Industrial Laser Solutions - May/June 2015

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Wire stripping Engraving rollers Drilling ceramics Cleaning aluminum Processing mobile devices Cutting brittle materials

Cutting abrasive discs

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Features technology report

CO2 lasers advance next-generation abrasive disc technology

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Process provides health and safety advantages for work environment JOHN DILLON

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Laser processing enables smaller, faster mobile devices

application report

Laser applications for printing and embossing

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application report

Femtosecond laser processing of brittle materials Process can machine glasses and sapphire with high quality VICTOR MAT YLITSK Y, FR ANK HENDRICKS, and R AJESH PATEL

application report

Fiber laser micromachining in high-volume manufacturing High beam quality allows for small spots MARCO MENDES, ROUZBEH SARR AFI, JOSHUA SCHOENLY, and ROY VANGEMERT

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application report

Micromachining technologies yield high-volume production HAIBIN ZHANG

What do a beverage can, a banknote, and the interior of a car have in common? MARKUS BOHRER

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A dual-laser abrasive processing system produces high-density micro-holes for abrasive discs. (Image courtesy of Preco, Inc./Tyler Belisle)

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application report

Laser stripping for the medical and electronics industries Automated laser processes increase production efficiency, process control, and product innovation GEOFF SHANNON

technology report

Laser cleaning prior to laser welding of aluminum alloys Process reduces porosity formation in laser-welded aluminum alloys LIN LI

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Departments Update

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New CO laser offers highpower, 5–6µm output 4 Cutting narrow bars in thin metal foils 8 Can your coders keep up? 10 EMLACS project explores laser-assisted cold-spray material deposition 39 C a l e n d a r

DABbling

A blog by DAVID A. BELFORTE David shares his insights and opinions on current activities affecting industrial laser materials processing. www.industrial-lasers.com/dabbling.html

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39 A d I n d e x 40 My View

Five decades of laser light in automotive

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update New CO laser offers high-power, 5–6µm output Carbon monoxide (CO) laser technology was developed in the mid-1960s at the same time as CO2 lasers. The CO laser was attractive because it had the potential to be 2x more effcient than CO2 lasers. However, the CO laser presented challenges: Early CO lasers needed to be cooled to get high powers with high effciency (very high-power versions were actually cryogenically cooled) and the laser output power would degrade quickly; typical lifetimes were tens of hours at most. Consequently, CO2 laser technology “won” and has been the gas laser standard since.

composites. Very low attenuation of the 5–6µm laser light in chalcogenide and heavy metal fuoride fbers open the potential for fber delivery. One application in which this difference in absorption coeffcient has a signifcant impact is in glass cutting. In CO2 laser-based glass cutting, the 10.6µm output is absorbed very strongly at the surface. The heat generated at the surface must then diffuse into the bulk material; subsequent water jet cooling is then used to produce a thermal shock, which creates a scribe line in the glass. For thicker glass substrates, this is followed by mechana) b) ical breaking. The overall process is the same with the CO laser; however, glass absorption of the 5µm output is much lower. Thus, the light penetrates directly and further into the bulk material, inducing heating more evenly throughout the thickness of the glass. Testing at Cross-sections of 0.7mm-thick Corning CT24 glass, taken with a Nomarski Coherent has shown this to differential interference contrast microscope. The piece cut with a CO2 (~10µm) produce several important laser (a) shows residual stress, while the cut produced with the CO (~5µm) laser benefts, including no surface (b) is defect-free. melting, no cracks, and zero residual stress in the glass. Coherent has developed proprietary technology that The result is a better-quality cut, stronger glass, and a allows for CO lasers that operate at very high output powers wider cut process window (FIGURE). in the 5–6µm range at high effciencies at room temperaAnother important advantage is that the CO laser ture and that last for thousands of hours. The new Coherenables radial (free-form) glass cutting. In contrast, CO2 ent CO laser employs much of the same technology devellasers can only produce straight-line cuts because the oped over many years for Coherent’s CO2 laser products. inherently round output beam must be reshaped into a Coherent is developing CO lasers using waveguide and long, thin line beam to distribute the heat generated at the slab designs that will offer high average continuous-wave surface. Curved cuts are particularly important in smart(CW) power and high-peak-power pulsed operation. Typphone display applications because curved corners or ical output power for CO lasers yields roughly 70 percent shaping to accommodate buttons and controls are often of that of a CO2 laser. The CO laser version of the Coherent required. Curved and free-form cuts are possible with the J-3 laser, for example, produces roughly 230W at ~5µm (at CO laser, where the round beam penetrates directly into room temperature), while the same J-3-based CO2 laser the glass without adverse heat effects caused by the difproduces 340W at 10.6µm. fcult-to-control diffusion process. There are many benefts when materials processing at The other major beneft of the 5µm wavelength of the 5µm, the foremost of which is the “light-material” interaction CO laser output is that it can focus to a much tighter advantage. At 5µm output, it will have signifcantly different spot size with a longer depth of focus that allows drillinteractions compared to CO2 laser output simply due to the ing smaller holes and cutting with narrower kerf widths. different absorption coeffcients. It has stronger absorption The smallest spot size that can practically be achieved in many flms, polymers, PCB dielectrics, ceramics, and for a CO2 laser is roughly 55µm, where the limit for the

