Patent Publication Number: US-5897720-A

Title: Aluminum-copper-magnesium-manganese alloy useful for aircraft applications

Description:
This application is a continuation of application Ser. No. 08/407,842, filed Mar. 21, 1995 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to aluminum alloys suitable for use in aircraft applications. More specifically, it relates to a method of making an improved aluminum product having improved damage tolerant characteristics, including improved fracture toughness, fatigue resistance, corrosion resistance, formability and surface roughness properties. 
     2. Description of the Related Art 
     The design of commercial aircraft requires different sets of properties for different types of structures. Depending on the design criteria for a particular airplane component, improvements in fracture toughness and fatigue resistance result in weight savings, which translate to fuel economy over the lifetime of the aircraft, and/or a greater level of safety. For example, a slower fatigue crack growth rate will require a longer time for a crack or flaw to grow to a size where it becomes &#34;critical&#34; leading to catastrophic failure; and higher fracture toughness means that a crack can grow to a longer length before it is critical. 
     Corrosion damage has been a perennial problem in today&#39;s aircraft, and the fuselage is the prime location for corrosion to occur. Improvements in corrosion resistance, therefore, are often sought with or without weight savings. 
     The issues of toughness, fatigue and corrosion all relate to structural integrity of the airplane. Formability, on the other hand, is a manufacturing concern. Sheets that are easier to bend, stretch and draw into various shapes reduce the cost of building an airplane because the scrap rate is reduced, and less labor is required. 
     For some time, heat treatable aluminum base alloy sheet and plate containing copper, magnesium and manganese has found considerable acceptance for various structural members. Such alloys generally contain 3.8 to 4.9 wt. % copper, 1.2 to 1.8 wt. % magnesium and 0.3 to 0.9 wt. % manganese and carries the Aluminum Association designation of 2024 alloy. This alloy is noted for its superior strength to weight ratio, its good toughness and tear resistance, and adequate resistance to general and stress corrosion effects. 
     Workers in the field have generally adapted the 2024 alloy for use in the construction of commercial aircraft. For example, one alloy used on the lower wing skins of some commercial jet aircraft is alloy 2024 in the T351 temper. Alloy 2024-T351 has a relatively high strength-to-density ratio and exhibits reasonably good fracture toughness, good fatigue properties, and adequate corrosion resistance. U.S. Pat. Nos. 4,336,075 to Quist et al. and 4,294,625 to Hyatt et al. disclose an alloy which has a higher strength to density ratio, improved fatigue and fracture toughness characteristics over alloy 2024 while maintaining corrosion resistance levels approximately equal to or slightly better than 2024. Quist et al. and Hyatt et al. achieve their improvements by homogenizing the alloy at a moderate temperature, carefully controlling the hot-rolling and extrusion parameters and then natural age-hardening to produce a highly elongated, substantially unrecrystallized microstructure. Similarly, U.S. Pat. No. 5,213,639 to Colvin et al. discloses an alloy which has at least a 5.0% improvement over 2024 alloy in T-L fracture toughness or fatigue crack growth rate by re-heating the alloy prior to hot rolling. 
     There still remains a need, however, for an improved alloy that has increased fracture toughness, fatigue resistance, corrosion resistance and formability over alloy 2024. Accordingly, it is the primary object of this invention to provide such an alloy. 
     SUMMARY OF THE INVENTION 
     The present invention provides a product comprising an aluminum base alloy including about 3.8 to 4.5 wt. % copper, about 1.2 to 1.6 wt. % magnesium, about 0.3 to 0.6 wt. % manganese, not more than about 0.15 wt. % silicon, not more than about 0.12 wt. % iron, not more than about 0.1 wt. % titanium, the remainder substantially aluminum, incidental elements and impurities, the product having at least 5% improvement over 2024 alloy in fracture toughness, fatigue crack growth rate, corrosion resistance, formability properties. 
