# Numerical Study on the Evolution Mechanism of the Crater under a Millisecond Laser
## 1 Introduction  
Laser texturing is a laser processing technology, which can produce a specific micromorphology on the surface of mechanical parts. Because of its advantages like a high processing speed, high efficiency, non-contact processing, and less surface damage, it has received much attention in recent years. The crater texture has both shapes-bulges and dimples where the melted bulges increase the contact strength and friction, while the polished dimples store lubricating oil to form hydrodynamic lubrication [1]. Hence, the crater-shape topography finds wide applications in industry, such as for rollers, piston rings, and bearings [2; 3]. [PERSON] et al. [4] studied the effect of the geometrical parameters of the donut-shaped bump on the hydrodynamic lubrication performance. However, unlike for mechanical processing, it was found to be challenging to control the microtexture forming process. Thus, it is important to research the laser melting mechanism and analyze the influence of laser processing parameters on microstructure formation, to realize a self-designed microstructure. A crater shaped texture can be created by using a millisecond laser. Due to a low peak power density and long pulse duration, the metal substrate mainly melts and solidifies again while interacting with the metal. Hence, many scholars have studied the influence of the Marangoni effect and surface tension on molten pool flow. Based on the equations of conservation of mass, momentum, and energy in the weld pool, [PERSON] et al. [5] studied the evolution of temperature and velocity fields during laser spot welding of 304 stainless steel by using a heat transfer and fluid flow model, and found thatthe liquid flow is mainly driven by the surface tension and to a much lesser extent, by buoyancy force. [PERSON] et al. [6] took gravity, electromagnetic forces, arc pressure, and the Marangoni effect into consideration and studied the heat transfer and fluid flow in the molten pool. Also, the research compared the influence of various pulsed welding parameters such as pulse frequency and current ratio on the weld quality. [PERSON] et al. [7] studied the Marangoni effect in the model of selective laser melting, and considered that it was the main driving force of the fluid flow and enhanced the heat transfer in the molten pool. In the physical model of [PERSON] et al. [8], while considering the fluid flow driven by the surface tension gradient and influenced by the O element, the terminated distribution state of the AlN reinforcing particle was simulated. Their results showed that the thermo-capillary convection pattern changes from an inward flow pattern to an outward one due to oxidation of the molten pool. [PERSON] et al. [9] established a two-dimensional computational model and a Volume-of-Fluid model to analyze the streamlines and velocity distribution during the laser treatment. [PERSON] et al. [10] established a two-dimensional axisymmetric finite element model to find the contribution of thermo-capillary stress and surface tension to the melt flow and the deformation of the corresponding free surface. The model shows that the Marangoni effect is dominant in the heating phase, while the normal stress is dominant in the cooling period due to the curvature of the free surface. However, these numerical simulations did not consider the influence of metal evaporation on the molten pool flow after the local temperature reaches the boiling point. It is found that even the millisecond laser can easily make the metal reach its boiling point and that metal evaporation has a significant influence on the melting process [11]. [PERSON] et al. [12] studied the formation mechanism of the microstructure from the bump to the dimple, and the crater morphology formation was attributed to the intense Marangoni shear stress and recoil pressure. Moreover, when the laser energy is high, recoil pressure is the major driving force. [PERSON] et al.