Appearance
A manufacturer of aircraft engine turbine blades (Inconel 718, requiring 150 cooling holes per blade, Ø0.5 mm × 8 mm, 20° surface angle) was using EDM drilling (brass electrode, 0.3 mm Ø, 30 s per hole) at 75 minutes per blade. The recast layer (15–25 µm) required removal by post-processing. Switching to millisecond fibre laser percussion drilling (1.5 kW average power, 5 ms pulse, 50 Hz, 10 bar O₂ assist gas) reduced cycle time to 12 minutes per blade, eliminated electrode cost, achieved recast layer of 20–35 µm accepted without post-processing for non-fatigue-critical regions, and reduced total manufacturing cost per blade by 60%.
Laser Drilling Mechanisms and Parameters
Laser Drilling Method Comparison
| Drilling Method | Mechanism | Typical Bore Ø (mm) | Max L/D Ratio | Typical Depth (mm) | Material Removal Rate | Typical Hole Quality | Relative Speed | Primary Application |
|---|---|---|---|---|---|---|---|---|
| Percussion drilling | Multiple laser pulses at same spot, progressively deeper | 0.1–1.0 | 30:1 | 0.5–15 | 0.1–10 mm³/min | Moderate; taper 10–30%, recast layer 10–50 µm | Fast (0.5–5 s per hole) | Cooling holes, injector nozzles, thin sections |
| Trepanning | Laser drills pilot hole, then beam orbits to enlarge | 0.3–5.0 | 20:1 | 1–20 | 0.05–5 mm³/min | Good; minimal taper, roundness < 5 µm | Moderate (5–30 s per hole) | Precision nozzles, dies, orifices |
| Helical (laser spiral) | Beam rotates in spiral path while advancing axially | 0.3–10.0 | 40:1 | 1–30 | 0.02–2 mm³/min | Excellent; taper < 2%, roundness < 2 µm, Ra < 0.5 µm | Slow (10–120 s per hole) | Highest-quality deep holes, medical stents |
| Water-jet-guided laser (WJGL) | Laser guided by total internal reflection in water jet | 0.03–3.0 | 40:1 | 1–40 | 0.5–20 mm³/min | Excellent; minimal thermal damage, HAZ < 5 µm | Moderate (2–30 s per hole) | CMC cooling holes, semiconductor, medical |
| Ultrashort pulse (ps/fs) trepanning | Non-thermal ablation by multiphoton absorption | 0.01–1.0 | 15:1 | 0.1–5 | 0.001–0.5 mm³/min | Highest; no recast, no HAZ, Ra < 0.2 µm | Very slow (10–600 s per hole) | Medical implants, nozzles, micro-holes |
Laser Type Comparison for Deep Hole Drilling
| Laser Type | Wavelength (nm) | Pulse Duration | Peak Power | Average Power (W) | Max Repetition Rate | Typical Hole Quality | Capital Cost | Operating Cost per Hole (Ø0.5 mm × 8 mm) | Material Suitability |
|---|---|---|---|---|---|---|---|---|---|
| Millisecond fibre laser | 1,070 | 0.5–20 ms | 2–20 kW | 500–3,000 | 50–500 Hz | Good; recast 15–50 µm, taper 10–20% | $80–250k | $0.02–0.08 | Metals, superalloys; not for ceramics |
| Nanosecond fibre laser | 1,070 | 10–500 ns | 10–100 kW | 20–200 | 20–200 kHz | Good; recast 5–20 µm, taper 5–15% | $50–150k | $0.05–0.15 | Metals, some ceramics, polymers |
| Picosecond laser | 1,064 / 532 | 1–100 ps | 0.1–10 GW | 10–100 | 50–500 kHz | Very good; recast < 5 µm, minimal HAZ | $200–500k | $0.20–0.80 | All materials including ceramics, CMCs |
| Femtosecond laser | 800 / 1,064 | 100 fs–1 ps | 0.1–10 TW | 5–50 | 1–500 kHz | Excellent; no recast, no HAZ, Ra < 0.2 µm | $300–800k | $0.50–3.00 | All materials; best for thermal-sensitive applications |
| CO₂ laser | 10,600 | 0.1–50 ms | 1–10 kW | 200–6,000 | 100–2,000 Hz | Poor; large HAZ, recast > 100 µm | $100–300k | $0.02–0.10 | Non-metals; limited metal absorption |
| DPSS UV laser | 355 | 10–100 ns | 10–50 kW | 5–50 | 10–300 kHz | Good; cold ablation with UV photons | $80–200k | $0.10–0.40 | Polymers, ceramics, thin metals |
Key Process Parameters and Their Effect on Hole Quality
| Parameter | Symbol | Unit | Typical Range | Effect on Hole Diameter | Effect on L/D Ratio | Effect on Recast Layer | Effect on Taper | Effect on MRR | Optimal for Deep Holes |
|---|---|---|---|---|---|---|---|---|---|
| Pulse energy | Ep | mJ | 0.1–50,000 | Increases Ø | Decreases achievable depth | Increases thickness | Increases taper | Increases | Moderate energy, multiple pulses |
| Pulse duration | τ | s | 100 fs – 20 ms | Minimal effect | Shorter pulses enable deeper (less HAZ) | Shorter = thinner recast | Shorter = less taper | Decreases | 0.5–5 ms for percussion; < 10 ps for quality |
| Repetition rate | frep | Hz | 1–500,000 | Increases Ø at high rates | Decreases at high rates (plasma shielding) | Increases at high rates | Increases at high rates | Increases | 30–100 Hz for percussion; > 50 kHz for helical |
