The Working through complex electrical erosion is a process of removal of the excess of  working through repeted action of electrical discharges within the impulse, in the limited space between an electrode connected at one pole and the object of working connected at the other pole, of a tension impulses generator. The paper presents the study of the elementary processes which unfold simultaneously within the elementary working space,  as  well as the knowing and  mastering of the physical mechanism of the  drawing material 

Alin NIOAŢĂ

Dan DOBROTĂ

Florin CIOFU

Directory of Open Access Journals

                                                  Fiabilitate si Durabilitate - Fiability & Durability     nr.1/2010                                 Editura “Academica Brâncuşi” , Târgu Jiu, ISSN 1844 – 640X  105  THE PRINCIPLE ON WHICH THE DIMENSIONAL WORKING IS BASED THROUGH COMPLEX ELECTRICAL EROSION  Lecturer dr. eng. Alin NIOAŢĂ,  Prof.dr.eng. Dan DOBROTĂ,  Lecturer dr. eng. Florin CIOFU,  Engineering Faculty, ”Constantin Brâncuşi” University,   Abstract: The Working through complex electrical erosion is a process of removal of the excess of  working through repeted action of electrical discharges within the impulse, in the limited space between an electrode connected at one pole and the object of working connected at the other pole, of a tension impulses generator. The paper presents the study of the elementary processes which unfold simultaneously within the elementary working space,  as  well as the knowing and  mastering of the physical mechanism of the  drawing material .  Keywords: erosion, cathode, anode, electrode, thermal effect.  The processing through complex, electrical and electrochemical erosion represents  a superposition and a sequence in time and space in the working areas of the activities for processing  through  electrical  erosion  and  electrochemical  erosion.  In  order  that  this superposition to be possible, a different structure of the elementary working area is necessary and also an adjustment determined by the necessary working conditions, specific to each and every activity.  The structure of the elementary working area is presented in the figure number 1. Thus, the material drawing is the consequence of the simultaneous action of two erosive agents. On the one hand the working environment, ML, the electrolyte formed of the aqueous solution of the ions RARB, which we find in the electrical field of I intensity and which determines a partial electrochemical dissolving of the anode (the object to be processed OP), and on the other hand the electrical discharges in impulses generated by the breaking of the micro-contacts crossed by the current, constituted at the level of the micro-asperities, of the anode (OP) and respectively of the cathode (the object to be transferred), and which determine the  thermal component of the drawing. Both the erosive agents are of “bridge” type.  The process is influenced by the formation on the anode of a passive film PF, insoluble, with  dielectric  properties  and  high  mechanical  resistance,  by  the  existence  of  a  pressure  p between the electrodes, combined with a relative tangent movement vr of OT and OP, with the aim of generating impulses of current and forced mechanical de-passivation for reactivating the anode and evacuation of the waste processing outside of the working area.   As a result of these actions, mainly the erosion OP takes place through the dislocation of the elements of material, as a consequence of the electrical discharges DE and of the effect of anodic dissolving.  The  cathode  is  subject  to  erosion,  in  a  smaller  extent,  through  the  dislocation  of  the elements of material, of the object to be transferred. Also, there is a possibility for the electrical discharges  to  appear  through  the  local  penetration  of  the  anodic  film,  which  dislocates supplementary particles of material.   Taking into account different values of the determinative factors of the working conditions (tension, current, pressure and the relative velocity between OT and OP) there is a possibility to become  preponderant  either  the  electrochemical  phenomenon  of  dissolving,  or  the  thermal phenomenon of erosion as an effect of the electrical discharges.                                                    Fiabilitate si Durabilitate - Fiability & Durability     nr.1/2010                                 Editura “Academica Brâncuşi” , Târgu Jiu, ISSN 1844 – 640X  106  Fig.1. The fundamental scheme of processing through complex erosion     In case the specific power introduced within the working area is low and the drawing takes place especially based on the anodic dissolving, the productivity is reduced, but the precision of processing and ruggedness of the surface are of superior levels. In increasing the specific power, the  erosion  of  the  material  is  owed  almost  exclusively  to  the  thermal  effect  of  electrical discharges.  