The present invention relates to methods of forming precision metal parts and, more specifically, to thixotropic forming of precision multi-alloy parts.
As performance criteria for turbine engines becomes more stringent, there is a need for an improved turbine rotor that exhibits maximum resistance to both fatigue and creep.
Die casting is a well-known process for producing complex components with excellent surface quality and good dimensional accuracy. However, the structural integrity of die castings is often compromised by air trapped in the casting upon injection of the liquid metal into the die casting cavity. The resultant porosity also compromises heat treatment of the casting that is often necessary to refine the grain structure and increase the strength of the casting.
Forging is also a well known process for producing relatively strong components having a desirable grain structure. However, forged products generally exhibit relatively low resistance to creep due to their fine grain structures.
Thixotropic, or semisolid, metal forming is a viable alternative to traditional casting and forging methods. This process lies somewhere between a casting and a forging process in that the metal to be formed is brought to a xe2x80x9cthixotropicxe2x80x9d state; that is, 30 or 40 percent of the mass exists in a liquid phase and the balance in a solid phase. The solid portion comprises small spherically-shaped nodules suspended within the liquid phase. Semisolid metals heated to a thixotropic state exhibit unique Theological properties due to their non-dendritic, or spherical, microstructure. The rheological properties of the semisolid metal range from high viscosities, like table butter, for alloys at rest, to low viscosities, such as machine oil, as the shearing rate of the semisolid slug is increased. By heating the metals to a semisolid range and then agitating the semisolid alloy, the dendritic microstructure normally found is eliminated and replaced by the spherical microstructure. Upon solidification, the alloys then exhibit a fine equiaxed microstructure.
Normally, a highly viscous thixotropic slug will retain its outer shape provided there are no external forces, other than gravity, applied to it. However, its butter-like consistency is easily deformed to a low viscosity, particularly by a shearing action such as high velocity impact, making it extremely suitable when driving the alloy into the mold during the manufacturing process. Because semisolid-formed alloys exhibit an intermediate-sized grain structure, larger than forged grains and smaller than cast grains, it is expected that semisolid forged or cast alloys will have improved creep rupture resistance over traditionally forged alloys and improved strength properties over traditionally cast alloys.
The thixotropic process has been extensively studied by others in relation to lighter metals or low melting point metals such as aluminum, magnesium, zinc, and copper alloys. In general, the low melting point metals have a melting point within the range of 750 to 1250 degrees Fahrenheit. On the other hand, lower melting point copper alloys (other than Cuxe2x80x94Ni alloys for example), have a melting point from 1200-1900 degrees Fahrenheit but are still within the scope of the present invention.
Very little research is available with regard to high temperature alloys commonly used in turbine rotors, including ferrous or nickel-based alloys. In general, high temperature alloys have a melting point range from 2000xc2x0 F.-2700xc2x0 F. One significant difference between semisolid production for lighter alloys and that for high temperature alloys involves the adaptation of the process to the problematic and high heating temperatures of 2500xc2x0 F. to 2700xc2x0 F. as opposed to alloys in the 750xc2x0 F.-1250xc2x0 F. melting point range. Designing a semisolid process compatible with such high heat has proven challenging. Generally, chrome-nickel alloys of, for example, 18% Cr and 82% Ni are used in turbine rotor forgings. This alloy has a solidus of 2550xc2x0 F., and a liquidus of 2640xc2x0 F. where the alloy is completely molten. The semisolid/thixotropic phase exists between the solidus and liquidus temperatures at temperatures ranging between 2550xc2x0 F. and 2640xc2x0 F. The alloy is commonly forged at temperatures below 2550xc2x0 F., in the solid phase, and cast at molten temperatures above 2640xc2x0 F., in the liquid phase.
Yet another problem is that current net-shape forging and die-casting equipment design includes permanent molds that often do not readily separate from the part interface when removing the turbine rotors and their intricate blades from the mold. This results in fractured or weakened blades and a corresponding number of rejected parts that do not meet design specifications. A need exists for semisolid manufacturing methods that facilitate ease of removal of the finished part, thereby improving the production volume and reducing the rejection rate of the finished parts.
Finally, precision metal assemblies are specifically designed to withstand various forces under uniquely stressful conditions. In certain applications, however, one part of a complete assembly may be exposed to stress and temperature loads significantly different from that of other parts integral to the same assembly. For example, the bore of a rotor may require good elongation, high strength, and good low cycle fatigue properties but may not require high temperature properties. In contrast, certain blade or rim portions of the rotor might require very high creep resistance and stress rupture strength at elevated temperatures. Formulating a single alloy capable of withstanding the variable stresses subjected to different locations within a precision metal assembly has also proven challenging.
