Laser Bending Forming Process for Titanium Alloy Skin

Abstract

The present invention provides a laser bending forming process for titanium alloy skins, belonging to the technical field of metal processing. The laser bending forming process for titanium alloy skins includes the following steps: calculating the developed size of the skin according to the theoretical size of the skin part and performing blanking; measuring the distance from the theoretical bending position of the skin to one edge of the skin, and making a scribed mark on the bending position of the skin part according to the measured size; placing the side with the scribed mark facing up and laying it flat on a horizontal platform, and pressing the skin part tightly with a heavy object; placing an introduction plate and an extraction plate at the start end and the end end of the scribed mark respectively, and the introduction plate and the extraction plate are respectively closely attached to and fixed with the side surface of the skin; selecting a laser device, and adjusting the laser bending power in three sections; after bending and forming, removing the heavy object on the surface of the skin, breaking the introduction plate and the extraction plate, and completing. The present invention is to solve the problems of cracks and inaccurate bending angles occurring in the bending process.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal processing, and particularly relates to a laser bending forming process for titanium alloy skins. Background Art

[0002] Due to excellent properties such as low density, high specific strength, and corrosion resistance, titanium and its alloys are widely used in the structures of aerospace aircraft. The titanium alloy skin framework is widely adopted because its lightweight structure meets the requirements of high Mach number, long endurance, and high maneuverability of the aircraft, and there is a trend of gradually developing towards the main structure of the aircraft. The titanium alloy skin framework is often welded by means of high-energy beam welding, and the skin needs to be closely attached to the internal framework. Therefore, as the structure surface of the framework changes at multiple angles, the skin needs to be bent.

[0003] At present, most conventional bending forming processes use mechanical cold bending, which is generally used for steel and aluminum alloys, and less for titanium alloys. For example, CN103736787A provides a cold bending forming method for titanium alloy skins. Through the calculation of the unfolded material, the structural design of the bending compensation area, and the bending positioning design, an ordinary bending machine can be used to achieve precise cold bending forming of a titanium alloy skin, replacing the hot forming process of the existing titanium alloy wing surface skin with a multi-faceted inclined surface and variable sweep angle shallow bending structure; the entire cold bending forming process is simple to operate, with high positioning and cutting accuracy, greatly improving production efficiency and reducing production and manufacturing costs. It is found in actual operation that although the cold bending process has its own advantages, the process operation is relatively cumbersome, and the process accuracy is not easy to fully control, which cannot well meet the current market demands of high efficiency and high quality.

[0004] It is well known that titanium alloys have the characteristics of low plasticity, high elasticity, low rigidity, poor thermal conductivity, and high hardness, and are prone to problems such as bending cracks and inaccurate bending angles during the bending process. In the prior art, many metal bendings are carried out using different types of bending machines. For example: the precision bending process of CN101085459A, a profile steel intelligent bending machine and bending process of CN113210470A, etc., which actually also use the cold bending process and cannot solve the problems of cracks and inaccurate bending angles that occur in the cold bending process of titanium alloy skins.

[0005] Therefore, it is imperative to propose a new method suitable for bending titanium alloy skins. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a laser bending forming process for titanium alloy skins to solve the problems of cracks and inaccurate bending angles that occur in the bending process, in view of the deficiencies of the prior art.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is: a laser bending forming process for titanium alloy skin, comprising the following steps:

[0008] S01: Calculate the unfolded size of the skin according to the theoretical size of the skin part, and perform blanking;

[0009] S02: Measure the distance from the theoretical bending position of the skin to one side edge of the skin, and mark the bending position of the skin part according to the measured size;

[0010] S03: Place the side with the marked line face up and flat on a horizontal platform, and press the skin part with a heavy object;

[0011] S04: Place an introduction plate and a lead-out plate at the start end and the end end of the marked line respectively, and the introduction plate and the lead-out plate are respectively closely attached to and fixed with the side surface of the skin;

[0012] S05: Select a laser device. First, perform laser welding and bending on the skin at a low power. The starting arc point and the ending arc point of the welding are respectively set on the introduction plate and the lead-out plate. Stop when the weld reinforcement is formed on the skin surface and the introduction plate and the lead-out plate are respectively connected to the skin; then increase the power by 100 - 300W and continue laser welding and bending until one end of the skin starts to deform and bend; finally, adjust to a high power for welding and bending. Measure the bending angle of the skin each time after welding until the angle meets the requirements;

[0013] S06: After bending and forming, remove the heavy object on the skin surface, break the introduction plate and the lead-out plate, and it is completed.

