By Mark Shortt
Editorial Director, Design-2-Part Magazine

Non-contact processing delivers greater speed, precision, and flexibility

It all happens in the blink of an eye: Photons are excited in a resonator, causing a CO₂ laser to emit a high-energy beam in the infrared spectrum. A water-cooled mirror reflects the high-density beam through a lens, which focuses the beam down to a small spot, 0.008 inch in diameter, on the surface of a stainless steel component.

As the intense heat of the highly focused beam melts the workpiece, cutting all the way through to the bottom, a gas jet blows the melt through the bottom of the cut zone. While a moving mirror directs the laser beam in one direction, the part is moved in the other direction to achieve the desired profile.

The sequence describes laser cutting-in this case, by a hybrid system that uses a CNC machine tool to move both the optics and the workpiece.

Compared to other profiling methods, the process cuts at high speed. According to the LIA Guide to Laser Cutting, a publication of the Laser Institute of America, a typical 1200-Watt CO₂ laser can cut 0.08-inch-thick mild steel at approximately 240 inches per minute. The same machine is capable of cutting thick acrylic sheet, 0.2-inch thick, at a rate of about 480 inches per minute. In both cases, the laser-cut components will be ready for service immediately after cutting, without need for subsequent cleaning, according to the Guide.

Although speed is easily its most apparent advantage, laser cutting is also favored for the power and precision that comes with the ability to focus light into a very small area.

"Consider a 1500-watt beam of light with a .400 inch diameter," said Steve Garcia, applications engineer for Rache Corp., Camarillo, California. "Now focus 1500 watts of energy into a .010-inch spot. The density of this power is roughly equal to 15 million watts per square inch," he explained.

Because the reflected beam passes through a lens that focuses it very finely, it produces a very narrow kerf (cut width) in the range of 0.004 to 0.04 inch (0.1 to 1.0 mm). As a result, it can hold extremely tight tolerances on small-geometry cuts. The finely focused beam minimizes the heat-affected zone, which generally prevents thermal distortion or damage to surrounding areas of the workpiece.

High repeatability is also ensured through use of CNC programs, which permit quick modifications to the program without needing to re-tool. The programs allow users to vary, depending on the application, functions such as laser power, feed rate, focus, type of gas, and nozzle gas pressure.

Numerous Uses

Lasers are capable of cutting numerous materials, including steel, stainless steels, and super alloys. They can also be used to cut plastics, rubber, composites, ceramics, quartz, glass, and wood. Frequently, the process is specified for cutting extremely hard materials, such as titanium, Kovar, hastelloy, and Inconel. A key benefit of laser cutting is that the lack of contact between the tool (the laser cutting head) and workpiece eliminates problems of tool wear and breakage.

Because of such advantages, the process is being used increasingly by job shops to replace more traditional cutting methods, especially for aerospace and automotive contract work. But cutting is not the only laser-based process making inroads at job shops across North America. Many manufacturers currently use lasers for another machining operation-drilling-and for welding, marking, etching, and engraving. Some of the more innovative applications are modeling, rapid prototyping, surface modification, and micro-fabrication.

Types of Lasers

The term "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. Several types of lasers are used in manufacturing operations, but the most common are CO₂ (gas) and Nd:YAG (solid-state) lasers. Both types are named after the molecule or atom that produces the laser light. CO₂ lasers produce laser light by the excitation of CO₂ molecules; the laser light of Nd:YAG lasers is generated by neodymium (Nd) ions that are held in a special crystal rod made from yttrium aluminum garnet (YAG).¹

Power levels for CO₂ laser cutters range from 50 to 6,000 Watts and higher. But until recently, when new applications for high-powered lasers began to emerge, a more typical power range was approximately 50-3500 Watts. Generally, CO₂ lasers generate higher powers than Nd:YAGs.

Although higher-powered lasers typically cut at faster speeds (resulting in higher productivity), the relationship between laser power and speed is not always simple. In some cases, high-powered lasers present focusing problems, which lead to larger focused spots and cut widths. The need to remove more material to produce the cut can actually slow the process to some extent. Another problem that often arises with higher-powered lasers is poor beam quality, which renders them more suitable for heat treating and welding than for cutting.

The wavelengths of light produced by CO₂ lasers and Nd:YAG lasers are 10.6 and 1.06 microns, respectively. Because of its shorter wavelength, the light of an Nd:YAG laser has several advantages over light from a CO₂ laser. It can be focused down to a smaller spot, permitting finer, more detailed work. Because it is more easily absorbed by metal surfaces, it can be used for work on highly reflective materials, such as copper or silver alloys. And, unlike CO₂ light, Nd:YAG laser light can travel through glass. Therefore, quartz optical fibers can be used to carry the laser beam relatively long distances to the workpiece, according to the LIA Guide.

