Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2002-08-05
2004-06-08
Stoner, Kiley (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S900000, C164S312000, C366S083000, C366S088000
Reexamination Certificate
active
06745818
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an apparatus and method for forming a shaped component from liquid metal alloy. In particular, it relates to a method and apparatus for converting liquid alloy into semisolid slurry which is injected subsequently into a die cavity to produce shaped components. The apparatus and method are applicable to light alloys, such as aluminum alloy, magnesium alloy, zinc alloy and any other alloy suitable for semisolid processing.
One of the conventional methods used to manufacture metallic components is die casting. In the conventional die casting process, the liquid metal is usually forced into a mold cavity at such a high speed that the flow becomes turbulent or even atomized. As a result, air is often trapped within the cavity, leading to high porosity in the final components, which reduces the component strength and can cause component rejection if holes appear on the surface after machining. Moreover, components with high porosity are unacceptable because they are usually not heat-treatable, thus limiting their potential applications.
Intuitively, the porosity due to turbulent or atomized flow could be reduced or even eliminated if the viscosity of the metal flow could be increased to reduce the Reynolds number sufficiently so trapped air is minimized, somewhat similar to the injecting molding of plastics. However, it was not clear how this could be achieved until the early 1970s when Metz and Flemings proposed the concept of semisolid material (SSM) processing. They suggested that, if metal solidification is carried out in the semisolid state, the porosity of castings could be reduced significantly. The study of Spencer et al showed that when molten metal is agitated during cooling below its liquidus temperature, the dendritic primary solid would be broken into near spherical particles suspended in the liquid metal matrix. The exponentially increased viscosity with the solid fraction of such a semisolid slurry can produce sound castings with die casting process. The SSM process improves upon the die casting method by injecting semisolid metal rather than fully liquid metal into a die cavity for component production. Compared with conventional die casting routes, SSM processing has the following advantages: (1) cost effectiveness over the whole manufacturing cycle; (2) near-net shape processing; (3) consistency and soundness of mechanical properties; (4) ability to make complex component shapes; (5) weight reduction through alloy substitution and more efficient use of materials; (6) high production rate; (7) enhanced die life; (8) less environmental cost. The enhanced mechanical properties result from the improved microstructural features, such as refined grain size, non-dendritic morphology and substantially reduced porosity level.
Although the concept of SSM processing seems promising, the major problem remains as how the slurry is produced and how the component is shaped efficiently and reliably. Since the early 1970s, a number of alternatives to the original MIT rheocasting process have been developed. One of the most popular processes currently used is thixoforming, in which pre-processed nondendritic alloy billet are reheated to the semisolid region prior to the shaping process. It is therefore a two-stage process. The high cost of pre-processed non-dendritic raw materials and of the re-heating process are by far the greatest obstacles to the development of the full potential of this approach. In addition, plastic injection molding techniques have recently been introduced into the SSM processing field. One process is “thixomolding” for Mg-alloys, which was developed by Dow Chemicals and currently marketed by Thixomat, the other one was developed at Cornell University (USA). However, the quality of both semisolid slurries and final components is not totally satisfactory.
During the last 20 years, the most active method of producing semisolid slurry is mechanical agitation. Unfortunately, most mechanical stirring methods have not gained popularity in industry because of the problems associated with erosion of the stirring device, problems with synchronisation of the stirring with the continuous casting process, and the inadequate shear rate to obtain fine particles.
A number of references disclose thixomolding processes, in which a solid or semisolid feed is first processed (for example by heating the feed to liquefy it whilst subjecting it to shear) and then injected into a mold to form a component. Examples of such references include: EP 0867246 A1 (Mazda Motor Corporation); WO 90/09251 (The Dow Chemical Company); U.S. Pat. No. 5,711,366 (Thixomat, Inc.); U.S. Pat. No. 5,735,333 (The Japan Steel Works, Limited); U.S. Pat. No. 5,685,357 (The Japan Steel Works, Limited); U.S. Pat. No. 4,694,882 (The Dow Chemical Company); and CA 2,164,759 (Inventronics Limited).
The disadvantage however with heating solid granules in order to convert them into the thixotropic state (thixomolding) rather than cooling liquid metal into the thixotropic state (rheomolding) is that it is very difficult to control particle size and particle size distribution in the sub-structure of the thixotropic slurry. Specifically, particle sizes of thixomolded slurries tend to be an order of magnitude larger than those of rheomolded slurries, and to have a wider sized distribution. This has negative implications for the structural properties of the casted components.
Furthermore, the above-mentioned references employ a standard single screw extruder for subjecting the thixotropic slurry to shear. The result is a component of low quality.
A number of references do disclose rheomolding processes. For example, WO 97/21509 (Thixomat, Inc.) relates to a process for forming metal compositions in which an alloy is heated to a temperature above its liquidus temperature, and then employing a single screw extruder to shear the liquid metal as it is cooled into the region of two phase equilibrium.
U.S. Pat. No. 4,694,881 (The Down Chemical Company) relates to a process in which a material having a non-thixotropic-type structure is fed in solid form into a single screw extruder. The material is heated to a temperature above its liquidus temperature, and then cooled to a temperature lower than its liquidus temperature and greater than its solidus temperature whilst being subjected to a shearing action.
WO 95/34393 (Cornell Research Foundation, Inc.) also discloses a rheomolding process in which super-heated liquid metal is cooled into a semisolid state in the barrel of a single screw extruder, where it is subjected to shear whilst being cooled, prior to being injection molded into a cast.
None of the thixomolding or rheomolding references describe a process which enables components of a sufficiently high structural integrity to be formed.
SUMMARY OF THE INVENTION
The primary objective of this invention is to provide an apparatus and method which converts liquid alloy into its thixotropic state and fabricates high integrity components by injecting subsequently the thixotropic alloy into a mold cavity in an integrated one-step process.
Another objective of the invention is to provide an apparatus and method which is specially adapted for producing semisolid metal alloys with a highly corrosive and erosive nature in their liquid or semisolid state.
Still another objective of the invention is to provide an improved die casting system suitable for production of high integrity components from semisolid slurry.
In a first aspect of the invention, there is provided a method for forming a shaped component from liquid metal alloy, comprising the steps of cooling the alloy to a temperature below its liquidus temperature whilst applying shear at a sufficiently high shear rate and intensity of turbulence to convert the alloy into its thixotropic state, and subsequently transferring the alloy into a mold to form a shaped component, wherein shear is applied to the alloy by means of an extruder having at least two screws which are at least partially intermeshed.
In a second aspect of t
Bevis Michael John
Fan Zhongyun
Ji Shouxun
Bieschko, Esq. Craig A.
Brunel University
DeWitt Ross & Stevens S.C.
Stoner Kiley
Tran Len
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