Thermomechanical ageing for 6xxx extrusions

Abstract

The present invention relates to extrusions for structural components, such as bumper, side impact beam, seat sill in vehicles and more particularly to a method for optimizing strength and energy absorption of 6XXX aluminium alloys extrusions by variations in thermomechanical ageing (TMA) consisting in i) an artificial preageing treatment with a duration t 1 at a temperature T 1 selected to increase the yield strength of said extrusion between 5% and 20%, said temperature T 1 being typically between 120° C. and 180° C. and said duration t 1 being typically between 1 and 100 hours, to obtain an artificially preaged extrusion, ii) a plastic deformation of said artificially preaged extrusion between 1% and 80% to obtain a deformed extrusion, iii) a final artificial ageing treatment of said deformed extrusion with a duration t 2 at a temperature T 2, said temperature T 2 being typically between 140° C. and 200° C. and said the duration t 2 being typically between 1 and 100 hours.

1 . A manufacturing process for obtaining extrusions made of 6xxx aluminium alloy, comprising the following successive steps:
 a) Casting a billet comprising, in weight %,
 Si 0.1-1.2; Cu≤1.2; Mn 0.05-1.2; Mg 0.1-1.2 
 one or more of Cr≤0.3%; Zn≤0.5; Ti≤0.21; Zr≤0.2; V<0.1, 
 rest aluminium and impurities, 
   b) Heat treating said cast billet;   c) Extruding said heat treated billet through a die to form a solid or hollow extrusion;   d) quenching said extrusion down to room temperature;   e) Natural ageing said extrusion during less than100 days;   f) ageing said naturally aged extrusion;   
       wherein said ageing treatment comprises at least three steps, which are successively
 i) an artificial preageing treatment with a duration t 1  at a temperature T 1  selected to increase the yield strength of said extrusion between 5% and 20%, optionally between 6% and 19%, and optionally between 8% and 18% compared to the yield strength obtained after step e), said temperature T 1  being optionally between 120° C. and 180° C. and said duration t 1  being optionally between 1 and 100 hours, to obtain an artificially preaged extrusion, 
 ii) a plastic deformation of said artificially preaged extrusion between 1% and 80% to obtain a deformed extrusion, 
 iii) a final artificial ageing treatment of said deformed extrusion with a duration t 2  at a temperature T 2 , said temperature T 2  being optionally between 140° C. and 200° C. and said duration t 2  being optionally between 1 and 100 hours. 
 
     
     
         2 . The manufacturing process according to  claim 1 , wherein step b) comprises pre-heating said cast billet, optionally between 410° C. and 530° C. 
     
     
         3 . The manufacturing process according to  claim 1 , wherein step b) comprises an homogenizing treatment of said cast billet, optionally performed at a temperature between 485° C. and 630° C. for a duration between 1 h and 12 h, to obtain an homogenized billet and a pre-heating of said homogenised billet between 410° C. and 530° C. 
     
     
         4 . The manufacturing process according to  claim 1 , wherein step b) comprises solution heat treating said billet at a temperature between Ts−60° C. and Ts, wherein Ts is the solidus temperature of the said aluminium alloy, and quenching until the billet mean temperature reaches a value between 400° C. and 480° C. while ensuring that the billet surface never goes below a temperature of substantially 400° C. 
     
     
         5 . The manufacturing process according to  claim 1 , wherein step b) comprises homogenizing said cast billet, optionally at a temperature between 485° C. and 630° C. for a duration between 1 h and 12 h, to obtain an homogenized billet and solution heat treating said billet at a temperature between Ts−60° C. and Ts, wherein Ts is the solidus temperature of the said aluminium alloy, and quenching until the billet mean temperature reaches a value between 400° C. and 480° C. while ensuring that the billet surface never goes below a temperature of substantially 400° C. 
     
     
         6 . The manufacturing process according to  claim 5 , wherein no additional solution heat treatment is performed on the extrusion. 
     
     
         7 . The manufacturing process according to  claim 1 , wherein said plastic deformation of step ii) is obtained by stretching or hydroforming or pressing or stamping or bending or roll bending or stretch bending or rotary stretch bending or pulse magnetic forming or flow forming or forging or rolling or drawing or deep drawing or impact or inverse extrusion or punching or blanking. 
     
     
         8 . The manufacturing process according to  claim 1 , wherein the alloy composition comprises the following elements in weight %:
 Si 0.6-1.1; Cu 0.2-1.0; Mn 0.3-0.9; Mg 0.4-0.9 and one or more of Cr≤0.3; Ti≤0.1; Zr≤0.2; V<0.1, rest aluminium and impurities.   
     
     
         9 . The manufacturing process according to  claim 1 , wherein the alloy composition comprises the following elements in weight %:
 Si 0.6-1.0; Cu 0.2-1.0; Mn 0.05-1.2; Mg 0.1-1.2 and one or more of Cr 0.01-0.3; Zn≤0.5; Ti≤0.21; Zr 0.01-0.2; V<0.1, rest aluminium and impurities.   
     
     
         10 . The manufacturing process according to  claim 1 , wherein the impurities have a maximum content of iron of 0.5%, and other impurities have a maximum content of 0.05% each and 0.15% total. 
     
     
         11 . The manufacturing process according to  claim 1 , wherein an extrusion speed of step c) is controlled such that a surface exit temperature be close to 550° C. 
     
     
         12 . The manufacturing process according to  claim 1 , wherein an extrusion speed of step c) is controlled such that a surface exit temperature be higher than 550° C. and lower than solidus temperature. 
     
     
         13 . The manufacturing process according to  claim 1 , wherein the deformed extrusion is obtained by performing ii) the plastic deformation of said artificially preaged extrusion between 2% and 7%. 
     
     
         14 . The manufacturing process according to  claim 1 , wherein the final artificial ageing treatment of step iii) is performed in multiple steps. 
     
     
         15 . The manufacturing process according to  claim 1 , wherein an equivalent time t(eq) at 170° C. of the final artificial ageing treatment is between 1 and 80 hours. 
     
     
         16 . The manufacturing process according to  claim 15 , wherein the equivalent time t(eq) at 170° C. is defined by the formula
     T (eq)=∫exp (− Q/RT ′) dt /exp  (−( Q/RT ref) 
 
 wherein T′ (in Kelvin) is an instantaneous treatment temperature which changes with time t′ (in hours), T ref  is a reference temperature set at 443 K (170° C.), t(eq) is expressed in hours, with the constant R=8.31 J/mol/K and the activation energy of the diffusion of Mg, Q=130400 J/mol.