Segmented variable-speed rotary friction welding method for round Chinese fir tenon

Abstract

The invention is suitable for the technical field of wood non-adhesive connection, and provides a China fir round tenon segmented variable-speed rotary friction welding method which is characterized in that aiming at the characteristics of China fir materials, a'preheating-main welding-stable welding 'three-stage variable-speed process is innovated, and a special tenon with a spiral guide groove and an annular stop boss is matched, so that the China fir round tenon segmented variable-speed rotary friction welding is realized. And the surface is coated with a lignin-nano silicon dioxide composite modifier. The method comprises the steps of mortise precision pretreatment, tenon modification preparation, segmented welding and heat preservation curing, specifically, gradient heating is conducted to 180-220 DEG C in the preheating stage, lignin is melted at a high speed in the main welding stage, and three-dimensional mechanical interlocking is formed at a low speed in the stable welding stage. The problems of high wood loss rate, insufficient joint strength and poor water resistance in the prior art are solved, compared with the prior art, the pulling resistance of the welded joint is greatly improved, the wood loss rate is remarkably reduced, the thickness swelling rate of water absorption in 24 hours is greatly reduced, the process is stable, the automation degree is high, and adhesive pollution is avoided; and the industrial green production requirements of furniture manufacturing and wood structure buildings are met.

Description

Technical Field

[0001] This invention relates to the field of glueless wood joining technology, and in particular to a method for segmented variable-speed rotary friction welding of cedar round tenons. Background Technology

[0002] As the most important fast-growing commercial timber species in southern my country, Chinese fir is widely used in furniture manufacturing and timber-framed construction due to its advantages such as rapid growth, uniform wood properties, ease of processing, and low cost. Traditional Chinese fir joining methods mainly rely on adhesive bonding or metal fasteners. Adhesives pose a risk of releasing harmful substances such as formaldehyde, polluting the environment and endangering human health; metal fasteners are prone to corrosion, affecting the durability of the connection. Furthermore, both methods suffer from low production efficiency, high timber wastage, and complex assembly.

[0003] With increasingly stringent environmental regulations and the promotion of green manufacturing concepts, glue-free rotary friction welding technology has become an ideal alternative for fir wood joints. However, existing rotary friction welding technology for fir wood dovetail joints has many shortcomings that urgently need to be addressed: First, the process design lacks specificity. Existing technologies mostly adopt a two-stage fixed speed process of "high-speed welding - low-speed penetration", which does not fully consider the material characteristics of cedar wood, such as low lignin content (about 25-30%), strong fiber toughness, and poor thermal stability. This results in insufficient lignin melting when preheating is insufficient, and excessive speed in the main welding stage can easily cause excessive carbonization of the wood. The wood loss rate generally exceeds 8%, and the joint strength is unstable. Secondly, the tenon structure design is unreasonable. Most existing tenons adopt axial straight groove or no groove structure. The molten wood fiber can only flow along the axial direction, making it difficult to form a three-dimensional mechanical interlock. The joint pull-out force is generally lower than 4000N, and the water resistance is poor. The thickness expansion rate of water absorption in 24 hours exceeds 8%, and it is easy to crack under the action of wet expansion and dry shrinkage. Third, the surface modification schemes have limited effects. Existing technologies mostly use single organic acids or ester modifiers such as citric acid-sucrose ester, which can only form simple cross-linked networks through in-situ esterification. The interfacial bonding is insufficient, and the modifier has poor compatibility with fir fiber, which easily leads to coating peeling. Fourth, there is a lack of precise control mechanisms. The existing process does not have real-time temperature feedback and dynamic pressure adjustment mechanisms. The rotation speed and pressure parameters are fixed and cannot be adapted to fir substrates with different moisture contents and different sizes, resulting in a narrow process window and a low product qualification rate in industrial production. Fifth, the mortise and tenon have poor compatibility. The mortise is not precision processed after processing, and the inner wall roughness is high and the moisture content fluctuates greatly. When it is matched with the tenon, uneven gaps are likely to occur, which affects the generation and distribution of frictional heat and further aggravates the instability of welding performance.

[0004] Existing literature on fir dove welding technology is limited to two-stage variable speed or constant speed process parameter optimization. There are no reports of adopting a three-stage variable speed process of "preheating-main welding-stabilized welding". There is also no comprehensive technical solution that combines spiral guide groove tenons, composite modifiers, temperature feedback and pulse pressure control. It cannot solve the core problems of traditional processes, such as high wood loss rate, insufficient joint strength, poor water resistance and weak adaptability, which restricts the industrial promotion and application of fir rotary friction welding technology.

