New welding techniques for new materials
Around one million rivets hold a large passenger jet together. Photo: Fotolia/DWerner
What holds a large passenger jet together? The answer: around one million rivets. Indeed, the riveting of aircraft components is a proven and safe technology. But it means additional weight: the rivets themselves have a certain mass, and in addition the plates have to overlap in order to be riveted. If these could be welded instead, then depending on the aircraft type, up to 20 percent if its weight could be saved – and thus reduce fuel consumption and CO2 emissions.
HZG researchers have the vision of rivet-free aircraft. Photo: HZG/Christian Schmid
One of the alternatives is laser welding. Some manufacturers, such as Airbus, are already using this, but only for specific components such as the base of the fuselage. HZG researchers are working on expanding the new technology to include the joining of other components. Their vision: a rivet-free aircraft.
The challenge is that the plates used in the aviation industry are just a few millimetres thick. In welding, unlike riveting, they should not overlap but instead be joined edge to edge. To do this, work has to be extremely accurate, and must rule out the possibility of any obstructive defects entering the material during the welding process. Amongst other things, gas cavities may occur due to heating. These cavities then leave pores in the welded seam – and thus are potential starting points for damaging cracks.
To suppress this pore formation, the HZG experts are experimenting with special light sources: in what are known as fibre lasers, the light is produced inside a glass fibre. The advantage of these devices is that they can generate high-quality laser beams with a wide diameter. In these laser beams, the melt can solidify from the outside inwards. This gives the gas that is forming sufficient time to escape from the melt, thus leaving no pores behind.
Photo: HZG/Christian Schmid
A further task is to join alloys that are considered unweldable using classic methods. This applies, for example, to high-strength aluminium alloys with high zinc content. This zinc easily evaporates during welding.
This leaves behind a low-zinc, low-strength weld – the material is literally welded to death! The use of fibre lasers can also help here: the size of the laser beam and the application duration can be so finely tuned that hardly any zinc evaporates during welding. The Helmholtz-Zentrum Geesthacht has now patented the procedure.
The Hall of the Multi-Talented
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Friction Stir Welding
Photo: HZG/Christian Schmid
Another method developed at the HZG is friction stir welding. Unlike laser welding, the material is not fused but only heated sufficiently to become ductile and pliable. This occurs through friction. A special rotating tool presses with some force on the workpieces being joined.
The materials become hot and plastic and can be “stirred” together. The advantage of this method is that it can connect completely different materials, such as aluminium with titanium, or even with plastic. The main difficulty, however, is controlling the welding process. The join location must not become too soft or hot, otherwise there is a risk that the tool will puncture the material.
The Brazilian aircraft manufacturer Embraer, with whom HZG has been collaborating for many years, is already using friction stir welding for components in the cockpits of its short-haul jets. In future, the company also wants to produce mechanical reinforcements using this innovative procedure, as well as window frames – and save many rivets in doing so.
A Live Experiment: Joining with X-Rays
The materials researchers at the HZG have developed a modular welding machine with which they travelled to the PETRA III X-ray source at DESY in Hamburg. There, during friction stir welding, they were able to investigate live how the thermomechanically influenced zone of the weld seam behaves.