Al-zn-cu-mg alloys and their manufacturing process

Abstract

The invention relates to a rolled aluminum-based alloy product having a thickness of at least 80 mm comprising, (in weight %): Zn 6.85-7.25, Mg 1.55-1.95, Cu 1.90-2.30, Zr 0.04-0.10, Ti 0-0.15, Fe 0-0.15, Si 0-0.15, other elements ≤0.05 each and ≤0.15 total, remainder Al, wherein at mid-thickness more than 75% of grains are recrystallized or at mid-thickness 30 to 75% of grains are recrystallized and non-recrystallized grains have an aspect ratio in a L/ST cross section less than 3. A process for the manufacture of a rolled aluminum-based alloy product comprises the steps of: (a) casting an ingot made in an alloy according to the invention, (b) conducting an homogenization of the ingot (c) conducting hot rolling of said homogenized ingot in one or more stages by rolling, (d) conducting a solution heat treatment a quench, (e) conducting stress relieving, and, (f) conducting an artificial aging treatment. The products of the invention are suitable for aircraft construction and have advantageous fatigue crack growth properties.

1 . A rolled aluminum-based alloy product having a thickness of at least 80 mm comprising, (in weight %):
 Zn 6.85-7.25,   Mg 1.55-1.95,   Cu 1.90-2.30,   Zr 0.04-0.10,   Ti 0-0.15,   Fe 0-0.15,   Si 0-0.15,   other elements ≤0.05 each and ≤0.15 total, remainder Al,   wherein at mid-thickness more than 75% of grains are recrystallized or at mid-thickness 30 to 75% of grains are recrystallized and non-recrystallized grains have an aspect ratio in a L/ST cross section less than 3.   
     
     
         2 . The product according to  claim 1  wherein Cu 1.95-2.25 and optionally Cu: 2.00-2.20. 
     
     
         3 . The product according to  claim 1  wherein during a fatigue crack growth rate test according to standard ASTM E647 the crack remains within a cone of ±20° and optionally of ±15°, which origin is at intersection of a line passing through specimen holes centers and a specimen axis of symmetry and da/dN at ΔK=15 MPa√m is less than 2.0 10 −4  mm/cycle, optionally less than 1.0 10 −4  mm/cycle and optionally less than 0.9 10 −5  mm/cycle, on a L-S C(T) fatigue specimen at mid-thickness with W=40 mm and B=10 mm. 
     
     
         4 . The product according to  claim 1 , wherein said product has at least one of the following properties:
 a) a minimum life without failure after Environmentally Assisted Cracking (EAC) under conditions of high stress, at a short transverse (ST) stress level of 80% of the product tensile yield strength in ST direction, and humid environment with 85% relative humidity at a temperature of 70° C., of at least 30 days and optionally of at least 40 days,   b) a conventional tensile yield strength measured in the L direction at quarter thickness of at least 515−0.279*t MPa and optionally of 525−0.279*t MPa and optionally of 535−0.279*t MPa, t being the thickness of the product in mm,   c) a K IC  toughness in the L-T direction measured at quarter thickness of at least 32-0.1*t MPa√m and optionally 34−0.1*t MPa√m and optionally 36−0.1*t MPa√m, t being the thickness of the product in mm.   
     
     
         5 . The product according to  claim 1  wherein the thickness thereof is from 80 to 200 mm, or optionally from 100 to 180 mm. 
     
     
         6 . A structural member suitable for aircraft construction wherein said structural member is used in wing ribs, spars and frames, comprising theft product according to  claim 1 . 
     
     
         7 . A process for the manufacture of a rolled aluminum-based alloy product comprising:
 a) casting an ingot comprising, (in weight-%)   Zn 6.85-7.25,   Mg 1.55-1.95,   Cu 1.90-2.30,   Zr 0.04-0.10,   Ti 0-0.15,   Fe 0-0.15,   Si 0-0.15,   other elements ≤0.05 each and ≤0.15 total, remainder Al;   b) homogenizing the ingot;   c) hot rolling said homogenized ingot to a rolled product with a final thickness of at least 80 mm;   d) solution heat treating and quenching the product;   e) stress-relieving the solution heat treated and quenched product;   f) artificially aging the stress-relieved product;   
       wherein the hot rolling starting temperature is controlled to obtain after f at mid-thickness more than 75% of recrystallized grains or at mid-thickness 30 to 75% of recrystallized grains and non-recrystallized grains with an aspect ratio in a L/ST cross section less than 3. 
     
     
         8 . The process according to  claim 7  wherein the hot rolling starting temperature is at least 145*Zr −0.313 −20 and optionally at least 145*Zr −0.313 −10. 
     
     
         9 . The process according to  claim 7  wherein the hot rolling starting temperature is at most 145*Zr −0.313 +20 and optionally at least 145*Zr −0.313 +10. 
     
     
         10 . The process according to  claim 7  wherein the equivalent aging time t(eq) is comprised between 8 and 30 hours and optionally between 12 and 25 hours,
 the equivalent time t(eq) at 155° C. being defined by the formula: 
 
       
         
           
             
               
                 t 
                 ⁡ 
                 
                   ( 
                   eq 
                   ) 
                 
               
               = 
               
                 
                   ∫ 
                   
                     
                       exp 
                       ⁡ 
                       
                         ( 
                         
                           
                             - 
                             16000 
                           
                           / 
                           T 
                         
                         ) 
                       
                     
                     ⁢ 
                     dt 
                   
                 
                 
                   exp 
                   ⁡ 
                   
                     ( 
                     
                       
                         - 
                         16000 
                       
                       / 
                       
                         T 
                         ref 
                       
                     
                     ) 
                   
                 
               
             
           
         
         where T is the instantaneous temperature in ° K during annealing and T ref  is a reference temperature selected at 155° C. (428° K). t(eq) is expressed in hours. 
       
     
     
         11 . The process according  claim 7  wherein the solution heat treatment temperature is from 460 to about 510° C. or optionally from about 470 to about 500° C.