Materials combinations that reduce product weights (and thus energy consumption) are high on the wish lists of car manufacturers. Such products include parts that are made of a combination of steel and the lighter aluminium. In order to bypass the effects of changes in materials that occur when they are fused by standard welding methods, the car industry and many other sectors are now looking into a welding approach known as a "cold marriage of materials".
The approach has given a boost to atomic-scale detective work.
"Cold" alternatives to welding
There exist many "cold" alternatives to standard welding methods, all of which are characterised by temperatures much lower than those used in traditional approaches. Common to these methods is that the atoms in the two materials being fused form bonds between themselves that act to combine the materials together.
When components combining steel and aluminium are fused in this way, a thin zone or "interface" (as specialists call it) is formed that is neither steel nor aluminium. Instead, the marriage creates hybrids of the two pure materials within the body of the overall product.
Observing atomic structure
Materials researchers in Norway are now able to observe the new materials and the hybrid relations between them created by "cold marriage" welding. All this thanks to a new "peephole" approach that allows us to observe the atomic structure at the interfaces.
This has given us hope that one day we will be able to show the industry the range of materials types, as well as the cold welding methods and procedures, required to weld interfaces that can withstand the forces to which they are subjected.
The "peephole" is made up of a series of computer models that constitute interconnected "mathematical microscopes" at different levels of magnification. Computer models are tools that use equations to provide somewhat simplified descriptions of components of the world around us, and especially how selected components react to external influences.
Our models are based on images taken with electron microscopes, which are like "glasses" that can see individual atoms. We have carried out our model development work under the auspices of two of the Research Council of Norway's centres for research-based innovation – SFI Manufacturing and SFI CASA.
Researchers from SINTEF and NTNU are working together with several industrial companies from Norway and abroad. These include vehicle and automotive parts manufacturers, as well as goods manufacturers from many other sectors. In fact, knowledge about interfaces is relevant to a number of different products such as boats, vehicle parts, vessels and aircraft, furniture and electronic components.
This knowledge can be transferred to processes involving the welding of metals other than steel and aluminium, and we are planning similar studies involving plastics.
Focusing on interface atoms
Metals are made up of atoms organised in more or less well-ordered stacks. However, this pattern is disrupted at interfaces where the outermost layers of atoms encounter those of the other material. It's just like neighbours of different strengths having to link arms to form a chain. One of the aims of our research is to look into how these links have to be designed so that interface atoms maintain their structure when subjected to external push and pull forces, such as when a vehicle component is exposed to stresses on the road.
Another of our aims is to understand the frictional forces that act at the interfaces between production equipment and product materials when the latter are being shaped and machined. The aim here is to develop the optimal methods for these processes.
When we observe our models in the greatest detail, we see how individual atoms organise themselves at the interfaces that we are studying. Then we zoom out and let the model demonstrate the fine details of the different zones that make up the interface in question.
Finally, we let the model observe the product from the outside. By identifying the geometric form of the product and the orientation of the external forces, we obtain the last piece in the puzzle that we need to predict what forces the links between neighbouring interface atoms can withstand before they break and generate incipient cracks.
If our formulae for robust interfaces are to succeed, we have to evaluate the significance of many factors. Everything from the alloys that make up the parent materials, to the pressures and temperatures under which welding takes place. No small puzzle indeed! However, the results are so promising that we believe we will be successful.
This article was first published in the business daily Dagens Næringsliv (DN) on Friday 6 July 2018, and is reprinted here with the permission of the paper.