Study Of Microstructure And Properties Of All-position Plasma Arc Welding Joint Of Titanium Alloy Pipelines

With the decreasing of the production and processing cost of titanium and its alloys recently,the applications are becoming increasingly extensive in aerospace,ocean engineering,medical treatment,petrochemical engineering and other fields.Because of the expandasion of titanium application range,it is urgently needed of a high efficient connection technology of titanium alloy pipeline.At present,argon tungsten arc welding can only be used as the field welding method of titanium alloy pipelines.The most difficult problems are low welding efficiency and complicated processing before welding.Based on the above reasons and market demand,a kind of plasma arc welding device was developped which can be used for all-position welding of pipelines to make all-position plasma arc welding of titanium pipelines come true through the experimental conditions of Guangdong Welding Institute(China-Ukraine E.O.Paton Institute of Welding).This topic set up and debugged a new plasma arc welding platform for pipelines,completed preliminary exploration of TA2 pipelines all-position plasma arc welding.Microstructure and properties of the girth welding joint were studied with OM,micro-sclerometer,SEM and XRD.Mechanical properties of tensile,bending,impact and macrohardness tests were done,and the tensile and impact fracture of welding joints were analyzed.The above conclusions were summarized as follows:(1)The diameter of tungsten is 4.0mm,the internal depth of tungsten is 2.5mm,the nozzle diameter is 3.0mm,the duty ratio is 60%,and the welding speed is 130 mm /s.The pulse peak current is 165-180 A,the base current is 80-85 A,the pulse frequency is 5.5-7Hz,and the plasma flow is 5.5-6.5L/min.With this process,a single-side welding joint without welding defects of the 8.8mm TA2 pipeline can be achieved.(2)Different welding positions are not affected seriously by different welding parameters.Different welding positions have the similar growth patterns and regularity.The organization welding joint area are α and α’ grain.(3)The tensile property of the all-position plasma arc welding joint is better than that of the base material;the bending angle can reach 180°;the impact energy is only 58.7% of the base material;hardening of the hexagonal martensite phase appearance at the heat-affected-zone and welding joint;the welding center is softened.The tensile fracture is ductile fracture while the impact fracture is cleavage fracture.