Mold temperature compensation process for improving interlaminar shear strength of thermoplastic composite material laminated plate

Abstract

The invention relates to the technical field of automatic fiber laying in-situ consolidation forming of thermoplastic composite materials, discloses a mold temperature compensation process for improving interlayer shear strength of a thermoplastic composite material laminated plate, and aims to solve the problems of heat loss and crystallization gradient of a mold to the surface of a component caused by increase of the thickness of the laminated plate. A strategy of dynamically adjusting the temperature of the mold layer by layer is adopted, and the consistency of the upper surface temperature and the cooling rate of each laying layer is maintained. According to the method, the crystallization gradient in the thickness direction of the laminated plate can be effectively reduced, and the overall crystallinity uniformity and the cross-interface interdiffusion and entanglement number of molecular chains are improved, so that the interlayer bonding quality of the CF / PAEK laminated plate is remarkably enhanced.

Description

Technical Field

[0001] This invention relates to the field of in-situ consolidation molding technology for automatic fiber placement of thermoplastic composites, specifically to a mold temperature compensation process for improving the interlaminar shear strength of thermoplastic composite laminates. Background Technology

[0002] Advanced fiber-reinforced composites, with their high specific strength, specific modulus, fatigue resistance, and designability, have become core materials for lightweight aerospace structural design. Compared to traditional thermoset composites, carbon fiber reinforced polyaryletherketone (CF / PAEK) offers mechanical advantages such as low density, high specific strength and modulus, strong impact resistance, and high damage tolerance. Compared to carbon fiber reinforced polyetheretherketone (CF / PEEK), CF / PAEK has a melt temperature that is approximately 40°C lower, while other mechanical properties remain essentially equivalent. Therefore, CF / PAEK exhibits significant advantages in automated fiber placement in situ consolidation (AFP-ISC) processes.

[0003] Due to the high viscosity of CF / PAEK and the rapid heating and cooling characteristics of the AFP-ISC process, the interlaminar properties of the laminates are insufficient. Therefore, improving the interlaminar shear strength (ILSS) of laminates processed in the AFP-ISC process is an urgent problem to be solved. Previous strategies for improving the ILSS of thermoplastic composites in the AFP-ISC process have mainly focused on optimizing process parameters, repeated heating and pressurization, and material pretreatment (such as adding carbon nanotubes). However, the above methods still have obvious limitations in practical applications: simple optimization of process parameters has a limited effect on improving interlaminar properties; the repeated heating and pressurization strategy sacrifices the high-efficiency molding advantages of automated layup processes; and while the introduction of modification methods such as carbon nanotubes can enhance interfacial bonding, it significantly increases the material preparation cost and process complexity.

[0004] In-depth investigation into the mapping relationship between the microstructure and mechanical properties of materials reveals that, for semi-crystalline thermoplastic composites like CF / PAEK, the crystallinity and crystal morphology of the matrix resin are key intrinsic factors determining the quality of interlaminar bonding. Higher crystallinity typically implies stronger molecular chain entanglement and a higher matrix modulus, which is beneficial for improving interlaminar shear strength. However, most existing studies only focus on the crystallization behavior at a single point within the laminate.

[0005] Adjusting the mold temperature above the glass transition temperature prolongs the relaxation time of molecular chains in the elastic state, thereby promoting interdiffusion of molecular chains between different layups and improving the interlaminar shear strength of the laminate. However, due to temperature loss during the upward conduction of mold temperature, the mold generates a bottom-up temperature gradient on the laminate. This affects the cooling rate of the PAEK matrix in the crystallization region, leading to inconsistent crystallinity among the layups and a gradually decreasing crystallinity gradient distribution along the thickness direction of the laminate from bottom to top. Since the crystallinity of the laminate is significantly positively correlated with the interlaminar bonding strength, the presence of a crystallization gradient reduces the interlaminar shear strength, making it difficult to meet the interlaminar strength requirements of aerospace and other fields. Summary of the Invention

