On high velocity impact of micro-sized metallic particles in cold spraying

In this study, a systematic examination of particle deformation behaviour in cold spraying was conducted for Cu particle using both the Lagrangian and Arbitrary Lagrangian Eulerian (ALE) methods. It is found that the meshing size in modelling by Largrangian method influences significantly the localized shear instability at interface areas. With refining the meshing size the onset velocity for interface shear instability decreases. The extrapolation of these results yields a reasonable critical velocity comparable to the actual one in cold spray practice. The results indicate that both the flattening ratio and compression ratio of the deformed particles increase with the increase in particle velocity, which are in good agreement with the experiment results. The ALE method provides a suitable way to examine the particle deformation in cold spraying. Moreover, the numerical results also show that there exists the similarity for the deformation of particles of different diameters.     

Numerical and experimental investigations of splat geometric characteristics during oblique impact of plasma spraying

Splats are obtained on the substrates inclined at different angles (0°, 20°, 40° and 60°) by plasma spraying process and characterized by SEM and WYKO^® optical surface profiler. Numerical model is developed using CFD software FLOW-3D^® to simulate the process of droplet impact, spreading and solidification onto the substrates. Splat characteristics such as spread factor, aspect ratio and fractional factor are defined and compared between simulation and experiment. Fair agreements are obtained. In addition, the impacting behavior including spreading and solidification are analyzed in details from the simulation results. The rates of reduction in droplet kinetic energy during impact, spreading and solidification are also compared between different inclination angles.     

SPH simulation of fuel drop impact on heated surfaces

The interaction of liquid drops and heated surfaces is of great importance in many applications. This paper describes a numerical method, based on smoothed particle hydrodynamics (SPH), for simulating n-heptane drop impact on a heated surface. The SPH method uses numerical Lagrangian particles, which obey the laws of fluid dynamics, to describe the fluid flows. By incorporating the Peng–Robinson equation of state, the present SPH method can directly simulate both the liquid and vapor phases and the phase change process between them. The numerical method was validated by two experiments on drop impact on heated surfaces at low impact velocities. The numerical method was then used to predict drop-wall interactions at various temperatures and velocities. The model was able to predict the different outcomes, such as rebound, spread, splash, breakup, and the Leidenfrost phenomenon, consistent with the physical understanding.     

Effect of substrate hardness on the deformation behavior of subsequently incident particles in cold spraying

A systematic finite element analysis (FEA) on the subsequently incident particles which refer to the particles depositing after the formation of the first layer coating is conducted in this study to clarify the bonding mechanism inside the cold sprayed coating. A simplified particle impact model is proposed and the simulated results based on this model demonstrate that substrate hardness exerts some effects on the deformation behavior of the subsequently incident particles. Hard substrate makes these particles deform intensively but using soft substrate enables them to achieve a slight deformation. At the same time, it is also found that substrate hardness plays its best role only when the formed coating is thin, as the development of the coating, the particle deformation behavior becomes more and more insensitive to the substrate hardness. The multi-particle impact simulation based on Eulerian method is also performed and reaches the same conclusion, confirming the accuracy of the simplified model. Besides, it is also found that when the velocity is increased to a hypervelocity, excessive deformation occurs in the formed coatings due to the impact of the subsequently incident particles.     

Study on impact fusion at particle interfaces and its effect on coating microstructure in cold spraying

This paper deals with the impact melting phenomenon at the interfaces between the deposited particles in cold-sprayed coatings and its effect on coating microstructure and particle bonding mechanism. Al-12Si, Al2319, Ti, Ti-6Al-4V, Ni and NiCoCrAlTaY powders were selected as feedstocks, which have various thermal and mechanical properties. The analytical results showed that most of the used materials possibly experienced the local melting at the contact interfaces of particles under certain impact conditions. Low melting point, relatively high gas temperature and chemical reaction with the atmosphere are the main factors contributing to the impact fusion during cold spraying. The results also indicated that the local melting would benefit the formation of a metallurgical bonding between the deposited particles and enhance the coating cohesion.     

