Examination of substrate surface melting-induced splashing during splat formation in plasma spraying

Impacting of a molten droplet with a melting point much higher than the substrate results in melting of the substrate around the impact area. Melting of the substrate surface to a certain depth alters the flow direction of the droplet. The significant change of fluid flow direction leads to the detaching of fluid from the substrate. Consequently, splashing occurs during the droplet-spreading process. In the current study, molybdenum (Mo) splats were formed on a stainless steel substrate under different plasma-spraying conditions. For comparison, Mo splats were also deposited on an Mo surface. The substrate surface was polished prior to deposition. The powders used had a narrow particle size distribution. The results show that the morphology of splats depends significantly on the thermal interaction between the molten particle and the substrate. The splat observed was only a central part of an ideal disk-like complete splat. The typical pattern of Mo splats was of the split type, presenting a small split structure on the surface of the stainless steel substrate. With Mo particles, the preheating of a steel substrate has no effect on splat morphology. On the other hand, a disk-like Mo splat with a reduced diameter of a dimple-like structure at the central area of the splat was formed on Mo substrates, and splashing can be suppressed through substrate preheating. Based on the experimental results, a surface melting-induced splashing model was proposed to explain the formation mechanism of the Mo splat on a steel surface. The influence of droplet condition on splat formation is discussed. 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.     

Numerical simulation of splat formation dynamics in plasma spray forming

Plasma spray forming shows overwhelming advantages in rapid fabricating parts and moulds. The coating microstructure is strongly dependent on the splat morphology and inter-splat contact nature. Therefore, it is necessary to investigate the splat formation mechanism in order to analyze the coating properties. A dynamical process of a single fully molten droplet impacting onto a smooth surface was investigated. At the same time, the interaction between the two molten droplets in the horizontal and vertical directions was also simulated. Finally, the simulations of impact of a molten droplet on an inclined plane and a sharp edge were presented. It is concluded that the relative distance of the two droplets strongly influences the dynamics of the two droplets interaction. The various surface conditions have direct effects on the dynamics of splat formation. When a droplet impacts onto an inclined surface and a sharp edge, the splat morphology changes obviously and the phenomenon of break up is observed.     

Review of impact and solidification of molten thermal spray droplets

The unique properties of coatings created by thermal spray deposition depend on the rapid solidification of individual splats created by impinging molten droplets. However, the impact process has been little studied because of the difficulty of measuring or numerically simulating the process, which occurs very quickly over a small area. Other scientific fields have investigated the impact of liquid droplets on solid surfaces. This paper reviews these studies, along with those conducted specifically on the thermal spray process. Modelers have almost universally ignored droplet solidification during impact; however, some experimental evidence suggests that the solidification process plays a significant role in splat formation. Splashing of impacting liquid droplets, another topic that has been largely ignored, affects deposition efficiency, porosity, and bond strength, and may also affect the amount of oxides incorporated in the coating. The scaling of data from impacting millimeter-size droplets traveling at low velocities to thermal spray conditions is questioned.     

Transient contact pressure during flattening of thermal spray droplet and its effect on splat formation

The flattening process of an isothermal droplet impinging on flat substrate in plasma spraying is studied numerically using “Marker-And-Cell” technique that enables the evolution of the droplet/substrate dynamic contact pressure. The distributions of the pressures upon substrate surface during flattening are calculated under different droplet conditions. The correlation of the distribution of the peak contact pressure along substrate surface with the observed splat morphology is examined. The results show that the transient contact pressure is initially high and concentrates at a small contacting area and then spreads quickly with droplet flattening. The maximum pressure is located at the front of the droplet at an early stage of deformation, which drives the fluid moving quickly along substrate and results in lateral flow. The contact pressure is mainly associated to droplet density and velocity. The peak pressure reduces monotonically with flattening and becomes negligible at the region where the flattening degree is larger than 2. The magnitude of the pressure resulting from evaporating gas by rapid heating of the adsorbed water on the substrate surface is comparable to that of the dynamic contact pressure at the region where the flattening degree ranges from 1.5 to 2. It is suggested that the reduced contact pressure at the late stage of spreading and disturbance by the evaporation-induced pressure resulting from rapid heating of the surface adsorbents by flattening droplet may contribute significantly to the splashing of flattening droplet and the formation of a reduced disk-like splat.     

