The Basic Investigation On Interaction Between Laser And Material In Water

Laser machining takes the usage of high energy density to gasification the material in short time. With advantages of great stability, high machining speed and high accuracy, it is suitable for micro-machining. However, there are several particular and complicated phenomenons, like cavitations, when the laser processing happens in water. It is necessary to comprehensive understand the basic theory of cavity and the effect on the laser processing itself in order to apply it in water. Meanwhile, experimental investigation is undergoing to find out the effect of the parameter of laser and the property of water surely on the final property of parts, such as the radius, the depth, the roundness and the roughness.In this paper, the Lattice Boltzmann Method is applied to set up the numerical model of cavitations. In order to simplify the model without losing the accuracy of simulation, the movement of the bubble wall is considered as the boundary condition for the gas and as initial condition for the fluid. In this way, the two phases (gas-fluid) physical phenomenon is divided into two simple models which are connected by the bubble wall. In the simulation, trough the analysis of the velocity and the pressure distribution, it is found out that the symmetric bubble is characteristic with pulsation and high speed shock that are limited in a small field with radius of 7mm during the bubble growth. And it noted that the bubble is circularly collapsed for the symmetric velocity distribution.The surface of the part is introduced into the symmetric bubble model to form the more realistic model called asymmetric bubble. With the help of simulation, no pulsation is detected but it is noticed that the pressure is no longer present uniform distribution for the four virtual surfaces symmetrically set around the bubble. Closer to the part's surface, the pressure is slightly higher and the velocity gradient is larger than those further away from the solid surface. And the distortion along the flow line is discovered at the end of the solid surface. Comparing with the symmetric bubble, the asymmetric bubble collapses in a complicated way thanks to the various velocities in different point of the bubble wall. What is more, a micro jet with radius of 10μm is generated which leads to the sharp reduction of momentum of the wall and the increase of the pressure near the bubble wall. According to the simulation, the impact on the workpiece peaks at 1.6GPaThe lower laser power is and the deeper in the water, the lower the energy density is at the focus. In addition, the laser runs with less power than the rated power, the spot size is decrease. All these result in less depth but more accuracy. However, when the laser pass through the water, the bubble in the water changes the position of the spot which increase the roundness. The important properties of the workpiece are the boiling point determining the minimum energy needed and the corresponding light absorption rate determining the maximum utilization of the laser's energy. Based on the orthogonal experiments, the optimal parameters are found as following, laser power 10W and 5mm under the water. With these parameters, the series of holes and slots are produced.