| Average power | Pavg | W | 5–6,000 | Proportional to √Pavg | Increases depth at same pulse energy | Increases | Minimal effect | Increases | Highest available within quality limits |
| Beam focal position (defocus) | Δz | mm | -3 to +3 | Smaller at focus | Deeper at slight defocus (0.5–1 mm) | Thinner at focus | Higher taper at defocus | Lower at defocus | 0 to +1 mm for deep holes |
| Assist gas pressure | Pgas | bar | 1–20 | Minimal effect | Higher pressure increases depth | Higher pressure reduces recast | Minimal effect | Increases | 5–15 bar (oxygen for metals, argon for ceramics) |
| Assist gas type | — | — | O₂, Ar, N₂, air | Minimal | O₂ increases depth (exothermic) | O₂ may oxidise surface | Minimal | O₂ highest | O₂ for metals; Ar for reactive materials |
| Number of pulses (percussion) | N | — | 10–5,000 | Slight increase | Increases depth (logarithmically) | Accumulates | Maintained | — | 100–1,000 for typical deep holes |
FAQ
How does laser deep hole drilling compare to mechanical deep hole drilling and EDM drilling?
Laser deep hole drilling, mechanical deep hole drilling (gun drilling, BTA), and EDM drilling each occupy distinct niches in the hole-making process landscape, and the optimal choice depends on hole geometry, material, quality requirements, and production volume. The comparison for a typical small-diameter deep hole (Ø0.5 mm × 10 mm in Inconel 718) illustrates the trade-offs. Laser drilling (millisecond fibre laser percussion) achieves a drilling time of 1–5 seconds per hole, recast layer of 15–50 µm, hole taper of 10–20%, and roundness of 5–15 µm. Equipment cost is $80,000–250,000 and per-hole cost (including capital amortisation, consumables, and maintenance) is $0.05–0.20 at production volumes above 10,000 holes per year. Gun drilling achieves a drilling time of 30–120 seconds per hole (depending on peck cycle requirements), surface finish Ra 0.2–0.6 µm, diameter tolerance ±0.005 mm, hole taper < 2%, and no recast layer. However, gun drilling requires a through-bore or sufficient clearance at the exit for the drill to pass, a starting bushing, high-pressure coolant, and mechanical access to both sides of the workpiece. The minimum practical gun drill diameter for Inconel 718 is approximately Ø1.5 mm — below this diameter, the drill tube is too fragile and tool life is impractical. Gun drilling of Ø0.5 mm holes in Inconel 718 is not feasible due to drill breakage and the inability to deliver adequate coolant through such a small drill. EDM drilling (small-hole EDM, also called fast-hole EDM) uses a rotating tubular brass electrode (Ø0.2–0.5 mm) with deionised water dielectric. Drilling time is 10–60 seconds per hole (depending on depth and material), recast layer is 5–25 µm (thinner than laser at low energy settings), taper is 5–15%, and roundness is 3–10 µm. The key limitations of EDM drilling are: electrode wear (the electrode is consumed, requiring automatic electrode feeding or replacement — typically 20–50 mm of electrode consumed per mm of hole depth); the requirement for electrical conductivity in the workpiece (non-conductive ceramics cannot be EDM-drilled); and the need for post-processing to remove the recast layer for fatigue-critical applications. The practical selection guide is: choose laser drilling when hole diameter is < 1.5 mm in superalloys or hardened materials, when the component surface is curved or angled, when drilling speed is the priority, or when the material is non-conductive (e.g., ceramics, CMCs). Choose gun drilling when diameter is > 1.5 mm, when hole quality (surface finish, roundness, taper) is the priority, when zero recast layer is required, or when production volume justifies the tooling setup. Choose EDM drilling when the workpiece must be drilled after heat treatment (no distortion), when the material requires very low cutting forces (fragile or thin-wall components), or when the aspect ratio exceeds laser drilling capability (> 30:1) but the diameter is below gun drilling capability. In production aerospace applications, the three processes are often used in combination: EDM or laser for film cooling holes in turbine blades, gun drilling for main shaft bores and oil passages, and EDM for wire-start holes in die components.
