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.     

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.     

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.     

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.     

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.     

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。     

Assessment of CFD Modeling via Flow Visualization in Cold Spray Process

The two-phase flow properties of copper particle laden nitrogen are computationally modeled and compared with the data obtained from the experiments, determining the achievable degree of consistency between model and reality. Two common, commercial nozzles are studied. A two-way coupled Lagrangian scheme along with the RSM turbulence model is used to track the particles and to model the interactions between the gas and the particulate phase. Significant agreement is found for the geometrical gas flow structure, the resulting particle velocities, and the dependence of the two-phase flow on the particulate phase mass loading. The particle velocities decrease with increasing mass loading, even for modest powder feed rates of <3 g/s. The velocity drop occurs even when the gas flow rate is kept constant. Adiabatic gas flow models neglecting the energy consumption by the particles are thus inaccurate, except for very dilute suspensions with low technical relevance. For the cases modeled, the experiments evidence the high predictive power of the chosen CFD approach.     

Method and Process Development of Advanced Atmospheric Plasma Spraying for Thermal Barrier Coatings

Over the last few years, global economic growth has triggered a dramatic increase in the demand for resources, resulting in steady rise in prices for energy and raw materials. In the gas turbine manufacturing sector, process optimizations of cost-intensive production steps involve a heightened potential of savings and form the basis for securing future competitive advantages in the market. In this context, the atmospheric plasma spraying (APS) process for thermal barrier coatings (TBC) has been optimized. A constraint for the optimization of the APS coating process is the use of the existing coating equipment. Furthermore, the current coating quality and characteristics must not change so as to avoid new qualification and testing. Using experience in APS and empirically gained data, the process optimization plan included the variation of e.g. the plasma gas composition and flow-rate, the electrical power, the arrangement and angle of the powder injectors in relation to the plasma jet, the grain size distribution of the spray powder and the plasma torch movement procedures such as spray distance, offset and iteration. In particular, plasma properties (enthalpy, velocity and temperature), powder injection conditions (injection point, injection speed, grain size and distribution) and the coating lamination (coating pattern and spraying distance) are examined. The optimized process and resulting coating were compared to the current situation using several diagnostic methods. The improved process significantly reduces costs and achieves the requirement of comparable coating quality. Furthermore, a contribution was made towards better comprehension of the APS of ceramics and the definition of a better method for future process developments.     

Modeling of Thermal Spraying Heat Transfer Processes by Exodus Stochastic Method

This article deals with the application of the stochastic Exodus method for modelling of thermal spraying heat transfer processes and for solving direct and indirect problems. The Exodus stochastic method has an advantage in straightforward solving of the transient inverse heat transfer multi-dimensional problems over other methods based on iterative fittings procedures used for example by finite element methods (FEM). Theoretical background of the method is introduced. Application capabilities of the method are shown on the example of high velocity oxygen fuel thermal spraying heat transfer process analysis. Comparisons with results of FEM computational method application are presented.     

镁合金表面冷喷涂技术研究进展

归纳和总结了国内外镁合金冷喷涂技术最新研究进展,介绍了镁合金冷喷涂Al-Zn、纯Al、Al基复合涂层、Al基非晶涂层、Zn基合金、Ni、不锈钢涂层等的耐腐蚀性.讨论了工艺参数(如压力、温度)以及喷涂材料的选择,涂层对镁合金防腐蚀性能的影响,展望了未来镁合金在汽车、航空航天轻量化方面的发展.     

冷轧管孔型设计的简便方法

提出了设计冷轧管孔型的Ks计算方法。计算结果表明：与现有计算法对比,Ks计算法是可行的,并且能够控制最后小段的Ksn值（它与变形量的意义是等价的）。可在铝管冷轧厂推广使用该方法。