Patent Publication Number: US-5292386-A

Title: Process for the manufacture of aluminum sheets

Description:
The invention is directed to a process for the manufacture of sheets or plates from aluminium alloys. 
     In the case of aluminium-lithium alloys, which are mainly used for structurally critical aerospace components, it is desirable for achieving adequate damage-tolerant properties and sufficient isotropy that the material should be recrystallized prior to the artificial aging stage when sheets or plates of a thickness of approximately 1 to 8 mm are rolled. It is unfortunate that normally, anisotropic mechanical properties result as compared with type 2024 T 351 aluminium alloys. In crack propagation tests, sheets made from the last-mentioned aluminium alloy exhibit fatigue cracks which extend macroscopically normal to the direction of the applied principal normal stress, which fact can be utilized for example in the construction of aircraft components. 
     It has been found, however, that in the case of aluminium-lithium alloys such a desirable material behaviour cannot be obtained by cold working and subsequent recrystallization prior to artificial aging, not even if the recrystallization process is promoted by conducting a conventional thermal treatment subsequent to hot rolling and prior to cold working, by which thermal treatment the material adopts a state of overaging. In the case of aluminium-lithium alloys it has not been possible subsequent to hot working, intermediate annealing, cold working and recrystallization to reproducibly achieve such properties in which fatigue cracks extend normal to the principal normal stresses. Rather, it has been found that fatigue cracks deviate in various ways from the crack propagation path which extends normal to the principal normal stresses, and that deviations of up to 70° may occur. Moreover, when such a thermomechanical process is employed the disadvantage of poor cold-workability has resulted which is marked by a strong tendency towards the formation of edge cracks. The spectrum of applicable process parameters is greatly limited thereby. 
     The invention is based on the object of improving the manufacture of sheet metal with simple means so that it will also be possible in the case of aluminium-lithium alloys to achieve sufficient isotropy of the manufactured sheets in which fatigue cracks extend substantially normal to the applied principal normal stresses. It is also desirable to achieve good cold-workability. 
     In summary, the invention is directed to a process of manufacturing aluminum sheets of thickness ranging from about 0.5 to 10 mm from aluminum alloys. The process comprises the steps of: 
     (a) shaping a bar made from said aluminum alloy into a sheet, strip or other similar semifinished product; 
     (b) subjecting said semifinished product to solution heat treatment; 
     (c) quenching said solution heat-treated semifinished product; 
     (d) forming said quenched product at a reduction of between about 2% to 60%; 
     (e) subjecting the annealed semifinished product to intermediate annealing in a temperature range of between about 250° C. to 475° C. for a period of 1-85 hours; 
     (f) subjecting the annealed semifinished product to cold working at a reduction between about 40% to 80%; 
     (g) solution heat treating the cold worked semifinished product at a temperature at which recrystallization occurs; 
     (h) quenching said solution-treated semifinished product; 
     (i) working said quenched semifinished product at a reduction of up to about 8% to provide a finished product; and 
     (j) aging said finished product. 
     Type AlLi 8090 aluminium alloys having the following composition are especially preferred: 
     
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lithium:           2.2-2.7% (by weight)                                   
copper:            1.0-1.6%                                               
magnesium:         0.6-1.3%                                               
zirconium:         0.04-0.16%                                             
iron:              ≦0.3%                                           
silicon:           ≦0.2%                                           
chromium:          ≦0.1%                                           
manganese:         ≦0.1%                                           
titanium:          ≦0.1%                                           
zinc:              ≦0.25%                                          
other ingredients  ≦0.05%                                          
individually:                                                             
total of other     ≦0.15%                                          
ingredients:                                                              
balance aluminium                                                         
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     The solution heat treatment is performed within a temperature range of from 500° C. to 550° C. and preferentially for a time period of t=10 min up to 2 h. Quenching is performed at quenching rates of ≧300° C./min. 
     Intermediate annealing of the semifinished material is performed in accordance with the instant invention within a temperature range of from 250° to 475° C. for a period of from 1-85 h, while working of the recrystallized semifinished material is performed especially as a cold-working step with an amount of deformation of only up to 8%, especially up to 5% and preferentially only up to 3.5%. In this connection it is recommended chiefly to perform stretching and/or stretch-forming, i.e. rolling should be avoided. 
     The first forming stage prior to intermediate annealing is appropriately conducted at a low amount of deformation of from 5% to 20%, while the second forming stage, i.e. cold working subsequent to intermediate annealing, is conducted at a high degree of cold working, e.g. cold rolling of from 40% and 90%. 
     The intermediate thermal treatment (intermediate annealing) is appropriately performed so that the formed material is initially held at an intermediate annealing temperature which corresponds to one of the following two formulae: 
     