     In an alternative embodiment, the invention provides a method of producing an aluminum product comprising providing stock including an aluminum alloy comprising about 3.8 to 4.9 wt. % copper, about 1.2 to 1.8 wt. % magnesium, about 0.3 to 0.9 wt. % manganese, not more than 0.30 wt. % silicon, not more than 0.30 wt. % iron, not more than 0.15 wt. % titanium, the remainder substantially aluminum, incidental elements and impurities; hot working the stock; annealing; cold rolling; solution heat treating; and cooling thereby producing an alloy having improved fracture toughness, fatigue resistance, corrosion resistance, and formability properties. 
     The foregoing and other objects, features, and advantages of the invention will become more readily apparent from the following detailed description of preferred embodiment which proceeds with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows composition-phase relations for an Al--Cu--Mg system at 930° F. 
     FIG. 2 is a graph showing fracture toughness (K app )as a function of iron content. 
     FIG. 3 is a graph showing tear strength--yield strength ratio (TYR) as a function of iron content. 
     FIG. 4 is a graph showing fracture toughness (K app ) as correlated with manganese and iron levels. 
     FIG. 5 is a graph showing tear strength--yield strength ratio (TYR) as correlated with manganese and iron levels. 
     FIG. 6 is a graph showing formability parameters as a function of iron and manganese levels. 
     FIG. 7 is a graph showing unit propagation energy of alloys having 0.54 wt. % and 0.98 wt. % Mn fabricated with and without an intermediate anneal 
     FIG. 8a is a photograph showing the improved alloy having 0.54 wt. % Mn without intermediate annealing and 
     FIG. 8b is a photograph of the same alloy with intermediate annealing according to the present invention. 
     FIG. 9a is a photograph showing the improved alloy having 0.98 wt. % Mn without intermediate annealing and 
     FIG. 9b is a photograph of the same alloy with intermediate annealing according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The fracture toughness, fatigue resistance, corrosion resistance, and formability properties of the present invention are dependent upon a chemical composition that is closely controlled within specific limits as set forth below and upon carefully controlled and sequenced process steps. If the composition limits or process parameters stray from the limits set forth below, the desired combination of fracture toughness, fatigue resistance, corrosion resistance, formability, and surface smoothness objectives will not be achieved. 
     The aluminum alloy of the present invention comprises about 3.8 to 4.5 wt. % copper, about 1.2 to 1.6 wt. % magnesium, about 0.3 to 0.6 wt. % manganese, not more than about 0.15 wt. % silicon, not more than about 0.12 wt. % iron, and not more than about 0.10 wt. % titanium, the balance being aluminum and impurity elements. For any remaining trace elements, each has a maximum limit 0.05 wt. %, with a total maximum of 0.15 wt. %. A preferred alloy would comprise about 4.0 to 4.4 wt. % copper, about 1.25 to 1.5 wt. % magnesium, about 0.35 to 0.50 wt. % manganese, not more than about 0.12 wt. % silicon, not more than about 0.08 wt. % iron, and not more than about 0.06 wt. % titanium, the balance being aluminum and impurity elements. 
     The chemical composition of the alloy of the present invention is similar to that of alloy 2024, but is distinctive in several important aspects. The alloying elements contained in the allowed range of variation for alloying elements contained in the invention alloy is less than for 2024. This is important because many mechanical and physical properties change as composition changes. To maintain the desired close balance of properties of the invention it is therefore necessary to restrict composition changes to a greater degree than is normally done. In addition, to the restricted ranges of copper, magnesium, and manganese, the silicon, iron, and titanium concentrations are reduced to the lowest levels commercially feasible for aluminum alloys of the present type in order to improve the fracture toughness. 
     Improved Fracture Toughness 
     The &#34;damage tolerant&#34; design philosophy being used today for commercial and military aircraft assumes that all structures contain flaws (cracks). The stress at the tip of a sharp crack is characterized by a stress intensity factor K, given by 
     
