Patent Application
20050194072
The present invention relates to metal alloys and more particularly to magnesium-based metal alloy compositions, as well as methods of making and using such compositions.
Magnesium-based alloys are generally classified into two distinct categories, cast or wrought alloys. Both types of alloys are in widespread use throughout many industries, including in the automotive industry. Magnesium-based alloy cast parts can be produced by conventional casting methods which include diecasting, sand casting, permanent and semi-permanent mold casting, plaster-mold casting and investment casting. Cast parts are generally formed by pouring a molten metal into a casting mold that provides shape to the molten material as it cools and solidifies. The mold is later separated from the part after solidification.
Cast alloy materials demonstrate a number of particularly advantageous properties that have prompted an increased demand for magnesium-based alloy cast parts in the automotive industry. These properties include low density, high strength-to-weight ratio, easy machinability and good damping characteristics. However, many of the compositions for casting alloys are not particularly well-adapted to use as a wrought alloy, where the alloy material is further worked by a deformation process after solidification. Further, many of the commercially available wrought magnesium-based alloys are not comparable to the performance capabilities of other metal wrought alloys (e.g., aluminum-based or stainless steel alloys). Therefore, there is a need for an improved magnesium-based alloy suitable for wrought alloy applications.
In one aspect, the invention provides a metal alloy comprising: aluminum (Al) from about 2.5 to about 3.5 percent by weight (hereinafter, “wt. %”); manganese (Mn) from about 0.2 to 0.6 wt. %; zinc (Zn) less than about 0.3 wt. %; one or more impurities collectively less than about 0.1 wt. %; and a balance of magnesium (Mg).
In another aspect, the present invention provides a magnesium-based wrought alloy has a composition comprising: aluminum (Al) of between about 2.5 to about 4.0 wt. %; manganese (Mn) and zinc (Zn) collectively present at less than about 1.0 wt. %; impurities collectively less than about 0.1 wt. %; and a balance of magnesium (Mg).
In yet another aspect, the present invention provides a method of forming a wrought alloy element comprising: forming a molten alloy material having a composition comprising aluminum (Al) of less than about 4.0 wt. %; manganese (Mn) and zinc (Zn) collectively present at less than about 1.0 wt. %; one or more impurities collectively less than about 0.1 wt. %; and a balance of magnesium (Mg), at a casting temperature. The alloy material is cooled to solidify. The solidified alloy material is processed by deformation, thereby forming the wrought alloy element.
The present invention further provides a component for use in a vehicle, the component comprising a magnesium-based wrought alloy comprising aluminum (Al) of less than about 4.0 wt. %; manganese (Mn) and zinc (Zn) collectively present at less than about 1.0 wt. %; one or more impurities collectively less than about 0.1 wt. %; and a balance of magnesium (Mg).
The present invention also provides a magnesium-based wrought alloy that has a composition comprising: aluminum (Al) of between about 2.5 to about 4.0 wt. %; manganese (Mn) and zinc (Zn) collectively present at less than about 1.0 wt. %; impurities collectively less than about 0.1 wt. %; and a balance of magnesium (Mg); wherein the wrought alloy has an elongation of greater than 8% at room temperature.
In another preferred aspect of the invention provides a magnesium-based wrought alloy has a composition comprising: aluminum (Al) of between about 2.5 to about 4.0 wt. %; manganese (Mn) and zinc (Zn) collectively present at less than about 1.0 wt. %; impurities collectively less than about 0.1 wt. %; and a balance of magnesium (Mg); wherein the wrought alloy having an extrusion speed of greater than 305 mm per minute at 360° C.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
In one aspect, the present invention provides a strong, corrosion-resistant, and lightweight magnesium-based alloy. By “magnesium-based” it is meant that the composition is primarily comprised of magnesium, generally greater than 80wt. % magnesium. As used herein, the term “composition” refers broadly to a substance containing at least the preferred metal elements or compounds, but which may also comprise additional substances or compounds, including additives and impurities. The term “material” also broadly refers to matter containing the preferred compounds or composition. The present invention further relates to methods of making preferred embodiments of the magnesium-based alloy, as well as to methods of making components with preferred embodiments of the inventive alloy.
