Abstract:
High strength aluminum alloys based on the Al—Zn—Mg—Cu alloy system preferably include high levels of zinc and copper, but modest levels of magnesium, to provide increased tensile strength without sacrificing toughness. Preferred ranges of the elements include by weight, 8.5-10.5% Zn, 1.4-1.85 % Mg, 2.25-3.0% Cu and at least one element from the group Zr, V, or Hf not exceeding about 0.5%, the balance substantially aluminum and incidental impurities. In addition, small amounts of scandium (0.05-0.30%) are also preferably employed to prevent recrystalization. During formation of the alloys, homogenization, solution heat treating and artificial aging processes are preferably employed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application contains subject matter that is related to the subject matter set forth in U.S. application Ser. No. 10/829,391, which was filed on Apr. 22, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates, in general, to a high strength aluminum alloy based on the Al—Zn—Mg—Cu alloy system and a process for forming the same. Although not limited thereto, the alloys are particularly suited for use in sporting goods and aerospace applications.  
         [0004]     2. Description of the Background Art  
         [0005]     The highest strength aluminum alloys known at this time are based on the aluminum-zinc-magnesium-copper system. Commercial high-strength alloys currently being produced include AA7055 (nominally 8% Zn-2% Mg-2.2% Cu-0.10% Zr), AA7068 (nominally 7.8% Zn-2.5% Mg-2.0% Cu-0.10% Zr) and a Kaiser Aluminum alloy designated K749 (nominally 8% Zn-2.2% Mg-1.8% Cu-0.14% Zr). From the published phase relationships at 860° F. for an alloy containing 8% Zn, one can note that K749 is near a phase boundary, while the other two alloys are in multiple phase fields. In the latter case all the alloying elements are not in solid solution at 860° F., and are not only unavailable for age hardening, but the undissolved phases remaining after heat treatment detract from toughness. Although solution heat treating at a higher temperature than 860° F. will dissolve more of the solute, care has to be taken to ensure that the alloy does not undergo eutectic melting, which is a common problem in commercially cast alloys that have locally enriched regions as a result of microsegregation that occurred during casting.  
         [0006]     There is a need in many applications, such as sporting goods and aerospace applications, for even stronger alloys based on the aluminum-zinc-magnesium-copper system that do not sacrifice toughness. However, this requirement presents a problem because, in general, as the tensile strength of an aluminum alloy is increased, its toughness decreases.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention addresses the foregoing need in a number of ways. More particularly, there are three distinct avenues for increasing an alloy&#39;s strength while maintaining its toughness: rich alloy chemistries; processing to maximize alloying effectiveness; and preventing recrystallization. Rich alloys provide more solute, which is potentially available for age hardening to higher strength levels; effective processing ensures that the solute is available for strengthening and not out of solution as second phases, which detract from fracture toughness; and maintaining an unrecrystallized microstructure optimizes both strength and toughness.  
         [0008]     To provide increased tensile strength without sacrificing toughness through the use of rich chemistries, the present invention comprises aluminum alloys based on the Al—Zn—Mg—Cu alloy system that preferably include high levels of zinc and copper, but modest levels of magnesium. As an option, small amounts of scandium can also be employed to prevent recrystallization. Each of the alloys preferably includes at least 8.5% Zn and 2.25% Cu by weight. Higher levels of each of these elements up to about 10.5% Zn and 3.0% Cu can be used. However, modestly lower amounts of Mg (max 1.85%) are preferably used to allow higher levels of the Cu. The preferred ranges of all elements in the alloys include by weight, 8.5-10.5% Zn, 1.4-1.85 % Mg, 2.25-3.0% Cu, and at least one element from the group Zr, V, or Hf not exceeding about 0.5%, the balance substantially aluminum and incidental impurities. In the preferred embodiments, 0.05-0.30% Sc is also included in the alloys to prevent recrystallization. Additionally, it has been found that toughness decreases as the total weight percentage of magnesium and copper increases. Experiments have established that the ideal range of these two elements be between 4.1 and 4.5% combined. Still further, the total weight percent of Zn, Cu and Mg is ideally between 13.0 and 14.5%.  
