Casting methods for making articles having a fine equiaxed grain structure

Abstract

Methods for casting a metallic material to form a component are described. The component can be a superalloy-containing turbine part, for example. The general method includes the step of pouring the metallic material, in molten form, into an investment mold; and then rapidly immersing the entire investment mold into a bath that contains a low-melting liquid coolant metal, so as to achieve substantially uniform, multi-directional heat transfer out of the molten material. The molten material that solidifies to form the component is characterized by a fine-grained, equiaxed grain structure. Related embodiments include the use of two ingots that constitute the superalloy material. One ingot includes the oxygen-reactive elements, and is prepared by a vacuum-melting technique. The other ingot includes the remainder of the elements, and can be prepared by a number of techniques, such as air-melting processes.

What is claimed: 
     
         1 . A method of casting a metallic material to form a component, comprising the following steps:
 (a) pouring the metallic material, in molten form, into an investment mold; and   (b) rapidly immersing the entire investment mold into a bath comprising a low-melting liquid coolant metal, so as to achieve substantially uniform, multi-directional heat transfer out of the molten material, thereby solidifying the molten material to form the component, and providing a fine-grained, equiaxed grain structure thereto.   
     
     
         2 . The method of  claim 1 , wherein the investment mold is pre-heated, in a vacuum or an inert atmosphere. 
     
     
         3 . The method of  claim 1 , wherein the investment mold comprises an interior surface that includes a nucleating agent for enhancing the formation of the equiaxed grain structure. 
     
     
         4 . The method of  claim 3 , wherein the nucleating agent comprises at least one cobalt-containing oxide. 
     
     
         5 . The method of  claim 1 , wherein the bath has a mass that is at least  4  times the total mass of the mold and the cast metal. 
     
     
         6 . The method of  claim 1 , wherein the rate of immersion is defined by a withdrawal rate of at least about 380 cm (150 inches) per hour. 
     
     
         7 . The method of  claim 1 , wherein the metallic material is at a temperature of about 50° C. to about 100° C. above its melting point, while being poured into the mold. 
     
     
         8 . The method of  claim 1 , wherein the liquid coolant metal is at a temperature of about 700° C. to about 1400° C. below the melting point of the metallic material, while the metallic material is poured into the mold. 
     
     
         9 . The method of  claim 1 , wherein the molten metallic material being poured into the mold includes dispersed solid particles of the metallic material; and the solid particles comprise less than about 2% of the total weight of the metallic material. 
     
     
         10 . The method of  claim 9 , wherein the solid particles within the molten material are obtained by subjecting the molten material to at least one solidification-melting cycle, prior to pouring the molten material into the mold. 
     
     
         11 . The method of  claim 1 , wherein multiple investment molds are immersed in the bath to form multiple components; and the investment molds are arranged in the bath to provide maximum, multi-directional heat transfer out of the molten material. 
     
     
         12 . The method of  claim 11 , wherein the investment molds are arranged in a general star-shape, relative to each other, along the longest access of each mold. 
     
     
         13 . The method of  claim 1 , wherein the metallic material comprises a superalloy based on nickel, cobalt, iron, or combinations thereof. 
     
     
         14 . The method of  claim 13 , wherein the component is a turbine engine part. 
     
     
         15 . The method of  claim 1 , wherein the metallic material for the component comprises a group of elements generally unreactive with oxygen; and also comprises at least one oxygen-reactive element. 
     
     
         16 . The method of  claim 15 , wherein the metallic material to be cast is produced by
 preparing a first ingot by a vacuum-melting technique, wherein the first ingot comprises all of the oxygen-reactive elements and at least one base element selected from nickel, cobalt, or iron;   preparing a second ingot by either an air-melting technique, an inert gas technique, or a vacuum-melting technique, wherein the second ingot comprises all of the generally unreactive elements;   attaching the two ingots together, or placing the two ingots together, to form a casting charge; and   melting the charge and pouring the molten material into the investment mold.   
     
     
         17 . The method of  claim 16 , wherein the vacuum-melting technique is selected from the group consisting of vacuum induction melting; vacuum arc re-melting; and non-consumable arc melting;
 and the air-melting technique is selected from the group consisting of air casting and argon oxygen decarburization.   
     
     
         18 . The method of  claim 16 ; wherein the first ingot comprises at least one of nickel, cobalt; and iron; and at least one of aluminum, titanium, zirconium, hafnium, and the rare earth metals. 
     
     
         19 . A method of casting a nickel-based superalloy to form a turbine engine component, comprising the steps of:
 (i) preparing a first ingot by a vacuum-melting technique, wherein the first ingot comprises nickel and all elements in the superalloy that are oxygen-reactive; and
 preparing a second ingot by either an air-melting technique, an inert gas technique, or a vacuum-melting technique, wherein the second ingot comprises the superalloy elements that are generally non-reactive with oxygen; 
   (ii) attaching the two ingots together, or placing the two ingots together, to form a casting charge; and melting the charge and pouring the molten material into an investment mold; and   (iii) rapidly immersing the entire investment mold into a bath comprising a low-melting liquid coolant metal, so as to achieve substantially uniform, multi-directional heat transfer out of the molten material, thereby solidifying the molten material to form the component, and providing a fine-grained, equiaxed grain structure thereto.   
     
     
         20 . The method of  claim 19 , wherein the investment mold comprises an interior surface that includes a nucleating agent for enhancing the formation of the equiaxed grain structure.