Optimization of spray conditions in cold spraying based on numerical analysis of particle velocity

The effects of the parameters involved in cold spray on the acceleration of particles are systematically investigated by a CFD code in order to reveal the main factors influencing significantly particle velocity. The parameters involved include nozzle geometry parameters, processing parameters and properties of spray particles. It is found that driving gas type, operating pressure and temperature are main processing parameters which influence particle velocity. As for nozzle geometry, the expansion ratio and divergent section length of spray gun nozzle show significant effects. Moreover, the density, size and morphology of powder also have significant effects on particle velocity. The effects of those main parameters are summarized in a comprehensive equation obtained through nonlinear regression of the simulated results for the estimation of particle velocity. The interactions of the parameters on particle acceleration can be examined through the equation. Moreover, the optimization of the dimensions of spray gun nozzle and spray parameters can be realized based on the obtained results.     

Simulation and Improvement of the Micro Abrasive Blasting Process

Micro abrasive blasting (MAB) is becoming an important machining technique for the fabrication of Micro Electro Mechanical systems. The process is based on the erosion of a mask-protected brittle substrate by an abrasive-laden air jet. Currently available blasting machines are relatively simple. However, to exploit this technique for applications of industrial interest a more efficient and controllable process is required. In this paper abrasive blasting is analysed by means of a set of models containing different sub-models for the particle beam, the material removal mechanism and the resulting blasting profile. A new line-shaped Laval nozzle has been developed, which is able to increase the particle velocity by 40% compared to a conventional round nozzle. The blasting-profile of this line-shaped nozzle is more uniform and the material removal rate is significantly higher compared to conventional nozzles, which in turn leads to higher achievable aspect ratios of the three-dimensional microstructures.     

A strategy for efficient and quality cutting of materials with abrasive waterjets considering the variation in orifice and focusing nozzle diameter

In Abrasive Waterjet (AWJ) cutting, orifice and focusing nozzle diameter undergo continuous change in their dimensions due to erosive nature of high velocity abrasive waterjet. This particular phenomenon can affect the efficiency and quality of the process. To achieve maximum efficiency and desired quality with this process, the parameters need to be optimally selected from time to time considering the changes in the dimensions of orifice and focusing nozzle. In an effort to develop strategies for this purpose and to build the knowledge base for adaptive control of the process, the present work aims to study the influence of orifice and focusing nozzle diameter variation on the performance of abrasive waterjets in cutting 6063-T6 aluminum alloy. The performance was assessed in terms of different parameters such as depth of cut, material removal rate, cutting efficiency, kerf geometry and cut surface topography. In order to maintain the desired performance, it is essential to monitor the condition of nozzles and suitably adjust the process parameters with a view to control the process. Towards the latter, the present work attempts to suggest a strategy that can aid in replacing the nozzles at an appropriate time for maintaining the performance of process within certain limits so as to maintain the precision in machining with abrasive waterjets.     

PIV测速技术分析异型坯连铸结晶器流场

通过建立1[∶]1异型坯结晶器物理模型，采用PIV粒子测速技术，研究断面尺寸为767.3 mm×383.1 mm×103.2 mm异型坯结晶器不同工艺参数条件和不同水口结构对结晶器内流场的影响。PIV实验结果表明，减小拉速和增大水口底部内径可以有效地减小冲击深度，结晶器深度860 mm处水口中心最大流股速度分别下降了27.96%和41.46%；增加拉速和减小水口浸入深度可以提高流场下旋涡上顶点位置。通过减小拉速和浸入深度，增大水口底部内径可以改善结晶器内流场。     

磨料射流喷嘴压力场与速度场的研究

为提高磨料射流抛光加工过程中的可靠性和效率,对在磨料射流加工中起到重要作用的喷嘴进行研究具有重要的意义。该文在分析喷嘴射流机理的基础上,针对典型的喷嘴进行仿真研究,得到了喷嘴内部压力场和速度场的分布情况,同时采用有限元对喷嘴结构参数进行数值模拟分析与比较,得出不同结构参数对于流体性能的影响。最后对该仿真方法的可靠性进行了实验验证,并给出了误差率,证明了在该文中所使用的仿真方法具有可靠性,为进一步研究磨料射流喷嘴提供了实用的价值和有效的方法。     

