Friction stir welding method with welding interface fiber filling

Abstract

The invention provides a friction stir welding method with a welding interface filled with fibers. The friction stir welding method comprises the steps that composite particles with carbon fibers are prepared; cleaning and clamping the fiber reinforced thermoplastic resin based composite board; a welding tool of the friction stir welding equipment is preheated to a set temperature, a stirring needle of the welding tool is provided with an axial through hole, the lower end of the axial through hole is located at the lower end of the stirring needle, a jacking mechanism is arranged at the top of the axial through hole, and the composite particles in the axial through hole can be jacked to the lower end of the stirring needle through the jacking mechanism; welding is conducted; and the workpiece is cooled. By the adoption of the scheme, the interface shear resistance and the connection strength of the FRP plate friction stir welding joint can be greatly improved, long fiber filaments can be implanted into base metal near the welding joint interface along with welding, and continuous fiber longitudinal strengthening and transverse strengthening of the welding joint are achieved; the friction stir welding method is particularly suitable for friction stir welding of thick plate lap joint and curved plate lap joint.

Description

Technical Field

[0001] This invention relates to the field of friction stir welding technology, and more specifically to a friction stir welding method with fiber filling at the weld interface. Background Technology

[0002] With the continuous development of lightweight technology, fiber-reinforced polymer (FRP) composites, due to their excellent performance and lightweight characteristics, have gradually become a widely used emerging material in aerospace, rail transportation, and vehicle manufacturing. Common FRP material joining methods include mechanical joining, adhesive bonding, and welding. Among these, mechanical joining requires the addition of additional structural components, which can easily lead to increased overall structural weight, stress concentration, and affect the service life of the sheet material; adhesive bonding has a relatively long curing time, and the joint strength is easily affected by the environment.

[0003] Friction stir welding (FSW), as a solid-state joining technology, has certain advantages in welding low-melting-point materials. Currently, some research has been conducted on the design of welding tools, material weldability, joining principles, and post-weld mechanical properties of FSW in thermoplastic resin-based composites. However, it remains largely in the laboratory research stage, with limited practical industrial applications. One key reason is that for conventional FRP materials, the properties of the interfacial material change after the welding thermal cycle, resulting in a significant decrease in joint strength compared to the base material and poor shear resistance at the weld joint interface. In existing methods, Ghasemi et al. used FSW to join carbon fiber reinforced polypropylene composites and found that the joint strength was only 25% of that of the base material. Therefore, improving the joint strength of FRP sheets, especially the shear resistance of the weld joint interface, is a problem that needs to be addressed. Summary of the Invention

[0004] At least in view of the technical problems mentioned in the background art, the present invention aims to provide a friction stir welding method with fiber filling at the weld interface.

[0005] The present invention adopts the following technical solution.

[0006] A friction stir welding method with fiber filling at the weld interface, comprising the following steps: Step 1: Prepare composite microparticles with carbon fiber filaments and load the composite microparticles into the microsphere feeding system of the friction stir welding equipment; Step 2: Clean and clamp the fiber-reinforced thermoplastic resin-based composite board; Step 3: Preheat the welding tool of the friction stir welding equipment to the set temperature, which is 50℃-80℃; wherein, the stirring pin of the welding tool has an axial through hole, the lower end of the axial through hole is located at the lower end of the stirring pin, and a pressing mechanism is provided at the top of the axial through hole, which can push the composite particles in the axial through hole to the lower end of the stirring pin. Step 4: Move the welding tool to the welding start point, set the friction stir welding parameters, and then start welding; during the welding process, the composite microparticles are continuously injected into the welding interface through the axial through hole and eventually solidify inside the weld. Step 5: After welding, cool the workpiece.

[0007] Preferably, the composite microparticles include thermoplastic resin spheres and carbon fiber filaments embedded in the thermoplastic resin spheres, wherein the length of a single carbon fiber filament in the thermoplastic resin sphere is not less than 20 cm.

[0008] Furthermore, the method for preparing composite microparticles with fiber filaments is as follows: first, carbon fiber filaments are rolled into clusters, then the clusters are placed in resin powder so that the resin powder fully coats the carbon fiber filaments, and then composite microparticles are formed through heating to melt, heat preservation, cooling (naturally cooling to room temperature) and curing processes.

[0009] The heating and melting process involves sending the resin powder-coated clusters into a curing oven for heating. The heating temperature is higher than the melting temperature (or viscosity temperature) of the resin material but lower than its thermal decomposition temperature. The holding time is determined based on the resin viscosity, with the optimal time being when the resin powder is observed to transform into a liquid state without dripping.

[0010] Further, the steps for forming carbon fiber filaments into clusters are as follows: the carbon fiber filaments are wound onto a needle with a diameter of no more than 0.5 mm, then the filaments are removed from the needle, compacted, dripped with molten resin, and shaped into a round shape to obtain a cluster.

