Abstract:
An Al/Cu/Mg/Mn alloy for the production of semi-finished products with high static and dynamic strength properties has the following composition: 0.3–0.7 wt % silicon (Si), max. 0.15 wt. % iron (Fe), 3.5–4.5 wt % copper (Cu), 0.1–0.5 wt. % manganese (Mn), 0.3–0.8 wt. % magnesium (Mg), 0.5–0.15 wt % titanium (Ti), 0.1–0.25 wt % zirconium (Zr), 0.3–0.7 wt. % silver (Ag), max. 0.05 wt. % others individually, max 0.15 wt. % others globally, the remaining wt. % aluminum (Al). The invention further relates to a semi-finished product made for such an alloy and a method of production of a semi-finished product made for such an alloy.

Description:
CROSS REFERENCE APPLICATIONS 
     This application is a national phase application claiming priority from PCT application no. PCT/EP2002/07193 filed on 29 Jun. 2002. 
     FIELD OF INVENTION 
     Subject matter of the invention is an Al/Cu/Mg/Mn alloy for the production of semi-finished products with high static and dynamic strength properties. The invention further relates to semi-finished products manufactured from such an alloy with high static and dynamic strength properties as well as to a method for the production of such a semi-finished product. 
     BACKGROUND OF THE INVENTION 
     Aluminum alloys having a high static and dynamic bearing capacity include the alloys AA 2014 and AA 2214. Drop-forged parts for wheel and brake systems of airplanes are manufactured from these Al alloys in the artificially aged state. The semi-finished products produced from the alloy intrinsically have the listed strength properties of the alloys, especially at lower temperatures. However, at temperatures of more than 100° C. these properties decrease more rapidly than is the case with alloys of the group AA 2618. 
     Semi-finished products of the alloys of group AA 2618 have better high-temperature stability and are utilized for a variety of uses such as compressor impellers for rechargeable diesel engines or for rotors of ultracentrifuges. However, at temperatures below 100° C., the aluminum alloys of the group AA 2014 and AA 2214 have greater bearing capacity. 
     In the wheel brake system of airplanes considerable heat is generated during the braking process. This leads to temperature increases even in the wheels, which typically are fabricated of an AA 2014 or AA 2214 alloy. These can cause early overageing of this alloy and lead to a severe limitation of the service life of the structural part. 
     In compressor impellers the transition to titanium alloys has been made to give the compressor impellers the necessary static and dynamic strength properties even at increased temperatures. However, employing titanium is expensive is therefore not suitable for the production of airplane wheels. Furthermore, titanium is less well suited as a material for wheels due to its limited thermal conductivity. 
     The problematic described above is not new. Therefore, for many years there has been the wish for an Al alloy, which combines the high strength properties of the alloys AA 2014 or AA 2214 at ambient temperature and the thermal stability of the alloys AA 2618 or 2618 A. 
     SUMMARY OF THE INVENTION 
     The invention therefore addresses the problem of providing such an alloy, a semi-finished product produced of such an alloy with high static and dynamic bearing capacity, high thermal stability, high fracture toughness and high creep resistance as well as a method for the production of such semi-finished products. 
     Other aspects of this invention will appear from the following description and appended claims, reference being made to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views. 
     This problem is solved according to the invention with an alloy that has the following composition:
         0.3–0.7 wt. % silicon (Si)   maximally 0.15 wt. % iron (Fe)   3.5–4.5 wt. % copper (Cu)   0.1–0.5 wt. % manganese (Mn)   0.3–0.8 wt. % magnesium (Mg)   0.05–0.15 wt. % titanium (Ti)   0.1–0.25 wt. % zirconium (Zr)   0.3–0.7 wt. % silver (Ag)   maximally 0.05 wt. % other, individually   maximally 0.15 wt. % other, total   remaining wt. % aluminum (Al).       