What is recast layer and how can it be controlled in laser deep hole drilling?
Recast layer (also called the melt layer or resolidified layer) is a thin film of material that has been melted during laser drilling and re-solidified on the hole wall. In the laser drilling process, a high-power laser pulse melts and vaporises the workpiece material. The molten material is partially ejected from the hole by the vapour pressure and assist gas flow, but a fraction — typically 10–30% of the melted volume — remains on the hole wall and re-solidifies as the hole cools between pulses. The recast layer has a distinct microstructure from the base material: it is usually microcrystalline or amorphous (depending on cooling rate), contains gas porosity from vapour entrapment, may be oxidised (if oxygen assist gas was used), and contains microcracks from thermal contraction stresses. For Inconel 718 drilled with a millisecond fibre laser, the recast layer is typically 5–50 µm thick with micro-hardness of 450–550 HV (vs 350–420 HV for the base material), oxygen content increased from < 0.05% to 2–8% (when using oxygen assist gas), and microcrack density of 2–10 cracks per mm. The recast layer affects component performance in three ways: (1) fatigue life reduction — the microcracks in the recast layer act as fatigue crack initiation sites, reducing the high-cycle fatigue life by 30–70% compared to a recast-free hole; (2) corrosion resistance degradation — the oxidised, microcracked recast layer provides pathways for corrosive media to reach the base material; (3) flow restriction — for fuel injector nozzles and cooling holes, the rough recast surface (Ra 2–5 µm) increases flow resistance compared to a smooth bore (Ra < 0.5 µm). Recast layer control methods ranked by effectiveness: (1) Use shorter pulse durations — reducing pulse duration from 5 ms to 0.5 ms reduces recast layer thickness from 30–50 µm to 10–20 µm in Inconel 718 because less molten material is produced per pulse. The trade-off is reduced material removal rate (longer drilling time). (2) Increase assist gas pressure — increasing oxygen pressure from 5 to 15 bar reduces recast layer thickness by 30–50% by improving melt ejection efficiency. The trade-off is increased oxygen embrittlement of the hole surface. (3) Use argon instead of oxygen — argon assist gas eliminates oxidation of the recast layer but produces a thicker recast layer (because argon provides no exothermal energy to assist melt ejection). Argon is preferred when corrosion resistance is more important than recast layer thickness. (4) Post-processing removal — the most reliable method for recast layer elimination. The recast layer can be removed by: abrasive flow machining (AFM), which reduces recast thickness from 30 µm to < 5 µm in 2–5 minutes per hole; electrochemical polishing (for conductive materials), which removes 10–30 µm uniformly in 30–120 seconds; or hand polishing with flexible abrasive tools (for accessible larger holes). (5) Use water-jet-guided laser (WJGL) — the water jet cools the hole wall during drilling, reducing recast layer thickness to < 5 µm even with millisecond pulses, because the molten material is quenched and flushed away by the water jet. (6) Use ultrashort pulse lasers (ps/fs) — femtosecond laser pulses ablate material by non-thermal mechanisms (multiphoton absorption and Coulomb explosion) that produce no molten phase and therefore no recast layer. The trade-off is 100–1,000× slower drilling speed.
What is the maximum aspect ratio (depth-to-diameter ratio) achievable with laser deep hole drilling?