[0005] Therefore, developing a segmented variable speed rotary friction welding method that is adapted to the properties of fir wood, has stable performance, high efficiency, and is environmentally friendly has important practical significance and industrial value. Summary of the Invention

[0006] This invention provides a segmented variable-speed rotary friction welding method for cedar wood dovetail joints, aiming to solve the technical problems of high wood damage rate, insufficient joint strength, poor water resistance, and weak adaptability in traditional processes.

[0007] This invention is implemented as follows: a method for segmented variable-speed rotary friction welding of cedar wood dovetail joints, comprising the following steps: S1. Pretreatment of fir substrate: Select fast-growing fir trees aged 20-30 years with an air-dry density of 0.40-0.45 g / cm³. 3 The substrate has a moisture content of 5-12%, free from visible defects such as knots, cracks, decay, and discoloration. After processing the substrate to the preset dimensions, it is equilibrated for 72 hours in a constant temperature and humidity environment (temperature 20-25℃, relative humidity 45-55%). A CNC drilling machine is used to pre-drill mortises at preset positions on the substrate. The mortise diameter is 0.10-0.25mm larger than the diameter of the dovetail, and the mortise depth is 1.2-1.5 times the length of the dovetail. The inner wall of the mortise is sanded circumferentially with 800-1000 grit sandpaper, resulting in a surface roughness Ra≤0.8μm. Immediately after sanding, the substrate is placed in a 50-60℃ hot air drying oven for 30-60 minutes to ensure that the difference between the moisture content of the inner wall of the mortise and the moisture content of the substrate is ≤1%. S2. Preparation of special round tenons: S2.1 The dovetail base material is made of fir wood from the same batch as the fir wood base material, with a moisture content of 5-12%, and is processed into a cylindrical shape with a diameter tolerance of h6 and a length of 35-50mm. S2.2 Machine an annular stop boss 3-6mm from the end of the tenon. The boss height is 0.6-1.2mm, and the outer diameter of the boss is 0.05-0.10mm smaller than the mortise diameter. Round the corner at the transition between the boss and the tenon body (round corner radius 0.3-0.5mm). S2.3 Below the boss, 3-8 spiral guide grooves are made along the axis of the round tenon. The groove depth is 0.4-1.0mm, the groove width is 1.2-2.2mm, the spiral angle is 15-20°, the lead is 15-25mm, the circumferential included angle between adjacent grooves is evenly distributed, the groove opening edge is rounded (rounded corner radius 0.2-0.3mm), and the bottom of the groove is rounded (rounded corner radius 0.1-0.2mm). S2.4 Preparation of composite modifier: Weigh lignin, nano-silica, and polyethylene glycol according to the following mass ratio, wherein the lignin:nano-silica ratio is 1:0.3-0.5, and the amount of polyethylene glycol added is 5-8% of the total mass of lignin and nano-silica, wherein the molecular weight of polyethylene glycol is 400-600; add the above raw materials to anhydrous ethanol, stir at a speed of 500-800 r / min for 30-45 min to form a uniformly dispersed composite modifier solution with a solid content of 15-20%; S2.5 The composite modifier is uniformly coated on the surface of the round tenon using a high-pressure electrostatic spraying method. The spraying pressure is 0.3-0.5MPa, the spraying distance is 15-20cm, and the coating thickness is 0.03-0.06mm. After coating, the round tenon is placed in a 65-70℃ forced-air drying oven for 30-40 minutes. After drying, it is cooled to room temperature for use, ensuring that the coating does not crack or peel off. S3. Segmented Variable Speed ​​Welding: A CNC friction welding machine with an infrared temperature measurement module and a pressure sensor is used. The infrared temperature measurement module is installed next to the welding machine spindle, with a temperature measurement range of 100-400℃ and a temperature measurement accuracy of ±5℃. The pressure sensor accuracy is ±0.1MPa. The temperature and pressure data are fed back to the welding machine control system in real time, and the speed and pressure parameters are automatically adjusted. Specifically, it is carried out in three stages: Preheating stage: Align the prepared dovetail with the center of the mortise, start with a speed of 3000 rpm, and gradually increase the speed by 500 rpm to 4000 rpm every 1 second. Slowly insert the dovetail into the mortise until the annular stop boss is completely in contact with the substrate surface. Apply 5-8 MPa axial pressure for 3-5 seconds until the infrared temperature measurement module detects that the temperature of the dovetail and mortise contact surface reaches 180-220℃. If the temperature does not reach 180℃ after 5 seconds of preheating, increase the maximum speed to 4500 rpm and extend the preheating time to 6-8 seconds. Ensure that the temperature meets the standard before proceeding to the next stage. Main welding stage: When the contact surface temperature reaches the preset threshold, the welding machine control system controls the servo motor and electromagnetic clutch to switch the speed of the dove to 6000-8000 rpm within 0.2-0.3s, while increasing the axial pressure to 10-16MPa for 5-8s. During this stage, the temperature is monitored in real time. If the temperature exceeds 280℃, the speed is automatically reduced by 500 rpm. If the temperature is below 240℃, the speed is automatically increased by 500 rpm to ensure that the main welding temperature is stable at 240-280℃, so that the lignin can be fully melted. Stable welding stage: After the main welding stage is completed, the rotation speed is switched to 1000-1500 rpm instantly, and an axial pressure of 10-16 MPa is applied in a pulse at a frequency of 0.5-1 Hz. The peak value of the pulse pressure is 14-16 MPa and the valley value is 10-12 MPa, which lasts for 10-15 seconds. This promotes the flow of molten lignin and wood fiber along the spiral guide groove to form a three-dimensional mechanical interlocking structure. S4. Heat preservation and curing: After the welding stabilization stage, stop the rotation of the dovetail, maintain axial pressure for 25-45 seconds, then release 50% of the pressure, retain a pre-tightening pressure of 5-8 MPa, tightly wrap the welded joint with aluminum foil, and place it in a constant temperature and humidity incubator at 60-70℃ and 40-50% relative humidity for 1-2 hours for heat preservation. After curing, allow it to cool naturally to room temperature, and then completely release the pressure to obtain the welded finished product.

