Age-hardenable aluminium alloys

Abstract

This invention concerns AA5000 series alloys with the addition of Cu that can be retained in a solution treated condition after hot working, for example by hot rolling on a hot mill or by hot extruding. There is described a method of producing an age-hardenable aluminium alloy comprising the steps of: a) casting an alloy of a composition comprising the following expressed in weight percent: Magnesium : 1.0 to 4.0, Cooper : 0.1 to 0.6, Manganese : up to 0.8, Iron : up to 0.5, Silicon : up to 0.3, Chromium : up to 0.15, Titanium : up to 0.15, Balance : Aluminium with incidental impurities b) optionally homogenising the cast alloy, c) hot working the casting at an initial temperature of at least 400 DEG C to form an intermediate product, wherein at least part of the hot working is carried out whilst the casting is at a temperature above the solvus temperature of the alloy, d) cooling the intermediate product either during hot working or in a subsequent step at a rate such that at least a partially recovered or recrystallised structure is formed ant that sufficient copper is retained in solid solution in the alloy to cause an age hardening effect on the alloy if phase precipitation takes place during the alloy's subsequent thermal history, and e) optionally allowing or arranging for phase precipitation to occur in the alloy. The described method is particularly suited to the production of can end stock and sheet for automotive applications.

1 . A method of producing an age-hardenable aluminium alloy comprising the steps of: 
 a) casting an alloy of a composition comprising the following expressed in weight percent:                                                    Magnesium   1.0 to   4.0         Copper   0.1 to   0.6         Manganese   up to   0.8         Iron   up to   0.5         Silicon   up to   0.3         Chromium   up to   0.15         Titanium   up to   0.15                       Balance   Aluminium with             incidental impurities                                                          b) optionally homongenising the cast alloy,    c) hot working the casting at an initial temperature of at least 400° C. to form an intermediate product, wherein at least part of the hot working is carried out whilst the casting is at a temperature above the solvus temperature of the alloy,    d) cooling the intermediate product either during hot working or in a subsequent step at a rate of less than 5° C./min such that at least a partially recovered or recrystallised structure is formed and that sufficient copper is retained in solid solution in the alloy to cause an age hardening effect on the alloy if phase precipitation takes place during the alloy's subsequent thermal history, and    e) optionally allowing or arranging for phase precipitation to occur in the alloy.    
     
     
         2 . A method as claimed in  claim 1  wherein the alloy has the following composition expressed in weight percent:  
       
         
           
                 
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   Magnesium 
                   2.0 to 
                   4.0 
                 
                     
                   Copper 
                   0.2 to 
                   0.5 
                 
                     
                   Manganese 
                   up to 
                   0.6, 
                 
                 
                 
                 
               
                     
                     
                   preferably 
                 
                 
                 
                 
                 
               
                     
                     
                   up to 
                   0.5 
                 
                     
                   Iron 
                   0.1 to 
                   0.3 
                 
                     
                   Silicon 
                   up to 
                   0.2 
                 
                     
                   Chromium 
                   up to 
                   0.15 
                 
                     
                   Titanium 
                   up to 
                   0.05 
                 
                     
                   Boron or 
                 
                     
                   Carbon 
                   up to 
                   0.01 
                 
                 
                 
                 
               
                     
                   Balance 
                   Aluminium with 
                 
                     
                     
                   incidental impurities 
                 
                     
                     
                 
                     
                     
                 
             
                
                
               
               
                
                
                
               
            
             
                
               
            
             
                
                
                
                
                
                
                
               
            
             
                
                
                
                
               
            
           
         
       
     
     
         3 . A method as claimed in  claim 2  wherein the magnesium content is 2.5 to 4.0%.  
     
     
         4 . A method as claimed in any one of  claims 1  to  3  wherein the intermediate product has a substantially fully recovered or recrystallised structure.  
     
     
         5 . A method as claimed in any one of the preceding claims wherein the casting is homogenised before hot working at a temperature of at least 480° C., preferably 500 to 600° C., so that substantially all of the magnesium and copper in the casting are in solid solution.  
     
     
         6 . A method as claimed in any one of the preceding claims wherein the casting is hot worked, optionally with re-heating of the casting to above the alloy's solvus temperature, and preferably at least 450° C., to take substantially all of the magnesium and copper present into solid solution.  
     
     
         7 . A method as claimed in any one of the preceding claims wherein the hot working step is carried out when the casting has an initial temperature of from 450° C. to 580° C.  
     
     
         8 . A method as claimed in any one of the preceding claims wherein the alloy is DC cast.  
     
     
         9 . A method as claimed in any one of the preceding claims including the step of cold rolling the hot worked casting, optionally with coiling.  
     
     
         10 . A method as claimed in any one of  claims 1  to  9  wherein the hot working is effected by extrusion.  
     
     
         11 . A method as claimed in any one of  claims 1  to  9  wherein the hot working is effected by hot rolling.  
     
     
         12 . A method as claimed in any one of the preceding claims wherein the hot worked casting is cooled at a rate of less than 1° C./min.  
     
     
         13 . A method as claimed in any one of the preceding claims wherein if after the said hot working step the temperature of the intermediate product exceeds the solvus temperature of the alloy then cooling of the intermediate product to a temperature below the alloy's solvus temperature is effected at a rate less than 2° C./sec.  
     
     
         14 . A method as claimed in  claim 12  or  claim 13  wherein the cooling is effected by forced air cooling.  
     
     
         15 . A method as claimed in any one of the prceeding claims wherein no separate annealing step is carried out after the hot working step (c)and before the cooling step (d).  
     
     
         16 . A method as claimed in any one of the preceding claims wherein the product of the method is can end stock.