Superalloys, ceramic shells examined.

The use of titanium and nickel alloys, and research centering on the production of ceramic shells highlighted the technical program offered by the AFS Investment Casting Div. this year. A newly developed method for directly producing ceramic shells for investment casting was the topic of Y. Uziel's presentation. Uziel's company, Soligen, Inc., helped develop the process called Direct Shell Production Casting (DSPC). With DSPC, according to Uziel, "ceramic shells are automatically fabricated directly from a CAD file of the desired part. The process produces a ceramic shell similar to the shells produced by investment casting, yet eliminates the need for tooling, wax and shell dipping."

Actual production of the shell involves spreading a thin layer of fine alumina powder over a piston. A printhead then moves over the layer, shooting tiny drops of colloidal silica onto the powder surface in a pattern. The piston is then lowered within a powder bin, making room for the next layer. The cycle is repeated until all layers have been printed and the entire shell is formed. The excess powder is then removed to yield a finished shell. According to Uziel, the process allows for fast design changes, eliminating the process of maintaining patterns or dies.

Investment casting of superalloys continues to draw much attention. D. Chong, McDonnell Douglas Government Aerospace--East, reports that the growing interest is because "casting has demonstrated to be a cost-effective means of manufacturing aerospace parts compared to other fabrication processes such as machining and forging." And she adds that "Despite the advances that have been made in titanium foundry technology, there is a reluctance to eliminate the casting factor because of the history of property variability in aluminum castings."

Chong discussed the goals of a Titanium Casting Task Force which designed a standard practice that was tightly controlled by a specification for titanium to reduce the variability in mechanical properties. "Only when the variability was reduced and meaningful allowable established, could reduction or elimination of the casting factor be considered."

She concluded that the program was able to successfully reduce the variability by applying a more restrictive chemical composition on the Ti parts, through the use of post-casting treatments and by writing a new AMS specification to provide better process control.

The simulation modeling of nickel-base superalloys was the subject of a paper by H. Wang and E. Perry, GE Corporate Research and Development in conjunction with J. Zou and R. Doherty of Drexel Univ. In their discussion they described a new modeling approach termed "multicomponent micro-enthalpy" for modeling the micro aspects in Ni-base superalloys which, they report, has the potential for predicting microsegregation.

According to the authors, "In this approach, a unique solution procedure for acquiring additional information, such as the temperature increments due to each solutal concentration change and the internal fraction of solid at the element level at each time step, has been developed through the use of the multicomponent growth model." To verify their results, the researchers compared experimental results and model predictions.