Porosity morphology and its evolution mechanism in laser mirror welding of 2219 aluminum alloy

Laser mirror welding, which inputs the symmetrical energy on the sides of the butt joint structure, has been employed to solve the problem of large angular distortion in single laser welding for medium-thick weldments. However, porosity defects is becoming a challenge for laser mirror welding due to dual laser beams and trans-verse welding direction. In this study, experimental analysis and numerical simulation modeling are used to reveal the characteristics and evolution mechanism of porosity in laser mirror welding of 2219 aluminum alloy. Compared with metallurgical porosity related to hydrogen, the size of process porosity is larger, the shape is more irregular, and the inner wall is rougher. The fluctuation of the coupled keyhole is the root cause of the formation of process porosity, manifesting as high-speed fluids on the wall. It is investigated that large process porosities mainly occur near the upper fusion line, which can be divided into two types: alpha region in the center and beta region near the weld surface. The bubbles, which are formed owing to the collapse of the central keyhole in alpha region and the detachment of keyhole hump in beta region individually, follow with upward melt flow by buoyancy. They are captured before crystallization resulting in the concentrated distribution of porosities in these regions.