On se pose quelquefois la question, lorsque les pièces présentent trop de défauts internes où lorsque les caractéristiques mécaniques sont insuffisantes. Faut-il passer en thixocoulée ?

Histoire
La thixocoulée est née dans les années 1970 aux USA au MIT (Massachusetts Institute of Technology/USA) sous l'impulsion du Prof. Merton C. Flemings.

Merton_Flemings
L'inventeur de la thixocoulée (MIT)

Coulée en phase pâteuse
Rappelons que la thixocoulée consiste, dans une technologie proche de la fonderie sous pression, à injecter l’alliage en phase pâteuse (intervalle semi-solide) dans un état thixotrope (globulaire et non plus dendritique).

Principe de la thixocoulée
En pratique, en thixocoulée (thixocasting), le fondeur achète de l'aluminium thixo, sous forme de barres, qu'il découpe en billettes (au poids de la grappe). Ces billettes sont réchauffées par induction (carrousel multipostes verticals ou système monoposte horizontal) et déposées dans le conteneur de la machine à l'état semi-solide pour être injectées. Un système de diaphragme au niveau du système d'alimentation permet à la peau d'oxyde en surface d'être scalpée et de rester dans le système d'alimentation.

Les alliages transformés sont en général de l'Al Si7Mg (A356) qui permettent d'obtenir de hautes propriétés mécaniques (Rp0.2, A%).

rhocasting_microstructure
Structure dendritique (gauche) et globulaire thixotrope (droite)

L'excellente santé interne des pièces moulées en thixocoulée (absence de porosités de type retassure et soufflure) permet de réaliser des traitements thermiques (souvent indispensables pour obtenir l'allongement souhaité).

Par rapport au moulage coquille gravité ?
La thixocoulée (et la rhéocoulée) sont en concurrence directe avec le moulage gravité moule métallique (coquille ou basse pression) car transformant les mêmes matérieux (type Al Si7Mg) pour des pièces nécessitant de l'allongement.

Qu'apporte donc "en plus" la transformation à l'état pâteux ? :
- Un temps de cycle réduit (plus faible même qu'en fonderie sous pression) assurant une meilleure productivité.
- Un meilleur état de surface de la pièce (très comparable à celui obtenue en fonderie sous pression).
- Une meilleure précision dimensionnelle (concept de "near net shape") permettant de limiter l'usinage ultérieur (souvent plus coûteux que l'opération de fonderion elle-même).

3_process_compar_s
Remplissage comparatif sur pièce test - Thixo, fonderie sous pression et fonderie sous pression avec sous vide - étude CTIF

globulaire_thixo_12013
Santé pièce (absence de pores en zone massive) en thixocoulée dans un bossage (20 x 25) en Al Si7Mg

Malheureusement, la technologie de thixocoulée, après de grands espoirs suscités dans les années 1990 et un début de développement industriel réussi techniquement un peu partout dans le monde (Stampal en Italie, ...), s’ai révélée très limitative (onéreuse, pointue, surcout matière important, process concurrent comme le moulage coquille gravité, ...). La plus grosse production fut celle des "rampes d'injection".

La rhéocoulée
La rhéocoulée, qui fait l’objet de nombreuses recherche (procédés NRC, SSR, SEED, …) pourrait relancer le developpement industriel de la transformation en phase pâteuse (technologie à mi-chemin entre la fonderie et la forge). La rhéocoulée à l’avantage de permettre la production de la matière thixotrope chez le fondeur alors que la thixocoulée exigeait un approvisionnement spécifique (une matière en barre). La rhéocoulée remplace dès à présent la thixocoulée pour la production de pièce à l'état semi-solide.

thixocoul_e__billette
Billette semi-solide (phase pâteuse)
L'image "Symbole" de la thixocoulée

Les différents procédés de Rhéocoulée :
1. NRC d'UBE
Le procédé de rhéocasting NRC (pour New Rhéocasting) a été développé et commercialisé par le constructeur de machien à couler sous pression japonnais UBE. Le NRC est le 1er procédé a être arrivé sur le marché. Au démarrage, il était commercialisé uniquement sur chantier UBE vertical (type HVSC 350). Depuis, UBE commercialise le carrousel de production de matière sans la machien, autorisant d'équiper ainsi une machine d'un autre constructeur. La matière liquide est refroidi par air sur les différents postes d'un carrousel

2. SEED d'Alcan
Le CTA-CNRC (canada) et Rio Tinto Alcan ont développé lanouvelle technologie SEED (brevet Alcan) qui permet d'équiper une machine existante de n'importe quel constructeur. L'alliage versé dans un godet est brassé par vibration et versé par le fond du godet dans le conteneur.

