Study On The Deformation And The Behaviour Of Plasma During Laser Welding Of TC4 Titanium Alloy

Because of its advantages(relatively high specific strength, good corrosion resistance, high temperature resistance and low weight), TC4 titanium alloy is widely used in many industries, including the aeronautics and astronautics industries. However, weld defects such as large deformations and lack of fusion are easily appear during TC4 titanium alloy laser welding. The laser induced plasma that generated during the laser welding process is closely related to the stability of welding and the weld quality. So that, this paper has been researched on the behaviour of plasma and deformation control during laser welding of TC4 titanium alloy.Firstly, to reduce longitudinal flexural deformations during the laser welding of TC4 titanium thin plates, our team has been presented a synchronous gas cooling method(SGCM) that uses liquid nitrogen-cooled argon gas with the laser source to actively control the cooling process during welding. However, the mechanism of reducing the welding deformation is still need to be studied. An SGC M laser welding model is developed. Simulations of the traditional laser welding process and the welding process with the SGCM show that the synchronous gas cooling affects the longitudinal flexural deformation and the longitudinal residual stress. The pos t-weld deformation results obtained from the experiments match those obtained from the simulations relatively well. When the heat sink distance is 10 mm and the mean heat transfer coefficient is 360 W/(m2·K), the deformation decreases by 71%, the longitudinal residual stress decreases by 21%, and the longitudinal plastic strain decreases by 14%. This test verifies the effectiveness of the SGCM laser welding. Beside,compared to the conventional welding, the prior β grains and the acicular α′ phase for the SGCM become fine on the morphology.Secondly, the visualization of vapor-plasma plume occurred during the laser welding of TC4 titanium was observed by high speed camera in this paper. The average centroid of vapor-plasma plume decreased first and then increased as the laser welding power increased from 700 W to 1200 W. The average centroid of vapor-plasma plume decreased as the side shielding gas increased, while the average tilt angle of vapor-plasma plume showed the opposite tendency. The periodic oscillation of the vapor-plasma plume was found below cycles of 4000 Hz. The vapor-plasma plume erupt from the keyhole. The area of the keyhole which observed horizontally also periodically changes. The covariance mapping method was applied to frequency for studying the correlation between the vapor-plasma plume and the keyhole. It is found that both the vapor-plasma plume and the keyhole were all had a high degree of correlation near the frequency of 2000 Hz. Therefore, the periodic oscillation of vapor-plasma plume can be proven to be caused by the keyhole from the point of view of statistics.Finally, optical signal of plasma during the laser welding was measured in this paper. In order to improve the calculation accuracy of the plasma’s parameter, this paper removed the background spectrum of the plasma and then normalized the rest of the spectrum. The electron temperature of the plasma was 6100 –7000 K, the electron density of plasma was 0.64×1016–1.4×1016 cm-3 as laser power increase from 650 W to 1000 W. Besides, this paper established a relationship between the electron temperature of the plasma and the depth of weld.The research results about the behaviour of plasma and deformation during laser welding of TC4 titanium alloy will provide a theoretical guida nce and theoretical basis for reducing weld defects such as deformations, lack of fusion and real-time monitoring weld quality.