Patent Application
20080305000
The present invention relates to aluminum-magnesium alloys with silver additions.
Aluminum alloys containing magnesium as the principal alloying element, also known as AA5XXX alloys, exhibit very good corrosion resistance but lack the tensile and compressive strength required to make them usable in structural parts of aircraft. In order to address this issue, scandium additions have been made to AA5XXX alloys. The role of scandium is to promote the formation of fine Al3Sc dispersoids that pin down dislocation and sub-grain movement and therefore increase the strength of the alloy. One of the disadvantages of using scandium dispersoids is the fact that Al3Sc precipitation takes place very rapidly below typical metal processing temperatures, therefore making such alloys susceptible to coarsening. Once coarsening of Al3Sc takes place, the strength increase due to scandium additions is irreversibly lost.
Examples of conventional alloys are disclosed in U.S. Pat. Nos. 4,927,470, 5,066,342, 5,597,529, 5,601,934 and 6,139,653, and Published U.S. Application No. 2004/0091386A1, all of which are incorporated herein by reference.
This invention relates to Al—Mg—Ag wrought products and methods of making the same useful in aircraft applications. Further, the invention relates to Al—Mg—Ag wrought products having improved strength when compared to traditional Al—Mg alloys.
In accordance with the present invention, silver additions can accomplish a strength increase in 5XXX aluminum alloys similar to those of Sc while providing a more “processing friendly” environment. The additional strengthening is caused by the formation of Ag2Al precipitates and disordered Ag clusters in the Al—Mg solid solution. The benefit of Ag additions results from the fact that Ag2Al precipitation is a more controllable process than Al3Sc precipitation. Furthermore, Ag2Al precipitation is reversible via solution heat treating. One of the common problems encountered in Sc containing AA5XXX alloys is the fact that Al3Sc starts precipitating very rapidly during the hot working operation and leads to substantial strain hardening of the matrix making the processing of such alloy very difficult and impossible for certain profiles like extrusions with a high extrusion ratio. On the other hand, the use of Ag in AA5XXX alloys allows for a much more friendly processing as Ag2Al precipitation during the hot working process is much slower therefore preserving lower metal flow stress and enhancing the workability of the alloy.
It is an object of the invention to provide an improved Al—Mg—Ag wrought product for use in aircraft.
It is another object of the invention to provide an Al—Mg—Ag wrought product having improved strength.
It is yet another object of the invention to provide a method for producing an Al—Mg—Ag wrought product having improved strength properties, fracture toughness and resistance to fatigue crack growth.
It is still another object of the invention to provide a method for producing an Al—Mg—Ag alloy having improved strength properties, fracture toughness, good levels of corrosion resistance.
It is another object of this invention to provide aerospace structural members such as extrusions from the alloy of the invention.
It is another object of this invention to provide aerospace structural members such as sheet and plate from the alloy of the invention.
In accordance with these objects, the present invention comprises alloys, and products made therefrom, comprising from about 3.5 to about 10 weight percent Mg, from about 0.05 to about 0.5 weight percent Ag, from about 0.01 to about 1.0 weight percent Mn, from about 0.01 to about 0.15 weight percent Zr, the remainder aluminum and incidental elements and impurities. In one embodiment, from about 0.01 to about 0.8 weight percent Cu as well as from about 0.01 to about 1.0 weight percent Zn may be added to the alloy. In another embodiment, from about 0.05 to about 0.2 or 0.4 weight percent Sc may be added to the alloy. In a further embodiment, the alloy may be substantially free of such Cu, Zn and/or Sc additions, i.e., such additions are not purposefully added to the alloys and are only present in trace amounts or as impurities.
The invention also includes an improved aluminum base alloy wrought product such as an extrusion or flat rolled product consisting essentially of from about 3.5 to about 10 weight percent Mg, from about 0.05 to about 0.5 weight percent Ag, from about 0.01 to about 1.0 weight percent Mn, from about 0.01 to about 0.15 weight percent Zr, from about 0.05 to about 0.2 weight percent Sc, max. 0.15 weight percent Si, max. 0.15 weight percent Fe, and the remainder aluminum and incidental elements and impurities.