In another aspect, the present invention provides a wrought magnesium-based alloy, which is designed for improved extrudability and formability, while still maintaining strength and corrosion resistance. As used herein, the terms “wrought” and “worked” are synonymous and refer to an alloy which is generally processed in two separate steps, as recognized by one of skill in the art. The first step comprises forming molten metal into an ingot, a billet, or a pre-form. The pre-form formed in the first step is then processed by working the pre-form in a second step, thereby forming a wrought product. The pre-form thus undergoes a physical deformation process in the second step, which may include for example, extrusion or rolling. The wrought product can then be used to form a part or a portion of a part.
On the other hand, “casting” as it is generally known, involves pouring a molten metal alloy into a casting mold to essentially form a solidified cast part in a near-finished state. The molten metal alloy is poured into a mold, where the metal alloy solidifies after cooling, to form a cast part. The physical requirements for cast alloys are different from the requirements for wrought alloys, due to the differences in physical processing. Thus, while a wrought alloy is first, in essence, cast as an ingot or pre-form, it must also further withstand the additional physical deformation and corresponding processing conditions. Hence, wrought alloys generally require additional optimization of a greater variety of physical properties than those properties needed for a cast alloy. For example, wrought alloys require higher ductility, extrudability and formability, while still requiring sufficient strength and castability for the initial casting process.
Reducing the weight of components in parts assemblies is important for improving efficiency in many different applications, but becomes of great importance for fuel efficiency in mobile applications, such as in automobiles. For example, current magnesium parts are generally made by die casting due to the high productivity and good castability of magnesium alloys. However, many metal parts can be made of wrought alloys for any given application, which can further improve efficiency. For example, tubular sections of steel and aluminum alloys are increasingly used in the automotive industry to replace stamped components, which potentially translates to weight savings, part consolidation, and improved vehicle performance. Such tubular components can be used to form support structures, such as truck frames, engine cradles, roof rails, cross-member supports, and instrument panel beams.
Presently, one of the best performing wrought magnesium alloys available is known as AZ31B (which per ASTM designation is a magnesium-based alloy having a composition of approximately 3 wt. % aluminum (Al), 1 wt. % zinc (Zn), and the balance magnesium and impurities, which is commonly expressed in the format: Mg-3 wt. % Al-1 wt% Zn), which offers the best combination of mechanical properties and extrudability from the wrought magnesium alloys available. However, all currently available magnesium-based wrought alloys have relatively poor extrudability and formability compared to available aluminum extrusion alloys, for example.
The magnesium-based alloys of the present invention are relatively low cost lightweight alloys that demonstrate improved ductility and extrudability, while maintaining relatively high strength and castability through a range of temperatures. (e.g., between ambient temperatures of approximately 26° C. to about 200° C.). The magnesium-based alloys of the present invention are particularly well suited to wrought alloy applications. Further, the inventive alloys are also corrosion resistance. As a result of such properties described above, the inventive alloys are suitable for use in a wide variety of applications including various automotive components including, for example, frames, support members, cross-members, instrument panel beams, roof rails, engine cradles, transfer cases, and steering components.
Preferred embodiments of the present invention comprise aluminum as an alloying element, which is generally believed to have a favorable effect on the physical properties of a magnesium alloy. Aluminum generally improves strength and hardness of a magnesium-based alloy, but it reduces the overall ductility. Generally, increasing aluminum content (i.e., above about 5 wt. %) widens the freezing range for the magnesium-based alloy, which makes it easier to cast. However there is a trade off, because an increased aluminum content makes the alloy more difficult to subsequently work, due to an increased hardness.
Thus, one aspect of the present invention is optimizing the aluminum content in the inventive alloy to maximize the ductility and extrudability, while maintaining reasonable strength and castability (for billet casting prior to working or extrusion). Thus, preferred embodiments of the present invention comprise an aluminum content of less than about 4% by weight, as will be discussed in more detail below. “About” when applied to values indicates that the calculation or the measurement allows some slight imprecision in the value (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If, for some reason, the imprecision provided by “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein indicates a possible variation of up to 5% of the indicated value. For example, a component of about 10 wt. % could vary between 10±0.5 wt. %, thus ranging from between 9.5 and 10.5 wt. %. Particularly preferred embodiments of the present invention comprise an aluminum content of from about 2.5 to about 3.5 wt. %, to optimize the strength and extrudability. In one preferred embodiment of the present invention, the aluminum content is about 3 wt. %.
