Material behaviors and mechanical features in friction stir welding process

This paper presents the 3D material flows and mechanical features under different process parameters by using the finite element method based on solid mechanics. Experimental results are also given to study the effect of process parameters on joining properties of the friction stir welds. Numerical results indicate that the tangent flow constitutes the major part in the material flow. The shoulder can accelerate the material flow on the top half of the friction stir weld. The distribution of the equivalent plastic strain can correlate well with the microstructure zones. Increasing the angular velocity of the pin, the material in the nugget zone can be more fully mixed, which improves the joining quality of the two welding plates. The increase of speeds, including the rotational speed and the translational speed, can both accelerate the material flow, especially in front of the pin on the retreating side where the fastest material flow occurs. The contact pressure on the pin-plate interface is decreased with the increase of the angular velocity. An erratum to this article can be found at     

基于弹流润滑的无级变速器滑移控制安全区域分析

针对无级变速器滑移模式下传动失效概率增大的问题，基于弹流润滑理论分析了不同传递转矩和相对滑动速度情况下，从动轮摩擦副油膜压力、厚度及其剪切应力变化情况，建立了安全裕度模型，基于油膜极限剪切应力与最大剪切应力计算了摩擦副的安全裕度，并确定了滑移控制的安全工作区域，为滑移控制策略的制定与优化提供了依据。研究结果表明：随着从动带轮传递转矩、相对滑动速度的增大，摩擦副的安全裕度逐渐减小；当传递转矩小于130 N·m时，在所有相对滑移速度范围内均可保证摩擦副油膜处于安全状态;当传递转矩大于130 N·m时，随着相对速度的增大，摩擦副油膜失效概率增大。     

Polyethylene friction stir welding parameter optimization and temperature characterization

The use of friction stir welding (FSW) to join thermoplastics has proven to produce strong welds with good surface quality when compared to conventional welding methods. In this study, a Teflon stationary shoulder was developed to weld 3-mm-thick plates of high molecular weight polyethylene in butt-joint configuration. Different sets of welding parameters were chosen and tested to evaluate their effect on the weld strength. Also, in order to increase joint performance, the temperature generated during welding was measured. For that purpose, thermocouples were located underneath of the weld nugget surface to measure the generated frictional heat for different tool diameters and parameters. Tool diameter and rotational and welding speeds are the most influential parameters regarding the welding temperature; however, all the input parameters had statistically significant effect on the weld quality. Unlike FSW in metals, using this tool, the heat is generated mainly by surface contact of the rotating probe and copper sleeve than the base material. The strongest welded joint was able to withstand 97% of the force that is necessary to fracture the base material, without using an external heating source.     

Effect of welding speed on microstructure and mechanical properties of friction-stir-welded aluminum

The weld properties remain an area of uncertainty with respect to the effect of different speeds of friction stir welding (FSW). For this purpose, hardened steel tool of FSW was used, which consists of the shoulder and pin. The shoulder of the tool not only provides additional heat generated by friction but also prevents plasticized material to escape. In the present investigation, aluminum welds were made at various welding speed using the FSW technique. The welds were characterized for mechanical properties and microstructural investigation. It is observed that good correlation exists between the mechanical properties and welding speeds. The best mechanical properties were obtained at lower welding speed.     

多输出齿轮泵供油条件下双定子马达的输出特性

以四输出齿轮泵和双作用双定子多速液压马达为基础建立传动系统,理论上探讨该传动中马达输出转速和转矩的多样性,根据泵可输出4种流量及马达多输入的特点,泵与马达在普通连接方式下,可以输出26种不同转速及8种转矩;在差动连接方式下,可输出14种不同转速和4种转矩。进一步扩展探讨了m输出泵与n作用双定子多速马达构成的传动系统中马达的转速及转矩,并初步总结出该类传动的输出规律：普通连接方式下可以实现m（2n+n2）种转速和2n+n2种转矩,差动连接方式下可以实现mn2种转速和n2种转矩。     

圆盘转子流体阻力分析与试验研究

分析了圆盘转子在紊态流体中旋转受到的流体阻力,利用边界层理论计算了转子旋转所需的扭矩与功率,设计了转子试验装置,得到了转子在不同转速下所需的扭矩和功率数据,应用最小二乘曲线拟合方法得到了转子扭矩、功率与转速的关系。分析结果表明：转子扭矩与转速的一次方和二次方有关,功率与转速的一次方、二次方、三次方均有关。尽管应用边界层理论计算时忽略了表面形状等因素的影响,计算结果较试验数据偏小但仍可作为选取电机的参考。另外,通过对摩擦阻力系数进行修正,可以得到与试验数据相符的计算结果。     

