Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1998-12-15
2001-04-10
Lin, Kuang Y. (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S122200, C164S361000
Reexamination Certificate
active
06213192
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the casting of metals to make thin-wall articles, and, more particularly, to the casting of such articles using metals which are subject to hot tearing.
Many articles are made by casting molten metal into a mold that closely defines the final shape of the article, and solidifying the molten metal in the mold. The solidified metal may need some final processing such as consolidating, crack repairing, cleaning, and/or final machining, but its gross as-cast shape is that of the desired final article.
This casting process is sometimes used because, for some metals, there is no practical alternative. These same metals have such low ductilities, even at elevated temperatures, that they cannot be fabricated by conventional metal working procedures. One casting application is the fabrication of parts used in the high-temperature portions of gas turbine (jet) engines. An example of such an article is the mixer used in some gas turbine engines to mix ambient air with exhaust gas.
Some of the metal alloys that would be desirably used in these castings are subject to hot tearing when cast by conventional procedures. Two examples are the nickel-base superalloys Rene' 108 and Mar-M247. These metals are qualitatively different from other nickel-base superalloys, in that they exhibit low grain boundary ductilities in the range from below the solidus temperature (about 2434° F. in these alloys) down to about 2100° F., as well as low polycrystalline grain boundary strengths in this same temperature range. During the cooling through this temperature range that follows casting of the molten metal and initial solidification, differential strains caused by the difference in thermal expansion coefficient between the metal and the surrounding casting shell mold create large differential stresses within the metal and cause grain boundary (intergranular) cracks to open. This cracking phenomenon is termed “hot tearing”. The hot-tearing cracks remain in the final article, in many cases causing it to be unacceptable and in other cases requiring expensive repair and reworking before the article is acceptable for service.
The problem is particularly troublesome when metals subject to hot tearing are used to cast large, thin-walled structural articles by conventional processes. Localized differential strains across the wall thickness may be quite large, and grain boundary hot tearing is often observed. One approach to alleviating the hot tearing and defects in the casting is to modify the composition of the alloy to improve grain boundary ductility, but then. the desirable final mechanical properties of the material would be lost as well.
There is a need for an operable approach to the casting of large, thin-walled and hollow articles made of metals that are subject to hot tearing. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a structural, thin-walled article made of a metal subject to hot tearing, and a method for its fabrication The wall of the article may be made quite thin, yet grain boundary hot tearing of the metal is minimized. The approach of the invention yields a high-quality final article with a well-controlled microstructure.
In accordance with the invention, an article is made by the steps of providing a mold structure defining an article having a first wall, an opposing second wall, and an interior space therebetween, and providing a molten base metal that is subject to hot tear cracking when cast as a non-directional structure. The approach further includes introducing the molten base metal into the mold structure, and directionally solidifying the base metal in the mold structure to form a directionally oriented structure.
The article may be described as hollow, although the wall need not be continuous around the periphery of the article. The invention is most beneficially applied when the walls are relatively thin. In one embodiment, the walls are less than about 2 percent of a total thickness of the article, and in another embodiment, the walls are less than about 0.100 inches thick. The mold structure for making such an article typically has an exterior mold wall that defines the outer surface of the article, and an interior mold wall that defines the inner surface of the article. A structure of particular interest to the inventor is a gas turbine engine mixer, but the invention is applicable to a wide variety of structures.
The base metal typically has a polycrystalline ductility of less than about 1 percent over the temperature range of from the solidus temperature (typically about 2434° F.) down to about 2100° F. Examples of such metal alloys subject to hot tearing, when cast in a non-directional form as a thin-wall, complexly shaped article, are the nickel-base superalloys Rene' 108 and Mar-M247.
The base metal alloy is directionally solidified to form a directionally oriented structure. The directional solidification may be performed to produce either a single crystal (monocrystal) or a directionally oriented polycrystal. In each case, the number of grain boundaries is reduced as compared with those found in generally equiaxed castings. In particular, the number of grain boundaries extending through the thickness of the walls is substantially reduced, to zero in some cases. The inventor has observed that the predominant failure mode in hot tearing of conventionally prepared, non-directional, fine-grained, generally equiaxed castings of the alloys subject to hot tearing is intergranular failure along grain boundaries extending through the thickness of the wall. By eliminating these failure paths, the incidence of failure by hot tearing is greatly reduced or eliminated.
The reason for the improvement in properties resulting from the present approach, as compared with the conventional approach, is believed to relate to the structure occurring at the solidifying interface. However, the operability and results obtained with the present invention are not dependent upon the correctness of the following possible explanation. The directional solidification process is a high gradient process, with a large ratio of G/R, where G is the temperature gradient at the solidifying front and R is the withdrawal rate of the casting from the furnace. Consequently, the gap between the all-solid and all-liquid regions of the solidifying casting is smaller than in conventional casting. This small gap effectively reduces the amount of constitutional supercooling at the solidifying interface, thereby reducing the amount of interdendritic and/or grain boundary segregation which contributes to reduced grain boundary strength and ductility as well as increased amounts of interdendritic and/or grain boundary porosity in conventional casting. During cooldown after solidification is complete, cracking tends to occur through the region of interdendritic segregation and grain boundary porosity in the conventional castings, but not in the castings produced by the present approach
The present invention is practiced using directional casting equipment sufficiently large for the article to be prepared. In a typical case, the furnace surrounding the mold is stationary. Directional solidification is accomplished by lowering the mold containing the base metal of the casting, so that the metal cools and solidifies substantially unidirectionally.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
REFERENCES:
patent: 4935072 (1990-06-01), Nguyen-Dinh
patent: 5297615 (1994-03-01), Aimone et al.
patent: 5320487 (1994-06-01), Walker et al.
patent: 5374319 (1994-12-01), Stueber et al.
General Electric Company
Hess Andrew C.
Lin Kuang Y.
Narciso David L.
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