Keeping It Together with Laser Welding

Laser welding is faster and cheaper than conventional joining technology, and it may even lead to lower greenhouse-gas emissions. Although laser welding is by no means easy to do, technological advances are making it an increasingly viable option within various industries, including transportation, electronics and sensing.

 

figureAutomotive taillights are just one of thousands of applications of laser welding.

The field of optics is contributing to the development of a sustainable future in all kinds of ways—some of which are less obvious than others. Laser welding, for example, will surely be a critical tool for improving automobile fuel efficiency and reducing exhaust pollution, primarily by reducing the weight of cars.

In the United States, the Obama Administration is currently negotiating with automakers the final details of regulations that will require American vehicles to achieve an average mileage of 54.5 miles per gallon by 2025. These regulations are expected to cut automotive emissions by millions of tons a year, and reduce U.S. oil imports by billions of barrels over the life of the program.

“We can build these cars now,” the Alliance of Automobile Manufacturers told The New York Times recently. But doing so will necessitate dramatic advances in “manufacturing technology and vehicle weight reduction, and laser welding will be crucial to achieving those goals.”

A laser makes a weld between two metals by vaporizing a “keyhole” in the upper metal and boring through the metal vapor to the metal below. “In some laboratory experiments, you can actually see right through the keyhole during the laser welding,” says Stan Ream, laser technology leader at the Edison Welding Institute in Columbus, Ohio, U.S.A. The metal vapor subsequently cools rapidly, forming a “nugget” of solidified metal that is the weld. Laser welds can be created in different geometries. Lap welds are easier to create because alignment is less critical, but butt welding is often required in order to meet the specifications of the finished part.

There are myriad uses of laser welding in many applications, from oil and gas exploration and development to power generation, aviation, automobiles and rail transportation. Welding accounts for maybe 15 percent of the total sales of industrial solid state and CO2 lasers, according to David Belforte, editor of Industrial Laser Solutions magazine. Altogether, roughly 40,000 of these lasers are sold in a year, so laser welding accounts for approximately 6,000 lasers annually. Between CO2 and solid-state lasers, the split is about three-quarters in favor of solid state for welding applications, Belforte says.

 

figureLaser welds are called “keyhole” welds because the laser vaporizes a narrow keyhole through both metals. The vapor subsequently resolidifies to form the weld nugget.

Automotive industry

Volvo has been one of the leaders in incorporating laser welding into its manufacturing strategy. The current version of the Volvo C70, the company’s small, retractable-hardtop convertible, has more than ten meters of laser welds in its body. Compared to the previous version of the C70, manufactured using conventional welding techniques, the torsional stiffness of the new model has been boosted by 250 percent.

Alignment of the parts prior to welding at the Volvo factory in Uddevalla, Sweden, takes significantly more time than the welding operation itself. The 4-kilowatt, lamp-pumped Nd:YAG laser is an expensive capital investment. Thus, to maximize its efficient usage, its output is shared among four fiber-optic delivery fibers, each feeding a separate workstation. While the laser power is being delivered to one workstation, parts are being aligned at the other three. Thanks to this arrangement, the laser is performing a welding operation 82 percent of the time.

ThyssenKrupp Tailored Blanks GmbH in Duisburg, Germany, has recently extended the concept of tailored blanks (see sidebar) to produce tubular parts instead of flat tailored blanks. Dubbed “tailored orbitals” by the company, these components are manufactured by first laser-welding several steel sheets together and then wrapping them into a tubular shape. Tailored orbitals are especially suited for exhaust systems because they can be designed to withstand the hot, corrosive gases in an automobile’s exhaust.

Elsewhere in automotive exhaust systems, catalytic converters also gain strength and corrosion resistance from laser welding. Eberspaecher North America, an auto-parts supplier in Brighton, Mich., U.S.A., has replaced its conventional TIG (tungsten-inert-gas) welding station with a system built around a CO2 laser. Although the laser’s capital cost is 30 percent greater than the TIG welder’s, its throughput is 80 percent greater and the finished converters are more reliable. Like the Nd:YAG system in Volvo’s Swedish facility, the Eberspaecher laser’s output is coupled to more than one work station, so a weld is made at one station while parts are loaded and aligned at another.

Electronics

Knowledgeable people tend to regard the automotive industry as the largest user of laser welders. However, according to data compiled at Industrial Laser Solutions, the electronics and medical-device industries far exceed the automotive industry. Dave Belforte, the magazine’s editor, notes that, even though Volkswagen is installing many new laser welders in its plants around the world, electronics is still a larger market, at least in terms of units. And for manufacturing electronics, lasers offer a precise and fast technique of joining components.

