1 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China. 2 Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, China.
Cite this article:
Bingbing YU,Zhiyong CHEN,Zibo ZHAO,Jianrong LIU,Qingjiang WANG,Jinwei LI. MICROSTRUCTURE AND MECHANICAL PROPERTIES OF ELECTRON BEAM WELDMENT OF TITANIUM ALLOY TC17. Acta Metall Sin, 2016, 52(7): 831-841.
Most titanium alloys have been designed for aeronautical applications, where their excellent specific properties are fully employed and weldability is a classic problem with Ti and its alloys. Microstructure and mechanical properties of the electron beam weldments of TC17 alloy were investigated in this work. The results showed that there exhibited three zones across the TC17 electron beam weldment: the fusion zone (FZ), heat affected zone (HAZ) and base metal (BM). It was also observed that the as-welded FZ consisted of metastable β columnar grains, while the HAZ consisted of acicular α/α′ phase, equiaxed α phase and metastable β phase. Furthermore, it was indicated that the transformation from metastable β phase to α+β phase happened when the FZ and HAZ were post-weld heat treated at 630~800 ℃, the coarsening of α laths and the grain boundary α were also observed when the heat treatment temperature increased. The increasing of 450 ℃ ultimate tensile strength of FZ was ascribed to the precipitation of secondary acicular α platelets during tensile testing in the as-welded and 800 ℃ heat treated conditions, which led to the low yield ratio of FZ. The tensile failure location of the weldments was found to occur in preference in the low tensile yield strength area, or in the low hardness area when the difference between yield strength across the weldments is very small. It was concluded that the optimal post-weld heat treatment for the TC17 alloy weldment was 630 ℃, 2 h, A.C., at which the weldments showed good combination of tensile strength and elongation.
Fig.1 Microstructure of TC17 titanium alloy forging
Heat treatment
Process
S-PWHT1
630 ℃, 2 h, A.C.
S-PWHT2
700 ℃, 2 h, A.C.
S-PWHT3
800 ℃, 2 h, A.C.
D-PWHT2
700 ℃, 2 h, A.C.+630 ℃, 2 h, A.C.
D-PWHT3
800 ℃, 2 h, A.C.+630 ℃, 2 h, A.C.
Table 1 Post-weld heat treatments (PWHTs) of TC17 alloy weldments
Fig.2 Schematics of location of the fusion zone (FZ) in the tensile specimen (unit: mm, BM—base metal) (a) tensile specimen of weldment (b) tensile specimen of FZ
Fig.3 Macrostructure of electron beam welding (EBW) weldment of TC17 titanium alloy (HAZ—heat affected zone)
Fig.4 OM (a), SEM (b) and TEM (c) images of FZ in as-welded TC17 EBW weldment (Inset shows the SAED pattern in the square region of Fig.4c)
Fig.5 XRD spectrum of FZ of as welded TC17 titanium alloy EBW weldment
Fig.6 XRD spectra of FZ of TC17 EBW weldment after different PWHTs (a) S-PWHT1 (b) S-PWHT2 (c) S-PWHT3 (d) D-PWHT2 (e) D-PWHT3
Fig.7 Microstructures of FZ of TC17 EBW weldment after different PWHTs (Insets show high magnified images)