Some aspects on 3D numerical modeling of high velocity impact of particles in cold spraying by explicit finite element analysis

Three-dimensional modeling of particle impacting behavior in cold spraying by using ABAQUS/Explicit was conducted for copper and other materials. Various combinations of calculation settings concerning material damage, Arbitrary Lagrangian Eulerian adaptive meshing, distortion control and contact interaction were examined. The effects of meshing size and particle size on the impact behavior were analyzed compared to the previous results. The results show that the simulations with material damage cope well with the element excessive distortion and the resultant output is more reasonable than that obtained without material damage. In addition, the meshing size has less effect on the output with the material damage than without material damage. Although particle size has little effect on the morphologies of the deformed particles, it has some effect on the failure of elements at contact interfaces. The critical velocity for particle deposition could be estimated given the appropriate material properties.     

Numerical simulation of deformation behavior of Al particles impacting on Al substrate and effect of surface oxide films on interfacial bonding in cold spraying

In this study, a comprehensive examination of the deformation behavior of Al particles impacting on Al substrate was conducted by using the Arbitrary Lagrangian Eulerian (ALE) method to clarify the deposition characteristics of Al powder and the effect of surface oxide films in cold spraying. It was found that the deformation behavior of Al particles is different from that of Cu particles under the same impact conditions owing to its lower density and thus less kinetic energy upon impact. The results indicated that a higher velocity was required for Al particles to reach the same compression ratio as that of Cu particles. On the other hand, the numerical results showed that the oxide films at particle surfaces influenced the deformation and bonding condition between the particle and substrate. The inclusions of the crushed oxide films at the interfaces between the depostied particles inhibit the deformation.     

高速碰撞数值计算中的SPH分区算法

针对高速碰撞问题大变形局部化的特点,提出一种分区计算的光滑粒子法.将整个计算域划分为若干子域,在可能发生大变形的子域布置较多的粒子,其它子域布置较少的粒子.分析子域交界面上产生计算不稳定的原因并提出解决方案.采用该方法对长杆弹侵彻厚靶板问题进行数值模拟,并与传统方法进行对比.结果表明,分区算法在保证计算精度的前提下,能显著提高光滑粒子法的计算效率.     

液滴溅落问题的光滑粒子动力学模拟

对传统的光滑粒子动力学方法进行了改进, 改进的光滑粒子动力学方法对传统粒子方法中的核近似式和粒子近似式进行了修正, 采用Riemann 算法求解光滑粒子动力学流体控制方程, 添加了表面张力的计算程序, 考虑了表面张力对液滴溅落的影响. 应用改进的光滑粒子动力学方法对液滴静止状态下冲击液面的飞溅过程进行了数值模拟. 计算结果表明, 改进的光滑粒子动力学方法能够有效地描述液滴溅落液面的动力学特性和自由表面变化特征, 能够得到稳定精度的结果.     

液滴冲击液膜问题的光滑粒子动力学模拟

本文对传统的光滑粒子动力学方法进行了改进．改进的光滑粒子动力学方法对传统粒子方法中的核梯度进行了修正,采用了一种新型的耦合边界条件,添加了表面张力和人工应力的计算程序．应用改进的光滑粒子动力学方法对液滴冲击液膜问题进行了数值模拟．得到了不同时刻液滴内部的压力变化特征,精细地捕捉了不同时刻的自由面,从机理上分析了液滴产生飞溅的条件,探讨了韦伯数,表面张力对液滴冲击液膜问题的影响．计算结果表明,改进光滑粒子动力学方法能够有效地描述液滴冲击液膜的动力学特性和自由表面变化特征,能够得到稳定精度的结果．     

Simulation of surface tension in 2D and 3D with smoothed particle hydrodynamics method

