Investigation On Mechanism Analysis For Pulsed Laser Hole-trepanning Using Magnetic Assistance

Laser hole-generating technology has gradually replaced the traditional mechanical drilling method,which has been widely used in various fields.However,there are still some problems in laser hole-generating technology.For hole-generating using long pulses,micro defects(such as the recast layer,micro cracks,taper)easily occurs due to the thermal effect.And the control of micro-hole size is not stable,and the hole-generating accuracy cannot be guaranteed.Meanwhile,for ultra-fast laser hole-generating,material removal efficiency is low.Unstable ablation particles and plasma occur in the hole-generating owing to the obvious plasma shield.Accordingly,it is difficult to solve the problem of the depth limit for ultra-fast laser hole-generating.Therefore,this paper proposes a method of magnetic field-assisted laser hole-generating to improve the efficiency and quality of deep hole generation.The numerical simulation and experimental study for pulsed laser hole trepanning in stainless steel 304 sheets were carried out using millisecond and femtosecond laser with and without magnetic field assistance.Firstly,the principle of magnetic field-assisted pulsed lase hole trepanning was presented through describing the characteristics of laser beam,the physical process of pulsed laser hole-generating,the formation of plasma and the influential mechanism of magnetic field on plasma.The configuration of experimental platform for millisecond/femtosecond pulsed laser hole trepanning drilling with magnetic field assistance were illustrated together with introducing the treatment method of sample and equipment for observation and measurement.Secondly,the numerical simulation of millisecond laser hole-trepanning with magnetic field assistance was carried out using the ANSYS analysis software.The temporal temperature field distribution of millisecond laser hole-trepanning was analyzed,and the cross-section profile of the micro-hole was simulated via using the birth-death element method.The changes of hole entrance and exit diameters underthe action of different single pulse energy were analyzed,which provided a reference for millisecond pulse laser hole-trepanning using magnetic field assistance.Thirdly,this work analyzed the experiment of Z-direction synchronous and asynchronous hole-trepanning using millisecond laser to optimize the experimental parameters.The optimized parameters of synchronous and asynchronous hole-trepanning using millisecond pulse laser were obtained.The experiments of millisecond laser Z-direction asynchronous hole-trepanning using magnetic field assistance were carried out,and the influence of magnetic field on millisecond laser hole-trepanning was studied.It was shown that the magnetic field enhanced the effect of melt expulsion with spattering and improved hole wall quality.Finally,the experiments of femtosecond layered ring trepanning using magnetic field assistance were carried out for analyzing the effect of magnetic field under different operations.The influential mechanism of magnetic field on femtosecond laser hole trepanning was systematically studied,in terms of the three aspects of geometrical morphology characteristics,hole wall roughness and hole wall composition analysis.It was shown that the external transverse magnetic field improved the hole geometry and hole wall quality while reducing the oxidation degree.Based on the orthogonal experiments,the optimized parameters for femtosecond laser layered ring trepanning of through holes were obtained.In this thesis,the thermal effect of femtosecond laser hole trepanning with high pulse repetition rate was studied from the aspects,by means of debris accumulation,heat affected zone and oxidation.It was indicated that the debris accumulation at the hole entrance decreases,the debris accumulation at the hole exit increases,and the through-hole taper was improved by using magnetic field.