The maximum achievable aspect ratio (L/D ratio) in laser deep hole drilling depends on the laser type, drilling method, material, and acceptable hole quality. The thermodynamic limit is set by the fact that laser light is attenuated as it propagates through the deepening hole — the laser beam diverges, reflects off the hole walls, and is absorbed by the vapour plume. The practical limits for production-quality holes are: (1) Millisecond fibre laser percussion drilling — maximum L/D of 20–30:1 in superalloys (Inconel 718, Hastelloy X) for holes where some taper (10–20%) and recast layer (20–50 µm) are acceptable. The limiting factor is the logarithmic relationship between hole depth and number of pulses — each additional increment of depth requires exponentially more pulses. For a Ø0.5 mm hole in Inconel 718 at 1.5 kW average power, the depth increases from 5 mm at 200 pulses to 8 mm at 1,000 pulses to 10 mm at 5,000 pulses — the incremental depth per 100 pulses drops from 2.5 mm to 0.5 mm to 0.1 mm as the hole deepens. The practical production limit is L/D = 20:1 for millisecond laser percussion. (2) Nanosecond laser trepanning — maximum L/D of 15–25:1 in metals and ceramics. The trepanning method removes material layer by layer from the sidewall rather than only from the bottom, providing better depth capability than percussion but at slower speed. The limitation is beam divergence at depth — the laser beam focused through a trepanning optic has a Rayleigh range that limits the depth over which the beam remains focused. (3) Helical drilling (laser spiral) — maximum L/D of 30–40:1 in metals and 20–30:1 in ceramics. The helical method combines axial feed with orbital beam motion, creating a cylindrical hole with parallel walls. The depth limit is set by the ablation rate at the bottom of the hole — material removal becomes less efficient as the hole deepens because the vapour plume and ejected particles attenuate the incoming beam. Helical drilling is the best method for achieving the highest L/D ratio with the best hole quality from a single process. (4) Water-jet-guided laser (WJGL) — maximum L/D of 40:1 in CMCs and metals, with demonstrated depths of 40 mm at Ø1 mm in SiC/SiC CMC. The water jet provides two advantages for depth capability: it continuously removes ablation debris from the hole, preventing beam attenuation by the vapour plume; and it maintains a straight, parallel-sided hole because the water jet is guided by the hole wall, naturally centering the laser beam. (5) Femtosecond laser drilling — maximum L/D of 10–15:1 for production-quality holes in metals, although L/D of 30:1 has been demonstrated in transparent materials. The depth limit for femtosecond drilling is more restrictive than for longer-pulse lasers because the non-thermal ablation mechanism requires that the laser pulses reach the bottom of the hole with sufficient intensity to induce multiphoton absorption — this intensity threshold is more difficult to maintain at depth than the thermal melting threshold for longer pulses. In practice, the maximum aspect ratio for femtosecond laser drilling in metals is limited to approximately 10–15:1 for holes with diameter < 1 mm. For comparison to non-laser methods: EDM drilling achieves L/D of 20–40:1 for small holes (Ø0.3–3 mm); gun drilling achieves L/D of 100–400:1 for holes > Ø2 mm; and BTA drilling achieves L/D of 100–400:1 for holes > Ø12 mm. Laser drilling is not competitive with mechanical drilling for L/D > 40:1 but is the preferred method for L/D of 10–40:1 in materials or geometries that mechanical drilling cannot access.
What assist gas should be used for laser deep hole drilling of different materials?
Assist gas selection for laser deep hole drilling depends on the material being drilled and the desired balance between drilling speed, hole quality, and surface chemistry. The assist gas serves three functions: (1) melt ejection — the gas pressure forces molten material out of the hole; (2) oxidation exotherm — reactive gases (oxygen) provide additional chemical energy that increases material removal rate; (3) shielding — inert gases protect the hole surface from oxidation. For nickel-based superalloys (Inconel 718, 625, Hastelloy X, Waspaloy) — the most common materials for laser-drilled cooling holes — oxygen assist gas at 6–15 bar is the standard choice. Oxygen reacts exothermically with the molten nickel and chromium, releasing additional energy that increases the drilling speed by 30–60% compared to argon at the same pressure. The oxygen also improves melt ejection by creating a more energetic vapour plume. The trade-off is that the recast layer contains 5–15% oxygen (as oxides of nickel, chromium, and aluminium), which makes the recast layer harder and more brittle. For non-critical cooling holes (combustor liners, exhaust nozzles), the oxygen-assisted recast layer is acceptable. For fatigue-critical turbine blade holes, oxygen-assisted recast layer may need removal by post-processing. For titanium alloys (Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo), oxygen assist gas can cause oxygen embrittlement of the heat-affected zone, reducing fatigue life. Argon at 5–10 bar is preferred for titanium laser drilling. The drilling speed with argon is 20–40% slower than with oxygen, but the hole surface is free of embrittlement. For stainless steels (304, 316, 17-4PH), oxygen assist gas at 5–10 bar provides good drilling speed and produces a recast layer with acceptable oxidation for most applications. Nitrogen can be used as an alternative when minimising surface oxidation is important. For aluminium alloys, oxygen provides minimal exothermic benefit because aluminium oxide has a high melting point (2,072°C), and the oxide layer actually hinders melt ejection. Compressed air at 5–10 bar is the most economical choice for aluminium laser drilling and provides adequate melt ejection without the risk of excessive oxidation. For ceramics (alumina, zirconia, SiC) and CMCs, argon at 5–15 bar is the primary choice. Oxygen does not provide exothermic benefits for ceramics (the materials are already oxidised), and CO₂ laser is often used instead of fibre laser because ceramics absorb CO₂ wavelength (10.6 µm) more efficiently than fibre laser wavelength (1.07 µm). For polymers and composites, compressed air or nitrogen at 2–5 bar is used. Higher pressures can cause delamination in composite materials. The general rule: use oxygen for maximum drilling speed in metals (where recast layer oxidation is acceptable); use argon for maximum hole quality and minimum surface alteration in all materials; use compressed air for cost-sensitive applications in materials where oxidation is not a concern.
