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Acta Metall Sin  2016, Vol. 52 Issue (7): 831-841    DOI: 10.11900/0412.1961.2015.00602
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Bingbing YU1,Zhiyong CHEN1(),Zibo ZHAO1,Jianrong LIU1,Qingjiang WANG1,Jinwei LI2
1 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China.
2 Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, China.
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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.

Key words:  TC17 titanium alloy      electron beam welding      microstructure      mechanical property     
Received:  20 November 2015     

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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)

(a) S-PWHT1 (b) S-PWHT2 (c) S-PWHT3 (d) D-PWHT2 (e) D-PWHT3

Fig.8  SEM images of HAZ in as-welded TC17 EBW weldment (Insets show high magnified images)

(a) far-HAZ (b) middle-HAZ (c) near-HAZ

Fig.9  TEM image and SAED pattern (inset) of near-HAZ in EBW weldment of TC17 alloy
Fig.10  SEM images of near-HAZ in TC17 EBW weldment after different PWHTs (Insets show high magnified images)

(a) S-PWHT1 (b) S-PWHT2 (c) S-PWHT3 (d) D-PWHT2 (e) D-PWHT3

Fig.11  Microhardness across the TC17 EBW weldments of as-welded condition and after different PWHTs

(a) as-welded (b) S-PWHT1 (c) S-PWHT2 (d) S-PWHT3 (e) D-PWHT2 (f) D-PWHT3

Fig.12  Average microhardness of FZ and BM in TC17 EBW weldment of as-welded condition and after different PWHTs
Fig.13  Pseudo-binary section through a β isomorphous phase diagram (schematically)[2] (Ms—martenite starting transformation temperature)
Fig.14  Low (a) and locally high (b) maginified fractographs of room temperature tensile specimens of FZ in TC17 EBW weldments after S-PWHT3
Fig.15  TEM image of as welded FZ after 450 ℃ tensile testing
Tensile temperature Heat treatment σ0.2 / MPa σb / MPa δ Failure location
Room temperature As-welded 840 996 6.0 FZ
S-PWHT1 1133 1188 9.8 BM
S-PWHT2 1017 1085 14.2 BM
886 904 16.3 BM
1041 1103 12.8 BM
D-PWHT3 1088 1159 4.8 FZ
450 ℃ As-welded 773 918 16.0 BM
S-PWHT1 770 903 14.0 BM
S-PWHT2 740 870 14.8 BM
S-PWHT3* 640 965 12.5 FZ
640 1000 20.0 BM
D-PWHT2 703 825 16.8 BM
D-PWHT3 763 893 8.3 FZ
Table 2  Room temperature and 450 ℃ tensile properties of TC17 titanium alloy EBW weldments after different PWHTs
Tensile temperature Heat treatment σ0.2 / MPa σb / MPa δ σ0.2/σb
Room temperature

As-welded 792 878 11.0 0.90
S-PWHT1 1341 1368 2.5 0.98
S-PWHT2 1054 1084 6.3 0.97
S-PWHT3 867 878 7.7 0.98
D-PWHT2 1073 1108 6.0 0.97
D-PWHT3 1034 1089 7.3 0.95
450 ℃ As-welded 890 1280 4.0 0.70
S-PWHT1 927 1040 8.8 0.89
S-PWHT2 780 858 6.5 0.91
S-PWHT3 658 900 9.7 0.73
D-PWHT2 810 907 7.3 0.89
D-PWHT3 763 882 10.0 0.87
Table 3  Room temperature and 450 ℃ tensile properties of FZ in TC17 EBW weldments after different PWHTs
Tensile temperature Heat treatment σ0.2 / MPa σb / MPa δ σ0.2/σb
Room temperature

As received 1165 1220 10.3 0.95
S-PWHT1 1086 1120 10.5 0.97
S-PWHT2 1004 1034 15.3 0.97
S-PWHT3 944 971 16.0 0.97
D-PWHT2 1030 1058 12.5 0.97
D-PWHT3 1217 1273 10.0 0.96
450 ℃

As received 785 905 18.3 0.87
S-PWHT1 763 870 19.0 0.88
S-PWHT2 695 785 20.8 0.89
S-PWHT3 725 1045 22.0 0.69
D-PWHT2 713 813 17.5 0.88
D-PWHT3 850 975 19.0 0.87
Table 4  Room temperature and 450 oC tensile properties of BM after different heat treatments (mass fraction / %)
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