Patent Grant
9999921
This invention relates generally to a method to make light-weight metal-matrix composite components through squeeze casting or semi-solid metal (SSM) forming, and more particularly to making such components from a reinforced metal matrix composite where the reinforcing phase or phases are generated in-situ during such casting or forming operations.
Casting has become the dominant form of metal-forming operations for the manufacture of repeatable (i.e., high-volume) components (particularly those that employ lightweight metal alloys such as aluminum or magnesium), and includes numerous variants, such as die casting, permanent mold casting, sand casting, plaster casting, investment casting or the like. Nevertheless, it is known that the mechanical properties of cast components are often inferior to their wrought counterparts, due in no small part to porosity and related defects that are inherent in (or at least hard to avoid) known casting processes. Unfortunately, high-volume production and shape complexity considerations may render wrought options cost-prohibitive, if not outright impossible.
SSM forming techniques have helped to bridge the gap by providing metallic alloys that deliver wrought properties with a forming process capable of the large-scale production of complex shapes. In particular, the slurried (i.e., thixotropic) microstructure of these SSM techniques makes it easy to perform the semi-solid shaping by casting, forging or other known forming processes. In a conventional SSM forming process, a cast billet is (1) heated to a temperature above its recrystallization temperature yet below its solidus temperature; (2) extruded into a generally columnar form; (3) cut into shorter segments; (4) heated into a semi-solid state; and (5) squeezed into a cavity that is formed in a die set to form a part. Despite advantages, porosity, outer skin microstructure and related incomplete part formation issues persist in conventional SSM, especially in articles formed with complex geometries with thin or otherwise small features. Moreover, the billets used in this and related thixotropic processes are a highly specialized (and therefore expensive) way to achieve the desirable non-dendritic (i.e., globular) microstructure.
In a related way, squeeze casting has been investigated as a way to prepare components from lightweight alloys. The process is also referred to by other names, such as liquid metal forging, liquid die forging, semi-solid casting and forming, extrusion casting, pressurized solidification and pressurized crystallization. A conventional squeeze casting process is defined by the following steps: (1) pre-quantifying an amount of melt to be poured into a preheated die cavity; (2) ramping down a punch close to the die cavity; (3) pressurizing the molten metal and holding it there for a short period (for example, a few seconds) until the punch is withdrawn; and (4) ejection of the part from the die cavity. Thus, in one form, squeeze casting (and related liquid forging approaches) is simpler than SSM forming in that it uses a pre-determined volume of molten metal that is poured into a die cavity and squeezed under pressure during solidification, thereby forming the alloy parts in a single operation. Moreover, squeeze casting makes it possible to use wrought aluminum (or magnesium) alloy in a liquid state to form complex parts with intricate features. High direct melt pressure helps eliminate hot tearing and creates products with superior mechanical properties and low porosity. As such, squeeze casting is seen as a hybrid of conventional casting and forging techniques to achieve the strength and confidence level of forging with the high-volume economics and shape capabilities of castings.
It is known that increased structural or mechanical properties (such as elastic modulus, strength, fatigue resistance, creep resistance or the like) of components may be achieved through the introduction of reinforcing phases into the bulk alloy. As such, the class of materials known as composites has been created to help satisfy increasingly these and other demanding engineering requirements. One of the difficulties associated with creating such engineered composites is the cost associated with introducing disparate materials in such a way that they achieve the desired structural benefits in the final product. Because the introduction of a discreet reinforcing phase into a bulk alloy is complex (and therefore prohibitively expensive), it is incompatible for high-volume component production techniques for engine components through one or more of the traditional forms of metal casting mentioned above.
Significantly, the present inventors have discovered that traditional SSM or squeeze casting techniques have not been able to fully exploit all of the mechanical or structural properties that the use of such materials would otherwise offer. Specifically, the present inventors have determined that there remains a need to develop low-cost, durable engine components through a cost-effective, high-volume manufacturing approach that uses SSM, squeeze casting or related fabrication techniques to better exploit the high specific properties made possible by lightweight metal matrix composites.