     Compared to the prior known alloys AA 2014 and AA 2214, the claimed alloy has higher static and dynamic thermal stability and improved creep resistance while also having very good mechanical fracturing properties. These properites are attained in particular at a copper-magnesium ratio between 5 and 9.5, in particular at a ratio between 6.3 and 9.3. The copper content is preferably between 3.8 and 4.2 wt. % and the magnesium content between 0.45 and 0.6 wt. %. The copper content is markedly below the maximum solubility for copper in the presence of the claimed magnesium content. As a consequence, the fraction of insoluble copper-containing phases is very low, also taking into consideration the remaining alloy and companion elements. Thereby an improvement is obtained with respect to the dynamic properties and the fracture toughness of the semi-finished products manufactured from such an alloy. 
     In contrast to the known AA alloys 2014 and 2219, a portion of the claimed alloy is silver with contents between 0.3 and 0.7 wt. %, preferably 0.45 and 0.6 wt. %. In the interaction with silicon (0.3–0.7 wt. %, preferably 0.4–0.6 wt. %) the hardening takes place via the same mechanisms as in silver-free Al/Cu/Mg alloys. However, it has been found that with lower silicon contents, the course of precipitation is different due to the addition of silver. 
     While the semi-finished products manufactured from such an alloy have good high-temperature stability and creep resistances under cooler conditions, they do not meet the desired requirements. Only silicon contents above 0.3 wt. % suppress the otherwise typical change of the precipitation behavior of Al/Cu/Mg/Ag alloys, such that unexpectedly higher strength values can be attained without having to give up the high-temperature stability and the creep resistance with the Cu and Mg contents according to the invention. 
     The manganese content of the claimed alloy is 0.1 to 0.5 wt. %, preferably 0.2–0.4 wt. %. In the case of alloys with higher manganese contents undesirable precipitation processes were found with long-term high-temperature stress, which led to a decrease of strength. For this reason the manganese content is limited to 0.4 wt. %. However, manganese is fundamentally required as an alloy component for the control of the grain structure. 
     To balance the reducing effect of manganese with respect to the grain structure control, the alloy contains zirconium between 0.10–0.25 wt. %, preferably 0.14–0.20 wt. %. The precipitating zirconium aluminides, as a rule, are developed even more finely dispersed than manganese aluminides. Moreover, it was found that the zirconium aluminides contribute to the thermal stability of the alloy. 
     For grain sizing 0.05–0.15 wt. %, preferably 0.10–0.15 wt. % of titanium is added. The titanium is usefully added in the form of an Al/5Ti/1B prealloy, whereby boron is automatically included in the alloy. Finely dispersed, insoluble titanium diborides are formed therefrom. These contribute to the thermal stability of the alloy. 
     The alloy can comprise maximally 0.15% iron, preferably 0.10%, as an unavoidable contamination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph showing the 0.2% yield strength and the tensile strength of the alloy according to the invention in state T6in comparison to prior known alloys, as a function of the test temperature. 
         FIG. 2  is a graph showing the long-time stress to rupture strength of the alloy according to the invention in state T 6  in comparison to known alloys. 
         FIG. 3  is a graph showing the 0.2% yield strength and the tensile strength of airplane wheels manufactured from the alloy according to the invention in comparison to such manufactured from known alloys. 
         FIGS. 4   a  and  4   b  are graphs showing the fatigue strength of the alloy according to the invention in comparison to a known alloy in state T 6  at ambient temperature and at a temperature of 200° C. 
     
    
    
     Before explaining the disclosed embodiment of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown, since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Table 1 reproduced below shows the chemical composition of four alloys (B, C, D, E) according to the invention as well as the composition of the alloys AA 2214 and AA 2618 examined as a comparison (data in wt. % (n.d.: not determined) 
                                                                                                                   TABLE 1               Alloy   Si   Fe   Cu   Mn   Mg   Ni   Zn   Ti   Ag   Zr   V                                B   0.47   0.08   4.40   0.200   0.58   0.003   0.048   0.135   0.45   0.150   0.018       C   0.47   0.08   3.64   0.210   0.59   0.003   0.015   0.115   0.52   0.150   0.017       D   0.47   0.08   3.87   0.200   0.61   0.003   0.015   0.117   0.52   0.150   0.019       E   0.52   0.08   4.14   0.200   0.61   0.003   0.02   0.115   0.44   0.150   0.018       AA 2214   0.77   0.17   4.29   0.883   0.57   0.003   0.031   0.024   0.003   0.007   n.d.       AA 2618   0.22   1.1   2.58   0.020   1.53   1.007   0.043   0.059   0.003   0.002   n.d.                    
From these alloys semi-finished products were manufactured following the method steps listed below:
     a) casting of an ingot from an alloy,   b) homogenizing the cast ingot at a temperature, which is as close under the incipient melting temperature of the alloy as is possible, for a length of time adequate to attain maximally uniform distribution of the alloy elements in the cast structure,   c) hot working of the homogenized ingot by forging at a block temperature of approximately 420° C.,   d) solution treatment of the semi-finished product worked by forging at temperatures sufficiently high to bring the alloy elements necessary for the hardening into solution such that they are uniformly distributed in the structure, with the solution treatment taking place in a temperature range of 505° C. over a time period of 3 hours,   e) quenching of the solution-treated semi-finished product in water at ambient temperature,   f) cold working of the quenched semi-finished products by cold upsetting by 1 to 2%, and   g) artificial ageing of the quenched semi-finished product at a temperature of 170° C. over time period of 20 to 25 hours. The open-die forged pieces produced in this manner were subsequently tested for their properties in the artificially aged state T 6 .   
     
