Effects of pulsating electrolyte flow in electrochemical machining
Introduction
Electrochemical machining (ECM) removes metal material by controlled anodic dissolution in an electrolytic cell. Compared to other typical machining technologies, ECM has superiority by having a high material removal rate, a good surface integrity, is stress free, and has no tool wear or metallurgical defects. ECM is a promising and low-cost process for yielding various components of difficult-to-machine materials, and has been well established in diverse applications, such as turbine blades, airfoils, and surgical implants (Rajurkar et al., 1999).
In ECM, the electrolyte with a velocity of 10–30 m/s is pumped into the inter-electrode gap to remove waste products (gases and metallic hydroxides) and Joule heat. The distributions of gases and Joule heat affect the electrolyte electrical conductivity and determine the machining accuracy. Therefore, many studies have focused on the disposal of metal hydroxide sludge and an efficient process simulation. Various approaches have been developed to modify the electrolyte flow regime and enhance the electrolyte refreshment in the inter-electrode gap. Rajurkar and Zhu (1999) found that orbital electrode movement reduces the flow disrupting phenomena and improves the machining accuracy and machining stability. Hewidy et al. (2001) proposed that orbital electrode movement eliminates the presence of the spikes and enhanced the ECM accuracy. Ruszaj et al. (2003) demonstrated that electrode ultrasonic vibration causes heterogeneous cavitation of electrolyte flow in the gap, and significantly improves the removal of heat and products out of the machining gap. Hewidy et al. (2007) shown that low frequency tool vibration provides a positive effect by changing the physical conditions in the inter-electrode gap, and enhances electrolytic renewal and the removal of sludge products. Wang et al. (2010) proposed that reverse electrolyte flow pattern with vacuum-extraction could prevent the occurrence of cavitations and diminishes sparking and formation of striations.
Furthermore, abundant computational methods have been used to analyse the characteristics of electrolyte flow, the distributions of gas and temperature, and acknowledge the anodic shaping rules. Analytical solutions (Hopenfeld and Cole, 1969) were obtained to describe the one-dimensional equilibrium-cutting gap along the flow direction. Fujisawa et al. (2008) established a multi-physics model, including multiphase flow, thermal fields and electric fields, to predict the final shape of a three-dimensional compressor blade. Van Tijum and Pajak (2008) used a multi-physics approach to support the design of the ECM process for machining the electric shaver. Lee et al. (2009) applied a multi-physics model, consisting of electric, convection and diffusion, to predict the parametric effects on machining accuracy. Deconinck et al. (2011) proposed the multi-ion transport and reaction model to describe the effects of water depletion, and temperature on the anodic process in ECM.
Havemann and Rao (1954) shown that periodic fluctuations of fluid flow create different hydrodynamic characteristics and alter the thickness of the boundary layer; therefore, the pulsating flow of optimised pulsating parameters is beneficial to the transfer process. Pulsating flow has been applied to heat exchange, ramjet combustion, solid fermentation, drip irrigation emitter (Benavides, 2009).
Recently, the low-frequency tool vibration has been introduced to ECM, and high precision was obtained. When the electrode vibrates, the gap dimension oscillates in the same amplitude, and an electrolytic fluctuation is observed. However, this pulsating flow generated in an oscillating gap is different from those in a constant gap, which have been well applied in heat and mass transfer. Research shows limited studies on this pulsating flow in electrochemical machining.
In this study, attempts have been made to generate the pulsating flow via a servo-valve in the electrolytic supply pipe. This work focuses on the improvement of the heat transfer and material removal rate in ECM. A multi-physics model coupling of electric, heat, transport of diluted species, and fluid flow is presented to study the variations of electrolytic velocity, electrolytic temperature, and ion concentration along the flow direction near the workpiece surface. Moreover, experiments have been conducted to verify the feasibility of the proposed process.
Section snippets
Theoretical models
ECM is a field-synergy electrolysis process, which consists of mass transfer, energy transfer, momentum transfer and chemical reactions (McGeough, 1974). When a voltage is applied across the cathode tool and the anodic workpiece, the metallic ions of the anodic dissolution migrate from the anode surface to the electrolyte by an electrical force and are formed to insoluble hydroxides in neutral solutions. At the same time, hydrogen and oxygen is generated on the cathode and anode surface,
Simulation results
The evolution procedures of process variables, such as gas void fraction and temperature, were simulated in the ECM process with pulsating electrolyte flow. Furthermore, the effects of pulsating parameters on the transfer of heat, material anodic dissolution rate and surface profile were studied. Simulations were performed in transient mode with initial expressions of zero.
Experimental procedures
An ECM system, as shown in Fig. 11, was constructed to study the effects of pulsating electrolyte flow on material removal and surface profile in the ECM process. The pulsating flow is modulated by a Get-type electro-hydraulic servo valve (RT6615E, Radk-Tech, China), which can quickly respond to a broadband 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 SS304, with
Conclusions
Pulsating electrolyte flow was introduced to enhance heat and mass transfer for ECM by a modified electrolyte flow approach. The conclusions can be summarised as follows:
- 1.
A multiple physics model was established to study the distribution of several physical variables along the flow direction, which were critical to ECM. The simulation results indicate that proper pulsating parameters would enhance the heat transfer and material removal rate. The predicted profiles agreed well in trend with the
Acknowledgements
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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