Material flow analyses of high-efficiency joint process in VPPA keyhole flat welding by X-ray transmission system

The consideration of environment-friendly and low energy consumption tends to control and guide the progress of metal joint manufacturing processes. Variable polarity plasma arc (VPPA) welding is an environment-friendly, high quality and efficiency joint method for aluminum alloys due to its characteristics of such as little welding fume, and high energy density etc. However, the three-dimensional morphology and physical phenomena of penetration weld pool in VPPA welding are not clearly understood, which slows down its process development and optimization. This work aims to clarify the three-dimensional keyhole detouring flow of VPPA welding for aluminum alloy. The liquid metal distribution of penetration weld pool was generally obtained by metallographic observation. The measurement results show that the liquid metal layer attached to the three-dimensional keyhole boundary is thin. And the liquid metal mainly distributes in the bottom side of the keyhole. In order to understand the weld pool dynamics, the stereo synchronous imaging of tungsten tracer particles using two sets of X-ray transmission system was adopted, for the first time, to measure the 3D velocity field of convection flow inside weld pool of VPPA welding of aluminum alloy in real time. The flow on the weld pool surface at different locations of keyhole boundary was also measured by tracing zirconia and slag particles. The liquid metal flows simultaneously to the top and around the keyhole. The metal at the bottom of keyhole's front wall, flows around the keyhole towards the backside, then separately flows to top side and bottom side, forming a vertical stationary point. The key to weld and avoid defect proofing are to ensure stable detouring flow on the keyhole bottom and proper stationary point position. (C) 2019 Elsevier Ltd. All rights reserved.