[0008] Preferably, in step S1, the drill bit speed of the CNC drilling machine is 1500-2000 rpm, the feed rate is 5-10 mm / min, and compressed air is used to blow away the wood chips in the mortise in real time during the drilling process to avoid the wood chips remaining and affecting the welding effect.

[0009] Preferably, in step S2, the number of spiral guide grooves is 4-6. When the diameter of the round tenon is ≤10mm, the number of spiral guide grooves is 4; when the diameter of the round tenon is >10mm and ≤15mm, the number of spiral guide grooves is 6. The ratio of the groove width to the groove depth of the spiral guide groove is 2:1-3:1 to ensure that the molten fiber can smoothly fill the groove and form an effective interlock.

[0010] Preferably, in step S2, the nano-silica has a particle size of 20-50 nm and a specific surface area of ​​100-200 m². 2 / g, before use, is modified with silane coupling agent (KH-550). The modification method is as follows: add nano silica to an ethanol aqueous solution (ethanol:water = 3:1, volume ratio), add silane coupling agent (the amount added is 3-5% of the mass of nano silica), ultrasonically disperse for 30min, and then dry at 80℃ for 2h to enhance the compatibility between nano silica and lignin.

[0011] Preferably, in step S3, the spindle runout of the CNC friction welding machine is ≤0.02mm, the rotation speed adjustment accuracy is ±50rpm, and the axial pressure adjustment accuracy is ±0.1MPa, to ensure the stability of the rotation speed and pressure during the welding process and avoid fluctuations in welding quality due to insufficient equipment precision.

[0012] Preferably, during the preheating stage of step S3, the speed at which the dove is inserted into the mortise is 2-5 mm / s to avoid damage to the inner wall of the mortise due to excessively fast insertion or reabsorption of moisture due to excessively slow insertion.

[0013] Preferably, in the stabilization stage of step S3, the duration of the pulse pressure is: peak pressure lasts for 0.3-0.5s, and valley pressure lasts for 0.5-0.7s. The pulse pressure promotes the uniform distribution of molten fibers in the spiral groove and reduces interface voids.

[0014] Preferably, in step S4, the thickness of the aluminum foil is 0.08-0.12mm, and when wrapping, it is ensured that the welded joint area and the surrounding 10-15mm range are completely covered to avoid cracking of the joint area due to rapid evaporation of moisture during the curing process.

[0015] Preferably, in step S2, after the round tenon is processed, the surface is lightly sanded with 1000-grit sandpaper to remove processing burrs. After sanding, the surface roughness Ra≤0.5μm, ensuring that the composite modifier can be uniformly adhered.

[0016] Preferably, in step S1, when the substrate size is greater than 500mm×500mm×500mm, stress relief treatment is performed on the substrate before and after the pre-tenoning. The treatment method is as follows: the substrate is placed in a heat preservation box at a temperature of 40-45℃ for 24 hours and then naturally cooled to room temperature to reduce the influence of internal stress of the substrate on welding accuracy.

