Enhancement of surface roughness in electrochemical machining of

Int J Adv Manuf Technol DOI 10.1007/s00170-013-5238-9

ORIGINAL ARTICLE

Enhancement of surface roughness in electrochemical machining of Ti6Al4V by pulsating electrolyte N. S. Qu & X. L. Fang & Y. D. Zhang & D. Zhu

Received: 26 March 2013 / Accepted: 29 July 2013 # Springer-Verlag London 2013

Abstract Electrochemical machining (ECM) is widely used in machining a variety of components used in aerospace, defence, automotive and medical applications. The surface roughness of the ECM process has become important because of increased quality demands. Considerable attention has been paid to achieving low surface roughness in ECM. Surface roughness is closely related to the distribution of gases and Joule heat produced during the ECM process, which affect the electrolyte electric conductivity and directly determine the surface roughness. In this report, a pulsating electrolyte, which is one of the unsteady flows that are characterized by periodic fluctuations of the mass flow rate and pressure, is first introduced to the ECM process. The ECM process is affected by the pulsating electrolyte because it can modify the heat transfer. The goal of this report is to present experimental results of the surface roughness obtained on Ti6Al4V samples using a developed pulsating electrolyte supply system in ECM. It is observed that a lower surface roughness and higher material removal rate could be obtained by using a pulsating electrolyte with proper pulsating frequency and amplitude. In direct current ECM, the surface roughness Ra is 5.7 μm, the material removal rate is 0.85 g/min at a constant electrolyte, the lowest surface roughness is 3.69 μm and the largest material removal rate is 0.92 g/min, which are obtained at a pulsating frequency of 10 Hz and amplitude of 0.2 MPa. In pulsed current ECM, the surface roughness Ra and material removal rate are 0.67 μm and 0.38 g/min at a constant electrolyte, respectively, and both the minimum surface roughness Ra of 0.53 μm and maximum material removal rate of 0.39 g/min are observed when the proper pulsating electrolyte flow frequency and amplitude are used.

N. S. Qu (*) : X. L. Fang : Y. D. Zhang : D. Zhu College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics & Astronautics, Nanjing 210016, China e-mail: [email protected]

Keywords Surface roughness . Pulsating electrolyte . Electrochemical machining . Material removal rate

1 Introduction Electrochemical machining (ECM) is an advanced machining technology that provides an economical and effective method to machine heat-resistant, high-strength materials into complex shapes. Therefore, it has been developed and applied in aerospace, aeronautics, defence and medical industries [1–4]. In recent years, ECM has been used in other industries, such as automobile and turbo-machinery, because of its advantages, which are its applicability regardless of material hardness, zero tool wear, high material removal rate and production of components of complex geometry [5]. Despite these advantages, research is still being conducted on various aspects of electrochemical machining. Many researchers focus on improving the machining accuracy and surface roughness [6, 7]. Surface roughness, which is used to determine and evaluate the quality of a product, is one of the major quality attributes of an electrochemical machining product. The surface roughness has become important because of increased quality demands. However, even if the dimensions of the component are well within the dimensional tolerances, there are still possibilities of rejecting the component for lack of required surface roughness [8]. The study of the effects of various machining parameters on surface roughness has been performed to analyse the suitable parametric combinations that can be created for achieving controlled surface roughness. In the literature, there are many relevant investigations of low surface roughness during ECM processes. The effects of electrolyte flow velocity and electric potential during the electrochemical machining process for generation of various surface characteristics have been successfully studied through experimentation, and the obtained surface roughness values on the titanium sample machined by ECM were in the range of 2.4 to 2.93 μm [9]. Senthilkumar

