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Fraunhofer LBF Strength of EMPT-Welded Aluminum-Steel Joints

Author / Editor: Anke Zeidler-Finse / MA Alexander Stark

Electromagnetic pulse technology is a new joining process for steel and aluminum alloys. The Fraunhofer LBF shows that the vibration resistance of these joints also meets high requirements.

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Steel-aluminum joint after cyclic peel stress: failure in the aluminum.
Steel-aluminum joint after cyclic peel stress: failure in the aluminum.
(Source: Fraunhofer LBF)

Mixed welded joints are of great importance in lightweight design to realize future innovative designs, as they can be used to produce components that combine high strength and low weight in a targeted manner. Electromagnetic pulse technology (EMPT) is an innovative process for producing such joints. The sheets to be joined are accelerated to high speeds by a pulsed electromagnetic field and firmly bonded.

Micrographs from metallographic investigations conducted by the Fraunhofer LBF show that the joined sheets of the same type of aluminum alloy bond very well. Due to the high relative displacements of the sheets during the joining process, a wave-like joining structure is formed in the joining zone. "In the case of aluminum-steel joints, no significant intermetallic phases in the joint area can be detected on the micrographs, as is typically the case with other material-locking mixed joints. This is probably the reason for the high strength of the joints," explains Dr. Jörg Baumgartner, who is responsible for the research project at the Fraunhofer LBF.

Joining Zone Withstands Fluctuating Load

In order to evaluate the joints with regard to fatigue strength, the scientists of the Fraunhofer LBF carried out tests on shear tensile and shear tensile specimens. The central issue was whether it is possible to evaluate and design EMPT-joined joints with the concepts already known from conventionally welded joints.

The fatigue strength tests showed that the actual joint zone withstood the fluctuating load. In all specimens, the cracks under cyclic stress were always initiated at the sharp root notches of the overlapping joints. In the case of aluminum compounds of the same type, crack propagation always occurred through the base material. "We were also able to observe this behavior in the aluminum-steel specimens under peel stress. Under shear stress, the joining zone of these specimens showed such a high strength that the failure in the aluminum base material occurred outside the joining zone," says Baumgartner.

Strengths Similar to Conventionally Welded Joints

In order to perform a fatigue assessment, the research team built finite element models of the samples. This model shows the cracked notches with a standardized reference radius of 0.05 mm. An evaluation was carried out using the notch stress concept, which is the standard method for the fatigue strength evaluation of welded joints in many areas. It showed that the local stress limits of all specimens with failure of the aluminum material are comparable to conventional, for example laser or MIG, welded aluminum joints.

Thus, the high cyclic strength of the joints was proven by the analysis. In addition, designers now have a method at their disposal which allows them to reliably design joints already udring the development phase.

The IGF project “Failure behaviour of mixed welded joints under multi-axis crash-like and fluctuating load based on the example of EMPT sheet metal welding”, IGF project no. 17883 N/1, of the DVS Research Association for Welding and Allied Processes, was funded by the Federal Ministry of Economics and Energy on the basis of a resolution of the German Bundestag via the AiF as part of the program for the promotion of Cooperative Industrial Research (IGF). The research is carried out together with the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg.

This article was first published by MM MaschinenMarkt

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