Micro-Casting of Titanium Alloys Using 3D-Printed Self-Boiling Molds

Northwestern University, Evanston IL

Investigators

Abstract

Because of their high specific strength and corrosion-resistance, titanium alloys are widely used in various applications such as high-value medical products, e.g., artificial joints and bones, implants, and surgical tools, etc. However, conventional machining and forging processes typically used in their manufacture are limited in mechanical properties and productivity. This award supports fundamental research of a novel casting method for titanium-alloyed parts and cast features ranging from microns to tens of millimeter by using a new type of functional molds, namely, self-boiling molds, produced by three-dimensional (3D) printing. The manufacturing innovation is accomplished through the mechanisms of boiling heat transfer that facilitate heat transfer control at all points of a mold wall to adjust the cooling rate, thus, manipulating the cast microstructures. The impact of this research on US industry will be a competitive technology for controlling key properties of cast titanium components, principally for medical applications, in conjunction with 3D printing functional molds. The spillover effects of this innovation will, as anticipated, have an impact on the casting industry as a whole. Equally important, the project will provide a multidisciplinary training platform in materials, casting and 3D printing for graduate and undergraduate students for next-generation workforce development. The overarching goal of this project is a fundamental understanding of the self-boiling mold methodology to actively control the temperature history of molten alloys at various locations of a cast part and, thereby, its microstructure and mechanical properties. The analytical foundation for predicting the relationship between the cooling rate of the molten metal influenced by the characteristics of a ceramic mold and boiling medium will be pursued and established along with methods for designing molds of the required cooling rates that assure the desired microstructure distribution. First, the characterization of different self-boiling mold topologies will be studied on a specially designed testbed to establish the relation between boiling modes and mold wall properties and construction, a key factor for determining the cooling performance of the mold wall. Second, the determination of the cooling rates for a given microstructure distribution will be addressed through the formulation of a numerical model based on the Cellular Automaton method. Specially designed sapphire molds will be used for model verification through the visualization of the nucleation process, molten metal flow and temperature measurements. Next, a model-based determination of the self-boiling mold’s properties for a designed cooling rate distribution will be devoted, verified by experiments using the testbed. At the end, self-boiling mold fabrications using 3D printing (based on direct ink writing), sintering and impregnation methods will be conducted followed by micro-casting performance evaluations, demonstrated on micro-laparoscopic forceps. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.