The methods for simulating surface tension with smoothed particle hydrodynamics (SPH) method in two dimensions and three dimensions are developed. In 2D surface tension model, the SPH particle on the boundary in 2D is detected dynamically according to the algorithm developed by Dilts [G.A. Dilts, Moving least-squares particle hydrodynamics II: conservation and boundaries, International Journal for Numerical Methods in Engineering 48 (2000) 1503–1524]. The boundary curve in 2D is reconstructed locally with Lagrangian interpolation polynomial. In 3D surface tension model, the SPH particle on the boundary in 3D is detected dynamically according to the algorithm developed by Haque and Dilts [A. Haque, G.A. Dilts, Three-dimensional boundary detection for particle methods, Journal of Computational Physics 226 (2007) 1710–1730]. The boundary surface in 3D is reconstructed locally with moving least squares (MLS) method. By transforming the coordinate system, it is guaranteed that the interface function is one-valued in the local coordinate system. The normal vector and curvature of the boundary surface are calculated according to the reconstructed boundary surface and then surface tension force can be calculated. Surface tension force acts only on the boundary particle. Density correction is applied to the boundary particle in order to remove the boundary inconsistency. The surface tension models in 2D and 3D have been applied to benchmark tests for surface tension. The ability of the current method applying to the simulation of surface tension in 2D and 3D is proved.     

应用SPH方法对平板铅飞层对碰凸起动载行为的数值模拟研究

采用光滑粒子流体动力学(SPH)方法,模拟研究了两点起爆圆柱炸药驱动组合飞层(铝+铅)的动载行为,得到了爆轰波对碰区铅飞层类似射流状超前凸起的计算图像。模拟结果表明,对碰凸起呈现与实验结果相同的分层结构,且凸起头部具有散碎特征,根部相对密实。因此,SPH方法能够较好地模拟铅这类低熔点、低强度金属在对碰区中出现的散碎等破坏现象。     

Significant influences of metal reactivity and oxide films at particle surfaces on coating microstructure in cold spraying

Based on large amount of experimental observations, the effects of metal reactivity and oxide films at particle surfaces on coating deposition behavior in cold spraying were presented and discussed. The oxygen contents in as-sprayed Ti, Ti-6Al-4V and Al coatings were higher than those in the corresponding starting powders. The obvious flashing jets outside nozzle exit during deposition of Ti and Ti-6Al-4V were caused by the reaction of the particles with oxygen in the entrained or the adopted air. For Ti and Ti-6Al-4V coatings, their porous structures are predominantly attributed to the surface reactivity (defined as reactivity with oxygen). This surface reaction could be helpful for formation of a metallurgical bonding between the deposited particles. For Al, even though it is more reactive than Ti, the oxide films at Al particle surfaces suppress the surface activity.     

The bond strength of Al-Si coating on mild steel by kinetic spraying deposition

Kinetic spraying (or cold gas dynamic spraying) works by accelerating small solid particles to supersonic velocities, and then impacting them onto a substrate. These high impact velocities, and low particle temperatures are the principal attributes of kinetic spraying technology. However, only recently has this technology's interfacial behavior, due to particle/substrate impaction, become well understood. In order to investigate the particle/substrate bond behavior, Al-Si feedstock was deposited onto mild steel, over a range of particle velocities; next, their respective coating bond strengths were measured by the stud pull coating adherence test. The effects of the particle velocity and the substrate surface roughness on the coating bond strength were presented, and a model of the particle/substrate bond generation was discussed in an effort to estimate the bond strength.     

Optimal design of a convergent-barrel cold spray nozzle by numerical method

A convergent-barrel (CB) cold spray nozzle was designed through numerical simulation. It was found that the main factors influencing significantly particle velocity and temperature include the length and diameter of the barrel section, the nature of the accelerating gas and its pressure and temperature, and the particle size. Particles can achieve a relatively low velocity but a high temperature under the same gas pressure using a CB nozzle compared to a convergent-divergent (CD) nozzle. The experiment results with Cu powder using the designed CB nozzle confirmed that particle deposition can be realized under a lower gas pressure with a CB nozzle.     

