Investigation On Mechanisms For Laser-Based Hole Generation Assisted By Transverse Electric Field And Lateral Airflow

With the development of society and industry,the need for high-quality,high-efficiency and high-precision micro-hole generation is becoming more and more urgent.Laser-based hole generation technology plays an important role in the field of micro-hole generation,resulting from its inherent characteristics and some advantages as compared to other hole-generating methods.However,laser hole-generating is essentially a thermophysical process.During a laser drilling process,some defects such as the recast layer and micro cracks usually occur.The laser-induced hybrid plume,which is composed of vaporized materials and plasma,will also affect the incident laser beam through an optical shielding effect.As a result,the quality and efficiency of hole generation will be negatively inflenced.The use of an ultrafast laser is not enough to solve the above-mentioned problems effectively,which in turn significantly reduces the hole-generation efficiency.As per the problems mentioned above,based on the influential characteristics of the airflow and electric field on the laser-induced hybrid plume,a new method of laser-based hole generation assisted by a transverse electric field and a lateral airflow is proposed in this thesis.The transverse E-field with a lateral airflow will weaken the shielding effect of the hybrid plume on the incident laser beam via improving its spatial morphology and distribution.Accordingly,the quality of micro-hole generation,the laser hole-generating efficiency and the microstructure and micro hardness performance surrounding the hole sidewall will be improved.In this thesis,at first,a five-axis high-precision millisecond laser hole-generating platform was designed and established using the assistance of a transverse E-field with a lateral airflow.The effect of a transverse E-field and a lateral airflow on the hole geometry,hole wall quality,microstructure and micro hardness performance was explored.At the same time,in order to further study the influence of a transverse E-field and a lateral airflow on the transient evolution of laser-based hole generation,the real-time monitoring of the millisecond laser percussion drilling processes were also carried out using a high speed camera with and without the assistance of transverse E-field and a lateral airflow.It was found that the E-field and lateral airflow accelerated the expansion and dispersion of the vapour-plasma hybrid plume,shortened the time duration for drilling through a hole,and improved the efficiency of laser hole-generating.When using a transverse E-field,a lateral airflow,a transverse E-field with a lateral airflow,the hole entrance diameter decreased by 9.1%,6.8% and 9.4%,respectively,while the hole depth increased by 5.9%,8.7% and 13.0%,respectively.As a result of the transverse E-field assistance with and without later air blowing,the hole wall quality was noticeably improved,attributing to the enhanced vertical plume motion and the weakened local thermal shock effect on the hole wall.In this work,the influence of the E-field intensity and airflow type on the femtosecond laser layered ring hole-cutting performance was also explored.It was shown that the hole entrance and exit diameters became smaller while the hole depth increased due to the use of a transverse E-field and a lateral airflow.With the increase of the E-field intensity,the hole diameter and hole wall surface roughness decreased,while the micro hardness increased.Meanwhile,the lateral airflow accelerated the air motion surrounding the hole and reduced the heat affected zone,while the E-field enhanced the plume motion and weakened the oxidation on the hole wall.