| With the advantages of low density, high strength-to-weight ratio and superior corrosion resistance, titanium alloys have been applied more and more widely in the fields of aviation and aerospace industries. As the laser welding has some characteristics such as high speed, good appearance and narrow HAZ, it becomes a main method for titanium alloys welding. The weld microstructures produced by traditional welding methods are abnormally coarse and the comprehensive properties of the weld are poor due to the large heat input. And the weld needs post-weld heat treatment, which will decrease the welding efficiency. The investigation of the technology of titanium alloys laser welding benefits improvement of the weld quality used for missile shells. It is very important to increase the national defense power.Titanium alloys absorb a large amount of N2, H2 and O2 gas at high temperature, which will badly destroy the weld performances; on the other hand, the intensive plasma appears during the titanium alloy laser welding, which influences the welding stability if the plasma is not controlled efficiently. The differences of the intensity of plasma light signal between titanium alloy and steel are studied, and the results indicate the light signal intensity produced during the laser welding for titanium alloy was greater than that for low-carbon steel. A special coaxial nozzle was designed to resolve the difficulties of the titanium alloys laser welding.The key point of designing coaxial nozzle is how to acquire the steady airstream, which can be used to control the plasma and protect the molten pool at high temperature at the same time. Based on the analysis of the disadvantage of control plasma by side flow gas, a scheme was proposed that a coaxial nozzle was chose to control plasma for titanium alloy laser welding. In order to acquire the steady airflow, an analysis of flow with in the nozzles was executed for the first time. The flow in different diameters of nozzle, cylinder cavum and "filter" cavum structure, with and without convergence pipe, different outlet diameters and different input gas flux was compared. The calculated results indicate that the maximum axial flow speed and the minimum turbulent kinetic energy can be attained in the "filter" cavum structure with convergence pipe when the inlet flux and outlet diameters are equal, by which the consumption of the shielding gas can be efficiently reduced. A welding experiment was executed to study the flow of gas in nozzle. The results indicate that the designed nozzle can be successfully used for suppressing the plasma; the silvery weld can be attained when the flux of gas from the coaxial nozzle is in the range of 0.25~1.25m3/h and that from the rear-cover is 1.0m3/h; the optimum flux of gas from the coaxial is 0.5m3/h. When the gas from coaxial nozzle over 1.5 m3/h, the weld was slightly polluted by oxygen; when over 1.75 m3/h,the weld was evidently polluted by oxygen. At that time, the kinetic energy of turbulent gas increases notably, which indicates the turbulence of gas is the reason why the weld was polluted by oxygen. The titanium alloy weld was produced with the coaxial nozzle to investigate the influence of the focus positions to the weld quality. The tensile strength and micro-hardness of the weld of BT20and TC1 are higher than of the base metal, but the elongation relatively low. The microstructure of weld is martensite with acicular α' phase. The tolerance of weld gap was determined and annular weld was produced during experiments. In comparison with TIG welding, laser welding for Titanium alloy has some advantages, such as the depth to width ratio is 6~7 times and the welding speed is 5~10 times as those of the TIG welding.The porosity is a common defect in laser welding for titanium alloy. Especially, the macro porosity weakens the strength of the cross-area, but also cause the stress concentrated. It is necessary to make clear the mechanism of the porosity formation. The results of the weld with a pad and the autogenous weld...
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