         [0009]     To maximize alloying effectiveness during formation of the alloys, a homogenization process is preferably employed after alloy ingot casting in which a slow rate of temperature increase is employed as the alloy is heated as near as possible to its melting temperature. In particular, for the last 20-30° F. below the melting temperature, the rate of increase is limited to 20° F./hr. or less to minimize the amount of low melting point eutectic phases and thereby further enhance fracture toughness of the alloy. Once the ingot is formed into finished shape using extrusion and rolling steps, for example, the product is preferably solution heat treated at 870 to 900 degrees F. and then artificially aged. The aging process can be carried out by exposing the product to a one, two or three step heat treatment process. In the first step, the product is exposed to a temperature range of 175-310 degrees F. for 3 to 30 hours. In the optional second step, the first step is followed by heating at 310 to 360 degrees F. for 2 to 24 hours. Finally, in the third optional step, the product is heated at 175 to 300 degrees F. for 1 to 30 hours. As a still further option, the second and third aging steps can be used without the first aging step.  
         [0010]     The foregoing alloys and processing operations enhance the properties of the Al—Zn—Mg—Cu alloy system, such that they can be more effectively employed in numerous applications. Specific products or items in which the subject alloys can be employed include, among others, sporting goods including baseball and soft ball bats, golf shafts, lacrosse sticks, tennis rackets, and arrows; and aerospace application including aerospace components such as wing plates, bulkheads, fuselage stringers, and structural extrusions and forgings; and ordnance parts such as sabots and missile launchers. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The features and advantages of the present invention will become apparent form the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings, in which:  
         [0012]      FIG. 1  is a graph depicting T6 strength (YTS and UTS) as a function of the total alloy content in weight percent for a number of sample alloys formed in accordance with the preferred embodiments;  
         [0013]      FIG. 2  is a graph depicting fracture toughness as a function of combined percentages of Cu and Mg for sample alloys formed in accordance with the preferred embodiments;  
         [0014]      FIG. 3  is an equilibrium diagram which depicts the phase relationships at 885° F. as a function of percentages of Cu and Mg for an alloy formed in accordance with the preferred embodiments that contains 9% Zn;  
         [0015]      FIG. 4  is a graph illustrating the effect of the ratio of Mg to Cu on fracture toughness for the alloys formed in accordance with the preferred embodiments;  
         [0016]      FIG. 5  is a graph depicting second phase volume percent as a function of heating rate in a formation process for Alloy AA7068; and  
         [0017]      FIG. 6  is a graph illustrating the effect of scandium on strength of an Al-8% Zn-2.2% Mg-1.9% Cu alloy. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     The following examples illustrate how alloy modifications and efficient processing operations can be used to enhance the properties of the Al—Zn—Mg—Cu alloy system in accordance with the preferred embodiments of the present invention, such that they can be more effectively utilized in sporting goods and aerospace applications.  
       EXAMPLE 1  
       [0019]     A heretofore unexplored region of the Al—Zn—Mg—Cu alloy system consists of compositions comprising about 9% to 10% zinc, 2.2% to 2.8% copper, and 1.6% to 2.0% magnesium. The alloy compositions listed in Table 1 were cast as 9-in. diameter billets: note that all these alloys contain about 0.05% scandium, an element which in combination with zirconium is effective in preventing recrystallization.  
