Gravity effects on temperature distribution and material flow in the keyhole pool of VPPA Al welding

• The heat and mass transfer, as well as the effect of gravity on the keyhole formation in the molten pool of VPPA welding are investigated. • The driving mechanism of the molten pool flow in VPPA keyhole welding of aluminum alloy is revealed, and the influence degree of different forces is quantitatively compared. • By comparing the flow behavior of the keyhole molten pool at different positions, it is found that gravity mainly changes the flow velocity of the molten metal, and has little effect on the flow pattern. • At different welding positions, the difference in material flow affects the temperature distribution of the molten pool surface, and at the same time makes the heat conduction and heat convection of the molten pool metal around the keyhole different. This study investigated heat and mass transfer as well as the effect of gravity on keyhole formation in the molten pool of keyhole plasma arc welding. The flow trajectory of the molten pool was obtained via high-speed camera images illuminated by a laser light system, and the temperature distribution was obtained using an infrared thermal camera. The experimental results showed that the molten metal flowed rearward and downward at the top surface of the weld pool and thereafter to the rear side of the keyhole. There are two types of reverse flow of molten metal on the rear side of the keyhole. The different welding positions had minimal effect on the flow pattern; however, the flow velocities were different. Further, for the temperature distribution, although the keyhole front side yielded a similar high-temperature area under different welding positions, the area of the high temperature on the keyhole rear side in the vertical position welding was much larger than that in the flat position. Consequently, the influence of the force on the keyhole molten pool was evaluated, and the flow pattern of the molten pool was found to be dominated by the sheer force. Moreover, the flow velocity and solidification morphology of the molten pool were affected by the gravity at different welding positions. In addition, the correlation mechanism of the temperature distribution, flow, and solidification morphology of the keyhole molten pool was analyzed using the combined action of shear force and gravity, which further revealed the mechanism of the influence of gravity on the keyhole welding process.