The Mechanics of Assist Gas in Laser Cutting

By August 6, 2015 Article, Technology No Comments

^ Assist Gas Ejects Molten Material in Laser Cutting (Source: http://www.industrial-lasers.com/articles/2010/10/fine-laser-cutting.html)

Aiding Laser Cutters

Choosing the correct laser cutting assist gas and using it at the appropriate pressure and flow-rate often makes the difference between an effective and economical cut and a fiasco. The choice depends largely on the type and thickness of materials, but other factors such as the required quality and quantity also influence the decision.

Experts believe we are on the threshold of revolutionary advances in laser cutting. Even otherwise, the process is internationally recognized as a fundamental, indispensable technology on account of its capacity to cut umpteen materials to nearly any shape.

Functions & Selection Criteria

Functions of assist gas:

  • (oxygen) adds to the laser’s cutting power
  • flushes off molten material
  • shields the cut zone
  • minimizes heat-affected zone
  • protects machine optics from fumes
Cutting Speed Vs Material Thickness Comparison between Nitrogen & Oxygen while Cutting Mild-Steel with 5.2KW Laser-Power  (Source: http://www.bocworldofwelding.com.au/media/pdf/file/library/WOWLibrary-Laser%20Gases.pdf)

Cutting Speed Vs Material Thickness Comparison between Nitrogen & Oxygen while Cutting Mild-Steel with 5.2KW Laser-Power
(Source: http://www.bocworldofwelding.com.au/media/pdf/file/library/WOWLibrary-Laser%20Gases.pdf)

Choice of assist gases depends on the balance between production costs and production cycle times:

  • Gas-Material Combinations:
  • oxygen, nitrogen, compressed air: all suited for mild-steel and stainless-steel
  • argon: titanium
  • oxygen: copper, aluminum
  • nitrogen and then air: brass
  • low pressure air (<5psi) and nitrogen: thermoplastics
  • never use oxygen to cut galvanized sheets, for they emit toxic fumes
  • Oxygen:
  • cuts thick material effectively and economically with low flow-rates and low laser-power because its exothermic (heat-releasing) reaction strengthens the beam’s cutting power
  • produces an oxidized, corrosion-prone edge that must be removed before subsequent painting or welding
  • purity is a huge concern, anything below 99.7% is unacceptable
  • Nitrogen:
  • no oxidized edge
  • has replaced oxygen for cutting stainless-steels as high-power nitrogen lasers cut faster and provide oxidized-edge-free cuts
  • cuts under-0.08inch-thick mild-steel faster than oxygen does
  • greater volumes needed to create the required high-flow and high-pressure because if suffocates the cut vis-à-vis oxygen’s amplifying effect
  • required volumes drop with decreasing material thicknesses
  • slowest assist gas except when cutting very-thin materials
Gas Consumption Vs Material Thickness Comparison between Nitrogen & Oxygen while Cutting Mild-Steel with 5.2KW Laser-Power (Source: http://www.bocworldofwelding.com.au/media/pdf/file/library/WOWLibrary-Laser%20Gases.pdf)

Gas Consumption Vs Material Thickness Comparison between Nitrogen & Oxygen while Cutting Mild-Steel with 5.2KW Laser-Power
(Source: http://www.bocworldofwelding.com.au/media/pdf/file/library/WOWLibrary-Laser%20Gases.pdf)

Oxygen and nitrogen are similarly priced. You need less oxygen for cutting because it adds to the laser’s power. Using oxygen is more economical, but generally gives inferior, less-accurate cuts vis-à-vis nitrogen cuts.

Compressed Air combines the merits of nitrogen and oxygen (air is mostly nitrogen):

  • most compatible with cutting aluminum, for it gives a clean edge
  • high investment costs for a setup that delivers clean air at elevated pressure and large flow-rates

Factors Affecting Quality & Troubleshooting

Assist gas pressure and flow-rate, material feed-rate, focal position of lens, and laser power all determine the quality. Setting these parameters incorrectly lowers cut quality and so does incorrect centering of the beam in the nozzle and discrepancies in path of the gas from its delivery point to storage, plumbing, cutting head, and nozzle.

