Laser cutting thick plate? Check the surface

A high-powered fiber laser cuts thick HRP&O plate. As laser powers climb, so do cutting speeds. Eliminate secondary operations, and the process becomes even more competitive. Images: Steel Warehouse

If you visit FABTECH this year in Orlando, you’ll likely see a continuation of the laser power arms race. Not too long ago, a 12-kW fiber laser was something fantastical. Now we’re seeing cutting powers of 30 kW and above, some with beveling capability. It’s highly likely that, when it comes to laser cutting power, the industry is just getting started.

Machine providers aren’t pushing laser cutting power just because they can. It turns out there’s a real market for the technology, especially in thick plate cutting. The ultrahigh-powered fiber laser can outpace competing thermal cutting technologies, of course, but the argument for the technology isn’t just about speed; it’s also about eliminating secondary operations. What was once cut and sent for secondary deburring or machining now can be processed “done-in-one” on the laser.

Various advances have converged to make all this possible, and raw fiber laser power is just one piece of the puzzle. Others include advances in assist gases, how those gases are mixed and delivered, as well as the beam’s shape and power-density profile.

One often overlooked piece of the puzzle is material quality. Cost-effective and formable, hot-rolled steel dominates the heavy fabrication arena, yet the material’s surface condition isn’t ideal for the laser. Pickling and oiling it, though, changes the scenario completely.

The thermal cutting benefits of hot-rolled pickled and oiled (HRP&O) material—descaled in a hydrochloric acid bath—have been known for a while. Steve Pugh, corporate director of laser engineering at Steel Warehouse, the service center based in South Bend, Ind., along with co-presenter Danny Lerman, creative manager, introduced research showing the benefits of laser cutting HRP&O plate almost a decade ago, at FABTECH 2016 in Las Vegas.

For years, Pugh has traveled the country, helping fabricators with their cutting operations, and since that initial presentation, cutting technology has seen some extraordinary advancements. “It’s amazing what’s changed in just the past five years,” Pugh said in a recent interview. He added that those changes have made material surface quality even more important, especially considering just how productive today’s laser cutting systems can be.

“We’re pickling material up to 2 in. now,” Pugh said. “That’s giving a surface condition really tailored for today’s fiber lasers. We’re seeing 30-kW systems cut thick plate faster than legacy thermal cutting technologies. The cutting speeds are just tremendous.” He emphasized, however, that without good material, those tremendous cutting speeds become a hit-or-miss affair.

Hot-rolled plate comes from a heated slab that’s flattened through rollers to achieve its final dimensions, after which it’s allowed to cool. It doesn’t undergo as much processing as cold-rolled steel, hence its lower cost, yet the way it’s made does create a scaly surface that isn’t ideal for cutting with a beam of light.

Pugh described traditional hot-rolled steel as having a “moon-like” surface, with the beam navigating through a landscape full of craters. That surface variation can affect the cutting head’s height sensing and, hence, the laser’s focus position. Pugh added that blasted material seems to have a similar affect, as the beam navigates the peaks and valleys on the material’s surface. That surface condition might be ideal for painting downstream, but it isn’t ideal for cutting with an ultrahigh-powered laser beam.

“We’ve seen some cutting on thick material with a blasted surface, but the machine is usually oxygen cutting,” Pugh said, referring to the use of oxygen as the assist gas. “And you can’t cut at the speed you can attain with straight nitrogen or an oxygen-nitrogen mix.”

A high-powered fiber laser initiates a pierce in thick HRP&O plate.

Some shops have experimented with laser cleaning (using a separate machine), but as Pugh explained, an ultracleaned surface still might not be ideal, depending on the material. Even more significant, at least when it comes to laser cutting thick plate, it turns out that a very thin scale jacket can help the beam cut better.

Pugh and others at Steel Warehouse have worked with various laser cutting operations to test a variety of material. Over the years they’ve discovered the benefits of a thin scale jacket, a signature characteristic of some of the latest HRP&O plates. Although HRP&O is descaled, the scale isn’t removed entirely—a fact that actually helps the high-power fiber laser cut better.

“By pickling and oiling the material, we get a very tight, very clean scale jacket,” Pugh said, “which we’ve found can help tremendously with fiber laser cutting.”

Picture a laser beam optimally shaped to slice through thick stock, with a mix of nitrogen and oxygen propelling (and burning) molten material out of the kerf. A thin, tightly controlled scale jacket helps establish the melt pool, making strategic use of impurities like copper, chromium, nickel, and other trace elements.

“These impurities actually can help you establish that cutting melt pool,” Pugh explained, adding that they promote smooth flow of molten material through the kerf.

At the same time, the material’s flat surface keeps the focus steady and the laser head’s height sensing operating as intended, both when cutting vertically and at a bevel. When combined with other dialed-in parameters like focus, beam density profile, speed, and acceleration, the operation often results in a clean cut that can flow directly to the next major manufacturing operation, no deburring or grinding required.

