Available online at Materials and Design 29 (2008)

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1 Available online at Materials and Design 29 (28) Materials & Design Effect of standoff distance on coating deposition characteristics in cold spraying W.-Y. Li a, *, C. Zhang a,b, X.P. Guo a, G. Zhang a, H.L. Liao a, C.-J. Li b, C. Coddet a a LERMPS, Université de Technologie de Belfort-Montbéliard, Site de Sévenans, 91 Belfort Cedex, France b State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi an Jiaotong University, Xi an 7149, PR China Received 27 September 26; accepted 12 February 27 Available online 1 March 27 Abstract In this study, the effect of standoff distance on coating deposition characteristics in cold spraying was investigated by the experiment and numerical simulation of particle acceleration. Al, Ti and Cu powders of different sizes were used as feedstocks. It was found that the deposition efficiency was decreased with the increase of standoff distance from 1 mm to 11 mm for both Al and Ti powders used in this study. However, for Cu powders, the maximum deposition efficiency was obtained at the standoff distance of 3 mm, and then the deposition efficiency decreased with further increasing the standoff distance to 11 mm. The standoff distance had a little effect on coating microstructure and microhardness for these three powders. Both the stain-hardening effect of the deposited particles and the shot-peening effect of the rebounded particles take the roles in coating hardness. It was also found that the surface of substrate or previously deposited coating could be exposed to a relatively high gas temperature at a short standoff distance. Ó 27 Elsevier Ltd. All rights reserved. Keywords: Cold spraying; Standoff distance; Aluminum coatings; Titanium coatings; Copper coatings; Deposition efficiency 1. Introduction Cold spraying is an emerging coating process. In this process, spray particles are injected into a high speed gas jet in a de Laval type nozzle and accelerated to a high velocity (3 12 m/s). The deposition of particles takes place through the intensive plastic deformation upon impact in a solid state at a temperature well below the melting point of spray material. Consequently, the deleterious effects such as oxidation, phase transformation, decomposition, grain growth and other problems inherent to conventional thermal spraying processes can be minimized or eliminated [1,2]. Therefore, it becomes more and more attractive for its high deposition efficiency and volume production of many metallic coatings and composites [2 18], * Corresponding author. Tel.: ; fax: addresses: wenyali_cn@hotmail.com (W.-Y. Li), hanlin. liao@utbm.fr (H.L. Liao). and even cermets [1,19] and nanostructured coatings [19,2]. It has been widely accepted that particle velocity prior to impact is one of the most important parameters. Generally, for a given material, there exists a critical velocity resulting in a transition from erosion of the substrate to deposition of the particle. Only those particles achieving a velocity higher than the critical one can be deposited to produce a coating. It has been reported that the critical velocity was associated with properties of spray materials [2,3,12,13,16,17,21] and substrate [3 5], particle conditions prior to impact such as particle temperature [5,6,13,18], size [16,21,22] and its surface oxidation state [6]. As particle velocity is higher than the critical one, the deposition efficiency increases with increasing the particle velocity [2,3,7 1,13,14,17,18,23]. Consequently, in order to realize a high deposition efficiency, the majority of spray particles have to be accelerated to a velocity higher than the critical one /$ - see front matter Ó 27 Elsevier Ltd. All rights reserved. doi:1.116/j.matdes

2 298 W.-Y. Li et al. / Materials and Design 29 (28) According to the reported results obtained by both the experiment and numerical simulation, many factors influence the particle velocity in cold spraying, including nozzle geometry, accelerating gas conditions and properties of particles [3,7,13,14,16 18,22 31]. For a converging diverging nozzle, the increase of nozzle divergent section length will lead to the significant increase of particle velocity [18,25]. There exists an optimal expansion ratio (area ratio of nozzle throat to exit) for particle acceleration under different spray conditions owing to the presence of show waves outside the nozzle exit [11 13,16]. As the nozzle dimensions are fixed, with increasing either