Further, preferred embodiments of the present invention comprise manganese as an alloying ingredient at less than about 0.6 wt. %. While manganese does not appear to have a large impact on tensile strength of a magnesium-based alloy, it does increase yield strength of the magnesium alloys. Further, manganese functions to improve corrosion resistance of a magnesium aluminum alloy system, by facilitating removal of iron and other heavy metal elements into relatively inert intermetallic components, some of which separate out of the alloy during melting. In preferred embodiments of the present invention, the alloy comprises manganese of from about 0.2 to about 0.6 wt. %, and most preferably from about 0.26 to about 0.6 wt. %. In one preferred embodiment of the present invention, manganese is added at about 0.4 wt. %, as recommended by ASTM Specification B93-94a.
Particularly preferred embodiments of the present invention comprise less than about 0.3 wt. % zinc as an impurity, and most preferably less than 0.22 wt. %. Zinc has typically been used as an alloying ingredient to strengthen magnesium-based alloys of the prior art, however, such alloys typically have significantly lower extrudability, ductility, and increased hot-shortness. Further, zinc containing magnesium alloy systems are generally prone to micro-porosity, and the zinc has been reported to increase surface cracking and oxidation of Mg—Al—Zn based alloys during extrusion, resulting in lower extrusion speed limits. Thus, in contrast to known wrought magnesium alloy systems, the present invention minimizes the amount of zinc present to an impurity level of less than about 0.3 wt. %.
As previously discussed, AZ31B is a known wrought alloy, which contains zinc. AZ31B has one of the fastest extrusion rates among known wrought magnesium-based alloys. Upon evaluation of the known wrought alloys, the performance of the AZ31B (which has a composition of about 3.0 wt. % Al, about 1.0 wt. % Zn, about 0.20 wt. % Mn and the balance Mg and impurities) was compared to the performance of another known wrought alloy, AZ61, (which has a composition of about 5.0 wt. % Al, about 0.30 wt. % Mn, and the balance Mg and impurities) and to a known cast magnesium-based alloy, AM50 (which has a composition of about 5 wt. % Al, about 0.30 wt % Mn, and the balance Mg and impurities). Such cast alloys are not generally known to be useful for wrought alloy applications.
By evaluating these results based on known alloy systems, preferred embodiments of the present invention optimize the aluminum content, by providing a sufficient amount of strength and castability, while minimizing the aluminum content such that it does not detrimentally impact the ductility and extrudability. In accordance with the results of the comparison, one preferred embodiment of the present invention is a novel magnesium-based alloy having an optimized aluminum content of between about 2.5 to about 4.0 wt. % of aluminum, with a most preferred range of aluminum between about 2.5 to about 3.5 wt. %. Other preferred embodiments of the present invention comprise aluminum of less than about 4 wt. %, based on predicted alloy system behavior. Another known cast alloy, AM20 (which has a composition of about 2 wt. % Al, about 0.30 wt % Mn, and the balance Mg and other impurities) having a lower aluminum content, is predicted to be useful as a wrought alloy for making components in certain preferred embodiments of the present invention, but with a relatively low strength due to it low aluminum content.
Thus, in accordance with the principles discussed above, preferred embodiments of the present invention provide a magnesium-based alloy that comprises aluminum (Al) between about 2.5 wt. % and about 4.0 wt. %; manganese (Mn) and zinc (Zn) collectively present at less than 1.0 wt. %; one or more impurities collectively less than 0.1 wt. %; and a balance of magnesium (Mg). In particularly preferred embodiments, the Mn is present at less than about 0.6 wt. % and Zn is present at less than about 0.3 wt. %.
One particularly preferred embodiment of the present invention provides a metal alloy comprising aluminum (Al) at about 3 wt. %; manganese (Mn) at about 0.4 wt. %; zinc (Zn) of less than about 0.22 wt. %; one or more impurities of less than about 0.1 wt. %, with a balance of magnesium (Mg). This embodiment of the inventive alloy of the present invention may be nominally represented by the ASTM formula for magnesium alloys, as “AM30”.
