The fiber laser changed the laser cutting game, not just for its speed but for its wavelength. The CO2 laser beam’s 10.6-micron wavelength had decades of success since the birth of the laser cutting industry, but when it came to nonferrous material, optical reflectivity reared its ugly head. This complicated laser cutting nonferrous material in a big way. Copper and brass cutting with a CO2 laser was (and still is) rare, though some tenacious fabricators accomplished the feat.
Cutting aluminum with a CO2 laser is of course quite common. But the CO2’s 10.6-micron wavelength still isn’t ideal, so the process remains a little like squeezing a small round peg in a larger square hole. It’s not impossible; the peg still fits through the hole, but securing it takes some effort.
Then at the beginning of this century, the fiber laser stepped into the fray with its 1-micron wavelength. Most common metals in the fab shop absorb more and reflect less of that 1-micron wavelength than the 10.6-micron wavelength. In the fiber laser arena, in fact, aluminum cuts very well, as do even copper and brass.
So when a fabricator achieves a clean cut in aluminum or other nonferrous material with the fiber laser, what exactly occurs in the kerf itself? To answer this question, The FABRICATOR spoke with Charles Caristan, PhD, a technical fellow and global market director, metal fabrication and construction, at Air Liquide’s Conshohocken, Pa., office. A longtime expert in laser cutting, Caristan is the author of Laser Cutting Guide for Manufacturing, published by SME.
As Caristan explained, there’s a lot more to the nonferrous cutting recipe than the beam wavelength. Other components include power density, beam focusing, kerf width, and the assist gas type and flow rate. Mix this all together in the right way, and you get the fiber laser’s eye-popping cutting speeds and clean cuts, even in a range of nonferrous materials that were once considered too reflective to be cut with a CO2 laser beam of light.
Note that what follows doesn’t cover specific cutting parameters, which for most cutting machines are set by the equipment-maker. Some fabricators use these factory settings, others adjust them depending on application requirements. What follows describes—in basic, “non-photonics-journal” terms— why those parameters work the way they do.
If someone says that something in laser cutting is impossible or impractical, chances are a fabricator somewhere has made it both possible and practical. For instance, Caristan recalled visiting a fabricator years ago that cut a 0.125-in.-thick copper alloy using a 2.5-kW CO2 laser. “The fabricator did this for years,” he said. “The cutting head moved slowly, and the operator had to stop the cutting cycle midway through to let it cool. It was not pretty, but it was doable.”
Laser cutting nonferrous material has a history of meeting and overcoming obstacles. As Caristan explained, early adopters of the CO2 laser experienced some serious growing pains when cutting reflective material. Early on, they saw the effects of aluminum’s low absorption characteristics yielding back-reflections.
“So not only was the laser cutting process less efficient,” Caristan said, “but they had to deal with back-reflection through the optical systems, going all the way back to the laser resonators’ cavities, often destroying them. We’ve learned a lot since then. Most machines, including fiber laser systems, have built-in optics and numerical controls that mitigate or prevent back-reflection.”
Tool- and diemakers pay attention to a material’s strength and shear properties. Engineers and technicians developing laser cutting parameters direct their focus elsewhere, including a material’s absorption and reflectivity characteristics; melting point; viscosity of the molten material; thermal conductivity; and material surface conditions, including films and coatings.
“The difficulty when cutting aluminum is to make a clean cut with minimal dross,” Caristan said. “With the proper assist gas, delivery, and flow, you can minimize the generation of dross.”
Viscosity plays a role here. All metal has a certain level of viscosity at melting temperature, but the viscosity isn’t constant as the metal heats further. The laser brings aluminum well past its melting temperature of a little more than 1,200 degrees F. As Caristan described in his book, aluminum’s viscosity actually reduces by more than half as its temperature rises between its melting temperature up to 1,328 degrees F—a difference of just a little more than 100 degrees F, a minute temperature change in the world of thermal cutting. As the low-viscosity material cools, its viscosity more than doubles as it gets closer to resolidifying—and evacuating it effectively before it solidifies becomes a complicated affair.
“Low viscosity becomes a major contributor to dross buildup,” Caristan stated, “particularly if the melting temperature of the material is relatively low, as with aluminum.”
The Aluminum Oxide Film
Some cutting challenges have to do with melting temperature, especially when it comes to the thin layer of aluminum oxide (Al2O3) film, which forms on aluminum’s surface as it’s exposed to the atmosphere. The film prevents further corrosion, but it also complicates the laser cutting process.