[0017] Compared with related technologies, the segmented variable-speed rotary friction welding method for fir wood dovetails provided by this invention has the following advantages: 1. Significantly improved process adaptability and stability: The innovative design of a three-stage variable speed process of "preheating-main welding-stable welding" combined with gradient speed increase, real-time temperature feedback and pulse pressure control is precisely adapted to the material characteristics of cedar wood with low lignin content and strong fiber toughness. The speed and pressure can be dynamically adjusted according to the real-time temperature of the tenon and mortise to avoid over-carbonization or insufficient melting. It is suitable for cedar wood substrates with a moisture content of 5-12%, with a wide process window. The product qualification rate in industrial production is ≥98%, solving the core problems of poor adaptability and large performance fluctuations of traditional processes. 2. Significantly optimized mechanical properties of the joint: The spiral guide groove of the special tenon guides the molten wood fibers to form a three-dimensional mechanical interlocking structure, which increases the mechanical interlocking area by more than 60% compared with the traditional axial straight groove; the annular stop boss ensures the positioning accuracy of the tenon and avoids insertion that is too deep or too shallow; the lignin in the composite modifier has good compatibility with the fir substrate, nano silica fills the interface voids, and polyethylene glycol enhances the interface flexibility. The synergistic effect of the three makes the joint pull-out force 50-65% higher than the existing technology, reaching up to 5500N, far exceeding the 3000-4000N of the traditional process, meeting the strength requirements of heavy furniture and wooden structure buildings; 3. Significantly reduced wood loss rate: The three-stage process precisely controls the welding temperature at 180-280℃, avoiding wood carbonization caused by local overheating; the spiral guide groove reduces the irregular flow of molten fibers, reducing fiber loss; the precise fit between the mortise and tenon reduces ineffective loss during the friction process, reducing the wood loss rate by 40-48%, with a minimum of only 2.8%, which is significantly lower than the 8-12% of the existing technology, improving wood utilization and reducing production costs; 4. Significantly improved water resistance: The composite modifier forms a dense three-dimensional cross-linked network at high temperature, covering the joint interface and effectively blocking water penetration; the mechanical interlocking structure formed by the molten fibers in the spiral guide groove reduces the channels for water intrusion; the constant temperature curing process avoids stress cracks inside the joint, and the 24-hour water absorption thickness expansion rate is ≤3.5%, which is more than 60% lower than the 8-12% of the existing technology, significantly improving the joint's resistance to humid environments and extending its service life; 5. High industrial production efficiency: The entire process is automated using CNC equipment, with automatic adjustment of parameters such as temperature, speed, and pressure, requiring no manual intervention. The welding time for a single joint is only 25-40 seconds, increasing production efficiency by more than 35% compared to traditional processes. No adhesives are used, avoiding the release of harmful substances such as formaldehyde, which meets the requirements of green manufacturing. The standardized design of tenons and mortises facilitates mass production, reduces assembly difficulty, and further improves production efficiency. 6. Scientific and rational structural design: The spiral guide groove's spiral angle, lead, groove width, and groove depth have been precisely optimized. Working in synergy with the three-stage process and pulse pressure, it ensures that the molten fiber can quickly fill and form a stable three-dimensional interlock. The height and outer diameter design of the annular stop boss ensures positioning accuracy while avoiding obstruction of the molten fiber flow. The composition ratio of the composite modifier has been optimized through numerous experiments, taking into account interfacial bonding, flexibility, and water resistance. All structural and process parameters are highly matched, forming a synergistic effect. Detailed Implementation

[0018] The present invention will be further described below with reference to the embodiments.

[0019] 1. Example 1 1.1 Pretreatment of fir substrate: 25-year-old fast-growing fir from Guizhou Province was selected, with an air-dry density of 0.42 g / cm³. 3The initial moisture content was 8%, with no defects such as knots or cracks. The substrate was processed into a 100mm×80mm×50mm cuboid and placed in a constant temperature and humidity chamber (temperature 22℃, relative humidity 50%) for 72 hours to equilibrate the moisture content, resulting in a moisture content of 7.8%. A German Proxxon Typ28 21 CNC drilling machine was used to pre-drill a mortise at the center of the substrate. The diameter of the dovetail was 10.00mm (h6 tolerance), so the mortise diameter was set to 10.20mm, and the mortise depth was 50mm (the dovetail length was 40mm, and the depth was 1.25 times the length). The inner wall of the mortise was sanded with 800-grit sandpaper along the circumference, resulting in a roughness Ra=0.6μm. Immediately after sanding, the substrate was placed in a 55℃ hot air drying oven for 45 minutes. After drying, the moisture content of the inner wall of the mortise was 7.0%, with a difference of 0.8% from the moisture content of the substrate itself.