Batteries represent a fast-developing technology that is becoming crucial, not only to power our many electronic gadgets—laptops, smart phones, e-readers and so on—but also to provide the electricity that will drive the thousands of electric cars that will appear on highways in the coming decade; indeed, an increase in these electric vehicles will contribute to the 54.5 miles-per-gallon goal. The United States is funneling hundreds of millions of dollars into battery research, and laser welding is emerging as a key technology in manufacturing new batteries.

 

figureIn manufacturing an automobile’s side panel, a flat tailored blank is created by laser-welding together steels of different strength and thickness. After the flat tailored blank is welded together, it is formed by stamping, bending and cutting into its final shape, in this case as an automotive side panel.

Tailored blanks: How laser welding results in more fuel-efficient cars

Lasers make many welds in automotive assembly lines, but they have probably had the biggest impact on the automobile industry in the widespread utilization of tailored blanks. A technique imported from Germany in the early 1990s, tailored blanks are now used by a vast majority of car makers, producing in excess of 30 million units annually.

A tailored blank is a flat sheet of several metals of differing thickness, strength and coating, laser-welded together to create a single unit. The blank is designed to have the heaviest, strongest steel where stress is greatest, and lighter metals where less strength is needed.

Laser welding of tailored blanks, rather than other welding techniques, is preferred in the automotive industry for several reasons. Because the welding energy is delivered with pinpoint accuracy, laser welding produces a smaller heat-affected zone, resulting in a flatter, stronger weld, according to Jim Degen of the Degen Development Group in Clarkston, Mich., U.S.A.

The tailored blank is more suitable for stamping and other forming operations that follow. Moreover, laser welding is capable of nonlinear welds, and it uses no consumables such as filler wire or cover gases, so it’s less expensive than conventional welding techniques. Fiber, CO2, Nd:YAG and disk lasers are all used in manufacturing millions of tailored blanks every month.

Tailored blanks result in vehicle weight reduction—and thus better fuel efficiency—because heavy materials are used only where they are required. Likewise, expensive metals are used only where necessary, reducing both the cost of materials and the amount of scrap. And because high-strength steels can be deployed exactly where they are most needed, overall vehicle strength and crash performance are improved.

Further cost reduction occurs because only one large part is stamped, instead of many smaller parts. This reduces the number of die sets needed, saving tens or hundreds of thousands of dollars per die set. And finally, because laser butt-welding reduces the number of overlapping joints, corrosion resistance is improved.

Lasers have proven particularly adept at joining dissimilar materials in batteries, whether they are electrodes, conductors or the battery casing itself. Copper-to-aluminum welds, for example, which are common in batteries and difficult to do, are straightforward with a fiber laser.

Efficiently manufacturing sophisticated batteries, each needing many hundreds of welds, requires high-speed welding. The low beam divergence of a fiber laser makes it particularly well suited for galvo-scanning welding, where a galvo-mounted mirror directs the beam to the parts being welded. Conventional welding speeds are limited by the welding head’s inertia. However, by scanning the beam from one weld to the next—without moving anything but the galvo mirror—one can attain welding speeds of hundreds of millimeters per second.

Nd:YAG lasers and most fiber lasers produce a beam whose wavelength is around a micrometer, but that wavelength is highly reflective from conductors like copper and silver. Because most of the light is reflected off the copper parts being welded, it can take many times more laser energy to weld a copper part than to make an equivalent weld with a steel part. Worse, the reflectivity decreases markedly when the conductor becomes molten. That sudden change in reflectivity makes it very easy to overheat a weld, vaporizing it and damaging surrounding components.

However, engineers at Miyachi Unitek in Monrovia, Calif., and elsewhere, have capitalized on the lower reflectivity of these conductors at green wavelengths. The reflectivity of copper, for example, drops from 90 percent at 1.06 µm to 45 percent at 532 nm. By frequency doubling a normal-mode Nd:YAG laser (i.e., one that’s pulsed by the flashlamp, without a Q-switch), they have demonstrated successful and repeatable welds of copper conductors. “The 532-nm pulsed Nd:YAG laser welds copper as well as a one-micron laser welds steel,” notes Miyachi engineer Geoff Shannon.

Laser micro-welding is also vital to the manufacture of the hand-held electronic devices powered by these batteries, as well as to the fabrication of the thousands of micro-sensors that are turning up in everything from automobile engines to refrigerator doors. The traditional techniques of joining conductors, such as soldering, crimping or brazing, become impractical as conductor dimensions shirk to a few thousandths of an inch.

