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High-power lasers: bringing new applications to light.

In today's metalworking field, the motivation for using high-power lasers (HPLs) is either attainment of heavy-section welding capability or enhanced processing speed for high-speed welding, surface treating, and heavy-section cutting. Potential applications include longitudinal or spiral seam welding of line pipe, welding of heavy-section structural components, field joining of line pipe, ship construction, welding of high-volume heavy-duty automotive and truck components, and similar uses. At present, practical, production-suitable component thicknesses range from approximately 6 mm at 5 kW, 16 mm at 14 kW, and 25 mm at 25 kW.

Lasers with average power of 9 kW or more have been available since the early 1970s, when both Avco Everett (no longer producing industrial lasers) and United Technologies offered lasers in the 9 kW to 25 kW power range. About 20 units were delivered to application laboratories around the world, but none were used in production.

The first high-volume production application of 9-kW [CO.sub.2] lasers was automotive component welding. Five 9-kW modular systems were delivered to General Motors in 1983 for fabrication of automotive air-conditioning components. These units, together with three 6-kW machines in the same installation, represented one of the most intensive, single-location uses of HPLs in industrial production. Based on the results of over nine years of operation, laser welding has proven to be a highly reliable and cost-effective industrial production process.

One of the first components that was designed specifically for HPL welding was a clutch drive-pulley assembly for automotive air conditioners. The low specific energy and small weld volume afforded by the laser permitted use of thin walled subassemblies facilitating substantial weight savings. In addition, high welding speeds (2.5 mpm) reduced the total number of weld stations and factory floor space required, when compared to conventional welding. The units installed in 1983 are still in high availability operation and some have logged more than 50,000 hours of production use in the nine years to date.

A second HPL production application was introduced in Japan by Kawasaki Steel (Chiba) in the early 1980s. Japanese-built, 10-kW laser systems were used for coil joining in a rolling mill. The laser welding process that replaced butt welding has proven to be extremely effective in production.

Current HPLs

In 1988, UTIL introduced a modular product line that includes 6, 14, 18, and 25 kW [CO.sub.2] lasers. Rated output is attained in a donut-shaped beam with "unstable resonator" cavity optics exhibiting a magnification of two (ratio of outer-to-inner donut diameters). Currently, this product line represents the only industrially rated units in the world at powers exceeding 10 kW. In these lasers, vane-axial blowers circulate the laser gas through modular flow loops comprising a discharge channel and water-cooled heat exchangers. A multi-element cathode yields a stable, high current DC discharge to a water-cooled copper anode. Optical cavity mirrors are supported on platforms at either end of the laser; the platforms are connected by an Invar (low coefficient of thermal expansion) alloy structure to minimize the effects of temperature variation. The optical assembly is supported from the laser frame by vibration isolation mounts. Hinged covers at each end of the laser swing back to facilitate optics inspection, cleaning, and change.

The beam exits the low pressure ( 0.1 atm) laser cavity through a patented aerodynamic window. The pressure difference across the window is maintained by supersonic flow of clean dry air of approximately 3 [m.sup.3] /min supplied at 5 atmospheres. The aerowindow replaces the solid transmitting window customarily used in low-power systems, yielding two significant benefits. First, elimination of the transmitting window improves output stability and endurance characteristics. Solid windows suitable for carbon dioxide laser beam transmission are costly, are subject to thermal lensing, which results in focal point variations, and lack adequate durability for use at 10 kW and above. Second, some solid windows for carbon dioxide are not transmissive to visible light, thereby preventing use of a visible laser alignment beam for final cavity adjustments. In contrast, the aerowindow of a UTIL laser permits the use of an internal HeNe alignment laser which replicates the invisible carbon dioxide beam and responds simultaneously to any motion in the cavity optics. No aerowindow failures have been experienced in more than a half million hours of production application.