[0020] 1.2 Preparation of Special Dovetails: ① The dovetail substrate is made of fir wood from the same batch as the substrate, with a moisture content of 7.8%, and is processed into a cylindrical shape with a diameter of 10.00 mm (h6 tolerance) and a length of 40 mm; ② A ring-shaped stop boss is machined 4 mm from the end of the dovetail, with a boss height of 0.8 mm and an outer diameter of 10.15 mm (0.05 mm smaller than the mortise diameter). The transition between the boss and the dovetail body is rounded with a radius of 0.4 mm; ③ Four spiral guide grooves are made along the axis of the dovetail below the boss, with a groove depth of 0.6 mm, a groove width of 1.8 mm, a spiral angle of 18°, a lead of 20 mm, a circumferential angle of 90° between adjacent grooves, a rounded edge of 0.25 mm at the groove opening, and a radius of 0.15 mm at the bottom of the groove; ④ Preparation of composite modifier: 100 g of lignin and 40 g of nano-silica (particle size 30 nm, specific surface area 150 m²) are weighed according to the following mass ratio. 2 / g, modified with KH-550), 8g of polyethylene glycol (molecular weight 400); add the above raw materials to 500mL of anhydrous ethanol and stir at 600r / min for 40min to form a uniform solution with a solid content of 18%; ⑤ use a high-voltage electrostatic spraying equipment (spraying pressure 0.4MPa, spraying distance 18cm) to uniformly coat the composite modifier on the surface of the dovetail, with a coating thickness of 0.04mm; after coating, put it in a 68℃ forced-air drying oven to dry for 35min, and after cooling to room temperature, check that the coating has no cracks or peeling and the surface is smooth.

[0021] 1.3 Segmented variable speed welding: Employs an infrared temperature measurement module (model OMEGA OS137) and a pressure sensor (model HBM). The self-made CNC friction welding machine (U9B) has a spindle runout of 0.01mm, a speed adjustment accuracy of ±50rpm, and a pressure adjustment accuracy of ±0.1MPa. ① Preheating stage: Align the dove with the center of the mortise, start at 3000rpm, increase the speed by 500rpm every 1s to 4000rpm, insert the dove into the mortise at a speed of 3mm / s until the boss is in contact with the substrate surface, apply 6MPa axial pressure for 4s, and the infrared temperature measurement module detects that the contact surface temperature reaches 200℃. ② Main welding stage: Switch to 7000rpm high speed within 0.25s, increase the axial pressure to 13MPa, and continue for 6s. During the process, the temperature is stable at 260℃ and does not exceed 280℃. ③ Stable welding stage: Instantly switch to 1200rpm low speed, apply 13MPa axial pressure in a pulsed manner at a frequency of 0.8Hz (peak value 15MPa for 0.4s, valley value 11MPa for 0.6s), and continue for 12s.

[0022] 1.4 Thermal curing: Stop rotating, maintain a pressure of 13MPa for 35s, then release 50% of the pressure (retaining a pre-tightening pressure of 6.5MPa), completely wrap the joint area and the surrounding 12mm area with 0.1mm thick aluminum foil, and place it in a constant temperature and humidity chamber at 65℃ and 45% relative humidity for 1.5h for constant temperature curing; after curing, allow it to cool naturally to room temperature and release the remaining pressure.

[0023] Test results: Using a WDS-50KN universal mechanical testing machine (loading speed 2.5mm / min), the joint pull-out force was 5500N; the wood damage rate was 2.8%; the 24-hour water absorption thickness expansion rate was 3.2%; SEM observation showed that the molten fibers formed a dense three-dimensional mechanical interlocking structure along the spiral guide groove, with no voids or carbonization at the interface; FTIR analysis showed that the composite modifier formed effective chemical bonds with the wood fibers.

