| Summary: | 碩士 === 國立臺灣海洋大學 === 材料工程研究所 === 97 === In this study, the influence of post-weld heat treatments (PWHTs) on the notched tensile properties of Ti-4.5Al-3V-2Fe-2Mo (SP700) and Ti-6Al-6V-2Sn (Ti-662) laser welds at different temperatures were investigated. Notched tensile tests of SP700 and Ti-662 welds with distinct PWHTs were carried out at 150, 300, and 450℃. The results were also compared with those from a mill-annealed base metal (MA) tested at the same conditions.
The weld with PWHT at 704℃ and MA specimens exhibited a reduced notched tensile strength (NTS) and an increased ductility with increasing temperatures. In contrast, the weld with PWHT at 482℃ (W-482), which revealed severe notch brittleness at room temperature, showed an increase in strength and ductility as the temperature was increased. When tested at 450℃, the as-welded SP-700 specimen was sensitive to notch brittleness due to fast response of age-hardening at the testing temperature. The fracture appearance of the MA specimen exhibited fine dimples which increased in size remarkably with increasing temperatures. For the as-welded and W-482 specimen, the extent of intergranular fracture increased significantly with increasing temperatures. Moreover, all Ti-662 welds had a trend of increasing NTS from room temperature to 150℃, but a gradual decrease in NTS with further rises in temperature.
Fatigue crack growth tests of SP700 welds were carried out at 300, and 450℃. The results were also compared with those of the MA specimen tested at the same condition. For both of the MA and welds, the identical fatigue crack growth behavior occurred at room temperature and 300℃. However, a slightly increased in crack growth rate was found at 450℃. The observation of fracture surface revealed that the MA and welds at room temperature exhibited the transgranular fatigue fracture. A pronounced rough fracture surface of the weld resulted from the inherent coarse structures of the weld, which promoted zig-zag crack path.
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