In  this  case  a  drawing  of  material  takes  place,  which  is  owed  to  the  anodic dissolving,  simultaneously  with  the  drawing  of  electrical  impulses,  but  outside  the  surface affected by these discharges. Under the circumstances of the preponderance of the effect of electrical erosion, the local concentrations  of  energy  may  lead  to  high  temperatures  within  the  channel  of  electrical discharges with values of 104… 105K, and may lead beside smelting and local vaporization of the  material  in  the  surface  layer  of  OP,  to  decomposition  of  the  electrolyte.  The  results  of dissolving and decomposing of the electrolyte work together to form a passive film as well to determine their properties (Me+ + RA- = Me RA;  RB- + e- = RB; see figure number 1 ).    Reactivation of the anode through forced removal of the film enables the maintenance of some simultaneous discharges or partially superposed and consequently also a well- marked Joule  thermal  effect  is  possible  due  to  the  concentration  of  the  current  on  small  surfaces. Densities of 103… 105 A/cm2 are reached, whereas the duration of a discharge is of 10-3… 10-5s. The greatest part of these energy is transmitted to the anode, which otherwise is subject to an intense erosion, erosion which constitutes the main effect of technological interest.   These local  concentrations of energy put the material to  great stresses and as a result, beside a well- marked global erosive effect, certain metallurgical phenomena take place which lead to some layers thermally modified to emerge, having special characteristics, as well as some  micro-fissures  in  these  layers.  These  alterations  testify  for  the  nature  preponderantly thermal of the processing through complex erosion.  Also  the  processing  through  erosion  means  a  complex  process,  which  includes  other characteristic elementary processes,  which takes place simultaneously  within the elementary working  area.  Studying these elementary processes, as  well as  knowing and mastering the physical mechanism of drawing the material “may and must help” the process of generalization the processing through complex electrical erosion.  The great number of parameters and factors in the process, the co-existence and great speed of variation of the elementary processes, as well as the reduced space in which these unfold, have led however to the lack of a unitary theory referring to the drawing of material, which still remains a problem un-clarified completely. Nevertheless most of the scientists agree with the theory of the thermal drawing, appraising as determinative the thermal effect caused by: the energy unfolded through Joule- Lenz process due to the reduction in area phenomenon  of the current lines at its crossing to the electrode which presents a larger section for the discharging channel with small section and then to the                                                   Fiabilitate si Durabilitate - Fiability & Durability     nr.1/2010                                 Editura “Academica Brâncuşi” , Târgu Jiu, ISSN 1844 – 640X  107 semi- finished product, also with a larger section(figure no. 2 a);  the kinetic energy of the charges in motion (electrons and ions) in the discharging channel of the un-stationary electrical resort, suddenly given way at the surface of the semi-finished product and electrode (figure no. 2.c).  Together  with  the  thermal  processes,  which  have  the  greatest  share  in  the  drawing  of material, as we have already mentioned, concurrently some mechanical actions determined by the relative movement between the electrode and the semi- finished product take place, as well as some chemical processes of decomposing of the working environment.  Due to the interaction of the dissociated elements with the smelted metal, on the surface of the semi- finished product (figure no. 2.b) and the electrode, a film of oxide will be formed. Consequently, electrical discharges in impulse may take place, caused by the easy penetration of the oxide film (figure 2.d) with low dielectric rigidity, or of the areas with impurities.  The special literature estimates that for the processing through complex electric erosion with breaking contact, the drawing of material is the consequence of unfolding two preponderant elementary processes such as: 1.  the electric contact; 2.   the un-stationary electric resort;                                Fig.2. The elementary working area a-electric contact; b-oxide film; c-discharge in un-stationary electric resort; d-discharge through penetration; e-erosive particle  The electric contact, as a thermal source in the process of processing through complex electric  erosion,  starts  the  moment  when  the  physical  contact  is  realized  between  the  most prominent micro- asperities, one of them pertaining to the processed object (OP), and the other to the transferred object (OT) (figure 1). The contact is realized in a kinetic way by the relative velocity  between  the  two  bodies,  by  the  pressure  of  contact,  as  well  as  by  the  depth  of processing.  