Welding is more reliable, but common welding techniques are problematic. Ultrasonic welding, where acoustic vibrations at hundreds of kilohertz cause a plastic deformation rather than melting, is of very limited utility because it requires access to both sides of the joint. Common resistance welding can damage the delicate parts being welded. However, laser welding requires access to only one side of the joint, and the laser energy delivered to the weld can be dispensed in precisely measured amounts.

Welding plastics

Joining nonmetals, and plastics in particular, is a new and growing application of laser welding. Today, plastic welding accounts for maybe 15 percent of the market for laser welders, estimates Belforte. However, plastics are replacing metal in a growing list of consumer goods, from automobiles to electronics, and the reasons are many: cost savings, weight reduction, corrosion resistance and electrical isolation, among others. As plastics appear more and more frequently in manufactured goods, the need for plastic welding will grow proportionally.

Nonmetals such as plastic have traditionally been welded by friction or hot-plate technology. A major drawback of these techniques is that they require contact between the tool and the part being welded. This contact often causes a mechanical load on the part, and also results, too often, in residual material adhering to the tool. Laser welding, which is of course contactless, avoids these problems.

Welding plastics is a fundamentally different process than welding metals. In metals, the laser beam vaporizes its way through the metals, and the vapor resolidifies to form the weld nugget. In welding plastics, the laser light passes through the transparent top plastic and is focused on the absorbent plastic underneath. The beam generates heat at the interface, which is conducted both upward and downward, softening both plastics. The plastics mix and, when they re-harden, form the weld.

Welding plastics can be a more delicate operation than welding metals, because plastics burn easily if they are overheated. Maintaining temperature within the appropriate range requires balancing the laser power and welding speed with the absorbance of the plastic. Often, this balance requires a closed-loop system where a pyrometer-galvoscanner monitors the weld temperature and generates a signal that controls the laser power applied to the weld.

The signal from the pyrometer can also provide a quality-control parameter by detecting defective welds. One of the more common failure mechanisms is a bubble formed during the heating process. The bubble will produce a spike in the pyrometer signal, an indication that the part should be rejected.

Another useful quality-control technique involves monitoring the Fresnel reflection from the index discontinuity between the two plastics being welded. Prior to welding, a low-power probe beam will be clearly reflected from the interface, but when the weld is made, the plastics mix and the index discontinuity disappears. “The use of reflection diagnostics will rise in the future, because that technique delivers very safe results,” says Marika Nitscher of LPKF Laser & Electronics in Erlangen, Germany.

Applying heat to the interface between the two plastics being welded requires that the top material be transparent and the bottom absorbent. That becomes challenging when both materials are the same color, or both are transparent. The European Union’s “PolyBright” program has done much to address this issue in recent years, developing plastics additives that absorb infrared radiation without changing the plastic’s apparent color or transparency. As an added advantage, the absorption depth in the plastic can be customized by adjusting the concentration of the additive. Another degree of freedom can be obtained by varying the wavelength of the diode laser making the weld, bringing diode lasers with wavelengths out to 1,940 nm into play.

 

 

figurePlastic and Globo welding. (Left) In plastic welding, the laser beam is absorbed at the interface between a transmissive plastic on top and an absorbing plastic beneath. Heat is conducted into both plastics, which soften and mix to form the weld. (Right) For complex, three-dimensional welds, the “Globo welding” technique forces the parts together with a pneumatically supported glass sphere, which also serves as a lens to focus the fiber-coupled, diode-laser radiation onto the joint. A computer-controlled robotic arm rolls the welding head along the joint.

The Gentex Corporation in Gross-Umstadt, Germany, has developed an alternate technique, applying an infrared-absorbing film to the plastic substrate. It typically takes the liquid, which is sprayed onto the surface at a rate of several nanoliters per square millimeter, several seconds to dry, but that delay can be reduced by pre-heating the part or by blowing air or nitrogen across the surface. The coating may have a visible tinge, but it decomposes when the laser hits it, and the color disappears.

Automotive taillights and other large, contoured, injection-molded parts pose a special challenge to laser welding. While conventional welds are one-dimensional or occasionally two-dimensional, these are long welds—typically a foot or more in length—in three dimensions.

Addressing spatial gaps

A further complication is introduced because dimensional tolerances in injection-molded parts cannot be held tightly, so there may be a significant spatial gap between parts being welded. The gap impedes heat conduction between the parts and undermines the quality of the resulting weld.

The conventional solution to the gap problem is to clamp the parts in a complex, many-fingered clamping jig. Such jigs are expensive to design and build, and the alignment of parts in the jig adds precious seconds to the manufacturing process. An appealing alternative has been developed by Huf Tools GmbH in Velbert, Germany. A technique they call “Globo welding” involves a glass sphere that is pneumatically forced against the part being welded. The force pushes the parts together, ensuring a good thermal contact when the weld is made. The sphere also serves as a final lens to focus the laser light onto the joint. This physical arrangement guarantees that the clamping force is always orthogonal to the weld direction, everywhere along the three-dimensional weld.

