The assist gas in laser cutting machines is just as important as the laser itself for cutting metal. It's not simply a supporting player that "blows away molten slag," but a core process factor that determines the success, quality, efficiency, and cost control of the cutting process. Its importance can be understood from the following aspects:
I. Core Functions: Four Main Functions
Removing Molten Material and Forming the Cutting Surface
This is the most basic function. After the laser melts or vaporizes the metal, the high-pressure assist gas blows the molten metal slag away from the bottom or sides of the kerf (depending on the material) at high speed, forming a clean, continuous cut. Without assist gas, the molten metal would re-solidify, blocking the kerf and interrupting the cutting process.
Participating in Chemical Reactions and Providing Additional Energy
For oxygen: This is a reactive gas. When cutting carbon steel, oxygen undergoes a vigorous exothermic oxidation reaction with the high-temperature iron (similar to combustion). The heat released by this chemical reaction (accounting for more than 60% of the total cutting energy) greatly supplements the laser energy. This allows for cutting thicker carbon steel with lower laser power, which is crucial for efficiency and cost when cutting carbon steel.
For nitrogen and argon: These are inert gases. They do not react with the metal, relying on a purely physical "melting-and-blowing" process. The cutting surface has no oxide layer, forming a clean, silvery-white or natural-colored surface, commonly used for stainless steel, aluminum, titanium alloys, etc.
Protecting Optical Components and Workpiece
Dispersing smoke and spatter: This prevents these substances from splashing upwards, contaminating and damaging the focusing lens above, extending the lifespan of expensive optical components.
Cooling the heat-affected zone: The airflow can provide a certain cooling effect on the cut and surrounding areas, reducing workpiece thermal deformation and the width of the heat-affected zone, which is especially important for precision machining.
Isolating from air: Inert gases can prevent certain reactive metals (such as titanium alloys) from undergoing harmful reactions with nitrogen and oxygen in the air at high temperatures, ensuring material performance.
Focus Protection and Energy Maintenance
During the initial stages of piercing and cutting, the assist gas helps stabilize and protect the "keyhole" focused by the laser, maintaining the energy absorption mechanism. High-speed airflow helps to blow away the plasma cloud at the cutting front (especially when cutting highly reflective materials), preventing the plasma from shielding the laser energy and ensuring that the laser energy can continuously and effectively act on the material.
II. Importance: Determining Process Success and Quality
Cutting Capability and Thickness Limits
Carbon Steel: Oxygen assistance determines the maximum cutting thickness and speed. With the energy supplementation from the oxygen combustion reaction, lasers above 10kW can easily cut carbon steel thicker than 40mm.
Stainless Steel/Aluminum: High-purity, high-pressure nitrogen is essential for achieving bright, oxide-free cutting. To achieve a perfect bright surface, sufficient gas pressure and flow are needed to quickly blow away the molten material in liquid form, preventing oxidation. The thicker the material being cut, the higher the required gas pressure (often up to 20-30 bar) and purity (99.99%+).
Cutting Section Quality
Perpendicularity and Roughness: The gas pressure, flow rate, and nozzle condition directly affect the way the slag is blown away. Insufficient pressure will lead to slag accumulation at the bottom and a rough cross-section; excessive pressure may cause streaks on the cross-section or widening of the upper part.
Oxide Layer: Using oxygen will produce a dense oxide layer (which affects welding), while using nitrogen can obtain a clean surface that can be directly used for welding or electroplating.
Processing Efficiency and Cost
Speed: Optimized gas parameters can maximize chemical reaction energy (oxygen) or slag removal efficiency (nitrogen), thereby increasing cutting speed.
Cost Composition: For bright, oxygen-free cutting, the cost of nitrogen consumption is one of the main components of the overall processing cost, sometimes even exceeding electricity costs and equipment depreciation. Therefore, gas utilization efficiency (such as using a nitrogen generator) is key to cost reduction.
Conclusion
When laser cutting metals, the assist gas is not merely "auxiliary," but a "main force" alongside the laser. It is a multi-functional process lever, integrating physical slag removal, chemical energy enhancement, cross-section shaping, and equipment protection. The precise selection and control of gas type, pressure, purity, flow rate, and nozzle design are at the core of laser cutting process optimization, directly determining the quality, capability, efficiency, and final cost of processing. Without the correct assist gas strategy, even the highest power laser cannot reach its full potential.