Abstract
Objective:
The interfacial interaction properties of femtosecond lasers with energetic materials (EMs) and metal shells were studied during disassembling of decommissioned EMs. Based on verifying the safety of femtosecond processing of a single type of explosives, analyzing the safety processing parameters of explosives bonded to tungsten alloys. This research will develop the theory of femtosecond laser-materials interactions, providing an efficient way to disassemble decommissioned munitions.
Methods:
The femtosecond laser used for explosive ablation and cutting in the experiment is a Ti: sapphire laser with a center wavelength of 800 nm, a pulse width of 35 fs, a pulse repetition frequency of 1000 Hz, and average output power of 2 W. The explosion-proof tank is carried out during the EMs cutting process, which can contain the pressure pulse, high temperature, and reaction debris generated by the explosion and filter the harmful gas during the laser cutting. The bottom three-dimensional motion control platform moves the EMs samples, and the charge coupled device (CCD) camera observes and records the reaction phenomenon of the sample ablation and cutting through the window of the explosion-proof tank. This work ensures the safety of the experimental process by strictly controlling the number of explosives (mg level).
Results and Discussions
The results of the ablation experiment were detected by microscopic detection methods, and the ablation characteristics of tungsten alloys and explosive were discussed. When the incident fluence of the femtosecond laser is 0.32 J/cm2, regular elliptical holes are formed on the surface of the TATB and the surface layer appears black. This morphology indicates that the femtosecond laser irradiation will not cause obvious thermal effects on the area outside the laser irradiation area at low fluence. The ablation area and contour are black when the incident laser fluence is in the lower range of 0.32 – 1.59 J/cm². With the high incident laser fluence, the ablation area expands rapidly, but the surrounding area yellowed by the influence of heat is not obvious. When the laser energy range is within 3.19 – 6.37 J/cm², we can find the ablation area is significantly larger than the laser spot area. Compared with the increase of laser fluence from 0.32 J/cm² to 6.37 J/cm², the ablation area of TATB has risen by about 30 times, and the surrounding heat-affected material is denatured, while the ablation area of HMX has increased by three times and the ablation morphology has no obvious change.
Conclusions
In summary, we demonstrated the feasibility and safety of femtosecond laser cutting of metal alloy of discarded ammunition by femtosecond laser single-pulse irradiation. Under laser single-pulse irradiation, HMX is more stable than TATB without significant thermal effect, even when the laser fluence reaches 6.37 J/cm². Femtosecond laser fluence between 0 – 3.19 J/cm² is a safe working range for cutting tungsten-bonded TATB and HMX. The research of femtosecond laser cutting provides a theoretical basis for the disassembly and precision machining of energetic materials, which can effectively reduce the occurrence of dangerous accidents in the process of discarded ammunition.
 

femtosecond laser,tungsten alloy,energetic materials,safety verification