Aluminum copper lithium alloy with improved resistance under compression and fracture toughness

Abstract

The invention relates to a manufacturing process for flat-rolled products made of an alloy containing aluminum, including the steps of production, casting, homogenization, rolling at temperature greater than 400° C., solution heat treating, quenching, stretching between 2 and 3.5% and aging. The invention also relates to flat-rolled products obtained by this process, which offer a favorable compromise of properties between mechanical resistance under compression and stretching and fracture toughness. The products according to the invention are useful in particular for the manufacture of upper wing skins.

1 . A process for manufacturing a flat-rolled product comprising an aluminum alloy, said process comprising the following performed in succession,
 a) producing a molten aluminum metal bath comprising 4.2 to 4.6% Cu by weight, 0.8 to 1.30% Li by weight, 0.3 to 0.8% Mg by weight, 0.05 to 0.18% Zr by weight, 0.05 to 0.5% Ag by weight, 0.0 to 0.5% Mn by weight, at the most 0.20% Fe+Si by weight, less than 0.20% of Zn by weight, at least one element chosen from Cr, Sc, Hf and Ti, the quantity of said element, if it is chosen, being from 0.05 to 0.3% by weight for Cr and Sc, 0.05 to 0.5% by weight for Hf and from 0.01 to 0.15% by weight for Ti, other elements at least 0.05% by weight each and 0.15% by weight in total, remainder aluminum;   b) casting a rolling slab from said molten metal bath;   said rolling slab is homogenized in order to reach a temperature ranging from 450° C. and 550° and for a period ranging from 5 to 60 hours;   d) hot rolling said rolling slab into a plate, maintaining a temperature of at least 400° C.   e) allowing said plate to undergo solution heat treatment at a temperature from 490 to 530° C. for 15 min to 8 hours and quenching said product;   f) allowing said plate to undergo controlled stretching with a permanent set of 2 to 3.5%,   g) performing aging such that said plate reaches a temperature ranging from 130 to 170° C. for 5 to 100 hours,   with the proviso that no significant cold working is carried out on said plate, between said hot rolling d) and said solution heat treatment e).   
     
     
         2 . The process according to  claim 1 , wherein the Cu content ranges from 4.3 to 4.4% by weight. 
     
     
         3 . The process according to  claim 1 , wherein the Li content is up to 1.15% by weight. 
     
     
         4 . The process according to  claim 1 , wherein the Li content ranges from 1.10 to 1.20% by weight. 
     
     
         5 . The process according to  claim 1 , wherein the Mg content ranges from 0.50 to 0.70% by weight. 
     
     
         6 . The process according to  claim 1 , wherein the Mn content is not more than 0.1% by weight. 
     
     
         7 . The process according to  claim 1 , wherein
 Fe and Si contents are each at the most 0.08% by weight and/or   the Ti content is from 0.01 to 0.10% by weight and the Cr, Sc and Hf content are at the most 0.05% by weight and/or   the Zn to content is at most 0.15% by weight.   
     
     
         8 . The process according to  claim 1 , wherein the permanent set is accomplished by controlled traction and is selected so as to obtain a compression yield stress at least equal to tensile yield stress. 
     
     
         9 . The process according to  claim 1 , wherein controlled stretching is realized directly after solution treatment and quenching. 
     
     
         10 . The process according to  claim 1 , wherein aging is under-aging close to peak of compression yield stress. 
     
     
         11 . A flat-rolled product of a thickness ranging from 8 to 50 mm and having a primarily unrecrystallized granular structure optionally obtained by said process according to  claim 1 , said product comprising at mid-thickness at least one of the following combinations of characteristics:
 (i) for thicknesses from 8 to 15 mm, at mid-thickness, a tensile yield stress R p0.2 (L)≧600 MPa, a compression yield stress R p0.2 (L)≧620 MPa and fracture toughness such that K 1C  (L−T)≧28 MPa√m and/or K app  (L−T)≧73 MPa√m, for 300 mm wide and 6.35 mm thick CCT test samples,   (ii) for thicknesses from 8 to 15 mm, at mid-thickness, a tensile yield stress R p0.2 (L)≧630 MPa, a compression yield stress R p0.2 (L)≧640 MPa and fracture toughness such that K 1C  (L−T)≧26 MPa√m and/or K app  (L−T)≧63 MPa√m, for 300 mm wide and 6.35 mm thick CCT test samples,   (iii) for thicknesses from 15 to 50 mm, at mid-thickness, a tensile yield stress R p0.2 (L)≧610 MPa a compression yield stress R p0.2 (L)≧620 MPa and fracture toughness K 1C  (L−T)≧22 MPa√m,   (iv) for thicknesses from 15 to 50 mm, at mid-thickness, a tensile yield stress R p0.2 (L)≧580 MPa a compression yield stress R p0.2 (L)≧600 MPa and fracture toughness K 1C  (L−T)≧24 MPa√m.   
     
     
         12 . An airplane structural element, optionally an upper wing skin, said element comprising said product according to  claim 11 . 
     
     
         13 . A product according to  claim 11  capable of being used for a structural element. 
     
     
         14 . The product according to  claim 11 , capable of being used for aeronautical engineering. 
     
     
         15 . The product of  claim 13 , capable of being used for aeronautical engineering.