Fenomenologia și optimizarea unor procedee de prelucrare hibridă, prin descărcări electrice asistate de ultrasunete

1. Metode hibride de prelucrare

Investigarea temei de cercetare propuse in prezentul proiect corespunde unui domeniu deosebit de important, ce este intr-o continua dezvoltare, si anume cel al tehnologiilor neconventionale.

Metodele hibride de prelucrare implica integrarea a doua sau mai multe metode diferite de prelucrare intr-un singur proces de prelucrare, capabil sa exploateze avantajele individuale ale metodelor in cauza si sa inlature/ diminueze eventualele efecte negative/ dezavataje specifice respectivelor metode.

Aparitia metodelor hibride de prelucrare a fost determinata in principal de dezavantajele individuale ce le prezinta unele dintre metodele neconventionale (de exemplu, productivitatea relativ mica a prelucrarii prin electroeroziune, precizia slaba a prelucrarii prin eroziune electrochimica etc.) ce au condus la cautarea unor modalitati de exploatare a avantajelor si minimalizare a dezvantajelor inregistrate in cadrul unor prelucrari bazate pe o singura metoda neconventionala.

Altfel spus, prin aplicarea unei metode hibride de preelucrare se urmareste nu doar reunirea efectelor pozitive ale metodelor individuale luate in consideratie, ci si sporirea unora dintre aceste efecte prin actiune sinergetica.

tinand cont de ponderea e care o are fiecare proces implicat in modificarea materialului semifabricatului, metodele hibride de prelucrare se impart in:

- metode hibride de prelucrare in care proceselor implicate se desfasoara simultan si ambele contribuie direct la indepartarea materialui din semifabricat (de exemplu, prelucrarea prin eroziune complexa, electrica si electrochimica);

- metode hibride de prelucrare in cadrul carora un proces are rolul de a facilita dezvoltarea in conditii mai bune a celuilalt proces implicat (de exemplu prelucrare prin eroziune electrochimica asistata de ultrasunete, unde rolul prezentei vibratiilor ultrasonice este de a impiedica formarea unei pelicule pasivante pe suprafata semifabricatului). in descrierea unor astfel de metode este dificil de stabilit care este procesul principal si care este cel secundar.

De mentionat ca sunt metode hibride de prelucrare bazate pe combinarea unei metode clasice de prelucrare cu una neconventionala (de exemplu, strunjire asistata de fascicul laser), metode hibride ce constau in aplicarea simultana a doua metode neconventionale (de exemplu, prelucrare prin eroziune complexa, electrica si electrochimica) si metode hibride ce unesc trei principii neconvetionale diferite, cum ar fi prelucrare prin eroziune complexa, electrica si electrochimica asistata de ultrasunete.

Continua si rapida dezvoltare a proceselor hibride de prelucrare se datoreaza atat utilizarii tot mai frecvente in domeniul constructiilor de masini a materialelor greu prelucrabile (cum ar fi ceramica, materialele compozite, materiale izolatoare, oteluri dure etc.), cat si introducerii unor piese de forme si suprafete tot mai complicate, de calitate superioara si de mica rugozitate.

Astfel de metode complexe de prelucrare (bazate pe combinarea unor diferite fenomene fizice, energetice, chimice, mecanice si electrice) sunt numite in diverse moduri; de exemplu, in limba engleza intalnim ,,cross machining processes [Rajurkar, K. P., Ross, R.F., Wei, B., Kozak, J., Williams, 1992, "The role of nontraditional manufacturing processes in future manufacturing industries," Manufacturing International, 1992, ASME, pp. 23-37], ,,hybrid machining" [Ebeid, S.J., Hewidy, M.S., El-Taweel, T.A., Youssef, A.H. Towards higher accuracy for ECM hybridized with low-frequency vibrations using the response surface methodology. Journal of Materials Processing Technology Elsevier Science S.A., S.U.A., 149, 2004; 432- 438], ,,combined process" [ Chuchro, M., Ruszaj, A., Zybura-Skrabalak, M., Altena, H., De Silva, A., McGeough, J.A., Kozak, J., Advantages of electrodischarge machining assisted by electrode ultrasonic vibrations, Proceedings of the Symposium Research on Clean Hybrid Micromachining (HMM) Processes, Krakow, 27 September 2001, p. A2-1 - A2-19].

in limba romana se folosesc conceptele de ,,procese hibride" sau ,,procese complexe" [ Herman, R.I.E. Prelucrarea prin eroziune electrica complexa, Timisoara: Editura Augusta, 1998].

