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Muscle YAGs: precision plus power.

With new muscle and fiberoptic delivery, the neodymium-doped yttrium aluminum garnet (YAG) solid-state laser is now challenging its older [CO.sub.2] gas-laser cousin in a manner, some claim, much like the way solid-state electronics displaced their gas and vacuum-tube predecessors. Once relegated to delicate and temperature-sensitive laboratory work, the new, more powerful YAG will also be soon challenging a lot of nonlaser ways of doing welding, cutting, and drilling.

A good example of the breadth of what YAG lasers are now doing is provided by Lumonics Corp, Industrial Products Div, Livonia, MI. They manufacture [CO.sub.2] lasers, and distribute excimer lasers built in Canada and YAG lasers made by their Rugby, England, group. The JK YAG-laser product line ranges in size from 20 W to 1 kW (average power). Introduced to the US last summer, the 1-kW unit has been received with particular interest by the automotive and research industries.

The 1-kW pulsed YAG laser is offered in three versions: the 706 for welding, the 707 for cutting, and the 708 for drilling. The basic differences are in beam quality and power-supply configuration.

Welding with YAG

The 706 is best for spot and short-seam welding in the auto industry and appliances, believes Steve Llewellyn, Lumonics's vice president. "Most auto welding is done in air," he points out, "whereas other industries might want to use inert shielding gases of either argon or nitrogen. But the 706 is good also for deep welding in aerospace applications, competing already on the lower end of electron-beam capability." The 706 is doing thin-metal welding of specialty alloys below 1/4", providing deep penetration (usually with an argon shielding gas).

An interesting Lumonics YAG application is at the Gillette Safety Razor Co, welding Sensor razor assemblies. All the lasers are in one corner of the plant, near the assembly and processing machines and connected by fiberoptics to individual work stations. This simplifies servicing of these machines, Llewellyn reports, and integrates the laser into the manufacturing process much more easily than if the laser generators had to be out on the production floor, using up valuable floor space and getting in the way.

Cutting

Configured for cutting, the 707 uses a coaxial gas jet, either to boost cutting power by reacting with the metal or to blast away combusted material to clear an optical path into the cut. In carbon steels, that would be oxygen, and an inert gas is used in aerospace for cutting stainless and high-temperature alloys. "You use inert gases to reduce contamination of the cut edge where you don't want oxides," Llewellyn explains. "Like the recast problem in EDM, this minimizes HAZ and recast, but what recast that does remain is cleaner and has greater integrity than a recast zone where the cutting gas was oxygen."

The key applications so far in automotive are a combination of relatively short cut lengths and difficult-access situations. While admitting that the [CO.sub.2] laser is still best for cutting out large floor-panel areas, he sees the YAG's edge is its fiberoptic delivery. "Optical alignment is much simpler, and the reliability of the overall system is greatly improved."

[CO.sub.2] requires several mirrors mounted inside the robot arm to get the laser beam to the workpiece, and its alignment degrades with robotic movement, reducing the efficiency of power delivery. "In a YAG system, the fiber is hooked up just like another electrical cable on that robot arm, and delivery is not affected by robot movement. We are hearing that the auto industry is having significant alignment problems with [CO.sub.2] cutting robots. If that is true, then the YAG laser may have even more of an advantage than we imagined. And once you integrate YAG into a processing line, the difference between using it for one or two functions and using it for all the laser functions on that line is very small."

Final body alignment is an ideal application for YAG laser cutting. Several dozen YAG lasers are being used in Japan to cut antenna holes and trim holes in auto bodies. The laser-cut holes can provide near-perfect alignment of trim, regardless of how that frame may have been distorted by the body-building process. The result is a better quality product. Floorboard holes can also be cut for various transmission options--easily, quickly, and accurately.

Drilling

The Lumonics 708 YAG laser is for deep drilling at high speed. The most interesting application so far is percussion drilling of cooling holes for aircraft-engine blades and vanes. These holes are blasted 0.020" to 0.030" dia and 1/8" to 1.5" deep. There is some gas assist, but the drilling is done by repeatedly pulsing the laser focused down into the hole. The percussive force is from the laser vaporizing material at the bottom of the hole, creating an explosion that blows molten metal up out of the hole. Holes 0.030" dia by 1.5" deep can be drilled by pulsing the laser, yielding an almost parallel-sided, clean hole. This is presently being done in the aerospace industry with 400-W YAG lasers. A 1.5" deep hole takes about 20 sec. With a 1-kW laser, drilling time can be cut in half.

