Physical mechanisms of fluid flow and joint inhomogeneity in variable-polarity plasma arc welding of thick aluminum alloy plates

Abstract

A variable-polarity plasma arc (VPPA) is an effective energy source for the welding of thick aluminum alloy plates. However, the mechanisms influencing the fluid flow and the inhomogeneous distribution associated with thick-plate VPPA welding remain unclear, restricting the application of this technology in welding of thick aluminum alloys. Here, the relationship between the microstructure of the weld bead, energy transfer, and fluid flow is clarified by combining in situ three-dimensional x-ray imaging and multi-physics modeling. We find that heat conduction at the keyhole wall is the main factor influencing the morphology of the weld pool. The plasma arc pressure hinders the upward flow of liquid metal, while shear forces promote this flow. This causes the metal close to the weld pool surface to flow slowly, while that inside the weld pool has much higher velocity. It is also concluded that the large crystal size observed in the lower layer of the weld is partly caused by heat treatment from the upper layer of the thick plate. An eddy with a high flow velocity to the rear of the weld pool destroys the crystal-growth process, and this is considered to be one of the reasons for fine crystals appearing in the upper part of the weld. The mechanisms revealed here will help us to guide the use of VPPA technology in the production of stable, high-quality welding of thick aluminum alloys.