Nanosecond laser transmission welding method based on liquid film assistance

Abstract

The invention discloses a nanosecond laser transmission welding method based on liquid film assistance. The nanosecond laser transmission welding method comprises the following steps that firstly, a light-transmitting to-be-welded part and a metal substrate are cleaned; 2, volatile liquid is added between the cleaned to-be-welded part and the contact interface of the metal substrate, so that a liquid film is formed between the cleaned to-be-welded part and the contact interface of the metal substrate; step 3, applying pressure to the to-be-welded part and the metal substrate to uniformly distribute a liquid film, and realizing optical contact between the to-be-welded part and the metal substrate by using Van der Waals' force; and 4, the to-be-welded part-metal substrate treated in the step 3 is placed on a sample table of a nanosecond pulse laser welding system, and infrared nanosecond laser is used for welding. The light-transmitting material and the metal can be efficiently and stably welded, the welding efficiency is improved, and the cost is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser welding, and particularly relates to a nanosecond laser transmission welding method based on liquid film assistance. Background Art

[0002] In the past few decades, the reliable connection of dissimilar materials by laser welding has received extensive attention from researchers. Especially in the fields of electronics, optics, medical devices, etc., the demand for high-precision connection of materials such as glass and metal is becoming increasingly urgent. However, due to the huge differences in physical and chemical properties between glass and metal, especially the mismatch of thermal expansion coefficients, conventional connection methods such as adhesives, brazing, electron beam welding, and arc welding have significant defects such as insufficient joint strength, large welding stress, wide heat affected zone, and low precision, making it difficult to meet the requirements of modern precision manufacturing processes.

[0003] In recent years, ultra-short pulse laser welding technologies (such as picosecond and femtosecond lasers) have been gradually applied to the welding of brittle and transparent materials such as glass and metal due to their extremely small heat affected zone and precision welding capabilities. These ultra-short pulse lasers can highly localize the welding energy through nonlinear optical effects, significantly reducing the defect risk caused by thermal stress concentration, and improving the welding strength. However, the high cost of ultra-short pulse laser equipment, the high complexity of the equipment, and the strict requirements for environmental and operating conditions have limited their large-scale industrial applications to a certain extent.

[0004] In contrast, nanosecond laser welding technology has gradually been used in the welding research of dissimilar materials, especially glass and metal, due to its advantages such as compact equipment structure, low cost, good stability, simple maintenance, and controllable heat influence. However, existing nanosecond laser welding technologies usually rely on electroplated layers or intermediate solders; in the absence of these additional materials, external pressure generally needs to be applied to achieve optical contact (the material gap must be less than one-quarter of the wavelength) and maintain stable welding quality. The realization of optical contact requires high-precision alignment of the workpieces, and stable optical transmission conditions are the premise for successful laser welding. If the gap between dissimilar materials is too large, the molten material will not be able to effectively fill, resulting in low welding quality. In addition, the introduction of external pressure increases the complexity of the welding equipment and the difficulty of precise pressure control, thereby reducing the welding efficiency and the finished product rate, and limiting the popularization and application in actual industrial production.

[0005] In summary, existing nanosecond laser welding technologies have problems such as a narrow range of processing process parameters, strict requirements for optical contact, complex process, and high cost. There is an urgent need to develop a new nanosecond laser welding method that does not require external pressure and has a simple, economical, and efficient process to achieve the reliable connection of dissimilar materials, especially glass and metal. Summary of the Invention

[0006] The object of the present invention is to provide a nanosecond laser transmission welding method assisted by a liquid film for the deficiencies of the prior art. This method can efficiently and stably weld light-transmitting materials and metals, improve welding efficiency, and reduce costs.

[0007] To solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A nanosecond laser transmission welding method assisted by a liquid film, comprising the following steps:

[0009] Step 1: Clean the light-transmitting workpiece to be welded and the metal substrate.

[0010] Step 2: Add a volatile liquid between the contact interfaces of the cleaned workpiece to be welded and the metal substrate, thereby forming a liquid film therebetween.

[0011] Step 3: Apply pressure to the workpiece to be welded and the metal substrate to make the liquid film evenly distributed, and utilize the van der Waals force to achieve optical contact between the workpiece to be welded and the metal substrate.

[0012] Step 4: Place the workpiece to be welded - metal substrate processed in Step 3 on the sample stage of the nanosecond pulsed laser welding system, and perform welding using an infrared nanosecond laser.

[0013] Further, in Step 2, the volatile liquid is one of ethanol, distilled water, and ionic solution.