To satisfy the above need, the present inventors have determined that the in-situ nucleation and growth of a reinforcing phase in a lightweight metallic alloy to make a composite may be triggered by an activating event of an added precursor material that occurs during a squeeze casting or SSM forming approach. One preferred form of such an activating event is a thermal one, where the precursor is exposed to an elevated temperature during the component-forming process. Regardless of the activating mechanism used to effect the thermally-based in-situ conversion of the precursors into the reinforcing phases that are dispersed throughout the lightweight metallic bulk alloy, the present inventors have discovered that the improved mechanical properties made possible by the composite-like nature of the formed components can be made in high volumes through SSM forming or squeeze casting in a manner similar to that of traditional die casting and other high-volume traditional casting approaches.
According to a first aspect of the present invention, a method of making a reinforced metal matrix composite component is disclosed. The method includes introducing one or more reinforcing phase precursors (also referred to herein as “nucleation site precursors”, or more simply “precursors”) into a bulk (i.e., feed) alloy, converting the precursors into reinforcing phases through an activation step and forming the component as a composite of the bulk alloy and the reinforcing phase or phases using squeeze casting or SSM forming in combination with optional post-forming heat treatment such that a linear dimension of the reinforcing phase is in the nanometer to micrometer range. The feed alloy is selected from the group consisting of aluminum-based alloys, magnesium-based alloys and so-called high-entropy alloys where in the present context, such “high-entropy” alloys are those that are made up of numerous (typically five or more) metals in approximately equal amounts. One such example is a combination of aluminum, lithium, magnesium, scandium and titanium. Such materials exhibit nanocrystalline configurations that possess high specific mechanical properties. Moreover, within the present context, such high-entropy alloys are deemed herein to be encompassed by the term “aluminum-based alloys”, “magnesium-based alloys” or the like so long as the respective aluminum or magnesium is one of the predominant constituents (even if not the majority constituent). Significantly, the presence of the reinforcing phases that are generated during the activation helps the bulk alloy to take on composite-like attributes so that increases in certain mechanical properties (such as the elastic modulus) of the formed composite are realized. Unlike conventional composites where a reinforcing phase is added in its substantially final form, the reinforced phases of the various aspects of the present invention disclosed herein are formed in-situ during one or more of the liquid-solid transformation or subsequent heat treatment of the material.
As mentioned elsewhere, the choice of using squeeze casting or SSM forming may depend on the component being fabricated, as well as the choice of the bulk alloy being used. In situations where SSM forming is employed, two additional options are possible, a first of which includes providing the bulk alloy in particulate (i.e., solid, examples of which include granular, powder or related) form, and a second of which includes providing the bulk alloy in a substantially liquid (i.e., melted) form.
According to another aspect of the present invention, a method of making a reinforced metal matrix composite component includes introducing one or more reinforcing phase precursors into a bulk alloy that is selected from the group consisting of high-entropy alloys, aluminum-based alloys, or magnesium-based alloys, catalyzing the reinforcing phase precursor (or precursors) such that a reinforcing phase will form and grow before or as part of shaping the component as a composite of the bulk alloy and the one or more reinforcing phases. The shaping includes heating the mixture of bulk alloy and precursors until it is in an at least partially melted form, placing it into a die cavity and imparting an elevated pressure on the composite until a shape of the component defined by the die cavity has substantially solidified. As with the previous aspect, the shaping uses an SSM forming or squeeze casting operation, while the bulk alloy may be in either the particulate or molten state, and more than one die cavity (for example, a preliminary die cavity and a final die cavity) may be used.
According to yet another aspect of the present invention, a method of making a reinforced metal matrix composite component is disclosed. The method includes introducing one or more reinforcing phase precursors into a bulk alloy that is selected from the group consisting of high-entropy alloys, aluminum-based alloys, or magnesium-based alloys, and then shaping the component as a composite of the bulk alloy and a reinforcing phase that is formed by activation of the reinforcing phase precursor. The shaping is achieved by either squeeze casting or SSM forming, and includes heating the composite until it is in an at least partially melted form, placing the at least partially melted composite into a die cavity and imparting an elevated pressure on the composite until a shape of the component defined by the die cavity has substantially solidified. In one optional form, the growth of the reinforcing phases is partially (or in some cases, substantially) achieved by one or more subsequent heat treatment steps such that upon a catalytic reaction taking place in the bulk alloy, the reinforcing phases grow out of the precursor sites. In this way, the presence of the reinforcing phases is achieved in a way that differs from the conventional addition and subsequent mixing of discreet reinforcing phase particles into the bulk alloy.
