Enhancing of low-temperature deformation compatibility of pure titanium welded tube by laser welding and cold-worked in welded zone

This study is aimed to improve the low-temperature deformation behaviour of titanium heat exchange tubes by optimizing welding method and weld treatment processes. The various welding processes and cold-worked in welded zone process are adopted, and the effects of macro-morphology, microstructure and mechanical properties on low-temperature deformation behaviour are studied. The results show that the width of the laser welded joint and welded zone is 3.8 mm and 2.9 mm respectively, which is 40% and 63% of TIG welded joint. Especially, the width of heat affected zone and the grain size of laser welded joint are obviously reduced due to its concentrated energy density. The cold-worked process in welded zone can relieve the macroscopical and microstructural discontinuity between base metal, heat affected zone and welded zone. The inner weld reinforcement is eliminated to improve smooth transition and the columnar α grains in welded zone are remarkable fragmented with plenty of twins. The hardness of the welded zone after cold-worked is significantly higher than that of the base metal, resulting in base metal being the main deformation area of the welded tube at low temperature, which is beneficial to improve the deformation performance of the whole welded tube. By adapting the cold-worked and laser welding process with smooth transition and narrower weld width, the pure titanium welded tube can meet the requirement of three times low-temperature deformations.