Research Of Numerical Simulation And Plasma Control During Laser Deep Penetration Welding

Laser welding technology is one of the important and advanced technologies for material connection in engineering and technical field. With the appearance of high power and power density laser beam, the technology of deep penetration laser welding is applied widely in the industry. The recent major research achievements and the hot issues of deep penetration laser welding have been summarized. Meanwhile, its characteristics and limitations have been discussed as well. And then many important advances have been analyzed and discussed in the field of deep penetration laser welding in this article.The different stages have been analyzed and summarized from the beginning of laser irradiating material to the keyhole formed during deep penetration laser welding and the interaction of the laser and material had been investigated by thermodynamic calculation and analysis in this article. Based on the basic theory of physics and metallurgical process, the three-dimensional model of deep penetration laser welding has been established. Combining with the characteristics of deep penetration laser welding, optimizing a dynamic heat source adapted with keyhole depth, using FLUENT software, selecting the models of solidification/melting, evaporation/condensation and the VOF, the relevant thermodynamic problems are studied and analyzed in the process of the deep penetration laser welding. The formation and evolution trend and feature of the molten pool and keyhole have been analyzed, and how the laser power and welding speed influence the shape of keyhole has been explored with an optical multicultural analyzer and a high-speed camera during deep penetration laser welding. In the initial phases of welding, with the temperature increasing, the keyhole becomes deeper and the surface of weld pool becomes bigger. In the stable welding stage, the keyhole size and the temperature field adjacent to weld pool change little, and the surface of weld pool increases gradually. When weld pool reach the balance of heating and cooling, the surface increase no more. With the increase of welding speed, the keyhole shape is leaned and leaned in welding direction and the other side is contrary to it. When the input power is higher, a dent appears in the middle to lower part of the keyhole in the side of condensation and a convex peak appears in the upper. The flow direction has changed around the concave and convex border. It is easy to make the keyhole collapse and form a pore. The calculation results are in good agreement with the experimental results. Besides, it can give reasonable explanation for some experimental phenomena.The thermodynamics behaviors of laser-induced plasma have been studied during laser welding. Based on the basic theory of aerodynamic and the simulation result of plasma pressure distribution above keyhole, a new welding nozzle has been designed to control the plasma during deep penetration laser welding. The flow state of the nozzle has been simulated and analyzed combining with the experiment through the schlieren photography. The experiment results demonstrate that the inverse bremsstrahlung absorption of plasma is considerably larger than Fresnel absorption of keyhole wall. The deviation between electronic temperature and ionization temperature of plasma is very obvious in the initial stage during laser welding and then tends to balance gradually. At the same time, temperature gradient of plasma becomes smaller. In the stable welding process, there is only a small influence from laser power increase on the temperature of the plasma, but the influence of the plasma’s size on weld area is important. When the plasma temperature increases sharply, with the pressure increasing in the keyhole, the plasma will up and down, it will induce in the interrupt of welding process or produce blowholes. Moreover, the size fluctuation from plasma will worsen the weld shaping. In a stable welding process,plasma oscillates with a certain frequency. The ups and downs frequency of plasma is consistent with its size fluctuation approximately. And the frequency is also basically in agreement with the surface stripe change of weld. Adjusting double gas flows and when the inner gas flow 30L/min and the outer gas flow 10 L/min, the plasma can be controlled well, then a stable welding process and well-formed weld without visible flaws can be obtained.The technologies of autogenous laser welding laser welding and hybrid laser-TIG welding are used on thick plate of high strength lower alloy structural steel 10 Cr Ni Mn Mo V in this article. The unique advantages of hybrid laser-TIG welding is summarized by comparing and analyzing the process parameters, welding joints of autogenous laser welding laser welding and hybrid laser-TIG welding. With the optimal process parameters of hybrid welding,the good welding joint without visible flaws can be obtained and its mechanical properties are tested according to industry standards. The results show that the hybrid welding technology has certain advantages and possibility in welding thick plates. It can reduce the demands of laser power, and it is significant for lowering the aspect ratio of weld during hybrid welding, so the gas in the molten pool can rise and escape easily when welding of thick plates. Therefore,the pores forming tendency decreases. At the same time, hybrid welding enhances welding speed, and optimizes the energy input. The transition and grain size of the microstructure of hybrid welding joint is better and its hardness is higher than base material. Furthermore, its tensile strength and impact toughness is as good as base material. Consequently, the hybrid welding joint can meet the industry needs completely.