Int J Adv Manuf Technol

et al. investigated the influence of process parameters, such as applied voltage, electrolyte concentration, electrolyte flow rate and tool feed rate, on the metal removal rate (MRR) and on surface roughness to fulfil the effective usage of electrochemical machining of Al/SiCp composites [10]. In fact, the quality of the electrochemically machined surface is related to that of the electrolyte flow field. In ECM, an electrolyte with a velocity of 10–30 m/s is pumped into the inter-electrode gap to remove the waste products (gases and metallic hydroxides) and Joule heat. The distribution of gases and Joule heat affect the electrolyte electric conductivity and directly determine the surface roughness. Therefore, many studies have focused on the disposal of gases and Joule heat to acquire low surface roughness. Fan et al. has presented that electrochemical–magnetic composite processing could be efficient in lowering surface roughness [11]. Ruszaj et al. acquired a better surface finish by using pulse electrochemical machining assisted by electrode ultrasonic vibrations and machining in a mixture of electrolyte and abrasive powder [12]. El-Taweel et al. investigated the feasibility of improving surface quality through a novel combined process of electrochemical turning with roller burnishing using the Taguchi fractional factorial experimental design technique, and indicated that the combined process effectively improved the surface roughness [13]. Additionally, an ECM process with rotating electrode movement is proposed to enhance the uniformity of electrolyte flow and reduce or eliminate the flow field disrupting processes; furthermore, a significant improvement in surface finish is observed [14]. Pulsating flow is one of the unsteady flows that are characterized by periodic fluctuations of the mass flow rate and pressure. Pulsating flow has been applied to processes such as heat exchange, ramjet combustion, solid fermentation, drip irrigation emitter etc. [15–18]. It has been proven that pulsating flow creates different characteristics of hydrodynamics and alters the thickness of the boundary layer, and pulsating flow with optimized pulsating parameters are beneficial to the transfer process [19]. However, pulsating flow has not been introduced to ECM until now. In ECM, the Joule heat plays an important role in machined surface roughness. It is believed that pulsating flow may enhance the Joule heat transfer during ECM. Thus, this report focuses on the enhancement of machined surface roughness by pulsating electrolyte. The influence of pulsating electrolyte frequency and amplitude on the surface roughness in the electrochemical machining of Ti6Al4V was experimentally investigated.

the waveform of the pulsating electrolyte flow is shown in Fig. 1. The schematic view of the experimental set-up is shown in Fig. 2a. The pulsating electrolyte is modulated by a Get-type electrohydraulic servo valve (RT6615E, Radk-Tech, China) that can quickly respond to a signal ranging from 0 to 100 Hz. A specific full-feedback control system was established to control the pressure and obtain the machining voltage and current. Samples of Ti6Al4V with dimensions of 12 mm×12 mm×10 mm, as shown in Fig. 2b, were ultrasonically cleaned and weighted by analytical balance (AE240, Mettler, China) before and after the experiments. The surface roughness of these samples is in the range of 1.0 to 1.4 μm. Based on pre-trials of the machining stability, the machining conditions were chosen and listed in Table 1. Before starting the experiments, an initial inter-electrode gap of 0.2 mm in the axial direction was set. Constant feed rate of 1.5 and 0.8 mm/ min were used in direct current ECM and pulsed current ECM, respectively. Furthermore, the averaged electrolytic pressure of pulsating flow was 0.4 MPa in all experiments. MRR is one of the most important criterions determining the machining operation. A higher rate is always preferred in such operations and is defined as follows: MRR ¼

m t

Where m is the mass removed in the ECM process and t is the machining time. The surface roughness of the machined surface was measured using a profilometer (T8000 SC, HOMMEL-ETAMIC, Germany), and 3D profiles of the machined surface were measured by a 3D video microscope (DVM5000, Lecia, Germany). In addition, a scanning electron microscope (S-3400N, Hitachi, Japan) was used to observe the machined surface.

3 Results and discussion 3.1 Effect of pulsating electrolyte flow on surface roughness in direct current ECM Both the morphology and 3D profile of the machined surfaces prepared in direct current ECM with a constant electrolyte of

2 Experimental procedures Pulsating electrolyte is one of the unsteady flows that are characterized by periodic fluctuations of the mass flow rate and pressure. Pulsating flow is defined as a continuous mass flow variation with a repeating waveform. In the current study,

ð1Þ

Fig. 1 Waveform of pulsating electrolyte


a

b

Cathode Workpiece blank

Machined surface

10 mm

Fig. 2 Experimental facility. a Experimental set-up schematic diagram. b Cathode and workpiece for the experiments

0.4 MPa and a pulsating electrolyte at the pulsating frequency of 10 Hz and amplitude of 0.2 MPa are shown in Figs. 3 and 4, respectively. The applied voltage is 22 V in this set of experiments. Figure 3 indicates that the smoother surface can be obtained by applying a pulsating electrolyte at a frequency of 10 Hz and amplitude of 0.2 MPa. Figure 4 indicates that the 3D profile for a pulsating electrolyte of 10 Hz and 0.2 MPa shows fewer high peaks and deep valleys and results in the generation of a better surface finish. Figure 5 presents the morphology and 3D profile of a machined surface produced in direct current ECM with a pulsating electrolyte at the frequency of 5 Hz and the amplitude of 0.4 MPa. When

Table 1 Machining conditions Electrolyte Electrolyte temperature Pulsating frequency (Hz) Pulsating amplitude (MPa) Averaged pressure (MPa) Machining time (s)

