Fiber laser systems can be used to cut difficult areas. Photo: LaserStar Technologies
Aluminum, copper, bronze, brass, stainless steel with a mirror polish.
What do these metals have in common? For one, they are all popular metals that are used in myriad industries. They are also highly reflective materials that present a challenge to fabricators who use lasers to process these metals. Challenges that can lead to issues with cut quality, material thickness, and damage to the laser system itself.
When the laser beam strikes the material, part of the energy is reflected and the rest is absorbed. Aluminum is a highly reflective material, redirecting around 92 percent of visible light and up to 98 percent of medium and far infrared radiation. Higher reflective materials typically require high peak powers to overcome these reflectivity issues.
Why is this an issue? The Welding Institute’s Website (www.twi-global.com), in an item on overcoming laser problems, finds that the more reflective material, the more potential damage can occur. For example: “Aluminum is more reflective than [carbon manganese] (C-Mn) steel or stainless steel and has the potential to cause damage to the laser itself. Most laser cutting machines use a laser beam aligned normal to a flat sheet of material. This means that should the laser beam be reflected by the flat sheet it can be transmitted back through the beam delivery optics, and into the laser itself, potentially causing significant damage.”
Here is a collection of materials that show the range of reflective materials that can be cut with a fiber-laser system. Photo: Trumpf
When it comes to drilling, cutting, and welding, there is essentially the choice between carbon dioxide lasers (CO2) and solid-state lasers such as fiber optic lasers. Many consider fiber to be the best choice for cutting reflective materials, but CO2 lasers with proper precautions can cut these materials.
David Dobson, sales engineer for Trumpf’s TruLaser product said that “solid state lasers are much better for cutting reflective materials because the 1um wavelength is more easily absorbed into the material, as much as four times more than a CO2.”
Fiber lasers can cut material and modern equipment has developed methods to protect itself against laser beam bounce back. Photo: BLM
Essentially, CO2 lasers emit at up to 10.6μm, while fiber optic lasers emit at about the 1μm level. At near-infrared wavelengths, metal reflectivity is significantly lower than at the longer emission wavelengths of CO2 lasers.
Shorter wavelengths create smaller, more focused spots and cut down on HAZ and other negative surface features. Todd O’Brien, North American Product Manager, Lasers, for the BLM Group USA, said, “Fiber optic laser beams run about 3 to 4 thou (0.003 to 0.004 in.), while CO2 lasers are in the 6 to 8 thou range (0.006 to 0.008 in.).”
While all reflective material can cause bounce back, the difference in how the beam is generated and delivered is a major factor. CO2 lasers uses resonators that send the laser beam by a series of mirrors and deliver the energy to a single spot. This can happen not just with the initial processing (cutting, drilling, welding), but also with the molten metal created during processing that can also be highly reflective and become a source of a damaging reflective beam, said O’Brien.
Fiber optics uses a resonator to excite electrons and then a beam is passed through a fiber optic cable to the cutting head. “You don’t have the mirrors to worry about,” he adds. “With the mirrors you can get a back reflection to the resonators, lens and other items. This (fiber optics) eliminates that factor in the process.”
Dave Bartholomew, global sales, and product specialist at Whitney, part of the Megafab group of companies, explained toShop Floor Lasers’ magazine why this works. “[With a] fiber delivery cable that is properly designed, if you get that back reflection into the cable, the cable actually absorbs it so that it never gets back to the power source.”
A robot-based laser cutting system works on a highly reflective material. Photo: Jenoptik
Torsten Reichl, product manager for Jenoptik-Votan BIM, who is with the company’s laser and materials processing division, said that bounce back can be a problem even in the best-case scenario. “The biggest drawback is that the process itself stops by getting a back reflection into the system. That means that the laser system stops operation due to (state-of-the-art,[shut-off]) protection systems or (in worst case) due to damage to the laser itself.”
Many lasers offers protection or “failsafe” systems to prevent damage to the cutting equipment such as sensors within the laser head that monitor radiation levels and automatically shuts the laser down if too much radiation is reflected back onto the lens.
“Each manufacturer has their own way of dealing with reflectivity issues,” said Dobson. The company’s TruDisk laser, for example, has much lower energy intensity when compared to a traditional fiber laser, said Dobson. “This is simply because of the architecture of its design, which has more surface area to dissipate the energy. The result is a significant decrease in damage risks.”
Jenoptik, which offers robotic fiber laser cutting technology, uses special moving strategies for piercing and laser processing to avoid back reflections, said Reichl. While not wanting to divulge proprietary information, he said that the “kind of beam path Jenoptik is using does minimize back reflections. The beam path is a completely sealed beam path inside of the robot, and there we made some adaption to minimize the risk of back reflections. During the piercing process, we use a special pulse mode to oxidize the material surface quickly in order to minimize the risk of back reflection.”
Todd Deslauriers, product manager for LaserStar Technologies, said that a magnet around the company’s BDO or beam delivery optics component protects it from laser bounce back by deflecting the laser away from the components.
Deslauriers adds that another method is placing the focusing head on a slight angle to prevent the beam from directly bouncing back. Tilting the cutting head at an angle is also a way to improve the cutting effectiveness when cutting thicker material, said O’Brien.
Cutting thicker material can be a challenge for fiber lasers, according to Richard Dennison, of UK-based Charles Day (Steel) Ltd. in his blog, “Overcoming Issues with Laser Cutting.” According to the metal cutting company, when using fiber laser cutting equipment on material thicker than 5mm, “the roughness of cut and cutting speed deteriorate considerably and waterjet cutting becomes a more viable option. They strongly reflect the laser beam; both cut quality and sheet thickness values are lower.”
In another online blog from a laser metal cutting company, IDA Control, the item “Laser Cutting Secrets Revealed,” uses mild steel and aluminum to illustrate this point. “For example, laser power of 1500W can cut 12mm (0.47 in) of mild steel but only 4mm (0.16 in) of pure aluminum.”
Fiber lasers are better for reflective materials, the company says, because the material better absorbs its wavelength. Photo: Trumpf
Traditionally, O’Brien said, in the laser world, when cutting a ¼ in. or 3/8 in. material you would want a CO2 resonator because it would do a better job on the thicker material. But, technology is challenging that tradition. “To solve that problem,” said O’Brien, “The laser world uses a collimator that expands the beam width to erode thicker material.”
Dobson adds that “When fusion cutting, such as with nitrogen assist gas, laser power and material thickness go hand and hand. The thicker the material, the higher the laser power that is required.”
Not everyone agrees that thickness is as big a dilemma. There are other important issues says Deslauriers. “The issue is typically not about the thickness of the material as long as the laser has enough energy to penetrate into the material,” he said. “The surface effect is typically what is focused on, such as is it a polished surface, machined finished surface, or dull finish (i.e. sandblasted).”
According to Deslauriers, good, clean surfaces are recommended so the material absorbs the laser energy constantly and the metal can flow evenly. For reflective materials, the surface can be blackened to aid in the absorption of the laser light.
In other applications, copper-based alloys can be nickel-plated to enhance laser absorption and reduce the power needed. Stainless steel can be supplied plastic coated on the cutting side to minimize the risk.
While all of these can help, sometime a quality cut comes down to the basics. Dobson adds that a paramount laser cutting consideration is using good parameters: “Correct focus, gas pressure, and power will ensure a quality cut.”