磨料有序排布电镀砂轮的制备及其把持力研究

为解决电镀砂轮磨削加工中容屑空间不足的问题，采用点胶微粘接的方法制备了磨料有序排布的电镀砂轮，分析了磨料粘接效果和镀层力学性能。通过SEM分析了磨料/镀层/导电胶的结合界面，并进行了干磨削试验。研究结果表明，直径约为磨料粒径40%的胶点可粘接住磨料，单个胶点上粘接多颗磨料的占比小于6%；双脉冲电镀工艺制备的镀层显微硬度大于500HV，表层残余应力小于100MPa，磨料/镀层/导电胶之间的界面贴合紧密，无明显缺陷；砂轮在磨削时没有出现磨料脱落现象。     

大涡模拟Smagorinsky模型用于磨粒流精密加工喷嘴的质量控制研究

为研究固液两相磨粒流加工喷嘴小孔过程中的流场分布、涡旋形成规律及涡旋的存在对磨粒流加工的影响机制，采用Smagorinsky亚格子模型对磨粒流加工喷嘴小孔的流道进行大涡数值模拟，并使用磨粒流对变直径喷嘴工件进行加工试验。数值模拟发现磨粒流流体中磨粒与壁面的碰撞与剪切作用随流体的速度增大而增大，同一截面的速度存在速度差，其中还伴随涡旋的存在；通过试验研究发现：经固液两相磨粒流加工后的喷嘴小孔表面质量得到明显提高，喷嘴经过四次不同入口速度的磨粒流加工后大孔处表面粗糙度Ra由1.24 μm降至0.542 μm，小孔处表面粗糙度Ra由1.21 μm降至0.437 μm。结论显示固液两相磨粒流加工技术可有效提高被加工喷嘴工件的内表面质量，加工时同一截面的速度存在速度差，速度差的存在利于涡旋的形成，涡旋的存在利于提高磨粒流加工过程的剪切作用，有助于获得高质量的喷嘴小孔内通道表面。     

车轮多边形磨耗统计规律及关键影响因素分析

针对列车车轮多边形磨耗问题广泛存在于轨道交通运输领域,会导致车辆/轨道系统产生高频的振动冲击,严重影响车辆和轨道系统零部件的使用寿命,危及行车安全这一问题,调查了大量车轮的多边形磨耗情况并进行统计分析,掌握了高速列车车轮多边形磨耗问题的现状和特点。以18～20阶多边形磨耗车辆为例,通过理论研究和试验分析(试验分析包括车辆系统振动特性测试和转向架模态特性测试),对车轮多边形磨耗的根本原因及诱导因素进行研究。研究发现,轮轨系统在580 Hz频率附近存在固有模态是导致车辆发生18～20阶多边形磨耗的根本原因,轮轨表面的各种不平顺能激发或者加剧轮轨系统在580 Hz频率附近的模态共振,从而诱发车轮多边形磨耗的产生。该结果可为高速列车车轮多边形磨耗问题的防止和进一步研究提供参考。     

Fabrication of composites from different carbon content ferrous powders adopting thermomechanical processes

In this research, deep consolidation technique is utilized for processing of low and high carbon content ferrous powders to manufacture composite material. High carbon content ferrous powders were used as reinforcing material. The powder particles were allowed to mix with low carbon content ferrous powders in 50:50 weight percent for fabrication of composites. The composites were heat treated at three different temperatures (i.e., 800°C, 900°C, and 1000°C). The changes in microstructure, microhardness, the grain size, and bonding among powder particles were studied. Refinement of grains was observed and this led to improved hardness enabling the processed material to be used as a suitable composite. Abrasive wear tests were carried out using a laboratory tribometer in dry reciprocating sliding contact against Sic abrasive paper. The outcomes demonstrate that the abrasive wear resistance was notably affected by the treatment temperature and hardness of the composites.     

Attitude control of spacecraft simulator without angular velocity measurement

In this paper the attitude control of a spacecraft simulator using Reaction Wheels (RW) as the actuators is investigated. The main goal of the current study is to bring the RWs to the rest at the end of the maneuver without angular velocity measurement. A modified feedback linearization controller is applied by considering the Euler angles of the simulator as the output and the RWs angular momentums as the internal state variables. The stability of the proposed controller and the internal dynamics is analyzed using Lyapunov theory. Two modified sliding mode observers are designed to estimate the angular velocities of the spacecraft attitude control subsystem simulator. The proposed observers do not use the control input and the detailed knowledge of the model and thus it can be implemented easily. The global stability of the system is proved. The proposed controller and observers are finally evaluated numerically and experimentally on an attitude spacecraft simulator.     

