Metal founding – Means to shape metallic material – Including ladle or crucible type melt receptacle
Reexamination Certificate
2002-09-03
2004-09-28
Elve, M. Alexandra (Department: 1725)
Metal founding
Means to shape metallic material
Including ladle or crucible type melt receptacle
C164S338100, C266S275000
Reexamination Certificate
active
06796362
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates in general to an apparatus constructed and arranged for producing an “on-demand” semi-solid material for use in a casting process. Included as part of the overall apparatus are various stations which have the requisite components and structural arrangements which are to be used as part of the process. The method of producing the on-demand semi-solid material, using the disclosed apparatus, is included as part of the present invention.
More particularly, one embodiment of the present invention relates to a thermal jacket for engaging the exterior of a forming vessel containing molten metal to control the heating/cooling rate of the molten metal during the semi-solid material forming process. Although the present invention was developed for use in the semi-solid forming of metals or metal alloys, certain applications of the invention may fall outside of this field.
The present invention incorporates electromagnetic stirring and various temperature control and cooling control techniques and apparata to facilitate the production of the semi-solid material within a comparatively short cycle time. Also included are structural arrangements and techniques to discharge the semi-solid material directly into a casting machine shot sleeve. As used herein, the concept of “on-demand” means that the semisolid material goes directly to the casting step from the vessel where the material is produced. The semi-solid material is typically referred to as a “slurry” and the slug which is produced as a “single shot” is also referred to as a billet. These terms have been combined in this disclosure to represent a volume of slurry which corresponds to the desired single shot billet.
Semi-solid forming of light metals for net-shape and near-net shape manufacturing can produce high strength, low porosity components with the economic cost advantages of die-casting. However, the semi-solid molding (SSM) process is a capital-intensive proposition tied to the use of metal purchased as pre-processed billets or slugs.
Parts made with the SSM process are known for high quality and strength. SSM parts compare favorably with those made by squeeze casting, a variation of die-casting that uses large gate areas and a slow cavity fill. Porosity is prevented by slow, non-turbulent metal velocities (gate velocities between 30 and 100 in./sec.) and by applying extreme pressure to the part during solidification. Both squeeze casting and SSM processes produce uniformly dense parts that are heat-treatable.
SSM offers the process economics of die casting and the mechanical properties that approach those of forgings. In addition, SSM capitalizes on the non-dendritic microstructure of the metal to produce parts of high quality and strength. SSM can cast thinner walls than squeeze casting due to the globular alpha grain structure, and it has been used successfully with both aluminum and magnesium alloys. SSM parts are weldable and pressure tight without the need for impregnation under extreme pressure that characterizes the squeeze-cast process.
The SSM process has been shown to hold tighter dimensional capabilities than any other aluminum molding process. That has intensified demand for SSM components due to the potential for significant cost savings, reduction of machining, and quicker cycle times for higher production rates. Besides high strength and minimal porosity, SSM parts exhibit less part-to-die shrinkage than die cast parts and very little warpage. It produces castings that are closer to the desired net shape, which reduces and can even eliminate secondary machining operations. Surface finishes on the castings are often better than the iron and steel parts they replace.
The SSM process requires higher final mold pressure (15,000 to 30,000 psi) than conventional die casting (7,000 to 12,000 psi), but modern die casting equipment provides the flexibility needed to produce SSM parts efficiently and economically. Real-time, closed-loop hydraulic circuits incorporated into today's die casting machines can automatically maintain the correct fill velocities of the SSM material alloy. Closed-loop process control systems monitor metal temperature and time, voltage feedback from electrical stator and other data to provide a very robust and precisely controlled operation that can maximize productivity of high quality parts and ensure reproducibility.
As described, it is well known that semi-solid metal slurry can be used to produce products with high strength and low porosity at net shape or near net shape. However, the viscosity of semi-solid metal is very sensitive to the slurry's temperature or the corresponding solid fraction. In order to obtain good fluidity at high solid fraction, the primary solid phase of the semi-solid metal should be nearly spherical.
In general, semi-solid processing can be divided into two categories; thixocasting and rheocasting. In thixocasting, the microstructure of the solidifying alloy is modified from dendritic to discrete degenerated dendrite before the alloy is cast into solid feedstock, which will then be re-melted to a semi-solid state and cast into a mold to make the desired part. In rheocasting, liquid metal is cooled to a semi-solid state while its microstructure is modified. The slurry is then formed or cast into a mold to produce the desired part or parts.
The major barrier in rheocasting is the difficulty to generate sufficient slurry within preferred temperature range in a short cycle time. Although the cost of thixocasting is higher due to the additional casting and remelting steps, the implementation of thixocasting in industrial production has far exceeded rheocasting because semi-solid feedstock can be cast in large quantities in separate operations which can be remote in time and space from the reheating and forming steps.
In a semi-solid casting process, generally, a slurry is formed during solidification consisting of dendritic solid particles whose form is preserved. Initially, dendritic particles nucleate and grow as equiaxed dendrites within the molten alloy in the early stages of slurry or semi-solid formation. With the appropriate cooling rate and stirring, the dendritic particle branches grow larger and the dendrite arms have time to coarsen so that the primary and secondary dendrite arm spacing increases. During this growth stage in the presence of stirring, the dendrite arms come into contact and become fragmented to form degenerate dendritic particles. At the holding temperature, the particles continue to coarsen and become more rounded and approach an ideal spherical shape. The extent of rounding is controlled by the holding time selected for the process. With stirring, the point of “coherency” (the dendrites become a tangled structure) is not reached. The semi-solid material comprised of fragmented, degenerate dendrite particles continues to deform at low shear forces. The present invention incorporates apparata and methods in a novel and unobvious manner which utilize the metallurgical behavior of the alloy to create a suitable slurry within a comparatively short cycle time.
When the desired fraction solid and particle size and shape have been attained, the semi-solid material is ready to be formed by injecting into a die-mold or some other forming process. Silicon particle size is controlled in the process by limiting the slurry creation process to temperatures above the point at which solid silicon begins to form and silicon coarsening begins.
It is known that the dendritic structure of the primary solid of a semi-solid alloy can be modified to become nearly spherical by introducing the following perturbation in the liquid alloy near liquidus temperature or semi-solid alloy:
1) Stirring: mechanical stirring or electromagnetic stirring;
2) Agitation: low frequency vibration, high-frequency wave, electric shock, or electromagnetic wave;
3) Equiaxed Nucleation: rapid under-cooling, grain refiner;
4) Oswald Ripening and Coarsening: holding alloy in semi-solid temperature for a long time.
While t
Lombard Patrick J.
Spiller George
Wang Shaupoh
Winterbottom Walter L.
Brunswick Corporation
Elve M. Alexandra
Tran Len
Woodard Emhardt Moriarty McNett & Henry LLP
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