A Numerical Study on Metallic Powder Flow in Coaxial Laser Cladding

In coaxial laser cladding, the quality and property of deposition products are greatly influenced by the powder flow, which is responsible to transport additive materials to the deposition point on a substrate precisely. The metallic powder flow in coaxial laser cladding is simulated by a numerical model based on the gas-solid flow theory. The characteristics of powder concentration distribution between coaxial nozzle and deposition point for a kind of nickel based alloy powder are studied by the proposed model. The relationship between the process parameters and powder flow characteristics, such as focus distance from the nozzle exit and maximum powder concentration, is analyzed to optimize the powder feeding process. In addition, the influence of substrate with different surface shapes on the powder flow is investigated. The results can be used as a guideline for the location of the substrate and the selection of proper processing parameters for coaxial laser cladding

Phonon and Elastic Instabilities in Zincblende TlN under Hydrostatic Pressure from First Principles Calculations

The lattice dynamic and elastic instabilities of zincblende (ZB) thallium nitride (TlN) under hydrostatic pressure are extensively studied to reveal the physically driven mechanism of phase transition from the ZB to a rocksalt structure using pseudopotential plane-wave density functional calculations within the local density approximation. Our calculated results shows that both transverse acoustic phonon mode softening behavior and elastic instability are responsible for the pressure-induced structural phase transition in ZB TlN

Microstructural and Mechanical Properties of a NiCoCrAlY Coating Prepared by Laser Cladding on a Compacted Graphite Cast Iron Surface

Laser cladding with synchronous powder feeding on the surface of compacted graphite cast iron (CGCI) was conducted using NiCoCrAlY powder in this research, with the aim of obtaining coatings mainly consisting of directional columnar crystals in the microstructure. A metallographic microscope, scanning electron microscope (SEM), X-ray diffractometer (XRD), and Vickers hardness tester were used to analyse the dilution rate, bonding properties, microstructure, and microhardness of the coating. The research results showed that: the alloy coating mainly consisted of the dendrites of gamma(Fe, Ni) and the carbide phases of Cr7C3 and Cr23C6 between grain boundaries. Meanwhile, it was discovered that the powder feed rate can significantly influence the crystalline state within the coating: with increasing powder feed rate, the microstructure of the coating was gradually converted from dendrites with poor directivity to columnar crystals with strong directivity, and the average microhardness was reduced owning to the decrease of the amount of Cr7C3 and Cr23C6 phases

Microstructural evolution and bonding characteristic in multi-layer laser cladding of NiCoCr alloy on compacted graphite cast iron

Laser cladding of NiCoCr alloy powder on cast iron was performed with a 1 kW Nd:YAG continuous laser. Despite numerous advantages, one of the most critical issues is the formation of martensite and a brittle layer in the vicinity of the bonding interface due to the inherent rapid cooling, which might result in cracking nearby and a decrease in bonding strength. The objective of this research is to produce well bonded NiCrCo alloy coating that is free of pores and cracks on cast iron without preheating the whole substrate. Considering the poor weldability of cast iron, the strategy of reciprocated deposition, namely multi-layer laser cladding, was applied. The microstructural evolution of the multi-layer coating on the cast iron was investigated with the emphasis on the variation of the bonding zone. The results showed that NiCoCr coatings with different layers consisted of fine solid solution dendrites surrounded by an inter-dendritic network of precipitates. The multi-layer clad coating had a gradual distribution of elements, which was different from that in the single layer coating. The martensite near the bonding interface was transformed into tempered sorbite under the thermal influence of multi-layer deposition, leading to a decrease in the high micro-hardness zone width. A bending test confirmed that the bonding brittleness was reduced and the mechanical properties were improved through the use of the multi-layer laser cladding approach. (C) 2016 Elsevier B.V. All rights reserved.</p

Frictional contact of a rigid punch on an arbitrarily oriented gradient half-plane

Frictional contact of a rigid punch on a half-plane with shear modulus varying according to an exponential gradient in an arbitrary direction is investigated in this paper. Fourier integral transform method is employed to reduce the current sliding contact problem to a Cauchy-type singular integral equation of the second kind. The contact pressure and the in-plane stress, as well as the stress singularity and the stress intensity factor near both contact edges are obtained, which are further analyzed for different friction coefficients, non-homogeneity parameters and gradient orientation angles. The moment in order to keep the punch moving perpendicularly to the contact surface is also evaluated. All the present results should be helpful for the design of surfaces with strong wear resistance and understanding the mechanical mechanism of graded materials in nature

Nonslipping Contact Between a Mismatch Film and a Finite-Thickness Graded Substrate

The contact behavior of an elastic film subjected to a mismatch strain on a finite-thickness graded substrate is investigated, in which the contact interface is assumed to be nonslipping and the shear modulus of the substrate varies exponentially in the thickness direction. The Fourier transform method is adopted in order to reduce the governing partial differential equations to integral ones. With the help of numerical calculation, the interfacial shear stress, the internal normal stress in the film and the stress intensity factors are predicted for cases with different material parameters and geometric ones, including the modulus ratio of the film to the substrate, the inhomogeneous feature of the graded substrate, as well as the profile of the contacting film. All the physical predictions can be used to estimate the potential failure modes of the film-substrate interface. Furthermore, it is found that the result of a finite-thickness model is significantly different from the prediction of a generally adopted half-plane one. The study should be helpful for the design of film-substrate systems in real applications

