A lattice-structure-oriented online z-pin reinforced additive manufacturing method

Abstract

The present application relates to a kind of online Z-pin reinforcement additive manufacturing methods for grid structure. It belongs to the technical field of additive manufacturing and composite interlaminar reinforcement. The method uses continuous fiber printing head, thermoplastic resin printing head and Z-pin implantation head to work alternately, by dividing the grid structure into several printing layers in Z direction, each layer is divided into continuous fiber printing area and discontinuous resin printing area, the fiber printing direction in odd layer and even layer is staggered with each other;First, continuous fiber pre-impregnated filament is extruded by continuous fiber printing head to fill fiber printing area, then pure resin is extruded by thermoplastic resin printing head to fill resin printing area, and then Z-pin is implanted by Z-pin implantation head at specified position;Repeat the above steps until the grid structure is formed. The present application ensures the continuous path of fiber by reasonable zoning processing, and combines online Z-pin implantation to reinforce interlaminar performance, avoids the stress concentration problem caused by through implantation, and improves the overall mechanical properties of grid structure.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing technology, specifically relating to an online Z-pin reinforced additive manufacturing method for grid structures. Background Technology

[0002] The grid structure has the advantages of high air permeability, lightweight, and high strength, and has broad application prospects in aerospace, automotive, and shipbuilding industries. Its advantages lie in its excellent topology optimization, resulting in better bending, buckling, and impact resistance compared to other composite material structures.

[0003] Currently, the main manufacturing methods for composite material grid structures in China are manual and automated layup. Continuous laying of the reinforcing ribs leads to prepreg accumulation and fiber bridging in the joint areas, resulting in reduced strength at the joints. In addition, the composite prepreg tapes required for both manual and automated layup are mostly thick, making it difficult to form grid parts with fine dimensions.

[0004] Continuous fiber additive manufacturing (3D printing) technology has become a new development trend and focus in the field of composite materials in recent years. Compared with processes such as lamination and fiber placement, this process can form more complex and intricate structures. In grid structures, a smooth transition between nodal areas can be achieved through reasonable printing path planning.

[0005] However, due to the "layer-by-layer" process characteristics of additive manufacturing, significant interlayer bonding problems exist in the formed parts, necessitating the introduction of additional interlayer strengthening methods. Patent CN109760336B proposes an additive manufacturing method for Z-axis reinforced continuous fiber composite materials with pre-placed fiber rods. This method improves interlayer performance by pre-placing Z-axis fiber rods during the printing process. However, to ensure no mechanical interference occurs during printing, the continuous fibers need to be interpolated to avoid the reserved positions of the Z-axis fiber rods. This reduces the fiber content in the structure and is unsuitable for manufacturing thin-walled structural parts such as gratings.

[0006] Z-pin reinforcement technology is a mainstream technique for improving the interlaminar properties of laminated composite materials. Compared with other three-dimensional reinforcement techniques (three-dimensional weaving, stitch toughening), this technology has advantages such as easier operation and control of process quality, and is particularly suitable for local reinforcement, lightweight high-strength sandwich structure preparation, and composite material bonding. This technology is based on the concept of discontinuous sutures, utilizing the "pinning" bridging effect of micro-diameter Z-pins. It is mainly used for thermosetting resin material systems. After the prepreg is laid up and before curing, needle-like reinforcing fibers are implanted through the thickness of the component using ultrasonic technology. However, while this through-hole implantation improves interlaminar properties, it also causes localized material damage, resulting in a loss of some in-plane properties.

[0007] For example, invention patent CN105196563A proposes a Z-pin reinforced composite material grid structure and its manufacturing method, which uses a "cut-and-lay" process to form the grid. However, this process leads to a decrease in strength at the nodes, thus requiring the introduction of Z-pin reinforcement technology to strengthen the nodes. However, in this solution, both the cutting of fibers at the nodes and the through-implantation of Z-pins cause a loss of in-plane properties of the component. Summary of the Invention

[0008] The purpose of this invention is to address the aforementioned problems of composite material grid structures. This invention proposes an additive manufacturing method for online Z-pin reinforcement of grid structures. The composite material grid is fabricated using a continuous fiber additive manufacturing process, and non-penetrating online Z-pin implantation is achieved during the additive manufacturing process, thereby strengthening the interlayer properties of the grid structure without sacrificing some in-plane properties.

