Study On The Process Behavior And Defects Control During Laser Deep Penetration Welding Of Aluminium Alloy

To ensure the long-weld qualities of large and complex structures by laser welding is the key issue in application of laser welding technology. As the production of power beam, vapor and plasma, keyhole, the metal, and there interaction in laser deep penetration welding is a complex process. Besides, because of its special physical properties, aluminum is easily lead to instability in the process of laser welding and results in welding defects.Research on laser deep penetration welding process behavior, the joint defects and related mechanism were carried out with the typical aeronautical aluminum alloy of 5A90 aluminum-lithium alloy. To ensure the quality of laser welding of aluminum alloy, the geometry characteristics of joints and the effects of welding parameters on them were studied; the defects of porosity and surface depression characteristics and their affecting factors in laser deep penetration weld were analysed; the dynamic characteristics and performance of the keyhole and pool in deep penetration laser welding were disscussed; Based on the knowledge of characteristics and behavior of keyhole and molten pool in the deep penetration welding, the generation mechanism of porosity and surface depression were revealed; the measures to effectively control the defects were also discussed.High-speed video observations show that: in non penetration welding process, the metal vapor / plasma over the keyhole in the irregular fluctuations. Keyholes show a positive opening and closing of unstable periodic and the size changes of keyholes are large. The size of droplets shaped metal pool changes in the oscillation. In the full penetration welding process, the metal vapor / plasma from the keyhole on the back of periodic eruptions, Keyhole presents a more stable periodic opening and closing. With the heat input increases, the periodic opening and closing of keyholes trend to stable and the cycle becomes longer, and metal pool size of the resulting longer and wider, clear pool surface subsidence comes out.R is the ratio of weld back width and weld top width, which is used for characterizing of the quality of aluminum weld during laser deep penetration welding. Defocusing distance, laser power, welding speed are the important parameters for effecting R, which can be explained by the laser power density and heat input. R= 0.6 is the control curve of porosity, and R=0.9 is the control curve of depression defects.Weld porosity in laser deep penetration welding of aluminum could be attributed into "Metallurgical feature-type" pores and "process -type"pores. The former were caused by the weld surface contamination or other relevant things; the "process -type" pores were were caused by the unstable behavior of keyhole opening and closing, these kind of pores can be controlled by getting full penetration welds. The surface defect of depression in laser deep penetration welding of aluminum was caused by strong evaporation from the burning of metal in the welding process. To aim at the defects of porosity and depression in laser deep penetration welds, the introduce of twin spot laser welding and YAG-MIG hybrid welding process measures to improve the stability of keyhole and pool behavior in order to suppress defects by combining the heat energy. The results showed that these two measures would greatly improve the surface forming quality and effectively inhibit the production of weld porosity in laser deep penetration welding of aluminum.