The Materials Testing Institute at the University of Stuttgart has developed a process, based on friction stir welding (FSW), for the high-strength welding of aluminum and steel plates. The plates can have different thicknesses and the seam is so stable that the hybrid plates can even be deep drawn. This enables lightweight yet sturdy hybrid metal components to be made for a variety of applications, such as in the automotive industry.
“Current estimates suggest that, without compromising safety, hybrid plates could be used in the automotive industry to reduce the weight of bodyshells by around 10%,” says Martin Werz from the Materials Testing Institute at the University of Stuttgart. Over the service life of a car, the reduced weight reduces fuel consumption and emissions.
FSW is a special type of welding process in which a rotating (stirring) tool traverses along the joint line with great force (friction) to intermix the two metal plates. The Stuttgart-based scientists have developed a modified FSW process and new tools for the high-strength joining of aluminum and steel sheets of different thicknesses. “Friction stir welding of aluminum with steel is like baking a marble cake – the dark and light batter must be blended together, without producing just a brown mix,” explained Werz. “In aluminum-steel connections the mixed batter from the cake analogy corresponds to the brittle intermetallic phases.”
According to Werz, intermetallic steel and aluminum compounds made by traditional arc welding are fragile. In contrast, FSW can be used to produce high-strength and stable hybrid plates, also known as “hybrid tailor welded blanks.” The special thing about the process developed by the Material Testing Institute is that “we can achieve a bigger cross-section area by butt and overlap welding the plates at the same time. This results in both higher strength and greater flexibility,” explained Werz. This enables the production of a single component providing both the advantages of thin sheet metal made of high strength steel and somewhat thicker aluminum sheets, which has greater buckling strength. Buckling strength refers to a component’s resilience to an elastic load perpendicular to the surface. In other words, the material bends like a feather and then returns to its original form once the load is removed, without buckling.
“Until now, hybridization in the process of constructing vehicle bodies has only been managed by assembling components made of different materials. Thanks to our development work it is now possible to combine aluminum and steel into single components,” said Werz. “This gives designers much more freedom in optimizing structures and hence reducing weight.”
The university has produced a number of parts using this process, which have held up well during testing phases. “We have tried out a range of tensile tests. The weld seam held even when the test piece material well away from the seam had already given way,” said Werz. The seam is so stable that it won’t burst even if the hybrid plates are worked by deep drawing. “Our welding seam even holds in complex geometries, such as curvatures.” Furthermore, the seam is so stable that the hybrid plates can even be deep drawn.
“As well as the welding process, we have also developed other parts of the process chain for our new process,” said Werz. This includes an innovative, energy-efficient heat treatment method and a special forming process which can be used with hybrid plates of different thicknesses.
Another application for hybrid components is in electric vehicles. “The technology we have developed could also be used to join copper and aluminum sheets of different thicknesses,” said Werz. “These can then be used to make pool connectors for e-mobility, for example.” By welding copper and aluminum of varying thicknesses together, the specific electrical resistance and cost of each material can be considered used when selecting plate thickness. “It would be possible, for example, to weld somewhat thicker but cheaper aluminum sheets with thinner copper sheets.”