An investigation on the hole quality during picosecond laser helical drilling of stainless steel 304

Abstract

Precision drilling with ultra-short pulse lasers (e.g., picosecond and femtosecond) has been advocated to significantly improve the quality of the micro-holes with reduced recast layer thickness and no heat-affected zone. However, a combination of high-power picosecond laser with helical drilling strategy in laser drilling has rarely been reported in previous studies. In the present study, a series of micro-holes with circular, triangular, rectangular, and rhombic shapes (diameter 0.6 mm) were manufactured on stainless steel 304 using a newly developed laser drilling system which incorporated a picosecond laser and a high-speed laser beam rotation apparatus into a five-axis positioning platform. The quality of the helical drilled holes, e.g., recast layer, micro-crack, circularity, and conicity, were evaluated using an optical microscope, an optical interferometer, and a scanning electron microscope. In addition, the microstructure of the samples was investigated following etching treatment. It was demonstrated that the entrance ends, the exit ends, and the side walls of the micro-holes were quite smooth without accumulation of spattering material and formation of recast layer and micro-crack. No tapering phenomenon was observed, and the circularity of the holes was fairly good. There was no distinctive difference with regard to the microstructure between the edges of the holes and the bulk material. Picosecond laser helical drilling can be an effective technique for manufacturing of micro-holes with very high quality. The development of high-power picosecond laser would promote picosecond laser drilling to be more industrial relevance in the future.

The features of the through-hole by picosecond pulse laser helical drilling on alloy

“…At 200kHz and 500kHz,when the laser power is 6W,which means the fluences were set at 0.6× 10 -2 J/mm 2 and 0.24× 10 -2 J/mm 2 .As shown in Fig.1(c) and Fig.1(f), the diameter of the periphery of the hole was about 40μm, and the inner diameter was about 30μm. In addition, the middle area was a whitish zone(width 5-10μm) along the perimeter of the holes.This whitish zone is of particular interest as potential recast layer that re-solidifies on the side wall of the holes during laser drilling,which had been reported that the difference in elemental composition between the whitish zone and the bulk material is not significant, there is only a slight increase in the content of C and O [8] .When machining the through-hole using higher pulse energy,some recast layer has emerged on the side of the hole as shown in Fig.1(c), however, the machining efficiency of this condition was more higher than the lower pulse energy which is shown in Fig.1(f), e.g.there is some um-removed material on the taper bottom.It demonstrates that the crack had emerged on the whitish is shown in Fig.1(f), this will become the source of the defect. So, the controlling of the repetition rate and the processing time are very important for the drilling holes, an optimal parameters will make the machining efficiency and the hole quality with optimization.…”

“…And then, Bruneau et al [7] studied the mechanisms of ultra-fast laser ablation during deep-drilling of metals using plasma diagnostics, and it was showed that the nanoparticles were important on the accuracy of micro-machining. One of the most efficient and accurate drilling technologies namely helical drilling was reported [8] .It was reported that a combination of picosecond laser with an optimal parameter setup and helical drilling method has been proposed to gain microholes with different shapes on stainless steel. Precision drilling with picosecond laser maybe a better method to obtain the high-quality holes with the optimal adjustment of parameters such as average power,repetition rate,and focal position [9] .…”

Femtosecond Laser Micro/Nano-manufacturing: Theories, Measurements, Methods, and Applications

“…For metal materials, due to the complex interaction environment and many influencing factors during femtosecond laser fabrication, the shape of micro-hole exit is irregular and the processing quality is poor, besides the processing results are not easy to observe. Therefore, it is more difficult to fabricate high-aspect-ratio microholes on metals by femtosecond laser [176][177][178][179].…”

Drilling of micron-scale high aspect ratio holes with ultra-short pulsed lasers: Critical effects of focusing lenses and fluence on the resulting holes’ morphology

“…They are more pronounced in stationary beam drilling methods such as percussion drilling, compared with lower throughput, moving beam methods like trepanning and helical drilling [12]. To address these issues, for both drilling categories, new methods and tools have been developed, in particular changing the focusing position [8,13], the use of assisted gases [14][15][16][17], beam rotation [18,19], drilling under water [20], two-side drilling [21,22], new drilling cycles [12] and also different modelling and optimizing approaches to identify the optimum processing window [23][24][25][26][27]. Furthermore, the effects of process settings were investigated, too, especially peak power and pulse width effects on holes' repeatability in regards to the entry diameters [28] the evolution of shallow craters' diameter and depth in regards to number of pulses (NoP) and fluence [29] and the effect of different laser parameters such as average power on circularity, HAZ and taper angle [30] Other parameters that affect the laser drilling process, especially pulse energy, fluence and intensity, have been investigated, too.…”