[0009] The technical solution of the present invention: According to a first aspect of the present invention, an additive manufacturing online Z-pin strengthening method for grid structures is provided, the method comprising the following steps: S1: Grating structure partitioning: The grating is divided into several printing layers in the Z direction. Each layer is divided into a continuous fiber printing area and an intermittent resin printing area. The fiber printing direction in odd-numbered layers and the fiber printing direction in even-numbered layers are interleaved.

[0010] S2: The continuous fiber printhead extrudes continuous fiber prepreg and fills the fiber printing area defined in step S1; the thermoplastic resin printhead extrudes pure resin material and fills the resin printing area defined in step S1.

[0011] S3: The macroscopic load-bearing strength requirement of the component is quantified step by step into the inter-story strength value of each target area. Then, the implantation angle and spatial distribution of the Z-pin are determined according to the inter-story strength requirement of the target area of ​​the structure, so as to achieve "precise reinforcement on demand" of inter-story performance. The Z-pin is implanted at the designated position using the Z-pin implantation head. After implantation to the specified length, the shearing system cuts the Z-pin.

[0012] S4: Repeat steps S2 and S3 until the complete grid structure is formed.

[0013] In one possible embodiment, in step S1, the printed layers include odd-numbered layers and even-numbered layers. In the odd-numbered layers, the fibers are continuously distributed along a first direction; in the even-numbered layers, the fibers are continuously distributed along a second direction different from the first direction. This design causes the fiber directions of each layer to intersect, enhancing the in-plane isotropic or directional load-bearing capacity of the grid structure.

[0014] In one possible embodiment, the first direction and the second direction are two extending directions of the grid bars, which are perpendicular to each other or form a preset angle to ensure that the fiber path is consistent with the geometry of the grid.

[0015] In one possible embodiment, in step S2, the fiber type in the continuous fiber prepreg is one of carbon fiber, glass fiber, quartz fiber, or aramid fiber, and the specific selection is determined according to the strength and stiffness requirements of the target structure.

[0016] In one possible embodiment, the prepreg resin in the continuous fiber prepreg filament and the pure resin extruded from the thermoplastic resin printhead are the same thermoplastic resin to ensure interfacial compatibility; the thermoplastic resin is one of polylactic acid, nylon, polyetherimide, and polyetheretherketone, and the specific selection is determined based on temperature resistance, toughness, or printing processability.

[0017] In one possible embodiment, in step S3, the implantation angle and spatial distribution of the Z-pin are determined according to the interlaminar strength requirements of the target structural region, specifically including the following sub-steps: S31. Obtain the target interlayer strength value S_target.

[0018] The target value is given based on the overall load-bearing strength requirement of the component. The macroscopic load-bearing strength requirement of the component is quantified step by step into the interlayer strength gap of each target area, such as interlayer shear strength or type I / II fracture toughness, as the performance index that Z-pin reinforcement design needs to achieve.

[0019] S32. Determine the initial surface distribution density ρ0.

[0020] Based on the mapping relationship between areal density and interlaminar strength under the same material system and process conditions, the initial areal density ρ0 (unit: pins / mm²) is inversely derived from S_target.

[0021] The mapping relationship can be obtained from a semi-empirical bridging model or a historical experimental database. When the mapping relationship data is insufficient, a cohesive finite element model with discrete Z-pin bridging law is established for simulation calculation to provide an initial estimated density. To provide a comparison benchmark, a backup density group of ρ0±Δρ is further set, which includes ρ1=0.8ρ0 and ρ2=1.2ρ0.

[0022] S33. Determine the injection direction and distribution array of Z-pins, and set staggered configurations under shared repeating layers.

[0023] The angle θ between the Z-pin injection direction and the printed layer plane is limited to 45°–90°. Preferably, when the primary failure mode is interlayer shear delamination, the angle θ is 60°–90°; when it is necessary to match the principal stress direction to suppress mixed delamination, the angle θ is optimized to 45°–60°. Under this angle constraint, the Z-pin can effectively penetrate multiple printed layers, providing bridging closure force.

[0024] Given a surface density ρ and a Z-pin implantation diameter d, a regular array is first generated, including a square array or an equilateral triangular array. Then, any two Z-pins sharing a common repeating layer are staggered. A common repeating layer refers to the intersecting layup sets of two Z-pins penetrating in the thickness direction. If the spacing between two Z-pins in the plane is too small, the fibers within the same layup will be cut multiple times, causing interference in the bridging regions and reducing reinforcement efficiency. Specific methods of staggering include at least: (1) Planar offset: The planar implantation points of adjacent Z-pins are offset along the main fiber direction within the layer or the printing path direction of the layer, so that the lateral projection distance between any two Z-pins in any printed layer they pass through is not less than a preset safety threshold. This threshold is preferably 3 times the Z-pin diameter, i.e., ≥3d. In implementation, the initial regular array is adjusted to a rhombic grid with row or column phase difference, or an interleaved array is used, so that the Z-pin holes do not overlap in the same fiber bundle path within the shared layer.

