Investigation on profile of microchannel generated by electrochemical micromachining
Introduction
Micro machining technology is progressing rapidly in the field of micro fabrication of micro devices used in communication, electronics, micro fluidics, MEMS and computer industry. The demand for micro parts in the above mentioned fields are very high as there is a trend in the society to have microproducts due to limitation of space, energy and material etc. One of the well known micromachining technology is the lithographic process used extensively for silicon machining. The material properties of silicon do not suit the requirement of micro surgery, biotechnology and high strength application at higher temperature. Micro EDM technology has also been used for the generation of microgrooves and micro holes etc. in electrically conductive advanced materials but inherent problem is heat affected zone in the work piece. There are specific advantages of Electrochemical micro-micromachining (EMM) over the other micro-machining techniques such as no heat affected zone, stress free smooth machined surface, machining capability of all advanced metals irrespective of hardness of material, no burr, no tool wear etc. EMM can be used for the micro fabrication of stainless steel for the possibility of wide application in the area of surgery, micro reactors, biotechnology etc. Lee et al. (2002) generated microgrooves with the variation of depth identified by machining current and inter-electrode gap size, on the air lubricated hydrodynamic bearing and thereby improving the possibility of wider use of microchannels or micro profiles in the high speed gas or air lubricated bearing. Bhattacharyya and Munda (2003) developed EMM set up and studied influence of machining process parameters on machining criteria such as material removal rate (MRR) and overcut phenomena. Li et al. (2003) applied side insulated tool to drill micro-hole using pulsed voltage and the improvement of machining accuracy was shown. Ahn et al. (2004) drilled micro holes on stainless steel plate using nanosecond pulses in H2SO4 electrolyte by fabricated tungsten carbide tools. Kozak et al. (2008) conducted EMM using ultra short (nanoseconds) pulses and presented a mathematical model of micro-shaping process, taking into consideration the unsteady phenomena in double layer. Kim et al. (2005) proposed taper reduction technique with disk type electrode and multi-holes were drilled at a single step in H2SO4 electrolyte. Ryu (2009) used citric acid as environment friendly electrolyte for the generation of micro square cavity and micro circular cavity on stainless steel. Schuster et al. (2000) used ultra short pulse voltage experimentally to fabricate an ultra size copper structure and copper tongue based on the finite time constant for double layer charging. Kirchner et al. (2001) fabricated three dimensional micro elements on stainless steel by applying nanosecond voltage pulses using a mixed electrolyte of HF and HCL. Hewidy et al. (2007) improved accuracy in ECM with the application of low frequency vibrations of tool and presented a model. Yang et al. (2009) used ultrasonic vibration of semi-cylindrical tool for the drilling of deep holes with reduced machining time and machining gap. Pattavanitch et al. (2010) applied boundary element method to model the slot milling by electrochemical machining and presented a 3D model with the discretized domain of the work piece and tool surfaces. Purcar et al. (2004) presented simulation model of 3D anodic shape change during electrochemical machining based on “marker” method. The work piece shape change was found by displacing each node proportional with, and in the direction of current density. Hocheng et al. (2003) presented a theoretical and computational model on the evolution of profile with the elapse of time while φ 100 μm holes were being machined by electrochemical machining. Wang et al. (2010) presented a simulation model and finite element based solution of current density distribution for predicting the shape evolution during through-mask EMM of surface patterning. Donglin et al. (2011) presented a mathematical model describing the EMM using inert metal mask (platinum plate) and simulation of shape evolution. Ghoshal and Bhattacharyya (2014) simulated machining zones with reversed taper tool and conical tool during EMM of microhole and predicted that both reversed taper and forward taper tool could be used for generation of taper less micro holes. Some of the above mentioned researchers dealt with shape evolution during drilling operation and very few described about the shape evolution during generation of microchannel. Moreover, all the shaping models reported by the researchers are related to straight tools only.
The cross sectional profile is an important attribute of microchannel from the point of view of application in micro fluidics, biomedical and engineering sector. Other attributes that define the electrochemical micromachining qualities of microchannel are size, overcut, taper angle and surface finish. In the present work, emphasis has been given to reduce taper angle of cross sectional profiles, produce good quality of surface and generate various shapes of micro-profile by the use of straight, conical and reversed taper micro tool during EMM. The major process parameters investigated were electrolyte concentration, vibration of micro tool and front end shape of micro tool on the two methods of blind microchannel generation i.e. the scanning machining layer by layer method (Malapati and Bhattacharyya, 2011) and the sinking and milling method (Ghoshal and Bhattacharyya, 2013).
Section snippets
Simulation models of machining zone with different shaped tools
Anode and cathode separated by a very narrow gap and having appropriate electrolyte between the electrodes results in the dissolution of anode and the shape generated depends on the current density distribution at the vicinity of anode surface which is going to be dissolved. In the case of pulsed power supply, there is pulse on time, Ton and pulse off time, Toff during each cycle as shown in Fig. 1. In electrochemical micromachining, the total current supplied during a pulse period is the sum
Planning of experiments
The experimental set up shown in Fig. 8 was used for the fabrication of micro tool and EMM purpose. H2SO4 electrolyte was used during experimentation as sludge is fully dissolved in H2SO4 electrolyte. The machining conditions are listed in Table 2. The concentrations of electrolyte were chosen from 0.05–0.3 M. EMM is almost impossible below 0.05 M concentration due to poor conductivity of electrolyte and material removal is caused by micro-sparking. High overcut limits the use of higher
Results and discussions
The experiments have been conducted in the present set up to analyze the effect of electrolyte concentration, front end shape of micro tool and vibration of micro tool on the cross sectional profile during the generation of microchannel.
Conclusions
In this paper, various types of cross sectional profiles generated with the help of straight micro tool, conical micro tool and reversed taper micro tool were investigated. The major process parameters investigated were electrolyte concentration and vibration of micro tool on surface finish. Based on the above investigations and analysis the following conclusions can be drawn:
- (i)
To reduce taper angle of blind microchannel, machining zones were simulated with straight, conical and reversed taper
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