The features of cold spray nozzle design

This article presents the peculiarities of the supersonic nozzle design for the cold gas-dynamic spraying. The procedure to produce the high particle velocity by correct choice of the geometrical dimensions of the accelerating nozzles is described. Numerical and experimental research of wedge-shaped nozzles shows that there is a nozzle with its particular dimensions for a given type of particles that produces the maximum possible particle velocity at the moment of impact on a target surface.     

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.     

Numerical simulation of the cold gas dynamic spray process

A computational fluid dynamic (CFD) model of the cold gas dynamic spray process is presented. The gas dynamic flow field and particle trajectories within an oval-shaped supersonic nozzle as well as in the immediate surroundings of the nozzle exit, before and after the impact with the target plane, are simulated. Predicted nozzle wall pressure values compare well with experimental data. In addition, predicted particle velocity results at the nozzle exit are in qualitative agreement with those obtained using a side-scatter laser Doppler anemometer (LDA). Details of the pattern of the particle release into the surroundings are visualized in a convenient manner. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Particle velocity and deposition efficiency in the cold spray process

Copper powder was sprayed by the cold gas-dynamic method. In-flight particle velocities were measured with a laser two-focus system as a function of process parameters such as gas temperature, gas pressure, and powder feed rate. Mean particle velocities were uniform in a relatively large volume within the plume and agreed with theoretical predictions. The presence of a substrate was found to have no significant effect on in-flight particle velocities prior to impact. Cold-spray deposition efficiencies were measured on aluminum substrates as a function of particle velocity and incident angle of the plume. Deposition efficiencies of up to 95% were achieved. The critical velocity for deposition was determined to be about 640 m/s for the system studied.     

Effect of processing conditions on porosity formation in cold gas dynamic spraying of copper

The cold gas dynamics process is a promising low-temperature spray process in which particles are accelerated in a supersonic flow before impacting with substrate to be coated. In this study the effect of spray temperature, spray pressure, and particle size on porosity formation in cold spray coatings are investigated. Results show that an increase in spray temperature and a decrease in particle size lead to a decline in volume fraction of porosity. Furthermore, particle velocity and particle temperature are determined to be the significant parameters for elimination of porosity. A model is proposed for estimation of the volume fraction of porosity for alloy of this study.     

Measurements of cold spray deposition efficiency

An analytical model of the kinetics of coating formation during cold spray is presented. The model is used to correct experimental data on deposition efficiency. The experimentally observed values are shown to be affected by experimental conditions, such as the velocity of substrate motion, the number of passes, the mass of a single portion of powder, and the exposure time of a given surface section. It is noted that experimental conditions can exert a significant effect on the consequences of the high-speed interaction of particles with a substrate. Relations are suggested that allow one to correct the results of deposition efficiency determined experimentally and to avoid mistakes in interpreting the data obtained.     

Numerical simulation on syphonage effect of laval nozzle for low pressure cold spray system

Instead of injected by high pressure powder feeder, powders can be drawn into the nozzle by syphonage effect generated by supersonic gas flow in low pressure cold spray. This characteristic makes low pressure cold spray conveniently for on-site operation. However, no data have ever been reported on the relationship between the nozzle structures and the gas flow in the powder feeder pipe. In this paper, a CFD software (STAR CCM+) was used to calculate the gas flow in nozzle of the DYMET 413 commercial low pressure cold spray system. Variation of structures and process parameters based on the commercial system were also investigated. The syphonage effect is strongly influenced by the powder feeding location, the temperature and pressure in prechamber has little effect on syphonage effect in powder feeder pipe. The syphonaged gas will decelerate the gas velocity and low down the gas temperature in nozzle, so it is best to control the mass flow rate of powder feeding gas by selecting the location. One of the disadvantages is that the particles will collide with the nozzle wall which makes the nozzle a short service life.     

Interface material mixing formed by the deposition of copper on aluminum by means of the cold spray process

The objective of this article is to present microstructural evidence of a bonding mechanism between copper, which has been deposited by the cold spray process, and an aluminum substrate. Deposition conditions are varied to determine their effects on the nature of the bond. Mechanical measurements, such as adhesion strength and hardness, as well as visual methods are used to characterize the process. A ballistic model is proposed to explain the process.     