[0014] Further, the pressing method in Step 3 includes mechanical pressing or vacuum negative pressure pressing, wherein the pressing pressure is less than the fracture strength of the workpiece to be welded.

[0015] Further, the cleaning method in Step 1 is: Place the workpiece to be welded and the metal substrate in a cleaning solution and perform ultrasonic cleaning for 5 - 10 minutes.

[0016] Further, the cleaning solution is one of anhydrous ethanol, acetone, and deionized water.

[0017] Further, the light-transmitting workpiece to be welded in Step 1 is silica glass.

[0018] Further, in Step 3, the contact gap between the workpiece to be welded and the metal substrate is made less than 200 nm by applying pressure and the van der Waals force of the liquid film.

[0019] Further, in Step 4, the laser power of the infrared nanosecond laser is 2 - 15 W, and the pulse repetition frequency is 20 - 80 kHz.

[0020] Further, the scanning mode of the infrared nanosecond laser is a linear filling square path.

[0021] Furthermore, the scanning pitch of the infrared nanosecond laser is 10 - 50 μm, and the scanning speed is 40 - 80 mm / s.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: A stable liquid film is obtained by adding a volatile liquid between the light-transmitting workpiece to be welded and the metal substrate. The optical contact is maintained by surface tension and van der Waals force, without relying on a plating layer or an intermediate solder, and without additionally adding a complex external force loading device. Only by gently pressing can the optical contact between the light-transmitting material and the metal be ensured, greatly simplifying the welding process, reducing the complexity and cost of the welding device. After welding, the liquid volatilizes, thus obtaining the required welded part; (2) Immediately after the liquid film is prepared, laser welding is carried out. The laser processing equipment uses an infrared nanosecond laser processing platform. The laser beam is emitted by a fiber infrared laser, passes through an attenuator and a beam amplifier, enters the galvanometer, and the control of the scanning path of the laser beam is realized by connecting the controller with computer software. According to the scanning path of linear filling rectangle, it is focused on the glass-metal interface, and the local area is melted through the transient heating effect to achieve a high-strength bond between the glass and the metal. The welding quality is good, realizing efficient and stable welding of glass and metal and improving the welding efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a schematic process flow diagram of nanosecond laser transmission welding according to Embodiment 1 of the present invention;

[0024] Figure 2 is the SEM microstructural diagram of the welding joint interface and the super-depth-of-field microscope photograph according to Embodiment 1 of the present invention; among them, (a1)-(a2) are the SEM diagrams of the inlay structure, (b1)-(b2) are the cross-sectional SEM diagrams at the glass-metal bonding surface, and (c1)-(c4) are the super-depth-of-field microscope photographs of the cross-sections of the glass and the metal;

[0025] Figure 3 is the process mechanism diagram of the weld formation according to Embodiment 1 of the present invention; among them, (a1)-(a3) are the schematic diagrams of the early thermal response on the laser welding surface, (a4) is the schematic diagram of the formation of the interface molten pool, and (a5) is the specific welding fracture model

[0026] Figure 4 is the EDS surface scan diagram and the EDS line scan diagram of the weld area according to Embodiment 1 of the present invention, where (a) is the EDS surface scan diagram and (b) is the EDS line scan diagram

[0027] Figure 5 is the schematic diagram of the action mechanism of the liquid film during the welding process according to the embodiment of the present invention; among them, (a) is the schematic diagram of the liquid film, (b) is the schematic diagram of the edge morphology of the liquid film, and (c), (d) are the weld construction diagrams under the synergistic action of thermal drive and recoil force. Detailed implementation manners

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0029] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments may be combined with each other.

[0030] The present invention will be further described below in conjunction with specific embodiments, but it is not limited to the present invention.

[0031] Embodiment 1:

[0032] As Figure 1 shown, a nanosecond laser transmission welding method based on liquid film assistance includes the following steps:

[0033] Step 1: Cut high-purity silica glass and 304 stainless steel substrates into 25 mm × 25 mm sizes, respectively, put them into deionized water and ultrasonically clean for 8 minutes to remove surface contaminants, and then dry them with absorbent paper for standby;

[0034] Step 2: Drop 0.5 μL of anhydrous ethanol on the contact surface between the glass and the stainless steel substrate to form a liquid film;

[0035] Step 3: Slightly mechanically press the two to make the interface reach optical contact by using the surface tension and capillary action of the liquid film, and the interface gap is less than 200 nm;

[0036] Step 4: Place the sample on a nanosecond laser processing platform for laser treatment. The laser treatment parameters are as follows: laser wavelength 1064 nm, pulse width 200 ns, pulse repetition frequency 40 kHz, power 6 W, scanning rate 60 mm / s, scanning area 20 mm × 20 mm, scanning pitch 20 μm, and use a linear filling rectangular path for transmission welding to obtain a high-strength welded joint. After shear strength testing, the joint strength is as high as 19.2 MPa, the welding interface is uniform and dense, and no obvious cracks and defects are observed.