Comparative Process Limitations

Despite its benefits, laser processing is subject to some limitations. For example, materials that tend to char, burn, or develop micro-cracks are generally not suitable for laser processing, according to Patricia Wyche, marketing and special projects manager for Directed Light, Inc., San Jose, California.

Electrical discharge machining (EDM) can cut thicker materials and hold tighter tolerances, and die cutting can cost less than laser cutting at high-volume quantities. Electron beam welding is capable of higher power than laser welding, and therefore, achieves deeper penetration in materials such as copper, aluminum, and brass, she said. (For more on laser welding, see Advances in Job Shop Welding.)

Technology Still Emerging

Many of the advanced processing methods that are now used by job shops, such as five- and six-axis cutting of 3D shapes, were first employed by OEMs from industries such as aerospace and automotive. Now, OEMs from a variety of high-tech industries, including medical devices and power generation, are routinely outsourcing the work. Despite this trend, however, laser processing is far from realizing its full potential in job shops.

"It is our opinion that lasers are under-utilized in job shop environments due to the cost of the equipment," said Patricia Wyche, marketing and project manager at Directed Light, Inc., San Jose, California. "Also, laser processing is still an emerging technology, so there is a lack of education on the process. Although the U.S. has the largest base of installed lasers, laser processing is actually used in more industries and applications throughout Europe and Japan," she explained.

At a power level of 500 Watts (0.5 kW), the price tag for an Nd:YAG laser cutter is on the order of $90,000 to $100,000, according to Richard Walker, president, Cutting Edge Optronics. Capability for fiber optic beam delivery costs an additional $10,000-$15,000, he said. The cost rises to about $160,000-$170,000 for a 1000-Watt (1-kW) Nd:YAG, and approximately $280,000-$300,000 for a 2000-Watt (2-kW) cutting system, Mr. Walker said. For CO₂ laser cutters, costs range from $65,000 for a 1000-Watt (1-kW) system to about $280,000 for a 6-8-kW system, according to Kevin Laughlin, PRC Laser.

Companies in industries that are more technologically advanced are more likely to recognize the advantages of laser processing, according to Kip Brockmyre III, vice president and general manager of Laser Fare, Inc., Smithfield, Rhode Island. Manufacturers in the aircraft/aerospace, electronics, power generation, and medical device industries, for example, are making extensive use of laser machining. "General sheet metal shops tend to use a fair amount of CO₂ laser work for flat plate, but otherwise tend to be unaware of the benefits of sophisticated laser processing," said Mr. Brockmyre. "Overall, lasers are used about 25% of the time they should be used," he added.

Drilling

Like laser cutting, laser drilling is a non-contact process that avoids tool wear or breakage and minimizes clamping requirements. It also offers the high levels of repeatability and flexibility permitted by computer control and programming. Popular applications include the drilling of cooling holes in jet engine components, and lubrication holes in automotive transmission hubs.

Precise control of heat input and the ability to work with extremely hard materials are additional advantages of laser drilling, which can produce shaped and shallow-angle holes, according to the July 1999 issue of LIA Today, a newsletter of the Laser Institute of America. According to the SME Tool and Manufacturing Engineers Handbook, however, the process can produce microscopic cracking and recasting. Efforts should be made to minimize or eliminate these issues, the Handbook advises.

Marking and Engraving

Integrated laser systems have made it possible to achieve fast, permanent, non-contact marking of a wide range of materials, including metals, plastics, semiconductors, ceramics, marble, and glass. Clean, crisp, markings can be made with high accuracy as a result of extremely small spot diameters, some as tiny as 0.003 inch.

New Applications for Existing Technology

Kevin Laughlin, PRC Laser, said that the technological advances he sees are not "new technology" as much as they are new applications of existing CO₂ technology. Because high-powered lasers in the 4000-6000 Watt (4-6 kW) range permit higher processing speeds, they are being used increasingly for faster cutting of thicker materials, he noted.

Driving the demand for the higher power levels is the fact that laser-cutting job shops are looking to become more diverse and flexible, with capabilities for cutting both thick and thin metals. This strengthens their ability to compete against shops that use punch presses, EDM, or waterjet cutting, he said. "A 6 kW laser can cut 1 1/4-inch-thick stainless steel faster than either a milling machine or EDM," Mr. Laughlin said.

Mr. Laughlin said that 4-kW lasers are also being increasingly used for free-form cladding, a process that resembles stereolithography except that it uses metal to mock up specialty parts and check them for form, fit, and function. The process involves spraying down a mixed, powder metal composition and directing a laser beam to it. As the laser beam hits it, the composition changes from a powder to a metal-based substrate.

Thin-Material Laser Cutting

Die stamping, EDM, and chemical etching-a process in which acids are dissolved around a desired shape-are the methods most commonly used to fabricate precise, intricate shapes from thin material. Rache Corp., however, offers laser cutting as an alternative process for manufacturing thin-material shapes. The company uses its Nd:YAG work center for cutting shapes in thin material up to .050-inch thick.