                                                                                             TABLE 1                           Alloy compositions                Percent by Weight            Alloy   Si   Fe   Cu   Mg   Zn   Zr   Sc                    179   0.04   0.07   2.47   1.83   8.87   0.14   0.06       180   0.04   0.09   2.71   1.89   8.95   0.13   0.06       189   0.04   0.08   2.14   1.89   8.60   0.12   0.05       190   0.03   0.09   2.31   1.86   9.21   0.13   0.05       191   0.03   0.11   2.35   1.81   9.63   0.13   0.05       192   0.04   0.10   2.33   1.87   10.13   0.12   0.05       200   0.04   0.09   2.58   1.64   8.84   0.12   0.05       202   0.04   0.12   2.46   1.66   8.87   0.13   0.05       203   0.04   0.10   2.69   1.78   8.94   0.13   0.05       204   0.03   0.10   2.88   1.58   8.78   0.12   0.05       209   0.04   0.08   2.64   1.49   8.78   0.14   0.05       213   0.03   0.07   2.42   1.63   9.65   0.13   0.05       214   0.03   0.09   2.56   1.44   9.50   0.14   0.05       215   0.04   0.09   2.57   1.73   9.82   0.12   0.05       216   0.03   0.10   2.81   1.60   9.65   0.13   0.05                  
 
         [0020]     The billets were homogenized at 880 F. (F means degrees Fahrenheit) and extruded to seamless 4-in. diameter tubes with a 0.305 in. wall thickness. The extrusions were solution heat treated at 880 F., quenched in cold water and “peak” aged to the T6 temper (24-hr soak at 250 F.). They were tested for tensile properties in the longitudinal direction and sections from all of the extrusions were cut and flattened to pieces about 12″ square, which were also solution heat treated at 880 F., quenched in cold water and peak aged. These flattened sections were tested for fracture toughness (ASTM B645) in the T-L orientation. The tensile and fracture toughness properties are listed in Table 2.  
                                                                                                 TABLE 2                           Tensile and fracture toughness properties                Strength       Toughness               (ksi)       (ksi rt.in)            % Zn   % Cu   % Mg   UTS   YTS   Kq   Kp                    8.60   2.14   1.89   97.1   88.5   25.2   30.5       9.21   2.31   1.86   100.1   93.9   22.4   27.5       9.63   2.35   1.81   99.9   94.2   20.9   25.4       10.13   2.33   1.87   103.2   97.8   21.2   24.0       8.87   2.47   1.83   101.1   92.2   20.9   23.9       8.95   2.71   1.89   102.9   93.7   20.1   20.5       8.84   2.58   1.64   98.6   93.8   23.1   25.8       8.87   2.46   1.66   98.4   92.8   25.3   22.2       8.94   2.69   1.78   100.0   94.2   24.2   22.4       8.78   2.88   1.58   99.1   93.8   24.8   21.9       8.78   2.64   1.49   96.4   91.9   24.8   22.9       9.65   2.42   1.63   100.3   96.3   24.7   21.3       9.50   2.56   1.44   98.5   94.9   26.2   21.2       9.82   2.57   1.73   102.6   98.2   21.9   18.2       9.65   2.81   1.60   100.6   97.1   20.0   18.4                  
 
         [0021]     As can be seen from Table 2, tensile yield strengths well in excess of 90 ksi were obtained in most of the alloys, with two compositions achieving about 98 ksi. As shown in  FIG. 1 , strength correlated well with the total alloy content, with each wt. pct. adding about 4.8 ksi to the yield strength. The equilibrium phase relations at the homogenizing and solution heat treatment temperature explain the reason for this behavior.  FIG. 3  shows how the compositions listed in Table 1 relate to the magnesium and copper solubility limits at 885F for alloys containing a nominal zinc level of 9%. Compositions lying below the demarcation line between the solid solution and the Al+S phase regions (i.e., the solvus) are single phase alloys, which have superior fracture toughness values for a given strength level, compared to those in the 2-phase region. The best combinations of strength and toughness are associated with alloys near the solvus line, which is why the 2.7% Cu/1.9% Mg composition has a relatively low toughness level. The preferred compositions therefore lie within the dashed lines that run approximately parallel to the solvus. These relationships are defined by controlling the total copper plus magnesium concentrations between 4.1 % and 4.5%.  
