2000 Series alloys with enhanced damage tolerance performance for aerospace applications

#This invention relates to an Al—Cu—Mg—Ag-alloy having improved damage tolerance, suitable for aerospace and other demanding applications. The alloy has very low levels of iron and silicon, and a low copper to magnesium ratio.

#In commercial jet aircraft applications, a key structural requirement for lower wing and fuselage applications is a high level of damage tolerance as measured by fatigue crack growth (FCG), and fracture toughness. Current generation materials are taken from the Al—Cu 2XXX family, typically of the 2X24 type. These alloys are usually used in a T3X temper and inherently have moderate strength with high fracture toughness and good FCG resistance. Typically, when the 2X24 alloys are artificially aged to a T8 temper, where strength is increased, there is a degradation in toughness and/or FCG performance.

#Damage tolerance is a combination of fracture toughness and FCG resistance. As strength increases there is a concurrent decrease in fracture toughness, and maintaining high toughness with increased strength is a desirable attribute of any new alloy product. FCG performance is often measured using two common loading configurations: 1) constant amplitude (CA), and 2) under spectrum or variable loading. The latter is intended to better represent the loading expected in service. Details on flight simulated loading FCG tests are described in J. Schijve, “The significance of flight-simulation fatigue tests”, Delft University Report (LR-466), June 1985. Constant amplitude FCG tests are run using a stress range defined by the R ratio, i.e. minimum/ maximum stress. Crack growth rates are measured as a function of a stress intensity range (? K). Under spectrum loading, crack growth is again measured, but this time is reported over a number of “flights.” Loading is such that it simulates typical takeoff, in flight, and landing loads for each flight, and this is repeated to represent typical lifetime loadings seen for a given part of the aircraft structure. The spectrum FCG tests are a more representative measure of an alloy's performance as they simulate actual aircraft operation. There are a number of generic spectrum loading configurations and also aircraft-specific spectrum which are dependent on aircraft design philosophy and also aircraft size. Smaller, single aisle aircraft are expected to have a higher number of takeoff/landing cycles than large, wide-bodied aircraft that make fewer but longer flights.

#The invention is further illustrated by the following drawings in which:

#FIG. 1 is a graph showing constant amplitude FCG data for 2524-T3 and Sample A-T8 sheet. Tests were conducted in the T-L orientation with R ratio equals 0.1.

#FIG. 2 is a graph showing constant amplitude FCG data for 2524-T3 and Sample A-T8 sheet. Tests were conducted in the L-T orientation with R ratio equals 0.1.

#Definitions: For the description of alloy compositions that follow, all references to percentages are by weight percent (wt %) unless otherwise indicated. When referring to a minimum (for instance for strength or toughness) or to a maximum (for instance for fatigue crack growth rate), these refer to a level at which specifications for materials can be written or a level at which a material can be guaranteed or a level that an airframe builder (subject to a safety factor) can rely on in design. In some cases, it can have a statistical basis, e.g., 99% of the product conforms or is expected to conform with 95% confidence using standard statistical methods.

#When referring to any numerical range of values herein, such ranges are understood to include each and every number and/or fraction between the stated range minimum and maximum. A range of about 3.0-4.0 wt % copper, for example, would expressly include all intermediate values of about 3.1, 3.12, 3.2, 3.24, 3.5, all the way up to and including 3.61, 3.62, 3.63 and 4.0 wt % Cu. The same applies to all other elemental ranges set forth below, such as the Cu:Mg ratio of between about 3.6 and 5.

#The present invention provides an aluminum-based alloy having enhanced damage tolerance consisting essentially of about 3.0-4.0 wt % copper; about 0.4-1.1 wt % magnesium; up to about 0.8 wt % silver; up to about 1.0 wt % Zn; up to about 0.25 wt % Zr; up to about 0.9 wt % Mn; up to about 0.5 wt % Fe; and up to about 0.5 wt % Si; the balance substantially aluminum, incidental impurities and elements, said copper and magnesium present in a ratio of about 3.6-5 parts copper to about 1 part magnesium. Preferably, the copper and magnesium are present in a ratio of about 4-4.5 parts copper to about 1 part magnesium.