The demand for composite materials will increase at a faster rate, and its high cost has become an important bottleneck restricting the widespread application of composite materials. Low-cost composite material manufacturing technology is a core issue in the field of composite materials research. To improve the performance-price ratio of composite materials, in addition to research and improvement in raw materials, assembly and maintenance, etc., it is more important to reduce the manufacturing cost of composite materials. In recent years, the emergence of 3D printing technology has opened up a whole new way of thinking for the manufacturing industry. Different from the subtractive manufacturing and isomaterial manufacturing of traditional manufacturing, it can quickly mold complex shaped products and achieve the maximum limit by adding materials. Use of raw materials. Studying the applicability of 3D printing technology in the manufacture of aerospace composite products has important engineering significance for promoting the low-cost manufacturing of aerospace composite products.
The launch cost of a spacecraft is high, and the quality of the payload has a huge impact on the launch cost. Therefore, the payload pays special attention to structural efficiency in the structural design and material selection. Carbon fiber composites have the characteristics of high specific strength, large specific modulus, good thermal stability, and strong designability. The excellent comprehensive performance is exactly what aerospace products need to pursue performance *, structure quality efficiency *. At present, carbon fiber composite products used in aerospace optical remote sensors cover various parts of remote sensors, such as camera lens barrels, camera brackets, hoods, trusses, and so on. The resin used is mainly epoxy resin and cyanate resin, and the reinforcing material is mainly continuous carbon fiber. According to the specific product characteristics and process characteristics, according to the product performance requirements and thickness requirements, the prepreg is laminated on the mold in a certain order and number of layers to form a green body, and then the green body is placed in an autoclave or The hot press performs high temperature and high pressure curing for several hours in a high temperature environment.
Aerospace remote sensor composite materials and manufacturing processes have the following characteristics:
1) To ensure the mechanical properties of the product, continuous fibers are used for the reinforcement;
2) The resin matrix epoxy resin and cyanate resin are both thermosetting resins, which need to be cured for a few hours at a specific curing temperature and pressure (chemical crosslinking reaction occurs) to form a stable network crosslinked polymer;
3) The inside of the prepreg laminated body is loose. In order to exhaust the air and other small molecules in the body, high pressure must be applied to the body while heating to increase the compactness of the product and ensure the mechanical properties of the product;
4) For complex structure products, in order to ensure its mechanical properties, the prepreg design often requires multiple planes or multiple parts to be continuously laminated. For thin-walled reinforced lens barrels, flange rings and lens barrel bodies must be guaranteed. In the multi-directional joint, it is necessary to ensure that the ends are continuous in the circumferential direction and the roots between the ends are continuous in the multi-directional joint.
1.3D printing technology
1.1 Principles of 3D printing technology
3D printing is also called additive manufacturing, which is different from traditional subtractive or isomaterial manufacturing methods. It uses the method of material accumulation layer by layer to manufacture solid parts. This technology is integrated and developed on the basis of modern CAD / CAM technology, laser technology, computer numerical control technology, information technology, precision servo drive technology and new materials and physical and chemical technology. Its working principle is to discretize the computer three-dimensional model of the physical entity into a series of two-dimensional laminations. Using precision nozzles or laser heat sources, according to the lamination information, under the control of digital control, the cladding molding material is reinforced by continuous physical lamination Material, layer by layer to generate 3D solid products.
1.2 Composite 3D printing technology
Among various 3D printing technologies, 3D manufacturing of composite materials mainly includes selective laser sintering (SLS), fused deposition modeling (FDM), layered solid manufacturing (l, LOM), and stereolithography (SL).
(1) SLS
The main method for manufacturing composite materials by SLS is the mixed powder method, that is, the matrix powder and the reinforcement powder are mixed. The laser heats the powder in a specific area according to the cross-sectional shape of the design drawing, so that the matrix powder with a relatively low melting point is melted, thereby melting the matrix The reinforcements are bonded together to achieve compounding of the components. The problem with this method is that the density of the two materials in the mixed powder is different, and it is easy to settle and make the product components uneven. By synthesizing a single composite material powder for technical improvement, the composite material powder produced can overcome the problems of easy sedimentation and unevenness of the mixed powder, and can thus produce higher quality products.
(2) FDM
The composite material produced by the FDM process is made of prepreg tow in advance with fibers and resin, and then the prepreg tow is sent to the nozzle. The tow is heated and melted at the nozzle and stacked on the platform according to the design trajectory to form a layer of material. The connection with the layer is formed by partial or complete melting of the resin. The composite prepreg tow used in FDM technology must meet the requirements of composition, strength, and low viscosity. Generally, a plasticizer is added to the composite to increase fluidity.
(3) LOM
The LOM technology is similar to the FDM. It is necessary to prepare unidirectional fiber / resin prepreg tows in parallel to make a weft-free cloth, that is, a prepreg strip. The prepreg strip is sent to the workbench via a conveyor belt. The contour line of each section of the model is cut into prepreg strips, which are layered together layer by layer to form a three-dimensional product.