[0006] To overcome the shortcomings of the existing technology, this invention provides a mold temperature compensation process for improving the interlaminar shear strength of thermoplastic composite laminates. This method involves adjusting the mold temperature (Tm) layer by layer during the layup process. tool ), to achieve the surface temperature (T) of the laminate. s The consistency of the CF / PAEK laminate is improved, thereby reducing the crystallization gradient in the thickness direction of the laminate and significantly improving the ILSS of the CF / PAEK laminate.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A mold temperature compensation process for improving the interlaminar shear strength of thermoplastic composite laminates includes the following steps; Step 1: Measure T with a thermal imager S and T tool Before proceeding, emissivity calibration is required. First, set an emissivity value in the software included with the thermal imager, and then measure the emissivity using both the thermal imager and a thermocouple. S and T tool The results from the thermal imager are compared with those from the thermocouple. If the results are inconsistent, the emissivity is readjusted until it matches the thermocouple measurement results. Step 2: Place the heatable aluminum mold 5. Using the measurement value of the K-type thermocouple as a reference, calibrate the emissivity of the CF / PAEK composite material and the surface of the aluminum mold 5. Determine the emissivity value set in the Optris 450i imager to ensure accurate T-measurement. S The accuracy of the measurements was then assessed using a thermal imager during the laying process. s and T tool Perform calibration; Step 3: CF / PAEK composite material is laid on a heatable aluminum mold 5, and the CF / PAEK composite material is compacted by pressure roller 1 under infrared laser 3 to form CF / PAEK laid layer 4. Step 4: During the automated fiber layup in situ consolidation (AFP-ISC) process, after each layer is laid, the temperature is adjusted by adjusting the mold temperature control module, thereby adjusting T. tool And use a thermal imager to verify T layer by layer S If T S If deviations occur, the temperature control module should be adjusted two or more times to ultimately ensure that the temperature of each layer is within acceptable limits. tool Consistency ensures a consistent cooling rate after each layer is laid, reduces the crystallization gradient in the thickness direction of the laminate, and significantly improves the ILSS of the CF / PAEK laminate.

[0008] In step 3, the laminate with CF / PAEK pre-laid layer 4 has dimensions of 38mm × 350mm × 3.51mm (length × width × height) [0°]. 26 Laminate. Infrared laser 3 and pressure roller 1 move at the same speed along the laying direction. The resin matrix of CF / PAEK layer 2 to be laid and CF / PAEK layer 4 to be laid after being irradiated by infrared laser 3 melts. Then, pressure roller 1 provides pressure to both of them, so that they are bonded together.

[0009] Step 3 includes T S Four variables: laying temperature, laying force of pressure roller 1, and laying speed; Among them, T S The temperature ranges from 150℃ to 250℃, the laying temperature is 380℃ to 480℃, the laying force of roller 1 is 300 N to 500 N, and the laying speed is 50 mm / s to 100 mm / s. These four variables are carried out under orthogonal combination conditions.

[0010] In step 4, T is performed at intervals of 4 layers. s Measurement, obtaining T s After the data is collected, T is adjusted in small steps. tool This allows the T measured by a thermal imager after the layer is laid out to be... s If the requirements are met, record the value at this time T. tool For the same layup, the experiment should be repeated at least 3 times and the average value should be taken; data on "layup number - T" should be collected within the complete layer domain. tool "Discrete datasets".

[0011] The small-step adjustment method involves adjusting the temperature by 3-5°C at a time.

[0012] The beneficial effects of this invention are: This invention compensates for T layer by layer tool Maintaining the T-value of the laminate during the laying process sThe consistency of crystallinity ensures that the cooling rates of each layer are nearly uniform after placement. Since crystallinity is negatively correlated with cooling rate, the near uniformity of crystallinity across layers reduces the crystallization gradient along the thickness direction of the laminate, thereby increasing the overall average crystallinity of the laminate and raising the number of interfacial interdiffusions and entanglements of chain segments. Therefore, this significantly improves the ILSS of the CF / PAEK laminate. Attached Figure Description

[0013] Figure 1 This is an overall schematic diagram of an embodiment of the present invention.