The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:
Referring first to
Referring next to
As mentioned above, SSM-based microstructures have superior flow characteristics when compared to those with dendritic microstructure, as the equiaxed microstructure of the billet feedstock can be heated to the semi-solid temperature range to convert the fine grained billet microstructure into the globulitic microstructure which allows a relatively free flowing (yet still viscous) fluid behavior. This in turn allows higher metal flow velocities without the attendant turbulence problems, which in turn significantly improves component production rates. In addition to SSM forming producing no turbulence during filling, it also uses a lower incoming metal temperature so that there less thermal shock to the tooling, employs shorter cycle times due to lower incoming metal temperature, and involves no handling of liquid metal, and produces a fine microstructure with low or no porosity and high mechanical properties. Squeeze casting affords similar advantages, including enjoying the benefit of: producing good surface finish (which contributes to reduced post-cast finishing), producing near net shape parts with almost no material waste, permitting on-site melting of any residual material as a way to reduce waste, and leaves the resulting components with fine microstructure, low or no porosity and high mechanical properties.
Referring with particularity to
Referring with particularity to
Regardless of which of the two parallel paths 300A, 300B are used, subsequent steps include transferring the solid preliminarily-shaped part to a final shaped die 350, heating the finally-shaped part and die in order to partially melt the part 360, applying elevated pressure to the partially-melted part 370 in order to help solidify it in a substantially final shape 380, ejecting the solidified part 390 and performing any optional post-ejection heat treatment 400. In one preferred form, the post-ejection heat treatment 400 may help to further develop the desired microstructures, including uniformly distributed reinforcing phases of different sizes ranging from nanometers to micrometers. With appropriate selection of the precursors to seed the nucleation sites, the particles of the reinforcing phase will be higher in elastic modulus than the bulk alloy, thereby providing additional stiffness of the resulting composite part. In a preferred form, the precursor would be soluble in the alloy at a temperature above which the alloy is solid such that a catalyzing activation arising from increases in temperature, pressure or other energy source (such as ultrasound, vibration or electromagnetism) will promote the formation of the nucleation sites so that the reinforcing phase particles will grow at the nucleation sites to micro size because of one or more of structure, size and composition at the site. The resultant reinforcing particles will themselves be insoluble in the alloy at some temperature below the temperature at which they nucleated, and may be in the form of compounds including (but not limited to) ceramics, intermetallics or dispersoids, as well as a combination of them. Such ceramics may include silicon carbide, silicon nitride, silicon oxide, boron carbide, boron nitride, titanium nitride, titanium carbide, titanium oxide, silicon aluminum oxynitride, steatite (magnesium silicates), aluminum oxide (alumina) and zirconium dioxide (zirconia, which can be chemically stabilized in several different forms, or in metastable structures that can impart transformation toughening, such as the less brittle partially stabilized zirconia). Likewise, suitable intermetallics may include FeAl, Fe3Al, FeAl3, FeCo, Cu3Al, NiTi, NiAl, Ni3Al, Ag3Sn, Cu3Sn, TiSi2, MgCu2, MgZn2, MgNi2, CuZn, Cu31Sn8, SbSn as well as others containing three or more elements. Compounds of low cost rare earth elements such as Ce and La may also be used. The precursors that lead to the reinforcing particles can be added separately or together during the process, depending on the need. Significantly, the precursor achieves two things: first, it provides nucleation sites at which the reinforcing phase particles can grow, and second, it provides the elements which will feed the reinforcing phase growth. As such, they may (or may not) be made from a single composition. Moreover, they may be coated (as discussed below) so that the outside composition is different from that of the core that is controlled by the growth of the various reinforcing phases.