3. SSR de BuhlerPrince
Le SSR (pour Semi-Solid Rheocasting) est une technologie mise au point au MIT et commercialisée par BuhlerPrince (initialement par Idra). Cette technologie utilise le principe d'un brassage mécanique par une canne en graphite pour passer progressivement l'alliage liquide en une gelée semi-solide versée dans le conteneur de la machine.

4. D'autres technologies
De nombreux brevets voient le jour et des nouveaux centres de R&D et des acteurs industriels développent des technologies de rhéocasting.

rhocasting__jap
Brevet japonnais de machine rhéocasting - la verra t'on un jour ?

Notre avis :
Pour répondre à la question initiale "faut-il produire en rhéocoulée ?", Explorer d’abord d’autres process, plus répandus (moulage gravité coquille ou sable, …) et donc plus opérationnels et moins couteux avant d’imaginer produire des pièces en rhéocoulée. Mais pour de la grande série type automobile, pourquoi pas ...

Info Merton Flemings
***************
Merton C. FlemingsProfessor of Materials Processing

Toyota Professor Emeritus
Director, Lemelson - MIT Program

SB Metallurgy, MIT, 1951
SM Metallurgy, MIT, 1952
ScD Metallurgy, MIT, 1954

4-415, 77 Mass. Ave., Cambridge, MA 02139
617-253-3233 (phone)
flemings@mit.edu

Merton C. Flemings received his S.B. degree from MIT in the Department of Metallurgy in 1951. He received his S.M. and Sc.D. degrees, also in Metallurgy, in 1952 and 1954, respectively. From 1954 to 1956, he was employed as Metallurgist at Abex Corporation, Mahwah, New Jersey, and in 1956 returned to MIT as Assistant Professor. He was appointed Associate Professor in 1961, and Professor in 1969. In 1970, he was appointed Abex Professor of Metallurgy. In 1975, he became Ford Professor of Engineering and, in 1981, Toyota Professor of Materials Processing. He established the Materials Processing Center at MIT in 1979 and was its director from 1979 to 1982. He served as Head, Department of Materials Science and Engineering, from 1982 to 1995 and thereafter returned to full-time teaching and research as Toyota Professor. He was Visiting Professor at Cambridge University in 1971, Tokyo University in 1986 and at Ecole des Mines de Paris in 1996. He served as MIT Co-Director of the Singapore-MIT Alliance, a major distance educational and research collaboration among MIT and two Singaporean universities from 1999-2001. He is currently Professor of Materials Processing and Director of the Lemelson-MIT Program, a program at MIT which has as its aims to honor inventors and to inspire inventiveness in young people.

Professor Flemings’ research and teaching concentrate on engineering fundamentals of materials processing, and on innovation of materials processing operations. He has been active nationally and internationally in strengthening the field of Materials Science and Engineering and in delineation of new directions for the field. He is a member of the National Academy of Engineering and of the American Academy of Arts and Sciences. He is author or co-author of 300 papers, 26 patents, and two books in the fields of solidification science and engineering, foundry technology, and materials processing. . He has worked closely with industry and industrial problems throughout his professional career, and currently serves on a number of corporate and technical advisory boards. He is member of the board of the Silk Road Project, a non-profit foundation established to illuminate contributions of the Silk Road to the arts and society broadly, and to support innovative collaborations among artists of the Silk Road and the West.