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Industrial Laser Solutions MAY/JUNE 2015

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update CO laser is less than 25µm. On the other hand, achieving the same spot size with both wavelengths can be accomplished with smaller optics at the shorter wavelength. This in turn allows for smaller/ faster galvo mirrors and other advantages in optical system design. The longer focal length that can be used with a shorter wavelength leads to a larger process window, allowing a larger feld of view for area processing. Another signifcant glass application is micro-hole drilling, such as that required in interposers for 3D circuit packaging. This application again takes advantage of both the focusability and controlled light-material interaction. Here, very small holes can be drilled with depth control and no heat damage/cracking. The introduction of this new laser technology, enabling a new wavelength, is expected to open brand-new applications and enhance many current applications across the materials processing market.

Aerotech’s New Nmark AGV-HPO

Cutting narrow bars in thin metal foils JENA, GERMANY –Long,

narrow structures in the micron scale in thin metal foils of 0.5mm) is deposited

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with no thermal defect in the substrate. The deposited layer can be directly machined or reworked. The main advantages of low-pressure cold gas spraying are the lack of heat input, high processing speed, and low investment cost. New material combinations are especially promising in automotive and aeronautics. The main challenge in this technology is the adherence of the frst layer on the workpiece. The aim of the EU research project EMLACS will improve adhesion on different substrates by using high-speed laser surface structuring with integrated nanosecond and picosecond lasers with low-pressure cold gas spraying. New material combinations can then be developed for industrial use. www.industrial-lasers.com

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update The deposition of metallic materials (copper or aluminum; FIGURE) on CFRP and glass fber-reinforced plastic (GFRP) sub-

strates is being investigated, which has already created signifcant interest in the aeronautic and automotive industries. In addition, the new technology can be applied in novel ways in electronics manufacturing. As an example, Cold Gas Spraying may deposit a copper layer on a non-conducting housing for fanless heat removal from electronic components. The project team is composed of French, Dutch, and German partners. Dycomet Europe (Netherlands) brings cold gas spraying expertise, Edgewave (Germany) delivers high-power short-pulsed laser technology, and Industrial Laser Systems (France) is acting as the system integrator and coordinator of the project. Research teams from Université de Technologie de Belfort-Montbéliard (UTBM; France) and the Fraunhofer Institute for Laser Technology (ILT; Germany) are developing the process. The EMLACS project (reference number 606567) has been running since June 2014 under Research for SMEs – FP7-SME-2013 and has been funded by the Research Executive Agency (REA) for 24 months.

High-speed deposition of copper on aluminum segments uses laser-assisted cold spraying. (Image credit: Dycomet)


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This article was written by Gail Overton, senior editor for Laser Focus World.

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CO2 lasers advance next-generation abrasive disc technology PROCESS PROVIDES HEALTH AND SAFETY ADVANTAGES FOR WORK ENVIRONMENT

JOHN DILLON

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n the early days of industrial CO2 lasers, a fair amount of attention was focused on pursuing laser cutting abrasive (sandpaper) discs (FIGURE 1) as a viable market. The logic was clear, as a laser solution would eliminate the need for hard tooling. However, the economics were not favorable, as the capital cost of implementing laser technology was just too high and the production rates were too slow when compared to existing die-cutting solutions. Additionally, those die-cutting processes were well established and there were no compelling reasons to pursue a laser solution because die cutting did not present challenges that a laser process would solve. In recent years, that has changed. This article reviews the history of producing abrasive sanding discs from well-established die-cutting practices to a laser process required to meet new demands placed on abrasive disc manufacturers. Abrasive disc production

Abrasive material is produced in 48-in.-wide or wider master rolls, with the abrasive grit on one side of the roll substrate and a means of attachment to the backup pad on the other. The attachment methods are typically a pressure-sensitive adhesive or, more commonly, a polymer ‘loop’ material that will physically bond to the ‘hook’ surface of the backup pad (think Velcro). While the material handling systems for die cutting and laser cutting are different, they all require the ability to unroll the incoming material, process the material by performing a periphery through-cut and, if needed, produce dust-extraction holes on the disc surface, singulating the disc from the roll, removing the unused waste skeleton, and stacking the discs into counted piles for insertion into boxes for purchase by the end user. www.industrial-lasers.com

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FIGURE 1. A laser-cut, multi-hole abrasive disc is attached to a backup pad for use on a dual-action pneumatic sander. (Photo: Norton|Saint-Gobain) Abrasive disc production has always been the domain of flat-bed die cutting, and for good reason. Even though lasers offered advantages, die cutting was (and in many cases remains) the most cost-effective means of production, especially for discs that only require a periphery cut or a periphery cut with large dust-extraction holes. Flat-bed die systems use an upper platen that is attached to a hydraulic ram. The steel rule die is secured to the platen and once the hydraulic ram is actuated, the steel rule penetrates the abrasive material and the part is produced. These systems are capable of production rates up to 60 strokes/min. Additionally, if the die board is steel-ruled with three 6-in.-diameter discs, it is possible to produce 180 discs/min, resulting in high production rates for a nominal investment. A steelrule die also has a long life and low cost because the die only costs a couple-hundred dollars and it self-sharpens while die cutting the abrasive. With this background, it is easy to see MAY/JUNE 2015 Industrial Laser Solutions