[0014] Preferably, the materials, specifications and states of the introduction plate and the lead-out plate are the same as those of the skin.

[0015] Preferably, the fitting gaps between the introduction plate and the lead-out plate and the skin are not greater than 0.1mm. After adjusting the fitting gaps, fix the introduction plate and the lead-out plate.

[0016] Preferably, the low power is 400 - 600W, and the high power is 700 - 1000W.

[0017] Exemplarily, the low power can be selected as 400W, 450W, 500W, 550W or 600W; the high power can be selected as 700W, 800W, 900W or 1000W.

[0018] The present invention adopts three-stage power regulation. Among them, the first stage is low power, the second stage is increased by 100-300W on the basis of the first stage, so as to effectively improve the bead edge generated by laser bending with appropriate low power or relatively low power laser welding and ensure the quality of the skin; the third stage is high power, which is moderately increased again on the basis of the second stage, so as to shorten the bending time with appropriate high power and improve production efficiency, and the overall effect is remarkable.

[0019] Preferably, in the laser welding and bending, 5 weldings are taken as a group, and 2-3 groups are welded at low power; after increasing the power by 100-300W, 1-2 groups are welded.

[0020] For example, the low power can choose to weld 2 groups or 3 groups, the increased power can choose to increase by 100W, 150W, 200W, 250W or 300W, and after increasing the power, 1 group or 2 groups can be chosen to weld.

[0021] Preferably, the laser device is an IPG YLS-6000-CT type fiber laser, and the operation is carried out by using a KUKA KR C4 robot control system.

[0022] It can be understood that other lasers and operating systems that can realize the laser bending forming of titanium alloy can also be commonly used in the present invention, and are not limited thereto.

[0023] Preferably, the maximum output power of the laser device is 6000W, the wavelength is 1030nm, and the optical fiber is 200nm.

[0024] Preferably, the parameter design of the laser welding and bending is: speed v = 0.04m / s, defocus amount +15mm, protective gas 99.9% Ar, air pressure 0.2MPa, and the laser program adopts a continuous light-emitting program.

[0025] Preferably, the laser bending forming process of the titanium alloy skin further includes: before step S02, the surface of the skin part is polished to remove the surface oxide layer, and the polished surface is cleaned with alcohol.

[0026] Preferably, the laser bending forming process of the titanium alloy skin further includes: after breaking the lead-in plate and the lead-out plate in step S06, the connection part between the lead-in plate and the lead-out plate and the skin is polished with sandpaper.

[0027] It is recorded in Document CN103736787A that for the forming of workpieces with high strength, large plate thickness, complex shape or large deformation amount in most alloys, a hot forming process is mostly adopted. However, the electric energy consumption, die cost, labor cost and production efficiency of the hot forming process are much higher than those of the cold forming process. At present, with the continuous changes in technology and requirements, for some difficult-to-form materials, such as titanium alloys, the efficiency of the cold bending process is relatively low, and the quality level is uneven, which cannot well meet the high-quality requirements of current materials in frontier fields such as aerospace. Therefore, it is necessary to break through the inherent limitations and explore a new hot forming process suitable for high-quality bending of titanium alloys.

[0028] Laser thermal forming technology belongs to a kind of hot forming process and is a flexible forming technology. It has the characteristics of high forming accuracy without springback, short production cycle, large production flexibility, and is easy to realize the in-situ compounding of processing procedures such as laser cutting and laser welding. It has great application potential in the aerospace, microelectronics and automotive industries. Laser thermal forming technology is especially suitable for high-hard and brittle materials, but the process design is difficult and there are many factors to consider. Laser thermal forming belongs to a transient thermo-elastoplastic problem and is a very complex dynamic deformation process. It is very difficult to comprehensively understand the temperature field distribution and deformation history during the heating process only by experiments, and the process parameters, geometric dimensions and thermophysical properties of the formed plate are all related to the deformation quality.