(4) SL
To make a composite material with SL, firstly, the photosensitive polymer and the reinforcing particles or fibers are mixed into a mixed solution, and the mixed solution stored in the liquid tank is rapidly scanned by an ultraviolet laser, so that the photosensitive polymer rapidly undergoes photopolymerization, thereby changing from a liquid state. It is solid, and then the table lowers the height of a layer of sheet, and performs a second layer of laser scanning curing, and so on to form the final product. SL manufacturing composite materials have problems such as the precipitation of reinforced particles leading to uneven distribution of particles, the generation of voids after curing caused by foam in the solution, and the reflection of particles to reduce the laser absorption energy and therefore require longer irradiation time.
2. Advances in 3D printing technology for composite materials
Thermoplastic resins have the characteristics of heating and softening and cooling and solidification, which is easy to achieve additive manufacturing. In the 3D printing market, thermoplastics are the mainstay. Similarly, in composite 3D printing technology, composites based on thermoplastic resins are also relatively important. As a research object, the reinforcing materials include chopped fibers and continuous fibers.
2.1 Chopped fiber / thermoplastic resin composite material
3D printing companies in Germany and the United States and 3D printing companies in China have developed chopped fiber / thermoplastic resin composite powders that can be used in SLS technology and commercialized them.
2.2 Continuous fiber / thermoplastic resin composite material
The American company developed a continuous carbon fiber reinforced thermoplastic composite 3D printing equipment in early 2014, and printed a carbon fiber reinforced nylon composite. The printer has two nozzles, one nozzle conveys thermoplastic resin (nylon or polylactic acid), one nozzle conveys continuous prepreg carbon fiber filament or prepreg glass fiber filament, the prepreg fiber filament is coated with thermoplastic resin specially developed for printer, two The nozzles work in turn, and the resin and prepreg tow are laid along the X / Y plane using the FDM-based process to achieve the fiber and resin composite. The fibers can be oriented as needed or placed only where needed. At present, this equipment can only achieve fiber orientation in the X / Y direction, and cannot achieve Z-direction.
The American company and the US Department of Energy's Oak Ridge National Laboratory have collaborated to develop mass-produced carbon fiber composite materials FDM manufacturing technology. The cooperation is divided into three phases. The first phase studies how to put broken fibers in the FDM process and how to adjust the various mechanical properties of the material. The second to third phases of research focus on manufacturing continuous carbon fiber composites on the center line and further Processing.
2.3 Chopped fiber / thermosetting resin composite material
Harvard University has developed an epoxy resin suitable for 3D printing, which enables 3D printing of thermosetting resins, as shown in Figure 3. To improve the viscosity of the resin, the researchers added nanoclay, dimethyl phosphate, silicon carbide whiskers, and chopped carbon fibers, and used imidazolium as a curing agent, which greatly expanded the printing window of the resin, making the resin in The viscosity does not increase significantly during the print window period. By controlling the fiber aspect ratio and nozzle diameter, the filler is oriented under the action of shear force and extrusion flow, and the orientation of the filler is controlled to obtain the oriented fibers. Printed parts are pre-cured at a lower temperature and then removed from the substrate for further high-temperature curing.
3. Analysis of 3D printing technology of aerospace resin-based composite materials
At present, composite fiber 3D printing technology is mainly short fiber / thermoplastic composite materials, materials and equipment have been commercialized, while thermosetting matrix composite materials have only realized 3D printing of chopped fiber reinforced composite materials in the laboratory. Combining with the product characteristics of aerospace remote sensor composite materials, continuous fiber-reinforced thermosetting composite 3D printing technology needs urgent breakthroughs in printing materials, multidimensional continuous printing, and pre-curing functions.
1) Develop adaptive printing materials. The 3D printing process of composite materials requires that the printing materials have appropriate viscosity, fluidity, long operating time, and short molding time. Therefore, it is necessary to adaptively develop the existing aerospace composite material system and improve the material system to provide satisfaction. 3D printing technology and materials required for aerospace applications.
2) Breakthrough fiber multi-dimensional continuous printing. Composite 3D printing equipment urgently needs to break through continuous stacking in multi-dimensional directions, such as setting up a five-axis / six-axis linkage printing platform to achieve multi-dimensional continuous printing by rotating the platform to meet the continuous layering of multiple planes and multiple parts of aerospace complex structure products Claim.
3) Achieve pre-compaction function. Thermosetting resin-based composite materials need to achieve curing of the resin matrix and densification of the product under high temperature and pressure. After printing a certain number of layers, the compact can be pre-compacted and heated in the equipment to improve the compactness in the middle of printing. After printing is completed, the blank is moved to a curing device for final curing. The use of low-cost technology is one of the effective ways to reduce the cost of composite materials. 3D printing technology can increase the use of materials to increase the use of materials to reduce the production cost of composite materials. In addition, for complex-structured composite products, 3D printing technology can also reduce dependence on tooling and shorten processing time. At the same time, it can achieve overall molding and reduce assembly time. It is of great engineering significance to study the application of 3D printing technology in aerospace composite materials. . For continuous fiber-reinforced thermosetting resin composite materials used in aerospace remote sensors, 3D printing needs to solve problems such as printing materials, continuous multi-dimensional printing of fibers, and pre-curing functions.