Perfect connections: IPSUS

The welding tool looks like a drill. Where the two materials to be welded meet, it plunges its elongated pin into the material and turns until its broad shoulder rotates on the material surface. The frictional heat causes the material to soften as it is stirred by the tool. The machine travels quickly along the interface of the two sheets. Behind, the material cools – and the welding seam is complete. Without additional filler material. However, the most important part remains hidden from the eye, namely the inner structure of the material in and around the joint. It decisively influences the material’s characteristics and, hence, its performance.

Instead of, as in the past, subsequently examining samples under the microscope, the researchers now want to observe in-situ how the microstructure of the various materials transforms as the joint is made. The Virtual Institute IPSUS was established in 2007 to observe in-situ metallurgical phenomena when joining. To achieve this goal the project leader Dr. Jorge dos Santos from the GKKS Research Centre in Geesthacht and partners from IPSUS have developed an array of joining and measurement instruments to be used at the synchrotron radiation source at the Helmholtz Centre DESY in Hamburg.

On the one hand, their insights will serve to optimise the joining process for each material: rotational speed, welding speed, axial force. On the other hand, and more importantly according to Jorge dos Santos, IPSUS also aims to support material developers. They can then directly design the alloys in such a way that they are weldable and so do not lose their excellent properties after fabrication. “Friction stir welding (FSW) has revolutionised production technology,” says Jorge dos Santos. This method, developed in the 1990s, also makes it possible to joint materials that were previously difficult or even impossible to weld. Aluminium alloys, for example, magnesium, or special steels. “The future belongs to these materials,” believes the scientist. For they form the basis of the lightweight engineering sector in aircraft, motor vehicles, and in the case of ODS steels would even be suitable for fusion reactors – if they can at some stage be jointed reliably and without any loss of quality.

The decisive advantage of friction stir welding is provided by the low process temperatures. This ensures that no fluid phase is formed, rather just a plastic phase. The material reorganises itself. Precipitates grow or coalesce, partially or completely dissolve in the matrix. These micro and nanostructures eventually determine the properties of the end product. How strong is it, how elastic, how resistant against corrosion or fatigue? The IPSUS researchers use high energy X-ray radiation capable of observing structures on the nanometre scale to take a look inside the materials. At their beam line at the HASYLAB in Hamburg, they have set up a special friction stir welding machine so that the synchrotron beam can sample the weld during its production. In the meantime, the beam detector delivers reliable (i.e. in situ) and very valuable results. In the future, the researchers want to use high-speed detectors in order to observe the processes with an even better temporal resolution.

However, the experimental work is not everything, far from it. “To gain the most complete possible understanding of the process it is supported by modelling work,” explains Jorge dos Santos. For the microstructure, the researchers are building on existing models for metallurgical processes which they feed with new data direct from the scattering experiments. “The hope is that we will, at the end of the day, perhaps no longer need any more experiments, but rather can run everything through on the computer,” he says. The modelling of the jointing process will also contribute to this. Its primary information lies in the temperature distribution and the visualisation of the material flow. This, too, has already been achieved, assures Santos.

Finally, the scientists continue to characterise the probes in order to compare these results with those produced by the models. It is clear that not all the required skills and expertise can be found under a single roof. This is why IPSUS is organised as a “Virtual Institute” so that the expertise existing at various institutions can be pooled. The GKSS is responsible for the process development and modelling as well as for the scattering studies and part of the characterisation. The KarlsruheInstitute of Technology is experienced in dealing with fusionmaterials, the Max Planck Institute for Iron Research knows all about so-called TWIP steels. The Universities of Manchester and Cranfield were consulted for modelling the microstructure and the process, while experts from the Ruhr University Bochum contributed their expertise in high-resolution electron microscopy. Last but not least, scientists at the TU Berlin can use the results for a similar jointing method. And industry is on board with an advisory panel.