Int J Adv Manuf Technol (2012) 60:611–623 DOI 10.1007/s00170-011-3642-6
ORIGINAL ARTICLE
Surface roughness optimization of cold-sprayed coatings using Taguchi method Tarun Goyal & Ravinderjit Singh Walia & T. S. Sidhu
Received: 11 October 2010 / Accepted: 13 September 2011 / Published online: 27 September 2011 # Springer-Verlag London Limited 2011
Abstract In this paper, Taguchi L 18 orthogonal array have been employed for depositing the electro-conductive coatings by varying various process parameters, i.e., substrate material, type of powder feeding arrangement, stagnation gas temperature, stagnation gas pressure, and stand-off distance. The response parameter of the coatings so produced is measured in terms of surface roughness. The optimum process parameters are predicted on the basis of analyses (ANOVA) of the raw data and signal to noise ratio. The significant process parameters in order of their decreasing percentage contribution are: stagnation pressure, stand-off distance, substrate material, stagnation temperature of the carrier gas, and feed arrangement of the powder particles, respectively. Keywords Low-pressure cold spray (LPCS) . Coating . Surface roughness . Taguchi optimization . Signal to noise ratio (S/N)
1 Introduction Surface treatment procedures are used to modify the surface properties of various materials without altering their bulk T. Goyal Punjab Technical University, Jallandhar, Punjab, India e-mail:
[email protected] R. S. Walia (*) PEC University of Technology, Chandigarh, Punjab, India e-mail:
[email protected] T. S. Sidhu SBSCET, Ferozepur, Punjab, India e-mail:
[email protected]
characteristics. These techniques are used within industrial environments to improve resistance to corrosion, wear, fatigue, and heat. A comparison of competitive spraying technologies clearly shows that cold gas dynamic spraying has been established as a viable coating technology in the thermal spray processes family. The dense and oxide-free coatings that may be produced through gas dynamic spraying (GDS) have brought about a multitude of new applications which, up to now, have not been feasible using traditional processes. Practical solutions may be readily observed within such industries as automotive and electronics manufacturing. Despite this great progress, however, the full potential of high-pressure and low-pressure GDS has not yet been fully realized [1]. Cold gas dynamic spray process is emerging as a boon in the thermal spray category for producing coatings so as to avoid material degradation in the field of surface engineering. The cold spray process was originally developed in the mid-1980s at the Institute of Theoretical and Applied Mechanics of the Russian Academy of Sciences in Novosibirsk by Dr. Anatolii Papyrin and his colleagues. They successfully deposited a wide range of pure metals, metal alloys, and composites onto a variety of substrate materials, and demonstrated the feasibility of the cold spray process for various applications. A US patent was issued in 1994, and the European patent in 1995 [2]. Cold spray coatings may be classified as high-pressure cold spray (HPCS) and low-pressure cold spray (LPCS) on the basis of the stagnation pressure of the working gas. Table 1 shows classification of the two versions of the cold spray process on the basis of their operating process parameters. Cold gas dynamic spray (or simply cold spray) is a process of applying coatings by exposing a metallic or dielectric substrate to a high velocity (300–1,200 m/s) jet of small (1–50 μm) particles accelerated by a supersonic jet of compressed gas. This process is based on the selection of
612
Int J Adv Manuf Technol (2012) 60:611–623
Table 1 Classification of cold spray process on the basis of their process parameters [3] Process characteristic
High-pressure system
Low-pressure ystem
Working gas
N2, He, air
N2, air
Gas pressure, MPa Gas preheat, °C Gas flow rate, m3/h
2.5–4.5 20–800 50–150
0.5–1.0 20–550 15–30
Maximum gas mach no. Powder flow rate, g/s
1–3 0.1–1.0
1–3 0.1–1.0
Particle size, μm
5–100
10–80
the combination of particle temperature, velocity, and size that allows spraying at the lowest temperature possible. In the cold spray process, powder particles are accelerated by the supersonic gas jet at a temperature that is always lower than the melting point of the material, resulting in coating formation from particles in the solid state. As a consequence, the deleterious effects of high-temperature oxidation, evaporation, melting, crystallization, residual stresses, debonding, gas release, and other common problems for traditional thermal spray methods are minimized or eliminated [4]. Eliminating the deleterious effects of high temperature on coatings and substrates offers significant advantages and new possibilities and makes cold spray promising for many industrial applications. Figure 1 shows the operating principle of low-pressure cold spray process. In this paper, surface coatings have been produced by cold spray process by selecting Taguchi L 18 OA design. This research work is an attempt for development of coatings by low-pressure cold spray process for electrotechnical applications. The process parameters selected for producing the coatings are: stagnation pressure, stagnation Fig. 1 Operating principle of low-pressure cold spray process [5]
temperature of the carrier gas, type of powder feeding arrangement, substrate material, and stand-off distance. The coatings so produced are analyzed on the basis of surface roughness obtained on the coated specimens. The aim of the present work is to optimize process parameters for surface roughness of low-pressure cold-sprayed copper coatings. An attempt has been made to do so in the present research by coating copper powder on the substrate— aluminum; brass and nickel alloy using low-pressure cold spray process so that the frequent contact of dissimilar materials may be avoided. The substrates have been selected keeping in view their actual applications in the industrial components. The information arising out from the investigation will be useful to explore the possibility of use of the low-pressure cold spray coatings on different materials so as to achieve a lower surface roughness value of the so-formed coatings.