[0025] (2) Depth Staggering: When process conditions allow for the implantation of Z-pins at different depths, adjacent Z-pins are designed to be implanted at alternating depths. For example, the first Z-pin penetrates the entire reinforcement layer, while the second Z-pin adjacent to the first only penetrates a portion of the layers, such as 2 / 3 of the full thickness. This staggers the bridging layers in the thickness direction, preventing them from sharing the same complete layup combination. When the design requires all Z-pins to penetrate the full thickness, planar staggering is mainly used to meet the spacing criteria.

[0026] After the above staggered adjustment, a Z-pin distribution coordinate file suitable for the current surface density is generated, and the injection angle is fixed at the selected θ.

[0027] S34. Prepare experimental pieces and conduct strength tests.

[0028] Based on the injection direction, distribution array, and staggered scheme determined in S33, Z-pins are implanted during the filament laying or printing process using additive manufacturing equipment to prepare standard mechanical property test specimens, such as short beam shear specimens or double cantilever beam fracture specimens. Simultaneously, control test specimens are prepared according to backup density groups ρ1 and ρ2. The actual interlaminar strength S_exp is tested on all test specimens, and the failure mode is recorded.

[0029] S35. Adjust the areal density based on the experimental strength and iterate.

[0030] Compare S_exp with S_target: If S_exp < S_target, it indicates insufficient bridging ability and the areal density needs to be increased. Use the current data point (ρ0, S_exp) and the data points of the higher density group (ρ2, S_exp2) for interpolation or extrapolation to calculate the new areal density ρ_new that is expected to meet the standard.

[0031] If S_exp is significantly higher than S_target and exceeds the preset strength margin, the areal density can be appropriately reduced to reduce the structural weight and fiber damage. Similarly, ρ_new is obtained by extrapolation based on the existing data.

[0032] If S_exp has reached S_target and is within a reasonable margin, the current areal density is the available value.

[0033] Repeat the staggered setting and distribution generation in S33 according to ρ_new, and prepare experimental specimens for testing again. Iterate in this way until the measured strength satisfies S_exp ≥ S_target and the margin is appropriate, and record the density at this time as the optimal areal density ρ_opt.

[0034] S36. Output the final number and distribution pattern of Z-pins.

[0035] According to the area A of the actual area to be strengthened of the component, calculate the total number N of Z-pins to be implanted = ρ_opt × A. The final output distribution pattern includes: injection angle θ (between 45° and 90°), array type, row pitch, column pitch, stagger phase, boundary avoidance rule, and the Z-pin implantation point coordinate file directly used for the additive manufacturing control system.

[0036] Through the above steps, the quantitative iterative determination of the number and distribution of Z-pins from the strength requirement is achieved, and the angle constraint and the principle of staggering common repeated layers are systematically observed in the distribution to ensure the interlayer strengthening effect and structural integrity.

[0037] In a possible embodiment, the material system of the Z-pin is the same as that in the continuous fiber prepreg to avoid weak interfacial bonding between dissimilar materials.

[0038] In a possible embodiment, the Z-pin implant head uses an ultrasonic system and a heating system to assist in implantation. The ultrasonic system reduces the implantation resistance through high-frequency vibration, and the heating system melts the local resin to promote the fusion of the Z-pin and the interlayer interface material, thereby enhancing the anchoring effect of the Z-pin.

[0039] In one possible embodiment, the Z-pin has a diameter of 0.4 mm to 1.0 mm, a size range that ensures sufficient insertion strength while avoiding excessive damage to the fiber path.

