# Nanosecond Laser Ablation of Ti-6 Al-4V under Di erent Temperature
## 3 Results and Analysis
### Ablation Analysis of Experiment B  
We observed and compared the ablation crater morphology of 1-32 pulses irradiation at different substrate temperatures, taking 50 \({}^{\circ}\)C as example, shown in Figure 8. The corresponding data are shown in Table 3.  
From Figure 8, it was found that at substrate temperature of 50 \({}^{\circ}\)C, after the first pulse irradiation, obvious ablation crater was not formed. After the 4 th pulse irradiation, an obvious ablation crater was formed on the surface of targets, and ablation depth increase rapidly between pulse number of 4 and 32. Figure 8: Ablation crater morphology after irradiation of 2\({}^{\text{n}}\) (n = 0, 1, 2, 3, 4, 5) pulses at substrate temperatures of 50 \({}^{\circ}\)C. (**a**) 1 pulse; (**b**) 2 pulses; (**c**) 4 pulses; (**d**) 8 pulses; (**e**) 16 pulses; (**f**) 32 pulses. In particular, the morphology of the ablation craters after the 4 th pulse irradiation at different substrate temperature are compared and shown in Figure 9. It was found that after the 4 th pulse irradiation, obvious ablation craters were formed in 50 \({}^{\circ}\)C group, however, with the increasing of substrate temperature, the ablation craters gradually became inapparent, in 400 \({}^{\circ}\)C group, there was only a shallow crater. In addition, we compared the ablation craters' cross section after irradiation of 32 pulses at different substrate temperatures, as shown in Figure 10. It was found that the morphology of the ablation craters were more slender in a lower substrate temperature. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Laser fluence** & **Pulse** & **Depth** & **Diameter** & **\(\omega\)** \\  & **(μJ/μ\({}^{2}\))** & **Number** & **(μm)** & **(μm)** & **(μm\({}^{3}\)/mJ)** \\ \hline
1 & & 1 & 3.83 & 0 & 0 \\
2 & & 2 & 5.33 & 35 & 25.64 \\
3 & 0.7143 & 4 & 14.55 & 52 & 77.25 \\
4 & & 8 & 25.40 & 69 & 118.72 \\
5 & & 16 & 54.12 & 81.5 & 176.46 \\
6 & & 32 & 112 & 90 & 222.66 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Ablation experiment data at 50 \({}^{\circ}\)C substrate temperature. Figure 11 presents the relationship between ablation crater's diameter and pulse number. It was found that when the pulse number are less and the ablation craters are not obvious, the ablation craters' diameter increases rapidly with the increasing of pulse number, when the pulse number are more and the ablation craters are obvious, the ablation craters' diameter is almost invariant. We took the diameter of the ablation crater after irradiation of 32 pulses, and drew the Diameter-Substrate temperature curve, as shown in Figure 12. Similar to Figure 5b of experiment A, the diameter of the ablation crater increased as substrate temperature increased, and the fitting curve is similar to Gaussian distribution curve. Figure 13 presents the ablation depth-pulse number curves at substrate temperatures of 50 \({}^{\circ}\)C-400 \({}^{\circ}\)C. It was found that after the 4 th ablation, the curve became straight. The results show that at the same temperature, the ablation of laser pulses after the 4 th pulse irradiation was similar with each other. In addition, as the substrate temperature increased, the slope of the curves showed a downward trend. In details, between the 1 st pulse irradiation and 8 th pulse irradiation, the slope of the curve was stable first and then rising, the rising point of 50 \({}^{\circ}\)C-200 \({}^{\circ}\)C was forward, and 250 \({}^{\circ}\)C-400 \({}^{\circ}\)C was backward. It shows that the ablation may have been greatly affected by the plasma-shielding effect  
Figure 11: Diameter of different pulse number. Figure 12: Diameter of ablation craters under different substrate temperature. when the substrate temperature was more than or equal to 250 \({}^{\circ}\)C. In combination with the phenomena shown in Figure 9, we think that this indicates that a phase explosion occurs at this pulse number (4 pulsed).