       
         
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
                 Fracture 
               
               
                 Strength values at ambient temperature 
                 toughness at ambient temp. 
               
             
          
           
               
                   
                 Sample 
                 R p02   
                 R m   
                 A 5   
                 Sample 
                 K IC   
               
               
                 Alloy 
                 direction 
                 (MPa) 
                 (MPa) 
                 (%) 
                 direction 
                 (MPa√m) 
               
               
                   
               
             
          
           
               
                 C 
                 L 
                 448 
                 485 
                 11.2 
                 T-L 
                 31.3 
               
               
                   
                 LT 
                 427 
                 471 
                 7.2 
                 S-L 
                 29.5 
               
               
                   
                 ST 
                 417 
                 479 
                 6.3 
                 S-T 
                 32.2 
               
               
                 D 
                 L 
                 456 
                 495 
                 10.7 
                 T-L 
                 28.3 
               
               
                   
                 LT 
                 434 
                 478 
                 8.0 
                 S-L 
                 29.1 
               
               
                   
                 ST 
                 429 
                 484 
                 5.5 
                 S-T 
                 29.6 
               
               
                 E 
                 L 
                 454 
                 494 
                 9.9 
                 T-L 
                 26.1 
               
               
                   
                 LT 
                 446 
                 493 
                 6.4 
                 S-L 
                 25.5 
               
               
                   
                 ST 
                 438 
                 494 
                 4.9 
                 S-T 
                 26.9 
               
               
                 AA 2214 
                 L 
                 444 
                 489 
                 9.7 
                 T-L 
                 24.2 
               
               
                   
                 LT 
                 439 
                 483 
                 6.4 
                 S-L 
                 25.9 
               
               
                   
                 ST 
                 429 
                 480 
                 5.8 
                 S-T 
                 27.3 
               
               
                 AA 2219 
                 L 
                 286 
                 408 
                 16.7 
                 T-L 
                 31.1 
               
               
                   
                 LT 
                 288 
                 403 
                 8.4 
                 S-L 
                 34.4 
               
               
                   
                 ST 
                 366 
                 455 
                 5.0 
                 S-T 
                 32.3 
               
               
                 AA 2618 
                 L 
                 389 
                 443 
                 5.1 
                 T-L 
                 19.2 
               
               
                   
                 LT 
                 383 
                 437 
                 4.7 
                 S-L 
                 16.7 
               
               
                   
                 ST 
                 376 
                 427 
                 4.1 
                 S-T 
                 19.3 
               
               
                   
               
             
          
         
       
     
