2. Description of the Background Art Aluminum alloys containing magnesium as the principal alloying element have two potential advantages for aircraft structures: they are lighter than the standard 2000 and 7000 series alloys; and unlike the latter materials, they are weldable by conventional fusion techniques, which could lower manufacturing costs by reducing the 2-3 million rivets typically used to assemble a commercial airliner.

A number of aluminum alloys have been developed in which magnesium is added to improve strength. These alloys are particularly suited for aerospace applications because of their strength and damage resistance. However, these alloys are not particularly suited for aerospace applications because their strength levels are not high enough. To address this problem, improved Al-Mg based alloys have been developed in which a dispersoid generating element, such as scandium, is added to the alloy. The addition of scandium to the alloys results in the formation of Al3Sc dispersoids, which are precipitates that are known to impart significantly greater strength and corrosion resistance to products made from the alloys.

However, the tensile properties of Al-Mg-Sc based alloys deteriorate rapidly with thermomechanical processing and high temperature operations, such as rolling, that are necessary to form aircraft fuselage components. The degradation in tensile properties occurs because the scandium dispersoids must be small in size and large in number to impart increased strength to the alloy, and high temperature operations cause them to grow too large.

Nevertheless, the desire remains to use Al-Mg-Sc alloys in aerospace applications because of their corrosion resistance, and also because these alloys are weldable, thus eliminating the need to use expensive rivet-based assembly procedures.

SUMMARY OF THE INVENTION The present invention relates to Al-Mg-Sc based alloy in which an additional dispersoid generating element, hafnium, is added to the alloys to substantially eliminate degradation of the tensile properties during rolling and other thermomechanical and high temperature operations. The addition of hafnium to the alloys has been found to reduce growth of the scandium dispersoid particles during these thermal operations, thus enabling the scandium particles to maintain their strength enhancing characteristics. More particularly, the present invention comprises alloys, and products made therefrom, whose wt. % composition comprises 1.0-8.0% Mg, 0.05-0.6% Sc, 0.05-0.20% HO and the balance aluminum and incidental impurities.

In a more preferred form of the invention, a small amount of manganese, preferably 0.1-0.8 wt. %, is added to the alloys to improve the strength characteristics even further. In an expriment on a sample alloy formed in accordance with the most preferred embodiment, the alloy's tensile properties were not degraded after rolling operations in which the sample was hot and cold rolled to a thin sheet suitable for use in an aircraft skin, and then annealed.

Alternatively, the alloy can be strengthened further by the addition of 0.05-0.20% Zr, either with or without the manganese.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS All of the embodiments of the present invention comprise Al-Mg-Sc based alloys, and products made therefrom, in which hafnium is added to the alloys to increase strength and corrosion resistance. In a first embodiment, the alloys preferably include 1.0-8.0% Mg, 0.05- 0.6% Sc, 0.05-0.20% Hf, and the balance aluminum and incidental impurities, with the most preferred ranges of the recited elements being 4.0-6.0% Mg, 0.2-0.4% Sc, and 0.08-0.15% Hf. Within these ranges, a composition of 5.0% Mg, 0.25% Sc, 0.12% Hf, and the balance aluminum and incidental impurities is believed to provide the best results.

The significance of each element in the subject alloys is as follows. Mg added to the alloys in the recited amount increases the strength of the alloy substantially. However, if Mg is added in amounts above approximately 8%, the resulting alloy becomes difficult to process.

Sc is added to generate Al3Sc dispersoids, which as stated previously, substantially increase the strength and corrosion resistance of the alloys.

Hf is the most significant element in the alloys of the present invention. This element, like Sc, is another dispersoid generating element that can be used in place of Sc to achieve improvements in strength and corrosion resistance. However, the inventor has discovered that when Hf is used in combination with Sc, the Hf acts to stabilize the Al3Sc dispersoids during rolling and thermal processing. More specifically, without the addition of Hf, the Al3Sc dispersoids will grow too large during thermal processing, and substantially diminish the alloys'tensile properties. Surprisingly, however, the inventor has discovered that the addition of Hf to the alloy limits the growth ofthe Al3Sc dispersoids. The amounts of Sc and Hf added to the alloys must not, however, be above the recited ranges to avoid primary formations in the alloys that would once again, diminish their tensile and other properties.

