LASER DRILLING


energy. Despite the higher number of pulses used in the SALaD process, the cumulative removal volume is substantially higher, indicating that each pulse remains productive over a longer drilling cycle.


Furthermore, the cooling action of the mist stabilises the process zone temperature, thereby preserving laser absorption efficiency over multiple pulses. In contrast, the SOTA dry process tends to suffer from cumulative thermal build-up, leading to increased plasma shielding and reduced laser-material coupling efficiency over time. The superior per-joule removal observed in the SALaD trials is therefore attributed not merely to thermal suppression, but to a dynamic synergy between evaporative cooling, impulsive melt expulsion, and consistent absorption conditions across the drilling cycle.


Figure 3: Comparison of surface roughness of SALaD vs SOTA driiled hole


The temperature history plot reveals a distinct difference between the two processes. Figure 5 shows high temperatures for the first few (~5) pulses (corresponding to high material removal), followed by a significant drop (~80%)


around pulse number 6 (after drilling the hole), and then a gradual reduction until pulse number 20 (due to laser interaction with the hole walls), after which no noticeable temperature is observed. In contrast, the SALaD process shows peak temperatures for the first three pulses (corresponding to high material removal) and then maintains relatively higher temperatures throughout all 25 recorded pulses. The relatively higher temperatures observed on subsequent pulses (from the fourth pulse) are more indicative of the plasma plume temperature rather than the material temperature.


Figure 4: Comparison of material removal efficiency (8 J at 40 NoP, 65 Hz, 1.5 ms, 6-4 bar; 16 J at 35 NoP, 105 Hz, 1.5 ms, 6-4 bar)


a b SUMMARY


This study has demonstrated that integrating a coaxial water- spray delivery system into a millisecond fibre laser drilling configuration markedly enhances the machinability of brittle ceramics such as aluminium nitride. The introduction of a fine, coaxially aligned water mist into the laser-material interaction zone serves multiple roles: it provides localised evaporative cooling, promotes symmetrical melt ejection, and significantly mitigates thermal damage mechanisms including recast layer formation, spatter deposition, and microcracking.


ACKNOWLEDGEMENT Figure 5: Thermal camera results of a) SALaD and b) SOTA laser drilling


(Figure 2a), and no visible HAZ or microcracking was present. The material interface remained sharp and clean up to the very edge of the hole, demonstrating that the water mist effectively reduces thermal accumulation, enables better melt ejection and prevents thermal damage of the base material.


The introduction of a coaxial water-spray into millisecond fibre laser drilling of AlN ceramics fundamentally transforms the process dynamics, yielding significant improvements in hole quality, thermal regulation, and process repeatability. SEM images confirmed the absence of subsurface damage and recast in the SALaD condition, while surface roughness measurements (Figure 3) showed more uniform roughness values (~0.4 µm Ra) compared to the dry-drilled case (1.6 µm Ra), in the lateral direction. In addition, SALaD method removes more material per unit of energy compared to the SOTA drilling method. Figure 4 reveals that the SALaD process consistently achieves higher material removal volumes per joule of input


The authors acknowledge support through UKRI Future Leaders Fellowship under grant number MR/V02180X/1. REFERENCE


[1] P Ghosh et.al., J. Manuf. Process 151, 251-266, 2025.


* Kiran Pick, Sundar


Marimuthu (MTC)


Priyanka.Ghosh@the-mtc.org the-mtc.org


Priyanka Ghosh is an award-winning engineer, pioneering various laser-based manufacturing


activities at the MTC, and is a Visiting Fellow at London South Bank University.


LASER USER 118 DECEMBER 2025 | 27


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