弹性壳结构静力与动力分析的光滑粒子法

本文通过采用移动最小二乘函数作为近似函数 和完全拉格朗日方程作为近似方程来改善光滑粒子法的稳定性和数值精度; 在此基础上, 提出了壳结构静力分析的光滑粒子法, 并完善了壳结构动力分析方法; 最后, 采用国际公认的壳结构的标准测试模型对静力和动力问题分别进行了验证, 所得结果与已有数据吻合良好, 证明了本文数值模型的有效性和可靠性, 为光滑粒子法进一步在裂纹、破碎等非线性壳结构中的应用提供参考. 关键词： 弹性壳 静力与动力分析 光滑粒子法 完备性和稳定性     

Influence of substrate hardness transition on built-up of nanostructured WC-12Co by cold spraying

The ability of cold spray process to retain the feedstock microstructure into coating makes it possible to deposit nanostructured WC-Co coatings. In the present study, the deposition behavior of nanostructured WC-12Co coating was examined through the surface morphology and cross-sectional structure of the deposited single WC-12Co particle impacting on the substrates with different hardness using a nanostructured WC-12Co powder. Substrates included stainless steel, nickel-based self-fluxing alloy coatings with different hardness. It was observed from the top surface and cross-section of individual WC-12Co particles that the penetration leading to particle deposition depends on substrate hardness. When the substrate surface is covered by WC-12Co particles, the hardness of the newly formed substrate, i.e. the coating, increases greatly. The significant increase of the surface hardness leads to the rebounding off of impacting particles and erosion of the deposited particles, which prohibits effective built-up of coating. However, it was found that with spray jet fixed, a deposit with a thickness up to over 700 μm can be built-up. A model involving in substrate hardness transition during deposition is proposed to explain such phenomenon, which can be employed to optimize the conditions to build up a uniform nanostructured WC-12Co coating.     

基于光滑粒子动力学方法的液滴冲击固壁面问题数值模拟

采用改进的光滑粒子动力学(SPH)方法对液滴冲击固壁面问题进行了数值模拟. 为了提高传统SPH方法的计算精度和数值稳定性, 在传统的SPH方法的基础上对粒子方法中的密度和核梯度进行了修正, 采用了考虑黎曼解法的SPH流体控制方程, 构造了一种新型的粒子间相互作用力(IIF)模型来模拟表面张力的影响. 应用改进的SPH方法对液滴冲击固壁面问题进行了数值模拟. 计算结果表明：新型的IIF 模型能够较好地模拟表面张力的影响, 改进的SPH方法能够精细地描述液滴与固壁面相互作用过程中液滴的内部压力场演变和自由面形态变化, 液滴的铺展因子随初始韦伯数的增大而增大, 数值模拟结果与实验得到的结果基本一致. 关键词： 液滴 固壁面 光滑粒子动力学 表面张力     

液滴在气固交界面变形移动问题的光滑粒子流体动力学模拟

为准确模拟液滴在气固交界面变形移动问题,对基于连续表面张力模型的表面张力光滑粒子流体动力学方法进行了改进.改进方法采用新的边界处理方式和界面法向修正方法,即将固体边界虚粒子色函数值根据液面的位置进行相应设定以保证气-液-固三相交界处流体粒子的界面法向沿接触线法线方向,引入Brackbill提出的壁面附着力边界条件处理方法,对在气-液-固三相交界处的流体粒子及部分固体边界虚粒子的界面法向进行修正,修正前后保持法向模值不变,得到了含壁面附着力边界条件的表面张力算法.模拟了受壁面附着力影响的水槽中液面的变化过程、液滴润湿壁面过程和剪切气流驱动液滴在固体表面变形脱落过程,并与流体体积函数方法进行了对比.结果表明,该方法在处理壁面附着力问题时精度较高,稳定性较好,适合处理工程中液滴在气固交界面变形移动问题.     

超高速碰撞碎片云特征的SPH方法数值分析

 采用光滑粒子流体动力学（SPH）方法对不同形状的弹丸超高速碰撞形成的碎片云特性作了模拟分析,给出了靶孔直径和碎片云宽度随碰撞速度的变化规律、同一速度下不同形状的弹丸累积碎片分布规律、同种弹丸不同速度下的累积碎片分布规律、弹丸初始半径范围内的碎片云无量纲向前总动量M_D/M₀随膨胀距离L_E的变化以及碎片云前端速度的变化规律等。通过数据拟合,进一步给出了累积碎片百分数与碰撞速度和碎片质量的近似函数关系,计算表明该函数关系与数值结果吻合的很好。