Field of the Invention
The invention relates generally to welded aluminum-copper-lithium alloy products, and more specifically such products in the form of sections intended to produce stiffeners in aeronautical design.
Description of Related Art
Ongoing research is carried out to develop materials that can simultaneously reduce weight and increase the efficiency of high-performance aircraft structures. Aluminum alloys containing lithium are very beneficial in this respect, as lithium reduces the density of aluminum by 3% and increase the modulus of elasticity by 6% for each percent by weight of lithium added. In order for these alloys to be selected in aircrafts, the performance thereof must reach that of the alloys commonly used, particularly in terms of compromise between the static mechanical strength properties (yield stress, fracture strength) and damage tolerance properties (toughness, fatigue-induced crack propagation resistance), these properties being generally antinomic. Said alloys must also display a sufficient corrosion resistance, be able to be shaped using usual methods and display low residual stress so as to be able to be machined integrally.
U.S. Pat. No. 5,032,359 describes a large family of aluminum-copper-lithium alloys wherein the addition of magnesium and silver, particularly between 0.3 and 0.5 percent by weight, makes it possible to increase mechanical strength. Said alloys are frequently referred to using the brand name “Weldalite™”.
U.S. Pat. No. 5,198,045 describes a family of Weldalite™ alloys comprising (as a % by weight) (2.4-3.5) Cu, (1.35-1.8) Li, (0.25-0.65) Mg, (0.25-0.65) Ag-(0.08-0.25) Zr. Welded products manufactured with said alloys combine a density less than 2.64 g/cm3 and a compromise between mechanical strength and advantageous toughness.
U.S. Pat. No. 7,229,509 describes a family of Weldalite™ comprising (as a % by weight) (2.5-5.5) Cu, (0.1-2.5) Li, (0.2-1.0) Mg, (0.2-0.8) Ag, (0.2-0.8) Mn—(up to 0.4) Zr or other refining agents such as Cr, Ti, Hf, Sc and V. Examples displayed exhibit an improved compromise between mechanical strength and toughness, but their density is higher than 2.7 g/cm3.
Published patent application WO2007/080267 describes a Weldalite™ alloy not containing zirconium intended for fuselage sheets (as a % by weight) (2.1-2.8) Cu, (1.1-1.7) Li, (0.2-0.6) Mg, (0.1-0.8) Ag, (0.2-0.6) Mn.
The patent EP1891247 describes a Weldalite™ alloy with a low alloy element content and also intended for the manufacture of fuselage sheets comprising (as a % by weight) (2.7-3.4) Cu, (0.8-1.4) Li, (0.2-0.6) Mg, (0.1-0.8) Ag and at least one element selected from Zr, Mn, Cr, Sc, Hf, Ti.
US Published Patent application WO2006/131627 describes an alloy intended to make fuselage plates comprising (wt. %) (2.7-3.4)Cu, (0.8-1.4) Li, (0.2-0.6) Mg, (0.1-0.8) Ag—and at least one element among Zr, Mn, Cr, Sc, Hf and Ti, wherein Cu and Li satisfy the condition Cu+5/3 Li<5,2.
U.S. Pat. No. 5,455,003 describes a method to make aluminum-copper-lithium alloys having improved mechanical strength and toughness at cryogenic temperature. This method applies notably to an alloy comprising (in wt. %) (2.0-6.5)Cu, (0.2-2.7) Li, (0-4.0) Mg, (0-4.0) Ag, (0-3.0) Zn.
Alloy AA2196 comprising (in wt. %) (2.5-3.3)Cu, (1.4-2.1) Li, (0.25-0.8) Mg, (0.25-0.6) Ag, (0.04-0.18) Zr and at most 0.35 Mn, is also known.
It was generally acknowledged in said patents or patent applications that severe homogenization, i.e. at a temperature of at least 527° C. and for a period of at least 24 hours would make it possible to achieve the optimal properties of the alloy. In some cases of alloys with low zirconium contents (EP1891247) or free from zirconium (WO2007/080267), much less severe homogenization conditions, i.e. a temperature below 510° C., were used.
However, there is still a need for Al—Cu—Li alloy products having a low density and further enhanced properties, particularly in terms of compromise between mechanical strength, on one hand, and damage tolerance, particularly toughness and fatigue-induced crack propagation resistance, on the other, while having other satisfactory usage properties, particularly corrosion resistance.