O metoda de prelucrare rezultata din combinarea diferitor procese electrofizice si electrochimice de prelucrare ofera noi posibilitati tehnologice privind precizia, calitatea suprafetei, productivitatea etc.

Una din cele mai cunoscute si mai vechi metode hibride de prelucrare o constituie prelucrarea prin eroziune complexa, electrica si electrochimica care are la baza procese de prelevare de material din semifabricat pe cale electrochimica cat si electrica. Aceste procese se desfasoara in acelasi timp, dar sunt diferentiate in spatiu. Din punct de vedere teoretic este posibila realizarea unei prelucrari prin eroziune complexa, electrica si electrochimica, asistata de ultrasunete datorita posibilitatii de suprapunere in timp a acestor trei procese (electroeroziune, dizolvare electrochimica si fenomen de cavitatie) [Jain, V.K., Choudhury, S.K., Ramesh, K.M. On the machining of alumina and glass, Elsevier 2002].

Posibilitati de obtinere a unor metode hibride de prelucrare

Luandu-se in considerare fenomenele fizico-chimice ce interfera in desfasurarea lor, se considera ca principalele tehnologii neconventionale de prelucrare sunt: prelucrarea prin electroeroziune (EDM - electrical discharge machining), prelucrarea prin eroziune electrochimica (ECM - electrochemical machining), prelucrarea prin eroziune chimica (CM- chemical machining), prelucrarea cu fascicul laser (laser beam machining - LBM), prelucrarea cu fascicul de electroni (electron beam machining- EBM), prelucrarea cu jet de apa (water jet machining - WJM), prelucrarea cu ajutorul ultrasunetelor (ultrasonic machining - USM).

In tabelul 1 sunt prezentate cateva din metodele hibride de prelucrare, obtinute prin combinarea a doua metode neconventionale, cunoscute in literatura de specialitate.

Astfel, se pot enumera:

- prelucrarea prin eroziune complexa, electrica si electrochimica (ECDM - electrochemical discharge machining);

- prelucrarea prin eroziune electrochimica asistata de ultrasunete (USMEC - ultrasonic machining with electrochemical assistance ( H. El-Hofy, B.A. Khairy- "A comparative study on produced geometry and surface integrity by electrochemical machining, electrodischarge machining and electroerosion-dissolution machining", IEMS -5 Conference, University of Central Florida, March 1995, 315-327);

- EDMLB - electroeroziune asistata de laser (denumita, in limba engleza, si arc laser machining (V.S. Kovalenko - "Ways to intensify laser hardening technology". Annals of the CIRP, Swissland, 47, 1, 1998, 133-136);

- EDMUS- prelucrarea hibrida prin electroeroziune asistata de ultrasunete, aplicata la prelucrarea electrozilor - scula din grafit, la obtinerea micro-orificiilor, a fantelor sau cavitatilor complexe. Se asigura o mai buna calitate a suprafetei prelucrate, cu un strat afectat termic de grosime mai mica;

- ECMUS - prelucrare hibrida prin eroziune electrochimica asistata de ultrasunete, presupunand utilizarea unui lichid electrolitic cu particule abrazive in suspensie ce asigura atat dizolvarea anodica a materialului semifabricatului, cat si aparitia fenomenului de cavitatie, in prezenta ultrasunetelor; acest fenomen impiedica formarea peliculei pasivante;

- CMLB - eroziune chimica asistata de laser;

- CMEB - eroziune chimica asistata de fascicul de electroni (de exemplu, electronolitografierea);

- LBMWJ - prelucrare cu fascicul laser asistata de un jet de apa (firma Laser - Microjet a produs un echipament de prelucrat cu fascicul laser ghidat de un jet de apa[Hoogstrate]);

- LBMUS - prelucrare cu laser asistata de ultrasunete (laser beam machining assisted by

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