# Surface Forming Criteria of Ti-6 AL-4V Titanium Alloy under Laser Loading
## 3 Results and Discussion
### Influence of Energy on Forming Effect  
The experimental parameters in Test 1: TC4 titanium alloy with a thickness of 0.03 mm was selected as the experimental object, the diameter of nanosecond laser spot was adjusted to 7 mm. Black tape was pasted on the surface of the target as the absorbent coating, and the impact position of the target was fully covered by water flow through manipulator. The single energy was 3 J, 4 J, 5 J, 6 J, 7 J, 8 J, 9 J and 10 J respectively. Figure 4a shows the back morphology of TC4 titanium alloy target with 0.03 mm thickness under different laser energies. In Test 2, the thickness of the target material was 0.08 mm, the diameter of the laser spot was 7 mm, and the pulse energy was 3 J, 5 J, 6 J, 7 J, 8 J, 9 J and 10 J. Figure 4b shows the deformation curves of the target material at different energies. Figure 4: (**a**) The morphologies of target material with different energies. (**b**) The outline of targets with different energies. Table 3 shows the experimental results of Test 1 and Test 2, from which the relationship between the surface morphology change of the target and the laser pulse energy can be summarized. The laser impacted a TC4 titanium alloy target with a thickness of 0.08 mm, under the conditions of one impact time and a laser spot diameter of 7 mm, the plastic deformation degree of the target gradually grew with the upward trend in the energy of laser. When the laser energy is in a lower range, the percentage of its energy loss in the total energy decreases with the gradual growth of the laser pulse energy, and the effect of the laser-induced shock wave on the free surface of the target is more obvious. The maximum deformation was almost at the center of the laser impact, and as the distance from the center of the spot became larger, the deformation of the target material would be relatively reduced. When the thickness was 0.03 mm, the back of the target was not a smooth surface with varying degrees of deformation. The increase of laser energy resulted in larger and larger irregular holes. Holes came in diverse shapes, such as thin lines, windmills, petals and so on, and the measured aperture was positively correlated with the pulse energy of the laser. The destruction of materials is a dynamic process that gradually accumulates: the formation of micro-cracks and micro-holes, the growth of cracks and holes, the growth of micro-cracks and micro-holes merges into large cracks and holes, large holes form one or multiple continuous fracture surfaces which penetrate the material, and the material fails and damages. When the laser spot diameter is 7 mm, the thickness of the titanium alloy target material is 0.03 mm. Because the target material is too thin, under the single pulse laser energy of 3 J to 10 J, the target material does not undergo the first three processes, and directly breaks under the laser shock. On the surface, failure damage occurs through the target material. For the titanium alloy target material with a thickness of 0.08 mm, under a single pulse laser energy of 3 J to 10 J, the target material shows a complete dynamic damage process, and finally it turns out to be deformed.