Keeping cars light on their wheels
Laser welding is an established and increasingly important tool within the car-making industry. The process provides improved production speeds, simplified design and the ability to meet the demands of an industry that is continually looking for lighter vehicles to give improved fuel efficiencies.
The focused beam used in laser welding generates extremely high intensities on the work piece, with focal points of around 0.4mm to 1mm in diameter. Because of this narrow working area, laser welding is suitable for nearly every weld geometry, including butt-welds and overlap welds of sheet metal panels. Laser welding also produces a strong joint, opening up new design possibilities.
Dr Jan-Philipp Weberpals, team coordinator for laser beam technology/sensors at Audi, told Laser Systems Europe: ‘The laser offers certain advantages as a tool that cannot be realised by other joining methods. They include, first of all, the ability to design applications such as a zero-gap roof joint with incredible precision in steel and aluminium bodies. For another, the very precise heat application enables small flange widths, such as on the door and windshield, which increases the visual field.’
One such new design, made possible by laser welding techniques at Trumpf, is the underbody of the StreetScooter Model ‘Work’, which has been developed for Deutsche Post DHL Group (German Postal Service) by RWTH Aachen University spin-off StreetScooter GmbH – now a 100 per cent subsidiary of Deutsche Post DHL Group.
The underbody of the electric vehicle is a completely flangeless design, which uses plug-in connections to replace the conventional flanges. This translates to lower weight and reduced running costs when compared with traditional car body designs. ‘The underbody was a great design to fully utilise the laser welding process, as the accessibility and joining geometries were not suitable for arc welding techniques – but perfect for laser welding processes,’ commented Marc Kirchhoff, industry manager automotive at Trumpf.
The laser welding process was developed using tests on sheet metals to perfect the new geometries required. Various material thicknesses were tested and the components were produced by cutting, bending and, finally, assembling and laser welding them together.
Laser welding brings many benefits to the production process, as Kirchhoff explained: ‘There is a significant reduction in the number of productions steps and this means less handling and logistic steps so the time to production is reduced.
‘Although a reduced production time is not a necessity for this specific project [StreetScooter] where only a few vehicles are required per week, it is an obvious benefit to the wider automotive industry,’ Kirchhoff added.
No saw cuts or holes were required in the StreetScooter’s underbody and the number of bends in the material was reduced from 102 to 55. There is also an 8 per cent reduction in the weight of the design even when the same material was used to produce it.
The need for improved production speeds is present across the car making sector, as Kirchhoff explained: ‘The laser welding process needs to be fast and work around the world as the automotive industry is a global one that demands speed. This is achieved by creating an easy to handle and automated laser welding process.
‘We need intelligent systems that can automatically detect the quality of the weld. We also need to design cost effective systems within the automotive industry to improve efficiencies and lower footprints,’ Kirchhoff added.
Aluminium, once the material of the niche premium car market, is now slowly being adopted within the mass automotive market due to its reduced weight and the reasonable additional costs of this metal, when compared with traditional materials such as zinc-coated steel.
Axel Luft, sales manager global automotive at Laserline, explained: ‘Resistance spot welding is a cheap and widely used technology to join steel components, but aluminium resistance spot welding is not a popular option because of the high conductivity of aluminium. Therefore, laser joining is a good alternative to join aluminium.’
Laser joining techniques have paved the way for new design opportunities for a vehicle’s outside appearance. Laser brazing of zinc-plated sheet metal or laser welding of aluminium on the exterior parts are two such unique joining technologies.
The joint is sealed and has a smooth surface, ready for coating. Luft said: ‘Laser joining, either the brazing of steel or welding of aluminium, on the outer skin is special because you can directly paint it – this you cannot do with other joining techniques.’
Luft added: ‘After colour coating, the seam is no longer visible, giving the vehicle a better appearance and also reduces manufacturing costs. Audi reported cost reductions of €10 per car due to brazing of roof to side panel joints.’
Remote laser welding
Remote laser welding is a trend sweeping the automotive industry due to the increased flexibility and high operational speed of such systems. The process uses the combination of a high power, high quality laser with a scanner unit and an automated control system.
Audi’s Weberpals said: ‘A new approach within the industry is remote laser beam welding of aluminium parts. The method enables effective heat management control during the weld and exact positioning of the beam in relation to the trimmed edge to prevent hot cracking because of the precise control of weld penetration. The gap dimension between parts can also be determined during the joining process, and it can be reduced by control strategies.’
