Numerical Study On Thermal And Mechanical Effects Of Combined Double Pulsed Laser Drilling

Aiming at solving recast layer and the pore blocking phenomenon of the hole during the millisecond laser drilling, this paper presented double pulse laser combined drilling conception. A millisecond laser with nanosecond laser combined effect of perforated aluminum target numerical model was established in this paper. The finite element method was used to analyze the effect of laser parameters on the hole shape as well as the migration of molten material during drilling process.In order to have a deep understanding to millisecond laser drilling process, first, the solid-liquid phase transition has been studied in the early stage of millisecond laser drilling of aluminum target from the perspective of the numerical simulation. The velocity distribution of Marangoni convection inside the pool was calculated. In order to explore the appropriate laser spot size, vaporization threshold corresponding different laser beam radius as well as the aspect ratio was also studied. The results showed that the smaller the radius of the laser spot the greater aspect ratio.After the occurrence of gas-liquid phase transition, the migration form of matter and the formation of the hole were studied when the energy of millisecond laser exceeded the vaporization threshold. The migration form of substances was obtained with different laser energy under the laser beam radius 0.3mm. When the energy is below 15J, in addition to material evaporated, some melt was expelled out and accumulated around of the hole surface. When the energy is greater than 15J, a strong ejection was observed with the speed of 8m/s approximately. Meanwhile, the higher the laser power is, the larger of the speed of the ejection, the greater the aspect ratio.The key effect of double pulse laser drilling is using shock wave, which is generated by nanosecond laser plasma to promote removal of molten material. The effect of shock wave on liquid was investigated numerically and experimentally. The experimental results showed that as the keyhole depth increasing from 0.5mm to 3mm, the removed mass ratio of water decreased from 88.9% to 26.2%. Further numerical results showed that when plasma shock wave was acting on water, the speed of ejection will be faster than 474m/s since the surface tension of the water is low. In the case of liquid aluminum with higher surface tension, the speed decreased to around 180m/s.Based on the study of the effect of plasma shock wave on the liquid, the model of shock wave was implemented into the process of millisecond laser drilling. Then the process of drilling by combined double pulse laser was simulated. A second ejection was observed with the help of plasma induced shock wave. However, the velocity of ejection was less than 100m/s because the hole was so deep that molten material was difficult to eject out. And the size of ejected droplet was small. The study of the time interval between plasma shock wave and ms laser showed that the later the plasma is loaded, the thinner the recast layer is.The research in this paper will provide theoretical references for comprehending the phenomenon in combined double pulse laser drilling. This will be also beneficial to the application of combined double pulse laser drilling in process of cooling hole.