What are the current limitations and future developments in laser deep hole drilling?
The current limitations of laser deep hole drilling are: (1) Depth limitation — even with optimised processes, laser drilling cannot approach the L/D ratios of mechanical deep hole drilling. The practical limit for production laser drilling is L/D = 40:1 (with WJGL), while gun drilling routinely achieves L/D > 100:1. This limits laser drilling to applications requiring holes shallower than approximately 40 mm for small diameters. (2) Hole taper — percussion-drilled holes typically have 10–30% taper (entry larger than exit), which is unacceptable for precision components where the hole serves as a bearing surface or sealing passage. Trepanning and helical drilling reduce taper to < 5% but at slower drilling speeds. (3) Recast layer and microcracks — even with optimised parameters, laser-drilled holes in superalloys have a recast layer of 5–50 µm containing microcracks. For fatigue-critical components (turbine blades, medical implants), the recast layer must be removed by post-processing, adding cost and cycle time. (4) High aspect ratio hole quality at depth — as hole depth increases, the hole quality deteriorates: taper increases, roundness degrades, and surface roughness increases. The bottom of a deep percussion-drilled hole is typically 20–50% smaller than the entry and has 2–3× higher surface roughness. (5) Material limitations — highly reflective materials (copper, gold, silver, aluminium) are difficult to laser drill because they reflect the laser wavelength, requiring higher pulse energy or shorter pulse durations to achieve coupling. Copper, for example, reflects > 95% of fibre laser wavelength (1.07 µm) at room temperature, making it one of the most challenging materials for laser drilling. (6) Capital cost — industrial laser drilling systems range from $80,000 (millisecond fibre laser) to $800,000 (femtosecond laser with beam delivery), compared to $50,000–300,000 for a small-hole EDM system with comparable hole-making capability. Key future developments that are expanding the capabilities of laser deep hole drilling include: (1) Beam shaping — diffractive optical elements and spatial light modulators that shape the laser beam into annular or multi-spot patterns, enabling parallel drilling of multiple holes (2–20 holes simultaneously) and improved hole geometry control. Multi-spot beam shaping has demonstrated 3–5× productivity improvement for turbine blade cooling hole drilling. (2) Polarisation control — radial polarisation (where the electric field vector points radially outward from the beam axis) improves coupling into deep holes by 20–40% compared to linear polarisation, enabling deeper drilling and reduced taper. (3) High-average-power ultrashort pulse lasers — industrial femtosecond lasers have progressed from 10 W average power in 2015 to 500+ W in 2026, enabling the non-thermal ablation advantages of femtosecond pulses at drilling speeds that approach nanosecond laser speeds. A 500 W femtosecond laser can drill a Ø0.5 mm × 5 mm hole in Inconel 718 in approximately 10 seconds — competitive with nanosecond drilling speed but with zero recast layer. (4) In-process quality monitoring — optical coherence tomography (OCT) integrated into the laser drilling head enables real-time measurement of hole depth and geometry during drilling, with closed-loop control of pulse parameters to maintain consistent hole quality. (5) Hybrid laser-mechanical drilling — laser pre-drilling followed by mechanical single-lip reaming combines the speed of laser drilling with the surface quality of mechanical finishing, eliminating the recast layer while maintaining drilling speeds faster than EDM or mechanical drilling alone.
This article provides an overview of laser deep hole drilling for industrial applications. Process selection, parameters, and quality control depend on the specific material, hole geometry, and application requirements. The technical data presented here reflects published research and documented industrial applications as of 2026.