Micro-Casting of Titanium Alloys in Self-boiling Molds

Overview: Titanium alloys because of their strength, being lightweight and corrosion-resistant are widely used for not only various industrial products but also medical parts including artificial joints, artificial bones, various implants and surgical tools. However, conventional machining and forging processes, typically used in their manufacture, are limited in their abilities to yield the necessary accuracy, mechanical properties and productivity. In this research, an innovative micro-casting method for micro-parts is proposed by using a new type of functional, namely, self-boiling mold, never used before, produced by 3D printing. The 3D printed topology and gradient structure of the mold provides for an unprecedented methodology for controlling the microstructure and mechanical properties of the cast products. This is accomplished through the mechanisms of boiling heat transfer that facilitate heat transfer control at all points of the mold wall and adjust the cooling rate (solidification speed) to the desired conditions after pouring the molten metal. By using such molds, it will be possible to cast micro-size cast products with controlled grain structure and mechanical characteristics constituting a capability that is impossible to achieve by machining, forging or other manufacturing processes. Intellectual Merit: The low thermal conductivity of titanium alloys in an issue in their casting, especially for parts with fine and complex shapes. In this work, an innovative concept of a self-boiling mold is proposed to actively control the temperature history of the molten alloy at various points of the cast part, which has been extremely difficult because of the alloy’s low thermal conductivity. To realize the proposed self-boiling mold methodology, the following unresolved questions will be addressed: (a) Formulation of predictive laws for casting solidification grain size/shape, phase transformation and for the mechanical properties of the cast parts; (b) Understanding and control of boiling mechanisms and cooling properties of molds composed of variable mold material mixtures; (c) Characterization of the self-boiling mold material constitution (particle density, particle size, composition of binder and boiling materials) and its relations to cooling performance; (d) Conception, design and implementation of high precision 3D multi-material printing methods for self-boiling molds; (e) Theoretical and experimental investigations of castability as a function of mold and heat transfer characteristics; (f) Evaluation of the effectiveness of the proposed casting method for medical product fabrication. The effects of solidification microstructure and phase transformation behavior on the mechanical properties of cast parts will be investigated and assessed from a metallurgical viewpoint. Broader Impacts: This project will deliver a newly conceived method and associated knowledge for micro-casting of titanium alloys with self-boiling molds fabricated by 3D printing. For this purpose, a high precision 3D mold printing system and mold design principles will be developed. From the scientific standpoint, fundamental contributions to physical mechanisms that relate grain-growth to cooling rates and to boiling mechanisms and thermal conduction theory, as the basis for instating more advanced and better-controlled casting processes, are anticipated. The proposed methodology, founded in the newly formulated models and methods, will find general applicability in casting other materials with a wide range of thermal conductivities that require preci