the gas temperature or pressure, particle velocity will be increased. When helium is used, the particle can reach to a higher velocity than that using nitrogen or air. Moreover, the particle velocity increases with the decrease of particle size and a higher velocity can be obtained for a particle of lower density under the same gas conditions. The previous study [25] and other results in the literature [7,1] also showed that when the nozzle dimensions are fixed, the standoff distance from nozzle exit to substrate influenced the particle velocity and thus the deposition efficiency [3,8]. However, there are few reports focusing on this issue. Therefore, in this study, the effect of standoff distance on coating deposition characteristics was investigated through both the experiment and numerical simulation of particle acceleration aiming at the optimization of cold spray process. 2. Numerical modeling Numerical modeling was performed by using a commercial software FLUENT (Ver. 6.1) to determine the flow field of driving gas inside and outside the nozzle, and subsequently the accelerating of particles in cold spraying. Due to the axisymmetric characteristic of flow in this study, a developed two-dimensional symmetrical model [11,25, 26,32] was adopted as shown in Fig. 1. The wall boundary is, by default in FLUENT, not moving and of a fixed heat flux of zero. The outside domain was a cylinder of 6 mm in radius and 2 mm in length from the nozzle exit as shown in Fig. 1. In some simulations, the substrate was also taken into consideration, which was a disc of 15 mm in radius and 3 mm in thickness. The dimensions of nozzle in simulation are determined according to the experiment nozzle used. The compressed air was taken as an ideal and compressible one. A coupled implicit method available in FLUENT was used to solve the flow field and the results in a steady state were obtained. The standard K e turbulence model was utilized in the simulation. The acceleration of particles was computed using discrete phase modeling (DPM) available in FLUENT [31]. In these simulations, the gas conditions are prescribed as those used in the experiment. As a first approximation, the powder particles are of spherical shapes. The initial temperature and axial velocity of particle are 27 C and 2 m/s, respectively. All the results illustrate the change of variables along the central axis of the nozzle. 3. Experimental procedures In this study, three types of powders, Al, Ti and Cu were used as feedstocks in order to investigate the effect of particle density on its acceleration at different standoff distances. The details of these three powders are given in Table 1. The morphologies of these three powders are shown in Fig. 2. The Al2319 ( 63 lm) and Cu ( 75 lm) powders are produced through a gas atomization process by LERMPS lab. (UTBM, France) and present a spherical morphology as shown in Figs. 2a and c. The commercially available Ti powder ( 325 mesh) is manufactured through a hydride dehydride (HDH) process by Northwest Institute for Non-ferrous Metal Research (Xi an, China) and exhibits an angular morphology as shown in Fig. 2b. The size distributions of these powders were characterized by a laser diffraction sizer (MASTERSIZER 2, Malvern Instruments Ltd., UK) as shown in Fig. 3. Mild steel plates of dimensions of mm were used as substrates and sandblasted using alumina grits prior to spraying. The cold spray system installed in LERMPS lab with a commercially available cold spray gun (CGT GmbH, Germany) was used for coating deposition. An optimal nozzle designed by LERMPS was employed in this study, which has a throat of 2.7 mm, an exit of 6 mm and a divergent section length of 17 mm according the previous study [11]. The high-pressure compressed air was used as the driving and powder carrier gases. The driving gas was operated at a pressure of 2.8 MPa and a temperature of about 52 C. The standoff distances from the nozzle exit to the substrate surface were 1, 3, 5, 7, 9, 11 mm. During spraying, the substrates were mounted on a specially designed fixture to deposit simultaneously the coatings at different standoff distances. The spray gun was mounted on a robot (ABB, Sweden) and moved with a traverse speed of 2 mm/s. Table 1 The details of Al, Ti and Cu powders used for cold spraying Powder Fabrication method Size distribution (lm) d(.1) d(.5) d(.9) Al2319 Gas atomization Ti HDH Cu Gas atomization Fig. 1. Schematic diagram of the computational domain and boundaries for gas flow and particle acceleration.