Predicting splat morphology in a thermal spray process

Splats formed during a thermal spray process may be either highly fragmented or intact and disk-like. To predict this change in splat morphology, a dimensionless solidification parameter (Θ), which takes into account factors such as the particle diameter and velocity, substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, has been defined. Θ is the ratio of the thickness of the solid layer formed in the splat while it is spreading, to the splat thickness. The value of Θ can be calculated from simple analytical models of splat solidification and spreading. If solid layer growth is very slow (Θ ? 1), the splat spreads out to a large extent. Once it reaches maximum spread, it becomes so thin that it ruptures, producing fragmented splats. If, however, the solid layer thickness is significant (Θ ∼ 0.1-0.3), the splat is restricted from spreading too far and does not become thin enough to rupture, resulting in disk splats. When solid layer growth is rapid (Θ > 0.3), it obstructs liquid from flowing outward during droplet impact, producing splats with fingers around their periphery. Predictions from the model are compared with experimental data and found to agree well.     

铸钢件内孔等离子弧喷焊修复技术

材料为ZG310-570的铸钢件因承受冲击载荷的频繁作用导致其内孔磨损失效,是该类铸钢件最主要的失效形式。利用粉末等离子弧喷焊技术对铸钢件受损内孔进行修复,并利用火花直读光谱仪、金相显微镜、布氏硬度计、冲击试验机和扫描电子显微镜分别对喷焊层外观质量、显微组织和力学性能进行分析。分析结果表明,采用粉末等离子弧喷焊修复方法使铸钢件内孔尺寸得到恢复;喷焊层与基体为冶金结合,结合线清晰;喷焊层的硬度值为267.5 HBW,优于基体材料的180.2 HBW,修复后的内孔冲击吸收功为15.40 J,满足技术要求;喷焊层的断口存在大量撕裂,说明喷焊层具有一定的冲击韧性。     

等离子熔射过程中射流与粒子流的加热效应

采用CCD图像采集系统与图像处理技术提取等离子射流长度;以红外测温仪检测的单位时间内基体温度变化来衡量加热效应,研究不同熔射距离与射流长度条件下射流和粉末粒子流对基体的加热效应特点.结果表明,当熔射距离不大于射流长度时,基体温升主要来至于射流加热效应;随着熔射距离增大,射流对基体的加热效应迅速减弱;当熔射距离大于射流长度时,粒子流加热效应比较明显.提出射流长度可以作为合理选择熔射距离的特征评价指标,并通过不同熔射距离条件下熔射皮膜的截面尺寸以及形貌进行验证.     

Modeling of particles impacting on a rigid substrate under plasma spraying conditions

Finite-element methods have been applied for the spreading process of a ceramic liquid droplet impacting on a flat cold surface under plasma spraying conditions. The goals of the present investigation are to predict the geometrical form of the splat as a function of process parameters, such as initial temperature and velocity, and to follow the thermal field developing in the droplet up to solidification. A nonlinear finite-element procedure has been extended to model the complex physical phenomena involved in the impact process. The dynamic motion of the viscous melt in the drops as constrained by elastic surface tensions and in interaction with the developing contact with the target has been coupled to transient thermal phenomena to account for the solidification of the material. A model is used to study the impact of spherical particles of liquid ceramic of given temperature and velocity on a flat, cool rigid surface. The deformation of the splat geometry as well as the evolution of the thermal field within the splat are followed up to the final state and require adaptive discretization techniques. The proposed model can be used to correlate flattening degrees with the initial process parameters.     

Real-time imaging of the plasma spray process—Work in progress

A LaserStrobe Control Vision system was employed to examine water-stabilized plasma (WSP), gas-sta-bilized plasma (GSP), and single-wire arc plasma (SWAP) technologies. Visualization of the plasma spray process in each of these technologies has been made possible, in some instances, for the first time. Parame-ter optimization for the three processes was accomplished. This technology has significantly added to the theoretical and scientific knowledge of plasma diagnostics and plasma processing. This summary of work in progress was presented at the NSF Design, Manufacturing and Industrial Innovation Grantees Meeting, San Di-ego, January 4–6,1995.     

One-dimensional mathematical model for selecting plasma spray process parameters

A simple, unified, one-dimensional model has been developed to relate the effects of plasma spray parameters on the temperature and velocity of the plasma and particles and on the void content in the coating. The torch, spray, and substrate regions in a plasma spray process were first modeled independently and then coupled so that the plasma and particle characteristics calculated in one region served as inputs for the subsequent region. Comparison of the model predictions with experimental data showed reasonable agreement. Deviations from the measured data were attributable to the simplifying assumptions used in modeling the different regions of the process. A parametric analysis of the unified one- dimensional model showed that, despite its simplicity, the model is well suited for optimizing process parameters in terms of particle type and size to obtain high- integrity coatings.     