10 % NaNO3 + 10 % NaCl 35 °C 1, 2, 5, 10, 20, 40 0.1, 0.2, 0.3, 0.4 0.4 30

compared with the machined surface generated with the pulsating electrolyte of 10 Hz and 0.2 MPa, more high peaks occur in the machined surface 3D profile created with the pulsating electrolyte of 5 Hz and 0.4 MPa. This observation signifies that both pulsating frequency and amplitude affect the machined surface finish. To investigate the effect of both pulsating frequency and amplitude on the machined surface roughness, a series of experiments were conducted. Figure 6 shows the variation of both the surface roughness Ra and MRR with varying pulsating frequency. At a constant electrolyte, the surface roughness Ra is 5.70 μm and the MRR is 0.85 g/min. When the pulsating electrolyte of 0.2 MPa amplitude was introduced to ECM instead of a constant electrolyte, both the surface roughness and MRR changed. At the pulsating frequency of 1 Hz, the surface roughness Ra increased to 6.26 μm and the MRR decreased to 0.83 g/min. While the pulsating frequency ranged from 2 to 10 Hz, the surface roughness decreased gradually and MRR increased gradually. The lowest surface roughness, 3.69 μm, is observed at the pulsating frequency of 10 Hz. Similarly, the biggest MRR, 0.92 g/min, occurs at the pulsating frequency of 10 Hz. When the pulsating frequency was further increased to 20 Hz, the surface roughness increased to 4.69 μm and the MRR decreased to 0.74 g/min. The effect of pulsating amplitude on both the surface roughness Ra and MRR is shown in Fig. 7. At the pulsating amplitude of 0.1 MPa, the surface roughness Ra and MRR are approximately 4.69 μm and 0.85 g/min, respectively. When the pulsating amplitude increased to 0.2 MPa, the surface roughness decreased to 3.91 μm and the MRR increased to 0.90 g/min. With an increase in pulsating amplitude from 0.2 to 0.4 MPa, the surface roughness increased and the MRR decreased. ECM is a field-synergy electrolysis process, which consists of mass transfer, energy transfer, momentum transfer and chemical reactions [16]. When a voltage is applied across the cathode tool and anode workpiece, the metallic ions of the anodic dissolution migrate from the anode surface to the electrolyte by electric force and are formed into insoluble hydroxides in neutral solutions. At the same time, hydrogen and oxygen are generated on the cathode and anode surface, respectively. Heat generated by the passage of current and electrochemical reactions will heat the electrolyte in the interelectrode gap. All of these incidents will interactively influence the electrolyte electric conductivity. The relation between the electric conductivity κ, electrolyte temperature T and gas void fraction βgas is given as follows: κ ¼ κ0 1−β gas

bp

ð1 þ αðT −T 0 ÞÞ

ð2Þ

Where κ0 is the electrolyte electric conductivity at the inlet, T0 is the electrolyte temperature at the inlet, α is the degree of temperature dependence and bp is Bruggeman's coefficient.

Int J Adv Manuf Technol Fig. 3 Morphology of the machined surface prepared by direct current ECM at a constant electrolyte and pulsating electrolyte. a Constant electrolyte. b Pulsating electrolyte of 10 Hz and 0.2 MPa

a constant electrolyte

b pulsating electrolyte of 10Hz and 0.2 MPa

Fig. 4 3D profile of the machined surface prepared by direct current ECM at a constant electrolyte and pulsating electrolyte. a Constant electrolyte. b Pulsating electrolyte of 10 Hz and 0.2 MPa


b pulsating electrolyte of 10 Hz and 0.2 MPa

According to Eq. (2), the local electrolyte conductivity is closely related to the gas void fraction and local electrolyte temperature. The gas bubbles and Joule heat generated in the inter-electrode gap cause varying local electrolyte conductivity. A change in electrolyte conductivity with flow path affects the potential distribution. In direct current ECM, an electrolyte with a constant velocity is pumped into the inter-electrode gap to remove the waste products (gases and metallic hydroxides) and Joule heat. The electrolyte conductivity at the inlet is bigger than that at the outlet; therefore, the inlet's electric potential is higher than the potential at the outlet. When a pulsating electrolyte is applied, flow separation occurs and creates regions of reverse flow, where regions of high mixing and turbulence are generated. The boundaries of this reverse flow are characterized by the creation and destruction of eddies of large turbulence energy and vortex shedding, which are expected to increase the heat and mass transfer rates. The pulsating electrolyte is beneficial for Fig. 5 Morphology and 3D profile of the machined surface prepared by direct current ECM at pulsating flow of 5 Hz and 0.4 MPa. a SEM. b 3D profile

