Metal founding – Means to shape metallic material – United particle type shaping surface
Reexamination Certificate
2002-04-24
2003-10-07
Elve, M. Alexandra (Department: 1725)
Metal founding
Means to shape metallic material
United particle type shaping surface
C164S023000, C164S034000, C249S061000
Reexamination Certificate
active
06629559
ABSTRACT:
BACKGROUND
This invention relates generally to casting and more specifically metal casting using ceramic containing molds.
A substantial number of metal castings are created by pouring molten metal into a ceramic mold. In sand casting, the mold is typically made of sand, held together with various binders. In investment casting, the mold is typically made of refractories, such as alumina powder, bound together by silica. A significant problem relates to a discrepancy in the change in the dimensions of the ceramic mold and the forming metal within the mold, as the casting cools.
The problem has two aspects. First, as most metals solidify from liquid, there is a significant volume change, generally a shrinkage, on the order of 5%, by volume. Second, the coefficient of thermal expansion of metals is typically substantially higher than that of ceramics. Thus, as the solidified casting continues to cool, the metal will shrink more than the ceramic.
Both these effects can lead to rupture or distortion of the casting. As the casting freezes, it is quite weak, and at that stage, can actually be torn apart by the constraints imposed by the ceramic mold. Such a failure is called a “hot tear,” or “hot crack.” As the casting continues to cool, it can be distorted by the constraints imposed by the ceramic mold.
Significant effort is expended in the casting art toward minimizing the difficulties imposed by the differential in change in dimension. Such efforts include control over the melt feeding pattern and freezing pattern through the use of risers and chills. In investment casting, the composition of the shell may be tailored, to promote its breakage during the cooling phase of the casting operation.
Typically a mold is made by providing a pattern whose outside shape is the shape of the object to be made. The pattern is made of a material that will burn away at a later stage. This pattern is dipped into a ceramic slurry, which forms a coating thereon. The coated shell is subsequently dipped in a different ceramic slurry, with different properties from the first ceramic slurry. The coated shell is dipped again and again into a succession of slurries, each being different from the previous slurry and the pattern is removed. The result is a hollow body coated with a succession of different coatings, much like layers of onion skin (except that the center of the body is hollow). The designer chooses the different coatings with the hope that they will themselves rupture, as the casting cools, thereby preventing the casting from distorting. Additional customization to promote or prevent mold rupture at specific locations can be provided by causing the shell to be thicker in certain locations.
Typically, the innermost layer is composed of a relatively fine grained ceramic slurry. Each subsequently applied slurry will typically be composed of coarser ceramic particles, such that each layer is successively coarser than the previous. The resultant body is porous. The distribution of particles and particle free void regions is uniform in each layer, and random within that layer, depending on the size and shape and uniformity of particles in the slurry, as well as the liquid content, both quantitative and qualitative. The voids are approximately the same size as the particles, and typically particles of any given layer can not pass through the voids in that layer. Further, the designer can not specify where in any layer, relative to the location of features in the casting, particles or voids will reside. Further, the designer has limited, if any, control over how, or where in the ceramic slurry coatings, relative to the features in the casting, the mold will break. The pattern of solid particles and voids within a layer is not predetermined or controllable, or repeatable in any way.
Typically if the mold is made, and it breaks prematurely, or in the wrong place, the designer will change the next attempt by using different coatings, or by increasing the thickness in different locations.
The following illustrates the problem quantitatively.
DIFFERENTIAL IN CHANGE IN GEOMETRY
Phase Change
As the molten metal solidifies, it contracts. It is important to note that this contraction results from the phase change, not a temperature change, which is discussed below. For example, aluminum contracts by between 3.5 and 8.5%, by volume (or equivalently, between about 1.2 and 3.8% in linear dimension) upon solidification. Nickel alloys contract approximately 3% by volume upon solidification. (ASM Handbook, Volume 15, p. 768 (aluminum) and p. 822 (nickel).)
In an extreme case, a casting may contract by this full amount upon solidification. More typically, a mold is designed with “risers,” which continue to feed molten metal to the casting as it cools, to minimize the impact of the solidification shrinkage. However, in castings of complex geometry, such approaches often do not fully compensate for the shrinkage and some dimension change of the casting results.
TEMPERATURE CHANGE
As the casting continues to cool, after solidification, it contracts further. For example, the coefficient of thermal expansion of aluminum alloys and nickel alloys are approximately 20×10
−6
/° C. and 14×10
−6
/° C., respectively. Ceramic mold materials contract far less. For example, in the range of 200-1200° C., the coefficient of thermal expansion of alumina is approximately 7×10
−6
/° C. and that of fused silica is approximately 1×10
−6
/° C. Thus, for example, when an aluminum casting cools in a ceramic mold made of alumina from a solidification temperature of 630° C. to room temperature, the linear shrinkage of the metal is approximately 1.25% (after solidification) but the ceramic only shrinks approximately 0.4%. Thus, for example, a casting of 10 inch dimension (25.4 cm) will shrink 0.09 in. (0.22 cm) more than the mold that is containing it.
Thus, the difference in the degree of shrinkage of the casting upon solidification and the shrinkage of the casting after solidification due to cooling on the one hand, at the same time as the degree of shrinkage in the mold on the other hand may cause ruptures or distortions in the casting or mold rupture.
Such failure of the mold is uncontrolled and usually harmful. For instance, it may occur while the molding material is still liquid, or flowable, thereby resulting in leakage of the molding material from the mold. Or, it may occur at a location that does not provide the stress relief to the casting that is required.
SUMMARY
In general, the present invention solves the problems that arise from the differential changes in geometry inherent to casting metal in a ceramic mold, by control of the internal morphology. By “internal morphology” it is meant, between the surfaces of the mold that face the casting, and that face the external environment. Specifically, layered fabrication techniques are used to create a ceramic mold. Control may be exercised, not just over the geometry of the inner and outer walls of the mold themselves, but also of the morphology of the structure between the walls. For example, an internal geometry composed of a cellular arrangement of skeletal elements and voids may be created within the mold wall. Through such control of the internal morphology, structures may be designed and fabricated so that the ceramic mold is virtually guaranteed to fail at an appropriate time during the solidification and/or cooling of the casting. Thus, the casting itself is not damaged. As used herein in the context of the mold, “to fail” means to break, rupture or bend past an elastic limit. If a structure “fails” under a loading condition, it will not return to its original form after the loads are removed. The goal of the present invention is to design and control the mold to fail (break, rupture or bend past an elastic limit) and thus to avoid rupture, or even distortion, of the casting.
Such collapsing molds of the invention typically consist of a thin layer of ceramic, which defines the casting cavity. This layer must be thin enough to fai
Bang Won B.
Cima Michael J.
Sachs Emanuel M.
Elve M. Alexandra
Massachusetts Institute of Technology
McHenry Kevin
Weissburg Steven J.
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