Ultra Fast Laser Processing: How Picosecond Pulses Shape Metal

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  What makes ultra fast laser processing different
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  Cutting and drilling with ultra fast laser processing
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  Micro-welding and dissimilar metal joints
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  Surface structures and texturing
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  Ultra fast laser processing versus other routes

Industrial lasers split into four families by emission time: continuous wave, quasi-continuous wave, short pulse and ultra-short pulse. Ultra fast laser processing sits at the last group, where the pulse width is around 10⁻¹² s, meaning a picosecond, or shorter. A picosecond is one trillionth of a second. The pulse ends before heat can travel far into the part.

The mechanism is not melting. Peak power density reaches 10¹² to 10¹³ W/cm², which strips electrons out of the lattice in a few picoseconds. The material turns into a plasma and leaves as vapor and fine debris. Because the pulse is shorter than the electron-lattice relaxation time, the bulk never sees a melt pool. That is why the cut edge stays clean and the recast layer is thin.

Compare this with a continuous wave (CW) fiber laser or a millisecond pulsed Nd:YAG. Those tools couple energy into the part for 10⁻³ to 10⁻⁶ s. Heat diffuses roughly 100 to 1,000 times deeper than the optical penetration depth. The result is a wide heat-affected zone (HAZ), dross on the underside, and microcracks in hardened alloys. On a 0.5 mm stainless shim, CW cutting can leave a HAZ of 50 to 150 μm. Ultrafast cutting on the same shim stays under 5 μm.

That single difference drives every application below: cutting stents and thin foils, welding dissimilar metals, drilling cooling holes, and writing surface structures that change friction or wetting. If your feature tolerates a 100 μm HAZ, a cheaper short-pulse laser is usually enough. If the part is a medical implant or a fuel injector nozzle, the ultrafast route is the one that keeps the metallurgy intact.

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  Pulse widthPicosecond (10⁻¹² s) or femtosecond (10⁻¹⁵ s); shorter than electron-lattice relaxation.
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  Peak power density10¹² to 10¹³ W/cm², enough to ionize the lattice directly.
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  Heat-affected zoneTypically under 5 μm, versus 50–150 μm for CW or ms-pulse lasers.

For cutting, the beam is focused to a spot of 10 to 30 μm and moved along the contour at 100 to 1,000 mm/s. Material is removed layer by layer, so the kerf is narrow, often 15 to 40 μm. On a 100 μm stainless foil this gives a clean edge with no dross and no need for a secondary deburring step. The same part on a CW fiber laser would need a nitrogen assist and a post-process acid etch to remove the recast layer.

The trade-off is speed and thickness. Ultrafast cutting removes material by ablation, so throughput scales with average power. Cutting 1 mm stainless on a 50 W picosecond source runs at roughly 1 to 5 mm/s. A 6 kW fiber laser cuts the same sheet at 20 to 40 mm/min, an order of magnitude faster. So ultrafast cutting is for thin sections, tight features, and heat-sensitive alloys, not for 10 mm plate.

Drilling follows the same logic. Trepanning with a picosecond beam produces holes of 50 to 300 μm diameter with an aspect ratio of 10:1 or better and no recast lip. Fuel injector nozzles, turbine cooling holes, and medical cannula bores are common parts. The hole wall taper is usually under 2°, and the entry and exit diameters differ by only a few micrometers.

One practical limit: debris and redeposition. Ablation throws nanoparticles that can settle back on the surface. Most ultrafast cutters run a coaxial gas assist, usually nitrogen or compressed air at 2 to 6 bar, to sweep the plume away. Parts that need a mirror finish after drilling still get a light polishing step, even if the HAZ is already negligible.

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  Kerf width15–40 μm on a 10–30 μm focus spot, thin sections only.
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  Cutting speed1–5 mm/s on 1 mm stainless at 50 W average power.
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  Drilling50–300 μm holes, aspect ratio 10:1, taper under 2°.
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  Debris controlCoaxial nitrogen or air assist at 2–6 bar.

Ultrafast welding works in a different regime from conventional laser welding. Instead of a melt pool that solidifies over milliseconds, the pulse creates a localized plasma that fuses a thin interface layer. Pulse energies are low, often 1 to 50 μJ, and the weld spot is 10 to 100 μm wide. The total heat input per joint is small enough that a 50 μm copper foil can be welded to a 50 μm aluminum foil without melting either one through.

This matters for battery tabs, sensor leads, and medical electrode assemblies. Copper and aluminum form brittle intermetallics when melted together. A melt-pool weld between them produces a joint that cracks under thermal cycling. Ultrafast welding limits the mixing zone to a few micrometers, so the intermetallic layer stays thin and the joint survives. The same applies to stainless-to-titanium and nickel-to-copper joints.

Weld strength is lower than a full-penetration melt weld. A micro-weld carries shear loads in the tens of newtons, not hundreds. Engineers use it for electrical continuity and hermetic sealing at small scale, not for structural brackets. If the joint carries a primary load, a resistance weld or a conventional laser weld is the right call.

The process also needs tight part fit-up. Gaps above 5 μm leave an incomplete bond. Fixtures must hold the two foils in contact across the whole weld seam, and the beam path is usually programmed as a series of overlapping spots at 50 to 80% overlap to build a continuous seam.

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  Pulse energy1–50 μJ, spot width 10–100 μm.
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  Typical jointsCu-to-Al battery tabs, sensor leads, medical electrodes.
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  Fit-up limitGap under 5 μm; spot overlap 50–80% for a seam.
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  Load capacityShear loads in the tens of newtons, not structural.

The same short pulse that cuts metal can also write structures on it. By scanning a defocused beam in a pattern, the surface develops laser-induced periodic surface structures (LIPSS) with a period close to the laser wavelength, typically 0.5 to 2 μm. These ripples change how the surface interacts with light, water and other surfaces.

Three effects are useful in production. First, hydrophobicity: a textured stainless or titanium surface can reach a water contact angle above 150°, which makes it self-cleaning and resists biofilm. Second, friction and wear: micro-dimples of 10 to 50 μm diameter act as oil reservoirs and reduce friction in sliding contacts. Third, optical absorption: blackened copper and aluminum surfaces absorb more than 90% of visible light, which helps in sensor and solar hardware.

Texturing is a finishing operation, not a shaping one. It removes only 1 to 20 μm of material, so it does not change part dimensions. It does change the surface finish reading, though. A part that was Ra 0.8–1.6 μm before texturing will read Ra 2–5 μm after, because the stylus follows the ripples. If the drawing calls for a specific Ra, specify it before texturing and note the texture separately.

Adhesion is the other common use. Bonding to PEEK, PTFE or carbon-fibre composites is difficult because these materials have low surface energy. Ultrafast texturing creates mechanical interlock and raises surface energy without a chemical primer. Lap shear strength on textured PEEK can double compared with an untreated surface, though the exact gain depends on the adhesive and the pattern.

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  LIPSS period0.5–2 μm, close to the laser wavelength.
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  Depth removed1–20 μm; dimensions stay unchanged.
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  Finish shiftRa 0.8–1.6 μm reads Ra 2–5 μm after texturing.
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  AdhesionRaises surface energy on PEEK, PTFE and composites.