Europäisches Patentamt *EP000938593B1* (19) European Patent Office Office européen des brevets (11) EP 0 938 593 B1 (12) EUROPEAN PATENT SPECIFICATION (45) Date of publication and mention (51) Int Cl.7: C22C 19/07, B22F 3/15 of the grant of the patent: 09.10.2002 Bulletin 2002/41 (86) International application number: PCT/US97/20185 (21) Application number: 97948184.3 (87) International publication number: (22) Date of filing: 03.11.1997 WO 98/020177 (14.05.1998 Gazette 1998/19) (54) POWDER METALLURGY, COBALT-BASED ARTICLES HAVING HIGH RESISTANCE TO WEAR AND CORROSION IN SEMI-SOLID METALS PULVERMETALLURGISCHE, AUF KOBALT BASIERTE GEGENSTÄNDE MIT HOHER VERSCHLEISSFESTIGKEIT UND KORROSIONSBESTÄNDIGKEIT IN HALBFESTEN METALLEN ARTICLES A BASE DE COBALT, PRODUITS DE LA METALLURGIE DES POUDRES, PRESENTANT UNE RESISTANCE ELEVEE A L’USURE ET A LA CORROSION DANS DES METAUX SEMI-SOLIDES (84) Designated Contracting States: (56) References cited: AT BE CH DE DK ES FI FR GB IT LI LU NL SE EP-A- 0 466 401 (30) Priority: 04.11.1996 US 743335 • KUMAR P.: "Properties of P/M Stellite Alloy no. 6" PROG. POWDER METALL., no. 41, 1986, (43) Date of publication of application: pages 415-437, XP002059744 01.09.1999 Bulletin 1999/35 • HAMIUDDIN M: "Development of wear resistant, strong and fully dense stellite by liquid phase (73) Proprietor: Thixomat, Inc. sintering" POWDER METALLURGY Ann Arbor, MI 48104 (US) INTERNATIONAL, 1987, WEST GERMANY, vol. 19, no. 2, ISSN 0048-5012, pages 22-26, (72) Inventors: XP002059745 • KENNETH, E., Pinnow • BETTERIDGE W.: "Cobalt and its Alloys" 1982 , Pittsburgh, PA 15237 (US) ELLIS HORWOOD , GREAT BRITAIN • RAYMOND, F., Decker XP002059746 Pages 119-125 Ann Arbor, MI 48104 (US) • PATENT ABSTRACTS OF JAPAN vol. 096, no. 008, 30 August 1996 & JP 08 109405 A (NIPPON (74) Representative: Patentanwälte Dr. Solf & Zapf STEEL CORP), 30 April 1996, Candidplatz 15 81543 München (DE) Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). EP 0 938 593 B1 Printed by Jouve, 75001 PARIS (FR) 1 EP 0 938 593 B1 2 Description further appreciated, the component construction of the present invention will find applicability as articles, not BACKGROUND AND SUMMARY OF THE INVENTION only in the construction of machines 10 practicing the above method, but also in machines practicing alterna- [0001] The present invention relates to cobalt-base 5 tive variations on the above process and other process- articles having high resistance to wear and corrosion in es. Such machines and articles include, without limita- semi-solid metal environments. More specifically, the in- tion, die casting, metal injection molding, plastic injec- vention relates to fully dense powder metallurgy articles, tion molding machines as well as tools and dies. made from a novel Co-Cr-W-C type alloy being particu- [0004] Because of contact with corrosive semi-solid larly suited for long term use in high wear, high temper- 10 metals (such as magnesium and zinc), the elevated op- ature machinery employing a variant process of semi- erating temperatures, oxidation, and the high wear na- solid metal molding (SSM). ture of the environment (contact between the various [0002] The metallurgical process referred to herein is operating parts of the machine and the semi-solid metal one where metals and metal matrix composites are is an extremely high wear and shock condition), compo- heated and stirred in the solid plus liquid phase region 15 nents of the above machinery are very demanding on and then injected into a mold or die at lower tempera- their materials of construction. Screw velocities, for ex- tures. This process has proven to result in parts having ample, involve acceleration from 0 to 3 meters/sec. and improved material characteristics, previously uncasta- deceleration back down to 0, all in 0.2 seconds. The cho- ble and unobtainable shapes, and reduced post forma- sen materials of construction must be resistant to cor- tion processing steps. Two versions of the above proc- 20 rosive attack by the semi-solid metal being processed, ess, also known as Thixomolding® (Thixomat, Inc., Ann must be highly resistant to wear, and must exhibit suffi- Arbor, Michigan), are generally disclosed in U.S. Patent cient strength and toughness to withstand the stresses Nos. 4,694,881 and 4,694,882, which are herein incor- imposed during long-term exposure at the relevant ele- porated by reference. The process generally involves vated temperatures under these severe thermal cycling the shearing of a semi-solid metal so as to inhibit the 25 and high shock conditions. growth of dendritic solids and to produce nondendritic [0005] From a corrosion standpoint, iron and some solids within a slurry having improved molding charac- cobalt-based alloys have been reported as satisfactory teristics which result in part from its thixotropic proper- for processing semi-solid