Metal founding – Means to shape metallic material – Pressure shaping means
Reexamination Certificate
2002-06-28
2004-05-18
Lin, Kuang Y. (Department: 1725)
Metal founding
Means to shape metallic material
Pressure shaping means
C164S113000
Reexamination Certificate
active
06736188
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a vessel for the production of molten materials. More specifically, the present invention is a vessel optimized for the handling the processing environment involved in the production of molten or liquid metals and their molding into articles of manufacture.
2. Description of the Prior Art
Metal compositions having dendritic structures at ambient temperatures conventionally have been melted and then subjected to high pressure die casting procedures. These conventional die casting procedures are limited in that they suffer from porosity, melt loss, contamination, excessive scrap, high energy consumption, lengthy duty cycles, limited die life, and restricted die configurations. Furthermore, conventional processing promotes formation of a variety of microstructural defects, such as porosity, that require subsequent, secondary processing of the articles and also result in use of conservative engineering designs with respect to mechanical properties.
Processes are known for forming metal compositions such that their microstructures, when in the semi-solid state, consist of rounded or spherical, degenerate dendritic particles surrounded by a continuous liquid phase. This is opposed to the classical equilibrium microstructure of dendrites surrounded by a continuous liquid phase. These new structures exhibit non-Newtonian viscosity, an inverse relationship between viscosity and rate of shear. The materials themselves, in this condition, are known as thixotropic materials.
One process for converting a dendritic composition into a thixotropic material involves the heating of the metal composition or alloy, hereafter just “alloy”, to a temperature which is above its liquidus temperature and then subjecting the liquid alloy to shear or agitation as it is cooled into the region of two phase equilibria. A result of sufficient agitation during cooling is that the initially solidified phases of the alloy nucleate and grow as rounded primary particles (as opposed to interconnected dendritic particles). These primary solids are comprised of discrete degenerate dendritic spherules and are surrounded by a matrix of an unsolidified portion of the liquid metal or alloy.
Another method for forming thixotropic materials involves the heating of the alloy to a temperature at which some, but not all of the alloy is in a liquid state. The alloy may then be agitated. The agitation converts any dendritic particles into degenerate dendritic spherules. In this method, it is preferred that when initiating agitation, the semisolid metal contain more liquid phase than solid phase.
An injection molding technique using thixotropic alloys delivered in an “as cast” state has also been seen. With this technique, the feed material is fed into a vessel where it may be further heated and at least partially melted. Next, the alloy is mechanically agitated by the action of a rotating screw, rotating plates or other means. As the material is processed, it is moved forward within the vessel. The combination of partial melting and simultaneous agitation produces a slurry of the alloy containing discrete degenerate dendritic spherical particles, or in other words, a semisolid state of the material and exhibiting thixotropic properties. The thixotropic slurry is delivered to another zone, which may be a second vessel, located adjacent a nozzle. The slurry may be prevented from leaking or drooling from the nozzle tip by controlled solidification of a solid metal plug of the material in the nozzle (by controlling the nozzle temperature). Alternatively, a mechanical or other valving scheme may be employed. The sealed nozzle provides protection to the slurry from oxidation, or the formation of oxide on the interior wall of the nozzle, that would otherwise be carried into the finished, molded part. The sealed nozzle further seals the die cavity on the injection side facilitating, if desired, the use of vacuum to evacuate the die cavity further enhancing the complexity and quality of parts so molded.
Once an appropriate amount of slurry for the production of the article has been accumulated in this zone, a piston, screw or other mechanism causes the material to be injected into the die cavity forming the desired solid article. Such casting or injection machines of the above or related varieties are herein referred to as semi-solid metal injection (SSMI) molding machines.
Currently, SSMI molding machines typically perform a substantial portion of the heating of the material in a barrel of the machine. Material enters at one section of the barrel while at a reduced temperature and is then advanced through a series of heating zones, where the temperature of the material is rapidly and, at least initially, progressively raised. The heating elements themselves, typically resistance or induction heaters, of the respective zones along the barrel may or may not be progressively hotter than the preceding heating elements. As a result, a thermal gradient exists both through the thickness of the barrel as well as along the length of the barrel.
Barrel construction for such machines has seen the barrels formed as long (up to 110 inches) and thick (outside diameters of up to 11 inches with 3 to 4 inch thick walls) monolithic cylinders. As the size and throughput capacities of these machines have increased, the length and thicknesses of the barrels have correspondingly increased. This has lead to increased thermal gradients throughout the barrels and previously unforeseen and unanticipated consequences. The primary barrel material, wrought alloy 718 (having a limiting composition of: nickel (plus cobalt), 50.00-55.00%; chromium, 17.00-21.00%; iron, bal.; columbium (plus tantalum) 4.75-5.50%, molybdenum, 2.80-3.30%; titanium, 0.65-1.15%; aluminum, 0.20-0.80; cobalt, 1.00 max.; carbon, 0.08 max.; manganese, 0.35 max.; silicon, 0.35 max.; phosphorus, 0.015 max.; sulfur, 0.015 max.; born, 0.006 max.; copper, 0.30 max. used in constructing these barrels is often in short supply and costly. Additionally, alloy 718 exhibits poor stress rupture properties, poor elongation and phase instability.
Fine grained alloy 718 of high quality is expensive and is available only as cast/wrought billet, which needs extensive boring and external machining to shape complex vessels. The scrap of alloy 718 generated by going this route can be as high as 50%. Additionally, alloy 718 is unstable at 600-700° C., tending to transform its fine gamma double prime hardening phase to a brittle delta phase. Impact energy (Charpy V-notch) and stress rupture strength can thus degrade.
HIPPING of complex net shapes of alloy 718 is desirable to increase yield and to apply liners. However, cast/wrought alloy 718 suffers grain growth to large grains of ASTM No. 00. Impact energy (Charpy V-notch) and stress rupture strength can again degrade. Powder metal alloy 718 retains finer grain size upon HIPPING but stress rupture properties (life and ductility) still suffer severely. Furthermore, Thixomolding®, semisolid metal injection molding of thixotropic alloys, is expanding into higher temperature alloys that impart additional instability to alloy 718.
In several cases, failed monolithic barrels have been analyzed and it determined that the barrels failed as a result of thermal stress and, more particularly, thermal shock in the cold or input end of the barrels. As used herein, the cold or input end of a barrel is that section or end where the material first enters into the barrel. It is in this section where the most intense thermal gradients are seen, particularly in an intermediate temperature region of the cold section, which is located downstream of where the material enters. Large grained alloy 718 has been especially prone to cracking under these high stress conditions.
During use of a SSMI molding machine, the solid material feedstock, which may be in a pellet and chip form, may be fed into the barrel while at ambient temperatures, approximately 75° F. Being long and thick, the barrels of these
Decker Raymond F.
Vining Ralph E.
Walukas D. Matthew
Brinks Hofer Gilson & Lione
Lin Kuang Y.
Thixomat, Inc.
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