of 100-200mm / s, and no transverse oscillation is superimposed on the scanning path.

[0081] Example 3:

[0082] This embodiment is basically the same as embodiment 1, the difference is that in the step of S2, the long axis direction of the elliptical light spot is set perpendicular to the weld direction, and the rest of the parameters such as the ratio of major and minor axes, power, speed and transverse oscillation are the same as embodiment 1.

[0083] Example 4:

[0084] This embodiment is basically the same as embodiment 1, the difference is that in the step of S2, the transverse oscillation superimposed on the scanning path is cancelled, and only the elliptical light spot is used for straight-line scanning preheating, and the rest of the parameters are the same as embodiment 1.

[0085] Example 5:

[0086] This embodiment is basically the same as embodiment 1, the difference is that in the step of S3, the spiral scanning motion at the laser focal point is cancelled, and only the circular light spot is used for straight-line scanning welding, and the rest of the parameters are the same as embodiment 1.

[0087] Comparative Example 1:

[0088] This embodiment is basically the same as embodiment 1, the difference is that in the steps of S2, S3, S4, the preheating stage, light spot shape switching, scanning trajectory modulation and gradual change of power and speed at the end of the stage are cancelled.

[0089] Specifically, after the PMMA workpiece is fixed, a circular light spot with a diameter of 0.65mm is directly used to perform straight-line scanning welding with the laser focal point aligned with the upper and lower layer interface at a laser power of 250W and a scanning speed of 1200mm / s, and the laser output is stopped immediately after welding, and the workpiece is forcedly cooled.

[0090] Experimental Example:

[0091] Experimental Purpose:

[0092] This experiment aims to verify the influence of the laser welding method described in embodiment 1-embodiment 5 and comparative example 1 on the quality of the poly methyl methacrylate (PMMA) workpiece welding joint.

[0093] Test method and evaluation index (the results are shown in Table 1):

[0094] The following tests were performed on the PMMA samples welded according to the process of Examples 1 to 5 and Comparative Example 1 :

[0095] Tensile strength (MPa): Lap shear samples were prepared and tested according to ASTM D638 standard using a universal material testing machine, the tensile speed was set at 5 mm / min, the maximum load before sample fracture was recorded, and the tensile strength was calculated. This index directly reflects the mechanical bearing capacity of the welded joint.

[0096] Weld area 400-700 nm average transmittance (%): The optical transmittance of the central area containing the weld in the visible light band (400-700 nm) was measured using a UV-visible spectrophotometer, and the average value was taken. This index evaluates the influence of the welding process on the optical transparency of PMMA material, and the higher the value, the smaller the thermal damage.

[0097] Longitudinal residual stress (MPa): A small hole was drilled at the center line and near the weld using a blind hole stress tester, and the longitudinal (along the welding direction) residual stress of the weld was calculated by measuring the released strain. This index evaluates the level of internal stress introduced by the welding heat process, and excessive stress is easy to cause deformation or cracking of the workpiece.

[0098] Standard deviation of fusion line width (pm): Metallographic samples were prepared on the cross section of the weld, and clear images were taken under an optical microscope. At least 10 measurement points were uniformly selected along the length direction of the weld, the fusion line width was measured, and the standard deviation was calculated. This index reflects the consistency of the weld width, and the smaller the standard deviation, the more stable the welding process and the more uniform the weld quality.

[0099] Maximum yellowing index of heat affected zone (HAZ) : The color difference between the weld heat affected zone (HAZ) and the original material was measured using a color difference meter, and the maximum color difference value was recorded. This index quantifies the degree of material aging and yellowing due to heat.

[0100] Interface bubble / carbonization defects: The interface area of the weld was observed by high-resolution optical microscopy or scanning electron microscopy, and the defects such as bubbles (pores) and carbonization (black spots) were qualitatively evaluated, divided into "none / slight", "moderate", and "severe".