[0029] The research on laser thermal forming technology at home and abroad mainly focuses on the analytical models, numerical simulations and processing process parameters of its single deformation, and certain achievements have been made. However, it is found in the research that in addition to the expected transverse bending deformation along the normal direction of the heating line, the workpiece will also generate unexpected deformations. Although these can be ignored in conventional hot forming, for workpieces with high forming accuracy requirements, such as parts in the aerospace, medical and microelectronics fields. These forming errors will not only affect the assembly accuracy of the workpiece, but also seriously affect the service life of the workpiece, restricting the application of laser thermal forming technology to workpieces with high forming accuracy requirements.

[0030] As is well known, laser forming technology relies entirely on thermal stress. To complete the deformation of a workpiece, dozens or even hundreds of scans are required. During the multiple heating processes, many factors such as thermal accumulation, material deformation, and property changes will affect the magnitude of the deformation. That is to say, as the number of scans increases, the material deformation will exhibit different patterns. The special thermophysical and mechanical properties of titanium alloys determine the characteristics of its laser forming technology. Li Liqun et al. (Li Liqun, Chen Yanbin, Zhang Liwen, et al. Laser Bending Forming Characteristics of Ti-6Al-4V Titanium Alloy [J]. The Chinese Journal of Nonferrous Metals, 2005.) took Ti-6Al-4V thin plate materials as the research object and studied the application characteristics of laser bending forming technology on titanium alloys. Through experiments, the influence law of different scanning parameters on the bending angle was described, and the dynamic characteristics of the plate bending under multiple scans were studied. The experimental results show that the bending angle increases approximately linearly with the number of scans. When the number of scans is certain, there is an optimal cumulative line energy to make the bending angle reach the maximum. Different material states exhibit different bending dynamic characteristics during the heating and cooling processes. In addition, the microstructure and mechanical properties of the material after laser bending were measured and analyzed. The results show that the selection of scanning parameters has a greater impact on the shape of the phase change region and the microstructure in the phase change zone. After multiple scans, the strength and hardness of the material in the heating zone both increase, and the plasticity decreases. This literature has made a series of studies on the application of laser bending forming technology on titanium alloys from a theoretical level, but has not proposed a technical idea and solution suitable for industrial application and capable of solving the technical problems of the present invention.

[0031] Based on the long-term exploration and research on the laser bending forming process of titanium alloys, the inventor of the present invention proposes a practical titanium alloy laser bending forming solution that can be promoted in the industry from a theoretical level. This process can not only improve the bending efficiency of titanium alloys and meet the needs of the current rapid development, but also is not prone to problems such as cold bending cracks and inaccurate bending angles.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] Based on the above technical solution, the present invention provides a laser bending forming process for titanium alloy skins. The steps are simple and the operation is easy. It can effectively solve the problems of cracks and inaccurate bending angles that occur in the traditional bending process of titanium alloy skins, making the quality of titanium alloy skins higher and the bending angles more accurate. The process of the present invention has been verified by a large number of practices. For the bending of titanium alloy skins with a thickness of 1-3 mm, its production efficiency can be increased by more than 2 times compared with the existing level, the crack incidence rate is zero, and the accuracy of the bending angle is higher than 99.9%.

[0034] The present invention provides a laser bending forming process for titanium alloy skins. The original three-stage laser welding process is adopted for bending. First, the skin is laser welded and bent with low power. The starting point and ending point of the welding are respectively set on the introduction plate and the lead-out plate. When the weld reinforcement is formed on the skin surface and the introduction plate and the lead-out plate are respectively connected to the skin, the low power is preferably 400-600W. Too high or too low power will have an unpredictable impact on subsequent deformation operations. Then, the power is increased by 100-300W, and the laser welding and bending continue until one end of the skin begins to deform and bend. Finally, the high power is adjusted for welding and bending. Each time after welding, the bending angle of the skin is measured until the angle meets the requirements. Through the refined operation of the above process, on the one hand, the generation of cold bending cracks is avoided, and on the other hand, by using different powers in segments for laser bending, the skin quality can be improved and the welding efficiency can be increased. In particular, the low-power laser welding effectively improves the undercut generated by laser bending, ensures the skin quality, and the high power shortens the bending time and improves the production efficiency, with a remarkable overall effect.

