Process limits for percussion drilling of stainless steel with ultrashort laser pulses at high average powers

Abstract

The availability of commercial ultrafast lasers reaching into the kW power level offers promising potential for high-volume manufacturing applications. Exploiting the available average power is challenging due to process limits imposed by particle shielding, ambient atmosphere breakdown, and heat accumulation effects. We experimentally confirm the validity of a simple thermal model, which can be used for the estimation of a critical heat accumulation threshold for percussion drilling of AISI 304 steel. The limits are summarized in a processing map, which provides selection criteria for process parameters and suitable lasers. The results emphasize the need for process parallelization.

Feasibility assessment of parallelized helical drilling

“…The heat accumulation that leads to significant quality issues by additional generation of excessive melt increases with increasing pulse energies Ep and increasing pulse repetition rate frep 8 , 9 . The upper limit frep,crit of the repetition rate imposed by heat accumulation and its dependence on the pulse energy was discussed and verified by more than 1000 percussion-drilling experiments 33 and leads to the conclusion that the detrimental influence of heat accumulation can only be avoided by corresponding parallelization approaches. The findings likewise also apply to helical drilling with the difference that parallelization is even more difficult to implement here.…”

“…When the pulse repetition rate per borehole is lowered to frep/n≤frep,crit by a suitable choice of n, the larger temporal and spatial separation of the incident pulses in the hole resolves heat accumulation effects without (the need for) a reduction of the pulse energy Ep incident on the individual boreholes. In contrast, when avoiding heat accumulation through the approach of simultaneous parallelization by beam-splitting, depending on the pulse repetition rate frep of the laser, the pulse energy Ep/n≤Ep,crit that is applicable at most to avoid heat accumulation may even be so low that a drilling process is not possible or cannot reach the desired depth 33 …”

“…In contrast, when avoiding heat accumulation through the approach of simultaneous parallelization by beam-splitting, depending on the pulse repetition rate f rep of the laser, the pulse energy E p ∕n ≤ E p;crit that is applicable at most to avoid heat accumulation may even be so low that a drilling process is not possible or cannot reach the desired depth. 33 Of course, in practice, n is also constrained by the finite dimensions of the workpiece and/or the limited scan field provided by the optical system, which in turn limits the maximum average laser power that can be used.…”

Ultrafast laser applications in the kW‐class

“…Although an average laser power of up to P = 570 W was available, the power used for the drilling process was limited to P = 84 W by setting the repetition rate to f = 30 kHz. As explained in [19], it is not possible to drill a high-quality microhole with an average power >100 W due to thermal defects. Drilling of multiple holes at the same time by means of parallelization as shown for 10 µm thin foils in [16] could be a potential solution for using kW average power for deep holes as well, if a high-energy beam source as demonstrated in [9] is used.…”

High speed laser drilling of micro-holes for hydrogen applications

“…In addition to the drilling strategy, a complex interaction between the laser pulse and material is heavily dominated by the specific type of laser utilized (continuous wave [22][23][24] , millisecond [25][26][27] , nanosecond [28][29][30] , and picosecond/femtosecond [31][32][33] ). Regardless, the drilling of metals is predominately a thermal process with the management of material melting and heat propagation being pivotal in defining the quality of the micro-holes.…”