13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of Post-Weld Heat Treatment on the Fatigue Properties of Dissimilar Titanium Alloy Joints S.Q. Wang1, 2, J.H. Liu2, D.L. Chen1,* 1Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, Ontario M5B 2K3, Canada 2State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China * Corresponding author: dchen@ryerson.ca Abstract The purpose of this study was to characterize the microstructural change and fatigue resistance of electron beam welded (EBWed) dissimilar joints between Ti-6Al-4V and BT9 (Ti-6.5Al-3.5Mo-1.5Zr) alloys after two types of post-weld heat treatment (PWHT), namely only aging and solution treatment followed by aging (STA). It was observed that no significant microstructural change occurred in the FZ after only aging and the high hardness in the fusion zone (FZ) remained, while coarse acicular α occurred in the FZ in the STA condition, leading to a reduced hardness. Both types of post-welded joints were cyclically stable at lower strain amplitudes of 0.2-0.6%, and exhibited cyclic softening at higher strain amplitudes of 0.8%-1.2%. As the total strain amplitude increased, cyclic stress amplitude increased, while the fatigue life decreased. Fatigue crack initiation occurred from the specimen surface or near-surface defects and propagation was characterized mainly by the formation of characteristic fatigue striations. Keywords Titanium alloy, Post-weld heat treatment, Microstructure evolution, Strain-controlled fatigue. 1. Introduction Welding is one of the most widely used and economical joining processes for titanium alloys, which makes it possible to create composite structures in the welded joints for various purposes. However, in the electron beam welding (EBW) and laser beam welding, the fusion zones are known to exhibit poor ductility due to the large prior β grain size and a wholly or partially martensitic microstructure [1]. The low thermal conductivity of titanium alloys causes superheating in the heat-affected zone (HAZ), increases the grain size and gives rise to residual stresses in the HAZ, exerting a negative effect [2]. It is necessary to adjust unstable microstructure of the as-welded joints [3]. The post-weld heat treatment (PWHT) is one of the commonly-used methods which could stabilize microstructure, decrease the inhomogeneity of the structure and increase the mechanical properties of the welded joint. Arenas and Acoff [4] reported that PWHT promotes the formation of γ phase in the fusion zone of a gas tungsten arc (GTA) welded gamma titanium aluminide alloy. Guo et al. [5] observed that there was no significant hardness increase in fusion zone (FZ) at Ti-64 side of a dissimilar linear friction weld between Ti-64 and Ti-6246 alloys after PWHT. Tuppen et al. [6] performed stress-controlled fatigue tests to determine S-N curves of diffusion-bonded dissimilar Ti-6Al-4V/Ti-4Al-4Mo-2Sn-0.5Si titanium alloy joints. Fu et al. [7] studied the effects of electron beam local heat treatment (EBLHT) on strain-controlled fatigue properties of EBWed Ti-6Al-4V alloy joints, and their results showed that Ti-6Al-4V alloy joints with and without EBLHT exhibited cyclic softening and the fatigue life of the joints after EBLHT could increase by 30%. Despite the extensive studies on the weldability of titanium alloys, it is unknown if the PWHT would have a significant effect on the microstructure and cyclic deformation behavior of electron beam welds between Ti-6Al-4V and BT9 (Ti-6.5Al-3.5Mo-1.5Zr) alloy. The aim of this study was, therefore, to identify the effect of two common PWHT processes, namely aging with or without solution heat treatment, on the microstructure and fatigue performance of welded joints. 2. Material and Experimental Procedure The materials used in the present study were forged 10 mm thick Ti-6Al-4V and BT9 titanium
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