应用CVD金刚石涂层工具研磨单晶蓝宝石

通过热丝化学气相沉积(HFCVD)法制备了具有球状晶结构、棱锥形晶结构和棱柱形晶结构等3种不同表面特征的化学气相沉积(CVD)金刚石涂层工具,以提高其研磨效率。通过正交实验法研究了金刚石涂层晶粒形态、载荷、工作台转速、研磨时间等4个工艺参数对蓝宝石材料去除率和表面粗糙度的影响。结果表明:金刚石涂层的晶粒形态对材料去除率和表面粗糙度影响较大;球状晶结构金刚石涂层切向力较小,棱柱形晶结构金刚石涂层切向力较大;选择棱柱形晶CVD金刚石涂层工具研磨蓝宝石,在研磨加工参数为载荷0.15 MPa、转速100 r/min、研磨时间3 min时,其材料去除率为0.397μm/min,表面粗糙度为0.354μm。结果表明:提出的CVD金刚石涂层工具可用于进一步加工、研磨蓝宝石切片,去除其表面划痕,从而改善工件表面质量。     

C/C材料表面ZrB2-SiC功能梯度涂层残余应力分析

目的以C/C复合材料为基体,设计ZrB_2-SiC功能梯度材料。方法利用Ansys软件对等离子喷涂ZrB_2-SiC功能梯度涂层在沉积过程中产生的残余应力进行数值模拟,分析成分分布指数p和梯度层厚度t对梯度涂层残余应力的影响;并通过基于悬臂梁理论的热应力解析,计算与基体接触的涂层在涂层与基体厚度比λ不同时的残余应力值。结果模拟分析结果表明,在涂层与基体的界面,梯度层的厚度对轴向压应力影响不大,径向压应力和切向应力均随厚度的增加而增大,在边缘区域应力集中较为严重,易产生层间破坏;纯ZrB_2层为表面层,其应力主要为径向压应力,且沿径向逐渐减小至0,到边缘处又突变为拉应力,并随p的增大而减小。对比解析法分析可得两者计算的与基体接触的涂层内部的残余应力随λ的增大都是逐渐降低的,这符合涂层内部的应力分布原理。根据优化设计,获得功能梯度材料在各梯度层厚度d为0.1~0.2 mm,成分梯度指数为4时的热应力变化缓和效果较好。结论基于悬臂梁理论的解析解可以很好地评估热应力,并验证了该模拟的正确性。     

Effect of prestressed ultrasonic peen forming parameters on bending curvature and spherical deformation of plate

The elastic prestressed ultrasonic peen forming (UPF) was adopted in order to solve problems of insufficient bending deformation and large spherical deformation of plate during free UPF. The theoretical analysis of prestressed UPF and the influence of elastic prebending moment on deformation were analyzed. Spherical deformation coefficient was defined to quantificationally describe the spherical deformation. Experiments were conducted to compare the differences between free UPF and prestressed UPF processes and the effects of processing parameters on bending curvature and spherical deformation coefficient were studied. The results show that peening trajectory in chordwise direction is beneficial to enlarging spanwise bending deformation and decreasing spherical deformation coefficient. Large prebending curvature is helpful to increase spanwise bending deformation and decrease chordwise deformation, thereby obviously decreasing spherical deformation coefficient. Large spanwise deformation can be obtained under large firing pin velocity, small plate thickness and small offset distance. Large firing pin velocity plays a positive role in decreasing spherical deformation, while plate thickness and offset distance have little effect on it. Above all, prebending curvature and peening trajectory are the most important factors during prestressed UPF process. This study provides guidance for parameters optimization of prestressed UPF for wing plate with large thickness.     

Low dielectric loss ceramics in the Mg4Nb2O9-ZnAl2O4-TiO2 ternary system

This study used a traditional solid-state reaction method to prepare a series of composite ceramics in the 0.7Mg₄Nb₂O₉-(0.3-x)ZnAl₂O₄-xTiO₂ ternary system. Crystalline phases and microstructure of Mg₄Nb₂O₉-ZnAl₂O₄-TiO₂ dielectric ceramic composites were investigated and correlated with the relevant dielectric properties. It was observed that the addition of Ti⁴⁺ substituted Nb⁵⁺ in the Mg₄Nb₂O₉ structure, which promoted the decomposition of Mg₄Nb₂O₉ to form the second phase, Mg₅Nb₄O₁₅, during sintering. The synergistic effect of ZnAl₂O₄-TiO₂ co-doping promoted the Mg₄Nb₂O₉ ceramic densification. The sample (0.7Mg₄Nb₂O₉-(0.3-x)ZnAl₂O₄-xTiO₂) with x = 0.15?0.2 exhibited dielectric constants of 13–14, larger than those of ZnAl₂O₄, Mg₄Nb₂O₉ and Mg₅Nb₄O₁₅, due to the NbO₆ octahedra distortion resulting from the substitution of Al³⁺/Ti⁴⁺ for Nb⁵⁺ in Mg₄Nb₂O₉ and Mg₅Nb₄O₁₅. The long-range order of the NbO₆ octahedra was enhanced by co-doping ZnAl₂O₄ and TiO₂, thereby enhancing the Qxf value. A dielectric constant of 13.1, Qxf value of 366,000 GHz and a τ_f of ?60.8 ppm/°C were obtained from 1300 °C sintered 0.7Mg₄Nb₂O₉-0.15ZnAl₂O₄-0.15TiO₂. These results show that 0.7Mg₄Nb₂O₉-0.15 ZnAl₂O₄-0.15TiO₂ ceramic is a good candidate for microwave electronic device applications.     

