Everybody is happy when there’s nothing to see: seams on the tailgate, water channel and vehicle roof must be durable but otherwise not noticeable. That’s what automotive manufacturers and their customers want. Rippled and frayed seams are completely taboo and perfect transitions the ideal. While welding usually has the upper hand in car body construction, it is no good for parts that will be visible once the vehicle is finished. The process does not meet optical requirements – welding seams are too thick, rippled and frayed. Laser brazing has established itself in this area of automotive manufacturing because it can do what laser welding can’t: namely, produce durable seams that no-one can see later.
Secure micro-level bonding
With laser brazing a solid-state laser heats up the filler material – usually a copper-based wire – to more than 900 degrees Celsius and thus to above its melting point. The soft material is evenly distributed in the joint gap, it wets the base material and bonds the two parts together after it has cooled off. Thanks to the even outflow of the copper wire, the seam can hardly be seen later.
Laser brazing is different than welding because the base material is not melted by the laser beam, it is simply wet by the filler material, which delivers a micro-level bond between the two mating parts. This is a key factor, because melting the two materials would increase corrodibility, as well as reducing the optical quality of the seams.
The two mating parts bond on a micro-level. (Photo | TRUMPF)
With Laser brazing the base material is not melted by the laser beam, it is simply wet by the filler material. (Photo | TRUMPF)
It’s not just because of the attractive seams that automotive manufacturers across the globe prefer this. Other benefits include its good gap bridging, low heat input and high seam durability.
In tensile tests, the base material is more likely to tear than the seam. That is due to the filler wire – usually a CuSi3 wire. On the one hand, this has a melting temperature below that of steel and lends itself to filling the joint gap, while on the other hand, it is also very durable. Moreover, laser brazing is a fast process: OEMs currently work at a process speed of between 1.8 and 4 meters in standard applications.
Proven method, new challenges
So laser brazing has firmly established itself in the automotive industry. But new materials currently entail new challenges: hot-dip galvanized metal sheets are currently very popular in body construction. In the foreseeable future, these will no doubt replace the electro-galvanized sheets used to date because they are more resistant to corrosion as well as being cheaper and more readily available worldwide.
However, the fact that hot-dip galvanizing is less uniform as a coating makes it harder to achieve the declared goal of laser brazing – namely a non-porous and spatter-free seam that is no longer visible after painting. A project team at TRUMPF was tasked with finding a solution. For their testing they used a six-kilowatt disk laser and adaptive laser processing optics with integrated seam tracking.
A special trick opened the door to the solution – shaping the laser beam. The developers divided it into three partial beams with different diameters and powers. Small focal points play an important role in this context. They prepare the metal sheet by removing the zinc coating and preheating the base material. Everything then runs smoothly in the following brazing process, meaning the hot-dip galvanized sheet can be joined without any spatters and pores – at soldering speeds of up to 4.5 meters per minute at present.
Yvonne Gürtler, Phone: +49 7156 303-31284, E-mail: email@example.com