         [0022]     Although the properties described above were obtained with a “standard” T6 temper aging treatment by exposing the shaped products to heat of between 175 and 310 F. for 3 to 30 hours (24 hr at 250 F. was specifically used), as with most Al—Zn—Mg—Cu alloys, other practices may also be advantageous, depending on the desired combination of properties. For example, a tube from composition #213, when drawn to a tube 2.625″ in diameter with a 0.110″ wall thickness and aged by a 2-step practice of 8 hr at 250 F. plus 4 hr at 305 F. had yield and tensile strengths of 100.9 ksi and 102.6 ksi, respectively. Similarly, the subject alloy can be over aged beyond peak strength in a second step at temperatures in the 310-360 F. temperature range for 2 to 24 hours to provide a desirable combination of strength and corrosion resistance. Another preferred embodiment includes a final aging treatment in a third step at a lower temperature in the range 175-300 F. for 1 to 30 hours, which provides an additional strength benefit with no loss in corrosion properties. As yet another alternative, the alloy can be subjected only to the aforementioned second and third aging steps by skipping the first step.  
       EXAMPLE 2  
       [0023]     To compare the invention alloy with other commercial high-zinc alloys such as AA7036, AA7056 and AA7449, which have higher Mg/Cu ratios in the range 1.0 to 1.4, the following alloys were prepared as described in Example 1.  
                                                                                             TABLE 3                           Compositions of Comparative Alloys                Percent by Weight            Alloy No.   Si   Fe   Cu   Mg   Zn   Zr   Sc                    36   0.03   0.06   1.91   2.17   9.02   0.15   0.05       39   0.04   0.05   1.28   2.74   9.02   0.13   0.06       43   0.03   0.03   1.44   2.62   9.04   0.13   0.05       47   0.04   0.06   1.59   2.34   8.95   0.14   0.06                  
 
         [0024]     The yield strengths and toughness values for these alloys are listed in the following table.  
                                     TABLE 4                           Mechanical Properties of Comparative Alloys                Mg/Cu       Yield   Kpmax       Alloy   Ratio   %(Mg + Cu)   Strength (ksi)   (ksi rtin.)               36   1.14   4.08   94.9   24.5       47   1.47   3.93   93.9   22.7       43   1.77   3.99   93.9   21.3       39   2.14   4.02   92.7   20.2                  
 
         [0025]      FIG. 4  compares the toughness levels of these alloys on the basis of Mg/Cu ratio with the invention alloys, using those compositions that have similar strength levels (93-95 ksi) and total Mg+Cu contents (4.0-4.2%).  
       EXAMPLE 3  
       [0026]     As noted earlier it is important that undissolved second phases do not remain after processing so that fracture toughness can be maximized. This is especially important in alloys that are rich in alloy content, and lie near an equilibrium solvus phase boundary. To illustrate how homogenizing practice can affect the amount of such undissolved phase(s), samples of as-cast AA7068 alloy billet were heated from 850 F. at various rates in a differential scanning calorimeter (DSC), and the energy associated with eutectic melting, which started at about 885 F. was measured. This energy measurement is directly proportional to the amount of undissolved second phase remaining at the incipient melting point, and the relationship between these factors has been determined by quantitative microscopy.  FIG. 5  shows how heating rate affects the amount of this phase as determined from the DSC data.  
         [0027]     Note that a slow heating rate of about 10 F./hr reduces the amount of second phase to a level below 1 vol.%. One would expect that a ˜5 F/hr heating rate would reduce the “soluble” portion to near zero. We also note that for heating rates of 10-20 F./hr, the volume fraction of undissolved eutectic is no greater than the amount of insoluble Fe-containing constituent (independent of heating rate or homogenization temperature) at a nominal 0.12% Fe level (approx. 1 vol.%).  
