Investigation On Laser Micro Cutting With/without Using E-Field Or Underwater Magnetic Field And Its Improvement For Surface Brazing

Compared with traditional mechanical machining,laser micro cutting has its unique technical advantages.However,laser micro cutting is a thermal process,in which some defects such as the recast layer,micro cracks,heat affected zone and residual stress normally occur.Accordingly,in this work,the E-field and the underwater magnetic field were proposed and utilized for assisting the laser micro cutting process,aiming for reducing or avoiding these defects.Furthermore,the laser micro cutting technique was also proposed and used for improving surface wetting and brazing performance for ceramic and super-alloy materials.On the basis of the established high-precision femtosecond laser micro-cutting facility assisted by a transverse E-field,the effects of pulse repetition rate,scanning cycles per layer,scanning speed,feed distance and E-field assistance on laser micro hole-cutting in super-alloy GH4133 and ceramic Al₂O₃ sheets were discussed.It was shown that the use of a transverse E-field improved the hole-cutting efficiency and quality.With the increase in E-field intensity,the hole entrance and exit diameters,hole wall surface roughness and oxidation decreased,while the blind-hole depth and micro hardness performance showed a reverse trend.Based on the developed experimental platform of high-precision millisecond laser micro-cutting assisted by an underwater unidirectional magnetic field,the effects of laser cutting speed,assist gas pressure and defocus distance on micro slot-cutting performance in silicon nitride ceramic sheets were studied.The influence of water,unidirectional magnetic field and underwater unidirectional magnetic field on laser micro cutting performance in Si₃N₄sheets was also analyzed.It was found that both water and magnetic field improved the quality of laser micro slot-cutting,whereas the underwater magnetic field performed further better.As per the non-wetting problem on the ceramic surface,in this thesis,based on femtosecond laser micro cutting,the effect of the microstructure array on the spreading and wetting performance for high-temperature filler metal on Si₃N₄ and GH4133 sheet surfaces was discussed.The performance for brazing Si₃N₄ and GH4133 improved by the laser cut microstructure array was also analyzed,which included the performance of the formation,microstructure characteristics,micro hardness and shear strength for the brazed joint.It was indicated that the laser cut microstructure array could improve the wettability of the melted filler metal on the ceramic surface.Compared to the circular hole array,the wettability performance for the nickel-based filler metal on the super-alloy surface improved by using a laser cut square hole array or slot hole array was better.It was shown that the fracture normally located at the ceramic side near the brazed joint of the ceramic and super-alloy sheets.The square slotted hole array patterned on the ceramic surface noticeably improved the shear strength of the brazed joint.