Investigation On Mechanisms And Fundamentals For Laser Hole-generating Assisted By Magnetic Fields With/without Ultrasonic Vibrations

In order to increase the thrust-to-weight ratio of the engine,the pulsed laser is normally utilized for generating a large number of microholes in its thermal components for improving its working performance and lifespan.Laser hole-generating is essentially a thermal physical process,some defects such as the recast layer,microcracks and residual stress usually occurred.In this work,a systematic investigation on laser hole-generating was carried out assisted by ultrasonic vibrations and/or external magnetic fields for reducing and avoiding these defects,improving the hole-generating quality and performance.In the present work,the numerical simulation on laser trepanning of square-slotted hole and hole array in stainless steel sheets with and without ultrasonic assistance was carried out for analyzing the stress fields on the workpieces trepanned.Based on reasonable modification and verification for the established models using corresponding experimental results,the transient and/or residual stress distribution characteristics,which were induced by single hole trepanning and hole array trepanning with and without ultrasonic assistance,were analyzed systematically and numerically using the finite element method(FEM)coupled with self-developed subprograms.The influence of ultrasonic assistance on stress reduction was also discussed.As a whole,the ultrasonic assistance resulted in a small change for the distributional trend of the transient/residual stress field predicted,while the influence of ultrasonic-induced softening effect on laser trepanned workpiece was primarily demonstrated by the effective reduction of the stress value.Using a self-developed real-time monitoring system,a systematic investigation was carried out for observing and analyzing the process and performance of millisecond laser percussion drilling with and without water-based ultrasonic vibrations and/or magnetic assistance based on high-speed photography.The systematic and corresponding comparisons between the effects of co-axial assist gas,magnetic assistance and water-based ultrasonic vibrations for millisecond laser drilling of both blind holes and through holes using high-speed photography were studied.Using a direct comparable visual method based oncomparable observations using high-speed camera,the effects of the co-axial assist gas,magnetic assistance and/or ultrasonic vibrations on millisecond laser drilling process,drilling efficiency and drilling quality were systematically analyzed by correspondingly comparing and correlating the real-time observations with the measurements for laser drilled specimens,including the characterizations and analyses for material removal,plume expansion with dispersion,specimen spatters,hole geometry/morphology,hole sidewall recast layer and micro hardness performance.The co-axial assist gas flow and magnetic field could effectively enhance plume dispersion and material removal rate,improve the quality of hole sidewall quality and increase micro hardness.Ultrasonic vibrations accelerated the upward ejection speed of hybrid plume and molten spatters,increased hole-drilling efficiency,and improved quality of holes.It was shown that ultrasonic vibrations made grains finer and improved the micro hardness performance.During laser hole-cutting process,the effects of ultrasonic vibrations and/or magnetic fields on the hole geometry,hole dimensions,hole wall quality,microstructure and micro hardness were also studied and analyzed.It was shown that the hole diameter,taper and roughness decreased with magnetic flux density,while the hole depth and micro hardness showed a reverse trend.During the process of helical laser hole-cutting assisted by ultrasonic vibrations and/or magnetic fields,laser-induced plasma was effectively suppressed,and the shield effect on the laser beam was weakened.The substrate absorbed more laser energy,and the flow of the molten metal was enhanced.Helical laser hole-cutting assisted by water-based ultrasonic vibrations and magnetic fields generated thinner recast layer with satisfied micro hardness performance.