Georgia Institute of Technology Materials Science and Engineering

Committee Members:   

Prof. Naresh Thadhani, Advisor, MSE/ME 

Prof. Joshua Kacher, Advisor, MSE 

Prof. David McDowell, ME/MSE 

Prof. Christopher Saldana, ME 

Prof. Hamid Garmestani, MSE 

Abstract: 

Additive Manufacturing (AM) via 3D printing offers the ability to tailor materials with microstructures along various length scales so that properties may be optimized for specific use cases. Though methods for producing metallic parts which are fully dense and homogenous on the macroscale have been largely resolved, AM metals fabricated through laser powder bed fabrication (LPBF) possess highly heterogeneous hierarchical microstructures which affect their mechanical properties and failure response. These microstructures result from a complex thermal processing history and can be influenced by anything from laser settings to platform heat or gas flow rate and gas type. While the quasistatic mechanical behavior of AM stainless steels has been extensively studied, the effects of these microstructural heterogeneities have not been well characterized in a dynamic loading environment, and thus, the failure mechanisms of AM materials under these conditions are poorly understood.

The first part of this dissertation investigates the role of local microstructure in Stainless Steel 316L (SS316L), through both the intentional control of fabrication process and as a byproduct of processing, in determining the spall behavior of AM materials and seeks to understand both spatially and temporally the heterogeneous failure modes which may be present. The second part explores the role of processing on the microstructure of direct ink write (DIW) fabricated metals (alloys) from their oxide components and seeks to understand the significance of rheological and thermodynamic factors which drive the process of successful printing, reduction, and sintering in alloyed and single element metals.

The spall properties for LPBF SS316L were measured in both the in-plane (IP) and through-thickness (TT) build directions for a fully dense as-built part. Additionally, the effects of mesoscale porosity on spall were measured in the IP direction. When random and intentional porosity was added throughout the LPBF SS316L material, the spall failure modes displayed local heterogeneities where observed damage depended on the amount of porosity as well as the distance from the pores. Nano-CT scans of select impacted samples reveal local strain accommodation through pore damage and solidification of SS316L powder that dampens or even locally eliminates the spall response. The overall results show that porosity plays a critical role in slowing the shock wave propagation, effectively shifting the spall plane towards the rear free surface, and in some cases eliminating it entirely.

Ferrous materials including elemental iron, SS316L, and the Cantor alloy were fabricated from their oxide constituents and 3D printed using DIW. The volume of particles in solution was optimized through the addition of a dispersant and the use of bimodal particle distributions. Reduction pathways which take advantage of the highly negative Gibbs Free Energy of mixing allow for reduction of Mn, and Cr oxides in both the Cantor alloy and SS316 to create alloys from stable oxides which would normally not be suitable for reduction. Alloys manufactured using DIW have an isotropic grain orientation and were fabricated with greater than 90% density. Demonstrating the capabilities of DIW as a solid-state processing test bed as well as a potential low-cost metal AM technique in addition to improving certain solid-state processing short falls, including minimizing the development of a core-shell microstructure.