Examination on Substrate Preheating Process in Cold Gas Dynamic Spraying

Substrate preheating always takes an important role in particle bonding and formation of the first layer coating in cold gas dynamic spraying (CGDS). In this study, a systemic investigation on substrate preheating process is conducted with Cu, Al, Steel, and Ti substrate by both numerical and experimental methods. The computational fluid dynamic (CFD) approach is adopted to simulate the heat exchange process between gas and solid substrate. The numerical results show that substrate can be significantly preheated by the high-temperature gas, especially by the gas at the near-wall zone behind the bow shock where the temperature is extremely high. Moreover, the comparison between different substrates implies that substrates with smaller thermal conductivity can achieve higher surface temperature and larger temperature gradient which may greatly contribute to the generation of residual stress, such as Ti substrate in this study. For the heat flux, Cu substrate obtains the largest value at the center zone of the substrate surface, followed by Al, Steel, and Ti substrate, but at the outer zone, the heat flux through the Cu substrate surface is smaller than the other three types of substrates. Besides, based on the experimental results, it is found that the substrate surface temperature amounts to the peak value only when the preheating time is sufficiently long.     

Impact Conditions for Cold Spraying of Hard Metallic Glasses

As compared to thermal spray techniques, cold spraying allows to retain metastable phases of the feedstock material like amorphous structures, due to lower process gas temperatures. Compared to crystalline metals, metallic glasses are brittle at ambient temperature but viscous at higher temperatures. Therefore, cold spray parameters must be optimized for conditions that allow softening of the amorphous spray material for successfully producing coatings. For this study, a FeCoCrMoBC metallic glass was used that in comparison to others offers advantages with respect to higher hardness, less costly feedstock powder, and minimum reactivity with the environment. Necessary impact conditions were investigated to meet the window of deposition in cold gas spraying. According to calculations and cold spray experiments, neither the glass transition temperature T _g nor the melting temperature T _m can describe required conditions for bonding. Thus, a so called softening temperature between the glass temperature and the melting temperature had to be defined to calculate the critical velocity of metallic glasses. With respect to the bonding mechanism, impact morphologies could prove that a transition to viscous flow gets more prominent for harsher spray conditions. By sufficiently exceeding the critical condition for bonding, coatings with rather dense microstructures can be processed at deposition efficiencies of about 70%. The coatings have a hardness of 1100?HV 0.3, but the results also demonstrate that further work is still needed to explore the full potential for bulk metallic glasses.     

Calculation of Particle Parameters for Cold Spraying of Metal-Ceramic Mixtures

Formation of metal-ceramic composite coatings by cold spray is one of the major directions in the development and application of the technology. As experiments showed, addition of a hard ceramic component into the mixture can shift the transition from substrate erosion to particles adhesion closer to adhesion. This effect may be induced by ceramic particles which not only erode, but also activate the target surface. Velocity and temperature of particles at their high-velocity impact onto the substrate are governing parameters in particles/substrate interaction. These parameters influence both the process of metal particles deposition and the process of erosion/activation of the substrate surface by ceramic particles. Metallic and ceramic particles collide with each other in the gas stream. These collisions can produce preactivation effect on metal particles by cleaning their surface. The level of activation depends on a typical velocity of collision which is the difference between velocities of metal and ceramic particles. Parameters of metallic and ceramic particles in the gas stream are estimated. Calculations show that components of mixtures with fine abrasive particles have greatly different velocities that influences preactivation of metal particles. At the same time, the substrate surface is activated by fine abrasive particles characterized by a high-impact velocity.     

Modeling Aspects of High Velocity Impact of Particles in Cold Spraying by Explicit Finite Element Analysis

In this study, an examination of cold spray particle impacting behavior using the ABAQUS/Explicit program was conducted for typical copper material (OFHC). Various combinations of calculation settings concerning element type, Arbitrary Lagrangian Eulerian adaptive meshing, contact interaction, material damage, etc., were examined with the main focus on the element excessive distortion and its effect on the resultant output. The effect of meshing size on the impact behavior was also clarified compared to the previous results obtained by using the LS-DYNA code. Some fundamental aspects on modeling of cold spray particle deformation are discussed.     