       EXAMPLE 4  
       [0028]     It has been recognized for a number of years that scandium in combination with zirconium is an effective recrystallization inhibitor. A Russian review article states “it is desirable to add scandium to aluminum alloys in a quantity from 0.1 to 0.3% together with zirconium (0.05-0.15%)”. However, “the greatest effect . . . is observed for alloys not containing alloy elements combining with scandium in insoluble phases . . . ; with a limited copper content [scandium combines with copper] alloying with scandium together with zirconium of Al—Zn—Mg—Cu and Al—Cu—Li alloys is possible”. As such, “commercial alloys based on Al—Zn—Mg—Sc—Zr (01970, 01975) have been developed”.  
         [0029]     Two potential drawbacks to scandium additions to 7XXX alloys containing about 2% copper are evident:  
         [0030]     1) the copper level is high enough to combine with scandium, thereby rendering it ineffective, and  
         [0031]     2) the high price of scandium; at the 0.2% level it would add about $10 a pound to the cost of the aluminum alloy.  
         [0032]     It would therefore be economically and technically attractive if scandium levels could be effectively used below those recommended in the Russian literature.  
         [0033]     Alloys of the compositions listed in the following table were prepared as 5″ diameter billets, which were processed as described below. Although the sample alloys contained more Mg and less Cu than the preferred alloys discussed previously, it is believed that the effect of Sc addition to the alloys would be essentially the same for the preferred alloys.  
                                                                                                       Alloy   % by wt.            No.   Si   Fe   Cu   Mg   Zn   Zr   Sc                    A   0.03   0.04   1.95   2.20   8.07   0.11   0.00       B   0.03   0.05   1.86   2.17   8.05   0.00   0.22       C   0.03   0.05   1.89   2.18   8.09   0.11   0.06       D   0.03   0.04   1.84   2.12   8.11   0.12   0.11       E   0.03   0.05   1.95   2.18   8.08   0.11   0.22                  
 
         [0034]     The ingots were homogenized at 875 F. using a 50 F./hr heating rate and air cool, and then reheated to 800 F. and extruded to a 0.25″ by 3″ flat bar. Sections of each extrusion were annealed at 775 F. for 3 hr, cooled 50 F./hr to 450 F., held 4 hr and cooled 50 F./hr to room temperature. The sections were then cold rolled to 0.040″ sheet using five pass reductions (84% total reduction). The sheets were solution heat treated at 885 F. for 30 min, quenched in cold water, and then aged to the peak strength condition (10 hr at 305 F.). The as-extruded bars were also heat treated similarly and both products were tested for transverse tensile properties, as listed below. The specific effects of scandium on strength are also shown in  FIG. 6 .  
                                                                                                       Alloy       UTS (ksi)   Yield Strength (ksi)            No.   % Zr   % Sc   Extrusion   Sheet   Extrusion   Sheet                    A   0.11   0   94.7   90.7   91.4   87.8       B   0   0.22   88.2   92.0   86.1   88.4       C   0.11   0.06   95.7   97.1   92.2   93.3       D   0.12   0.11   95.2   96.6   92.2   93.3       E   0.11   0.22   94.5   96.5   91.1   92.5                  
 
         [0035]     A number of points are evident from these results: 
    1. The strongest alloy in both extrusion and sheet form contains 0.06% Sc (with 0.11% Zr)     2. At the 0.1% Zr level, 0.06% Sc is effective in raising the strength of the sheet product by about 6 ksi.     3. 0.22% Sc in the absence of zirconium raises the strength of the sheet product by only 1 ksi, and lowers the extrusion strength by about 6 ksi. The effectiveness of only 0.06% Sc in preventing recrystallization was confirmed by comparing the microstructures of the sheet products containing (a) 0.11% Zr, (b) 0.11% Zr+0.06% Sc, and (c) 0.22% Sc (no Zr). In view of the foregoing, the preferred range in the alloys for Sc is 0.05-0.30%, with a more preferred range of 0.05-0.10%.    
 
         [0039]     Although the present invention has been described in terms of a number of preferred embodiments and variations thereon, it will be understood that numerous additional variations and modifications may be made without departing from the scope of the invention. Thus, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.