[0011] In this invention, the composite microparticles are plasticized under the action of the stirring needle during the welding process, and some segments of the carbon fiber filaments in the composite microparticles are embedded in the base material (the base material above and below the welding interface).

[0012] Preferably, the parameters for friction stir welding include: welding tool tilt angle of 1-3°, shoulder pressure of 0.1-0.3 mm; welding tool rotation speed of 500 rpm-3000 rpm; and welding speed of 500 mm / min-3000 mm / min.

[0013] Preferably, the insertion depth of the lower end of the stirring needle is exactly at the interface of the overlap of the base material.

[0014] Beneficial effects: The solution of this invention can not only significantly improve the interfacial shear resistance and connection strength of the friction stir welded joint of FRP sheet, but also implant long fiber filaments into the base material near the weld joint interface during welding, so as to realize continuous fiber longitudinal and transverse reinforcement of the weld joint; this invention is particularly suitable for friction stir welding of thick plate lap joints and curved plate lap joints. Attached Figure Description

[0015] Figure 1 This is a schematic diagram illustrating the process of preparing composite microparticles with carbon fiber filaments in the example. Figure 2 This is a schematic diagram of the composite microparticles in the embodiment; Figure 3 This is a partial schematic diagram of the friction stir welding state of the FRP sheet in the embodiment; Figure 4 This is a partial schematic diagram of the welded joint of the FRP sheet in the embodiment. Detailed Implementation

[0016] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example 1

[0017] A friction stir welding method with fiber filling at the weld interface, comprising the following steps: Step 1, prepare composite microparticles 3 with carbon fiber filaments (e.g. Figure 2 As shown, the composite microparticles 3 include thermoplastic resin spheres and carbon fiber filaments embedded in the thermoplastic resin spheres (the length of a single carbon fiber filament in the thermoplastic resin sphere is 30cm), and the composite microparticles 3 are loaded into the microsphere feeding system of the friction stir welding equipment. Combination Figures 1 to 3 As shown, the method for preparing composite microparticles 3 with fiber filaments is as follows: first, carbon fiber filaments are rolled into clusters, then the clusters are placed in resin powder to fully coat the carbon fiber filaments with resin powder, and then composite microparticles 3 are formed through heating, sintering, and heat preservation processes; wherein, the step of rolling carbon fiber filaments into clusters is as follows: the carbon fiber filaments are wound around a needle with a diameter of no more than 0.5 mm (e.g., Figure 1 (as shown in part a), then remove the silk roll from the needle body (as shown in part a). Figure 1 (As shown in part b), after compaction, drip molten resin onto it and round it (as shown in part b). Figure 1 As shown in section c), the cluster body is obtained; Step 2: Clean the fiber-reinforced thermoplastic resin-based composite board (upper board 2 with a wall thickness of 4mm, lower board 5 with a wall thickness of 4mm, referred to as FRP board, the width of the FRP board is 15cm and the length is 35cm) (wipe the FRP board with 75% alcohol solution, and dry it with cold air after cleaning) and clamp it (clamp the cleaned FRP board on the welding machine with an overlap width of 25mm, use a special clamp to fix the board to be welded in the area to be welded by the friction stir welding machine, install the special friction stir welding tool of this invention on the handle of the welding machine and fix it with screws). Step 3: Preheat the welding fixture of the friction stir welding equipment to a set temperature of 70℃. The stirring pin 1 of the welding fixture has an axial through-hole. The lower diameter of the stirring pin 1 is 8mm, the length of the stirring pin is 5mm, and the diameter of the axial through-hole is 5mm. The lower end of the axial through-hole is located at the lower end of the stirring pin 1. A pressing mechanism 4 is provided at the top of the axial through-hole. The pressing mechanism 4 can push the composite microparticles 3 inside the axial through-hole to the lower end of the stirring pin 1. In this scheme, the axial through-hole is part of the microsphere feeding system. In step 1, the composite microparticles 3 are pre-loaded into the axial through-hole. Step 4: Move the welding tool to the welding start point, and control the insertion depth of the lower end of the stirring pin 1 precisely at the lap interface of the base material. Set the friction stir welding parameters (welding tool tilt angle 1°, shoulder pressure 0.2mm; welding tool speed set to 600rpm; welding speed set to 800mm / min), and then begin welding. During the welding process, the composite particles 3 are continuously injected into the welding interface through the axial through-hole and eventually solidify inside the weld. Figure 4 As shown, during the welding process, the composite microparticles 3 are plasticized under the action of the stirring needle 1. Some segments of the carbon fiber filaments 6 in the composite microparticles 3 are embedded in the base material (the base material above and below the welding interface layer 7). That is, some segments of the carbon fiber filaments 6 are embedded in the base material above the welding interface layer 7, some segments of the carbon fiber filaments 6 are embedded in the base material below the welding interface layer 7, and some segments of the carbon fiber filaments 6 are embedded in the welding interface layer 7. The excess plasticized material in the weld moves upward to the shoulder of the stirring tool and overflows. Step 5: After welding, place the welded FRP sheet workpiece in the air to cool, controlling the humidity of the air to 20%~30% to prevent some of the FRP matrix material from absorbing moisture at high temperature, and to allow the interface resin material to solidify during cooling. Example 2

[0018] A friction stir welding method with fiber filling at the welding interface, referring to Example 1, the main difference between the two is: using an upper plate 2 with a wall thickness of 6mm and a lower plate 5 with a wall thickness of 4mm; the preheating temperature of the welding tool is 80℃, the length of the stirring pin is 6mm; the friction stir welding parameters are: shoulder pressure 0.1mm; welding tool rotation speed is set to 800rpm; welding speed is set to 600mm / min.

