Process parameter dependencies of continuous and pulsed laser modes on surface polishing of additive manufactured aluminium AlSi10Mg parts

Laser powder bed fusion is a well-established 3D printing technique for metal alloys, but exhibits a poor surface quality. Laser polishing provides the possibility of a fast contact-free and fully-automatable surface treatment. This paper deals with the experimental investigation of laser polishing of laser powder bed fusion parts made of aluminium AlSi10Mg. Laser polishing is done with a 4 kW solid state disc laser in combination with a multi-axis system and a one dimensional scanner optic. The laser is operated at continuous and pulsed operation mode. The parameter study reveals a high dependency of the achievable roughness on the laser beam intensity, the track and pulse overlap, the energy density and the number of polishing passes and polishing directions. Pulsed laser polishing mode with up to four passes from different directions revealed the lowest surface roughness of 0.14 μm Ra. With respect to the initial average surface roughness of Ra = 8.03 μm a reduction of the surface roughness of greater than 98 % could be achieved. Polishing with continuous laser radiation at one polishing pass resulted in Ra = 0.23 μm at an area rate of 20 cm²/min. Laser polishing using four passes achieved a further improvement up to Ra = 0.14 μm.     

Effects of cavity-induced mixing enhancement under oblique shock wave interference: Numerical study

In this research, the effects of oblique shock on the mixing characteristics in a supersonic combustor equipped with a cavity is numerically investigated. To reveal the flow structure of the supersonic flow field under oblique shock wave interference, three-dimensional steady RANS equations and SST k-ω turbulence model are adopted. The current work focuses on comparing the interaction effects between oblique shock wave and bow shock wave, which are formed by fuel jet on fuel mixing under different conditions. The numerical analysis demonstrates that an optimal angle exists for the mixing efficiency of the ramp. The optimal angle diminishes as the jet-to-crossflow pressure ratio increases. The oblique shock wave in a certain range is conducive to enhance the penetration depth of ethylene. The smaller angle of the ramp does not cause large stagnation pressure losses.     

桩体刺入全过程褥垫层的工作特性

褥垫层是复合地基的核心构成,能协调桩、土之间的差异变形。在桩体刺入褥垫层的过程中,褥垫层的工作特性在发生变化,这方面研究还较少见。通过室内模型试验,观测桩体刺入褥垫层不同深度的桩顶压力、桩顶位移量、褥垫层的变形等试验结果,来研究褥垫层的全过程工作特性,并分析桩间土压力、桩径等对褥垫层的影响。结果表明:在桩体刺入过程中,褥垫层存在压密变形、剪切变形、压碎变形3个阶段;桩间土压力对褥垫层的整个变形过程均有影响,在其他条件相同时,褥垫层抵抗的桩顶压力、褥垫层的承载厚度均随着桩间土压力的增大而增大;桩径对褥垫层的影响主要在压碎变形阶段,褥垫层的承载厚度随桩径的增大而增大,且基本呈线性增长。     

消费空间生产视角下杭州市美丽乡村发展特征——以下满觉陇、龙井、龙坞为例

在新型城镇化背景下,传统乡村的更新升级日益与大都市消费空间扩张、农业现代化紧密联系在一起。以杭州为例,其外围乡村地域已经形成动力机制各异的不同消费空间圈层模式,研究选取3个典型美丽乡村为案例分析对象,构建美丽指数对其现状特征进行评价,并探索各自不同的动力机制。结论是,3个乡村均具有生产与消费兼具的地域空间特征,在人居环境、社会文化、产业经济方面具有一定的差异性,这是由于受不同的动力主体推动所致,由此也形成了都市化背景下乡村空间的圈层模式。     

浅析《花间集序》的词学观

《花间集序》是我国词学史上的第一篇词学理论,该序对词学批评理论的建树做出了很大的贡献,对词的审美特征、价值功能以及关系当时文风的雅俗问题都进行了有益的探索,给后世学者提供了参考的依据。     

Zircaloy-U02 and -Water Reactions and Cladding Temperature Estimation for Rapidly-Heated Fuel Rods under an RIA Condition

Zircaloy cladding chemical reactions with coolant water and UO₂ fuel at elevated temperatures under a reactivity initiated accident (RIA) condition were studied from a metallurgical point of view on the basis of the nuclear safety research reactor (NSRR) experiments. The cladding-fuel chemical reaction was extensively analyzed and found to be explainable from equilibrium phase diagrams. The systematic estimation methods of maximum cladding temperature were proposed and examined from metallographies. Maximum cladding temperature can be estimated from measured oxidation thicknesses in the temperature range of 1,000~1,600°C, from melting microstructures in the range of 1,600~1,950°C and also from the volume fraction of the precipitates, (U, Zr)0_2-x, in once-molten oxygen-stabilized α-zircaloy in 1,950~2,400°C. The estimation by the method proposed in the paper is more valid than thermocouple indications at high temperatures, since thermocouples perturb the temperatures they are measuring or fail at the extremely high temperatures. The results are thought to be applicable also to understand general fuel rod behavior under hypothetical accident conditions which cause severe fuel damage.     

背景典型斑点特征的聚类分析

迷彩是一项重要的目标防护手段,斑点形状设计是进行迷彩设计的基础。首先自动提取背景图像中的明显斑点,然后分析背景斑点周长、面积、伸长度、矩形度、细长度、分散度等特征的统计特性,最后将长度、矩形度、细长度、分散度作为斑点特征向量对提取出的背景斑点进行HCM聚类分析,进而得到背景典型斑点的特征参数。该方法能得到大样本背景图像斑点特征的统计特征,对自动、定量化的迷彩设计具有一定意义。     

Activation characteristics analysis on concealed fault in the excavating coal roadway based on microseismic monitoring technique

In order to effectively monitor the concealed fault activation process in excavation activities, based on the actual condition of a working face containing faults with high outburst danger in Xin Zhuangzi mine in Huainan, China, we carried out all-side tracking and monitoring on the fault activation process and development trend in excavation activities by establishing a microseismic monitoring system. The results show that excavation activities have a rather great influence on the fault activation. With the working face approaching the fault, the fault activation builds up and the outburst danger increases; when the excavation activities finishes, the fault activation tends to be stable. The number of microseismic events are corresponding to the intensity of fault activation, and the distribution rules of microseismic events can effectively determine the fault occurrence in the mine. Microseismic monitoring technique is accurate in terms of detecting geologic tectonic activities, such as fault activations lying ahead during excavation activities. By utilizing this technique, we can determine outburst danger in excavation activities in time and accordingly take effective countermeasures to prevent and reduce the occurrence of outburst accidents.     

Numerical and experimental studies on hydrodynamic characteristics of sleeve regulating valves

The sleeve regulating valve is a kind of control valve, which widely used in natural gas transportation. Jet flow generated in valve throats will impact on the surface of the valve body and the valve clack, and then cause the destruction of the valve. In this work, two regulators were designed to reduce the hydraulic impacts, and the effects of regulators on the hydraulic performance of the sleeve regulating valve are investigated in detail. Numerical and experimental studies were applied. Results indicate that the introduction of a regulator reduces the value of flow coefficient (K_V), and the reduction is greater with larger valve opening. Meanwhile, the flow coefficients for the valve with regulator I is a bit larger than that with regulator II. The results also demonstrate that the employment of regulator decreases the entropy production rate (EPR) of high EPR regions, reduces the pressure drop of valve throats, and slows down the velocity in the valve throats. Thus the impacts of the fluid on the throat walls are weakened by regulators. It is noted that the reduction of impact forces with regulator I (has small round holes) is larger than that with regulator II (with two rows of oval holes).