Water-based helical laser hole-cutting in nickel super-alloy GH4049 assisted by longitudinal and transverse magnetic fields with/without ultrasonic assistance

Abstract

A novel helical laser hole-cutting (HLHC) technology based on coupled assistance of water-based ultrasonic vibrations and external longitudinal/transverse magnetic fields is reported for cutting holes in nickel super-alloy GH4049 sheets. Effects of external unidirectional transverse and longitudinal magnetic fields on HLHC performance were correspondingly analyzed and compared with and without water-based ultrasonic assistance. The influence of the loading direction of an external magnetic field and the magnetic field intensity of an external longitudinal/transverse magnetic field on HLHC performance is first reported based on systematic corresponding comparisons with and without water-based ultrasonic assistance. The effects of external longitudinal/transverse magnetic fields and beam expanding ratios on actual laser focal spot sizes were analyzed and discussed. It was demonstrated that the latest reported HLHC technology assisted by water-based ultrasonic vibrations and external longitudinal magnetic fields could generate deeper holes with relatively high cutting efficiency, smaller hole taper, higher hole aspect ratio, smaller hole circularity deviation, better hole wall quality, much better performance for relieving residual stress, thinner recast layer, more uniform refinement of grains, and better performance for improving micro hardness. Generally, a stronger external longitudinal magnetic field coupled by water-based ultrasonic vibrations was more helpful for cutting deeper and higher-quality holes. It was shown that the change of loading direction of an external magnetic field with/without ultrasonic assistance did not greatly alter recast layer formation and micro hardness performance for the workpiece helically cut. Compared to the transverse magnetic assistance, a corresponding longitudinal magnetic field coupled with water-based ultrasonic vibrations was more effective for micro hardness improvement for this reported HLHC technology. When using the same water-based ultrasonic vibration assistance, a stronger longitudinal magnetic field was more effective for relieving residual stress for the workpiece helically cut (a stress reduction percentage of around 96.5% was reported in this work). The average laser focal spot size obviously decreased with laser beam expanding ratio, but the external longitudinal/transverse magnetic assistance did not greatly alter the actual laser focal spot size.