Laser Technology Supplement: Manufacturing with Light
Lasers for industrial fabrication are getting more powerful, sharper focused, and easier to use
By Bruce Morey
While some laser technologies, like CO2 or Nd:YAG lasers, are becoming more reliable and easier to use, newer technologies like disk lasers, diode lasers, and fiber lasers may very well change the way lasers are used in industry by providing sharper focus and higher efficiencies.
Workhorses of the laser world, CO2 lasers have increased their power in the last few years, while wall plug efficiencies have remained typically at approximately 7–13%. "Up to about three years ago, 4 kW was considered a highwattage CO2 laser," says Jason Hillenbrand, laser product manager for Amada Inc. (Buena Vista, CA). "Today, 6-kW lasers are becoming more of a mainstream product." The higher-power systems Amada offers are more affordable and easier to maintain than in years past, in part because of common components used in their 4-kW and 6-kW systems. Higher-power systems are beginning to compete with plasma-cutting systems, according to Hillenbrand. There are advantages to laser cutting. "With laser cutting, the edge is clean, no secondary processes are required, and the fit-up is nice, with low rake angle along the edge of the part," says Hillenbrand.
He cautions, however, that a higher-power laser is not all there is to cutting thicker materials, or cutting the same thickness faster. "Beam mode defines how that laser power is going to be used." (See sidebar Laser Beam Modes.)
Controlling the size of the laser spot (as opposed to beam shape within the spot) on the workpiece has never been easier. "Controlling beam divergence in flying optic lasers is very simple these days. I can put any mirror I want in the system and get the spot size I want," asserts Hillenbrand.
Today, CO2 systems are often embedded in large cantilever or gantry-style CNC machines. The Syncrono, manufactured by Prima North America (Chicopee, MA), illustrates how CO2 laser systems have evolved towards greater speed and ease of use.
Using a patented laser head, Prima developed a fast-cutting machine by creating a "machine within a machine." According to Pieter Schwarzenbach, vice president of laser technology for Prima, the fast-flow CO2 laser used, which is available in power ranges to 4 kW, does not move. It sits on a stationary portion of the machine—mirrors direct the beam to the patented cutting head. Two additional small, linear-motor-driven parallel kinematic axes on the moving cantilever move the laser head, which has less mass than an attached CO2 laser. Within 58 sec, the machine cuts 1000 2-mm-diam holes pitched every 3 mm in 1-mm mild steel or aluminum. Advances in linear drives and CNC controls made this development possible, according to Schwarzenbach, rather than any particular advance in the CO2 laser.
"Although other laser types could be used in place of the CO2, right now it does not make sense," he states. "We believe that the CO2 laser is still by far the most cost-effective tool for our clients."
The Nd:YAG laser, also employed in industrial applications for decades, uses a solid-state resonator that produces laser light in the near-infrared portion of the spectrum. Despite the Nd:YAG's lower wall plug efficiency (up to 5%), there are reasons to choose an Nd:YAG laser: these include the ability to produce high peak power and high energy, and the laser's flexibility, which enables delivery of the beam via fiber-optic cables. For instance, a Nd:YAG laser by GSI Laser Div. (Novi, MI), rated at 300-W average power, can produce up to 16-kW peak power. The Nd:YAG is well-suited for applications such as welding or drilling and cutting with lower heat-affected zones and lower total heat input.
Fiber optic delivery means flexibility—laser beams can be delivered on the end effectors of existing robots, while keeping bulky laser cabinets separated from the point of use. Fiber delivery eliminates aligning optics and mirrors, though getting the laser light into the fiber can sometimes be a bit tricky. Better fiber-optic engineering and laser-system controls to provide plug-and-play use is the trend in this category of lasers.
Plug-and-play fiber-optic delivery translates into ease of setup and equipment safety. "We have a patented fiber-optic design that prealigns the beam into the cable, and has built into both ends of the fiber a mechanism that protects against back-reflection," says Andrew Dodd, a director with GSI Group Laser Division. Back reflection can catastrophically damage the fiber-optic cables or the laser system's resonator. "For example, this can be an issue with aluminum welding, which tends to reflect a lot of light," says Dodd. GSI's Luminator Fiber Optics solves this problem either by channeling back-reflection into a beam dump via a capillary tube, or by automatically shutting down the system to protect against damage. With peak pulse powers as high as 30 kW, potential for damage is significant.
Other fiber-optic innovations include beam splitting and time-sharing a single laser through multiple fiber-optic delivery mechanisms, providing more flexibility and potential cost sharing.