2 Literature review The cold spray process was developed in 1994 and is still in infancy but various researchers and scientists all over the world have been contributing significantly towards the improvement in the process for wide range of application area. The research on the process is focused on process attributes, design and modeling, process parameter effects, nozzle design parameters, bonding mechanism, bow-shock phenomenon, adiabatic shear instability, post spraying treatments, and exploring new combinations and advancements of the process to suit wide-spread application areas. Some of the important research contributions in regard to the effect of various process parameters on the improvement of the process are presented in this section.
Int J Adv Manuf Technol (2012) 60:611–623
2.1 Effect of carrier gas temperature Lee et al. [6] presents the effects of gas temperature on critical velocity and deposition characteristics in kinetic spraying. This study was carried out to determine the influence of process gas pressure and temperature on the kinetic spray deposition of bronze powder onto aluminum and mild steel substrates. It was found that increasing the gas pressure caused an increase in particle velocity, while increasing the gas temperature not only affected the particle velocity but also the particle temperature. Increasing the particle temperature could enhance thermal softening, which is important for bonding. They found that there exists two critical velocities—particle bonding on substrate (Vcr1) and particle–particle bonding (Vcr2). The critical velocity decreased by 50 m/s when the process gas temperature was increased by 100°C. The difference between the critical velocity for particle deposition onto the aluminum substrate (Vcr1) and for particle–particle bonding (Vcr2) was about 160 m/s. The difference between the critical velocity for particle deposition onto the mild steel substrate (Vcr1) and for particle–particle bonding (Vcr2) was higher than about 330 m/s. Bond strength at the substrate/coating interface mainly depends on the first stage critical velocity (Vcr1). 2.2 Effect of carrier gas pressure Li et al. [7] studied the effect of gas pressure on Al coating by cold gas dynamic spray. They found that gas pressure as one of the processing parameters can have an influence on the coating properties. They suggested that the peening effect of low-pressure cold spray be suitably considered. 2.3 Coating powder properties Ning et al. [8] studied the effects of powder properties on in-flight particle velocity and deposition process during low-pressure cold spray process. The results showed that the irregular shape particle presents higher in-flight velocity than the spherical shape particle under the same condition. Critical velocities of about 425 m/s and more than 550 m/s were estimated for the feedstock copper powder with spherical and irregular shape morphology, respectively. For irregular shape particles, the in-flight velocity decreased from 390 to 282 m/s as the particle size increases from 20 to 60 mm. For the irregular shape particles, the critical velocity decreased from more than 550 to 460 m/s after preheating at 390°C for 1 h. It was also found that the larger size powder presents a lower critical velocity in this study. Li et al. [9] presented significant influence of particle surface oxidation on deposition efficiency, interface
613
microstructure, and adhesive strength of cold-sprayed coatings. They found that gas and particle temperature and particle oxygen content significantly affect the deposition efficiency which increases with increasing gas and particle temperature and decreases with increase in oxygen content. The hardness for low and high oxygen content copper powder was comparable but the oxide inclusions at the coating interface was more with high oxygen content powder. The adhesive strength of the coatings were found to decrease with increasing oxygen content as it inhibited the effective bonding of fresh metals. 2.4 Process conditions Shin et al. [10] presented a study on the influence of process parameters such as feed rate, spray distance, and particle velocity on deposition characteristics of soft/hard composite coating in kinetic spray process. The results showed that the high diamond fraction in the coating can be achieved using a low feed rate, intermediate spray distance, and high impact particle velocity. The possibility of impact between hard brittle diamond particles is the main factor affecting the diamond fraction in the coating. Although the deposition efficiency, diamond fraction, and bond strength of the coating increase with particle velocity, a slight decrease of cohesive strength between diamond particle and bronze base was also observed. 