reducing the difference of the electrolyte conductivity in the inter-electrode gap with the proper pulsating electrolyte parameters. Thus, the difference in the electric potential in the interelectrode gap using the pulsating electrolyte is smaller than that using the constant electrolyte. When a constant applied voltage is applied in ECM, the difference in electrolyte conductivity and potential in the inter-electrode is smaller, which may lead to a more homogeneous machined surface profile. Conversely, when compared with a constant electrolyte, a higher MRR is observed in the pulsating electrolyte with appropriate pulsating electrolyte parameters. This illustrates that the higher potential is obtained when the correct pulsating electrolyte is used. It has been shown that the surface roughness decreases as the electric potential voltage increases [20]. Therefore, a better surface profile may be acquired with the correct pulsating electrolyte. It should be noted that the lowest surface roughness is observed when the MRR is highest in these direct current ECM experiments.

a SEM

b 3D profile


a

b

7

1.0

6 0.9 5

MRR /gmin-1

Roughness Ra /µm

Fig. 6 Effects of pulsating frequency on both surface roughness and material removal rate in direct current ECM with pulsating amplitude of 0.2 MPa. a Variation of roughness Ra with pulsating frequency and b variation of MRR with pulsating frequency

4 3

0.8

0.7

2 0.6

1 0

0

5

10

15

20

0.5

25

0

5

Pulsating frequency /Hz

0.95

4

0.90

3 2 1

25

0.4

0.5

0.85 0.80 0.75

0.1

0.2

0.3

0.4

Pulsating amplitude /MPa

It is well known that the pulsating flow frequency and amplitude affect the heat and mass transfer. In the current experiment, it is believed that 10 Hz is the most favourable frequency for the enhancement of both heat and mass transfer at the amplitude of 0.2 MPa. Meanwhile, the highest MRR is observed at a frequency of 10 Hz. Similarly, the amplitude of 0.2 MPa is most favourable to improving both heat and mass transfer at a frequency of 5 Hz. However, a negligible influence of the pulsating electrolyte occurs on the mass transfer process at a frequency of 1 Hz and amplitude of 0.2 MPa, and at a frequency of 5 Hz and amplitude of 0.4 MPa. This lack of pulsating electrolyte leads to the roughest surface and lowest MRR. In fact, many studies have reported that a pulsating flow Fig. 8 3D profile of machined surface prepared in pulsed current ECM at a constant electrolyte and pulsating electrolyte. a Constant electrolyte and b pulsating electrolyte of 10 Hz and 0.2 MPa

20

1.00

5

0 0.0

15

b

6

MRR /gmin-1

a Roughness Ra /µm

Fig. 7 Effects of pulsating amplitude on both surface roughness and material removal rate in direct current ECM with pulsating frequency of 5 Hz. a Variation of roughness Ra with pulsating amplitude b variation of MRR with pulsating amplitude

10


0.5

0.70 0.0

0.1

0.2

0.3


of optimized pulsating parameters is beneficial to the transfer process [21]. 3.2 Effect of pulsating electrolyte flow on surface roughness in pulsed current ECM To investigate the effect of pulsating electrolyte flow on the machined surface roughness created by pulsed current ECM, a series of experiments were conducted. In the current experiment, an applied voltage of 22 V, frequency of 1,000 Hz and 40 % duty cycle is used. Figure 8 presents the 3D profile of samples machined in pulsed current ECM with a constant electrolyte and a pulsating


b pulsating electrolyte of 10 Hz and 0.2 MPa


a

b

1.2

0.40

1.0 0.35 0.8

MRR /gmin-1

Roughness Ra /µm

Fig. 9 Effects of pulsating amplitude on both surface roughness and material removal rate in pulsed current ECM. a Variation of roughness Ra with pulsating frequency and b variation of MRR with pulsating frequency