Computer fluid dynamics analysis for efficient spraying of oil mist in finish-turning of Inconel 718

A compressible turbulence analysis of the flow of oil mist in MQL finish-turning of Inconel 718 was conducted using a finite volume method for optimizing the spraying conditions of oil mist and reducing the oil consumption. First, a domain of the analysis including a rotating workpiece and a single point cutting tool with a spraying nozzle was discretized, and then, changes in the velocity, pressure and vorticity of compressed air, and mass flow rate of oil mist near the cutting edge were calculated for three types of spraying nozzles. It was found that when the distance from the outlet of a nozzle to the tool tip was decreased using cover-type nozzles, mass flow rate of oil mist to near the cutting edge increased drastically. Among the three types of nozzles, the mass flow rate was the largest for a cover-type nozzle for oblique spraying. These results were consistent with the performance of the nozzles in cutting experiments. Finally, the flight of an oil droplet and the lubrication mechanism in MQL machining were investigated using analysis results.     

船舶喷涂机器人扇形喷嘴内射流模拟对比分析

通过对目前船厂船体喷涂工艺的分析,设计船舶喷涂机器人射流系统原理结构。扇形喷嘴是机器人喷涂新方案的关键部件,其内部流场特性的研究非常重要。以船厂目前常用的4种扇形喷嘴为例,应用CFD手段,根据流体理论建立数学模型,并根据应用现场实际,简化了物理模型,分别进行4种扇形喷嘴腔体内流场的数值模拟,并对比分析了4种扇形喷嘴轴心线上的压力变化曲线和速度变化曲线,得到4种扇形喷嘴各自的特性,为后续船舶喷涂机器人扇形喷嘴深度模拟提供参考。     

High-Speed Additive Manufacturing Through High-Aspect-Ratio Nozzles

The feasibility of layer-by-layer manufacturing through high-aspect-ratio (HAR) nozzles for microextrusion of paste to deposit planes has been investigated. Various conditions for paste extrusion, including nozzle moving speed, piston speed, extrusion rate, and distance between the nozzle tip and substrate, have been evaluated. By linking various microextrusion parameters together with the aid of a critical distance concept derived from microextrusion using circular nozzles and addressing the extrusion delay in response to the change of the piston speed and air pocket problems properly, we successfully microextruded single planes, multilayer objects, and larger planes made of multiple smaller planes side by side through HAR nozzles. It is further demonstrated that the X–Y dimensions of an extruded plane in the steady-state extrusion stage are determined by the nozzle travel distance and the length of the HAR nozzle opening if microextrusion is conducted with proper conditions. However, the height of the extruded plane is not only determined by the microextrusion conditions, but also affected by the drying shrinkage of the paste after microextrusion. This demonstration of the feasibility of using a HAR nozzle machine opens the door to manufacture of multimaterial, multilayer devices with high productivity in the near future.     

Fluid dynamic study of direct current plasma jets for plasma spraying applications

Recently, direct current (dc) plasma torches equipped with converging-diverging (Laval) nozzles, instead of standard cylindrical ones, have been shown to present several advantages for both vacuum and atmospheric plasma spraying, such as diminishing the gradients of temperature and velocity and reducing the turbulence intensity in the jet fringes. The present study was concerned with the diagnostics of the plasma jets produced by three nozzles of various contours: a standard cylindrical anode and a Mach 2.5 and Mach 3 Laval nozzle. Emission spectroscopy (absolute intensity) and enthalpy probe techniques were used to measure temperature and velocity fields. Special attention was given to the effects of spray chamber pressure on flow regime inside the nozzles and to the distribution of the temperature and velocity fields in the plasma jet. Results showed that under the chamber pressure conditions used (vacuum), for which Laval nozzles originally were designed, the generated plasma jets had greater centerline velocities and larger high temperature zones compared to standard cylindrical nozzles. The results showed significant improvement in the deposition efficiency by using nozzles with these computed contours.     