[0024] 2. Example 2 The difference from Example 1 is as follows: 2.1 The moisture content of the cedar wood substrate is 12%, and the moisture content of the dovetail is 12%; 2.2 The spiral guide groove has 6 grooves, with a groove depth of 0.5 mm, a groove width of 1.5 mm, a helix angle of 16°, and a lead of 18 mm; 2.3 The composite modifier contains lignin: nano silica = 1:0.3 and polyethylene glycol 5% (molecular weight 600). 2.4 The preheating stage lasts for 5 seconds, with the temperature reaching 190℃; the main welding stage has a rotation speed of 6500 rpm and a pressure of 12 MPa, lasting for 7 seconds; the stabilization welding stage has a rotation speed of 1300 rpm and a pulse frequency of 0.7 Hz, lasting for 14 seconds. 2.5 Insulation and curing temperature: 60℃, time: 2h.

[0025] Test results: Pull-out force 5100N; wood damage rate 3.2%; 24h water absorption thickness expansion rate 3.5%; joint microstructure shows tight fiber interlocking with no obvious defects, meeting the requirements for industrial use.

[0026] 3. Example 3 The difference from Example 1 is as follows: 3.1 The diameter of the round tenon is 12.00 mm (h6 tolerance), and the length is 45 mm; the diameter of the mortise is 12.30 mm, and the depth is 56 mm; 3.2 The annular stop boss is 5mm from the end, 1.0mm high, and has an outer diameter of 12.20mm; 3.3 The composite modifier contains lignin: nano silica = 1:0.5, and polyethylene glycol is added at 8% (molecular weight 500). 3.4 During the preheating stage, the initial rotation speed is 3000 rpm, which is gradually increased to 4500 rpm (due to the 5% moisture content of the substrate, the initial temperature rises slowly) and is maintained for 5 seconds, reaching a temperature of 185℃. 3.5 During the main welding stage, the rotation speed is 7500 rpm, the pressure is 14 MPa, and the duration is 5 seconds; during the stabilization welding stage, the rotation speed is 1400 rpm, the pulse frequency is 1.0 Hz, and the duration is 10 seconds. 3.6 Insulation and curing temperature: 70℃, time: 1 hour.

[0027] Test results: Pull-out strength 5300N; wood damage rate 3.0%; 24h water absorption thickness expansion rate 3.3%; joint strength and water resistance are excellent, suitable for fir wood substrates with low moisture content.

[0028] 4. Comparative Example 1 (Existing Technology and Process) The process employs a publicly available two-stage "high-speed-low-speed" method, with the following specific parameters: 4.1 The tenon is a grooveless cylinder with a diameter of 10.00 mm and a length of 40 mm. The surface is coated with a citric acid-sucrose ester mixed modifier (mass ratio 1:1, thickness 0.03 mm). 4.2 The mortise diameter is 10.10 mm, and the inner wall is not sanded; 4.3 Welding process: The first stage is high-speed rotation at 5000rpm for 5s, and the second stage is low-speed rotation at 1000rpm for 10s. The axial pressure is constant at 12MPa, and there is no temperature feedback control. 4.4 The remaining conditions (substrate, equipment, testing methods) are the same as in Example 1.

[0029] Test results: Pull-out force 3667N (only 66.7% of Example 1); Wood damage rate 8.5% (3.0 times that of Example 1); 24h water absorption thickness expansion rate 8.8% (2.75 times that of Example 1); SEM observation showed that there were obvious gaps at the joint interface, insufficient fiber winding, and local carbonization.

[0030] 5. Comparative Example 2 (without composite modifier) The difference from Example 1 is that the surface of the dovetail is not coated with a composite modifier, while the other process parameters are completely the same.

[0031] Test results: Pull-out strength 4125N (only 75% of Example 1); 24h water absorption thickness expansion rate 6.2% (1.94 times that of Example 1); FTIR analysis showed that there was no obvious chemical bond at the interface, mainly relying on mechanical interlocking, and the bonding strength and water resistance were significantly reduced.

[0032] 6. Comparative Example 3 (without spiral guide groove) The difference from Example 1 is that the dove has axial straight grooves (4 grooves, 0.6 mm deep and 1.8 mm wide) on its surface, without a spiral structure, while the other process parameters are completely the same.

[0033] Test results: Pull-out force 4400N (only 80% of Example 1); Wood loss rate 3.9% (1.39 times that of Example 1); SEM observation showed that the molten fibers flowed only along the axial direction, forming a two-dimensional interlock, with local voids at the interface, and the mechanical interlocking effect was weaker than that of Example 1.

[0034] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on these embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still combine, add, delete, or otherwise adjust the features of the various embodiments of the present invention according to the circumstances without conflict or creative effort, thereby obtaining different technical solutions that do not fundamentally depart from the concept of the present invention. These technical solutions also fall within the scope of protection of the present invention.

