Marangoni flow patterns of molten pools in multi-pass laser cladding with added nano-CeO2

Abstract

Adding rare-earth elements has been proven to modify the microstructure and enhance the mechanical properties of the laser cladding layer. However, the effect of such elements on the flow pattern in molten pools has not been extensively determined, especially for multi-pass laser cladding. The surface tension of T15 high speed steel and nano-CeO₂ mixed powders is measured using the sessile drop method. Comparing the results from the high-speed photography of the molten pool flow and the multicomponent multiphase flow model, various Marangoni flows driven by surface tension are elucidated, and the cross-section of the multi-pass cladding is further analyzed. The surface tension increases with increasing CeO₂ content. When CeO₂ is less than 0.6 wt%, the surface tension monotonically increases with increasing temperature, and the molten pool flows inward. When CeO₂ is greater than 0.6 wt%, the surface tension decreases at high temperatures, and the molten pool exhibits double flows. In multi-pass laser cladding, the inclination of the molten pool increases on adding CeO₂. Consequently, the inclined angle of the cladding pass interface increases, and the fusion line on the substrate tends to be straightened. At 0.9 wt% CeO₂, the added powders remain at the center of the molten pool because of a low velocity, creating a low-temperature zone. The temperature coefficient of surface tension demonstrates a positive–negative–positive distribution from the center to the surrounding pool surface, thus forming an outflow. The solidus point of the cladding alloys is lower than that of the substrate, resulting in a small indentation at the remelted region between passes, which increases the interfacial zone.