CuSn预合金对金属粉末选区激光烧结过程及烧结体质量的影响

研究了CuSn预合金对Cu基金属粉末和Ni基金属粉末选区激光烧结的影响。结果表明,对于Cu-CuSn-CuP和Ni-CuSn-CuP两类合金粉末,通过合理控制激光工艺参数(特别是激光功率和扫描速率),能顺利实现粉末烧结成形,且无明显的"球化"效应和翘曲变形。对于Cu-CuSn-CuP合金,由于结构金属Cu的熔点相对较低,在一定的工艺条件下,可以通过调整CuSn的配比,得到致密的显微组织均匀的烧结件;而对于Ni-CuSn-CuP合金,结构金属Ni的熔点相对较高,虽然也可以得到较为致密的烧结件,但其微观组织存在明显的不均匀性。     

Dawn′s exploration of Vesta

On 16 July 2011, after completing nearly four years of interplanetary flight, Dawn entered orbit around (4) Vesta, the second most massive body in the main asteroid belt. Dawn used solar electric propulsion to spiral to six different orbits to accomplish its science campaign. Although the transfers to progressively lower orbits presented significant challenges, all were executed smoothly. During its nearly 14 months in orbit, Dawn spiraled down to 210 km above the surface and back up before initiating the gradual departure to travel to dwarf planet (1) Ceres for a 2015 rendezvous. Dawn′s exploration of Vesta has shown it to be geologically complex and fascinating, resembling terrestrial planets more than typical asteroids. Among the principal features is a 500-km-diameter impact basin within which is the second tallest mountain known in the solar system. This paper presents Dawn′s operations at Vesta and summarizes the principal findings.     

燃烧室内壁数字化加工技术研究

针对火箭发动机燃烧室内壁型槽复杂、加工效率低和质量不稳定的难题,利用加工中心数字仿真技术,对加工中心整体装配建模,进行数字化加工工艺过程整体仿真,研究五轴棒铣刀加工技术,革新了原有的螺旋槽片铣刀加工工艺方案,提出薄壁螺旋槽燃烧室内壁零件加工中心高效加工工艺技术方案,加工质量显著提高,加工效率提高近3倍,工艺改进后的零件经过装配热试车考核满足要求.     

Cf/SiC陶瓷基复合材料车削加工工艺研究

为满足高性能、轻质化的设计要求,开展了Cf/SiC基复合材料一金属连接工艺试验件的试验工作,阐述了Cf/SiC陶瓷基复合材料喷管车削加工难点和解决方案.理论分析和试验验证表明,通过选择合理的机械加工方法、人造金刚石聚晶车刀及冷却润滑方法,采用合适的切削用量和刀具几何参数,能够实现陶瓷基复合材料喷管连接部位的车削加工,保...     

非接触式表面电位探针设计与研制概述

在月球环境地面模拟实验室中,月表土壤的模拟带电情况测试需要用到非接触式表面电位探针。文章设计了一种非接触式表面电位探针,利用压电陶瓷外加驱动电压可产生形变的原理,以正弦电压信号驱动压电陶瓷周期振动,引起陶瓷片与被测带电体表面间电容变化从而产生感应电流,通过对感应电流的测量计算得出被测表面的静电电位。在实际工作中,其测量范围可达到0~4000V,测量精度优于5%。该仪器可广泛应用于表面静电带电测量及其他空间静电带电测量项目。     

钎焊式离心轮加工工艺研究

离心轮水力性能的优劣对泵的效率影响最大,离心轮加工质量好坏直接影响泵的特性。针对钎焊式离心轮加工的技术要求与技术难点进行分析、研究,结合现有数控设备对离心轮的加工工艺进行改进、优化及数控加工转化。     

精密薄壁支架零件高效防变形工艺研究

开展了精密薄壁支架零件的结构特点和技术难点分析,通过对薄壁支架类零件结构加工过程进行仿真分析、加工工艺优化等技术研究,实现了弱刚性结构零件的高效防变形加工,提高了弱刚性结构零件的加工精度和表面质量,从而满足了高精度薄壁支架类零件的质量要求。     

空间次生环境研究及探测方法概述

文章基于空间次生环境及其效应定义,分别结合不同次生环境阐述了国内外在磁场作用下带电粒子对航天器的影响、航天器非金属材料出气的影响、航天器与空间等离子体相互作用影响、航天器发动机羽流效应和航天器舱内电子环境及效应等的研究状况,并有针对性地开展了典型的发动机羽流效应、放电监测系统和航天器自身磁场分布探测研究。     

On high-altitude electric fields in the auroral downward current region and their coupling to ionospheric electric fields

The downward field-aligned current region plays an active role in magnetosphere–ionosphere coupling processes associated with aurora. A quasi-static electric field structure with a downward parallel electric field forms at altitudes between 800 km and 5000 km, accelerating ionospheric electrons upward, away from the auroral ionosphere. Other phenomena including energetic ion conics, electron solitary waves, low-frequency wave activity, and plasma density cavities occur in this region, which also acts as a source region for VLF saucers. Results are presented from high-altitude Cluster observations with particular emphasis on the characteristics and dynamics of quasi-static electric field structures. These, extending up to altitudes of at least 4–5 Earth radii, appear commonly as monopolar or bipolar electric fields. The former occur at sharp boundaries, such as the polar cap boundary whereas the bipolar fields occur at softer boundaries within the plasma sheet. The temporal evolution of quasi-static electric field structures, as captured by the pearls-on-a-string configuration of the Cluster spacecraft, indicates that the formation of electric field structures and of ionospheric plasma density cavities are closely coupled processes. A related feature of the downward current is a broadening of the current sheet with time, possibly related to the depletion process. Preliminary studies of the coupling of electric fields in the downward current region, show that small-scale structures are typically decoupled from the ionosphere, similar to what has been found for the upward current region. However, exceptions are also found where small-scale electric fields couple perfectly between the ionosphere and Cluster altitudes. Recent FAST results indicate that the degree of coupling differs between sheet-like and curved structures, and that it is typically partial. The electric field coupling further depends on the current–voltage relationship, which is highly non-linear in the downward current region, and still unrevealed, as to its specific form.