河床模型地形的无接触快速测量技术

针对河床模型地形测量的需要，介绍了两种无接触快速地形测量技术，即利用超声波实现水下地形无接触快速扫描测量和利用激光与CCD光学成像原理实现水上地形的无接触快速测量，并介绍了测量系统的组成、工作原理及有关主要技术问题的解决方法。     

超厚不锈钢钢带成形工艺研究与应用

钢铝接触轨是城市轨道交通供电系统的主体,接触轨采用不锈钢钢带与铝合金复合的方式,接触轨接触表面采用不锈钢,可提高接触轨的耐磨性,钢铝接触轨中不锈钢钢带的成形是关键技术,是难点之一.通过研究不锈钢钢带的下料、成形、铆接孔加工工艺及方法,滚挤成形出合格不锈钢钢带,其与铝轨结合面完全满足技术要求.     

耐热不锈钢电磁成形过程的稳定性

本文介绍了一种适于制备中小尺寸特种合金铸件的完全无接触电磁成形技术,分析了影响耐热不锈钢无接触电磁成形过程稳定性的因素,并在此基础上,选择合理的工艺参数,试验获得了表面质量良好的不锈钢样件。     

特种合金感应熔炼电磁约束成形技术若干进展

感应熔炼电磁约束成形是一种无容器金属材料处理加工技术,具有无器壁污染、短流程等优点。评述了几种感应熔炼电磁约束成形技术的原理和特点,并对其存在问题及发展趋势进行了讨论。指出电磁约束成形定向凝固技术集金属材料的加热熔化、无接触约束成形及组织定向凝固于一体,特别适合于高熔点、易氧化、高活性的特种合金坯件的无污染制备。     

软接触电磁连铸技术的研究进展

介绍了软接触电磁连铸技术及其原理,该技术可有效提高铸坯的表面质量,着重分析了实现钢的软接触电磁连铸在结晶器结构、材质以及电磁场参数等方面需要解决的问题,并介绍了该技术的最新研究成果:高频调幅磁场及无结晶器振动的电磁连铸技术.     

钢的软接触电磁连铸技术

介绍了软接触电磁连铸技术并分析了其作用原理,该技术可有效提高铸坯的表面质量,着重分析探讨了工业上实现钢的软接触电磁连铸在结晶器结构、材质以及电磁场参数等方面需要解决的关键问题,并介绍了该技术的最新研究成果:高频调幅磁场电磁连铸技术和无结晶器振动的电磁连铸技术.     

基于激光定位技术的机器人非接触无编程示教方法与实现

针对现有机器人示教系统操作复杂、示教编程难度高及需操纵机器人等问题,提出一种基于激光定位技术的机器人非接触无编程示教方法。以示教笔代替机器人末端工具绘制示教轨迹,通过激光定位技术获取示教笔的示教路径和动作,并通过机器人逆运动学求解及轨迹规划得到各个关节的控制轨迹,实现机器人的快速非接触无编程示教。最后通过搭建桁架式六轴机器人硬件和软件系统,实现了基于激光定位技术的机器人非接触无编程示教,验证了提出的示教方法的可行性。所提示教方法简化了示教过程,降低了操纵机器人带来的安全风险,且无需操作人员具备编程能力,使机器人示教工作具有良好的适应性,具有广阔的应用空间。     

TC4钛合金Zn-Al合金涂层的抗接触腐蚀性能研究

为了有效解决钛合金-铝合金之间的接触腐蚀问题,采用无铬达克罗技术在TC4钛合金表面制备了Zn-Al合金涂层,并检测了涂层的组织和厚度,对有涂层的TC4合金试样进行了盐雾试验,以研究涂层的防蚀性能。结果表明,TC4合金的Zn-Al涂层均匀致密,厚约35μm,与基体结合良好。在盐雾试验29 d后,无涂层钛合金-铝合金接触偶对的腐蚀程度要比有涂层的钛合金-铝合金接触偶对严重得多。电化学测试结果表明,涂层钛合金-铝合金接触偶的自腐蚀电位差比无涂层钛合金-铝合金接触偶的自腐蚀电位差低315 mV左右,前者的开路电位差则比后者低244 mV,并且接触偶对的阴、阳两极发生了转换,即原先作阳极的铝合金变成了被保护的阴极。Zn-Al合金涂层的存在能有效降低相互接触的钛-铝合金之间的电位差,具有一定的防腐蚀作用。     

数字化控制技术在电弧炉供电系统上的应用

针对35 kV电弧炉熔炼生产过程中供电系统存在着人工操作的过补偿与欠补偿,谐波引起系统电压不稳定与功率因数低等问题,利用目前现有设备,确定技术改造方案,合理地将数字化静止型动态滤波及无功补偿技术应用于电弧炉供电系统中.经过技术改造方案分析并实施,有效解决了系统中存在的诸多问题,收到了较好的技术经济效益.     