Regardless of how the parts are forced together while the weld is being made, there can be significant residual stress in the final welded part. A hybrid welding system, developed at LPKF Laser & Electronics AG and the Bayerisches Laserzentrum GmbH, both in Erlangen, Germany, can reduce that stress by thermally softening the parts as the weld is being made.

 

figurePlastics to ceramics. Welding plastics to ceramics is a two-step process. First, a pulsed laser ablates pits in the ceramic, then the plastic is laser heated and flows into the pits, creating a strong weld.

In this system, a pair of halogen lamps is nestled into the welding head alongside the laser’s beam-delivery optics. The infrared radiation from the lamps is focused on the weld with elliptical reflectors. Although the lamps deliver 160 W compared to the laser’s 5 W, the intensity of the laser at the joint is an order of magnitude greater than that of the lamps. Without the lamps, laser welding cannot be accomplished reliably when the gap between parts is greater than 0.1 mm. But with this hybrid welding system, gaps as large as 0.4 mm can be tolerated.

Joining plastics and ceramics

Joints between plastics and ceramics are required in a host of devices, from microelectronics to piezo actuators, and in all sorts of sensors, including those that measure conductivity, pressure, pH, resistance and mechanical stress. Historically, joints between these materials have been made with adhesives or with mechanical fasteners such as screws or clips.

However, both of these approaches have disadvantages: Adhesives can be chemically active in some environments, and they have long-term stability issues. Screws and clips, on the other hand, require extra steps such as drilling and tapping holes. Moreover, mechanical fasteners require gaskets for sealed—i.e., liquid- or gas-proof—joints, and gaskets have inherent issues with long-term stability.

At first glance, laser welding plastics and ceramics would seem impossible, because the materials have vastly different melting temperatures. Most plastics melt at several hundred degrees Centigrade, and decomposition begins at 400° C or 500° C. Ceramics don’t melt until their temperatures are well over 1,000° C.

But investigators at the Fraunhofer Institute for Material and Beam Technology in Dresden, Germany, have developed a novel and apparently successful technique for laser welding plastics to ceramics. They first pattern the ceramic, using a pulsed laser to ablate tiny holes and pits in the surface. The plastic part is then positioned on top of the patterned ceramic and laser-heated so it melts and flows into the pits in the ceramic, forming a strong weld. In Fraunhofer’s tests, such welds have shown tensile strength up to 25 N/mm2.

Challenges associated with laser welding

Despite the numerous benefits of laser welding, it finds far fewer applications than any other industrial laser process, including drilling, cutting, marking or heat treating. Why? Not mincing words, Ream explained recently in Industrial Laser Solutions magazine, “Laser welding is hard to do.” There are two primary difficulties, Ream says. First, there are seemingly straightforward mechanical problems: The joint must be adequately prepared and aligned, often with more precision and speed than in other welding techniques.

The second difficulty has to do with how the welded materials react to the intense heat and the rapid heating and cooling rates associated with laser welding. Some metals, especially conductors, reflect too much laser light, while some plastics absorb too little. The absorption of some plastics depends on which direction the laser is moving across the surface, and the absorption of some metals changes as a function of temperature.

Looking forward

Despite the inherent difficulty of laser welding, it still holds much promise for the future. It can make many welds faster and cheaper than conventional joining technology. And as laser technology in general, and fiber-laser technology in particular, advances—leading to cheaper, more efficient lasers producing higher-quality beams—the usefulness and acceptance of laser welding can only increase.


Breck Hitz is the president of Photonetics Associates, which provides professional training in laser and fiber optics technology. He is located in the San Francisco Bay Area.

References and Resources

>> S. Ream. “Why so little laser welding?,” Industrial Laser Solutions, Feb. 20, 2009.
>> G. Shannon and P. Severloh. “Laser micro-welding of conductive materials,” Industrial Laser Solutions, June 1, 2011.
>> A closer look a tailored blanks: www.twbcompany.com/Applications.html; Also see: http://weldingdesign.com/consumables/news/wdf_10713/.
>> Introduction to laser welding thermoplastics.
>> The basics of laser welding.
>> EU research aims to perfect laser-stabilized plasma welding, welding design & fabrication, July 12, 2011.

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Keeping It Together with Laser Welding

Laser welding is faster and cheaper than conventional joining technology, and it may even lead to lower greenhouse-gas emissions. Although laser welding is by no means easy to do, technological advances are making it an increasingly viable option within various industries, including transportation, electronics and sensing.

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