Numerical simulation of powder transport behavior in laser cladding with coaxial powder feeding

Laser cladding with coaxial powder feeding is one of the new processes applied to produce well bonding coating on the component to improve performance of its surface. In the process, the clad material is transported by the carrying gas through the coaxial nozzle, generating gas-powder flow. The powder feeding process in the coaxial laser cladding has important influence on the clad qualities. A 3D numerical model was developed to study the powder stream structure of a coaxial feeding nozzle. The predicted powder stream structure was well agreed with the experimental one. The validated model was used to explore the collision behavior of particles in the coaxial nozzle, as well as powder concentration distribution. It was found that the particle diameter and restitution coefficient greatly affect the velocity vector at outlet of nozzle due to the collisions, as well as the powder stream convergence characteristics below the nozzle. The results indicated a practical approach to optimize the powder stream for the coaxial laser cladding.</p

Identification and anti-oxidative potential of Milk Fat Globule Membrane (MFGM)- derived bioactive peptides released through in vitro gastrointestinal digestion

This study investigated the stability of milk fat globule membrane (MFGM) protein under simulated gastrointestinal conditions using an in vitro enzymatic digestion method. The optimal hydrolysis conditions were determined by monitoring the changes in particle size and zeta-potential of MFGM protein hydrolysates over time. Furthermore, the distribution of small molecular weight peptides with antioxidant activity was explored through DEAE-52 combined with in vitro cell experiments. Two novel antioxidant peptides (TGIIT and IITQ) were identified based on molecular docking technology and evaluated their potential scavenging activity against 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2'-Azinobis-(3-ethylbenzthiazoline-6-sulphonate) (ABTS+) radicals. TGIIT and IITQ also demonstrated remarkable abilities in promoting mitochondrial biogenesis and activating Keap1/Nrf2 signaling pathway, which can effectively counteract skeletal muscle dysfunction induced by oxidative stress. Thus, MFGM-derived antioxidant peptides have the potential to be employed in food to regulate muscle protein metabolism and alleviate sarcopenia

生态调水前后塔河下游湿地时空变化监测/Wetland Dynamic Monitoring and Change Analysis in Downstream of the Tarim River by Remote Sensing Around Ecological Water Diversion[J]

湿地作为干旱区一种重要而特殊的生态系统,具有很强的环境指示性.为了准确地认识和评估人工生态输水对塔里木河下游湿地生态环境修复的成效.文章通过遥感技术手段,动态地监测了人工应急生态输水前后塔里木河下游的湿地景观变化.研究结果表明:通过10余次的人工生态输水,在总体上改变了流域湿地长期持续减少的状态,流域范围内湿地总面积呈现稳中有增,湿地生态环境得到了明显改善.其中,河流湿地和湖泊湿地变化过程呈现“V”型逆转,面积有着显著的增加;沼泽湿地减少的速度得到明显遏制,动态度由1990-2000年的-5.35％降低到2000-2010年-1.09％;此外,下游河道断流现象得到改善,尾闾湖——台特玛湖在干涸了近30年后重现恢复往日生机.说明了塔河人工生态调水对下游的湿地生态环境拯救起到了积极的作用.以上研究可为塔河的综合治理提供有益的科学依据和决策支持

新疆干旱区湿地景观格局遥感动态监测与时空变异/Remote sensing detection and spatio-temporal change analysis of wetlands in Xinjiang arid region[J]

为了解和掌握全球变化环境下中国干旱区湿地的分布现状、变化过程和时空格局特征，该文通过多源遥感技术手段，采用面向对象分类方法和目视解译相结合，完成了干旱区湿地信息的多期监测，实现了新疆地区2000、2005和2010年的湿地分布制图，并利用动态度模型和地统计方法计算了近10 a时空变化情况，分析了湿地变化的时空特征。研究结果表明：1）近10 a来全疆干旱区湿地约增加了10.1%，2010年新疆湿地总面积达到22438 km2，较2000年约增加了2065 km2；2）在类型构成上，2010年河流湿地、湖泊湿地、沼泽湿地和人工湿地所占比例分别为42.73%、29.43%、17.98%、8.92%和0.94%；3）在空间分布上，南北疆呈现不均衡，南疆湿地约占总体的76%，北疆湿地仅占24%；主要分布在塔里木流域，以河流湿地为主，空间分布差异显著；4）新疆湿地面积总体上呈增多的趋势，不同类型的湿地面积变化幅度差异较大。沼泽湿地和人工湿地的动态度分别为：2.58%、2.43%，河流湿地与湖泊湿地的动态度为：0.59%、0.43%；净增量最大的是沼泽湿地和河流湿地，分别增加了827.92和531.86 km2，占湿地净增量的65.85%。在变化原因上，全球变暖、新疆区域气温升高和降水增多是湿地近年来增加的主要原因；人类活动的干扰，加剧了湿地变化的复杂性和空间差异性。研究结果可为区域生态环境变化研究以及湿地资源的保护规划提供数据支持和结论参考