High tensile and strong bending SLS composites
Thermosetting short fiber reinforced polymer (SFRP) composites have excellent thermal stability, dimensional stability, rigidity and corrosion resistance, and are very promising lightweight materials. They have been widely used in military, automotive and aerospace industries. . At present, the main preparation methods are compression molding and injection molding using sheet molding compound (SMC) and block molding compound (BMC) as raw materials. The production efficiency is high and the product accuracy is good, but the preparation cycle is long, the production cost is high, and it is difficult to manufacture. Complex shaped parts.
Huazhong University of Science and Technology Rapid Manufacturing Center proposed for the first time an integrated process for the preparation and forming of carbon fiber / epoxy thermosetting resin based on powder bed laser additive manufacturing (SLS), which can overcome the above-mentioned shortcomings. ) / Resin (EP) ternary structure and exhibits higher tensile and flexural strength than most reported SLS materials.
Its preparation process is shown in Figure 1. In the first step, the carbon fiber (CF) is surface-treated and mixed with PA12, so that the surface of the CF is coated with a thin layer of PA12 to obtain a PA12 / CF composite powder for SLS; in the second step, the SLS process is used for printing : PA12 melts under the high temperature of the laser, and acts as a binder to connect the CFs to each other to form a network structure and print the porous preform. The third step is to use high-performance epoxy resin under high temperature and negative pressure conditions. (EP) infiltrate the preform; finally, the composite material is cured to prepare a CF / PA12 / EP ternary composite material.
It can be seen from Figures 2.a and b that after the SLS process, the PA12 polymer binder has been completely melted, and the CF is connected to form a porous structure, which facilitates the subsequent infiltration and filling of the liquid epoxy resin. As shown in Figures 2.c and d, after filling, the EP matrix and CF reinforcement penetrate into each other, forming a three-dimensional continuous structure dispersion. Among them, the thin PA12 polymer coating on the CF surface has two functions: (1) in the SLS process, under the laser irradiation, as a binder to connect the discrete CF into a porous CF preform; (2) as an intermediate layer , Increase the chemical interaction and wettability between CF and EP matrix. The relative content of binder PA12 in the composite powder determines the initial strength and porosity of the SLS blank. The more binder, the higher the strength of the blank, the lower the porosity, and the less the amount of epoxy resin penetrating into the composite. Studies have shown that on the premise of sufficient strength post-treatment, the optimal content of PA12 to maximize porosity is 25 Vol%.
Table 1. Performance comparison of CF / PA12 / EP ternary composites with several other polymer-based composites prepared by SLS
Table 1 shows that the ternary composites prepared by this method have higher tensile and flexural strengths than the polymer-based composites prepared by several other SLS, reaching 101.03 MPa and 153.43 MPa, respectively.
Figure 3. Tensile fracture surface of ternary composite material, magnifications are (a) 500 ×, (b) 2000 × As can be seen from Figure 3.a, the composite material shows typical brittle failure behavior, and the fracture surface has shear deformation. Rough form. The deformation and cracks of the EP matrix propagate in different directions (as indicated by the arrows). The crack propagation is blocked by the CF / PA12-rich region and forced to change the trajectory, which improves the fracture toughness and strength of the composite. Three fiber failure mechanisms: fiber pull, interfacial exfoliation and matrix failure can be observed on the failure surface. The higher tensile and flexural strengths of polymer-based composites compared to several other SLS can be attributed to: (1) the uniform distribution of CF; (2) the CF and EP caused by mechanical interlocking and chemical interactions Good interface combination, as shown in Figure 3.b.