Picosecond Laser Drilling of Carbon Fiber Reinforced Composite Laminates by Helical Drilling Process
Objective For the drilling of carbon fiber reinforced polymer(CFRP)laminates,there are some disadvantages such as hole taper and large heat affected zone(HAZ)when the traditional two-dimensional galvanometer scanning method is adopted.Herein,a helical drilling process is developed for drilling the CFRP laminate with large diameter holes.Theoretical analysis and experimental validation are carried out using a one-factor test method to reveal the generation mechanisms of hole taper and HAZ in helical drilling.The influence of process parameters(wedge prism deflection angle,mirror translation distance,rotary speed,laser power,and repetition frequency)on the HAZ and hole taper,and the influence of fiber orientation on the quality of drilled holes during laser drilling are analyzed.The results show that the high-quality hole of CFRP with a hole taper of 0.5° and a HAZ size of less than 50 μm can be obtained by rationally optimizing the helical drilling process parameters.Finally,we may provide a theoretical reference and experimental basis for further engineering of high-quality drilling of CFRP laminates by lasers.Methods We develop a novel helical drilling process for CFRP laminates by picosecond lasers,which enables arbitrary adjustment of the hole diameter and is particularly suitable for drilling large holes in CFRP laminates.For this helical drilling process,the laser beam rotates along the circumference of the target diameter,and meanwhile,the laser beam rotates at high speed around its beam axis.The high-speed rotation of the laser beam is similar to the rotation of the drill bit in mechanical drilling,which can achieve a larger machining width.Among them,R is the eccentricity distance of the incidence point of the laser beam.The rotary cutting radius r is the amount of material removed,which is controlled by the helical drilling optical system.Additionally,the helical drilling experiments are conducted with compressed air.After the experiments,a super depth of field three-dimensional microscope(VHX-6000)is employed to observe the HAZ and the taper of the drilled holes.By combining experimental verification with theoretical analysis,a quantitative evaluation of the quality of helical drilling is conducted.A single-factor experimental method is utilized to explore the influence of various process parameters(the deflection angle of wedge prism,mirror translation,helical rotating speed,excitation power,and repetition frequency)on the drilling quality.Results and Discussions Under the deflection angle of 30°,the HAZ size at the entrance and exit of the hole is below 50 μm[Fig.5(a)],and the diameters of the hole entrance and exit are about 4653 μm and 4580 μm respectively.The hole taper does not change significantly with the deflection angle increase of the wedge prism[Fig.5(b)].Therefore,the deflection angle variation of the wedge-shaped prism has little effect on the hole taper.When the mirror translation is 4 mm,the minimum taper is about 0.36°(Fig.8).Therefore,the mirror translation(M2)mainly affects the hole taper and HAZ size.As the mirror translation increases,the hole taper and the HAZ gradually decrease and tend to stabilize.The hole taper shows no significant change with the increasing helical rotating speed[Fig.11(b)].The taper of the drilled holes does not change significantly with the rising laser influence,and basically remains in the range of 0.5° to 0.6°(Fig.12).Therefore,the laser influence has little effect on the taper of the drilling process,mainly affecting the HAZ and hole size.The variation of hole taper with the increasing repetition frequency is not significant and remains at around 0.5°(Fig.13).The fiber notches and serrated edges of the epoxy resin at the hole exit can be found(Fig.14).Conclusions We propose a helical drilling method for preparing large holes in CFRP laminates.Theoretical analysis and experimental investigations are conducted based on the preparation of large holes using picosecond laser helical drilling of CFRP laminates with 4 mm thickness.The results show that the deflection angle of the wedge-shaped prism in the helical drilling optical system can change the radius of the laser beam and control the hole diameter.The mirror translation mainly controls the incidence angle of the laser beam,and adjusting the incidence angle can effectively reduce the hole taper and HAZ,which is caused by the obstruction of the hole wall.Meanwhile,we reveal the influence of laser energy density,pulse repetition frequency,and helical scanning speeds on the size and taper of the hole.By optimizing the process parameters of helical drilling,high-quality holes with a hole taper of 0.5° and a HAZ size below 50 μm can be obtained.As a result,these findings may provide a theoretical reference and experimental basis for further engineering application of laser high-quality drilling of holes in CFRP laminates.
optical fabricationcarbon fiber reinforced polymerpicosecond laserhelical drillinghole taperheat affected zone
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