[0014] In the diagram: 1 is the pressure roller; 2 is the CF / PAEK layer to be laid; 3 is the infrared laser; 4 is the CF / PAEK layer already laid; 5 is the heatable aluminum mold. Detailed Implementation

[0015] The present invention will now be described in further detail with reference to the accompanying drawings.

[0016] like Figure 1 As shown, the present invention discloses a mold temperature compensation process for improving the interlaminar shear strength of thermoplastic composite laminates. During the laminate laying process, infrared laser 3 and pressure roller 1 move at the same speed along the laying direction. After being irradiated by infrared laser 3, the resin matrix on the upper surface of the CF / PAEK layer to be laid 2 and the CF / PAEK layer already laid melts. Then, pressure roller 1 provides pressure to both, so that they are bonded together.

[0017] A heatable aluminum mold 5 is placed at the bottom to provide structural support for the forming component. Heating the mold can reduce warping deformation and avoid laying termination caused by excessive warping, so that the component can be laid smoothly.

[0018] The function of aluminum mold 5 is: to serve as a T tool The carrier relies on the high thermal conductivity of aluminum to rapidly and evenly transfer temperature and dynamically adjust T. tool It provides a rigid reference surface that can withstand a 500N bonding force without deformation, ensuring precise wire laying and eliminating the temperature difference between the aluminum mold 5 and the first layer of prepreg to avoid initial warping; it acts as a thermal buffer layer to slow down the cooling rate of the bottom layer, reduce the difference in cooling rate between the upper and lower layers, avoid excessive crystallization gradient in the thickness direction, and help improve the overall crystallization uniformity.

[0019] Example: Step 1: The CF / PAEK unidirectional prepreg (CF / PAEK layer 2) was provided by the Beijing Institute of Aeronautical Materials, China Aero Engine Corporation. Its width and thickness were 6.35 mm and 0.135 mm, respectively, with a resin content of 33%. The fiber's tensile strength and tensile modulus were 5880 MPa and 294 GPa, respectively, with an elongation of 2% and a density of 1.8 g / cm³. A 38 mm × 350 mm × 3.51 mm [0°] was prepared. 26 The laminate meets the interlaminar shear strength test requirements according to standard ASTM D2344.

[0020] Step 2, Experiment at T S The laying process is carried out under the orthogonal combination of laying force (300N-500N) and laying speed (50mm / s-100mm / s) of pressure roller 1, with a laying temperature of (150℃-250℃) and a laying temperature of (380℃-480℃). The purpose of orthogonal experiments is to systematically analyze the independent effects of single variables and the coupling effects of multiple variables by combining four variables according to an orthogonal array, while reducing the number of experiments and covering the key levels of each variable. The benefits include: reducing the number of experiments while covering key parameter combinations, clarifying the influence patterns of variables through experiments, and selecting optimal initial parameters.

[0021] Step 3: Using the measurement value of the K-type thermocouple as a reference, calibrate the emissivity of the CF / PAEK composite material and the aluminum mold 5 surface to determine the emissivity value set in the Optris 450i imager, ensuring the measurement of T... S The accuracy of the measurements was then assessed using a thermal imager during the laying process. s and T tool Perform calibration.

[0022] The aluminum mold 5 serves as a support. The aluminum mold 5 is stationary; material is laid on top of it. Because subsequent temperature compensation and adjustment of the aluminum mold 5 are required, its emissivity also needs to be calibrated. Step 4: During the sample laying process, T is performed at intervals of "every 4 layers". s The selected number of layers was 1, 5, 9, 13, 17, 21, and 25. This setting can avoid increasing the process complexity due to excessively dense spacing (such as 1 layer per test) and prevent the influence of excessively sparse spacing (such as 6 layers per test or 8 layers per test) on the Ts effect due to the increased number of layers, resulting in inaccurate fitting.