[0101] Table 1:

[0102] Experimental Groups Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Tensile Strength (MPa) 28.75 18.34 22.16 24.58 20.47 9.83 400-700 nm Average Transmittance (%) 91.23 85.67 88.45 89.81 86.92 78.36 Longitudinal Residual Stress (MPa) -8.46 -15.82 -12.73 -10.25 -14.31 -22.54 Fusion Line Width Standard Deviation (pm) 12.38 28.91 19.64 16.52 25.73 45.17 Heat Affected Zone Maximum Yellowing Index (AE) 2.15 4.78 3.42 2.89 3.96 7.25 Interfacial Bubble / Carbonization Defects None / Mild Moderate (Bubbles) Mild None / Mild Mild (Bubbles) Severe (Carbonization & Bubbles)

[0103] From Table 1, we can see that:

[0104] The excellent comprehensive performance of embodiment 1 is due to the regulation of the thermodynamic behavior of the PMMA molecular chain. In the preheating stage, the use of an elliptical light spot with the long axis parallel to the weld seam combined with transverse oscillation realizes uniform and gentle heating of a large area, so that the PMMA molecular chain obtains sufficient energy and space to slowly stretch and reorient at the glass transition temperature, establishing a uniform pre-relaxation molecular conformation basis for subsequent welding and avoiding local stress concentration.

[0105] In the main welding stage, the instantaneous switching to a small circular spot with high energy density and superimposing spiral scanning concentrates energy on the interface, so that the kinetic energy of the molecular chain at the interface increases sharply, rapidly breaks through the glass transition temperature and enters the high-elastic state or even the viscous flow state.

[0106] The strong spiral stirring effect produces effective inter-chain entanglement and diffusion at the micro level, promotes the mutual penetration and entanglement of molecular chain segments between different PMMA plate interfaces, and forms a firm interfacial metallurgical bond.

[0107] The subsequent gradual ending allows the molten interface molecular chain to have sufficient time to relax before solidification, reducing the residual internal stress and molecular chain orientation frozen due to rapid cooling;

[0108] Therefore, the joint exhibits the highest tensile strength, excellent optical transmittance, the lowest residual stress and defects, indicating that the molecular chain realizes full and gentle fusion and solidification.

[0109] In embodiments 2 to 5, embodiment 2 directly uses a small circular spot in the preheating stage, which concentrates too much energy, increasing the risk of local molecular chain overheating degradation, and failing to achieve uniform preheating in a wide area, so that the molecular chain at the interface is in an uneven initial state during subsequent welding, the chain segment diffusion and entanglement are insufficient, resulting in low strength and bubbles;

[0110] In embodiment 3, the long axis of the elliptical light spot is perpendicular to the weld seam, which weakens the effect of dispersing thermal stress along the weld seam direction, the preheating uniformity decreases, resulting in poor pre-relaxation effect of the molecular chain and high residual stress;

[0111] In embodiment 4, the transverse oscillation is cancelled, the temperature field uniformity during preheating is further reduced, and the preheating activation degree of the molecular chain is not as good as that of embodiment 1, affecting the integrity of subsequent fusion;

[0112] In embodiment 5, the spiral scanning is cancelled in the main welding stage, which means that there is a lack of mechanical stirring force in the molten pool, and the PMMA molecular chain melted at the interface mainly relies on the slow diffusion of the concentration gradient, the inter-chain entanglement efficiency and uniformity are greatly reduced, which is easy to form local poor fusion or bubbles, resulting in the deterioration of mechanical strength and optical performance.

