Trans Indian Inst Met (2014) 67(3):351–357 DOI 10.1007/s12666-013-0353-8
TECHNICAL PAPER
TP 2764
The Influence of Cathode Surface Velocity on Friction Aided Jet Electrodeposition Dazhi Huang • Lida Shen • Jinsong Chen Jun Zhu
•
Received: 29 April 2013 / Accepted: 21 August 2013 / Published online: 21 November 2013 Ó Indian Institute of Metals 2013
Abstract In friction aided jet electrodeposition (FAJED), nozzle outlet width, flow of electroplating solution passing through the nozzle, and the distance between nozzle and cathode could influence the flow field on the cathode surface. In this research, a flow field mathematical model of FAJED was constructed using FLUENT to simulate the velocity on the cathode surface and investigate the influence of cathode surface velocity on the quality of nickel film produced experimentally. Results illustrated that in the area influenced by deposition on the cathode surface, moderate and well-distributed velocity on the cathode surface was conducive to the production of low-defect count nickel films. Combined with the friction of free particles on the deposition layer, moderate velocity and its even distribution on the cathode surface were chosen to obtain flat, compact deposition layers and process rotational parts. Keywords Jet electrodeposition Friction Cathode surface Velocity Free particles
1 Introduction
cathode surface and then flows over this cathode surface at high speed. The thickness of the diffusion layer is thereby reduced: this process increases the limiting current density, and subsequently improves the rate of electro-deposition speed [1–3]. However, in this process, there are numerous defects induced in the deposition layer such as pockmarks, pitting, accumulated burl, etc. due to factors such as: hydrogen inclusion evolution, electroplating solution impurities, etc. To improve the structure of the deposition layer, based on JED, free particles are introduced to apply friction across the deposition layer. Such friction can improve the structure of the deposition layer and refine its grain sizes thereby improving deposition quality. Therefore, friction aided jet electrodeposition (FAJED) technology can be applied more broadly in the preparation of bulkmass nano-crystalline materials at high speed and low cost [4–6]. In the process of FAJED, deposition quality is closely associated with size and hardness of free particles, and other jetting process parameters. Among these process parameters, the velocity over the cathode surface plays an important role in deposition quality.
In the process of jet electrodeposition (JED), the electroplating solution is jetted from a nozzle onto the 2 Experiments D. Huang L. Shen (&) J. Zhu College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautic, Nanjing 210016, Jiangsu, China e-mail:
[email protected] D. Huang J. Chen School of Mechanical Engineering, Huaihai Institute of Technology, Lianyungang 222005, Jiangsu, China
2.1 Principle of FAJED As shown in Fig. 1, based on ordinary JED, FAJED is a special processing technology which alternates electrodeposition, and friction, processes by adding free particles such as ceramic balls or marbles around the cathode to rotate it.
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Trans Indian Inst Met (2014) 67(3):351–357 Table 2 Process conditions of FAJED Parameter
Value
Nozzle size (mm)
1, 2, 3 9 20 mm
Solution flow rate (L/h)
200, 250, 300
Jet distance (mm)
1,2,4
Cathode rotating speed (min)
6
FAJED time (min)
120
pH
4
Temperature (oC)
50
Current densities (A/dm2)
80
Free particles diameter (mm)
2
solution and meanwhile prevent hydroxide generated in the electrochemical process from being deposited on the cathode. The structure of the final deposition layer was thus improved. Other process conditions are listed in Table 2. Fig. 1 Schematic diagram of experimental apparatus, 1 nozzle; 2 power; 3 motor; 4 cathode; 5 free particles; 6 magnetic pump; 7 isothermal reservoir