Simultaneous independent measurement of splat diameter and cooling time during impact on a substrate of plasma-sprayed molybdenum particles

In thermal spray processes, the coating structure is the result of flattening and cooling of molten droplets on the substrate. The study of the cooling time and evolution of the splat size during impact is then of the highest importance to understand the influence of the spray parameters and substrate characteristics on the coating structure. Measurement of particle temperature during impact requires the use of a high-speed two-color pyrometer to collect the thermal emission of the particle during flattening. Simultaneous measurement of the splat size with this pyrometer is difficult since the size of the particle can change as it cools down. To measure the splat size independently, a new measurement technique has been developed. In this technique, the splat size is measured from the attenuation of the radiation of a laser beam illuminating the particle during impact. Results are presented for plasma-sprayed molybdenum particles impacting on a glass substrate at room temperature. It is shown that the molybdenum splat reaches its maximum extent about 2 μs after the impact. In this work, we show that this increase of the splat surface is followed by a phase during which the splat size decreases significantly during 2 to 3 μs.     

Effect of particle morphology on flow characteristics of a composite plasma spray powder

The effects of BaF₂-CaF₂ particle morphology on National Aeronautics and Space Administration (NASA) PS304 feedstock powder flowability were investigated, BaF₂-CaF₂ eutectic powders were fabricated by comminution (producing an angular morphology) and by gas atomization (producing a spherical morphology). The fluoride powders were added incrementally to the other powder constituents of the NASA PS304 feedstock, (Ni-Cr, Cr₂O₃, and Ag powders). A linear relationship between flow time and concentration of the BaF₂-CaF₂ powder was found. The flow of the powder blend with spherical BaF₂-CaF₂ was better than that with angular BaF₂-CaF₂. The flowability of the powder blend with angular fluorides decreased linearly with increasing fluoride concentration. However, the flow of the powder blend with spherical fluorides was independent of fluoride concentration. The results suggest that for this material blend, particle morphology plays a significant role in flow behavior, offering potential methods to improve powder flowability and enhance the commercial potential. These findings may be applicable to other difficult-to-flow powders such as cohesive ceramics.     

等离子弧喷焊颗粒增强铁基合金涂层组织的分析

采用等离子弧喷焊技术,利用后送粉装置在45钢表面制作了WC颗粒增强铁基合金复合涂层,并采用扫描电子显微镜(SEM)、X射线衍射仪(XRD)和能谱仪(EDS)分别对涂层组织结构及成分进行了分析.结果表明,WC颗粒主要分布在涂层顶部,在中部和底部有少量的WC,分布比较均匀;从涂层与基体的熔合区到涂层顶部分别出现了平面晶、胞状晶、树枝晶和等轴晶;涂层表面组织中化合物主要由WC,Fe23B6,Cr23C6,Cr7C3,W2B5,FeW2B2等组成;在喷焊过程中,WC发生了溶解,涂层顶部WC的溶解量比较大;从基体到涂层表面,硬度值基本呈上升趋势,最高达2 218 HV.     

Calibration of a two-color imaging pyrometer and its use for particle measurements in controlled air plasma spray experiments

Advances in digital imaging technology have enabled the development of sensors that can measure the temperature and velocity of individual thermal spray particles over a large volume of the spray plume simultaneously using imaging pyrometry (IP) and particle streak velocimetry (PSV). This paper describes calibration, uncertainty analysis, and particle measurements with a commercial IP-PSV particle sensor designed for measuring particles in an air plasma spray (APS) process. Yttria-stabilized zirconia (YSZ) and molybdenum powders were sprayed in the experiments. An energy balance model of the spray torch was used to manipulate the average particle velocity and temperature in desired ways to test the response of the sensor to changes in the spray characteristics. Time-resolved particle data were obtained by averaging particle streaks in each successive image acquired by the sensor. Frame average particle velocity and temperature were found to fluctuate by 10% during 6 s acquisition periods. These fluctuations, caused by some combination of arc instability, turbulence, and unsteady powder feeding, contribute substantially to the overall particle variability in the spray plume.     

Influence of particle parameters at impact on splat formation and solidification in plasma spraying processes

A measurement system consisting of two high- speed two- color pyrometers was used to monitor the flattening degree and cooling rate of zirconia particles on a smooth steel substrate at 75 or 150 °C during plasma spray deposition. This instrument provided data on the deformation behavior and freezing of a particle when it impinged on the surface, in connection with its velocity, size, and molten state at impact. The results emphasized the influence of temperature and surface conditions on particle spreading and cooling. When the substrate temperature was 150 °C, the splats had a perfect lenticular shape, and the thermal interface resistance between the lamella and the substrate ranged from 10^{− 7} to 10^{− 8} W/m² · K. The dependence of the flattening degree on the Reynolds number was investigated.     