         K=Yσ√ c. 
    
     where σ is the average applied stress on the structure (pounds per square inch), Y is a dimensionless parameter dependant on the geometry of the structural member, and c is the crack length. The stress intensity factor at which the crack begins to extend, generally resulting in catastrophic failure, is known as the fracture toughness of the material. 
     I now consider the factors that affect the fracture toughness of heat treated aluminum alloys, i.e., those alloys that derive their strength from thermal treatments such as a &#34;solutionizing&#34; operation that dissolves the alloying elements, followed by rapidly cooling to room temperature (quenching), and then &#34;aging&#34; at room temperature or higher (250 to 375° F.) to precipitate the alloying elements as small discrete particles within the aluminum matrix. Two major controls must be maintained on the chemical composition of these alloys. First, the iron impurity level must be kept to a minimum, because it is insoluble in aluminum and forms coarse, brittle intermetallic particles that contribute to crack extension and fracture. Second, the amounts of the major alloying elements, such as copper and magnesium, should be controlled to ensure that they are dissolved into the aluminum matrix during the solutionizing operation. Any intermetallic particles that are left undissolved reduce fracture toughness as do those resulting from high iron levels. 
     FIG. 1 graphically illustrates an equilibrium phase diagram for the aluminum (Al)--copper (Cu)--magnesium (Mg) system at 930° F. Specifically, FIG. 1 defines the copper and magnesium concentrations that can be dissolved. If the limits defined by the alpha aluminum region are exceeded, undissolved particles of Al 2  CuMg (commonly designated as &#34;S&#34; phase) and Al 2  Cu (commonly designated as &#34;θ&#34; phase), remain after solution heat treatment. This situation is complicated by the presence of iron, which can combine with copper to from an insoluble Al 7  Cu 2  Fe intermetallic constituent. The copper level in FIG. 1 therefore must be adjusted upwards by an amount equal to approximately twice the iron concentration because the Al 7  Cu 2  Fe constituent contains about two times as much copper as iron. 
     A third compositional factor is the role of sparingly soluble alloying elements such as chromium, manganese and zirconium. One or more of these alloying elements are intentionally added to aluminum to form &#34;dispersoids,&#34; which are small intermetallic particles that are useful in controlling the crystallite, or &#34;grain&#34; structure of aluminum alloys. All metallic products are comprised of numerous crystallites, or grains, which should not be allowed to grow to a large size during any of the thermal processing operations, because strength and good fracture toughness are favored by small grains. The dispersoid particles act to &#34;pin&#34; the grains and prevent their growth. 
     The dispersoid forming element in Al-Cu-Mg alloy 2024 is manganese in the range of 0.3 to 0.9%. Unexpectedly, I have discovered a significant effect of manganese on fracture toughness as measured by two test methods. In one method, I tested to failure 16-inch wide by 36-inch long panels with a 4-inch long through--thickness sharp crack in the orientation transverse to the rolling direction (T-L). Using an equation similar to K=Yσ√ c, above, I calculated values of K (apparent) or K app . It is noteworthy that K app  determined in this manner is only an indicator of the true fracture toughness, because the stress required to cause failure exceeded the elastic limit of the material. A much wider panel would be required to obtain the actual fracture toughness, and is beyond the capability of most test laboratories. In the second method, a 1.5-inch wide by 2.25-inch long panel with a sharp notch on one side was pulled to failure and the tear strength (maximum load divided by the cross sectional area) was measured. The tear strength divided by the yield strength as determined in a standard tensile test, commonly called the tear-yield ratio (TYR), is known to correlate with fracture toughness. Table 1 illustrates a number of production lots of 2024 alloy sheets having various iron and manganese contents which I tested for toughness by the aforementioned methods. Table 2 illustrates the results of these tests. 
     
                       TABLE 1
______________________________________
CHEMICAL COMPOSITIONS OF PRODUCTION LOTS OF 2024-T3
SHEETS
(Core alloy with cladding removed)
% by wt.sup.a
Alloy
     Si       Fe     Cu     Mn   Mg     Ti   Zn
______________________________________
1    &lt;0.1     0.035  4.31   0.33 1.37   0.02 0.02
2    &lt;0.1     0.04   4.21   0.46 1.28   0.02 0.02
3    &lt;0.1     0.07   3.99   0.32 1.37   0.02 0.06
4    &lt;0.1     0.07   3.99   0.44 1.28   0.04 0.22
5    &lt;0.1     0.17   4.21   0.39 1.44   0.03 0.03
6    &lt;0.1     0.16   4.17   0.77 1.21   0.03 0.07
7    &lt;0.1     0.19   4.43   0.54 1.48   0.01 0.01
______________________________________
 .sup.a By inductively coupied plasma spectroscopy.
 
