The Effect of Temporal Spacing on the Microstructure and \\Mechanical Properties of Titanium during Pulsed Laser-Powder Bed Fusion

Abstract

Additive manufacturing (AM) has established itself among the major technologies of the 21st century. Its ability to fabricate extremely complex near-net shape parts in short amounts of time stands out as one of its key features. Critical for full adoption of the technology is a complete understanding of the underlying phenomena governing melting and solidification processes. Therefore, researchers have sustained extensive efforts to build and modify AM machines for better access to the build platform. This work presents the successful development and fabrication of a low-cost Laser-Powder Bed Fusion test bed at Auburn University. The machine aims to provide a learning platform for investigation involving microstructural modification through varying exposure strategies, process monitoring, characterization, and machine development. Machine design utilizing an open architecture allows for full control of relevant laser parameters and process atmospheres.

To explore some of the essential limiting factors in L-PBF, a process parameter study was carried out to establish ideal processing conditions for commercially-pure titanium (CP-Ti) using a Concept Laser MLab. Fully dense samples were successfully fabricated and characterized using optical microscopy and X-ray CT. The fully dense materials were also characterized through polarized light microscopy and EBSD. Heat treatment was employed to remove the highly-textured microstructure and establish a strong, yet ductile bimodal microstructure consisting of part recrystallized $\alpha$-grains and part lamellae-alpha grains. Lastly, tensile testing was conducted to evaluate the strength of the AM samples and compare them to conventionally wrought samples.

Herein, the last chapters describe a new pathway for alteration of the microstructure of CP-Ti using low-duty-cycle pulsed laser exposure. Keyhole porosity was identified as a substantial issue with the processability of CP-Ti through pulse wave L-PBF. However, the obtained microstructure showed significant differences in the alpha' lath-size and the prior-beta grains. A peak shift of ca. 2% in the grain boundary axis misalignment was observed for continuous laser exposure, indicating the transformation of some of the variant grain boundaries towards twin boundaries. This could be reduced through the use of pulsed laser exposure. Furthermore, heat treatment of the pulsed CP-Ti significantly reduced the grain size and degree of recrystallization when compared to a sample with continuous laser exposure under the same energy input. Lastly, the severe keyhole porosity was reduced through the use of a Ramp-down pulse shape with the overall same heat introduction, allowing backfill of the keyhole with metal before solidification while retaining the unique microstructure that enabled the faster and more complete recrystallization.