Aluminum melts at about 950 K, or a little more than 1,200 F; the aluminum oxide melts at roughly 2,000 K, or at more than 3,000 F. “The high melting point of the aluminum oxide film formed on the surface of molten aluminum droplet causes it to solidify very quickly around the still molten droplet, so it is very important for the assist gas to flush it out quickly before it resolidifies,” Caristan explained. “If it is not flushed quickly enough, it forms stalactites on the bottom edge, also known as dross.” He added that the good news is that, compared to dross from material like stainless, aluminum dross is generally soft, so soft that many operators can comb it away with their thumb.
Aluminum’s thermal conductivity is many times that of carbon steel, and that thermal conductivity accelerates heat loss; that is, heat conducts away from the kerf into the main body of the workpiece. The more heat conduction loss you have, the less heat actually stays in the kerf, and the less efficient laser cutting is.
Differences in thermal conductivity contributes to different cutting characteristics between grades, especially in thicker materials. As Caristan published in his book, 6XXXX series aluminum experienced much higher heat conduction loss than the 5XXXX aluminum; the two cut similarly in gauge thicknesses, but very differently in thicker stock.
Historically, operators cutting aluminum with a CO2 laser faced several challenges that made cutting more inefficient: high reflectivity of the 10.6-micron light beam, as well as aluminum’s high thermal conductivity leading to more heat conduction loss. In fact, all the heat loss forced many operations to deal with thermal expansions in the sheet, sometimes writing the cutting program so the head moved alternately from one quadrant of the sheet to another, equalizing the heat effects.
All this said, the fiber laser’s power density and, again, 1-micron wavelength have really changed the game. Aluminum’s thermal properties haven’t changed; it still has high thermal conductivity. But it also absorbs more and reflects less of the energy from the 1-micron laser beam. This, combined with the high power levels, power densities, and speeds offered by the modern fiber laser beam, has improved laser cutting performance substantially.
Assist Gas Flow and the Laser Focus
Laser cutting aluminum with nitrogen assist gas or compressed shop air (which can work for thin stock) fosters a similar cutting action as other alloys cut with nitrogen. Vastly oversimplified, it’s all an interplay among thermal energy from the beam, feed rate, the resulting kerf width, and the assist gas flow that flushes the molten material out of the kerf. Perfect the assist gas flow so it plays well with the heat (focus and beam characteristics), cutting speed, and kerf width, and you achieve a quality cut edge with minimal striations and dross.
Traditionally, aluminum usually calls for a beam focus that’s deep under the surface of the material, particularly as the material gets thicker. This helps flush the material out the bottom of the kerf. To understand how and why this occurs, visualize material being melted at the top of the kerf, this time with the focus spot at or near the material surface.
“The material melts quickly and then flows through the kerf, where the beam diverges and the energy density drops quadratically,” Caristan said. Hence, less energy is available at the bottom of the kerf to the molten metal, making the metal oxides freeze into dross.
Set the focus low beneath the material’s surface and the power density situation changes. As the molten material from near the material surface travels down the kerf, it passes through the brightest section of the beam and, hence, remains a liquid until it is evacuated out the bottom.
The balancing act is just getting started. “There is a window of opportunity for the cutting speed,” Caristan said. “If you cut too quickly, you produce dross. But if you cut too slowly, you also produce dross.”
Dross from a fast cut is intuitive; the assist gas didn’t have time to flush the molten material before the heat source (the beam) moved forward, so the molten material “froze” at the bottom of the cut as dross.
But what about dross from cutting too slowly? Caristan said he doesn’t have hard science to back this up, “but I believe it has to do with the heat input on the metal and the ability of the assist gas to remove all the molten metal at once.”
Travel speed also affects the kerf width. A slower travel speed creates a wider kerf, while a faster beam creates a narrow kerf. “As your kerf narrows, you have difficulty bringing the assist gas through, and you don’t achieve as much flushing power,” Caristan said. This in turn affects cut quality, including dross.
Edge striations also change with cutting speed, along with other variables. Cut aluminum (and other material) too slowly, and you see deep striations. “Those represent evidence of a stream of gas pushing and flushing liquid metal,” Caristan said.
Supersonic Effects, Standoff, and Nozzle Centering
All this interplays with yet another variable that isn’t considered very often: the speed of the gas flowing out of the nozzle. It’s supersonic and, like anything that travels faster than sound, generates small shock waves. “This shock wave can deflect the flow of assist gas from where you intended it to flow,” Caristan said, “and it can disrupt the amount of gas you have flowing through the kerf.”
