# Surface Forming Criteria of Ti-6 AL-4V Titanium Alloy under Laser Loading
## 3 Results and Discussion  
### Influence of Thickness on Forming Effect  
The spot diameter was selected to be 7 mm, and a 10 J single pulse laser was used to impact TC4 titanium alloy targets of different thicknesses. The target thicknesses were 0.01 mm, 0.03 mm, 0.05 mm, 0.08 mm, 0.3 mm, 0.5 mm, 0.8 mm, 1.0 mm, 1.2 mm and 1.5 mm respectively. The thickness of TC4 titanium alloy targets ranging from 0.01 mm to 0.5 mm had obvious plastic deformation, whereas the targets larger than 0.5 mm had no visible deformation. Figure 2a shows the back outline of targets with different thicknesses under the impact of 10 J laser pulses. Figure 2b exhibits the morphologies of the target material after a single impact deformation.  
As shown in Figure 2, when the thickness of the target material was 0.01 mm, the target underwent plastic deformation and a petal-shaped small hole was present on the back of the target. As the laser spot was not placed at the center of the target material during experimental operation, the target material only presented its shape, and the specific  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Material** & \(\rho\)**(g cm\({}^{-3}\))** & **E/Gpa** & **v** & **A/Mpa** & **B/Mpa** & **C** & **n** & **m** \\ \hline Ti-6 AL-4V & 4.51 & 110 & 0.34 & 875 & 793 & 0.011 & 0.387 & 0.71 \\ \hline \end{tabular}
\end{table}
Table 2: Johnson-Cook parameters of Ti-6 AL-4V.  
Figure 2: **(a)** The outline of targets with different thicknesses. (b) The morphologies of target material with different thicknesses.  
value could not be measured. For 0.03 mm thickness target, there was a petal-shaped hole appearing on the back of the target, and the maximum tear size was about 20.1 mm. Compared with the target with 0.01 mm and 0.03 mm thickness, the 0.05 mm TC4 alloy target seemed to be different; it had a 6.4 mm aperture circle hole after laser impact, and the size was almost equal to the laser spot diameter. When the thickness of the target material was 0.08 mm, 0.3 mm and 0.5 mm respectively, the deformation profile curve of the target showed a gradually shrinking trend, and the corresponding maximum deformation decreased from 2.6915 mm to 0.9675 mm to 0.4334 mm. Since there was no deformation behind the target with a thickness of 0.5 mm, the deformation profile of the impact surface was measured. As is demonstrated in Figure 2, no plastic deformation occurs when the thickness of the target material is greater than 0.5 mm.  
The laser irradiates the absorption layer on the surface of the target. The absorption layer quickly absorbs the laser energy and then vaporizes quickly. In the process of continuing to absorb the laser energy, the steam is easily heated and ionized into plasma, and the plasma propagates into the target to form a shock wave. As the thickness of the target material increases, the propagation distance of the laser-induced shock wave in the radial direction of the target material rises, causing the time for the shock wave to reach the free surface of the target material to increase. Then the attenuation of the shock wave in the process of propagation inside the target material increases, resulting in a gradual decrease in the deformation of the target material. Therefore, for the TC4 titanium alloy target, it is more difficult for the surface of the target to undergo plastic deformation with the increase of the target thickness, and when the thickness reaches a certain value, no significant plastic deformation is found on the surface of the target.