Absorption Rate Variations of Brass and Pure Copper in Fiber Laser Cutting

Pure copper absorbs only 5–8% of 1070 nm fiber laser energy at room temperature, while brass (CuZn37) absorbs 12–18% due to zinc’s lower reflectivity and higher absorptivity. This 2–3× difference in absorption means brass can be cut with a 1–2 kW fiber laser at 3–5 m/min, whereas pure copper requires 3–6 kW at 1–2 m/min for the same thickness. The absorption rate of both materials increases dramatically once a melt pool forms — reaching 30–50% — but the initial coupling efficiency determines whether the cut starts at all. For copper, a pulsed “pre‑heating” strategy or a green laser (515 nm) is often required to overcome the initial reflection barrier.

You need to cut 2 mm copper busbars for an electrical panel. Your 2 kW fiber laser cuts 2 mm brass beautifully — clean edges, minimal dross. On copper, the beam reflects. The material heats slowly. After 10 seconds of dwell, it finally melts, but the cut is wide, rough, and covered in spatter. You increase power to 100% — the copper glows red, but the cut still fails.

The problem is not the laser. The problem is the absorption rate of copper at the fiber laser wavelength. Copper reflects approximately 92–95% of 1070 nm light. Brass, containing 30–40% zinc, absorbs significantly more. This article quantifies the absorption rates of pure copper (C11000) and common brass alloys (C26000, C27200, C28000) as functions of temperature, surface condition, and wavelength. It provides parameter tables for cutting both materials and explains strategies to improve coupling into copper, including surface texturing, pulsed pre‑heating, and wavelength selection.

Why Absorption Matters for Laser Cutting

Laser cutting requires that the beam energy be absorbed by the material to create a melt pool. The fraction of incident power absorbed is the absorption rate $A$A. The rest is reflected.

The net heat input to the workpiece is: $$P_{\text{absorbed}}=A\times P_{\text{laser}}$$Pabsorbed=A×Plaser

For a 2 kW fiber laser, if $A=0.05$A=0.05 (5%), only 100 W is absorbed — barely enough to melt 2 mm of copper. If $A=0.15$A=0.15 (15%), 300 W is absorbed — sufficient for stable cutting.

The absorption rate depends on:

• Wavelength: Metals absorb more at shorter wavelengths (green, blue) than near‑IR.
• Material: Alloying elements change the electronic band structure.
• Temperature: Absorption increases as the material heats toward melting.
• Surface condition: Oxidized or textured surfaces absorb more than polished ones.
• Polarization: P‑polarized light at oblique incidence absorbs more.

For copper and brass, the difference in absorption at room temperature is dramatic and determines whether cutting is possible with a given laser power.

Absorption Rates of Pure Copper and Brass – Quantitative Data

1 Room Temperature Absorption (Solid State)

Measured values for 1070 nm fiber laser (source: IPG Photonics, 2021; Laser Institute of America, 2019):

Material	Composition	Absorption (%)	Reflectivity (%)	Notes
Pure copper (C11000)	≥99.9% Cu	5–7	93–95	Polished surface
Copper (oxidized)	Cu + Cu₂O	10–15	85–90	After 1 hour at 150°C
Brass C26000 (cartridge brass)	70% Cu, 30% Zn	12–15	85–88	Polished
Brass C28000 (Muntz metal)	60% Cu, 40% Zn	15–18	82–85	Polished
Bronze (90% Cu, 10% Sn)	CuSn10	10–12	88–90	Lower than brass
Pure zinc	100% Zn	20–25	75–80	For reference

The key observation: zinc absorbs 2–3× more than copper at 1070 nm. As zinc content increases, brass absorption increases nearly linearly. For a 30% Zn brass, absorption is approximately 0.07 (Cu) + 0.23 (Zn) × 0.3 = 0.07 + 0.069 = 0.139 (14%), matching measured values.

2 Temperature‑Dependent Absorption

Absorption increases as the metal heats. At the melting point, absorption rises significantly because the free electron relaxation time changes.

Material	Temperature	Absorption (%)	Change from RT
Pure copper	25°C	6	Baseline
Pure copper	300°C	10	+67%
Pure copper	600°C	18	+200%
Pure copper	1083°C (melting)	30–40	+500%
Brass (70/30)	25°C	14	Baseline
Brass (70/30)	300°C	20	+43%
Brass (70/30)	900°C (melting)	40–50	+250%

Practical implication: Once the material reaches a few hundred degrees, absorption improves significantly. The challenge for copper is the initial coupling — getting past the high‑reflectivity “barrier” at room temperature.

