Date

8-1-2018

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Misiolek, Wojciech Z.

Abstract

Titanium alloys are widely used in the aerospace industry. For example, most components of the Boeing 777 landing gear are made of the Ti-10V-2Fe-3Al (Ti1023) alloy due to its lighweight and superior mechanical strength. In addition, because Titanium is chemicaly active element, when jointing the Ti alloy components, the laser beam welding (LBW) has been selected for avoiding the eventual contaminations and narrowing the heat affected zone by the presence of its protected atmosphere and low heat input. However, there is a lack of reported research results about welded Ti1023 alloy, particularly laser beam welding. Therefore, the microstructure and mechanical properties of LBW Ti1023 were first investigated in this study.Three zones were formed on the LBW Ti1023: base material (BM), heat affected zone (HAZ), and fusion zone (FZ). The bimodal (spherical and lath) distribution of primary α phase dispersed in a matrix of β phase was observed on the BM and HAZ. However, only β phase was observed in the fusion zone (FZ). This whole β phase structure was formed due to two steps: melting and retaining. The melting temperature was higher than β-transus temperature and transformed the α phase into β phase. Then, fast cooling can avoid α martensite formed and retain the β phase. In addition to the microstructure, the experimental results from the hardness and tensile tests showed lower properties in the FZ. Moreover, the residual stress of BM and FZ were measured separately in this study.During analysis of the entire manufacturing process, post welding heat treatment (PWHT) was applied to improve mechanical properties of the welded components. Two subgroups of the heat treatment conditions were set and investigated. The experiment results of both subgroups presented a different extent of strength improvement. The heat treatment condition of one subgroup is annealing+aging. The hardness of all three zones (base material, heat affacted zone, and fusion zone) in this subgroup increased and are almost equal to each other due to their structure consisted of primary α (αp) and secondary α (αs) phase. From the hardness profile and tensile test results, the optimization heat treatment condition of this subgroup is chosen as 750oC annealing for 1 hour and following by water quenching, then through 500oC aging for 4 hours and following by air cooling. Another subgroup heat treatment condition consists only of aging. The FZ in this subgroup showed the highest hardness because only αs was observed. Also, the optimization heat treatment condition is chosen as 500oC aging for 4 hours and following by air cooling.

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