Wet abrasive wear behavior of WC-based cermet coatings prepared by HVOF spraying

In this study, three kinds of WC-based cermet coatings including WC–CoCr coating, WC–Ni coating and WC–Cr₃C₂–Ni coating were prepared by the high-velocity oxygen-fuel (HVOF) spraying process. Scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and Vickers hardness tester were used to analyze the microstructure and mechanical properties of these coatings. The WC–CoCr coating presented the highest average microhardness of 1205 HV_0.3, and then followed by the WC–Cr₃C₂–Ni coating (1188 HV_0.3) and the WC–Ni coating (1105 HV_0.3). The abrasive wear behavior of the WC-based coatings under the conditions of different applied loads and sediment concentrations were studied by a wet sand-rubber wheel tester. The results indicated that the abrasive wear loss rates of all the coatings increased with the increment of applied load or sediment concentration. In addition, the coatings with higher microhardness appeared to have higher abrasive wear resistance. The abrasive wear resistance of the WC-based coatings was 4–90 times higher than that of AISI 304 stainless steel under the same testing condition. The abrasive wear mechanism of the WC-based coatings was deduced to be the extrusion and removal of binder phases, as well as the fragmentation and peel-off of hard phases.     

强化研磨加工中喷射角度对GCr15轴承钢板塑性变形的影响

利用ABAQUS有限元仿真软件研究不同喷射角度强化研磨加工GCr15轴承钢板过程的塑性变形,得到不同喷射角度与等效塑性应变沿真实路径距离的关系,并通过拉伸试验进行验证。喷射角度为90°的强化研磨加工伸长率为1.4%,相比未经过强化研磨加工的伸长率数值缩减了2.0%。断口收缩率由空白组的4.9%降低到喷射角为90°的2.2%。结合仿真和试验可知,强化研磨加工中喷射角度越大,塑性变形能力越低,材料加工越难。     

Simulation of media behaviour in vibration assisted abrasive flow machining

Abrasive flow machining (AFM) is an abrasive-based precision finishing process used for achieving surface finish in micro and nano-level. The AFM polishes surfaces by extruding a visco-elastic media in contact with the workpiece. The media, also called a ‘flexible tool’, plays a key role in the performance of the process. Ultrasonic assisted abrasive flow machining (UAAFM) is a new variant of the AFM process in which the workpiece is subjected to mechanical vibration orthogonal to the media flow direction. In this process a high frequency, in the range of about 5–20 kHz, is given to the workpiece with the help of a piezo actuator and a specially designed fixture. The present work highlights on the possible behaviour of the tool (media) during UAAFM and its effect on the machining process through a computation based approach. Commercially available simulation tool was used to study the effect of the media in response to different set of machining conditions. The responses were evaluated in terms of changes in the fluid pressure, velocity profile of the fluid, temperature distribution in the working fluid and the possible wall shear on the work surface. A three-dimensional model was constructed for simulating the UAAFM process. The simulation shows that the abrasive particles tend to hit the target surface at an angle ‘θ’ which significantly affects the basic mechanisms involved and enhances the effectiveness of the process. The computed wall shear explains that the process will have higher finishing rate and hence the performance. The enhanced interaction of abrasive media in UAAFM while compared to simple AFM could be explained by the resultant pressure–velocity phenomena. Results show that while changes in the amplitude of applied vibration (10 μm and 50 μm) significantly affect the wall shear, the media velocity and pressure profiles are only marginally sensitive to this parameter. The simulation results also confirm that the rise in temperature during the process will not affect the media stability. Results have been discussed vis-a-vis the basic mechanism of the process through suitable illustrations.     

柴油机喷油器结构参数与磨料流加工效果关联的研究

基于磨蚀模型和多相流离散相模型建立了柴油机喷油器磨料流加工过程的CFD计算模型。在验证基础模型的有效性后,分别针对喷孔直径、喷孔锥角和交点位置三种结构参数进行变参数模拟计算。通过对加工域颗粒流速、湍流强度和磨蚀率的分析,获得了喷油器主要结构参数对液力研磨加工的影响机理:喷孔直径和喷雾锥度的变化对喷孔壁面磨蚀量的影响较大,随着两者的增大,喷孔壁面的磨蚀量有所增加;交点位置由上至下的变化对喷孔壁面磨蚀量的影响较小,但是喷孔入口处磨蚀量有所增加。     

磨料粒径及形状对磨料水射流切割钛合金表面微观形貌的影响

采用不同粒径与形状的磨料颗粒，在磨料水射流切割平台上切割钛合金，用超景深显微镜等设备分析，以研究不同的磨料粒径及形状对钛合金表面微观形貌的影响。研究表明:磨料粒径越大，钛合金表面所形成的划痕越长，80目磨料颗粒形成的微划痕长度约为160目磨料的5倍。且球形磨料颗粒所形成的划痕末端堆积较少，其表面线粗糙度低于9.5 μm；具有棱边的磨料颗粒所形成的划痕有唇状或鳞片状金属堆积，其表面线粗糙度在9.5～13.0 μm间；柱体形的磨料颗粒冲击形成的划痕带有尖锐的棱角，其表面粗糙度大于13.0 μm。