____________________ * Corresponding author: Daniel GHICULESCU, University “Politehnica” of Bucharest, 313 Splaiul Independentei, sector 6, Bucharest, Romania, +40724209377, email: daniel.ghiculescu@nsn.pub.ro, liviudanielghiculescu@yahoo.com MODELLING ASPECTS OF REMOVAL MECHANISM AT ULTRASONIC AIDED ELECTRODISCHARGE MACHINING D. Ghiculescu * , N. I. Marinescu, S. Nanu University “POLITEHNICA” of Bucharest, ROMANIA ABSTRACT: The paper deals with researches concerning modelling of material removal mechanism at microgeometric level by computer aided finite element method (FEM) of ultrasonic aided electrodischarge machining (EDM+US). Simplifying hypotheses, boundary conditions and the method of meshing are presented. The approached microgeometries modelling can be assimilated as results of previous machinings: frontal milling, grinding, rough EDM etc. The microgeometries are parameterized to highlight the influence of various elementary surfaces and material characteristics on temperature distribution produced by discharges under conditions of ultrasonically aided EDM. At classic EDM finishing, FEM results emphasized the advantage of working with relaxation pulses, against commanded ones. When ultrasonic aiding of EDM finishing, commanded pulses become more effective compared to relaxation pulses. Moreover, commanded pulses can be produced within the ultrasonic semiperiod of liquid stretching, when working conditions facilitate the machining rate increase. In all cases, ultrasonic contribution within removal mechanism enlarges 4-5 times the volume removed by a single discharge. The comparison between FEM results and previous experimental data pointed out that the removal mechanism modelling, taking into account disposal and machined microgeometry dimensions, is essential in progress of understanding the complex process of EDM+US. KEYWORDS: electrodischarge machining, ultrasonics, removal mechanism, microgeometry, modelling. 1 INTRODUCTION The electrodischarge machining aided by ultrasonic longitudinal oscillations of electrode-tool (EDM+US) is based on a very intricate mechanism of material removal. Cavitational phenomena, ultrasonically induced within the working gap play an important part. At classic EDM finishing, the process presents high instability mainly due to very narrow working gap, lower than 10 μm. At EDM+US finishing, the main output technological parameters are remarkably improved: machining rate (V W ) up to 500% [1,2,3], surface roughness (R a ) and volumetric relative wear (ϑ) up to 50% [4,5]. Basically, these are the results of the cumulative microjets (cumulative implosion of gas bubbles from working gap developing pressure of MPa order) action produced at final of each ultrasonic oscillation period. They are parallel to machined surface and remove much more material in liquid state (melted by electrical discharges) and solid state (the microgeometry peaks with lower shear resistance) [5]. 2 MICROGEOMETRY MODELLING 2.1 MATERIAL CHARACTERISTICS As input data for computer FEM microgeometry modelling, the thermo physical characteristics of main constituents of materials to be machined are considered. In case of steels micromachining, thermo-physics characteristics – specific heat, density, thermal conductivity, melting point - of ferrite and cementite were inserted. They have dimensions of μm order, the same as EDM spots produced on machined surface. 2.2 BOUNDARY CONDITIONS The types of boundary conditions were: (a) constant surface temperature; (b) constant output thermal flux. These were based on Utsumi’s measurements concerning the spot temperature and Conn’s hypothesis related to narrowing of the plasma channel in the cathode zone, previously experimentally confirmed [6], also in high agreement with our experimental data [3,4,5]. Consequently, the following boundary conditions related to (a) are inserted: at EDM micromachining with relaxation pulses, negative polarity (electrode-tool is cathode), the temperature on anodic spot, was t aspot =2500 o C and its radius, R aspot =10 μm; at EDM micromachining with commanded pulses, with positive polarity (electrode-tool is anode), cathode spot temperature is t cspot =2550 o C and its dimension, R cs =2.5 μm. For condition (b), a constant output thermal flux of Φ = 0.1 W/mm 2 was taken into account.