Evaluation of coatings produced via kinetic and cold spray processes

An analysis of physical and mechanical properties of coatings produced by kinetic and cold spray processes is presented. Adhesion, hardnesses, porosities, critical velocities, and other properties of aluminum and copper coatings from both spray methods are analyzed and discussed, including scanning electron microscopy and optical micrographs. Similarities and differences between each of the coating methods and their effects on the resulting coatings are presented. A brief history and discussion of the bonding mechanisms for the larger particle coatings produced by the kinetic spray method is provided.     

金属材料喷雾淬火过程的界面热交换分析

通过末端淬火试验,对铝合金试样喷雾淬火过程的界面热交换进行了研究。采用反热传导法求解了所有试验的界面热流密度(q)和界面传热系数(h),重点分析了喷雾压力、喷嘴直径和试样表面粗糙度对界面热交换的影响。结果表明:喷射压力对整个淬火界面换热过程均有影响,但对过渡沸腾阶段影响更大,且喷射压力越大,q及其峰值q_max越大,进入核沸腾阶段的时间越短;喷嘴直径越大,q、q_max越大,越早进入核沸腾阶段,但增大喷嘴直径对界面换热的影响存在上限;随表面粗糙度增大,q、q_max先减小后增大;在本试验条件下,上述喷射压力、喷嘴直径和表面粗糙度对界面热交换的影响规律均不受另外两个参数取值的影响。此外,由于喷射的微小液滴均匀覆盖了整个热表面,产生了剧烈的核沸腾,导致在部分试验中,q曲线在核沸腾阶段出现了二次升高现象。     

New developments in cold spray based on higher gas and particle temperatures

In cold spraying, bonding is associated with shear instabilities caused by high strain rate deformation during the impact. It is well known that bonding occurs when the impact velocity of an impacting particle exceeds a critical value. This critical velocity depends not only on the type of spray material, but also on the powder quality, the particle size, and the particle impact temperature. Up to now, optimization of cold spraying mainly focused on increasing the particle velocity. The new approach presented in this contribution demonstrates capabilities to reduce critical velocities by well-tuned powder sizes and particle impact temperatures. A newly designed temperature control unit was implemented to a conventional cold spray system and various spray experiments with different powder size cuts were performed to verify results from calculations. Microstructures and mechanical strength of coatings demonstrate that the coating quality can be significantly improved by using well-tuned powder sizes and higher process gas temperatures. The presented optimization strategy, using copper as an example, can be transferred to a variety of spray materials and thus, should boost the development of the cold spray technology with respect to the coating quality. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Gas dynamic principles of cold spray

This paper presents an analytical model of the cold-spray process. By assuming a one-dimensional isentropic flow and constant gas properties, analytical equations are solved to predict the spray particle velocities. The solutions demonstrate the interaction between the numerous geometric and material properties. The analytical results allow determination of an optimal design for a cold-spray nozzle. The spray particle velocity is determined to be a strong function of the gas properties, particle material density, and size. It is also shown that the system performance is sensitive to the nozzle length, but not sensitive to the nozzle shape. Thus, it is often possible to use one nozzle design for a variety of operational conditions. Many of the results obtained in this article are also directly applicable to other thermal spray processes.     

Impact of high velocity cold spray particles

This article presents experimental data and a computational model of the cold spray solid particle impact process. Copper particles impacting onto a polished stainless steel substrate were examined in this study. The high velocity impact causes significant plastic deformation of both the particle and the substrate, but no melting was observed. The plastic deformation exposes clean surfaces that, under the high impact pressures, result in significant bond strengths between the particle and substrate. Experimental measurements of the splat and crater sizes compare well with the numerical calculations. It was shown that the crater depth is significant and increases with impact velocity. However, the splat diameter is much less sensitive to the impact velocity. It was also shown that the geometric lengths of the splat and crater scale linearly with the diameter of the impacting particle. The results presented will allow a better understanding of the bonding process during cold spray.     

Analysis of tantalum coatings produced by the kinetic spray process

Tantalum (Ta) coatings have been produced using a relatively new process, kinetic spray. Ta starting powders having particle diameters greater than 65 μm are injected into a de Laval-type nozzle, entrained in a supersonic gas stream, and accelerated to high velocities due to drag effects. The particles’ kinetic energy is transformed via plastic deformation into strain and heat on impact with the substrate surface. Particles are not thermally softened or melted, producing relatively low oxide, reduced residual stress, high adhesion and low porosity coatings. Analysis of the mechanical and physical properties of these Ta coatings demonstrated increasing hardness, cohesive adhesion, and decreasing porosity as a function of particle velocity. Comparison between kinetically sprayed coatings and coatings produced using conventional coating methods will be discussed.     

