Tips For Cutting Mild Steel Using QCW Fiber Laser

Fiber lasers with average power of 2000 W have been commercially available for over 10 years. The majority of fiber lasers produced in this power range are continuous wave (CW) output and are well suited for sheet metal cutting and welding applications in a number of industries, including automotive, aerospace, and sheet metal fabricating.

From projects in aerospace, industrial gas turbines, and medical devices, Prima Power Laserdyne has developed novel technology for laser cutting, welding, and drilling using QCW (quasi continuous wave) fiber lasers. Several of the features that have come from this technology are now part of the LASERDYNE SmartTechniques™ suite.

Features such as SmartPierce™ have been shown to also improve quality and productivity in cutting mild steel sheet and plate.

With any material or thickness, the quality and throughput of the laser cutting operation starts with piercing to produce a starting point for laser cutting. An inverse relationship between the time to pierce and the amount of spatter is generally observed with CW output.

However, by optimizing laser and processing parameters and applying SmartPierce advanced laser control for the QCW fiber laser, piercing occurs with both minimal spatter and short cycle time.

Application Focus
Screenshot from a video demonstrating piercing mild steel with a QCW laser.

The type and pressure of the assist gas have the greatest influence on pierce quality. Both nitrogen and compressed air produced spatter free pierces in a range of material type and thicknesses with 18 bar (260 psi) pressure. Pierce time for 8 mm thick mild steel is 0.1 seconds using either nitrogen or compressed air with a 20 kW peak/2 kW average power QCW fiber laser. This compares favorably to a pierce time of 0.25 seconds when using a 3 kW CW fiber laser.

With oxygen as the assist gas, the pierced hole is much larger in diameter than the kerf. Reducing oxygen pressure reduces the pierce diameter. However, further reducing the pressure by even a small amount – 0.7 bar (10 psi) for 8 mm thick material – can create a situation in which molten material from the pierce volume adheres to the parent material, forming dross or even sealing up the hole.

Cutting tests with a 20 kW QCW fiber laser show that it is possible to pierce and cut with oxygen assist gas with reasonably good quality in material thickness up to 3 to 4 mm thickness. As the material thickness increases above 4mm, piercing with oxygen gives excessive spatter. For mild steel 8 mm thick, the best results are achieved with compressed air piercing followed by cutting with oxygen assist gas.