Agitating – Stirrer within stationary mixing chamber – Magnetic stirrer
Reexamination Certificate
2000-06-01
2002-06-11
Soohoo, Tony G. (Department: 1723)
Agitating
Stirrer within stationary mixing chamber
Magnetic stirrer
C366S147000, C164S504000
Reexamination Certificate
active
06402367
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to metallurgy, and, more particularly, to a method and apparatus for controlling the microstructural properties of a molded metal piece by efficiently controlling the temperature and viscosity of a thixotropic precursor metal melt through precisely controlled magnetomotive agitation.
BACKGROUND OF THE INVENTION
The present invention relates in general to an apparatus which is 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 specifically, the present invention incorporates electromagnetic stirring techniques and apparata to facilitate the production of the semi-solid material within a comparatively short cycle time. As used herein, the concept of “on-demand” means that the semi-solid 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.
It is well known that semi-solid metal slurry can be used to produce products with high strength, leak tight and 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.
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. Solid phase particle size is controlled in the process by limiting the slurry creation process to temperatures above the point at which the solid phase begins to form and particle 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 the methods in (
2
)-(
4
) have been proven effective in modifying the microstructure of semi-solid alloy, they have the common limitation of not being efficient in the processing of a high volume of alloy with a short preparation time due to the following characteristics or requirements of semi-solid metals:
High dampening effect in vibration.
Small penetration depth for electromagnetic waves.
High latent heat against rapid under-cooling.
Additional cost and recycling problem to add grain refiners.
Natural ripening takes a long time, precluding a short cycle time.
While most of the prior art developments have been focused on the microstructure and rheology of semi-solid alloy, temperature control has been found by the present inventors to be one of the most critical parameters for reliable and efficient semi-solid processing with a comparatively short cycle time. As the apparent viscosity of semi-solid metal increases exponentially with the solid fraction, a small temperature difference in the alloy with 40% or higher solid fraction results in significant changes in its fluidity. In fact, the greatest barrier in using methods (
2
)-(
4
), as listed above, to produce semi-solid metal is the lack of stirring. Without stirring, it is very difficult to make alloy slurry with the required uniform temperature and microstructure, especially when the there is a requirement for a high volume of the alloy. Without stirring, the only way to heat/cool semi-solid metal without creating a large temperature difference is to use a slow heating/cooling process. Such a process often requires that multiple billets of feedstock be processed simultaneously under a pre-programmed furnace and conveyor system, which is expensive, hard to maintain, and difficult to control.
While using high-speed mechanical stirring within an annular thin gap can generate high shear rate sufficient to break up the dendrites in a semi-solid metal mixture, the thin gap becomes a limit to the process's volumetric throughput. The combination of high temperature, high corrosion (e.g. of molten aluminum alloy) and high wearing of semi-solid slurry also makes it very difficult to design, to select the proper materials and to maintain the stirring mechanism.
Prior references disclose the process of forming a semi-solid slurry by reheating a solid billet formed by thixocasting or directly from the melt using mechanical or electromagnetic stirring. The known methods for producing semi-solid alloy slurries include mechanical stirring and inductive electromagnetic stirring. The processes for forming a slurry with the desired structure are controlled, in part, by the interactive influences of the shear and solidification rates.
In the early 1980's, an electromagnetic stirring process was developed to cast semi-solid feedstock with discrete degenerate dendrites. The feedstock is cut to proper size and then remelt to semi-solid state before being injected into mold cavity. Although this magneto hydrodynamic (MHD) casting process is capable of generating high volume of semi-solid feedstock with adequate discrete degenerate dendrites, the material handling cost to cast a billet and to remelt it back to a semi-solid composition reduces the competitiveness of this semi-solid process compared to other c
Lu Jian
Norville Samuel M. D.
Wang Shaupoh
AEMP Corporation
Soohoo Tony G.
Woodard Emhardt Naughton Moriarty & McNett
LandOfFree
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