In thin-material drilling--0.060" to 1/4"-thick combustors, for example--holes can be drilled "on the fly." A large-diameter combustor shell can be rotated and drilled at rates of 100 to 200 holes/sec. This is also being done with 400-W YAG lasers, and a 1-kW laser would permit faster drilling or doing thicker materials on the fly, notes Llewellyn. "A 1-kW laser will produce a 0.027" dia hole 1/4" deep in a single pulse. To avoid a high degree of ovality, the laser's on-time must be quite short--0.65 to 1 millisec at surface speeds of 6 to 20 ipm. Compared to indexing, drilling, and indexing, this is at least six times faster." Which means a lot when a 4-ft- dia part has 100,000 holes in it.

In the auto industry, some are looking at drilling oil-feed holes in bearings with YAG to reduce microcracking and HAZ. Hole sizes range from a few thousandths to 0.030", beyond which, you have to switch to trepanning because the beam becomes too big to be effective in removing material.

Compared to EDM or ECM for putting holes in difficult materials, the YAG laser is faster and more flexible, and even large holes in thick materials can be done in reasonable production times.

Energy sharing

Another plus for higher-power YAG lasers is being able to use fiber optics for time sharing of one energy source between several work stations, or the ability to make multiple welds at the same time at a single station. For example, one laser can be used to weld motor laminations by splitting the beam three ways and making three simultaneous welds to save indexing time. Or, on round parts, you could weld with two beams 180-deg opposed to spread out heat-distortion effects more evenly across the part. These kinds of things have been done before with [CO.sub.2] lasers and complicated mirror systems, but are now much easier to do with YAG and fiberoptic delivery.

Importance of beam quality

Richard Walker, vice president, Rofin Sinar, Plymouth Rock, MI, agrees that the move is certainly toward high YAG power, but feels the key is maintaining the same beam quality. "In the past, that was not always the case."

Their RSY 500P, a 500-W YAG laser, will soon be joined by a 1-kW model. "I think 1-kW YAG lasers will be used primarily with fiberoptics, and therein lies a problem. A lot of vendors don't have good enough beam quality to put it through an optic fiber efficently.

"When fibers first came along, a lot of people tried to put their laser beam into a fiber, and it burned up. Beam quality was inconsistent, the beam moved around, it was too big or too unpredictable to get into the fiber.

"Although most use a 0.040" -dia fiber for beam delivery, we use fibers half that size and can put significantly more than a kilowatt in that 0.020" dia."

The problem is that as soon as the beam comes out the other end, it diverges at a high rate, and to get any efficiency, you must add at the least a two-lense system--one to make the beam parallel and the second to focus it to a useful spot size, roughly the same diameter as the fiber. "You can go smaller," Walker notes, "but your working distance gets smaller, and the lenses become significantly higher cost. With a kilowatt of power, this means a lot of spatter in the welding mode, and a severe danger of damage to the optics."

The other problem is that a small depth of focus requires maintaining the focal point very, very accurately. That, of course, is not good with robotic delivery, where YAG's edge over [CO.sub.2] is a more reliable beam delivery. "Any laser must be focused to be useful," Walker explains. "So we've designed our laser with a very high beam quality, without compromising power, to get it into a much smaller fiber. This gives us a small spot size, a large working distance, and a good depth of focus. We end up with a fairly efficient and forgiving system on the end of the fiber."

Compared to a 0.040" fiber and spot size, a 020" fiber and spot provide four times the energy density for the same kilowatt of laser-power input. This can mean cutting or welding four times as fast. "The secret is not in the fiber or the optics on the delivery end. It's in the design of the laser. We designed our kilowatt lasers specifically for fiber-optic delivery. Others can get a 0.020" spot, but at a loss in focusing depth and working distance, and they require a very accurate delivery system. Robotic delivery is not that accurate, and the stamped or formed part to be laser cut or welded is not always perfect.

"In automotive--the market that's driving this technology--we're cutting 1 mm (0.040") double-sided galvanized steel in excess of 3.5 m/min. That's a fair lick for a robot, and we're doing this with a high degree of accuracy."

Pushing the robot too far?

Faster cutting sppeds sacrifice accuracy and may require going to five-axis Cartesian gantry systems. "I can remember," he laughs, "he very first [CO.sub.2] laser on the first robot, six or eight years ago. It was a disaster! A that time, they were taking the most unreliable energy source and trying to put it on the most inaccurate motion system.

"Now, robots have improve; lasers have improved; and fiber optics have made things a lot better: better cut quality, better consistency, higher speeds, and a more efficient system. The accuracy of the part you cut is limited only by the accuracy of the robot."

Rofin Sinar has developed a special end effector--a combined trepanner, autofocus, and height-sensing device. The robot comes in, sets its arm position at the right height to focus the beam on the surface, and the trepanner cuts the hole without using robot-arm movement. "You move a much smaller mass at much higher speed and higher accuracy than the robot arm could possibly achieve," explains Walker.