Two production welding applications have been implemented within the past year. Creusot-Loire Industries in France is using a 14-kW laser system for armor-steel welding. In another application, nominal 30 cm diam, plain carbon steel cylindrical segments of 12.7 mm wall thickness are being laser welded. Due to production speed requirements, a 25-kW laser was selected for this application. To date, more than 20,000 radiographically-sound production welds have been generated.

Continuous-seam welding

A high-power application nearing production implementation is high-speed longitudinal seam welding of 400 Series stainless-steel tubing with wall thicknesses ranging from 1.5 mm to 3.0 mm. Minimum welding speeds of 12 mpm to 18 mpm are required for cost effectiveness. Although energy delivery requirements are readily met for the desired speeds, the onset of fluid dynamic weld metal instability, commonly referred to as "humping," at speeds above 14 mpm has impeded development. A twin-focus technique has been developed which delays the instability to higher speeds and enhances weld characteristics. This technique is the basis for a 14-kW [CO.sub.2] system that will be used by Schoeller Werk GmbH in Germany in a pioneering application of high-speed longitudinal seam welding of stainless-steel automotive tubing.

Another promising high-power longitudinal seam-welding application involves welding of line pipe. A 25-kW laser with a linearly polarized output beam is being used to explore a unique "wedge shape" welding process for this work. This technique, being developed jointly by the Fraunhofer ILT and Hoesch Rohr AG in Germany, utilizes a line-focused beam that is directed into the wedge formed by the joining surfaces as they are forced together in a tube mill. Appropriate polarization orientation facilitates beam energy reflection into the contact point. Picture closing a zipper with continuous welding as the two surfaces meet; the result is an extremely narrow, controlled fusion zone.

Because only a small quantity of material is fused in the process, very high-speed welds can be achieved in comparison to those obtained by conventional deep penetration. Further, due to the very short liquid time of the fusion zone, chemistry changes during welding are minimal. Following post-weld heat treatment, it is almost impossible to distinguish the fused region from parent metal.

Torque converter welds

Recently, two 14-kW [CO.sub.2] lasers were manufactured for use in a single workstation to join automatic transmission torque converter shells at Ford. The dual laser approach is being used to minimize welding time and thermally-induced distortions. Simultaneous dual beam welding permits the nominal one meter long weld to be formed in less than 20 seconds.

In addition, locating the welding beams 180 deg apart on the torque converter circumferential seam results in balancing the thermal stresses induced during the welding process. The dual beam approach minimizes the requirements for complicated tooling to offset these stresses.

Sheet-metal welding

An area of intense current interest for HPLs is in custom blank and three-dimensional sheet-metal welding. Although much automotive sheet metal is typically only 0.8 mm thick and can be readily penetrated at reasonable speeds with medium-power lasers, HPLs are useful for some applications. These include welding of custom blanks formed of material of similar as well as different thickness, and segments having specific sizes and shapes designed to decrease material waste. Multiple layers, thicker material, and higher welding speeds favor the use of high power for the attainment of cost-effective operation.

A similar requirement occurs in the formation of fabricated sheet metal components in which two or three layers of material may be involved at the weld joint. This work requires sophisticated beam or component manipulation; it is only in the last few years that processors have been available to control the system with the speed and accuracies required. This also requires good fixturing for sound fitup. A 14-kW system will be used by Audi in Germany for such an application.

Camshaft remelting

A line-focused, 14-kW [CO.sub.2] laser beam is being used to remelt the critical wear surface of a cast-iron cam lobe. This process is being pursued because solid-state transformation hardening does not provide adequate cam surface wear resistance for modern, high-performance engines. With remelting, an iron carbide (Ledeburitic) structure is obtained that yields more durable wear characteristics.

The camshaft remelting process has been of critical interest to European automotive companies for some time and has been the subject of many R&D efforts. Recently, Mauser-Werke GmbH Germany has undertaken intense development of the process using a 14-kW UTIL laser system. Production-suitable process parameters enabling cam lobes to be treated in less than ten seconds each have been established, and qualification camshaft lots have been generated that have passed stringent evaluation.

