Research On Penetration Deepening And Porosity- Defect Suppression Mechanism In Laser Welding Of Al-alloy Under Subatmospheric Pressures

Aluminum alloys are widely used in aerospace equipment,shipbuilding and rail transportation field due to the advantages of low density,strong corrosion resistance,high specific strength and good formability.Compared to ordinary laser welding method,the laser welding under subatmospheric pressures has broader prospect in the formation and joining of the thick-plates,because the penetration can be deepened and the weld porosity defects can be effectively suppressed.In this study,the research was carried out on 5083 aluminum alloy with thickness of 30 mm.The influence of reduced ambient pressure on the formation of weld seam and the porosity defects was comprehensively studied by combination of simulation with experiment.The keyhole dynamics and the molten flow characteristics in deep-penetrated laser welding under different ambient pressures were compared.The deepening mechanism of penetration and the reasons for restraining porosity-defects under subatmospheric pressures were analyzed.It provides reference for optimizing welding process and improving welding quality of aluminum alloys.Firstly,laser welding of aluminum alloys were experimentally conducted.The influence of laser power and ambient pressure on the weld penetration,the weld aspect ratio and the porosity defects were analyzed.The results showed that when all other process conditions were kept same,the weld penetration depth increased with increase of laser power under same ambient pressure,and it increased with decrease of ambient pressure at same laser power.When keeping the ambient pressure constant,with the increase of laser power,the change of the weld aspect ratio depended on the increase of weld penetration caused by the more intense laser and the increases of surface weld width caused by stronger plasma metal vapor plume.When keeping the laser power constant,the weld aspect ratio increased with the decrease of the ambient pressure.When the laser power was low(less than 5 kW),little porosity was found on the longitudinal section,and it changed little with the decrease of ambient pressure.When the laser power was high,the porosity was reduced with the ambient pressure decreasing.The weld formation and porosity defects were closely related to the keyhole formation process and molten flow in deep-penetrated laser welding process.In order to study the keyhole formation process and molten flow in laser welding process and to reveal their effect on weld formation and porosity defects,a three dimensional mathematical model was established based on the theory of computational fluid dynamics,taking into account the physical process of energy transfer,mass transfer and phase change.Multiple reflection and Fresnel absorption were realized in the heat source model.At the beginning of the deep-penetrated laser welding process,the multiple reflection plays a small role,so the absorption of the laser energy is low.As the keyhole depth increased,the laser energy absorption increased and finally stabilized,and periodically fluctuate at a certain value.The keyhole depth increased first and then remained fluctuant.The solidification of the molten pool began at the bottom of the fusion zone and proceeded to the top and center of the molten pool.On above basis,the ambient pressure was taken into consideration in the model and the keyhole formation process and molten flow under different ambient pressures was compared.It was found that the keyhole was deepened in each stage of the keyhole formation and ultimately led to the increase of weld penetration under reduced ambient pressure,this was because the plasma metal vapor plume was restrained under reduced ambient pressure,thus to result in enhancing the laser energy density and decreasing the laser refracted angle.At the same time,the boiling point of the material was lowered under reduced ambient pressure.The laser energy distribution along the keyhole wall was uniform under reduced ambient pressure.Besides,it was easier for the bridge between the front and rear wall to be reopened.Therefore,the possibility for bubble formation was reduced.The generated bubbles were easier to escape from the molten pool under reduced ambient pressure.In conclusion,the porosity defects can be reduced under reduced ambient pressure.The simulation results are in good agreement with the experimental ones,indicating that the established numerical model in this paper is reasonable and can explain the keyhole dynamics and molten flow as well as the mechanism of porosity formation of aluminum alloys under subatmospheric pressures.