Research On The Formation Mechanism Of The Keyhole Induced Porosities During Laser Welding T-type Jiont

The steel sandwich structures panels, which are constituted by faceplates and web plates, are widely used in the field of shipbuilding andoffshore platform for their advantages of light weight, high strength andhigh stiffness. However, it is difficult to carry out the manufacturingprocess of the sandwich structure, especially the welding process betweenface plates and web plate. Laser welding with advantages such as highefficiency, little welding distortion and stable welding quality offers aefficient way to manufacture the sandwich structures. However, theporosity is a severe issue in the deep penetration laser welding of T-joints.The existence of porosity leads to the reduction of bearing area of the weldand the degradation of the properties, that is, the strength, toughness,anti-fatigue and corrosion-resistant. So it is necessary to carry outinvestigations to make out the formation process and mechanism ofporosities, and then provide effective methods to reduce porosity defects.In this study, porosity’s formation process, porosity characterizationand the influencing factors of porosity were systematically studied throughsimulation and experiments to make out the formation mechanism ofporosity and find ways to reduce porosity. Firstly, the laser was used toweld the T-joint of the high-strength low alloy steel and the influences ofthe laser welding parameters on the formation of the porosities were alsosystematically studied. Then the welding structure with porosities wasdetected by destructive testing method and X-ray radiography testing.However, it was a challenge to characterize its inner features, especiallythe size and distribution of the defects based on these conventional detection methods. So the three-dimensional Nano-CT technique wasintroduced to observe the three-dimensional size, morphology anddistribution of the porosities. And the Nano-CT characterization couldreveal much more details of porosity and could also be helpful to estimatethe influence on the properties of the welding structure.Mathematical models for simulating the laser stake welding of theT-joint were established based on the computational fluids dynamics (CFD)and Volume-of-fluid(VOF) methods. The solidification model andadiabatic bubbles model were incorporated in to simulate the laser keyholewelding process and the formation of the porosities. The numerical resultsshowed that the keyhole was unstable during the laser welding process,which could cause the collapse of keyhole, and therefore bubbles occurredin the molten pool. The bubble trying to escape from the molten poolmoved following with the fluid flow in the molten pool. Some bubblescould escape out of the molten pool under the competition of flow andsolidification speed. But some bubbles, captured by solidified wall duringtheir migration process in the molten pool, would evolve to be porosities.Another phenomenon was that some bubbles forming adjacent to fusionline were easier to be captured, which could explain the phenomenon thatsome porosities appear close to the fusion line.The influences of welding parameters on the formation of porositieswere systematically studied based on the established models. The resultsindicated that the keyhole became more unstable with the increase of thelaser power and the decrease of spot diameter and welding velocity,leading more porosities to form. In addition, the gap between web plateand face plate also had important effects on the formation of the porosities.The existence of the gap could change the profile of the welding pool andaggravate the instability of the keyhole, leading to the formation of thekeyhole-induced porosities.The results of simulation and experimental research indicated that themismatching between welding parameters and material physical parameters led to the formation of porosities. In addition, the interactiveeffect between porosities and surrounding melt and the competitionbetween porosities’ floating up speed and solidification rate had greatimpacts on the transformation from bubbles to porosities.