         T≧(78-KW.sub.2)·6+360                      (1) 
    
     
         (KW.sub.2 -78)·7+300≦T≦(78-KW.sub.2)·2.35+340 (2) 
    
     The temperature is measured in °C. and the amount of cold forming (KW 2 ) (subsequent to intermediate annealing) is measured in percent (based on the initial thickness of the material). 
     The material is maintained at approximately this holding temperature for a period of between 1 and 85 hours. Thereafter the material is preferentially cooled at a cooling rate of not more than 40° /h down to the temperature range of from 325° to 275° C. 
     In another preferred embodiment the material during intermediate annealing, after having been held at the intermediate annealing temperature, is cooled at a cooling rate of V&gt;300° /min and subsequently cold-formed. In this case the following relationship 
     
         t≧8·e.sup.15,000(1/T-1/670)                ( 3) 
    
     should appropriately be observed between the holding time t at the holding temperature and the target holding temperature T; t is the holding time in terms of hours and T is the holding temperature in terms of °K. 
     Especially preferred final sheet thicknesses are between 1 and 9 mm. 
     The process steps g, h and i disclosed herein, i.e. solution heat treatment, quenching of the solution heat treated semifinished material, and forming of the quenched semifinished material, may also be repeated, either as a whole or with partial steps being arbitrarily omitted. 
     It has been found that the sheets which have been produced in accordance with the instant invention by using the alloy AlLi 8090 and which have a thickness of from 4 to 7 mm, exhibit fatigue cracks in CT-cuts--through the entire range of fatigue crack propagation--which are macroscopically normal to the applied principal normal stresses. Therefore the material is highly isotropic. Moreover, the process is marked by excellent cold rolling behaviour of the semifinished material in the second forming stage, i.e. forming subsequent to intermediate annealing, as compared with the tendency towards edge crack formation. 
     This will be described in detail with reference to the following examples; the alloys were composed as follows: 
     
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lithium:           2.32% (wt. %)                                          
copper:            1.02%                                                  
magnesium:         0.77%                                                  
zirconium:         0.07%                                                  
iron:              0.06%                                                  
silicon:           0.036%                                                 
balance aluminium and unavoidable impurities.                             
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     EXAMPLE 1 
     In the following example, the excellent cold-rolling property resulting from the use of the thermomechanical process of the instant invention is to be demonstrated as compared with the conventional thermomechanical treatment. 
     Table 1 lists examples of thermomechanical treatments which have been conducted. 
     Table 2 illustrates for these treatments the depth of the occurring edge cracks in dependence on the amount of cold rolling employed as measured during cold rolling subsequent to intermediate annealing. 
     