[0113] Comparative Example 1 uses the most simplified direct welding method, completely lacks the fine control of the thermal history of the PMMA molecular chain, resulting in serious molecular chain damage and poor bonding, the high energy density laser acts on the cold state PMMA interface instantaneously, the molecular chain segment undergoes a sharp and uneven heating and cooling cycle, this thermal shock is extremely easy to cause local molecular chain to break or decompose due to overheating, which seriously damages the molecular structure integrity of PMMA;

[0114] At the same time, due to the absence of preheating process, the molecular chain has no time to stretch and rearrange and is directly heated and melted, the diffusion time of the chain segment at the interface is extremely short, and the entanglement is extremely insufficient;

[0115] Fast cooling freezes these imperfect molecular entanglement structures and huge thermal stress in the joint, forming high residual stress and a large number of micro defects;

[0116] Therefore, from the molecular point of view, the welding process of Comparative Example 1 is full of chain rupture, decomposition and insufficient entanglement, and the joint shows extremely low strength, serious optical performance degradation and a large number of defects.

[0117] The above is based on the ideal embodiment of the present application, through the above description, relevant personnel can make various changes and modifications without deviating from the technical idea of the present application. The technical scope of the present application is not limited to the contents of the specification, and the technical scope must be determined according to the scope of claims.

Claims

1. A laser welding method, characterized in that, Includes the following steps: S1. Fix the polymethyl methacrylate (PMMA) workpiece and prepare for welding using a laser with a wavelength of 1900-2000nm; S2. Adjust the beam shaping module to form an elliptical beam spot, making its major axis parallel to the weld direction, and use the beam spot to preheat the workpiece with the first power and the first scanning speed, while superimposing transverse oscillations on the scanning path. S3. Switch the laser spot to a circular spot, increase the laser power to the second power, increase the scanning speed to the second scanning speed, and simultaneously align the laser focus with the upper and lower interface and superimpose the spiral scanning motion. S4. During the transition time, the circular light spot is continuously transitioned back to the elliptical light spot, while the laser power is linearly attenuated from the second power to zero, the scanning speed is linearly reduced from the second scanning speed, and the laser focus is moved upward from the interface. S5. Forced air cooling is applied to the workpiece. The cooling air temperature is 20-30℃, the flow rate is 2-4m / s, and the cooling duration is 3-7s.

2. The laser welding method according to claim 1, characterized in that: In S2, the ratio of the major axis to the minor axis of the elliptical light spot is 1.5-2.5, and the equivalent diameter is 1.5-2mm; the first power is 80-120W, and the first scanning speed is 100-200mm / s; the transverse oscillation is a sinusoidal oscillation with a frequency of 8-12Hz and an amplitude of 0.2-0.4mm.

3. The laser welding method according to claim 2, characterized in that: In step S3, the diameter of the circular light spot is 0.5-0.8 mm; the second power is 230-270 W; and the second scanning speed is 1000-1500 mm / s.

4. The laser welding method according to claim 1, characterized in that: In S4, the transition time is 80-120ms; during the transition, the ratio of the major and minor axes of the light spot increases linearly from 1 to 2, the equivalent diameter increases linearly, and the focal position moves upward from the interface by 40-60μm.

5. The laser welding method according to claim 1, characterized in that: In S2, the beam shaping module is used to switch and transform the shape of the light spot; the beam shaping module is one of a deformable mirror, a diffractive optical element, or a spatial light modulator.

6. The laser welding method according to claim 5, characterized in that: When a deformable mirror is used, its surface deformation accuracy is λ / 8 to λ / 12, and the response time is 0.5-1ms.

7. The laser welding method according to claim 5, characterized in that: When diffractive optical elements are used, the diffraction efficiency is 90-95% and the switching time is 40-60ms.

8. A laser welding method according to claim 5, characterized in that: When a spatial light modulator is used, the number of pixels is 1900×1000-2000×1100, and the refresh rate is 100-150Hz.

9. The laser welding method according to claim 1, characterized in that: In step S2, the workpiece temperature is monitored in real time by an infrared thermal imaging system. When the average temperature of the central area reaches 80-90℃ and the standard deviation of the temperature difference along the weld direction is less than 5℃, the process proceeds to step S3.

10. A laser welding method according to claim 1, characterized in that: In S3, the spiral radius of the spiral scan is 10-20 μm, the pitch is 0.4-0.6 mm, and the rotation frequency is 500-700 Hz.

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