                                                                                                                             TABLE 3                   Alloy   E   AA 2214   AA 2618            R test     T hold     R p02     R m     A 5     R p02     R m     A 5     R p02     R m     A 5         (° C.)   (h)   (Mpa)   (Mpa)   (%)   (Mpa)   (Mpa)   (%)   (Mpa)   (Mpa)   (%)                    20   1   454   494   9.9   444   489   9.6   380   434   6.5       50   1   453   493   12.6   443   485   9.8   382   433   6.1       100   1   449   474   13   425   458   11   374   423   6.5       150   1   404   417   14.3   403   424   13.6   366   404   7.6       170   1   403   416   16.3   382   400   13.6   382   389   9.6       200   1   355   372   18   348   368   13.8   340   359   12.2       220   1   340   351   18   324   344   14.2   301   332   12.4       250   1   268   282   19   250   268   16.1   282   300   14.7                    
Definitions Sample Directions:
     L=longitudinal direction: parallel to the main form change direction   LT=long transverse direction: parallel to the width direction   ST=short transverse direction: parallel to the thickness direction   
     The improved strengths of the alloy according to the invention (for example alloy E) is clearly evident in Tables 2 and 3. For example, while the prior known alloy AA 2214 shows good strength values at ambient temperature, it does not at higher temperatures. Moreover, the creep resistance and the fracture toughness are markedly better at ambient temperature and at higher temperatures in the claimed alloy compared to the prior known alloys. This comparison makes clear that the tested prior known alloys have good properties only with respect to a single strength parameter. In no case do the prior alloys have good properties in all relevant strength values at ambient temperature as well as at increased temperatures. Just as is the case with the fatigue properties, the creep resistance of this prior known alloy is not satisfactory. Very good properties over all tested strength parameters could only be determined in the case of the alloy according to the invention. 
       FIG. 1  also makes graphically clear the better strength properties of the alloy (alloy E) according to the invention compared to the known alloys (AA 2214 as well as AA 2618). The results showed unexpectedly that the strength values of alloy E are better even at temperatures below 100° C. than those of the known alloy AA 2214, which is known for its especially high strength values in this temperature range. 
     Additionally, the creep resistance of the semi-finished products was tested. Table 4 shown below provides the test results (LMP: Larson Miller parameter) in summary: 
                                                                                                                                       TABLE 4                   Alloy            E   AA 2214   AA 2618            T test     σ test     t fracture     LMP   T test     σ test     t fracture     LMP   T test     σ test     t fracture     LMP       (° C.)   (MPa)   (h)   (—)   (° C.)   (MPa)   (h)   (—)   (° C.)   (MPa)   (h)   (—)                    180   185   2513   10.60   205   200   30   10.27   205   183   10   10.04           167   4762   10.82       190   50   10.38       179   50   10.38                           181   100   10.52       175   100   10.52                           130   500   10.85       163   500   10.85                           100   800   10.95       159   1000   11.00                    
Plotted graphically, the markedly better long-time stress to rupture strength of the alloy in the T 6  state in comparison to the known alloys AA 2214 and AA 2618 in the T 6  state is apparent. This is shown in  FIG. 2  as time-compensated temperature representation. The especially good creep resistance of the alloy according to the invention could not be foreseen making this result surprising.
 
     Within the scope of testing the method steps for the production of these semi-finished products, it was found that comparable material properties of the produced semi-finished products can be attained if the step of hot working is carried out at a block temperature between 320° C. to 460° C. The hot working can be either forging or rolling. The step of quenching of the solution treated semi-finished product can take place in a temperature range between ambient temperature and 100° C. (boiling) in water. It is also possible to utilize a water-glycol mixture for the quenching, the temperature of which-should not exceed 50° C. 
     A cold working step of a drawing out by 1% to 5% can be carried out in the case of extruded or rolled products for the purpose of reducing the intrinsic stresses due to the quenching instead of the previously described step of cold working through cold upsetting during forging. The step of artificial ageing can be carried out over a time period of 5 to 35 hours, preferably between 10 and 25 hours, in a temperature window between 170° C. and 210° C. 
     During further tests strand-cast ingots were produced as described above and airplane wheels manufactured by drop forging in the preforge die and finish forge die at a temperature of 410 to 430° C. These wheels were subsequently solution treated at 505° C., quenched in a mixture of water and glycol of ambient temperature and thermally age-hardened at 170° C. for 20 hours. These were compared to mass-produced airplane wheels of the alloy AA 2214. Samples were taken from the wheels produced of the claimed alloy and of the conventional alloy at sites distributed over the circumference, and tested for their tensile strength. The results are shown in  FIG. 3 . It can clearly be seen that the alloy E according to the invention yields better values compared to the known alloy AA 2214. 
     Fatigue tests in comparable samples of the two cited alloys also show that the wheels produced from the claimed alloy attain markedly better values than the wheels produced from the alloy AA 2214. This applies to the fatigue tests carried out at ambient temperature (cf.  FIG. 4   a ) as well as to the fatigue tests carried out at a test temperature of 200° C. (cf.  FIG. 4   b ). 
     The description of the claimed invention makes clear that surprisingly the claimed alloys have not only high dynamic and static strength values, but that they have an especially good high-temperature stability, fracture toughness and creep resistance. This alloy is therefore particularly suitable for the production of semi-finished products, which must meet precisely these requirements, such as airplane wheels or compressors. 
     Although the present invention has been described with reference to the disclosed embodiments, numerous modifications and variations can be made and still the result will come within the scope of the invention. No limitation with respect to the specific embodiments disclosed herein is intended or should be inferred. Each apparatus embodiment described herein has numerous equivalents.