| Summary: | 碩士 === 國防大學中正理工學院 === 兵器系統工程研究所 === 94 === This study is focused on the influences of various post-weld heat treatments ﹙PWHT﹚on the microstructures and the mechanical properties of the TIG-welded Ti-6Al-4V weldment. Metallographic observation shows that the fusion zone of the weldment with no PWHT was a relatively larger grain-size microstructure. The enhanced grain-size microstructure of fusion zone was identified by TEM to be a nanostructure of a+b duplex phase whose orientation was random. For the weld with a PWHT of annealing at 704°C for 2 hours and air cooling, the a2-Ti3Al metallic compound was found to precipitate in the fusion zone. This type of metallic compound contributed a significant strengthening effect together with its decreased elongation. For the weld with a PWHT of solutioning at 954°C for 15 minutes, water-quenching, and following ageing at 704°C for 4 hours and furnace cooling, a great amount of r-TiAl compounds in size of 2~5 nm was found on the fusion zone. The tensile test, shows that the precipitation of r-TiAl metallic compounds of the 704°Cx4 hrs-aged weld increased the yield strength from 802 to 838 MPa and decreased the elongation from 11.6 to 2.8%.
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