等离子喷涂钼层片组织和晶体取向的EBSD分析

在抛光1Cr13不锈钢基底上采集了大气等离子喷涂的钼层片。利用台阶仪、扫描电子显微镜（SEM）、能谱仪（EDS）、电子背散射衍射（EBSD）等表征了层片的形貌、微观组织和晶体学取向分布。结果表明，钼层片呈“花瓣状”，并伴有铺展飞溅。基底在层片中心处因熔化/撞击形成“凹坑”，平均深度0.7μm。层片中心区域为细小的柱状晶，晶粒尺寸范围0.75~9.25μm，平均值为2.30μm；边缘区域为沿着径向拉长的晶粒，晶粒尺寸范围4.25~18.25μm，平均值8.50μm。层片边缘区域因翘曲，接触热阻较中心区域大，造成晶粒粗大。无论是层片中心区域还是边缘区域晶粒取向都随机分布，没有明显织构。     

Numerical simulation of impingement of molten Ti,Ni, and W droplets on a flat substrate

Thermal spraying has been widely applied to process thin protective coatings on preshaped parts and to manufacture metallic preforms of a variety of geometries. The quality of materials produced by thermal spraying depends critically on the impact conditions of the droplets. In the present study, the deformation behavior and interaction of molten droplets impinging onto a flat substrate during thermal spraying have been numerically simulated. The calculated results reveal that a droplet spreads uniformly in the radial direction during impingement and eventually forms a thin splat with final diameter and thickness up to 11.3 times and down to 0.02 times the impact diameter, respectively. The final splat diameter increases rapidly with increasing impact velocity and melt density or decreasing melt viscosity. For the processing conditions of interest, the final splat diameter and the spreading time may be approximated by correlations: d_s/do = l.04Re^0.2 and t_s/(do/uo) = 0.62Re^0.2, where ds/do is the dimensionless splat diameter; ts/(do/uo) is the dimensionless spreading time; and Re is the Reynolds number. A fully liquid droplet impinging onto a flat solid substrate leads to good contact between the splat and the substrate. Multiple fully liquid droplets striking simultaneously onto other flattening, fully liquid splats cause ejection and rebounding of the liquid, as well as formation of voids within the liquid.     

On the reproducibility of air plasma spray process and control of particle state

Controlling particle state is important to not only achieve the required microstructure and properties in coatings but also to clearly isolate and understand the role of other clusters of variables (such as the various substrate and deposition conditions) on the aforementioned attributes. This is important to design coatings for high performance applications and in the ongoing efforts toward achieving prime reliance. This study examines the variability in particle state and explores a few strategies to control them for improved reproducibility with the aid of in-flight particle and plume sensors. The particle state can be controlled by controlling the torch parameters or by directly controlling the particle state itself via feedback from particle and plume sensors such as DPV 2000 (Tecnar Automation Ltd, Quebec, Canada) and torch diagnostic system-spray plume trajectory sensor (TDS-SPT) (Inflight Ltd, Idaho Falls, ID). There exist at least a few control protocols to control the particle state (predominantly temperature and velocity) with judicious choice of critical parameters. In the present case particle state has been controlled by varying the critical torch parameters (primary gas flow and arc current) in a narrow range using 8% YSZ of angular morphology (fused and crushed) with 10–75 μm size distributions in conjunction with a N₂-H₂ laminar (nonswirl) plasma. Two important results emerge: (a) The particle state resulting from averaged individual particle measurements (DPV 2000) is surprisingly stable with variability in T<1% and variability in V of <4%. Ensemble approaches yield a somewhat higher variability (5% in temperature). Despite this the variability in basic coating attributes such as a thickness and weight is surprisingly large. (b) Applying a much simpler control strategy to only control the particle injection and hence the particle trajectory results in reduced variability in coating attributes. 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.     

大功率等离子喷涂枪的设计

针对等离子喷涂大功率化的要求，设计了大功率等离子喷涂枪，利用二次喷嘴，功率可达100kW以上，喷嘴管型采用拉筏儿管，使等离子焰流速度达到超音速，出口的焰流为匀直流，利用双水内冷，分别调节钨极与喷嘴的冷动水流量，从而控制等离子流的热焓，有效提高了能源利用率，延长了喷枪的使用寿命。     

基于SoC的等离子体喷涂送粉器设计

在此提出一种新的设计方法，能够实时在线改变送粉器粉末配比，通过称重传感器检测粉末质量来保证配比精度，实现配比比例与配比的粉末种类，性质无关。该送粉系统尤其适合于小批量复合材料的试制，为进一步的实验研究打下了良好的基础。