Cold Spraying of Armstrong Process Titanium Powder for Additive Manufacturing

Titanium parts are ideally suited for aerospace applications due to their unique combination of high specific strength and excellent corrosion resistance. However, titanium as bulk material is expensive and challenging/costly to machine. Production of complex titanium parts through additive manufacturing looks promising, but there are still many barriers to overcome before reaching mainstream commercialization. The cold gas dynamic spraying process offers the potential for additive manufacturing of large titanium parts due to its reduced reactive environment, its simplicity to operate, and the high deposition rates it offers. A few challenges are to be addressed before the additive manufacturing potential of titanium by cold gas dynamic spraying can be reached. In particular, it is known that titanium is easy to deposit by cold gas dynamic spraying, but the deposits produced are usually porous when nitrogen is used as the carrier gas. In this work, a method to manufacture low-porosity titanium components at high deposition efficiencies is revealed. The components are produced by combining low-pressure cold spray using nitrogen as the carrier gas with low-cost titanium powder produced using the Armstrong process. The microstructure and mechanical properties of additive manufactured titanium components are investigated.     

高速电弧喷涂雾化过程的数值模拟

建立了高速电弧喷涂枪喷嘴结构气体流动的三维CFD模型,模拟了喷嘴的自由射流、以及气流喷向基体时的射流分布特征.在此基础上,计算了雾化熔滴在气体射流作用下的飞行轨迹,并通过对比实验测量了喷涂粒子的雾化性能.数值模拟结果显示,气体射流并非呈完全的轴对称分布,在丝材相交的平面和垂直于该平面的方向上,气流速度分布差别明显;在距基体约25 mm的射流中心附近位置,气体速度迅速下降,这为雾化熔滴的高速沉积提供了条件.另外,雾化粒子的数值模拟与实验结果相一致,都呈现出了中间高边缘低的速度分布特征.     

真空冷喷涂铜颗粒加速特性数值研究

基于Fluent气固两相流数值模拟,研究真空冷喷涂铜颗粒的加速特性,分析了环境压力、喷涂距离、入口总温和颗粒粒径等参数对真空环境下颗粒撞击速度的影响。结果表明:环境压力是决定颗粒撞击速度的关键因素,随环境压力的变化,小直径颗粒(dp≤1μm)撞击速度的变化曲线呈抛物线状态,但大直径颗粒无显著变化;采用合适的喷涂距离,才能获得最大的颗粒撞击速度;增加入口总温可提高小直径颗粒的撞击速度,但对大直径颗粒无明显加速效果;真空冷喷涂颗粒的尺寸可从微米级减小至亚微米级,但过小的颗粒仍难达到足够高的撞击速度。     

电火花喷涂技术应用与工艺方法研究

为了提高某些重要零件或有特殊要求的零部件的硬度、耐磨性、红硬性等力学性能,或改变零件表面的合金成分和组织结构.用电火花喷涂技术使得一种硬度较低的导电材料附着于另外一种硬度较高的导电材料表面,从而改变或提高硬度较高的导电材料的耐磨性或其他力学性能和物理性能.     

基于材料性能的冷喷涂临界速度预测窗口研究取得新进展

正冷喷涂过程中,喷涂粒子被高速气流加速到较高的速度(200~1 200m/s),在固态下碰撞基体,通过粒子强烈的塑形变形沉积在基体上形成涂层。由于喷涂材料和碰撞速度不同,粒子或者从基体上反弹或者沉积于基体上,使得粒子开始沉积到基体上的速度被称为临界速度,它是冷喷涂技术的一个     

A Generalized Critical Velocity Window Based on Material Property for Cold Spraying by Eulerian Method

In this paper, the previously developed Eulerian model (Yu et al., J Therm Spray Technol 21(3):745-752, 2012), which could well predict the critical velocity and erosion velocity, was extended to other commonly used materials such as aluminum, iron, nickel, stainless steel 316, and Inconel718 for studying the influence of material property and establishing a generalized window of critical velocity. Results show that the deformation behavior of the used materials could be classified as coordinated deformation (copper, iron, nickel) and uncoordinated deformation patterns (aluminum, stainless steel, and Inconel718). However, it was found that the steady maximum equivalent plastic strain values at the critical velocity for each material concentrate in the extent of 2.6-3.0 regardless of deformation pattern. Dimensionless analysis shows that, the calculated critical velocity increases with the increase of material characteristic velocity, and this relationship can be primarily used to predict the critical velocity.     