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why die cutting has been the process of choice for most abrasive disc-processing applications. Laser processing advantages

So, what is causing the shift to laser processing? A couple of reasons include

FIGURE 2. Three abrasive discs illustrate the progression from no holes, to 10mm-diameter holes, to high-density, 1.5mm-diameter micro-holes for dust extraction. (Photo: Norton|Saint-Gobain)

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the requirement for improved work environments in addition to improved debris extraction for critical finishing applications. This is especially true for the automotive body shop market. As the formulations for automotive finishes evolved, the

use of dust-free holes in the abrasive disc has been a primary focus of abrasive manufacturers. The first-generation dust-free holes were fairly large in diameter, being the same size and number as the extraction holes in the backup pad, typically 6 to 8

FIGURE 3. The latest-generation, high-density micro-hole abrasive discs with matched backup pad technology. (Photo: Norton|Saint-Gobain)

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t e c h n o l o g y holes that are 10mm in diameter (FIGURE 2). The 10mm slugs are removed by allowing the slugs to be captured in the punch that produces the hole, then extracted through the punch. While this strategy works well, it is not 100% effective and slugs can remain in the disc, which requires manual inspection and removal. In use, the 10mm-diameter holes offer a means of dust extraction that was not available before; however, they are far from ideal, as dust is only extracted in the area around the holes and not the entire disc surface. Additionally, alignment of the abrasive disc holes to the extraction holes in the backup pad is critical and any misalignment by the operator will have a detrimental impact on dust-extraction efficiency. As the number of dust-extraction holes across the abrasive disc increased, not only did the dust extraction rate improve, but improved cut performance and longer disc life resulted. The drive to produce even smaller holes resulted in the only viable production option left—industrial

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CO2 lasers. The results have been dramatic, resulting in strong market acceptance of this product offering by abrasive

manufacturers. The latest-generation discs have well in excess of 300 holes per disc, with each micro-hole being approximately 1.5mm in diameter (FIGURE 3). There are a multitude of advantages with laser-processed holes. The dust-extraction rate is almost 90% more effective than the original 6- or 8-hole design, so the particulate extraction provides an improved health environment as well as reduced surface contamination for the operator. An inherent drawback of the die-cutting process with abrasive discs is the deformation induced by the die-cutting process into the abrasive surface (FIGURE 4). This deformation results in reduced cutting area, as the disc surface is not flat. The non-contact laser process

FIGURE 4. The photo on the left shows a die-cut disc after use with the inherent deformation caused by die cutting. The disc on the right shows how the laser-cut disc remains flat after use for maximum life, performance, and dust extraction. (Photo: Norton|Saint-Gobain)

The new 4th generation Digi-Cube II is the UK designed and manufactured Digital Laser Scan Head from Laser Control Systems that's not just ahead of its time - it's the future of laser marking. This new compact version of the original and innovative Digi-Cube sets whole new standards for speed and reliability and comes with a No Hassle, 2 Year Warranty as standard – and its IP55 rated. The Digi-Cube II offers enhanced digital, super fast, scan head performance for less than the cost of most analogue models and even the much more expensive digital scan heads cannot begin to match its performance. Its DSP technology rapidly computes the exact drive impulses required to achieve the smallest possible mirror movements to deliver 1000 impressions per second. The new unique Digi-Struct Software, developed specifically for the Digi-Cube, offers users a choice of up to 256 programmes when downloaded on to a key board operated control box, dispensing with the need for a laptop on the shop floor - saving time, eliminating programming error and preventing any unauthorised interference with the laser marking programmes. For more details on this remarkable and innovative Digital Scan Head and Digi-Struct Software, see us at Laser World of Photonics, Munich, Stand 135/3 Hall B2 or call +44 (0) 1462 813236 or e-mail [email protected]

