# Nanosecond Laser Ablation of Ti-6 Al-4V under Di erent Temperature
## 3 Results and Analysis
### Ablation Analysis of Experiment A
We use cone to approximate the shape of the ablation crater, so the volume of the ablation crater can be estimated as:  
\[V=\frac{1}{12}\pi D^{2}h \tag{3}\]  
where \(V\) is the volume of ablation crater, in unit of \(\upmu\)m\({}^{3}\); \(h\) is the ablation depth, in unit of \(\upmu\)m; \(D\) is the diameter of ablation crater, in unit of \(\upmu\)m.  
Thus, we can get ablation efficiency \(\omega\), in unit of \(\upmu\)m\({}^{3}\)/m\(\}\):  
\[\omega=\frac{V}{nE} \tag{4}\]  
where \(n\) is pulse number. Figure 7 presents the relationship between \(\omega\) and substrate temperature. It was found that between 50 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, the slope of the curve increased with the increase of substrate temperature. When the temperature reaches 250 \({}^{\circ}\)C, the slope decreased greatly; when reaches 400 \({}^{\circ}\)C, the slope decreased to a negative value, and the ablation efficiency \(\omega\) obviously decreased. On the whole, it showed a trend of rising first, and then declining. We use the following theory to explain the above phenomena:  
The phase-explosion effect: When the target's temperature is extremely high due to laser irradiation, phase explosion instead of normal evaporation become the main coefficient affecting ablation, explosion boiling will happen and mainly manifested by explosive spattering of melt and  
Figure 6: Relationship between ablation depth and substrate temperature. Figure 7: Relationship between ablation efficiency \(\omega\) and substrate temperature. molten droplets. Existing research has confirmed that the occurrence of phase explosion will greatly accelerate the removal of materials [5; 6; 10; 11; 13]. The plasma-shielding effect: The influence of plasma was investigated in many studies; the formation of plasma will decrease the absorption rate of laser [8]. In addition, [PERSON] took plasma shield effect into consider to correct the energy distribution of the incident laser and found that the increment of laser fluence leads to a slower nonlinear increment in diameter of melt zone [12]. [PERSON] pointed out in his study that at high laser fluence, plasma shielding will efficiently attenuates the incoming Laser radiation and reduces the ablation rate [24]. We think that the substrate temperature can be considered as the initial accumulative energy, and it can be further equivalent to laser fluence. Thus, high substrate temperature may strengthen phase explosion, as well as cause plasma-shielding effect to occur in advance. Because of the inverted cone shape of the ablation pit and the Gaussian distribution of the laser pulse energy, the plasma is easier to gather at the ablation center and less likely to form or thinner at the ablation edge. Therefore, the earlier the plasma formed, the more ablation depth will be reduced, and the final ablation depth will be shallower, but the radius of ablation pit will not be affected, finally result in the phenomenon in Figures 5 and 6. Phase explosion effect and the plasma-shielding effect can also explain the phenomenon showed in Figure 7, \(\omega\) increase rapidly between substrate temperature of 150 \({}^{\circ}\)C and 200 \({}^{\circ}\)C indicated that phase explosion may occurred, while the decrease between 350 \({}^{\circ}\)C and 400 \({}^{\circ}\)C may indicated that a strong plasma shielding occurred. Further studies will be described in Experiment B.