    
     
                       TABLE 2
______________________________________
EFFECT OF IRON AND MANGANESE CONTENTS ON TOUGHNESS
OF ALCLAD 2024-T3 SHEET.sup.a
                       T-L K.sub.app    YS.sup.c
Alloy No.
       % Fe    % Mn    (ksi.check mark.in)
                                T-L TS/YS.sup.b
                                        (ksi)
______________________________________
1      0.035   0.33    89       1.76    45.5
2      0.04    0.46    87       1.70    46.3
3      0.07    0.32    85       1.65    45.4
4      0.07    0.44    82       1.59    44.4
5      0.17    0.39    83.5     1.60    43.4
6      0.16    0.77    77.5     1.52    44.5
7      0.19    0.54    79.5     1.53    44.2
______________________________________
 .sup.a Also: 4.0-4.5% Cu and 1.2-1.5% Mg; all 0.063&#34; thick.
 .sup.b Tear strengthyield strength ratio (TYR).
 .sup.c Transverse tensile yield strength.
 
    
     I used data from Table 2 to plot the toughness measurements as a function of wt. % iron in FIGS. 2 and 3. As expected, FIGS. 2 and 3 demonstrate a correlation of fracture toughness with decreasing concentrations of iron. Surprisingly, however, the lots with relatively low manganese levels exhibit higher toughness values for a given iron content. Table 3, which compares the toughness levels at two manganese levels for a number of iron concentrations, also demonstrates this phenomenon. Table 3 also lists copper contents for each alloy, because high levels of copper can reduce toughness by the presence of undissolved Al 2  Cu and Al 2  CuMg phases. Notably, the copper levels of the alloys being compared in each case are almost equivalent. 
     
                       TABLE 3
______________________________________
EFFECT OF MANGANESE AT VARIOUS IRON LEVELS ON
TOUGHNESS OF ALCLAD 2024-T3 SHEET
ALLOY   % MN      % CU     T-L K.sub.app
                                  T-L TS/YS.sup.b
______________________________________
0.035-0.04% Fe
1       0.33      4.31     89     1.76
2       0.46      4.21     87     1.70
        Δ(%)         2.3    3.50
0.07% Fe
3       0.32      3.99     85     1.65
4       0.44      3.99     82     1.59
        Δ(%)         3.5    3.80
0.16-0.17% Fe
5       0.39      4.17     83.5   1.6
6       0.77      4.21     77.5   1.52
        Δ(%)         7.7    5.3
        Ave. Δ/0.1%  2.2    2.4
        Mn
______________________________________
 .sup.a Tear strengthyield strength ratio (TYR).
 
    
     A linear regression analysis of the K app  data showed that manganese has approximately half the detrimental effect that iron has. This discovery is particularly important because of the relatively high levels of manganese in alloys such as 2024. Specifically, FIG. 4 demonstrates toughness, K app , as a function of iron and manganese concentrations, producing the correlation: 
     
         K.sub.app =93.2-29.2 (% Fe+0.50 % Mn) 
    
     Similarly, toughness, expressed as tear-yield ratio or TYR, is represented by the correlation: 
     
         TYR=1.81-0.27(2% Fe+Mn) 
    