For inferior cuts, slow down feed-rate only as a last resort, for this might negates the very purpose of investing in a laser cutter – rapid cutting. Common issues include:

  • Dross in oxygen and nitrogen cutting:
  • Beady Dross: indicates excessive flushing due to too-low focal-point or too-high gas flow
  • Spiky Dross: material solidifies before it is flushed. It indicates too-high focal-point or too-low gas flow
  • Side-Burning in oxygen cutting
  • Unequal Cutting in X-Y plane

Troubleshooting Checklist

Issue Corrective Action Time for Corrective Action (Minutes)
Nozzle Contamination wipe dirt or spatter on nozzle

replace damaged nozzle

1-2
Cutting Gas use correct gas in correct quantity

check flow and pressure with flow meter

check nozzle for blockages

1-2
Cutting Speed compare with successful setting

try increasing and decreasing speed by 10% and 20%

1-2
Nozzle Stand-Off replace nozzle with non-identical nozzle

change nozzle-lend gap

1-2
Laser Power & Pulsing Conditions compare with successful setting

laser may need servicing, tuning, or warming-up

clean internal mirrors

1-5
Material Specification & Condition use correct material-gas combination

check for scratches, rust, and coatings

1-5
Nozzle Type, Condition, & Alignment check the nozzle’s diameter, centering, and condition (scratches) 1-10
Beam Steering Mirror Condition & Alignment clean mirrors and align them square-central 5-60 per mirror
Lens Type, Condition, & Alignment check the lens’ focal length, fitting, condition (scratches), temperature, and alignment 10-20
Laser Mode Quality & Polarization tune and clean the cutter 20-40

 

Position of Focus: Flawed focal position is often the prime and most-overlooked culprit. The focal point is the hottest and narrowest point on the beam. Considerations:

  • Use long focal-length lens for thicker materials and vice versa
  • Locate the focal point just inside the material when cutting with nitrogen because nitrogen cools the cut zone

 

 

Edge with Dross

Edge with Dross (Source: http://www.twi-global.com/news-events/case-studies/dross-free-laser-cutting-stainless-steel-040/)

Dross-Free Edge (Source: http://www.twi-global.com/news-events/case-studies/dross-free-laser-cutting-stainless-steel-040/)

Dross-Free Edge
(Source: http://www.twi-global.com/news-events/case-studies/dross-free-laser-cutting-stainless-steel-040/)

 

Excessive nitrogen can cause plasma indicated by a purple-white arc. This way, you are plasma cutting with a laser and creating a rough edge

  • Locate the focal point just over the material when cutting with oxygen because oxygen adds to the laser’s power
  • Changing the focal-point changes the kerf-width as the beam is narrowest at its focus. The combined setting of the kerf-width and feed-rate determines the value of all other cutting parameters

Kerf is the zone on the material where the laser makes the cut

Gas Pressure: Cutting finer details on the workpiece requires greater pressure (about 60psi). To maintain sufficient pressure use:

  • high-pressure gas supply tank
  • external device for pressurized flow of the liquidized gas
  • sufficient gas volume
  • pipes, hoses, regulators, connectors, and fittings of sufficiently large diameter
  • sweeping curves instead of 900 elbows on pipes
  • nitrogen purging (inerting the atmosphere by using nitrogen to remove moisture and oxygen) of brazed pipes

Gas Volumes: are critically important when cutting narrow kerf-widths. Fiber lasers with large-diameter nozzles delivering larger volumes are better at cutting narrow kerfs. Larger nozzles increase throughput and more than compensate the cost of additional gas.

Assist gas columns have oscillating waves at their peripheries due to their friction with the stationary, ambient air. For narrow columns, these waves move into the kerf and damage the cut. Wider columns keep these waves at their outskirts, away from the kerf. Larger nozzles improve cut edges, allow using larger range of cutting speeds, and enable low-pressure operations. However, they add to the gas costs.

Finally

On account of the interplay between multiple factors, there are no hard-and-fast rules on the choice of assist gases and setting of their various parameters. There are only guidelines. Operators need good judgment for making the perfect cut.

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