Product made of an AlCuMg alloy for aircraft structural elements

Abstract

Rolled, extruded or forged product made of an AlCuMg alloy processed by solution heat treatment, quenching and cold stretching, to be used in the manufacture of aircraft structural elements, with the following composition (% by weight): Fe<0.15 Si<0.15 Cu:3.8-4.4 Mg:1-1.5 Mn:0.5-0.8 Zr:0.08-0.15 other elements: <0.05 each and <0.15 total with a ratio R m (L)/R 0.2 (L)>1.25. The invention is particularly applicable to the manufacture of lower wings, and has a set of properties (toughness, crack propagation rate, fatigue strength, residual stress level), that are better than alloy 2024.

1 . Rolled, extruded or forged product made of an AlCuMg alloy processed by solution heat treatment, quenching and cold stretching, to be used in the manufacture of aircraft structural elements, with the following composition (% by weight): 
 Fe<0.15 Si<0.15 Cu:3.8-4.4 (preferably: 4.0-4.3) Mg:1-1.5 Mn:0.5-0.8 Zr:0.08-0.15    other elements: <0.05 each and <0.15 total with a ratio R m (L)/R 0.2 (L)>1.25 (and preferably >1.30).    
     
     
         2 . Product according to  claim 1 , characterized in that Fe+Si<0.15%  
     
     
         3 . Rolled product between 6 and 60 mm thick according to one of claims  1  or  2 , with an ultimate tensile strength R m(L)  in the quenched and stretched temper>475 MPa and yield stress R 0.2 (L)>370 MPa.  
     
     
         4 . Rolled product between 6 and 60 mm thick according to any one of  claims 1  to  3 , with a plastic range between the ultimate tensile strength R m  and the yield stress R 0.2  in the L and TL directions in the quenched and stretched temper>100 MPa.  
     
     
         5 . Rolled product between 6 and 60 mm thick according to any one of  claims 1  to  4 , for which the critical intensity factor (L-T direction) K c  in the quenched and stretched temper>170 MPa{square root}m and K co >120 MPa{square root}m measured according to ASTM standard E 561 on notched test pieces sampled at a quarter thickness with parameters W=500 mm, B=5 mm and 2 B0 =165 mm).  
     
     
         6 . Rolled product according to any one of  claims 1  to  5 , characterized in that it has a critical intensity factor (L-T direction) K c  or K c0  at least 10% higher than alloy 2024 under the same conditions.  
     
     
         7 . Rolled product according to any one of  claims 1  to  6 , characterized in that the crack propagation rate (L-T direction) da/dn in the quenched and stretched temper, measured according to ASTM standard E 647 on notched test pieces sampled at a quarter thickness with parameters W=200 mm and B=5 mm) is as follows: 
 <10 −4  mm/cycle for ΔK=10 MPa{square root}m  
 <2.5 10 −4  mm/cycle for ΔK=15 MPa{square root}m  
 and <5 10 −4  mm/cycle for ΔK=20 MPa{square root}m  
 
     
     
         8 . Rolled product according to any one of  claims 1  to  7 , characterized in that deflection f is measured in the L and TL directions after machining a bar supported on two supports separated by a length l to its mid-thickness<(0.14 l 2 )/e, where f is measured in microns, e is the thickness of the plate and l is the length measured in mm.  
     
     
         9 . Rolled product according to any one of  claims 3  to  8 , characterized in that the average fatigue life measured on a notched sample taken at mid-thickness in the L direction is more than 20% better than with the 2024 alloy.  
     
     
         11 . Process according to any one of  claims 1  to  9  for manufacturing a product comprising the following steps: 
 cast a plate with the indicated composition,  
 homogenize this plate between 450 and 500° C.,  
 hot transformation, and possibly cold transformation by rolling, extrusion or forging, until the required product is obtained,  
 solution heat treatment at a temperature of between 480 and 505° C.,  
 quench in cold water,  
 cold stretching to at least 1.5% permanent deformation,  
 natural aging under ambient conditions.  
 
     
     
         12 . Process according to claim  10 , characterized in that the hot transformation takes place with an exit temperature>420° C. and preferably >440° C.  
     
     
         13 . Use of plates according to one of  claims 3  to  9  for manufacturing the skin of an aircraft lower wing.  
     
     
         14 . Use of profiles according to one of claims  1  or  2 , for manufacturing aircraft lower wings or fuselage stringers.