Thick plates made of al-cu-li alloy with improved fatigue properties

Abstract

The invention relates to a rolled product having a thickness of at least 50 mm made of aluminium alloy comprising, in % by weight, 2.2% to 3.9% of Cu, 0.7% to 1.8% of Li, 0.1% to 0.8% of Mg, 0.1% to 0.6% of Mn; 0.01% to 0.15% of Ti, at least one element chosen from Zn and Ag, the amount of said element, if it is chosen, being 0.2% to 0.8% for Zn and 0.1% to 0.5% for Ag, optionally at least one element chosen from Zr, Cr, Sc, Hf, and V, the amount of said element, if it is chosen, being 0.04% to 0.18% for Zr, 0.05% to 0.3% for Cr and for Sc, 0.05% to 0.5% for Hf and for V, less than 0.1% of Fe, less than 0.1% of Si, the remainder being aluminium and inevitable impurities, having a content of less than 0.05% each and 0.15% in total; characterized in that its granular structure is predominantly recrystallised between ¼ and ½ thickness. The invention also relates to the process for manufacturing such a product. The products according to the invention are advantageously used in aircraft construction, in particular for the production of an aircraft wing spar or rib.

1 . Rolled product with a thickness of at least 50 mm made of aluminum alloy comprising as a percentage by weight 2.2 to 3.9% Cu, 0.7 to 1.8% Li, 0.1 to 0.8% Mg, 0.1 to 0.6% Mn; 0.01 to 0.15% of Ti, at least one element chosen from Zn and Ag, the amount of said element if it is chosen being 0.2 to 0.8% for Zn and 0.1 to 0.5% for Ag, optionally at least one element chosen from Zr, Cr, Sc, Hf, and V, the amount of said element if it is chosen being 0.04 to 0.18% for Zr, 0.05 to 0.3% for Cr and for Sc, 0.05 to 0.5% for Hf and for V, less than 0.1% of Fe, less than 0.1% of Si the rest aluminum and unavoidable impurities, with a content of less than 0.05% each and 0.15% in total; comprising a grain structure predominantly recrystallized between ¼ and ½ thickness. 
     
     
         2 . Rolled product according to  claim 1 , comprising a thickness between 80 and 130 mm. 
     
     
         3 . Rolled product according to  claim 1 , wherein the maximum Li content is 1.5% by weight. 
     
     
         4 . Rolled product according to  claim 1 , wherein the sum of the content of elements Zr, Cr, Sc, Hf and V is less than 0.08% by weight. 
     
     
         5 . Rolled product according to  claim 1  having
 (i) for a thickness of between 50 and 75 mm, at quarter thickness, a yield strength R p0.2 (LT)≥435 MPa and optionally R p0.2  (LT)≥455 MPa and a toughness K 1C  (T−L)≥28 MPa√m and advantageously such that K 1C  (T−L)≥30 MPa√m, 
 (ii) for a thickness of between 76 and 102 mm, at quarter thickness, a yield strength R p0.2 (LT)≥435 MPa and optionally R p0.2  (LT)≥455 MPa and a toughness K 1C  (T−L)≥25 MPa√m and advantageously such that K 1C  (T−L)≥27 MPa√m, 
 (iii) for a thickness of between 103 and 130 mm, at quarter thickness, a yield strength R p0.2 (LT)≥428 MPa and optionally R p0.2  (LT)≥448 MPa and a toughness K 1C  (T−L)≥23 MPa√m and advantageously such that K 1C (T−L)≥25 MPa√m, 
 (iv) for a thickness greater than 130 mm, at quarter thickness, a yield strength Rp0.2(LT)≥428 MPa and optionally Rp0.2(LT)≥448 MPa and a toughness K 1C  (T−L)≥21 MPa√m and advantageously such that K 1C  (T−L)≥23 MPa√m, 
 and having a fatigue crack growth rate measured according to standard ASTM E647 on CCT test pieces, with a central crack, of width 100 mm and thickness 6.35 mm taken at mid-thickness in the direction L-S of less than 10-4 mm/cycle for a ΔK=20 MPa√m. 
 
     
     
         6 . Rolled product according to  claim 1  having a low propensity for crack bifurcation wherein the fracture during a fatigue test in the direction L-S at a maximum load of at least 3000 N, R=0.1, on a batch of at least 6 CT test pieces of thickness 10 mm and total width 50 mm, is mainly via the rear face. 
     
     
         7 . Rolled product according to  claim 1  having a low propensity for crack bifurcation wherein during a fatigue test in the direction L-S at a maximum load of at least 3000 N, R=0.1, on a CT test piece of thickness 10 mm and total width 50 mm the distance d over which the crack is neither in the initial direction S nor in the direction L is at least 5 mm and optionally at least 10 mm. 
     
     
         8 . Manufacturing process for a plate according to  claim 1  comprising:
 a) the casting of a slab, made of aluminum alloy comprising, as a percentage by weight, 2.2 to 3.9% Cu, 0.7 to 1.8% Li, 0.1 to 0.8% Mg, 0.1 to 0.6% Mn; 0.01 to 0.15% of Ti, at least one element chosen from Zn and Ag, the amount of said element if it is chosen being 0.2 to 0.8% for Zn and 0.1 to 0.5% for Ag, optionally at least one element chosen from Zr, Cr, Sc, Hf, and V, the amount of said element if it is chosen being 0.04 to 0.18% for Zr, 0.05 to 0.3% for Cr and for Sc, 0.05 to 0.5% for Hf and for V, less than 0.1% of Fe, less than 0.1% of Si the rest aluminum and unavoidable impurities, with a content of less than 0.05% by weight each and 0.15% in total; 
 b) homogenizing said slab at a temperature of at least 490° C., 
 c) hot rolling said plate to obtain a plate of at least 50 mm thick, 
 d) solution heat treatment between 490° C. and 540° C., 
 e) quenching with cold water, 
 f) controlled stretching of the said plate with a permanent set of 1 to 7%, 
 g) artificial aging of said plate by heating between 130° C. and 170° C. for 5 to 60 hours, 
 wherein the sum of the content of elements Zr, Cr, Sc, Hf, and V is less than 0.08% by weight and/or in that in b) the homogenization comprises at least one step for which the temperature is at least 520° C., the time during which the temperature is greater than 520° C. being at least 20 hours and in c) the hot rolling exit temperature is less than 390° C. 
 
     
     
         9 . A plate according to  claim 1  for producing an aircraft wing spar or an airplane wing rib. 
     
     
         10 . A plate according to  claim 9  for the lower part of a welded spar.