The influence of nozzle design on HVOF exit gas velocity and coating microstructure

A simple device was constructed for determining a value for the average combustion gas velocity at the exit plane of a high-velocity oxyfuel gun. This device was used to measure the velocities of a standard factory-made barrel nozzle and a specially designed de Laval nozzle as a function of the fuel/oxygen ratio and the total mass flow rate. The Mach number of the de Laval nozzle was 1.42. The maximum combustion gas exit velocities determined for the standard and the de Laval nozzles were 1100 and 1550 m/s, respectively. The maximum velocity depends on the fuel/oxygen ratio but is independent of the total flow rate. The effect of increased combustion gas velocity on coating quality is demonstrated.     

Particle behavior in supersonic flow during the cold spray process

The cold spray process is a relatively new process that uses high velocity metallic particles for surface modifications. Metallic powder particles are injected into a converging-diverging nozzle and accelerated to supersonic velocities. In this study two-dimensional temperature and velocitiy distributions of gas along the nozzle axis are calculated and the effects of gas pressure and temperature on particle velocities and temperature inside and outside the nozzle are investigated. It was found that acceleration of the gas velocity takes place in the area of the nozzle throat, and it increases and reaches a maximum value at the nozzle exit. Due to compression shocks, irregular changes of the gas jet properties were found in the area after the nozzle and these resulted in the experience of the maximum particle velocity by the change of the particle size at a given gas pressure and temperature.     

Supervision and Measuring of Particle Parameters During the Wire-Arc Spraying Process with the Diagnostic Systems Accuraspray-g3 and LDA (Laser-Doppler-Anemometry)

Due to low cost of operation, high deposition rates and efficiency, wire arc spraying has become one of the most important thermal spray technologies, especially as a tool for coatings used to improve corrosion and wear protection. In order to obtain high-quality coatings, the flow characteristics of the atomizing gas have to be optimized. Thus, the nozzle design as well as the properties of the gas used need to be adjusted and the resulting particle parameters have to be quantified. Employing the Accuraspray-g3 system in combination with Laser Doppler Anemometry (LDA), the particle size distribution and velocity have been measured for a wide range of parameters, including different materials, different gas pressures and nozzles resulting in design suggestions and offering the possibility to compare the two different diagnostic systems.     

针型喷嘴射流特性研究

运用FLUENT流体仿真软件的VOF两相流模型对针型喷嘴在不同入口压力、不同打击距离的喷嘴射流流场进行数值分析,得到针型喷嘴射流流场的速度和被清洗件承受的压力分布云图和压力分布曲线,研究针型喷嘴射流参数的变化对射流流场速度及压力的影响规律。结果表明:随着入口水压的增加,射流最大速度和被清洗件上承受的压力值增加,但增加趋势逐渐减小;随着打击距离的增加,清洗范围略微增大,被清洗件上承受的压力值减小,但减小程度逐渐下降;进而得出当入口压力为30 MPa,打击距离在90～100 mm范围内的针型喷嘴具有最佳清洗效果。     

Analysis and modelling of particle velocities in micro-abrasive air jet

Abrasive jet micromachining (AJM) is a non-traditional technology that can effectively remove hard and brittle materials at high cut quality. A key requisite in modelling the AJM process is to determine the velocities of abrasive particles. In this paper, a theoretical analysis for particle velocities within a micro-abrasive air jet is presented and the associated particle velocity models are developed. The particle velocities at the nozzle exit are determined based on the nozzle length, particle mean diameter, particle density, air density and air flow velocity. The distribution of particle velocities along the jet centerline downstream from the nozzle and the particle velocity profile at a jet cross-section are also modelled considering surrounding air entrainment and air-particle interaction. A numerical solution to the models is developed to determine the particle velocities by dividing the nozzle and the jet flow in air into small segments along the jet axial direction. The developed models are finally verified by comparing the calculated particle velocities with those from a particle image velocimetry (PIV) measurement of the velocity distribution in micro-abrasive air jets. It is shown that the model calculations and the corresponding experimental results are in good agreement with less than 4% average errors.     

