查看更多>>摘要：Recently,the preparation of ultra-high temperature HfC ceramic coating has gained sig-nificant attention,particularly through the application of the HfCl4-CH4-H2-Ar system via Chem-ical Vapor Deposition(CVD),which has been found widely applied to C/C composites.Herein,an analysis of the reactions that occur in the initial stage of the CVD-HfC coating process is presented using Density Functional Theory(DFT)and Transition State Theory(TST)at the B3LYP/Lan-l2DZ level.The results reveal that HfCl4 can only cleave to produce hypochlorite,which will further react with methyl to synthesize intermediates to form HfC.According to the analysis of the energy barrier and reaction constant,HfCl preferentially reacts with methyl groups to form complex adsorptive intermediates at 1573 K.With a C-Hf bond production energy of 212.8 kcal/mol(1 kcal=4.18 kJ),the reaction rate constant of HfCl+CH is calculated to be 2.15 × 10-18 cm3/s at 1573 K.Additionally,both the simulation and experimental results exhibit that the upward trend of reaction rate constants with temperature is also consistent with the deposition rate,indi-cating that the growth curve of the reaction rate constants tends to flatten out.The proposed reac-tion model of the precursor's decomposition and reconstruction during deposition process has significant implication for the process guidance.
查看更多>>摘要：Burrs generated during the machining of Aramid-Fiber-Reinforced Composites(AFRPs)pose a challenge for the production efficiency of aircraft and helicopter housing parts.Existing studies have generally attempted to suppress burrs by referring to delamination suppres-sion methods.In contrast to stratification,burrs are remediable machining defects.As such,a mechanochemical method with burrs trimming technological strategy are implemented to effec-tively combat burrs.Herein,we clarify the mechanism by which aramid fibers cannot be cut off using analytical and numerical models.In addition,the mechanism of fiber fracture with Modified Polyurethane Reactive Polymer(M-PUR),and development of anti-burr devices(thermostatic adhesive sealed generator)are discussed.Finally,the experimental results show that the reduction rate in burr length is 87％-91％through the mechanochemical method.The method not only opens a new avenue to solve the burr problem of aramid fibers but also builds an interdisciplinary bridge between polymer science and composite machining.
查看更多>>摘要：To further improve the performance of binders,a SiHfBCN-based high-temperature resistant adhesive was successfully synthesized by Polymer-Derived Ceramics(PDC)route using TiB2,Polysiloxane(PSO)and short SiC nanowires as fillers.The effect of short SiC nanowires on the adhesive strength at room temperature and high temperature,as well as the reinforcing mechanism was studied.Compared with the adhesive without SiC nanowires,after curing(at 170 ℃)and pyrolysis(at 1000 ℃)in air,the appropriate adding of SiC nanowires upgrades the room temperature and high temperature(at 1000 ℃ in air)adhesive strength to(12.50±0.67)MPa(up by about 32％)and(13.11±0.79)MPa(up by about 106％),respectively.Attractively,under the synergistic impact of the nanowire bridging,nanowire breaking,nanowire drawing and crack deflection,the optimized adhesive exhibits multi-stage fracture,causing the increscent fracture displacement.
查看更多>>摘要：Superalloy thin-walled structures are achieved mainly by brazing,but the deformation process of brazed joints is non-uniform,making it a challenging research task.This paper records a thorough investigation of the effect of brazing parameters on the microstructure of joints and its mechanical properties,which mainly inquires into the deformation and fracture mechanisms in the shearing process of GH99/BNi-5a/GH99 joints.The macroscopic-microscopic deformation mech-anism of the brazing interface during shearing was studied by Crystal Plasticity(CP)and Molecular Dynamics(MD)on the basis of the optimal brazing parameters.The experimental results show that the brazing interface is mainly formed by(Ni,Cr,Co)(s,s)and possesses a shear strength of approximately 546 MPa.The shearing fracture of the brazed joint occurs along the brazing seam,displaying the characteristics of intergranular fracture.MD simulations show that dislocations dis-associate and transform into fine twinning with increased strain.CP simulated the shear deforma-tion process of the brazed joint.The multiscale simulation results are consistent with the experimental results.The mechanical properties of thin-walled materials for brazing are predicted using MD and CP methods.
查看更多>>摘要：On-machine tool setting is a pivotal approach in achieving intelligent manufacturing,and laser tool setters have become a crucial component of smart machine tools.Laser tool setters play a crucial role in precisely measuring the dimensions of cutting tools during the part machining process,focusing on tool length and diameter.As a measuring instrument,the positions of the laser axis of the laser tool setter need to be accurately calibrated before use.However,in actual calibra-tion scenarios,traditional calibration methods face challenges due to installation errors in the tool setter and geometric errors in the measuring rod.To address this issue,this study proposes a novel calibration method.Initially,the calibration mechanism of the laser beam axis is established.Based on the accurate mathematical model of the laser beam and the measuring rod,and using the poly-gon clipping algorithm,the mathematical mechanism of the laser tool setter's work is established.Then,a novel method is introduced to calculate the compensation distance between the laser beam reference point and the rod bottom center point at each moment during calibration.Furthermore,by utilizing the kinematic chain of the tool setter calibration system,a new calibration method is developed to accurately calibrate the position of the laser beam axis in the machine tool coordinate system.Finally,the accuracy of the calibration method is verified through simulation experiments and calibration tests.This method improves the calibration accuracy of the tool setter,and the mathematical model of the laser tool setter can be extended to the measurement of tools,thereby improving the precision of tool measurements.This research significantly improves the efficient pro-duction performance of smart machine tools.
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