Heat treatment of aluminum alloys containing silicon and scandium

Abstract

Heat treatment of aluminum alloys containing Si with Sc additions improves mechanical properties of the alloys by the formation of Sc-containing dispersoids. The Al—Si—Sc alloys may include Zr or other refractory metals, and may also include Mg, Cu and/or Cr. Depending upon the cooling rate used during casting, at least some of the Sc-containing dispersoids may be present upon casting and cooling of the Al—Si alloy, in which case it may be desirable to reduce or eliminate coarsening of the as-cast dispersoids upon subsequent heat treatment. The subsequent heat treatment may also generate fresh Sc-containing dispersoids. Alternatively, the as-cast Al—Si alloy may not include Sc-containing dispersoids upon cooling, in which case the Sc-containing dispersoids may form during the heat treatment process.

What is claimed is: 
     
         1 . A method of heat treating a 4XXX or a 6XXX aluminum alloy comprising silicon in an amount of at least 0.2 weight percent, scandium and magnesium, the method comprising:
 heating an as-cast billet of the 4XXX or 6XXX aluminum alloy to a homogenization temperature and cooling the homogenized billet; and   heat treating the as-cast and homogenized billet of the 4XXX or 6XXX aluminum alloy to a dispersoid precipitation temperature sufficient to precipitate Sc-containing dispersoids in the aluminum alloy, wherein the heat treating comprises performing in first and second stages, the first stage heat treatment is performed at a lower temperature than a higher temperature of the second stage heat treatment, the higher temperature of the second stage heat treatment is achieved by ramping up from the lower temperature of the first stage heat treatment to the higher temperature of the second stage heat treatment, and the higher temperature of the second stage heat treatment is below the homogenization temperature.   
     
     
         2 . The method of  claim 1 , wherein the heat treating is performed at a substantially constant heat up rate. 
     
     
         3 . The method of  claim 1 , wherein the heat treating is performed at varying heat up rates. 
     
     
         4 . The method of  claim 1 , wherein the higher temperature of the second stage heat treatment is 500° C. or less. 
     
     
         5 . The method of  claim 1 , wherein the homogenization temperature is greater than 500° C. 
     
     
         6 . The method of  claim 1 , wherein the homogenization temperature is above the heat treating dispersoid precipitation temperature and causes dissolution of at least a portion of any Sc-containing dispersoids contained in the as-cast billet. 
     
     
         7 . The method of  claim 1 , wherein the 4XXX or 6XXX aluminum alloy further comprises zirconium, the first stage heat treatment causes precipitation of the Sc-containing dispersoids, and the second stage heat treatment causes precipitation of Zr-containing shells on the Sc-containing dispersoids. 
     
     
         8 . The method of  claim 7 , wherein the first and second stage heat treatments are performed at substantially the same heat up rates. 
     
     
         9 . The method of  claim 7 , wherein the first and second stage heat treatments are performed at different heat up rates. 
     
     
         10 . The method of  claim 1 , wherein the billet is not subjected to a working step prior to the heat treating. 
     
     
         11 . The method of  claim 1 , further comprising heating the as-cast and homogenized billet to an aging temperature prior to the heat treating. 
     
     
         12 . The method of  claim 11 , wherein the aging temperature is below the first stage heat treatment temperature and causes breakdown of aluminum solid solution within the billet. 
     
     
         13 . The method of  claim 1 , wherein the as-cast and homogenized billet comprises as-cast Sc-containing dispersoids. 
     
     
         14 . The method of  claim 13 , wherein at least a portion of the as-cast Sc-containing dispersoids are retained during the heat treating step, and the heat treating step reduces or eliminates coarsening of the as-cast Sc-containing dispersoids while also precipitating the Sc-containing dispersoids. 
     
     
         15 . The method of  claim 13 , wherein at least a portion of the as-cast Sc-containing dispersoids are dissolved prior to or during the heat treating step. 
     
     
         16 . The method of  claim 1 , wherein the as-cast and cooled billet is substantially free of Sc-containing dispersoids. 
     
     
         17 . The method of  claim 1 , wherein the 4XXX or 6XXX aluminum alloy comprises from 0.001 to 0.2 weight percent scandium. 
     
     
         18 . The method of  claim 1 , wherein the 4XXX or 6XXX aluminum alloy comprises less than 0.1 weight percent scandium. 
     
     
         19 . The method of  claim 1 , wherein the 4XXX or 6XXX aluminum alloy further comprises a refractory metal selected from the group consisting of Zr, Ti, Hf and V, or a rare earth element. 
     
     
         20 . The method of  claim 1 , wherein the 4XXX or 6XXX aluminum alloy further comprises Zr. 
     
     
         21 . The method of  claim 20 , wherein the Zr comprises up to 0.25 weight percent. 
     
     
         22 . The method of  claim 1 , wherein the 4XXX or 6XXX aluminum alloy comprises Mg. 
     
     
         23 . The method of  claim 22 , wherein the Mg comprises from 0.1 to 2 weight percent. 
     
     
         24 . The method of  claim 20 , wherein the 4XXX or 6XXX aluminum alloy further comprises Cu, Cr or a combination thereof. 
     
     
         25 . The method of  claim 1 , wherein the 4XXX or 6XXX aluminum alloy comprises a 4XXX alloy. 
     
     
         26 . The method of  claim 1 , wherein the 4XXX or 6XXX aluminum alloy comprises a 6XXX alloy. 
     
     
         27 . The method of  claim 1 , wherein the lower temperature of the first stage heat treatment is from 225 to 350° C. 
     
     
         28 . The method of  claim 1 , wherein the higher temperature of the second stage heat treatment is from 400 to 500° C. 
     
     
         29 . The method of  claim 1 , further comprising cooling the heat treated billet. 
     
     
         30 . The method of  claim 1 , further comprising working the billet after the heat treating. 
     
     
         31 . A heat treated billet of a 4XXX or a 6XXX aluminum alloy containing silicon and scandium having Sc-containing dispersoids therein produced by the method of  claim 1 .