For sheet metal cutting, laser technology has long been a mature and efficient production method, with thousands of cutting systems around the world. In the past 10 years, solid-state fiber-coupled lasers represented by fiber and disk systems have become the first choice, basically replacing CO2 lasers in the metal processing market. Both solutions work in a similar way, using a cutting head to focus the laser beam on the metal to be processed through a conical nozzle of coaxial high-pressure auxiliary gas.

So far, commonly used lasers have used fixed beam quality, with a wide range of output power to reflect cutting performance, and a suitable range of focal spot sizes to produce good cutting results. The standard of "one size fits all ranges" is to select a specific focal spot size for a given laser cutting system, and use a lens combination to handle all metal types and thicknesses. This solution works well for a low-cost, simple device and defines the cutting effect for a given power for all types of metal sheets.

However, this approach always requires different degrees of compromise when facing the different characteristics of different metal materials. When cutting stainless steel with nitrogen (melt cutting), a smaller divergence angle and focus spot are required, while when cutting mild steel with oxygen (oxidation cutting), a larger divergence angle and focus spot are required. If this "one size fits all" approach is used, it is difficult to take into account the effects of both melt cutting and oxidation reaction cutting at the same time.

For laser cutting equipment manufacturers, it is a good choice to choose a cutting head with an electric zoom collimation system and a variable beam expansion system, which can change the focal spot size to a certain extent. However, as shown in Figure 1, as long as the beam quality is constant, it cannot provide the best combination of focal spot size and divergence angle, because a small focal spot always has a larger divergence angle than a large focal spot. To improve the quality of melt cutting and oxidation reaction cutting, the industry needs lasers with variable beam quality.

Against this backdrop, SPI has developed a new laser, the variMODE laser, which can control the beam quality by controlling the output beam divergence. This is all done inside the laser, without any external optics. SPI installed a ?100μm delivery fiber inside the laser, allowing the beam quality to be selected from 3.2 mm.mrad (M2 is 9.5) to 5.8 mm.mrad (M2 is 17). The poorer beam quality of 5.8 mm.mrad can also be matched to the NA of a standard commercial cutting head, ensuring that all the laser power can be delivered to the sheet metal without causing light shearing in the cutting head. The switching time from poor beam quality to good beam quality is typically 30ms, which is sufficient to achieve fast switching between perforation and cutting.

In a 3kW cutting test, SPI set the variMODE laser to high beam quality (3.2 mm.mrad) and low divergence, and compared it with the beam quality of the standard product (4.5 mm.mrad). The test used the same cutting head with the same focal spot size in both cases.

The test results are shown in Figure 3. The cutting speed using high beam quality is significantly faster than that using standard beam quality. In 2mm stainless steel cutting, the high beam quality cutting speed is 45% faster, and in 4mm stainless steel cutting, the high beam quality cutting speed is 25% faster.

During the test, the laser output power of both lasers was kept constant at 3kW, and the speeds used were all the speeds that can actually be produced, with a high tolerance for changes in the focus position and nozzle separation height.

When cutting low carbon steel with oxygen, there is not much difference in cutting speed within the effective beam quality range, but there is a big change in quality. When using a lower beam quality (5.8 mm.mrad), the cutting edge quality and processing window are significantly improved. Especially for thick carbon steel (20 mm), the average roughness (Rz) of the low beam quality cut surface is < 30μm. The higher the beam quality, the rougher and more unstable the cutting edge.

In addition, the experiment shows that the cutting temperature with low beam quality is much lower than that with higher beam quality. For example, when cutting 20 mm mild steel with a 3 kW laser, the sample with high beam quality (3.2 mm.mrad) reaches 190°C, while the sample with low beam quality (5.8 mm.mrad) reaches only 110°C for the same cut end face, assuming other parameters remain unchanged. Lower processing temperatures also produce lower end face roughness.

While improving edge quality, mild steel can also be cut to a thinner, higher aspect ratio. As shown in Figure 4, a 3 kW variMODE laser cuts a 2.5 mm wide connecting rod shape on 20 mm mild steel with a beam quality of 5.8 mm.mrad and a cutting speed of 0.7 m/min.

Another benefit of using a variable mode laser is that different beam qualities can be used for perforation and cutting, respectively. To ensure a smooth cut end face, perforation is required in the metal sheet before cutting begins, which is particularly important for oxygen cutting. Pulsed lasers can improve perforation quality, but it takes a long time to perforate thick carbon steel. If a higher beam quality is used, the perforation time can be significantly reduced.

As can be seen from the results, using the variMODE laser, it is possible to perforate with a high beam quality to obtain faster and higher quality holes, and then switch to a lower beam quality hole for cutting. This process also benefits from the variMODE laser's fast beam quality switching, while also avoiding pauses during cutting to ensure smooth processing.

Summary

Compared with lasers with fixed beam quality, laser systems with variable beam quality have obvious advantages for sheet metal processing. At the same laser power, laser systems with variable beam quality perform better than systems with fixed beam quality. When set to high beam quality, stainless steel melting cutting speeds are significantly increased; when set to lower beam quality, thick carbon steel cutting speeds are the same, but due to lower workpiece temperatures, end face quality is improved, and narrow contour cutting can also be performed