If shock waves deflect above the kerf, they form a partial barrier that hinders the column of assist gas, which in turn alters the gas dynamics in the cut and could affect the ability of the gas to evacuate the molten metal effectively—hence, you get poor cut quality. The likelihood of this becomes greater as the kerf width narrows.
Because gas flow is supersonic, laser cutting operators can’t eliminate the shock waves, but they can make them less detrimental to the cut by setting the nozzle standoff distance properly. “The rule of thumb is that the standoff distance should be equal to or less than the nozzle orifice diameter,” Caristan said. Go higher, and you exacerbate the shock wave deflections that could yield less gas to actually make it into the kerf.
Also, make sure the focused beam is centered in the nozzle aperture. “You need to make sure that the nozzle aperture’s center is always perfectly aligned with the centerline of the kerf,” he said. “A misalignment shows in different cutting performance each time you change cutting direction.”
Beam and Nozzle Advances
The focused beam propagation and energy distribution are complex subjects, but when thinking about focus, imagine the laser beam as two cones, one on top of the other. Where the cones’ tips meet is the focus spot. The shorter the focal length of the focusing optic, the fatter the cones, the smaller the focus spot size, and the greater the power density at the focus point.
The focus point size changes with the wavelength, so when the beam itself is made of a shorter wavelength, the power density at the focus point increases quadratically. That high focusability, and how well various metal grades absorb energy from fiber laser beams, is part of what makes the fiber laser so effective.
“It’s one reason you have the rule of thumb—that for certain materials and material thicknesses, each kilowatt of a fiber laser has cutting performance equivalent to double that of the CO₂ laser of the same power,” Caristan said.
In laser cutting, more power density creates more heat energy, and how much energy depends on how well a metal, including aluminum, absorbs the energy from the laser beam. But this is just part of the equation.
Molten metal needs to be evacuated. A short focal length of the focusing optic means that the power density drops dramatically as you move away from the focus point position. This narrows the kerf, and it also means the focus spot has to be in just the right place, especially as the metal gets thicker. The narrow kerf can make it difficult for the assist gas to cleanly evacuate the molten metal.
“A focusing optic with a short focal length makes the beam diverge quickly past the focus point,” Caristan said, “so by the time you get to the bottom of the kerf, you have very little power density, relatively speaking.” This is one reason that setting a deeper focus point position (within and not on top of the material) on thicker aluminum has been a common practice.
One of the advances the industry has seen in recent years is to decrease the effect of this drop in power density. You can’t change the physics of laser beams; they all converge into and diverge out of a focus point. Even so, other beam characteristics can be changed to produce a better cut edge.
As Caristan explained, some offer an oscillating focus that adjusts its behavior with the thickness of material. Others change the beam’s energy distribution or mode based on material grade and thickness. For instance, a beam in a gaussian mode, with concentrated energy in the very center that dissipates over the beam profile, has lower energy density away from the center, which makes for a narrow kerf. A doughnut distribution concentrates energy around the beam perimeter, maintaining the highest energy closer to the walls of the cut.
But again, energy from the beam is only half the equation; the effectiveness of the assist gas flow is the other half. Here nozzle technology has played an important role. Some nozzles now actually have components that touch the workpiece surface. These reduce the wasted assist gas that never makes it into the kerf, which is a particular issue with the narrower kerf produced by the fiber laser.
“In a typical nozzle, the gas flow expands as soon as it exits the orifice,” Caristan said, “and a big portion of it never sees the kerf. With these touch nozzles that kiss the workpiece surface, you have less gas wasted on the workpiece surface and more gas going directly into the kerf.”
The Ideal Cut
A shop using a laser to oxygen-cut thick mild steel takes advantage of the chemical reaction between oxygen and iron. For cutting thicker aluminum and other nonferrous material with nitrogen, it’s all about melting and evacuating the material cleanly.
Caristan described an “ideal” laser setup, with assist gas flowing in a perfect laminar fashion into the cut, with a beam that removes and flushes material cleanly—no “freezing” prematurely at the bottom (dross) or on the edge (striations). The supersonic shock waves are there, but they move in a fashion that they don’t deflect away or obstruct the flow of gas into the kerf.
These days high-powered lasers cut extraordinarily quickly, but all that speed doesn’t have an impact if the resulting parts need to be reworked or scrapped. The industry has come a long way in understanding exactly how the laser cuts metal, and work continues. The better that understanding gets, the better cutting parameters can become, and the better chance an operator has of achieving a clean-cut part on the first try.
By Tim Heston