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November 17th, 2011

Solid freeform fabrication of one-piece investment casting molds for airfoils

Published on November 17th, 2011 | Edited by: Eileen De Guire

 

Integrally cored ceramic mold fabricated by ceramic stereolithography. The vertical cross-section on the left shows the core and shell parts. Five horizontal cross-sections of individual layers in the build-up are shown on the right. The mold is for investment casting of superalloy airfoils for gas turbine engines. Credit: Bae and Halloran; ACT.

Much of the research in materials science journals focuses on nanostuff, atomic resolution characterization and other gee-whiz science, so it’s refreshing to come across an article offering a new approach to making bread-and-butter ceramic components.

The theme of the November/December issue of the International Journal of Applied Ceramic Technology is “Ceramics Manufacturing Technologies,” and the lead article is about fabrication of investment casting molds by stereolithography techniques.

In particular, the University of Michigan research group that wrote the article is interested in making one-piece molds for casting of superalloy airfoils for gas turbine engines. Airfoils have intricate, hollow, interior cooling passages that include a fair amount of fine structure.

There are two parts to an investment casting mold—the core and the shell. The core mold—the piece that will form the part—is set into the shell mold. The two molds are made separately by very different processes, which requires separate tooling and production lines, before they are assembled into the final mold.

In the paper, Bae and Halloran report on a single-step process for fabricating an integrated core-shell mold of fused silica in a single patternless construction. Their process is an adaptation of stereolithography, a solid freeform fabrication technique that builds up three dimensional objects layer-by-layer. Conventional SLA uses lasers to construct polymeric parts based on CAD programs. This paper describes the adaptation, called CerSLA, as “a repeated layered manufacturing process, where thin liquid layers of ceramic-monomer suspension are solidified by photopolymerization with a UV laser, thereby “writing” the design for each slice.”

The integrated mold is built on a platform that drops in 100 µm increments through a suspension of acrylate monomers, fused silica powders, dispersant and photoinitiator. The laser scans the surface in a prescribed pattern and solidifies each layer in turn. The regions of the suspension that were subject to the laser are cured, but still green, and composed of fused silica and polyacrylate binder. Each layer takes about five seconds to write, with most of the time going to recoating the cured part with new suspension for the next layer. After all layers are written on, the mold is rinsed free of residual suspension, the binder is burned out and the part is sintered.

The choice of processing parameters is critically important. Any lack of integrity between the layers can cause the mold to crack or delaminate during sintering. Consistency and accuracy are also matter. For example, the outer shell mold walls need to be a constant thickness to ensure proper strength and thermal properties to avoid failure during metal casting. To become a viable option, CerSLA must also meet design tolerance specifications for investment casting molds, which are in the range of ±0.15-0.25 mm/25 mm.

The researchers built a 104 mm tall integrated core-shell mold in 1,047 layers. They successfully fabricated fine features of the airfoil core mold, however the finest features of the trailing edges were lost. It’s possible that they were not fully formed, or they may have broken off during the rinsing step.

The horizontal and vertical dimensions of the green mold were within 1 percent of the design specification, and shrinkage on sintering was about 10 percent in both directions.


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