| The continuous fiber reinforced thermoplastic composites(CFRTPCs)have excellent properties such as the high specific stiffness and strength,light weight,great impact and corrosion resistance,and long fatigue life.Meanwhile,they are green and environmentally friendly advanced materials with significant application value in the fields of high-precision machining equipment,aerospace,automobile and high-speed rail.This paper explored the new composite molding technology named 3D printing additive manufacturing technology.In view of the problems of 3D printing of continuous fiber reinforced composites including the printing defects and poor mechanical performance,the key technologies including the defect control and mechanical property strengthening were studied,laying a theoretical and methodological foundation for realizing the structural design,performance regulation and integrated rapid prototyping of 3D printed CFRTPCs.In this paper,a 3D printing experimental platform suitable for continuous fiber reinforced thermoplastic composites was built and the integrated debugging and iterative optimization were carried out.The typical kinematic structure mechanism of the 3D printing equipment was discussed.The fiber cutting and wire feeding mechanisms and the printing platform called hot bed were designed based on composite properties.The surface of hot bed was optimized by coatings according to the warping deformation defects.The open-source control system of 3D printing equipment for CFRTPCs was designed including the upper software design system,the lower hardware driven system and the human-computer interaction system.Functional modules such as the slicing and path planning algorithms were written.The printing speed and temperature control algorithm were optimized.Secondly,the basic physical and chemical properties,mechanism of mechanical properties and forming process of continuous fiber reinforced composites were analyzed.The causes of forming defects of composites were analyzed,providing theoretical support for 3D printing of composites.The interfacial stress transfer and bond strength models of composites were constructed.The real-time impregnation and pre-impregnation printing process of continuous fiber were compared,and the impregnation process of molten resin in the 3D printing process was modeled and analyzed.The integrated dual-nozzle module was designed.The synergistic control relationship of the printing processes was discussed.An evaluation system for 3D printed composites was established.The internal pores and failure damage were modeled and simulated.Combined with the actual pore and failure morphology of the printed composites,the simulation results of the evaluation models were verified to be valid.Based on the analysis of the composite defects,the 3D printing platform was used to carry out the experiments of composite materials printing,and the influence of key process parameters on the defects and mechanical properties was discussed.The printed surface defects including scratches,redundancy,stacking,path and interlayer gaps,warpage deformations,as well as internal gaps and cracks were summarized according to the printing experiments of CFRTPCs.The formation reasons of printing defects were analysed based on the microscopic morphologies.The printing accuracy and stability were evaluated.The mathematical models between warping deformation and hot bed temperature,porosity and nozzle temperature were established to quantitatively analyze typical printing defects of composites.The microporous composite and solid glue coatings of the hot bed with 60 to70 ℃ were designed to improve the surface flatness and the printing stability,reducing warpage deformations,printed size errors and internal porosity with 290 to 295 ℃ nozzle temperature,effectively improving tensile and flexural properties.Aiming at the tensile and fatigue properties of printed composites,a mathematical model was established and modified to study the quantitative influence law between continuous fiber reinforcement and mechanical properties of composites.And the enhancement effect of continuous fiber arrangement on mechanical properties of composites was analyzed,providing reference for the design of mechanical properties.The tensile experiments proved that the average distribution of continuous fiber reinforced layers and the increase of continuous fibers helped to improve the elastic modulus and ultimate tensile strength.The fatigue resistance of composites with more continuous fibers was stronger at the same tensile stress level.When the volume fraction of continuous fiber was less than 0.57%,the continuous carbon fiber played a positive role in improving the fatigue life within the fatigue period range of less than 10000.When the fatigue period number exceeded 10000,the pore defects caused by continuous fiber reduced the enhancement effect on the fatigue life.The tensile and fatigue failure mechanisms of the printed CFRTPCs were analysed based on the micromorphology observation.In order to further reduce the printing defects and strengthen the mechanical properties of composites,the coupling optimization process and auxiliary strengthening process of 3D printing were designed and verified by experiments.The coupling optimization of key printing parameters was designed and the results showed that in order to obtain the best tensile modulus and ultimate tensile strength,the optimum nozzle temperature,layer thickness,feed speed and printing speed were respectively 295 ℃,0.5 mm,6.5 r/min,500 mm/min.In order to obtain the minimum cross-sectional porosity,the above processes should be285-295 °C,0.4-0.5 mm,6.0-6.5 r/min and 550 mm/min respectively.The mechanisms of strengthening processes including the hot wind,ultrasonic vibration and laser melting were analysed.The experiments verified the improved mechanical properties of 3D printed CFRTPCs under the independent and coupling actions of strengthening processes.The tensile modulus,ultimate tensile strength and elongation at break of continuous carbon fiber reinforced PPS composites were increased by 39.48%,144.28%,62.62%.The flexural modulus,strength and maximum load were increased by 25.09%,21.06%,21.05%. |
