Porosity in thick section alloy 690 welds - Experiments, modeling, mechanism, and remedy

Laser and hybrid laser-arc welding present significant opportunities for thick section welding of nickel-based alloys during the construction and repair of nuclear power plant components. However, the impact of these welding processes on the fusion zone geometry and defect levels in Alloy 690 are not well understood. A series of laser and hybrid laser-gas metal arc welds were fabricated with varying laser powers and welding speeds. The internal macroporosity remaining after welding was characterized using x-ray computed tomography. While the porosity levels attributed to keyhole instability and collapse remained high in the laser welds for all power levels, the addition of the arc in the hybrid laser-arc welds inhibited the formation of porosity at laser powers in excess of 4 kW. A well-tested three-dimensional heat transfer and fluid flow model was used to determine both the geometry of the fusion zone and the region of mixing of the filler metal within the weld pool for various welding variables. By correlating the geometries of the weld pool and the volume of the filler metal mixing region with the experimentally determined porosity, significant insight can be obtained about the mechanism of porosity reduction in hybrid laser-arc welding. At lower laser powers, a combination of high-speed filler metal addition and small pool size prevented the bubbles from escaping. The experimental and calculated results show that porosity in Alloy 690 hybrid welds can be eliminated if the laser heat input and arc conditions are properly selected to avoid the bubble being trapped in the weld pool. The mechanistic understanding uncovered in the work is used to develop a process map showing the important combination of welding variables for producing porosity-free hybrid welds.