ThinKing June 2019: Stirred not shaken - a new process connects aluminium and steel plates to high-strength hybrid components



Save at least ten percent of weight with hybrid components.

The Materials Testing Institute at the University of Stuttgart has developed a process, based on friction stir welding, for the high-strength welding of aluminium 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 light yet sturdy components to be made for application in the automotive industry, for example.

The Development Agency for Lightweighting Baden-Württemberg presents this innovation in the June 2019 edition of the ThinKing. Leichtbau BW GmbH uses this label to showcase excellent lightweight technology products or services from Baden-Württemberg every month.

At a glance:
- High-strength connections between steel and aluminium
- The hybrid metal plates can be deep drawn
- Applications: Bodywork → 10 percent lower body weight (reduced fuel consumption / emissions) or pool connector for e-mobility → use of cheaper aluminium rather than copper

“Friction stir welding of aluminium with steel is like baking a marble cake – the dark and light batter must be blended together, without producing just a brown mix. In aluminium-steel connections the mixed batter from the cake analogy corresponds to the brittle intermetallic phases,” explains Martin Werz from the Materials Testing Institute (MPA) at the University of Stuttgart. Friction stir welding 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 process and new tools for the high-strength friction stir welding of aluminium and steel sheets of different thicknesses. According to Werz, intermetallic steel and aluminium compounds made by “normal” arc welding are fragile. In contrast, “friction stir welding” can be used to produce high-strength and stable hybrid plates, says Werz. These are 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,” explains Werz.

The seam doesn’t give way in tensile testing
“We have tried out a range of tensile tests. The weld seam held even when test piece material well away from the seam had already given way,” explains 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,” says Werz proudly. According to Werz, the automotive industry in particular is a field in which the high-strength aluminium and steel hybrid structures could be used. As he puts it: “the advantages of thin sheet metal made of high strength steel and somewhat thicker aluminium sheets which has greater buckling strength can then be used in a single component by combining both materials, saving resources in the process.” 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, hybridisation 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 aluminium and steel in single components. This gives designers much more freedom in optimising structures and hence reducing weight,” says Werz.

Own process chain
“As well as the welding process, we have also developed other parts of the process chain for our new process,” says 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. According to Werz “the technology we have developed could also be used to join copper and aluminium sheets of different thicknesses. These can then be used to make pool connectors for e-mobility, for example.” The advantages: copper and aluminium of varying thicknesses could be welded together and the specific resistance of each material used when selecting plate thickness. “It would be possible,” for example, “to weld somewhat thicker but cheaper aluminium sheets with thinner copper sheets,” says Werz.

The environmental benefits: lower fuel consumption and reduced emissions
“Current estimates suggest that, without compromising safety, hybrid plates could be used in the automotive industry to reduce the weight of bodyshells by around ten percent,” says Werz. “Over the entire service life and distance driven by a passenger car the lower weight reduces fuel consumption and emissions,” explains Dr. Wolfgang Seeliger, Managing Director of Leichtbau BW GmbH.