4 September 2013

BMW use TRUMPF short pulsed lasers for the welding preparation process

The laser has proven to be a powerful cutting tool and can perform centimetre-deep welds – but its remarkable ability to process surfaces shows off its gentler side.

Apply a laser to five centimetre thick stainless steel and it will burn a hole right through it. Whenever laser light comes into contact with a material, it takes just fractions of a second for it to produce a glowing micro-inferno that eats its way through the material at a rate of several metres a minute. That is the laser we know and love: Light transformed into fire.

Yet the very first commercially successful application of laser light was in eye surgery. That’s because the real strength of a laser lies in its controllability — there is no other tool that releases so much energy yet can be so precisely metered. Penetration depth and heat input can be controlled with enough precision to allow lasers to ablate or activate surfaces right down to the micron in a process that is reliable, reproducible and automatable. While the 1980s and 1990s saw industry learning to appreciate the powerful side of the laser, it is its gentler side that has increasingly been sparking interest in the new millennium — its ability to prepare for joining processes, clean materials, remove coatings, and refurbish and smooth oxidized surfaces.

The cleaning skills of short pulse lasers
The BMW plant in Dingolfing, Germany, provides a good example: A few years ago, the Bavarian automaker was developing a process for welding axle differentials using lasers when the project team came up against one of the typical challenges of laser welding. The components used in the differential have a manganese phosphate coating, which shortens the running-in period. The welding laser has no difficulty evaporating the coating, but the high process speed does not provide enough time for all the vapour to escape evenly and cleanly from the joining gap. Instead, it mixes with the steel melt and introduces turbulence into the process, causing spatter and inclusions. “As long as you get the parameters right, there is no adverse effect on component functionality,” explains Oliver Müllerschön, automotive key account manager at TRUMPF Laser- und Systemtechnik GmbH. “But BMW works at a very fast pace and is looking to increase the output rate even more, so there is no time to reset the parameters every time there are variations in the process” Yet that was exactly what they were forced to do. The manganese phosphate coating, which is applied in chemical baths, varies in thickness so that the laser parameters had to be manually adjusted at regular intervals.

TRUMPF proposed the use of short-pulse lasers for the welding preparation process. The light from these lasers, guided by a scanner system, passes over the surface of the stationary crown gear. The high pulse peak power removes the coating from the weld joint without heating up the steel to any significant degree. “Our tests have shown that the laser ablation does not cause any structural changes whatsoever — and that’s exactly how our client wanted it,” says Müllerschön. BMW is now integrating the short pulse laser systems directly into its welding stations, further boosting the degree of automation in its production line.

Better bonding
The goal of weight reduction is not only boosting the use of laser welding, as shown by the BMW example, but is also increasing the use of lightweight construction materials such as aluminum and composites. These new materials require new joining techniques — and the laser is perfectly equipped to provide them. Aluminium is becoming more and more popular, and joining techniques, particularly in the aviation industry, increasingly include gluing as well as welding. However, the gluing process requires a bare metal surface — yet materials such as aluminium quickly acquire an oxide layer when exposed to air, and adhesives struggle to bond with the resulting “passivation layer”. Conventional methods use sandblasting or shot peening to remove this layer, before cleaning and degreasing the aluminium by chemical means and applying the adhesive. In contrast, laser systems integrated in the production line evaporate the unwanted passivation layer and vacuum it up just before applying the adhesive. This eliminates the need for subsequent cleaning and means that nobody has to provide and dispose of blasting abrasives and chemicals.

And that’s not all. The fact that the pulsed beams can be so precisely controlled and metered means that the workpiece can also be checked in the same pass. The laser uses a spot-by-spot technique to profile the surface with high-density squares, just microns deep. The melt accumulates on the edges of these squares. Adhesives and plastic sprays work their way into this pattern, which gives them plenty of space for bonding. And this method is not limited to flat sheets: By incorporating a multi-axis system or robot to guide the laser, it is possible to apply the method to virtually any component geometry.

Even modern fiber composite materials bond better after being exposed to laser light. In carbon fiber reinforced plastic (CFRP), the carbon fibers are embedded in a thermoset matrix, such as an epoxy resin, for example. To maintain the material’s strength, it is important to ensure that the adhesive preparation does not damage the carbon fibers. This is no problem for the laser thanks to its precise depth control. It simply roughens and activates the epoxy resin covering the fibers while cleaning and degreasing the underlying surface in the same step.
For the laser, roughening and smoothing are only a question of adjusting the parameters. To create a more aesthetically pleasing look for aluminium, for example, the laser can melt and smooth the surfaces prior to painting. The same principle allows lasers to create perfect plastic surfaces — an indirect effect achieved by polishing the interior of injection moulds through remelting with laser light. The procedure in each instance is the same, involving carefully metered doses of energy directed at a precisely defined series of points or surfaces.

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