[0019] Compared with Example 1, the difference is that the composite microparticles 3 with carbon fiber filaments are omitted, and a solid stirring needle 1 is used.

[0020] Compared with Example 1, Example 2 is different in that FRP spherical particles without long fibers and of uniform particle size are used instead of composite microparticles 3 in Example 1.

[0021] Compared with Example 1, Example 3 differs in that the stirring needle is 6mm long.

[0022] Mechanical property tests were performed on the weld joints obtained in the examples and comparative examples. The results showed that: In Example 1, the shear strength of the weld joint was 28.9 MPa; in Example 2, the shear strength of the weld joint was 26.7 MPa; in Comparative Example 1, the shear strength of the weld joint was 20.3 MPa; in Comparative Example 2, the shear strength of the weld joint was 22.6 MPa; in Comparative Example 3, the shear strength of the weld joint was 21.5 MPa. (Analysis suggests that the main reason for the lower shear strength of the weld joint in Comparative Example 3 compared to the corresponding weld joint in Example 1 is that the lower end of the stirring pin is below the weld interface, resulting in greater material resistance at the lower end of the stirring pin, preventing the long carbon fiber filaments in the composite microparticles 3 from moving longitudinally into the base material above the weld interface.)

[0023] One of the key technologies of this invention is to introduce thermoplastic resin spheres (particles) with built-in long carbon fiber filaments into the weld interface through a specific friction stir welding structure and method. Under the combined action of stirring by the stirring needle and the flow of plasticizer, the continuous fibers are guided to grow longitudinally and laterally, which ultimately greatly improves the interfacial shear resistance and connection strength of the friction stir welded joint of FRP sheet, as well as the continuous longitudinal and transverse reinforcement of the weld joint. This invention is particularly suitable for friction stir welding of thick plate overlaps and curved plate overlaps.

Claims

1. A friction stir welding method with fiber filling at the weld interface, characterized in that the steps include... include: Step 1: Prepare composite microparticles with carbon fiber filaments and load the composite microparticles into the microsphere feeding system of the friction stir welding equipment; Step 2: Clean and clamp the fiber-reinforced thermoplastic resin-based composite board; Step 3: Preset the welding tool of the friction stir welding equipment to a set temperature of 50℃-80℃; wherein, the stirring pin of the welding tool has an axial through hole, the lower end of the axial through hole is located at the lower end of the stirring pin, and a pressing mechanism is provided at the top of the axial through hole, which can push the composite particles in the axial through hole to the lower end of the stirring pin. Step 4: Move the welding tool to the welding start point, set the friction stir welding parameters, and then start welding; during the welding process, the composite microparticles are continuously injected into the welding interface through the axial through hole and eventually solidify inside the weld. Step 5: After welding, cool the workpiece.

2. The friction stir welding method according to claim 1, characterized in that: The composite microparticles include thermoplastic resin spheres and carbon fiber filaments embedded in the thermoplastic resin spheres, with each individual carbon fiber filament in the thermoplastic resin sphere having a length of not less than 40 cm.

3. The friction stir welding method according to claim 2, characterized in that, The method for preparing composite microparticles with fiber filaments is as follows: first, carbon fiber filaments are rolled into clusters, then the clusters are placed in resin powder so that the resin powder fully coats the carbon fiber filaments, and then composite microparticles are formed through melting, heat preservation and curing processes.

4. The friction stir welding method according to claim 3, characterized in that, The steps for forming carbon fiber filaments into clusters are as follows: the carbon fiber filaments are wound onto a needle with a diameter of no more than 0.5 mm, the filament rolls are then removed from the needle, compacted, dripped with molten resin, and shaped into a round shape to obtain a cluster.

5. The friction stir welding method according to any one of claims 1-4, characterized in that: During the welding process, the composite microparticles are plasticized under the action of the stirring needle, and some segments of the carbon fiber filaments in the composite microparticles are embedded into the base material.

6. The friction stir welding method according to claim 5, characterized in that, The parameters for friction stir welding include: welding tool tilt angle 1-3°, shoulder pressure 0.1-0.3mm; welding tool rotation speed set to 500rpm-3000rpm; welding speed set to 500mm / min-3000mm / min.

7. The friction stir welding method according to claim 6, characterized in that: The insertion depth of the stirring needle is exactly at the interface of the base material.

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