The un-stationary electric resort. The breaking of the bridges is important because it represents the previous stage of the electric resort, if the operating conditions allows its start, that is if the tension applied overcomes the potential of ionization of the metal vapours (9V). This method of starting the resort, through breaking the contact between the metallic bodies crossed by the current, has a large applicability in practice since it has the advantage not to need the penetration of the environment between the electrode- tool and the semi-finished product. Thus, the sources of supply are no longer needed to provide high tensions, but only values to start and support the resort. Although, it is necessary for the same polarity to be maintained for at least the same interval of time as for ions displacement toward the cathode. Consequently in order to realize the processing reduced sources of tension are enough (to smelt the metal) and low  inductive  circuits  (low  self-inductance  to  realize  short  time  heating).  The  un-stationary electric resort as a thermal source of process represents the second stage of unfolding of the process.                                                    Fiabilitate si Durabilitate - Fiability & Durability     nr.1/2010                                 Editura “Academica Brâncuşi” , Târgu Jiu, ISSN 1844 – 640X  108 This stage is formed and developed when the electric contact is broken, when the free electrons in the peripherical area of the metal accumulated supplementary energy, due to the thermal effect of heating (Joule-Lenz effect leads to temperatures of 5000…6000 0C), which is sufficient to overcome the energetic necessity of leaving the metal and to produce the thermo- electronic emission.  Then classical processes of forming the conductor channel take place in an electric field between two irregularities with the emergence of the micro- resort, a strong ionization (the metal vapuors being present) and eventually the emergence of the thermal plasma in the column of discharging dependent of the local conditions and the resort length. The un- stationary electric resort is formed the moment when the distance cathode- anode is of 10-4 cm or less (of the size of medium way of the electrons which displace freely without collision, between the cathode and anode to whom are given way energies), having the duration of between 10-3 and 10-5s and is characterized by high tensions of current of 10…200A/mm2 and temperatures of 104…105K. The relative movement between the semi- finished product and the electrode- tool will lead to the  elongation  of  the  resort  and  consequently  to  modifying  of  the  discharging  parameters, lending to the phenomenon the character of limited impulse in time and space, having as a result the compression of the resort the moment when the speed of the ionization process of the resort space is higher than that of the process of ionization itself.  If the processing conditions are changed, this unfolding of the elementary processes of erosion  are  modified,  too:  for  instance,  if  the  tension  is  too  low,  then  the  contact  may  be interrupted without the consequence of discharging in the resort, or if the smelted metal is not removed in the elementary working area, then the interruption of a single process is followed by a new discharging in the resort. Consequently, there are three main possible types of erosion: -  contact erosion, at low tensions: U = 6…10V;  -  resort- contact erosion, at medium tensions: U 10..20V; -  erosion through resort, at high tensions: U =20…30V. Since the processing through complex electric erosion is not a process of stationary nature, in fact all the types of erosion mentioned may be developed, and depending on the values of the process  parameters  one  of  them  will  be  predominant,  determining  the  efficiency  of  the processing through electric erosion with breaking of contact. The erosion through contact being mainly uni-polar, it is the less efficient; in the case of the resort- contact erosion, the efficiency increases and the erosion through the resort is efficient only if the duration of the impulses is limited. In  conclusion,  during  the  process  of  processing  using  the  complex  electric  erosion proceeding, heating and evacuation of the metal at the level of the irregularities represents the result of accumulating the elementary processes, dependent directly on the local conditions and the way of distributing the energy between the semi- finished product and the electrode.  References [1] Gavrilaş, I. ş.a. - Prelucrări neconvenţionale în construcţia de maşini -Editura Tehnică, Bucureşti, 1991. [2]  Nioaţă  A.,  ş.a.-  Cloratul  de  sodiu  (naclo3)  –  lichid  anorganic  folosit  la  prelucrările dimensionale    prin  eroziune  complexă  electrică  şi  electrochimică,  Revista  „Fiabilitate  şi durabilitate” Nr.1, Editura „Academica Brâncuşi”, Tg-Jiu, 2008 [3]  Nioaţă  A,  ş.a.-  Rugozitatea  funcţie  de  putere  la  debitarea  oţelului  40CrMoV5  prin eroziune complexă cu introducerea electrolitului prin obiectul de transfer Revista „Fiabilitate şi durabilitate” Nr.4, Editura „Academica Brâncuşi”, Tg-Jiu, 2009 [4] Nioaţă, A – Cercetări teoretice şi experimentale privind optimizarea unor parametri ai prelucrării prin eroziune complexă, Teză de doctorat, Sibiu, iulie 2007 

THE PRINCIPLE ON WHICH THE DIMENSIONAL WORKING IS BASED THROUGH COMPLEX ELECTRICAL EROSION

Abstract

Similar works

Full text

Available Versions

Directory of Open Access Journals