Cutting and drilling of materials, especially metals, is well-established in industry. Because laser processing is typically slower and more costly than conventional processes, it is typically used in high value applications where precision is required. These applications are important drivers for laser source development, motivating improvements in cost, throughput, and efficiency of short pulse lasers. The desire to improve the throughput and efficiency of laser drilling and cutting also drives the exploration of new approaches which optimize the laser material interactions involved in the ablation process itself.
Both pulsed and continuous wave (CW) lasers are employed in metal processing. When a laser pulse interacts with a metal surface, it vaporizes a thin (10 to 100 nm) layer of material leaving behind a melt layer. The energetic cost of removing material as a vapor is high, since much of the energy of the laser pulse goes into heating the vapor and into the enthalpy of vaporization which is in general high. Thus, short pulsed lasers have low removal efficiency. For longer pulses (nanosecond and longer) or in the case of CW excitation, in which the laser intensity is much less than for the shorter pulses, vaporization can be reduced and more energy goes to creating a deeper melt layer. The most efficient processes avoid vaporization, setting up conditions to remove material as a lower temperature melt. However, the melt is held tightly to the surrounding material by surface tension, and in deep channels created by high aspect ratio drilling, the melt can be very difficult to remove. This limitation can be overcome in some industrial processes through use of high-pressure gases, however, the effectiveness of this approach is reduced in high aspect ratio channels, for very fast drilling or cutting, for thick materials, or in cases where some stand-off is required such that the laser is not proximate to the surface.
Livermore Lab researchers have developed two new methods for improving the efficiency of laser drilling. The first method is based on multi-pulse laser technology. Two synchronized free-running laser pulses from a tandem-head Nd:YAG laser and a gated CW laser are capable of drilling through 1/8-in-thick stainless-steel targets at a standoff distance of 1 m without gas-assist. The combination of a high-energy laser pulse for melting with a properly tailored high-intensity laser pulse for liquid expulsion results in the efficient drilling of metal targets. The improvement in drilling is due to the recoil pressure generated by rapid evaporation of the molten material by the second laser pulse.
The second method is based on periodically modulating the CW laser light intensity with a sinusoidal frequency. The modulation frequency is chosen to match the frequency of oscillation of the melt pool. As a result of this resonant excitation, the melt removal rate is enhanced dramatically. Typically, the resonant frequencies are in the kHz regime.
LLNL's enhancement of laser material processing uses modulation of the processing laser intensity, or multi-pulse laser technology, to increase melt removal efficiency and to enhance the drilling and cutting process. Using these methods, LLNL researchers have demonstrated greater than 10x enhancement in removal rate efficiency.
The practical applications of the method include the enhanced drilling with high aspect ratio and possible increase in the cutting rate for laser metal processing.
- Laser cutting of metal sheets
- Accelerated drilling of high aspect ratio channels, with good walls quality
- Possible applications to processing of dielectrics and semiconductors
Two patent applications have been filed for this technology; PCT/US2019/023558 (IL13305) and PCT/US2019/023523 (IL13306).