Experimental characterization and real-time monitoring for laser percussion drilling in titanium alloy using transverse electric field assistance and/or lateral air blowing

Abstract

A novel laser percussion drilling technology assisted by an electric field with lateral air blowing is reported. The lateral air blowing is first proposed for assisting the E-field assisted laser drilling for improving hole-drilling performance, quality and efficiency by blowing away the vapor plume and cooling the target workpiece. A high-speed camera is first used for observing and characterizing the transient laser percussion drilling process with and without an electric field and/or a lateral airflow. It was indicated that the transverse electric force and/or the side air blowing expedited the spatial motion with dispersion and density reduction of the vapor-plasma plume for enhancing laser drilling efficiency via weakening the plume-induced shield effect. The recast layer was reduced by using the transverse E-field assistance with and without lateral air blowing, and the combined assistance of a transverse E-field together with a lateral airflow was more effective for reducing the recast layer. Interestingly, as a result of the dynamic air cover across the sample surface induced by the lateral air blowing, the laser-vapor interaction inside a drilling hole was greatly enhanced together with the laser-material interaction, leading to the rapidly-accumulated expansion and then the sudden explosion of the laser-induced plume inside a drilling hole. The enhanced vertical scouring and flowing effect resulting from the accelerated vertical plume-motion inside a drilling hole, which was caused by the accumulated pressure difference inside and outside of the hole due to the enhanced outside motion-dispersion effect (especially induced by the electric force) of the plasma-vapor plume, expedited the uniform molten material removal and distribution along the hole sidewall. Importantly, a transverse E-field with and without a side airflow effectively refined the grains of the heat affected zone (HAZ) near the fusion zone of a through hole via increasing the average grain size number. The micro hardness was improved after laser hole-drilling by loading a transverse E-field with and without a side airflow due to a grain refinement effect with and without an airflow cooling effect. It was demonstrated that the lateral air blowing used was more effective to increase laser drilling efficiency for drilling through the workpiece than the E-field used, and the hole-drilling efficiency was further increased if using the combined assistance of a transverse E-field and a side airflow. After the workpiece was drilled through, the hybrid plume signal was effectively suppressed primarily above the hole entrance by using a transverse E-field and/or a side airflow, especially in case of using the transverse E-field assistance.