Research Of Manufacturing Micro-convex Points On 45# Steel Surface By Long Pulse-width Laser

The metals which have surface micro-convex morphology are widely used in industrial, such as roller, stamping die, cutting tool and so on. In recent years, laser texturing has been researched a lot in the laboratory and actual engineering field. But so far, there are still so many of different perspectives about the forming mechanisms about laser-induced convex texturing. Therefore, in this thesis, pulsed laser was used to manufacture micro-convex points on the surface of 45# steel. Scanning electron microscopy and 3D profiler were used for measuring the morphologies of the micro-convex points. Three typical morphologies(spherical-cap shape,Mexican-sombrero shape, and M-type donut shape) were observed in the experiments.The numerical methods has been conducted to evaluate the temperature field and gasification recoil pressure. And the influences of pulse width and peak power density on the morphology have been researched systematically through the experimental and theoretical analysis. The following are the main contents.(1) Based on the 3D model of interaction between pulsed laser and 45# steel the laser-induced temperature fields have been simulated. The effect of laser pulse width and peak power density on the temperature field have been researched systematically. Combined with the theory of fluid field, the evolution trend of the micro-convex morphology has been predicted with the changing of the laser parameters when the material melts. The results indicate that, within a range of the laser parameters, laser will cause material melting and forming molten pool. Because the surface tension at the center of the molten pool is bigger than that at the edge,the liquid metal in the molten pool flow from the edge to the center, which produce micro-convex finally. Spherical- cap shape tends to form when peak power density is small and pulse width is short. With the increase of the pulse width and peak power density, the diameter of the central convex point become smaller and its height increases. So, micro-convex points evolve into Mexican-sombrero shape.(2) The gasification recoil pressure model of interaction between pulsed laser and 45# steel has been used to research the recoil pressure changing with the laser pulse width and peak power density. And the plasma shock wave model has been usedto analyze the effect of plasma shock wave pressure and shielding actions on micro-convex morphology. It turns that, within a range of the laser parameters, the temperature at the central of the molten pool exceeds the gasification temperature,gasified materials can form recoil pressure up to 107 or 108 Pa, exerting on the molten pool, which press down the top of the convex. Thus, M-type donut shape is formed. As the laser pulse width or peak power density increase furtherly, the recoil pressure grows accordingly. And the liquid material is completely pushed to the edge of the molten pool and eventually evolved into the crater morphology. Gaseous metal material absorb laser energy producing plasma and plasma shock wave pressure. We must notice that, in laser texturing, the peak power density is exactly around the threshold of plasma formation, so that the plasma shock wave pressure is small, less than the gasification recoil pressure 2 or 3 orders of magnitude. So in our research,plasma-induced shock wave pressure is a secondary factors influencing the micro-convex morphology. But, the shielding actions of plasma can not be ignored for most of the applications. And on those occasions, inert gas can be used to suppress the generation of plasma.(3)Laser texturing has been studied experimentally by choosing the appropriate laser parameters. Three typical morphologies(spherical-cap shape, Mexican-sombrero shape, and M-type donut shape) were observed in the experiments. Scanning electron microscopy and 3D profiler were used for measuring the morphologies of the micro-convex points. The height and the diameter of the convex changing with laser pulse width and peak power density, and the evolution law of the micro-convex morphology have been researched. Most of the experimental results validated the theoretical prediction.