Study of molten pool dynamics and porosity formation mechanism in full penetration fiber laser welding of Al-alloy

To study molten pool dynamics, root hump formation, periodic keyhole behavior in full penetration laser welding (FPLW) of aluminum alloy, a numerical simulation was carried out, in which volume of fluid (VOF) method and ray-tracing algorithm were adopted. A varied metallic vapor shear stress model was considered. Meanwhile a series of welding experiments on specimens being composed of aluminum alloy and quartz glass were conducted to analyze the porosity formation process. It was found that the overheated sagging with low surface tension stagnated at the bottom side and solidified to form root hump. The dominant backward heat convection at the lower part can well explain the extreme spreading of molten pool at bottom surface. Periodic behavior of the keyhole was identified during quasi-steady stage. The porosity can be effectively suppressed in FPLW. Both the porosity ratio and average porosity number were reduced obviously. The main mechanism of porosity formation in FPLW is the collapse of the rear keyhole wall, mainly caused by bulges on the front keyhole wall. Bubble coalescence is responsible for large porosity size and coalescence efficiency depends on bubble size difference. (C) 2019 Elsevier Ltd. All rights reserved.