Effect of Substrate and Process Parameters on the Gas-Substrate Convective Heat Transfer Coefficient During Cold Spraying

The final quality of cold-sprayed coatings can be significantly influenced by gas-substrate heat exchange, due to the dependence of the deposition efficiency of the particles on the substrate temperature distribution. In this study, the effect of the air temperature and pressure, as process parameters, and surface roughness and thickness, as substrate parameters, on the convective heat transfer coefficient of the impinging air jet was investigated. A low-pressure cold spraying unit was used to generate a compressed air jet that impinged on a flat substrate. A comprehensive mathematical model was developed and coupled with experimental data to estimate the heat transfer coefficient and the surface temperature of the substrate. The effect of the air total temperature and pressure on the heat transfer coefficient was studied. It was found that increasing the total pressure would increase the Nusselt number of the impinging air jet, while total temperature of the air jet had negligible effect on the Nusslet number. It was further found that increasing the roughness of the substrate enhanced the heat exchange between the impinging air jet and the substrate. As a result, higher surface temperatures on the rough substrate were measured. The study of the effect of the substrate thickness on the heat transfer coefficient showed that the Nusselt number that was predicted by the model was independent of the thickness of the substrate. The surface temperature profile, however, decreased in increasing radial distances from the stagnation point of the impinging jet as the thickness of the substrate increased. The results of the current study were aimed to inform on the influence and effect of substrate and process parameters on the gas-substrate heat exchange and the surface temperature of the substrate on the final quality of cold-sprayed coatings.     

On Parameter Selection in Cold Spraying

For cold spraying, a method for the construction of the window of deposition and the selection of optimum process parameters is presented. Initially, particle impact velocity and the critical particle velocity for bonding are worked out and expressed explicitly in terms of key process and material parameters. Subsequently, the influence of particle velocity on coating characteristics is examined in view of the results of experiments and simulations. It has been found that main coating characteristics can be described as a unique function of the ratio of particle velocity to critical velocity, here referred to as ??. Finally, coating properties are linked directly to primary process parameters via parameter selection maps, where contours of constant ?? are plotted on a plane of gas temperature versus gas pressure. Inferences of the presented method and the resulting parameter selection maps are discussed for the example of copper as feedstock material.     

Shock-Wave-Induced Spraying: Modeling and Physics of a New Spray Process

A two-dimensional axisymmetric transient model for the shock-wave-induced spraying process (SISP) is developed. SISP is a new cold spray process used to apply coatings of various metallic materials onto a wide range of different substrates. The model is validated with reference to a simplified one-dimensional approximation of the flow field. The model solves equations for mass, momentum, energy, ideal-gas law, as well as turbulence. The valve is represented as a ball-seat-type valve. The results are presented as contours of flow variables in a space-time domain. Values of pressure, axial velocity, Mach number, as well as static and total temperature are examined. The effects of varying supply pressure and temperature on these flow variables are investigated in detail. Additionally, air and helium are compared as the driving gas.     

A Morphological Approach to the Modeling of the Cold Spray Process

A coating buildup model was developed, the aim of which was simulating the microstructure of a tantalum coating cold sprayed onto a copper substrate. To do so, first was operated a fine characterization of the irregular tantalum powder in 3D, using x-ray microtomography and developing specific image analysis algorithms. Particles were grouped by shape in seven classes. Afterward, 3D finite element simulations of the impact of the previously observed particles were realized. To finish, a coating buildup model was developed, based on the results of finite element simulations of particle impact. In its first version, this model is limited to 2D.     

计算冷挤压力的模拟试验法

提出了一种估算冷挤压力的方法－模拟试验法。文中阐述了这种方法的原理和试验过程，表明在冷挤压中用模拟方法预测挤压成型压力是很有用的。