基于声发射技术的接触疲劳失效检测应用与研究

接触疲劳是齿轮、轴承等长期承受交变载荷的重要旋转零部件的主要失效形式。采用声发射技术对接触疲劳失效过程进行监测,对损伤程度进行检测以及揭示接触疲劳失效机理具有重要的意义。随着声发射技术不断发展及先进声发射信号处理技术的出现,其在接触疲劳失效检测中的研究也越来越深入。文章回顾了声发射技术的发展现状,综述了声发射技术应用于块体零件和涂层零件的接触疲劳失效检测的研究进展与存在的问题,探讨了进一步研究的方向和解决问题的思路。     

Numerical Simulation of Columnar to Equiaxed Transition for Directionally Solidified Ti-44Al Alloy

Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition （CET） during directional solidification of Ti-44Al alloy. The simulation results show that the solutal interactions from growing equiaxed grains play an important role on CET. The effects of the applied thermal gradient and pulling velocity, the equiaxed seed spacing and nucleation undercooling on the CET are investigated in the present simulation. The simulated results indicated that the columnar branch spacing depends not only on the thermal gradient and the pulling velocity, but also on number of the seeds. A spacing adjustment can occur through initiation of seeds that develop into new columnar grains. The dependence of the CET on the thermal gradient and pulling velocity, qualitatively agrees with the analytical CET model of Hunt,     

NUMERICAL SIMULATION OF COLUMNAR TO EQUIAXED TRANSITION FOR DIRECTIONALLY SOLIDIFIED Ti-44Al ALLOY

Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition (CET) during directional solidification of Ti-44Al alloy. The simulation results show that the solutal interactions from growing equiaxed grains play an important role on CET. The effects of the applied thermal gradient and pulling velocity, the equiaxed seed spacing and nucleation undercooling on the CET are investigated in the present simulation. The simulated results indicated that the columnar branch spacing depends not only on the thermal gradient and the pulling velocity, but also on number of the seeds. A spacing adjustment can occur through initiation of seeds that develop into new columnar grains. The dependence of the CET on the thermal gradient and pulling velocity, qualitatively agrees with the analytical CET model of Hunt.     

微波烧结镀铜TiB_2增强铜基复合材料的膨胀性能

应用化学镀铜方法对TiB2颗粒进行表面镀铜,采用微波烧结技术制备了含TiB2体积分数不同的TiB2/Cu复合材料,测试了试样在50~300℃区间的膨胀系数,探讨了TiB2含量及其表面改性对TiB2/Cu复合材料热膨胀系数的影响。结果表明:随着TiB2含量的增加,复合材料的膨胀系数降低;TiB2颗粒表面镀铜后,在TiB2相同含量条件下,TiB2/Cu复合材料的膨胀系数进一步降低;理论模型计算结果表明,TiB2未镀铜的TiB2/Cu复合材料的膨胀系数与ROM模型计算值相符合,而TiB2镀铜的TiB2/Cu复合材料的膨胀系数与Kerner模型计算值相符合,反映了TiB2颗粒镀铜后能很好地改善颗粒与基体的结合。     

Effect of thermal convection on columnar-to-equiaxed transition during solidification of Al-Cu alloy

To investigate the effect of three-dimension(3 D) thermal convection on columnar-to-equiaxed transition(CET), the CET transition during the solidification of an Al-Cu alloy was simulated by 3 D cellular automaton model coupled with the finite element method(CAFE). The thermal convection in the liquid phase was considered. The results show that the thermal convection in the liquid phase promotes the CET. When the convection is present, the temperature gradient at the start position of CET increases and the growth velocity of columnar dendrite decreases. The convection influences the formation of elongated equiaxed grain through changing the local temperature gradient and dendritic growth velocity.     