     This correlation is illustrated in FIG. 5. 
     Improved Fatigue Resistance 
     As noted in the previous section, the &#34;damage tolerant&#34; design philosophy assumes that flaws (cracks) are present in all structural materials. If these cracks are permitted to grow to a &#34;critical&#34; size such that the stress intensity factor at the crack tip exceeds the fracture toughness of the material, catastrophic failure occurs. Cracks can grow as a result of cyclic loads (fatigue) caused by takeoff and landing or cabin pressurization and depressurization. Fatigue crack growth rates for the projected cyclic loading stresses are therefore desirably low. 
     Experimentally, I have found that the velocity of cracks growing under fatigue conditions, i.e., the fatigue crack growth rate, is dependent on the stress intensity factor difference (ΔK) associated with the minimum and maximum load. The stress intensity factor increment (ΔK) must therefore, be specified when comparing fatigue crack growth rates for different materials 
     In addition to improved toughness, I also discovered that higher purity alloys with relatively low manganese levels also had low fatigue crack growth rates. I determined this by running tests at a stress intensity increment (ΔK) of 30 ksi√in. and a load ratio (maximum load divided by minimum load) of 0.1 on several of the alloys listed in Tables 1-3. For example, alloys 1 and 2 had average crack growth rates of 7.0×10-5 and 7.5×10-5 inches/cycle, compared to a nominal value of 20×10-5 inch/cycle for standard 2024 alloy typified by alloy 7. Thus, the alloy of my invention has about a 50% decrease in crack growth rate over standard 2024 alloy at a ΔK of 30 ksi√in. Similarly, I discovered fatigue benefits at lower values of ΔK. For example, at a ΔK of 5 ksi√in., alloys 1,2,3, and 4 had crack growth rates of 1.5 to 2.2×10-7 inches/cycle compared to 1.7 to 4.0×10-7 inches/cycle for standard 2024 alloy. Or stated another way, my new alloy had about a 25% decrease in crack growth rate in the low ΔK regime. 
     Improved Corrision Resistance 
     Yet another benefit of the new alloy of my invention is improved corrosion resistance. As noted earlier, good corrosion resistance is of prime concern in aircraft fuselage structures. Corrosion of aluminum alloys is usually aggravated by salt (sodium chloride) containing environments such as can be present near oceans. Sheet samples from alloys 3 and 7 (of Tables 1-3) were therefore exposed to a marine atmosphere at Daytona Beach, Fla. for one year. The protective cladding was removed from one surface so that the inherent corrosion resistance of the core alloy could be assessed. This also simulates the practical situation where one side of a fuselage panel is chemically milled to a thinner section size. After the one-year exposure period, tensile specimens were machined from the samples, and as recommended in the Corrosion Handbook (edited by H. H. Uhlig, John Wiley &amp; Sons, p. 956), the corrosion damage was quantified by loss in ductility. This method is particularly suited to materials that are susceptible to pitting and intergranular corrosion. Table 4 summarizes tensile elongation measurements before and after the exposure to the marine atmosphere. Metallographic examination revealed that ductility loss corresponded with the depth of pitting corrosion attack on the exposed and corroded alloys. It is apparent that alloy 3, which has lower iron and manganese contents, is superior in corrosion resistance. 
     
                       TABLE 4
______________________________________
EFFECT OF MARINE EXPOSURE ON DUCTILITY LOSS
          Elongation,
          % in 1 inch
Alloy  Before       After  % Loss in Ductility
______________________________________
3      23.5         19.1   19
7      22.5         14.5   36
______________________________________
 
    
     Improved Formability 
     Another advantage of my invention is improved formability. Good formability is important to the aircraft manufacturers because of lower costs because of reduced scrap rates and manpower requirements. Two indicators of formability are (1) ball punch depth as determined by indenting the sheet with a 1-inch diameter steel ball until it cracks (also known as Olsen cup depth), a measure of a material&#39;s capability of being stretched in more than one direction, and (2) minimum bend radius, a measure of a material&#39;s ability to be bent without cracking. Note that there is some uncertainty in minimum bend radius measurements because the determination of surface cracking is somewhat subjective, and the method involves bending sheet samples around dies of incremental (not continuously varying) radii. Table 5 lists bend radius and ball punch depth of alloys 1, 2, 4, 6 and 7. As FIG. 6 illustates, both of these indicators correlate with % Fe+1/2% Mn, i.e., alloys with about 0.1 % Fe and less than about 0.5% Mn have superior formability. 
     
                       TABLE 5
______________________________________
FORMABILITY OF 2024-T3 SHEET
                          Olsen Cup
                                 180° Min. Bend
Alloy   % Fe   % Me       Depth, in.
                                 Radius, in.
______________________________________
1       0.035  0.33       0.336  0.025-0.032
2       0.04   0.46       0.319  0.025-0.032
4       0.07   0.44       0.333  0.032-0.064
6       0.16   0.77       0.287  0.080-0.100
7       0.19   0.54       0.309  0.080-0.100
______________________________________
 
    
     Improved Surface Roughness 
     Three lots each of standard 2024 and the invention composition were chemically milled to half thickness in a buffered 14% NaOH solution. The roughness of the milled surfaces was measured in a direction perpendicular to the rolling direction using a profilometer with a 2 μm (2×10 -6  meters) diamond stylus. The results listed in Table 6 show a 10 to 45% improvement for the invention product. 
     