Wear mechanisms of gradient ceramic nozzles in abrasive air-jet machining

The nozzle is the most critical part in abrasive air-jet machining equipment. Ceramics, being with high wear resistance, have great potential as abrasive air-jet nozzle materials. In this paper, a (W,Ti)C/SiC gradient ceramic composite was developed to be used as nozzle material. The erosion wear behavior of the (W,Ti)C/SiC gradient nozzle was investigated and compared with a conventional ceramic nozzle. Results showed that the gradient ceramic nozzles exhibited an apparent increase in erosion wear resistance over the conventional ceramic nozzles. The mechanism responsible was found to be that the tensile stresses at the entry region of the nozzle were greatly reduced when compared with the conventional nozzle. This effect may lead to an increase in resistance to fracture, and thus increase the erosion wear resistance of the gradient nozzle. It is indicated that gradient structures in ceramic nozzles are effective to improve the erosion wear resistance of conventional ceramic nozzles in abrasive air-jet machining.     

Performance and wear characteristics of ceramic, cemented carbide, and metal nozzles used in coal–water–slurry boilers

Ceramics, cemented carbides, and metals were prepared to be used as nozzles in CWS boilers. CWS burning tests in a boiler with these nozzles were carried out. The erosion wear resistance of these nozzles was compared by determining their erosion rates and hole diameter variation. Results showed that the life of the ceramic nozzles is about 30 times high than that of the metal nozzles. The wear types at the nozzle wall surface differed in various positions. The nozzle center wall section suffers form abrasive impact under low impact angles, and the damage at the center wall mainly occurs by plowing and plastic deformation for metals, and by polishing action for carbides and ceramics. The primary wear mechanisms at the exit of ceramic nozzle exhibited thermal shock damage with chipping owing to the greater thermal stresses.     

高压二氧化碳载送干冰喷射去污流场分析及关键参数优化

为提升核设施退役去污能力,提出高压二氧化碳载送干冰喷射去污方法。基于Fluent软件建立了喷射去污系统中关于喷嘴与去污箱体内部流场的分析模型,通过流场分析获得了干冰颗粒速度分布。以靶距、喉半径、喷嘴长度、干冰粒径、压力为关键参数,以干冰颗粒对污染表面的冲击应力与分布直径的乘积为去污评价指标,分别开展了1.5、3 mm口径喷嘴流场分析的正交试验。通过均值响应分析和主效应分析掌握了各关键参数对去污性能的影响规律,获得最优参数组合。分析结果表明：关键参数优化后,1.5 mm口径喷嘴去污性能提升了5.64%,3 mm口径喷嘴去污性能提升了15.8%,后者去污性能是前者的1.3倍,但前者的二氧化碳利用率是后者的3倍,具有更好的经济性。该分析结果可为喷嘴选型和喷射去污系统参数计算提供支撑。     

Design of a new nozzle for direct current plasma guns with improved spraying parameters

A new design is proposed for direct current plasma spray gas-shroud attachments. It has curvilinearly shaped internal walls aimed toward elimination of the cold air entrainment, recorded for commercially available conical designs of the shrouded nozzle. The curvilinear nozzle design was tested; it proved to be capable of withstanding high plasma temperatures and enabled satisfactory particle injection. Parallel measurements with an enthalpy probe were performed on the jet emerging from two different nozzles. Also, corresponding calculations were made to predict the plasma flow parameters and the particle parameters. Adequate spray tests were performed by spraying iron-aluminum and MCrAlY coatings onto stainless steel substrates. Coating analyses were performed, and coating qualities, such as microstructure, open porosity, and adhesion strength, were determined. The results indicate that the coatings sprayed with a curvilinear nozzle exhibited lower porosity, higher adhesion strength, and an enhanced microstructure.     

新型内吹环的设计与多喷嘴内吹环流场分析

针对车间喷嘴吹扫镀锌钢管内部多余锌粒时,出现的消耗大、喷管出口速度低、吹扫效率低等问题,设计一种新型的内吹环结构。这种结构由4个喷嘴组成,每个喷嘴的结构内轮廓为B样条曲线,喷嘴出口采用椭圆形结构。对原内吹环与新型内吹环进行流场仿真,仿真结果表明:新型内吹环的吹扫效果要高于原内吹环。并且研究了内吹环出口到管口的距离、喷嘴压力以及喷嘴出口直径对于流场的影响。其结果表明:靶距为40 mm的吹扫效果要优于20和30 mm,内吹环压力为0.6 MPa时射流速度大于压力为0.3和0.15 MPa,出口截面积越小吹扫效率越高。