2.5 Spray parameters Steenkiste et al. [11] studied kinetic spray coatings. They found that new high-velocity spray apparatus has provided the capability of controlling process parameters important to the kinetic spray process. This process forms coatings via conversion of particulate kinetic energy to mechanical and thermal deformation of particles upon impact with a substrate. These coatings were found to have relatively low porosity values, hardness comparable with the corresponding bulk materials, adhesive strengths as high as 68–82 MPa, and oxide contents essentially the same as in the powders. In DOE the nozzle air inlet pressure, inlet air temperature, nozzle-substrate stand-off distance, and powder feed rate were varied. Al, Cu, and Fe powders were sprayed onto Al and brass substrate. Substrate effects appeared to be relatively weak in this experiment. Porosity and maximum coating thickness in a single pass were measured. Porosity values were found to be less than 1% for Cu and Fe, but in the case of Al porosity was sensitive to the spray parameters. Al and Fe had similar average single-pass thicknesses, while Cu thicknesses were much higher. In fact, Cu thicknesses were sensitive primarily only to powder feed rates.
614
2.6 Incidence angle Gang et al. [12] discuss the effect of different incidence angles on bonding performance in cold spraying. It was found that the contact area between the deformed particle and substrate decreases and the crater depth in the substrate reduces with increasing the tilting angle at the same impact velocity. The normal component of impact velocity takes an important role in the impacting process and formation of bonding. Li et al. [13] presented numerical investigations of the effect of oblique impact on particle deformation in cold spraying. They showed that the tangential component of incident velocity create a gap between deformed particle and substrate, decreasing contact area, and deteriorating bonding. They felt that the maximum deposition efficiency may not be found at normal angle; however, at an angle range in which the deposition efficiency may be promoted under the combined positive effect of shear friction and negative effect of tangential movement. 2.7 Stand-off distance Li et al. [14] studied the effect of stand-off distance on coating deposition characteristics in cold spraying. It was found that the deposition efficiency was decreased with the increase of stand-off distance from 10 to 110 mm for both Al and Ti powders used in this study. However, for Cu powders, the maximum deposition efficiency was obtained at the stand-off distance of 30 mm, and then the deposition efficiency decreased with further increasing the stand-off distance to 110 mm. The stand-off distance had a little effect on coating microstructure and micro hardness for these three powders. Both the strain-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 stand-off distance. Pattison et al. [15] discuss the effect of stand-off distance and bow-shock phenomena in cold spray process. The bow shock formed at the impingement zone plays a critical role in the cold spray process; not only does it reduce the velocity of the gas, but also that of the entrained particles. Therefore at small stand-off distances, when the strength of the bow shock is high, deposition performance is reduced. While at large stand-off distances, when the bow shock has disappeared, deposition can continue unhindered. 2.8 Impact velocity analysis Grujicic et al. [16] discusses the analysis of the impact velocity of powder particles in the cold gas dynamic spray
Int J Adv Manuf Technol (2012) 60:611–623
process. They concluded that using a nonlinear regression analysis and a numerical solution for the one-dimensional isentropic gas flow in a cold gas dynamic spray nozzle, a relatively simple function is defined which relates the gas velocity at the nozzle exit with the nozzle expansion ratio and the carrier gas stagnation properties. Further, they found that to compute the velocity at which particles impact the substrate surface, deceleration of the particles in a stagnant subsonic region adjacent to the substrate surface must be considered. Wu et al. [17] present the measurement of particle velocity and characterization of deposition in aluminum alloy by kinetic spraying process. The results showed that particle velocity increases with increasing the process gas pressure and temperature. At higher temperature (or pressure), gas pressure (or temperature) do more effect particle velocities. They found that the substrate surface roughness do not have great effect on deposition efficiency. In this research, parameters such as particle and substrate temperatures effect were not studied. Helfritch et al. [18] in their paper gave a model study of powder particle size effect in cold spray deposition. It has often been thought that the