Doctoral Dissertations
Date of Award
12-2024
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Mechanical Engineering
Major Professor
Tony L. Schmitz
Committee Members
Sudarsanam S. Babu, Bradley H. Jared, Eric A. Lass
Abstract
Closed-loop temperature-controlled AFSD of Ti-6Al-4V was conducted at a series of temperature setpoints including 700 ºC, 750 ºC, 800 ºC, and 850 ºC. Oxygen pickup in the bulk material did not exceed ASTM standards for the material. The resulting deposits were sectioned and characterized. Based on the temperature setpoint, it was found that the resulting microstructure morphologies in the stir zone varied from equiaxed, as in the 700 ºC temperature setpoint, to lamellar, as in the 850 ºC temperature setpoint. The intermediate 750 ºC and 800 ºC temperature setpoints exhibited mixed microstructures with a mix of both equiaxed and lamellar characteristics. It was also found that microstructure morphology and microhardness varied as a function of the build height for all deposition conditions. These variations were attributable to the processing conditions with the order of the variation linked to both process stoppages for material reloads as well as the individual layers during deposition.
Subsize tensile bars were extracted from the deposits in the weld, transverse, and normal directions before being quasi-statically tensile tested. Despite drastic differences in microstructure morphology, all tensile samples exhibited similar mechanical properties with only slight differences among the temperature setpoint conditions and orientations. An additional set of normal direction tensile bars was polished and etched to investigate the effects of microstructure morphology on resulting mechanical properties. It was found that samples from all temperature setpoint conditions exhibited multiple-yielding behavior with initial yielding observed in lamellar regions, though failure was sometimes observed in equiaxed regions. Tungsten contamination was observed in some tensile samples that exhibited lowered ductility, especially in samples near the periphery of the deposits. This shows the potential for poor bonding in the flash regions of the deposit.
Recommended Citation
Heikkenen, Eric Michael, "Process-Structure-Property Relationships in Closed-Loop Temperature-Controlled Additive Friction Stir Deposition of Ti-6Al-4V. " PhD diss., University of Tennessee, 2024.
https://trace.tennessee.edu/utk_graddiss/11359