Clean metal nucleated casting

Abstract

Nucleated casting is a method of casting metal below its liquidus temperature, casting as a semisolid rather than as a liquid. In nucleated casting, a stream of liquid metal is directed into a gas atomizer, which converts it into a spray and accelerates the spray droplets from the atomization zone toward a collection mold. Because of the high surface-to-volume ratio, high relative velocity, and a large temperature difference between the gas and droplets, the droplets lose heat very efficiently to the surrounding gas.

Droplet diameter and process parameters such as gas-to-metal flow ratio, process rate, and spray distance have significant impact on the solid fraction of the metal at the collection zone. Small particles lose all of their heat of fusion and are fully solid when they strike; they will likely remelt in the semi-solid pool. Large particles are fully liquid when they strike. Medium-size particles are partially solid. When the particles strike the surface of the pool, a large number of nucleation sites are available for further solidification. Each nucleation site grows until it strikes an adjacent site and all local liquid is consumed and a fine-grained metallurgical structure results. When operated at high gas-to-metal ratio the semisolid will have small (<30%) fraction liquid, and free-form casting is possible. This process is commonly called spray forming. When operated at low gas-to-metal ratio the semisolid will have a high (>75%) fraction liquid, and a mold is required to contain the metal as it continues to solidify. This process is termed nucleated casting.

Alloy 718 metal has been cast in a prototype nucleated casting system for demonstrating the feasibility of the new process. Metallographic examination shows that the as-cast material possesses a uniform, equiaxed 0.075 mm (ASTM 4.5) grain structure. Macrosegregation-related defects were not found in the casting or in forgings that were obtained from the ingots.