Claims

1. A method for segmented variable-speed rotary friction welding of fir wood dovetail joints, characterized in that, Includes the following steps: S1. Pretreatment of fir substrate: Select fast-growing fir trees aged 20-30 years with an air-dry density of 0.40-0.45 g / cm³. 3 The substrate has a moisture content of 5-12% and is free from visible defects such as knots, cracks, decay, and discoloration. After processing the substrate to the preset dimensions, it is equilibrated in a constant temperature and humidity environment for 72 hours. A CNC drilling machine is used to pre-drill mortises at preset positions on the substrate. The mortise diameter is 0.10-0.25 mm larger than the diameter of the dovetail, and the mortise depth is 1.2-1.5 times the length of the dovetail. The inner wall of the mortise is sanded circumferentially with 800-1000 grit sandpaper, resulting in a surface roughness Ra ≤ 0.8 μm. Immediately after sanding, the substrate is placed in a 50-60℃ hot air drying oven for 30-60 minutes to ensure that the difference between the moisture content of the inner wall of the mortise and the moisture content of the substrate is ≤ 1%. S2. Preparation of special round tenons: S2.1 The dovetail base material is made of fir wood from the same batch as the fir wood base material, with a moisture content of 5-12%, and is processed into a cylindrical shape with a diameter tolerance of h6 and a length of 35-50mm. S2.2 A ring-shaped stop boss is machined 3-6mm from the end of the round tenon. The boss height is 0.6-1.2mm, and the outer diameter of the boss is 0.05-0.10mm smaller than the mortise diameter. The transition between the boss and the round tenon body is rounded. S2.3 Below the boss, 3-8 spiral guide grooves are opened along the axis of the round tenon. The groove depth is 0.4-1.0mm, the groove width is 1.2-2.2mm, the spiral angle is 15-20°, the lead is 15-25mm, the circumferential included angle between adjacent grooves is evenly distributed, the groove opening edge is rounded, and the groove bottom is rounded. S2.4 Preparation of composite modifier: Weigh lignin, nano-silica, and polyethylene glycol according to the following mass ratio, wherein the lignin:nano-silica ratio is 1:0.3-0.5, and the amount of polyethylene glycol added is 5-8% of the total mass of lignin and nano-silica, wherein the molecular weight of polyethylene glycol is 400-600; add the above raw materials to anhydrous ethanol, stir at a speed of 500-800 r / min for 30-45 min to form a uniformly dispersed composite modifier solution with a solid content of 15-20%; S2.5 The composite modifier is uniformly coated on the surface of the round tenon using a high-pressure electrostatic spraying method. The spraying pressure is 0.3-0.5MPa, the spraying distance is 15-20cm, and the coating thickness is 0.03-0.06mm. After coating, the round tenon is placed in a 65-70℃ forced-air drying oven for 30-40 minutes. After drying, it is cooled to room temperature for use, ensuring that the coating does not crack or peel off. S3. Segmented Variable Speed ​​Welding: A CNC friction welding machine with an infrared temperature measurement module and a pressure sensor is used. The infrared temperature measurement module is installed next to the welding machine spindle, with a temperature measurement range of 100-400℃ and a temperature measurement accuracy of ±5℃. The pressure sensor accuracy is ±0.1MPa. The temperature and pressure data are fed back to the welding machine control system in real time, and the speed and pressure parameters are automatically adjusted. Specifically, it is carried out in three stages: Preheating stage: Align the prepared dovetail with the center of the mortise, start with a speed of 3000 rpm, and gradually increase the speed by 500 rpm to 4000 rpm every 1 second. Slowly insert the dovetail into the mortise until the annular stop boss is completely in contact with the substrate surface. Apply 5-8 MPa axial pressure for 3-5 seconds until the infrared temperature measurement module detects that the temperature of the dovetail and mortise contact surface reaches 180-220℃. If the temperature does not reach 180℃ after 5 seconds of preheating, increase the maximum speed to 4500 rpm and extend the preheating time to 6-8 seconds. Ensure that the temperature meets the standard before proceeding to the next stage. Main welding stage: When the contact surface temperature reaches the preset threshold, the welding machine control system controls the servo motor and electromagnetic clutch to switch the speed of the dove to 6000-8000 rpm within 0.2-0.3s, while increasing the axial pressure to 10-16MPa for 5-8s. During this stage, the temperature is monitored in real time. If the temperature exceeds 280℃, the speed is automatically reduced by 500 rpm. If the temperature is below 240℃, the speed is automatically increased by 500 rpm to ensure that the main welding temperature is stable at 240-280℃, so that the lignin can be fully melted. Stable welding stage: After the main welding stage is completed, the rotation speed is switched to 1000-1500 rpm instantly, and an axial pressure of 10-16 MPa is applied in a pulse at a frequency of 0.5-1 Hz. The peak value of the pulse pressure is 14-16 MPa and the valley value is 10-12 MPa, which lasts for 10-15 seconds. This promotes the flow of molten lignin and wood fiber along the spiral guide groove to form a three-dimensional mechanical interlocking structure. S4. Heat preservation and curing: After the welding stabilization stage, stop the rotation of the dovetail, maintain axial pressure for 25-45 seconds, then release 50% of the pressure, retain a pre-tightening pressure of 5-8 MPa, tightly wrap the welded joint with aluminum foil, and place it in a constant temperature and humidity incubator at 60-70℃ and 40-50% relative humidity for 1-2 hours for heat preservation. After curing, allow it to cool naturally to room temperature, and then completely release the pressure to obtain the welded finished product.