                       TABLE 1                                                     
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Thermomechanical treatments performed                                     
Sample No.                                                                
         Thermomechanical Treatment                                       
                                  Rem.                                    
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TMT1     HR + TT + CR             1                                       
TMT2     HR + SHT + TT + CR       2                                       
TMT3     HR + SHT + PW (40%) +    3                                       
         TT (300 C/8 h) + CR                                              
TMT4     HR + SHT + PW (25%) +    3                                       
         TT (300 C/8 h) + CR                                              
TMT5     HR + SHT + PW (15%) +    3                                       
         TT (300 C/8 h) + CR                                              
TMT6     HR + SHT + PW (15%) +    3                                       
         TT (350 C/8 h) + CR                                              
TMT7     HR + SHT + PW (15%) +    3                                       
         TT (400 C/8 h) + CR                                              
TMT8     HR + SHT + PW (15%) +    3                                       
         TT (450 C/8 h) + CR                                              
TMT9     HR + SHT + PW (15%) +    3                                       
         TT (300 C/81 h) + CR                                             
TMT10    HR + SHT + PW (15%) + TT (425 C/                                 
                                  3                                       
         8 h +  WQ) + CR                                                  
TMT11    HR + SHT + PW (15%) +    3                                       
         TT (425 C/1 h) + CR                                              
TMT12    HR + SHT + PW (15%) + TT (375 C/                                 
                                  3                                       
         53 h + WQ) + CR                                                  
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 HR = hot rolling                                                         
 SHT = solution heat treatment                                            
 TT (x°  C./y h) = intermediate annealing at a holding temperature 
 of x°  C. for y hours                                             
 PW (z %) = preforming prior to intermediate annealing by z %             
 CR = cold rolling                                                        
 1 = conventional thermomechanical treatment                              
 2 = experimental thermomechanical treatment                              
 3 = thermomechanical treatment of the invention                          
 
    
     
                       TABLE 2                                                     
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Edge crack depth on cold rolling in dependence                            
on the degree of cold rolling for the thermomechanical                    
treatments listed in Table 1.                                             
Degree of Cold Rolling                                                    
TMT    30%      40%    50%  60%    70%  80%   85%                         
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TMT1   23       55     75   --     --   --    --                          
TMT2   0        14     45   78     --   --    --                          
TMT3   0        0      8    15     23   36    42                          
TMT4   0        0      7    16     22   37    45                          
TMT5   0        0      0    14     22   32    40                          
TMT6   0        0      0    11     36   65    --                          
TMT7   0        0      0     0      6   27    34                          
TMT8   0        0      0     0      0    9    16                          
TMT9   0        22     22   22     45   61    *                           
TMT10  0        0      0     0      0    0    *                           
TMT11  0        0      0    16     32   32    *                           
TMT12  0        0      0     0      0   *     *                           
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 -- = sample cracked through completely                                   
 x=  not measured                                                         
 
    
     EXAMPLE 2 
     Here, there are results of measurements of fatigue crack propagation for samples produced in accordance with the thermomechanical process of the instant invention as compared with conventionally produced samples (Table 3). 
     The fatigue crack propagation tests were made with so-called CT-cuts in the propagation direction T-L which is particularly critical as to fatigue crack deviations. The examined range of variation of the stress intensity was ##EQU1## 
     The criterion selected to describe deviations of the fatigue cracks from the desired direction normal to the applied principal normal stress was--in case of a deviation--the angle of the crack front in relation to the vertical to the principal normal stress, measured in degrees. 
     
