Investigate on the porosity morphology and formation mechanism in laser-MIG hybrid welded joint for 5A06 aluminum alloy with Y-shaped groove

Laser-MIG hybrid welding is an advancing technology for the joining of aluminum alloy with the medium-thickness owing to the high efficiency and excellent gap bridging ability. In this paper, the laser-MIG hybrid welding experiment for 5A06 aluminum alloy with the thickness of 6.9 mm is carried out. The process porosities and metallurgical porosities in the weld cross-section and tensile fracture are investigated. Furthermore, it is novelty that the formation mechanism of process porosities and metallurgical porosities are comprehensively analyzed based on the porosity morphology after welding, as well as characteristics of molten pool and keyhole during laser-MIG hybrid welding. It is found that the formation of process porosity is owing to the fact that the tip of the keyhole is easily closed by the molten metal stream and separated to form a process bubble. The formation of the metallurgical porosity is attributed to the atomic hydrogen. The inner wall of the metallurgical porosity with the second phase is observed. It is inferred that the formation of second phase is accompanied by the atomic hydrogen. Besides, the area with dense metallurgical porosities and the aggregate metallurgical porosity are formed under the driven force of the liquid metal flow behavior. The formation process of aggregate metallurgical bubble goes through three states, separation state, contact state and aggregate state.