Multi-scale Modeling and Finite Element Analyses of Thermal Conductivity of 3D C/SiC Composites Fabricating by Flexible-Oriented Woven Process

Thermal conductivity is one of the most significant criterion of three-dimensional carbon fiber-reinforced SiC matrix composites(3D C/SiC).Represent volume element(RVE)models of microscale,void/matrix and mesoscale proposed in this work are used to simulate the thermal conductivity behaviors of the 3D C/SiC composites.An entirely new process is introduced to weave the preform with three-dimensional orthogonal architecture.The 3D steady-state analysis step is created for assessing the thermal conductivity behaviors of the composites by applying periodic temperature boundary conditions.Three RVE models of cuboid,hexagonal and fiber random distribution are respectively developed to comparatively study the influence of fiber package pattern on the thermal conductivities at the microscale.Besides,the effect of void morphology on the thermal conductivity of the matrix is analyzed by the void/matrix models.The prediction results at the mesoscale correspond closely to the experimental values.The effect of the porosities and fiber volume fractions on the thermal conductivities is also taken into consideration.The multi-scale models mentioned in this paper can be used to predict the thermal conductivity behaviors of other composites with complex structures.

Interfacial Bonding Mechanism and Mechanical Performance of Continuous Fiber Reinforced Composites in Additive Manufacturing

The additive manufacturing of continuous fiber composites has the advantage of a high-precision and efficient forming process,which can realize the lightweight and integrated manufacturing of complex structures.However,many void defects exist between layers in the printing process of additive manufacturing;consequently,the bonding performance between layers is poor.The bonding neck is considered a key parameter for representing the quality of interfacial bonding.In this study,the formation mechanism of the bonding neck was comprehensively analyzed.First,the influence of the nozzle and basement temperatures on the printing performance and bonding neck size was measured.Second,CT scanning was used to realize the quantitative characterization of bonding neck parameters,and the reason behind the deviation of actual measurements from theoretical calculations was analyzed.When the nozzle temperature increased from 180 to 220℃,CT measurement showed that the bonding neck diameter increased from 0.29 to 0.34 mm,and the cross-sectional porosity reduced from 5.48%to 3.22%.Finally,the fracture mechanism was studied,and the influence of the interfacial bonding quality on the destruction process of the materials was determined.In conclusion,this study can assist in optimizing the process parameters,which improves the precision of the printing parts and performance between the layers.

Study on the Distribution of Frictional Forces on Z-yarn Continuous Implanted Preforms and Their Applications

To improve the quality and efciency of Z-directional 3D preform forming,the Z-yarn frictional force distribution model of the preform and its wear mechanism were investigated.In this study,a tensile force measuring device was designed to measure the force required to replace the guide sleeve,which is equivalent to the Z-yarn frictional forces.The frictional force is proportional to the number of preform layers and is applied to the preform decreased from the corner,edge,sub-edge,and middle in order.A back propagation neural network model was established to predict the friction at diferent positions of the preform with diferent layers,and the error was within 1.9%.The wear of Z-yarn was studied at diferent frictional positions and after diferent times of successive implantation into the preform.The results showed that with an increase in the number of Z-yarn implantations and frictional forces,the amount of carbon fber bundle hairiness gradually increased,and the tensile fracture strength damage of the fber was increasingly afected by the frictional forces.In the corner position of the preform,when the number of implantations was 25,the fber fracture strength decreased non-linearly and substantially;in order to avoid fber fracturing in the implantation process,the Z-yarn needs to be replaced in time after 20–25 cycles of continuous implantation.This study solves the problem of difculty in measuring the force required for individual replacements owing to the excessive number of guide sleeves,puts forward the relationship between fber wear,preform position,and implantation times,solves the phenomenon of fracture in the preform during Z-direction fber implantation,and realizes the continuous implantation of fbers.

Research on the macro-and meso-mechanical properties of frozen sand mold based on Hertz-Mindlin with Bonding model

In this study,macro-and meso-mechanical properties of frozen sand molds were discussed based on the Hertz-Mindlin with Bonding(HMB)model.Plackett-Burman,steepest ascent,and central composite designs were utilized to propose a parameter calibration methodology.The effects of mesoscopic parameter variations on the compressive strength and average gradient of stress-strain were investigated through response surface method analysis.Results showed that the relative error between the simulated and measured repose angle is 3.1%under calibrated intrinsic contact parameters.The compressive strength and average stress-strain gradient primarily depend on the normal and shear stiffness per unit area,as well as the particle size and porosity of the silica sand.Furthermore,taking load-displacement curves of three frozen sand molds with different geometric characteristics as the target value,the reliability and effectiveness of the frozen sand mold HMB model were verified through uniaxial compression tests and discrete element simulations.

