ELECTROCHEMICAL MACHINING UNDER PULSED CURRENT CONDITIONS M. DATTA and D. LANWLT Material8 Department, Swiss Federal Institute of Technology, Lausanne, Switzerland zyxwvutsrqponmlkjihgfed (Received 29 Ocrober 1980) Abstnct-The possibility of using pulsed furrent in electrochemical machining at low electrolyte flow rate has been inve$igated. Theoretical aspects of predicting electrolyte heating and limiting rate of mass transport are discussed in terms of simplified models. High rate dissolution of nickel in sodium chloride solutions under pulsed current conditions was investigated in a Bow channel cell by studying the influence of different pulse parameters on anode potential, surfaa microtexture, surface roughness and current efficiency of metal dissolution. Obtained results indicate that ancdc potential and surface finish are controlled by nwu transport in agreement with steady state behavior. Maximum current density applicable under pulsed current conditions is limited by the occurrence of sparking. F L iI NOMENCLATURE bulk concentration concentration at x = 6, specific heat of the electrolyte surf= conaatrntion satuaratioa cotuxntration effective diffusion wefficient of the trans- port limiting species Faraday constant average pulse current density steady state limiting current density peak current density pulse limiting current density electrode length pulse time pulse period duty cycle temperature linear flow velocity of the electrolyte instantaneous tetnpcmture rise average temperature rise average steady state diGsion layer thickness pulsating diffusion layer thickness dectrolyte conductivity electrolyte. deasity current efficiency of metal dissolution current efficiency of metal dissolution at i = i,, i = i,, i = i,, INTRODUCTION In electrochemical machining (ECM), dissolution rate, surface finish and precision of the machined piece are related to the kinetics and stoichiometry of the elec- trode reactions. These parameters are strongly in- Buenced by prevailing mass transport conditions[ 1,2]. It is well zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA known that the influence of local variations within the inter-electrode gap of mass transport rates and of electrolyte conductivity due to the presence of gas and to the joule heating can be minim&d by working at high electrolyte flow velocity. On the other hand, high electrolyte flow rates require an elaborate pumping system and a heavy machine frame to maintain rigidity. The resulting installation and opcr- sting cost present a major limitation to the rider application of the ECM process. An alternate possibility is to use a pulsed current.In principle, this allows one to apply a high instantaneous current density without the necessity of high eIec- trolyte flow rate since each current pulseis followed by an off time such that the system relaxes between two consecutive pulses. One of the attractive features of pulse ECM (PECM) is that three parameters, namely, peak current density, pulse-on time and pulse-off time can be varied independently while in conventional ECM only average current can be chosen. In the present paper a theoretical analysis of the influenceof different PECM parameters on electrolyte heating and mass transport is presented together with experimental studies aimed at testing the theory by investigating the high rate dissolution behavior of nickel in 5M NaCl solution under pulsed current conditions. THEORETICAL CONSIDERATIONS In principle, either current or voltage pulses of any shape can be applied. For simplicity, the present discussion will be restricted to rectangular aMstapt current pulses separated by intervals of zero current. Figure 1 shows schematically the current pulses to be considered. Here i,, is the peak currept density, c, is the pulse-on time, t; IS the pulse-off time, t, is the pw period. The ratio cJc,,~ is called the duty cycle. PECM then takes place at an average current density given by 4 = ip&.ltrV) (1) Among different problems encoun&red during hjgb rate dissolution in narrow gapqelectrolyte zyxwvutsrqponmlkjihgfed heating aud 899