Decreased cracking susceptibility of 7xxx series direct chill (dc) cast ingots

Abstract

Process control of intense stirring along a solidification front and adjustments in casting speeds during direct chill casting of 7xxx series alloys can decrease an ingot’s cracking susceptibility. Intense stirring control is used to reduce the thickness of the solidification front, promote agglomeration of hydrogen gas rejected at the solidification front, remove impurities rejected at the solidification front, and improve grain size. Intense stirring control is used to operate at faster casting speeds without risk of increasing the thickness of the solidification front. Optional reheating during casting to promote dispersoid formation is used to generate a high-strength zone of dispersoid-strengthened solidified metal in the outer periphery of the ingot, which can further decrease the ingot’s susceptibility to cracking.

1 . A method, comprising:
 delivering molten metal from a metal source to a metal sump of an embryonic ingot being cast in a mold;   forming an external solid shell of solidified metal by extracting heat from the metal sump, wherein a solidifying interface is located between the external solid shell and the metal sump;   advancing the embryonic ingot in a direction of advancement away from the mold at a casting speed while delivering the molten metal and forming the external solid shell;   determining an intensity of stirring using the casting speed, wherein the intensity of stirring is suitable to achieve a target solidification interface profile at the casting speed; and   inducing stirring within the molten sump at the determined intensity, wherein inducing stirring within the molten sump induces the solidification interface to take on the target solidification interface profile at the casting speed.   
     
     
         2 . The method of  claim 1 , wherein inducing stirring comprises applying stirring forces to the molten metal in the metal sump using a non-contact magnetic stirrer. 
     
     
         3 . The method of  claim 1 , wherein delivering molten metal comprises delivering molten metal at a mass flow rate through a plurality of nozzles, and wherein inducing stirring comprises increasing a flow rate of molten metal through at least one of the plurality of nozzles while maintaining the mass flow rate through the plurality of nozzles. 
     
     
         4 . The method of  claim 1 , further comprising:
 modifying the casting speed;   determining an updated intensity of stirring using the updated casting speed, wherein the updated intensity of stirring is suitable to achieve the target solidification profile at the updated casting speed; and   inducing stirring within the molten sump at the updated intensity, wherein inducing stirring within the molten sump at the updated intensity induces the solidification interface to take on the target solidification interface profile at the updated casting speed.   
     
     
         5 . The method of  claim 1 , wherein the molten metal is a 7xxx series aluminum alloy. 
     
     
         6 . The method of  claim 1 , further comprising measuring a temperature of the embryonic ingot, wherein determining the intensity of stirring using the casting speed comprises using the measured temperature. 
     
     
         7 . The method of  claim 1 , wherein the target solidification interface profile is predetermined to minimize a risk of cracking. 
     
     
         8 . The method of  claim 1 , further comprising:
 continuously forming a high-strength zone within the external solid shell at a cross section of the embryonic ingot that is perpendicular to the direction of advancement and that intersects the internal molten core, wherein the high-strength zone is located between an outer surface of the external solid shell and the internal molten core, and wherein forming the high-strength zone includes reheating the external solid shell at the cross section to induce dispersoid precipitation in the external solid shell.   
     
     
         9 . The method of  claim 1 , wherein inducing stirring within the molten sump comprises controlling delivery of the molten metal into the metal sump such that a jet of molten metal erodes a depression into the solidifying interface at a bottom of the metal sump, the depression having a diameter sized to match a diameter of the bottom of the metal sump. 
     
     
         10 . A method, comprising:
 delivering molten metal from a metal source to a metal sump of an embryonic ingot being cast in a mold;   forming an external solid shell of solidified metal by extracting heat from the metal sump, wherein a solidifying interface is located between the external solid shell and the metal sump;   advancing the embryonic ingot in a direction of advancement away from the mold at a casting speed while delivering the molten metal and forming the external solid shell; and   controlling delivery of the molten metal into the metal sump to generate a jet of molten metal sufficient to erode at least a portion of the solidifying interface at a bottom of the metal sump.   
     
     
         11 . The method of  claim 10 , wherein controlling delivery of the molten metal comprises controlling delivery of the molten metal such that the jet of molten metal erodes the solidifying interface to a thickness that is at or less than 10 mm. 
     
     
         12 . The method of  claim 10 , wherein delivering the molten metal comprises delivering the molten metal at a mass flow rate through a plurality of nozzles, and wherein generating the jet of molten metal comprises increasing a flow rate of molten metal through at least one of the plurality of nozzles while maintaining the mass flow rate through the plurality of nozzles. 
     
     
         13 . The method of  claim 10 , further comprising applying stirring forces to the molten metal in the metal sump using a non-contact magnetic stirrer. 
     
     
         14 . The method of  claim 10 , further comprising modifying the casting speed, wherein controlling delivery of the molten metal includes dynamically adjusting delivery of the molten metal based on the modified casting speed such that the jet of molten metal continues to erode at least the portion of the solidifying interface at the bottom of the metal sump. 
     
     
         15 . The method of  claim 10 , wherein the molten metal is a 7xxx series aluminum alloy. 
     
     
         16 . The method of  claim 10 , further comprising measuring a temperature of the embryonic ingot, wherein controlling delivery of the molten metal comprises dynamically adjusting delivery of the molten metal based on the measured temperature such that the jet of molten metal continues to erode at least the portion of the solidifying interface at the bottom of the metal sump. 
     
     
         17 . The method of  claim 10 , further comprising:
 continuously forming a high-strength zone within the external solid shell at a cross section of the embryonic ingot that is perpendicular to the direction of advancement and that intersects the metal sump, wherein the high-strength zone is located between an outer surface of the external solid shell and the metal sump, and wherein forming the high-strength zone comprises reheating the external solid shell at the cross section to induce dispersoid precipitation in the external solid shell.   
     
     
         18 . (canceled) 
     
     
         19 . An embryonic ingot, comprising:
 a solidified shell of aluminum alloy extending from a solidifying interface to a bottom end in a casting direction; and   a liquid molten core of the aluminum alloy extending from an upper surface to the solidifying interface, wherein the liquid molten core includes a jet of the aluminum alloy impinging the solidifying interface at a bottom of the liquid molten core to form a depression in the solidifying interface.   
     
     
         20 . The embryonic ingot of  claim 19 , wherein the liquid molten core includes re-suspended grains from the solidifying interface. 
     
     
         21 . The embryonic ingot of  claim 19 , wherein the liquid molten core includes re-suspended hydrogen from the solidifying interface. 
     
     
         22 . The embryonic ingot of  claim 19 , wherein the solidified shell comprises a high-strength zone disposed between an outer surface of the solidified shell and a centerline extending in the casting direction through a center of the liquid molten core and a center of the solidified shell, wherein the high-strength zone has a higher concentration of dispersoids than a remainder of the solidified shell. 
     
     
         23 . The embryonic ingot of  claim 19 , wherein the aluminum alloy is a series 7xxx aluminum alloy.