                       TABLE 6
______________________________________
SURFACE ROUGHNESS OF CHEMICALLY MILLED SHEET
                       Roughness
Gage, in.       Alloy  (× 10.sup.-6 in.)
______________________________________
0.125           IP.sup.a
                        58
                2024   107
0.160           IP     107
                2024   119
0.190           IP     139
                2024   186
______________________________________
 .sup.a Invention Product
 
    
     Improved Physical Properties by Intermediate Annealing Step 
     I have also discovered that I can further improve the properties that I have discussed above by an intermediate thermal treatment. Specifically, I introduce an intermediate annealing step after hot rolling but before cold rolling to the final gage to produce an improved alloy. 
     For purposes of the present invention, I prefer a method which includes providing stock comprising an aluminum alloy having about 3.8 to 4.5 wt. % copper, about 1.2 to 1.6 wt. % magnesium, about 0.3 to 0.6 wt. % manganese, not more than 15 wt. % silicon, not more than 12 wt. % iron, not more than 0.1 wt. % titanium, the remainder substantially aluminum, incidental elements and impurities; hot working the stock; annealing; cold rolling; solution heat treating; and cooling. 
     Optionally, before the hot working step, I homogenize the stock to produce a substantially uniform distribution of alloying elements. In general, I homogenize by heating the stock to a temperature ranging from about 900 to 975° F. for a period of at least 1.0 hour to dissolve soluble elements and to homogenize the internal structure of the metal. I caution, however, that temperatures above 935OF are likely to damage the metal and thus I avoid these increased temperatures if possible. Generally, I homogenize for at least 4.0 hours in the homogenization temperature range. Most preferably, I homogenize for about 6.0 to 12.0 hours at about 920° F. 
     As discussed above, my preferred aluminum alloy comprises about 4.0 to 4.4 wt. % copper, about 1.25 to 1.5 wt. % magnesium, about 0.35 to 0.5 wt. % manganese, not more than 0.12 wt. % silicon, not more than 0.08 wt. % iron, not more than 0.06 wt. % titanium, the remainder substantially aluminum, incidental elements and impurities. 
     For hot working, I prefer a hot rolling step where the stock is heated to a temperature ranging from about 750 to 925° F. for about 1.0 to 12.0 hours. Most preferably, I heat the stock to a temperature ranging from about 825 to 900° F. for about 1.0 to 2.0 hours to obtain a gage thickness ranging from about 0.1 to 0.25 inches. I generally perform hot rolling at a starting temperature ranging from about 600 to 900° F., or even higher as long as no melting or other ingot damage occurs. When the alloy is to be used for fuselage skins, for example, I typically perform hot rolling on ingot or starting stock 12 to 16 or more inches thick to provide an intermediate product having a thickness ranging from about 0.1 to 0.25 inches. 
     After hot rolling, I next anneal the stock. Preferably, I anneal at a temperature ranging from about 725 to 875° F. for about 1.0 to 12.0 hours. Most preferably, I anneal the stock at a temperature ranging from about 750 to 850° F. for about 4.0 to 6.0 hours at heating rate ranging from about 25 to 100° F. per hour, with the optimum being about 50° F. per hour. 
     After annealing, I next cold roll the intermediate gage stock. Preferably, I allow the annealed stock to cool to less than 100° F. and most preferably to room temperature before I begin cold rolling. Preferably, I cold roll to obtain at least a 40% reduction in sheet thickness, most preferably I cold roll to a thickness ranging from about 50 to 70% of the hot rolled gage. 
     After cold rolling, I next solution heat treat the stock. Preferably, I solution heat treat at a temperature ranging from about 900 to about 940° F. for about 10 to 30 minutes. It is important to rapidly heat the stock, preferably at a heating rate of about 100 to 2000° F. per minute. Most preferably, I solution heat treat at about 920 to 930° F. for about 15 minutes at a heating rate of about 1000° F. per minute. 
     If the temperature is substantially below 920° F., then the soluble elements, copper and magnesium are not taken into solid solution. This circumstance can be illustrated by reference to FIG. 1. As the temperature is decreased, the lines encompassing the aluminum solid solution region shift to the left as depicted by the arrows. When copper and magnesium are not taken into solution, two undesirable consequences result: (1) there are insufficient alloying elements to provide adequate strength upon subsequent age hardening; and (2) the copper and magnesium-containing intermetallic compounds (Al 2  Cu and Al 2  CuMg) that remain undissolved detract from fracture toughness and fatigue resistance. Similarly, if the time at the solution heat treatment temperature is too short, these intermetallic compounds do not have time to dissolve. The heating rate to the solutionizing temperature is important because relatively fast rates generate a fine grain (crystallite) size, which is desirable for good fracture toughness and high strength. 
     After solution heat treatment, I rapidly cool the stock to minimize uncontrolled precipitation of secondary phases, such as Al 2  CuMg and Al 2  Cu. Preferably, I quench at a rate of about 1000° F./sec. over the temperature range 750 to 550° from the solution temperature to a temperature of 100° F. or lower. Most preferably, I quench using a high pressure water spray at room temperature or by immersion into a water bath at room temperature, generally ranging from about 60 to 80° F. 
     EXAMPLE 
     To demonstrate the present invention, I first homogenized two 3&#34;×9&#34; ingots having the composition listed in Table 7 at a temperature of about 910° F. for about 15 hours. 
     
