Heat treatable aluminium alloy with improved mechanical properties and method for producing it

Abstract

The present invention relates to a method for processing extrusion billets or billets produced from an aluminium 6xxx alloy to finished or semi-finished products such as profiles or blanks, comprising a) Homogenizing the extrusion ingot or billetb) soft annealing operation followed by preheating and extrusion of the billet to form a profile or blank, whereafter the profile or blank is exposed to a solutionising operation, where the 6xxx alloy contains Mg in the range 0.85-1.15 wt % and Si in the range 0.60-0.75 wt %, Fe in the range 0-0.5 wt %, Cu in the range 0-0.30 wt %, Cr in the range 0-0.10 wt %, Mn in the range 0-0.20 wt %, Zn in the range 0-0.5 wt %, Ti in the range 0-0.15 wt %, V in the range 0-0.15 wt % and other elements not specified below in the range 0-0.05 wt %, with balance Al, the Mg/Sieff ratio of the alloy being above 1.6, where Sieff=Si−½ wt % (Fe+Cr+Mn+Zr).

1 . A method for processing extrusion billet produced from an aluminium 6xxx alloy containing
 Mg 0.85-1.15 wt %,   Si 0.60-0.75 wt %,   Fe 0-0.5 wt %,   Cu 0-0.30 wt %,   Cr 0-0.10 wt %,   Mn 0-0.20 wt %,   Zn 0-0.5 wt %,   Ti 0-0.15 wt %,   V 0-0.15 wt %, and   other elements not specified below 0-0.05 wt %, with balance Al,   the Mg/Si eff  ratio of the alloy being above 1.6,   where Si eff  Si−⅓ wt % (Fe+Cr+Mn+Zr),   the method comprising the steps:   a. homogenizing the extrusion billet,   b. soft annealing the extrusion billet at a temperature between 35° and 450° C.,   c. preheating the extrusion billet,   d. extruding the extrusion billet to form a profile,   e. cooling the profile down to room temperature,   f. exposing the profile to a solutionizing and quenching operation,   g. optionally stretching the extruded profile,   h. artificially ageing the profile,   where a cooling rate from the homogenizing temperature in step a. to the soft annealing temperature in step b. is at least 100° C. per hour.   
     
     
         2 . The method for processing extrusion billets according to  claim 1 , wherein the soft annealing is taking place at a temperature of between 38° and 420° C. 
     
     
         3 . The method for processing extrusion billets according  claim 1 , wherein the cooling rate from the homogenizing temperature to the soft annealing temperature is at least 200° C. 
     
     
         4 . The method for processing extrusion billets according to  claim 1 , where the homogenizing step a. is taking place at a temperature of between 540 to 590° C. and 
     
     
         5 . The method for processing extrusion billets according to  claim 1 , where the preheating step c. is taking place at a temperature of between 35° and 450° C. 
     
     
         6 . The method for processing extrusion billets according to  claim 5 , where the preheating step c. is taking place at a temperature of between 38° and 420° C. 
     
     
         7 . The method according to  claim 1 , the minimum concentration of Mg is 0.90 wt %, or 0.95 wt %. 
     
     
         8 . The method according to  claim 1 , the concentration of Si is 0.63-0.72 wt %, or 0.65-0.70 wt %. 
     
     
         9 . The method according to  claim 1 , the minimum concentration of Ti is 0.05 wt %, or 0.07 wt %. 
     
     
         10 . The method according to  claim 1 , the maximum concentration of Cu is 0.20 wt %, or 0.15 wt %, or 0.10 wt %. 
     
     
         11 . The method according to  claim 1 , the solutionizing temperature of step f. is above 555° C., or above 560° C., or above 565° C. 
     
     
         12 . An extrusion billet processed by the method according to  claim 1 , wherein an average Mg 2 Si particle diameter of a microstructure of the extrusion billet is in the range 2-5 μm, or in the range 2-4 μm, where the particle diameter is calculated based on a measured area in LOM in a plane in a polished cross section of the billet converted to a circle. 
     
     
         13 . A semi-finished or finished extruded product produced according to  claim 1 , having a yield stress (Rp0.2) of at least 300 MPa, preferably above 310 MPa, more preferably above 320 MPa.