Why Brass Cuts Easily and Copper Does Not

For a 2 mm-thick sheet, the minimum absorbed power required to maintain a cut is approximately 200–300 W per mm of thickness (empirical for copper alloys). Therefore:

• 2 mm brass: need 400–600 W absorbed.
• 2 mm copper: need 400–600 W absorbed.

With a 2 kW fiber laser:

• Brass: $A=0.14$A=0.14, absorbed = 280 W. Marginally sufficient. In practice, brass cuts well at 2 kW, 2 mm thickness.
• Copper: $A=0.06$A=0.06, absorbed = 120 W. Insufficient. The material heats slowly, and heat conduction (copper’s thermal conductivity is 400 W/m·K) dissipates the energy faster than it accumulates.

The result: brass cuts; copper does not.

1 Cutting Parameter Comparison (2 kW Fiber Laser, 1070 nm)

Material	Thickness (mm)	Speed (m/min)	Pressure (bar N₂)	Cut quality
Brass C26000	1.0	8–10	10	Clean, bright edge
Brass C26000	2.0	3–4	12	Clean, minimal dross
Brass C26000	3.0	1.5–2.0	15	Good, slight dross
Pure copper C11000	1.0	1–2	15	Poor, heavy dross
Pure copper C11000	2.0	<0.5	20	Unstable, often fails
Pure copper (4 kW)	2.0	2–3	18	Acceptable
Pure copper (6 kW)	2.0	4–5	20	Good

For pure copper, a 2 kW laser is insufficient for reliable cutting above 1 mm. Many fabricators use 4–6 kW or switch to a green laser (515 nm), where copper absorption exceeds 40%.

Strategies to Improve Copper Absorption for Fiber Laser Cutting

If you only have a fiber laser (1060–1080 nm) and need to cut copper, several techniques can raise the effective absorption.

1 Surface Texturing (Pre‑Oxidation)

Heating the copper surface in air before cutting creates a thin oxide layer (Cu₂O, CuO). Oxides absorb more than pure metal. A 0.1–0.5 µm oxide layer increases absorption from 6% to 12–15%.

Method: Pass a low‑power defocused beam over the cutting path (e.g., 100 W, 100 mm/s, 5 mm defocus) to create a dark oxide line. Then cut with full power. The oxide layer absorbs the initial beam, heats the underlying copper, and once melting begins, absorption becomes self‑sustaining.

Effect: Reduces required power by 30–40% for the same thickness.

2 Pulsed Piercing with Power Ramping

Instead of a continuous wave (CW) at full power, use a pulsed sequence:

Step	Power (%)	Duration (ms)	Action
1	20	10	Initial heating, oxide formation
2	40	10	Further heating
3	60	5	Partial melting
4	80	5	Full melt
5	100	CW	Cut

The pulses allow the material to heat between pulses, reducing reflected power spikes. This is particularly effective on thin copper (≤1.5 mm).

3 High Nitrogen Pressure

Higher gas pressure (20–25 bar vs. 10–15 bar for brass) helps eject molten material before it resolidifies. For copper, the keyhole is less stable; high pressure keeps it open.

4 Negative Focus (Deeper Focus)

Set focus 2–3 mm below the surface (negative focus). This increases the beam diameter at the top surface, reducing peak power density (and reflection) while maintaining energy at the bottom.

5 Switch to Green or Blue Laser

Green wavelength (515 nm) has an absorption of 40–45% on copper at room temperature. Blue (445 nm) is even higher (50–60%). If copper cutting is a major part of your business, a green fiber laser or a direct diode blue laser is the correct solution. For occasional copper cutting, the techniques above can work with a 3–4 kW fiber laser.

Absorption Rate Table for Common Copper Alloys (1070 nm Fiber Laser)

Alloy	UNS	Cu (%)	Zn (%)	Sn (%)	Absorption (%) (RT, polished)	Recommended min power (2 mm)
Pure copper	C11000	99.9	—	—	5–7	4 kW
OFHC copper	C10100	99.99	—	—	5–6	4 kW
Cartridge brass	C26000	70	30	—	13–15	2 kW
Yellow brass	C26800	65	35	—	14–16	2 kW
Muntz metal	C28000	60	40	—	15–18	2 kW
Naval brass	C46400	60	39	1	14–16	2 kW
Phosphor bronze	C51000	95	—	5	10–12	3 kW
Silicon bronze	C65500	97	—	—	8–10	3.5 kW

For alloys with high zinc content (≥30%), standard fiber laser cutting is practical. For high‑copper alloys (>90% Cu), significantly more power or a different wavelength is required.