0.6 0.4

0.30

0.25 0.2 0.0

0

5

10

15

20

0.20

25

0

5


electrolyte at the pulsating frequency of 10 Hz and amplitude of 0.2 MPa. The figure indicates that the 3D profile for pulsating electrolyte of 10 Hz and 0.2 MPa obtains a better surface finish. Figure 9 shows the variation of both the surface roughness Ra and MRR with varying pulsating frequency. With a constant electrolyte at 0.4 MPa, the surface roughness Ra and MRR are 0.67 μm and 0.38 g/min in pulsed current ECM, respectively. When the pulsating electrolyte at 1 Hz and 0.2 MPa was introduced to ECM instead of a constant electrolyte, the surface roughness Ra increased to 0.98 μm and the MRR decreased to 0.36 g/min. When the frequency increased to 2 Hz, the surface roughness Ra abruptly decreased to 0.64 μm while the MRR remained at 0.36 g/min. When the frequency varied from 5 to 10 Hz, both the surface roughness and MRR curves became flattened. Additionally, both the minimum surface roughness Ra of 0.53 μm and maximum MRR of 0.39 g/min are observed at the frequency of 10 Hz. When the frequency further increased to 20 Hz, the surface roughness Ra increased to 0.75 μm and the MRR decreased to 0.37 g/min. The influence of pulsating amplitude on both surface roughness Ra and MRR is shown in Fig. 10. At the pulsating amplitude of 0.1 MPa, the surface roughness Ra and MRR are 0.75 μm and 0.34 g/min, respectively. When the pulsating amplitude increased to 0.2 MPa, the surface roughness Ra suddenly dropped to 0.54 μm and the MRR increased to 0.38 g/min. The surface roughness Ra gradually increased

a

15

20

25

and the MRR gradually decreased when the pulsating amplitude increased to 0.4 MPa. In pulsed current ECM, each current pulse is followed by a relaxation time of zero current, which allows for removal of the reaction products and heat generated by the joule effect from the inter-electrode gap. The periodic replacement of the electrolyte allows for a small conductivity change. Therefore, the better surface finish may be generated by pulsed current ECM as compared with direct current density. However, the electrolyte conductivity of the inlet is slightly better than that of the outlet. When a pulsating electrolyte is applied, the difference of the electrolyte conductivity in the interelectrode gap with the correct pulsating electrolyte parameters would decrease further. Thus, the difference of the electric potential in the inter-electrode gap using the pulsating electrolyte is smaller than that using the constant electrolyte, which may lead to more homogeneous machined surface profile. Alternatively, when compared with a constant electrolyte, the higher MRR is observed in pulsating electrolyte with the correct pulsating electrolyte parameters. It should be noted that the lowest surface roughness is observed when the MRR is the highest in the pulsed current ECM experiments. This indicates that the higher potential is obtained at the outlet when the proper pulsating electrolyte is used. The pulsating flow frequency and amplitude affect the heat transfer and mass transfer. In the present pulse current ECM

b

1.2

0.40

1.0 0.35 0.8

MRR /gmin-1

Roughness Ra /µm

Fig. 10 Effects of pulsating amplitude on both surface roughness and material removal rate in pulsed current ECM. a Variation of roughness Ra with pulsating amplitude and b variation of MRR with pulsating amplitude

10


0.6 0.4

0.30

0.25 0.2 0.0 0.0

0.1

0.2

0.3


0.4

0.5

0.20 0.0

0.1

0.2

0.3


0.4

0.5


experiment, negligible influence of the pulsating electrolyte occurs on the mass transfer process at a frequency of 1 Hz and amplitude of 0.2 MPa. This leads to the roughest surface and lowest MRR.

4 Conclusions In this report, electrochemical machining with a pulsating electrolyte is presented as a way to machine Ti6Al4V. When a pulsating electrolyte with the proper parameters is applied, high mixing and turbulence are generated in the fluid and heat boundary, which increases the heat transfer rate and is beneficial for reducing the difference of the electrolyte conductivity in the inter-electrode gap. The smaller difference in electrolyte conductivity and potential leads to a more homogeneous and smoother surface profile. Based on the experimental investigations, the conclusions can be summarized as follows: 1. In direct current ECM, the better surface profile and higher MRR could be obtained by using pulsating electrolyte. The significant machining parameters for ECM with pulsating electrolyte are pulsating electrolyte flow frequency and amplitude. The roughness decreased from 5.7 μm at a constant pressure of 0.4 MPa to 3.7 μm at a pulsating pressure of 10 Hz in frequency and 0.2 MPa in amplitude, and the material removal rate increased from 0.85 to 0.92 g/min. 2. In pulsed current ECM, the pulsating electrolyte is similarly beneficial in improving the machined surface roughness and MRR when the proper pulsating electrolyte flow frequency and amplitude are used. The roughness decreased from 0.67 μm at a constant pressure of 0.4 MPa to 0.53 μm at a pulsating pressure of 10 Hz in frequency and 0.2 MPa in amplitude, and the material removal rate increased from 0.37 to 0.39 g/min. However, the effects of pulsating flow on pulsed current ECM are less efficient than those on direct current ECM. 3. ECM with pulsating electrolyte is a promising topic in the enhancement of surface roughness. Acknowledgments The authors wish to acknowledge the financial support provided by the China Natural Science Foundation (51175258) and the Funding of Jiangsu Innovation Programme for Graduate Education (CXZZ11_0195).

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