Enter the world of green cars

July 1, 2011
One of the eyecatchers at the 2011 Laser World of Photonics was the beautiful Tesla Roadster V2.5 Sport shown on the cover of this issue.

Munich, Germany - One of the eyecatchers at the 2011 Laser World of Photonics was the beautiful Tesla Roadster V2.5 Sport shown on the cover of this issue. Fraunhofer ILT (Aachen) chose this battery-powered sports car – capable of 0 to 60 mph in a little over 3 seconds – to make a point. This performance was achieved lugging around a half-ton of laser-welded batteries.

FIGURE 1. The author inside the Tesla Roadster V2.5 Sport.

I couldn't resist the offer to sit in the Tesla, even though it was a bit of a problem getting my 6-foot-plus frame into the cockpit. Once inside, I was surprisingly comfortable. The real challenge was getting out (FIGURE 1), which was accomplished with the assistance of Dr. Reinhardt Proprawe, Director of ILT. Only a scheduling conflict prevented me from trying the Tesla out later on a test track outside the Messe halls.

FIGURE 2. The battery cells are in between the aluminum cooling plates. The cell connectors are welded together using 3 weld seams, which are welded after each other. The connector on top is always the aluminum connector with the nickel plated copper connector being below, so it is a serial connection.

This car shows the possibilities of existing e-cars (electromobile cars) and also demonstrates the challenges of laser applications in processing of batteries and CFK / GFK lightweight components. The parts used in the Tesla are mainly hand-made, therefore, the car is very expensive. Also, since the batteries in all e-cars today are very heavy, ILT is investing in R&D to join and cut fiber reinforced materials and to weld Cu/Al contacts in batteries to improve the power/weight ratio for batteries and help to realize lightweight parts in the body area of e-cars.

More than 30 Fraunhofer Institutes are involved in developing alternative transportation systems in a joint project called "Fraunhofer System Research for Electromobility (FSEM)". The aim is to develop prototypes for hybrid and electric vehicles to support the German automotive industry as it makes its first inroads into electromobility. The German Federal Ministry of Education and Research (BMBF) is funding this project to the tune of 44 million euros from the federal economic stimulus programs I and II.

Fraunhofer ILT researchers are working on the Frecc0 (FRaunhofer EConcept Car 0), where they are welding the battery cell connectors of a vehicle based on the Artega GT (www.artega.de/d/) of Artega Automobil GmbH & Co. KG, a small German car manufacturer.

Fraunhofer ILT has been tasked with making the electrical contacts between the lithium-ion batteries. The high energy density of these rechargeable lithium-ion batteries has prompted an increasing number of car manufacturers to use them as energy storage systems in electric vehicles. Vehicle battery packs consist of multiple cells, which have to be interconnected via electrical contacts. Cells in flexible casings known as pouch cells are frequently used in such applications. They have strips of copper and aluminum foils, respectively, leading from the electrode layers and extending from the envelope to form positive and negative terminals.

To build a battery pack, the terminals of the individual cells have to be connected in series by linking the anode of one cell to the cathode of the next cell, and so on. This is normally done using screw connections because aluminum and copper are dissimilar metals and thus difficult to bond reliably. Conventional welding and soldering techniques are not a practical alternative because the flexible cells risk being damaged by the heat.

Using a joining method to connect the terminals, instead of screw connections, offers the advantage of a much lower electrical resistance and hence increased battery efficiency. Laser welding is well suited as a joining method because it allows the process to be automated, and it can be performed with a relatively low energy input if the process parameters are set appropriately. As part of the FSEM project, laser welding is therefore being qualified as a joining technique for the described task.

In experimental tests, process parameters make it possible to produce reproducible welded connections between the terminals with an adequate current-carrying capacity (FIGURE 2). Current research is focused on qualifying the long-term stability of the connections.

Rechargeable lithium-ion batteries are at present the most promising energy-storage technology for electromobility applications. The availability of efficient energy-storage systems that can be manufactured at low cost is a vital enabling factor for this fast-growing branch of industry. Laser welding can help manufacturers produce the high-performance batteries needed for mobile applications more cost effectively.

ILS thanks Dipl.-Ing. Benjamin Mehlmann ([email protected]) of Fraunhofer ILT (www.ilt.fraunhofer.de) for his contribution to this Update item.
– David A. Belforte

Laser welding thermoplastics with customized beam intensity

Dortmund, Germany and Sarnen, Switzerland - When laser polymer welding, the typical laser beam profile has a Gaussian shape. Consequently, more energy is deposited in the middle of the laser spot than at its edges, leading to an eventual overheating and burns in the center of the weld seam or to inaccurately defined weld seam widths. To avoid such situations and to increase the process robustness, alternatives, based on the use of advanced beam shapers and high brilliance laser sources, have been developed and demonstrated recently by LIMO Lissotschenko Mikrooptik GmbH (Germany) and Leister Process Technologies (Switzerland) as part of the European POLYBRIGHT project.