2.2 Experimental Device The chosen cathode was a 20 mm diameter graphite rod, while the anode consisted of a nickel rod in the anode cavity which was surrounded by nickel bead both having a purity of 99.9 %. The additional free particles were 2 mm diameter ceramic balls. Before the experiments, the cathode was buffed with fine sand paper and submerged in an alcohol solution to eliminate pollutants. According to the principle of FAJED, the kit was set-up as shown in Fig. 1, and it included: a nozzle, a power supply, a motor, the cathode, free particles, a magnetic pump, an constant temperature supply reservoir, etc. 2.3 Electroplating Solution Compositions and Electrodeposition Process Parameters Table 1 shows the electroplating solution compositions used experimentally. Among the compositions, nickel sulphate was the main source of nickel ions; nickel chloride was the activator of the anode. Besides, as a buffer, boric acid was used to maintain the pH of the electroplating
Table 1 Composition of electrolyte and process parameters Chemical
q (g/L)
NiSO4.6H2O
250–300
NiCl2.6H2O
35–40
H3BO3
40–45
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2.4 Flow Simulation FAJED technology differs from friction aided electrodeposition technology [7, 8]. As the cathode rotates, the friction between free particles and the electrodeposition in the former on its cathode alternate, and its cathode is partially submerged by free particles. The exposed part sees JED, while the submerged part is affected by the friction of those free particles. So, JED and friction are two relatively independent processes; while, for the latter, free particles are placed between the anode and cathode: as the cathode rotates, apart from friction on the cathode’s surface, free particles also affect the flow, and electric, fields on the cathode surface to further influence the processing and production. The nozzle, made of PMMA, was a cylinder (dia. 30 mm 9 50 mm length) and the nozzle cavity was an inverted quadrangular frustum pyramid with the size of upper end (inlet) being 20 mm 9 10 mm and its lower end (outlet) 20 mm 9 2 mm. The cathode took the form a cylinder with a diameter of 20 mm. As shown in Fig. 2, this device using a FAJED can be simplified into a two-dimensional model. FLUENT was used to develop mathematical flow field models of the FAJED. A standard k–e equation for turbulence models was chosen. As Fig. 2 indicates, the nozzle’s upper end was the inlet boundary, set as its velocity inlet. Other parts of the nozzle cavity and exterior were set as fixed walls, while the cathode was set as a rotatable wall. Besides, the base flow field was a free particle surface, set as a fixed wall; and the others set as pressure outlets were thus outlet boundaries. The whole flow field was influenced by gravity.
Trans Indian Inst Met (2014) 67(3):351–357
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Fig. 2 Flow field two-dimensional model
3 Results and Analysis
Fig. 3 Fluid velocity on cathode surface for different w
In the process of FAJED, the size of the nozzle’s orifice, flow variation of electroplating solution passing through nozzle and the distance between nozzle and the cathode all affected the cathode surface velocity. Consequently, the processing of FAJED was also influenced. In natural convection, the diffusion layer thickness ranged from 0.2 to 0.5 mm [9]. Therefore, as shown in Fig. 2, a survey line was chosen on an arc segment some 0.5 mm from the cathode surface and parallel thereto in the X-direction between -5 and 5 mm to observe the velocity of the electroplating solution. In addition, in JED, the cathode surface corresponding to the nozzle outlet was the area most influenced by electrodeposition which had little, to no effect elsewhere [10]. Hence, velocity and its variation in this key area formed the focus for this work.
