How to laser cut metal without burning it?

Cut edges that look scorched lose trust. I once stood on a shop floor watching a flawless stainless bracket come off the table, only to fade into a bruised rainbow under the cooling fan. That moment cost the customer and the margin.

Metal can be cut cleanly when laser power, speed, focus, and assist gas work in harmony. Dial each knob toward the lowest heat input that still pierces, sweep the kerf with high‑pressure nitrogen, and bright edges will replace every burn mark.

My team at Kirin Laser has tuned thousands of recipes for OEM partners. Below I unpack the same playbook we use when we onboard a new distributor, so you can move straight to silver edges and zero rework.

I will dig deeper into four common questions, break the myths, share real parameter ranges, and add tables you can copy onto the shop wall.

How to prevent burn marks when laser cutting?

Streaked edges trigger warranty calls. Operators slow the line, costs rise, and profit shrinks. I felt that pain at an automotive supplier last year. Their stainless brackets showed brown tint along every slot. Within a week we tuned the machine and burn marks vanished.

Burn marks form when heat input exceeds the metal’s ability to shed energy and when oxygen feeds the flame. Cut surfaces stay bright when I lower power, ramp speed, center focus, and flush the cut with nitrogen. Even 304 stainless shines like a mirror at 3 kW when parameters match thickness.

Parameter matrix for burn‑free edges¹

Material	Thickness (mm)	Power (kW)	Speed (mm/s)	Focus Offset (mm)	Gas	Gas Pressure (bar)
Stainless 304	1	1.0	75‑90	0	N₂	12
Stainless 304	6	3.0	9‑12	+0.3	N₂	14
Mild Steel	6	2.5	15‑18	+0.2	O₂	0.8
Aluminum 5052	3	3.0	45‑55	0	Air	6

Common pitfalls

1. Over‑powered pierce – Operators love to crank power during piercing. The crater soaks heat, travels with the beam, and stains the edge for the next 20 mm.

2. Dirty nozzle – A single speck skews the jet, bends the melt, and leaves tiger stripes.

3. Wrong standoff – If nozzle gap doubles, gas velocity halves. Oxide rises instantly.

Automotive client case

We cut 4 mm 304 at 2.5 kW / 18 mm ⁄ s with oxygen. Edge was straw‑colored. We switched to 1.6 kW, 30 mm ⁄ s, focus 0 mm, 13 bar nitrogen. Scrap fell from 6 % to below 1 %. Turnaround improved by one full shift.

Can a laser burn through metal?

First‑time buyers fear that a 10 kW beam will ruin parts if it dwells. That fear is healthy. A fiber laser can reach 10⁸ W ⁄ cm² at the focus. Leave it in one spot and copper will glow white. Yet the same beam can cut jewelry‑grade lines once motion and gas are tuned.

Yes, any laser that cuts can also burn, but burn is optional. Control dwell time, keep optics clean, and pick the right assist gas, and you turn destructive heat into productive heat.

Thermal budget vs. dwell time

Variable	Low‑risk range	High‑risk trigger
Linear Energy (J/mm)	<20	>40
Pierce Dwell (ms)	<80	>150
Nozzle Gap (mm)	0.8‑1.0	>1.5
Sheet Flatness (mm)	<0.2	>0.5

Rule of thumb: Dwell time = burn time². Keep the beam moving or modulate power during cornering.

Reflective metals and back‑reflection

Why copper bites back

Copper reflects \~95 % of 1.06 µm light at room temp. A stray 5 kW bounce can pit a $3 000 collimator lens.

Mitigation table

Step	Method	Effect
1	Black marker or graphite spray	Increases absorption \~30 %
2	Low‑power pre‑heat spiral	Drops reflectivity before full power
3	5 ° Beam tilt with adaptive head	Sends reflection off optical path

Practical story

A bus‑bar maker asked if 6 kW would burn 4 mm copper. I set a two‑stage pierce: 1 kW spiral 120 ms, then 4 kW cut at 40 mm ⁄ s with 15 bar nitrogen. Edge roughness hit 10 µm Ra, no lens alarms in six months.

How do you laser engrave without burning?

When a cutting head shifts to mark QR codes, the process gets delicate. Burning shows as halos around each glyph. Customers hate it because scanners misread codes. The fix is to trade raw power for pulse finesse and airflow.

To engrave cleanly I drop power below 30 %, raise frequency above 40 kHz, and scan at 1000 mm ⁄ s or faster. Air assist at 3 bar sweeps vapor. Three shallow passes beat one deep pass every day.

Pulse strategy chart

Mode	Pulse Width (ns)	Peak Power (kW)	Frequency (kHz)	Typical Use
CW	‑	‑	0	Deep cutting
Q‑switched	80	22	20‑40	Black marking
MOPA burst	10×4	8	50‑150	Color marking

Heat‑affected zone (HAZ)³ vs. pass count

Passes	Power (%)	HAZ (µm)	Mark Depth (µm)
1	60	45	40
3	25	12	38
5	15	8	37

Multiple light passes cut heat by 4× while hitting the same depth.

Engraving airflow guide

Nozzle ID (mm)	Flow (L/min)	Effect
1.0	25	Removes fumes only
2.5	60	Cools zone + clears debris
3.5	90	Adds slight material removal

Story from my bench

I etched a polished brass logo at 30 % power and saw black rings. Reset to 15 % power, 60 kHz, 1200 mm ⁄ s, and three passes. Background stayed mirror bright. Total cycle time rose from 8 s to 12 s but polishing time fell to zero. Net win.

How can a laser cut through metal?

Light cuts when intensity exceeds the metal’s melting enthalpy at the spot. The assist gas then plays police and janitor: it protects the melt from oxygen and sweeps it out of the kerf.

I tell every trainee: Power forms the melt, pressure removes it, motion shapes it. Get those three nouns right and the verb—cut—just happens.

Energy balance breakdown

Energy term4	Symbol	Typical Value 3 kW	Role
Laser Power	P	3 000 W	Input
Absorption	η	0.35	Input × η
Latent Heat fusion	Lf	250 kJ/kg	Consumption
Removal Rate	ṁ	0.001 kg/s	Consumption × Lf

The equation P × η ≥ ṁ × Lf sets the minimum continuous speed. Anything slower adds sensible heat, widens kerf, and risks burn.

Assist gas decision tree⁵

Is alloy carbon steel?

├─ Yes: need oxidation boost? ─┬─ Yes → Oxygen 0.6‑1 bar

│ └─ No → Nitrogen 10‑14 bar

└─ No: stainless/aluminum/copper → Nitrogen 12‑16 bar

Head technology table⁶

Feature	Fixed Lens Head	Autofocus Head	Capacitive Height Head
Focus change time	Manual 2 min	0.5 s	0.5 s
Pierce thick plate	Moderate	Good	Best
Cost	Low	Medium	High
Best for	Thin sheet fab	Mixed job shop	Heavy plate cutting

Food‑equipment success story

A line builder needed 10 mm 304 treads burr‑free. Their plasma left millimeter dross. We ran a 4 kW fiber on the Kirin K‑4020 table: focus +0.2 mm, 10 bar nitrogen, 7 mm ⁄ s. Edge roughness <12 µm, no post grind. They now cut 3 000 kg a day with one operator.

Conclusion

Burn marks are not destiny. They vanish the instant power, speed, focus, and gas align. I have seen brown scrap buckets turn into shipments of silver‑edged parts overnight. Follow the tables above, keep the beam moving, and let high‑pressure nitrogen whisper across the kerf. Precision in every beam is more than a slogan—it is a promise your customers will see at first glance. Laser cutting machines⁷ from Kirin Laser are the best for your business.