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eliminates this drawback and the motion of flat-bed presses. The Die punched holes cause deformation result is increased surface area challenges with the laser process d contact with over 30% productivare with the web handling equipity improvement when compared ment and developing a stable prod0 to lower-density die-cut holes cess to produce 100% slug-free (FIGURE 5). An added benefit is holes, registering the disc periphSurface area that the operator no longer needs ery to the holes and handling the to take time to align the abrasive discreet discs once they are cut Multi-air cyclonic laser-cut discs are fat and true disc extraction holes with the holes from the web of abrasive. d in the backup pad. The high-denProducing 100% slug-free holes sity hole pattern allows the operais critical because any ‘hanging tor to place the disc anywhere on chad’ left on the top or bottom disc Surface area the backup pad, resulting in greater surface has the potential to damproductivity. age the surface being prepared FIGURE 5. This illustration shows the additional effective Producing these next-gen- surface area that is gained with a laser solution. (Illustration: for painting, resulting in excessive eration abrasive discs is a fairly Norton|Saint-Gobain) reworking of the area. With proper straightforward process from a management of the slug removal laser and motion system standprocess, it is possible to attain this point. High-powered, diffusion-cooled CO2 lasers integrated with demanding requirement. The dust-extraction holes are produced large-area galvanometer motion systems provide an ideal platform first in a cutting zone specifically designed to keep the raw, abrafor processing the discs. The abrasive rolls are typically produced sive stock flat during processing, but still allowing for efficient slug in a wide web, so it is typical to have multiple laser/motion sources removal in the zone. Registration of the abrasive web is maintained across the roll to increase productivity. An additional advantage with as the material transitions into the periphery cutting zone, where laser technology is its ability to process the abrasive discs with the the laser through-cuts the disc periphery and separates it from web in continuous motion instead of the less-productive indexing the abrasive web. As the now-completed disc is separated from the web, it can drop onto a conveyor for removal from the cutting zone. The singulated discs can then be offloaded from the conveyor manually and boxed; however, this is a labor-intensive process that can be easily automated with robots. A single high-speed robot and vision system can rapidly identify the disc on the conveyor for picking and placing onto a counted stack of discs. The only manual process required is to box the counted discs and they are ready for distribution.

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Conclusion

Lasers have often been referred to as a solution waiting for an application. In the case of using lasers to produce advanced abrasive discs, this was certainly the case. For many years, lasers were a possible production solution—just not an economically viable one. However, as the demands for product performance advance, many times the solution can be found in the unique advantages that a laser process can offer. In this case, a laser now provides a solution that results in greatly improved performance while improving the health and safety of the work environment. This application follows the path of many industrial laser applications, where demands for increased product performance can only be found with a laser solution. Growth in the laser market is being driven not only in the metals market but also in the paper- and polymer-converting market, where lasers offer a cost-effective solution for next-generation, high-performance products entering the market. ✺ ACKNOWLEDGEMENT

Velcro is a registered trademark of Velcro Industries B.V. JOHN DILLON ([email protected]) is VP of marketing/key accounts at Preco, Inc., Somerset, WI; www.precoinc.com. www.industrial-lasers.com

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Laser applications for printing WHAT DO A BEVERAGE CAN, A BANKNOTE, AND THE INTERIOR OF A CAR HAVE IN COMMON?

MARKUS BOHRER

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he industrial market for processing large-scale films has seen dramatic changes since the 1980s and has almost completely been replaced by lasers and digital processes. A commonly used technology for engraving screens in the printing industry, well known since then, is the use of RF-excited CO2 lasers with a beam power up to about 1kW modulated in accordance to the pattern to be engraved (FIGURE 1). A mesh is covered with a thin polymer layer and the modulated laser beam engraves this layer, where holes in the mesh have to be opened. This is a very efficient way to produce printing plates and cylinders, especially when it comes to high-volume printing. Almost all printed textiles, carpets, wallpapers, and some features of banknotes use this technique. Direct modulation of CO2 lasers is limited to about 10kHz, which is mainly due to metastable nitrogen—a major part of the laser gas mixture. Current printing technologies used in tube and can printing demand a much higher pulse frequency of some-hundred kilohertz. The reason for that is, to a lesser extent, the higher resolution rather than the need for real 3D structures in the material. Whereas engraving meshes is basically a 2D process, engraving printing plates and rollers of polymer or rubber is a 3D engraving process with complex structures. Each direct engraved structure needs a solid foundation for stabilization during printing and may have a sophisticated geometry on top, such as a well-defined plateau and an undercut to compensate for dot gain. Future needs for high-security printing (banknotes, security papers, passports, etc.; FIGURE 2) will require at least half a megahertz or even more, and industry now wants photorealistic pictures in packaging design, which requires a similar performance. Acousto-optic modulators (AOMs) offer the possibility to control the laser beam in a much faster way than by direct modulation of the discharge of the laser with the RF sources. But AOMs have limits due to their absorption in the germanium

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FIGURE 1. Laser engraving screens, where print color is squeezed through the mesh.

FIGURE 2. Laser-engraved print rollers for banknotes. www.industrial-lasers.com

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and embossing crystal and their damage threshold. To get the best out, the AOMs, the laser source, and the beam path have to be well designed. All state-of-the-art lasers are tested, especially concerning pulse behavior, power stability, pointing stability, and mode. The rise and fall time determines the pulse behavior and, thus, the engraving speed. The nitrogen in the gas mix slows down the pulse frequency to around 10kHz. This was sufficient for many applications in the past, but it is not enough for the future. A typical laser power vs. time diagram shows deviation values between ±5 and 10%. This is absolutely not suitable for a controlled 3D engraving into the depth of the material. Laser pointing stability is surprisingly good for various tested lasers and has a direct impact on the use of AOMs, which are sensitive on incident angles. Near the power limit of the AOM, the germanium crystal is very sensitive to bad laser modes. Hot spots cause a distortion of the outgoing beam and can easily destroy the crystal. A behavior of many CO2 lasers is the bad mode in the near-field. Typically, the distance between the output coupler and the AOM should be around 2 m or more, which results in a much better mode (FIGURE 3). This is sometimes difficult to realize, especially in a compact engraving machine. The new CO2 laser project

