TIG process, microstructure and properties of hot-rolled high-strength titanium alloy thick-walled pipe

For the thick-walled high-strength seamless titanium alloy pipe produced by the new hot continuous rolling process, the circular weld butt welding process test was carried out by the two welding methods of TIG automatic wire feeding and manual wire filling, and the joints under the two processes were compared and analyzed from aspects of formation of weld, microstructure and mechanical properties. The results showed that the surface of the weld pass under the two welding methods appeared bright silver without obvious oxidation. The surface of the joint under automatic wire feeding was smoother and beautiful, while the surface of the joint under manual wire filling had irregular fish scales. The β phase in the heat-affected zone got coarsened to a certain extent, and a certain amount of fine needle-like α’ martensite phase formed. The microstructure of the weld zone was mainly lath and block α phases formed along the β phase boundary, as well as a small amount of acicular α’ martensite formed in the crystal grains. Hardness of the TIG joint under both automatic wire feeding and manual wire filling were the highest in the heat-affected zone, hardness of the weld was the lowest and hardness of the base material was in the middle. The tensile test fracture positions all located in the weld, and the average tensile strength was 603.8 MPa and 571.7 MPa respectively. The impact energy absorption of the heat-affected zone reached 41.7 J and 78.5 J respectively, and the heat-affected zone under manual wire filling showed better impact toughness than the base metal.Highlights: (1) Microstructure and mechanical properties of new titanium alloy pipe under hot rolling process were studied under manual TIG/automatic TIG process, both processes adopted multi-layer and multi-pass welding technology.(2) When pure titanium wire was used for filling in the two processes, tensile strength of the two joints was both higher than that of the parent metal, and impact toughness of weld and heat affected zone was higher than that of the parent metal. Due to the low alloying element of pure titanium welding wire, the content of martensite formed at the weld position was low after cooling.(3) The complex thermal process of manual TIG/automatic TIG multi-layer and multi-pass process played an appropriate role in heat treatment, and a large number of overlapping α microstructure formed in the heat affected zone, which greatly improved the impact toughness of the heat affected zone and solved the problem of low impact toughness of the heat affected zone under the conventional welding process of this titanium alloy.