                       TABLE 3                                                     
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Examples of thermomechanical treatments and results                       
of fatigue crack propagation tests.                                       
                                 (de-                                     
                                 grees)                                   
                                 Crack                                    
                                 Path                                     
Sample                           De-                                      
No.   Thermomechanical Treatment viation                                  
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1     HR + SHT + PW (15%) + TT (250 C/8 h) +                              
                                 4*                                       
      CR (60%)                                                            
2     HR + SHT + PW (15%) + TT (300 C/8 h) +                              
                                 0*                                       
      CR (60%)                                                            
3     HR + SHT + PW (15%) + TT (325 C/8 h) +                              
                                 0*                                       
      CR (60%)                                                            
4     HR + SHT + PW (15%) + TT (325 C/8 h) +                              
                                 0*                                       
      CR (77%)                                                            
5     HR + SHT + PW (15%) + TT (350 C/8 h) +                              
                                 0*                                       
      CR (60%)                                                            
6     HR + SHT + PW (15%) + TT (375 C/8 h) +                              
                                 0*                                       
      CR (77%)                                                            
7     HR + SHT + PW (15%) + TT (400 C/8 h) +                              
                                 0*                                       
      CR (77%)                                                            
8     HR + SHT + PW (15%) +  TT (425 C/8 h) +                             
                                 0*                                       
      CR (77%)                                                            
9     HR + SHT + PW (15%) + TT (450 C/8 h) +                              
                                 0*                                       
      CR (77%)                                                            
10    HR + SHT + PW (15%) + TT (450 C/8 h) +                              
                                 0*                                       
      CR (68%)                                                            
11    HR + SHT + PW (15%) + TT (375 C/8 h +                               
                                 0*                                       
      WQ) + CR (60%)                                                      
12    HR + SHT + PW (15%) + TT (425 C/8 h +                               
                                 0*                                       
      WQ) + CR (60%)                                                      
13    HR + SHT + PW (15%) + TT (425 C/8 h +                               
                                 0*                                       
      WQ) + CR (68%)                                                      
14    HR + SHT + PW (15%) + TT (375 C/53 h +                              
                                 0*                                       
      WQ) + (CR 65%)                                                      
15    HR + TT + CR               0-37.sup.                                
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 *=  according to the invention                                           
 .sup. = conventional                                                     
 WQ = quenched                                                            
 
    
     EXAMPLE 3 
     In the third example, some technological properties of metal sheets produced in accordance with the instant process are compared with metal sheets produced according to conventional thermomechanical processes and with metal sheets produced from conventional alloys. 
     Table 4 lists examples for the static mechanical properties. Table 5 compares typical crack propagation rates under load in T-L. 
     Finally, FIGS. 1 to 4 illustrate material textures of sheets produced in accordance with the instant invention from the alloy 8090, compared with sheets produced along conventional thermomechanical lines, based on their &lt;111&gt;pole figures. Whereas the conventional thermomechanical process results in recrystallized sheets whose material textures mainly comprise the typical positions W (cube), Ms (brass), Goss and R, the recrystallization texture of the sheets produced in accordance with the process of the invention in the sheet interior mainly comprises the A position and in the sheet exterior the W-BN position (cube/sheet-normal position) in addition to a high background. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIGS. 1, 2: (111)pole figures of AlLi 8090 sheets produced by a conventional process: sheet interior FIG. 1, sheet exterior FIG. 1. 
     FIGS. 3, 4: (111)pole figures of AlLi 8090 sheets produced by the process of the invention: sheet interior FIG. 3, sheet exterior FIG. 4. 
     
                       TABLE 4                                                     
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Typical mechanical properties                                             
           (MPa)      (MPa)      (%)                                      
Sample No. σ.sub.02                                                 
                      σ.sub.MAX                                     
                                 ε.sub.8                          
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6          308        425        14.6.sup.1)                              
7          314        427        15.4.sup.1)                              
8          315        432        14.6.sup.1)                              
9          310        430        15.1.sup.1)                              
15         305-315    409-412    10.2-10,4.sup.2)                         
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 .sup.1) flat bars, thickness × 10, measured length 35 mm, process  
 according to invention                                                   
 .sup.2) flat bars, thickness × 12.5, measured length 51 mm,        
 conventional process                                                     
 
    
     
                       TABLE 5                                                     
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Typical values of crack propagation rate in the fatigue                   
crack propagation test, measured as 0.0001 mm/cycle for                   
sheets produced according to the inventive process as                     
compared with 8090 sheets produced by the conventional                    
thermomechanical process and with the conventional                        
alloys 2024T351 and 7075T351.                                             
Alloy Process                                                             
ΔK                                                                  
        8090T851    8090T851                                              
(N/mm.sup.1,5)                                                            
        conventional                                                      
                    invention                                             
                             2024T351                                     
                                     7075T7351                            
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400     5 · 10.sup.-1                                            
                    5 · 10.sup.-1                                
                             1.4     1.9                                  
600     2           2        4.5     7.1                                  
800     6           5        12      20                                   
1000    14          13       24      50                                   
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