[0035] In the laser welding and bending of the present invention, 5 weldings are taken as a group, and 2-3 groups are welded with low power. After the power is increased by 100-300W, 1-2 groups are welded. The number of welding times has a comprehensive impact on the heating degree, deformation and performance of the material. Therefore, it is very crucial to control the number of laser weldings.

[0036] The laser equipment of the present invention is an IPGYLS-6000-CT type fiber laser, and the KUKA KRC4 robot control system is used for operation. In this way, the controllability of laser welding is high, the accuracy is high, the instability of manual operation can be optimized, the quality can be improved, and the level can be kept consistent.

[0037] The parameter design of the laser welding and bending of the present invention is: speed v = 0.04m / s, defocus amount +15mm, protective gas 99.9% Ar, air pressure 0.2MPa, and the laser program adopts a continuous light output program. Under the control of the above parameters, the present invention obtains high-quality titanium alloy bending products. As is well known, the combination of laser welding parameters and parameters such as the number of welding times also has a comprehensive impact on the heating degree, deformation and performance of the material. Therefore, controlling reasonable laser welding and bending parameters also helps to improve the bending quality and stability of the titanium alloy skin.

[0038] In addition, by adopting the segmented laser bending process, the back of the skin will not be oxidized due to too high power, eliminating the back protection measures and making the operation convenient. Description of the Drawings

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0040] Figure 1 It is a schematic diagram of a titanium alloy skin part;

[0041] Figure 2 It is a schematic diagram of the bending and clamping of a titanium alloy skin;

[0042] Among them, 1 - skin part, 2 - introduction plate, 3 - lead-out plate, 4 - heavy object, 5 - digital display inclinometer, 6 - scribing, 7 - start end, 8 - end end, 9 - skin edge, 10 - starting arc point, 11 - ending arc point. Specific embodiments

[0043] To better understand the present invention, the following further clearly elaborates the content of the present invention in combination with embodiments, but the protected content of the present invention is not limited to the following embodiments. In the following description, a large number of specific details are given to provide a more thorough understanding of the present invention. However, it is obvious to those skilled in the art that the present invention can be implemented without one or more of these details.

[0044] Embodiment 1

[0045] Refer to Figure 1 and Figure 2 Take the bending of a 1.5 mm titanium alloy skin as an example for illustration.

[0046] 1) Calculate the unfolded size of the skin according to the theoretical size of the skin part 1, and perform blanking;

[0047] 2) Grind the surface of the skin part 1 to remove the surface oxide layer, and clean the surface with alcohol;

[0048] 3) Taking the skin edge 9 as the reference, measure the distance at the theoretical bending position of the skin. According to the measured size, mark at the bending positions (91.75 mm and 217.82 mm) of the skin part 1 with a caliper or height gauge, and use the scribing 6 tool to make scribing 6;

[0049] 4) Place the side with the scribing 6 mark facing up and flat on a horizontal platform, and use the heavy object 4 to press the horizontal side of the skin part 1;

[0050] 5) Place the inlet plate 2 and the outlet plate 3 at the starting end 7 and the ending end 8 of the scribed line 6. The material, specifications, and state of the inlet plate 2 and the outlet plate 3 are the same as those of the skin. The inlet plate 2 and the outlet plate 3 are closely attached to the side of the skin, and the fitting gap is not greater than 0.1 mm. After adjustment, fix the inlet plate 2 and the outlet plate 3.

[0051] 6) Selection of laser equipment: KUKA KR C4 robot control system, IPG YLS-6000-CT fiber laser, with a maximum output power of 6000 W, a wavelength of 1030 nm, and a fiber of 200 nm; Design of laser bending process parameters: speed v = 0.04 m / s, defocus amount +15 mm, protective gas 99.9% Ar, air pressure 0.2 MPa, and the laser program uses a continuous light output program.