As lasers become easier to work with, customers have become more sophisticated in how to use the laser beam. "Some of our customers want us to change the output of the laser from one pulse to the next. We have to understand and control the laser, so that we have high repeatability from pulse to pulse. We have control software and hardware that allow our lasers to be more repeatable with faster parameter changes," says Tom Kugler, applications engineering manager for GSI Laser Division. Speed of communication between the laser and the control system is the key enabler. "We have had to build controls into the hardware of the laser itself," says Kugler.
Another company that has evolved its control and monitoring systems for its lasers is Trumpf (Plymouth, MI). Over 650 sensors are employed in just one of their YAG lasers, feeding a host of information parameters into a laser-control system. The control system adjusts power output for consistent laser power delivery, or alerts the operator. Trumpf's service department can view this information remotely via Telediagnostics. Faults are solved, or advice provided for many incidents, without a service call. Alternatively, the problem can be diagnosed prior to a visit.
Direct diode lasers use semiconductor material to convert electricity directly into laser light, in the same near-infrared wavelengths as Nd:YAG lasers. They have high wall-plug efficiency (30–45%) but lesser beam quality, and they cannot be pulsed. The trend for these systems is higher power, and suppliers are now offering fiber-optic delivery. More importantly, engineers now better understand which applications these lasers are uniquely suited for, such as brazing or powdermetal cladding.
Direct diode lasers once delivered too large a spot size to couple to a fiber-optic cable, according to Klaus Kleine, vice president of US operations for Laserline GmbH (Los Gatos, CA and Germany). Now, he says, "in a 450-µm fiber, we can deliver up to 3 kW; in a 1-mm-thick fiber, we can produce up to 6 kW." Laserline advertises beam parameter products (BPP) of 150 mm-mrad for the higher power 6-kW system. Without fiber-optic delivery, Laserline now offers a 10-kW directdiode system with a rectangular beam shape of 60 x 300 mm-mrad.
The wider, nonround beam shape is not necessarily bad. "Our 4000L produces a spot that is a line 12 by less than 0.5 mm," states John Haake chief operating officer for Nuvonyx Inc. (Bridgeton, MO). "We actually take advantage of that in applications like heat treating, cladding, seam welding, wire-feed welding, cladding, and brazing." He says direct diodes are opening new applications for lasers. "Direct diode's technical competition is not other laser technologies, but other heating technologies, like induction for heat treating, or direct flame."
First introduced commercially in 2000, disk lasers are one of the newer trends in industrial lasers. Disk lasers are made with Yb:YAG solid-state materials and produce near-infrared light that's delivered by fiber-optics. In addition, they boast wall-plug efficiencies as high as 25%, and up to 10 times better beam quality than a comparable Nd:YAG laser. Higher beam quality results because the disk dissipates heat better than the rod of a Nd:YAG-type laser, thereby eliminating thermal lensing.
Trumpf, a pioneer in disk lasers, has seen decreasing costs at the same time the power available has increased. In December 2006, Trumpf introduced an 8-kW disk laser, their TruDisk 8002 model. "This model produces twice as much power at about two-thirds the cost per kW, as compared to disk lasers available just two years ago," says David Havrilla, YAG laser product manager for Trumpf. Projections are that by 2008 their 8-kW system will be about half the cost per kW of their 4-kW system delivered in 2005.
The measured beam parameter products (BPP) are less than 8.5 mmmrad at the workpiece for Trumpf's high-powered disk lasers, as compared to 30 mm-mrad for Trumpf's high-power rod-type Nd:YAG lasers. Better beam quality means more flexibility for the process engineer, and higher throughput—plus smaller spot sizes, longer focal lengths, and larger depths of focus, as well as the ability to transmit on smaller fiber-optic cables. They offer lower-power disk lasers with a BPP of 2.5 mm-mrad at 1-kW power, which are well-suited for cutting steel up to a few millimeters thick, according to Havrilla.
Havrilla describes disk lasers as robust for industrial use. "For instance, disk lasers are field-repairable by the customer, and are architecturally insensitive to backreflection," he says, obviating the need for specialized factory repair or for isolators or devices to protect the laser. This also means that there is no limit on the length of the fiber-optic delivery cable.
Fiber lasers, more recently introduced in industrial applications, use a doped optical fiber instead of CO2 gas or solid-state resonators. They operate in near-infrared wavelengths like Nd:YAG or disk lasers. Delivered with a fiber-optic cable, they have a simple, low-maintenance architecture. Some have described it as a "laser engine." Fiberoptic lasers provide high-quality beams, stable operation, low maintenance costs through ease of use, small physical size, and wall-plug efficiencies to 25% or more.