smallest particles attain highest velocities in the cold spray process, thus achieving good deposition efficiency. While it is true that small particles exit the nozzle at high velocity, their velocity at impact can be significantly lower. Modeling efforts showed that the low gas velocity following the bow shock wave decreases particle velocities, especially for the smallest particles. It was shown that impact velocity increases as the particle diameter decreases until a diameter of 4 to 8 μm is reached. Impact velocity then decreases as the particle diameter is further reduced. Schmidt et al. [19] present development of a generalized parameter window for cold spray deposition. Calculations and experimental results demonstrate that size effects in impact dynamics can have a significant influence on the critical velocity, which has to be exceeded for bonding, and thus on the amount of bonded area and coating quality. The results obtained for copper and steel 316 L clearly demonstrate that critical velocities decrease with increasing particle size, which can be attributed to effects by heat conduction or strain rate hardening, respectively. Based on that analysis, a simple expression was supplied which allows an easy estimation of critical velocities for different metallic materials. The generalized approach developed in this study supplies the tools needed to predict optimum spray conditions and required powder size cuts for successful cold spraying of various materials. Li et al. [20] studied the high velocity impact of micro size particles in cold spraying. They concluded that both the compression ratio and flattening ratio of the particles increase with increase in particle velocity. They further
Int J Adv Manuf Technol (2012) 60:611–623
615
Table 2 Nominal chemical composition of the substrate materials chosen [23] Nominal chemical composition of ASTM B 221 (Al alloy) %Si
%Fe
%Cu
%Mn
%Mg
0.4–0.8 0.7 0.15–0.40 0.15 0.8–1.2 Nominal chemical composition of ASTM B 36 (Brass) % Cu 64–68.5
%Cr
%Zn
%Ti
%Others
%Al
0.04–0.35
0.25
0.15
0.15
Rem.
% Pb 0.15
% Fe 0.05
% Zn Remainder
Nominal chemical composition of ASTM B 435 (Ni alloy) %Fe 31
%Ni 20
%Co 18
%Cr 22
%Mo 3
%W 2.5
found that the compression ratio is more convenient for characterizing the extent of particle deformation than the flattening ratio owing to easy estimation and its dependency on meshing size. 2.9 Impact phenomena Klinkov et al. [21] analyzed the significance of particle impact phenomenon. In this study, a classification of impact phenomena is made based on particle size and impact velocity. The study showed that results of particle impacts depend not only on impact velocity but also on particle size. One can separate some typical ranges of impact velocities and particle sizes, where features of impacts are similar. Impacts result in two contrasting phenomena: destruction (also called erosion, cratering etc.) and adhesion (also called attachment, sticking). At hyper-velocity impacts, cratering and destruction are typical features which exhibit minor scale effects. At low impact velocities, there is a transition from erosion to adhesion (sticking) when the particle size decreases. Here, the nature of adhesion is due to Van der Waals and electrostatic forces. At high velocities, the results of impact depend not only on size and velocity but also on other parameters (e.g., plasticity, Fig. 2 Principle scheme of low-pressure portable cold spray machine (SST, Centreline, Windsor, Canada) [1]
%Mn 1
%Si 0.4
%Ta 0.6
%Al 0.2
%C 0.1
%N 0.2
%Zr 0.02
%La 0.02
particle flux concentration, etc.). In the regime of cold spray there is a competition between adhesion and erosion. Under these conditions a temperature variation in a rather minor range can result in a transition from erosion to adhesion. Wu et al. [22] studied the rebound phenomenon in kinetic spraying deposition. They concluded that a rebound phenomenon was observed in which a high particle velocity caused a high fraction of rebound particles. A maximum impact velocity was found for the particle deposition onto the substrate. The deposition of individual particles was controlled by the adhesion energy and the rebound (elastic recovering) energy. The impacting particles could only be attached to the substrate when the adhesion energy was higher than the rebound energy.
3 Experimental procedure 3.1 Substrate material The substrate materials selected were Al (ASTM B 221), brass (ASTM B 36), and Ni (ASTM B 435) in the rolled sheet form. The substrate materials selected for the study finds application in the manufacture of electrical contact points, fuse element of
616
Int J Adv Manuf Technol (2012) 60:611–623
Table 3 Design parameters and their chosen levels considered for the Taguchi experiment
Nozzle type, converging– diverging; carrier gas, air; powder size,