3 W.-Y. Li et al. / Materials and Design 29 (28) Volume fraction (%) 12 Al (34.6 μm) Ti (22.4 μm) 1 Cu (37.9 μm) Particle diameter (μm) Fig. 3. Size distributions of the Al, Ti and Cu powders used. Gas and particle velocity (m/s) V g Al 5μm 2μm 4μm 6μm X axial distance from nozzle exit (mm) Gas and particle velocity (m/s) V g Ti 5μm 2μm 4μm X axial distance from nozzle exit (mm) Fig. 2. Morphologies of (a) Al, (b) Ti and (c) Cu powders used. The weight gains of coatings were estimated through measuring the weights of samples before and after deposition. Relative deposition efficiency (RDE) was used to characterize the deposition efficiency (DE) under different conditions, which is defined as follows: W c RDE ¼ 1% ð1þ W c max where W c is the coating weight at certain standoff distance, W c max is the maximum weight among the coatings deposited at different standoff distances. The cross-sectional microstructure of the as-sprayed coatings was examined by optical microscope (OM) (Nikon, Japan). The microhardness of the coatings was tested by a Vickers harness indenter (Leitz, Germany) with a load of.981 N for 15 s. More than 15 values randomly tested in the polished cross-section were averaged to evaluate the coating hardness. Gas and particle velocity (m/s) V g Cu 2 5μm 6μm 2μm 8μm 4μm X axial distance from nozzle exit (mm) Fig. 4. Changes of gas and particle velocities along the nozzle axis without the presence of substrate for (a) Al, (b) Ti and (c) Cu powders under the spray conditions in this study.

4 3 W.-Y. Li et al. / Materials and Design 29 (28) Results and discussion 4.1. Effect of standoff distance on particle velocity Numerical simulations were conducted for Al, Ti and Cu particles of different sizes according to the spray conditions in this study. Fig. 4 shows the simulation results on the changes of gas and particle velocities along the nozzle without the presence of substrate for Al, Ti and Cu powders. It is seen that the gas velocity increases significantly after the nozzle throat. Some shock waves are generated near the nozzle exit indicated by the oscillation of gas velocity. And then, the gas velocity is decreased quickly after the shock waves at some standoff distance of about 6 mm. For the particles, their velocities also increase remarkably after the throat and are further accelerated to a maximum value outside the exit at some standoff distance ranging from 5 to 1 mm, which is dependent on the particle density and size. For the same powder, this optimal standoff distance increases with increasing the particle size. While for different powders of the same particle size, this optimal standoff distance is longer for the powder of higher density. These results suggest that for a lighter particle the optimal standoff distance is shorter, which is consistent with the reported results that the lighter particles are more readily influenced by the gas flow [25,29]. However, it should be pointed out that with this optimized nozzle, the velocity change of a particle, no matter the particle density and size, is not so significant at the standoff distances from to 1 mm as shown in Fig. 4. This fact could explain the evolution of coating microstructure and microhardness as function of the standoff distance, which will be discussed below. RDEs of Al, Ti and Cu powders on the standoff distance under the spray conditions in this study. It is found that for Al and Ti powders, RDE decreases obviously with increasing the standoff distance. Steenkiste et al. [3] reported the decrease of deposition efficiency for Al powder ( 45 lm) as the standoff distance changing from 19 to 38 mm. Karthikeyan et al. [8] also reported the decrease of deposition efficiency for Ti powder ( 4 mesh) as the standoff distance changing from 5 to 2 mm. While for Cu powder as shown in Fig. 5, RDE firstly increases 4.2. Effect of standoff distance on deposition efficiency After the experiment, the relative deposition efficiencies of different powders at different standoff distances were calculated using Eq. (1). Fig. 5 shows the dependence of Relative deposition efficiency (%) Air, 2.8MPa, 52 o C Al Ti Cu Standoff distance (mm) Fig. 5. Changes of relative deposition efficiencies of Al, Ti and Cu powders with standoff distance under the spray conditions in this study. Fig. 6. OM micrographs of Al coatings deposited at the standoff distances of (a) 1 mm, (b) 3 mm, (c) 5 mm, (d) 7 mm, (e) 9 mm, and (f) 11 mm.