[0023] Get T s After the data is collected, T is adjusted in small steps. tool This allows the T measured by a thermal imager after the layer is laid out to be... sIf the requirements are met, record the value at this time T. tool For the same layup, the experiment should be repeated at least 3 times and the average value should be taken; data on "layup number - T" should be collected within the complete layer domain. tool "Discrete datasets".

[0024] The small-step adjustment method involves adjusting 3-5℃ at a time to avoid large steps (such as ≥10℃) that could lead to temperature fluctuations. tool A sudden rise / fall triggers T s It deviates significantly from the target value.

[0025] Step 5: To ensure that T is applied after each layer of laminate is laid... s Consistent, based on Origin software, the laminate's relationship with "number of layers - T" is... tool The discrete dataset is fitted using a suitable fitting method (such as linear regression or nonlinear fitting) to generate a continuous "layer number - mold temperature" relationship curve; this curve represents the actual temperature during processing. tool The adjustment provides a theoretical basis, thereby ensuring the uniformity and consistency of the surface temperature of the laminate. Through the above process, the mold temperature (T) is adjusted layer by layer. tool ), to achieve the surface temperature (T) of the laminate. s The consistency of the laminate reduces the crystallization gradient in the thickness direction.

[0026] The specific reaction mechanism is: After each layer is laid up, the cooling rate of each layer is made nearly uniform, thereby reducing the crystallization gradient in the thickness direction of the laminate and increasing the overall average crystallinity of the laminate. The number of inter-diffusion and entanglement of chain segments across interfaces also increases. All work aims to improve the overall average crystallinity of the laminate, i.e., to increase the number of inter-diffusion and entanglement of chain segments across interfaces, thereby improving ILSS6. The study also investigated the crystallization distribution in the thickness direction of laminates treated with mold temperature compensation (TTC) and those without TTC treatment. A Mettler Toledo UV-DSC differential scanning calorimeter was used to analyze the crystallization behavior of the laminate material in the thickness direction. The test was conducted in a nitrogen environment, heating from room temperature to 500°C at a rate of 10°C / min. By testing the changes in endothermic or exothermic changes during phase transitions such as melting and crystallization, the relevant enthalpy values ​​were accurately determined, providing a basis for studying the crystallization behavior and thermal properties of the material. Crystallinity was calculated using the following formula: In the formula: X c It refers to the crystallinity of the material; Δ H m It is the melting point and enthalpy of fusion; Δ H cIt is the enthalpy of cold crystallization; Δ H f This refers to the enthalpy of melting when the polymer is fully crystallized. Referring to LM-PAEK, the enthalpy of melting is 130 J / g. This invention selects 130 J / g as the enthalpy of melting for PAEK when it is fully crystallized. f It is the mass fraction of carbon fiber in the sample.

[0027] Step 6: Investigate the interlaminar shear strength (ILSS) of laminates treated with TTC and those not treated with TTC. According to standard ASTM D2344, 20 mm × 6 mm × 3 mm samples were cut from the center of the prepared specimens, and the interlaminar shear strength of the CF / PAEK laminates was tested by the short beam shear test (SBS). The short beam shear test was conducted on a Xi'an Licheng PLD-5 mechanical testing machine equipped with a 50 kN load cell and a three-point bending fixture. The span of the shear specimen was 12 mm, the loading roller was 6 mm, and the support roller was 3 mm.

[0028] Five sets of repeated experiments were conducted, and the average value was taken.

[0029] Laminates treated with the TTC process showed improved ILSS compared to laminates not treated with the TTC process.

