The car of the future will have to meet a growing number of expectations from society in terms of energy efficiency, fuel consumption, environmental protection, safety, driving comfort, and sustainability. One key focus will be reducing vehicle weight by using innovative materials and introducing multi-material systems to the production process. To exploit the full potential to reduce weight, thyssenkrupp Steel Europe has developed a state-of-the-art composite material called Litecor®. A team of experts at the Steinbeis Innovation Center for Intelligent Functional Materials, Welding and Joining Techniques, Implementation has now created a process for joining the materially thermally.
Litecor® is a combination of ultra-thin body plates with a polymer filler material (a core layer of plastic of varying thicknesses in between). As a composite material, not only does it weigh essentially the same as aluminum, it also offers a number of further advantages:
The key priority at the start of the project was to develop a production technique so that the material could be used in applications compatible with industry – and to do this quickly. This was because even when using the very latest cold metal transfer welding methods to carry out thermal joining, the plastic between the two steel layers was being damaged. These layers are 0.2 and 0.25 mm thick and the plastic was separating from the steel. The team at the Dresden-based Steinbeis Innovation Center set themselves the goal of developing a new thermal joining technique. Their aim was to use a defined joining geometry, such as a flange seam, not just with steel-plastic-steel composites, but also in combination with steel sheets. The project also involved developing a compatible burner technology with an automation option, plus suitable soldering materials. For the steel-plastic-steel composites, development work focused on applications like producing linear joins around the 2 by 3 meter connection between the vehicle roof and side parts. The experts had to address a number of factors: the concave nature of the seal, the narrowness of the metal weld, corrosion resistance, and a significant reduction in post-process finishing.
An innovative development approach was taken based on a non-transferring light arc process. This kind of energy source was primarily chosen because of the filler material. One of the key priorities was to avoid layer separation – delamination of the polymer – during joining. There was also a need to prevent damage to the zinc coating on parts that were going to be joined, mainly because zinc coating provides corrosion protection. It was, however, permitted to subject the materials to temperatures of up to 250°C. The phases formed in the area along the joins on the base material, and the joins themselves, had to be thermally stable at temperatures of up to 220°C. The key priorities with the seams were the flanged seams and the fillet seams welded on the lap joint in standard positions. The target joining speed was ≥ 1.0 m/min.
The problem faced by the Steinbeis experts from Dresden was highly specific to the material involved, so as a result, they decided to design and develop a kind of “tungsten inert gas (TIG) cold wire burner head.” A central aspect of their concept is the TIG burner. The filler material is fed into the plasma ray of the burner through a separate cold wire insertion device. The system consists of a burner head fitting, which includes the integrated inert gas supply and the filler material feeder, and a tungsten electrode integrated into the burner head. Unlike conventional TIG welding processes, the light arc is ignited between the tungsten electrode and the fusing filler material.
Working in collaboration with manufacturing partners and a committee overseeing the project, the team succeeded in using the new burner system to invert the polarity of the burner, thus significantly reducing the work function of the light arc. The project teams managed to orientate the burner and its energy source to explore a whole variety of parameters until they could control the heat input in such a way that the filler material within the composite received sufficient wetting and connection to the surface. This avoided delamination and composite imperfections.
The new TIG cold wire burner prototype was used by the team to produce samples that were then put through rigorous testing. The result: The samples fulfilled all requirements, meaning there is now nothing to prevent the technique from being used to produce flanged seams on Litecor ® with DC grade steel, offering joins of the quality required for car roofs. The new joining process and the equipment it requires are easy to implement and automate.
[Translate to English:] Dr.-Ing. habil. Khaled Alaluss, Oleg Nuss, Prof. Dr.-Ing. Gunnar Bürkner
Intelligent Functional Materials, Welding and Joining Techniques, Implementation (Dresden))