magnesium-base alloys. Nick- ties (a semi-solid nondendritic material which exhibits a el-base alloys, such as Alloy 718, are of interest as con- viscosity which is proportional to the applied shear rate 30 struction materials because of their good strength at el- and lower than that of the same alloy when in a dendritic evated temperatures and lower cost when compared to state). most cobalt-base alloys. However, because molten [0003] A machine adapted to employ the above type magnesium attacks nickel containing alloys, some SSM of processes and to which the present invention has par- processors have specified that alloys which come into ticular applicability is schematically shown in FIG. 1. The 35 contact with molten magnesium must contain less than construction of the molding machine 10 is, in some re- about three percent nickel. Prior machines avoided this spects, similar to that of a plastic injection molding ma- problem by using Alloy 718 in their barrel constructions chine. In the illustrated machine 10, feed stock is fed via while incorporating a shrink-fitted barrel insert made of a hopper 12 into a heated, reciprocating screw injection a cobalt-base alloy such as Stellite 6 (nominally 28 Cr, system 14 which maintains the feedstock under a pro- 40 4.5W and 1.2C) or Stellite 12 (nominally 30Cr, 8.3W and tective atmosphere 16, such as argon. As the feed stock 1.4C), which are commercially available from the Cabot is moved forward by the rotating motion of a screw 18, Corporation, Kokomo, Indiana. While generally per- it is heated by heaters 20 and stirred and sheared by forming well with respect to corrosion, they are deficient the action of the screw 18. This heating and shearing is in toughness and have exhibited cracking and fracture done to bring the feedstock material into its solid plus 45 in the machines of the above type. Under the high tem- liquid temperature range. The thixotropic slurry formed perature fatigue conditions of the machines, cracks in by this action passes through a nonreturn valve 22 in Stellite liners have been seen to propagate into the Alloy the forward part of the injection system 14 of the ma- 718 barrel resulting in total failure of the barrel assembly. chine 10 into an accumulation chamber 24. Upon accu- This is unsafe and necessitates costly repairs and re- mulation of the needed amount of slurry in the accumu- 50 placements. It has come to be determined that articles lation chamber 24, the injection cycle is initiated by ad- of an alternative material, having greater toughness, vancing the screw 18 with a hydraulic actuator and caus- would be more desirable in that they would provide for ing the mold 26 to fill through a nozzle 28. As opposed longer wearing components. to other methods of semi-solid molding, the above de- [0006] The selection of materials for processing semi- scribed method has the advantage of combining slurry 55 solid aluminum-base alloys is much more complex. This generation and mold filling into a single step. It also min- is particularly true because most iron, cobalt, and nickel- imizes the safety hazards involved in melting and cast- base alloys are readily attacked by aluminum alloys. In ing reactive semi-solid metals. Obviously, and as will be addition to these concerns and those recited in connec- 2 3 EP 0 938 593 B1 4 tion with processing magnesium, other important con- ing) of gas atomized prealloyed powders. In general, pri- cerns relate to the availability, cost and manufacturing or studies have shown that PM processing of these ma- characteristics of the construction material. terials produces a material with higher hardness, higher [0007] In the injection molding of magnesium-base al- tensile strength, and higher ductility than is achieved by loys, the maximum operating temperatures within the 5 casting the alloys and that these improvements are still barrel typically range between about 593°C and 649 °C retained at elevated temperature. The abrasive wear re- with the temperatures sometimes ranging to 816 °C. sistance of these PM materials is somewhat lower then Most common AISI iron-base hot work tool steels (such their cast counterparts owing to the smaller sizes of the as H-10 and H-13, and even more highly alloyed hot primary carbides. For the same reasons, their machina- work tool steels such as H-19 and H-21) lose strength, 10 bility has been seen to improve. hardness and wear resistance at these temperatures. [0011] With regard to the properties of the prior Co-Cr- As a result, a number of very specialized materials for W-C-type alloys, it has often been assumed that these machine construction have been used, in particular alloys are at their maximum hardness in the cast or these alloys include Stellite 6 and 12 (mentioned above) welded condition and that their properties cannot be and similar Co-Cr-W-C-type alloys.