旋转冲击作用下凿岩机水封密封性能分析*

为探讨液压凿岩机冲洗机构水封在高频冲击和旋转运动复合作用下的泄漏情况,在分析钎尾冲击速度随时间变化规律基础上,通过求解广义雷诺方程分析密封区的流体力学特性,建立旋转冲击复合作用下凿岩机水封密封区膜厚及泄漏量的计算模型。基于ABAQUS分析冲击周期内变速度下的水封接触应力变化规律,分析不同流体压力、冲击速度幅值和转速对密封性能的影响。结果表明:最大接触压力的变化和内外行程有关,外行程最大接触压力随冲击速度幅值的增大而增大,随转速的增大而减小,内行程则相反;同时无转速时的最大接触压力明显小于有转速时的最大接触压力,外行程的整体的最大接触压力大于同位置内行程的最大接触压力;冲击速度幅值越大,一个周期内的实时泄漏量和净泄漏量越大;不同转速下的实时泄漏量比较接近,转速越大,净泄漏量越小,表明在一定的范围内提高转速有利于提升水封密封性能;随液体压力增大最大接触压力变大,实时泄漏量变小,而净泄漏量先变小再变大,表明在中等压力下水封的密封性能更好。     

精密钢球传动减速机构的瞬时接触热特性分析

基于Hertz接触理论,以精密钢球传动减速机构为研究对象,建立了减速机构啮合副齿面接触分析模型。分析了钢球与摆线槽齿面间的瞬时接触域,在此基础之上,分析输入轴转速、负载对减速机构热特性的影响,得到钢球与齿面间接触域的瞬时生热率和瞬时摩擦温升。分析结果表明：钢球与摆线槽齿面的瞬时接触域是时变接触椭圆;输入轴转速和负载对瞬时生热率和瞬时摩擦温升都有明显影响,但转速的影响更大;瞬时摩擦温升在常规转速下随转速单调递增,在超低转速下随转速先递减再递增。研究结果为精密钢球传动的三维热力耦合分析奠定了理论基础。     

基于流固热多场耦合的高速旋转唇形密封性能研究

针对一种橡胶材料高速旋转唇形密封,结合其自身结构及应用工况,通过方程离散化计算等方法,建立定量分析密封系统密封性能的数值仿真模型。基于所建立的模型对密封实际服役状态下界面流体力学、宏观固体力学、表面粗糙峰微观接触力学、唇口摩擦生热等多物理过程之间的耦合关系进行分析,借助实验验证并完善数值仿真模型的准确性。通过所建立的数值仿真分析模型研究油封唇口接触压力分布、唇口温度、摩擦力矩及泄漏率等衡量密封性能的关键参数随转速的变化规律。结果表明：在高转速条件下考虑摩擦生热时油封径向力减小,接触宽度增加,摩擦力矩减小,说明摩擦热对油封密封性能产生较大影响。     

Effects of welding speed on microstructures and mechanical properties of AA2219-T6 welded by the reverse dual-rotation friction stir welding

Reverse dual-rotation friction stir welding (RDR-FSW) is a novel FSW technology in which the tool pin and the assisted shoulder rotates reversely, thus it has the capability to obtain appropriate welding conditions through adjusting the rotating tool pin and surrounding assisted shoulder independently. In the present study, a RDR-FSW tool system was designed and successfully applied to weld high strength aluminum alloy 2219-T6, and the effects of welding speed on microstructures and mechanical properties were investigated in detail. At a constant rotation speed of 800 rpm for both the rotating tool pin and the reversely rotating assisted shoulder, defect-free joints were obtained at welding speeds ranging from 50 to 150 mm/min, while a cavity defect appeared at the three-phase confluction on the advancing side when the welding speed increased to 200 mm/min. With increasing of the welding speed, the width of the softened region decreased, but the minimum microhardness value increased gradually. When compared with the joints welded by the conventional FSW, there is only a minor variation of the Vickers hardness across the stirring zone in the joint welded by the RDR-FSW. The maximum tensile strength 328 MPa (73.7 % of the base material) was obtained at the welding speed of 150 mm/min, while the elongation reached its maximum 7.0 % (60.9 % of the base material) at the welding speed of 100 mm/min. All defect-free joints were fractured at the weakest region with the minimum Vickers hardness, while for the joint with cavity defects the fracture occurred at the defect location. The tensile fracture was in the ductile fracture mode.     