Weberpals added: ‘Process efficiency is boosted by eliminating the need for supplemental welding wire and protective gases, and this reduces ongoing production costs. This reduces the amount of heat input to the part by nearly one-half, which has beneficial effects in terms of reducing heat-related distortion. The increased feed rate and the lower power required by the laser beam reduces CO2 emissions by around 25 per cent.
‘All of these advantages pay off, especially the ability to implement lightweight designs with smaller flanges. The welds in the door frame of the Audi A8 illustrate this well. Audi is the first premium car manufacturer to achieve the USP of being able to use remote laser beam welding to join conventional aluminium alloys,’ Weberpals added.
The StreetScooter welding system also used remote optics, which incorporated a new scan-line system with integrated online seam tracking. Seam tracking, also known as joint tracking, involves real-time tracking just ahead of where the weld is being deposited. This allows for robot or machine trajectory shifts, and also allows for adaptive control so that adjustments to power and focal position, or travel speed can be made to change the weld bead formation. These intelligent optical systems increase productivity and quality.
A versatile tool
There are plenty of applications for laser welding within car production. ‘Laser welding is applied throughout today’s automotive industry. It [the laser] is used to create the gears and engine of a vehicle, clean and then join vehicle components like car body parts and even mark those parts with unique numbers so they are traceable,’ Kirchhoff commented.
Manufacturing powertrain components, car parts such as gears that generate power and deliver it to the road surface, is one area where the laser is used heavily. Dr Wolfram Rath, product manager of laser sources at Rofin-Sinar Laser, explained: ‘The fast and precise [nature of laser welding] technology enables the car maker to increase the torque on the individual powertrain element and design it [to be] lighter and smaller. The result is, for example, a lighter transmission with an increased number of speeds and fast dual clutch techniques for high comfort and less fuel consumption.’
Laser welding is used to manufacture a diverse range of parts, from motor to air bag components. Rath commented: ‘Modern motors use high pressure injection systems for precise fuel injection for homogeneous ignition and combustion. These complex injection systems consist of many single parts assembled, joined and laser welded step by step with high speed, minimum distortion and highest quality in mass production.
‘All modern cars use a manifold of airbags to protect passengers in case of an accident. Lasers are used to weld inflator components, but also to cut the airbag textiles, and to weaken the interior by laser perforation, which enables a faultless function and the invisibility from the [car] interior,’ he added.
This popularity for laser welding is a result of the demands of the automotive industry, as Andreas Büchel, product manager laser machines for plastics welding in Jenoptik’s lasers and material processing division, explained: ‘Nowadays the trend in the automotive industry is for lighter materials and cost reduction. Laser technology is a very valuable manufacturing tool, because it allows contact-free processing. Welding strengths are also comparable or even higher than competing technologies.
‘Welding speeds are sufficiently high to meet customer specifications and will further increase due to the development of high-power laser diodes. And, last but not least, laser welding supplies outstanding costs of ownership to the customers,’ Büchel added.
As the automotive industry demands lightweight vehicles, there is a move to designs that incorporate a mixture of materials. For example, instead of the conventional and simple joining of two steel components, the industry is now investigating how to join dissimilar materials such as steel and aluminium, or even metals with plastics.
Kirchhoff said: ‘The industry and OEMs are pushing us really hard for lightweight vehicles to reduce fuel consumption and laser welding is the key to achieving an easy and cost effective process for joining dissimilar materials.’
The joining of dissimilar materials presents considerable challenges. In principle, a laser can weld any material that can be joined by conventional processes. The ability to weld dissimilar metals depends on many factors, including the physical properties which have a high influence on the amount of energy coupled in and the heat transfer across the weld.
When welding dissimilar metals, good solid solubility is essential for sound weld properties. This is only achieved with materials that have compatible melting temperature ranges. If the melting temperature of one material is near the vaporisation temperature of the other, poor welds are obtained and often form brittle intermetallics. The joining of dissimilar materials is a process that is still being developed within the industry.
And there are many new developments to come, thanks to the possibilities laser welding will introduce to the automotive industry. Büchel said: ‘The advantages of this processing method will open up new production methods and enable new products.’
This also means that the laser welding technologies and methods behind the new designs will have to keep pace with the demands of the automotive industry, as Büchel added: ‘The products are getting more complex and gain more and more functions. Therefore the processing machines must get more and more flexible.’
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