5 W.-Y. Li et al. / Materials and Design 29 (28) to a maximum value at the standoff distance of about 3 mm and then decreases gradually. However, Steenkiste et al. [3] reported the decrease of deposition efficiency for Cu powder ( 325 mesh) as increasing the standoff distance from 19 to 38 mm. The previous study with bronze powder (Cu 8Sn, 25 lm) also showed that the deposition efficiency decreased gradually with increasing the standoff distance from 1 to 11 mm. Therefore, it can be considered that those differences are caused by the various acceleration behavior and critical velocities of different powders. For Al and Cu powders in this study, they are of the similar particle sizes but big difference in density. Therefore, when compared with Cu powder, RDE of light Al powder decreases more quickly with increasing the standoff distance. It is similar for Ti powder. On the other hand, for heavy Cu or bronze powders of different particle sizes, the particle size takes an important role in the particle acceleration and thus the deposition efficiency. With large Cu particles, the deposition efficiency increases firstly with increasing the standoff distance and then decreases with further increasing the standoff distance as shown in Fig. 5 in this study. Fig. 7. OM micrographs of Ti coatings deposited at the standoff distances of (a) 1 mm, (b) 3 mm, (c) 5 mm, (d) 7 mm, (e) 9 mm, and (f) 11 mm. Fig. 8. OM micrographs of Cu coatings deposited at the standoff distances of (a) 1 mm, (b) 3 mm, (c) 5 mm, (d) 7 mm, (e) 9 mm, and (f) 11 mm.

6 32 W.-Y. Li et al. / Materials and Design 29 (28) Effect of standoff distance on coating microstructure Figs. 6 8 show the cross-sectional OM micrographs of the as-cold-sprayed Al, Ti and Cu coatings under the spray conditions mentioned above and at different standoff distances. It is found that for Al and Ti coatings, the coating thickness decreases with the increase in the standoff distance. While for Cu coating, the thickest coating is that deposited at the standoff distance of 3 mm. These results are consistent with the observation of deposition efficiency in this study. On the other hand, it is found that the standoff distance has a little effect on coating microstructure except the coating thickness for three types of coatings. However, Al and Ti coatings present a porous structure, especially Ti coating, but Cu coating exhibits a dense structure. For Al and Ti coatings, the porosities are 1 3% and 9 18%, respectively. This porous structure for Al and Ti coatings have been reported in the literature [3,8,9,12]. Generally, the deformation extent of deposited particles accounts mainly for the porosity of cold sprayed coatings. The deformation extent of a particle is determined by its strength as well as its density, which will influence the kinetic energy of particle at the same velocity [12]. Therefore, it is difficult to form a dense Al coating due to its low density [3,12]. But for Ti and its alloy coatings, the recent results showed that the surface reactivity of Ti and its alloy powders takes an important role in the formation of this kind of porous coating [12]. The obvious flashing jet observed outside the nozzle exit during the experiment with Ti or its alloy powder suggests Microhardness (Hv.1 ) Al Ti Cu Standoff distance (mm) Fig. 9. Effect of standoff distance on the microhardness of Al, Ti and Cu coatings. that the powder particles react with the oxygen in the entrained air [12]. Therefore, the particles can be adhered together with a little contact area and without obvious plastic deformation thanks to the local melting of contact interface under high temperatures resulting from both the reaction and adiabatic shear impacting process [12]. This point of view can explain well why the coating was porous but still with a relatively high deposition efficiency [8,9,12] Effect of standoff distance on coating microhardness Fig. 9 shows the effect of standoff distance on the microhardness of the as-cold-sprayed Al, Ti and Cu coatings under the spray conditions mentioned above. It can be observed that the microhardness changes not so significantly with the increase in the standoff distance for these three coatings. It is also found that for Cu and Ti coatings the microhardness at the standoff distance of 7 mm is relatively high. These results are not consistent with the results of deposition efficiency. However, taking the velocity changes of in-flight particles into consideration as shown in Fig. 4, it can be considered that the