[0052] 7) First, perform laser welding and bending with a power of 500 W. The starting arc point 10 and the ending arc point 11 of the welding are respectively set on the inlet plate 2 and the outlet plate 3. Welding 5 times is a group, and weld 3 groups with 500 W until a weld reinforcement is formed on the surface of the skin and the inlet plate 2 and the outlet plate 3 are connected to the skin 1; Increase the welding power by 100 W and perform welding and bending again. Weld 2 groups until one end of the skin starts to deform and bend. Finally, perform welding and bending with 700 W. Use a digital display inclinometer 5 to measure the bending angle of the skin every time welding is performed until the angle reaches the required value.

[0053] 8) After bending and forming, remove the heavy object 4 on the surface of the skin, break the inlet plate 2 and the outlet plate 3 with pliers, and polish the connection between the inlet plate 2 and the outlet plate 3 and the skin with sandpaper until it is bright.

[0054] The above method provided by this embodiment is applied to the bending of 100 batches of 1.5-mm titanium alloy skins, with a crack generation rate of 0 and a bending angle accuracy of 100%.

[0055] Example 2

[0056] Refer to Figure 1 and Figure 2 for an illustration with the bending of 2-mm titanium alloy skins as an example.

[0057] The difference from Example 1 is: 7) First, perform laser welding and bending with a power of 550 W. The starting arc point 10 and the ending arc point 11 of the welding are respectively set on the inlet plate 2 and the outlet plate 3. Welding 5 times is a group, and weld 2 groups with 550 W until a weld reinforcement is formed on the surface of the skin and the inlet plate 2 and the outlet plate 3 are connected to the skin 1; Increase the welding power by 150 W and perform welding and bending again. Weld 2 groups until one end of the skin starts to deform and bend. Finally, perform welding and bending with 800 W. Use a digital display inclinometer 5 to measure the bending angle of the skin every time welding is performed until the angle reaches the required value.

[0058] The above method provided in this embodiment is applied to the bending of 2mm titanium alloy skins in 120 batches, with a crack generation rate of 0 and a bending angle accuracy of 99.99%.

[0059] Example 3

[0060] Refer to Figure 1 and Figure 2 , and taking the bending of 1mm titanium alloy skin as an example for illustration.

[0061] Different from Example 1: 7) First, perform laser welding and bending with a power of 450W. The starting arc point 10 and the ending arc point 11 of the welding are respectively set on the introduction plate 2 and the extraction plate 3. Five weldings are taken as a group, and three groups are welded at 450W until the weld reinforcement is formed on the skin surface and the introduction plate 2 and the extraction plate 3 are connected to the skin 1; increase the welding power by 120W and perform welding and bending again. Weld one group until one end of the skin starts to deform and bend. Finally, perform welding and bending at 700W. Measure the bending angle of the skin with a digital display inclinometer 5 every time a welding is performed until the angle meets the requirements.

[0062] The above method provided in this embodiment is applied to the bending of 1mm titanium alloy skins in 150 batches, with a crack generation rate of 0 and a bending angle accuracy of 100%.

[0063] Example 4

[0064] Refer to Figure 1 and Figure 2 , and taking the bending of 2.5mm titanium alloy skin as an example for illustration.

[0065] Different from Example 1: 7) First, perform laser welding and bending with a power of 550W. The starting arc point 10 and the ending arc point 11 of the welding are respectively set on the introduction plate 2 and the extraction plate 3. Five weldings are taken as a group, and three groups are welded at 550W until the weld reinforcement is formed on the skin surface and the introduction plate 2 and the extraction plate 3 are connected to the skin 1; increase the welding power by 200W and perform welding and bending again. Weld two groups until one end of the skin starts to deform and bend. Finally, perform welding and bending at 900W. Measure the bending angle of the skin with a digital display inclinometer 5 every time a welding is performed until the angle meets the requirements.

[0066] The above method provided in this embodiment is applied to the bending of 2.5mm titanium alloy skins in 110 batches, with a crack generation rate of 0 and a bending angle accuracy of 99.99%.

[0067] Example 5

[0068] Refer to Figure 1 and Figure 2 , and taking the bending of 3mm titanium alloy skin as an example for illustration.