after its maximum velocity was reached. Besides, different nozzle outlet widths provided with different areas of depositional influence (double the nozzle width [10]): -0.0005 B X B 0.0005 was the main area influenced by deposition when w = 1 mm whilst –0.001 B X B 0.001 was the main area influenced by deposition when w = 2 mm, and at w = 3 mm, the main area influenced by deposition was such that –0.0015 B X B 0.0015. As shown in Table 3, although the minimum velocity for different w was approximately equal, as w increased, the mean fluid velocity increased as did the ratio of the maximum to the minimum velocities at any given section (i.e. the flow was non-uniform). According to the aforementioned technological requirements, nozzles with different values of w were chosen to prepare three group specimens and their deposition layer surfaces were observed using scanning electron microscopy (SEM). Fig. 4 shows the surface morphology of the deposition layer at a magnification of 7509. When w = 1 mm (Fig. 4a), the surface morphology of the deposition layer grew in a spinal shape with many scratches. When w = 2 mm (Fig. 4b), in addition to several small accumulated burls, a deposition layer with fewer pinholes and accumulated burls was seen and nickel film surface was compact and flat; when w = 3 mm (Fig. 4c), there were numerous accumulated burls and cellular particles of various sizes on the deposition layer surface therefore the surface was unacceptably rough. Simulation and experiments indicate that different outlet widths had an impact on the structure of the deposition layer surface. When w was small, despite the well-distributed velocity, the low mean velocity resulted in the compensation speed of the nickel ions being slower than
3.1 The Influence of Nozzle Outlet Width The distance h between nozzle and cathode was set to 2 mm. To ensure the same velocity at the nozzle outlet, the flow q of electroplating solution was set to 125, 250, and 375 L/h to simulate conditions on the cathode surface at nozzle outlet widths w = 1, 2, and 3 mm respectively: data are shown in Fig. 3 and Table 3. The velocity distribution of the electroplating solution over the cathode surface for different nozzle outlet widths w is shown in Fig. 3. Similar velocities were found when X was close to zero (the centre of the jet) and their distribution was bilaterally symmetrical along X = 0. When w was small, the maximum velocity was reached in the centre of the jet and decreased in every radial direction therefrom. When w was increased, and away from the jet’s centre, the velocity accelerated rapidly and then gradually declined
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354 Table 3 Comparison table of velocity (m/s) for different w
Trans Indian Inst Met (2014) 67(3):351–357
w (mm)
Vmax
Vmin
Vmax/Vmin
Mean velocity
1
0.88
0.86
1.02
0.87
2
1.22
0.86
1.42
1.04
3
1.49
0.88
1.69
1.18
Fig. 4 Surface morphology of nickel casting layers for different w
their deposition speed. In addition, there were more defects on the deposition layer due to severe hydrogen evolution. In contrast, when w was large, the velocity was high but its distribution was uneven, which caused the compensation speed of the nickel ions to exceed their deposition speed. Thus, atomic adsorption and growth of the deposition layer were influenced negatively. There therefore appeared to be more cellular particles present on the finished surface. When w was moderate, a balanced velocity was obtained and the compensation speed of nickel ions approached their deposition speed which was conducive to growth of a low defect count deposition layer and an equilibrium state between its growth and the friction with any free particles was achieved. 3.2 The Influence of Flow of Electroplating Solution Passing Through the Nozzle The distance h between nozzle and cathode was set to 2 mm and the nozzle outlet width w = 2 mm. The flow velocity was simulated at flowrates q = 200, 250, and 300 L/h respectively: the results are given in Table 4.
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As q increased gradually, the maximum: minimum velocity ratio decreased slightly which indicated that the velocity, was related to the nozzle outlet width w. The same w was provided with nearly the same proportionality of velocity, but the mean velocity increased stably as q increased. As shown in Fig. 5a–c were surface morphologies of nickel films at flowrates of 200, 250 and 300 L/hr respectively. Apparently, when the nickel film surface presented different surface growth morphology at q = 250 L/hr (Fig. 5b) this corresponded to the critical velocity of this electroforming solution. Below 250 L/hr (Fig. 5a), there were defects on the nickel film surface such as pitting, accumulated burl, etc. At 250 L/hr, the nickel film exhibited pyramid suppression and small grain sizes. Moreover, at over 250 L/hr (Fig. 5c), the surface quality of the deposition layer was once more poor because the surface was covered with small particles. Simulation and experiments showed that the flow q of electroplating solution at the nozzle outlet influenced the structure of the final deposition layer surface. When q was small, the compensation speed of nickel ions on cathode
Trans Indian Inst Met (2014) 67(3):351–357 Table 4 Velocity (m/s) in the influenced area with different q