                       TABLE 7
______________________________________
CHEMICAL COMPOSITIONS OF LABORATORY INGOTS
% by wt.
Alloy   Si     Fe       Cu   Mn      Mg   Ti
______________________________________
A       0.07   0.07     3.84 0.54    1.24 0.02
B       0.07   0.09     3.83 0.98    1.22 0.02
______________________________________
 
    
     I then reheated the ingots to a temperature of about 800° F. and hot rolled them to an intermediate gage thickness of about 0.200&#34; having a final temperature of about 550° F. I then divided each hot rolled sheet into two sheets. I annealed one of the sheets at a temperature of about 835° F. for about 2.0 hours using heating and cooling rates of about 50° F./hr. The other control sheet was not annealed. Then, I cold rolled all four sheets to a gage of about 0.063&#34; and solution heated treated them at about 920° F. for about 30 minutes. Finally, I quenched all four sheets in room temperature water. I then tested all four sheets after naturally aging them at room temperature for greater than one month (T4 temper) for tensile properties and Kahn tear energy, which are listed in Table 8. 
     
                       TABLE 8
______________________________________
PROPERTIES WITH AND WITHOUT INTERMEDIATE ANNEAL
                                      UPE, in-
Alloy
     Anneal    UTS, ksi YS, ksi
                               Elong, %
                                      lb/in.sup.2
______________________________________
A    Yes       68.3     42.6   23.5   845
A    No        66.4     40.8   24.5   755
B    Yes       68.9     42.3   22     705
B    No        68.4     41.2   21     650
______________________________________
 
    
     I used the data from Table 8 to plot yield strength versus unit propagation energy in FIG. 7. Surprisingly, the intermediate-annealed variants of both alloys were not only somewhat stronger, they were also significantly tougher than their un-annealed counterparts. The lower manganese Alloy A also had higher toughness values than the high manganese Alloy B, as expected based on FIGS. 5-7 and my previous discussion, above. 
     In addition to improvements in mechanical properties, the sheets produced with intermediate annealed had improved formability as evidenced by deeper ball punch depths shown in Table 9. 
     
                       TABLE 9
______________________________________
BALL PUNCH DEPTHS
WITH AND WITHOUT INTERMEDIATE ANNEAL
                      Olsen Cup
Alloy         Anneal  Depth, in.
______________________________________
A             Yes     0.330
A             No      0.304
B             Yes     0.295
B             No      0.265
______________________________________
 
    
     Notably, the lower manganese alloy also had superior forming behavior as would be expected based on my previous discussion. 
     In addition, when I examined the grain structures of Alloys A and B, I discovered considerably finer grain sizes in the intermediate-annealed alloys. FIGS. 8a and 9a compared to FIGS. 8b and 9b, respectively, illustrate the phenomenon of finer grain size that I observed. 
     Having illustrated and described the principles of my invention in a preferred embodiment thereof, it should be readily apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. I claim all modifications coming within the spirit and scope of the accompanying claims.