[0037] Embodiment 2:

[0038] A nanosecond laser transmission welding method based on liquid film assistance includes the following steps:

[0039] Step 1: Ultrasonically clean high-purity silica glass and H62 brass substrates in deionized water for 8 minutes respectively, and then dry them with absorbent paper for standby;

[0040] Step 2: Drop 1.0 μL of deionized water between the glass and the metal substrate to form a liquid film;

[0041] Step 3: Slightly press with a mechanical pressing device to make the liquid film evenly distributed and achieve a good optical contact state;

[0042] Step 4: Use a nanosecond laser with a power of 2 W, a scanning speed of 40 mm / s, a scanning area of 30 mm × 30 mm, a scanning pitch of 50 μm, and a pulse repetition frequency of 20 kHz for welding, successfully achieving a high-strength bond between the glass and the brass, with a shear strength as high as 21.5 MPa.

[0043] Example 3:

[0044] A nanosecond laser transmission welding method based on liquid film assistance, comprising the following steps:

[0045] Step 1: Ultrasonically clean the silica glass and the 304 stainless steel substrate, remove surface contaminants and dry for standby;

[0046] Step 2: Drop 0.8 μL of ionic solution between the cleaned substrates to form a liquid film;

[0047] Step 3: Slightly press with a vacuum negative pressure device to form a stable optical contact with a uniform liquid film thickness;

[0048] Step 4: Perform transmission welding using a nanosecond laser with a wavelength of 1064 nm, a pulse width of 200 ns, a power of 15 W, and a pulse repetition frequency of 80 kHz, with a scanning speed of 80 mm / s, a scanning area of 35 mm × 35 mm, and a scanning pitch of 10 μm, to obtain a welded sample with a weld strength as high as 18.5 MPa.

[0049] To better illustrate the effects of the embodiments of the present invention. Take the joint interface after welding in Example 1 for electron microscopy scanning to obtain the SEM as shown in Figure 2 below. Figure 2 In (a1)-(a2) below are SEM images of the inlaid structure formed between the welding materials, (b1)-(b2) are the cross-sections (glass side) at the glass-metal bonding surface, and (c1)-(c4) are the super-depth-of-field microscope images of the glass and metal cross-sections. As shown in Figure 2 in (a1)-(a2), on the contact interface between the glass and the metal, a periodic "S-shaped" inlaid structure of 15 - 30 μm will be generated. The inlaid structure will tightly bond the glass and the metal like a "lock head", significantly enhancing the welding strength. Figure 2 In (b1)-(b2) are the cross-sections at the glass-metal bonding surface, where a large smooth area in the middle of the glass and brittle fractures around it can be observed. Combining Figure 2(c1)-(c4) Super-depth-of-field three-dimensional view observation. The smooth area in the middle is due to the fact that during the shearing process, the strength of the weld is greater than that of the glass interior, resulting in tearing within the glass and forming a smooth deep pit, as shown in (c3)-(c4). From the stainless steel part, the glass torn off adheres to the stainless steel surface, forming a large central convex part, as shown in (c2).

[0050] Figure 3 It is the process mechanism diagram of the weld formation in Example 1. Based on the previous SEM images and super-depth-of-field images, a fracture mechanism model was constructed. The energy of the nanosecond laser is first absorbed by the stainless steel surface material with a thickness ranging from several hundred nanometers to several micrometers, and then plasma and tiny preliminary molten pools are formed, specifically as Figure 3 (a1)-(a3) shown. The behavior of the plasma is crucial during the weld formation process, which promotes material mixing and the formation of strong joints. During the ablation process, due to intense plasma activity, a large number of ablation particles of glass and stainless steel fly. Under the conditions of optical contact and limited gap, the compression between materials makes the plasma behavior disordered, resulting in heat accumulation and being filled with a large amount of recoil steam, thus triggering molten pool sputtering and shock waves, and forming a partial heat-affected zone around. Figure 3 (a4) presents the schematic diagram of the formation of the interface molten pool. The upper-layer glass and the recast layer splash downward onto the stainless steel surface under the impact of the nanosecond laser and their own gravity. In addition, the fused quartz glass at the interface within the laser modification area becomes opaque and starts to absorb more laser energy. In the surrounding laser overlapping area, more molten materials are formed and gathered together. Under the action of the recoil pressure and Marangoni force, the molten materials flow into the gaps in the surrounding areas that are not fully scanned, and then cool and solidify. First, small solder joint fusion nuclei are formed, and then the fusion nuclei grow, and finally a weld with an inlaid structure is formed, as Figure 3 (a5) shown. Figure 3 (a5) shows the specific welding fracture model. In the middle part, the materials fully form an inlaid structure, and the welding strength mainly depends on this part for support. When a shear experiment is carried out, the joint tears from the interior of the upper-layer glass. While the welding strength on both sides is relatively low, the inlaid structure is not fully formed, and the fracture extends along the interface sub-interface, and the basic fracture surface only shows the fracture of the brittle phase, with only a small amount of molten pool in a small part.

