Optimizing CO₂ laser use: Part II

Photo courtesy of TRUMPF Inc.

Part I of this series discussed the production of the CO₂ laser beam and the interaction of the gases required to produce a quality beam. This part discusses beam delivery to the workpiece and the gases used to process the material.

CO₂ laser beams used in materials processing must be maintained and delivered to the workpiece according to specific requirements. The basic ones are that the beam must:

1. Maintain a constant power level over great distances.
2. Produce consistent results.
3. Travel to the workpiece accurately.
4. Focus precisely at the desired point of contact.

Beam Delivery

A laser beam delivery system comprising a mirror telescope, beam-bending mirrors, and a phase shifter delivers the beam properly to the laser cutting head. Once the beam is delivered to the laser head, a focusing lens centers the point correctly to the work area.

The laser beam typically is enclosed in a protective pathway that must be filled with a gas to slightly above atmospheric pressure. Any particulate matter or particles that may absorb the laser light must be removed from this pathway. Compressed air is a common choice for this application, but air can contain moisture, particles, and oils that can decrease the laser beam's power and shorten the bean-bending mirror's life. Nitrogen is preferable because it is contaminant-free and does not affect the mirrors adversely.

Mirrors have finite lives that vary depending on the process and the required power level. The typical life is 200 to 600 hours. The beam power from the resonator outlet to the workpiece should be measured at regular intervals to determine mirror and beam degradation. Loss of mirror efficiency negatively affects the process. It also increases costs by delivering the beam at less than the required power level. A 10 percent decrease in mirror efficiency can increase electrical power consumption for the same operation by as much as 15 percent.

Cutting

Once the beam has been properly delivered to the laser head, the beam's focus coupled with the assist gases at the proper pressure and flow completes the laser cutting process. Each material type requires the right combination of laser power, focusing lenses, and assist gas pressures and flows to do the job successfully. The laser manufacturer can supply this information in a format similar to the charts in Figure 1 and Figure 2. As shown in these charts, the parameters required to produce a quality laser-cut part vary with the material type and thickness.

Undesirable factors to consider in laser cutting are:

1. Burr formation, which normally results from improper laser beam focus and travel speed.
2. Kerf, a groove or notch that usually is wider at the top than the bottom or the exit of the cut.
3. Pitting, irregular erosions in the cut surface normally caused by a slow cutting speed with oxygen or air assist gas.
4. Surface roughness, different from pitting in that the surface is measured as an average and not a single point.

Laser cutting should produce consistent cut surface quality and groove depth. Irregularities normally are caused by using incorrect assist gas pressure and flow or a cutting speed that is too fast.

Selecting the Gas and Supply Method

The assist gases and supply equipment must be able to supply the gas to the laser head at the desired pressure and flow. Selecting the proper assist gas system is not always as simple as it might seem.

Among the items to consider when selecting the proper assist gases and supply system are:

Gas Required. Most lasers today can process various materials using different assist gases. Air, oxygen, and nitrogen are the most common. Argon and helium are required for titanium and some other materials. Each assist gas has pros and cons for each material. Consult your gas supplier for the most cost-effective system for your application.

Material Type and Thickness. Oxygen is the typical choice for carbon steel; air and nitrogen can be used to cut carbon steel, stainless steel, and aluminum.

Work Schedule. The more hours the laser is online, the greater the demand on the laser assist gas delivery system.

Once the assist gases have been selected, the supply method must be determined (see Figure 3). Gases must be supplied continuously, without interruption. High-pressure compressed-gas cylinders can be used, but these systems often have a high unit cost. Portable cryogenic cylinders, commonly referred to as "liquid" cylinders, have lower unit cost but limited pressure and flow capabilities. Cryogenic technology, commonly referred to as microbulk or minibulk, supplies both the required pressure and flow for laser cutting with oxygen, argon, and nitrogen assist gases.

Always remember that the CO₂ process involves using gases for generating the laser beam, protecting beam delivery to the workpiece, and for cutting. Each step in the process is an independent application with unique gas requirements; the final finished part depends on the combination of all three. The higher the beam quality, the higher the part quality, and the more cost effectively the part is produced.