[0040] In one possible embodiment, the implantation length of the Z-pin is at least twice the thickness of the current printed layer, allowing the Z-pin to penetrate the multi-layer structure and form a "through" or "semi-through" anchoring effect, effectively connecting adjacent layers.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention addresses common problems in the manufacturing of laminated composite materials, such as low interlaminar strength and easy delamination under load. It proposes an online Z-Pin reinforced additive manufacturing method that significantly improves the mechanical properties of materials by embedding reinforcing fibers or needles (Z-pins) in real time during 3D printing. This technology can optimize the integrity and strength of the structure in real time, maintaining lightweight characteristics while improving tensile, compressive, and fatigue resistance. It is applicable to various composite materials and allows for performance customization by adjusting the position, density, and orientation of the Z-pins. Furthermore, its precise positioning of the reinforcing material improves resource utilization efficiency and reduces waste. It also shows broad application potential in aerospace, automotive, and medical device manufacturing, making it a highly efficient and flexible additive manufacturing technology. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the grid structure partitioning process in this invention; Figure 2 This is a schematic diagram showing the fiber distribution and fiber direction of odd-numbered and even-numbered layers in the grid structure partitioning process of this invention; Figure 3 This is a schematic diagram of the Z-pin distribution in the grid structure zoning process of this invention; Figure 4 This is a schematic diagram of the collaborative printing process of the grid structure in this invention.

[0043] Wherein, 1—continuous fiber prepreg printing area, 101—continuous fiber prepreg printed along direction 1, 102—continuous fiber prepreg printed along direction 2, 2—pure resin printing area, 3—Z-pin, 4—Z-pin implant head. Detailed Implementation

[0044] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention and not for limiting the claims of the present invention.

[0045] Example 1 refer to Figure 1 , Figure 2 , Figure 3 and Figure 4 In this specific embodiment, the target part is... Figure 1 The grid structure shown uses continuous fiber prepreg 1, which is T300 continuous carbon fiber reinforced PA (nylon) composite prepreg 1, and thermoplastic resin 2, which is pure PA (nylon) material. The material system of Z-pin 3, which is used to enhance interlayer performance, is also T300 continuous carbon fiber reinforced PA (nylon) composite prepreg 1, and the diameter of Z-pin 3 is about 0.6 mm.

[0046] The steps for forming the grid structure using a continuous fiber printhead and a thermoplastic resin printhead in this embodiment are as follows: Step 1: Reference Figure 1 and Figure 2 The target grating part has a total height of 1.8 mm in the Z direction. The grating structure is divided into a total of 6 printing layers in the Z direction, each with a height of 0.3 mm. In the odd-numbered printing layers, the continuous fiber prepreg filament 101 is printed in the Y-axis direction of the coordinate system, and the remaining grating structure areas are filled with pure resin 2. In the even-numbered printing layers, the continuous fiber prepreg filament 102 is printed in the X-axis direction of the coordinate system, and the remaining grating structure areas are filled with pure resin 2. Step 2: Reference Figure 4 The printing temperature of the continuous fiber printhead was set to 270℃ and the printing speed to 5mm / s, while the temperature of the thermoplastic resin printhead was set to 260℃ and the printing speed to 40mm / s. The printing layer height was 0.3mm. The first and second layers of the grid structure were printed in sequence. Step 3: Reference Figure 4 The length of each implanted Z-pin is set to twice the layer height, i.e., 0.6mm. The Z-pin implantation head 4 is heated to 180℃. Figure 4 The Z-pin was implanted at the indicated location, and after implantation of 0.6mm, the cutting system cut the Z-pin.

[0047] Step 4: Continue printing layers three through six above the implanted Z-pin structure. Using a continuous fiber printhead, a thermoplastic resin printhead, and Z-pin implantation head 4, the printing is performed collaboratively according to their respective action information. The collaborative printing process is as follows: Figure 4 As shown, this continues until the complete grid structure is formed.

[0048] Example 2 This embodiment provides an online Z-pin reinforced additive manufacturing method for grating structures, used to manufacture a triangular mesh grating structure with a grating width of 3mm, a grating angle of 60°, a total height of 20mm, a printing layer thickness of 0.4mm, and a total of 50 printing layers.

[0049] The specific steps are as follows: Step 1: Layering and Printing Direction Definition. The grid is divided into 50 printing layers along the Z-axis. In odd-numbered layers, the fiber printing direction is continuously distributed along one direction of the grid, corresponding to one side of the triangular mesh (e.g., the 0° direction). In even-numbered layers, the fiber printing direction is continuously distributed along two other directions, corresponding to the other side of the triangular mesh (e.g., the 60° direction). The fiber printing area covers the continuous grid strips, and the resin printing area fills the gaps between the grid strips.

[0050] Step 2: Printing parameter settings and material filling. A continuous fiber printhead is loaded with glass fiber prepreg, the prepreg being polylactic acid (PLA); a thermoplastic resin printhead extrudes pure PLA resin. The printhead temperature is set to 210℃, and the printing speed is 8mm / s. The fiber and resin areas of each layer are filled separately.