基于机器视觉的工程机械行驶速度的研究

工程机械行驶速度是预测机器的牵引力、滑转损失、牵引效率和滑转率等牵引性能参数时所需要的基本变量，传统的接触式测量方法由于行驶路面条件等因素的影响，所测量的速度值并非是工程机械的实际行驶速度，利用工程机械行驶路面的纹理随机性，提出了基于机器视觉技术的工程机械行驶速度测量方法，该方法通过安装在车体上的CCD摄像机，在连续成像的条件下，通过匹配跟踪得到路面同一纹理特征在相邻两帧图像中的位置，利用摄像机内外参数和成像帧频实时计算得到工程机械的瞬时行驶速度，通过采集路面纹理图像进行模拟实验验证了该方法的可行性。     

Influences of rotation speed on microstructures and mechanical properties of 6061-T6 aluminum alloy joints fabricated by self-reacting friction stir welding tool

A 6061-T6 aluminum alloy was self-reacting friction stir welded by using the specially designed tool with unequal shoulder diameters at a constant welding speed of 150 mm/min to investigate the effect of rotation speed on microstructure and mechanical properties of the joints. Excessive flash on the bottom surface of the joint and groove defects on both surfaces of the joint were formed when the lower shoulder diameter was much smaller. The suitable shoulder sizes were determined as 16 and 18 mm in lower shoulder diameter and upper shoulder diameter, respectively. The grain size and the dislocation density in the weld nugget zone (WNZ) increased with increasing rotation speed. The tensile strength of joints first increased with increasing rotating speed and then decreased remarkably as a result of the formation of void defect. The joints welded at lower rotation speeds were fractured in the thermal mechanically affected zone (TMAZ). However, the fracture locations of the defect-free joints were changed to the heat affected zone (HAZ) at higher rotation speeds.     

Thermally activated phenomena in nanoscopic sliding friction

The role of thermal activation for a nanoasperity sliding on a flat surface with low speed is discussed. If the asperity slides in a vacuum, thermal vibrations may excite slipping from an equilibrium position to the next one along the sliding path. This effect is reduced if the velocity increases and leads to a logarithmic increase of friction with the sliding velocity. This is not always the case in a humid environment, where the formation of capillaries between tip and surface acts as an obstacle for thermally activated jumps. Depending on the wettability of the surface, different behaviors are observed. Copyright © 2006 John Wiley & Sons, Ltd.     

Numerical simulation and experimental research on the wheel brush sampling process of an asteroid sampler

To examine the environmental characteristics of the microgravity force and the weathered layer on an asteroid surface, a symmetric wheel brush asteroid sampler is proposed for the collection of particles on the asteroid surface. To study the influence of the wheel brush rotation speed on the sampling efficiency and the driving torque required for the wheel brush, the contact dynamics model between particles and sampling wheel brushes is established and a simulation and experimental verification of the sampling process are conducted. The parameter calibration of the sampled particles is studied first, and the calibrated particle parameters are used in the numerical simulation of the sampling process. The sampling results and the particle stream curves are obtained for the working conditions of different rotation speeds, and the effects of different parameter settings on the sampling efficiency are analyzed. In addition, a set of rotating symmetrical sampling wheel brush devices is built for the ground test, and the dynamic torque sensor is used to test the torque change of the wheel brush during the sampling process. The relationship between the speed of the wheel brush and the driving torque of the wheel brush motor is determined by comparing the simulation results with the test results. Results indicate that when the rotating speed of the wheel brush is faster, the sampling efficiency is higher, and the driving torque required for the sampling wheel brush is greater. Moreover, a numerical simulation analysis of the sampling process of the wheel brush sampler in a microgravity environment is conducted to determine the optimal speed condition, and the brushing test of the wheel brush sampler in the microgravity environment is verified with the drop tower method. This research proposes the structural optimization design and motor selection of a wheel brush asteroid sampler, which provides important reference value and engineering significance.     