Compare this with cutting traditional hot-rolled material. The flakes change the heat characteristics at the point of cut, which in turn affects how molten metal flows through the kerf width. Good operators might be able to dial in parameters to make it work, and some laser cutting systems now tout intelligent sensors that aim to counteract the negative effects of bad material. Regardless, no one argues that a poor material surface works against you, not for you, when trying to create that perfect cut with minimal dross.

Still, traditional hot roll doesn’t have oil on the surface, and HRP&O does, of course, so how does that oil affect the cut?

“Years ago, low-powered CO2 lasers were slow and hot, and a lot of machines didn’t have fume removal systems,” Pugh said. “Now, everything has downdraft systems, so you don’t see smoke.” He added that the oil isn’t rolled on manually. Modern HRP&O has oil electrostatically applied to a thickness of about 0.001 in.

“Also, with today’s faster speeds and narrower kerfs, combined with the oil on top [of the plate], together help push the heat out faster,” Pugh said. “You’re not getting the terrible heat-affected zones like you did when plate cutting with a CO2 laser. The heat dissipates, especially when cutting pickled and oiled material. Yes, the focus is very hot, but the heat-affected zone is so much smaller.”

An edge comparison shows cut quality for (from left) blasted, HRP&O, and hot-rolled plate (right). These were made with a 12-kW fiber laser, and quality differences have become even more significant with higher laser powers.

With thick-plate laser cutting in general, Pugh sees numerous puzzle pieces coming together, and he illustrated them by describing a complex plate shape with jaws on the edge. In the past, such parts often led to laborious days at the denesting station, as workers with magnetized tools struggled to lift pieces out of a thick skeleton. Now, workers can lift those pieces out with ease. What makes this possible?

One ingredient is a laser head with good acceleration. “You absolutely need good acceleration to get out of those corners,” Pugh said, adding that if the laser dwells too long, the melt pool increases, which leads to excessive dross and poor cut quality.

Another ingredient: the width and power distribution of the beam itself. This gives a wider kerf and makes for easy denesting. The beam works in concert with other key ingredients: assist gas and nozzle technologies, with nitrogen evacuating metal and doses of oxygen giving that chemical-reaction boost, allowing for some unprecedented cutting speeds.

Then comes the material quality, which is where material like HRP&O will continue to play an important role. “The material needs to be consistent,” Pugh said. “If you don’t have a good, tight scale jacket all the way across flat material, the laser cutting process could be a mess.”

The issues at the cut all occur at the microscopic scale, Pugh said, but operators can still look for signs of trouble by watching the plume of sparks emerging from under the material. A consistently conical plume is a good sign that the laser is producing a consistent cut edge. If the plume underneath goes off to one side or trails behind the beam, there’s a good chance something (like flaky scale) is causing problems with heat distributions.

Sparks may be bouncing on the leading edge of the kerf simply because it’s not melting all the way through, thanks to the loss of heat. The bouncing can get so bad that material may start to blow back up through the cut path. Eventually the laser can lose its cutting ability and start to weld. At this point, operators can stop cutting and restart to regain the cut path.

Of course, all this hinders productivity in a big way. Observing the cut is always a good practice, but it’s reactive, not preventive. And with ever-increasing production potential of today’s lasers, that loss of productivity starts to cost more. In the world of high-powered laser cutting, tiny flakes on the material surface can have profoundly detrimental ripple effects.

Imagine a cutting machine with multiple heads cutting 1-in. plate simultaneously. The setup has helped make other thermal cutting processes, especially oxyfuel, competitive for decades. The approach overcomes throughput constraints from slow cutting speeds and helps feed a steady stream of heavy plate downstream. Dialed in correctly, the kerf usually is wide enough so that workers can lift pieces out without issue.

Of course, the significant heat-affected zone at the edge could cause issues downstream. And the pieces usually required grinding, deburring, and—for pieces that require extra precision or specific features, like threaded holes—even some machining. Those secondary processes are just part of the heavy-fab landscape. Sure, the plate could be waterjet-cut and bypass grinding and deburring, but that process might be too slow—and the last place a fabricator wants a constraint is in the primary cutting process.

Conversely, laser cutting’s “done-in-one” capability has been a key strength. Even more exciting, Pugh said, is the potential of solid-state, 1-µm-wavelength laser cutting machines with multiple cutting heads. In the automotive business, high-powered laser power sources have been shared simultaneously between various welding work centers. Now, power from one laser source is being split on one large-format cutting bed. Here, the scale-up potential of ultrahigh laser powers really becomes apparent.

Beyond this, at least one laser cutting machine is now offering machining capabilities. Think of a multifunction thermal cutting machine, only now, instead of combining oxyfuel, plasma, and machining, the system combines a solid-state laser with the ability to drill and tap. Tapping is especially valuable. Today’s lasers can cut incredibly small, accurate holes in thick plate, but they can’t create threaded holes.

Such technology is just emerging, Pugh said, and early adopters are taking a serious look. But as operations start adopting the technology over the next few years (which The Fabricator will be covering, of course), the cutting calculus in heavy fabrication will start to change. Optimizing an operation for high-power laser cutting will become more critical than ever.