Shusen Zhao

Capillary convection in pulsed – butt welding of miscible dissimilar couple:

Pulsed laser butt welding of miscible dissimilar couple has been studied numerically based on a three-dimensional heat and mass transfer model. Driving forces of molten metal flow in the case of stainless steel and nickel are compared using dimensional analysis. Thermo-capillary convection due to surface temperature gradient is recognized as the dominating fluid flow. Temperature coefficients of surface tensions are varied to investigate the influences of the convection on heat and mass transfer phenomena in weld pool. The temperature fields at stainless steel and nickel sides are found to be asymmetric and are greatly affected by different Marangoni convections. Concentration of Ni increases as the temperature coefficient of surface tensions changes from negative to positive. The temperature coefficient of surface tension of stainless steel has greater impact on the weld pool configuration and element concentration than that of nickel. The obtained weld pool configurations are found to be dependent on Prandtl number and convection patterns.

Surface cleaning of hot-rolled sheet steel by laser ablation of oxide layer using a 100-ns high-repetition frequency pulsed laser

Abstract. An investigation of laser cleaning of the oxide layers on the surface of the hot-rolled steel sheets was presented both experimentally and theoretically. By using a ∼100-ns pulsed laser, the optimized experimental parameters, such as the scanning speed, spatial overlap between two adjacent scans, and laser power density, were studied to obtain efficient and high-quality cleaning and avoid the formation of new oxide layer due to the heating by the laser. The temperature elevation and ablation depth were simulated and the data agree well with the experimental results. The change of composition of the surface layer, the surface roughness, and hardness due to laser ablation were also measured.