[0051] Figure 4 (a) is the EDS surface scan diagram of the weld area in Example 1, which intuitively shows the element distribution of both sides of the glass (Si, O) and stainless steel (Fe, Cr, Mn). From Figure 4It can be clearly observed that, taking the weld seam as the boundary, the glass side is dominated by Si and O elements, while the stainless-steel side is dominated by metal elements such as Fe, Cr, and Mn. Moreover, there are obvious regions of mutual penetration and overlap at the bonding interface, indicating that local melting and diffusion of materials occurred between the glass and the stainless steel during the welding process. The existence of this element mixing zone is formed through the processes of laser-induced high-temperature melting, flow, and re-solidification of the interface materials, reflecting the thermal accumulation effect induced by nanosecond lasers and the dynamics of the interface molten pool. Further observation Figure 4 The EDS line scan results of (b) can quantitatively analyze the variation of elements along the scanning path. The scanning line extends from the glass side towards the stainless steel direction, crossing the interfaces of the two welding materials. It can be clearly seen from the line scan curve that at the interface position (approximately 6 - 7 μm), the contents of Si and O elements representing the glass side gradually decrease, while the contents of Fe, Cr, and Mn elements representing the stainless-steel side increase significantly. The elements on both sides show a gradient change and form an obvious interaction region at the welding interface. It is worth noting that at a position close to the center of the weld seam, the Si element exhibits obvious mutation and fluctuation characteristics, indicating that the glass material is locally melted and fully mixed with the stainless steel, forming a stable weld interface. The gradient characteristics of this element interaction region further confirm that the nanosecond laser transmission welding process assisted by a liquid film can effectively achieve the bonding of glass and stainless steel, rather than just mechanical lapping or simple physical attachment.

[0052] Figure 5 is the modeling analysis of the specific principle of the liquid film. A thin layer of anhydrous ethanol liquid film is added between the glass and the stainless-steel plates. Since the anhydrous ethanol liquid film can be well infiltrated with the glass and the stainless steel, after applying sufficient pressure, it can be considered that the liquid film is in full contact with the glass and there is no air layer in the middle. Its schematic diagram is as shown in Figure 5 (a). The liquid film in the middle can be regarded as a liquid bridge between the plates. Denote the internal pressure of the liquid layer as P w , the atmospheric pressure as P 0 , the contact area between the glass sheet, the stainless steel, and the liquid bridge as S, and the thickness of the water layer as h. The influence of the surface tension between the gas phase and the liquid phase on the formation of the liquid bridge can be directly calculated using Young's equation, specifically as formula (1):

[0053]

[0054] In the formula, Δp is the Laplace pressure, which refers to the pressure difference between the internal pressure of the liquid bridge and the pressure on the external atmospheric interface, that is, the surface tension between the liquid and gas phases. R 1 and R 2 are the principal radii of the surface curvature, R 1 is the radius of curvature of the intersection line of the open contact surface between the liquid film edge and the air and the vertical line, R 2 is the intersection line with the horizontal plane. In Figure 5Marked in (a).

[0055] Since the order of magnitude of R 1 is much larger than that of R 2 , the reciprocal of R 1 is much smaller than the reciprocal of R 2 , and the formula (1) can be simplified. The contact angles θ 1 and θ 2 represent the contact angles of the alcohol solution with the upper glass and the lower stainless steel respectively. Therefore, the value of R 2 is affected by these contact angles. Since ethanol can wet glass and stainless steel well, an internal depression will be generated when the liquid bridge is formed, and the schematic diagram of the edge morphology is shown in Figure 5 (b). Based on this, the simplified Young's equation can be expressed as formula (2):

[0056]

[0057] Analyzing the liquid surface at the edge, the liquid surface at the edge is in equilibrium under the internal pressure P w , the atmospheric pressure P 0 and the additional pressure Δp generated by the surface tension. So it satisfies formula (3):

[0058]