An obvious choice was to achieve a highly stable resonator and a close-to-perfect beam mode, using a “classic” folded CO2 laser a)

b)

16128 14336 12288 10240 8192 6144 4096 2048 0

FIGURE 3. A CO2 laser at full power at a distance of 1 m (a) and a distance of 6 m (b), represented in 2D (left) and 3D (right). www.industrial-lasers.com

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FIGURE 4. A carbon-fiber CO2 laser set up with two IR cameras (blue) before and after the AOM (yellow). with modern materials like carbon fiber for the resonator structure. Carbon fiber tubes can have a very small thermal expansion coefficient (less than 1µm per meter and kelvin), especially when well designed. FIGURE 5. A CO laser with a carbon-fiber 2 Design includes resonator (background) and hexapod for intensive finite-ele- optimizing the AOM (left) on top. ment method (FEM) calculations to optimize thermodynamical behavior [1]. Beam path optimization. A custom-built carbon fiber optics table is used for high precision of the laser resonator and the entire setup for the beam path with the AOM and the infrared (IR) cameras (PyroCams) to visualize the beam mode online. A precise measurement of the influence of the germanium crystal—especially regarding distortions—can be made with two PyroCams before and after the AOM (FIGURE 4). Hexapod. The AOM with a germanium crystal offers a comparatively good performance with up to or even more than 600W of CO2 laser power, provided that the mode of the laser beam is close to the Gaussian shape. If the power gets too high and especially if hot spots on the crystal surface occur, it is easily damaged. Optimizing an AOM means shaping the laser beam to a balance between a small beam spot and an intensity that is still suitable for the crystal. The smaller the beam spots, the higher the pulsing frequency. Lateral and rotational optimization has to be achieved, which is quite an effort unless the pivot point for the two translational and three rotational movements can be shifted to the centroid of the incident beam on the crystal surface. A hexapod provides this feature in a perfect way and allows movement MAY/JUNE 2015 Industrial Laser Solutions

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in the range of some-tenths of nanometer. FIGURE 5 shows the central resonator distance bar of the carbon-fiber CO2 laser with the hexapod and the quick-change telescope. The beam path is lit up with a green laser pointer beam for pre-alignment. Experiments proved the comparatively large tolerance for AOM shifts and tilts and the high sensitivity for the beam mode. A bad mode immediately leads to severe distortions of the beam, which can be detected by comparing mode results from the PyroCam after the AOM with the previous one. Power stability is improved by a factor of 10 without any closed-loop regulation. This is a good basis for high-precision applications. Thermoelectric cooling directly for the AOM and for the resonator temperature regulation unit even improves stability.

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Industrial applications

After having optimized the laser source regarding power stability (less than ±1%) and beam mode to a perfect Gaussian shape, engraving results are very accurate and repeatable (FIGURE 6). The better the performance, the more applications can now be served effectively—especially for high-speed laser engraving rollers for embossing high-quality dashboards for the car industry, producing artificial leather, and for the previously mentioned high-security printing applications. Ongoing research was done to optimize the beam shape for the AOM to achieve faster pulses. Also, the fully closed loop control of the laser signal will be optimized and developed to an industrial state to ensure a trusted pulse. Now, what are the applications?

Cans. To get an idea of the can industry’s huge impact in the beverage market, one can look at the production of cans, which exceeds 1.4 billion cans per year—just in Germany! All major multi-billion can producers have changed designs from film exposed to high-speed, laser-engraved printing plates. This gives them the ability to adapt new designs in less than a day, from the idea to being ready to print dry offset plates. On the other hand, the high resolution of direct-engraved, 3D-structured dry offset plates allows photorealistic printing on cans, which will be seen in the stores worldwide in the near future. Typically, four plates for CMYK colors and some additional spot colors are used. And in spite of a breathtaking 3000 cans per minute produced and in-line printed in multi-color mode, engraving structures around 10µm

FIGURE 6. Laser engraving with high accuracy and repeatability.


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of the car, whether it’s an SUV or a sports car. And besides the leatherette dashboard application, cloth for door inserts as well as seat applications are an additional focus (FIGURE 8). Outlook

