Material removal in laser chemical processing with modulated laser power

Laser chemical machining (LCM) is a method of laser processing based on gentle material removal by means of thermal induced chemical dissolution. Since LCM depends predominantly on the surface temperature of the workpiece, the process window is restricted by the appearance of gas bubbles at higher laser powers and their associated shielding effect. In order to extend the process understanding, the influence of the laser power modulation on the removal behavior is investigated in the present work. The experiments were conducted on titanium grade 1 and with phosphoric acid. Based on the response time in experiments with a single step function of the laser power, a spatial frequency threshold was determined above which a constant removal depth could be expected. Afterward, the laser power was modulated rectangularly in time, resulting in combination with the process velocity in different spatial modulation frequencies varying from 1 to 20 mm-1. The investigations showed that the removal cavity exhibited sinusoidal oscillation in depth along the machining direction with a spatial frequency corresponding to the spatial frequency of the laser power. When the spatial frequency exceeds the determined threshold frequency, the cavity depth is constant. This established the basis for generating complex removal profiles by varying the power in the range below the threshold frequency.