The result, he says, is extremely good hole quality, much better than [CO.sub.2] could do on the arm of a robot. "The [CO.sub.2]'s multimirror system is not only expensive, it requires high maintenance and is unrealiable. When it tries to cut at high speed, the inertia effects of the robot arm tend to distort the mirrors and create problems in both speed and cut quality. Some robot delivery systems use up to six mirrors, and each adds horrific magnifying error."

Challenging EBW

Is there a competition now between YAG and electron-beam welding? "Not at the lower power," Walker feels, "but this will happen when YAG gets into higher power levels--several kilowatts. That would yield a head-on competition. They both produce similar weld profiles, although YAG is not going to produce weld thicknesses of several inches depth as EBW does. But in 0.5" penetrations, high-power YAG lasers will be very competitive because they don't require vacuum chambers and are much more flexible in delivery.

"With 1-kW YAG, you can weld stainless-steel pipe to depths up to 0.5". A pulsed 1-kW laser can weld 6 mm depths at 30 ipm. At 10 ipm, you can get about 0.5" penetration. But a 2-kW laser could weld that same 10mm thickness at 12 to 15 ipm--or weld 25-mm (1") material much slower.

"When you can deliver 1.5 to 2kW on the end of a fiber, you have a system that could replace resistance and spot welding on the automotive line. It would be noncontacting, nonwearing, highly reproducible, nonconsumptive of electrodes, need to end-effector maintenance, and deliver programmed welds wherever you want them. You can't do that with alternative system."

High-power YAGs will be used for welding, he notes not cutting because above 1 kW you lose beam quality. "With welding, that's not as critical and once you get to 4 kW, for example, that would yield a phenomenal increase in weld penetration, and electron-beam welding would be totally outmoded."

Some negatives

But there are some negatives that will slow down the spread of YAG laser use:

Safety. All laser require eye-safety precautions. An errant laser or even the reflection off a metallic surface can quickly blind an observer, whether [CO.sub.2] or YAG. While [CO.sub.2] can be stopped by glass (eye-glasses) or plexiglass, YAG's shorter wavelength requires an opaque barrier (usually metal, although drywall will do) and special (expensive) viewing windows.

Thus, the laser-robotic assembly line must be completely sectioned off. A room surrounds the laser cell, a door opens when parts move in, closes while the laser is operating, and opens again for finished parts to move out.

Energy loss. The energy losses in YAG fiberoptic delivery systems are at either end. Focusing down from the laser-generated beam diameter of 3/8" into the flat end of the fiber creates a 2% or so loss. A similar reflective-surface loss occurs at the other end of the fiber. The loss within the fiber itself, even for lengths over 100 m, is less than 2% (One installation problem, though, is weaving long lengths of fiber into a plant without splicing it.)

Life is not a problem, either for the fiber or the YAG-generating crystals. "We have hundreds of fibers in the field running at 250 W and up without life problems," says Lumonics Llewellyn," and we have laser crystals that have been in use for 10 years." The key is keeping fiber ends clean, not baked with contamination, and avoiding exposing the YAG crystal coatings to industrial atmospheres."

Equipment cost. The premium for YAG over [CO.sub.2] remains large--approximately double, although there has been some improvement in the key consumable, the flash lamps, Rofin Sinar's Walker reports. "A few years ago, 5 million shots per lamp was considered great. Now, we get 50 million. Consumable costs for cutting are more than those for welding because the high peak power is more harsh on the flash lamps, and this reduces flash-lamp life."

Newer designs are being worked on to reduce that problem, Walker says, but are not yet available. "The big jump," he predicts, "will be the use of diode light sources instead of flash lamps. Their efficciency will be an order of magnitude better because the deliver light in a well-defined wavelength band compatible with the pumping band of the crystal. Diode-pumped lasers--presently limited to a few watts--are being used in medical and scientific applications. The problem is making a large array of diodes commercially affordable."

Operating cost. "Including process gas," Llewellyn estimates, "a 1-kW [CO.sub.2] cutting laser has about a $4/hr to $5/hr running cost, including maintenance. A 400 W YAG laser in a welding mode costs about $9/hr and, in a cutting mode, $10 to $11.A 1-kW YAG laser is probably $15/hr to $16/hr running cost for welding."

A key factor is that the energy conversion factor for YAG is only 4%--you must buy 25 kW of electric power to get 1 kW in laser energy. But, Llewellyn points out, [CO.sub.2] lasers have to be warmed up--you may only cut or weld for half a shift, yet pay energy costs for the whole shift. With YAG lasers, you only pay when you're pulsing.
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:yttrium aluminum garnet solid-state laser
Author:Sprow, Eugene
Publication:Tooling & Production
Article Type:Cover Story
Date:May 1, 1991
Words:2783
Previous Article:Technology is key to world class status.
Next Article:Multiaxis EDM makes large dies.
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