A real time machining error compensation method based on dynamic features for cutting force induced elastic deformation in flank milling

During the machining process, cutting forces cause deformation of thin-walled parts and cutting tools because of their low rigidity. Such deformation can lead to undercut and may result in defective parts. Since there are various unexpected factors that affect cutting forces during the machining process, the error compensation of cutting force induced deformation is deemed to be a very difficult issue. In order to address this challenge, this article proposes a novel real time deformation error compensation method based on dynamic features. A dynamic feature model is established for the evaluation of feature rigidity as well as the association between geometric information and real time cutting force information. Then the deformations are calculated based on the dynamic feature model. Eventually, the machining error compensation for elastic deformation is realized based on Function Blocks. A thin-walled feature is used as an example to validate the proposed approach. Machining experiment results show that the errors of calculated deformation with the monitored deformation is less than 10%, and the thickness errors were between ?0.05 mm and +0.06 mm, which can well satisfy the accuracy requirement of structural parts by NC (Numerical Control) machining.     

Optimal design of a six-axis vibration isolator via Stewart platform by using homogeneous Jacobian matrix formulation based on dual quaternions

A six-axis vibration isolation system is essential to high-precision space systems for attenuating vibrations on precise instruments. The kinematic optimal design is researched for the space six-axis vibration isolator via Stewart mechanism. Jacobian matrix is the basis of the kinematic performance index. However, the conventional Jacobian matrix is not usually dimensionally homogeneous due to the inhomogeneous physical units, caused by the different mathematical representations of the rotation and translation. In this paper, we propose a dual quaternion approach to derive the dimensionally homogeneous Jacobian matrix of a general six-axis parallel mechanism. Two quaternions are used to parameterize the rotation and translation of the platform. The dimensionally scaling factor for the generalized Jacobian matrix is defined as the ratio of the norms of the two quaternions. The dimensionally homogenous Jacobian matrix is then obtained and applied to the optimal design of the six-axis vibration isolator. The performance index of isotropy is considered to make the isolator minimum kinematic coupling in its working configuration.     

Comparative investigation on grindability of K4125 and Inconel718 nickel-based superalloys

Nickel-based superalloy is a typical hard-to-machining material in the aero-engine manufacturing industry. The grindability difference of two kinds of nickel-based superalloys, i.e., equiaxed cast nickel-based superalloy K4125 and wrought nickel-based superalloy Inconel718, are discussed in this article. The influence of grinding parameters (e.g., grinding speed, workpiece speed, and depth of cut) on the grinding force, grinding temperature, and ground surface quality are explored. The results illustrate that under the given grinding conditions, grinding K4125 generates higher forces than Inconel718. The temperature from the K4125 grinding process is beyond 400 °C, while the temperature from grinding on Inconel718 is below 200 °C. Moreover, because of the chip adhesion on the wheel surface when grinding K4125, not only the wheel wear is more severe but also the ground surface of K4125 is worse than that of Inconel718. Accordingly, it could be inferred that the grindability of K4125 is worse than that of Inconel718.     

Real-time cutting tool state recognition approach based on machining features in NC machining process of complex structural parts

Cutting tool state recognition plays an important role in ensuring the quality and efficiency of NC machining of complex structural parts, and it is quite especial and challengeable for complex structural parts with single-piece or small-batch production. In order to address this issue, this paper presents a real-time recognition approach of cutting tool state based on machining features. The sensitive parameters of monitored cutting force signals for different machining features are automatically extracted, and are associated with machining features in real time. A K-Means clustering algorithm is used to automatically classify the cutting tool states based on machining features, where the sensitive parameters of the monitoring signals together with the geometric and process information of machining features are used to construct the input vector of the K-Means clustering model. The experiment results show that the accuracy of the approach is above 95% and the approach can solve the real-time recognition of cutting tool states for complex structural parts with single-piece and small-batch production.     

Calculation of the Fe-C-Ti ternary phase diagram

A thermodynamic analysis of phase equilibria in the Fe-Ti, Ti-C, and Fe-C-Ti systems has been carried out. The Gibbs energy has been expressed by the two-sublattice model being separated into the paramagnetic and ferromagnetic terms. In particular, the equilibrium between austenite and NaCl type Ti-carbide has been treated as a portion of the miscibility gap in the fcc phase. The thermodynamic parameters for each phase were evaluated on the basis of experimental phase equilibrium and activity data. The calculated phase boundary of austenite in equilibrium with Ti-carbide exhibited a characteristic shape, not only in the isothermal but also in the vertical section.