# Nanosecond Laser Ablation of Ti-6 Al-4V under Di erent Temperature
## 3 Results and Analysis  
### Ablation Analysis of Experiment A  
Figure 3 presents the top-view optical microscope images of ablation craters under substrate temperature of 50 \({}^{\circ}\)C-400 \({}^{\circ}\)C, after irradiation of 50 pulses. It can be seen from figures that the morphology of the ablation craters changed little under different substrate temperatures (50-400 \({}^{\circ}\)C), and there were heat-affected zones with different color and small craters near the ablation crater. In order to observe the ablation crater more clearly, the top-view scanning electron microscope images of ablation craters are shown in Figure 4. From the ablation craters figure of SEM images, it can be seen that the ablation craters present a stepped shape, the craters' diameter of the outermost ring was the largest and the craters' diameter was reduced in a downward circle; with the increase of substrate temperature, the diameter of the outermost ring of the ablation craters also gradually grew larger, and the splashing structure of the ablation slag also increased. We explain this phenomenon in this way: In this experiment, focused laser was used and applied on the target upper surface. However, because limited by focusing accuracy, the focus was actually below the upper surface. After each pulse, the ablation depth increased, and the distance between the  
Figure 4: Top-view scanning electron microscope images of ablation craters. (**a**) 50 \({}^{\circ}\)C; (**b**) 100 \({}^{\circ}\)C; (**c**) 150 \({}^{\circ}\)C; (**d**) 200 \({}^{\circ}\)C; (**e**) 250 \({}^{\circ}\)C; (**f**) 300 \({}^{\circ}\)C; (**g**) 350 \({}^{\circ}\)C; (**h**) 400 \({}^{\circ}\)C.  
ablation plane (spot) and the focal plane (focus) decreased. Therefore, the spot size (ablation radius) decreased step-by-step with the increasing of pulse number--resulting in the above phenomenon of ablation craters' stepped shape. Figure 5b illustrates the relationship between ablation-crater diameter and substrate temperature. It was found that the diameter of the ablation crater increase as substrate temperature increased. Moreover, the diameter increased fastest in the middle and slowest at both ends, and the fitting curve was similar to Gaussian distribution curve. Energy accumulation effect is used for explaining the multi-pulse ablation of metals in existing researches [11; 22; 23]. Since laser pulse energy was accumulated in the form of heat, substrate temperature could be considered as the initial accumulative energy. As the substrate temperature increased, less energy was absorbed from heating to ablation, and the ablation threshold \(l_{\text{th}}\) decreased. The relationship between laser fluence (energy density) and cross section radius is [23]:  
\[l(r)=l_{\text{max}}e^{-\frac{\omega^{2}}{\omega^{2}}}, \tag{1}\]  
where \(r\) is the distance (\(\upmu\)m) to the center of the beam, \(l_{\text{max}}\) is the peak-on-axis fluence (J/\(\upmu\)m\({}^{2}\)) of the laser beam, and \(\omega\) is the beam waist radius (\(\upmu\)m). Let \(l(r)\) be equal to the ablation threshold \(l_{\text{th}}\), the ablation diameter \(d\) is:  
\[d=\sqrt{-2\omega^{2}\ln\frac{l_{\text{th}}}{l_{\text{max}}}}, \tag{2}\]  
Since \(l_{\text{th}}\) decreased with the increasing of substrate temperature, the diameter of the ablation crater was positive to the substrate temperature, while the Gaussian distribution of the curve may be due to phase explosion and plasma shielding: The rapid growth of radius between 150-250 \({}^{\circ}\)C may be  
Figure 5: (**a**). How we measured diameter of ablation craters; (**b**) diameter of ablation craters under different substrate temperature.