He received the Simpson Gold Medal from the American Foundrymen’s Society in 1961, the Mathewson Gold Medal of TMS in 1969, the Henry Marion Howe Medal of ASM International in 1973, and became a Fellow, ASM International in 1976. In 1977, he was awarded the Henri Sainte-Claire Deville Medal by the Societe Francaise de Metallurgie. In October 1978, he received the Albert Sauveur Achievement Award from ASM International. In 1980, he received the John Chipman Award from AIME. In 1984, he was elected an honorary member of the Japan Foundrymen’s Society, and in 1985 received the James Douglas Gold Medal from the AIME. The Italian Metallurgical Association awarded him the Luigi Losana Gold Medal in 1986, and he was elected honorary member of The Japan Iron and Steel Institute in 1987. He was elected a TMS Fellow in 1989. In 1990, he received the TMS Leadership Award, the Henry Marion Howe Medal, and delivered the Edward DeMille Campbell Memorial Lecture of ASM International. In 1991, he received the Merton C. Flemings Award from Worcester Polytechnic Institute. Sigma Alpha Mu elected him a Distinguished Life Member in 1992. In 1993, he received the TMS 1993 Bruce Chalmers Award and was elected Councillor of Materials Research Society. He was served on the ASM International Board of Trustees, 1994-1997.

Professor Flemings received the Acta Metallurgica J. Herbert Holloman Award in 1997 for “contributions to materials technology that have had major impact on society”. Also in 1997 he was appointed David Turnbull Lecturer of the Materials Research Society for “outstanding contributions to understanding materials phenomena and properties”. He received the Educator Award of TMS in 1999. Also in 1999 he received the FMS (Federation of Materials Societies) National Materials Advancement Award, for “outstanding capabilities in advancing the effective and economic use of materials and the multi-disciplinary field of materials science and engineering generally, and for contributions to the application of the materials profession to national problems and policy.” He was awarded the ASM and TMS Distinguished Lectureship in Materials and Society for the year 2000, for the “invention of numerous new solidification technologies which are widely used industrially, and for leadership in defining the national agenda in materials science and engineering.”

In 2002, he was appointed Honorary Foreign Member, Korean Academy of Science and Technology, and two endowed chairs were established in his name at the Massachusetts Institute of Technology. He was awarded an honorary doctorate by the Swiss Federal Institute of Technology in Lausanne, in 2004 for “his role as pioneer and for his exceptional scientific contributions in the field of solidification and foundry.” In 2005 he received the Gold Medal of the Japan Institute of Metals. He also received in 2005 the Albert Easton White Distinguished Teacher Award from ASM, “in recognition of unusually long and devoted service in teaching as well as significant accomplishments in materials science and engineering and an exceptional ability to inspire and impart enthusiasm to students.”

He was elected Honorary Member of AIME in 2006, “for his pioneering work in solidification processing, for the development of novel processes which are used commercially, for his leadership in expanding the field of metallurgy to materials engineering, and to materials science and engineering, and for his leadership in establishing a national agenda for the field of materials.”

Selected Publications :
- Solidification Processing, McGraw-Hill, New York, (1974).
- “Solidification Science and Engineering Practice,” MRS David Turnbull Lecture, MRS Bulletin, 23 (1998) 30–36.
- “Materials Education for the New Century.” MRS Bulletin, 26 (November 2001) 918–924. With S. Suresh.
- Encyclopedia of Materials Science and Technology, Elsevier, London (2001). Edited with K.H.J Buschow, R.W. Cahn, B. Ilschner, E.J. Kramer, S. Mahajan.
- “Evolution of Particle Morphology in Semisolid Processing.” R.A. Martinez and M.C. Flemings” Metallurgical and Materials Transactions, 36A ( 2005) 2205.
- “Traveling Technologies,” Along the Silk Road, E. ten Grotenhuis Ed., University of Washington Press (2002) 107–121.

Teaching Involvements :
- Spring 2007 3.048 Advanced Materials Processing
- Spring 2007 3.52 Materials Processing

An article in the Boston Globe, of June 16, 2006, “Inventing by trial, error and teamwork” by Naila F. Moreira read: “We see it very much as MIT’s mission to promote inventiveness and creativity among kids,” said Merton C. Flemings, an engineering professor at the Massachusetts Institute of Technology, and director of the Lemelson-MIT Program, which recognizes and supports invention.

On November 13, 2005, the New York Times summarized Prof. Flemings concerns about America’s future innovativeness. In the Jan. 23, 2005 “Starts and Stops” column of the Boston Globe, Prof. Flemings described the “a-ha” moment when he understood what determines a microstructure in cast metals. Prof. Flemings was quoted in a Tech Talk article on Americans’ perception of technology—most Americans value the toothbrush over other, more recent, innovations. He was interviewed by the Wisconsin State Journal for a Jan. 14, 2004 story on historic firsts.

Source :  Blog acier-Flemings



Février 2009