[0030] The above description is a further detailed explanation of the present invention in conjunction with specific preferred embodiments. It should not be considered that the specific embodiments of the present invention are limited to this. Any simple deductions or substitutions made without departing from the concept of the present invention should be considered as falling within the scope of patent protection determined by the submitted claims.

Claims

1. A mold temperature compensation process for improving the interlaminar shear strength of thermoplastic composite laminates, characterized in that, Includes the following steps; Step 1: Measure T with the thermal imager S and T tool Before proceeding, emissivity calibration is required. First, set an emissivity value in the thermal imager, then measure the emissivity using both the thermal imager and a thermocouple. S and T tool The results from the thermal imager are compared with those from the thermocouple. If the results are inconsistent, the emissivity is readjusted until it matches the thermocouple measurement results. Step 2: Place the heatable aluminum mold (5), and calibrate the surface emissivity of the CF / PAEK composite material and the aluminum mold (5) based on the measurement value of the K-type thermocouple to determine the emissivity value set in the thermal imager and ensure the measurement of T S The accuracy of the measurements was then assessed using a thermal imager during the laying process. s and T tool Perform calibration; Step 3: CF / PAEK composite material is laid on a heatable aluminum mold (5), irradiated by infrared laser (3), and the CF / PAEK composite material is compacted by pressure roller (1) to form CF / PAEK laid layer (4). Step 4: During the automated fiber placement and in-situ consolidation (AFP-ISC) process, after each layer is placed, the temperature is adjusted by adjusting the mold temperature control module, thereby adjusting T. tool And use a thermal imager to verify T layer by layer S If T S If deviations occur, the temperature control module should be adjusted two or more times to ultimately ensure that the temperature of each layer is within acceptable limits. tool Consistency ensures a consistent cooling rate after each layer is laid, reduces the crystallization gradient in the thickness direction of the laminate, and improves the ILSS of the CF / PAEK laminate.

2. The mold temperature compensation process for improving the interlaminar shear strength of thermoplastic composite laminates according to claim 1, characterized in that, In step 3, the laminated CF / PAEK pre-laid layer (4) has dimensions of 38mm × 350mm × 3.51mm (length × width × height) [0°]. 26 Laminated board.

3. The mold temperature compensation process for improving the interlaminar shear strength of thermoplastic composite laminates according to claim 1, characterized in that, In step 3, the infrared laser (3) and the pressure roller (1) move at the same speed along the laying direction. The resin matrix on the upper surface of the CF / PAEK layer to be laid (2) and the CF / PAEK layer already laid (4) after being irradiated by the infrared laser (3) melts. Then the pressure roller (1) provides pressure to both of them, so that they are bonded together.

4. The mold temperature compensation process for improving the interlaminar shear strength of thermoplastic composite laminates according to claim 1, characterized in that, Step 3 includes T S The four variables are: laying temperature, laying force of pressure roller (1), and laying speed; Among them, T S The temperature ranges from 150℃ to 250℃, the laying temperature is 380℃ to 480℃, the laying force of the pressure roller (1) is 300 N to 500 N, and the laying speed is 50 mm / s to 100 mm / s. The four variables are carried out under orthogonal combination conditions.

5. The mold temperature compensation process for improving the interlaminar shear strength of thermoplastic composite laminates according to claim 1, characterized in that, In step 4, T is performed at intervals of 4 CF / PAEK layers (4). s Measurement, obtaining T s After the data is collected, T is adjusted in small steps. tool This allows the T measured by a thermal imager after the layer is laid to be properly positioned. s If the requirement is met, record the value of T at this point. tool For the same layup, the experiment should be repeated at least 3 times and the average value should be taken; collect data on "layup number - T" within the complete layer domain. tool "Discrete datasets".

6. The mold temperature compensation process for improving the interlaminar shear strength of thermoplastic composite laminates according to claim 5, characterized in that, The small-step adjustment method involves adjusting the temperature by 3-5°C at a time.

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