# Numerical Study on the Evolution Mechanism of the Crater under a Millisecond Laser
## 3 Results and Discussion
### Effect of Laser Duration on Morphology  
The effects of the ablation rate on microstructure morphology were compared when the power density was \(3\times 10^{9}\) W/m\({}^{2}\) and the laser duration was 1-3.5 ms. Similarly, the experimental and simulated results showed the same variation trend. It can be seen from Figure 17 that the depth, diameter, and melted zone of the microstructure increased with the increase of the laser pulse width and as the depth increased from 1.78 \(\upmu\)m to 9.45 \(\upmu\)m, the radius increased from 110 \(\upmu\)m to 186 \(\upmu\)m and the melted zone increased from 0.00198 mm\({}^{2}\) to 0.01249 mm\({}^{2}\), while the height of the edge bulge, in the linear diagram of Figure 17b, is shown to decrease with the increase of pulse width after 1.5 ms.  
Figure 16: Crater morphology under different peak power density for \(t=10\) ms, \(\tau=1\) ms.  
Figure 17: _Cont._  
The surface morphology of the molten pool at the end of heating under a different pulse width was compared, as shown in Figure 18. It should be noted that when the pulse width changes, the central depth of the molten pool changes more sharply, and the height of the edge bulge increases more slowly in comparison. The reason for this is that the maximum temperature of the molten pool and the evaporation time of materials increases with the increase of laser duration, leading to more material being ablated and removed. Due to the clear difference between the pool depth and bulge height, more molten reflow is performed during cooling under the effect of Young-Laplace stress, resulting in a decrease of the edge bulge after cooling.  
In engineering applications of lubrication and friction, a deep crater is usually required, and a larger melted zone is beneficial for improving the strength of the contact surface. Upon comparing the microstructures with a peak power density of \(5\times 10^{9}\) W/m\({}^{2}\), a pulse width of 1 ms, a peak power density of \(3\times 10^{9}\) W/m\({}^{2}\), and pulse width of 3 ms, as shown in Figures 15 and 17, it can be found that the two microstructures have similar depths after cooling. The surface morphology of the molten pool at the end of heating is presented in Figure 19. It can be seen that the depths of the two microstructures are quite different at 18 \(\upmu\)m in Figure 19a and 25.2 \(\upmu\)m in Figure 19b, whereas the height of the edge bulge is similar, at 13 \(\upmu\)m in Figure 19a and 12.5 \(\upmu\)m in Figure 19b. The edge bulge width of the latter is more extensive, which makes more melt flow back, resulting in similar depths after cooling. Moreover, it can be seen that the melting area is larger under the high pulse width. Therefore, it can be concluded that when the laser pulse width is between 1 ms and 3.5 ms, peak power density is between 1 ms and 3.5 ms, and peak power density is between 1 ms and 3.5 ms.  
Figure 17: Variation of the molten pool under different laser durations for \(I=3\times 10^{9}\) W/m\({}^{2}\), \(t=10\) ms. (**a**) Radius of the crater, (**b**) Height of the crater, (**c**) Depth of the crater, (**d**) Melted zone area of the crater.  
Figure 18: Crater morphology under a different laser duration for \(t=10\) ms, \(I=3\times 10^{9}\) W/m\({}^{2}\).  
\(3\times 10^{9}\) W/m\({}^{2}\) and \(5\times 10^{9}\) W/m\({}^{2}\), z higher pulse width will lead to a larger melted zone than a higher power density upon obtaining microstructures with the same depth.