An introduction to laser cleaning and laser texturing

A robotic laser system cleans an optical tray inside an enclosed workcell. Laser Photonics

Precision sheet metal fabrication simply wouldn’t be where it is today without lasers. Laser cutting has become ubiquitous, and laser welding—both automated and manual—is rapidly growing in popularity. But lasers do more than just cut and weld. They can also clean.

Laser cleaning isn’t quite ubiquitous, but it’s gaining a foothold for applications like paint and rust removal as well as various specialty cleaning applications in aerospace, automotive, and beyond. Most laser cleaning setups use scanning optics. These move the beam at multiple meters per second to project a desired shape, such as a circle or rectangle, onto the surface.

But what is laser cleaning, really? In truth, the term covers a range of distinct processes that fall into two broad categories. One removes surface contaminants while the other “sculpts” or “textures” the surface to suit specific coating, adhesive bonding, or other application requirements. Both surface contaminant removal (referred to here as “cleaning”) and laser texturing can use similar or even identical equipment, but the two processes are distinct. Which category a manufacturer uses depends on what it needs to accomplish.

Laser cleaning involves the removal of surface contaminants, like rust and unwanted paint. Setups incorporate fume extraction systems that capture whatever contaminants the laser is removing. A few applications, like titanium cleaning for critical aerospace applications, call for a shielding gas of some sort to prevent the formation of surface oxides.

In many common applications, the laser ablates surface contaminates. Ablation—when a solid turns immediately into a gas—works well when the surface material’s ablation threshold is much lower than that of the base metal. This way, the laser can atomize the contaminants without affecting the metal surface.

Energy delivered by the laser performs the cleaning action in various ways, depending on what’s being removed. “‘Cleaning’ is a generic word we use for several processes. In every case, we’re removing something from the metal surface, but how we remove it changes. Sometimes the laser creates a thermal shock effect on the surface.”

So said Dmitri Novikov, director of business development at IPG Photonics, Marlborough, Mass. He added that in these cases, the different coefficient of thermal expansion between surface debris like rust and the base metal creates a shock that, in effect, “shakes” the rust particles off the surface. He added that in other cases, heat from the laser burns the material to be removed—usually paint or other organic coatings.

What about removing surface oil? “Oils are transparent to laser light,” Novikov explained, so they aren’t removed via ablation or thermal shock. In this case, the laser “boils” individual sections of oil. Specifically, the laser heats a small area of base metal, which causes droplets of oil to jump off the surface and into the air, where a fume evacuation system captures them. “In these applications, the extraction system is just as important as the laser itself.”

Ablation, thermal shock, burning, and boiling represent the most common ways a laser cleans a metal surface, but even these aren’t the only ways. For instance, some specialty laser cleaning processes use what Novikov called an “exfoliation,” a method used to remove certain kinds of ceramic coatings, especially on turbine blades. Here, the laser light isn’t absorbed but is instead transmitted through the layer of ceramic coating. When that light reaches the metal substrate, it creates incredibly intense but also incredibly localized heat, which in turn forms a plasma that “lifts” the ceramic coating from the surface.

The exact beam parameter and energy profile depends on the laser cleaning application, but the most common is a top hat beam-energy profile. “Think of it like a dull tool. You don’t want to shape or texture the surface. You want to preserve it in a pristine way.”

A laser removes rust and other surface contaminants from an aerospace turbine. IPG Photonics

That was Antoine Nguyen, marketing supervisor at Laserax Inc., Quebec City, Quebec, Canada. He added that cleaning applications often use a multimode laser with, say, a 1.5-mm spot size.

The laser offers a level of precision rarely matched by other tools. Laser power, pulse duration, and beam profile all can be tweaked so the process removes only what’s intended and nothing else. This applies to laser cleaning, where the laser removes contaminants while leaving the base metal untouched, but it also applies to laser texturing.

In these cases, a laser beam—usually Gaussian, with high energy in the center of the profile—ablates a layer of metal and causes a near instantaneous state change, from solid to liquid and back to solid, for metal just beneath the ablation zone. Nguyen added that the process often uses a single-mode laser that can achieve a very small spot to create a very precise texture.

Imagine a blasting process in which every grit particle is precisely sized and controlled. That’s roughly analogous to laser texturing, though the process textures the surface in an entirely different way—with heat from the laser instead of physical impact from grit or shot.

Some metal manufacturing applications involve laser texturing precise patterns to change the surface properties, such as making the surface hydrophobic. These precision texturing applications often use lasers with extremely short pulse durations, each measured in pico- or even femtoseconds. Many other texturing applications prepare a metal surface for coating—no grit or cleaning chemicals required.

If you’ve visited FABTECH in recent years, you might have seen hand-held laser welding systems that also perform laser cleaning. These continuous-wave (CW) systems have very small scanning optics inside the gun itself, allowing the beam to wobble in a pattern. In cleaning mode, the laser delivers a thermal shock that forces oxides and other impurities off the surface and into a fume collection system.

After laser welding, another pass from the laser serves two purposes. First, it creates a thermal shock that again forces oxides from the weld off the surface. Second, it allows alloying elements like chromium to move toward the surface, passivating the material to restore its corrosion resistance. Traditional passivation systems apply a chemical that helps initiate the natural passivation process. Lasers accomplish something similar with heat—no chemicals required.

“It’s been shown that corrosion pitting is drastically reduced if you do laser passivation,” Novikov said.