Today, direct laser engraving has certain limits regarding resolution and productivity. The combination of AOM and CO2, with its current FIGURE 7. Cans are a multi-billion production limitation of laser power (around industry. 600W), allows the production of around 1 m2 per hour for printing allow finest details—especially when it plates or rollers (at a 3D depth of 500µm), comes to kiss print (when the finest strucenough for many applications—but industures on the printing plate just smoothly try will demand more. And there will be contact the can in order to print the light more in the near future (e.g., multi-beam). colored areas). Engraving embossing rollers to achieve a pressed-in structure is one more trend in this industry. Amazingly enough, there is an additional trend from cans to bottles for wines. The three big can producers (Rexam, Ball Packaging, and Crown) together have more turnover than the whole laser industry; just imagine the impact of laser engraving applications within FIGURE 8. Synthetic leather on a dashboard. this single branch of consumer industry (FIGURE 7). Good-old film material will retire and lasers Cans are not the only application in the will take its place. food and beverage industry by far. Just Results from recent leatherette matelook at your next yogurt cup or its lid—it’s rial and embossing research will be spread very likely that a laser is responsible for its over many products in daily life (bags, walstate-of-the-art high-resolution print. lets) so that it will not be difficult to get into Automotive applications. The use of touch with laser engraving technology lasers for engraving in the automotive every day and even almost every hour. industry dates back to the early laser days. Think about it when you open the next The day/night design for many dashboard bottle of mineral water, as the banderole elements is a well-known and commonly is likely printed with a high-speed, laser-enused application. A current trend is to apply graved printing plate. ✺ sophisticated artificial animal patterns to the dashboard (usually a scan from real REFERENCE 1. M. Bohrer, “Ultra stable carbon fibre high power CO2 laser skins, expanded with complex algorithms with high quality laser beam and AOM implementation,” to a perfect-looking, non-repetitive surSPIE Photonics West 2015, paper 9343-61 (2015). face). Every car manufacturer has a distinct design and even the various series DR. MARKUS BOHRER ([email protected]) is the within an automotive company have very CEO of Dr. Bohrer Lasertec GmbH, Neusiedl am See, Austria. typical designs corresponding to the type

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Fiber laser micromachining in high-volume manufacturing HIGH BEAM QUALITY ALLOWS FOR SMALL SPOTS

MARCO MENDES, ROUZBEH SARRAFI, JOSHUA SCHOENLY, and ROY VANGEMERT

A

new generation of fiber lasers operating in the near-infrared (NIR) at 1070nm has unique properties such as high pulse energy with high peak power, high average power, and very good beam quality. Consequently, extreme high-power densities are possible, allowing for adequate coupling and high-quality machining of materials that are typically transparent at these wavelengths. These lasers, known as quasi-continuous-wave (QCW) ytterbium fiber lasers, can operate with variable pulse length in pulsed mode at high peak power and high repetition rate, as well as in continuous-wave (CW) mode at high average power. This translates into high-throughput machining, from drilling to scribing and cutting. QCW fiber lasers can be either single- or multi-transverse mode, which allows adjustment of the focused beam size as needed for the process. The beam quality of a single-mode fiber (14µm diameter) is very fine, with an M2 100 holes/s. (a) represents the exit, while (b) is a smooth cross-section.

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Drilling and scribing

High-speed via drilling can be done using a single laser pulse per hole by combining both the high peak power and somewhat long pulse duration of the QCW laser. FIGURE 1 shows ~20µm-diameter exit holes drilled on alumina (96% Al203) 635µm thick, at the rate of 300 holes/s and a pitch of 150µm, with the part moving under the beam at a linear speed of 45mm/s. The part was coated prior to machining and cleaned/polished after processing. Laser processing was done using the single-mode QCW fiber laser, with pulse duration around 200µs. The shorter the laser pulse duration used, the higher the maximum scanning speed can be without hole elongation; therefore, the higher the maximum drilling rate. However, there is an optimum peak power that leads to best hole quality, with optimum pulse energy and pulse duration. Typically, thicker materials require higher pulse energy and/or longer pulses needed to drill through. For example, drilling rates of 750 and 3000 holes/s are achieved on 381- and 100µm-thick alumina (99.6% Al203), respectively, for 20µm exit holes. For a similar via size, a drilling rate of 300 holes/s is achieved for 381µm-thick AlN. Due to its higher thermal conductivity, AlN requires a higher peak power and longer pulse duration to be used to drill holes compared to alumina at the same thickness. Note that similar drilling rates and high quality can be achieved for various types of materials in addition to ceramics, from semiconductors such as silicon to metals such as stainless steel. Drilling requires good coupling, but also the ability to adjust the size of the holes drilled. High power densities typically result in consistent coupling and hole drilling in materials that couple poorly, such as alumina. However, significantly increasing peak power simply to promote coupling could impact hole quality. In addition, an application may desire larger hole sizes, requiring increased focused spot sizes that reduce the maximum achievable power density. Traditionally, absorbing coatings are used to enhance surface coupling on alumina. IPG Microsystems has developed new methods in which enhanced coupling can be achieved by use of modified laser techniques and no coating is needed to www.industrial-lasers.com

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achieve coupling enhancement. However, depending on the particular situation, coatings might still be used to help overall quality by minimizing splatter and dross accumulation.

a)

39.5µm

303.2µm 32

The hole size machined can be adjusted by changing the diameter of the process fiber (for example, by changing laser and its fiber or by using a beam switcher/coupler connecting the laser feed fiber to

b)

38.6µm

280.7µm 07

FIGURE 3. Scribing at 300mm/s for both alumina and AlN (381µm thick). (a) represents the side view for alumina, while (b) is the side view for AlN.