These and other objects of the present invention will be more apparent from the following description.
The present invention provides Al—Mg—Ag based alloys, and products made therefrom, in which additional elements are added to the alloys to increase strength. It has been discovered previously that the addition of Sc to Al—Mg alloys (also known as AA5XXX alloys) increases the strength of these alloys and improves their ability to retain their strength after creep annealing. In the Al—Mg—Sr alloy systems, the additional increase in strength is achieved via Al3Zr dispersoid precipitation. The Al3Zr dispersoids pin down the dislocations and sub-grain boundaries, thereby increasing the strain hardening behavior of the alloy and ultimately increasing the strength of the alloy. However, one of the major drawbacks of the Sr containing AA5XXX alloys is the fact that the Al3Zr dispersoid precipitation occurs at temperatures lower than the metal processing temperature. As a result, Al3Zr will precipitate during the hot plastic deformation process. This can lead to un-uniform metal properties across the finished product. Al3Zr precipitation during the metal fabrication process may also lead to severe work hardening of the material which may make it impossible to be processed thru all the manufacturing steps or may limit the applicability of the process to a limited number of profiles that can be manufactured.
One other difficulty encountered by Sc containing AA5XXX alloys is the coarsening of Sc during the hot plastic deformation process. During hot plastic deformation of Al—Mg—Sc alloys, the temperature of the material being processed can rise above its starting temperature and reach values that will favor the coarsening of the Al3Zr, and therefore a degradation in mechanical properties. This phenomena occurs quite frequently in situations where the hot plastic deformation is more severe.
In accordance with the present invention, the role of Sc is replaced by Ag. There is, however, a fundamental difference between the formation mechanism of the two types of precipitates. Al3Zr is a dispersoid type precipitate, and its formation is characterized by a fast aging kinetics and the impossibility of re-solutionizing the precipitate once formed. In contrast, Ag2Al precipitates at a slower rate, and these precipitates can be dissolved in the matrix by heating the alloy at temperatures below the melting point, typically in temperature ranges between 860° F. and 1,000° F. Al3Zr precipitates will only dissolve at temperatures above the melting temperature of the alloy. An advantage presented by the Ag additions is the ability to better control the precipitation of Ag2Al and the ability to dissolve the precipitate and re-precipitate it in a controlled manner.
Al—Mg alloys also known as AA5XXX alloys are conventionally known as non-heat treatable alloys, i.e., strength in this family of alloys is not achieved via precipitation strengthening, but rather via work hardening. Furthermore, exposing AA5XXX alloys as well as Sc containing AA5XXX alloys to temperatures of 860° F. to 890° F. will lead to a degradation in mechanical properties. In contrast, the present invention provides heat treatable Al—Mg alloys via Ag additions.
The alloys have the following chemical composition: 3.5 to 10 weight % Mg; 0.05 to 0.5 weight % Ag; 0.01 to 1 weight % Mn; 0.01 to 0.15 weight % Zr; and the remainder Al and incidental impurities.
Silver additions to aluminum-magnesium alloys provide improved corrosion resistance and strength. The formation of AlAg2 inside the grains acts as nucleating sites for A145Mg28 precipitates. The silver additions stabilize the alloy at elevated temperatures and prevent migration and re-precipitation of alloying elements at the grain boundaries, thereby improving inter-granular corrosion resistance. Silver additions also produce a precipitation hardening effect, thereby enhancing strength of the alloys.
Manganese and zirconium act as grain refiners and may also serve as recrystallization inhibitors.
The Al—Mg—Ag alloys of the present invention are distinct from conventional 5XXX series alloys because they are susceptible to heat treatment. Under normal conditions, traditional 5XXX series alloys are not considered to be heat treatable. However, the present Al—Mg—Ag alloys exhibit improved properties when subjected to solution heat treatment, quenching, working such as stretching, and aging. For example, the following production path may be used: casting; homogenizing; extrusion or rolling; heat treatment followed by rapid cooling; cold working; and age hardening.
Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.
This application claims the benefit of U.S. Provisional Application No. 60/917,445 filed May 11, 2007, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60917445 | May 2007 | US |
