Unidirectionally-solidification process and castings formed thereby

Abstract

A process capable of producing large metallic castings having lengths of one hundred centimeters or more and a unidirectional crystal structure substantially free of freckle defects. The process includes pouring a molten metal alloy into a preheated mold within a heating zone, withdrawing the mold from the heating zone, through a heat shield, and into a cooling zone to directionally solidify the molten metal alloy, and then cooling the mold to produce the casting and the unidirectional crystal structure thereof. The heat shield operates as a barrier to thermal radiation between the heating zone and the cooling zone, and the mold is withdrawn at a rate that, in combination with the heat shield, maintains a thermal gradient to solidify the molten metal alloy and form primary dendrite arms having an average spacing therebetween of about 150 to about 500 micrometers.

1 . A process of producing a metallic casting having a length greater than one hundred centimeters and a unidirectional crystal structure that is substantially free of freckle defects, the process comprising:
 pouring a molten metal alloy into a cavity in a preheated mold located within a heating zone, the cavity having the shape of the casting;   withdrawing the mold from the heating zone, through a heat shield, and into a cooling zone to directionally solidify the molten metal alloy, the heat shield operating as a barrier to thermal radiation between the heating zone and the cooling zone, the mold being withdrawn at a rate that, in combination with the heat shield, maintains a thermal gradient of greater than 50° C./cm in the molten metal alloy to solidify the molten metal alloy and form primary dendrite arms having an average spacing therebetween of about 150 micrometers to about 500 micrometers; and then   cooling the mold to produce the casting and the unidirectional crystal structure thereof that is substantially free of freckle defects having a size greater than one hundred centimeters.   
     
     
         2 . The process according to  claim 1 , wherein the casting has a mass of at least about 18 kg. 
     
     
         3 . The process according to  claim 1 , wherein the thermal gradient is greater than 80° C./cm. 
     
     
         4 . The process according to  claim 1 , wherein the withdrawal rate is greater than 1.25 mm/minute. 
     
     
         5 . The process according to  claim 1 , wherein in combination the thermal gradient and the withdrawal rate result in a cooling rate of at least 20° C./minute. 
     
     
         6 . The process according to  claim 1 , wherein the average spacing between the primary dendrite arms is about 325 micrometers up to about 450 micrometers. 
     
     
         7 . The process according to  claim 1 , wherein the casting is characterized by a ratio of the average spacing of the primary dendrite arms to the length of the casting of about 0.75 to about 5.0 micrometers per centimeter. 
     
     
         8 . The process according to  claim 1 , wherein the unidirectional crystal structure has a columnar single crystal microstructure. 
     
     
         9 . The process according to  claim 1 , wherein the unidirectional crystal structure has a columnar polycrystalline microstructure. 
     
     
         10 . The process according to  claim 1 , wherein the metal alloy is chosen from the group consisting of nickel-base superalloys and intermetallic alloys. 
     
     
         11 . The process according to  claim 1 , wherein the casting is a component for a gas turbine. 
     
     
         12 . The process according to  claim 11 , wherein the component is a last-stage bucket of a land-based gas turbine. 
     
     
         13 . The casting produced according to the process of  claim 1 . 
     
     
         14 . The casting according to  claim 13 , wherein the unidirectional crystal structure has a columnar single crystal microstructure. 
     
     
         15 . The casting according to  claim 13 , wherein the unidirectional crystal structure has a columnar polycrystalline microstructure. 
     
     
         16 . The casting according to  claim 13 , wherein the metal alloy is chosen from the group consisting of nickel-base superalloys and intermetallic alloys. 
     
     
         17 . The casting according to  claim 13 , wherein the average spacing between the primary dendrite arms is about 325 micrometers to about 450 micrometers. 
     
     
         18 . The casting according to  claim 13 , wherein the casting is characterized by a ratio of the average spacing of the primary dendrite arms to the length of the casting of about 2.25 to about 3.25 micrometers per centimeter. 
     
     
         19 . The casting according to  claim 13 , wherein the casting is a component for a gas turbine with a mass of at least about 18 kg. 
     
     
         20 . The casting according to  claim 19 , wherein the component is a last-stage bucket of a land-based gas turbine.