[0069] Differing from Example 1: 7) First, laser welding and bending are carried out at a power of 600W. The starting arc point 10 and the ending arc point 11 of the welding are respectively set on the introduction plate 2 and the lead-out plate 3. Five weldings are taken as a group, and two groups are welded at 600W until the weld reinforcement is formed on the surface of the skin and the introduction plate 2 and the lead-out plate 3 are connected to the skin 1; then the welding power is increased by 220W, and welding and bending are carried out again for two groups until one end of the skin begins to deform and bend. Finally, welding and bending are carried out at 1000W. Each time after welding, the bending angle of the skin is measured with a digital display inclinometer 5 until the angle reaches the required value.

[0070] The above method provided in this embodiment is applied to the bending of 100 batches of 3mm titanium alloy skins, with a crack generation rate of 0 and a bending angle accuracy of 99.98%.

[0071] In summary, the above method provided by the present invention is applied to the bending of titanium alloy skins with a thickness of 1 - 3mm. Compared with the cold bending forming method of titanium alloy skins provided by CN103736787 A, the production efficiency is increased by more than 2 times, the average crack generation rate is reduced from 50% to zero, and the bending angle accuracy is higher than 99.9%, showing significant progressiveness.

[0072] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Any other modifications or equivalent replacements made by those of ordinary skill in the art to the technical solutions of the present invention shall be covered by the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solutions of the present invention.

Claims

1. Laser bending forming process for titanium alloy skin, characterized in that: It includes the following steps: S01: Calculate the unfolded size of the skin according to the theoretical size of the skin part, and perform blanking; S02: Measure the distance from the theoretical bending position of the skin to one side edge of the skin, and mark the bending position of the skin part according to the measured size; S03: Place the side with the marked line facing up and flat on a horizontal platform, and press the skin part with a heavy object; S04: Place an introduction plate and an extraction plate at the starting end and the ending end of the marked line respectively, and the introduction plate and the extraction plate are respectively closely attached to and fixed with the side of the skin; S05: Select a laser device. First, perform laser welding and bending on the skin with a low power. The low power is 400 - 600W. The starting point and the ending point of the welding are respectively set on the introduction plate and the extraction plate. Stop when the weld reinforcement is formed on the skin surface and the introduction plate and the extraction plate are respectively connected to the skin; Then increase the power by 100 - 300W and continue laser welding and bending until one end of the skin starts to deform and bend; Finally, adjust to a high power for welding and bending. The high power is 700 - 1000W. Measure the bending angle of the skin once for each welding until the angle meets the requirements; Among them, the parameters of the laser welding and bending are designed as: speed v = 0.04m / s, defocusing amount +15mm, shielding gas 99.9% Ar, air pressure 0.2MPa, and the laser program adopts a continuous light output program; S06: After bending and forming, remove the heavy object on the skin surface, break the introduction plate and the extraction plate, and it is completed.

2. The laser bending forming process for titanium alloy skin according to claim 1, characterized in that: The materials, specifications and states of the introduction plate and the extraction plate are the same as those of the skin.

3. The laser bending forming process for titanium alloy skin according to claim 2, characterized in that: The fitting gaps between the introduction plate and the extraction plate and the skin are not greater than 0.1mm. After adjusting the fitting gaps, fix the introduction plate and the extraction plate.

4. The laser bending forming process for titanium alloy skin according to claim 3, characterized in that: In the laser welding and bending, 5 weldings are taken as a group, and 2 - 3 groups are welded with low power; After increasing the power by 100 - 300W, 1 - 2 groups are welded.

5. The laser bending forming process for titanium alloy skin according to claim 1, characterized in that: The laser device is an IPG YLS - 6000 - CT type fiber laser, and it is operated by using a KUKA KR C4 robot control system.

6. The laser bending forming process for titanium alloy skin according to claim 1, characterized in that: The maximum output power of the laser device is 6000W, the wavelength is 1030nm, and the optical fiber is 200nm.

7. The laser bending forming process for titanium alloy skin according to claim 1, characterized in that: It further includes: Before step S02, the surface of the skin part is polished to remove the surface oxide layer, and the polished surface is cleaned with alcohol.

8. The laser bending forming process for titanium alloy skin according to claim 1, characterized in that: It further includes: After breaking the lead-in plate and the lead-out plate in step S06, use sandpaper to polish the connection between the lead-in plate and the lead-out plate and the skin.

Images