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 theinhomogeneous 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 fluidflow is clarified by combining in situ three-dimensional x-ray imagingand multi-physics modeling. We find that heat conduction at the keyhole wall is the mainfactor influencing the morphology of the weld pool. The plasma arc pressure hinders theupward flow of liquid metal, while shear forces promote this flow. This causes the metalclose to the weld pool surface to flow slowly, while that inside the weld pool has muchhigher velocity. It is also concluded that the large crystal size observed in the lowerlayer of the weld is partly caused by heat treatment from the upper layer of the thickplate. An eddy with a high flow velocity to the rear of the weld pool destroys thecrystal-growth process, and this is considered to be one of the reasons for fine crystalsappearing in the upper part of the weld. The mechanisms revealed here will help us toguide the use of VPPA technology in the production of stable, high-quality welding ofthick aluminum alloys.