Laser cutting of mild steel and stainless steel has a long history and has been one of the primary applications for CO2 lasers. However CO2 lasers have not traditionally offered a good solution for cutting highly reflective materials.
Fiber lasers have an emission wavelength of around 1.07 µm, compared to 10.6 µm for traditional CO2 alternatives. Not only is the 1.07 µm laser light reflected less, and therefore absorbed more easily, but the shorter wavelength can be focused into a spot that is around 1/10th of the diameter of a CO2 beam. This provides a dramatically higher power density, making metal penetration easier. At such high power density levels, metals such as copper and brass quickly go through a phase change into molten state, therefore the laser beam rapidly overcomes the reflectivity barrier of such metals to initiate an efficient cutting process. Cutting such metals have shown to be challenging using CO2 lasers or near-IR lasers that have low peak power.
Copper, brass, bronze, silver, gold, and aluminum are highly reflective of infrared light in their solid state. Aluminum, however, is not considered a reflective metal for the practical purpose of fiber laser cutting.
Back off from the maximum feed rate the process can support by about 10 – 15% to avoid any risk that the cut will extinguish, thereby applying high levels of beam energy to a material in its most reflective state. If in doubt, start at a slower rate than you know the process can support. Allow sufficient dwell time to ensure pierce hole is through before moving the beam to start the cut.
For both piercing and cutting, set the focus position as close to the top surface as the cut quality allows. This minimizes the amount of surface material that interacts with the beam at the beginning of the process, thereby maximizing the power density of the beam, which leads to quicker melting.
|Minimum Peak Power Needed
Using the maximum peak power available for the piercing and cutting reduces the time in which the material is in its most reflective condition. The chart above can be used as a conservative guide to start the process development.
When piercing and cutting copper, using high-pressure oxygen (100-300 psi depending on the thickness) is typically used as the cutting gas to increase the process reliability. When oxygen is used, the formation of copper oxide on the surface reduces the reflectivity. For brass, nitrogen cutting gas works fine.