Variable-Diameter Deployable Structure Composite Preforms Made by Braiding and Needle-Punching Integrated Forming Technology

Variable-diameter deployable carbon fiber reinforced polymer(CFRP)composites possess deformation and load-bearing functions and are composed of stiff-flexible coupled preforms and matrix.The stiff-flexible coupled preform,serving as the reinforcing structure,directly determines the deployable properties,and its forming technology is currently a research challenge.This paper designs a braiding and needle-punching(BNP)composite preform forming technology suitable for stiff-flexible coupled preforms.Before forming,the preform is partitioned into flexible and rigid zones,with braiding and needle-punching performed layer by layer in the respective zones.A retractable rotating device is developed to form the stiff-flexible coupled preform,achieving a diameter variation rate of up to 26.6%for the BNP preform.A structural parameter model is also established to describe the geometric parameter changes in the deformation and load-bearing areas of the preform during deployment as a function of the braiding angle.Based on experiments,this paper explains the performance changes of BNP composites concerning the structural parameters of the preform.Experimental analysis shows that as the braiding angle increases,the tensile performance of BNP composites significantly decreases,with the change rate of tensile strength first decreasing and then increasing.Additionally,when the braiding angle is less than 21.89°,the impact toughness of BNP composites remains within the range of 83.66±2 kJ/m^(2).However,when the braiding angle exceeds 21.89°,the impact toughness of BNP composites gradually decreases with increasing braiding angle.Furthermore,a hybrid agent model based on Latin hypercube sampling and error back-propagation neural network is developed to predict the tensile and impact properties of BNP composites with different structural parameters,with maximum test relative errors of 1.89%for tensile strength and 2.37%for impact toughness.

Collaborative manufacturing technologies of structure shape and surface integrity for complex thin-walled components of aero-engine:Status,challenge and tendency

Presently,the service performance of new-generation high-tech equipment is directly affected by the manufacturing quality of complex thin-walled components.A high-efficiency and quality manufacturing of these complex thin-walled components creates a bottleneck that needs to be solved urgently in machinery manufacturing.To address this problem,the collaborative manufacturing of structure shape and surface integrity has emerged as a new process that can shorten processing cycles,improve machining qualities,and reduce costs.This paper summarises the research status on the material removal mechanism,precision control of structure shape,machined surface integrity control and intelligent process control technology of complex thin-walled components.Numerous solutions and technical approaches are then put forward to solve the critical problems in the high-performance manufacturing of complex thin-wall components.The development status,challenge and tendency of collaborative manufacturing technologies in the high-efficiency and quality manufacturing of complex thin-wall components is also discussed.

Digital high-efficiency print forming method and device for multi-material casting molds

Sand mold 3D printing technology based on the principle of droplet ejection has undergone rapid development in recent years and has elicited increasing attention from engineers and technicians.However,current sand mold 3D printing technology exhibits several problems,such as single-material printing molds,low manufacturing efficiency,and necessary post-process drying and heating for the manufacture of sand molds.This study proposes a novel high-efficiency print forming method and device for multi-material casting molds.The proposed method is specifically related to the integrated forming of two-way coating and printing and the shortflow manufacture of roller compaction and layered heating.These processes can realize the high-efficiency print forming of high-performance sand molds.Experimental results demonstrate that the efficiency of sand mold fabrication can be increased by 200%using the proposed two-way coating and printing method.The integrated forming method for layered heating and roller compaction presented in this study effectively shortens the manufacturing process for 3D-printed sand molds,increases sand mold strength by 63.8%,and reduces resin usage by approximately 30%.The manufacture of multi-material casting molds is demonstrated on typical wheeled cast-iron parts.This research provides theoretical guidance for the engineering application of sand mold 3D printing.

Effects of Low-pressure Annealing on the Performance of 3D Printed CF/PEEK Composites

The interlayer bonding properties are normally unsatisfying for 3D printed composites owing to the layer-by-layer formation process.In this study,low-pressure annealing was performed on 3D printed carbon fiber reinforced polyether ether ketone(CF/PEEK)to improve the interlayer bonding strength.The effects of annealing parameters on the mechanical properties and microstructure were studied.The results showed that the interlaminar shear strength(ILSS)of CF/PEEK improved by up to 55.4%after annealing.SEM and𝜇-CT were also applied to reveal the reinforcing mechanism.This improvement could mainly be attributed to the increased crystallinity of the CF/PEEK after annealing.Additionally,annealing reduced the porosity of the printed CF/PEEK and improved the fiber-resin interface.This resulted in a reduction in the stress concentration areas during loading,thereby enhancing the interlayer bonding strength of CF/PEEK.

Effect of contact forms on the wear of hard silicon surfaces by soft polymers

The mechanism of hard surfaces worn by soft polymers is not clearly understood.In this paper,a new hypothesis has been proposed,it holds that the stress acting on the hard surface under certain working conditions is the main reason for wear of the hard surface by a soft polymer.The hypothesis was investigated by changing the contact form between tribo-pairs.For this,friction tests between six polymer spheres and smooth,rough,and inclined monocrystalline silicon surfaces were carried out.The results show that for the same tribo-pair,the silicon surface will not be worn in some contact forms,but in other contact forms it will be worn.We believe the wear of hard surface by a soft polymer is the result of the combined stress state action on the hard surface.

Hybrid forming mechanism of patternless casting and laser cladding

In accordance with the requirement of manufacturing dies quickly and economically,a hybrid forming method of stamping dies for automobile panels is proposed.The method combines digital patternless casting and high-power laser cladding.An experimental study is conducted on the hybrid forming process and its trial production and application in the manufacturing of stamping dies for typical panels.Results prove that the laser cladding layer exceeds HRC60(Rockwell hardness)and thus meets the production efficiency requirement of automobile dies.The rate of defects is well controlled.Compared with traditional technology,this technology has remarkable advantages and advancement.