2. The method for segmented variable-speed rotary friction welding of fir round tenons according to claim 1, characterized in that, In step S1, the CNC drilling machine has a drill bit rotation speed of 1500-2000 rpm and a feed speed of 5-10 mm / min. During the drilling process, compressed air is used to blow away the wood chips in the mortise in real time to avoid wood chip residue affecting the welding effect.

3. The method for segmented variable-speed rotary friction welding of fir round tenons according to claim 1, characterized in that, In step S2, the number of spiral guide grooves is 4-6. When the diameter of the round tenon is ≤10mm, the number of spiral guide grooves is 4; when the diameter of the round tenon is >10mm and ≤15mm, the number of spiral guide grooves is 6. The ratio of the width to the depth of the spiral guide groove is 2:1-3:1 to ensure that the molten fiber can smoothly fill the groove and form an effective interlock.

4. The method for segmented variable-speed rotary friction welding of fir round tenons according to claim 1, characterized in that, In step S2, the nano-silica has a particle size of 20-50 nm and a specific surface area of ​​100-200 m². 2 / g, modified with silane coupling agent before use. The modification method is as follows: add nano silica to ethanol aqueous solution, add silane coupling agent, ultrasonically disperse for 30min, and then dry at 80℃ for 2h to enhance the compatibility between nano silica and lignin.

5. The method for segmented variable-speed rotary friction welding of fir round tenons according to claim 1, characterized in that, In step S3, the spindle runout of the CNC friction welding machine is ≤0.02mm, the rotation speed adjustment accuracy is ±50rpm, and the axial pressure adjustment accuracy is ±0.1MPa, to ensure the stability of the rotation speed and pressure during the welding process and avoid fluctuations in welding quality due to insufficient equipment precision.

6. The method for segmented variable-speed rotary friction welding of fir round tenons according to claim 1, characterized in that, During the preheating stage of step S3, the speed at which the dove is inserted into the mortise is 2-5 mm / s to avoid damage to the inner wall of the mortise due to excessively fast insertion or reabsorption of moisture due to excessively slow insertion.

7. The method for segmented variable-speed rotary friction welding of fir wood dovetails according to claim 1, characterized in that, In the stabilization stage of step S3, the duration of the pulse pressure is: peak pressure lasts for 0.3-0.5s, and valley pressure lasts for 0.5-0.7s. The pulse pressure promotes the uniform distribution of molten fibers in the spiral groove and reduces interface voids.

8. The method for segmented variable-speed rotary friction welding of fir round tenons according to claim 1, characterized in that, In step S4, the aluminum foil has a thickness of 0.08-0.12mm. When wrapping, ensure that it completely covers the welded joint and the surrounding 10-15mm area to avoid cracking of the joint due to rapid evaporation of moisture during the curing process.

9. The method for segmented variable-speed rotary friction welding of fir round tenons according to claim 1, characterized in that, In step S2, after the round tenon is processed, the surface is lightly sanded with 1000-grit sandpaper to remove burrs. After sanding, the surface roughness Ra ≤ 0.5 μm, ensuring that the composite modifier can be uniformly adhered.

10. The method for segmented variable-speed rotary friction welding of fir wood dovetails according to claim 1, characterized in that, In step S1, when the substrate size is greater than 500mm×500mm×500mm, stress relief treatment is performed on the substrate before and after the pre-tenoning. The treatment method is as follows: the substrate is placed in a heat preservation box at a temperature of 40-45℃ for 24 hours and then naturally cooled to room temperature to reduce the influence of internal stress of the substrate on welding accuracy.