Study On Gas Shielding Technology For Titanium Alloys Laser Welding

Shielding gas is an important parameter in laser welding, because it is used to protect the high temperature zone without pollution and suppress the plasma. Particularly to titanium alloys, whose chemical character is very active, the shielding gas is the key to ensure quality of laser welding. The project studied the thermal field and airflow field during the process of laser welding by method of numerical simulation, and analysed the influence rules of attitudes, sizes of side nozzles and gas parameters on the protective effect, providing the theoretical basis for optimizing the side nozzle. Designing a coaxial nozzle that more flexible than side nozzle and suitable for the high thermal input laser welding.Based on Marc and hybrid heat source of gauss plane heat source and columnar heat source, thermal simulation model was built for TC4 laser welding in T-joints. Through the comparison with the actual welding seam and the result of thermocouple, the accuracy of the model was substantiated. The study found that the critical protection temperature of TC4 in laser welding is 480℃, the color of the weld surface is silvery; if only protecting the high temperature zone in 520℃,700℃and 760℃, the color of weld surface would be yellow, purple and blue.Based on Fluent and standard k-ε equations, simulation model was built for research of the shielding gas flow behavior. Through comparison with the observation result of Schilieren, the accuracy of the model was substantiated. Studying the influence rules of attitudes and sizes of side nozzles and gas parameters on the effect of protective effect. The study found that the distance between nozzle and workpiece, nozzle’s angle and size were major factors influencing the high temperature zone’s protective effect; nozzle’s size and gas flow rate were major factors influencing the effect of suppressing plasma. Optimizing the paramaters of Φ8mm, Φ10mm and Φ16mm to improve the protective effect.The range of high temperature zone in high thermal input laser welding calculated by the simulation is the theoretical basis for the design of coaxial nozzle’s size. The twochannel coaxial nozzle was designed and manufactured. Based on the analysis of nozzle internal and external shielding gas flow characteristics, optimizing the structure and size of coaxial nozzle, clarifying the effct of the airflow constraints wall, sieve and lateral part injection angle’s changment to improve the air flow characteristics. They could improve the stability and uniformity of gas flow.Studying the influence rules of gas flow rate and the distance between nozzle and workpiece on the effect of high temperature zone’s protection and suppressing plasma. The results showed that when the gas flow rates of central part and lateral part were all 10L/min, the distance between nozzle and workpiece was 5mm, the process of P=1800W, v=0.6m/min laser welding was stablest, the effect of suppressing the plasma was best, and the scope of protection was the largest. The protective effects of laser welding in line and circle were both better than side nozzle’s, verifying the superiority of coaxial nozzle in the protection of titanium alloy laser welding.