Study On The Mechanism Of Penetration Mode For Thin Sheet Laser Deep Penetration Welding

The interaction between the keyhole and weld pool is the core problem of the laser keyhole welding. By analysis of the keyhole, weld pool and plasma temperature character and electron density character, a coaxial visual sensing system was established to detect the optical emission of the keyhole and weld pool in deep penetration laser welding process. Using the optical character of the keyhole and the weld pool, incorporate with the weld cross section character, the penetration mode of the thin sheet laser welding was studied. And the variation of the keyhole, weld pool and weld character with the processing parameter was analyzed, the physical mechanics of the penetration mode transition and its effect on the weld shape was represented theoretically.In this paper, the optical emission character of the keyhole and the weld pool was related to the weld cross section character, and the penetration mode of the laser full penetration welding was classified. Based on the situation of the keyhole whether full penetrated the workpiece, two penetration modes of the laser full penetration welding was represented, as open keyhole penetration mode and close keyhole penetration mode. The typical weld cross section of the open keyhole penetration mode is"X"type, the typical weld cross section of the close keyhole penetration mode is"V"type. With the transition of the penetration mode, the variety of the force equilibration at keyhole bottom was discussed. Combined with the relation between the process parameter to the keyhole existing and expansion, the effect of the process parameter on penetration mode transition was illuminated, and the weld formation mechanics for different penetration mode was explained by the heating situation and fluid driving force of the weld pool.For the coaxial visual sensing image in experiments, the Media filter, homomorphic filter and Linear gray stretch were used to eliminate noise and enhance processing. The Canny operator was applied to detect the edge of the keyhole and the weld pool. Finally, the Hough transformation was utilized to obtain shape character of the keyhole and the weld pool surface. The result of the keyhole, weld pool and the weld cross section shape characters according to the process parameter shown that the welding speed is the governing factor of the keyhole and weld pool shape angle, the included angle between undersides fusion-line and laser beam and the weld width ratio can be the characters to identify the penetration mode. In close keyhole penetration mode, the included angle between undersides fusion-line is positive, as the penetration mode transit to open keyhole penetration mode, the corresponding included angle is negative. As the workpiece thickness is 1.4mm, the weld width ratio greater than 0.55 indicates that the stable open keyhole penetration mode was obtained.The detection of the weld obverse width was achieved by the weld pool edge characters, which is represented in the coaxial visual sensing image. The detection result was agreed with the experimental result fairly. The character of the keyhole rear opening was applied to detection of the weld rear width. As the smaller keyhole rear opening size, the relation between the keyhole rear opening size and the weld rear width is approximate lineally. As the increasing of the keyhole rear opening size, the weld rear width tends to stable.Based on the technological experiment, the effect of the keyhole mechanics on welding process was considered, and the keyhole characters of the coaxial visual sensing image were introduced to the numerical simulation process, a three-dimensional mathematic model of the heat and mass transfer with the moving heat source in laser keyhole welding was developed. The temperature field and fluid flow vector field for different penetration mode was calculated, and the effect of the fluid flow driving forces was analyzed quantificational. The result of the numerical simulation demonstrated that the difference of the keyhole shape lead to the difference of the weld pool fluid flow pattern, which is the internal reason for the weld shapes formation in different penetration mode. For the driving force of the fluid flow, as the flow speed caused by the buoyancy is feeble, the effect of the buoyancy on the weld pool flow can be neglected. The fluid flow caused by the vapor friction is limited around the keyhole, which drive the fluid from the keyhole wall flow to the outer edge of the weld pool. The surface tension is the main driving force in the weld pool. Under the govern of the surface tension, the liquid metal of the weld pool surface flowed from the weld pool center to the edge, and an eddy flow was formed at the back of the weld pool. In open keyhole penetration mode, the surface tension is the key factor for the formation of the"X"type weld cross section.