Key Takeaways
- Oxygen (O₂): Reacts exothermically with steel — boosts cutting speed and enables thick plate cutting, but leaves an oxide edge. Required for mild steel >6 mm at lower power levels.
- Nitrogen (N₂): Inert — blows the melt away without reaction. Produces a clean, oxide-free, weld-ready edge. Mandatory for stainless steel and aluminium where oxide layer is unacceptable.
- Compressed air: Cheapest option — 21% O₂ content gives moderate cutting speed. Edge quality between O₂ and N₂. Best for mild steel parts that will be painted or powder-coated and don't require weld-quality edges.
- N₂ from on-site generation costs $0.03–0.08/m³ vs $0.50–$2.00/m³ for bottled N₂ — a nitrogen generator pays for itself quickly above 1,000 cutting hours/year.
The Role of Assist Gas
Assist gas in laser cutting serves three functions: it removes molten material from the kerf (blowing melt downward), it shields the lens and nozzle from backspatter, and — in the case of oxygen — it adds exothermic energy to the cut by reacting with the iron in the steel. The choice of gas determines edge quality, cutting speed, maximum thickness, and operating cost more than almost any other parameter apart from laser power.
Gas-by-Gas Comparison
| Parameter | Oxygen (O₂) | Nitrogen (N₂) | Compressed Air |
|---|---|---|---|
| Cutting mechanism | Laser + exothermic reaction | Laser only (inert) | Laser + partial reaction (21% O₂) |
| Edge oxidation | Heavy oxide layer (blue/brown) | Oxide-free, bright | Light to moderate oxide |
| Weld-ready edge? | No — must clean/grind | Yes | Marginal — usually OK for MIG/fillet welds |
| Paint/powder adhesion | Poor (oxide interferes) | Excellent | Good |
| Cutting speed vs N₂ | Faster on steel >6 mm | Baseline | 80–90% of N₂ speed on thin steel |
| Max thickness | Higher (reaction energy) | Lower (laser energy only) | Moderate |
| Nozzle pressure | 0.3–1.5 bar | 6–20 bar | 4–12 bar |
| Cost | Low (O₂ is cheap) | Medium–high (depends on supply) | Very low (compressor only) |
| Burr tendency | Some (oxide slag) | Minimal | Some |
Mild Steel: O₂ vs N₂ vs Air
For mild steel, the assist gas choice depends primarily on thickness and downstream application:
- 0.5–3 mm: All three gases work well. N₂ gives the best edge quality; air is cheapest; O₂ gives a slightly faster cycle time but with an oxide edge that often requires cleaning before welding.
- 4–8 mm: O₂ becomes advantageous for speed and clean kerf on a low- to mid-power laser. N₂ still works but at lower speeds, especially below 6 kW. Air is viable for general work.
- 10–25 mm: O₂ is the practical choice at 3–6 kW. High-power machines (12–20 kW) can cut thick steel with N₂ at commercially viable speeds — producing an oxide-free edge on thick plate, which is unusual and valuable for structural weldments.
Stainless Steel and Aluminium: N₂ Only
For stainless steel, oxygen creates a thick, tenacious chromium oxide layer on the cut edge. This layer cannot be painted, welded without preparation, or used in food/medical/corrosion applications without extensive post-processing. N₂ is the only sensible choice for stainless steel in a production environment.
For aluminium, oxygen causes severe burning and inconsistent cuts due to the highly exothermic Al + O₂ reaction. N₂ (or argon for ultra-clean edges) is required. High-pressure N₂ (12–20 bar) is standard for aluminium cutting.
| Material | Recommended Gas | Pressure Range | Reason |
|---|---|---|---|
| Mild steel, general fabrication | Air or O₂ | O₂: 0.5–1.5 bar; Air: 6–10 bar | Cost-effective; downstream painting acceptable |
| Mild steel, weld-quality edge | N₂ | 8–16 bar | Oxide-free edge ready for welding without preparation |
| Mild steel, thick (>10 mm) | O₂ | 0.5–1.0 bar | Exothermic energy essential for thick plate at lower wattage |
| Stainless 304/316 | N₂ | 10–20 bar | Chrome oxide prevention mandatory |
| Aluminium all alloys | N₂ | 12–20 bar | Prevents burning; oxide-free edge |
| Copper / brass | N₂ or air | 8–14 bar | Reactive gases cause rough cuts |
Nitrogen Supply Options
N₂ supply method has a major impact on operating cost at the throughput levels of a production laser shop:
| Supply Method | Approx Cost/m³ | Break-even Hours/Year | Notes |
|---|---|---|---|
| Bottled N₂ (cylinders) | $0.80–$2.50 | Baseline | Highest flexibility, highest per-m³ cost; practical only for very low usage |
| Liquid N₂ tank (bulk) | $0.15–$0.40 | ~500 hrs/year | Good mid-range solution; supplier delivers bulk liquid |
| On-site N₂ generator (PSA) | $0.02–$0.08 | ~1,000–1,500 hrs/year | Best long-term cost; requires compressor air supply; purity 95–99.999% |
PSA (pressure swing adsorption) generators separate N₂ from compressed air using molecular sieve material. They require clean, dry compressed air and produce N₂ at 95–99.5% purity for cutting (laser cutting usually requires 99.5%+ for stainless). Higher-purity generators cost more but eliminate the risk of oxidised edges from trace O₂ contamination.
Nozzle and Pressure Settings
N₂ cutting uses high pressure (8–20 bar) through a single or double nozzle to physically blow melt from the kerf. A key rule: if the pressure is too low, dross adheres to the bottom of the cut. If too high on thin material, the gas blows through the kerf before the laser fully melts it, causing roughness or striation.
O₂ cutting uses much lower pressure (0.3–1.5 bar) — the gas supports the combustion reaction without blowing the melt away before the reaction completes. High O₂ pressure causes burning and rough edges on thin material.
A general starting point: increase N₂ pressure until dross is eliminated, then back off 1–2 bar. Reduce O₂ pressure if edges burn or are rough at the bottom; increase it if dross (slag) sticks to the underside of the cut.