ANALYSIS: DISSIMILAR METAL WELDING
An electromagnetic approach to dissimilar metal welding
Jennifer Heßmann, Marcel Bachmann and Kai Hilgenberg, of BAM, explain a new joining process that could help reduce the weight of vehicles
The automotive industry is facing the necessary reduction of CO2 emissions. Due to increasing safety requirements, customer demands in comfort and motor performance, as well as the use of heavy batteries in e-mobility, the weight of vehicles has grown rapidly in recent years. To save weight without affecting the functional properties of vehicles, heavy materials must be replaced with lighter alternatives. Modern lightweight concepts have therefore been developed, featuring multi-material designs that require the joining of dissimilar materials. Besides steel, aluminium is one of the most important construction materials for the mass production of automobiles. The joining of this material combination, especially by thermal
Figure 2: a) Example of a spot-shaped lap joint, b) formation of intermetallic phases at the interface between steel and aluminium alloy after melt displacement
processes, is a challenge to overcome. Issues are caused by different material properties such as melting temperatures and thermal expansion coefficients. Furthermore, steel and aluminium are not dissolvable within each other, thus leading to the formation
Figure 1: Single process steps of the new approach for joining dissimilar metals based on laser welding and melt displacement by induced electromagnetic forces
12 LASER SYSTEMS EUROPE AUTUMN 2021
of brittle intermetallic phases. These phases reduce the load capacity of the joint and often act as crack initiation points. It is therefore still necessary to develop joining technologies to reduce these problems. Electromagnetic melt pool manipulation offers many possible ways of optimising laser welding. At the Bundesanstalt für Materialforschung und-prüfung (BAM) in Berlin, many possible use cases were investigated. For example, supporting the mixing process when using filler materials in thick plate welding or the degassing of pores while welding aluminium die cast. Another application is melt pool support, when welding thick plates to avoid the drop out of
the melt pool due to its own weight.
One new possible approach
for the joining of dissimilar metals is based on laser welding and melt displacement by contactless induced electromagnetic forces. This joining technology is still in the experimental stage but has already shown promising results. The process steps are illustrated in figure 1. The joining partners, in this case steel and aluminium sheet metal, are placed in an overlap configuration whereby the upper joining partner needs to have the higher melting point, as well as a hole leading to the lower joining partner. The laser beam melts the lower sheet through this hole. An oscillating
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