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q (L/h)
Vmax
Vmin
Vmax/Vmin
Mean velocity
200
0.98
0.68
1.44
0.83
250
1.22
0.86
1.42
1.04
300
1.47
1.06
1.39
1.27
Fig. 5 Surface morphology of nickel films produced at different q
surface was slower than their deposition speed, which led to more hydrogen evolution and a poorer quality of deposition layer. By comparison, when q was large, the compensation speed of nickel ions on cathode surface exceeded their deposition speed, which had a detrimental effect on atomic adsorption and growth of the deposition layer. Consequently, more cellular particles appeared. It was only when h was moderate that the compensation speed of the nickel ions on the cathode surface were close to their deposition speed. This was favorable to growth of a more defect-free deposition layer and an equilibrium state between layer growth free particle friction was achieved. 3.3 The Influence of the Distance Between Nozzle and Cathode Workpiece The flow q of electroplating solution passing through the nozzle was set to 250 L/h and a nozzle outlet width w = 2 mm was used. The flow field was simulated when the distance h between nozzle and workpiece was 1, 2, and 4 mm respectively: results are given in Table 5. With h increasing gradually, the maximum: minimum velocity ratio tended to decline initially but then
subsequently increase. This indicated that apart from nozzle outlet width w, the maximum: minimum velocity ratio, was related to the distance h between the nozzle outlet and cathode surface: excessively large or small h values were detrimental to the development of a well-distributed velocity profile. In addition, the mean velocity gradually decreased as h increased. As shown in Fig. 6, when h = 2 mm (Fig. 6b), a level, smooth, nickel film with fine, dense, grains and few defects was obtained. When h = 4 mm (Fig. 6c), a deposition layer surface with poor quality resulted. In contrast, the worst surface quality appeared when h = 1 mm (Fig. 6a) as the nickel film surface was covered with numerous defects such as pitting, pockmarks, etc. Simulation and experiments indicated that different distance h between the nozzle outlet and cathode surface affected the structure of the deposition layer surface. When h was small, the velocity distribution on the cathode surface was uneven and the compensation speed of nickel ions exceeded their deposition speed. Thus, atomic adsorption in, and growth of, the deposition layer were negatively influenced. Therefore, there were more defects on the deposition layer surface such as pitting, pockmarks, etc.
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356 Table 5 Velocity (m/s) in the influenced area for different h
Trans Indian Inst Met (2014) 67(3):351–357
h (mm)
Vmax
Vmin
Vmax/Vmin
Mean velocity
1
1.87
1.19
1.57
1.53
2
1.22
0.86
1.42
1.04
4
0.94
0.54
1.74
0.74
Fig. 6 Surface morphology of nickel film for different h
When h was very large, the most uneven velocity distribution on the cathode surface resulted. Besides, the compensation speed of the nickel ions was slower than their deposition speed, which caused more hydrogen evolution and degraded the quality of the deposition layer. It was only when h was moderate that the velocity was well-distributed and the compensation speed of nickel ions approached their deposition speed. Moreover, an
equilibrium state between growth of the deposition layer and free particle friction was achieved which resulted in more defect-free deposition layer being produced. When w = 2 mm, q = 250 L/h, and h = 2 mm, a 1 mm thick nickel film was produced. As shown in Fig. 7, the casting layer’s surface was flat and bright, and its section structure was compact. This shows that rotational parts could be processed by FAJED technology by controlling and ensuring a moderate velocity on the cathode’s surface.
4 Conclusions (1)
Fig. 7 Nickel film with a thickness of 1 mm
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On the cathode surface, with increased nozzle outlet width, the mean velocity increased. Similarly, the mean velocity also increased with the flow of electroplating solution passed through the nozzle, however, with increasing distance between the nozzle outlet and the cathode surface, the mean velocity decreased. Experiments indicated that excessively high, or low, velocities on the cathode surface were not conducive to the production of defect-free nickel films.
Trans Indian Inst Met (2014) 67(3):351–357
(2)
(3)
In the area influenced by deposition, uniformity of velocity distribution was related to flow of electroplating solution passing through nozzle and the distance between the nozzle outlet and cathode surface. Besides, uniformity of velocity distribution also had an effect on deposition quality and a welldistributed velocity was conducive to the formation of a low-defect count deposition layer. A moderate velocity on the cathode surface and friction of free particles on the deposition layer contributed to flat, compact final products. This indicated that FAJED technology can be used to process rotational parts.
Acknowledgments The work described in this paper was supported the National Science Foundation of China (No. 51105204 and 51105162) and fund for the doctoral program of higher education of china (No. 20113218120022).
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