Investigation and characterization of Cr3C2-based wear-resistant coatings applied by the cold spray process

The purpose of this study was to explore the potential of the cold spray (CS) process in applying Cr₃C₂-25wt.%NiCr and Cr₃C₂-25wt%Ni coatings on 4140 alloy for wear-resistant applications. This article discusses the improvements in Cr₃C₂-based coating properties and microstructure through changes in nozzle design, powder characteristics stand off distance, powder feed rate, and traverse speed that resulted in an improved average Vickers hardness number comparable to some thermal spray processes. Cold spray process optimization of the Cr₃C₂-based coatings resulted in increased hardness and improved wear characteristics with lower friction coefficients. The improvement in hardness is directly associated with higher particle velocities and increased densities of the Cr₃C₂-based coatings deposited on 4140 alloy at ambient temperature. Selective coatings were evaluated using x-ray diffraction for phase analysis, optical microscopy (OM). and scanning electron microscopy (SEM) for microstructural evaluation, and ball-on-disk tribology experiments for friction coefficient and wear determination. The presented results strongly suggest that cold, spray is a versatile coating technique capable of tailoring the hardness of Cr₃C₂-based wear-resistant coatings on temperature sensitive substrates.     

真空退火处理对冷喷涂铝基复合材料组织与性能的影响

采用冷喷涂增材制造工艺制备了Al-25Al2O3、Al-50Al2O3和Al-75Al2O3(体积分数,％)具有不同体积含量Al2O3颗粒的铝基复合材料,并采用SEM、EBSD、硬度测试和拉伸测试等测试方法分析了真空退火处理对冷喷涂铝基复合材料的微观结构和力学性能的影响.结果表明,Al2O3颗粒的加入增加了冷喷涂增材材...     

Characterization of Cold Spray Titanium Supersonic Jet

Titanium is widely used in aerospace, highly corrosive environments, and implants due to unique properties such as high strength to weight ratio and excellent corrosion resistance. Cold gas dynamic spray (cold spray) technology, in contrast to current fabrication technologies, has provided the potential for titanium to be utilized in broader industrial applications and at lower cost. Particle velocity is the most important parameter in the cold spray process that leads to successful deposition of titanium at supersonic speeds. In this study, particle image velocimetry (PIV) is utilized to characterize supersonic flow field for a commercially pure (CP) titanium powder. The results represent experimentally determined velocity for titanium particles under supersonic conditions with respect to propellant gas, spray temperature, and stagnation pressure. The high velocity flow region outside of the cold spray nozzle was significantly extended using helium. An increase in stagnation temperature results in a high velocity region close to the axis of the cold spray nozzle. In contrast, an increase in pressure expands the high velocity regions in the cold spray plume. The PIV that is a whole-flow-field process is a practical characterization technique for optimization of parameters and validation of the future models for the cold spray process.     

Arc spray process for the aircraft and stationary gas turbine industry

Technological advances in arc spray have produced a system that competes favorably with other thermal spray processes. In the past, arc spray was thought of as a process for very large parts that need thick buildups. However, an attachment device known as the arc jet system has been developed that focuses the pattern and accelerates the particles. This attachment device, coupled with the in-troduction of metal-cored wires that provide the same chemistries as plasma-sprayed powders, pro-vides application engineers with a viable economic alternative to existing spray methods. A comparative evaluation of a standard production plasma spray system was conducted with the arc spray process using the attachment device. This evaluation was conducted by an airline company on four major parts coated with nickel-aluminum. Results show that, for these applications, the arc spray process offers several benefits.     

往复式喷射沉积管坯制备中喷射高度的闭环控制

分析了往复式喷射沉积制备大壁厚管坯的工艺原理,研究了喷嘴喷射高度在线检测及闭环控制方法及技术。喷射高度控制系统包括漏包提升执行机构、沉积层厚度在线测量、基于PLC的喷嘴高度控制。提升执行机构采用伺服电机驱动的丝杠螺母机构,针对沉积层间断增长的特点,采用间断提升控制方式;研究了沉积层厚度在线测量方法,分析了收集基底形状误差对测量及控制精度的影响并提出多点测量方案。理论分析表明,采用三点测量法可消减基底形状误差的影响。对不同内径及壁厚的管坯进行了喷射实验,喷射高度累积误差低于5%,较好地满足了大壁厚管坯制备对稳定的喷射高度的要求。