In one preferred form, activation that results in forming the reinforcing phases at the nucleation sites includes catalyzing the one or more precursors through increasing the temperature of the bulk alloy above its solidus temperature. Once the precursor has been catalyzed, the resulting reinforcing phase avoids reverting back in the presence of the liquid melt by virtue of their relatively high melting temperatures in conjunction with their nucleation taking place at temperatures around or above the liquidus temperature TL of common aluminum and magnesium die casting alloys. In fact, these reinforcing phases (preferably in the form of particles) actually can be nucleated over a fairly wide range of temperature (for example, between about 200° to 800° C.), depending on the solution in which the nucleation happens, as well as upon the size of the reinforcing phase. For example, smaller radius particles form at lower melting temperature due to their high surface energy. In situations where use of an aluminum-based material is contemplated, the present inventors believe an activation temperature range of about 500° C. to 800° C. would be sufficient, while an activation temperature range of about 425° C. to 700° C. would be proper for a magnesium-based material. Within the context of the present invention, it is expected that the nucleation to occur around the liquidus temperature TL of typical casting alloys examples of which are shown in the table below.
Proper choice of the reinforcing phases will ensure that they remain solid even in the very hot bulk alloy due to their high melting point. For example, the melting temperature of one typical reinforcing oxide particle, titanium dioxide TiO2, is 1843° C. or 3350° F. As is understood by those skilled in the art, liquidus temperature TL and solidus temperature Ts such as those depicted in the table above are a function of materials compositions based on phase diagrams. Thus, a good solidus Ts temperature range for aluminum would be between about 500° C. and 700° C., while a desirable liquidus temperature TL range would be between about 550° C. and 750° C. Likewise, a preferred solidus temperature TS range for magnesium alloys would be between about 425° C. and 600° C., with a corresponding liquidus temperature TL range of about 550° C. to 700° C.
Moreover, activation of the precursors through the catalyzation steps discussed herein improves wettability through reduced interfacial energy; this in turn produces improvements in the desired reinforcing phases. Thus, in addition to controlling the size of the reinforcing phase, the precursors can be coated (especially when in ceramic form) with metals that generally have low-melting points, or compound particles by mechanical milling, as well as by mixing them in a solvent and then dried. The solvent or carrier (which may remain or be removed after processing) may be used to improve transformation process efficiency or effectiveness by helping reduce the interfacing energy between the surfaces of particles, as well as to avoid particle clustering. The solvent or carrier can be organic or inorganic chemicals, such as alcohol, chlorinated solvents, or commercially-available industrial solvent, as well as solid lubricant such as boron nitride powder, molybdenum disulfide (MbS2) powder or the like.
A significant benefit to using squeeze casting or SSM forming with the present composite-generating approach is that nontraditional compositions of aluminum or magnesium casting alloys may be used, including those with significant non-eutectic compositions that—while possessing valuable attributes for engine blocks and related automotive components—have hitherto been avoided due in part to the difficulty in casting such alloys into repeatable, high-quality finished products. Likewise, alloys traditionally associated with forged materials (such as aluminum-copper, aluminum-magnesium (either with or without additional alloying ingredients) may be used with the present invention, thereby opening up the range of usable materials to ones deemed hitherto inappropriate for low-cost, high-volume component manufacture. By way of example, the hypereutectic Alloy 390 is traditionally difficult to use because of the inability to maintain a desirable microstructure as a way to control the size and distribution of primary silicon during the casting process. By reducing the influence of the high heat of fusion associated with formation of primary silicon, the combination of squeeze casting or liquid forging with the in-situ composite formation discussed herein avoids traditional long cycle times and concomitant shortened tool life that previously limited the applicability of this (as well as other) alloys. The possibility of using hard-to-cast alloys (such as from the Al/Cu class of alloys) is especially desirable in the formation of engine block 100 in that the cylinder bores defined therein may be produced in a “bare-bore” configuration where no separate iron-based cylinder liners or other inserts would be needed. Moreover, traditional hypoeutectic alloys (such as Alloys 319 and 356, each with roughly 6 to 7 percent Si) that the Assignee of the present disclosure currently uses for engine blocks, as well as near-eutectic alloys (such as Alloy 380, with roughly 9% Si) may be beneficially used with the methods disclosed herein.
It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. Moreover, the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. As such, it may represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.
Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4592405 | Allen | Jun 1986 | A |
| 5289388 | Dahroug et al. | Feb 1994 | A |
| 5803154 | Sugawara et al. | Sep 1998 | A |
| Number | Date | Country |
|---|---|---|
| 1273244 | Nov 2000 | CN |
| Number | Date | Country | |
|---|---|---|---|
| 20160361764 A1 | Dec 2016 | US |