FIGURE 1. M-shaped beam profile generated using DOEs for a fiber laser.

Customized laser beam profiles for polymer welding enable an optimal energy input into the joining area and lead to enhanced process performance. These beam profiles can be achieved using diffractive or refractive optical elements. In a first approach, Leister, located in Sarnen, developed a set of diffractive optical elements (DOE) in order to achieve different beam profiles such as top-hat or M-shapes.

FIGURE 2: Bi-convex lens (left) and lens arrays (below) for generation of M-shaped beam profiles.

These DOEs take advantage of the high focusability of fiber lasers, and theoretically they act as a beam splitter. The small spot of the fiber laser is replicated many times across a desired beam diameter to achieve the desired profile. Furthermore, different weights are possible for each beam copy. As shown in FIGURE 1, an M-shaped beam profile with a diameter of 1 mm can be achieved with a DOE that copies the fundamental beam spot of the fiber laser within a 40 µm spaced grid. The replicas at the edge of the beam have a higher weighting factor than in the middle. Therefore, the laser intensity is higher in this area. This beam shaping approach works not only with fiber lasers but also with normal diode lasers. In this case the copies of the fundamental beams overlap and give a continuous intensity profile similar to the one shown in FIGURE 3.

LIMO, in Dortmund, demonstrated the possibility to achieve an M-shaped beam profile using refractive optical elements (ROE) such as bi-convex lenses or radial symmetric lens arrays (FIGURE 2). In the first case, the developed bi-convex lens is mounted inside a LIMO diode laser module and generates the recorded M-shape profile shown in FIGURE 3. For the second possibility to generate such a beam profile, the lens arrays can be used for any laser source available. Nevertheless, the same beam profile can be achieved for both concepts.

FIGURE 3. M-shaped beam profile generated using ROEs.

First experimental results were performed by each company. The DOEs indicate clearly that for the flat-top profile (FIGURE 4, left) the process window is narrower compared to the optimized M-shape profile (FIGURE 4, right). Additionally, the weld seam width for the flat-top profile changes with power, whereas for the M-shaped profile the weld seam width stays nearly constant. Similarly, the ROEs-generated M-shape confirmed this result.

FIGURE 4. The flat-top profile (left) with a narrower process window than the optimized M-shape profile (right).

For more information from Leister Process Technologies or LIMO Lissotschenko Mikrooptik GmbH, contact Ulrich Gubler at [email protected] or from Fraunhofer Institute for Laser Technology ILT, contact Andrei Boglea at [email protected].

Compressed air laser cutting

Lüneburg, Germany - Compressed air laser cutting fulfills several important functions at the same time during the laser cutting procedure. The targeted air flow drives the material melt out of the cutting gap at the same time as it cools the heat influence zone at the focal spot. The air also protects the optical lens from soiling and avoids the ignition of aerosol cutting emissions

These effects provide high-quality cutting results and are evident due to: neat material surfaces, fewer oxidization marks, smaller cutting widths, uniform cut edges, and the back of material is free of smoke deposits.

Clean, dry compressed air is delivered to the material that is being processed during the cutting procedure at a preset pressure of between 1 and 6 bar through a special copper cutting nozzle. The cutting nozzles, which have different opening cross-sections, are easy to change and optimize the cutting results considerably. Compressed air is the most cost-effective cutting gas for industrial applications. Pure, technical gases would be an additional cost factor and are not usually required in this case. A vacuum table concept, located beneath the material support, completes the process chain with the compressed air/cutting gas mixture systematically collected and efficiently led away.

The advantages of the gas jet assist nozzles are: exact process gas guidance above or into the cutting gap; focus lenses protected from flying sparks, vapors, and particles; and easy-to-change nozzles that sow no wear.

Different effects are achieved depending on the nozzle diameter for applications such as technical foils, textiles and non-woven fabrics, wood, and MDF, where a fine jet of compressed air achieves particularly neat cut edges and material surfaces. They are also appropriate for applications in cutting acrylics and other thermoplastics and laser engraving where a wide jet generates a cooling effect on the surfaces. A weak wide jet is beneficial to engraving applications and makes it possible to have polished edges in acrylic glass.

High-quality optical lenses, safe purging gas, copper cutting nozzles, clean compressed air and a suitable table and extraction concept are added value for critical laser users only if the correct combination is used. These important technical special features form the framework for successful and safe laser applications.

ILS thanks eurolaser GmbH www.eurolaser.com for sharing this technical information with readers.

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