Doctoral Dissertations

Date of Award

5-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Hahn Choo

Committee Members

Yanfei Gao, Zhili Feng, Claudia J. Rawn

Abstract

Friction Stir Spot Welding (FSSW) is a newly developed solid-state joining technique with considerable merits over conventional spot-welding techniques, such as relatively simple procedure and excellent welding properties. It has been successfully implemented for the joining of light-weight structural materials, such as Al- and Mg-based alloys, with superior weldability and reduction of the manufacturing costs and energy consumption. In addition, by removing the pin from the friction stir spot welding tool, the pinless FSSW (p-FSSW) has minimized the formation of welding defects such as keyhole and hooking, which resulted in further improvements in the mechanical properties of weldments. However, the application of p-FSSW to high-temperature structural alloys has not been as successfully demonstrated to date. In particular, difficulties related to the selection of welding tool materials that can be used for the high-strength and refractory Ti-based alloys have been a major challenge. Also, achieving strong metallurgical bonding has been challenging due to the relatively low thermal conductivity and refractory nature of the titanium alloys.

In Ti-6Al-4V alloy, the basic understanding of the relationships among the welding variables, processing temperature distribution during a spot welding, its influence on metallurgical bonding quality, and the resulting joint integrity and strength are particularly important issue. Moreover, establishing the correlation between the effect of p-FSSW on the microstructure development in the weld and its influence on mechanical performance is critical. Due to the microstructural evolution complicated by phase transformations between BCC ( ) and HCP ( ) occurring during/after the processing of Ti-6Al-4V alloy, the microstructure of the base material is known to significantly altered, creating various characteristic processing zones including the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ). Therefore, a fundamental study correlating the thermo-mechanical processes to the ensuing phase transformation and microstructure development is of primary importance.

To this end, in this work, a basic processing – microstructure – performance relationship study has been conducted on a novel p-FSSW processing of Ti-6Al-4V alloy. The main goal is to help facilitate further development of p-FSSW technique and its broader applications in aerospace structural components. The key research outcomes include:

(1) The welding parameters of the tool rotation and plunge speed were optimized. The measured plate interface bond length showed a good agreement with a theoretical calculation. Also, the mechanical properties of the lap-jointed Ti-6Al-4V plates were characterized and the results showed an excellent bond strength and a failure mode of base-material tearing.

(2) The characterization of weld nugget was performed for the development of texture and microstructural as a function of the thermo-mechanical input distribution during the processing.

(3) The bond mechanism of p-FSSWed Ti-6Al-4V alloy was identified as the diffusional bond with limited plastic deformation. The bonding sequence could be as follows: (i) contacting of the asperity, (ii) interfacial boundary bonding by plastic deformation, and (iii) grain boundary migration and void shrinkage.

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