| Titanium and its alloys have plenty of advantages, such as low density, high specific strength, excellent mechanical properties and corrosion resisting, nontoxic and nonmagnetic, applied to many fields like space flight and aviation, navigation, petrochemical engineering, motor vehicle, medical and energy. Titanium alloys possess good weldability. Welding processes such as TIG welding, laser beam welding, electron beam welding, friction stir welding are used to weld titanium alloys. As a new welding process with good properties, laser-arc hybrid welding process have been well applied to weld metals like steel, aluminium alloy and magnesium alloy. It has great superiorities in productive efficiency, weld bead performance and weld seam protection using laser-TIG hybrid welding process to weld thin-walled structure parts of titanium alloys.Laser-TIG hybrid welding process is used in this project for welding of TA15 titanium sheet with cold filler wire. Process of welding, wire feeding and weld seam protection are studied and a comparison to TIG welding process is made. Mechanical properties of weld bead are tested, and the influence of weld seam protection on weld bead performance is studied. Weld bead profile is analyzed, and control of weld bead profile is realized. These works are done to provide process specification and performance data for the wide use of laser-TIG hybrid welding process in joining of titanium and its alloys.Results show that welding process with high efficiency and quality can be achieved on TA15 titanium alloy by laser-TIG hybrid heat source. Welding process shows different characteristic with different heat source arrangement. Heat source possesses greater penetrability with TIG-leading arrangement, while a more stable welding process, a better weld bead profile, and a better protection to weld seam can be achieved with a laser-leading arranged heat source.Pollution of weld seam aggravates and mechanical properties fall rapidly when shielding time to weld seam is shorten. Compared with TIG welding process, laser-TIG hybrid welding process possesses higher productive efficiency and lower heat input which result in narrower weld bead width and finer grain size welding titanium alloys. Cooling rate of laser-TIG hybrid weld bead is higher than which of TIG weld bead. Better protection to weld seam and better mechanical properties of weld bead like ductility, tensile strength and fatigue performance can be achieved with the same shielding time. Shorter time for weld seam shielding is needed to achieve a desirable weld bead performance by laser-TIG hybrid welding process (12s) than by TIG welding process (20s).Power density distribution of weld heat source is concentrated when laser-TIG hybrid welding process is used for the welding of TA15 titanium alloys. Under the force of arc pressure and surface tension, the molten metal flow towards the end of molten pool, piling up and getting solidified, forming a ridge shaped weld with a high reinforcement. Weld bead profile can be improved through the use of increased laser defocusing distance and tungsten electrode tip angle. But these two method will reduce the penetrability of laser-TIG hybrid weld heat source in a certain extent. Adding 2% content active gas CO2 to the weld shielding gas can improve the weld bead profile remarkably. Adding of CO2 content of 2% to shielding gas will not lead to an increase in oxygen and carbon content in weld bead and will not reduce the joint strength. The main reason for the improvement of the weld bead profile is the change of surface tension gradient and weld metal flow when active element act on the molten pool surface. |
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