In the investment casting of high melting point metals and alloys, silica bonded ceramic shell molds conventionally have been used to contain and shape the molten material Bulging and cracking of conventional silica bonded ceramic shell molds have been experienced in the investment casting of recently developed high melting point alloys at casting temperatures above 2700.degree. F. because of the low flexural strength and low creep resistance of such shell molds at the higher casting temperatures. When the ceramic shell mold bulges, the dimensions of the resultant casting are not accurate. Significant cracking can result in failure of the ceramic shell mold and runout of the molten material.
To achieve better performance than conventional silica bonded ceramic shell molds provide at higher casting temperatures, ceramic shell molds having an alumina, mullite, or other highly refractory oxide bond have been used. These bond materials normally are incorporated into the shell molds via slurries or suspensions of the ceramic material. Ceramic shell molds bonded with highly refractory oxides, however, suffer from one or more of the following disadvantages. The required ceramic slurries typically are difficult to control with respect to suspension stability, viscosity, and drainage. Further, the slurry coatings are difficult to dry and cure. These shell molds must be fired to a high temperature to achieve adequate sintering or chemical bonding. The shell molds also may be too strong during post-cast cooling, thereby inducing hot tears and/or recrystallization in the cast metal. In addition, such shell molds can be too strong and chemically inert at room temperature to be easily removed from the casting.
Attempts also have been made to strengthen conventional silica bonded ceramic shell molds by reinforcing with a ceramic bracing network. Other efforts to overcome the inadequate high temperature properties of conventional silica bonded ceramic shell molds have focused on redesigning the part to be cast or changing the manner in which it is cast. These methods, however, are expensive, labor intensive, and, in most instances, impractical.
Accordingly, it is an object of the invention to provide a ceramic shell mold having improved mechanical properties at high temperatures.
Another objective of the invention is to provide a ceramic shell mold which facilitates improved control of casting dimensions and which can be easily removed from the casting.
A further objective of the invention is to provide a method for making a ceramic shell mold having improved mechanical properties at high temperatures.
Additional objects and advantages will be set forth in part in the description which follows, and in part, will be obvious from the description or may be learned by practice of the invention.