Joining Technology

Modified Friction Stir Welding Creates Deep-Drawable Steel-Aluminum Hybrid Sheets

| Editor: Alexander Stark

The Materials Testing Institute of the University of Stuttgart has developed a method based on friction stir welding which can be used to combine aluminum and steel sheets of different thicknesses.

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With the modified friction stir welding developed by the MPA, thin but high-strength steel sheets and slightly thicker aluminum sheets can be joined. According to the MPA, this results in higher buckling stiffness of the overall structure. Here, the agitating friction head has already covered a few centimetres and opened the view on the seam.
With the modified friction stir welding developed by the MPA, thin but high-strength steel sheets and slightly thicker aluminum sheets can be joined. According to the MPA, this results in higher buckling stiffness of the overall structure. Here, the agitating friction head has already covered a few centimetres and opened the view on the seam.
( Source: MPA Stuttgart )

The seam is so stable that the hybrid blanks can even be deep-drawn. According to the researchers, this is a good opportunity to produce lightweight and resilient components that offer new approaches to automotive engineering. The State Agency for Lightweight Design Baden-Württemberg presents this innovation in its monthly Thinking for June 2019.

Friction Stir Welding is Like Baking a Marble Cake

“When friction stir welding aluminum to steel, it's like baking a marble cake - the light and dark dough should form a bond, but must not be mixed to such an extent that only brown, mixed dough is produced," explains Martin Werz of the Material Testing Institute (MPA) at the University of Stuttgart. The mixed dough from the cake analogy is called intermetallic phases in aluminum-steel compounds, which are actually brittle, the expert explains. Friction stir welding, however, is a special joining process in which a rotating tool (the "stirrer", if you like) moves along the gap to be joined with a lot of force (hence the friction) and thus connects the blanks.

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Simultaneous Butt and Overlapping Welding

The scientist has succeeded in modifying the process and developing new tools for friction stir welding, which enables the high-strength welding of aluminum and steel sheets of different thicknesses. If steel and aluminum are welded using the well-known arc welding process, fragile intermetallic compounds occur. “Friction stir welding, on the other hand, produces high-strength and stable hybrid blanks," specifies Werz. These are also called "Hybrid Tailor Welded Blanks”. The special feature of the MPA process is that the sheets can be butt welded and overlap welded simultaneously, thus achieving a larger cross-sectional area, which ensures greater strength and consequently also greater formability, explains Werz.

Weld Seam Withstands Even Complex Deep-Drawing Effects

“We've done several tensile tests. The weld seam has held, and the material of the specimens has already yielded far away from the seam - that's the way it should be," emphasizes Werz. The seam is so stable that the hybrid blanks can also be formed by deep drawing without the seam failing. “Even with complex geometries such as curves, the weld seam holds, says Werz. The researcher sees an area of application for the high-strength aluminum-steel hybrid structures above all in the automotive sector, where the advantages of thin sheets of high-strength steels and aluminum sheets with a slightly higher thickness can be exploited in one component. "The result are components that are joined together to conserve resources and are significantly stiffer to buckling," he explains.

Friction Stir Welded Hybrid Sheets Are Less Likely to Get a Dent

Werz defines: “Dent stiffness is the resistance of a component to a load in the elastic area that acts perpendicular to the surface - that is, the material yields like a spring and returns to its original shape when the load is relieved, without leaving a permanent "dent".

Until now, hybridization in body design has only been achieved by assembling components made of different materials with extra fasteners, such as rivets. This causes additional material and time expenditure. And MPA development can already combine aluminum with steel in individual components. "This opens up additional degrees of freedom in structural optimization that reduce weight on the vehicle," Werz is certain.

It also Works with Copper and Aluminum as Joining Partners

“In addition to the welding process, we have also developed other parts of the process chain for our new process," notes Werz. This includes, for instance, a novel, energy-efficient heat treatment method and a special forming method which can take into account the difference in thickness of friction stir welded hybrid blanks. "In addition, the technology we developed could also be used for welding copper and aluminum sheets of different thicknesses. This could become a new approach for manufacturing pool connectors for e-mobility," Werz imagines. The advantage: One could take advantage of the specific electrical resistances of the two metals when selecting the sheet thickness: For example, combining somewhat thicker but cheaper aluminum sheets with thinner copper sheets.

Body-in-White Weight Reduced by 10 %

“According to current estimates, it can be assumed that the use of appropriate hybrid circuit boards in automotive design will reduce the weight of the bodyshell by around 10 % while maintaining safety," predicts Werz. "If we consider the entire service life and mileage of a passenger car, the weight reduction noticeably reduces fuel consumption and emissions," adds Dr. Wolfgang Seeliger, Managing Director of Leichtbau BW GmbH, during the Thinking June 2019.

This article was first published by MM MaschinenMarkt.

Original by Peter Königsreuther / Translation by Alexander Stark

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