Process, alloy enhancements focus of precision casting program.

With the continuing push from casting users for net shape parts and improved overall quality, the growth trend of investment cast components has shown a steady climb in recent years. Continued interest in the process has brought with it both process and alloy improvements. Several of these were described during this year's Casting Congress program.

The CLV process, developed for the casting of reactive metals, principally nickel base super alloys, was described by G.D. Chandley, Metal Casting Technology, Inc, and D.L. Cargill, Hitchiner Manufacturing (90-82). According to the authors, use of the process has grown and become the "sole source for many jet engine parts," because of its ability to cast very thin wall parts.

In the process, the metal is melted in a lower chamber and the mold is placed in an upper chamber. Air is evacuated from both chambers and they are flooded with argon. The open end of the mold is submerged in the metal and a vacuum is created in the upper chamber, drawing metal into the mold. The vacuum is held until the casting solidifies. The process allows for clean metal only to enter the mold in a way that minimizes turbulence, and allows for the filling of very thin mold sections.

Solidification modeling of single-crystal investment castings was another topic addressed in an investment casting session (90-53). This work was aimed at correlating the solidification conditions, microstructures and defects in single crystal castings using finite-elements thermal analysis.

Research showed that this method "can accurately model the casting process with relatively short computer times. And correlations of casting microstructures and defects with calculated solidification conditions indicate that finite-element modeling is a powerful tool for predicting microstructures and defects in single-crystal investment castings."

Two other papers described work being performed on alloys for investment casting. In studying ways to improve the ductility of BeCu C825000 alloy (90-41) without sacrificing part strength, A. Torok, T. Walsh, Sr., C. Hershberger and Y.V. Murty of Arwood Corp and NGK Metals Corp, focused on the use of compositional modifications and heat treatments. Their work showed that "by proper selection of heat treatment and chemistry, mechanical properties with ultimate tensile strength as high as 180 ksi and elongation as high as 14% can be achieved."

In other alloy work, B. Closset, Timminco Metals, and D. Fay, Uni-Cast Div/Strum, Ruger & Co, looked at strontium modification of aluminum investment castings (90-90). Their research showed that Sr levels of between 0.005-0.025% are adequate to achieve good modified structures even at relatively slow cooling rates. The positive results of modifying include increases in elongation and impact strengths that approached nearly 100%. The electrical conductivity of as-cast samples also improved by 5-10%. In addition, they demonstrated that thermal analysis is an effective method in controlling, nondestructively, the quality of A356.0 melts.