Columnar and Equiaxed Solidification of Al-7 wt.% Si Alloys in Reduced Gravity in the Framework of the CETSOL Project

During casting, often a dendritic microstructure is formed, resulting in a columnar or an equiaxed grain structure, or leading to a transition from columnar to equiaxed growth (CET). The detailed knowledge of the critical parameters for the CET is important because the microstructure affects materials properties. To provide unique data for testing of fundamental theories of grain and microstructure formation, solidification experiments in microgravity environment were performed within the European Space Agency Microgravity Application Promotion (ESA MAP) project Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL). Reduced gravity allows for purely diffusive solidification conditions, i.e., suppressing melt flow and sedimentation and floatation effects. On-board the International Space Station, Al-7 wt.% Si alloys with and without grain refiners were solidified in different temperature gradients and with different cooling conditions. Detailed analysis of the microstructure and the grain structure showed purely columnar growth for nonrefined alloys. The CET was detected only for refined alloys, either as a sharp CET in the case of a sudden increase in the solidification velocity or as a progressive CET in the case of a continuous decrease of the temperature gradient. The present experimental data were used for numerical modeling of the CET with three different approaches: (1) a front tracking model using an equiaxed growth model, (2) a three-dimensional (3D) cellular automaton–finite element model, and (3) a 3D dendrite needle network method. Each model allows for predicting the columnar dendrite tip undercooling and the growth rate with respect to time. Furthermore, the positions of CET and the spatial extent of the CET, being sharp or progressive, are in reasonably good quantitative agreement with experimental measurements.     

Industrial effects associated with the columnar to equiaxed transition in remelt ingots

Abstract

There have been many studies of the mechanisms involved in causing the solidification mode of an alloy to change from a columnar dendritic to an equiaxed structure, the columnar to equiaxed transition (CET). The parameters which cause the change are alloy segregation characteristics, freezing rate, undercooling and the presence or absence of nucleating sites for the equiaxial crystals. These parameters have been collected into theories which describe the CET and which generally satisfy the results found in laboratory or model systems. Although the effect has considerable scientific interest, the impact of a CET on the structure of an industrial ingot or casting is less widely recognised. In this report, we comment on the CET in relation to the structure of superalloy castings and ingots and on the formation of segregation defects in titanium alloy ingots. We conclude that one of the more important results of accurately modelling the solidification of these systems is that we may predict the casting conditions leading to the CET and also assess its impact on cast structure.     

The solidification path related columnar-to-equiaxed transition in Ti–Al alloys

Columnar-to-Equiaxed Transition (CET) of binary Ti–Al alloys and multi-component Ti–48Al–2Cr–2Nb alloys is studied using Bridgman solidification technique. The effect of aluminum concentration and growth rate on CET is determined. It is found in Ti–46Al and Ti–50Al alloy ingots equiaxed grains develop ahead of the moving solid–liquid interface with a growth rate of 500 μm/s; microstructures in Ti–49Al alloy stay columnar dendrites with the same growth rate. CET in Ti–Al alloys are not only influenced by growth rate, but also by the solidification path that is related to alloying composition. CET in Ti–Al alloys is predicted using the dendritic growth model based on the criterion of growth at marginal stability. According to the calculated results and directionally solidified microstructures, values of the nucleation undercooling for α and β phases are given. The growth rates to avoid CET in Ti–48Al–2Cr–2Nb alloy are suggested.     

Observations on the numerical prediction of the columnar to equiaxed grain transition in castings

The two approaches that have been made to the study of the columnar to equiaxed grain transition (CET) in castings using numerical computer models are considered. The first involves modelling the simultaneous growth of columnar and equiaxed grains. The second employs a simple macroscopic model of heat transfer and columnar solidification to predict the values of certain parameters at CET interfaces observed in experimentally solidified alloys. This paper questions conclusions that have been reported by researchers from a study employing the second approach and, also, the validity of the approach as a means of obtaining meaningful data on the CET.     

大学英语四、六级考试的史与思

本文主要从英语教学大纲要求的变化角度回顾了大学英语四、六级考试的历史,并指出大学英语四、六级考试现存的三大问题:影响了学生综合素质的提高,演变成高校最为典型的应试教育,与大学英语教学或学位证书直接挂钩,并就此提出了在课程设置、教学评价、分数解释、教育技术手段和教师素质方面的改革思路。     

Directional solidification processing on CET in Al-based alloys

The control of the transition from columnar to equiaxed (CET) dendrite microstructure is an important point to obtain desired final properties of industrial products. The objective is to understand how the formation and the evolution of the CET are influenced by the processing parameters and natural convection with Al - 3.5 wt.% Ni and Al - 7.0 wt.% Si alloys. Various experiments are carried out in a Bridgman furnace for which the thermal gradient and pulling velocity can be independently controlled. We concentrate our interest on the CET tendency, added particle effects and the evolution of dendrite grain structures under different test conditions. On the other hand, in-situ and real time observation of the solid-liquid interface is used to reveal the dynamics of the phenomena that occur, thus deepening our understanding. To achieve this objective, Synchrotron X-ray Radiography has been designed and performed at the European Synchrotron Radiation Facility.