Cutting Parameter Recommendations

1 Brass (C26000, 2 mm)

Parameter	Value	Notes
Laser power	2000 W	CW
Cutting speed	3.5 m/min	Adjust ±0.5 m/min
Assist gas	Nitrogen	Purity ≥99.995%
Gas pressure	12 bar
Nozzle diameter	1.5 mm
Standoff	1.0 mm
Focus position	–1.0 mm (below surface)
Expected edge	Bright, minimal dross

2 Pure Copper (C11000, 2 mm) – 4 kW Fiber

Parameter	Value	Notes
Laser power	4000 W	CW
Cutting speed	2.0 m/min
Assist gas	Nitrogen	High purity
Gas pressure	20 bar
Nozzle diameter	2.0 mm
Standoff	1.5 mm
Focus position	–2.0 mm
Pre‑heating	500 W defocused scan	Recommended

Without pre‑heating, expect heavy dross and possible cut failure. With pre‑heating, acceptable cuts are possible.

Material Composition Verification

Before cutting, verify the alloy composition. “Brass” from different suppliers can vary from 60% Cu to 85% Cu. Lower copper content cuts more easily. Use a handheld XRF analyzer or check the mill certificate.

Per ASTM B36 (brass plate/sheet), common compositions:

Alloy	Cu %	Zn %	Other
C23000 (red brass)	85	15	—
C24000 (low brass)	80	20	—
C26000 (cartridge brass)	70	30	—
C26800 (yellow brass)	65	35	—
C28000 (Muntz)	60	40	—

Red brass (85% Cu) cuts more like copper than brass. Expect 10–20% lower cutting speed than yellow brass.

Conclusion

The absorption rate of copper alloys in fiber laser cutting is the dominant factor determining cut feasibility and quality. Pure copper absorbs only 5–7% of 1070 nm light, requiring 4–6 kW for 2 mm thickness. Brass, with 30–40% zinc, absorbs 14–18% and cuts well with 2 kW.

For fabricators who occasionally cut copper, surface pre‑oxidation, pulsed piercing, and high gas pressure can improve coupling. For production copper cutting, a 4–6 kW fiber laser or a green laser is recommended.

Always verify the alloy composition. Red brass (85% Cu) behaves more like copper than yellow brass. Adjust parameters accordingly. Protect the laser from back‑reflection with proper isolators and monitoring.

With correct parameters, both brass and copper can be cut successfully on fiber lasers — but the power and technique differ by a factor of 2–3×.

Read More： Differences in Laser Cutting Parameters Between Brass and Copper

Why Does Cutting Brass Generate Highly Toxic Zinc Fumes?

Frequently Asked Questions

Q: Can I cut copper with a 1 kW fiber laser?

A: Only very thin copper (<0.5 mm) with pre‑oxidation. For 1 mm copper, 1 kW is marginal — expect heavy dross and slow speed. For 2 mm, not feasible. Upgrade to 3–4 kW or use a green laser.

Q: Why does my brass cut with a blue edge (discoloration)?

A: The blue discoloration is due to zinc oxide formation. Brass contains zinc, which oxidizes at high temperatures. Reduce nitrogen pressure slightly (from 12 bar to 10 bar) and increase speed by 10–15% to minimize oxide formation. For cosmetic parts, use a higher purity nitrogen (Grade 5.0).

Q: What is the maximum thickness of brass I can cut with a 2 kW fiber laser?

A: Approximately 4–5 mm for yellow brass (70/30) at 1–1.5 m/min. For red brass (85% Cu), limit to 3 mm. Thicker sections require more power or oxygen assist (but oxygen causes oxidation).

Q: Does the surface finish affect absorption?

A: Yes. Polished surfaces reflect more than matte or brushed finishes. A light sanding (400 grit) or chemical etching increases absorption by 2–5%. For difficult copper cuts, abrade the surface along the cut path.

Q: My laser trips a back‑reflection alarm when cutting brass. Why?

A: Brass reflects less than copper, but at high power (e.g., piercing), reflection can still be significant. Ensure the nozzle is clean and centered. Reduce pierce power to 50% for 10 ms before ramping up. If the alarm persists, check the isolator.

References

1. IPG Photonics. (2021). Fiber Laser Cutting of Copper and Brass Alloys. IPG Application Note AN‑CU‑2021.
2. Laser Institute of America. (2019). Absorption Coefficients of Metals at 1 µm Wavelength. LIA Technical Guide TG‑ABS‑2019.
3. ASTM B36/B36M. Standard Specification for Brass Plate, Sheet, Strip, and Rolled Bar. (Composition reference)
4. ISO 16162:2012. Cold‑rolled steel sheet products — Dimensional and shape tolerances. (Thickness reference)