particle impact velocity is attributed to the coating microhardness. Both the strain-hardening effect of the deposited particles in the coating and the shot-peening effect of the rebounded particles take the roles in coating hardness. Therefore, because the amount of particles is almost the same at different standoff distances, the coating harness changes little with the variation of the standoff distance Effect of standoff distance on gas temperature close to substrate According to the above results, the coating should be deposited at a shorter standoff distance for a higher deposition efficiency. However, for Cu coatings, it is found that their surfaces present different colors as shown in Fig. 1.It can be attributed to the different surface temperatures of coatings deposited at different standoff distances. A relatively high temperature causes the oxidation. It is more obvious at the standoff distance of 1 mm as shown in Fig. 1. Fig. 11 shows the simulation results on gas velocity and temperature distributions near nozzle exit both with and without the substrate. The substrate stands at 3 mm from nozzle exit. It is seen clearly that the gas velocity and tem- Fig. 1. Surface macrographs of Cu coating deposited at different standoff distances.

7 W.-Y. Li et al. / Materials and Design 29 (28) Fig. 11. Contours of gas velocity (a,b) and temperature (c,d) near the nozzle exit under the spray conditions in this study. (a,c) Without substrate; (b,d) with substrate at the standoff distance of 3 mm. Temperature ( C) Air, 2.8MPa, 52 o C T g T p, 4μm Cu Standoff distance (mm) Fig. 12. Effect of standoff distance on the calculated gas and Cu particle (4 lm) temperatures close to substrate under the spray conditions in this study. perature have been changed completely close to the substrate surface. There exists a region in the stagnation state, where the gas velocity is close to zero and the temperature is relatively high. The bow shock presents close to substrate surface, which was also observed by other researchers [27,29,3]. However, the previous study indicated that the presence of substrate has little effect on particle velocity and temperature as the particle size is larger than about 5 lm for Cu [25]. For the gas temperature, as shown in Fig. 12, it decreases significantly with increasing the standoff distance. That means the substrate surface or the previously deposited coating will be exposed to a higher temperature at a shorter standoff distance, which is proved by the experimental result in this study. Therefore, the selection of standoff distance should be careful with compromising the deposition efficiency and possible oxidation for temperature susceptive materials. 5. Conclusions According to the experiment and simulation results obtained in this study, the following conclusions can be drawn. (1) The deposition efficiency was decreased with the increase of standoff distance from 1 mm to 11 mm for both Al and Ti powders under the spray conditions in this study. For Cu powders, the maximum deposition efficiency was obtained at the standoff distance of 3 mm, and then the deposition efficiency decreased with further increasing the standoff distance. (2) With the increase of the standoff distance the coating thickness decreased for both Al and Ti coatings. For Cu coating, the thickest coating was obtained at the standoff distance of 3 mm. These results are consistent with the deposition efficiency. However, the standoff distance had a little effect on coating microstructure.

8 34 W.-Y. Li et al. / Materials and Design 29 (28) (3) The standoff distance had almost no influence on coating microhardness for these three coatings. This could be attributed to no significant change in the particle velocity outside the adopted nozzle. Both the stain-hardening effect of the deposited particles and the shot-peening effect of the rebounded particles control the coating microhardness. (4) At a short standoff distance, the substrate surface or previously deposited coating could be exposed to a relatively high gas temperature. Therefore, in cold spray practice, the selection of standoff distance should compromise between high deposition efficiency and low oxidation of coating for temperature susceptive materials. Acknowledgements This work was financially supported by Franche-Comte Regional Council of France. The authors thank Lucas Dembinski for the supply of Al and Cu powders. References [1] Papyrin A. Cold spray technology. 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