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at a linear speed of 250mm/s (FIGURE 4). A L1 coating was applied Length: 43.35µm prior to the process and removed afterwards, helping to protect from spatter and recast. These results b) demonstrate why the QCW fiber laser is disL1 Length: 23.910µm placing the CO2 laser in the machining of ceramics such as alumina and AlN, allowing for higher throughputs and, because it can FIGURE 4. High-speed cutting of alumina (99.6% Al203) with a thickness of 381µm at 250mm/s. (a) represents easily be focused to the top view of the entrance, (b) is the top view of the exit, and (c) is the side view of the cut wall. spot sizes below 50µm in diameter, machining a larger process fiber), the beam delivFIGURE 3 shows scribing of alumina of smaller vias and finer structuring. On ery (varying the collimator and/or objec(99.6% Al203) and AlN 381µm thick, both the other hand, compared against shorttive focal lengths) and process parameat a speed of 300mm/s, and using a pulse er-pulse-duration lasers operating in the ters used such as pulse duration and/or d u r a t i o n below 50µs. Scribing of alunanosecond or picosecond regime, the pulse energy (i.e. peak power), or relaQCW laser often allows for much higher tive position of focus plane vs. work-piece throughput due to its higher surface plane. removal rates, while still achievFIGURE 2 shows scanning electron ing adequate machining quality. microscopy (SEM) images of ~90µm Recently, cutting of sapphire has exit holes drilled using the multimode been the focus of considerable attenQCW fiber laser in 381µm-thick AlN, tion because of its use in mobile phones. at a rate of over 100 holes/s. FIGURE 5 shows some of the typical shapes Similar results are obtained cut for the consumer electronics market for 381µm-thick alumina using the QCW fiber laser. with typical entrance of Thicknesses up to several millimeters ~100µm in diameter, can be cut at reasonably high speeds and ~70µm in diamewith good cut quality, avoiding cracks ter for the exit for over and chip-out and with an average surface 20,000 holes/part in packaging roughness typically below 2µm (FIGURE 6). applications. Sapphire parts with thicknesses of 0.4, 1, Typical positional accuracy and 3mm thick were cut at speeds around FIGURE 5. Examples of various shaped achieved is within ±5µm over 12, 9, and 3mm/s, respectively, with final parts cut in sapphire with a QCW fiber an area of 150 × 150mm, with speeds depending on geometry and quallaser as used for consumer electronics. the hole diameter variation better than 15% ity requirements. of nominal hole size for 100% of the holes. Both periodic and non-periodic hole patmina 635µm thick can be done using a lonLaser workstation terns can be machined at high speed with ger pulse duration around 100µs at a speed Depending on the material and the applihigh positional and dimensional accuof 200mm/s with individual pulses each cation, process development establishes racy, by using external encoder-based leading to a depth over 350µm. which laser and laser technique is better laser triggering. suited to meet manufacturing goals, thus A similar setup used for drilling can also Cutting with QCW fiber laser allowing for specification of equipment be used for high-speed scribing on these The QCW fiber laser also allows for highoptions. In addition to required machining ceramics, where a single pulse is used speed and -quality cutting of ceramics with quality, including dimensional and posito machine a blind hole into the material, no dross or chip-out. High-speed cutting tional specifications, additional considwith an appropriate pulse-to-pulse spacof 635µm-thick alumina was demonstrated erations for high-volume manufacturing ing needed to allow for a follow on breakat 140mm/s using the single-mode QCW include throughput needs as well as cost ing operation. laser, while 381µm-thick alumina was cut of ownership. c)

a)

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a)

b)

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thicknesses of 0.4, 1, and 3mm were cut at speeds around 12, 9, and 3mm/s, with final speeds depending on geometry and quality requirements. QCW fiber lasers can be combined with advanced workstations to drive high-volume, high-accuracy laser manufacturing in a variety of markets, from packaging to consumer electronics. ✺

c)

L L1 Length: 401.54µm L d)

500µm

FIGURE 6. Examples of cut quality in sapphire parts for various thicknesses when using the QCW fiber laser. (a) represents a 0.4mm-thick cross-section cut at 12mm/s, (b) is a 1mm-thick cross-section cut at 9mm/s, (c) is a 2.8mm-thick cross-section cut at 3mm/s, and (d) is the top view of a cut with no cracks or chip-out. To address these multiple and at times very distinct requirements, the laser workstations are tailored to the final specifications needs, namely in terms of positional and dimensional specifications and throughput. IPG Microsystems provides a series of workstations in which multiple laser types and beam delivery systems can be installed and are available for immediate sequential use. For example sapphire can be cut using a QCW fiber laser coupled to a high-gas-pressure cutting head, followed immediately by processing with a pulsed picosecond laser using a galvanometer-based machining approach to bevel or polish the edges of the machined piece.

Edge of cut

ACKNOWLEDGEMENTS

200µm

demonstrated on 635- and 381µm-thick alumina at 140 and 250mm/s, with negligible dross and no chip-out. Similar results were shown for AlN, but typically with a relatively lower throughput. Sapphire with

The authors express their appreciation to the following people for their assistance in preparing this article: Cristian Porneala, Xiangyang Song, Mathew Hannon, Dana Sercel, Sean Dennigan, Heath Chaplin, John Bickley, and Jeff Sercel. MARCO MENDES ([email protected]), ROUZBEH SARRAFI, JOSHUA SCHOENLY, and ROY VANGEMERT are all with IPG Photonics–Microsystems Division, Manchester, NH; www.ipgphotonics.com/ microprocessing.htm.