Thus, one preferred embodiment of the present invention comprises a magnesium-based alloy which also contains standard levels of impurities that are commonly found in magnesium alloys, such as, silicon (Si), copper (Cu), nickel (Ni), iron (Fe), as well as other trace impurities. In preferred embodiments of the present invention, the additional impurities collectively comprise less than a maximum of about 0.1 wt. % of the alloy. In alternate preferred embodiments of the present invention, the alloy comprises the following impurities: less than about 0.01 wt. % Si, less than about 0.01 wt. % Cu, less than about 0.002 wt. % Ni, less than about 0.002 wt. % Fe, and less than 0.02 wt. % of all other trace impurities.
An alloy according to one preferred embodiment of the present invention was prepared as follows: 900 kg of melt was prepared and cast into billets having a dimension of 178 mm wide by 406 mm long, the alloy herein identified as “AM30”. For purposes of comparison, a prior art alloy sample of the AZ31B alloy was likewise prepared by casting a melt of 900 kg into billets having the same dimensions as the alloy of the present invention. Table 1 shows the specifications for the present inventive alloy (AM30) and the prior art alloy (AZ31B), as prepared.
The balance of both alloys comprises trace impurities typically found in magnesium alloys. The billets were both heated to 360° C. and tubes were extruded using a 1400 ton press to form tubes having dimensions of a nominal outside diameter of 70 mm and a nominal thickness of 4 mm. For each alloy, a maximum extrusion speed was determined at the onset of surface cracking of the tubes. Approximately 200 meters of tubes were made at the maximum extrusion speed for each alloy.
Tensile properties (i.e., tensile yield strength, ultimate tensile strength and elongation) were determined by testing performed on the prepared tensile specimens made from extruded tube samples. The tubes samples were machined along the longitudinal axis/direction of the tubes. Only the grip sections of the samples were flattened and the curved gage sections remained intact. Tensile strength testing was then carried out at ambient conditions (i.e., room temperature) and five elevated temperatures: 93° C., 121° C., 149° C., 177° C., and 204° C., per ASTM E21-92 specification for tensile strength testing of wrought alloys. ASTM standard specimens of 2″ gauge length were used for tests at an initial strain rate of 0.001 s−1 (i.e., 0.001/second). For each condition, at least three specimens were tested and the measured values were averaged.
The present invention further provides a method of forming a wrought alloy element comprising forming an alloy material having a composition comprising aluminum (Al) of less than about 4.0 wt. %; manganese (Mn) of less than 0.6 wt. %; zinc (Zn) of less than about 0.22 wt. %; one or more impurities of less than about 0.1 wt. %; and a balance of magnesium (Mg) at a casting temperature. The casting temperature is generally above the liquidus temperature of the alloy, but is at least at the point where the metal is molten and is in a substantially liquid-state. It is preferred that the casting temperature is greater than 600° C., most preferably greater than 640° C. The alloy material is cooled to ambient conditions.
The alloy material is then processed by a deformation process, which thereby forms the wrought alloy element. Such deformation processing of the alloy material may include a hot-working process, a cold-working process, or both. Hot-working processes generally include deformation processes conducted at elevated temperatures, which is generally preferred to be above about 200° C. Hot-working deformation processes include both die extrusion or sheet rolling. Cold-working deformation processes are generally conducted at lower temperatures, generally below 200° C. It is preferred that the cold-working is conducted at ambient room temperature conditions.
The present invention is particularly well-suited for automotive components and parts. Certain preferred automotive parts comprise a wrought alloy according to the present invention formed into a tubular structure, or in the alternative, into a sheet or plate structure. Such components are manufactured by undergoing a deformation process, such as extruding, rolling, bending, hydroforming, stamping, superplastic forming, gas forming, electromagnetic forming, including combinations thereof, or any other metal forming processes known to one of skill in the art. The alloy can be formed into a variety of automotive parts and components including, for example, frames, support members, cross-members, instrument panel beams, roof rails, engine cradles, transfer cases, and steering components.
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