Hand-held laser cleaning and texturing systems require personal protective equipment (PPE)—a welding helmet and laser safety glasses appropriate for the laser’s wavelength—designed to protect the skin and eyes. Invisible to the human eye, the laser’s 1-µm wavelength can enter the eye unnoticed and cause permanent damage. For this reason, hand-held laser cleaning and texturing, like hand-held laser welding, must occur in an interlocked, light-safe enclosure, and any window glass must be rated to protect against the laser light being used.

Automated laser cleaning has been gaining a presence in low-mix, high-volume settings. “A prime example is cleaning for battery welding,” Nguyen said. “These systems process millions of parts, cleaning surfaces for a process that welds several workpieces a second.”

A laser textures the edge of a battery housing before an adhesive bonding process. Laserax

“Lubrication removal after stamping is another growing application,” Novikov said. “Previously, these operations relied on large wash lines to prepare the stampings for coating. These used an enormous amount of water, which became contaminated with metals and other debris. Disposal can be difficult and expensive.”

Adhesion prep is another major application area, especially for laser texturing. An enclosure might be assembled with an adhesive designed to mate with a surface having a specific pattern or texture. Brake pad manufacturing uses a similar technique, with the laser texturing the metal surface before the pad is placed on top. “A brake pad is a simple part with a flat surface,” Nguyen said. “It’s a controllable process too, capable of texturing a specific area.”

Whether cleaning or texturing, automated setups often involve parts with simple shapes, like those brake pads. This way, the laser can access the surface easily, with no complicated manipulation of the workpiece or laser to achieve the optimal beam approach angle.

Laser welding’s speed and quality advantages are well known, which is why it’s showing up on more shop floors. But what about laser texturing as a replacement for blasting?

Automated setups can work in high-volume, low-mix situations, especially for simple part geometries. As parts become more complex and the part mix becomes greater, however, automation becomes more challenging. This has to do with the nature of laser texturing. The beam ideally should be perpendicular to the metal surface, or at least close to perpendicular as possible.

“When the surface is angled and the scan head is positioned vertically above it, a portion of the pattern is still in focus,” said Anna Vodopyanova, technical editor at Laser Photonics, Lake Mary, Fla. “But it may take a longer time to process compared to when the laser source is positioned perpendicular to the surface.” She added that multiaxis robotic workcells, including cobot options, help address the issue.

What about manual situations? Could a shop replace manual blasting with hand-held laser texturing? Again, safety is critical. Some videos online show the laser’s amazing potential, but they also show some less-than-ideal safety practices—especially outdoors.

In a fab shop, the laser texturing booth should be light-safe and interlocked, and any window needs to have safety glass rated to protect passersby from 1-µm-wavelength laser light. The operator needs laser safety glasses and the proper PPE as well. Coverage rates, surface preparation requirements, and overall process efficiency weigh into the equation as well.

Still, the process has potential. Laser texturing eliminates the need for blast consumables, saving manufacturers the cost of buying, storing, and disposing of abrasive media. It also opens the possibility of texturing only a portion of the metal surface in an extremely controlled fashion, no masking required.

And as Novikov pointed out, manual blasting has all sorts of indirect costs that often get overlooked. This includes increased health insurance, considering the potentially dangerous nature of the job. Manual blasting isn’t pleasant, and staff turnover costs can be high. “The job is physically demanding, loud, and dirty.”

A hand-held laser cleans a pipe surface before welding. IPG Photonics

To be sure, laser cleaning and texturing do not work everywhere. It really depends on what contaminants need to be removed and what the surface preparation requirements are. For instance, lasers don’t excel at removing mill scale from thick hot-rolled plate, especially in an automated setting that demands high levels of throughput.

“It’s quite difficult to remove, even with a powerful laser,” Nguyen said, “because the scale’s ablation threshold is very high. And the mill scale thickness correlates with the plate thickness.” Removing a thick layer of mill scale might require multiple passes with the laser.

Removing mill scale is certainly possible, and it can make business sense for the right application. Nguyen described a welding process in train track fabrication that benefits from laser cleaning. An automated, high-powered laser cleans the metal surface of mill scale before an automated welding process. “This is a 24/7, low-mix, high-volume operation,” he said. “In this case, investing in a high-powered laser made sense.”

In manual settings, mill scale removal can make sense for the right application. “Removing mill scale is still a slow process,” Novikov said, “but for hand-held solutions, speed usually isn’t critical anyway.”

“Since laser cleaning remains a new technology, it has not been tried on everything,” Vodopyanova said. “We continue to discover new applications.” She added that the list of coatings laser cleaning systems can remove continues to grow, and some manufacturers continue to push the envelope. For instance, certain extreme rust removal applications have involved two lasers used in succession, a higher-powered CW system followed by a low-powered pulsed laser.

Though new, the process shows potential, especially in an industry always looking for skilled workers. Blast grit and chemical cleaning agents tend to make a less-than-pristine environment.

“Many want to see more environmentally friendly and more sustainable processes in manufacturing,” said Miller Cordeiro, content manager at Laser Photonics. “Laser cleaning and similar technologies are giving people options and help them increase their ROI at the same time.”

Replace all that grit and all those chemicals with lasers—using the right safety protocols (PPE; light-safe, interlocked enclosures with the appropriate laser safety glass)—and a fab shop can become a cleaner, more attractive place to work. For an industry continually looking for new ways to attract workers, creating a better work environment is never a bad idea.