Dynamic keyhole behaviors and element mixing in paraxial hybrid plasma-MIG welding with a gap

•The Fe vapor ejected from the wire surface decreases the MIG arc temperature.•The molten metal flow and pressure imbalance near the keyhole bottom contribute to the keyhole collapse.•The element mixing behaviors are driven by the molten metal flow inside the molten pool. A paraxial hybrid plasma-MIG welding process is developed to join middle-thick high strength steel plates with a butting gap. The arc and metal vapor characteristics are investigated by an advanced three-dimensional spectroscope measurement system. The dynamic keyhole behaviors and element mixing are studied by a three-dimensional numerical model. The numerical and experimental results show that the Fe vapor ejected from the wire surface, other than the plasma keyhole, has a great influence on arc characteristics. The butting gap affects the keyhole stability and molten metal flow, as well as the element mixing. With a gap, the keyhole opens and closes periodically. The opposing flows at the front and rear keyhole bottom walls, and the pressure imbalance in the keyhole bottom contribute to the keyhole collapse. The gap promotes the downward flow below the MIG arc center, which helps to transport the Ni element from the top surface of the MIG pool region to the bottom. The “Pull-Push” molten metal flows in the top molten pool suppress the Ni element to flow from the MIG pool region to the plasma pool region. As a result, the Ni content is inhomogeneous in the hybrid pool, and higher in the bottom of the MIG pool region.