Fiber lasers have become a market force in low-power applications. "There are two areas in materials processing where fiber lasers have been successful," says John Tinson, a vice president for SPI Lasers (Southampton, United Kingdom), a supplier of fiber-laser systems. "One is the marking industry, such as indelible parts numbering or computer keyboards—fiber lasers in general are now about one out of every four lasers in the marking industry. Another is the micromachining area, processing materials that are less than 2-mm thick. These applications could include machining metals, silicon, or alumina." SPI produces lasers rated to about 200 W for these applications.
High-power materials processing and fabrication using fiber lasers is increasing in importance, led by IPG Photonics (Oxford, MA). IPG offers standard lasers rated to 10 kW, and advertises systems to 50 kW. "We can deliver higher-power systems by combining our existing systems like building blocks," says Mike Klos of IPG Photonics. Pricing for this technology is trending down as well. "We can deliver a 1-kW system for about half the price it was just a few years ago," says Klos.
Not only can fiber lasers deliver power, they deliver focus as well. The IPG 10-kW YLR-10000 delivers a beam measured in BPP as small as 6 mm-mrad with a fiberoptic delivery as small as 100 µm. Their 1-kW system specifies a BPP as small as 0.34 mm-mrad.
"Our applications have been growing as the technology becomes more accepted," says Bill Shiner, vice president industrial for IPG Photonics, "for instance, in areas like welding of tailored blanks using lasers that are typically rated at 4–5 kW." He believes the ease of use and low maintenance of the fiber laser is partly responsible for its acceptance.
Manufacturers are still trying to understand how these lasers best fit into particular applications. Some recent research, presented by Stuart Woods, a consultant formerly employed by SPI, found that "fiber lasers cutting twodimensionally with nitrogen as the assist gas, are more economical than CO2 lasers for cutting steels in thicknesses to 6 mm." He supports his statements with data developed by the Fraunhofer Institute for Material and Beam Technology (Dresden, Germany) and IPG Photonics. Additionally, because fiber lasers are versatile and can be attached to existing robots, he expects them to become more common in multiaxis 3-D cutting.
Because it transmits a laser beam through a fiber-optic cable connected directly to the output, it has been suggested that fiber lasers may be particularly sensitive to catastrophic failure due to back-reflection. IPG Photonics was aware of this as an issue when developing its systems. "The fiber-optic cables we supply are equipped with a quartz block on the end. If there were any back-reflections, they wouldn't damage the laser," says Klos. "In addition, within the device itself we have a redundant protection system, if anything does get back through, this protective system will shut down the system before any damage occurs." He says that the back-reflection protection system has been fully tested and proven to be reliable.
Laser Beam Modes
The power output of a laser should not be the only characteristic of a laser a process engineer considers. There are various ways of measuring laser beam quality. Two that many find useful are beam parameter product (BPP) and beam mode.
The beam parameter product is defined as the product of beam radius, measured at the beam waist, and the beam divergence half-angle, measured in the far field. The usual units are mm-mrad (millimeters times milliradians). The BPP measures the focusability of the beam—in general, the lower the number the better the beam quality, which translates into more flexibility for the process engineer.
Beam mode describes the distribution or cross-sectional intensity of the laser light within the radius of its spot size.
Not all beams are created equal—nor should they be. Engineers are finding that beam shapes can be matched to the application at hand, especially in cutting. A fine, knife-like beam is good for cutting thinner materials, but presents problems when extended to thicker materials.
Laser engineers in general talk about two different kinds of beam modes; the TEM00 provides a sharp Gaussian beam shape, with the power concentrated in the center (see image). The TEM01* mode provides a wider, flatter intensity distribution. The sharp TEM00 beam provides a higher power concentration and a faster cut—up to a material thickness of about ¼" (6.35 mm), according to Jason Hillenbrand, laser product manager for Amada, Inc. (Buena Vista, CA). Sharp beams lead to small kerf sizes, which can make it difficult to evaporate material out of the cut.
For materials thicker than about ¼", the flatter shape of the TEM01* beam mode provides a wider power distribution and a wider kerf, and actually allows a faster cut in thicker materials. Although TEM00 beams can be produced by diffusion-cooled resonators, Amada's RF excitation resonator technology can also produce the sharper TEM00 beams. Exploiting these beam modes has been a major advance that has taken place over the last few years, according to Hillenbrand.
This article was first published in the May 2007 edition of Manufacturing Engineering magazine.