In a more preferred embodiment of the present invention, manganese and/or zirconium are added to the alloys to improve their tensile properties further. The proportions of the other elements in the alloys remain the same, and the preferred range in wt. % for the manganese is 0.1-0.8, and for the zirconium is 0.5-2.0. The composition of the alloys is thus 1.0-8.0% Mg, 0.05-0.6% Sc, 0.05-0.20% Hf, 0.1-0.8% Mn and/or 0.05-0.20% Zr, and the balance aluminum and incidental impurities, with the most preferred ranges of the recited elements being 4.0-6.0% Mg, 0.2-0.4% Sc, 0.08-0.15% Hf, and 0.3-0.7% Mn and/or 0.08- 0.15% Zr. Within these ranges, a composition of 5.0% Mg, 0.25% Sc, 0.12% Hf, 0.6% Mn and/or 0.12% Zr, and the balance aluminum and incidental impurities is believed to provide the best results.

EXAMPLE 1 To test the tensile properties of an alloy formed in accordance with the present invention, a rolled sheet sample alloy was prepared as follows, and subjected to testing. First, a 3"x 9"ingot was cast of an alloy having the following wt. % composition: 5% Mg, 0.2% Sc, 0.12% Hf 0.5% Mn, the balance Al, and incidental impurities. This ingot was then subjected, without homogenization, to conventional rolling operations until it was formed into a sheet of 0.063" thickness. The sheet was then annealed at 550° F for 8 hours. For comparison purposes, additional rolled sheet samples were prepared in the same manner, but with different alloy compositions. A first of the comparison alloys did not contain hafnium and manganese so that this sample's alloy composition was 5% Mg, 0.2% Sc, the balance Al, and incidental impurities. The second and third comparison alloy compositions included

0.11% zirconium, with the third sample also containing 0.5% manganese. The Zr containing samples were employed because it is known that Zr also stabilizes the Al3Sc dispersoids, and thus improves the tensile properties of the rolled sheets.

The annealed sheets were then subjected to conventional tests to determine their tensile properties. The results of these tests are set forth in Table 1: TABLE 1 TENSILE PROPERTIES OF Al-Mg-Sc (No Homogenization, 0.063", 550 F/8 hrs.) Alloy(A1+ 5.0%Mg + 0.2%Sc0.12% Hf + 0.11% or 0.11% Zr + +)0.5% Mn 0. 5% Mn UTS(UltimateTensileStrength),56.349.456.460.1 ksi YS(YieldStrength),ksi42.632.642.846.7 EL(Elongation),%11.413.712.0 11.4 The test results indicate that substantial improvements in strength properties are obtained when hainium and manganese are added to an Al-Mg-Sc alloy. In particular, improvements of over 10% and 30% were achieved for the ultimate tensile strength and yield strength, respectfully, over the comparison sheet sample comprised of an Al-Mg-Sc alloy without hafnium and manganese. The obtained values for all three parameters also compare favorably with the comparison sample containing 0.11% Zr, although they were somewhat less than the comparison sample containing both Zr and Mn.

EXAMPLES 2-4 The tensile properties of the two zirconium containing samples indicate that the addition of manganese to the alloy provides further improvements to the observe tensile properties. However, the sample containing zirconium without manganese still provides substantial improvements in the rolled sheet's tensile properties over the sheet sample containing only Al-Mg-Sc. These results suggest that an alloy containing 5% Mg, 0.2% Sc, 0.12% HO the balance Al, and incidental impurities would also provide substantial improvements in ultimate tensile strength, yield strength and elongation over a conventional Al-Mg-Sc alloy. Furthermore, addition of zirconium to the alloys is expected to provide

improved strength properties as well, with a composition containing 5% Mg, 0.2% Sc, 0.12% Hf, 0.12% Zr, the balance Al, and incidental impurities expected to provide the best results.

In addition, 0.6% Mn may be added to this alloy as in the Example 1 alloy to further enhance its properties.

The values achieved for the tensile properties of the Al-Mg-Sc-Hf-Mn alloy of Example 1, and the values expected for the other Al-Mg-Sc-Hf alloys of Examples 2-4 indicate that the alloys can readily be employed in rolled sheet form for various aerospace applications, such as for aircraft fuselage skins, etc. As stated previously, these applications for the subject alloys are particularly attractive because of the superior corrosion resistance and weldability of Al-Mg-Sc alloys.

Although the present invention has been disclosed in terms of a number of preferred embodiments, it will be understood that modifications and variations could be made thereto without departing from the scope of the invention as defined in the following claims.