[0051] Step 3: Determining Z-pin Implantation Density and Distribution. Based on the strength requirement assessment, the Z-pin implantation density was determined to be 5 pins per square centimeter, mainly distributed at the grid intersections and in the middle area of ​​the grid. The Z-pins are made of glass fiber / polylactic acid composite filaments with a diameter of 0.8 mm. The implantation head implants the Z-pins under heating and ultrasonic assistance, with a local temperature of 200°C and an ultrasonic frequency of 25 kHz; the implantation length is 2.0 mm, which is 5 times the thickness of the printed layer. Subsequently, the cutting system cuts off the ends of the Z-pins, making them flush with the surface of the current printed layer.

[0052] Step 4: Repeat printing and implantation. Repeat steps 2 and 3 until all 50 layers are completed.

[0053] In this embodiment, Z-pins are implanted at the triangular mesh nodes, effectively connecting fiber layers in different directions. Simultaneously, due to the small diameter and moderate implantation depth of the Z-pins, they do not significantly interfere with the continuous fiber path, maintaining in-plane performance. The resulting grid structure exhibits excellent interlayer shear resistance, making it suitable for lightweight load-bearing structures.

[0054] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the inventive concept of the present invention and the description and drawings, or direct or indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. An online Z-pin reinforced additive manufacturing method for grid structures, characterized in that, The online forming and interlayer strengthening of the grid structure are achieved by alternating the operation of a continuous fiber printhead, a thermoplastic resin printhead, and a Z-pin implantation head, including the following steps: S1: Grating structure partitioning: The grating is divided into several printing layers in the Z direction. Each layer is divided into a continuous fiber printing area and an intermittent resin printing area. The fiber printing direction in odd-numbered layers and the fiber printing direction in even-numbered layers are interleaved. S2: The continuous fiber printhead extrudes continuous fiber prepreg and fills the fiber printing area; the thermoplastic resin printhead extrudes pure resin material and fills the resin printing area. S3: The macroscopic bearing strength requirement of the component is quantified step by step into the inter-story strength value of each target area. Then, the implantation angle and spatial distribution of Z-pin are determined according to the inter-story strength requirement of the target area of ​​the structure. Z-pin is implanted at the specified position using the Z-pin implantation head. After implantation to the specified length, the cutting system cuts off the Z-pin. S4: Repeat steps S2 and S3 until the complete grid structure is formed.

2. The online Z-pin reinforced additive manufacturing method for grid structures according to claim 1, characterized in that, The printed layer includes an odd-numbered layer and an even-numbered layer. In the odd-numbered layer, the fibers are continuously distributed along a first direction. In the even-numbered layer, the fibers are continuously distributed along a second direction different from the first direction.

3. The online Z-pin reinforced additive manufacturing method for grid structures according to claim 2, characterized in that, The first direction and the second direction are two extending directions of the grid bars, which are perpendicular to each other or form a preset angle.

4. The online Z-pin reinforced additive manufacturing method for grid structures according to claim 1, characterized in that, The fiber type in the continuous fiber prepreg is one of carbon fiber, glass fiber, quartz fiber, or aramid fiber.

5. The online Z-pin reinforced additive manufacturing method for grid structures according to claim 1, characterized in that, The prepreg resin in the continuous fiber prepreg and the pure resin extruded from the thermoplastic resin printhead are both the same thermoplastic resin; the thermoplastic resin is one of polylactic acid, nylon, polyetherimide, and polyetheretherketone.

6. The online Z-pin reinforced additive manufacturing method for grid structures according to claim 1, characterized in that, The material system of the Z-pin is the same as that of the continuous fiber prepreg.

7. The online Z-pin reinforced additive manufacturing method for grid structures according to claim 1, characterized in that, The Z-pin implant head is implanted with the aid of an ultrasonic system and a heating system to reduce implantation resistance and promote the bonding of the Z-pin with the surrounding material.

8. The online Z-pin reinforced additive manufacturing method for grid structures according to claim 1, characterized in that, The diameter of the Z-pin is 0.4 mm to 1.0 mm.

9. The online Z-pin reinforced additive manufacturing method for grid structures according to claim 1, characterized in that, The implantation length of the Z-pin is at least twice the thickness of the current printed layer.

10. The online Z-pin reinforced additive manufacturing method for grid structures according to claim 1, characterized in that, The strength requirement assessment includes: determining the implantation angle and spatial distribution of Z-pins through finite element analysis or topology optimization based on the target load-bearing capacity of the grid structure.

Images