Last year, Rache received approval from a major medical company to manufacture the firm's facial and cranial implants, titanium components that had previously been made using EDM. A concern of the company during the qualification phase was that the heat-affected zone might become a problem for the implant, a complicated part with many holes. However, it was found not to affect the form, fit, or function of the implant. Rache was also able to produce the implants at a lower cost by using laser cutting, Mr. Garcia said.

According to Mr. Garcia, Rache has been able to design multiple hole diameters, ranging down to .002 inches, into the same shape while holding corner radii to .002 inches regardless of thickness. The laser-cut thin materials exhibit smooth, square edges without the deformation resulting from stamping, he said. And by not using chemicals in the process chain, the company has eliminated the waste stream created by chemical milling.

Technological Advances in Laser Processing

Dr. Richard P. Martukanitz, of the Applied Research Laboratory (ARL) of Penn State University, sees technological advances in several areas. Diode-pumped Nd:YAG lasers are an improved technology that can be used for deeper drilling applications, he said. Widely seen as a growth market, these lasers use the diode as a catalyst to produce laser light-it supplies the energy to excite the crystal.

Another advancement, he said, are high-power, direct-diode lasers, currently being developed by Nuvonyx Corporation. Compact and portable (a 3-kW direct-diode YAG weighs about 15 lbs., according to Kevin Laughlin, PRC Laser), these lasers can be easily mounted on the end of a robotic arm. Unlike diode-pumped systems, they use a diode array to generate the laser light directly; the diode functions as a source rather than a catalyst.

Dr. Martukanitz also said that laser free-forming of titanium parts, an area in which ARL is heavily involved, is a promising method for rapidly fabricating components in titanium alloys. An extension of the concept of stereolithography, the process uses a laser and metal powder to form near net-shape components directly from CAD/CAM data. A potential application for the process is to fabricate forging pre-forms with properties tailored to specific applications, according to ARL. Forging pre-forms, for example, could be fabricated for turbine engine disks, which could be custom designed for each engine.

Laser free-forming might also be used to produce airfoil structures from Nb and Mo alloys, which are difficult to cast because of their high melting points. According to ARL, however, a number of issues need to be addressed before the process can be fully developed and implemented. These include refinement of the laser process to maximize deposition rate, and development of the process to fabricate alloys from elemental blends.

A new technology in surface modification recently became available when Laser Applications, Inc. (LAI) Southwest, located in Phoenix, Arizona, introduced its Laser Improved Surface Modification (LaserMod) process. The process is reported to transform metal surfaces to resist wear, by permitting the formation of a transition alloy within a base metal.

3D Cutting

Greater flexibility is the hallmark of five- and six-axis cutting machines, which, besides being able to cut flat sheet parts, can be used to profile three-dimensional shapes out of preformed parts. "Innovations in 3D cutting started with automotive and aerospace components made by companies like GM, Ford, and Boeing," said Mr. Laughlin. "A lot of that technology has been integrating its way down into the job shop environment."

The machines are of two basic designs. According to the LIA Guide to Laser Cutting, one is a robotic arm that has mirrors at each bend to direct the beam to the cutting head. The other is a gantry system consisting of a large, box-like structure. Within the frame, the cutting head is suspended from a moving x-y system; it is a wrist-like mechanism that contains mirrors to direct the beam to the nozzle, the Guide states.

For More Information

The Laser Institute of America, 13501 Ingenuity Drive, Suite 128, Orlando, FL 32826; (407) 380-1553; fax (407) 380-5588; e-mail: lia@LaserInstitute.org; Web address: www.LaserInstitute.org.

The Applied Research Laboratory (ARL) at Penn State University has formed a Laser Processing Consortium that conducts basic and applied research, as well as engineering development, for the purpose of advancing the technology of laser processing. A goal of the Consortium is to assist in the commercialization of advanced laser processing technology by helping to move it from the development lab into the hands of manufacturers. For more information from the Laser Processing Consortium of ARL, visit its Web site at www.arl.psu.edu/mm_lp.php

References

1. Cubberly, W.H., and Bakerjian, R., editors, 1989. Tool and Manufacturing Engineers Handbook. Desk edition. Dearborn, Michigan: Society of Manufacturing Engineers.

2. Kalpakjian, S., 1995. Manufacturing Engineering and Technology. 3d edition. Reading, Massachusetts: Addison-Wesley Publishing Co.

3. Powell, J., 1999. LIA Guide to Laser Cutting. 1^st edition. Orlando, Florida: Laser Institute of America.

4. ¹Powell, J., 1999. LIA Guide to Laser Cutting, p.51. 1^st edition. Orlando, Florida: Laser Institute of America.

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