[0059] According to Bernoulli's principle, when the thin water layer is in a static state, the internal pressure is equal everywhere. Therefore, the force balance equation of the upper glass sheet is formula (4), and the specific force diagram is on the left side of Figure 5 (a):

[0060] T + p w S = mg + p 0 S (4)

[0061] where T is the recoil pressure generated by the heating material after the start of laser radiation. The glass moves slowly upward with an unbalanced force. By combining (2), (3) and (4), the expression formula (5) of the recoil pressure T can be obtained:

[0062]

[0063] From formula (3), since Δp is positive, when there is no recoil pressure T, the upper glass sheet is compressed by the combined action of the surrounding atmospheric pressure and the internal pressure of the liquid film, and then the optical contact state is maintained. This pressure difference Δp helps to inhibit the diffusion and evaporation of the liquid film and enhance the welding effect. The capillary force of the liquid film can pull the liquid to expand along the interface and fill the micro-irregularities between the glass and the stainless steel, thereby improving the uniformity of the welding interface. The nanosecond laser heating of the liquid film will generate a local temperature difference, which will cause a temperature difference-driven convection and steam inside the liquid, and a recoil force will break out, asFigure 5 (c). Under the heating of the laser, the material expands, increasing the gap. However, when the laser reaches the interface between the glass and the stainless steel, the laser is linearly absorbed by the metal through free electrons, causing avalanche ionization and ultimately leading to the formation of a high-density plasma state. The energy of the multi-pulse laser and the significant heat accumulation form a large molten pool. The melted material and plasma are ejected to fill the gap between the two materials, forming a weld. The solidified weld is tightly bonded and provides tension to counteract the recoil pressure T, thus maintaining optical contact, as shown in Figure 5 (c)(d), which promotes the subsequent welding process.

[0064] The above are only the preferred embodiments of the present invention, and thus do not limit the implementation manners and protection scope of the present invention. For those skilled in the art, it should be realized that all the solutions obtained by equivalent substitution and obvious changes made by using the content of the specification of the present invention should be included in the protection scope of the present invention.

Claims

1. A nanosecond laser transmission welding method based on liquid film assistance, characterized in that: The following steps are involved: Step 1: Clean the transparent welded parts and metal substrate; Step 2: Add a volatile liquid between the contact interface of the cleaned workpiece to be welded and the metal substrate to form a liquid film between the two; Step 3: Apply pressure to the workpiece to be welded and the metal substrate to make the liquid film evenly distributed, and use van der Waals force to achieve optical contact between the workpiece to be welded and the metal substrate; Step 4: Place the metal substrate to be welded after the processing in step 3 on the sample stage of the nanosecond pulse laser welding system, and weld it using an infrared nanosecond laser.

2. The method of nanosecond laser transmission welding based on liquid film assistance according to claim 1, characterized in that: In step 2, the volatile liquid is one of ethanol, distilled water, and ion solution.

3. The method of nanosecond laser transmission welding based on liquid film assistance according to claim 1, characterized in that: The pressurizing method in step 3 includes mechanical pressing or vacuum negative pressure pressing, wherein the pressure applied is less than the fracture strength of the parts to be welded.

4. The method of nanosecond laser transmission welding based on liquid film assistance according to claim 1, characterized in that: The cleaning method in step 1 is: placing the workpiece to be welded and the metal substrate in a cleaning solution and performing ultrasonic cleaning for 5-10 minutes.

5. The method of nanosecond laser transmission welding based on liquid film assistance according to claim 4, characterized in that: The cleaning solution is one of anhydrous ethanol, acetone or deionized water.

6. The method of nanosecond laser transmission welding based on liquid film assistance according to claim 1, characterized in that: The light-transmitting parts to be welded in step 1 are made of silica glass.

7. The liquid film-assisted nanosecond laser transmission welding method according to claim 1, characterized in that: In step 3, pressure and van der Waals force of the liquid film are applied to make the contact gap between the workpiece to be welded and the metal substrate less than 200 nm.

8. The method of nanosecond laser transmission welding based on liquid film assistance according to claim 1, characterized in that: In step 4, the laser power of the infrared nanosecond laser is 2 to 15 W, and the pulse repetition frequency is 20 to 80 kHz.

9. The method of nanosecond laser transmission welding based on liquid film assistance according to claim 1, characterized in that: The infrared nanosecond laser scans in a way that straight lines fill a square path.

10. The method of nanosecond laser transmission welding based on liquid film assistance according to claim 1, characterized in that: The scanning interval of the infrared nanosecond laser is 10 to 50 μm, and the scanning speed is 40 to 80 mm / s.

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