Conclusions

A new generation of QCW fiber lasers enables high throughput drilling, scribing, and cutting of materials up to several millimeters thick with high quality typical of micromachining needs. The high beam quality allows for small spots on target with high power densities, leading to coupling in materials that are typically transparent at NIR wavelengths, such as alumina and sapphire. If larger spots are required, then there are processing techniques that still allow for 100% coupling to these materials. High drilling rates of 300, 750 and 3000 holes/s were achieved on 635-, 381-, and 100µm-thick alumina, respectively; highspeed scribing at 200 and 300mm/s was demonstrated for 635- and 381µm-thick alumina; and high cutting speeds were www.industrial-lasers.com

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Laser cleaning prior to laser weld PROCESS REDUCES POROSITY FORMATION IN LASER-WELDED ALUMINUM ALLOYS

LIN LI

Other surface organic contaminants such as oil and grease can also contribute to the hydrogen gas trapping in the weld zones. Porosities generated by hydrogen trapping are normally small (3mm) Al alloys, keyhole welding is necessary to enable the deep penetration. Keyholes are formed as a result of high-pressure vapor/plasma generation during laser welding that result in a deep vapor hole due to the high recoil pressure as the vapor leaves the melt pool. Such a keyhole allows the laser beam to be absorbed in much deeper part of the material compared with that at lower-power-density, conduction-based laser welding. If the keyhole is not stable (when the material surface

A

luminum (Al) alloys are typically used in the automotive, aerospace/aeronautical, and sport industries due to their light weight, ease of forming/machining, and acceptable strength properties. Joining Al alloy panels and sheets is typically realized by mechanical riveting, arc welding, brazing, friction-stir welding, laser welding, and hybrid laser/arc welding. Laser welding can be very fast and generates low thermal distortions. However, laser welding of Al alloys often results in significant amount of porosity, thus very low joint strengths—typically 50–75% that of the parent material. There are a number of reasons for porosity generation in laser welding of Al alloys: 1. Surface contamination, including hydrogen in the surface oxide layer. Hydrogen (H2) is typically trapped in the surface oxide layer on the Al alloys’ surfaces. During laser No gap welding, they can be released and dissolved in the melt pool. Hydrogen Porosity solubility in liquid Al is much higher 20 than that in the solid. In liquid, the H2 solubility [1] is: log S = (2760) + 1 log P+1.356 2 T In solid, the H2 solubility [1] is: log S = (2080) + 1 log P – 0.652 2 T where S is the solubility of hydrogen (mL/100 g Al) at 273°K and 760 torr, T is the temperature (K), and P is the partial pressure of hydrogen (1 torr =0.133 mbar) [2]. From the above, it can be seen that the H2 solubility in Al liquid is about 20 times of that in solid. Thus, Al alloys can take in large amount of H2 in the molten pool and release them as porosity during solidification.

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Welding speed 35 (mm/s)

Fillet edge joints 0.2mm gap

2000 1mm

1mm

Crack Power 3500 (W) 1mm Pores

1mm

Crack 5300

50

1mm

1mm

FIGURE 1. The effect of laser power, welding speed and sheet gap on porosity formation in laser welding of AA6014 aluminium alloy [7].



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ing of aluminum alloys tension is greater Number of pores than the vapor 80 recoil pressure, key70 hole collapse) or the 60 50 material solidifies 40 too quickly, vapor 30 could be trapped 20 in the fusion zone 10 and forms large 0 0 20 40 60 80 100 120 140 160 porosities (typiPore size (µm) cally >0.5mm). 3. Vaporization of alloy elements. Material alloy elements such as magnesium (Mg), manganese (Mn), copper 100µm (Cu), and silicon (Si) can be vaporized and trapped in the fusion zone, forming porosity. These alloy elements can also play a role of modifying the material surface tension and viscosity. There have been a number of methods that can be used to reduce porosity in laser welding of light alloys; for example, the use of controlled gas jet [3], the use of an oscillating beam [4], welding with twin beams [5], and surface cleaning [6]. Porosity is usually less for conduction-limited laser welding with low laser-power densities, but the process efficiency is usually lower. In this article, a case study is reported on the use of a laser cleaning technique for the reduction of weld porosity in laser welding of automotive 6000 series alloy (AA6014) or commonly known as AC-170PX that has 0.5–0.7% Ai, 0.35 (max.) F, 0.2 (max.) Cu, and 0.05–0.2 Mn. During the Al alloy sheet forming process, the Al surfaces were coated with titanium and zirconium (4mg/m2) and a dry lubricant (AlO70 at 1.5g/m2) and left on the surface after their forming. The welding was carried out with a filler wire and a 5.3kW disk laser.

FIGURE 2. The typical porosity size distribution in laser welding of AA6014 alloy at a 5300 W laser power and a 50 mm/s (i.e. 3 m/ min.) welding speed [7].

porosity, but still too high. These porosities are typically