干摩擦工况下Si3N4/PTFE配副材料摩擦磨损特性与转移膜形成分析

为使全陶瓷轴承在干摩擦工况下可靠运转,选用四氟乙烯(PTFE)材质的保持架为全陶瓷轴承提供润滑.利用Rtec销/盘摩擦磨损试验机,以PTFE盘与氮化硅(Si3 N4)销为摩擦副,研究Si3 N4/PTFE在不同载荷和转速条件下的摩擦磨损性能,通过SEM对Si3 N4表面的转移膜形貌进行观察,分析转移膜形成原因.结果表明...     

Friction stir welding characteristics of 2219-T6 aluminum alloy assisted by external non-rotational shoulder

Friction stir welding (FSW) of 2219-T6 aluminum alloy assisted by external non-rotational shoulder was carried out, and effects of the welding speed on microstructures and mechanical properties were investigated in detail. Defect-free joints were obtained in a wide range of welding speeds from 50 to 300 mm/min. The microstructural deformation and weld formation were dominated by the rotating tool pin and subsize concave shoulder but the non-rotational shoulder exerted very little effects for all joints. Compared with the weld obtained by conventional FSW, less intense stirring effects in FSW assisted by external non-rotational shoulder can only generate a narrower thermomechanically affected zone, whose width decreased with increasing of the welding speed. Microstructures and Vickers hardness distributions showed that this new welding process is beneficial to improving the asymmetry and inhomogeneity, especially in the weld nugget zone. The maximum tensile strength was up to 69 % of the base material.     

The distribution of friction heat between a stationary pin and a rotating cylinder

The knowledge of the distribution of friction heat between two bodies in sliding contact is necessary for the calculation of the elevated temperatures at the contact area. An analytical model has been devised for the calculation of the coefficient for the friction heat distribution between a rotating cylinder and a stationary pin. This model covers a wide field of practical applications. The results of this analysis are in very good agreement with the experimental results existing in the bibliography. Graphs are included to facilitate the numerical calculations. The proposed model takes into consideration the specific geometric configurations and the surface heat transfer conditions of the bodies coming in sliding contact.     

The mechanism of formation of wear particles of polytetrafluoroethylene

To obtain information about the basic processes involved in the formation of wear particles of polytetrafluoroethylene (PTFE), the rubbing surface of a PTFE pin was observed through a glass disk.In the initial stages of rubbing, small PTFE fragments transferred to the glass disk from the PTFE pin but did not necessarily adhere strongly to the glass disk. Some wear fragments were observed moving against the PTFE pin at a speed lower than the sliding speed of the glass disk. The fragments at higher travelling speeds combined with the fragments at lower travelling speeds and increased in size. The fragments also increased in size by scratching the surface of the PTFE pin.In steady state rubbing, large and small fragments were observed and the enlarged fragments were successively detached from the glass disk. The variation in the volume and the travelling speed of the fragments with rubbing was also examined. The total increase in the volume of transferred fragments in a definite sliding distance was found to be similar to the volume of removed fragments.     

车辆传动装置产热建模与计算分析

全面掌握传动装置的产热情况是对传动装置进行全工况热管理的必要前提.根据车辆各传动部件的结构特点,考虑离合器摩滑过程产热,建立了一种车辆传动装置产热模型.模型由弹性联轴器、液力变矩器、齿轮、轴承、离合器带排转矩、行星机构、搅油损失和离合器摩滑过程产热模型组成.对车辆不同工况条件下传动装置的产热流量进行了计算,给出了稳态工况传动装置产热流量计算值与设计定型时的试验结果的对比;得到了不同工况条件下传动装置总产热流量和各产热方式产热流量随时间的变化关系,并对计算结果进行分析.结果表明：传动装置计算结果与试验结果对比最大相对误差小于10％.     

基于STAR-CCM+的高速角接触球轴承喷油润滑研究

喷油润滑系统广泛应用于高速滚动轴承,喷油润滑条件下轴承温升特性是影响轴承动态工作稳定性的重要因素。基于两相流理论,以71904C角接触球轴承为研究对象,建立全轴承模型,采用旋转坐标系描述各组件运动,分析滚动轴承在不同参数下喷油润滑的两相流与传热效率的影响规律。结果表明：随着轴承转速增加,轴承搅拌力矩也相应增加,导致轴承内部温度升高;润滑油运动黏度增加,轴承内部流场搅拌力矩增加,导致轴承温度升高;轴承喷油速度增加,内部流场温度呈现先增加后降低趋势,因此存在一个最佳喷油速度使得轴承温升最低。