Automated paint stripping

All aircraft with painted surfaces must be stripped and repainted on an average four-year cycle. This necessitates the use of toxic chemicals and considerable hand scraping, resulting in an enormous expenditure for labor and serious air and water pollution. The resulting waste disposal problem has become increasingly important in recent years.

Problems like these led the US Navy to award a contract to engineer and build two Automated Laser Paint Stripping (ALPS) systems sized to strip aircraft such as the F-14 Fighter and the H-3, H-46, and H-53 helicopters. Funded by Naval Air Systems Command, Washington, DC, the ALPS systems are being developed for mid-1994 installation at the Naval Aviation Depots at Cherry Point, NC and Norfolk, VA.

Each system is designed to remove paint at a rate of at least 0.45 [cm.sup.2]/min and to selectively remove coatings. The latter capability is provided by a 256-color camera system using a patented real-time vision-feedback control. ALPS paint removal, process control, and color discrimination will allow for the removal of topcoats, paints, rain erosion coatings, textured anti-skid walkways, sealants, primers, and other surface coatings from both metallic and composite substrates without damage to the substrate material. Waste reduction by a factor of 200 over present chemical stripping techniques is expected, with the concentration and containment of the waste products in an environmentally acceptable form. The laser literally vaporizes the layer(s) to be removed, leaving solid waste with the paint volatiles removed of only about 10% to 50% of the original paint volume.

More HPLs coming

More than three decades have elapsed since the first successful operation of a laser. During that period, many systems and applications have been developed, and the word "laser" has become synonymous with high technology.

Presently, only UTIL offers industrial lasers at continuous powers exceeding 10 kW, with an operational 45-kW unit to be available this year. However, other manufacturers are moving toward marketing HPLs. Cambridge Power Beams Limited has been formed to market a 25-kW laser that is under development at the Laser Center in the United Kingdom with multinational Eureka funding. Trumpf Lasers has announced its plan to develop a 40-kW system in conjunction with the Fraunhofer ILT, with government funding (North Rhine Westphalia) to augment this effort.

In Japan, both Toshiba and Mitsubishi have experimented with HPLs. It would be expected that they would move to industrialize such systems if the market warrants. With the end of the Cold War, it is anticipated that Russian technology will also become available to industry.

It is not unreasonable to assume that production laser power trends will mirror those of electron beams following their introduction to welding in 1951. Early electron beam units were, typically, of 3 kW rating. Rapid development led to 6, 12, and 25 kW machines. Now, 35-kW units are commonplace, especially for out-of-vacuum welding applications, and R&D devices to 300 kW have been developed.

What maximum laser power is suitable for production? Welding tests conducted by the United Technologies Research Center in 1976 with a gas dynamic laser demonstrated full penetration, single-pass, 38 mm thick, alloy steel welding at 3 m/min and 90 kW. Higher powers may be available today by combining lower power beams as suggested by Arata at Osaka University and others. Higher power individual units are technically feasible. It may be assumed, therefore, that the international laser manufacturing community will respond to whatever power challenge industry requires.

For more information from United Technologies Industrial Lasers, South Windsor, CT, circle 245.

Lasers in industry

During the past twenty years, many [CO.sub.2] lasers have been, applied to cost-effective production materials processing tasks. These lasers may be grouped into three areas:

* Lasers with average power, rated to approximately 2 kW, and used mostly for two- and three-dimensional sheet metal cutting. There are more than 9000 laser cutting systems in operation around the world. Many of these systems incorporate sophisticated, multi-axis contouring capabilities.

* Lasers in the mid-power range to 6 kW used primarily for high-volume automotive component welding. Such laser systems number in the hundreds.

* Lasers exhibiting average powers above 6 kW. The number of such lasers in production use is currently in the tens of units.
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Banas, Conrad; Duhamel, Raymond F.
Publication:Tooling & Production
Date:Jun 1, 1992
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