| Medical implants can be used to replace the damaged organs and tissues of human beings.3D printing technology has the advantages of short production process,short manufacturing cycle and easy to form complex structures,which provides an opportunity for the porous characteristics of bionic natural bone structure.However,in the manufacturing process,rapid heating,cooling and polymer shrinkage of materials will lead to the generation and redistribution of residual stress,which will have a significant impact on the fatigue strength and fracture toughness of formed parts.In order to deeply explore the generation mechanism of 3D printing residual stress,reduce residual stress and effectively reduce printing deformation,cracking and other defects,taking medical implants as the object,this paper studies the production mechanism and control method of residual stress based on stereolithography apparatus(SLA).Through 3D printing single point impact simulation,the propagation and attenuation law of stress wave on the surface and inside of the workpiece were revealed;The theoretical model of 3D printing residual stress of multi-layer solid and porous structural parts was established.Through the 3D printing experiments of single-layer solid,multi-layer solid and porous structural parts,the influence mechanism of 3D printing process parameters(laser power and scanning speed)on residual stress in implant forming process was clarified step by step.The regulation method of 3D printing residual stress was designed to improve the generation accuracy of 3D printing ceramic medical implants with porous structure.The main research contets are as follows:The high fidelity simulation model was established,the action mechanism of SLA-3D printing stress wave on residual stress was clarified,and the influence law of SLA-3D printing parameters on residual stress was revealed.The propagation of SLA-3D printed stress wave(single point impact)was simulated and analyzed by finite element software.The wave velocity of laser light wave was basically consistent with the theoretical calculation value,which verified the correctness and effectiveness of the simulation model.The results showed that the residual stress of SLA-3D printing decreased with the increase of scanning speed and increases with the increase of laser power.The attenuation function of SLA-3D printed stress wave was obtained by fitting,which revealed that there was a positive correlation between SLA-3D printed stress wave attenuation and residual stress.The simulation study of SLA-3D printing residual stress of single-layer solid parts showed that the surface residual tensile stress was the smallest at the scanning curing center and gradually increased to the outside,so the edge of the printed parts was prone to warping deformation.The correctness of the simulation model was verified by SLA-3D printing experiment.Aiming at the problems of part deformation and cracking caused by SLA-3D printing residual stress,the residual stress SWB(Shrink,Wave,Bending)model of SLA-3D printing of multilayer solid parts based on stress wave theory was established and the quantitative relationship between SLA-3D printing residual stress and stress wave was clarified.Through SLA-3D printing experiment,the correctness of the theoretical model was verified from the perspectives of laser power and scanning speed.The theoretical model and experimental results showed that the SLA-3D printing residual stress increased with the increase of laser power and decreased with the increase of scanning speed.When the laser power increases in the range of 117-208 MW,the errors between the theoretical and experimental values of residual stress were 2.8%,1.4%,4.3%,5.3%and 4.2%respectively.When the scanning speed increases in the range of 2000-6000 mm/s,the errors between the theoretical value and the experimental value were 2.5%,3.9%,2.8%,2.4%and 3.8%respectively.Considering the residual stress of porous structure of medical implant,a theoretical model of SLA-3D printing residual stress of porous structure was established.It showed that the variation trend of residual stress was consistent with the stress concentration factor of hole with the size of hole.Based on the SWB model of SLA-3D printing residual stress and the stress concentration phenomenon of porous structure,a theoretical model of SLA-3D printing residual stress of porous structure was established.The porous samples of zirconia ceramics with two hole types(square hole and round hole)were printed by SLA.The correctness of the theoretical model was verified by measuring the surface residual stress of the printed parts.The control methods of 3D printing residual stress were studied by optimizing degreasing sintering process and blending material components.First,the surface residual stress of SLA-3D printed ZrO2 ceramic samples was regulated by optimizing the degreasing sintering process.When the sintering temperature was 1450?,the holding time was 0 min and 90 min,and the heating rate was 4?/min,the surface residual tensile stress was the smallest.Sintering temperature and holding time have a great influence on the surface residual tensile stress,but the effect of heating rate was not obvious.The residual stress can be effectively controlled by adjusting sintering temperature and holding time.The second was to control the residual stress and optimize the mechanical properties by adding new components.The carbon material graphene oxide(GO)was added to the zirconia(ZrO2)ceramic material,and the ZrO2-GO composite ceramic powder was sintered by hot pressing to optimize the ZrO2-GO composite ceramic material with better mechanical properties(GO content was 0.1?0.15 wt.%).The comparison of surface residual tensile stress between ZrO2 ceramic and ZrO2-GO composite ceramic SLA-3D printing showed that the GO composition of the preferred component made the combination between ZrO2 ceramic particles and matrix resin more uniform,and significantly reduced the surface residual tensile stress of SLA-3D printing. |
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