European Pat. No. 0 574 141 A1 entitled, xe2x80x9cThixoformable Layered Materials and Articles Made From Themxe2x80x9d, discloses a method of sequentially applying layers of substantially metallic material in a thixoforming process. The reference discloses a rotatable cylindrical collector that collects molten metal cooled by inert gases prior to concentric deposition on the collector. A thixotropic layer comprising 30-70% liquid is thereby formed as the atomized metal is sprayed onto the collector. Additional layers are then added in the same way that may or may not comprise the same alloy as found in the first layer. At least two of the layers have different properties. The layers may also comprise reinforcing materials such as ceramic, metallic, and intermetallic materials in spherical, fibrous, or any other shape. The reinforcing materials may be added by simply spraying them into the atomized melt spray during that stage.
EP 0 574 141 A1 discloses that a layered composition thixoformed by this method exhibits enhanced toughness and damage resistance due to the layered 3-dimensional structure. However, the method is labor intensive, for each layer must at least be melted, sprayed, cooled by inert gases, and then collected on the surface of the cylinder. The preforms formed by this method must then be cooled and cut to accommodate a thixoforming forging process, wherein a multiproperty component is manufactured.
U.S. Pat. Nos. 5,832,982, 5,878,804, and 6,003,585 to Williams et al. describe metal forming processes that form a semisolid slug or thixotropic solution containing 30-40% liquid and 60-70% solids. The slug is inductively heated to destroy the dendritic microstructure and when hardened results in a preferred fine equiaxed microstructure. A disadvantage of the processes, however, is that the slug must be heated to a point wherein the liquid percentage exceeds 40%, thereby ensuring destruction of the dendrites. As the liquid percentage increases, containment of the slug becomes exceedingly difficult and therefore complicates the process. Increased amounts of energy and time are also required to heat the slugs and destroy the dendrites.
U.S. Pat. No. 5,878,804 obviates the containment problem by heating the slug within a mold. Nevertheless, increased amounts of energy and time are still required for dendrite destruction. Cooling of the finished product is also more difficult when using a heated mold.
Finally, U.S. Pat. No. 6,003,585 also requires the manufacture of a multialloy slug to similarly accomplish the objects of the present invention.
Therefore, a need exists for a simplified and cost-effective thixotropic manufacturing method that can be modified to vary the properties of different parts integral to a complete assembly.
The present invention solves the aforementioned problems by implementing a thixotropic process under vacuum for the production of turbine rotors and other parts of intricate design that comprise high melting point alloys. The mechanical properties of semisolid forgings are tailored by microstructure or metallurgical chemistry to achieve optimized properties in specific locations of the final product.
Initially, two or more high temperature slugs are first machined or preformed to fit within a heater in the metal forming process. The slugs are generally comprised of the same or different alloys and may be heated to a semisolid or solid state depending on design criteria.
In a first embodiment of the process, a first slug is heated under vacuum but retained as a solid. A second slug is also under vacuum and is heated to a thixotropic or semisolid state. Once the desired liquid/solid thixotropic ratio is attained within the second slug, the solid and semisolid slugs are forged into the mold. Upon actuation of a piston or plunger, the solid slug is driven into the semisolid slug. The semisolid slug thus enters the mold and is then exuded into predetermined areas upon the subsequent entry of the solid slug. The areas receiving the semisolid slug thereby benefit from its respective properties once the completed assembly hardens, and the areas occupied by the solid slug(s) benefit from its respective properties once the assembly is completely forged.
The process described is particularly well suited for manufacturing turbine assemblies comprised of integrated bore, rim, and blade components. For example, to form a rotor assembly, a first slug containing a bore alloy may be heated but remain in the solid state within a heater. A second slug, axially aligned with the first slug, contains a blade and rim alloy and is simultaneously and independently heated to a thixotropic state within the heater. After the heating step, a piston or other means drives the solid (first) slug into the thixotropic (second) slug, whereby the thixotropic slug enters the mold and is then followed by the solid slug. The solid slug thus forces the semisolid slug to exude into predetermined outer rim and blade areas of the mold. The solid slug, on the other hand, remains within the central or bore region of the finished part. It should be appreciated that one or more solid slugs may be accelerated into one or more semisolid slugs depending on design criteria.
Modified equipment design may be utilized in alternate embodiments of the high melting point semisolid process.