Sathyanarayan Sairam Jaishankar
Advisor: Prof. Donggang Yao

will defend a doctoral thesis entitled,

Generating Fine Thixotropic Microstructures Using Rapid Instability Mechanisms For Shaping Semi-Solid Alloys

On

Monday, July 7th at 10:00 a.m.
MRDC 3515
(Zoom meeting link, Passcode: 252753)

Committee:

Prof. Donggang Yao – School of Materials Science and Engineering
Prof. Preet Singh – School of Materials Science and Engineering
Prof. Meisha Shofner – School of Materials Science and Engineering
Prof. Josh Kacher – School of Materials Science and Engineering
Prof. Gongyao Zhou – Department of Mechanical Engineering and Mechanics, Drexel University

Summary:

Molten metal shaping operations are limited to low-viscosity processing techniques such as casting. Meanwhile, molten thermoplastic polymers are compatible with a wide range of processing techniques owing to their tunable viscosity (by modulating shear rate, molecular weight, filler content, etc.). Therefore, semi-solid metal processing has gained traction to overcome the limitations of conventional metal processing. A thixotropic (globular) microstructure is desired to process semi-solid materials since it imparts a solid-like behavior to the material at rest but yields to flow at higher applied shear. A small globular grain size in this microstructure is essential to promote unhindered material flow through small openings and minimize liquid segregation.

In this thesis work, we addressed specific research gaps on rapidly generating small globular grains in the semi-solid state. We showed that the globularization mechanism during heating is strongly influenced by the initial microstructure, with cast materials undergoing phase coarsening while materials deformed through compression globularized through a gradual fragmentation from the outside in. Contrastingly, materials with oriented phases rapidly globularized through a novel Rayleigh instability mechanism and produced smaller globules. We devised a set of fundamental criteria to screen alloy compositions that may globularize through this instability mechanism. Particularly, alloys that retained a two-phase microstructure from room temperature to semi-solid temperature were desired.

We also showed that the globularization kinetics of the Rayleigh instability mechanism are influenced by the rate of transient temperature rise and the internal phase widths of the microstructure. The instability mechanism can be accelerated by reducing the phase widths, enhancing the heat transfer rate, increasing temperature differences, or reducing the material form. The ex-situ tests were conducted using a molten salt bath to facilitate rapid heat transfer to the material. Noteworthily, a combination of high heat transfer rate, reduced internal phase widths, and smaller material form allowed us to successfully generate a fine thixotropic microstructure with an average globular grain size of ~5 µm. Such small globular grains can improve the resolution of metal Fused Deposition Modeling (FDM) by enabling smaller nozzles, critical for precision components.

Our in-situ tests inside a custom-built semi-solid metal extrusion system highlighted the differences in microstructural evolution between ex-situ and in-situ conditions. We showed that a material may suffer from incomplete globular evolution and liquid segregation when heat transfer issues exist. Such issues may arise due to a large thermal mass of material or poor contact with the heated inner wall of the processing equipment. To ensure uniform heating and achieve the desired thixotropic microstructure, emphasis must be placed on minimizing temperature gradients by reducing the thermal mass of the material. Overall, our research contributed to the understanding of the structure-processing relationship in materials and expanded the scope of semi-solid metal processing toward high-viscosity techniques.